Oxide surfaces saturation

Above room temperature a fraction of the hydrogen taken up is slowly adsorbed. This fraction increases with temperature. A zinc oxide surface saturated in hydrogen at 1 atm. at 300°C. and cooled down to 0°C. in hydrogen is covered with hydrogen to the extent of 6 to 45% depending on the nature of pretreatment (4). The hydrogen taken up above room temperature is also reversibly adsorbed. 

Confusion in SMO literature can arise because there is no generally accepted method for determining surface density. As the metric that characterizes the surface oxide of supported metal oxide catalysts, surface density allows one to consider the various structures of the surface oxide on a common scale, independent of total oxide content, preparation method, calcination treatment, and surface area of the support oxide. Surface saturation and monolayer coverage are important threshold surface density values, at which surface oxide crystals form and at which complete consumption of surface hydroxyl groups of the support oxide occurs, respectively. Inconsistencies in these values come about because of (1) differences in their definitions, (2) difficulties in compatibilizing data from different characterization techniques, and (3) the use of support surface area instead of the overall composite SMO. These inconsistencies can make structural comparison of the same SMO composition, such as WO /ZrOj, difficult across different research groups. Calculated properly, however, the surface density metric provides the most simple and useful basis for understanding the relationship between surface nanostructure and catalytic and surface properties. 

Desorption of reversibly and irreversibly adsorbed C02 is accelerated by the removal of adsorbed oxygen with H2. The adsorption isotherm for C02 obtained on an oxidized surface reaches saturation for Pco2=0.15 atm. The addition of Pco2=... 

While it is well established that HO—ONO can be involved in such two-electron processes as alkene epoxidation and the oxidation of amines, sulfides and phosphines, the controversy remains concerning the mechanism of HO-ONO oxidation of saturated hydrocarbons. Rank and coworkers advanced the hypothesis that the reactive species in hydrocarbon oxidations by peroxynitrous acid, and in lipid peroxidation in the presence of air, is the discrete hydroxyl radical formed in the homolysis of HO—ONO. The HO—ONO oxidation of methane (equation 7) on the restricted surface with the B3LYP and QCISD methods gave about the same activation energy (31 3 kcalmol" ) irrespective of basis set size . ... 

Charge alteration on the surfaces of nanosized metallic silver particles has been investigated by simultaneously monitoring absorption and conductivity changes during pulse-radiolytic experiments [506]. Pulse radiolysis of a nitrous-oxide-(N20) saturated aqueous solution of 3.0 nm diameter metallic silver particles containing 0.2 M 2-propanol resulted in electron injection to the colloid. NzO functions to double the yield of hydroxyl radicals ( OH) generated in water... 

The work of Stone, and Tiley (61) on cupric oxide surfaces is also interesting from an electronic view point. They found that the amount of oxygen adsorbed on this oxide, after its previous saturation with carbon monoxide, corresponded to the ratio in agreement... 

Surface modification of LDPE film can also be brought about by chemical treatment [118] with an aqueous solution of ammoniacal ammonium persulphate in the presence of Ni+2 ions under variable reaction conditions. The investigation of treated surface showed the presence of polar groups (viz. carbonyl and hydroxyl) in the infrared (IR) spectroscopy, with characteristic bands at 1700, 1622 and 3450 cm-1. It is known that the persulphate ion attacks the double-bond-producing epoxy or diol group. However, the destructive oxidation of saturated hydrocarbons does not occur with persulphate alone, but requires the presence of the nickel (II) ion. The authors have proposed the following mechanism of chemical treatment ... 

Devlin JF, Morkin M, Repta C. Incorporating surface saturation effects into iron wall design calculations. In Wickramanayake GB, Gavaskar AR, Chen ASC, eds. Chemical Oxidation and Reactive Barriers Proceedings of the Second International Conference on Remediation of Chlorinated and Recalcitrant Compounds, Monterey, CA, 20-25 May 2000. Columbus, OH Battelle Press, 2000 C2 6 393 400. 

In conclusion, the use of amines for the characterization of adsorption and active sites raises the same problems as ammonia. The interaction of the amines, saturated aliphatic amines in particular, with oxide surfaces is certainly still less specific than that of ammonia because of their higher basicity and their larger molecular size. The influence of steric effects on amine adsorption has been discussed (172, 201b). Thus, Medema et al. (172) came to the conclusion that the adsorbed amount of an amine on 7-Al2 03 primarily depends on its molec-... 

Watzlaf, G. R. 1992. Pyrite oxidation in saturated and unsaturated coal waste. In Proceedings of 9th National Meeting of the American Society for Surface Mining and Reclamation. June 14-18,1992. Duluth, MN, pp. 191-205. 

Polyphosphates also play a role in the chemistry of natural metal oxides. For example, if the concentration of polyphosphates is smaller than that needed to saturate the available sites of a solid oxide surface, their presence enhances the sorption of surrounding metal ions by contrast, if their concentration exceeds that needed to saturate the surface, their presence decreases metal ion sorption due to two effects ... 

An oxide surface in water is covered with a layer of highly structured, chemisorbed water molecules (34, 35). When exposed to an electric field caused by charging of the surface, these water molecules approach dielectric saturation. As a consequence, the dielectric strength (e) of the solvent within the compact layer of the interface is lowered. The dependency of log K (stability constant) at constant temperature varies linearly with the reciprocal of the dielectric constant (e) of the media (36, 37), resulting in a displacement of the equilibrium towards the adduct in association reactions (32). As e decreases, the stability of complexes... 

The model system comprising the reaction sequence from initial 0-atom attack to steady-state erosion of a hydrocarbon surface can serve as a benchmark for fundamental atom-surface interactions at hyperthermal collision energies and for etching mechanisms of materials. Within this model system, there is still much to learn. It is likely that when a hyperthermal oxygen atom strikes a saturated hydrocarbon surface, it will either abstract a hydrogen atom or it will scatter inelastically. The subsequent reaction sequence becomes murky. Very little is known about the mechanisms of oxidation, surface roughening, or material loss. In fact, even the sticking... 

The apparent saturation capacity of an oxide surface for hydrogen adsorption at a given temperature and the large change to a new apparent saturation at another temperature, facts familiar to all who have studied the slow sorption processes on oxides, should be re-studied in reference to Volkenstein s assumption that the sites available for adsorption, the thermal sites, vary with temperature. On this view the measurements of Shou-Chu Liang and the writer would gain new significance. In brief, measurements of slow sorption on oxide surfaces need to be... 

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