Autothermal reforming of methane over Pt/ZrO2/Al2O3 catalysts

Applied Catalysis A: General 281 (2005) 19–24

www.elsevier.com/locate/apcata

Autothermal reforming of methane over Pt/ZrO2/Al2O3 catalysts

Mariana M.V.M. Souzaa,b, Martin Schmala,b,*

aNUCAT/PEQ/COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, 21945-970, Rio de Janeiro, BrazilbEscola de Quımica, Universidade Federal do Rio de Janeiro, C.P. 68542, 21940-900, Rio de Janeiro, Brazil

Received 9 July 2004; received in revised form 28 October 2004; accepted 5 November 2004

Available online 15 December 2004

Abstract

Autothermal reforming of methane, combining steam reforming and partial oxidation was carried out with Pt/Al2O3, Pt/ZrO2 and Pt/ZrO2/

Al2O3 catalysts, in the temperature range of 400–900 8C. The Pt/ZrO2/Al2O3 catalyst was found to be the most active and stable at 800 8C due

to the higher resistance to coke formation. The reaction occurs in two simultaneous stages: total combustion of methane and reforming of the

unconverted methane with steam and CO2, with the O2 conversion of 100% starting from 450 8C. The addition of O2 to the feed increases

methane conversion and the catalyst stability, decreasing the H2 and CO yields due to the enhancement of methane combustion. By

manipulating the O2/CH4 ratio of the feed it is possible to achieve the H2/CO ratio that is optimal to the GTL processes.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Methane; Steam reforming; Partial oxidation; Autothermal reforming; Pt/ZrO2/Al2O3 catalysts

1. Introduction

There has been substantial interest in recent years in

alternative routes for conversion of natural gas (methane) to

synthesis gas, a mixture of CO and H2, which can be used to

produce chemical products with high added values, such as

hydrocarbons and oxygenated compounds. In GTL (gas-to-

liquid) plants, where natural gas is firstly converted to

synthesis gas, which is the feedstock for Fischer–Tropsch

synthesis of hydrocarbons, above 60–70% of the cost of the

overall process is associated with syngas production [1,2].

Therefore, reduction in syngas generation costs would have

a large and direct influence on the overall economics of these

downstream industrial processes.

Steam reforming is the established process for converting

natural gas into synthesis gas. SRM is a very energy-

intensive process because of the highly endothermic

property of the reaction and the H2/CO ratio obtained

(about 3) is only suitable for processes requiring a H2-rich

feed (such as ammonia syntheis and petroleum refining

process), but it is too high for fuel synthesis via Fischer–

* Corresponding author. Tel.: +5521 2562 8352; fax: +5521 2562 8300.

E-mail address: [email protected] (M. Schmal).

0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2004.11.007

Tropsch reaction [2,3]. Moreover, steam reformers are

inadequate for hydrogen generation in fuel cell electric

vehicles because of their high thermal inertia for frequent

start-up and shut-down operation condition [4,5].

Autothermal reforming (ATR), a combination of steam

reforming and partial oxidation reactions, is an advanta-

geous route for syngas production for both economical and

technical reasons. It has low-energy requirements due to the

opposite contribution of the exothermic methane oxidation

and endothermic steam reforming. The combination of these

reactions can improve the reactor temperature control and

reduce the formation of hot spots, avoiding catalyst

deactivation by sintering or carbon deposition. Moreover,

ATR allows the production of syngas with a wider range of

H2/CO ratio by manipulating the relative concentrations of

H2O and O2 in the feed [6–8]. All these advantages indicate

that ATR should be the technology of choice for large-scale

GTL plants [9]. In addition, a fuel processor based on

autothermal reforming of methane could provide a low cost

and compact system, with fast start-up and capability to

follow load variations, more adequate for fuel cell electric

vehicles [10,11].

We have previously reported that Pt/ZrO2/Al2O3 systems

are effective formulations for CO2 reforming of methane

M.M.V.M. Souza, M. Schmal / Applied Catalysis A: General 281 (2005) 19–2420

[12,13], partial oxidation [14,15] and steam reforming [15].

The aim of this work is to investigate the coupling between

the steam reforming of methane and partial oxidation, with

varying O2/CH4 ratio in the feedstream, over the Pt/ZrO2/

Al2O3 catalyst, comparing with the catalytic behavior of

Pt/A2O3 and Pt/ZrO2.

