Partial Oxidation

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Hydrogen production by partial oxidation of

methane over Co based, Ni and Ru monolithic

catalysts

Halit Eren Figen* , Sema Z. Baykara

Yildiz Technical University, Chemical Engineering Department, Davutpasa Campus, Topkapi 34210, Istanbul,

Turkey

a r t i c l e i n f o

Article history:

Received 30 August 2014

Received in revised form

21 February 2015

Accepted 23 February 2015

Available online xxx

Keywords:

Hydrogen production

Methane

Partial oxidationCPOM

Catalyst

Monolith support

a b s t r a c t

Fossil fuels which supply most of the world energy demand are depletable, and they cause

greenhouse gas emissions which eventually lead to global warming and climate change.

Hydrogen, a clean and versatile energy carrier, can be converted into useful forms of en-

ergy in several ways. Catalytic partial oxidation of methane is a very promising process for

hydrogen and synthesis gas production, besides steam reforming of methane, the leading

technology. In the present work, catalysts for partial oxidation of methane have been

developed and studied in terms of structural properties and chemical performance. For this

purpose Co, CoeNi, CoeRu, CoeNieRu, and Ni catalysts loaded onto cordierite ceramic

monolithic supports were prepared via modified sol-gel-impregnation method. The cata-

lysts were characterized by, SEM-EDS, XRD, BET, and ICP-OES techniques. Activity tests of

the catalysts were performed in a tubular reactor at 450 ml/min total flow rate from 600 Cto 850 C. CoeNieRu was the most successful catalyst, with selectivity values of 93.10% H 2

and 93.81% CO, and CH4 conversion of 98.71%, and hydrogen production efficiency of

95.89% at 850 C. During the activity tests of this catalyst 2.13% CO2 was present in the

product stream.

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Production of hydrogen (H2) from sources with established

infrastructures such as natural gas, which is mostly methane

(CH4), via catalytic processes is expected to facilitate transi-

tion to clean energy systems and sustainable development

[1e5]. In recent years, catalyst development and determina-

tion of optimum operating conditions for fuel processors have

been the main areas of investigation contributing to the

progress of catalytic H2 production [6e11]. Catalytic process-

ing of CH4 at large scale is mostly carried out by steamreforming, partial oxidation, and dry reforming [12e14]; where

synthesis gas containing H2 and CO is produced from which

H2 and other fuels can be obtained. Catalytic partial oxidation

of methane (CPOM) yields 2 mol of H2 and ~36 kJ/mol energy

and ~30% cost reduction in comparison to steam reforming.

Moreover, NOx formation is avoided, smaller reactors [6e13]

can be used, and H2 /CO ratio (~2) is suitable for methanol

production and motor fuel synthesis (FischereTropsch) [12].

* Corresponding author. Tel.: þ90 533 311 5112, þ90 212 383 4757; fax: þ90 212 383 4725.

E-mail addresses: [email protected], [email protected] (H.E. Figen).

Available online at www.sciencedirect.com

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3

http://dx.doi.org/10.1016/j.ijhydene.2015.02.109

0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Figen HE, Baykara SZ, Hydrogen production by partial oxidation of methane over Co based, Niand Ru monolithic catalysts, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.02.109

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Main reaction of CH4 partial oxidation (R1), accompanied with

full oxidation of CH4 (R2) and with side Reactions R3 and R4,

can be assumed and stated as (at 25 C and 1 atm):

R1: CH4 þ ½ O2/ CO þ 2H2, DH ¼ 35.59 kJ/mol (1)

R2: CH4 þ 2O2/ CO2 þ 2H2O, DH ¼ 802.0 kJ/mol (2)

R3: CH4 þ3∕2 O2/ CO þ 2H2O, DH ¼ 519.33 kJ/mol (3)

R4: CH4 þ O2/ CO2 þ 2H2, DH ¼ 318.66 kJ/mol (4)

Reaction R1 is the main reaction step and it produces

synthesis gas (CO þ H2). Reaction R2 is a full combustion re-

action of CH4 while R3 is a side reaction that increases selec-

tivity of CO and decreases selectivity of H2. On the other hand,

side Reaction R4 increases selectivity of H2 and decreasesselectivity of CO [15e17].

Catalytic partial oxidation of methane reaction has been

studied in presence of heterogeneous catalysts with or

without noble metal constituents. Although noble metal cat-

alysts have high activity and stability, due to their high cost

and low accessibility, non-precious metal catalysts are often

preferred in industrial applications [12,18].Various catalysts

containing metals, noble and otherwise, have been developed

and studied previously, and results involving metals such as

B, Ca, Ce, Co, Ir, La, Ni, Pd, Pt, Rh, Ru, Sr, Th, Y, and Zr are

available [12,18e23] for applications using powder or mono-

lithic catalysts.

