Surface formation

On a fresh surface the metal has a steely lustre but rapidly tarnishes in air as a result of surface formation of oxide and carbonate species. For protection against oxidation the metal is usually stored in a light mineral oil. When made finely divided, eg, on being cut, it can be strongly pyrophoric, and, for this reason is used, as the ferro-alloy mischmetal, in lighter flints and ordnance. Cerium reacts steadily with water, readily dissolves in mineral acids, and is also attacked by alkafl it reacts with most nonmetals on heating. 

The reaction time depends on the quality of the potassium hydroxide employed. An induction period is often observed when older potassium hydroxide samples are used, possibly because surface formation of carbonates reduces the solubility of the salt in acetonitrile. An attempt was made to monitor the cinnamonitrile reaction by GLC, following loss of starting... 

Here A Gx is the free energy of chain break and formation of new bonds Gm is the free energy of chain surface bond formation Gs is the free energy of the surface formation Gex.s is the excessive combinatorial free energy stipulated by different disposition of chain molecules on the surface ziGcom.s is the combinatorial free energy stipulated by different disposition of intermolecular chain surface bonds on chain molecule. The rest of the G terms possess the abovementioned physical sense. Index ( ) relates to the end state of the system. 

The stove pipe is usually driven to sufficient depth (15-60 ft) to protect loose surface formation and to enable circulation of the drilling fluid. This pipe is sometimes cemented in predrilled holes. 

For steel, passivation is achieved by the surface formation of a tough, adherent mixture of oxides. The passive film is primarily gamma-magnetite (y-Fe203) but also contains gamma-hydrated ferric oxide (y-FeOOH). The film thickness is perhaps 15 A to 30 A (angstrom units). 

Kinetic studies of the decomposition of metal formates have occasionally been undertaken in conjunction with investigations of the mechanisms of the heterogeneous decomposition of formic acid on the metal concerned. These comparative measurements have been expected to give information concerning the role of surface formate [522] (dissociatively adsorbed formic acid) in reactions of both types. Great care is required,... 

Choi, J., Ishida, T, Kato, T, and Fujisawa, S., Self-Assembled Monolayer on Diamond-Like Carbon Surface Formation and Friction Measurements, Tribol. Int., Vol. 36, 2003, pp. 285-290. 

The WGS reaction is a reversible reaction, that is, it attains equilibrium with reverse WGS reaction. Thus the fact that the WGS reaction is promoted by H20(a reactant), in turn, implies that the reverse WGS reaction may also be promoted by a reactant, H2 or CO2. In fact the decomposition of the surface formates produced from H2+CO2 is promoted 8-10 times by gas-phase hydrogen. The WGS and reverse WGS reactions can conceivably proceed on different formate sites of the ZnO surface unlike usual catalytic reaction kinetics, while the occurrence of the reactant-promoted reactions does not violate the principle of microscopic reversibility[63]. 

Considering that the standard enthalpies of formation of various oxides, sulfides, and halides of As, Sb, and Bi are very close to each other, the similarity between this value and the standard enthalpy for the surface formation of As(OH)3, A//f = —680 + 20 kJ/mol, was taken as an indication that, indeed, the As redox process on Pt(l 11) involves the formation of hydroxide species [Blais et al., 2001]. For comparison with A/7 for the surface formation of Bi(OH)2, the strategy followed... 

This allows a direct influence of the alloying component on the electronic properties of these unique Pt near-surface formations from subsurface layers, which is the crucial difference in these materials. In addition, the electronic and geometric structures of skin and skeleton were found to be different for example, the skin surface is smoother and the band center position with respect to the metallic Fermi level is downshifted for skin surfaces (Fig. 8.12) [Stamenkovic et al., 2006a] owing to the higher content of non-Pt atoms in the second layer. On both types of surface, the relationship between the specific activity for the oxygen reduction reaction (ORR) and the tf-band center position exhibits a volcano-shape, with the maximum... 

SCHEME 19.1 Surface formate mechanism over Pt/Zr02. 

While the above results appear to strongly favor a surface formate associative mechanism for low-temperature water-gas shift over the Pt/Zr02 catalysts, methods to provide direct support for the mechanism have remained elusive. 

Jacobs, G., Khalid, S., Patterson, P.M., Sparks, D.E., and Davis, B.H. 2004. Water-gas shift catalysis Kinetic isotope effect identifies surface formates in rate limiting step for Pt/ceria catalysts. Appl. Catal. A Gen. 268 255-66. 

A dissociative adsorption of methanol forming surface methoxy groups is suggested as the initial step. This is followed by the slow step, the formation of some form of adsorbed formaldehyde species. Evidence.for the bridged species is not available, experiments with °0 labeled methanol are expected to clarify this. Continued surface oxidation leads to a surface formate group and to carbon monoxide. All the byproducts can be obtained by combination of the appropriate surface species. 

Cuf1101-HC00 The decomposition of formic acid on metal and oxide surfaces is a model heterogeneous reaction. Many studies have since shown that it proceeds via a surface formate species. Thus on Cu 110) adsorbed formic acid is found at low temperature. On heating to 270 K deprotonation occurs, giving rise to the surface formate, which in turn decomposes at 450 K with evolution of H2 and C02- In previous studies, particularly with vibrational spectroscopy, it had been demonstrated that the two C-0 bonds are equivalent and that the symmetry is probably C2v [19]. A NEXAFS study by Puschmann et al. [20] has subsequently shown that the molecular plane is oriented perpendicular to the surface and aligned in the <110> azimuth. 

