New Mechanistic and Reaction Pathway Insights for Oxidative Coupling of Methane (OCM) over Supported Na2 WO4 /SiO2 Catalysts

The complex structure of the catalytic active phase, and surface-gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na₂ WO₄ /SiO₂ catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H₂ -TPR, TAP and steady-state kinetics) experiments, that the long speculated crystalline Na₂ WO₄ active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na-WO_x sites. Kinetic analysis via temporal analysis of products (TAP) and steady-state OCM reaction studies demonstrate that (i) surface Na-WO_x sites are responsible for selectively activating CH₄ to C₂ H_x and over-oxidizing CH_y to CO and (ii) molten Na₂ WO₄ phase is mainly responsible for over-oxidation of CH₄ to CO₂ and also assists in oxidative dehydrogenation of C₂ H₆ to C₂ H₄ . These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na₂ WO₄ /SiO₂ catalysts.

Understanding Reaction Networks through Controlled Approach to Equilibrium Experiments Using Transient Methods

We report a combined experimental/theoretical approach to studying heterogeneous gas/solid catalytic processes using low-pressure pulse response experiments achieving a controlled approach to equilibrium that combined with quantum mechanics (QM)-based computational analysis provides information needed to reconstruct the role of the different surface reaction steps. We demonstrate this approach using model catalysts for ammonia synthesis/decomposition. Polycrystalline iron and cobalt are studied via low-pressure TAP (temporal analysis of products) pulse response, with the results interpreted through reaction free energies calculated using QM on Fe-BCC(110), Fe-BCC(111), and Co-FCC(111) facets. In TAP experiments, simultaneous pulsing of ammonia and deuterium creates a condition where the participation of reactants and products can be distinguished in both forward and reverse reaction steps. This establishes a balance between competitive reactions for D* surface species that is used to observe the influence of steps leading to nitrogen formation as the nitrogen product remains far from equilibrium. The approach to equilibrium is further controlled by introducing delay timing between NH₃ and D₂ which allows time for surface reactions to evolve before being driven in the reverse direction from the gas phase. The resulting isotopic product distributions for NH₂D, NHD₂, and HD at different temperatures and delay times and NH₃/D₂ pulsing order reveal the role of the N₂ formation barrier in controlling the surface concentration of NH_x* species, as well as providing information on the surface lifetimes of key reaction intermediates. Conclusions derived for monometallic materials are used to interpret experimental results on a more complex and active CoFe bimetallic catalyst.

Active Site and Electronic Structure Elucidation of Pt Nanoparticles Supported on Phase-Pure Molybdenum Carbide Nanotubes

We recently showed that phase-pure molybdenum carbide nanotubes can be durable supports for platinum (Pt) nanoparticles in hydrogen evolution reaction (HER). In this paper we further characterize surface properties of the same Pt/β-Mo₂C catalyst platform using carbon monoxide (CO)-Pt and CO-Mo₂C bond strength of different Pt particle sizes in the <3 nm range. Results from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temporal analysis of products (TAP) revealed the existence of different active sites as Pt particle size increases. Correlation between the resultant catalyst activity and deposited Pt particle size was further investigated using water-gas-shift (WGS) as a probe reaction, suggesting that precise control of particle diameter and thickness is needed for optimized catalytic activity.