ABSTRACT

In the context of heterogeneous catalysis, oxygen on metals and oxygen on metal oxides are often considered as two different phenomena that are little related to each other. The former is mainly investigated in the field of Surface Science and the calculation chemistry, while the latter in the field of conventional catalysis. In this review we performed a joint analysis of the literature data obtained in both fields, which showed that in many aspects these two types of oxygen are chemically similar. It is usually assumed that, as in the case of oxides, the reactivity of oxygen on metals is determined by its binding energy to the surface. Trying to confirm this idea by comparing the catalytic activity of metals with the heat of oxygen adsorption (Q_O2), we came across a paradox of excessive metal activity, which was associated with the presence of “hot” or super active (SA) oxygen. This oxygen is capable of oxidizing methane and other substances at cryogenic temperatures but does not make a significant contribution to Q_O2. A comparison of SA oxygen on metals with the O^•– radicals on oxides revealed a clear similarity of these species. This allows one to conclude that the oxygen on metals is a radical, which well explains its super high reactivity without using the idea of an energetically excited state. A hypothesis is proposed that not only O^•–, but also the surface O^2- species have a certain degree of the radical nature. Results of low-temperature reactions of SA oxygen offer a more precise interpretation of the selectivity rule. Not the high or low reactivity of surface oxygen in itself is essential for the selective catalyst. A consent between the rates of the product formation and its desorption from the surface is of vital importance. This concept opens the possibility for new approaches in the development of selective catalysts, including those based on metals.

摘要

在多相催化的背景下，金属上的氧和金属氧化物上的氧通常被认为是两种不同的现象，彼此之间几乎没有联系。前者主要研究表面科学和计算化学领域，后者主要研究常规催化领域。在这篇综述中，我们对两个领域获得的文献数据进行了联合分析，结果表明这两种氧在许多方面在化学上是相似的。通常假设，就氧化物而言，氧对金属的反应性取决于其与表面的结合能。试图通过比较金属的催化活性与氧吸附热 (Q _O2)，我们遇到了过度金属活性的悖论，这与“热”或超活性 (SA) 氧的存在有关。这种氧气能够在低温下氧化甲烷和其他物质，但对 Q _{O2 的}贡献不大。金属上的 SA 氧与氧化物上的 O ^•–自由基的比较揭示了这些物种的明显相似性。这使得人们可以得出结论，金属上的氧是一种自由基，这很好地解释了它的超高反应性，而无需使用能量激发态的概念。提出了一个假设，不仅 O ^•–，而且表面 O ^2-物种具有一定程度的激进性。SA 氧的低温反应结果为选择性规则提供了更精确的解释。表面氧本身的高或低反应性对于选择性催化剂不是必不可少的。产品形成速率与其从表面解吸的速率之间的一致性至关重要。这一概念为开发选择性催化剂（包括基于金属的催化剂）的新方法开辟了可能性。