2. Experimental

2.1. Catalyst preparation

The g-Al2O3 support (Harshaw, Al 3996) was calcined in

flowing air at 550 8C for 2 h (BET surface area = 200 m2/g).

The ZrO2 support was prepared by calcination of zirconium

hydroxide (MEL Chemicals) using the same conditions

(BET surface area = 62 m2/g). The 10% ZrO2/Al2O3 was

obtained by impregnation of alumina with a nitric acid

solution of zirconium hydroxide (BET surface area =

180 m2/g), as described elsewhere [13]. The supports were

impregnated with an aqueous solution of chloroplatinic acid

(H2PtCl6, Aldrich) by incipient wetness technique. The

catalysts were subsequently dried overnight at 120 8C and

calcined in air at 550 8C for 2 h. The platinum content was

around 1 wt.%. The prepared catalysts will be referred to

as PtAl for Pt/Al2O3, PtZr for Pt/ZrO2 and Pt10ZrAl for

Pt/ZrO2/Al2O3.

2.2. Catalyst testing

The reaction was carried out in a U-tube fixed-bed flow-

type quartz reactor, with length of 15 cm and 6 mm of

external diameter (the bed height is 4–5 mm), loaded with

20 mg of catalyst, under atmospheric pressure. The catalyst

bed is supported by quartz wool. The total feed flow rate was

held constant at 200 cm3/min (WHSV = 160 h�1), with

flowing He. Steam was added to the system using a saturator

at controlled temperature. The gas compositions are listed in

Table 1. The activity tests were performed at different

temperatures, ranging from 400 to 900 8C in steps of 50 8Cthat were kept for 30 min at each temperature. The loss in

catalyst activity at 800 8C was monitored up to 60 h on

stream. Blank experiments were also carried out but did not

show conversions above 5% even at high temperatures. The

reaction products were analyzed by on-line gas chromato-

graph (CHROMPACK CP9001), equipped with a Hayesep D

column and a thermal conductivity detector.

Table 1

Flow rates for activity and deactivation tests (cm3/min)

O2/CH4 H2O/CH4 CH4 O2 H2O He Total

0.25 0.5 40 10 20 140 200

0.5 0.5 40 20 20 120 200

1.0 0.5 20 20 10 150 200

2.0 0.5 10 20 5 165 200

2.3. Carbon deposition measurements

The amount of coke formed over the catalysts after

deactivation tests at 800 8C was examined by thermogravi-

metric analysis (TGA), using a RIGAKU thermoanalyzer

(model TAS 100). The samples were pretreated at 150 8Cunder flowing nitrogen and then heated at a rate of 10 8C/min

to 800 8C in a flow of 15% O2/N2 (50 cm3/min).

3. Results and discussion

3.1. Effect of temperature

The comparison of catalyst activities for autothermal

reforming of methane is displayed in Fig. 1A and B, in terms

of CH4 conversion and H2/CO product ratio, respectively.

The O2 conversion is 100% starting from 450 8C for PtAl

and Pt10ZrAl catalysts and 400 8C for PtZr. The PtZr

showed the highest activity at 400 8C, with 20% of CH4

Fig. 1. Catalytic activities in terms of CH4 conversion (A) and H2/CO

product ratio (B) of Pt catalysts for autothermal reforming of methane as a

function of temperature.

M.M.V.M. Souza, M. Schmal / Applied Catalysis A: General 281 (2005) 19–24 21

Fig. 2. Composition profiles for autothermal reforming of methane as a

function of temperature over the Pt10ZrAl catalyst.

conversion, maintaining this conversion up to 550 8C. The

Pt10ZrAl was the most active catalyst over the mid-

temperature range (450–600 8C) while at higher tempera-

tures PtZr exhibited slightly better activity than PtAl and

Pt10ZrAl.

