Reactions between single crystals

Reactions between single crystals and powder reactions 

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Reactions between single crystals and powder reactions 101... 

Two reactions of this kind, which were studied many years ago by Tammann as powder reactions, and which have more recently been extensively examined as reactions between single crystals and dense pressed sinters, are [26, 27] ... 

The reactions of small organic molecules on titanium dioxide surfaces provide the central focus of these examples. Comparisons with reactions on single crystal surfaces of other oxides illustrate some of the key structural and electronic issues in oxide surface reactivity and the importance of local interactions between surface sites and adsorbed molecules. 

While these results show a relationship between single crystals and supported metals when used for structure sensitive reactions, similar relations for those reactions promoted by a single atom site may not be as straightforward since there are more different types of single atom sites possible on a metal surface than there are groups of face atoms. 

The objective of this chapter is to show that particles in the mesoscopic regime have very different properties to the bulk phase and, specifically, to demonstrate how in-situ STM and FTIR spectroscopy have been successfully employed to determine information on the structure of model catalysts based on modification of substrate electrodes with metal particles of mesoscopic dimensions, and the effect of this structure on reactivity. It will be shown that studying these model electrodes helps provide a link between single-crystal electrodes, which have provided a wealth of useful information, and electrodes for real application. FTIR has long been invaluable as a probe for localized particle reaction on surfaces in electrochemical processes, and the present work will show how it can complement STM in providing excellent characterization of mesoscopic properties. 

Although a particular compound, in crystallizing, will usually adopt the same overall lattice, each real sample of the compound will have a unique set and distribution of imperfections of various kinds. Different specimens of a particular solid may be closely similar, but no two will be identical. Imperfections are relatively energy-rich sites of local distortion and so are important participants in solid state reactions. These deviations fi-om the ideal lattice may exert a greater influence on the thermal stability of a solid than the crystal structure. This is evident fi om the differences in kinetic behaviour often observed between single crystals and powdered samples of the same reactant. (The reaction interface is itself an atypical distortion of the crystal structure.) Traces of impurities, or radiation damage, can also significantly enhance reactivity. 

There have been correspondingly few papers describing this reaction on single crystal surfaces of metals. ° Four concern the reaction on various surfaces of platinum, but their value is unfortunately somewhat limited two of them used both Pt(lOO) and Pt(lll), but TOFs were given at quite different temperatures with another pair it is only possible to compare results on Pt(l 10). While activation energies (43 10 kJ mol ) and order of reaction where determined were broadly comparable (Table 8.4), values of TOF sometimes showed a marked variation between different surfaces, and it is not possible to define a unique hierarchy of activity. Most disappointingly there is little information on product selectivi-ties apart from Siot (Table 8.4) in one publication only partial information was... 

A severe limitation for practical applications is the large pressure difference between single crystal experiments under UHV conditions and the real reaction conditions of heterogeneous catalysis and other surface processes. Most surface science techniques are restricted to pressures below 10 Torr because they require hot filaments and high mean free paths for the probing particles. IR spectroscopy as an optical technique can be used in normal atmosphere as well as with liquid surroundings and is therefore ideally suited for in situ studies of surface processes under real reaction conditions. 

Stottlemyer AL, Ren H, Chen JG (2009) Reactions of methanol and ethylene glycol on Ni/Pt Bridging the materials gap between single crystal and polycrystalline bimetallie surfaees. Surf Sci 603 2630-2638... 

In this chapter, we have attempted to show that in situ structural analysis techniques and traditional electrochemical measurements can be combined to obtain detailed insight into electrochemical reactions on single-crystal electrode surfaces. Thus, it is possible to establish a link between macroscopic electrochemical phenomena and atomic-scale surface structures. X-ray diffraction is a relatively new technique that takes advantage of the high brightness of synchrotron radiation sources to... 

EPR spectroscopy can only be applied to systems in which the balance between radical decay and radical formation keeps the free-radicals concentration above the detection limit of the spectrometer used. This can be a particularly severe problem in studying reactions in liquids. An alternative approach is to slow down reactions by studying samples held at cryogenic temperatures, such as 77 K (liquid nitrogen) or 4.2 K (liquid helium). An example of this work is the study of radical reactions in single crystals of amino acids exposed to x-rays, work that sometimes leads to activation energies and rate constants for radical reactions. 

Single crystals of Fe with evaporated overlayers of K and/or A1 have been studied as models for the structure of the industrial catalyst. After heating of an Fe(llO) crystal with an A1 overlayer, the surface looks rough under the electron microscope [227]. The reaction leading to a dispersion of A1 and Fe is primarily a reaction between oxidic phases [221]. The activity increases after reaction between the crystal and the Al-overlayer [227], possibly due to the formation of facets [227]. After heating of an Fe(llO) crystal in 5 atm of N2 to 450 C, no increase is found in the activity of the crystal [227]. The presence of A1 stabilizes the Fe(lll) and Fe(211) surfaces during NH3 synthesis at 20 atm [221]. The heating of Fe(llO) and Fe(lOO) with an A1 overlayer causes a recrystallization of... 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

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