Nickel (Ni) is the most widely used catalyst presently.Although Ni containing catalysts have high activity, the

exothermic CPOM reaction leads to carbon deposition, sin-

tering of Ni due to coke formation; and deactivation of the

catalyst by forming NiAl2O4 phase especially when Ni is used

with Al2O3 support [12,18e23]. Studies with Ni catalysts in the

literature indicate reduction in coke formation and increase in

activity upon addition of Co, Cr, Sn, Mg, Ca, Ce, Gd, Y, Zr and

even trace amounts of noble metals [18e23]. For example, Ce-

NixOy has been prepared by precipitation method and was

used in partial oxidation of CH4 for H2 production; CH4 con-

version and H2 selectivity values of 75% and 52% were ob-

tained at 600 C respectively, while 70% CH4 conversion and

49% H2 selectivity were obtained at 200

C [6]. Catalysts Ni andCu in various compositions were deposited on Al2O3 by

impregnation method and they were tested in CH4 partial

oxidation reactions. When the CH4 to O2 ratio was 2.0 to 1.0,

50% conversion of CH4 was attained with Ni(5%)Cu(5%)/Al2O3

at 300 C [18]. In another work, partial oxidation of CH4 cata-

lyzed by NiO/Al2O3 including PteCeO2 ataC H4 to O2 ratio of 2.0

to 1.0 and at 800 C, hydrogen concentration was 40%; and

when the space velocity was set to 320 l. (h.g cat)1, this value

reached 74%. Catalyst selectivity to H2 was obtained as 87%

under the same conditions [19]. In a study where Ni/a-Al2O3,

NiSn/a-Al2O3, NiMn/a-Al2O3, NiMo/a-Al2O3 catalysts were

prepared by impregnation and tested for partial oxidation of

CH4, the results of the catalytic tests carried out at 800

C

indicated that the most promising catalyst was NiMo/a-Al2O3

with 90.6% CH4 conversion and 93.9% H2 selectivity [20].

Catalysts incorporating 3% (Ni þ Co) and supported on

CaAl2O4 /Al2O3 were prepared via impregnation method. The

most active catalyst in this work was (2%Ni þ 1%Co) on

CaAl2O4 /Al2O3 exhibiting 97% CH4 conversion and 97% H2

selectivity at 800 C [12]. Addition of Co can result in reduction

in the rate of carbon formation [24e

26], however Co catalystscan be strongly affected by factors such as the nature of

support, calcination temperature, and metal loading [27].

The activity of various Group VIII metals for hydrogen

production from methane by steam reforming is known to

follow the relative order: Ruz Rh > Ni > Ir > Pdz Pt >> Coz

Fe. Ruthenium (Ru) and Rhodium (Rh) have better stability on

stream, in the long term than Ni [28]. Furthermore, Ru is a lot

less expensive than other precious metals [29]. Precious

metals like Ru are better catalysts than Ni for methane com-

bustion and partial oxidation reactions as well [28]. Alumina

supported Ru catalysts (1% w/w) have good activity and

selectivity towards partial oxidation of methane [30]. For

example, a catalyst with as little as 0.015% (w/w) Ru on Al2O3

can display higher synthesis gas selectivity than a catalyst

with Ni on SiO2 [28].However, hot spot formation in the cata-

lyst bed and coking (leading to catalyst deactivation) are

possible with supported Group VIII metals (Rh, Pt, Ru, Ni) as

catalysts for partial oxidation of methane [6,12]. Coking is

more likely with Ni based catalysts, although Ni is much

cheaper, and is a good catalyst for synthesis gas production

[12].

In the present work, partial oxidation of CH4 over mono-

lithic catalysts with oxides of Co, CoeNi, CoeRu, CoeNieRu

and Ni for H2 production was investigated. Catalysts were

prepared and tested in terms of structural properties and

chemical performance.

Experimental

Materials and characterization

All reagents used were of analytical grade. Nitrate salts of Co,

Ni and Al, (Co(NO3)2$6H2O, purity:>99%, from Carlo Erba,

Ni(NO3)2$6H2O, purity:>99%, from Carlo Erba, Al(NO3)3$9H2O,

purity: >99%, from Merck), citric acid monohydrate

(C6H8O7$H2O, purity: >99% from Carlo Erba), and Ruthenium

Chloride Ru$Cl2$xH2O(Merck) were used as received. Honey-

comb type cordierite monolithic structures (400 CPSI),48.3 mm 48.3 mm 150 mm blocs, Rauschert Technical

Ceramics) were used as supports in the preparation of cata-

lysts. The Honeycomb type cordierite monoliths are referred

to as M-0 in the manuscript. Instrumental analysis with X-ray

diffraction (XRD), scanning electron microscopy-energy

dispersive spectroscopy (SEM-EDS), inductively coupled

plasma-optical emission spectroscopy (ICP-OES) and Brun-

nereEmmetteTeller (BET) surface analysis techniques were

used for structural study of the samples.

Characterization of crystal structure and determination of

crystallographic parameters of the catalysts were performed

by XRD analyses. Samples were ground in an agate mortarand

settled in an aluminum sample holder and XRD analyses were









carried out at ambient temperature using a Philips Panalytical

X'Pert-Pro diffractometer in a diffraction angle range of

10e90 with CuKa radiation (l ¼ 0.15418 nm) at operating

parameters of 40 mA and 45 kV with a step size of 0.02 and

speed of 1 /min. Phases were identified with reference to

powder diffraction file (PDF) database.

Specific surface area of the catalysts were characterized by

using BET technique under N2 adsorptive gas and He carriergas at 77 K after outgassing at 0.6 Pa and 473 K, using Quan-

tachrome, Autosorb Instrument.

To quantify metal contents present in the samples ICP-OES

measurements were performed using Perkin Elmer Optima

2100DV. Beforethe ICP-OES readings, a few milligrams of each

catalyst sample was ground into a powder which was dis-

solved in a mixture of certain strong acids (H3PO4, HCl, HNO3,

HF). The sample was then treated in a microwave digester

(Berghof Speedwave 3þ, Microwave Digestion System). During

the ICP-OES analysis; each treated catalyst sample was

divided into three portions, and according to the standard

procedure, 3 parallel readings were obtained and their average

was taken as the final value of the elemental analysis of theloaded metal content for the specific sample [31].

