Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde

Abstract

Bimetallic catalysts of the type Ni–M with M=Cu, Ag and Au, and supported on TiO2-P-25 have been prepared by chemical reduction using glucose as the reducing agent. Hydrogenation of cinnamaldehyde (CAL) to yield hydrocinnamaldehyde (HCAL), cinnamyl alcohol (COL) and hydrocinnamyl alcohol (HCOL) has been studied on the catalysts in the temperature range 60–140°C and at 20kg/cm2 pressure, with methanol as solvent. Ni crystallite sizes, measured by X-ray line broadening analysis (XLBA), H2 pulse chemisorption and Transmission Electron Microscope (TEM) techniques, are in the range 8–12nm. Temperature Programmed Reduction (TPR) and Diffuse Reflectance Spectroscopic (DRS) studies indicate the formation of Ni–Cu alloys, while Ni–Ag and Ni–Au exist as bimetallic nanoparticles. High-resolution HRTEM studies show that the bimetallic nanoparticles are in close contact, forming hetero junctions. Changes in the XPS binding energy values for Ni 2p1/2 and Ni 2p3/2 levels reveal that Cu/Ag/Au, tend to increase electron density around Ni, which retards the adsorption of CAL via olefinic bond and weakens NiH bond strength. H2 TPD measurements also indicated weakening of NiH bond. Bimetallic catalysts display higher CAL conversion and selectivity to COL vis-à-vis the corresponding monometallic catalysts at lower reaction temperatures, 60–80°C. But, selectivity to COL decreases at higher temperatures, 100–120°C. Mode of adsorption of CAL and nature of adsorbed hydrogen on bimetallic catalysts influence their activity and selectivity.