Investigations of Sulfur Tolerance of Water-Gas Shift Catalysts

摘要

The interaction of H sub 2 S with Cu(111) has been investigated. The adsorption proceeds dissociatively, with evolution of H sub 2 gas and deposition of S/sub a/. At 300K, the sticking probability (0.02) is constant up to nearly saturation coverage. By 650K, the dissociative sticking probability has decreased by a factor of 3.5. Removal of S/sub a/ using H sub 2 pressures up to 1000 torr was impossible, indicating a very slow reverse reaction. This helps explain why such low levels of sulfur can completely poison Cu-based catalysts. Figure 1 shows the influence of pre-adsorbed sulfur on the activity of Cu(111) for WGS at 10 torr H sub 2 O, 26 torr CO and 625K. There is a linear decay in rate up to saturation coverage, where the rate drops to zero. This linear decay indicates a simple site-blocking mechanism for sulfur poisoning, with only a small ensemble of free Cu sites (less than or equal to2) required for the WGS reaction. The Cu(111) surface was dosed with Cs from a droplet of CsOH solution in ultra-pure water, which was dried by heating mildly (375K, 1 min) in air. The cesium coverage was controlled by the initial Cs concentration in solution. The Cs-dosed catalyst has been characterized by XPS and ISS. Under WGS reaction conditions, the surface was reduced to a state involving, at most, one oxygen atom per Cs adatom. Figure 2 shows the influence of Cs upon the WGS kinetics at 625K, 10 torr H sub 2 O and 26 torr CO. There is a nearly linear increase in catalyst activity with Cs coverage until a p(2 x 2) LEED pattern appears after reaction (theta/sub Cs/ = 0.25). At this point, the activity is some 15x higher than for unpromoted Cu(111). Pure Cs on Cu(111) also gives a p(2 x 2) LEED pattern at saturation (theta/sub Cs/ = 0.25). For the optimally-promoted surface, the reaction order with respect to H sub 2 O was the same as for clean Cu(111), and the activation energy increased slightly to 23 kcal/mole sup -1 . (ERA citation 11:033552)