Surface reduction

G.G. Amatucci et al. Bellcore, A. Blyr (University ofAmiens, France) Higher temperature performance of LiMn204 after surface reduction and treatments, Ref. 18], Abstract 870. 

The stepwise heating/cooling cycle was conducted on the Ti02 wafer (8H rutile after being heat treated in O2) and a typical correlation of 0/Ti ratio versus temperature is shown in Figure 6 for one particular run. Surface reduction is facilitated by ESD, and additional cycles continually reduced both maximum 0/Ti ratio obtained at high temperature and the minimum 0/Tl ratio observed at room temperature, and a final value of 0.7 was measured at the com-... 

Daturi, M., Finocchio, E., Binet, C. et al. (1999) Study of bulk and surface reduction by hydrogen of CexZr, x02 mixed oxides followed by FTIR spectroscopy and magnetic balance, J. Phys. Chem. B, 103, 4884. 

Lavalley and coworkers—detailed characterization of impact of Ce surface reduction on the nature of the adsorption of H20, H2, CH3OH, CO, and C02 observed by IR spectroscopy—formates observed adsorption of CO on partially reduced ceria. 

Type II intensity increased upon surface reduction due to the formation of anion vacancies. 

In 1999, Binet et al.395 published a review on the response of adsorbed molecules to the oxidized/reduced states of ceria. In light of recent infrared studies on ceria, the assignments for OH groups, methoxy species, carbonate species, and formates are highly instructive. The OH and methoxy species have been briefly discussed. Characteristic band assignments of carbonate and formate species are provided below, the latter formed form the dissociative adsorption of formic acid, the reaction of CO with H2-reduced ceria surface, or via selective oxidation of methanol. Formate band intensities were a strong function of the extent of surface reduction of ceria. 

Table 74 Linear combination fits of XANES spectra with Ce3+ and Ce4+ reference compounds. Note that complete surface reduction (bold) is complete at lower temperatures with increasing metal loading. Ceria BET SA = 125 m2/g428 ... 

Li et al. reported first on the decoration of hydrothermal carbon spheres obtained from glucose with noble metal nanoparticles [19]. They used the reactivity of as-prepared carbon microspheres to load silver and palladium nanoparticles onto then-surfaces, both via surface binding and room-temperature surface reduction. Furthermore, it was also demonstrated that these carbon spheres can encapsulate nanoparticles in their cores with retention of the surface functional groups. Nanoparticles of gold and silver could be encapsulated deep in the carbon by in situ hydrothermal reduction of noble-metal ions with glucose (the Tollens reaction), or by using silver nanoparticles as nuclei for subsequent formation of carbon spheres. Some TEM images of such hybrid materials are shown in Fig. 7.4. 

Carbon-supported platinum (Pt) and platinum-rathenium (Pt-Ru) alloy are one of the most popular electrocatalysts in polymer electrolyte fuel cells (PEFC). Pt supported on electrically conducting carbons, preferably carbon black, is being increasingly used as an electrocatalyst in fuel cell applications (Parker et al., 2004). Carbon-supported Pt could be prepared at loadings as high as 70 wt.% without a noticeable increase of particle size. Unsupported and carbon-supported nanoparticle Pt-Ru, ,t m catalysts prepared using the surface reductive deposition... 

As with methanol desorption, a net weight loss was observed for the FeHo catalyst after ammonia desorption. This was caused by oxidation of the ammonia substrate to nitrogen and consequent catalyst reduction. The relative number of oxygen atoms removed was ca. 20 less than with methanol surface reduction. 

Pihl et al. (2006) explained the absence of NH3 as a by-product of the NOx reduction during the first part of the regeneration phase by a surface-reduction front moving downstream the reactor, in front of which the formed NH3 can be re-oxidized back to N2. Thus, an NH3 peak in the exhaust is expected to occur after the surface-reduction front reaches the monolith outlet. Cumaranatunge et al. (2007) proved experimentally that ammonia is an active intermediate in the regeneration of NSRC with H2., i.e. H2 can react with NOx producing NH3, which in turn is able to reduce the remaining NOx stored downstream the reactor. When NH3 is used directly at the reactor inlet instead of H2, the NOx reduction process is equivalent and equally effective. 

