A reinvestigation of the CO vibration frequency as a probe to determine the species of Au in the Au/MOR catalyst

Abstract

ONIOM calculations have been carried out to determine geometries, adsorption energies and vibrational frequencies of CO on Au2 and Au4-exchanged mordenite catalysts (Au2/MOR and Au4/MOR) using DFT with the B3LYP and ωB97X-D functionals. We considered structures with two Al atoms, MOR(TT), with at least one of them at the preferential site T4. To investigate the CO adsorption on Au2/MOR(TT) we examined three different situations: the adsorption of one single CO molecule bridged to two Au atoms (case I), one CO molecule adsorbed on a single Au atom (case II), and two molecules of CO absorbed on two Au atoms, one CO per Au atom (case III). From all the previous theoretically and experimentally proposed species of Au inside the MOR channels, including the ones considered in this work, only the μ2-CO-Au2/MOR configuration, with one CO adsorbed in a bridge form on two Au atoms, exhibits ν(CO) frequencies within the experimental range of 1949–2120cm−1 (low frequencies). Increasing the size of the Au particles shifts the ν(CO) to higher frequencies. The values of ΔHads decrease when the number of adsorbed CO molecules increases, similarly to what is experimentally observed when CO is adsorbed on Au/TiO2 and Au/Al2O3. Charge analysis shows that the Au2 in the Au2/MOR and the Au4 cluster in the Au4/MOR are cationic. Based on the present results we conclude that the use of ν(CO) as a probe to determine the Au species in the channels of MOR can only clearly identify Au2δ+ species inasmuch as the μ2-CO-Au2/MOR configuration is the only one among several others to exhibit ν(CO) frequencies within the experimental low frequency region.