Microstructure and surface morphology of the catalysts

were observed by field-emission gun scanning electron mi-

croscopy (CamScan Apollo 300 FEG-SEM). The samples were

covered with Au and made ready for analysis by fixing to the

device's sample holder with carbon sticky band.

Carbon deposition on catalysts were detected using the

thermal gravimetric analysis/Fourier transform infrared

spectroscopy (TGA/FTIR) technique, by measuring the CO2

evolved during thermal analysis [32,33]. A thermogravimetric

analyzer (Perkin Elmer Diamond TG/DTA) was coupled with a

Fourier transform infrared spectrometer (Perkin Elmer Spec-

trum One). Placing approximately 10 mg of sample on TGAbalanceand using a purge gas (pure O2) rate of 200ml/min, the

gases from TGA unit were transferred into the IR spectrom-

eter; and the interface was maintained at 200 C. A tempera-

ture program from ambient to 900 C, increasing at a rate of

5 C/min was used. Peaks within 2358e2344 cm1 bands were

investigated for CO2 asymmetric stretching.

Preparation of monolithic catalysts

Monolithic catalysts with oxides of Co, CoeNi, CoeRu,

CoeNieRu, and Ni were prepared by the sol-gel impregnation

technique. Preparation procedure can be divided into three

steps. (1) Preparation of monolithic ceramic supports: Beforecoating with catalysts the ceramic supports were prepared in

cylindrical shape with 13 mm diameter and 20 mm length. (2)

Coating of monolithic ceramic supports by alumina: Before

coating the monoliths with active catalysts, alumina was

wash coated [34] on thesurface of ceramic supports in order to

form a surface which would enable metals to adhere easily.

For this purpose 5 M aluminum nitrate solution was prepared

to begin wash coating of alumina onto the substrates. Weight

gain after each deposition was recorded until desired loadings

were achieved. Monoliths which were impregnated with

alumina nitrate solution were then calcinated at 600 C for3h.

During calcination of the monolithic supports, aluminum ni-

trate decomposes into nitrates and has high surface area after

the formation of cubic crystal gamma-alumina (g-Al2O3)

phase. (3) Coating of monolithic ceramic supports by sol-gel

impregnation method: Ceramic supports which had been

coated with alumina were sol-gel impregnated by using so-

lutions of Al(NO3)3$9H2O, Ni(NO3)2$6H2O, Co(NO3)2$6H2O, and

RuCl2$xH2O according to their composition. Weight gains

were again monitored to obtain desired loadings fallowed by

calcination at 800 C for 5 h. Synthesized catalysts werelabeled as M (Table 1) and were used for referring to support

and catalyst samples.

Performance tests of monolithic catalysts

Monolithic catalysts with oxides of Co, CoeNi, CoeRu,

CoeNieRu, and Ni were tested for H2 production from CH4

partial oxidation in a catalytic fuel processor system (HyGear-

Hexion located in TUBITAK-Marmara Research Center, Energy

Institute) equipped with Agilent 6890 gas chromatography

including a thermal conductivity detector (TCD) and a flame

ionization detector (FID). MolSieve, Plot-Q and GasPro col-

umns were used for separation of gasses. Before the experi-

ments, gas chromatography device was calibrated with a

certificated standard gas mixture suitable for feed andproduct

gases. In terms of determination of gas composition by gas

chromatography (GC), sources of uncertainty described in Ref.

[35] were taken into consideration. In the HyGear experi-

mental set-up the accuracy levels (as provided by the sup-

pliers) of mass flow controllers (Bronkhorst El-Flow),

temperature controller (Enda EUC442 PID Controller) with K-

type thermocouples (NiCreNi, 0e1200 C), and pressure

gauges (Swagelok, BG10) were ±0.5%, ±0.2% of full scale and

±1 C; and ±1.5% respectively.

The partial oxidation reaction was carried out in a tubular

stainless steel reactor with an inside diameter of 1.35 cm,

operated at atmospheric pressure. Reactants CH4 (99.500%),

O2 (99.999%), N2 (99.999%), H2 (99.999%) and calibration gases

for GC were supplied by HABAS and Air Products Company,

respectively. A gas mixture of CH4 and O2 (with a CH4 to O2

ratio of 2.0 to 1.0) was passed through the monolithic cata-

lysts at a gas hourly space velocity (GHSV) of 1 104 h1. The

GHSV was defined as the ratio of the reactant gas flow rate at

25 C and 1 atm to the total volume of the catalyst. The

catalysts were reduced in situ for 8 h at 300 C with hydrogen

gas (1%) before reaction. Water content of the reacting

mixture was condensed. Calculation of conversions and

yields were carried out according to the general equations

below [34,36]:

CH4 conversation : xCH4 ð%Þ ¼

FCO þ FCO2

FCO þ FCO2 þ FCH4

$100 (5)

H2 selectivity : SH2ð%Þ ¼

FH2

2$

FCO þ FCO2

$100 (6)

CO selectivity : SCOð%Þ ¼ ðFCOÞFCO þ FCO2

$100 (7)

H2=CO ratio : H2

CO

¼ FH2

FCO

¼2$SH2

SCO

(8)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 3 3








If H2 /CO > 2, partial oxidation process occurs according to

reactions R1, R2 and R4 [34].

R1 ð%Þ ¼ FCO

FCO þ FCO2

$100 ¼ SCO (9)

R2 ð%Þ ¼

FCO þ FCO2

FH2

2

FCO þ FCO2

$100 ¼ 100 SH2 (10)

R4 ð%Þ ¼

FH2

2 FCO

FCO þ FCO2

$

100 ¼ SH2 SCO (11)

If H2 /CO < 2, partial oxidation process occurs according to

reactions R1, R2 and R3 [34].

