Surface Catalytic Oxidation Research Articles

Redox processes controlling manganese fate and transport in a mountain stream.

The biogeochemical processes controlling the speciation and transport of manganese in a Colorado mountain stream were studied using a conservative tracer approach combined with laboratory experiments. The study stream, Lake Fork Creek, receives manganese-rich inflows from a wetland contaminated by acid mine drainage. A conservative tracer experiment was conducted on a 1300-m reach of the stream. Results indicate that manganese was progressively removed from the stream, resulting in a loss of 64 +/- 17 micromol day(-1) m(-1). Laboratory experiments using streamwater, mine effluent, and rocks from the stream showed the importance of surface-catalyzed oxidation and photoreduction on the speciation of manganese. The field and modeling results indicate that light generally promotes oxidation and removal of manganese from the stream, presumably through a photosynthetically enhanced oxidation process. Differences in Mn speciation within the stream suggest that reductive processes affect Mn speciation within the water column. These results identify the rapid oxidation and precipitation of MnOx as a dominant process within this freshwater stream.

Stochastic resonance in surface catalytic oxidation of carbon monoxide

Stochastic resonance is a nonlinear cooperative effect between external signal and noise, in which the noise can play a constructive role to increase the signal-to-noise ratio in the detection of a weak signal. A surface catalytic reaction model, to describe oxidization of carbon monoxide carrying out far from equilibrium, was adopted to study the stochastic resonance. By computer simulation, we found noise can induce state-to-state transitions, and stochastic resonance behavior may appear at narrow bistable states or near discontinuous Hopf bifurcations, while a weak periodic signal riding on noise is input controlling.

Catalytic Combustion Effects in Internal Combustion Engines

A review of the brief history of catalytic combustion in internal combustion (IC) engines suggests that catalytic combustion may aid in fuel ignition, but can also cause increased flame-quenching because of surface catalytic oxidation of unburned fuel in the gas boundary layer. The effect of catalytic combustion on heat transfer measurements in engines is also discussed, with the controversial Woschni effect, wherein thermal insulation appears to increase heat rejection from the combustion chamber, being shown to be probably only a manifestation of catalytic combustion.

Oxygen-18 incorporation in NICI mass spectrometry: the role of O 2, H 2O and rhenium oxides in surface-assisted oxidation reactions

Surface-assisted oxidation reactions in the chemical ionization (CI) source have been studied using oxidation yield determinations and oxygen-18 incorporation measurements with 18O 2 or H 2 18O added to the negative ion chemical ionization buffer gas (N 2 or Ar). Oxidation yield measurements show that O 2, but not H 2O, affects the oxidation yield. Using 18O 2, 18O incorporation was found to be consistently lower in the ionized products of surface-assisted reactions compared with the products of gas-phase ion/molecule reactions. In addition, the degree of 18O incorporation was influenced by ion source temperature, with an increase in ion source temperature reducing the degree of incorporation. Although H 2O has a negligible effect upon the efficiency of surface-assisted oxidation reactions, water-derived oxygen is efficiently incorporated by the products of surface-assisted oxidation reactions. In experiments with H 2 18O + O 2 mixtures a correlation between the 18O content of the oxidized products and the 18O content of filament-derived ReO 4 − was observed. This correlation, and the detection of rhenium oxides on ion source surfaces, suggest that filament-derived rhenium oxides may play a role in the activation of ion source surfaces towards the selective surface-catalyzed oxidation of aromatic hydrocarbons.

Oxidation reactions of dibenzothiophene subjected to negative chemical ionization with oxygen

Dibenzothiophene ( 1) suffers surface-catalyzed oxidation under CI(O 2) conditions. While in the positive mode M +· is the only major ion and those of the oxidation products are of minor importance in the negative mode, essentially only ions stemming from oxidized species can be seen, the sulfone ion [M + O 2] −· being the most important one. The ion m/z 184 previously attributed to M −· is actually the anion of 2-sulfobenzoic acid cyclic anhydride. Structures for the various oxidation products are proposed and the mechanisms leading to their formation are discussed.

High‐Temperature Carbon Monoxide Potentiometric Sensor

A high‐temperature potentiometric sensor was developed by coating a mixture of over one of the platinum electrodes of a cell with a yttria‐stabilized zirconia solid electrolyte. It showed good sensing characteristics over the temperature range of 450–550°C and concentration range of 0–10,000 ppm of in air. In particular, high sensitivity coupled with reasonably fast response and baseline recovery characteristics were observed at 450°C for concentrations below 3,000 ppm. The sensing mechanism and operating system were adequately described by a proposed mathematical model. The model relates the induced cell EMF to the various rate phenomena occurring in the oxide layer such as diffusion, adsorption, and surface catalytic oxidation of the adsorbed carbon monoxide by the gaseous oxygen.

