Partial Oxidation of Methane to Syngas Over γ‑Al₂O₃‑Supported Rh Nanoparticles: Kinetic and Mechanistic Origins of Size Effect on Selectivity and Activity

A series of supported Rh/γ-Al₂O₃ catalysts with an overall metal loading of 0.005 wt % was synthesized by impregnation of γ-Al₂O₃ with a toluene solution containing colloidally prepared well-defined (1.1, 2.5, 2.9, 3.7, and 5.5 nm) Rh nanoparticles (NP). The size of NP was not found to change after their deposition on γ-Al₂O₃ and even after performing partial oxidation of methane (POM) to synthesis gas at 1073 K for 160 h on stream. Apparent CO formation turnover rates and CO selectivity strongly decrease with an increase in this size. Contrarily, the overall scheme of POM is size-independent, i.e. CO and H₂ are mainly formed through reforming reactions of CH₄ with CO₂ and H₂O at least under conditions of complete oxygen conversion. The size effect on the activity and selectivity was related to the kinetics of interaction of CH₄, O₂, and CO₂ with Rh/γ-Al₂O₃ as concluded from our microkinetic analysis of corresponding transient experiments in the temporal analysis of products reactor. The rate constants of CH₄, O₂, and CO₂ activation decrease with an increase in the size of supported Rh NP thus influencing both primary (methane combustion) and secondary (reforming of methane) pathways within the course of POM

The Role of Adsorbed and Lattice Oxygen Species in Product Formation in the Oxidative Coupling of Methane over M₂WO₄/SiO₂ (M = Na, K, Rb, Cs)

MnOx–Na2WO4/SiO2 is one of the best-performing catalysts in the oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H6 and C2H4). The current mechanistic concepts related to the selectivity to the desired products are based on the involvement of crystalline Mn-containing phases, the molten Na2WO4 phase, surface Na–WOx species, and the associated lattice oxygen. Using in situ X-ray diffraction, operando UV–vis spectroscopy, spatially resolved kinetic analysis of product formation in steady-state OCM tests, and temporal analysis of products with isotopic tracers, we show that these phases/species are not categorically required to ensure high selectivity to the desired products. M2WO4/SiO2 (M = Na, K, Rb, Cs) materials were established to perform similarly to MnOx–Na2WO4/SiO2 in terms of selectivity–conversion relationships. The unique role of the molten Na2WO4 phase could not be confirmed in this regard. Our alternative concept is that the activity of M2WO4/SiO2 and product selectivity are determined by the interplay between the lattice oxygen of M2WO4 and adsorbed oxygen species formed from gas-phase O2. This lattice oxygen cannot convert CH4 to C2H6 but oxidizes CH4 exclusively to CO and CO2. Adsorbed monoatomic oxygen species reveal significantly higher reactivity toward overall CH4 conversion and efficiently generate CH3 radicals from CH4. These reactive intermediates couple to C2H6 in the gas phase and are oxidized, to a lesser extent, by the lattice oxygen of M2WO4 to CO and CO2. Adsorbed diatomic oxygen is involved in the direct CH4 oxidation to CO2. The electronegativity of alkali metal in M2WO4 was established to affect the catalyst ability to generate adsorbed oxygen species from O2. This knowledge opens the possibility to influence product selectivity by controlling the coverage by adsorbed and lattice oxygen via reaction conditions or catalyst design