R1 ð%Þ ¼ FH2

2

FCO þ FCO2

$100 ¼ SH2 (12)

R2 ð%Þ ¼

FCO2

FCO þ FCO2

$100 ¼ 100 SCO (13)

R3 ð%Þ ¼

FCO FH2

2

FCO þ FCO2

$100 ¼ SCO SH2 (14)

Based on product compositions, CH4 conversion, H2 and CO

selectivity, and H2 /CO ratio were calculated at different tem-

peratures for Co,CoeNi, CoeRu, CoeNieRu, and Ni monolithic

catalysts and also reaction steps and realization ratios (extent

of completion of the reaction, %) of these reactions (R1, R2, R3

and R4) were determined. Increase in CH4 conversion, H2 and

CO selectivity values were observed with increase in temper-

ature (Table 3).

Results and discussion

Results pertaining to phase and elemental identification,

morphology, surface area, optimum reaction temperature and

activity rating of the catalysts have been obtained.

Structural characterization of monolithic catalysts

By using impregnation method, ceramic supported catalysts

incorporating various Ni, Co and Ru based oxides were pre-

pared. Crystal phase properties, elemental analysis, specific

surface area and microstructure properties were determined

by XRD, ICP-OES, BET and SEM methods.

Fig. 1 e XRD patterns of monolith support and monolithic catalyst: (a) Blank Monolith M-0, (b-1) M-1 Fresh, (b-2) M-1 Spent,

(c-1) M-2 Fresh, (c-2) M-2 Spent (d-1) M-3 Fresh, (d-2) M-3 Spent, (e-1) M-4 Fresh, (e-2) M-4 Spent, (f-1) M-5 Fresh and (f-2) M-5

Spent.









Fig. 1 shows the XRD patterns of the ceramic monolith and

prepared monolithic catalysts in fresh and spent states. The

ceramic support cordierite (Mg 2Al4Si5O18) has hexagonal

crystalline structure with 01-084-1221 ICDD file number. Ac-

cording to the XRD analysis results, oxides of Co, CoeNi,

CoeRu, CoeNieRu, and Ni monolithic catalysts have cordi-

erite as the main phase. The XRD patterns verified the exis-

tence of Co3O4 (PDF: 01-074-1656) as the metal oxide phase in

all Co containing monolithic catalysts. The detailed crystal

phase analysis results of the other catalysts (fresh and spent)

are listed in Table 1. As expected, following impregnation, Co,

Ni, and Ru metal phases were crystalized as their oxide forms

(Co3O4, NiO, RuO2) on the ceramic support. Based on XRD

analyses of spent catalysts samples elemental Ni was found inM-5 (Fig. 1f, Table 2). Possible phases of CoNiO2, Co2RuO4,

NiAl2O4, CoAl2O4, NiO2, NiRuO2, Ru, Ni, Co were not encoun-

tered in remaining samples M-1 to M-4, indicating that new

metallic and oxide phases were not formed in those samples

within 18 h of exposure.

Crystal sizes of metal oxide phases found on ceramic

supports were calculated using Scherrer equation based on

XRD results [37]. At first, the most intense peaks of metals and

their locations (2q) were determined and the “Full Width at

Half-Maximum (FWHM)” values were specified. After “Fit

Profile” calculations [38], it was seen that Co3O4; NiO; Co3O4

and NiO; Co3O4 and RuO2; Co3O4, NiO and RuO2 oxide crystals

on ceramic supports of Co, Coe

Ni, Coe

Ru, Coe

Nie

Ru, and Ni

monolithic catalysts, had relatively same crystal sizes be-

tween 23.45 nm and 75.56 nm. It can be emphasized that

although each catalyst had different composition, the metal

oxide crystals formed at relatively same size since they were

prepared using the same procedure.

The specific surface areas of monolithic catalysts with

oxides of Co, CoeNi, CoeRu, CoeNieRu, and Ni were

measured to be 29.90 m2 /g, 25.42 m2 /g, 34.76 m2 /g, 38.68 m2 /g,

and 39.90 m2 /g respectively while the specific surface area of

ceramic support was 4.20 m2 /g. Although the monolithic cat-

alysts had quite similar specific surface areas, the sample

containing Ni, Co and Ru oxides had the largest, which is

consistent with literature [39]. Elemental composition of Co,

Coe

Ni, Coe

Ru, Coe

Nie

Ru, and Ni monolithic catalysts weredetermined by ICP-OES analysis. Table 1 gives the metal

content of the metal oxide phases in the catalyst samples.

Fig. 2a(50 magnification) and Fig. 2b (1000 magnifica-

tion) show the SEM images of horizontal cross sections of

interior surfaces of blank ceramic monolith and prepared

monolithic catalysts, respectively. According to SEM image of

ceramic monolith taken at 50 magnification (Fig. 2a1), it is

easily seen that corners of the blank ceramic monolith are

clear. Fig. 2a2e2a6 show increasing metal oxide accumulation

and regions of metal oxide agglomerates at the corners of the

square channels. Similar results are reported in literature [40].

According to SEM images taken at 1000 magnification

(Fig. 2b1e

6), comparison of horizontal cross sectional views of

Fig. 1 e ( continued ).









the interior channels of blank ceramic supports and metal

oxide loaded supports show that the coating process of poreswith metal oxides by impregnation method fills the pores in

between crystal like structures on the surface. From SEM im-

ages, crystal structures smaller than 1 mm can be observed.

Fig. 3 shows the elemental mapping analysis of the blank

ceramic monolith and prepared monolithic catalysts.

