On the Surface Nature of Bimetallic PdZn Particles Supported on a ZnO–CeO₂ Nanocomposite for the Methanol Steam Reforming Reaction

Abstract

CO adsorption—as a molecular probe—was studied by transmission IR spectroscopy on pre-reduced Pd and bimetallic PdZn nanoparticles. Palladium was supported (2 wt% Pd) on pure CeO₂, ZnO and a ZnO–CeO₂ composite (atomic ratio Zn:Ce = 1:2). The Pd 3d_5/2 binding energy shift, together with the formation of metallic zinc were consistent with the development of a PdZn alloy over the zinc-containing supports at increasing reduction temperature, as revealed by XPS. Following H₂ reduction at 623 K the bimetallic particles showed only linear CO adsorption (CO_L) at initial contact time (10 Torr CO, 298 K), giving rise to a convoluted IR band ascribed to different Pd sites, where it was assumed that the Pd–Pd distances were larger than for pure Pd crystallites, indicating the presence of a PdZn alloyed surface. However, for longer exposure time to CO and/or higher superimposed pressure, the appearance of bridge and hollow coordinated CO (CO_B and CO_H, respectively) on the Pd sites suggested the degradation of the PdZn surface alloy, most likely due to the segregation of Pd surface patches. The temperature-programmed, dynamic isobaric adsorption of CO (TPA-CO), under flowing CO(1%)/He on the catalysts pre-reduced at 623 K (that is, for similar conditions to those found in the methanol steam reforming—MSR-process) showed faster desorption of CO_L as compared to CO_B + CO_H species for supported Pd/CeO₂, as expected. However, the TPA-CO results on Pd/ZnO–CeO₂ were atypical: even under the superimposed, low CO partial pressure, and for a temperature range similar to those found at high methanol conversion in the MSR reaction, the PdZn bimetallic surface nature was recovered, which could be an explanation of the good selectivity to CO₂ of Pd/ZnO-based catalysts and—in particular—of the catalytically stable Pd/ZnO–CeO₂ materials.

Graphical Abstract