Co‐Adsorption of O₂ and CO on Au₂⁻: Infrared Photodissociation Spectroscopy and Theoretical Study of [Au₂O₂(CO)_n]⁻ (n=2–6)

Abstract

The co-adsorption of O and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au O (CO) ] (n=2-6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au O (CO) ] (n=2-6) complexes were characterized to have a core structure involving one CO and one O molecule co-adsorbed on Au with the other CO molecules physically tagged around. The CO stretching frequency of the [Au O (CO)] core ion is observed around ν˜ =2032-2042 cm , which is about 200 cm higher than that in [Au (CO) ] . This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co-adsorbed O and CO on Au may have relevance to the elementary reaction steps on real gold catalysts.

Binding of multiple SO2 molecules to small gold cluster anions (AuN−, AuNOH−, N = 1‐8)

“…Oxide‐supported gold nanoparticles catalyze a variety of reactions, and recent work has been focused on the potential of these catalysts to react with SO 2 , as well as to understand how SO 2 can hinder the ability of the catalysts to perform other reactions . Attempts have also been made to gain a more basic understanding of the catalytic activity by relating the reactivity of gas‐phase gold clusters to that of the supported gold catalysts . Previous studies with gas‐phase gold clusters have shown that the global charge is related to the adsorption ability, with anions showing particularly interesting activity .…”

Probing the binding and activation of small molecules by gas-phase transition metal clustersviaIR spectroscopy

“…Similar to hydride, the oxygen adatoms lead to a localization of electron density and reduce the extent of π backdonation to CO. 63 b For anionic Au 2 O 2 (CO) n − complexes, the large blue shift of ν (CO) of about 200 cm −1 compared to Au 2 (CO) 2 − indicates the presence of a formally neutral Au 2 entity in the mixed complex, which is in line with the formation of a superoxo species (O 2 − ). 44 Another example for using ν (CO) to gauge local charging has been CO bound to Mo doped Pt clusters, where the influence of Mo doping on the reactivity has been discussed above. Here, a combination of quantum chemical calculations, experimental studies of CO binding energies, and IR spectroscopy revealed that electron transfer between Mo and Pt atoms within the cluster depends on the cluster's size.…”

“…, by Duncan 198,228 towards binuclear or even larger (up to tetramer) clusters. 44,128,131,132,135,140,142,144,146,229 These measurements, however, are so far limited to the range of ν (CO).…”

“…In studies of CO bound to oxidized copper or gold dimers, Cu 2 O 2 (CO) n + ( n = 3–7) 146 and Au 2 O 2 (CO) n − ( n = 2–6), 44 there have been no indications for CO oxidation reactions, neither within the complexes nor induced by IR excitation, although Au 2 − is well-known to catalyse CO oxidation by O 2 in the gas phase. 42 This discrepancy has been explained by a much longer reaction time in the latter experiment that is performed in an ion trap allowing for the formation of thermodynamically more stable carbonate complexes which have been proposed as intermediates of the CO oxidation.…”

“…Also co-adsorbates of the form Au 2 (O 2 )(CO) m − have been characterized by their IR spectrum giving structural information about these possibly relevant intermediates in the CO oxidation catalyzed by Au 2 − . 44 Far-infrared spectroscopy even has provided detailed information on the internal geometrical structures of gold clusters. 7 b ,45…”