CO₂ reduction on adatom decorated platinum stepped surfaces

The rate of CO formation from CO₂ reduction on Pt(111) vicinal surfaces containing (100) steps, Pt(S)[n(111) × (100)], has been investigated using cyclic voltammetry. To obtain further information about the different roles of terrace and step atoms in this reaction, selective modification of step sites with either bismuth or copper has been performed. In this way, two different mechanistic regimes have been differentiated, depending on the potential range. In the high potential region, between 0.2 and 0.4 V RHE, CO₂ is activated on steps and proceeds to the formation of adsorbed CO even when there is no hydrogen adsorbed on the terrace. We suggest that protonation of the activated CO₂ uses protons from the solution. In this potential range, the activity decreases after the selective blockage of step sites with bismuth, while the deposition of copper on steps increases the activity. Contrarily, in the low potential region, below 0.2 V RHE, the presence of copper on the steps does not increase the amount of CO formed from CO₂ reduction. In fact, the amount of CO formed attains the same saturation value with or without copper. In addition, the CO formed in this potential region remains adsorbed near step sites as shown in the voltammetric profile. We rationalize these observations considering that, in this potential region, activated CO₂ reacts with adsorbed hydrogen and the reactions stop when hydrogen near the steps is depleted.