Elemental mapping image of the blank ceramic monolith

(Fig. 3a) shows that the interior channel surface also reveals

the homogeneous dispersion of Al, Mg and Si oxides in the

cordierite structure. Results from elemental mapping of inte-

rior channels of prepared monolithic catalysts (Fig. 3bef)

indicate homogeneous dispersion of elements over the inte-

rior surfaces and deposition of metal oxides at the corners

along the channel length. This situation is in agreement withSEM images (Fig. 2b).

Aging (sintering) is promoted by prolonged exposure to

high gas-phase temperatures. Rates of metal sintering can be

greatly minimized by choosing reaction temperatures lower

than 0.3-0.5 times the melting point of the metal, since metal

crystallite growth is highly thermally activated [41]. Although

water vapor in the reaction atmosphere may accelerate the

crystallization and structural modification of oxide supports,

it is possible to lower sintering rates by adding thermal sta-

bilizers to the catalyst. Thermal stability of a base metal such

as Ni can be increased via addition of a higher melting noble

metal such as Rh or Ru [42]. In the present study, maximum

reaction temperature was not higher than 850 C. Among the

catalysts used, Ni and Co have relatively lower melting points

(1455 C, 1495 C) compared to those of Ru and Mo (2334 C,2623 C), and the temperature ratios were 0.58, 0.57, 0.36 and

0.32 respectively.

Catalyst performance experiments were carried out in a

temperature range of 650e850 C, using a freshsample in each

run, and the exposure time did not exceed 18 h. Methane

conversions with present catalyst samples were quite close to

the values obtained by thermodynamic calculations [16].

Consequently, possibility of significant levels of sintering

was quite low, especially in samples containing Ru, as it has

been confirmed in Ref. [42].

Carbon or coke results from a balance between the re-

actions that produce atomic carbon or coke precursors and

the reactions of these species with H2, H2O or O2 that removethem from the surface.

Methods for lowering formation rates of precursors of

carbon or coke relative to their gasification rates vary with the

mechanism of formation and the nature of the active catalytic

phase [43]. Formation and growth of species of carbon or coke

on metal surfaces can be minimized by choosing reaction

conditions that minimize their precursors and by introducing

gasifying agents (ie, H2, H2O). Introduction of modifiers which

change surface metal ensemble sizes or which lower the sol-

ubility of carbon (i.e. Pt in Ni) can be effective in minimizing

deactivation due to fouling by carbon and coke [43].

In a previous study [43], investigating carbon deposition on

supported metal catalysts during partial oxidation of methane

Fig. 1 e ( continued ).









to synthesis gas at 1050 K (777 C) using a CH4 to O2 ratio of 2.0

to 1.0, it was found that the relative rate of carbon deposition

followed the order of Ni > Pd >> Rh, Ru, Irz Pt.

Very little carbon deposition was observed over the noble

metal catalysts, even after 200 h; demonstrating that macro-

scopic carbon deposition was independent of the mechanismfor synthesis gas production and that it was possible to

kinetically avoidcarbon deposition by using suitable catalysts.

In the present work, the exposure time of thecatalysts was

shorter (~18 h maximum) and CH4 conversion was close to

equilibrium.As canbe seen from Table 3, since reactions R1, R3

and R4 involve species (CO2 and H2O) that prevent carbon

deposition, significant deactivation was not expected espe-

cially in Ru containing catalysts. Carbon deposition on the

catalystswas investigatedin thepresent study viaTG-FTIR and

IR absorbancevalues of CO2 gaswithinthe wave numberrange

of 2344e2358 cm1 were obtained in the temperature range

45e680 C. Time scale being proportional to the oxidation

temperature, the amount of CO2 is interpreted in view of Lambert BeerLaw assumingthat thevariation of absorbance of

CO2 is directly proportional to its concentration. The maximum

absorbancevalues (absorbance/g. catalyst) of the catalysts M-1

to M-5 were 0.507 (at 346 C), 0.576 (at 440 C), 0.285 (at 360 C),

0.175 (at316 C)and0.972 (at 501C). Consequently, the relative

rate of carbondeposition on thecatalysts fallowed theorderM-

5 > M-2 > M-1 > M-3 > M-4 (Fig. 4).

Performance tests of monolithic catalysts

Variation of product compositions with temperature for cata-

lysts with Co, CoeNi, CoeRu, CoeNieRu, and Ni can be seen in

Fig. 5. For Co monolithic catalyst, generation of H2 and CO

gasses was not observed until 600 C. The product gas also

contained 29.39% CO2. Hydrogen (H2) generation started at

650 C and increased continuously to 29.65%, 52.43%, 62.486%

and 63.26% at 700 C, 750 C, 800 C and 850 C, respectively.

After 800

C there was no considerable increase in H2 compo-sition. While the percentage of H2 and CO increased, percent-

age of CH4 and CO2 were decreased with increasing

temperature. Values of CH4 conversion, H2 selectivity, CO

selectivity and H2 /CO ratio were calculated based on product

compositions at different temperatures in the range of

650e850 C. In addition, reaction steps and realization ratios of

these reactions were determined. As can be seen in Table 3,

CH4 conversion, H2 selectivity, and CO selectivity increased

with temperature. Methane full combustion reaction took

place only with 100% realization ratio at 600 C, accompanied

by both partial and full oxidation reactions of methane and the

second side Reaction R4, (Table 3)at650 C and 800 C. Methane

partial oxidation reaction (R1) started becoming dominant witha realization ratio of 71.30% at 750 C. The realization ratios of

methane partial oxidation reaction, defined as the main reac-

tion, were determined as 88.69% and 91.51% as temperature

increased to 800 C and 850 C, respectively.