Figure 7 0 1s NEXAFS from the surface formate species on Cu 110. The E vector is aligned in (a) the <110> azimuth and in (b) the <100> azimuth. After [20].

Figure 8 Photoelectron diffraction data (normal emission) for the surface formate species on (a) Cu 100] and (b) Cu 110). Insets A) The aligned atop site and B) the aligned bridge site. After [51.

Edwards and Schrader—IR investigations support common formate intermediate in water-gas shift and methanol synthesis over Cu/ZnO. Edwards and Schrader,234 using careful reduction procedures (95%N2/5%H2), were able to obtain direct evidence by infrared spectroscopy of the formation of active OH groups on Cu/ZnO, that formed surface formates on the surface of the zinc phase (1576, 1381, 1366, 2970, and 2878 cm-1, respectively for OCO asymmetric, OCO symmetric, and C-H stretching bands) upon exposure to CO. In the presence of CO and H20, the formate intensity initially increased, followed by the production of C02, indicative of water-gas shift. A carbonyl band was also observed at 2093 cm-1. The authors... 

The water-gas-shift (WGS) reaction (HzO + CO -> H2 I C02) on MgO, ZnO, and Rh/CeOz is another example of a surface catalytic reaction that is assisted by gas-phase molecules. It is known that the WGS reaction proceeds via surface formate intermediate (HCOO-), which can be monitored by FT-IR. The behavior of the surface intermediates (HCOO-) (Cat-X in Figure 8.1a) is remarkably influenced by weakly coadsorbed water molecules (A in Figure 8. lb). The characteristic aspect of the WGS reactions on ZnO and Rh/Ce02 are as follows ... 

It was found that the rate constant of the forward decomposition of the surface bidentate formate (DCOO ) to produce D2 and C02 increased from 0.34X10 4 sec-1 under vacuum to 5.3 X10-4 sec-1 under ambient water. Electron donors such as NH3, CH3OH, pyridine, and THF also increased the decomposition rate the rate constants of the forward decomposition of the surface formates at 553 K were determined to be 28.0X10 4, 7.7X10 4, 8.1X10-4, and 6.0X10 4 sec-1 under NH3, methanol, pyridine, and THF vapors (0.4 kPa), respectively. It is likely that the driving force for the forward decomposition of the formate is electron donation of the adsorbed molecule to the Zn ion on which the bidentate formate adsorbs. The reactant-promoted mechanism for the catalytic WGS reaction on ZnO is illustrated in Scheme 8.2. 

The WGS reaction is a reversible reaction that is, the WGS reaction attains equilibrium with the reverse WGS reaction. Thus, the fact that the WGS reaction is promoted by H20 (a reactant), in turn implies that the reverse WGS reaction may also be promoted by a reactant, H2 or C02. In fact, the decomposition of the surface formates produced from H2+C02 was promoted 8-10 times by gas-phase hydrogen. The WGS and reverse WGS reactions conceivably proceed on different formate sites of the ZnO surface unlike usual catalytic reaction kinetics, while the occurrence of the reactant-promoted reactions does not violate the principle of microscopic reversibility. The activation energy for the decomposition of the formates (produced from H20+CO) in vacuum is 155 kJ/mol, and the activation energy for the decomposition of the formates (produced from H2+C02) in vacuum is 171 kJ/mol. The selectivity for the decomposition of the formates produced from H20+ CO at 533 K is 74% for H20 + CO and 26% for H2+C02, while the selectivity for the decomposition of the formates produced from H2+C02 at 533 K is 71% for H2+C02 and 29% for H20+C0 as shown in Scheme 8.3. The drastic difference in selectivity is not presently understood. It is clear, however, that this should not be ascribed to the difference of the bonding feature in the zinc formate species because v(CH), vav(OCO), and v/OCO) for both bidentate formates produced from H20+C0 and H2+C02 show nearly the same frequencies. Note that the origin (HzO+CO or H2+C02) from which the formate is produced is remembered as a main decomposition path under vacuum, while the origin is forgotten by coadsorbed H20. 

Ce02 is contrasted with ZnO the surface formate on CeOz is stabilized by the coexistence of water vapor, where the selectivity to H2 and C02 only increases as a result of suppression of the backward decomposition of formate more than that of the forward decomposition by water vapor. This property of the Ce02 surface was modified by doping with a small amount (0.2 wt%) of Rh. The Rh/Ce02 catalysts have been commonly used as automobile exhaust gas-cleaning catalysts, on which the WGS reaction proceeds. 

In the catalytic WGS reaction on Rh/Ce02, linear OH groups reacted with CO to produce bidentate formates. In vacuum, 65% of the surface formates decomposed backwardly to H20+C0, and 35% of them decomposed forwardly to H2+C02. When water vapor coexisted, 100% of the formates decomposed forwardly to H2+C02 as shown in Table 8.1. The activation energy for the forward decomposition of the formate decreased from 56kJ/mol in vacuum to 33 kJ/mol due to the presence of water(D20) vapor. By addition of a small amount of Rh (0.2 wt%) to Ce02, the rate of the WGS reaction increased tremendously, and the value of the forward decomposition rate constant (k ) was promoted about 100-fold by the coexistence of gas-phase water (Table 8.1). 

Carbocations on Surfaces Formation of Bicyclobutonium Cation via Ionization of Cyclopropylcarbinyl Chloride over NaY Zeolite... 

AntimonyCni), surface formation, 30 113 Antimony-oxygen system, 30 101 Apolar solute, 32 432... 

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