Revealing the anti-sintering phenomenon on silica-supported nickel catalysts during CO2 hydrogenation

The CO2 catalytic hydrogenation represents a promising approach for gas-phase CO2 utilization in a direct manner. Due to its excellent hydrogenation ability, nickel has been widely studied and has shown good activities in CO2 hydrogenation reactions, in addition to its high availability and low price. However, Ni-based catalysts are prone to sintering under elevated temperatures, leading to unstable catalytic performance. In the present study, various characterization techniques were employed to study the structural evolution of Ni/SiO2 during CO2 hydrogenation. An anti-sintering phenomenon is observed for both 9% Ni/SiO2 and 1% Ni/SiO2 during CO2 hydrogenation at 400°C. Results revealed that Ni species were re-dispersed into smaller-sized nanoparticles and formed Ni0 active species. While interestingly, this anti-sintering phenomenon leads to distinct outcomes for two catalysts, with a gradual increase in both reactivity and CH4 selectivity for 9% Ni/SiO2 presumably due to the formation of abundant surface Ni° from redispersion, while an apparent decreasing trend of CH4 selectivity for 1% Ni/SiO2 sample, presumably due to the formation of ultra-small nanoparticles that diffuse and partially filled the mesoporous pores of the silica support over time. Finally, the redispersion phenomenon was found relevant to the H2 gas in the reaction environment and enhanced as the H2 concentration increased. This finding is believed to provide in-depth insights into the structural evolution of Ni-based catalysts and product selectivity control in CO2 hydrogenation reactions.

Catalytic combustion of lean methane over different Co3O4 nanoparticle catalysts

Three types of Co3O4 catalyst, namely Co3O4 nanoparticles (denoted as Co3O4-NPs, ∼12 nm in diameter), Co3O4 nanoparticles encapsulated in mesoporou s SiO2 (denoted as Co3O4@SiO2), and Co3O4 nanoparticles inside microporous SiO2 hollow sub-microspheres (denoted as Co3O4-in-SiO2), were explored to catalyze the combustion of lean methane. It was found that the methane conversion over the three catalysts has the order of Co3O4-NPs ≈ Co3O4@SiO2 > Co3O4-in-SiO2 due to the different catalyst structure. The comparison experiments at high temperatures indicate the Co3O4@SiO2 has a significantly improved anti-sintering performance. Combined with the TEM and BET measurements, the results prove that the presence of the mesoporous SiO2 layer can maintain the catalytical activity and significantly improve the anti-sintering performance of Co3O4@SiO2. In contrast, the microporous SiO2 layer reduces the catalytical activity of Co3O4-in-SiO2 possibly due to its less effective diffusion path of combustion product. Thus, the paper demonstrates the pore size of SiO2 layer and catalyst structure are both crucial for the catalytical activity and stability.

A new insight into the theoretical design of highly dispersed and stable ceria supported metal nanoparticles

How to design and develop ceria supported metal nanoparticles (M/CeO₂) catalysts with high performance and sintering resistance is a great challenge in heterogeneous catalysis and surface science. In the present work, we propose two ways to improve the anti-sintering capability of M/CeO₂ catalysts. One is to introduce Ti atom on CeO₂ (1 1 1) to form monatomically dispersed Ti, TiO_x or TiO₂-like species on ceria. Density functional theory calculations show that the much stronger interactions between Au and Ti modified CeO₂ (1 1 1) occur compared with that on CeO₂ (1 1 1). According to the electronic analysis, the strong interactions are attributed to the electron transfer from the Ti modified ceria substrate to Au. The other is to dope Ti into CeO₂ (1 1 1) to form Ti_xCe_1-xO₂. This also leads to the interaction enhancement between Au and Ti_xCe_1-xO₂ (1 1 1). Electronic analysis indicates that the charge protuberance of surface O atoms near Ti atom results in the strong interactions between metal and ceria. This work provides new ideas for preparing M/CeO₂ catalysts with high dispersity and stability, and sheds light into the theoretical design of catalysts.