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.