Advanced Catalytic Systems For The Partial Oxidation of Hydrocarbons: Improving Sulphur Tolerance of Rh Based Catalysts

Abstract

The catalytic partial oxidation (CPO) of methane over precious metal catalyst has been shown to be an attractive way to obtain syngas (CO and H2) or H2 which can be converted to clean fuels by Fischer–Tropsch synthesis or employed in fuel cells. However, the presence of sulphur bearing compounds naturally occurring in the fuel, or added as odorants to pipe-line natural gas (approximately up to 10 ppm), can have a detrimental effect on the CPO activity. In this work the effect of sulphur addition on the catalytic partial oxidation (CPO) of methane in the low to moderate temperature regime (300-800 °C) and under self-sustained high temperature (>800 °C) condition was investigated on Rh-based catalysts supported on either La2O3 or SiO2 stabilised γ-Al2O3. Based on the results of catalytic activity measurements and in-situ FT-IR/DRIFT spectroscopic characterisation, as well as TPR/TPD studies, it has been shown that the presence of sulphur can severely suppress the formation of synthesis gas by inhibiting the steam reforming (SR) reactions during the CPO of methane. It was demonstrated that the support material plays a crucial role in the CPO of methane in the low to moderate temperature regime. In the presence of a sulphating support such as La2O3-Al2O3 the partial oxidation reaction was much less inhibited than a less sulphating support such as SiO2-Al2O3. The sulphating support acts as a sulphur storage reservoir, which minimises the poison from adsorbing on or near the active Rh sites where reactions take place. However under the typical operating conditions of methane CPO i.e. at high temperatures and short contact times over structured reactors, sulphur in the feed inhibits the SR reaction by directly poisoning the active Rh sites thus preventing the sulphur storage capacity of the support from showing any beneficial effect on the S-tolerance. Both steady state and transient operation of the CPO reactor were investigated particularly with regards to poisoning/regeneration cycles and low temperature light-off phase. The analysis of products distribution in the effluent and heat balance demonstrated that sulphur reversibly adsorbed on Rh selectively inhibits the SR reaction path to syn-gas production. The extent of SR inhibition is greater when operating in air and diminishes at lower CH4/O2 feed ratios. The poisoning effect was also shown to be independent from the type of sulphur bearing compound and only indirectly affected by the type of catalyst support (La2O3 or SiO2 stabilised alumina) through the value of Rh dispersion. In fact by using in situ DRIFTS experiments of adsorbed CO at room temperature it was found that sulphur acts as a selective poison by preferentially adsorbing on smaller well dispersed Rh crystallites whilst larger metallic Rh sites are mostly unaffected. The adsorption of CO at room temperature before and after S poisoning is schematically represented below. Partial substitution of Rh/La-Al2O3 monolith catalysts with either Pt or Pd did not influence the way S adsorbs on highly dispersed Rh sites. Pd was found to have a detrimental effect on the overall catalytic activity and to be ineffective at improving the S-tolerance. On the other hand the partial substitution of Rh with Pt reduced the detrimental impact of S, which strongly inhibits the SR reaction on dispersed Rh sites but has a much smaller impact on Pt active sites. The improved tolerance of the bimetallic Rh-Pt catalyst against sulphur is due to its higher operating temperature related to the high oxidation activity of Pt which facilitates sulphur desorption from the catalyst and reduces its accumulation.