Methane combustion on Pd-based model catalysts: Structure sensitive or insensitive?

Formation of C1 oxygenates by Activation of Methane on B, N Co-Doped Graphene Surface Decorated by Oxygen Pre-Covered Ir13 Cluster: A First Principles Study

The environmentally friendly conversion of methane to value-added chemicals is studied by ab-initio calculations. We focus on the adsorption and dehydrogenation of methane were investigated on Ir13 cluster supported by boron nitrogen co-doped graphene (BNG). We show that the BNG-Ir13 cluster with low oxygen coverage exhibits more negative adsorption energy of methane (-0.44 eV) and lower activation energy barrier for its dissociation (1.24 eV for the second dehydrogenation step) compared to the cluster with high oxygen coverage. Next, assuming abundant CH3 and CH2 species due to a proper temperature control preventing further dehydrogenation, we study competitive C-O coupling reactions leading to the formation of value-added chemicals. The activation energy barriers for the formation of methanol and formaldehyde are on BNG-Ir13 are once again lower at a lower oxygen coverage. Furthermore, we study hydrogen recombination and confirm that H2 molecules can be formed on these surfaces. Based on these findings, the low-oxygen-coverage BNG-Ir13 cluster emerges as a promising catalyst for the selective conversion of methane to methanol and formaldehyde, as well as for hydrogen production.

Redox dynamics and surface structures of an active palladium catalyst during methane oxidation

Catalysts based on palladium are among the most effective in the complete oxidation of methane. Despite extensive studies and notable advances, the nature of their catalytically active species and conceivable structural dynamics remains only partially understood. Here, we combine operando transmission electron microscopy (TEM) with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT) calculations to investigate the active state and catalytic function of Pd nanoparticles (NPs) under methane oxidation conditions. We show that the particle size, phase composition and dynamics respond appreciably to changes in the gas-phase chemical potential. In combination with mass spectrometry (MS) conducted simultaneously with in situ observations, we uncover that the catalytically active state exhibits phase coexistence and oscillatory phase transitions between Pd and PdO. Aided by DFT calculations, we provide a rationale for the observed redox dynamics and demonstrate that the emergence of catalytic activity is related to the dynamic interplay between coexisting phases, with the resulting strained PdO having more favorable energetics for methane oxidation.

Reconstruction and catalytic activity of hybrid Pd(100)/(111) monolayer on γ-Al2O3(100) in CH4, H2O, and O2 dissociation

The importance of the "heterogeneity" of a Pd monolayer induced by interaction with a semi-ionic support in catalysis was evaluated. The geometry of the Pd monolayer was optimized on the (100) plane of γ-Al2O3 at fixed unit cell parameters defined by the oxide. Simulation of the deposition of a whole Pd monolayer in the flat Pd(100) form cut from the bulk led to the formation of a slightly distorted Pd(111) monolayer. The subsequent chemisorption or dissociation of CH4 or H2O on the Pd(111) layer resulted in a new hybrid Pd(100)/(111) structure containing alternating elements of (100) and (111) planes (the parallel bands of squares and triangles), which are similar for both CH4 and H2O reactions, and two isolated Pd mono-vacancies, respectively. The hybrid Pd(100)/(111) layer without chemisorbed species was found to be more stable than the initial distorted Pd(111) layer. The catalytic capabilities of these monolayer structures were investigated for the dissociation of methane and the water-gas shift reaction (WGSR) due to the lower predicted activation barriers for CH4, H2O, and O2 dissociation on the hybrid Pd(100)/(111) layer compared to that on the pure (bulk) Pd(100) surface. Moreover, the exothermic heats of these reactions were calculated to be moderate instead of endothermic heats on the Pd(100) or Pd(111) surfaces. The heats of H2O and NH3 adsorption on various monolayers were tested, revealing their dependence on Pd atomic charges. The relevance of the model of the heterogeneous Pd monolayer for explaining the maximum reaction rate experimentally observed at different Pd coverages was discussed. The transferability of the geometry and the extent of charge inhomogeneity of the hybrid monolayer without vacancies were also tested on the same γ-Al2O3(100) support for Pt, Rh, and Ag.

An In situ TEM study of the surface oxidation of palladium nanocrystals assisted by electron irradiation

The surface oxidation of palladium nanocrystals plays an important role in changing the active sites and subsequently influencing the catalytic reactivity. Such a microscopy study on surface oxidation, down to the atomic scale, is essential for understanding the structure-property correlations of palladium nanocrystal based catalysts. Herein, we present an in situ atomic scale study on the surface oxidation behavior of palladium nanocrystals, which is induced by electron beam irradiation under low oxygen partial pressure and at room temperature inside an environmental transmission electron microscope. We found that: (i) surface oxidation initially started at the edge sites with atomic steps or vertex sites, which served as active sites for oxidation; (ii) the oxidation reaction proceeded with a much faster rate on the {111} surface, indicating a certain crystallography preference; (iii) nanometer-sized palladium monoxide islands were formed on the surfaces eventually. The results from our in situ studies provide insightful knowledge, and will be of certain importance for the design of improved functional catalysts in future.

The effect of water on methane oxidation over Pd/Al2O3 under lean, stoichiometric and rich conditions

Methane conversion during cooling ramp: water inhibits methane conversion more severely in high oxygen concentration.

Cation doping size effect for methane activation on alkaline earth metal doping of the CeO2 (111) surface

Alkaline earth metal doping of CeO₂ improves the dissociative adsorption of methane with the behaviour showing a correlation to dopant ionic radius.

Oxygen-assisted water partial dissociation on copper: a model study

The strong acid–base interaction, corresponding to the high activity in water dissociation.

Chemisorbed oxygen atom on the activation of C–H bond in methane: a Rh model study

The pre-adsorbed oxygen has little effect on the catalytic activity of methane dehydrogenation on Rh-ad-atom catalysts.

Adsorption and decomposition of methylamine on a Pt(100) surface: a density functional theory study

The decomposition of methylamine was investigated on a Pt(100) surface using DFT, and it was found that the C–N bond was not broken.