Reaction Kinetics and Mechanism for the Catalytic Reduction of Propionic Acid over Supported ReOx Promoted by Pd

Influence of Domain Size and Support Composition on the Reducibility of SiO2 and TiO2 Supported Tungsten Oxide Clusters

Supported tungsten oxides are widely used in a variety of catalytic reactions. Depending on the support, the cluster size, oxidation state, reducibility and speciation of the tungsten oxides can widely differ. When promoted with a platinum group metal, the resulting spillover of hydrogen may facilitate the reduction of supported tungsten oxide species, depending on the support. High resolution scanning transmission electron microscopy imaging showed nanometer scale WO x clusters were synthesized on SiO2 whereas highly dispersed species were formed on TiO2. Results from H2-temperature-programmed reduction showed the presence of Pd lowered the initial reduction temperature of SiO2-supported WO x species but interestingly did not affect that of TiO2-supported WO x . X-ray photoelectron and absorption spectroscopies showed the W atoms in SiO2-supported WO x species reduce from a +6 oxidation state to primarily +5 after thermal treatment in 5% H2, while the fraction of W in the +5 oxidation state was relatively unaffected by reduction treatment of TiO2-supported WO x . The unusual behavior of TiO2-supported WO x was explained by quantum chemical calculations that reveal the lack of change in the oxidation state of W is attributed to charge delocalization on the surface atoms of the titania support, which does not occur on silica. Moreover, modeling results at <600 K in the presence of H2 suggest the formation of Brønsted acid sites, and the absence of Lewis acid sites, on larger aggregates of WO x on silica and all cluster sizes on titania. These results provide experimental and theoretical insights into the nature of supported tungsten oxide clusters under conditions relevant to various catalytic reactions.

Ester Reduction with H2 on Bifunctional Metal-Acid Catalysts: Implications of Metal Identity on Rates and Selectivities

Esters reduce to form ethers and alcohols on contact with metal nanoparticles supported on Brønsted acidic faujasite (M-FAU) that cleave C-O bonds by hydrogenation and hydrogenolysis pathways. Rates and selectivities for each pathway depend on the metal identity (M=Co, Ni, Cu, Ru, Rh, Pd, and Pt). Pt-FAU gives propyl acetate consumption rates up to 100 times greater than other M-FAU catalysts and provides an ethyl propyl ether selectivity of 34 %. Measured formation rates, kinetic isotope effects, and site titrations suggest that ester reduction involves a bifunctional mechanism that implicates the stepwise addition of H* atoms to the carbonyl to form hemiacetals on the metal sites, followed by hemiacetal diffusion to a nearby Brønsted acid site to dehydrate to ethers or decompose to alcohol and aldehyde. The rates of reduction of propyl acetate appear to be determined by the H* addition to the carbonyl and by the C-O cleavage of hemiacetal.

Pt1 enhanced C-H activation synergistic with Ptn catalysis for glycerol cascade oxidation to glyceric acid

The selective oxidation of glycerol to glyceric acid, an important value-added reaction from polyols, is a typical cascade catalytic process. It is still of great challenge to simultaneously achieve high glycerol activity and glyceric acid selectivity, suffering from either deep oxidation and C-C cleavage or poor oxidation efficiency from glyceraldehyde to glyceric acid. Herein, this work, inspired by nature, proposes a cascade synergistic catalysis strategy by atomic and low-coordinated cluster Pt on well-defined Cu-CuZrO_x, which involves enhanced C-H activation on atomic Pt₁ and O-H activation on cluster Pt_n in the oxidation of glycerol to glyceraldehyde, and cluster Pt_n for C=O activation followed by O-H insertion and atomic Pt₁ for C-H activation in the tandem oxidation of glyceraldehyde to glyceric acid. The enhanced C-H activation in the cascade process by atomic Pt₁ is revealed to be essential for the high glycerol activity (90.0±0.1%) and the glyceric acid selectivity (80.2±0.2%).

The selective oxidation of glycerol to glyceric acid is an important value-added cascade catalytic process. Here the authors report a cascade synergistic catalysis strategy by atomic and low-coordinated cluster Pt on well-defined Cu-CuZrOx to simultaneously achieve high activity and selectivity.