Study on the Mechanism of Ethanol Dry Reforming via Thermodynamic and Kinetic Investigations: Clarifying the Contribution of Tandem and Parallel Reactions in Syngas Production

Abstract

Ethanol dry reforming is an efficient method in CO₂ utilization and syngas production, with transition-metal-supported reducible supports as the active components. However, side reactions and intricate reaction networks hinder its industrial applications. In this study, Pt/NiAl₂O₄ was applied as a probe to systematically investigate the ethanol dry reforming (EDR) reaction network, where pressure-dependent studies were conducted on three tandem reactions, including: (i) ethanol dehydrogenation, (ii) acetaldehyde decomposition, and (iii) methane dry reforming; and one parallel reaction of (iv) ethanol dehydration. Independent rate equations have been selectively established, and interestingly, all the reaction was controlled by an independent C–H bond rupture elementary step, including (i) C_α–H bond rupture within ethanol, (ii) C_α–H bond rupture within acetaldehyde, (iii) C–H bond rupture inside methane, and (iv) C_β–H bond rupture within ethanol/ethoxide. Furthermore, as an intact molecule, CO₂ was found to facilitate C–H bond rupture in (i) acetaldehyde decomposition and (ii) methane dry reforming. The active species were primarily composed of NiAl₂O₄ and the interface between Pt and the support, with the support primarily covered by dissociated hydrogen, formate, and ethoxy groups, while Pt was mainly covered by CO, and the most abundant surface intermediates (MASIs) were observed to vary with changes in reaction conditions. All of these findings were further supported by in situ FTIR analysis, parity plot fittings, and systematic kinetic isotopic effects (KIE) analysis. Based on the thermodynamic investigations, this reaction is eventually controlled by the ΔG values. The energetic potential of the tandem reaction is stepwisely increased in the order of ethanol dehydrogenation (181.3 kJ/mol), acetaldehyde decomposition (195.3 kJ/mol), and methane dry reforming (196.6 kJ/mol), while the parallel reaction, ethanol dehydration, possesses the highest energetic potential (214.2 kJ/mol), which was strongly modified by the carbocation formation. This research provides critical insights into the reaction mechanism of ethanol dry reforming by ruling out the previously discussed oxidative and aldol condensation processes on the Pt/NiAl₂O₄ catalyst and establishes a theoretical basis for the rational design of catalysts in selective syngas production.