A Comparative Study of Methodologies for the Incorporation of Ge into HISiO2-Supported, Pt(Acac)2-Derived, Pt−Ge Catalysts and the Effect of Internal Oxide Structure on Particle Morphology and CO Adsorption

Abstract

Five synthetic approaches to the incorporation of germanium into materials based upon Pt(acac)(2) supported on HISiO2 have been assessed by using Pt L-III- and Ge K-edge extended X-ray adsorption fine structure (EXAFS), powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), and diffuse reflectance infrared spectroscopy (DRIFTS) of adsorbed CO. After reduction in 10% H-2/N-2 (T>673 K), two of these methods (utilizing GeBu4 (Bu = C4H9) and GePh4 (Ph = C6H5)) appear to result in the uniform formation of PtGe alloy particles though their capacities For CO chemisorption are markedly different. The successful formation of uniformly alloyed PtGe particles is determined by a competition between processes resulting in the formation of stable GeOx species, and those that result in intimate contact between the Ge precursor and evolving Pt particles. The latter processes are promoted by mobile precursors that interact with the support via physical interactions alone. However, the final character of the catalyst is also determined by the character of the ligands present in the Ge precursor; increasingly stable and aromatic ligands result in increased carbon retention and subsequent loss of adsorptive capacity. Further, it is found that the use of a mesoporous SiO2 support results in new CO adsorptions in DRIFTS that may not be ascribed to the formation of conventional Chini type Pt-carbonyl complexes or CO adsorption upon typical Pt particles. The origin of new absorptions is discussed in terms of the formation of Pt or PtGe particles with a morphology that has been directed by the internal structure of the mesoporous support.