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U N I V E R S I TAT I S O U L U E N S I S

ACTAC

TECHNICA

OULU 2007

C 283

Virpi Krger

POISONING OF AUTOMOTIVE

EXHAUST GAS CATALYST

COMPONENTS

THE ROLE OF PHOSPHORUSIN THE POISONING PHENOMENA

FACULTY OF TECHNOLOGY,

DEPARTMENT OF PROCESS AND ENVIRONMENTAL ENGINEERING,

MASS AND HEAT TRANSFER PROCESS LABORATORY,UNIVERSITY OF OULU

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A CTA UNIVE RS ITA T I S O

C Techn i c a 2 83

VIRPI KRGER

POISONING OF AUTOMO

EXHAUST GAS CATALYS

COMPONENTS

The role of phosphorus in the poisoni

Academic dissertation to be presented,

the Faculty of Technology of the Univ

public defence in Kuusamonsali (Au

Linnanmaa, on November 10th, 2007, a

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Copyright 2007Acta Univ. Oul. C 283, 2007

Supervised by

Professor Riitta L. Keiski

Reviewed by

Professor Edd Anders BlekkanProfessor Jose Luis Garca Fierro

ISBN 978-951-42-8607-0 (Paperback)ISBN 978-951-42-8608-7 (PDF)

http://herkules.oulu.fi/isbn9789514286087/ISSN 0355-3213 (Printed)ISSN 1796-2226 (Online)

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Krger, Virpi, Poisoning of automotive exhaust gas catalyst compo

phosphorus in the poisoning phenomena

Faculty of Technology, University of Oulu, P.O.Box 4000, FI-90014 Univers

Department of Process and Environmental Engineering, Mass and He

Laboratory, University of Oulu, P.O.Box 4300, FI-90014 University of Oulu, F

Acta Univ. Oul. C 283, 2007Oulu, Finland

Abstract

The aim of this thesis project was to gain new knowledge on the effect of phosp

activity and characteristics of automotive exhaust gas catalyst components. Th

of phosphorus and calcium were also studied.

The first test series of powdery catalyst samples contained Rh and oxide (Tsecond, Pt and oxide or ZSM-5 (Test series 2). The catalysts were analyzed wh

ageing and phosphorus poisoning procedures developed in this work. The pro

adding poison via impregnation in an aqueous solution (for Test series 1) and

under hydrothermal conditions (for Test series 2). The poison compounds form

the washcoat were studied by using physisorption analyses, SEM, TEM, XRD,

poison content of the samples was determined by ICP-OES and XRF. Lab

measurements were done to investigate the catalytic activity. Thermodynamic c

to obtain information about ageing conditions and phosphorus compounds formPhosphorus decreased the catalytic activity and the characteristic surface a

Addition of calcium to a phosphorus-poisoned catalyst was found to have even

on the catalysts' activity. The poisoning methods developed in this study

phosphorus compounds as can be found in vehicle-aged catalysts. Phospho

cerium, zirconium, aluminium, and titanium phosphates. Phosphorus was det

phosphorus-containing compounds were not observed. Phosphorus poisoning

phase at high operating temperatures and with high oxygen and water contents.

the role of phosphorus poisoning was more pronounced than the role of hydroPhosphorus poisoning mainly affects the oxide components used in this study,

The results can be utilized in the development of catalytic materials and cata

can better tolerate phosphorus poisoning under hydrothermal conditions. Th

applied in evaluating the effects of phosphorus on different catalyst compositi

the age of commercial catalysts.

Keywords: calcium, catalyst activity, deactivation, diesel catalysts, pho

poisoning, rhodium,three-way catalysts, zeolite catalyst

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To my

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Preface

This study was carried out in the Department of Process and En

Engineering at the University of Oulu during the years 20032006.

time the author was also a student in the Graduate School on Function

In addition, the work was a part of the Catalytical Materials: Cha

and Control of the Surface Poisoning Phenomena (POISON) projec

the Academy of Finland. The study was carried out in co-operation Oy, the Department of Chemistry at the University of Oulu, and the

Materials Science at Tampere University of Technology. Part of th

performed in the School of Chemical Engineering and Industrial Che

University of New South Wales, Australia, during a research visit in 2

I cordially thank Prof. Riitta Keiski, the supervisor of this thesis

the opportunity to work in her research group in the Mass and H

Process Laboratory. I also highly appreciate her expert knowledge

and the support she gave me during the work.

My warm thanks are due to the advisor of the work, Prof. Ulla L

help and encouragement were invaluable. M.Sc. Marko Hietikko,

Kanerva, Dr. Minnamari Vippola, Prof. Toivo Lepist, Prof. Risto L

Katariina Rahkamaa-Tolonen, M.Sc. Aslak Suopanki, Dr. Kauko K

Dennys Angove, Dr. David French, Lic.Tech. Juha Ahola, andTurpeinen are acknowledged for their successful co-operation in

work. I also express my thanks to Prof. David Trimm, Prof. Noel C

Yun Lei for increasing my knowledge in the field of catalysis

unforgettable time I spent in Australia.

I thank Mr. Jorma Penttinen and Ms. Hannele Nurminen for the

help in developing the test procedures and in carrying out the experimthank M.Sc. Katri Kynknniemi for her assistance in the laboratory w

I present my gratitude to Prof. Edd Anders Blekkan and Pro

Garca Fierro, who reviewed the manuscript of my thesis. Mr. Kei

acknowledged for linguistic corrections.

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Finally, I present loving thanks to my family. I thank my spouse

for bringing joy and a positive challenge into my life. I most cordialmother Aira, father Heikki, sister Anne, and brother Vesa for their con

and support. This thesis is dedicated to my parents, who always encou

my studies.

The work was funded by the Graduate School on Functional Surf

Academy of Finland. I also gratefully acknowledge the financial supp

the Faculty of Technology at the University of Oulu; Fortum FounUniversity Scholarship Foundation; Foundation of Technology; Jenn

Wihuri Foundation; The Finnish Cultural Foundation, The Regio

Northern Ostrobothnia; Finnish Konkordia Fund; and Finnish Catal

Ecocat Oy is acknowledged for donating catalysts for the laboratory t

Oulu, August 2007

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List of abbreviations

A/F ratio Air-to-fuel ratio

ATR Attenuated Total Reflection

BET Brunauer-Emmet-Teller theory

BJH Barrett-Joyner-Halenda theory

DOC Diesel Oxidation Catalyst

EDS Energy Dispersive X-ray Spectrometer

EDTA Ethylene Diamine Tetraacetic Acid

EGR Exhaust Gas Recirculation

EU European Union

FTIR Fourier Transform Infrared Spectroscopy

HC Hydrocarbon

ICP-OES Induced Coupled Plasma Optical Emission Spectroscopy

OSC Oxygen Storage Capacity

PGM Platinum Group Metal

PM Particulate Matter

SCR Selective Catalytic Reduction

SEM Scanning Electron Microscopy

SOF Soluble Organic Fraction

T50 The temperature of 50% conversion of the feed componeTEM Transmission Electron Microscopy

TPD Temperature Programmed Desorption

TWC Three-way Catalyst

XRD X-ray Diffraction

XRF X-ray Fluorescence

ZDDP Zinc Dialkyldithiophosphate

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List of original papers

This thesis is based on the following publications, which are referred

by their Roman numerals:

I Krger V, Hietikko M, Lassi U, Ahola J, Kallinen K, Laitinen R & Kei

Characterization of the effects of phosphorus and calcium on the ac

containing catalyst powders. Topics in Catalysis 30/31: 469473.

II Krger V, Lassi U, Kynknniemi K, Suopanki A & Keiski RL (2006)

development for laboratory-scale exhaust gas catalyst studies on

poisoning. Chemical Engineering Journal 120: 113118.

III Krger V, Hietikko M, Angove D, French D, Lassi U, Suopanki A,

Keiski RL (2007) Effect of phosphorus poisoning on catalytic activ

exhaust gas catalyst components containing oxide and Pt. Topics in Ca

409413.

IV Krger V, Kanerva T, Lassi U, Rahkamaa-Tolonen K, Vippola M & Kei

Characterization of phosphorus poisoning on diesel exhaust gas catalyscontaining oxide and Pt. Topics in Catalysis 45: 153157.

V Krger V, Kanerva T, Lassi U, Rahkamaa-Tolonen K, Lepist T & Kei

Phosphorus poisoning of ZSM-5 and Pt/ZSM-5 zeolite catalysts in diese

conditions. Topics in Catalysis 4243: 433436.

VI Kanerva T, Krger V, Rahkamaa-Tolonen K, Vippola M, Lepist T

(2007) Structural changes in air aged and poisoned diesel catalys

Catalysis 45: 137142.

The manuscripts for the publications (Papers IV) were written by t

this thesis. In Paper VI, the author was responsible for the chem

procedure, activity measurements, and surface area studies. The ma

written in close collaboration with the first author.

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Contents

Abstract

Preface

List of abbreviations

List of original papers

Contents

1 Introduction

1.1 Background ..............................................................................

1.2 Purpose of the work ................................................................

2 Automotive exhaust gas catalysis

2.1 General .....................................................................................

2.2 Three-way catalysis.................................................................

2.3 Diesel catalysis.........................................................................

2.3.1 Oxidation catalysis .......................................................2.3.2 NOx reduction ...............................................................

2.3.3 Particulate filtration ......................................................

3 Catalyst deactivation

3.1 Overview..................................................................................

3.2 Chemical deactivation .............................................................

3.2.1 Poisoning by phosphates ...............................................3.2.2 Poisoning by sulphur compounds.................................

