251544 Deactivation of Precious Metal Steam Reforming Catalysis: Effect of Sulphur

Wednesday, October 31, 2012: 4:15 PM
315 (Convention Center )
S. David Jackson1, Claire Gillan2, Martin Fowles3 and Sam French3, (1)WestCHEM, Department of Chemistry, University of Glasgow, Glasgow, United Kingdom, (2)Dept of Chemistry, University of Glasgow, Glasgow, United Kingdom, (3)Johnson Matthey, Billingham, United Kingdom

Deactivation of Precious Metal Steam Reforming Catalysis:

Effect of Sulphur.

Claire Gillan1, Martin Fowles2, Sam French2 and S David Jackson*1

1Centre for Catalysis Research, WestCHEM, Dept. of Chemistry, University of Glasgow, Glasgow, G12 8QQ Scotland.  David.jackson@glasgow.ac.uk

2Johnson Matthey plc, Belasis Ave, Billingham TS23 1LB UK

Introduction

Sulphur is known to poison catalysts for many reactions, in particular steam reforming.  Even when present in the hydrocarbon feedstock in small quantities, ppb levels, sulphur can have detrimental effects with increasing time on stream.  Currently, most of the work regarding sulphur poisoning has been carried out on nickel catalysts; however advances in steam reforming could mean that precious metal catalysts are an attractive option for the future.  This study looks at sulphur poisoning of precious metal catalysts, namely Pt/Al2O3.

To determine how the identity of the sulphur species affected the catalyst two different poisons were chosen, hydrogen sulfide and methanethiol.  Steam reforming of ethane has been studied to determine how the sulphur molecules affect activity and selectivity.

Experimental

The catalyst was prepared by impregnating the pre-dried support (alumina heated to 1173 K for 2h, S.A. 104 m2g-1) to incipient wetness with an aqueous solution containing the precursor salt (H2PtCl6) to give a weight loading of 0.2 %.  The catalyst had a BET area of 107 m2g-1and a Pt dispersion of 18 %.  Steam reforming experiments were carried out using a fixed bed micro reactor.  Ethane and steam were fed into the reactor at a ratio of 1:2.5 to limit carbon laydown.  The reaction pressure was 20bar and temperature 600oC. On-line G.C. was used to analyse the products.  The sulphur species were introduced into the system by dissolving the relevant gas into the feed water.  The concentrations of sulphur in water were 11.2ppm and 5.6ppm.

Results and Discussion

When methanethiol is introduced into the system (between dotted lines in Fig.1) the catalyst deactivates rapidly.  However once the poison is removed from the feed, the activity recovers to the expected level (Fig.1).  The solid line shows the normal deactivation profile.

When hydrogen sulfide is introduced deactivation of the catalyst is apparent, but to a lesser extent than methanethiol. This is evident with all the products (table 1).

From analysis of the deactivation of each product it is clear that methane formation is most affected, followed by hydrogen and carbon dioxide, which deactivate at the same rate, whilst, carbon monoxide formation showed very little deactivation. This suggests that some reactions are being poisoned more readily than others, leading to the proposal of the following deactivation order:

Hydrogenolysis/methanation > Water-gas shift > Steam reforming

Figure 1.  Effect of 11.2ppm methanethiol on the rate of H2 formation.

Product

H2S

CH3SH

H2

8

11

CO

2

9

CO2

9

14

CH4

16

24

Table 1.  Comparison of deactivation rate constants (x10-4) for each product

The study shows that the rate of Pt/Al2O3 deactivation is dependent on the identity of the poison, with methanethiol being more deleterious than hydrogen sulphide probably due to sulphur and carbon laydown.  The poisons also affect the selectivity of the process.  The overall deactivation process can be rationalized on the basis of the different reactions that are occurring and their different susceptibility to poisoning.


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