CO-Induced Diffusion of Ni Atoms to the Surface of Ni–Au Clusters on TiO₂(110)

Abstract

It has been well established that bimetallic systems can exhibit activity different than that of pure metals, and there are many examples in the catalysis literature illustrating the ability of a second metal to promote the desired catalytic activity and selectivity. 1À6 Consequently, there is much interest in basic understanding of the chemical activity on bimetallic surfaces in order to develop catalysts with properties that can be tuned by changing compositions. In some cases, reactions are promoted via a bifunctional mechanism, in which the reaction requires the different activities provided by each metal. 7À12 Furthermore, electronic effects associated with the formation of new me-talÀmetal bonds may alter surface chemical properties, such as CO adsorption strength, 6,13À16 hydrogenation activity, 3,17À19 dehydrogenation activity, 20,21 and reforming selectivity. 3,22,23 Bimetallic surfaces may also provide mixed-metal sites with activity different from that of the pure metal sites, such as on the SnÀPt alloy surfaces. 24À26 In addition, interactions between the metal clusters and the oxide support may also be used to control surface chemistry on the clusters, with lattice oxygen participating in reactions on the oxide-supported clusters. For example, atomic carbon on Ni clusters recombine with lattice oxygen from the titania support to produce gaseous CO, 27,28 and gaseous products containing lattice oxygen are observed in reactions on metal clusters supported on ceria. 29À34 Also for noble metals on ceria supports, ceria plays an important role in oxygen storage in the three-way catalysts for the conversion of CO, NO x , and hydrocarbons into CO 2 , water, and N 2 . 35À37 In other cases, it has been reported that chemical activity occurs at metal clusterÀoxide interfacial sites. 29,38À42 In order to probe the nature of metalÀmetal and metalÀsupport interactions, we have chosen to study a model system consisting of vapor-deposited NiÀAu bimetallic clusters supported on rutile TiO 2 (110). In this model system, the relationships between morphology, composition, and chemical activity can be explored on a fundamental level. The AuÀtitania system is one of chemical interest due to the unusual catalytic properties of

Growth and characterization of Au, Ni and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh

“…Tenney et al [32] stated that exposing Au-Ni clusters to 0.5 L CO at 22°C would induce the diffusion of Ni to the surface. However, no such CO-induced segregation of Ni has been observed in our work under the similar conditions.…”

“…Very recently, Chen's group studied the growth, surface composition, and chemical activity of Ni-Au clusters on TiO 2 (110) with surface science techniques together with density functional theory (DFT) calculations and ab initio molecular dynamics simulations [31,32]. They found that bimetallic Au-Ni clusters can be grown by nucleating Au onto the existing Ni clusters.…”

Growth and Structure of Ni–Au Bimetallic Particles on Reducible CeO2(111)

“…However, both Au and Ni can sinter at high temperatures and thus lose their activity. Furthermore, Ni can experience encapsulation by reducible oxide supports like ceria and titania [11][12][13]. During the steam reforming process, it can deactivate due to the coke formation [14,15].…”

“…The bimetallic Ni-Au systems could provide unique properties compared to individual Ni and Au counterparts due to the synergistic effects between the two metals as well as the interaction between the metal and oxide. Research has shown that the presence of Ni in Au could inhibit its sintering due to the formation of stronger Ni-Au bonds [12,16]. Incorporation of Au in Ni can also hinder the surface poison by carbon [17,18].…”

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

“…Moreover, the adsorbate-induced segregation of metal alloys under the reaction conditions and thus the changes in local atomic composition and surface structure have been predicted and demonstrated to occur for a number of gold alloy systems [12,13]. In particular, for Au-Pd nanoalloys (that will be more detailed bellow) although the gold surface enrichment is predicted to be thermodynamically favourable under vacuum conditions [14], a reversed segregation of Pd as a more active component to the surface is reported to occur in the presence of adsorbates [15][16][17].…”