Deactivation of precious metal steam reforming catalysts.
PhD thesis, University of Glasgow.
Full text available as:
Steam reforming is a well established industrial process for the formation of synthesis gas. It takes place in two reversible stages: the reforming reaction (1) followed by the water-gas shift reaction (2).
(1) CnH2n+2 + nH2O nCO + (2n+1)H2
(2) CO + H2O CO2 + H2
Reaction (1) is highly endothermic and is favoured at high temperatures and low pressure, while reaction (2) takes place at as low temperature as possible. One of the major problems affecting the steam reforming industry is catalyst deactivation due to sulphur poisoning. Sulphur is present in the hydrocarbon feedstock and even after desulphurisation steps some sulphur still remains, from ppb to ppm levels. Such low levels of sulphur can still poison the catalyst due to the catalyst having a significant time on stream. It is therefore desirable to produce a catalyst that exhibits sulphur resistance to prolong the lifetime of the catalyst.
In this project the behaviour of precious metal catalysts (Rh and Pt supported on La-ZrO2, Al2O3 and SiO2) towards sulphur was examined. Two major aspects were studied, the catalysts’ adsorpitive behaviour towards sulphur and the effect of sulphur during the steam reforming reaction.
Low pressure pulses of H2S and CH3SH over the catalysts followed by gas chromatography revealed that sulphur is a very strong adsorbing species, similar to CO, and could not be displaced by the adsorption of another species as there was no mechanism to desorb the sulphur species. The alumina supported catalysts offered some protection from sulphur poisoning, evidenced during co-adsorption experiments with CO, because the support was acting as a ‘sulphur sink’. Adsorption mechanisms were proposed for H2S at the different adsorption conditions tested: a 3-site adsorption mechanism producing surface sulphides at room temperature and at higher temperatures bulk sulphides were formed.
High pressure steam reforming reactions of ethane were carried out at three different temperatures (600oC, 550oC and 500oC) to act as standards to the poisoned reactions, but also revealed interesting insights into the reforming of ethane. Each catalyst produced a unique reaction profile during steam reforming, with the La-ZrO2 catalyst exhibiting greatest activity. The Rh catalysts showed high selectivity towards the formation of CH4 due to the hydrogenolysis of C2H6, which was not occurring over the Pt catalysts. The Pt catalysts were the least active and deactivated considerably as the result of carbon formation.
Sulphur species, hydrogen sulphide and methanthiol, were introduced into the reaction by dissolving them in the feed water. It was found the identity of the sulphur species had a significant impact upon the extent of catalyst deactivation, with methanthiol having the most detrimental effect, which was attributed to the molecule decomposing and laying down carbon. None of the catalysts tested exhibited particularly high sulphur resistance, particularly with regard to methanthiol, however Rh/ZrO2 did recover a lot of its original activity once the poison was removed from the feed. This was due to the removal of surface carbon rather than the removal of sulphur from the catalyst, because La-ZrO2 has a faster rate of oxygen transfer and therefore a mechanism to remove surface carbon.
Actions (login required)