A Comparative Study of Methodologies for the Incorporation of Ge into H_ISiO₂-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 H I SiO 2 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 GeBu 4 (Bu ) C 4 H 9 ) and GePh 4 (Ph ) C 6 H 5 )) 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 GeO x 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 SiO 2 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.

Suppressed surface alloying for a bulk miscible system: Ge on Pt(100)

“…[1][2][3][4] In many cases, the metal salts are reduced and combine with Pt to form bimetallic alloy clusters. 5,6 Fundamental surface science studies on well-defined single crystal surfaces are helpful in understanding this alloying process and the structure of the alloy clusters.…”

Particle development and characterisation in Pt(acac)2 and Pt(acac)2/GeBu4 derived catalysts supported upon porous and mesoporous SiO2: effect of reductive environment, and support structure

“…3 shows PXRD patterns derived ex situ from various elemental Pt samples after reduction/decomposition to 673 K; the corresponding pattern derived from equivalent treatment of the sample GeBu 4 /Pt(acac) 2 /H 1 SiO 2 can be found in ref. 14. These bear out the implications of the EXAFS measurements with considerable variation being observed in the intensity/halfwidth of the Pt reflections indicating a considerable variation in average crystallite size in each system.…”

“…8 shows aspect ratio plots derived from analysis of the particle distributions obtained from reduction (in H 2 /N 2 ) and decomposition (under N 2 ) of Pt(acac) 2 samples supported upon SiO 2 and H 1 SiO 2 to 673 K. Representative TEM images, and the derived particle distributions (PSD), obtained from analogous treatments of the Pt(acac) 2 and Pt(acac) 2 /GeBu 4 sample on the mesoporous silica have been presented elsewhere. 14 As noted in ref. 14 the mesoporous support yields a bifurcated PSD with particles characterised by an aspect ratio of ca.…”

“…The PXRD pattern derived from the reduced Ge/Bu 4 /Pt(acac) 2 /H 1 SiO 2 system has been presented elsewhere 14 and shows new reflections that may be indexed to the formation of Pt 3 Ge. 14,24 In this case, and in contrast to systems derived from GePh 4, 14 no evidence for the formation of a Pt 3 Ge 2 phase is observed.…”

“…Recently we have started to delineate the effects of varying the manner in which Ge is introduced to the evolving Pt(acac) 2 based catalyst system and the effect of using different, organogermanium precursors. 14 These studies have indicated that the mobility and stability of the organo-germanium species is crucial in avoiding the formation of GeO x species, and therefore the promotion of PtGe alloy formation.…”