LAUSR

Ethanol steam reforming (ESR) offers a sustainable route for hydrogen production, yet its complex reaction network and elusive intermediates hinder catalyst optimization. Here, we identify the vinyloxy radical (CH2CHO) as the critical chain-carrying intermediate in ESR over Ni/La2O3 catalysts, challenging the conventional view of acetyl radical (CH3CO) dominance. Through in situ synchrotron vacuum ultraviolet photoionization mass spectrometry with molecular beam sampling (SVUV-PI-MBMS), combined with density functional theory (DFT) calculations and microkinetic modeling, the dynamic speciation of gas-phase radicals and stable products are resolved across 473–1073 K. Experimental results reveal CH2CHO as the predominant intermediate, absent CH3CO detection. DFT calculations provide a theoretical foundation that supports the experimental observations, demonstrating that the CH2CHO-mediated pathway has a kinetic advantage over the CH3CO pathway. This finding aligns with kinetic simulation results, which reveal that CH2CHO controls 75% of the formaldehyde conversion flux. This work redefines the ESR mechanistic framework, offering a strategy to tailor catalytic pathways via intermediate control. Ethanol steam reforming (ESR) promises green H2, but tangled networks hide controlling intermediates. In-situ SVUV-PI-MBMS with DFT/microkinetics on Ni/La2O3 pinpoints vinyloxy (CH2CHO), not acetyl, as the chain carrier, steering most flux and redefining ESR mechanisms.