Variation of product compositions with temperature is

given in Fig. 5 for CoeNi catalyst. At 600 C 21.98% CO2 was

present in product gas mixture. Hydrogen production started

at 650 C and its composition increased to 29.81% (700 C),

51.33% (750 C), 59.15% (800 C) and 62.49% (850 C). By 850 C,

H2 and CO amounts increased substantially parallel to the

significant increase in methane conversion. At 600 C, only

methane full combustion reaction took place with 99.48%

Table 1 e Structural properties of fresh monolithic catalysts.

Code Catalyst Crystal phases Crystal sizes (nm) Elemental composition (Weight, %)

M-0 Monolith support Mg 2Al4Si5O18 (01-084-1221) e 35.1e14.4e50.5 (Al2O3eMgOeSiO2)

M-1 Co Co3O4 (01-074-1656) 65.58 2.33

(Co)

M-2 CoeNi NiO (01-089-7130) 34.20 1.22e1.20

Co3O4

(01-076-1802) 35.75 (CoeNi)

M-3 CoeRu Co3O4 (01-078-1969) 39.27 2.44e0.05

RuO2 (00-018-1139) 75.56 (CoeRu)

M-4 CoeNieRu Co3O4 (01-076-1802) 38.07 0.30e0.30e0.02

NiO (01-071-1179) 23.45 (CoeNi e Ru)

RuO2 (00-021-1172) 43.72

M-5 Ni NiO (01-089-3080) 39.85 1.65

(Ni)

Table 2 e Crystal phases of fresh and spent catalyst samples.

Code Catalyst Fresh catalyst crystal phases Spent catalyst crystal phases

M-0 Monolith support Mg 2Al4Si5O18 (01-084-1221) Mg 2Al4Si5O18 (01-084-1221)

M-1 Co Co3O4 (01-074-1656) Co3O4 (01-074-1656)

M-2 CoeNi NiO (01-089-7130) NiO (01-089-7130)

Co3O4 (01-076-1802) Co3O4 (01-076-1802)

M-3 CoeRu Co3O4 (01-078-1969) Co3O4 (01-078-1969)

RuO2 (00-018-1139) RuO2 (00-018-1139)

M-4 CoeNieRu Co3O4 (01-076-1802) Co3O4 (01-076-1802)

NiO (01-071-1179) NiO (01-071-1179)

RuO2 (00-021-1172) RuO2 (00-021-1172)

M-5 Ni NiO (01-089-3080) NiO (01-089-3080)

Ni (01-078-0712)









realization ratio. At 650 C, both partial and full oxidation of

methane and also second side reaction (R4, Table 3) were in

progress. As from 750 C, methane partial oxidation reaction

(R1) became dominant with a realization ratio of 69.50%. At

temperatures 800 C and 850 C realization ratios of the re-

action were 84.70% and 90.60%. Conversion of methane and

composition of reaction products increased from 800 C to

850 C. While partial oxidation main reaction and full oxida-tion reaction occured along with second side reaction ( R4) up

to 800 C, R4 replaced the first side reaction (R3) at 850 C. Also

selectivity of CO increased and CO2 level decreased apparently

in parallel with the extent of full oxidation reaction at 850 C.

Product compositions at different temperatures for CoeRu

catalyst are given in Fig. 5. In the product mixture at 600 C

29.32% CO2 was found. Starting from 650 C, 23.66%, 50.44%,

62.93% and 63.64% H2 was found in product gas at 700 C,

750 C, 800 C and 850 C respectively. When the temperature

reached 850 C, there was a slight increase in H2 and CO

amounts in parallel with a significant increase in methane

conversion. At 600 C, methane full oxidation reaction was in

progress with 95.02% realization ratio. After 700 C, partialoxidation of methane and second side reaction (R4, Table 3)

started along with full oxidation reaction. From 700 C,

methane partial oxidation became dominant with 62.60%

realization ratio and remained as the main reaction with

realization ratiosof 76.84%,87.86% and91.44% at 750 C,800 C

and 850 C respectively. Methane conversion increaseed sub-

stantially in parallel with hydrogen from 800 C to 850 C.

Partial oxidation main reaction and full oxidation reaction

along with second side reaction R4 continued at 850 C, par-

allel to increase in hydrogen selectivity.

Change in product compositions with temperature for

CoeNieRu catalyst is given in Fig. 5. At 600 C, 23.10% CO2 was

found in product gas mixture. At 650 C, hydrogen productionstarted and became: 18.78% (650 C), 48.03% (700 C), 57.93%

(750 C), 62.52% (800 C) and 63.92% (850 C). There was no

apparent increase in hydrogen amount after 800 C. In parallel

with temperature dependent H2 percentage increase, CO

amount also increased; however, CH4 and CO2 amounts

decreased. At 600 C, only methane full oxidation reaction

occured with 100% realization ratio. From 650 C, partial

oxidation of methane and second side reaction (R4, Table 3)

were in progress along with full oxidation reaction. From

700 C, methane partial oxidation became dominant with

64.30% realization ratio and remained so at 750 C, 800 C and

850 C with realization ratios of 81.92%, 90.85% and 93.10%.

From 800 C to 850 C there was substantial increase inmethane conversion in parallel with hydrogen amount. While

partial oxidation main reaction and full oxidation reaction

continued along with second side reaction (R4)upto800 C, R4

replaceed first side reaction (R3) at 850 C. Selectivity of CO

increased while CO2 level decreased.

