The significance of the oxidation state of gold in heterogeneous catalysis

我们的目标是构建一个详细的过程，中间体和活性位点参与乙烯的氢氯化在碳表面上的金氯化纳米颗粒。乙烯的氢氯化为氯乙烯提供了一种替代途径，氯乙烯目前是通过两步工艺生产的，包括乙烯的氧氯化生成1,2-二氯乙烷，然后裂解。氯乙烯聚合成聚氯乙烯（PVC）是一种重要的聚合物，在建筑和电子工业中有着广泛的应用。60年前，氯乙烯几乎完全是由煤中提取的乙烯在氯化汞催化剂上直接加氢氯化法生产的，但从石油中获得廉价的乙烯导致了生产路线的改变，尽管产量较低，产品纯度较低，废物处理成本较高。现在，日益减少的石油储量和日益增强的环保意识要求人们寻找替代氧化氯化路线的方法。HgCl2带来的安全和环境问题阻碍了前催化剂的回归，但哈钦斯和他的同事已经证明，碳负载的金是一种可行的替代品。我们的项目将为这类催化剂的发展提供信息，同时也将为研究金催化提供一个重要的新窗口。虽然金纳米颗粒的催化作用是一种既定的现象，特别是与选择性氧化过程有关，但尽管进行了大量的工作，但活性位点的关键点和反应机制仍然存在很大的争议。不同的研究人员分别强调了金催化剂不同方面的重要性，例如：金颗粒的大小；金与载体之间的界面以及金的氧化态。Fierro-Gonzalez和Gates将这一争议归因于这样一个事实，即人们的注意力主要集中在CO氧化的机理研究上，并认为这种实验上吸引人的反应并不像它看起来那样直截了当。他们建议，如果要在了解金催化活性部位的性质方面取得进一步进展，从而为开发更好的金催化剂提供方向，就必须在基础水平上研究更多不同的系统。与金/氧化物催化剂相比，金/碳催化剂很少受到表面科学界的关注，因此，对该体系基础化学的研究不仅为推动未来重要催化剂的发展提供了前景，而且为金催化的一些基本原理提供了新的思路。结合表面光谱学、表面显微镜和表面动力学方法，结合计算化学，将重点放在识别和表征氢氯化催化剂理想模型中金的活性状态。然而，在实践中，Au/C氢氯化催化剂由高面积活性炭组成，在表面表现出各种官能团，这可能影响吸附的金卤化物的沉积和稳定性。为了解决这个问题，该项目涉及三个独立的平行链，研究在环境条件下从改性HOPG和活性炭的水溶液中沉积的金，为HOPG上卤化金的模型表面科学/计算研究和实际催化之间提供桥梁。

We aim to construct a detailed picture of the processes, intermediates and active sites involved in the hydrochlorination of ethyne over gold chloride nanoparticles supported on carbon surfaces. The hydrochlorination of ethyne provides an alternative route to vinyl chloride which at present is manufactured by a two step process involving the oxychlorination of ethene to form 1,2-dichloroethane followed by cracking. Vinyl chloride is polymerised to polyvinyl chloride (PVC) an important polymer with widespread applications in the construction and electronics industries. Sixty years ago vinyl chloride was almost exclusively made by the direct hydrochlorination of ethyne, which derives from coal, over a mercury chloride catalyst but the availability of inexpensive ethene from oil led to a change of manufacturing route despite the lower yields, lower product purity and higher costs for waste treatment. Now, dwindling oil reserves and increasing environmental awareness is dictating a search for to the alternatives to the oxychlorination route. The safety and environmental issues posed by HgCl2 prevent a return to the former catalyst but Hutchings and co-workers have proven that gold supported on carbon can be a viable alternative. Our project will inform the development of such catalysts but will also provide an important new window on gold catalysis.Whilst catalysis by gold nanoparticles is an established phenomenon particularly in relation to selective oxidation processes, the key points of the active site and the reaction mechanism remain very controversial despite extensive work. Different researchers have separately emphasised the importance of different aspects of gold catalysts for example: the size of the gold particles; the interface between the gold and the support and the oxidation state of the gold. Fierro-Gonzalez & Gates ascribe this controversy to the fact that attention has largely focussed on mechanistic studies of CO oxidation and suggest that this experimentally appealing reaction is not as straight forward as it appears. They recommend that more diverse systems be investigated at a fundamental level if further progress is to be made in understanding the nature of the active site in gold catalysis and thus provide direction to the development of better gold catalysts. Au/C catalysts have received remarkably little attention from the surface science community (in contrast to the gold/oxide catalysts) and a study of the fundamental chemistry underlying this system therefore offers the prospect not only of advancing the development of an important future catalyst but also of shedding new light on some of the fundamental principles of gold catalysis. A powerful combination of surface spectroscopy, surface microscopy and surface kinetic methods applied in conjunction with computational chemistry will be focussed on identifying and characterising the active states of gold in ideal models of the hydrochlorination catalyst. However, in practice, the Au/C hydrochlorination catalyst consists of a high area activated carbon exhibiting a variety of functional groups at the surface which may influence the deposition and stability of the adsorbed gold halides. To address this issue the project involves three separate parallel strands with studies of gold deposited from aqueous solutions on modified HOPG and activated carbon under ambient conditions providing the bridge between model surface science/computational studies of gold halides on HOPG and the practical catalysis.