The increasing levels of carbon dioxide (CO2) emissions and their detrimental effects on the environment have spurred a growing interest in developing sustainable strategies for carbon utilization. Electrocatalytic CO2 conversion has emerged as a promising approach, offering a viable pathway to mitigate CO2 emissions and produce value-added products simultaneously [1,2]. The utilization of electrocatalysts is pivotal in the electrochemical conversion of CO2 as they enable efficient and selective reactions, leading to the valuable transformation of CO2. Consequently, the development of highly efficient and selective electrocatalysts becomes a fundamental aspect in enhancing the energy efficiency of this emerging technology in the era of energy transition from fossil fuel to renewable resources. 

Understanding the reaction mechanism is crucial as it provides insights into the underlying processes and enables the design of more effective electrocatalysts tailored towards producing the desired products. By unraveling the reaction mechanism, researchers can identify key factors influencing catalytic performance and make informed choices in electrocatalyst design, leading to improved efficiency and selectivity in CO2 reduction reaction (CO2RR). Metals have been widely investigated for the CO2RR, with extensive research conducted both experimentally and computationally. Among them, copper has gained significant attention as a unique electrocatalyst for the production of hydrocarbon fuels and chemicals such as methane, ethylene, and ethanol. However, achieving high selectivity at industrially relevant high current densities, typically greater than 1 A/cm2, remains a significant challenge [3,4]. 

The reaction mechanism for CO2 electroreduction on copper-based electrocatalysts is primarily elucidated by integrating density functional theory (DFT) calculations on large surface slab models and in-situ spectro-electrochemistry techniques using single crystals or large polycrystalline particles. However, the reaction mechanism on extremely small nanoparticles, approximately 1 nm in size, may differ from observations on larger particles due to their lower coordination number and higher reactivity. To date, there is a lack of systematic studies in the literature specifically examining the changes in the reaction mechanism over extremely small nanoparticles. The focus of research has predominantly been on larger particles, and there is limited understanding of how the behavior and reaction pathways may differ for nanoparticles on the order of 1 nm. 

In this study, we conducted experimental electrocatalysis combined with DFT computations to systematically analyze the changes in the reaction mechanism as the particle size becomes extremely small. Our investigation focused on two case studies involving copper and tin electrocatalysts. The results demonstrate that utilizing small nanoparticles with sizes of 1 nm or smaller leads to a shift in the reaction pathway, enabling the production of products that were challenging to achieve with conventional catalysts. Moreover, we observed a substantial increase in the electrocatalytic activity, and we successfully achieved partial current densities greater than 1 A/cm2. These findings underscore the importance of nanoparticle size in manipulating the reaction mechanism and unlocking improved performance in electrocatalysis. This advancement brings us closer to the realization of sustainable chemical and fuels production, ultimately contributing to the development of net-zero emission technologies. By leveraging the potential of small nanoparticles, we can pave the way for a more efficient and environmentally friendly approach to address the global challenges of carbon emissions and promote a greener future.