1, Brookhaven National Laboratory, Upton, New York, United States
Catalytic conversion of CO2 has attracted extensive attention worldwide in recent years, aiming at alleviating the global warming and ocean acidification. Ni-based catalysts are identified as the most promising catalysts for CO2 hydrogenation due to their considerable catalytic activity and lower cost compared to precious metal catalysts. However, Ni-based catalysts are generally favorable for the Sabatier methanation reaction for CH4 rather than the reverse water gas shift (RWGS) reaction for CO. In many cases, CO is more desirable than CH4 as it offers high flexibility to produce long-chain hydrocarbons and synthetic fuels via the methanol synthesis or Fischer–Tropsch reactions. Thus, it is important to promote the selectivity of CO2 hydrogenation to CO over Ni-based catalysts. In situ characterization methods play an important role in the research of CO2 hydrogenation over Ni-based catalysts, as they can provide an insight into the material properties and the catalytic mechanisms dynamically. Thus functional catalysts can be designed and applied accordingly.
In this study the LaNiO3 and LaFe0.5Ni0.5O3 perovskite-type catalysts were synthesized and used for CO2 hydrogenation. The catalytic results demonstrated that the products selectivity were very different over the two catalysts: LaNiO3 mostly produced CH4 while the product over LaFe0.5Ni0.5O3 was almost CO. Ni-related nanoparticles were found over the catalysts after reaction by TEM characterization, demonstrating that partial of the Ni ions were reduced out of the perovskite lattice. Various in situ measurements, including in situ XRD, in situ XAFS, and in situ AP-XPS, were applied to characterize the evolution of the catalysts under reaction conditions. The results of in situ XRD showed that LaNiO3 was transformed to LaNiO2.5 during the reaction while the perovskite structure of LaFe0.5Ni0.5O3 could be maintained. Both of the in situ XAFS and in situ AP-XPS demonstrated that metallic Ni was formed over LaNiO3 while no metallic Ni was founded even in a more reducing atmosphere (40% H2/Ar) at 673 K over LaFe0.5Ni0.5O3. The more stable perovskite structure of LaFe0.5Ni0.5O3 was able to stabilize the Ni species in a higher oxidation state. Since CO is the key intermediate for CH4 formation over Ni-based catalysts the reason for different product selectivity might be related to the adsorption and activation of CO over Ni-related species. Density functional theory (DFT) calculations revealed that metallic nickel is responsible for highly selective CH4 production, while nickel with higher valence state weakens the binding of CO and increases the activation barrier for further CO hydrogenation, leading to a higher selectivity for CO. These findings not only give an understanding of La-Fe-Ni perovskite structure but also establish new correlations between the catalytic performance and structural properties of Ni-based catalysts, providing catalyst synthetic strategies for the application.