Combining Atomic Layer Deposition with Surface Organometallic Chemistry to Enhance Atomic-Scale Interactions and Improve the Activity and Selectivity of Cu–Zn/SiO₂ Catalysts for the Hydrogenation of CO₂ to Methanol

Abstract

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 gCu –1 and selectivity to methanol of 83%, with a space-time yield of 0.073 g h–1 gcat –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.

Boundary Conditions for Promotion versus Poisoning in Copper–Gallium-Based CO2-to-Methanol Hydrogenation Catalysts

“…Over the Cu–SiO 2 catalyst (Figure d), two bands at 2933 and 2854 cm –1 appeared within the first 10 min after the switch and stabilized. In agreement with previously reported assignments, − we presume that these bands are associated with formate species adsorbed on the Cu 0 surface. Despite poor quality IR signal below 2000 cm –1 , a broad peak is visible around 1600 cm –1 (Figure S65), a characteristic ν­(C–O) band of the formate species .…”

“…The absence of formate bands for more active Cu 0.93 Ga 0.07 –SiO 2 (Figure e) can be related to the accelerated hydrogenation (shorter lifetime or lower steady-state concentration) of the corresponding formate intermediate to methanol, due to the presence of promoter GaO x in the vicinity of Cu. The accelerated rate of methanol formation could instead result in two bands at 2959 and 2857 cm –1 ascribed to methoxy species adsorbed on silica, previously observed for silica-supported Pd–Ga, Ni–Ga, and Cu–Zn catalysts. These methoxy species also detected over Cu 0.93 Ga 0.07 –SiO 2 are not absorbed directly on the active bimetallic surface but on the neighboring Si–OH groups.…”

“…These methoxy species also detected over Cu 0.93 Ga 0.07 –SiO 2 are not absorbed directly on the active bimetallic surface but on the neighboring Si–OH groups. They can be considered as secondary species whose concentration slowly increases over time due to the formation of methanol on the catalyst surface . This can explain why we detect more intense IR bands of these methoxy species for more active Cu 0.93 Ga 0.07 –SiO 2 in comparison to those for less active Cu–SiO 2 .…”

A systematic theoretical study of CO2 hydrogenation towards methanol on Cu-based bimetallic catalysts: role of the CHO&CH3OH descriptor in thermodynamic analysis

“…Herein, the bimetallic alloy model was constructed by substituting the copper atoms with other transition metal (TM) atoms (Au, Ni, Pd, Pt and Zn). The primary rationale for selecting TM atoms was based on a literature review, which identified Au–Cu, 4,36,37 Ni–Cu, 6,38,39 Pd–Cu, 7,40–42 Pt–Cu 43,44 and Zn–Cu 45–47 as extensively documented copper-based catalytic models. Choosing these models may help ensure that the combination of transition metals represented by Au, Ni, Pd, Pt and Zn demonstrates reliable catalytic activity when paired with copper.…”

An atomic layer deposition diffusion–reaction model for porous media with different particle geometries

“…[6][7][8][9][10] In catalyst preparation, ALD can be used to introduce active metals, promoters and other modifiers with tuneable coating thicknesses and penetration depths inside a porous particle. 5,[10][11][12][13][14][15][16][17] In order to achieve the desired reactant deposition result with different reactants and porous substrates, diffusion-reaction modelling can be used to design and optimize the ALD process. In the diffusion-reaction model, the particle geometry has an effect on the reactant transport inside the porous particle.…”