AMMONIA-SYNTHESIS OVER IRON SINGLE-CRYSTAL CATALYSTS - THE EFFECTS OF ALUMINA AND POTASSIUM
AMMONIA-SYNTHESIS OVER IRON SINGLE-CRYSTAL CATALYSTS - THE EFFECTS OF ALUMINA AND POTASSIUM
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DOI：
10.1021/j100411a003
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发表时间：
1986-09-25
影响因子：
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通讯作者：
SOMORJAI, GA
中科院分区：
文献类型：
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作者：
BARE, SR;STRONGIN, DR;SOMORJAI, GA
由氨元素催化合成氨是最重要的工业反应之一。现代氨合成催化剂在基本组成上与原始工业催化剂相似。它是由氧化铁(Fe304)制成的，其中添加了几个重量百分比的其他氧化物作为促进剂。尽管催化剂的整体组成有很大的不同(取决于所采用的特定合成条件)，但通常使用的氧化物是铝、钾和钙的氧化物。此外，现在添加了硅和镁的氧化物，而它们只作为杂质存在于早期催化剂的原材料中。尽管促进铁催化合成氨的研究已有80多年的历史，但对促进剂在反应中所起作用的详细了解还不清楚，但多年来已经提出了许多想法。2促进剂一般分为两类：(1)结构促进剂和(2)电子促进剂。由于这不是对所提议的各种促进者机制进行详细的批判性审查的地方，因此将不作评论地简要审查影响的类型。铝、镁和硅的氧化物被认为是结构修饰剂。例如，在氧化铝的情况下，在将Fe304还原为活性金属相(？-Fe)的过程中，氧化铝的存在抑制了铁在磁铁矿晶格中的扩散。因此，与其生长速度相比，小铁粒的形核进行得更快。这会导致形成细小的微晶，从而形成具有高比表面积(10-20m2g-1)的非常多孔的结构。1·3氧化铝还能抑制催化活性高的晶面(Fe(111)晶面4)向活性较低的晶面((100)和(110)晶面4)的转化。氧化钾被认为是一种电子促进剂，可以增加铁表面的比活性(每个铁原子的活性)。事实上，在超高真空(UHV)中钾和氮在铁单晶上的一系列化学吸附研究中，Ertl能够证明，亚单层含量的元素钾的存在增加了氮的解离粘着几率(氨中的限速步骤
The catalytic synthesis of ammonia from its elements is one of the most important industrial reactions. The modern ammonia synthesis catalyst is similar to the original industrial catalyst in regard to the basic composition. 1 It is made from iron oxide (Fe304), to which a few percent by weight of other oxides are added as promoters. Although there is considerable variation in the bulk composition of the catalysts (dependentupon the particular synthesis conditions employed), the oxides usually employed are those of aluminum, potassium, and calcium. In addition, oxides of silicon and magnesium are now added, whereas they were only present as impurities in the raw materials in the early catalysts. Even though the synthesis of ammonia catalyzed by promoted iron has been studied for over 80 years, a detailed understanding of the effectthat the promoters play in the reaction is not known, but therehave been many ideas put forward over the years. 2 The promoters are generally divided into two classes:(i) structural promoters and (ii) electronic promoters. Since this is not the place for a detailed critical review of the various mechanisms of the promoters that have been proposed, the types of effects will be briefly reviewed without comment. Theoxides of aluminum, magnesium, and silicon are thought of as structural modifiers. For example, in the case of aluminum oxide, during the reduction of the Fe304 to the active metallic phase («-Fe), the presence of aluminum oxide inhibits therate of diffusion of ironin the magnetite lattice. Thus, nucleation of small iron particles proceeds quickly compared with their growthrate. This leads to the formation of small crystallites and thereby a very porous structure with high surface area (10-20 m2 g-1). 1· 3 It has also been pos-tulated that alumina inhibits the conversion of crystallographic planes with high catalytic activity (the Fe (lll) planes4) into those less active ((100) and (110) planes4). Potassium oxide is considered as an electronic promoter that can increase the specific activity of the Fe surface (activity per Fe atom). 2 Indeed, in a series of chemisorptionstudies of po-tassium and nitrogen on Fe singlecrystals in ultrahigh vacuum (UHV), Ertl was able to show that the presence of submonolayer amounts of elemental potassium increased the dissociative sticking probability of nitrogen (the rate-limiting step in the ammonia