Pd2Ga-Based Colloids as Highly Active Catalysts for the Hydrogenation of CO2 to Methanol

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• 作者：Andrés García-Trenco ; Edward R. White ; Anna RegoutzDavid J. Payne ; Milo S. P. Shaffer ; Charlotte K. Williams
• 刊名：ACS Catalysis
• 出版年：2017
• 出版时间：February 3, 2017
• 年：2017
• 卷：7
• 期：2
• 页码：1186-1196
• 全文大小：764K
• ISSN：2155-5435

文摘

Colloidal Pd₂Ga-based catalysts are shown to catalyze efficiently the hydrogenation of CO₂ to methanol. The catalysts are produced by the simple thermal decomposition of Pd(II) acetate in the presence of Ga(III) stearate, which leads to Pd⁰ nanoparticles (ca. 3 nm), and the subsequent Pd-mediated reduction of Ga(III) species at temperatures ranging from 210 to 290 °C. The resulting colloidal Pd₂Ga-based catalysts are applied in the liquid-phase hydrogenation of carbon dioxide to methanol at high pressure (50 bar). The intrinsic activity is around 2-fold higher than that obtained for the commercial Cu-ZnO-Al₂O₃ (60.3 and 37.2 × 10^–9 mol_MeOH m^–2 s^–1), respectively, and 4-fold higher on a Cu or Pd molar basis (3330 and 910 μmol mmol_Pd or Cu^–1 h^–1). Detailed characterization data (HR-TEM, STEM/EDX, XPS, and XRD) indicate that the catalyst contains Pd₂Ga nanoparticles, of average diameters 5–6 nm, associated with a network of amorphous Ga₂O₃ species. The proportion of this Ga₂O₃ phase can be easily tuned by adjusting the molar ratio of the Pd:Ga precursors. A good correlation was found between the intrinsic activity and the content of Ga₂O₃ surrounding the Pd₂Ga nanoparticles (XPS), suggesting that methanol is formed by a bifunctional mechanism involving both phases. The increase in the reaction temperature (190–240 °C) leads to a gradual decrease in methanol selectivity from 60 to 40%, while an optimum methanol production rate was found at 210 °C. Interestingly, unlike the conventional Cu-ZnO-Al₂O₃, which experienced approximately 50% activity loss over 25 h time on stream, the Pd₂Ga-based catalysts maintain activity over this time frame. Indeed, characterization of the Pd/Ga mixture postcatalysis revealed no ripening of the nanoparticles or changes in the phases initially present.