Fundamental Sulphur-Chemistry of Molybdenum Carbide Surfaces: Towards Catalytic Exploitation of Transition Metal Carbides

现代世界中许多最重要的化学反应都是通过使用催化剂才得以实现的。这些物质要么加速化学反应，要么提高对所需产品的选择性--最好是两者兼而有之--同时不会在过程中耗尽自身。经典的例子包括使用铁催化剂从氮气和氢气合成氨，以及使用钴催化剂由一氧化碳和氢气制造合成汽油或柴油。这两个过程对未来的经济发展都至关重要(用于化肥生产的氨和用于碳中性运输的合成燃料)，幸运的是，用作催化剂的金属供应充足。如今使用的催化剂更多地是基于昂贵和稀有的贵金属，如铂、钯或铑(例如，所有这些都用于催化转化器，在排放之前去除汽车尾气中的有害气体)，因此寻找更便宜或更好的替代品相应地迫在眉睫。除了某些催化剂的成本和稀缺性之外，另一个长期存在的问题是催化剂表面的污染物积聚导致的逐渐失活。尽管催化剂本身没有用完，但实际发生化学反应的微观活性中心可能会被非活性原子堵塞，而移除这些活性中心以逆转催化剂的“中毒”往往需要付出相当大的努力或费用。同样，寻找不太容易中毒的非传统催化剂是非常紧迫的。在本项目中，我们将研究一种特别非传统催化剂的基本表面化学，碳化钼在许多类型的催化中作为铂和类似贵金属的廉价替代品非常有希望。这种化学最有趣的方面之一与硫的行为有关，硫是一种臭名昭著的催化剂毒物，在含硫分子分解时沉积。众所周知，碳化钼特别耐硫中毒，确实可以通过加强硫化合物与氢在加氢脱硫过程中的反应，从炼油厂(加工传统化石燃料或绿色碳中性生物燃料)产生的分子混合物中脱除硫。这不仅对减少汽车和发电厂的硫排放(造成酸雨)具有重要意义，而且还可以极大地提高炼油厂产品用作大宗化学品生产原料的适宜性，因为在大宗化学品生产中，硫化合物的存在会毒害许多常见的催化剂。目前，加氢脱硫的这一重要功能是通过含有钴和硫化钼的催化剂来实现的，但从经济和环境角度来看，碳化钼在工业实践中代表着一种飞跃。为了了解硫化物与碳化钼之间的相互作用，我们将进行红外光谱测量，能够识别最终结合到表面的各种分解产物，以及超声波分子束测量，使我们能够确定反应速度作为表面条件和进入的分子状态的函数。我们将在超高真空条件下工作，并使用特性极好的样品，以便使用最先进的技术尽可能获得最详细的结果。我们通过这种方式收集的信息将对学术界和工业界的其他科学家有用，他们寻求基于这种材料优化工作催化剂。

Many of the most important chemical reactions underlying the modern world are made possible only by the use of catalysts. These are substances that either speed up a chemical reaction or improve its selectivity towards the desired product - preferably both - whilst not themselves being used up in the process. Classic examples include ammonia synthesis from nitrogen and hydrogen, using an iron catalyst, and the manufacture of synthetic petrol or diesel from carbon monoxide and hydrogen, using a cobalt catalyst. Both of these processes are crucial to future economic development (ammonia for fertiliser production, and synthetic fuels for carbon-neutral transportation) and it is indeed fortunate that the metals involved as catalysts are in plentiful supply.More often, the catalysts in use today are based on expensive and rare precious metals, such as platinum, palladium or rhodium (all used, for example, in the catalytic converters that remove harmful gases from car exhausts prior to emission) and the search for cheaper or better alternatives is correspondingly urgent. Another perennial issue, besides the cost and scarcity of certain catalysts, is one of gradual deactivation by the build-up of contaminants at the surface of the catalyst. Although the catalyst itself is not used up, the microscopic active sites where the chemical reactions actually occur can become blocked by unreactive atoms, and removing these to reverse the 'poisoning' of the catalyst can often involve considerable effort or expense. Again, the search for unconventional catalysts that are less prone to poisoning is extremely pressing.In the present project, we will study the fundamental surface chemistry of a particularly unconventional catalyst, molybdenum carbide, which is extremely promising as a cheaper alternative to platinum and similar precious metals in many types of catalysis. One of the most interesting aspects of this chemistry relates to the behaviour of sulphur, which is a notorious catalyst poison that is deposited upon the decomposition of sulphur-containing molecules. Molybdenum carbide is known to be particularly resistant to sulphur poisoning, and indeed can be used to remove sulphur from the mixture of molecules produced by oil refineries (processing either traditional fossil fuels or green carbon-neutral biofuels) by enhancing the reaction of sulphur compounds with hydrogen in a process known as hydrodesulphurisation. Not only can this be of importance in reducing automotive and power-plant sulphur emissions (responsible for acid rain) but it can also massively improve the suitability of refinery products for use as feedstocks in the production of commodity chemicals, where the presence of sulphur compounds would poison many of the common catalysts. At present, this important function of hydrodesulphurisation is carried out with a catalyst containing cobalt and molybdenum sulphide, but molybdenum carbide could represent a leap forward in industrial practice from both the economic and the environmental perspectives.In order to understand the interaction of sulphur compounds with molybdenum carbide, we will carry out infra-red spectroscopic measurements, capable of identifying the various products of decomposition that end up bound to the surface, and supersonic molecular beam measurements that allow us to determine reaction rates as a function of surface conditions and the state of incoming molecules. We will work under ultra-high vacuum conditions, and with extremely well-characterised samples, so as to obtain the most detailed results possible with state-of-the-art techniques. The information we can gather in this way will be of use to other scientists, in both academia and industry, who seek to optimise working catalysts based on this material.