3.2.3 Poisoning by other compounds .....................................

3.3 Thermal deactivation................................................................

3.4 Mechanical deactivation .........................................................

3.5 Catalyst regeneration................................................................

4 Experimental 4.1 Catalysts ...................................................................................

4.2 Phosphorus poisoning procedures...........................................

4.2.1 Poisoning by impregnation............................................

4.2.2 Gaseous phase poisoning..............................................

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4.3.6 Fourier Transform Infrared-Attenuated Total Reflectio

4.3.7 X-ray Photoelectron Fluorescence.................................4.3.8 Activity measurements .................................................

4.3.9 Thermodynamic calculations........................................

5 Poisoning-induced changes in the catalyst components

5.1 Loss in catalytic surface area ..................................................

5.2 Accumulation of phosphorus ..................................................

5.2.1 Phosphorus content.......................................................5.2.2 Phosphorus compounds ................................................

5.3 Loss of catalytic activity .........................................................

5.3.1 Effect of phosphorus.....................................................

5.3.2 Effect of phosphorus and calcium ................................

5.4 Changes in the active metal particles and washcoat grain size

6 Evaluation of the poisoning procedures

6.1 Integration of the results .........................................................

6.2 Evaluation and utilization of the results..................................

6.3 Suggestions for further research..............................................

7 Summary and conclusions

References

Original papers

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1 Introduction

1.1 Background

Motor vehicles play a major role in urban air quality problems.

burning of petrol or diesel leads to formation of pollutants that h

harmful effects on the environment and human health. These pollut

carbon monoxide (CO), hydrocarbons (HCs), nitrogen oxides particulate matter (PM) (Kalantar Neyestanaki et al. 2004, Walli

2006).

Catalysis is one of the key technologies used in controlling a

Catalytic abatement of exhaust gas components has been used

automobile emissions since the 1970s, and it has been a success

development of environmental catalysts. The increasing motor vehicl

and tightening emission standards create a major challenge for the

of more durable and effective automotive catalysts. Demand

performance catalyst and requirements for long catalyst life have i

amount of research activity focused on automotive exhaust ga

(Greening 2001, Kaparet al. 2003, Twigg 2007)

Catalytic materials used in future three-way catalyst (TWC)

catalyst applications will have to be more effective in controllingThermal stability and good tolerance of poisons such as sulphur

originates from fuel, and phosphorus (P), zinc (Zn), calcium (Ca), and

(Mg), which originate from lubricating oils, are also needed. (Heck

1997, Farrauto & Heck 2000, Shelef & McCabe 2000, Lassi 2

challenges remain before researchers and the catalyst industry can

demands. Robust research efforts are required to clarify the functionmetals and washcoat components and to control deactivation phenom

improvements are also needed in the efficiency and optimization o

system of engine, fuel, and exhaust gas after-treatment. (Wallington e

Phosphorus and sulphur are present in aged catalytic converte

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Nowadays, over 95% of the worlds transportation fuel originate

fuels (Wallington et al. 2006). However, due to the quest to reduce defossil fuels and to control climatic change, interest in forms of energ

replace traditional crude oil-based fuels is growing (Hamelinck &

Romm 2006, Mittelbach 1995, Sagar 1995). According to E

2003/30/EC, the EU countries are supposed to replace the use of g

diesel with a renewable form of energy to the extent that by the year 2

of energy, calculated according to energy content, would come fromsources (EU 2003). Therefore, production and use of biodiesel and

derived from biomass is rapidly increasing (Mittelbach 1995). This

the role of phosphorus in exhaust gas catalysis. When using tradi

fuels, lubricating oils are normally a major source of phosphorus. In

biodiesel, phosphorus as well as potassium (K) and other pos

compounds may be present in various concentrations, depending on

the biomass (Grosser & Bowman 2006).

Despite the increasing risk of phosphorus poisoning in automoti

the number of publications dealing with the effect of phosphorus

catalyst components is still very limited. Therefore, this study was

with simplified model catalysts in order to obtain detailed knowl

poisoning phenomena of catalyst components. Research work in

essential in order to meet emission limits in the future.Research on deactivation of exhaust gas catalysts is widely ca

studying vehicle-aged, engine-aged, and oven-aged catalysts on th

scale (Koltsakis & Stamatelos 1997, Xu et al. 2004, Uy et al. 20

Galisteo et al. 2004). Vehicle- and engine-ageing methods prov

information about the overall deactivating effect caused by oil- and

contaminants and high operating temperatures. Their disadvantages are relatively expensive and time-consuming methods (Koltsakis &

1997). It may also be difficult to determine the relevance of a single

of deactivation to changes in catalytic activity and characteristics

procedures for studying chemical ageing on the laboratory scale wer

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known, and therefore, phosphorus was chosen to be the poisoning c

ageing. The purpose of this thesis project was to examine the effect ofon the catalytic activity and characteristics of components wid

automotive exhaust gas catalyst applications. As a special case, th

effect of phosphorus and calcium was also studied. To understand

induced changes in the washcoat, alterations in the surface of powd

components, caused by phosphorus poisoning and hydrothermal a

studied using several characterization techniques. In addition, differenphosphorus poisoning of catalysts with and without a precious

investigated. The characteristics of aged samples containing different

also compared.

Vehicle-ageing and engine-ageing procedures are widely used

poisoning phenomena of exhaust gas catalysts. However, these metho

relatively expensive and slow. In addition, studying the effects

deactivation mechanism on a decrease in catalytic activity may be

The mechanism of phosphorus poisoning is not completely known, a

studies carried out with simplified model components are required. O

of this study was to develop a methodology for accelerated chemical

studies that can be carried out in a time-saving, affordable, safe, an

way on the laboratory scale. In this research, deactivation of automot

caused by phosphorus poisoning was studied by developing twchemical ageing procedures on the laboratory scale. To obtain detailed

about the mechanism of poisoning, the studies were carried out wit

model components. Information about the formation and stability

compounds was gained by using thermodynamic calculations. These

were also used as a tool for planning ageing conditions.

Another purpose of the work was to study the mechanism ofpoisoning with different catalyst components used in present-d

converters. The work also focused on obtaining information about th

in which poisoning takes place and finding out whether some com

more sensitive to phosphorus than others. A further purpose of the

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automotive catalysts are also discussed. Chapter 4 presents the exper

of the work. The catalyst samples, ageing procedures, and chatechniques are described. The research results of the work are p

Chapter 5 and further integrated and evaluated in Chapter 6. Ch

discusses suggestions for further studies. Finally, the results are sum

conclusions are drawn in Chapter 7.

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2 Automotive exhaust gas catalysis

2.1 General

Automotive exhaust catalysts were introduced in the 1970s.

converters were used only to oxidize HC and CO emissions (Heck

2001). The first automotive catalysts to convert HC, CO, and NOx w

systems in which NOx was reduced in the first bed. Secondary air between the beds, enabling oxidation of HC and CO in the seco

stricter NOx standards in the 1980s led to development of three-way c

simultaneously catalyze three types of reactions: oxidation of CO

reduction of NOx. (Gandhi et al. 2003)

Modern automotive catalytic converters are very effective

gaseous pollutants in exhaust gas to levels that meet emissions s

converter (Figure 1) typically consists of a metallic or ceramic

substrate that provides a high geometric surface area with a low pr

The monolithic substrate is coated with a high-surface-area carrier m

as aluminium oxide (Al2O3). In order to maintain the high surfac

varying temperatures and gas compositions, the carrier is stab

lanthanum (La), barium (Ba) or silicon (Si) oxides, for instance. Prec

i.e. platinum (Pt), rhodium (Rh), and palladium (Pd), are added to thvarying compositions to provide catalytic reduction and oxidation of

more harmless compounds. (Heck & Farrauto 2001, Wallington

Seymor & O'Farrelly 2005)

Rare earth metal oxides are used to stabilize the precious me

sintering. Different oxides like cerium (Ce) and zirconium (Zr) oxide

improve catalytic efficiency by storing oxygen in lean conditions, i.e. oxygen, and releasing it when the engines operating conditions cha

oxygen phase. (Farrauto & Heck 1999, Wallington et al. 2006)

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Fig. 1. Typical structure of a metallic monolithic catalyst.

An exhaust gas after-treatment system consists of a catalytic conv

electronically controlled air-fuel management system. Air intak

injection are controlled to provide an appropriate ratio between oxyg

(air-to-fuel ratio, A/F). The oxygen content of the exhaust gas is me

oxygen or lambda sensor placed immediately before the catalyst in

manifold. (Farrauto & Heck 1999)

2.2 Three-way catalysis

Automotive exhaust gases formed in gasoline engines normally con

monoxide (CO), nitrogen oxides (NOX) and hydrocarbons (HCs). composition of gasoline engine exhaust gases is presented in Table 1.

catalyst is used to simultaneously remove these three major pollut

gasoline engines exhaust gases (Heck & Farrauto 2001). Under lea

h l i id i i f CO d h d b f

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Table 1. Composition of gasoline engine exhaust gas (Heck & Farrauto 20

CO

(vol-%)

HC

(ppm)

NOx

(ppm)

H2

(vol-%)

H2O

(vol-%)

CO2

(vol-%)

O2

(vol-%)

0.5 350 900 0.17 10 10 0.5

In ideal burning of gasoline, the following simplified reaction takes pl

2 2 2( )gasoline O air CO H O heat+ + +

Combustion in a real engine is incomplete, leading to formation of

combustion products. An automotive catalyst promotes reactions be

compounds according the following equations (Heck & Farrauto 2001

Oxidation of HC:

2 2( )4 2

y n

nC H y O yCO H O+ + + 2

n

Oxidation of CO:

