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

Abstract

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

Theoretical insights on the effect of reactive gas on the chemical ordering of gold-based alloys

“…Despite a considerable progress of the application of the previously described DFT methodologies in the identification of equilibrium alloy structures under reactive gas conditions [see the recent reviews 32,38], only very few studies at this level of theory have considered Au-based bimetallic nanosystems. In the following, we briefly review the main theoretical insights in this topic by presenting some examples of gold-based alloy systems.…”

“…Furthermore, as small unit cells periodically repeated in space cannot accommodate large deformations that might be induced by the addition of impurities, size mismatch and stresses cannot be considered. Detailed description of the basic features of this approach can be found in [32] for the case of bimetallic alloy in thermodynamic equilibrium with single gas molecule and in [33,34] for the case of multiple gas species.…”

“…Cluster expansion method (or lattice-gas-Hamiltonian) Cluster expansion (CE) method is a formalism that uses a set of DFT calculations of large number of ordered and disordered fixed structures to extract many-body interaction terms that describe the Hamiltonian of the alloy system [32]. A CE of the Hamiltonian is then termed by a polynomial equation involving the neighbouring energy interactions that, in principle, includes an infinite number of terms and summations.…”

LEEM and PEEM as Probing Tools to Address Questions in Catalysis

“…In multicomponent systems, like in multimetallic nanoparticles, each component may have different adsorptive properties towards one or even more reactants, characterized by significantly different energies of adsorption. As a consequence, a surface enrichment of one of the elements may occur over time under reactive [23] conditions, thus changing the original ability of the material to conduct the chemical reaction in the most efficient and low energy demanding path [32].…”

Combined UHV/high-pressure catalysis setup for depth-resolved near-surface spectroscopic characterization and catalytic testing of model catalysts

“…A rapidly increasing number of surface science studies [3][4][5][6][7] reflects the actuality of this research, that has been initiated and influenced for years by the groups of Somorjai et al 8 , Hayek et al 9 , Goodman et al 10 and Ertl et al 11 . The second step is the systematic testing of the different model catalysts at higher pressures, i.e.…”