The under-coordinated two-dimensional layered Ni clusters on molybdenum nitride support generated from the Ni-4nm/γ-Mo2N has been demonstrated to be a thermally stable catalyst in 50 h stability test in CO2 hydrogenation, and exhibits a remarkable catalytic selectivity reverse compared with traditional Ni particles-based catalyst, leading to a chemo-specific CO2 hydrogenation to CO. A Ni-4nm/γ-Mo2N model catalyst is synthesized and used to further study the reverse sintering effect by the combination of multiple in-situ characterization methods, including in-situ quick XANES and EXAFS, ambient pressure XPS and environmental SE/STEM etc.

Here we report that the dispersion of metallic Ni particles into under-coordinated two-dimensional Ni clusters over γ-Mo2N is a thermodynamically favorable process based on the AIMD simulation. Reversing the thermal induced sintering phenomenon and forming high temperature stable fine dispersed metallic centers with unique structural and electronic properties is one of the ever-lasting targets of heterogeneous catalysis. Our results therefore open up new routes for tuning the restructuring behavior of supported metal catalysts and designing catalysts with controlled catalytic structures and reactivities.

Thus, the Pt SAs on anatase TiO2 preferentially activated selective hydrogenation of phenylacetylene (21.22 x 10-2 s-1 at 50 ✬), while planar Pt NPs on rutile significantly enhanced the combustion of methane (3.11 x 10-2 s-1 at 310 ✬). As a result, the reverse reactivity was achieved by tunning their distinct restructuring behaviors. Environmental transmission electron microscopy (ETEM) and density function theory (DFT) calculations were employed to directly visualize the dynamic transformation of Pt NPs and reveal the specific role that TiO2 supports play in promoting the stability and diffusion of Pt SAs.

Utilizing a variety of state-of-the-art characterizations, we showed that rutile TiO2 favors the reshaping of 2D planar Pt NPs, whereas the anatase surface facilitates the redispersion of Pt NPs to SAs upon calcination in the air up to 400 ✬. Here we show a crystal phase mediated restructuring of Pt NPs on TiO2, as a simple approach for fabricating either atomically dispersed single atoms (SAs) or reshaped planar NPs of Pt catalysts with tunable reactivities. Restructuring of supported metal nanoparticles (NPs) e.g., reshaping and redispersion are of tremendous interest for the rational design of high-efficiency catalyst materials with precise particle sizes, shapes, and reactivities. The result also highlights the importance of the metal-carbonyl complexes as intermediates for Ostwald ripening and disintegration of supported metal catalysts in the presence of CO. The theory is applied to TiO$_$(110) supported Rh particles in the presence of carbon monoxide, and reproduces well the broad temperature, pressure, and particle size range over which the sintering and redispersion occurred in such experiments. This allows for the rapid exploration of sintering and disintegration of supported metal particles in huge phase space of structures and compositions under various reaction environments. Most energetics and kinetics barriers in the theory can be obtained conveniently by first-principles theory calculations. The thermodynamic criteria for the reactant assisted Ostwald ripening and induced disintegration are formulated, and the influence of reactants on sintering kinetics and redispersion are mapped out. It includes: (1) energetics of supported metal particles, (2) formation of monomers (both the metal adatoms and metal-reactant complexes) on supports, (3) corresponding sintering rate equations and total activation energies, in the presence of reactants at arbitrary temperature and pressure. To achieve a quantitative description of these complicated processes, an atomistic and generic theory taking into account of the reaction environment, particle size and morphology, and metal-support interaction is developed. Understanding Ostwald ripening and disintegration of supported metal particles under operating conditions have been of central importance in the study of sintering and dispersion of heterogeneous catalysts for long term industrial implementation.

Metal disintegration machine