Cu-Ga Interactions and Support Effects in CO2 Hydrogenation to Methanol Catalyzed by Size-Controlled CuGa Nanoparticles Deposited on SiO2 and ZnO

Abstract

Growing environmental concerns have led to a need for the reduction of CO₂ emissions and the search for alternative fuels. The synthesis of methanol via the CO₂ hydrogenation reaction provides a promising approach for these tasks. Promoting the existing Cu-based catalysts with Ga might be an option to create more effective catalysts. Here, size-controlled bimetallic CuGa nanoparticles (NPs) supported on either SiO₂ or ZnO were synthesized to study the nature of the interaction of Cu and Ga. Operando spectroscopy and diffraction characterization methods (XPS, XAS, XRD) were employed to establish structure, chemical composition, and reactivity correlations. We find that Ga stays oxidized under the reaction conditions and segregates to the surface. For the CuGa NPs/ZnO, the dominating interaction of Cu with ZnO inhibits the promoting effect of Ga. Only on the inert SiO₂ support, the beneficial influence of Ga is visible. Furthermore, high pretreatment temperatures were found to result in a favorable Cu-Ga interaction by partially reducing Ga, which is beneficial for methanol selectivity.

Keywords: CO2 hydrogenation; Cu; EXAFS; Ga; NAP-XPS; methanol.

1

AFM images of the Cu₇₀Ga₃₀ NPs supported on (A–C) SiO₂/Si­(111) and (D–F) ZnO(0001), after O₂-plasma for polymer removal (A, D), after CO₂ hydrogenation during NAP-XPS measurements (B, E), and the corresponding NP height histograms (C, F). The histograms were compiled from the analysis of multiple AFM images.

2

(A–D) STEM-HAADF images and corresponding EDX maps of (E–H) Cu and (I–L) Ga of CuGa NPs deposited on SiO₂ and ZnOAl. The samples were measured as-prepared and after CO₂ hydrogenation (H₂ + CO₂, 250 °C). The size of the scale bar in all images is 50 nm. The corresponding EDX spectra are shown in Figure S3.

3

(A) STEM and (B) corresponding EDX map of the CuGa NP/ZnOAl catalyst after reaction (H₂ + CO₂, 250 °C). The corresponding EDX spectra are shown in Figure S4.

4

Comparison of bimetallic micellar Cu₉₀Ga₁₀, Cu₆₂Zn₃₈ and pure Cu NPs, all supported on nanocrystalline SiO₂. The reactivity was measured at 40 bar and 250 °C in a feed gas mixture of 60% H₂, 20% CO₂, and 20% He. The methanol yield is normalized to the total amount of Cu in the catalyst. The flow rate for these measurements was 17 mL/min. The data points were obtained at 40 bar and 250 °C. The contribution of CO is excluded from the selectivity plot.

5

(A) Methanol yield and (B) product selectivity and CO₂ conversion of CuGa/SiO₂ and CuGa/ZnOAl catalysts. The product selectivity and the CO₂ conversion in (B) are shown only at 40 bar. (C) Temporal evolution of the methanol yield after reduction pretreatments at 250 and 500 °C. All reactivity measurements were conducted at a reaction temperature of 250 °C. Lines in (B) and (C) serve as a guide for the eye.

6

Selected (A, B) Cu K-edge and (C, D) Ga K-edge XANES and Fourier-transformed EXAFS spectra of the CuGa/SiO₂ catalyst in the as-prepared state, after reduction (H₂, 500 °C) and after reaction (H₂ + CO₂, 250 °C). Additionally, reference spectra of CuO, metallic Cu, Ga₂O₃, and metallic Ga are shown. The reference lines in (B) indicate the peak positions of the Cu–O and Cu–Cu bonds obtained from the CuO and metallic Cu references, respectively. The reference lines in (D) indicate the peak positions of the Ga–O and Ga–Ga bonds obtained from the Ga₂O₃ and metallic Ga references, respectively.

7

Linear combination analysis (LCA) of Ga K-edge spectra in CuGa NPs/SiO₂. (A) In-situ XANES spectra of the Ga K-edge that were used for LCA. (B) Evolution of the fractions of metallic Ga (black squares) and oxidized Ga (blue circles) under different H₂ and CO₂ + H₂ treatments at various temperatures (all at 1 bar) as obtained from the LCA using reference spectra.

8

Representative NAP-XPS spectra of the (A, D) Cu 2p and (B, E) Ga 2p regions for the Cu₇₀Ga₃₀ NPs supported on SiO₂/Si­(111) and ZnO(0001) measured with a photon energy of 1540 eV. The spectra shown here were acquired under ultrahigh vacuum (UHV), oxidizing (450 °C, O₂, 0.9 mbar), reducing (300 °C, H₂, 1 mbar), and reaction conditions (250 °C, H₂ + CO₂ (3:1), 1 mbar). The relative atomic fractions of Cu and Ga, acquired for two different probing depths, show segregation trends for the NPs supported on (C) SiO₂/Si­(111) and (F) ZnO(0001). These values were calculated from the 2p_3/2 peak areas of the corresponding spectra, and only the contributions from Cu and Ga were included.

9

Schematic representation of the proposed evolution of the CuGa NPs and their interaction with the support of the different materials upon exposure to different thermal treatments and chemical environments.