Reversing sintering effect of Ni particles on γ-Mo2N via strong metal support interaction

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. 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. 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. 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.

Reversing sintering effect of Ni particles on γ-Mo2N via strong metal support interaction

The reverse sintering effect of Ni particles under thermal treatment has been observed in the Ni/γ-Mo2N catalysts. The ab initio molecular dynamic simulation has demonstrated the redispersion of metallic Ni particles into under-coordinated two-dimensional Ni clusters over γ-Mo2N is a thermodynamically favorable process. Utilizing pre-synthesized 4 nm Ni nanoparticles as the loaded particles, a Ni-4nm/γ-Mo2N model catalyst was synthesized and used to 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. The theoretical and experimental studies both confirmed the reverse sintering effect in the Ni-γ-Mo2N system is driven by the strong metal-support interaction between Ni and γ-Mo2N. The potential application of the reverse sintering effect in heterogeneous catalysis has been realized using the high temperature favored CO2 hydrogenation reaction. 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, and exhibits a remarkable catalytic selectivity reverse compared with traditional Ni based catalyst, leading to a chemo-specific CO2 hydrogenation to CO.

Reaction of nitric oxide molecules on transition-metal-doped silver cluster cations: Size- and dopant-dependent reaction pathways

We report size- and dopant-dependent reaction pathways as well as reactivity of gas-phase free AgnM+ (M = Sc–Ni) clusters interacting with NO. Reactivity of AgnM+, except for M = Cr...

Ni Nanoparticles on CeO2 (111): Energetics, Electron Transfer and Structure by Ni Adsorption Calorimetry, Spectroscopies and DFT

<div>The morphology, interfacial bonding energetics and charge transfer of Ni clusters and nanoparticles on slightly-reduced CeO_2-x (111) surfaces at 100 to 300 K have been studied using single crystal adsorption calorimetry (SCAC), low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and density functional theory (DFT). The initial heat of adsorption of Ni vapor decreased with the extent of pre-reduction (x) of the CeO_2-x (111), showing that stoichiometric ceria adsorbs Ni more strongly than oxygen vacancies. On CeO_1.95 (111) at 300 K, the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Ni clusters on stoichiometric steps, followed by the Ni particles spreading onto less favorable terrace sites. At 100 K, the clusters nucleate on terraces due</div><div>to slower diffusion. Adsorbed Ni monomers are in the +2 oxidation state, and they bind by ~45 kJ/mol more strongly to step sites than terraces. The measured heat of adsorption versus average particle size on terraces is favorably compared to DFT calculations. The Ce 3d XPS lineshape</div><div>showed an increase in Ce³⁺/Ce⁴⁺ ratio with Ni coverage, providing the number of electrons donated to the ceria per Ni atom. The charge transferred per Ni is initially large but strongly decreases with increasing cluster size for both experiments and DFT, and shows large differences between clusters at steps versus terraces. This charge is localized on the interfacial Ni and Ce atoms in their atomic layers closest to the interface. This knowledge is crucial to understanding the nature of the active sites on the surface of Ni-CeO₂ catalysts for which metal-oxide interactions play a very important role in the activation of O−H and C−H bonds. The changes in these interactions with Ni particle size (metal loading) and the extent of reduction of the ceria help to explain how previously reported catalytic activity and selectivity change with these same structural details.</div>

Ni Nanoparticles on CeO2 (111): Energetics, Electron Transfer and Structure by Ni Adsorption Calorimetry, Spectroscopies and DFT

<div>The morphology, interfacial bonding energetics and charge transfer of Ni clusters and nanoparticles on slightly-reduced CeO_2-x (111) surfaces at 100 to 300 K have been studied using single crystal adsorption calorimetry (SCAC), low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and density functional theory (DFT). The initial heat of adsorption of Ni vapor decreased with the extent of pre-reduction (x) of the CeO_2-x (111), showing that stoichiometric ceria adsorbs Ni more strongly than oxygen vacancies. On CeO_1.95 (111) at 300 K, the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Ni clusters on stoichiometric steps, followed by the Ni particles spreading onto less favorable terrace sites. At 100 K, the clusters nucleate on terraces due</div><div>to slower diffusion. Adsorbed Ni monomers are in the +2 oxidation state, and they bind by ~45 kJ/mol more strongly to step sites than terraces. The measured heat of adsorption versus average particle size on terraces is favorably compared to DFT calculations. The Ce 3d XPS lineshape</div><div>showed an increase in Ce³⁺/Ce⁴⁺ ratio with Ni coverage, providing the number of electrons donated to the ceria per Ni atom. The charge transferred per Ni is initially large but strongly decreases with increasing cluster size for both experiments and DFT, and shows large differences between clusters at steps versus terraces. This charge is localized on the interfacial Ni and Ce atoms in their atomic layers closest to the interface. This knowledge is crucial to understanding the nature of the active sites on the surface of Ni-CeO₂ catalysts for which metal-oxide interactions play a very important role in the activation of O−H and C−H bonds. The changes in these interactions with Ni particle size (metal loading) and the extent of reduction of the ceria help to explain how previously reported catalytic activity and selectivity change with these same structural details.</div>