UC Berkeley

It has been a long-standing challenge in electrocatalysis to measure product formation with kinetically relevant time resolution. Here, we couple a real-time gas analyzer mass spectrometer with a modified sampling inlet to a gas diffusion electrode cell, achieving sub-second time resolution for both reactant consumption and product formation. We focus on CO 2 /CO reduction on oxide-derived Cu, motivated by the continuing interest in its reaction mechanism and its high selectivity to ethylene (C 2 H 4 ) formation. Potential step experiments for CO reduction reveal that the initial consumption of CO is fast (< 1 s), but the appearance of C 2 H 4 products can be 10 times slower, which we attribute to the time required for CO storage on the Cu surface and subsequent reconstruction into the active, C 2+ -forming, configuration. Previous studies have reported that the co-feed of 12 CO 2 / 13 CO results in the enhancement of 13 C-containing ethylene isotopologues, but the mechanism causing this remains unknown. Isotopic-labeled gas switching ( 12 CO 2 - 13 CO) experiments yield compelling evidence that 13 C species continue to exist for more than a minute at near proximity to or on the catalyst, even after the supply source has been cut off, evidenced by the significant enhancement of 13 C-containing ethylene isotopologues: pure ( 13 C 2 H 4 ) and mixed ( 13 C 12 CH 4 ), formed through a CO 2 (g)-*CO pathway. Importantly, the size of the C 1 reservoir, the amount of 13 C 12 CH 4 made when CO 2 reacts with it, and the amount of Cu + that is removed are all in the micromole range. Thus, the picture emerges that a monolayer is stripped off from the copper catalyst surface during the CO-CO 2 switch. Tracking of dynamic Cu surface insights (i.e., activation, dissolution and deactivation) provides valuable mechanistic insights for optimizing reaction conditions.