The activity was almost constant in temperature range

between 450 and 650 8C, which can be related to the

combustion of methane to CO2 and H2O, a very exothermic

reaction:

CH4 þ 2O2 ,CO2 þ 2H2O;

DH298 K ¼ �802 kJ=mol(1)

The observed high H2/CO ratio (>6.0) at low tempera-

tures, especially for PtZr catalyst, suggests that water–gas

shift (WGS) reaction occurs to a great extent with reforming

of methane, as already reported in [7,16]. The WGS

equilibrium was reached, according to thermodynamic

calculations. At the same time, the decrease in H2/CO ratio

with increasing reaction temperature is consistent with the

fact that WGS reaction is thermodynamically unfavorable at

higher temperatures:

CO þ H2O,CO2 þ H2; DH298 K ¼ �41 kJ=mol (2)

At high temperatures, the H2/CO ratio lower than 2.0 (this

ratio is 1.4 for PtZr and 1.8 for Pt10ZrAl at 800 8C) indicates

that the reverse of WGS reaction takes place simultaneously

with reforming of methane:

CH4 þ H2O,CO þ 3H2; DH298 K ¼ 206 kJ=mol (3)

CH4 þ CO2 ,CO þ H2; DH298 K ¼ 247 kJ=mol (4)

Fig. 3. Deactivation test in terms of CH4 conversion as a function of time on

stream at 800 8C.

Several authors have proposed a two-step mechanism for

the partial oxidation of methane. In the first step combustion

of methane takes place, producing CO2 and H2O; in the

second step, synthesis gas is produced via CO2 and steam

reforming reactions of unreacted methane [17–19].

This indirect mechanism is also clearly evidenced for

autothermal reforming, as can be seen by the composition

profiles shown in Fig. 2 for the Pt10ZrAl catalyst. The

O2 conversion is 100% starting from 450 8C. At this

temperature, there is the onset of CO2 production and the

water conversion is lower than methane conversion due to

the combustion of methane, which produces CO2 and H2O.

The low production of H2 and CO up to 600 8C is also related

to the occurrence of total combustion. The H2 and CO

contents increase sharply with increasing temperature from

650 8C, which can be explained in terms of the contribution

of methane reforming with steam and CO2, in agreement

with thermodynamic calculations.

The reaction mechanism, according to the literature, was

related to the change in the nickel valence state during the

partial oxidation of methane. The rapid transformation of the

reaction from complete oxidation of methane to partial

oxidation was associated to the reduction of NiO to Ni0 [17].

However, the surface modification for the platinum catalysts

can be disregarded, because after pretreatment with H2 flow

up to 700 8C the catalytic activity did not change (not

shown).

3.2. Effect of time on stream

Comparison of catalyst stability at 800 8C is shown in

Fig. 3. The equilibrium conversion at this temperature is

95%. The catalysts exhibited different initial activities and

deactivation rates. The PtZr showed the highest initial

activity, but deactivated at a mean rate of 0.86%/h during

28 h on stream. PtAl was the least active catalyst and the

mean deactivation rate was 0.42%/h. The catalyst with

10 wt.% of ZrO2 exhibited the best performance, with CH4

conversion decreasing from 82 to 68% in the first 10 h,

maintaining this conversion for over 60 h (mean deactivation

rate of 0.24%/h).

The deactivation is related to the deposition of inactive

carbon over the active surface and the amount of coke on

these catalysts was quantified by thermogravimetric


Fig. 4. TGA of Pt catalysts after deactivation test at 800 8C. Conditions:

15% O2/N2, 10 8C/min, and feed flow rate = 50 cm3/min.

analysis, carried out in an oxygen-containing atmosphere

(Fig. 4). The stability of the Pt10ZrAl catalyst is really

associated with the observation of little coke formation

during the reaction (about 0.2 mg coke/gcat h). On the other

hand, TGA experiment showed a weight loss of about 17%

on treating the PtAl and PtZr catalysts in oxygen, indicating

a significant amount of carbon deposition during 30 h on

stream (approximately 5.7 mg coke/gcat h).

The deposition of inactive carbon during methane

reforming can be originated from either methane decom-

position (reaction (5)) or CO disproportionation (Boudouard

reaction (6)), which are thermodynamically favorable below

900 8C [3,20]:

CH4 ,C þ 2H2; DH298 K ¼ 75 kJ=mol (5)

2CO,C þ CO2; DH298 K ¼ �172 kJ=mol (6)

Thermodynamic calculations showed that the extent of

carbon deposition during reforming decreases at higher reac-

tion temperatures, in agreement with several experimental

Fig. 5. CH4 conversion as a function of time on stream, at 800 8C, varying

the O2/CH4 feed ratio, for Pt10ZrAl catalyst.

observation [21,22]. These results suggest that CO dispro-

portionation is the main contributor to carbon deposition

because it is exothermic and the equilibrium constant

decreases with increasing temperature.

