Surface alloying

The following mechanisms in corrosion behavior have been affected by implantation and have been reviewed (119) (/) expansion of the passive range of potential, (2) enhancement of resistance to localized breakdown of passive film, (J) formation of amorphous surface alloy to eliminate grain boundaries and stabilize an amorphous passive film, (4) shift open circuit (corrosion) potential into passive range of potential, (5) reduce/eliminate attack at second-phase particles, and (6) inhibit cathodic kinetics. 

There is a large assortment of alloys available for abrasive service in the forms of wrought alloys, sintered metal compacts, castings, and hard-surfacing alloys. They can... 

The experimental investigation was performed by depositing copper films on the (100) -surface of a platinum single crystal. It was found that the reconstruction of the Pt surface was lifted upon Cu adsorption. The system was then heated to different temperatures and the formation of different ordered surface alloys was evidenced by... 

Laser and electron beam processing are effective methods for preparing amorphous surface alloys covering conventional crystalline bulk metals... 

Ion implantation and ion mixing produce amorphous alloys as thin as only several tens of nanometres. Implantation of metalloids such as phosphorus in austenitic stainless steel has been known to produce amorphous surface alloys having high corrosion resistance" ". 

Temperatures well in excess of 400°C can be used for processing in this case much deeper coatings are obtained, but the iron content of the surface alloy is higher and the diffusion layer is very brittle and less corrosion-resistant. This effect is easily explained when it is remembered that the rate of interdiffusion is far more rapid when the temperature is above the melting point of zinc (420°C). 

Scheme 5.1. Alloy formation and segregation in bimetallic systems with one of the metals present as a minority. The scheme qualitatively predicts whether two elements form a surface alloy or a solid solution. The results are valid in vacuum. As soon as an adsorbing gas is...

They provide materials with considerable improved catalytic properties (vide infra) either by the presence of the organometalHc fragment itself [ 120, 125-127], by the presence of the adatoms [128,129] or by the formation of the surface alloys (vide infra) [116,130-138]. 

Further annealing induces additional Ag overlayer enrichment with Pd atoms, causing a substantial intensity increase of the Pd resonant state, while the intensity at the Fermi level remained very small. This is a clear indication of the localized character of the Pd 4d state. The annealing of the Ag multilayer produces a surface alloy with a composition very close to Ago.sPdo.s which has a DOS at the Fermi level substantially smaller than the pure palladium. The annealing at higher temperature produces a Pd(l 10) surface with very small but very persistent amount of silver, which is in the form of three-dimensional clusters, located most probably below the first Pd(l 1 0) layer. 

Using periodic DFT calculations [Greeley and Nprskov, 2007], we calculate the value of the HER descriptor AGh on all of the surface alloys of interest. The results are... 

Figure 3.18 Spectrum of free energies of hydrogen adsorption, AGh, on binary surface alloys at r = 298K. The vertical axis shows the number of elements with free energies within 0.1 eV windows (O.O-O.l eV, 0.1-0.2 eV, etc.). The sohd vertical line indicates AGh = 0- The dashed vertical line gives the hydrogen free energy adsorption for pure Pt. AU free energies are referenced to gas phase H2. Adapted from [Greeley and Nprskov, 2007] see this reference for more details.

For screening purposes, the most important result to emerge from the data in Fig. 3.17 is that there is a very large number of surface alloys with AGh values roughly equal to zero (and hence a large number of such alloys with near-optimal values of the HER descriptor). This fact can be seen clearly in Fig. 3.18 the distribution of alloys with particular values of AGh is peaked near AGh = 0. 

All binary surface alloys of high predicated activity... 

Figure 3.19 Schematic representation of surface alloy stability tests. White spheres denote adsorbed hydrogen, black spheres denote solute metal atoms, and gray spheres denote host metal atoms. Adapted from [Greeley and Nprskov, 2007] see this reference for more details.

Using the raw data in Fig. 3.20, we can identify the Pareto-optimal set for the HER activify/stabilify criteria. This set represents the best possible compromise between activity and stability criteria for the surface alloys that we have considered the alloys in the set are, thus, logical choices for further consideration. The presence of pure Pt on the Pareto-optimal set is, in effect, a sanity check for our computational screening procedure. Pt is well known to be the most active and stable pure metal for the HER in acidic conditions. The alloys seen on the Pareto-optimal set include RhRe and BiPt. 

Figure 3.21 Hydrogen evolution after each stage of BiPt surface alloy synthesis, (a) Pt film after deposition and anneal (b) immediately after Bi-UPD (c) after second anneal to form the BiPt surface alloy. Adapted from [Greeley et al., 2006a] see this reference for more details.

Besenbacher F, Chorkendorff I, Clausen BS, Hammer B, Molenbroek AM, Nprskov JK, Stensgaard I. 1998. Design of a surface alloy catalyst for steam reforming. Science 279 1913-1915. 

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. 

Greeley J, Nprskov JK. 2005. A general scheme for the estimation of oxygen binding energies on binary transition metal surface alloys. Surf Sci 592 104-111. 

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. 

Ge Q, Desai S, Neurock M, Kourtakis K. 2001. CO adsorption on Pt-Ru surface alloys and on the surface of Pt-Ru bulk alloy. J Phys Chem B 106 9533-9536. 

Figure 14.6 Charge in the cathodic peak between 0.11 and 0.06 V as a function of Pt surface content diamonds, PC submonolayers on Ru(OOOl) circles, PcRui-j,/Ru(0001) surface alloys the lines are predicted trends for linear or polynomial correlations between charge and Pt surface...

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