The analogy goes further if the activity of NiO as an oxidation catalyst is examined closely. In the case of the H2-D2 exchange reaction on ZnO, there was an activation period corresponding to a surface reduction. For NiO, there is a deactivation period also corresponding to surface reduction during the catalytic oxidation of carbon monoxide. 

Different but still comparatively easy to understand is the reaction over the Sn/Sb catalyst. If it is in excess, the conversion of butene—90% to butadiene—is constant and independent of the composition indicating that reduction is fast and limited by the amount of oxygen available on the surface. Isomerization is absent, further proof of the speed of the reduction. At lower hydrocarbon pressures the rate is proportional to butene pressure. Consumption of. is never complete, showing that the reoxidation is slow, and it is only when 02 is in excess that oxygen consumption manages to keep pace with reduction. Here we encounter a situation where the surface reduction is fast, as it should be according to... 

The vast majority of reactions studied with the Hg electrode are reductions. At a Pt surface, reduction of H1 competes with reduction of many analytes ... 

In these reactions, metallic nickel is formed under conditions of net carbon deposition, indicative of mobility of one (or both) elements during the catalytic process. When small amounts of hydrogen were included in the gaseous reactant mixtures, the values of (log A, E) were significantly closer to the compensation line characteristic of reactions on the metal, as expected for displacement of catalyst equilibria in the direction of an increase in the area of the active metal phase present, presumably due to surface reduction. 

The kinetic data obtained were interpreted on the basis of a redox mechanism, with two main steps (1) dissociative adsorption of 02 (surface oxidation) and (2) interaction of NH3 with oxygen adsorbed in atomic form (surface reduction). Both steps are complex, comprising several elementary reactions. This mechanism is again consistent with the rate equation (16.1). 

Agglutination of RBCs by polycations (e.g. polylysines) involves electrostatic attraction between the positively charged lysine groups of the polylysine molecule and the negatively charged N-acetylneuraminic acid on the RBC surface. Reduction of RBC surface charge by neuraminidase treatment markedly decreases the effectiveness of RBC agglutination by polylysine. 

Because this is a surface reduction, the peaks are similar to those for adsorption. 

Hydrogen is also a species that can produce surface reduction of transition metal oxides by attacking the most reactive surface oxygen species, with formation of water and reduced metal centers. 

The products of reduction of salt anions are typically inorganic compounds like LiF, LiCl, Li20, which precipitate on the electrode surface. Reduction of solvents results, apart from the formation of volatile reaction products like ethylene, propylene, hydrogen, carbon dioxide, etc., in the formation of both insoluble (or partially soluble) components like Li2C03, semicarbonates, oligomers, and polymers.281 283 359 A combination of a variety of advanced surface (and bulk) analytical tools (both ex situ and in situ) is used286-321 332 344 352 353 360-377 to gain a comprehensive characterization... 

He found that below 573 K, the rate of surface reduction of C03O4 is much slower than the rate of CO. oxidation. Thus, participation of Ol in CO oxidation at 488 K is unlikely. If Ol were consumed in CO oxidation at steady state, one would expect regeneration of 0 to be fast. Experimental evidence suggests the opposite, l.e. that catalyst regeneration is slow. Formation of mono-dentate carbonate ([Ol] C02 in step 2) along with bidentate carbonate had been spectroscopically observed on C03O4 surfaces upon CO adsorption by Hertl (6) and Goodsel (7). Formation of these species rather than carbonyl type species indicate the roles played by O and 0qo. Step 2 could proceed via an intermediate step in which bidentate carbonate is formed as shown ... 

Although the surface models for anatase and rutile, as proposed by different authors, are idealized and differ from each other in details, it can certainly be concluded that coordinatively unsaturated Ti4+cations, O2- ions, and OH groups in widely varying configurations should be exposed on partially hydrated and/or hydroxylated surfaces. Depending on the local environments of these sites, a wide spectrum of possible intermolecular interactions should be the consequence which may render specific adsorption processes possible. Finally, the ease of the surface reduction of titanium dioxides due to hydrocarbon contamination (19) leads to the formation of new types of surface sites and to drastic changes of the surface properties. 

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