A density functional study of carbon monoxide adsorption on small cationic, neutral, and anionic aluminum phosphide clusters

The chemisorption of CO on aluminum phosphide (AlP)n (n = 1–12) clusters are studied with density functional theory. Among various possible CO adsorption sites, the on-top site is identified to be the most favorable chemisorption sit. The Al-top sites are the preferred one in the most cases for one CO adsorption in (AlP)n (n = 3–12) clusters, irrespective of the charge state of the clusters. The Al–P bond lengths decrease generally as the size of the cluster increases. There is a slight increase in the mean Al–P bond lengths after CO adsorption on the lowest-energy sites of the most AlP clusters. In general, adsorption energies of CO are found to decrease with an increase in the cluster size for both neutral and ionic clusters. The adsorption energies of CO on the cationic (AlP)nCO+ (n = 2–12) clusters are greater than those on the neutral and anionic complexes. The result shows that large adsorption energies of CO on neutral and ionic AlP clusters and large highest occupied and lowest unoccupied molecular-orbital gaps for (AlP)CO, (AlP)CO+ and (AlP)CO− make these species behaving like magic clusters. The CO vibrational frequencies νCO in all the cationic complexes are always larger than the corresponding quantities in the neutral and anionic clusters. All of them are smaller than those in the free CO molecule.