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

Abstract

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 molMeOH m–2 s–1), respectively, and 4-fold higher on a Cu or Pd molar basis (3330 and 910 μmol mmolPd 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

Relative Roles of Methanol Synthesis and Solid Acid Catalysts in the Direct DME Synthesis from Syngas

“…Several publications have focused on the activity of methanol dehydration of the admixed catalysts for direct DME synthesis. 6,[8][9][10]15,16 However, the effects of the solid acid catalysts can be discussed with the relative activity of methanol synthesis catalysts. If relatively more active methanol synthesis catalysts as compared with the solid acid catalysts were used, the activity of the solid acid catalysts would be very important for the activity of the direct DME synthesis catalysts (HMLD and HMHD).…”

“…On the other hand, it was reported that the overall rate of the STD reaction was determined by the acid properties of solid acid catalysts in methanol dehydration. 6,15,16 Therefore, STD synthesis rate can be determined by the admixed ratio of methanol synthesis and methanol dehydration catalysts.…”

Influence of reduction temperature on the formation of intermetallic Pd2Ga phase and its catalytic activity in CO2 hydrogenation to methanol

“…Intermetallic compounds are considered as promising materials for heterogeneous catalysis because of their several intrinsic characteristics, including high selectivity to particular reactions, and superior stability compared to pure or alkali promoted metals . Recent studies have found that Pd–Ga intermetallic compound, more specifically Pd 2 Ga phase, is a highly selective hydrogenation catalyst . Under identical reaction conditions, the intrinsic activity of Pd 2 Ga catalyst in CO 2 hydrogenation to methanol is found to be greater than that of the traditional Cu/ZnO/Al 2 O 3 catalyst owning to its lower desorption energy for the product than the adsorption energy for the reactant .…”

Investigation of the Phase Equilibria of CO2/CH3OH/H2O and CO2/CH3OH/H2O/H2 Mixtures

“…The current research on the CH 3 OH synthesis covers a broad pressure range. The pressure varies from atmospheric pressure processes with the aid of a plasma [8], over mid-pressure (30-50 bar) processes [9][10][11][12][13][14] to high-pressure processes [15][16][17][18][19][20][21]. Mostly, heterogeneous catalysts based on copper, gold, or palladium are applied.…”