Highly Oxidized Platinum Nanoparticles Prepared through Radio-Frequency Sputtering : Thermal Stability and Reaction Probability towards CO Dmitry A. Svintsitskiy, [a, b] Lidiya S. Kibis, [a, b] Andrey I. Stadnichenko, [a, b] Sergei V. Koscheev, [a, b] Vladimir I. Zaikovskii, [a, b] and Andrei I. Boronin* [a, b] 1. Introduction In spite of the high cost, Pt-based catalysts are some of the most in-demand and studied systems in heterogeneous cataly- sis. Supported platinum particles are widely applied for various reactions of oxidation (CO, [1, 2] hydrocarbons, [3] alcohols [4] ), hy- drogenation, [5] cracking, [6] isomerization, [7] and so forth. The in- vestigation of model systems, [8, 9] frequently applied in the case of platinum-containing catalysts, [10–13] plays a crucial role in un- derstanding reaction mechanisms on the surface of real cata- lysts. The combustion of CO is one of the most widely used cata- lytic reactions that models complete [14] and selective oxidation processes. [15] Moreover, owing to the high toxicity of CO, its catalytic oxidation is in demand for various ecological applica- tions. The mechanism of CO oxidation on platinum-based cata- lysts has been studied intensively in view of its importance in fuel cells [16] and three-way catalysts. [17] However, the exact mechanism of CO oxidation and the structure of active sites on the surface of platinum catalysts are still under discus- sion. [14, 18–25] First of all, there is a dispute about the reaction properties of different oxygen species, that is, adsorbed oxygen on a platinum surface or oxygen in oxide structures such as PtO 2 or PtO. Some researchers attribute the highest re- activity value towards CO oxidation to adsorbed oxygen spe- cies, [14, 19] whereas others associate it with oxide forms. [18, 25, 26] Miller et al. explained this discrepancy by the fact that there might be some areas free from oxides on the platinum surface, even with excess oxygen in the reaction mixture. [14] Such metal-like areas with adsorbed oxygen species could be re- sponsible for CO oxidation. Hedriksen and Frenken noted that platinum oxides were key structures for catalytic CO oxidation at high pressures (ca. 0.5 bar); however, the authors did not ex- trapolate their conclusions for the low-pressure range. [26] Com- parison of the reaction properties for different platinum oxides (PtO, Pt 3 O 4 , PtO 2 , etc.) is another object of scientific discussion. Li and co-workers reported that one-dimensional PtO 2 -like spe- cies were the most reactive structures on Pt(332) and Pt(110) surfaces. [18, 25] Other researchers found that oxygen from PtO in- teracted with CO more effectively than PtO 2 oxygen. [14] To con- sider the properties of oxide species, the real defect structure of oxides should be taken into consideration. The alteration of a defect structure often results in changes in the reaction properties. Seriani et al. showed, theoretically, that the absence of any defects on the surface of PtO 2 inhibited the adsorption of CO molecules, whereas CO adsorption on a deficient Pt 3 O 4 - like surface resulted in instantaneous CO 2 desorption. [23] The formation of defective PtO 2-x structures under the reaction con- ditions and their importance for catalytic CO oxidation are also emphasized in other work. [21] In contrast to single-crystal platinum surfaces, the reaction properties of supported nanoparticles can be influenced signif- icantly by their morphology/size [20, 27] and the nature of interac- tions with the support. [1, 28] The size effect is known to be im- portant in heterogeneous catalysis, as a decrease in particle size might result in the appearance or intensification of catalyt- ic activity. [29, 30] It should be noted that particle size and the nature of the supports strongly influence the thermal stability of platinum oxides. [27, 28] The reaction probability and thermal Platinum-oxide nanoparticles were prepared through the radio-frequency (RF) discharge sputtering of a Pt electrode in an oxygen atmosphere. The structure, particles size, electronic properties, and surface composition of the RF-sputtered parti- cles were studied by using transmission electron microscopy and X-ray photoelectron spectroscopy. The application of the RF discharge method resulted in the formation of highly oxi- dized Pt 4 + species that were stable under ultrahigh vacuum conditions up to 100 8C, indicating the capability of Pt 4 + –O species to play an important role in the oxidation catalysis under real conditions. The thermal stability and reaction proba- bility of Pt 4 + oxide species were analyzed and compared with those of Pt 2 + species. The reaction probability of PtO 2 nano- particles at 90 8C was found to be about ten times higher than that of PtO-like structures. [a] Dr. D.A. Svintsitskiy, Dr. L. S. Kibis, Dr. A. I. Stadnichenko, S. V. Koscheev, Dr. V. I. Zaikovskii, Prof. A. I. Boronin Boreskov Institute of Catalysis Pr. Lavrentieva 5, Novosibirsk, 630090 (Russia) E-mail : boronin@catalysis.ru [b] Dr. D.A. Svintsitskiy, Dr. L. S. Kibis, Dr. A. I. Stadnichenko, S. V. Koscheev, Dr. V. I. Zaikovskii, Prof. A. I. Boronin Novosibirsk State University Pirogova St. 2, Novosibirsk, 630090 (Russia) ChemPhysChem 0000, 00,0–0 # 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 & These are not the final page numbers! ÞÞ These are not the final page numbers! ÞÞ Articles DOI: 10.1002/cphc.201500546