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To address these challenges, a series of silica- supported Ni-Ga-based catalysts with varying Ni:Ga ratios (NixGa(100–x)/SiO2) was prepared via a surface organometallic chemistry approach [2], allowing for precise control over the metal loading. This method yielded Ni-Ga nanoparticles with different Ni-Ga ratios but uniform particle size (ca. 2 nm). The structure of the catalysts was probed using operando X-ray absorption spectroscopy (XAS) at the Ni and Ga K edges (acquisition time ca. 3 minutes per scan) at beamline BM31 and X-ray total scattering at beamline ID15A (acquisition time ca. 5 minutes per scan). The catalysts were characterized both after in-situ activation in H₂ at 600°C and under CO₂ hydrogenation conditions at 230°C and 20 bar (using a CO₂:H2:N2 gas mixture in a ratio of 1:3:1). These experiments were conducted in a capillary cell reactor with online off-gas analysis via gas chromatography. Additionally, differential pair distribution function analysis (d-PDF) of the X-ray total scattering data was performed, subtracting the scattering contribution from the amorphous SiO₂ support from the total scattering pattern of the NixGa(100–x)/SiO2 (Figure 50a).

Modelling of the d-PDF revealed a face-centred cubic (fcc) Ni-Ga random alloy for all catalyst compositions (Figure 50b). Based on Vegard’s law, the Ga content in the Ni-Ga alloy was determined and found to be lower than the nominal Ni:Ga ratio determined by ICP-OES, indicating that some Ga was not incorporated into the fcc structure. Ga K-edge XAS experiments further revealed that the unincorporated Ga existed as GaOx (Figure 50d).

Combining the structural and performance data (Figure 51), several key observations were made: (i) the presence of an fcc Ni-Ga alloy is crucial for achieving high methanol selectivity; (ii) small quantities of GaOx in addition to the Ni-Ga alloy promote methanol formation, as evidenced by comparing two catalysts with similar Ni:Ga ratios in the alloy but differing GaOx content; (iii) the catalyst with the highest methanol selectivity (Ni65Ga35/SiO2) contained the most Ga-rich fcc alloy with a Ni:Ga molar ratio of approximately 3:1, along with a minor quantity of oxidized Ga species.

Additional Ni K-edge XAS experiments revealed that (i) Ni was fully metallic in all catalysts studied, and (ii) alloying Ni with Ga modified the electronic structure of Ni, forming

Niδ- species (Figure 50c). Tracking the structure of the most active Ni65Ga35/SiO2 catalyst under CO2 hydrogenation conditions over ~ 4 hours of time on stream showed that the Ni-Ga alloy nanoparticles retained their structure and composition, with no evidence of dealloying or further alloying, correlating with a stable methanol production rate. Complementary operando DRIFTS experiments suggested that GaOx species facilitate CO2 activation and stabilize formate species on the catalyst surface, which are reaction intermediates in CO2 hydrogenation to methanol.

These findings offer fundamental insights into the reactivity of Ni-Ga-based catalysts and provide guidelines for further enhancing catalytic performance. The combination of operando X-ray scattering, XAS, and DRIFTS enables the formulation of quantitative structure- performance relationships. Overall, optimizing both the Ni:Ga ratio in the alloy and the quantity of GaOx species is key to obtaining catalysts with high methanol selectivity and production rates.

PRINCIPAL PUBLICATION AND AUTHORS

Structure and Role of a Ga-Promoter in Ni-Based Catalysts for the Selective Hydrogenation of CO2 to Methanol, N.K. Zimmerli (a), L. Rochlitz (b), S, Checchia (c), C.R. Müller (a), C. Copéret (b), P.M. Abdala (a), JACS Au 4 (1), 237-252 (2024); https:/doi.org/10.1021/jacsau.3c00677 (a) Department of Mechanical and Process Engineering, ETH Zürich, Zürich (Switzerland) (b) Department of Chemistry and Applied Biosciences, ETH Zürich, Zürich (Switzerland) (c) ESRF

REFERENCES

[1] F. Studt et al., Nat Chem. 6, 320-324 (2014). [2] C. Copéret, Acc. Chem. Res. 52, 1697-1708 (2019).

Fig. 51: a) Methanol formation rate and methanol selectivity (averaged over the first 180 minutes) as a function of the catalyst

composition, determined by elemental analysis and denoted as xICP in NixGa(100–x)/SiO2. b) Gaalloyed and GaOx contents as a

function of xICP. Shaded lines serve as visual guides.