Catalytic Functionalities of Palladium Supported on Zirconia Catalysts for Reductive-Amination of Phenol to Aniline

Abstract

A series of active palladium species supported on zirconia, with varying Pd loadings (0.5-6 wt.%), were synthesised using the impregnation method and characterised through XRD, surface area and pore analysis, UV-DRS, XPS, H₂-TPR, CO₂-TPD and CO chemisorption. XRD patterns indicated the formation of a crystalline PdO phase for Pd loadings exceeding 2 wt.%. Pore size distribution analysis showed a decrease in both pore volume and diameter as Pd content increased. Moreover, the intensity ratio of the Pd 3d_5/2 and Zr 3d_5/2 XPS peaks exhibited a strong correlation with Pd dispersion values derived from CO chemisorption. UV-DRS and TPR analyses revealed the presence of two distinct palladium species on the ZrO₂ support, including highly dispersed PdO species reduced at lower temperatures and bulk PdO species reduced at comparatively higher temperatures. The basicity of the catalysts increased with Pd loading, followed by a reduction at higher loadings. The findings indicate that the number of exposed surface Pd sites increases with increasing Pd loading, reaches a maximum at 2 wt.% Pd, and remains nearly constant at higher loadings. The catalytic activity for the vapour-phase reductive amination of phenol to aniline increased with Pd loading up to 2 wt.% and decreased at higher loadings, consistent with the observed trends in active Pd sites and surface basicity. The higher turnover frequency (TOF) for smaller Pd particles decreases with increasing particle size up to 3 nm and remains nearly constant for larger particles. The strong dependence of catalytic activity on Pd crystallite size confirms that the reductive amination of phenol proceeds as a structure-sensitive reaction.