The electrochemical carbon dioxide reduction reaction (CO2RR) is a promising pathway for converting waste CO2 into valuable chemicals. However, the development of CO2RR catalysts with high selectivity towards C2 products remains a significant challenge. Herein, we systematically doped Sn into copper Cu in concentrations ranging from 3% to 80% to tune its selectivity during the electrochemical CO2RR. Electrolyzer testing reveals a shift in product selectivity with varying Sn content. At a Sn doping level of 3%, Cu-Sn catalysts favor the production of ethanol and ethylene as the major products. In contrast, higher Sn concentrations, exceeding 3%, predominantly result in the formation of the C1 product such as formic acid. To elucidate the underlying mechanisms, in-situ Surface Enhanced Raman Spectroscopy (SERS) was performed on 3%, 50%, and 80% Sn-doped Cu catalysts. Raman indicates that at 3% Sn doping, the *COOH intermediate is stabilized, which promotes *CO dimerization and favors C2 products formation. In contrast, at 50% and 80% Sn, the HCOO* intermediate is observed, leading to formic acid as the major product. Prior density functional theory calculations suggest that Sn atoms in proximity to a Cu atom withdraw electrons from the Cu, resulting in the formation of Cuδ+. The decreased electron density at Cu weakens the π-back bonding with adsorbed *CO, thereby rendering the carbon more electrophilic. This promotes C-C coupling and favors C2 products formation. The convergence of electrolyzer testing, Raman intermediate detection, and DFT offers a synergistic workflow for further optimization of more selective catalysts.