Variation of product compositions with temperature is

given in Fig.5 for Ni catalyst. At 600 C 26.25% CO2 was present

in the product gas mixture. Hydrogen production started at

650 C and its composition increased to 43.39% (700 C),47.54%

(750 C), 56.87% (800 C) and 63.32% (850 C). By 850 C, H2 and

CO amounts increased substantially parallel to the significant

increase of methane conversion. At 600 C, only methane full

oxidation reaction occured with 100% realization ratio. At

Fig. 2 e XRD SEM images of monolith support and

monolithic catalysts: (a) Cross sections at 50X

magnification, a1: M-0, a2: M-1, a3: M-2, a4: M -3, a5: M-4,

a6 : M-5; (b) Inner channels at 1000X magnification, b1: M-

0, b2: M-1, b3: M-2, b4: M-3, b5: M-4, b6 : M-5.









650 C, both partial and full oxidation of methane and also

second side reaction (R4, Table 3) were taking place. As from

700 C, methane partial oxidation reaction (R1) became

dominant with a realization ratio of 53.41%. At temperatures

750 C, 800 C and 850 C realization ratios of the reaction were

62.99%, 78.75% and 91.42%. Conversion of methane and

composition of reaction products significantly increased from

800 Cto850 C. While partial oxidation main reaction and full

oxidation reaction were in progress along with second side

reaction (R4) up to 800 C, R4 replaces the first side reaction

(R3) at 850 C. Also selectivities of CO and H2 remarkably

increased while CO2 level decreased at 850 C.

Methane (CH4) conversion, H2 selectivity and CO selec-

tivity are the most important parameters for determining

Fig. 3 e Inner channel elemental mapping of monolith support and monolithic catalysts: (a) M-0, (b) M-1, (c) M-2, (d) M -3, (e)

M-4, (f) M-5.









the catalyst performance. Type of support, promoter mate-

rials, and textural properties of the catalyst, reaction tem-

perature and GSHV affect the performance parameters.

Values of CH4 conversion, H2 selectivity and CO selectivity

reported in this study compared favorably with other re-

ported results for CH4 partial oxidation with monolithic

catalysts (Table 4) [15,33,35e38]. It can be seen that oxides of

Co (M-1), CoeRu (M- 3), CoeNieRu (M- 4), and Ni (M-5)

monolithic catalyst demonstrated high catalytic activity for

CH4 partial oxidation.

The values of CH4 conversion in the present work obtained

with Co, Coe

Ni, Coe

Ru, Coe

Nie

Ru, and Ni, monolithic cata-lysts were higher than the results with a Ni monolith (81.18%)

[45], metallic Ni monolith-Ni/MgAl2O4 (85.30%) [46] and Ni-

aAl2O3 (80.20%) [15]. However these values were lower than

the results with a Rh based monolith (99.80%) [39]. When

compared with Ni monolith (88.71%) [45], metallic Ni mono-

lith-Ni/MgAl2O4 (91.50%) [46] and Rh based monolith (91.60%)

[33] H2 selectivity of the CoeRu (M-3) and CoeNieRu (M-4)

monolithic catalyst was better. On the other hand, Ce eZr/Ni

monolith (98.61%) [45] and Ni-aAl2O3 with Ce promoter

(98.10%) [15] catalysts have shown higher H2 selectivity when

compared with CoeNieRu monolithic catalyst. Table 4 pro-

vides a comparison of values of CH4 conversion, H2 and CO

selectivity obtained in this work with some other publishedvalues [15,39,44e47].

Catalytic partial oxidation of methane (CPOM) includes

several reaction equilibria and the resulting product compo-

sition is defined by the global thermodynamic equilibrium of

all the species involved [16].

Considering the results of a study on thermodynamic

equilibrium of CPOM at 1 bar pressure and a temperature

range of 700e1200 K, with a CH4 to O2 ratio of 2.0 to 1.0. using

HYSYS 3.2 [16]; methane conversion values obtained at 850 C

temperature and 0.36 s space time with the catalysts (M-1 to

M5) developed in the present study were comparable to

equilibrium conversions by 94.34, 93.40, 94.27, 95.98, and

94.28% for Reaction R1 (Table 5). T

a b l e 3 e

R e a l i z a t i o n r a t i o o f r e a c t i o n s ( R 1 , R 2 , R 3 , R 4 ) a t d i f f e r e n t t e m p e r a t u r e f o r C H 4

p a r t i a l o x i d a t i o n o v e r m o

n o l i t h i c c a t a l y s t s .

(

C )