2

1

2CO O CO+

2

22 2CO H O CO H + +

Reduction of NOx:

2 2

1/

2NO NO CO N CO+ +

2

2 2 2 2

1/

2NO NO H N H O+ +

2 2 2(2 ) / (1 )2 4y nn n

NO NO C H N yCO H O+ + + + +

22

n

In addition to the oxidation and reduction reactions mentioned a

significant reactions also take place in a TWC. These reactions includ

shift and steam reforming reactions (Kaparet al. 2003):

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2.3 Diesel catalysis

Unlike a spark-ignited engine, a diesel engine employs a compre

process in which the fuel is injected into a highly compressed ch

Therefore, oxygen is always in excess in a diesel engine. (Zelenka

Zelenka et al. 1996, Twigg 2006, Frost & Smedler 1995) Furth

requirements for diesel exhaust gas after-treatment are different fr

gasoline vehicles due to the unique demands of emissions standa

application, and high durability (Clerc 1996). The primary targemission control is reduction of particulates and NOx (Clerc 1996, Tw

Diesel emissions include all three phases of matter. Solid emissio

dry carbon and soot. The liquid phase includes hydrocarbons d

unburned fuel and engine lubricating oil (collectively Soluble Organ

SOF), as well as liquid sulphates. (Zelenka et al. 1996) The size

exhaust gas particle is between 0.01 and 1 m in diameter. Especia(

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dispersed form. Zeolites are used to adsorb HCs, thus preventing

inhibiting active Pt sites. (Twigg 2006)Some zeolite catalysts, such as platinum-containing ZS

demonstrated their effectiveness in reducing NOx with hydrocarbo

exhaust gas with a high oxygen content (Guo et al. 1995). The tem

diesel exhaust gas is normally relatively low, leading easily to a dec

operating temperature of a catalyst. Therefore, addition of a HC

needed. Zeolites have the ability to adsorb HCs, which also im

conversion with increasing temperature. (Shinjoh 2006, Farrauto &

With zeolite-containing catalysts, HC removal is effective over the en

temperature range with very low platinum loading. At low catalyst t

(below 200C), removal of gas phase HCs is attributed exclusively

However, at higher temperatures (300C and above), HC conversion

how the platinum is functioning in the catalyst. (Farrauto & Voss 1996

The desired main oxidation reactions in diesel exhaust catalysis Farrauto 2001):

Oxidation of SOF:

2 2SOF O CO H O+ + 2

Oxidation of HC:

2 2(1 )

4 2y n

nC H O yCO H O+ + +

2

n

Oxidation of CO:

2

1

2CO O CO+

2

Depending on the sulphur content of the fuel, the exhaust gas contaiamount of sulphur oxides (SOx). Sulphuric acid (H2SO4) is formed in

between sulphur trioxide (SO3) and water. Therefore, oxidation of

should be minimized in the oxidation catalyst (Heck & Farrauto 2001

1

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2.3.2 NOxreduct ion

An improved combustion process and exhaust gas recirculation (EGR

reduce NOx content in diesel exhaust gas (Zelenka et al. 1996). Deco

NO to its elements is thermodynamically favoured between a

relatively high temperatures (Goralski & Schneider 2002):

2

1 1

2 2

NO N O+U2

However, under lean conditions the catalytically active metal surfa

covered by strongly adsorbed oxygen, thus preventing NO adsorp

2006). Formation of NO2 becomes significant at moderate temperatu

excess oxygen (Goralski & Schneider 2002):

2

1

2NO O NO+ U

2

O

+

In order to comply with emission standards under lean conditions, NO

catalysis, i.e. selective catalytic reduction (SCR), or NOx trapping

exhaust gas after-treatment (Twigg 2006).

In SCR, a reducing agent such as alcohol, hydrocarbon, ammon

urea ((NH2)2CO) is required (Lox & Engler 1997, Zelenka et al. 1996

conversion rates can be achieved by adding the reducing agent to thebefore the catalyst (Zelenka et al. 1996). Using HC as a reducing ag

an advantage compared with other reductants because for instance, th

can be employed as a source of the reducing agent. The following

NO with HC or alcohol are possible (Lox & Engler 1997):

2 2 2 2 2(6 2) 2 (3 1) 2 (2 2)

m mm NO C H m N m CO m H

++ + + + + +U

2 1 2 2 26 2 3 2 (2 2)

m mm NO C H OH m N m CO m H O

++ + +U

Besides HC and alcohol, NH3 and (NH2)2CO can be used as reduci

SCR (Koebel et al. 2000, Madia et al. 2002). (NH2)2CO is a compoun

NH i h i i h (K b l l 2000)

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With equimolar amounts of NO and NO2, the reaction rate is faster

of reaction (16) (Koebel et al. 2000):

3 2 24 2 2 4 6NH NO NO N H O+ + + 2

In contrast, the reaction with NH3 and NO2 (18) is slower than reacti

(17) (Koebel et al. 2000):

3 2 2 2

74 3 6

2

NH NO N H O+ +

Formation of N2O is possible at temperatures above 400C (Koebel et

3 2 24 4 3 4 6NH NO O N O H O+ + +

2

At very high temperatures, an undesired reaction in which NH3 is oxi

may take place (Koebel et al. 2000):

3 2 24 5 4 6NH O NO H O+ +

In the trapping method, NOx is storaged as NO3 during the lean dri

When the trapping material is saturated, the stored NOx is released

the exhaust gas composition to be slightly reducing for a short period

is reduced to N2 over a Rh-containing component. (Twigg 2006)

2.3.3 Particu late fi l trat io n

Soot is present in exhaust gas as an aerosol with a concentration that

be burned in a catalytic converter. Furthermore, the residence time o

catalyst is not long enough for oxidation. (van Diepen et al. 1

advances in the combustion processes of modern passenger car di

have resulted in a reduction of PM emissions. Despite that, eliminatiofiltration is still needed to prevent possible health effects of diesel

2006) Therefore, soot is collected in particulate filters from which

removed (Koltsakis & Stamatelos 1997, van Diepen et al. 199

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Fig. 2. A wall-flow monolith particulate filter (Bissett 1984).

Trapping material is based on ceramic or metallic substrates that allocapture of dry soot. To maintain engine efficiency, the filtering surf

be sequentially regenerated. Regeneration methods can be divid

categories: active and passive processes. In active regeneration the tra

with a fuel burner or an electrical heater. Passive regeneration meth

catalytic fuel additives and catalytic coating before or inside the trap

al.1996, Stamatelos 1997)

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3 Catalyst deactivation

3.1 Overview

Deactivation of automotive catalysts is a problem that has not been to

with current catalyst technology. Catalyst deactivation is a

phenomenon due to the several possible factors causing it. Catalyst

their activity as a result of a number of unwanted chemical and physi(Butt & Petersen 1988, Forzatti & Lietti 1999, Bartholomew 2001, M

2001, Richardson 1989) The types of deactivation phenomena can be

three categories: chemical, thermal and mechanical ageing (Barthol

Bartholomew 2004). Table 3 shows one possible way to categ

deactivation types and the related mechanisms of deactivation.

Table 3. Types and mechanisms of catalyst deactivation and their

(Bartholomew 2004).

Deactivation type Deactivation mechanism Description of the mechanism

Poisoning Strong chemisorption of compo

catalytic surface, blocking of th

sites.

Vaporization Reaction of compounds with th

to produce volatile compounds

Chemical

Vapour-solid and solid-solid

reactions

Reaction of vapour, support, or

the catalytic phase to produce

phase.

Thermal Thermal degradation Thermally induced decrease in

surface, support surface area,

phase-support reactions.

Fouling Physical deposition of species

phase onto the catalytic surfac

Mechanical

Attrition/crushing Loss of catalytic material due to

internal surface area caused by

crushing of the catalyst.

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Figure 3 presents deactivation of a diesel catalyst in different temper

Mechanical deactivation, which takes place for instance in the formbreakage or crushing, is also a relevant deactivation phenomenon (

Stamatelos 1997). Deactivation can be induced by one cause or a com

several deactivating factors. In any case, the net effect is a r

catalytically active sites. (Forzatti & Lietti 1999)

Exhaust gastemperature range

0

200

400

600

800

1000

T (C)

Adsorption of H2O and particulates

Adsorption of SO2

Formation and chemisorption of SO3

Reaction of Zn and P with washcoat

Washcoat phase changes

Catalysttemperature range

Operarang

Fig. 3. Deactivation of diesel catalysts in different temperature ranges

modified from Lox & Engler 1997).

Deactivation detected in present-day catalytic converters during nor

operation is typically a result of chemical and thermal mechanisms, wand mechanical factors are more rarely the main reason for

(Koltsakis & Stamatelos 1997, Taylor 1984) This thesis focu

mechanisms of chemical deactivation, i.e. poisoning.

O h t t l t h b d ti t d ti

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3.2 Chemical deactivation

Exposure to catalyst poisons aggravates the loss of activity in automo

gas catalysts. Deactivation by poisoning is often related to hig

temperatures and sudden temperature changes. Poisons contaminate t

and precious metals and reduce the catalytically active surface by blo

sites (Butt & Petersen 1988, Angelidis & Sklavounos 1995). The a

automotive catalyst declines gradually over the course of time as

components of fuels, lubricants and other impurities accumulate on tsurface (Koltsakis & Stamatelos 1997). Poisoning is defined as a loss

activity caused by chemisorption of impurities on the active sites of

Poisons are substances whose interaction with active sites is stron

irreversible. (Butt & Petersen 1988, Forzatti & Lietti 1999)

Catalyst poisons can also be classified as selective or n

depending on whether the poisoning is universal or takes place onsites or crystal faces or affects only certain adsorbed species. Poi

certain compound can also be a reaction-specific phenomenon. In suc

poison is selective in one reaction, but non-selective in another. (But

1988)

Poisoning of an exhaust gas catalyst caused by accumulation of i

the active sites is typically a slow and irreversible phenomenaccumulated on the catalyst surface block the access of reactants t

sites. (Butt & Petersen 1988) Catalyst poisoning is possible even with

of impurities (Forzatti & Lietti 1999).

