Facet-dependent peroxo species regulate product distribution and H2O2 utilization in CeO2-catalyzed aniline oxidation

Abstract

Although the development of solid catalysts for H₂O₂ synthesis has recently gained considerable attention, optimizing its utilization and product selectivity in a given reaction is equally important but less studied. The design of solid catalysts for H₂O₂ activation in aniline oxidation to value-added nitrosobenzene and azoxybenzene has gained considerable attention over the past decade. However, distinct solid catalysts are often required to obtain the target compound in literature. The heterogeneity of materials used makes it very difficult to provide in-depth guidance for the design of catalysts. In this study, we showed that one can easily obtain the target compound in this reaction by regulating H₂O₂ activation on pristine CeO₂ enclosed by distinct but well-defined surfaces. The bridging peroxo species found on the (100) surface preferentially generates radicals, which non-selectively oxidizes H₂O₂ and aniline, resulting in poor H₂O₂ utilization and low nitrosobenzene yield in aniline oxidation. In comparison, the side-on peroxo species formed on the (110) surface displays a certain degree of selectivity towards aniline and hence gives a much higher nitrosobenzene yield with improved H₂O₂ utilization. To our surprise, the end-on peroxo species of the (111) surface is inert to H₂O₂ in the solution and can stoichiometrically convert aniline to phenylhydroxylamine (Ph-NHOH), which is the key intermediate to produce azoxybenzene. The structure–selectivity correlation established in this study is believed to guide the rational design of catalysts with high H₂O₂ utilization and product selectivity in other oxidation reactions.