Probing the Oxidation of Carbon Monoxide Utilizing Aun-
A. Welford Castleman, Jr.,
Due to its toxicity, increasing attention has been focused on the development of catalytic materials and methods to promote the removal of carbon monoxide.[1,2,3] Recent work by Haruta and coworkers[4,5,6] has prompted interest in the use of highly dispersed gold clusters or nanoparticles supported on metal-oxide surfaces for the oxidation of carbon monoxide.[7-10] From these investigations, it is reasoned that the particle size and the support play considerable roles in the activity of the gold catalysts. However, the mechanisms and processes responsible for the interactions between nanosized gold species and carbon monoxide and oxygen are still not well understood.
It is becoming increasingly recognized that gas phase cluster investigations can be employed to complement surface science studies by offering a model for the interactions that are taking place at the active sites on catalytic surfaces. Furthermore, the collection of atoms on supported catalysts have been described as “clusterlike.” This description, as well as the proposal that the charge state and a charge trasfer13 play critical roles in the activity of gold catalysts, has encouraged studies utilizing gas phase cluster species in the expectation that the observations will shed light on the processes responsible for the oxidation of carbon monoxide by gold nanoparticles.
Of late, our research has focused on the effect of size in the catalytic oxidation of carbon monoxide. Utilizing a fast flow reactor mass spectrometer, the interactions of carbon monoxide and oxygen on gold cluster anions have been probed. Preoxidized gold clusters are formed in a laser vaporization source by passing oxygen seeded in helium over the metal plasma formed by ablating a rotating and translating gold rod. Once formed, the preoxidized gold clusters travel down a flow tube, and carbon monoxide is introduced at a reactant gas inlet. The products are analyzed by a quadrupole mass analyzer and detected utilizing a channel electron multiplier.
The activity of supported gold clusters has been shown to rely on the size of the gold cluster that is dispersed on the surface. This has led to many studies aimed at elucidating the reason why the size has such a drastic effect and what size species are responsible for the increased activity of gold. Landman and coworkers have found that Au8 is the smallest size aggregate to catalyze the oxidation of carbon monoxide. Their studies reveal that electron transfer from the support to the cluster and the presence of F-center defect sites are essential in the activation of gold in the oxidation reaction.7 Wallace and Whetten, however, have reported that, through cooperative coadsorption, Au6- will promote the oxidation of carbon monoxide.
Our approach to this system is somewhat different than those employed by others in that, as mentioned previously, our gold clusters are preoxidized in the source. This is achieved by passing the oxygen across the plasma formed from laser ablation of a gold rod. This likely causes the dissociation of the oxygen molecule to take place prior to binding to the gold clusters due to the energy from the laser. Therefore, we observe products that correspond to oxygen atom addition with up to 3 oxygen atoms binding to Au-, Au2- adsorbs up to 4 oxygen atoms, and up to 5 oxygen atoms adsorbing to Au3-. This is in contrast to the even-odd binding behavior reported by others. Lee and Ervin, Wallace and Whetten, and Cox and coworkers all report single O2 addition to Aun- for even n only. [15-17] Our pattern of oxygen binding also led to different results for the oxidation of carbon monoxide. Due to our ability to adsorb oxygen atoms on the monomer and trimer of gold, we are able to probe the oxidation reaction on species that were not observed by others. From these studies, it was found that the monomer of gold was the only species from the cluster distribution to take part in the oxidation reaction. In fact, AuO- was the only cluster that promoted the oxidation of carbon monoxide. In Figure 1, the experimental results from these studies are shown. Figure 1a represents the addition of oxygen atoms to the gold monomer, and Figures 1b-1e correspond to the addition of increasing amounts of carbon monoxide to the gold oxides. The peak of importance in these spectra is that labeled AuO-. As the flow of CO is increased, this peak begins to diminish in intensity until it ultimately disappears. We believe that this species is oxidizing the carbon monoxide that is added at the reactant gas inlet to produce neutral CO2 and Au-. This is further demonstrated in the branching ratios of Figure 2. As the relative intensity of AuO- decreases with increasing CO flow in 2b, that of Au- increases, as shown in 2a. Branching ratios of the other species present in the oxide spectrum (Figure 1a) do not show any increase or decrease in relative intensity with CO flow. Therefore, we believe that AuO- is the only species to oxidize carbon monoxide under the present experimental conditions. Therefore, we have found that a smaller preoxidized gold species than those predicted by others[7,15] will promote the oxidation of carbon monoxide. Additional experimental studies and theoretical investigations are underway to further elucidate the reaction mechanism.
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Gold, clusters, gas phase, Carbon monoxide