Search for: All records

Abstract. Rate-driving force relationships, known as Brønsted-Evans Polanyi (BEP) relations, are central to many methods for predicting the performance of heterogeneous catalysts and electrocatalysts. Methods such as Tafel plots and ‘volcano’ analyses often assume the effect of adsorbate coverage on reaction rate across different materials is constant and known. Here we use UV-visible spectroscopy to test these assumptions, by measuring rates of net hydrogen atom transfer from colloidal cerium oxide nanoparticles (nanoceria) to organic reagents at varying surface CeO–H bond strengths and surface coverages. The resulting rate constants follow a linear BEP relationship, ∆log(k)=[alpha]∆log(Keq), across two sizes of nanoceria, two organic reagents, and a ~10 kcal mol-1 range of CeO–H bond strengths. Interestingly, the Brønsted slope is only 0.2, demonstrating that the rate constants are far less insensitive to CeO–H bond strength, than would commonly be assumed for a heterogeneous nanomaterial. Furthermore, we observe a Brønsted slope >1 when altering the reaction driving force via the organic reagent bond strength instead of that of CeO–H. The implications of these Brønsted slopes for either concerted or stepwise mechanisms are discussed. To our knowledge, these are the first solution-phase measurements of BEP relationships for hydrogen coverage on a (nano)material. 

Cerium oxide (ceria, CeO 2−x ) has been traditionally used as a catalyst support functionalized with metal nanoparticles or synthesized with metal dopants for a variety of applications ranging from catalytic converters to solid oxide fuel cells. In a departure from these typical heterogeneous motifs, we explore the interactions of nano-CeO 2−x systems with organometallic oxidation catalysts in organic solvents. Ceria is used here both as an organically-capped colloid and as an uncapped insoluble nanopowder. Both the colloid and nanopowder act as terminal oxidants by accepting hydrogen atoms from a ruthenium Noyori–Ikariya hydride complex. To our knowledge, this is the first demonstration that CeO 2−x can oxidize an organometallic hydride. Building on this concept, we show the uncapped CeO 2−x powder also acts as the terminal acceptor in catalytic alcohol dehydrogenation reactions, utilizing iridium pyridine sulfonamide catalysts under anaerobic and aerobic conditions. The coupling of homogeneous oxidation catalysts with cerium oxide demonstrates the versatility of CeO 2−x and a bridging of concepts in homogeneous and heterogeneous catalysis.