S, P, Zn, Ca, and Mg are typical oil- and fuel-derived contamina

used catalytic converters. These contaminants have been wid

(Williamson et al. 1984, Williamson et al. 1985, Inoue et al. 1992,

1993, Angelidis & Sklavounos 1995, Culley et al. 1996, Rokosz et al.

al. 2003, Cabello Galisteo et al. 2004, Fernndes Ruiz et al. 2002)

generation of catalytic converters, lead (Pb) was the major cause o

(Williamson et al. 1979a, Williamson et al. 1979b, Monroe 1980). No

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in separate washcoat layers. (Laurikko 1994, Laurikko 1995) F

precious metals used to control exhaust gas emissions have differeresistances against poisoning. For instance, it has been shown that P

less sensitive than Pd to poisoning by sulphur and lead. (Taylor 1

Engler 1997)

Despite the numerous studies carried out with poisoned c

individual and combined effects of poisons as well as the i

simultaneous poisoning and thermal deactivation are still not sufficie

Especially knowledge about effects related to deactivation of zeo

lacking. It is known that deactivation of zeolite-containing catalysts

both low and high temperatures, especially in moist conditions (van K

2000).

3.2.1 Poiso ning by phosp hates

Zinc dialkyldithiophosphate (ZDDP) is an antiwear, antioxidant, an

inhibiting additive that is used in engine lubricating oils, hydraulic oi

lubricants. ZDDP is produced by first letting an alkyl or aryl alcoho

phosphorous pentasulphide (P2S5), and then neutralizing the resulta

zinc oxide. The frictional heat formed by two steel surfaces rubbing

the engine causes ZDDP to react chemically with the surfaces. As astrong wear-resistant lubricating films are formed. The additiv

secondary oxidation inhibitor that decomposes hydroperoxides in th

step of hydrocarbon oxidation. In addition, ZDDP is an effec

corrosion inhibitor. (Dickey 2005)

In automotive applications ZDDP is a common source of typ

poisons, phosphorus and zinc (Kumaret al.

2004). Table 4 shcharacteristics of ZDDP. Caracciolo & Spearot (1976) have reported t

activity decreases linearly as the amount of phosphorus detected in

increases. The phosphorus content of modern lubricating oils is typ

0.15 wt-% (Bunting et al. 2004). According to Kalantar Neyestanaki e

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Table 4. Typical characteristics of zinc dialkyldithiophosphate (Researc

Petroleum Industry 2002).

Characteristics Typical value

Density (g/ml) at 15.5C 1.09

Viscosity (c.st) at 100C 10.3

Sulphur content (wt-%) 19

Phosphorus content (wt-%) 10

Zinc content (wt-%) 11

pH 56

The chemical composition in which phosphorus can be found

converters depends on many factors, such as the chemical compositio

oil additives, the age of the oil used, and conditions in the motor (Xu

Phosphorus contamination can be observed on the surface of catalyti

as several different compounds (Carduneret al. 1988, Xu et al. 2004

illustrated in Figure 4, phosphorus can form a phosphate-containing ois a mixture of Zn, Ca, and Mg phosphates on the washcoat (Liu &

Rokosz et al. 2001). These compounds are very stable on the cata

(Andersson et al. 2007). Low exhaust temperatures are favoura

formation of zinc pyrophosphate (Zn2P2O7) (Williamson et al. 1985

to Liu and Park (1993), formation of a glassy, amorphous phase of Pb

phosphates on the catalyst surface has been observed. Ca and Mg ha

reported to prevent phosphorus poisoning. Ueda et al. (1994) demo

phosphorus poisoning of the catalyst and the oxygen sensor can be r

using Ca and Mg sulphonates as oil additives. Furthermore, Willia

(1980) reported that fuel P (derived from cresyl diphenyl pho

neutralize Pb deactivation by the formation of lead phosphates.

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Fig. 4. Scanning electron microscope (SEM) image of a layer containing

Mg phosphates on a catalytic surface, magnified 5000 times (Rokosz et al

Secondly, phosphate compounds can also be formed directly wi

components. In that case, the resulting compounds are, for instance

and cerium phosphates (Rokosz et al. 2001, Uy et al. 2003, Cabello G

2004). In the case of aged diesel catalytic converters, the adsorbed

species have also been identified as phosphates (Beckmann et al. 19

et al. 2005). For instance, the interaction between phosphorus

resulting in the formation of AlPO4, is expected to be partially resdeactivation observed in aged diesel catalysts (Cabello Galisteo e

Furthermore, formation of CePO4 is also responsible for deterior

oxygen storage and release properties of cerium oxide (Battistoni

Lpez Granados et al. 2006, Larese et al. 2003, Larese et al. 2004a,

2004b).

Phosphorus is usually concentrated in the forward-most section oconverters (Culley et al. 1996, Becket al. 1997a, Becket al. 1997b

Cant 2000, Webb et al. 2003). A decrease in catalytic activity and

characteristics, such as a loss of surface area, in the front section of

h b i t d ith t i h h d iti (B k

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3.2.2 Poisoning by sulphur compo unds

Sulphur poisoning is a complex phenomenon that can involve signifi

in the structural, morphological, and electronic properties of a catalys

& Hrbek 1999). Gasoline and diesel contain sulphur in small amou

cumulative effect of poisoning during the catalysts life can be subst

case of TWCs, fast poisoning by sulphur can be to some extent rever

poisoned catalyst can be regenerated. (Yu & Shaw 1998) The rol

poisoning is especially substantial in diesel catalysts due to the compaoperating temperatures involved.

Despite the reduction in the sulphur content of fuel that has

during the past decades, sulphur is still present in diesel oil and gaso

amounts (Yu & Shaw 1998). Sulphur has a negative effect on the

oxygen storage capacity (OSC) of the catalyst (Boaro et al. 2001,

1998). The presence of sulphur can cause formation of new inactiveon the catalysts surface. Furthermore, morphological changes in the

also possible. (Yu & Shaw 1998)

Depending on the temperature and partial pressure of oxygen, su

present in an exhaust gas catalyst in the form of sulphates, s

oxysulphides (Karjalainen et al. 2005). In the fuel combustion proce

place in the engine, sulphur oxidizes to SO2

and SO3, which are then

the precious metal sites on the catalysts surface at temperatures belo

the reaction with aluminium oxides, aluminium sulphates are for

compounds deactivate the catalyst by reducing the active surfa

poisoning is a severe problem at low temperatures, whereas at temper

1000C adsorption of sulphur species is almost negligible. In additio

of sulphur poisoning on TWCs has been observed to be more s

oxidizing than in reducing conditions. (Butt & Petersen 1988, Heck

1996) Under reducing conditions sulphur forms H2S, which po

surfaces and negatively affects oxidation of HCs (Rabinowitz et al. 2

case of NOx storage, Fridell et al. (2001) have reported that sulphur

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3.2.3 Poisoning by other com pou nds

Automotive catalysts can also be deactivated by compounds comm

the structure of an engine. For instance, iron (Fe) is a poison for pla

metals (Kalantar Neyestanaki et al. 2004). Iron detected on the cataly

often assumed to originate from the corrosion of metal components i

(Cabello Galisteo et al. 2004). Other similar impurities may be c

nickel (Ni), and chromium (Cr). Chromium and nickel are ad

construction materials in order to improve the thermal stability Copper originates from engine bearings, for example. (Angelidis & S

1995)

3.3 Thermal deactivation

High temperatures and uncontrollable temperature changes causformation of new compounds, phase changes in existing comp

volatilization of components, leading to a loss of active surface

catalytic materials and the washcoat (Richardson 1989, Mooney

operating temperature of light-duty diesel catalysts is normall

moderate (below 500C), and therefore the contribution of therm

deactivation of these catalysts is of lesser importance compared

Depending on the load, exhaust gas temperatures may vary from

700C. Therefore, the influence of thermal ageing cannot be complete

(Cabello Galisteo et al. 2004, Koltsakis & Stamatelos 1997)

Sintering is a thermally activated process in which the surf

decreased by structural modification of the catalyst. As a resu

modifications, active catalytic materials and also the washcoat can

(Butt & Petersen 1988) Two different models have been proposintering of metal crystallites: the atomic migration model and th

migration model. In atomic migration atoms migrate from one c

another via the surface or the gas phase, causing an increase in crystal

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surface area. In addition, changes in the catalyst structure can cause e

of active metal particles. (Butt & Petersen 1988, Lassi 2003)

3.4 Mechanical deactivation

A catalytic converter can be damaged by a strong mechanical impa

case, the monolith is crushed and as a result, the back pressure of the

is increased. If the damage is not noticed in time, local overheatin

thermal deactivation and finally melting of the honeycomb. Metallic

endure mechanical deactivation better than ceramic ones. (Laurikko 1

Mechanical failure of the catalyst may also be a result of high te

Two types of factors can cause high catalyst temperatures. The first o

operating conditions. Secondly, as the result of misfiring, non-combu

mixtures undergo catalytic combustion, which can produce

exothermic reactions within the catalyst. At about 1200C, the metalto soften and probably shrink. If the catalyst temperature exceeds 140

will melt, after which it is destroyed. (Mooney 2005)

3.5 Catalyst regeneration

It is usually easier to prevent catalyst deactivation than to restore activity of the catalyst (Bartholomew 2001). However, some studie

carried out to investigate the possibilities of regenerating activ

decreased due to thermal ageing and poisoning.

