Micro-kinetics of non-oxidative methane coupling to ethylene over Pt/CeO2 catalyst

Abstract

A single atom Pt/CeO2 catalyst for the direct non-oxidative methane coupling (NOCM) operation to C2 hydrocarbon product species was prepared, mechanisms were evaluated, and kinetics based on elementary step reaction scheme equations were determined. The catalyst was characterized by a number of techniques; scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were implemented. CO DRIFTS showed the presence of single Pt atoms. Temperature, CH4 flow and functional weight hourly space velocity were varied in the ranges of 780–910 °C, 0.9–2.6 mL min-1 and 0.094–0.29 h−1, respectively. The applied total pressure, used was 1.5 bar. The highest reactant conversion achieved was 4.3 mol.%, the selectivity to C2 was 60 mol.%, and to ethene alone, maximally 80 mol.% among C2 products. Also some coke was forming, but processing remained stable for 245 h of the time on stream. System’s kinetic parameters were fitted to quantitative data values, non-linear regression analyses were being reiterated, and a relatively good agreement with the space–time point sets for CH4, ethylene, acetylene and hydrogen was reached. The sensitivity analysis of fixed rate constants showed which reaction steps influence reacting CH4, the reactivity towards transformed C2 and yields the most. It was shown that the important physico-chemical transformations are CH4 adsorption, the abstraction of the first H• from its neutral CH4 molecule on the surface and the sorption/desorption of H2. Fast C–C bond processes only have a limited measurable effect on CH4 converted. C2 are primarily affected by the Pt adhesion/release of CH4, synthesized C2 and H2. Modelling may be translated to other catalytic NOCM understanding, designing and optimizing, as an alternative to steam methane reforming.