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Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment

机译：催化中的合金：反应性环境的相分离和表面隔离现象

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摘要

Alloys play a crucial role in several heterogeneous catalytic processes, and their applications are expected to rise rapidly. This is essentially related to the vast number of configurations and type of surface sites that multi-component materials can afford. It is well established that the chemical composition at the surface of an alloy usually differs from that in the bulk. This phenomenon, referred to as surface segregation, is largely controlled by the surface free energy. However, surface energy alone cannot safely predict the active surface state of a solid catalyst, since the contribution of other parameters such as size and support effects, as well as influence of the adsorbates, play a major role. This can lead to numerous surface configurations as for example over the length of a catalytic reactor, as the chemical potential of the gas phase changes continuously over the catalyst bed and hence different reactions may prevail at different catalyst bed segments. Thanks to the rapid improvement of the analytical surface science characterization techniques and theoretical methodologies, the potential effects induced by alloyed catalysts are better understood. For catalysis, the relevance of measurements performed on well-defined surfaces under idealized ultrahigh vacuum conditions has been questioned and studies in environments that closely resemble conditions of working alloy catalysts are needed. In this review we focus on experimental and theoretical studies related to in situ (operando) observations of surface segregation and phase separation phenomena taking place on the outermost surface layers of alloy catalysts. The combination of first principles theoretical treatment and in situ observation opens up new perspectives of designing alloy catalysts with tailored properties.
机译：合金在几种异质催化过程中起着至关重要的作用，预计其应用将迅速上升。这与多组分材料所能负担的大量配置和表面位点类型有关。众所周知，合金表面的化学成分通常与大体中的化学成分不同。这种现象称为表面隔离，在很大程度上由表面自由能控制。但是，仅表面能不能安全地预测固体催化剂的活性表面状态，因为其他参数（例如大小和支撑效应）的贡献以及吸附物的影响起着主要作用。这可能会导致许多表面构型，例如在催化反应器的长度上，因为气相在催化剂床上不断变化，因此不同的反应可能在不同的催化剂床段中盛行。得益于分析表面科学表征技术和理论方法的快速改善，可以更好地理解由合金催化剂引起的潜在影响。对于催化，在理想化的超高真空条件下，在定义明确的表面上进行的测量的相关性已受到质疑，并且在需要相似工作的合金催化剂条件的环境中进行研究。在这篇综述中，我们着重于与原位（Operando）观察的实验和理论研究，对合金催化剂最外面的表面层进行了表面分离和相分离现象。第一原理理论处理和原位观察的结合开辟了设计具有量身定制特性的合金催化剂的新观点。

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