Particle Size or Electronic Effect? An XAS Study of Re@Pd Overlayer Catalysts

Abstract

Re@Pd (core@overlayer) catalysts are analyzed via X-ray absorption spectroscopy to correlate structural and electronic properties with previous reactivity results. The ethylene hydrogenation turnover frequency of the Re–Pd bimetallic and the Re@Pd catalysts is much lower than that of the pure Pd. A reduction of activity agrees with the computationally predicted properties of the overlayer catalysts and indicates that the overlayer catalysts have been synthesized. The FEFF fitted EXAFS yields a Pd–Pd inter-atomic distance in the Re@Pd SD small particle (2.7 nm) catalyst of 2.79 A and in the Re@Pd SD large particle (6.3 nm) catalyst of 2.76 A. The fitted Pd–Pd inter-atomic distance of Pd foil is 2.74 A. The small particle overlayer Re@Pd SD catalyst is also longer than the Pd–Pd inter-atomic distance seen in the, as synthesized, structureless bimetallic Re–Pd (2.73 A). The disparity in calculated inter-atomic distances indicates an electronic effect is being exerted upon the disperse Pd atoms by the larger Re crystals and vice versa. The Pd K-edge and Re LIII-edge white line data also show the electronic interaction between the Pd overlayer and the core Re atoms. The Pd white line of the overlayer catalysts shifted up relative to Pd foil indicating Pd d-band broadening as a result of the interaction of Pd with Re. The Re d-band of the overlayer catalysts narrowed, which is evidenced by the decreased white line absorption relative to Re foil. These observed changes in the Pd and Re d-band are consistent with computational predictions for catalysts with overlayer structures.