Cluster Model DFT Study of CO Adsorption to Gallium Ions in Ga/HZSM-5

Cluster model DFT calculations of CO adsorption on various possible forms of gallium in Ga/HZSM-5 zeolites have been performed. CO was found to only weakly interact with Ga +, (GaO) +, and (Ga(OH) 2- n H n ) + ( n = 1, 2) cationic clusters. The resulting shifts of the CO stretching frequency (Δν CO ) are only very small. On the other hand, CO coordination to small mononuclear and binuclear Ga 3+ hydride/hydroxide/oxide species results in positive shifts in the stretching frequency in the range Δν CO = 20−40 cm -1 . Larger shifts (Δν CO = 70−90 cm -1 ) are associated with CO coordination to Ga ions at the corners of small three-dimensional Ga−oxide clusters. The experimentally observed changes in the infrared spectrum of adsorbed CO over Ga/HZSM-5 zeolites upon reductive and oxidative treatments are interpreted with these insights. Possibilities for the formation of such polynuclear oxide species in the zeolite micropore space are discussed. On the basis of recent literature insights, we suggest that large shifts derive from CO coordination to oligomeric Ga cationic complexes stabilized by the negative zeolite charge.