Effect of zeolite protons on Pd particle size and H2 chemisorption

Abstract

Zeolite protons enhance metal dispersion by chemically anchoring small reduced Pd particles; they also markedly reduce the propensity of Pd to chemisorb H₂. Whereas the true metal dispersion, derived from Pd–Pd coordination numbers determined by EXAFS is higher for Pd/HY than for Pd/NaHY or Pd/NaNH₄Y, conventional H₂ chemisorption gives lower H : Pd ratios for the samples with higher dispersion. This apparent discrepancy is confirmed by static and dynamic adsorption measurements and by rapid isotope exchange between gaseous D₂ and zeolite protons. Remarkably, the mononuclear Pd proton adduct [Pd₁—H_x]^x+ does not catalyse D–H isotope exchange at 300 K, because it apparently does not chemisorb D₂ or H₂.