Near-surface alloy

Greeley J, Mavrikakis M. 2005. Surface and subsurface hydrogen adsorption properties on transition metals and near-surface alloys. J Phys Chem B 109 3460-3471. 

Knudsen J, Nilekar AU, Vang RT, Schnadt J, Kunkes EL, Mavrikakis M, Dumesic JA. 2007. A Cu/Pt near-surface alloy for water-gas shift catalysis. J Am Chem Soc 129 6485-6490. 

We note that other systems not resembling the simple diatomic molecules considered here may follow a different relationship [86]. There may be other classes of reactions, dehydrogenation or —C bond breaking that may follow other similar relationships and thus form another universality class. We also note that there are exceptions to the relations, most notably for H2 dissociation on near-surface alloys [87]. These deviations from the rules are still describable within the d band model, though [87]. 

Greeley, J. and Mavrikakis, M., Near-surface alloys for hydrogen fuel cell applications, Catal. Today, 111, 52, 2006. 

PM A Cu/Pt Near-Surface Alloy for Watr-Gas Shift Catalysis Studied by STM, XPS, TPD, and DFT Ronnie T. Vang, Jan Knudsen, Joachim Schnadt, and Flemming Besenbacher. Interdisciplinary Nanoscience Center (iNANO and Department of Physics and Astronomy), University of Aarhus, Denmark. 

PM Near-surface alloys and Core-shell nanocatalysts for reactions involving hydrogen ... 

Fig. 1 Side view of slab models of various bimetallic structures often used in computational studies. In each case, the bottom layers of the material are defined using the structure of a specified bulk material. The number of surface and bulk layers varies in different studies, (a) In the sandwich structure the surface is one component, often the same component as the bulk material and the second layer is another component. This structure is often used to determine ligand effects, (b) The pseudomorphic monolayer structure combines strain and ligand effects in one structure by placing a second component on top of a bulk material, (c) The near surface alloy combines strain, ligand and ensemble effects in one structure by considering an alloy film defined by just a few atomic layers on top of an ordered bulk material.

Theoretical and experimental studies of model bimetallic catalysts in recent years have distinguished between thermodynamically stable bulk alloys and so-called near surface alloys. Near surface alloys are materials where the top few surface layers are created in a chemically heterogeneous way, for example, by depositing a monolayer of one metal on top of another metal. These structures are often not the thermodynamic equilibrium states of the material. To give one example, Ni and Pt form an fee bulk solid solution under most (but not all) conditions,73 so if a monolayer of Ni is deposited on Pt and the system comes to equilibrium, all of the deposited Ni will dissolve into the bulk. There is, however, a considerable kinetic barrier to this process, so the near surface alloy of a monolayer on Ni on Pt(lll) is quite stable provided a moderate temperature is used.191 If the deposited monolayer in systems of this type has a tendency to segregate away from the surface, a common near surface alloy structure is the formation of a subsurface layer of the deposited metal.85 The deposition of V on Pd(lll) is one example of this behavior.192... 

Fig. 9. Schematic plot of oxygen solubility vs. alloy composition showing the effect of Si on the solubilities of MgO and A1203 and hence the oxygen gradient across the near surface alloy layer. Vlach et al. [41],...

Near-surface alloys (NSAs) are alloys wherein a solute metal is present near the surface of a host metal in concentrations different from the bulk (e.g., Pt monolayers on base metals). The understanding and development of NSAs is a promising area of research in catalyst design [11, 19-22]. For example, it has been... 

Andersson et al. further showed the driving force for outer shell layers of metals from the adsorption of gas molecules (Fig. 2.16) [65]. Upon adsorption, CO molecule may induce surface reconstructions of CuPt alloys to change from a surface rich in Pt to a well-ordered CuPt surface alloy [66]. This transformation is attributed to observation of the fact that near-surface alloy of CuPt becomes unstable at elevated CO pressures. 

So-caUed near-surface alloys (NS As) provide a template system for studying such effects. NS As or skins have been extensively studied as oxygen reduction reaction (ORR) catalysts in PEM fuel cells. If one could reduce the contents of the expensive elemental metal Pt by forming a stable thin layer of the metal on top of a cheap and abundant metal host like Fe, Co, or Ni and improve or maintain its high activity and low overpotential for ORR, it would be a remarkable achievanent. 

FIGURE 11.15 Dashed lines indicate the limiting potential as a function of the OH adsorption energy, while the full line is the kinetic activity map at 0.9 V versus RHE based on the free energy diagram in Figure 11.12. Experimental data for (111) facets measured at the same potential are included. All data are shown relative to Pt( 111). Theoretical values are from Hansen et al. (2014). Experiments labeled Cu/Pt(lll) are Pt overlayers on CuPt near-surface alloys from Stephens et al. (2011). Other experimental data are from Blizanac et al. (2004) Wang et al. (2004) for Pt(lll) Shao et al. (2006) for Pd(lll) Blizanac et al. (2006) for Ag(lll) and Ag(lOO) Stamenkovic et al. (2007), 315,493 for Pt Niflll). Adapted from Hansen etal. (2014). 

Figure 12.14 Ordered Type II near-surface alloy Cu/(Cu-F lr)/Cu(100). (a) STM image of Cu(lOO) after deposition of 0.5 ML Ir at 520 K. (b) Top view and (c) side view of a structure model of the Cu(100)-(2 x l)-lr subsurface alloy. (From Ref [40].)...

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