According to van Kooten et al. (2002), catalytic activity can

restored by heating the catalyst under oxygen-rich conditions, wh

some of the contaminant species (coke, HCs, nitrogen). However, va

al. (2002) reported that depositions of P, Zn, Ca, and S were very har

with this method, and therefore they are associated with irreversible

Reactivation of sulphated oxidation catalysts has been performed

Galisteo et al. (2007), who decomposed aluminium sulphate in a

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Washing a poisoned catalyst with organic acids has also been te

et al. (2001) and Christou et al. (2006) have shown that it is possible t

remove phosphorus accumulated in the catalyst using oxalic acid

method, catalytic activity was restored to a level corresponding to o

deactivation (Rokosz et al. 2001). Similar results have been reported b

& Sklavounos (1995), who treated catalysts containing Ca, P, S, Pb

compounds with acetic acid. According to Christou et al. (2006), b

catalyst with EDTA (ethylene diamine tetraacetic acid), depositions of

Mn, Fe, Cu, Ni, and S, but not P, could be removed. Washing a po

with a dilute solution of citric acid has been demonstrated to rem

amount of P and most of the S accumulated in the catalyst (Cabello G

2005a). Birgersson et al. (2006) showed in their study that combined

liquid chlorine treatments can increase the number of catalytically

metal sites on the washcoat surface. Furthermore, P and S could

removed, which led to regaining of micropores. As a result, catalyticrestored.

Catalyst regeneration is also possible in the case of thermal

Catalytic activity lost because of chemical interaction between cataly

can sometimes be restored by using suitable reaction conditions. F

oxidation of Rh/Al2O3 at 800 C caused diffusion of Rh into the alum

resulting in a decrease in CO oxidation activity, which was reported McCabe (1989). The activity could be restored with a reductive trea

800C) followed by reoxidation at 800C (Wong & McCabe 1989

Velasco et al. (2000) have presented results that are congruent with th

& McCabe. Birgersson et al. (2004) and Anderson et al. (2006) demo

a TWC that has been deactivated in terms of activity and metal dispe

regenerated by adding chlorine to an oxygen stream. This oxy

treatment also seems to be a potential procedure for regenerating DOC

ageing and sintering are the main causes of deactivation (Cabello G

2005b)

In general, it should be pointed out that the interactions betwe

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4 Experimental

4.1 Catalysts

This work consists of two test series and two different sets of catal

powders containing Rh (2.0 wt-%) or Pt (1.5 wt-%), which represent

used in commercial exhaust gas catalysts. In the latter test serie

without a precious metal were also studied. The catalysts were donate

Oy. Figure 5 shows examples of the powdery components used in thi

samples are listed in Table 5. The samples were studied when fresh

ageing and phosphorus poisoning procedures described in Chapter 4.2

Pt/Al2O3

Pt/CeO2 Al2O3

Pt/ZrO2CeO2

Fig. 5. Examples of powdery components used in automotive catalysts.

In addition to the samples presented in Table 5, three vehicle-a

catalyst monoliths were used as industrial reference catalysts for the

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is most significant in the forward-most section of monolithic

Therefore, the front zones of the commercial catalysts were used

material in this study.

Table 5. List and description of the powdery catalyst samples.

Catalyst sample Description

Test series 1

Rh/Zr-CeO2 Rh 2.0 wt-% in Zr-stabilized Ce oxide

Rh/Ce-ZrO2 Rh 2.0 wt-% in Ce-stabilized Zr oxide

Rh/-Al2O3/OSC Rh 2.0 wt-% in the OSC material, 0.5 wt-% in the entire washco

Test series 2

Al2O3

TiO2

CeO2

ZrO2

Zr-CeO2 Zr-stabilized Ce oxideCe-ZrO2 Ce-stabilized Zr oxide

ZSM-5 zeolite Si/Al2 29

Pt/Al2O3 Pt 1.4 wt-%

Pt/TiO2 Pt 1.5 wt-%

Pt/ZSM-5 zeolite Pt 0.85 wt-%

Pt/CeO2 Pt 1.5 wt-%

Pt/ZrO2 Pt 1.5 wt-%

Pt/Ce-ZrO2 Pt 1.5 wt-% in Ce-stabilized Zr oxide

Pt/Zr-CeO2 Pt 1.5 wt-% in Zr-stabilized Ce oxide

4.2 Phosphorus poisoning procedures

Two different methods were utilized for phosphorus ageing. In the f

the methodology development, the test series consisted of Rh-loapowders representing components used in TWC applications. Phosph

a special case, calcium, were added via impregnation. (Papers III)

poison treatment, the ageing procedure was further develope

d t th t diti i t l ti t d i

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4.2.1 Poison ing by imp regnat ion

The catalysts were pre-treated in a H2 flow at 900C for 24 hopoisoning, which was performed with an incipient wetness impregna

Aqueous salt solutions of (NH4)2HPO4 and Ca(NO3)24H2O were use

of phosphorus and calcium, respectively. The final concentrations of

and calcium in the catalyst powders were 1 wt-%. For XRD analysi

concentration was increased to 5 wt-% to obtain better detectability. (P

After poisoning with one poison, the catalyst powders were calcin30 minutes at 100C, followed by another 30 minutes at 200C and

one hour at 500C. Subsequently, some of the catalysts were poison

other poison (P+Ca or Ca+P) in order to study the combined effect

and phosphorus. (Paper I)

4.2.2 Gaseous phase po ison ing

The ageing temperature of 700C was chosen to correspond t

temperature in diesel catalytic converters (Koltsakis & Stamatelos

phosphorus content detected in vehicle-aged diesel catalysts varies d

the literature source and the method used for the detection. The

content detected in the front part of converters is normally less than 4

& Angove 1998). The amount of phosphorus fed into a catalyst sa

ageing was set to correspond to the typical phosphorus content dete

catalysts.

The aim was to develop an ageing procedure that is safe and

Therefore, different equipment constructions and materials were teste

development process. Tubular quartz, steel and ceramic reactor ma

tested. The steel and ceramic reactors were severely corroded by exposure to hydrothermal conditions and phosphorus addition. The q

was found to be resistant to corrosion even after several ageing

Vertical and horizontal reactor positions were tested in the construc

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Furnace

Sample 1.5gPeristaltic

pump

Mass flowcontroller

Air 250ml/min

700C / 4 hours

Temperatureindicator

Quartzreactor

Aqueous salt solution of(NH4)2HPO4 0.01ml/min

Fig. 6. Schematic illustration of the equipment used for gaseous phas

studies.

In order to simulate phosphorus poisoning in lean hydrothermal co

aqueous salt solution of (NH4)2HPO4 was fed into the quartz rea

peristaltic pump. It was demonstrated that less than 70% of the

adhered to the sample. In addition, the final phosphorus content in

varied depending on the catalyst. To ensure sufficient poisoning, the

(4 hours) as well as the (NH4)2HPO4 content (0.13 mol/l) and feedinsolution (0.1 ml/min) were set to correspond to a 6 wt-% target

phosphorus in the sample. One aim of the studies for which this agein

was developed was to compare the effects of phosphorus poisoning

containing different oxides. Therefore, ageing was carried out with all

according to the same procedure without changing the ageing time o

of phosphorus in the feed.

Air (250 ml/min) was used as a carrier gas for the (NH 4)2HPO4stream was controlled by a mass flow controller (Brooks 5850TR)

hours of ageing the (NH4)2HPO4 feed was turned off and the sample

the air stream at 500C for one hour

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and the poison compounds formed were studied using physisorpti

SEM, TEM, XRD, and FTIR-ATR. The poison content of the s

determined by chemical analyses (ICP-OES) and XRF. Laboratory-s

measurements were applied to study catalytic activity. The

calculations were used to obtain information about ageing con

phosphorus compounds formed during ageing.

4.3.1 Physiso rpt ion analyses

To characterize the catalysts before and after ageing, physisorption m

were carried out with a Micromeritics ASAP 2020 unit (Figure 7) a

Omnisorp 360CX unit. Specific surface areas (m2/g) were calculated

the BET theory. The BJH theory was used to evaluate the total p

(ml/g) and average pore size (nm) of the samples. The internal surfac

determined using the t-test method. The calculations were carried outadsorption isotherms at 196C and assuming that the pores had

shape. It should be mentioned that, for the calculations of the nume

certain approximations were made concerning pore shape, for instanc

the pores may have different structures. Furthermore, they m

interconnected with one another. (Webb & Orr 1997)

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4.3.2 Scannin g Electron Micros cop y

SEM-EDS was used to detect contaminant levels in the poisonePossible spatial differences in poison accumulation were also s

samples were cast in epoxy resin and polished and coated with

equipment used for the SEM studies was Jeol JSM-6400.

4.3.3 Transm ission Electro n Micros cop y

Transmission electron microscopy (TEM) was used to char

composition and microstructure of the samples and to determine ch

washcoat and Pt particle sizes after the ageing treatments. The m

were carried out with an analytical transmission electron microscope

from Jeol, operated at 200 kV and equipped with a Noran Vant

dispersive spectrometer. The TEM samples were prepared from samples by crushing the samples between glass slides. The crushed

then dispersed onto a perforated carbon-covered Cu grid with ethano

studies were carried out at Tampere University of Technology.

