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

Autor: Marco Antonio Chaer Nascimento, Lenin J. Díaz Soto, Anibal Sierraalta, Rafael Añez
Rok vydání: 2016
Předmět:
Zdroj: Applied Catalysis A: General. 526:53-61
ISSN: 0926-860X
Popis: ONIOM calculations have been carried out to determine geometries, adsorption energies and vibrational frequencies of CO on Au 2 and Au 4 -exchanged mordenite catalysts (Au 2 /MOR and Au 4 /MOR) using DFT with the B3LYP and ω B97X-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 Au 2 /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 μ 2 -CO-Au 2 /MOR configuration, with one CO adsorbed in a bridge form on two Au atoms, exhibits ν(CO) frequencies within the experimental range of 1949–2120 cm −1 (low frequencies). Increasing the size of the Au particles shifts the ν(CO) to higher frequencies. The values of Δ H ads decrease when the number of adsorbed CO molecules increases, similarly to what is experimentally observed when CO is adsorbed on Au/TiO 2 and Au/Al 2 O 3 . Charge analysis shows that the Au 2 in the Au 2 /MOR and the Au 4 cluster in the Au 4 /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 Au 2 δ+ species inasmuch as the μ 2 -CO-Au 2 /MOR configuration is the only one among several others to exhibit ν(CO) frequencies within the experimental low frequency region.
Databáze: OpenAIRE
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