Elsevier

Applied Catalysis A: General

Volume 454, 15 March 2013, Pages 53-58
Applied Catalysis A: General

Catalytic activity of Au/TiO2 and Pt/TiO2 nanocatalysts prepared with arc plasma deposition under CO oxidation

https://doi.org/10.1016/j.apcata.2012.12.049Get rights and content

Abstract

We report the catalytic activity of Au/TiO2 and Pt/TiO2 nanocatalysts under CO oxidation fabricated by arc plasma deposition (APD), which is a facile dry process involving no organic materials. Using APD, the catalyst nanoparticles were well dispersed on the TiO2 powder with an average particle size of 2–4 nm, well below that of nanoparticles prepared by the sol–gel method (10 nm). We found that the average particle size of the dispersed gold nanoparticles can be controlled by changing the plasma discharge voltage of APD. Accordingly, the amount of gold loaded on the TiO2 powder increased as the discharge voltage increased, but the specific surface area of the Au/TiO2 samples decreased. As for catalytic reactivity, Au/TiO2 showed a higher catalytic activity than Pt/TiO2 in CO oxidation. The catalytic activity of the Au/TiO2 samples showed size dependence where higher catalytic activity occurred on smaller gold nanoparticles. This study suggests that APD is a simple way to fabricate catalytically active nanocatalysts.

Highlights

► Novel synthesis of nanocatalysts fabricated by arc plasma deposition is reported. ► The particle size of the gold nanoparticles can be controlled by APD voltage. ► The catalytic activity of the Au/TiO2 samples showed size dependence.

Introduction

Development of nanocatalysts with maximal catalytic activity and good stability is the essential goal in heterogeneous catalysis. Synthesis of oxide-supported metal catalysts using colloid nanoparticles with controlled size and composition or wet-chemical routes have created new model catalytic systems that allow us to bridge the materials gap that exists between two- and three-dimensional model catalytic systems and industrial catalysts [1], [2], [3], [4], [5].

Synthesis of oxide-supported metal catalysts by wet-chemical routes is well known in heterogeneous catalysis. In general, oxide-supported metal catalysts produced via wet-chemical routes need to undergo an annealing process at elevated temperature in order to eliminate any organic materials, such as solvent, surfactant, or capping agents, as those organic materials may exist at the interface of the metal particles and oxide support and change catalytic reactivity [6], [7], [8]. However, the annealing process can cause oxidation of the catalyst metal particles and deteriorate the catalytic activity of the metal particles. In order to overcome the shortcomings of wet-chemical methods, dry synthesis processes for nanocatalyst fabrication, including laser vaporization [9], [10] or arc plasma deposition (APD) [11], [12], have recently received great interest due to their simplicity, high reproducibility, and the possibility for large-scale production.

It is well known that metal-oxide hybrid catalysts have shown good catalytic activity due to the strong metal-support interaction (SMSI) effect [13], [14], [15]. Particularly, gold nanoparticles deposited on metal oxide are known to have high activity for CO oxidation [16], [17]. In this study, we employed 1–5 nm Au and Pt nanoparticles fabricated using APD for metal catalysts without organic capping layers. We have investigated the catalytic activity of Au/TiO2 and Pt/TiO2 under CO oxidation. We studied the effect of the size of the Au nanoparticles, which was tuned by changing the plasma discharge voltage of APD, on the catalytic activity of CO oxidation.

Section snippets

Fabrication of catalysts

Au and Pt nanoparticles (1–5 nm in size) were deposited on TiO2 powder (DT-51 of Cristal Global, anatase phase) via arc plasma deposition. The arc pulses were generated with a period of 0.2 ms, frequency of 2 Hz, arc discharge condenser capacity of 1800 μF, and arc discharge voltage of 200 V or 300 V. The plasma from the cathode entered a container that contained TiO2 substrate powder. 10,000 pulses of Au and Pt plasma were deposited while the powder was stirred.

For comparison, Au/TiO2 nanoparticles

Catalyst characterization

Fig. 1 shows TEM images of the Au/TiO2 and Pt/TiO2 samples. The metal nanoparticles are evident as small dark spots, while the TiO2 substrate is brighter and larger. According to the images, the metal nanoparticles are well dispersed on the TiO2 powder. The Au/TiO2 samples produced using 300 V of arc discharge voltage showed a higher size distribution and larger gold nanoparticles than those produced using 200 V of arc discharge voltage (Fig. 1(a) and (b)). Metallic nanoparticles generated by APD

Conclusion

In conclusion, catalytically active Au/TiO2 and Pt/TiO2 powder were fabricated by APD, a dry process with no organic materials involved. Morphology of nanocatalysts, crystallographic structures, and surface area of the Au/TiO2 and Pt/TiO2 powders were measured using TEM, XRD, and BET, respectively. Using APD, the catalyst nanoparticles were well dispersed on the TiO2 powder with an average particle size (2–4 nm) well below that of nanoparticles prepared by the sol–gel method (10 nm). For the

Acknowledgments

This work was supported by the WCU (World Class University) program (31-2008-000-10055-0) and 2012R1A2A1A01009249 through the National Research Foundation, and the Research Center Program (CA1201) of IBS (Institute for Basic Science) of Republic of Korea, and by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

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    These authors equally contributed to this work.

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