4.3.4 Chem ical analys es

The phosphorus content of the samples was determined with ICP-OE

containing cerium were digested by heating them in a solution of (NH

salt, H2SO4, and HCl. The other samples were mixed with a solutio

HCl, and HF and digested by using microwaves. After digestion,

were diluted with water and analyzed with a GBC Integra XM

analyzer. In order to avoid a systematic error in the results, th

backgrounds of the above-mentioned solutions were measured.

4.3.5 X-ray diff ractio n

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supplied by International Centre for Diffraction Data (PDF-2 Powde

File Database).

4.3.6 Fourier Transfo rm Infrared-Attenuated Total Ref lect io

FTIR-ATR studies were carried out to identify phosphorus compo

detect possible ageing-induced changes in the catalyst stru

measurements were carried out with a Perkin Elmer Spectrum One FT

4.3.7 X-ray Photoelectron Fluoresc ence

X-ray photoelectron fluorescence was used to determine the phosph

of the poisoned catalyst samples. The XRF measurements were carrie

Philips MagiX apparatus containing Super Q Analytical software.

4.3.8 Ac t iv i ty measu rements

Activity measurements were carried out by using laboratory-sc

experiments to study poisoning-induced changes between fresh and a

powder samples.

Catalytic activities were determined in lean reaction conditiomodel reactions. The test gas mixtures are shown in Table 6. The gas

Test series 1 (samples containing oxide and Rh) was used to si

gasoline exhaust gas conditions. The gas mixture for Test series

containing oxide and Pt and ZSM-5) simulated diesel exhaust gas

Both gas mixtures were balanced with N2.

Table 6. Composition of the test gas mixtures for the activity measuremen

Gas component Concentration

Test series 1

CO 800 ppm

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Before the measurements the samples in Test series 1 were reduc

ml/min hydrogen flow for 10 minutes at 500C, followed by 15 minut

The samples in Test series 2 were oxidized in a 50 ml/min air flow fo

at 500C. The pre-treatments and measurements were carried out at

pressure in a tubular furnace using a quartz reactor.

The weight of a powder sample was 0.25 g for oxides and 0.10 g

The samples were mixed with quartz sand to prevent agglomera

catalyst, and therefore, to prevent diffusional effects in the cataly

samples were placed in the reactor tube with a support of quartz wowas controlled by mass flow controllers (Brooks 5850TR), and the to

during the experiments was 1 l/min. The temperature of the catalyst w

from room temperature up to 500C, with a linear heating rate of 1

The concentrations of the feed and product gases were measured as a

temperature every 5 seconds with a FTIR gas analyzer (GasmetT

Figure 8 illustrates the experimental setup.

1

2

3

6

1. Mass flow controllers2. Thermocouple3. Tubular furnace4. Quartz reactor5. Temperature controller

6. FT-IR Gasmet Analyzer

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4.3.9 Thermo dyn amic calculat ions

Thermodynamic equilibrium calculations were carried out using commChemistry 5.11 software. The software can be used as a tool for

chemical equilibriums between pure components in ideal solutions a

cases, in non-ideal solutions. Equilibrium compositions are calcu

solver that uses the Gibbs energy minimization method. The HSC

5.11 software includes a thermochemical database that contains entha

and heat capacity values. Thermodynamic modelling does not pinformation on reaction kinetics, but physically possible phenom

estimated by using thermochemical calculations. Phase stability diagr

the stability areas of condensed phases in ternary systems, either as a

temperature or in isothermal conditions. These diagrams are useful

estimation of the prevailing phases.

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5 Poisoning-induced changes in the cat

componentsThe research results and the changes caused by poisoning and h

ageing detected in the catalyst components are classified in this chap

sections. The sections are: loss in catalytic surface area, accu

phosphorus, loss of catalytic activity, and changes in the active me

and washcoat grain size.

5.1 Loss in catalytic surface area

As a result of poisoning and ageing, alterations in the specific surface

catalyst samples were detected. The BET surface areas of all the

poisoned catalyst powders were significantly diminished compared w

samples. Figures 9 and 10 present the specific surface areas of theTest series 2 (samples containing oxide and Pt and ZSM-5).

203

67

288

6

188

58

156

25

93

2

6241

101

15

6138 28

0

100

200

300

400

Pt/Al2O3 Pt/TiO2 Pt/CeO2 Pt/ZrO2 Pt/Zr-CeO2 Pt/Ce-ZrO2

Surface area (m2/g)

Fresh

H2O

P+H2O

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217

79

294

6

201

61

166

24

68 6040

145

2146 42 33

0

100

200

300

400

Al2O3 TiO2 CeO2 ZrO2 Zr-CeO2 Ce-ZrO2

Surface area (m2/g)

FreshH2O

P+H2O

Fig. 10. Specific surface areas of fresh, hydrothermally aged (H2O), and

poisoned (P+H2O) catalyst samples without Pt.

The effect of the ageing treatments was most significant for the surf

the ceria-based samples, i.e. CeO2, Pt/CeO2, Zr-CeO2, and Pt/Zr-CeO

and 10), and Rh/Zr-CeO2 (not shown). For instance, the specific sur

the poisoned Pt/Zr-CeO2 was one fifth of the surface area of the frFurthermore, for the samples in Test Series 1 (samples containing ox

the surface area of the phosphorus-treated Rh/Zr-CeO2 was less than o

m2/g) of the surface area of the fresh sample (191 m2/g). The respectiv

Rh/Zr-CeO2 were 41 m2/g and 55 m2/g (Papers II and IV). The spe

areas of Pt/ZrO2 and ZrO2 (Figures 9 and 10) were also strongly

ageing. As a result, the surface area of the phosphorus-poisoned Pt/Zas well as the hydrothermally aged and phosphorus-poisoned ZrO2 sa

not be determined. (Paper IV)

In the case of zeolites, the BET results showed that the fresh

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observed specific surface areas of the samples could be explained by

the pore volumes and increases in the pore sizes detected in the

Figures 1112 illustrate the total pore volumes and Figures 1314

pore sizes of the catalysts in Test series 2 (samples containing oxide

ZSM-5).

0.49

0.27

0.19

0.01

0.18 0.20

0.25

0.49

0.12

0.17

0.01

0.14

0.19

0.33

0.080.13 0.10 0.13

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Pt/Al2O3 Pt/TiO2 Pt/CeO2 Pt/ZrO2 Pt/Zr-CeO2 Pt/Ce-ZrO2 P

Total pore volume (ml/g)

F

H

P

Fig. 11. Total pore volumes of fresh, hydrothermally aged (H2O), and

poisoned (P+H2O) catalyst samples with Pt.

0.49

0.29

0.19 0.190.22

0.

0.50

0.11

0.17 0.17 0.19

0.41

0 100.13 0.13 0.14

0.2

0.3

0.4

0.5

0.6

Total pore volume (ml/g)

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10

16

3

8

4

1413

20

7

13

9

19

14

21

8

11

20

0

10

20

30

Pt/Al2O3 Pt/TiO2 Pt/CeO2 Pt/ZrO2 Pt/Zr-CeO2 Pt/Ce-ZrO2 P

Average pore size (nm)

Fresh

H2O

P+H2O

Fig. 13. Average pore sizes of fresh, hydrothermally aged (H2O), and

poisoned (P+H2O) catalyst samples with Pt.

9

15

3

7

1

15

12

19

1011

20

12

19

1113

18

0

10

20

30

Al2O3 TiO2 CeO2 ZrO2 Zr-CeO2 Ce-ZrO2

Average pore size (nm)

Fresh

H2O

P+H2O

Fig. 14. Average pore sizes of fresh, hydrothermally aged (H2O), and

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smallest pore volume was, however, calculated for the poisoned sa

was in agreement with the specific surface area determined for t

samples. In the case of Al2O3 and TiO2, the average pore s

hydrothermally aged and poisoned samples were equal. The effect of

was clearly observed in the pore volumes of the Al2O3 and Pt/Al2which did not decrease as a result of hydrothermal ageing. This phen

not observed in the specific surfaces or average pore sizes of the

samples. After phosphorus poisoning, a decrease in the pore volum

samples was detected. (Paper IV)For ZSM-5 and Pt/ZSM-5 the total pore volumes of the fresh s

not affected by hydrothermal ageing, as can be seen in Figures 11

contrast, phosphorus poisoning caused a decrease in the pore volu

samples, which was congruent with the results from the BET a

activity measurements. Any further explanation of the changes in

surface areas or activity could not be found in the pore size distributhe same average pore width for all six samples (3 nm) (Figures

(Paper V)

5.2 Accumulation of phosphorus

XRF, SEM-EDS and elemental analyses were used to detect

accumulation in the aged catalyst powders. Phosphorus compounds f

samples were identified with XRD and FTIR-ATR. In addition, the

calculations were used to support these methods.

5.2.1 Pho sph oru s con tent

The XRF analyses showed that phosphorus was adsorbed almost quan

the powdery samples during the incipient wetness impregnation m

semi-quantitative amount of phosphorus measured by XRF for Test

Rh-loaded powder samples) was 1 wt-%, which was the target

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tendencies of different materials to accumulate impurities. The very

surface areas of the fresh ZrO2 and Pt/ZrO2 are probably the reason f

phosphorus content measured in these samples. However, no correlat

the characteristics of the fresh samples and the phosphorus content de

samples after ageing was observed in this study. Further studies ar

explore the possible attributes that give a correlation betw

characteristics and the catalysts tendency to adsorb poisoning compo

IV)

3.9

4.4

2.1

0.4

2.3

3.8

0.0

1.0

2.0

3.0

4.0

5.0

Pt/Al2O3 Pt/TiO2 Pt/CeO2 Pt/ZrO2 Pt/Zr-CeO2 Pt/Ce-ZrO

Phosphorus content (wt-%)

Fig. 15. Phosphorus contents of the samples with Pt, treated with the ga

poisoning method.

