Facet-dependent peroxo species regulate product distribution and H 2 O 2 utilization in CeO 2 -catalyzed aniline oxidation

We showed that the coordination structure of the Ce sites affects the configuration (and reactivity) of the surface peroxo species in H 2 O 2 activation, which determines the product distribution and H 2 O 2 utilization in the aniline oxidation reaction. Although the development of solid catalysts for H 2 O 2 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 H 2 O 2 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 H 2 O 2 activation on pristine CeO 2 enclosed by distinct but well-defined surfaces. The bridging peroxo species found on the (100) surface preferentially generates radicals, which non-selectively oxidizes H 2 O 2 and aniline, resulting in poor H 2 O 2 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 H 2 O 2 utilization. To our surprise, the end-on peroxo species of the (111) surface is inert to H 2 O 2 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 H 2 O 2 utilization and product selectivity in other oxidation reactions.