The direct synthesis of methanol via the hydrogenation of CO₂, if performed efficiently and selectively, is potentially a powerful technology for CO₂ mitigation. Here, we develop an active and selective Cu-Zn/SiO₂ catalyst for the hydrogenation of CO₂ by introducing copper and zinc onto dehydroxylated silica via surface organometallic chemistry and atomic layer deposition, respectively. At 230 °C and 25 bar, the optimized catalyst shows an intrinsic methanol formation rate of 4.3 g h^-1 g_Cu^-1 and selectivity to methanol of 83%, with a space-time yield of 0.073 g h^-1 g_cat^-1 at a contact time of 0.06 s g mL^-1. X-ray absorption spectroscopy at the Cu and Zn K-edges and X-ray photoelectron spectroscopy studies reveal that the CuZn alloy displays reactive metal support interactions; that is, it is stable under H₂ atmosphere and unstable under conditions of CO₂ hydrogenation, indicating that the dealloyed structure contains the sites promoting methanol synthesis. While solid-state nuclear magnetic resonance studies identify methoxy species as the main stable surface adsorbate, transient operando diffuse reflectance infrared Fourier transform spectroscopy indicates that μ-HCOO*(ZnO_x) species that form on the Cu-Zn/SiO₂ catalyst are hydrogenated to methanol faster than the μ-HCOO*(Cu) species that are found in the Zn-free Cu/SiO₂ catalyst, supporting the role of Zn in providing a higher activity in the Cu-Zn system.

On page 452, column 1 (lines 12?23) reads: H2 and CO chemisorption show an uptake of 0.91 molH2 molPd?1 and 0.61 molCO molPd ?1, respectively (Table 1, Supporting Information S6). Considering a 1:1 CO/Pd stoichiometry,32 the dispersion from CO chemisorption (D?CO) equals 61%, in a reasonable agreement with the dispersion from TEM (D?TEM 70%; Supporting Information S9).32 While H2 chemisorption is not effective for a determination of the metal dispersion of Pd nanoparticles due to the formation of a stable bulk hydride with larger particles (2.6 nm),32 a comparison of the H2 uptake and D?CO would correspond to approximately three hydrogen atoms per surface Pd.

Three γ/β-Ga₂O₃ nanoparticle catalysts that differ in the relative ratio of γ-Ga₂O₃ to β-Ga₂O₃ were prepared to evaluate the effect of H₂ treatment (500 °C, 2 h) on the coordination environment of bulk and surface Ga sites, Lewis acidity and catalytic activity in propane dehydrogenation (PDH). Independent of the H₂ treatment, the initial PDH activity of the γ/β-Ga₂O₃ catalysts increases with the fraction of the β-Ga₂O₃ phase. This is explained by the presence of weak Lewis acid sites (LAS) in β-Ga₂O₃ while such sites are absent in γ-Ga₂O₃. Treatment with H₂ increases the catalytic activity of all three γ/β-Ga₂O₃ catalysts but for different reasons. For catalysts with higher fractions of β-Ga₂O₃, H₂ treatment increases further the relative abundance of weak LAS, likely by generating coordinatively unsaturated Ga sites (such as tricoordinated Ga sites nearby oxygen vacancies). In contrast, H₂ treatment of a catalyst containing a predominant fraction of γ-Ga₂O₃ phase induces disorder in the sub-surface structure of the nanoparticle, that is, it forms gallium and oxygen vacancies in the bulk and favors migration of gallium, and likely also of oxygen, to the surface. This induces a surface reconstruction that notably increases the fraction of strong LAS (and proportionally decreases the fraction of medium LAS), while creating no weak LAS in γ-Ga₂O₃-H₂. Therefore, the increase in the catalytic activity of H₂-treated γ-Ga₂O₃ is explained by the higher density of surface Ga sites in γ-Ga₂O₃-H₂ relative to calcined γ-Ga₂O₃. H₂-treated catalysts that contain a higher relative amount of weak LAS also feature a higher relative abundance of gallium hydride species associated with a low frequency FTIR band at ca. 1931–1939 cm⁻¹, that is, weak LAS likely give weakly-bound hydrides in β-Ga₂O₃. Our results highlight that weak LAS in unsupported Ga₂O₃ catalysts are more active in PDH than mild or strong LAS.

