Is Surface Complexity Needed for Industrial High-Performance Oxidation Catalysts?

The present study bridges the gap between academic research on monolayer-type catalysts and industrial high-performance catalysts. It investigates industrial high-performance oxidation catalysts for o-xylene to phthalic anhydride conversion. The optimization of the active mass shell thickness enhances pore utilization and thus allows improving catalyst selectivity and activity without influence of diffusion limitation. A design of experiments study focusing on the optimization of the elemental catalyst composition reveals that optimal catalysts require surface VO4 and SbO4 monolayer coverages far beyond typical monolayer catalysts as studied in academic literature. The statistical data evaluation shows that optimum element ratios in these catalysts are far from any known stoichiometric compounds. Hence, the role of the well-known antimony promoter cannot be explained by literature concepts like site isolation or mixed oxide formation. Comprehensive analysis of surface evolution during catalyst calcination and equilibration, combined with DFT studies, provides a detailed understanding of surface species formation and stability: the Sb promoter in such catalysts is not located at the outermost catalyst surface. It is incorporated into the surface of anatase maybe in form of VSbOx dimers. This yields catalysts which activate at higher rates during equilibration, and which are characterized by higher activity without sacrificing selectivity. The Cs promoter can readily be used to tune catalyst activity to the task of the catalyst layer within the graded bed, but it also affects selectivity. By shedding some light onto this complex interplay between the active component V, and the promoting elements, Sb and Cs, in determining catalyst activity and selectivity, and demonstrating the benefits of an Sb-doped anatase support, this study opens new avenues for catalyst optimization and design in the industrial, selective o-xylene oxidation reaction.