Crystal faces, chemical reactivity

Low Miller index surfaces of metallic single crystals are the most commonly used substrates in LEED investigations. The reasons for their widespread use are that they have the lowest surface free energy and therefore are the most stable, have the highest rotational symmetry and are the most densely packed. Also, in the case of transition metals and semiconductors they are chemically less reactive than the higher Miller index crystal faces. 

Chemical activity (functionality). Some crystal facets may be significantly more chemically reactive than others. For example, the (100) face of vanadyl... 

Chemically reactive because Fe2+ and Mg2+ are readily exposed at crystal faces... 

Well-defined metallic surfaces offer a unique way to draw relationships between the atomic level surface stmcture and the chemical reactivity. Two kinds of materials can be considered i) well defined faces of single-crystal alloys and ii) controlled overlayers of a foreign metal on a metallic substrate. 

The theory of solid state reaction kinetics includes no consideration of surface properties other than the recognition that crystal faces are the most probable location for nucleation in many reactions. Dehydration studies have provided evidence that, in many such processes, all surfaces are modified soon after the onset of chemical change [20,21], This is ascribed to a surface reaction that is limited in extent and can continue only at local sites of special reactivity where the recrystallization required for nucleation is possible. In other decompositions there is evidence that the modified reactivity associated with surfaces may influence the overall reaction [11] and may also preserve the identity of crystals. This, incidentally, masks the occurrence of melting during decomposition [22],... 

The symmetry of a material is also only partially revealed by the shape of etch pits on a crystal surface. These are created when a crystal begins to dissolve in a solvent. Initial attack is at a point of enhanced chemical reactivity, often where a dislocation reaches the surface. A pit forms as the crystal is corroded. The shapes of the pits, called etch figures, have a symmetry corresponding to one of the of 10 two-dimensional plane point groups. This will be the point group that corresponds with the symmetry of the face. An etch pit on a (100) face of a cubic crystal will be square, and on a (101) face of a tetragonal crystal will be rectangular. 

Geometric effects are known firom earlier work on catalysis. One expects that such surface imperfections as steps or terraces would be sites of preferential reactivity. Even well-ordered planes of a single crystal of a metal differ in their reactivity for example, different crystal faces of Pt, which have different arrangements of their surface atoms, catalyze the formation of quite different chemical products. It is therefore reasonable that clusters of different sizes, which are packed differently, differ in their reactivity. We can hope that at some point these differences in reactivity of clusters can be used to design better catalysts. However we must recognize that the atoms comprising small clusters usually do not pack in arrangements characteristic of the bulk. 

Are there further manifestations of a polar axis in crystals Obviously, the morphology,the crystal growth speed,the form of etch figures,and the chemical reactivity of faces are properties that can express a polar symmetry. 

Pressure tends to increase the chemical reactivity of nitromethane as well as the rate of thermal decomposition. It was observed, quite accidentally, that a pressure-induced spontaneous explosion of single crystals of nitromethane at room temperature can occur. Further study revealed that single crystals grown from the liquid with the (111) and either the (001) or the (100) crystal faces perpendicular to the applied load direction in the DAG, if pressed rapidly to over 3 GPa, explode instantaneously accompanied by an audible snapping sound. The normally transparent sample becomes opaque instantly. Visual examination of the residue revealed a dark brown solid which was stable when heated to over 300 C. Subsequent x-ray analysis showed the material to be amorphous. Mass spectral analysis of the residue was inconclusive because no well defined spectra were observed. Because most of the sample is recovered as solid residue after the explosion and is stable to over 300°C, the material may be amorphous carbon. This stress-induced explosion occurs only in protonated nitromethane because similar attempts on the deuterated form did not result in explosion. Shock experiments on oriented pentaerythritol (PETN) crystals have shown similar type behavior [25]. In this case it was suggested that the sensitivity of shock pressures to crystal orientation is the result of the availability of slip planes or system of planes in the crystal to absorb the shock, thereby increasing the threshold to explosion. A similar explanation may be applicable to the nitromethane crystals as well. The deuteration effect must play a role in the initiation chemistry. An isotope effect has been observed previously in the sensitivity of HMX and RDX to shock and thermal conditions [23]. 

Chemisorption, consisting of a chemical reaction confined to the solid surface, does involve rearrangement of electrons of both adsorptive molecules and surface atoms, yielding new surface terminations. Adsorptive and adsorbate being chemically different species in dissociative chemisorption, spectroscopic and/or ab initio modeling methods are required to assess the nature of surface species formed upon contact of the adsorptive with the reactive surface atoms [25, 26, 29]. Further, chemisorption is structure-sensitive in that the features of the process depend on the solid crystal structure (see for instance anatase vs. rutile, [56, 101] and amorphous silica vs. crystalline quartz, [15, 85, 102]) and on the crystal faces exposed by the solid material [103],... 

In bulk form cerium is a reactive metal that has a high affinity for oxygen and sulfur. It has a face centered cubic crystal stmcture, mp 798°C, bp 3443°C, density 6.77 g/mL, and a metallic radius of 182 pm. Detailed chemical and physical property information can be found in the Hterature (1,2). 

Similarly to the kinetics of the electroless etching of GaP in alkaline Fe(CN)6 solutions, the kinetics of the chemical etching of [lll]-oriented GaP depend upon the crystal orientation whereas the etching rate at the (lll)-face is kinetically controlled, the etching rate at the (TTT)-face is diffusion-limited. This difference in reactivity of the GaP-face towards the electron acceptor Br2 is explained by the difference in the Ga-P surface dipole orientation [72]. 

Asymmetric alkylation of carboxylic acid derivatives has been studied intensively for about 20 years. [1] Numerous auxiliaries, tailor-made structures with high steric demands for effective RelSi face differentiation, have been synthesized and their efficiency tested. [1, 2] In recent years besides the preparative aspects of enolates, physico-chemical investigations into their structure-reactivity relationships have gained interest. [3] Crystal structure analyses, osmometric measurements, and NMR studies in solution are helpful in the investigation of the factors that may control enolate reactions. [3-5]... 

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