In Situ DRIFTS and NAP-XPS Exploration of the Complexity of CO2 Hydrogenation over Size-Controlled Pt Nanoparticles Supported on Mesoporous NiO

Pt nanoparticles (4.8 nm) were anchored onto the surface of mesoporous nickel oxide (NiO) supports. Pt/NiO samples were compared with pristine NiO and Pt/SBA-15 silica catalysts in CO2 hydrogenation to form carbon monoxide, methane, and ethane at 473–673 K. Pt/NiO (1%) samples were ∼20 times and ∼1.5 times more active at 493 K compared with Pt/SBA-15 and NiO catalysts, respectively. However, the Pt-free NiO support has an activity of 120% compared to that of Pt/NiO catalysts at 673 K. In the case of 1% Pt/SBA-15 catalyst, the selectivity toward methane was 13%, whereas it was 90 and 98% for NiO and 1% Pt/NiO at 673 K, respectively. Exploration of the results of the reactions was performed by near-ambient pressure X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy. In the case of pure NiO, we found that the surface of the support was mainly covered by elemental Ni under reaction conditions, where the Ni/NiO x system is responsible for the high activity of the Pt-free catalyst. In the case of Pt/NiO, Pt improves the reduction of NiO x toward metallic Ni. In the case of 1% Pt/NiO catalysts, the presence of limited amount of Pt resulted in an optimal quantity of the oxidized Pt fraction at 673 K, showing the presence of a Pt/PtO x /Ni/NiO x mixed phase where the different interfaces may be responsible for the high activity and selectivity toward methane. In the case of pure NiO under reaction conditions, small amounts of formaldehyde and hydrogen-perturbed CO [H n CO (n = 1, 2)] were detected. However, in the case of 1% Pt/NiO catalysts, besides the absence of formaldehyde, a significant amount of H n CO (n = 2–3) was present on the surface, responsible for the high activity and methane selectivity.