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2.4 2.3

3.5

0.4

1.92.2

0.0

1.0

2.0

3.0

4.0

5.0

Al2O3 TiO2 CeO2 ZrO2 Zr-CeO2 Ce-ZrO2

Phosphorus content (wt-%)

Fig. 16. Phosphorus contents of the samples without Pt, treated with

phase poisoning method.

In contrast, in the impregnation ageing procedure, the final phosphoru

the sample is equal to the amount of phosphorus impregnated into

Therefore, the gas phase ageing procedure can be used as a tool

possible distinctions in the behaviour of different catalyst components

ageing conditions. (Paper II)

Poisoning can be considered a relevant deactivation mechanism f

reference catalysts (Rh/Pd/Al2O3/La2O3/OSC catalyst aged for 100,

petrol-driven vehicle). According to the XRF analyses, the reference

poisoned by phosphorus, calcium and zinc. However, sulphur was n

The first 2 cm from the inlet of the catalyst comprised the most sever

zone. The elemental analyses (XRF) showed that the catalyst contain

of phosphorus. Figure 17 presents the elemental analysis data fro

region of the TWC, including a back-scattered electron image a

elemental distribution maps (X-ray maps) obtained by EDS As can

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A)

C)

B)

D)

Fig. 17. Elemental analyses (X-ray maps) taken at the inlet of the vehic

catalyst; A) backscattered electron image showing the metal foil, w

contaminant overlayer; EDS elemental maps of B) Al, C) P, and D) Ca (Pap

XRF analyses showed that the phosphorus content of the diesel refer

(Pt/Al2O3/La2O3/OSC aged for 80,000 km in a diesel-driven vehic

wt-%. The catalyst was mainly poisoned by sulphur. Small conce

calcium and zinc were also detected. No contaminated overlayer was

the SEM-EDS studies (Figure 18). Instead, phosphorus had accumutop overlayer of the catalyst. Due to the overlapping peaks of pho

zirconium, quantitative EDS analyses were used to indicate that pho

located only on the upper layers of the catalyst. In contrast, zir

d t t d th h t th t l t U lik h h l h h d

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A)

C)

B)

D)

Fig. 18. Elemental analyses (X-ray maps) taken at the inlet of the vehicle

catalyst; A) backscattered electron image showing the metal foil and

washcoat; EDS elemental maps of B) Al, C) P, and D) S (Paper II).

It should be pointed out that the deactivation of the reference cat

result of a combination of several ageing phenomena and not

poisoning alone. However, based on the activity measurements carr

the vehicle-aged TWC reference (Lassi 2003), poisoning co

approximately 20% of the total deactivation, with phosphorus bein

contaminant.

5.2.2 Pho spho rus com poun ds

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Rh/Ce-ZrO2 and the TWC reference samples (Rh/Pd/Al2O3/La2O3/O

aged for 100,000 km in a petrol-driven vehicle) (Figure 19). F

CeAlO3 was also observed in the TWC reference due to the reactiowith the pure CeO2 in the catalyst's washcoat. (Paper I) According

studies, phosphorus had adhered to the ceria-based samples (CeO

Pt/CeO2, Pt/Zr-CeO2) as CePO4 (monazite-Ce). CePO4 was also found

but not in Pt/Ce-ZrO2. (Paper III)

Intensity(a.u.

)

0

100

200

300

2-Theta - Scale

13 20 30 40 50 60

Rh

a a

aa

a

b

d

a

d

d

d

d

c

cc

c

b&c

b&cb&c

b&c

a

a

Fig. 19. XRD diffractograms of (i) P-poisoned Rh/Al2O3/OSC, (ii) P-poi

CeO2, and (iii) P+Ca-poisoned Rh/Zr-CeO2. a = Ce(PO4), b = Zr-rich CeX

oxide, c = ZrO2 (monoclinic), d = Ce-rich CeXZr1-XO2 mixed oxide, and R(Paper I).

The Zr-rich mixed oxides (Ce-ZrO2 and Pt/Ce-ZrO2) contained

phosphate (Zr(P2O7)). However, zirconium phosphates were not det

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phosphorus appeared as aluminium phosphate AlPO4, as illustrated i

By contrast, no AlPO4 or any other phosphorus-containing compoun

observed in the Al2O3 sample. Phosphorus was detected in both TiO2as titanium pyrophosphate (TiP2O7). (Paper III)

Intensity

(a.u.

)

0

100

200

300

400

500

600

2-Theta - Scale

10 20 30 40 50 60

a

a

a

PtPt

b b

c

c

c

c

cdd

dd

i

i

i

i

Fig. 20. XRD diffractograms of (i) fresh Pt/Al2O3, (ii) P-poisoned Pt/Al2

Pt/CeO2, and (iv) P-poisoned Pt/CeO2. a = Al2O3, b = AlPO4, c = CeO2, d = C

= platinum.

The presence of calcium phosphate Ca3(PO4)2 in the Test series 1

Rh-loaded powders) treated with both phosphorus and calcium (Ca+was also considered. However, it could not be verified due to the sma

and overlapping peaks of CePO4. (Paper I)

The diffraction peaks of the ZSM-5 structure and Pt were identifi

JCPDS Fil C d 44 0003 d 04 0802 i l N i

l t i d b t ti l t f l h (3 2 t %) hi

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sample contained a substantial amount of sulphur (3.2 wt-%), whic

covered phosphorus on the surface of the catalysts. It is also poss

catalyst used in this study did not reach temperatures high enoughphosphorus oxide to phosphates with the washcoat components. (Pape

FTIR-ATR studies

FTIR-ATR studies were carried out in addition to XRD to produce su

information about the poison compounds. Phosphate bands can be11001000 cm-1 in the IR spectra. The presence of AlPO4 was dete

the Pt/Al2O3 (Figure 21) and Al2O3 samples, although phosphorus

could not be detected with XRD in the latter sample. In the case

Pt/TiO2 (Figure 21), a difference between the spectra of fresh a

samples was clearly observed, since phosphate bands were identif

cm

-1

. In the same way with XRD, phosphates were observed in thesamples (CeO2, Zr-CeO2, Pt/CeO2, Pt/Zr-CeO2). Phosphates that c

detected with XRD were detected with IR in the ZrO2 and Pt/Zr

(Paper III)

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110016002100260031003600

cm-1

d)

c)

b)

a)

e)

1078

29883666

1066

1394

Fig. 21. FTIR-ATR spectra of a) fresh Pt/Al2O3, b) P-poisoned Pt/Al2O3, c)

d) P-poisoned Pt/TiO2 and e) vehicle-aged reference catalyst (Paper III).

No phosphorus or any ageing-induced structural changes could be de

ZSM-5 and Pt/ZSM-5 samples with FTIR-ATR. This is an interestinmay differentiate the phosphorus poisoning mechanism of these z

other catalyst components, such as Zr-CeO2 mixed oxides, in which

was detected as phosphates after the same poisoning procedure. Still,

completely exclude the possibility of the presence of phosphates in t

ZSM-5 and Pt/ZSM-5 samples. (Paper V)

In the case of the diesel reference catalyst (360,000 km), phosp

not be detected with certainty with IR. The presence of phosphorus

was expected on the basis of the elemental analyses and XRD studie

not be confirmed due to the overlapping bands of the metal oxides an

The latter phases show an IR adsorption band at around 1078 cm -1 (P

information about the stability of the phosphorus compounds The

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information about the stability of the phosphorus compounds. The

reaction conditions on the poisoning phenomena was also s

calculations with CeO2 and Al2O3 in the presence of a (NH4)2Hmixture, which all were considered bulk materials, were carrie

commercial HSC Chemistry 5.11 software. (Krgeret al. 2007)

Equilibrium compositions and phase stability diagrams were ca

function of temperature and partial pressures of phosphorus and o

oxidizing conditions were chosen to simulate the conditions of t

mixture in the laboratory-scale chemical ageing procedure developed(Paper II). In the thermodynamic calculations the input corresponded

amount of solid oxide, aqueous salt solution of (NH4)2HPO4 and air

system during the four hours of ageing. The following mixture w

simulate the feed gas: 0.405 ml (NH4)2HPO4 (in the liquid phase), 23

(in the liquid phase), 13.2 l O2, 46.8 l N2. The solid input included

oxide or aluminium oxide. The results from the thermodynamic calculations were given as figures that present the mole fract

compounds as a function of temperature within a range of 0C to 100

and aluminium oxides were also studied with phase stability diagram

the partial pressures of oxygen and phosphorus at different temperatu

et al. 2007)

Mole fractions of phosphorus-containing compounds as a

temperature in the presence of Al2O3 or CeO2 and the feed gas mixtur

(NH4)2HPO4, H2O, O2 and N2 are presented in Figs. 22 and 23, resp

can be seen in the figures, formation of AlPO4 and CePO4 starts at ap

70C and the levels of these compounds are balanced at tempera

300C. According to the calculations, the relative amount of CePO4 f

chosen ageing temperature (700C) is higher than the amount of A

same temperature. This can be seen by comparing the molar prCePO4 with CeO2 and AlPO4 with Al2O3, which are 0.54 and 0.23,

At temperatures above 100C, the gas phase is composed almost ex

gaseous water and air (N2 and O2) and only very small mole fractions

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Fig. 22. Mole fractions of phosphorus-containing compounds as a

temperature in the presence of Al2O3 and the feed gas mixture containing

H2O, O2 and N2 (Krgeret al. 2007).

