Carbon-supported single-atom catalysts exhibit outstanding performance and broad application potential in electrocatalytic CO 2 reduction reactions (CO 2 RR). [1,2] Identifying the CO 2 RR reaction intermediates during electrocatalysis could improve our understanding on the catalytic mechanism for better design of more efficient catalysts. in-situ Raman spectroscopy could detect the transient intermediate over the catalyst surface under the reaction condition, therefore serving as important tool in revealing CO 2 RR reaction pathways. However, conventional in-situ Raman spectroscopy faces significant challenges in effectively detecting key reaction intermediates and structural evolution at catalytic active sites over carbon supported catalyst due to the intense fluorescence background, strong optical absorption of carbon materials, and the extremely weak Raman signals from single-atom active sites. To overcome these limitations, we employed Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy (SHINERS) [3] , which significantly enhances Raman signals through electromagnetic field enhancement by plasmonic nanoparticles coated with an inert shell. Using SHINERS, we successfully monitored the catalytic reaction intermediates of Cu single-atom catalysts supported on carbon substrates with different morphologies during CO 2 RR. Our results revealed distinctly different CO 2 RR reaction pathways mediated by atomic-scale dispersed Cu active sites on carbon supports of varying morphologies, highlighting the critical role of substrate structure in influencing catalytic pathways. Additionally, we extended the application of SHINERS to successfully observe the catalytic processes of differently sized Sn nanoparticles and their oxides supported on carbon substrates, effectively identifying essential reaction intermediates and their structural transformations. This study confirms SHINERS as a powerful analytical technique, providing deep insights into the catalytic mechanisms of single-atom and nanoscale catalysts, thus promoting the rational design and development of advanced catalysts for CO 2 reduction. ACKNOWLEDGMENT The work performed by JW and D.L. were partially supported by Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC02-06CH11357. J.W. and D.L were also supported by Joint Task Force Initiative of University of Chicago. [1]. Xu, H.; Rebollar, D.; He, H.; Chong, L.; Liu, Y.; Liu, C.; Sun, C.-J.; Li, T.; Muntean, J. V.; Winans, R. E.; Liu, D.-J.; Xu, T. Highly Selective Electrocatalytic CO₂ Reduction to Ethanol by Metallic Clusters Dynamically Formed from Atomically Dispersed Copper. Nat. Energy 2020 , 5 (8), 623–632. [2]. Xu, H.; Wang, J.; He, H.; Hwang, I.; Liu, Y.; Sun, C.; Zhang, H.; Li, T.; Muntean, J. V.; Xu, T.; Liu, D.-J. Modulating CO₂ Electrocatalytic Conversion to the Organics Pathway by the Catalytic Site Dimension. J. Am. Chem. Soc . 2024 , 146 (15), 10357–10366. [3]. Li, J.-F.; Huang, Y.-F.; Ding, Y.; Yang, Z.-L.; Li, S.-B.; Zhou, X.-S.; Fan, F.-R.; Zhang, W.; Zhou, Z.-Y.; Wu, D.-Y.; Ren, B.; Wang, Z.-L.; Tian, Z.-Q. Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nature 2010 , 464 (7287), 392–395.