The higher stability of Pt10ZrAl has already been verified

for steam reforming and partial oxidation separately [14,15]

as well as for CO2 reforming [13]. The higher stability is

closely related to its coking resistivity, which has been

attributed to Pt–Zrn+ interactions at metal-support interface.

The number of interfacial sites is higher on the Pt10ZrAl

catalyst as compared to PtZr itself, because the particle sizes

of ZrO2 on Al2O3 are very small compared to the bulk ZrO2

particles. Therefore, the Pt particles are better dispersed on

highly dispersed ZrO2 particles over AlsO3 [13,14]. The

interfacial sites on Pt-support promote CO2 dissociation,

improving the catalyst stability by shifting the Boudouard

reaction. Our previous FT-IR analysis [13] showed that the

Pt–Zrn+ interface is active for CO and CO2 adsorption, with a

decrease in the Pt–CO bond strength, inhibiting C–O bond

breaking and consequently producing less carbon formation

Fig. 6. Catalytic selectivities as a function of time on stream, at 800 8C, in

terms of H2/CO (A) and CO2/CO (B) product ratios, for Pt10ZrAl catalyst.

M.M.V.M. Souza, M. Schmal / Applied Catalysis A: General 281 (2005) 19–24 23

on the catalyst surface. Moreover, zirconia is a well-know

oxygen supplier, and its oxygen mobility is fast, which helps

to keep the metal surface free of carbon.

3.3. Effect of O2/CH4 feed ratio

Fig. 5 presents the CH4 conversion at 800 8C as a function

of time on stream, varying the O2/CH4 ratio and at a constant

H2O/CH4 ratio of 0.5, for the Pt10ZrAl catalyst. The O2/CH4

ratio of 0.25 corresponds to the stoichiometric amount of

autothermal reforming. The increase in the O2/CH4 feed

ratio not only increases CH4 conversion but also decreases

the catalyst deactivation. When the O2/CH4 ratio is 0.5 there

is a slight activation of the catalyst with time on stream and

when this ratio is increased to 1.0 the CH4 conversion is

100% throughout the duration of the experiment (60 h).

The increase in the catalytic stability with the addition of

O2 to the feed has already been observed in the literature, for

Pt/ZrO2 catalysts, during either steam reforming [16] or CO2

reforming of methane [23]. The excess of O2 probably favors

the reoxidation of carbonaceous residues formed over the

catalytic surface, avoiding the catalyst deactivation. The

increase in CH4 conversion with O2 addition can be related

to two distinct effects: the fast combustion of part of

methane with all O2 in the feed and the increase in the

Fig. 7. Composition profiles as a function of time on stream at 800 8C for Pt10Z

(D) 2.0.

reforming reaction rate due to the increase in temperature

caused by the exothermicity of combustion reaction. This

second effect is less significant in our case because all the

streams are greatly diluted by He (Table 1).

The comparison of catalyst selectivities at 800 8Cwith different O2/CH4 feed ratios is displayed in Fig. 6A

and B, in terms of H2/CO and CO2/CO product ratios,

respectively. The H2/CO ratio is minimum (about 1.5) when

O2/CH4 = 0.5; this ratio reaches the value of 2.0 when O2/

CH4 = 1.0. The CO2/CO ratio is about 0.2 for O2/CH4 � 0.5

and 0.9 for O2/CH4 = 1.0.

The role of oxygen addition on the catalytic selectivity is

fairly complicated due to the different contributions of

reactions (1)–(4). When O2/CH4 � 0.5 the CO2/CO ratios

have about the same value, which indicates that the total

combustion proceeds to the same extent, but the reforming

reaction is enhanced by decreasing O2/CH4 ratio due to the

higher relative availability of methane under these condi-

tions. When the ratio O2/CH4 is 1.0, the H2/CO ratio

obtained (about 2–2.1) has the optimum value for GTL

processes; however, the H2 and CO yields are lower, with

relative higher formation of CO2 and H2O by methane

combustion. The composition profiles of H2, CO, CO2 and

H2O as a function of O2/CH4 feed ratio are represented in

Fig. 7. It can be clearly observed that as O2/CH4 ratio

rAl catalyst in different O2/CH4 feed ratios: (A) 0.25, (B) 0.5, (C) 1.0, and


increases, the relative formation of CO2 and H2O increases

until the limit case (O2/CH4 = 2) when the production of H2

and CO is completely inhibited because of methane total

consumption by combustion.

