Vapour phase formic acid decomposition over PdAu/γ-Al ₂O₃ catalysts: Effect of composition of metallic particles

A series of 1.8 wt.% PdAu/γ-Al₂O₃ catalysts with a mean metal particle size of more than 10 nm and different Pd/Au ratios were prepared by impregnation followed by hydrazine reduction and tested in the decomposition of vapour phase formic acid (2.2 vol.%, 1 bar) with and without added CO to the gas mixture. The catalytic activity was correlated with the bulk (AAS, XRD, EDS/TEM) and surface (XPS) composition of PdAu particles. A novel result was obtained that the surface composition of the particles did not change despite a considerable variation of the Au atomic fraction and it was found to be close to that of the Pd₂Au alloy. The activity of the Au/Al ₂O₃ catalyst was negligible compared with that of the Pd/Al₂O₃ catalyst being a factor of 65 lower per gram of metal at 403 K. The surface electronic properties (XPS) and the catalytic activity of the Pd₂Au layer calculated per one surface Pd atom (TOF) did not depend on the bulk composition of the metal particles. A lower activity of the Pd₂Au layer as compared with the pure Pd layer (2-3 times) was observed, but the activation energies for the Pd and PdAu catalysts were similar (40 kJ mol^-1) and were lower than that for the Au/Al ₂O₃ catalyst (62 kJ mol^-1). Addition of CO led to a considerable inhibition of the reaction for all PdAu catalysts, this being accompanied by an increase in the activation energy to 69 kJ mol^-1.