Adsorption and Activation of CO on Small-Sized Cu-Zr Bimetallic Clusters.

Adsorption and activation of CO is a key step in any chemical reaction, which aims to convert it to other useful chemicals. Therefore, it is important to understand the factors that drive the activation process and also search for materials that promote the process. We employ the density functional theory to explore the possibility of using small-sized bimetallic Cu-Zr clusters, CuZr, with = 1-3 for the above-mentioned key step. Our results suggest that after adsorption, a CO molecule preferably resides on Zr atoms or at the bridge and triangular faces formed by Zr atoms in bimetallic Cu-Zr clusters accompanied with its high degree of activation. Importantly, maximum activation occurs when CO is adsorbed on the CuZr cluster. Interestingly, we find that the adsorption energy of CO can be tuned by varying the extent of the Zr atom in Cu-Zr clusters. We rationalize the high adsorption of CO with the increase in the number of Zr atoms using the d-band center model and the concept of chemical hardness. The strong chemisorption and high activation of CO are ascribed to charge migration between Cu-Zr clusters and the CO molecule. We find an additional band in the infrared vibrational spectra of CO chemisorbed on all of the clusters, which is absent in the case of free CO. We also observe that the energy barriers for the direct dissociation of the CO molecule to CO and O decrease significantly on bimetallic Cu-Zr clusters as compared to that on pure Cu. In particular, the barrier heights are considerably small for CuZr and CuZr clusters. This study demonstrates that CuZr and CuZr clusters may serve as good candidates for activation and dissociation of the CO molecule.