M - 1

M - 2

M - 3

M - 4

M - 5

R 1

R 2

R 3

R 4

R 1

R 2

R 3

R 4

R 1

R 2

R 3

R 4

R 1

R 2

R 3

R 4

R 1

R

2

R 3

R 4

6

5 0

1 0 . 6 7

7 8 . 2

9

e

1 1 . 0 4

5 . 3

8

8 8 . 7 4

e

5 . 8

8

1 8 . 9 1

6 1 . 3

2

e

1 9 . 7 7

1 9 . 8 1

6 8 . 3

2

e

1 1 . 8 7

2 8 . 8

8

4 3 . 6

9

e

2 7 . 4 4

7

0 0

3 1 . 4 7

5 2 . 9

9

e

1 5 . 5 4

3 3 . 3

2

5 0 . 7

2

e

1 5 . 9 7

6 2 . 6

0

2 2 . 6

9

e

1 4 . 7 1

6 4 . 3

0

2 5 . 4

2

e

1 0 . 2

8

5 3 . 4 1

3 2 . 9 1

e

1 3 . 6

8

7

5 0

7 1 . 3 1

2 0 . 7 7

e

7 . 9

2

6 9 . 5

0

2 1 . 8 7

e

8 . 6

3

7 6 . 8 4

1 3 . 7

2

e

9 . 4 4

8 1 . 9

2

1 3 . 2 7

e

4 . 8 1

6 2 . 9

9

2 7 . 9

8

e

9 . 0

2

8

0 0

8 8 . 6

9

8 . 8

8

e

2 . 4

3

8 4 . 7

0

1 2 . 7

3

e

2 . 5

6

8 7 . 8

6

7 . 2

9

e

4 . 8 5

9 0 . 8 5

8 . 1

9

e

0 . 9

6

7 8 . 7 5

1 7 . 2

6

e

3 . 9

9

8

5 0

9 1 . 5 1

7 . 2 5

0 . 8 7

e

9 0 . 6

0

8 . 5

0

0 . 8

9

e

9 1 . 4 4

8 . 0

2

e

0 . 5 4

9 3 . 1

0

6 . 1

9

0 . 7 1

e

9 1 . 4

2

7 . 2 5

1 . 3

3

e

Fig. 4 e Study of carbon deposition on the catalysts in

terms of change in absorbance of CO2 with temperature by

TG/FTIR technique.









Fig. 5 e Product stream composition rates of CH4 partial oxidation over monolithic catalysts: (a) CH4 conversion (%), (b) CO

selectivity (%), (c) H2 selectivity (%), (d) H2 /CO ratio.

Table 4 e CH4 conversion (%), H2 (%) and COselectivity (%) of CH4 partial oxidation over monolithic catalyst with oxides of Co(M-1), CoeNi (M-2), CoeRu (M-3), CoeNieRu (M-4), and Ni (M-5) compared with various monolith supported catalysts inliterature.

Catalyst CH4 conversion,% H2 selectivity,% CO selectivity,% Temperature, C GHSV, h1 Reference

Rh monolith 89.0 90.0 95.0 e e [35]

CeeZr/Ni monolith 91.88 98.61 95.84 800 1*105 [36]

Ni monolith 81.18 88.71 92.38 800 1*105 [36]

Metallic Ni monolith-Ni/MgAl2O4 85.3 91.5 93.0 800 1*105 [37]

Ni-aAl2O3 80.20 92.60 89.2 e 1*105 [15]

Ni-aAl2O3 with La promoter 93.90 100.0 93.70 e 1*105 [15]

Ni-aAl2O3 with Ce promoter 91.80 98.10 91.90 e 1*105 [15]

Pd based metal monolith 90.00 89.00 92.00 700 1*105 [38]

Rh based monolith (non-adiabatic) 99.0 91.6 90.5 ~ 800 1*104 [33]

Rh based monolith (adiabatic) 99.8 91.4 91.9 ~900 1*104 [33]

M-1 94.04 91.12 88.69 800 1*104 In this study

M-2 85.01 87.27 84.70 800 1*104 In this study

M-3 94.34 92.71 87.86 800 1*104 In this study

M-4 93.96 91.81 90.85 800 1*104 In this study

M-5 81.49 82.72 78.75 800 1*104 In this study

Table 5 e Comparison of catalyst performance experimentally obtained in the present study in terms of CH4 conversion byR1 (%) with thermodynamic equilibrium values calculated by HYSYS 3.2 simulation [16] for T: 650e850 C.

T (C) T(K) M-1 (Co) M-2 (CoeNi) M-3 (CoeRu) M-4 (CoeNieRu) M-5 (Ni) Equilibrium (HYSYS 3.2) [16]

650 10.67 5.38 18.91 19.81 28.88 75

923

700 31.47 33.32 62.60 64.30 53.41 84

973

750 71.31 69.50 76.84 81.92 62.99 90

1023

800 88.69 84.70 87.86 90.85 78.75 94

1073

850 91.51 90.60 91.44 93.10 91.42 97

1123









Conclusion

A significant advantage of CPOM over steam reforming is that

the endothermic heat is generated internally (in situ) in the

monolith thereby minimizing the high endothermic heat that

must be added for steam reforming. This is especially true for

ceramic monoliths which have low heat transfer properties.In the present work, effect of addition of Ni and Ru oxides into

Co based catalysts, and its contribution to hydrogen produc-

tion efficiency have been studied. Performance tests of

monolithic catalysts Co, CoeNi, CoeRu, CoeNieRu, and Ni

oxide were carried out at 600, 650, 700, 750, 800 and 850 C in a

tubular reactor. Complete combustion reaction was dominant

within 600e650 C. After 700 C, partial oxidation reaction

became dominant. Since hydrogen production for CoeNieRu

oxide catalyst had increased by only 0.98% from 800 to 850 C,

the optimum reaction temperature was accepted as 800 C.

Efficiency values of hydrogen production by partial oxidation

of methane at 800 C using Co, CoeNi, CoeRu, CoeNieRu, and

Ni as oxide catalysts have been calculated as 93.73%, 88.72%,94.40%, 93.78%, and 85.30%, which were higher than those

reported in the literature. At 850 C, these values increased to

94.90%, 93.74%, 95.47%, 95.89%, and 94.83%, respectively.

Further tests are planned for the following phase of the study,

to explore the long term stability of the catalysts developed in

the present study.

Acknowledgment

Financial support of the Yildiz Technical University (YTU-

BAPK: 27-07-01-07) and the technical support extended by theEnergy Institute of Marmara Research Center (MRC) of the

Scientific and Technological Research Council of Turkey

(TUBITAK) during performance testing of the catalysts are

gratefully acknowledged.

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