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

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 Pd-0 nanoparticles (ca. 3 nm), and the subsequent Pd-mediated reduction of Ga(III) species at temperatures ranging from 210 to 290 degrees 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 x 10(-9) mol(MeOH) m(-2) s(-1)), respectively, and 4-fold higher on a Cu or Pd molar basis (3330 and 910 mu mol mmol(PdorCu)(-1)h(-1)). Detailed characterization data (HR-TEM, STEM/EDX, XPS, and XRD) indicate that the catalyst contains Pd2Ga nanoparticles, of average diameters 56 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 degrees C) leads to a gradual decrease in methanol selectivity from 60 to 40%, while an optimum methanol production rate was found at 210 degrees 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.