The Formation of PdC_xover Pd-Based Catalysts in Vapor-Phase Vinyl Acetate Synthesis: Does a Pd–Au Alloy Catalyst Resist Carbide Formation?

Abstract

The formation of Pd carbide (PdC x ) during the synthesis of vinyl acetate (VA) was investigated over Pd/SiO 2 catalysts with two different Pd particle sizes, as well as over a Pd-Au/SiO 2 mixed-metal catalyst. XRD data show that PdC x was produced in the pure Pd catalysts after reaction based on the downshift of the Pd (111) and (200) XRD features. The smaller Pd particles showed greater resistance to the formation of PdC x . The XRD and XPS data are consistent with formation of a PdC x species at the surface of the Pd-Au catalyst, however, the primary contributor to the downshift of the Pd(111) feature subsequent to reaction in the mixedmetal catalyst is believed to arise from reaction-induced alloying of Au with Pd. The alloying of Au with Pd is apparently very effective in preventing PdC x formation in Pd-based catalysts for VA synthesis.

Shape-Dependence of Pd Nanocrystal Carburization during Acetylene Hydrogenation

“…Han et al have suggested, employing ex situ XRD, that Pd nanocrystals below 2.4 nm were more resistant to carbide formation than their larger counterparts due to the metal nonmetal transition that could lead to drastically altered chemical properties. 44 This does not conflict with our findings and suggests that there is probably a critical size (ca. 2.5 nm 44 ) above which size does not significantly impact carbide formation.…”

The role of carbon species in heterogeneous catalytic processes: an in situ soft x-ray photoelectron spectroscopy study

“…The influence on the catalytic properties of carbon residing in the subsurface region was described previously [14][15][16]. Since these are all ex situ studies the question of stability of intermediates is an issue.…”

Photon Emission Spectroscopy of Single Oxide-Supported Ag-Au Alloy Clusters

“…1,2 The outstanding properties of alloys in heterogeneous catalysis result from a number of effects, e.g., a lowered barrier for a specific chemical reaction, an increased selectivity, 3,4 and an enhanced resistance against contamination and poisoning. 5,6 The formation of alloy particles on thin oxide films has recently been characterized using scanning tunneling microscopy ͑STM͒ and electron and infrared spectroscopy. [7][8][9] In contrast to bulk alloys, the intermixing in small particles depends on several additional parameters, such as diffusivity, nucleation and growth of the compounds on the oxide support, their surface free energy and miscibility, and the thermodynamic alloying conditions.…”