Single crystal metal

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

In the following discussion, heavy emphasis is made of examples from studies of adsorbed layers on metal single-crystal samples. These illustrate the power of the HREELS technique and represent the main use of HREELS historically. Certainly HREELS has been used outside of the single-crystal world, and mention is made concerning its use on practical materials. This latter use of HREELS represents a true frontier. 

It is worth while, now, to analyse the motivation for making metallic single crystals and how, in turn, their production affected physical metallurgy. Initially, metallurgists were concerned to prevent the accidental generation of coarse grains in parts of objects for load-bearing service, and studied recrystallisation with this objective in view. To quote Keith, Iron crystals... were achieved subsequently by... 

While for many years, metal single crystals were used only as tools for fundamental research, at the beginning of the 1970s single-crystal gas-turbine blades began to be made in the hope of improving creep performance, and today all such blades are routinely manufactured in this form (Duhl 1989). 

Potentials of Zero Charge of Pt-Group Metal Single-Crystal Faces in Aqueous Solutions... 

Relative and Absolute Interfacial Parameter for Metal Single-Crystal Faces in Contact with Water... 

Almost all that is known about the crystal face specificity of double-layer parameters has been obtained from studies with metal single-crystal faces in aqueous solutions. Studies in nonaqueous solvents would be welcome to obtain a better understanding of the influence of the crystallographic structure of metal surfaces on the orientation of solvent molecules at the interface in relation to their molecular properties. 

Hamelin, A. Double-Layer Properties at sp and sd Metal Single-Crystal Electrodes 16... 

Reaction Kinetics and Mechanism on Metal Single Crystal Electrode Surfaces AdiiC, R. 21... 

Silver monolayer deposited on foreign metal single crystal. 

How does a support affect the morphology of a particle on top of it Which surface planes does the metal single crystal expose The thermodynamically most stable configuration of such small crystallites is determined by the free energy of the surface facets and the interface with the support, and can be derived by the so-called Wulff construction, which we demonstrate for a cross section through a particle-support assembly in two dimensions (Fig. 5.13). 

Metal carbonyls are also convenient for growth of certain METAL single crystals, vis-... 

Studies of the Kinetics and Mechanisms of Ammonia Synthesis and Hydrodesulfurization on Metal Single-Crystal Surfaces... 

Reaction Kinetics and Mechanisms on Metal Single Crystal Electrode Surfaces Recent Developments in Faradaic Rectification Studies... 

Figure 3.16 Volcano plot for the hydrogen evolution reaction (HER) for various pure metals and metal overlayers. Values are calculated at 1 barof H2 (298K) and at a surface hydrogen coverage of either 0.25 or 0.33 ML. The two curved lines correspond to the model (3.24), (3.25) transfer coefficients (not included in the indicated equations) of 0.5 and 1.0, respectively, have also been added to the model predictions in the figure. The current values for specific metals are taken from experimental data on polycrystalline pure metals, single-crystal pure metals, and single-crystal Pd overlayers on various substrates. Adapted from [Greeley et al., 2006a] see this reference for more details.

Figure 9.14 Kinetic current density (squares) at 0.8 V for O2 reduction on the Pt monolayer deposited on various metal single-crystal surfaces in a 0.1 M HCIO4 solution, and calculated binding energies (circles) of atomic oxygen (BEq), as a function of calculated d-band center (relative to the Fermi level, ej — sp) of the respective surfaces. The data for Pt(lll) were obtained from [Markovic et al., 1999] and are included for comparison. Key 1, PIml/ Ru(OOOl) 2, PtML/Ir(lll) 3, PtML/Rh(lH) 4, PtML/Au(lll) 5, Pt(lll) 6, PIml/ Pd(lll). (Reproduced with permission from Zhang et al. [2005a].)...

Similarly to the Pt monolayer catalysts, a series of Pd monolayers deposited on different metal single crystals were tested for ORR activity. The results are shown in Fig. 9.22. The ORR activity increases in the order PdML/Ru(0001) < PdML/fr(lll) < PdMi,/ Rh(lll)[Pg.299]

Reflection-Absorption IR spectroscopy (RAIRS) where the linearly polarized IR beam is specularly reflected from the front face of a highly reflective sample, such as a metal single crystal surface (Figure 3.1(a)). This is also sometimes referred to as IRAS (IR reflection absorption). The IR beam comes in at grazing angle (i.e. almost parallel to the surface), and although absorption bands in RAIRS have intensities that are some two orders of magnitude weaker than in transmission studies on... 

Preparation and Imaging of Metal Single-Crystal Surfaces... 

Kibler, L.A. Preparation and Characterization of Noble Metal Single-Crystal Electrodes, http //www.uni-ulm.de/ echem/index.html id=1023001. 

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