(735f) In-Situ Characterization of Intermetallic Ni5Ga3 Catalyst for CO2 Reduction to Methanol
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.
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