4. Conclusions

The support plays a decisive role on the catalytic behavior

during autothermal reforming of methane, in which steam

reforming and partial oxidation take place simultaneously.

The Pt/10% ZrO2/Al2O3 catalyst showed the highest activity

and stability during 70 h on stream at 800 8C, when

compared to Pt/Al2O3 and Pt/ZrO2. The composition

profiles when the feed is in stoichiometric ratio showed

that the reaction proceeds via a two-step mechanism with the

total combustion of methane followed by reforming of

unreacted methane with CO2 and H2O; the heat released by

combustion favors the reforming reaction. The higher

stability of the Pt/10% ZrO2/Al2O3 catalyst is closely related

to its coking resistance due to Pt–Zrn+ interactions at metal-

support interface.

The increase in O2/CH4 feed ratio not only increases the

CH4 conversion but also improves the catalyst stability. The

excess of O2 avoids the deposition of carbonaceous residues

over the catalytic surface; however, the H2 and CO yields

also decrease due to the enhancing of total combustion.

The H2/CO product ratio can be manipulated by varying the

O2/CH4 feed ratio and it is possible to achieve the optimum

ratio for GTL processes (H2/CO = 2 when O2/CH4 = 1.0).

References

[1] J.R. Rostrup-Nielsen, Catal. Today 21 (1994) 257.

[2] M.A. Pena, J.P. Gomez, J.L.G. Fierro, Appl. Catal. A 144 (1996) 7.

[3] S.C. Tsang, J.B. Claridge, M.L.H. Green, Catal. Today 23 (1995) 3.

[4] L. Pino, V. Recupero, S. Beninati, A.K. Shukla, M.S. Hegde, P. Bera,

Appl. Catal. A 225 (2002) 63.

[5] L. Pino, A. Vita, M. Cordaro, V. Recupero, M.S. Hegde, Appl. Catal. A

243 (2003) 135.

[6] S. Liu, G. Xiong, H. Dong, W. Yang, Appl. Catal. A 202 (2000) 141.

[7] Z.-W. Liu, K.-W. Jun, H.-S. Roh, S.-E. Park, J. Power Sources 111

(2002) 283.

[8] S. Ayabe, H. Omoto, T. Utaka, R. Kikuchi, K. Sasaki, Y. Teraoka, K.

Eguchi, Appl. Catal. A 241 (2003) 261.

[9] D.J. Wilhelm, D.R. Simbeck, A.D. Karp, R.L. Dickenson, Fuel Proc.

Technol. 71 (2001) 139.

[10] S.H. Chan, H.M. Wang, Int. J. Hydrogen Energy 25 (2000) 441.

[11] S. Ahmed, M. Krumpelt, Int. J. Hydrogen Energy 26 (2001) 291.

[12] M. Schmal, M.M.V.M. Souza, D.A.G. Aranda, C.A.C. Perez, Stud.

Surf. Sci. Catal. 132 (2001) 695.

[13] M.M.V.M. Souza, D.A.G. Aranda, M. Schmal, J. Catal. 204 (2001)

498.

[14] M.M.V.M. Souza, M. Schmal, Appl. Catal. A 255 (2003) 83.

[15] M.M.V.M. Souza, M. Schmal, Stud. Surf. Sci. Catal. 147 (2004) 133.

[16] M.E.S. Hegarty, A.M. O’Connor, J.R.H. Ross, Catal. Today 42 (1998)

225.

[17] D. Dissanayaki, M.P. Rosynek, J.H. Lunsford, J. Phys. Chem. 97

(1993) 3644.

[18] E.P.J. Mallens, J.H.B. Hoebink, G.B. Martin, Catal. Lett. 33 (1995)

291.

[19] F. van Looij, E.R. Stobbe, J.W. Geus, Catal. Lett. 50 (1998) 59.

[20] A.M. Gadalla, B. Bower, Chem. Eng. Sci. 43 (1988) 3049.

[21] M.C.J. Bradford, M.A. Vannice, Catal. Rev. Sci. Eng. 41 (1) (1999) 1.

[22] J.T. Richardson, S.A. Paripatyadar, Appl. Catal. 61 (1990) 293.

[23] A.M. O’Connor, J.R.H. Ross, Catal. Today 46 (1998) 203.

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Autothermal reforming of methane over Pt/ZrO2/Al2O3 catalysts