In-Situ Characterization of Intermetallic Ni5Ga3 Catalyst for CO2 Reduction to Methanol

Recently, a NiGa intermetallic catalyst for the reduction of CO₂ to methanol gathered great attention because it showed better activity and selectivity than CuZnAl-based conventional catalysts^1,2. DFT calculation and catalytic activity tests pointed out how the intermetallic nature of this catalyst plays a major role for activity and selectivity to methanol. However, a more detailed in-situ characterization that confirms the theoretical results is still missing. In fact, it is well know that nanoparticles morphology and oxidation state are strongly related to the gas environment, temperature and pressure and nanoparticles may undergo dynamic changes under catalytic conditions³.

We recently performed operando HERFD-XAS tests on Ni₅Ga₃/SiO₂. The tests were conducted under the same conditions employed in a labscale tubular reactor (CO₂/H₂=1; P=1-10bar; T= 200-300 ^oC; GHSV=6000 h^-1) and comparable catalytic performances were revealed. The right intermetallic phase was obtained by ex-situ reduction at 700 ^oC, as confirmed by XRD, and the samples were pre-reduced at 400 ^oC in-situ before the analysis. After in-situ reduction at 400 ^oC the analysis of the XANES region showed that while Ni was completely reduced, Ga was only partially reduced (as also evidenced by ETEM analysis;). Remarkably, we can conclude that upon reduction at 400 ^oC the Ni is in a reduced metallic state and when the sample is exposed to the CO₂/H₂gas mixture both Ni and Ga show a minimal change in oxidation state. These results obtained through bulk sensitive technique, such as HERFD-XAS, for bimetallic nanoparticles of ~8 nm may indicate a surface sites oxidation change during catalysis. Moreover, surface segregation effects cannot be ruled out. Promising preliminary APXPS data on the reducibility of the catalyst clearly showed the presence of oxidized Ga upon exposure to air, while no signals related to Ni was detected, confirming that the surface is covered by a layer of Ga oxide.

References

1) F. Studt et al. Nature Chemistry 6, 320-324 (2014).

2) I. Sharafutdinov et al. J. Catal. 320, 77-88 (2014).

3) C Holse, J. Phys. Chem. C 119, 2804-2812 (2015).