(538d) Deactivation of Precious Metal Steam Reforming Catalysis: Effect of Sulphur

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 600^oC.
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.

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.