Fig. 23. Mole fractions of phosphorus-containing compounds as a

CePO4 starts already at a lower partial pressure of oxygen than does

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CePO4 starts already at a lower partial pressure of oxygen than does

AlPO4. (Krgeret al. 2007)

Fig. 24. Phase stability diagrams of the Al-O-P system at 700C as a

phosphorus and oxygen partial pressure (Krgeret al. 2007).

According to the calculations, formation of phosphates with th

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g , p p

components is thermodynamically favourable in the oxidizing con

with the oxide samples and phosphorus source used in this study. Inwas shown that aluminium and cerium phosphates can be formed

temperature of diesel exhaust gas catalytic converters, which was cho

temperature used in the ageing procedure developed and evaluated i

(Krgeret al. 2007)

5.3 Loss of catalytic activity

5.3.1 Effect of phosp hor us

In activity measurements, the light-off temperatures (T50) of the fre

catalyst components were compared with each other. T50 is def

temperature of 50% conversion of the feed component. Phosphorus pa significant deactivating effect on the activities of most of the samp

the aged samples were strongly deactivated and their exact light-off t

could not be determined due to the vicinity of the autoignition tempe

feed gas.

Figures 26 and 27 illustrate that the C3H6 light-off temperatur

phosphorus-treated samples were significantly higher than those catalysts in Test series 2 (samples containing oxide and Pt and

addition, it was observed that already a low phosphorus con

considerable deactivating impact on the activity of the catalysts even

poison was evenly distributed in the catalyst powders and not only on

(Papers III and V)

T50 (C)

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179 185

269

214259

199228 227

292261 297

247243 240

309290 304

262

0

100

200

300

400

500

Pt/Al2O3 Pt/TiO2 Pt/CeO2 Pt/ZrO2 Pt/Zr-CeO2 Pt/Ce-ZrO

Fresh

H2O

P+H2O

Fig. 26. C3H6 light-off temperatures of the fresh, hydrothermally aged, a

samples with Pt.

228

350 362 364369

346

383 388

370

0

100

200

300

400

500

T50 (C)

Fresh

H2

OP+H2O

In the case of Test series 1 (samples containing oxide and R

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temperature of CO for Rh/Zr-CeO2 was 186C when fresh and 2

poisoned and for Rh/Ce-ZrO2 173C and 229C, respectively. Whethe results from Test series 1 and 2, it can be seen that the increase in

temperatures for the cerium-rich Rh/Zr-CeO2 and Pt/Zr-CeO2 (s

samples after phosphorus poisoning was smaller compared with the

the zirconium-rich Rh/Ce-ZrO2 and Pt/Ce-ZrO2 (see Fig. 26) sample

it can be concluded that the cerium-rich mixed oxides studied in th

better sustain the deactivating impact of phosphorus than can the zirmixed oxides in terms of catalytic activity. The CO light-off temper

Rh/Ce-ZrO2 and Rh/Zr-CeO2 samples were approximately the

phosphorus impregnation, but the relative decrease in the activ

phosphorus-treated Rh/Zr-CeO2 and Pt/Zr-CeO2 was almost equal. (P

The activities in the propene oxidation reaction (Test series 2, Fig

27) were significantly higher in the case of samples containing pl

were the activities of the same catalyst components without the pre

The propene light-off temperatures of the samples without Pt were

high even with the fresh samples, the only exception being Al2O3. T

temperatures of the fresh TiO2, CeO2, ZrO2, Zr-CeO2, and Ce-ZrO2 s

all between 350 and 370C. Hydrothermal ageing considerably d

activities of the samples. Most of the samples without Pt we

deactivated, and the exact propene light-off temperatures of these sa

not be determined due to the vicinity of the autoignition temperatur

(460C). The most substantial effect was detected in the case of Al2O

propene light-off temperature (>400C, could not be determined) h

by over 170C compared with the fresh sample. After phosphorus t

parent samples were strongly deactivated and the light-off temperatu

samples without the precious metal were above 400C. (Paper III)The Al2O3-based sample showed the highest activity of all t

including those containing Pt. In addition, Pt/TiO2 as well as the zi

samples, Pt/ZrO2 and Pt/Ce-ZrO2, had low propene light-off tempera

propene (460C). After phosphorus ageing, the increase in t

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temperature was highest for Pt/ZrO2 and lowest for Pt/CeO2 compa

fresh samples. In the hydrothermally aged case, the most substantphosphorus poisoning also occurred in the Pt/ZrO2 sample, but u

comparison with fresh samples, the smallest increase in the light-off

occurred with the Pt/Zr-CeO2 sample. (Paper III)

0

20

40

60

80

100

0 100 200 300 400T (C)

C3H

6conversion(%)

Fresh,

T50 = 214C

P+H2O, T50 =

H2O, T50 = 262C

Fig. 28. C3H6 conversion as a function of temperature and light-off temp

of the fresh, hydrothermally aged, and phosphorus-poisoned Pt/ZrO2 sa

III).

With zeolites, the activities in propene oxidation were higher in th

samples than were the activities of the samples without the precious

propene light-off temperature of fresh Pt/ZSM-5 was significantly lowof fresh ZSM-5 (Figure 26 and 27). The fresh zeolite samples showe

to adsorb propene at low temperatures. The presence of a precious m

affect the amount of HC adsorbed. Propene desorbed in the temperat

100

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-20

0

20

40

60

80

0 100 200 300 400

T (C)

C3H6

conversion(%) Fresh,

T50 = 231C

P+H2O, T50 =

H2O, T50 = 302

Fig. 29. C3H6 conversion as a function of temperature and light-off temp

of the fresh, hydrothermally aged, and phosphorus-poisoned Pt/ZS

(Paper V).

The ageing treatments had a deactivating effect on the catalytic ac

zeolite samples. Hydrothermal ageing increased the light-off tem

Pt/ZSM-5 and the effect of phosphorus ageing was even more substhydrothermal and phosphorus ageing, the ZSM-5 samples w

deactivated and the exact propene light-off temperatures of these sa

not be determined due to the vicinity of the autoignition temperatur

(460C). The ageing treatments also affected the hydrocarbon adsorpt

of the zeolites. After both hydrothermal and phosphorus ageing th

behaviour of the Pt/ZSM-5 and ZSM-5 samples for propene, observfresh samples, was almost completely lost. (Paper V)

NO and NOx (combined NO and NO2) conversions remained l

test gas mixture and diesel catalyst samples used in this work. After h

In summary, the phosphorus treatment deactivated the catalyst sa

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than the hydrothermal treatment alone. Deactivation appears to be ass

phosphorus compounds formed with the washcoat components.

5.3.2 Effect of phos pho rus and calc ium

The role of calcium as a catalyst poison has been of a lesser int

literature compared with, for instance, phosphorus and sulphur. H

described next, calcium has a deactivating effect on some Rh-contaipowders.

Calcium poisoning carried out with the impregnation method

Rh/Ce-ZrO2 and Rh/Zr-CeO2 to a small extent, whereas in the cas

Rh/Al2O3/OSC, no deactivation was observed. In all the samples the

effect of calcium was clearly smaller than that caused by P poisoni

light-off temperatures (T50

) of the fresh, P, P+Ca, Ca, and Ca+P-tre

CeO2 and Rh/Al2O3/OSC samples, measured in lean reaction con

presented in Figure 30. (Paper I)

186

163

232216

196

158

196

157179

0

100

200

300

Rh/Zr CeO2 Rh/ Al2O3/OSC

Fresh

P

P+CaCa

Ca+P

T50 (C)

also reported congruent results according to which the addition of

b d t d th i i ff t f h h b f

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observed to reduce the poisoning effect of phosphorus by form

phosphates that prevent accumulation of phosphorus on the surfcatalysts.

Poisoning treatments carried out in a reversed order (Ca+P

showed that phosphorus treatment also had a regenerating effect on

Rh/Ce-ZrO2 and Rh/Zr-CeO2. However, this was not the

Rh/Al2O3/OSC, and phosphorus treatment of calcium-poisoned Rh

reduced catalytic activity. It is possible that the more complex comthe presence of Al2O3 in Rh/Al2O3/OSC affect the reaction between

phosphorus. (Paper I)

As demonstrated in this study, calcium can decrease the catalyti

components used in automotive catalysts. Therefore, the role of

catalyst deactivation should be considered more profoundly, and furth

poisoning mechanisms are needed.

5.4 Changes in the active metal particles and washcoat g

The TEM analysis indicated considerable structural changes in

washcoat and Pt particles after both ageing treatments. The grain

Pt/Al2O3 washcoat had not increased and no transition from the -ph

phase had occurred. The platinum particles had grown in both treatmlarge particles (>100 nm) were also detected. The grain size of

washcoat had grown from about 1015 nm to 3040 nm after the

Detection of platinum was very difficult, and no clear indications o

growth were observed in the treated catalysts. The grain size of both

and Pt/Zr-CeO2 washcoats had increased from under 10 nm to

Detection of platinum was very difficult also in these two sampleoccasionally were large Pt particles observed in the aged catalysts. A

the analysis, the washcoat of the zirconia-based Pt/ZrO2 and Pt/Ce-Z

had gone through only minor changes due to the treatments. Once aga

than in the hydrothermally aged Pt/ZSM-5. The hydrothermal

h h i i t t t did t l h

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phosphorus poisoning treatments did not cause any clear chan

microstructures of the ZSM-5 and Pt/ZSM-5 zeolite washcoats. Hplatinum particle size distribution measurements of the Pt/ZSM-5 ze

an increase in Pt particle size after both hydrothermal ageing and po

number of small Pt particles (diameter of 025 nm) is higher in the fr

5 than in the treated samples. The growth of the Pt particles in the hy

aged Pt/ZSM-5 occurred at the expense of small Pt particles. Cluster