Atmospheric carbon dioxide (CO 2 ) levels, the main constituent of greenhouse gases, have been rising steadily since the industrial revolution, creating a global warming effect, with widespread impending environmental consequences. To reduce the concentration of CO 2 in the atmosphere, besides decreasing the emission, conversion of CO 2 to other carbon-based products is an economically and environmentally attractive strategy to reduce greenhouse gas levels. One of the most used conversion

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... ques is the hydrogenation of captured CO 2 , which not only reduces the concentration of CO 2 but also results in the production of fuels and other value-added products [1, 2] . Catalysts play a key role in this hydrogenation reaction to accelerate the reaction kinetics. Among different kinds of metal catalysts, Cu is particularly promising, considering its low cost and high natural earth abundance (67 ppm). To date there are limited high spatial resolution investigations on the degeneration behavior of Cu catalyst particularly under operando conditions. Since the CO 2 gas can be regarded as an oxidant, one critical failure mechanism of Cu catalyst is believed to be oxidation. Even though there are numbers of studies related to Cu oxidation in an oxygen environment [3, 4] , the failure mechanism of Cu catalysts during the catalysis process under CO 2 environment remains unclear, in part due to the difficulty in achieving sufficient spatial and temporal resolution for an in-situ process. By creating a sealed gas environment in a (scanning) transmission electron microscope (S/TEM) column, the details of the degradation process of the catalyst can be studied in real-time under different gas conditions with sufficiently high spatial and temporal resolution. The thermal stability of catalysts in different gas environments can be mimicked, which provides fundamental insights into understanding the failure mechanism of the catalyst. Herein, we address the failure mechanism Cu nanowires (NWs) under CO 2 atmosphere [5] . By means of operando S/TEM observations, the degeneration processes of Cu NW were monitored during exposure to 760 Torr CO 2 pressure and a temperature of 200 ℃. We reveal that the oxidation initiates from one end and gradually expands to the other end along the growth direction for a clean Cu NW with a smooth surface (negligible native oxide). When there is significant native oxide and increased surface roughness, this oxidation proceeds from outside to the center along the normal direction of the broad surface. The side-to-side oxidation is comprised of numerous end-toend oxidation paths at a very localized area as the oxidation process initiates at multiple defect areas. For both the large-scale end-to-end oxidation and localized end-to-end oxidation, the evolution of the Cu/void interface and growth of the oxide layer approximately follows linear kinetics. This work demonstrates how an experiment with an in-situ gas holder in a TEM can be used to study the evolution of a nanoscale catalyst, which has implications for many other catalytic systems [6, 7] .