Kinetics of surface-catalyzed oxidation of sulphur(IV) by dioxygen in aqueous suspensions of cobalt(II) oxide

A comparative rate analysis of autoxidation of sulphur(IV) in CoO suspension, leachate solution and blank reaction suggests the reaction to be surface catalyzed. In acetate buffers, the experimental rate law is: R obs= R un + ( k 3 + k 4 [H +] )[CoO][S(IV)][O 2] where R un is the rate of uncatalyzed reaction. The suggested mechanism requires the adsorption of both S(IV) and O 2 on the particle surface. Subsequently, the adsorbed species are activated resulting in the oxidation of sulphur( IV), probably through intervention of multiple oxidation states of cobalt.

Kinetic phase transitions in a model for surface catalysis

We consider a kinetic model, recently proposed by Ziff, Gulari and Barshad, that reproduces some features of the surface-catalyzed oxidation of CO; in particular the model shows transitions between poisoned and active states of the catalytic surface. The behavior of the model in mean field theory was studied by Dickman. It becomes analogous to that of known models for equilibrium and non-equilibrium phase transitions when non-reactive desorption of reactants is allowed; this also eliminates spurious stationary states. It then becomes possible to determine the relative stability of the two remaining stable stationary states, and hence to locate the phase transition, by studying an interface between the two phases, using a version of mean field theory for inhomogeneous systems. This new method for locating the phase transition in the context of a mean field treatment is the main new result of our paper. The adequacy of mean field theory can be estimated by comparison with a treatment of Bethe-Peierls type, a variant of mean field theory in which nearest neighbor correlations are taken into account. Mean field treatments seem particularly suitable for obtaining a quick impression of the consequences of changes in the model designed to include further effects known to be important in real catalysts.

An electrode-supported oxidation catalyst based on ruthenium(IV). pH "encapsulation" in a polymer film

The known oxidative catalytic behavior of the ions (bpy)/sub 2/(py)RuO/sup 2 +/ (bpy is 2,2'-bipyridine; py is pyridine) and (trpy) (bpy)RuO/sup 2 +/ (trpy is 2,2',2''-terpyridine) has been successfully transferred to electrode surfaces. The electrodes were chemically modified by deposition of thin polymeric films containing the (bpy)/sub 2/(H/sub 2/0)Ru/sup II/ group bound to poly-4-vinylpyridine. Upon oxidation to Ru/sup IV/ on the surface catalytic oxidation currents are observed for the modified electrodes in the presence of 2-propanol, p-toluic acid, and the xylenes with an added surfactant. The effective catalytic lifetimes of the films are limited ( > 30 turnovers per redox site) by a competing reaction within the polymer film which changes the nature of the redox sites. An unusual pH effect has been observed in the polymer films. Below pH 4, potentials measured by cyclic voltammetry for the bound ((bpy)/sub 2/(OH)Ru/sup II/(py)-)/sup 2 +//((bpy)/sub 2/(OH/sub 2/)Ru/sup III/(py)-)/sup 2 +/ and ((bpy)/sub 2/(O)Ru/sup IV/(py)-)/sup 2 +//((bpy)/sub 2/(OH)Ru/sup III/(py)-)/sup 2 +/ couples change with pH as expected. Above pH 4, where the unbound pyridyl groups in the polymer become deprotonated, up to pH 9.2 the redox couples no longer respond to pH changes in the bulk solution. The difference in pH behaviormore » between the polymer in solution and on the electrode surface has allowed us to observe the deposition process by cyclic voltammetry. In acid solution, the polymer film is open and large amounts of ferri- or ferrocyanide ions can be incorporated reversibly into the films by ion exchange. Over a longer time period, the cyano ions are permanently bound in the films, apparently by the formation of a cyano-bridged Fe-Ru dimer.« less

A computational analysis of a chemical switch mechanism. Catalysis-inhibition effects in a copper surface-catalyzed oxidation

A reaction scheme was designed for the tert.-butyl hydroperoxide-initiated autoxidation of hexadecane at 100/sup o/C, which was inhibited by small amounts of copper and accelerated by copper in excess of a critical ratio of initial hydroperoxide concentration to copper surface area. The model consisted of a homogeneous reaction set of 53 steps based on well-known free-radical oxidation, and a surface reaction set of 34 steps based on a proposed competition for trapping of hydroperoxide at inhibition and catalysis surface sites. A computational procedure for numerical solution of the mass-action differential equations by the Gear integration technique was used. The homogeneous mechanism correctly predicted the observed oxidation rates and insensitivity to initial hydroperoxide concentrations in the absence of copper. The surface mechanism, tested with various initial conditions and rate parameters, gave results in agreement with experiment for the following phenomna: location of the switch point of the mechanism, the shape of the reaction curve, and the insensitivity of the rate to the hydroperoxide concentration above the switch point. Graphs, tables, and 15 references.