Adsorption of carbon monoxide on smaller gold-cluster anions in an atmospheric-pressure flow-reactor: temperature and humidity dependence

Abstract

In the absence of moisture and at room temperature, the activity and saturation of CO on gold cluster anions, Au(N)-, are known to be highly dependent on the size of the cluster. Small Au(N)- clusters (N = 2,3) showed no adsorption activity, and the saturation CO adsorption values did not increase proportionately to cluster size or area. Here, we report on the effects of water vapor and temperature on the ability of Au(N)- clusters to adsorb CO in a high-pressure, fast-flow reactor. In contrast to all earlier reports, our results using this method show that smaller gold-cluster anions bind single and multiple CO groups at ambient temperature and above. In particular, species previously unseen at room temperature, corresponding to Au2(CO)-, Au3(CO) and Au4(CO)2, have been observed. Apparently, the presence of water vapor facilitates the adsorption of CO on the smaller clusters, possibly by aiding in the release of adsorption energy. As the number of studies concerning gold catalysis has continually increased over the past decade, these results provide important new information on the possible role of moisture in gold catalysis.

Gas Phase Formation, Structure and Reactivity of Gold Cluster Ions

“…This contradicted Ervin's results that these small clusters react with CO, albeit slowly [273]. This was later explained when they reported that preadsorbed water on Au 2 À and Au 3 À allowed the subsequent adsorption of the carbonyl and displacement of water [278].…”

“…Wallace and Whetten have published several studies on the reaction of CO with size-selected gold clusters at room temperature [276,[278][279][280]. For instance, in a study aimed at understanding the size-dependent reactivity of Au x À (x ¼ 4-19) [280], they concluded that initial products formed seem to correspond mainly to clusters with a gold electron shell filling at 8, 14, 18 and 20 electrons (i.e.…”

CO Oxidation Promoted by NO Adsorption on RhMn2O3– Cluster Anions

“…Gas-phase studies of isolated atomic clusters that can be considered as ideal models of active sites on related condensed-phase catalysts provide a unique approach to permeate the mechanistic details of elementary steps involved in the catalytic cycle. − CO oxidation is one type of extensively studied reactions in the gas phase, − while the studies are focused on the reactivity of bare metal clusters. , Available examples have emphasized the crucial roles of preadsorbed small molecules in promoting the reactivity of the resulting products in CO adsorption or oxidation. − For example, the preattached CO was identified to modify the electronic structure of the Ni 2 VO 4 – cluster . The Ni 2 CO moiety that behaves like a noble-metal atom in the product Ni 2 VO 4 – [CO] is responsible for the storage of electrons during CO oxidation and drives the facile desorption of CO 2 .…”