也曾读过很多综述,感到很多综述虎头蛇尾,引言部分牛皮吹得很震天响,可是到了最后越写越粗糙,到了结尾部分竟然几句话草草收尾了!读了如此综述,感到通篇就是“谁谁谁做了什么,他们发现了什么”,只有罗列事实,没有评述。正如K.R. Seddon在Inorganic Liquids in Synthesis (Second Edition)一书的序言中所说:"How many papers within this annual flood of reviews say anything critical, useful, or interesting? How many add value to a list of abstracts which can be generated in five minutes using SciFinder ot the ISI Web of Knowledge? How many of them can themselves be categorised as garbage? It is the twenty-first century----if a review is just an uncritical list of papers and data, what is its value?"
下面我介绍我写的Z. Ma, S.H. Overbury, S. Dai*, Gold Nanoparticles as Chemical Catalysts, in: Nanomaterials: Inorganic and Bioinorganic Perspectives, C.M. Lukehart, R.A. Scott (eds.), John Wiley & Sons, Chichester, in press. 一文的结论部份。
本文作为Nanomaterials: Inorganic and Bioinorganic Perspectives一书的一章,从催化剂设计角度综述了黄金催化剂结构设计的几种策略和具体的方法。和“市场”上其它综述的区别是:其它综述多描述传统的黄金催化剂,综述反应机理、一氧化碳氧化、有机反应和金催化剂的其它应用,但是很少有综述从“高级催化材料设计”的角度综述金催化剂设计的进展。本文的目的是填补这样的“市场空白”。其结论部份及其讲解如下:
8. Concluding remarks
In this chapter, we summarized recent advances in the preparation of nano gold catalysts from the perspective of inorganic materials synthesis. These materials synthesis techniques (e.g., chemical grafting, co-synthesis, and surface-sol-gel method) not only furnish new means to deposit uniform and well-dispersed gold nanoparticles on various supports, but also produce surface-functionalized supports and uniform nanostructured supports for loading gold particles. In addition, after gold particles are supported, the catalyst may be further modified to tune the catalytic performance. These catalysts provide new opportunities for the study of the relation between structure and catalytic performance, and aid in the rational design of gold catalysts.
To put the information in perspective, it should be mentioned that many methods introduced above are not specially invented by researchers working on gold catalysis, but have their origins in other contexts. For example, the post-grafting of mesoporous SiO2 by organosilanes and the one-pot synthesis of organosilane-functionalized mesoporous SiO2 (Section 3.2) are well known in materials chemistry.106,107 The co-synthesis of SiO2 in the presence of a soluble metal salt to prepare supported metal catalysts (Section 3.3) is also known.108,109 The surface-sol-gel or chemical grafting method (Section 4.1) was initially used to modify flat surfaces and powders for other applications.63 The solvated metal atom impregnation method (Section 7) was initially used by inorganic chemists,110 although they did not report gold catalysis at that time. The post-modification of metal catalysts by SiO2 matrix was reported for platinum catalysts used for car-emission control.111 Therefore, many inorganic synthesis methods can be used for the synthesis of novel gold catalysts, and these methods are also expected to be extended to the preparation of other metal catalysts.
One valid question is whether the gold catalysts synthesized by advanced, demanding, and often tedious, synthetic methods or using unique nanostructured supports (e.g., nanotubes and nanobelts) are better than those synthesized via conventional methods or using commercialized supports. As commented in a recent book,12 “Many methods of preparation have been used, but one wonders why some people have laboured to develop very sophisticated methods while others have been content with a much simpler method, apparently giving the same result, namely, the desired small gold particles.” The answer to that question is certainly ambiguous. On the one hand, modern nanotechnology can indeed help with the design of many gold catalysts with improved catalytic performance that can not be achieved by using conventional methods. On the other hand, one can find many publications focusing on the synthesis part, with the catalytic performance not reported or very low.12 One pitfall may be that some gold catalysts may contain residual capping agents and/or organic fragments. These organic fragments, if not sufficiently removed by special treatments, may poison CO oxidation, although gold catalysts with organic fragments may still show some activity in certain organic reactions. This point was sometimes overlooked, and thus undermining the real performance of these advanced gold catalysts. How to properly remove the organic fragments while avoiding the sintering of gold nanoparticles is a challenge. Even if this challenge is overcome, many factors still have to be considered before there can be large-scale utilization of gold catalysts synthesized by advanced technology.
In the future, it is important to study the structure-property correlation with the aid of an array of characterization methods.112 One may want to systematically design “model catalysts”, the objective of which is to seek deeper insights into mechanistic roles played by nanostructures instead of achieving high catalytic activity. One idea in this regard is to load gold colloids with identical sizes on different supports to compare their performance, and rank different supports.41,42 Another idea is to systematically build up supports with complex artificial structures. For instance, Au/TiO2/TiO2/SiO2, Au/Al2O3/Al2O3/SiO2, Au/TiO2/Al2O3/SiO2, and Au/Al2O3/TiO2/SiO2 catalysts were synthesized based on SiO2 support sequentially modified by two “layers” of metal oxides.65 However, these “metal/coating/coating/support” catalysts may potentially contain multiple support-coating, coating-coating, metal-support, and metal-coating interfaces, and the difference in catalytic activity in CO oxidation is sometimes too subtle to interpret. Close cooperation between synthetic chemists and those who characterize catalyst properties is needed to better develop functionalized gold catalysts and to elucidate catalytic and structural details on the molecular level.
Finally, it should be mentioned that due to the emphasis of this inorganic materials book, we mainly focus on the use of supported gold nanoparticles as catalysts, rather than on the chemical reactions catalyzed by these materials. So far, the most frequently adopted catalytic reaction to evaluate the performance of gold catalysts is CO oxidation. This probe reaction is easy to carry out and is of some practical values in environmental control. However, in the near future, it is expected that researchers in this area will pay more attention to the applications of gold catalysts in other reactions, such as selective reduction of NOx, water-gas shift, catalytic combustion, selective hydrogenation, selective oxidation, and carbon-carbon coupling of organic molecules.16 These reactions are more complex and more demanding than CO oxidation, and are certainly more interesting from an industrial perspective. Currently most gold catalysts used in these reactions are prepared using common methods and simple components. With the fast development of nanotechnology, it is expected that novel gold catalysts with advanced structures and multiple components will also play a role in these reactions in the future.