(455c) Influence of Particle Size and Sulfur Tolerance in ATR Over Ni Catalysts

Particle size is an important factor in the development of stable catalysts. In general it is beneficial to formulate catalysts with small metallic particle sizes. This has the dual effects of increasing the concentration of metal atoms at the surface and also decreasing the average coordination number of those surface atoms due to the larger proportion of step and kink sites relative to terrace sites. Such particle size effects have been specifically demonstrated in reforming systems, in that catalysts of higher dispersion have been shown to demonstrate both a higher hydrogen production activity and a lower deposition rate of carbon. The work presented here is unique in two important aspects. First, it has applied the subject of particle size to another key problem of catalytic reforming, namely the deactivation of active sites due to sulfur poisoning. Second, it has separated the two effects of varied catalyst loading (particle size and number of active sites) by preparing a series of Ni-based catalysts which had identical surface areas as measured by H₂ Chemisorption. Characterization of as-prepared catalysts via XRD and STEM coupled with EDS validated the differences in particle sizes, while DRIFTS analysis of CO adsorbed on the catalyst helped to describe the differences in the surface chemistry of the catalysts. The activities of these catalysts were compared during the autothermal reforming of sulfur-free isooctane and thiophene-doped isooctane. These experiments demonstrated that for this complex reaction system small particle sizes are actually less desirable under sulfur-free conditions (Figure 1a). However, when thiophene is present in the feed there is a less significant effect of particle size (Figure 1b). Post-reaction analysis of the catalysts using XPS and TPR suggest that this effect may be due to reducibility of small particles versus larger ones.