α-Ga2O3, β-Ga2O3, and I -Ga2O3 as well as the silica-supported catalysts I -Ga2O3/SiO2, β-Ga2O3/SiO2, and Ga(NO3)3-derived Ga/SiO2 were prepared, characterized, and evaluated for propane dehydrogenation (PDH) at 550 °C. The coordination environment and acidity of surface sites in stand-alone and SiO2-supported Ga2O3 catalysts were studied using FTIR, 15N dynamic nuclear polarization surface-enhanced NMR spectroscopy (15N DNP SENS), and DFT modeling of the adsorbed pyridine probe molecule. The spectroscopic data suggest that the Lewis acidic surface Ga sites in I -Ga2O3 and β-Ga2O3 (the latter obtained from colloidal nanocrystals of I -Ga2O3 via thermal treatment at 750 °C) are similar, except that β-Ga2O3 contains a larger relative fraction of weak Ga3+ Lewis acid sites. In contrast, α-Ga2O3 features mostly strong Lewis acid sites. This difference in surface sites parallels their difference in catalytic activities: I.e., weak Lewis acid surface sites are more abundant in β-Ga2O3 relative to α-Ga2O3 and I -Ga2O3 and the increased relative abundance of weak Lewis acidity correlates with a higher initial catalytic activity in PDH, 0.41 > 0.28 > 0.14 mmol C3H6 m-2 (Ga2O3) h-1 at 550 °C, for respectively β-, α-, and I -Ga2O3 with initial propene selectivities of 86, 83, and 88%. Dispersion of I -Ga2O3 or β-Ga2O3 on a silica support introduces strong as well as abundant weak Brønsted acidity to the catalysts, lowering the PDH selectivity. The I -Ga2O3/SiO2 catalyst was slightly more active than β-Ga2O3/SiO2 in PDH (Ga normalized activity) with initial propene formation rates of 11 and 9 mol C3H6 mol Ga-1 h-1 (sel = 76 and 73%, respectively). However, these catalysts deactivated by ca. 55% within 100 min time on stream (TOS) due to coking. In contrast, Ga/SiO2, with mostly tetracoordinated surface Ga sites and abundant, strong Brønsted acid sites, gave a lower activity and selectivity in PDH (3.5 mol C3H6 mol Ga-1 h-1 and 49%, respectively) but showed no deactivation with TOS. DFT calculations using a fully dehydroxylated oxygen-deficient model β-Ga2O3 surface show that tetra- A nd pentacoordinated Ga Lewis acid sites bind pyridine more strongly than tricoordinated Ga sites and a higher relative fraction of strong Lewis acid sites correlates with increased coking. Overall, our results indicate that weakly Lewis acidic, tricoordinated Ga3+ sites are likely driving the superior PDH activity of β-Ga2O3.

The direct conversion of CO₂to CH₃OH represents an appealing strategy for the mitigation of anthropogenic CO₂emissions. Here, we report that small, narrowly distributed alloyed PdGa nanoparticles, prepared via surface organometallic chemistry from silica-supported Ga^IIIisolated sites, selectively catalyze the hydrogenation of CO₂to CH₃OH. At 230 °C and 25 bar, high activity (22.3 mol_MeOHmol_Pd^-1h^-1) and selectivity for CH₃OH/DME (81%) are observed, while the corresponding silica-supported Pd nanoparticles show low activity and selectivity. X-ray absorption spectroscopy (XAS), IR, NMR, and scanning transmission electron microscopy-energy-dispersive X-ray provide evidence for alloying in the as-synthesized material. In situ XAS reveals that there is a dynamic dealloying/realloying process, through Ga redox, while operando diffuse reflectance infrared Fourier transform spectroscopy demonstrates that, while both methoxy and formate species are observed in reaction conditions, the relative concentrations are inversely proportional, as the chemical potential of the gas phase is modulated. High CH₃OH selectivities, across a broad range of conversions, are observed, showing that CO formation is suppressed for this catalyst, in contrast to reported Pd catalysts.

Silica-supported silver nanoparticles exhibit outstanding efficiency in the CO₂ hydrogenation to methyl formate in the presence of methanol under high pressure. Here, we show that ZrO₂ and Al₂O₃ supports significantly increase the catalyst activity, in line with their higher Lewis acidity. The weight time yield of methyl formate over Ag/ZrO₂ is up to 16.2 g_MF g_Ag h⁻¹ without detectable side-products, 25 times higher compared to Ag/SiO₂ at the same temperature. Transient in situ and operando DRIFTS studies uncover spillover processes of formate species from Ag onto the acidic support materials and show that the surface formates can further react with adsorbed methanol at the sites near the perimeter between Ag and the support to yield methyl formate.