Effects of chloride precursors on the palladium valency and surface structures of Pd [sbnd]Mg 2+/SiO 2catalysts for carbon monoxide hydrogenation

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The effects of chloride precursors on the Pd valence states and surface structures of Pd Mg 2+/SiO 2 catalysts for carbon monoxide hydrogenation have been studied by means of electron-spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) techniques. The Cl-containing catalyst exhibits a much more positive Pd valency and a much more stable Pd surface structure than the Cl-free one. This is attributed to the stronger ability of chlorine anions to stabilize Pd structures and to transfer electrons from Pd to Mg 2+, rather than to a better Pd dispersion of the Cl-containing catalyst. Over the Cl-containing catalyst, methanol formation and a Pd + ESR signal are found to increase concurrently with reaction time. Accordingly, Pd + ions are claimed to be essential for methanol formation. However, only those Pd + ions which are located in a suitable structure are supposed to be the active center for methanol synthesis, because oxygen or hydrogen-water pretreatments are found to cause (i) a marked decrease of the turnover frequencies (TOF) for methanol formation, (ii) a probable transformation of Pd(Cl,X)Mg 2+ (X=Cl or O) bridge structures to oxychloropalladium and MgO, and (iii) the formation of more Pd + ions. Furthermore, the Pd + ions formed during the induction period are found not to be mainly due to the catalyst being modified by water or methanol, but probably due both to the high-pressure Pd reconstruction and the oxidation of an HCOO intermediate. In addition, the metal-promoter-support interactions are discussed.