Electrochemical CO₂ reduction has the vast potential to neutralize CO₂ emission and valorizes this greenhouse gas into chemicals and fuels under mild conditions. Its commercial realization hinges on catalyst innovation as well as device engineering for enabling reactions at industrially relevant conditions. Copper has been widely examined for the selective production of multicarbon chemicals particularly ethylene, while there is still a substantial gap between the expected and the attainable. In this work, we report that the surface promotion of copper with alumina clusters is a viable strategy to enhance its electrocatalytic performance. AlO_x-promoted Cu catalyst is derived from Cu-Al layered double hydroxide nanosheets after alkali etching and cathodic conversion. It can catalyze CO₂ to ethylene and multicarbon products with great selectivity and stability far superior to pristine copper in both an H-cell and a zero-gap membrane electrode assembly (MEA) electrolyzer. The surface promotion effect is understood via computational simulations showing that alumina clusters can stabilize key reaction intermediates (*COOH and *OCCOH) along the reaction pathway.