Ni–Pt Alloy Nanoparticles with Isolated Pt Atoms and Their Cooperative Neighboring Ni Atoms for Selective Hydrogenation of CO₂ Toward CH₄ Evolution: In Situ and Transient Fourier Transform Infrared Studies

Abstract

We elucidated the cooperative active sites of Ni–Pt alloy nanoparticles composed of Pt atoms isolated by Ni atoms and their neighboring Ni atoms for the efficient and selective hydrogenation of CO2 toward CH4. The stepwise methanation dynamics were revealed by in situ observations and transient changes in the infrared spectra during the hydrogenation of CO2. It was found that the hydrogenation of CO species attached to the isolated Pt atoms proceeded through a bridging CO species between isolated Pt atoms and their neighboring Ni atoms, leading to an excellent selectivity toward CH4. Kinetic studies revealed that the high H2 dissociation ability of the Pt species accelerated the hydrogenation of the carbon species over the surface of the Ni–Pt alloy, thereby avoiding the rate limitations of CH4 formation which are common for Ni catalysts. The bifunctional role of the isolated Pt atoms therefore allowed the efficient formation of CH4 while maintaining the excellent selectivity of the Ni catalyst toward CH4, despite Pt catalysts tending to favor CO production.

Bimetallic Ni-Based Catalysts for CO2 Methanation: A Review

“…These chemisorbed CO species on Pt δ-could then be easily hydrogenated into CH4. As a result, Ni95Pt5 supported on Al2O3 with 5% Pt atoms in the NiPt alloy led to a higher CH4 formation rate and CH4 yield compared to the monometallic Ni catalyst and other NiPt alloys with a higher In a later work [122], it was shown, based on in-situ FTIR observations, that, on such NiPt alloy catalysts (iso-Pt), the adsorbed intermediate CO species that form CH 4 were rather bridging CO between isolated Pt atoms and nearby Ni atoms. On the monometallic Pt catalyst, strongly anchored CO species were mainly found as on-top CO (terminal mode, Pt-C≡O), which were difficult to be hydrogenated into CH 4 .…”

“…In a later work [ 122 ], it was shown, based on in-situ FTIR observations, that, on such NiPt alloy catalysts ( iso -Pt), the adsorbed intermediate CO species that form CH 4 were rather bridging CO between isolated Pt atoms and nearby Ni atoms. On the monometallic Pt catalyst, strongly anchored CO species were mainly found as on-top CO (terminal mode, Pt-C≡O), which were difficult to be hydrogenated into CH 4 .…”

“…Furthermore, the rate determining step (RDS) for CO 2 methanation over such NiPt alloy catalysts with isolated Pt atoms was suggested to be the chemisorption of H 2 , based on kinetic studies [ 122 ]. That could be explained by the fact that although Pt sites favoured the dissociation of H 2 at low temperatures, isolated surface Pt atoms were preferentially covered by CO rather than H 2 , due to the enhanced Pt-CO bonding.…”

“…As a result, this limited availability of H 2 dissociation sites on isolated Pt atoms became the factor that restricted the formation of CH 4 over NiPt alloys. Therefore, the RDS for CO 2 methanation over NiPt alloys was suggested to be H 2 dissociation rather than H-assisted CO dissociation or hydrogenation of surface carbon species, which are commonly indicated as the RDS over monometallic Ni methanation catalysts [ 7 , 122 ]. The experimental results and reaction mechanism for these two works are summarized in Figure 14 [ 31 , 122 ].…”

“…(d) Illustration of the adsorbed species and the reactions taking place over Ni/Al2O3, the iso-Pt catalyst, and Pt/Al2O3 during the hydrogenation of CO2. Reproduced with permission from[31,122]. Copyright: American Chemical Society, Washington, DC, USA, 2019, 2020.…”