ISSN 1998-0124 131 CN 11-5974/O4
Nano Res. Res. 2011, 2011, 4(1): 4(1): 131–142 131–142 Nano DOI 10.1007/s12274-010-0083-8 Research Article
Theoretical Investigations of the Catalytic Role of Water in Propene Epoxidation on Gold Nanoclusters: A HydroperoxylMediated Pathway Chun-Ran Chang, Yang-Gang Wang, and Jun Li ( ) Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China Received: 14 August 2010 / Revised: 16 October 2010 / Accepted: 17 November 2010 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2010
ABSTRACT We report a comprehensive theoretical investigation of the catalytic reaction mechanisms of propene epoxidation on gold nanoclusters using density functional theory (DFT). We have shown that water acts as a catalytic promoter for propene epoxidation on gold catalysts. Even without reducible supports, hydroperoxyl (OOH) and hydroxyl (OH) radicals are readily formed on small-size gold clusters from co-adsorbed H2O and O2, with energy barriers as low as 4–6 kcal/mol (1 cal = 4.186 J). Propene epoxidation occurs easily through reactions between C3H6 and the weakened O–O bond of the OOH radicals on the surfaces of gold clusters.
KEYWORDS Gold nanocluster, propene epoxidation, hydroperoxyl radical, density functional theory
1. Introduction The discovery of gold nanoparticles and nanoclusters as catalysts is one of the most important developments in current catalysis science [1, 2]. Low-temperature oxidation reactions of CO and hydrocarbons using gold catalysts are among the most widely investigated due to their potential applications in fuel cells and other industrial processes. Propylene oxide (PO) is a hydrocarbon oxidation product that is used in the commercial production of many commodity chemicals. PO is usually produced by chemical oxidation processes involving hydrochlorination, oxidation with oxygen, or oxidation with hydroperoxide, which all have undesirable drawbacks due to environmental concerns and economical reasons. A new approach proposed Address correspondence to
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by Haruta et al. involves direct propene oxidation with an O2/H2 mixture, but the direct use of H2 and the low conversion rate means that significant improvements in this process are still required [3, 4]. Recently, significant progress has been made in using an O2/H2O mixture for propene epoxidation reactions, which gives an eco-friendly new way to manufacture PO [5–7]. Molina and co-workers reported the catalytic formation of PO using immobilized Aux (x = 6–10) subnanometer clusters prepared by softlanding cationic gold clusters on an alumina support [5]. Ojeda and Iglesia reported the catalytic epoxidation of propene with H2O and O2 on gold nanoparticles (~3.3 nm ± 0.7 nm) supported on a TiO2 substrate [6]. Haruta et al. found that by using water vapour as gasphase promoter, highly selective propene epoxidation
132 occurs with O2 as oxidant and gold nanoclusters (
