Gas-phase species

As with the other surface reactions discussed above, the steps m a catalytic reaction (neglecting diffiision) are as follows the adsorption of reactant molecules or atoms to fomi bound surface species, the reaction of these surface species with gas phase species or other surface species and subsequent product desorption. The global reaction rate is governed by the slowest of these elementary steps, called the rate-detemiming or rate-limiting step. In many cases, it has been found that either the adsorption or desorption steps are rate detemiining. It is not surprising, then, that the surface stmcture of the catalyst, which is a variable that can influence adsorption and desorption rates, can sometimes affect the overall conversion and selectivity. 

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

In order to prepare thin fdms of (SN) on plastic or metal surfaces, several processing techniques have been investigated, e.g., the electroreduction of [SsNs]" salts. Powdered (SN) is prepared by the reaction of (NSC1)3 with trimethylsilyl azide in acetonitrile/ The sublimation of (SN) at 135°C and at pressure of 3 x 10 Torr. produces a gas-phase species, probably the cyclic [SsNs] radical, that reforms the polymer as epitaxial fibres upon condensation/... 

A powerful and readily applied method for the estimation of thermochemical properties for gas phase species is that of group additivity developed by Benson and his coworkers15 21-22. The method is based on the observation that the thermochemical properties of a molecule can be represented as a sum of contributions from the individual groups which make up the molecule. The method of defining groups and arriving at group... 

Measurement of the very low pressures associated with many of the Pu-intermetallics requires the high sensitivity afforded by target collection. To identify the gas phase species which are in the effusates, a mass spectrometer is needed. The excellent agreement between the mass spectrometric and target collection results confirms the predominance of the Pu gaseous species in the sublimation of these intermetallics. 

If the reaction involves gas-phase species and the rate law is expressed in terms of molar concentrations, then instead of K use Kc. 

If the reaction is between two gas-phase species, then this reaction could be a source of cloud condensation nuclei, or simply a means to neutralize an acidic aerosol. Although there are some questions concerning the measurement of atmospheric HNO3, (Lawson, 1988) most measurements indicate that gaseous HNO3 concentrations predominate over particle NOi". 

In these equations the independent variable x is the distance normal to the disk surface. The dependent variables are the velocities, the temperature T, and the species mass fractions Tit. The axial velocity is u, and the radial and circumferential velocities are scaled by the radius as F = vjr and W = wjr. The viscosity and thermal conductivity are given by /x and A. The chemical production rate cOjt is presumed to result from a system of elementary chemical reactions that proceed according to the law of mass action, and Kg is the number of gas-phase species. Equation (10) is not solved for the carrier gas mass fraction, which is determined by ensuring that the mass fractions sum to one. An Arrhenius rate expression is presumed for each of the elementary reaction steps. 

We begin this chapter with a discussion of the variabies that characterize gases. Then we develop a molecular description that expiains gas behavior. Next, we expiore additional gas properties and show how to do stoichiometric caicuiations for reactions invoiving gas-phase species. Finally, we return to the Earth s atmosphere and describe some aspects of its composition and chemicai reactions. 

Translation of the cell left and right permits the achievement of each of the three objectives listed above, using appropriate data subtraction procedures to remove contributions from gas phase species If present during measurements. 

This section treats the plasma physics and plasma chemistry of the typical silane-hydrogen RF discharge, with occasional examples that employ a somewhat higher excitation frequency. Electrical characterization of the discharge is followed by an analysis of the silane chemistry. An appropriate set of gas phase species is presented, which are then used in the modeling of the plasma. A comparison is made between modeling results and experimental work in ASTER. Extension to 2D modeling is presented as well. 

Fig. 16. Variation in a stationary cycling state of catalyst temperature, S03, and complex concentrations in the melt phase and the concentration of gas phase species with time in a half cycle in the forward flow portion of a reactor operating under periodic reversal of flow direction with r = 40 min, SV = 900 h (Csodo = 6 vol%, (Co2)o = 15 vol%, Ta = 50°C. Curves 1, just after switching flow direction 2,1 min 3, 6.6 min 4, 13.3 min, and 5, 20 min after a switch in flow direction. (Figure adapted from Bunimovich et at., 1995, with permission, 1995 Elsevier Science Ltd.)...

The effect of oxidizing atmospheres on the reduction of NO over rhodium surfaces has been investigated by kinetic and IR characterization studies with NO + CO + 02 mixtures on Rh(lll) [63], Similar kinetics was observed in the absence of oxygen in the gas phase, and the same adsorbed species were detected on the surface as well. This result contrasts with that from the molecular beam work [44], where 02 inhibits the reaction, perhaps because of the different relative adsorption probabilities of the three gas-phase species in the two types of experiments. On the other hand, it was also determined that the consumption of 02 is rate limited by the NO + CO adsorption-desorption... 

When a molecule is adsorbed on a surface, the symmetry of the combined adsorbate-substrate system is very likely to be reduced compared to that of the isolated gas-phase species or the bare adsorption site. This raises the possibility that, if mirror planes present in the isolated achiral molecule and those at the relevant... 

A great deal of success was attendant on the early application of PM-IRRAS to the gas/solid interface. Golden et ai (1981) reported the development of instrumentation, using conventional dispersive optics, able to record detailed infrared reflection-absorption spectra from molecules adsorbed on single-crystal Pt without any interference from the gas-phase species. 

In the gas phase species are isolated, far from each other. It follows that their behaviour is not influenced by solvation, as occurs in solution, or by reticular forces, as in the crystalline state. Thus, the gas phase allows the study of the intrinsic properties of a given species, that are only dependent on its chemico-physical properties, i.e. its molecular weight, type of atoms (C, N, O,...) involved, connections between them, etc. 

During steady-state isotopic transient kinetic analysis, the 12CO was switched to 13CO and the carbon-containing adsorbed and gas phase species were monitored in the IR as they exchanged from the 12C to the 13C label. Particular attention was made to those species that exchanged on a timescale similar to that of the exchange of the product C02, as that species could be a likely intermediate to the water-gas... 

Elementary reactions on solid surfaces are central to heterogeneous catalysis (Chapter 8) and gas-solid reactions (Chapter 9). This class of elementary reactions is the most complex and least understood of all those considered here. The simple quantitative theories of reaction rates on surfaces, which begin with the work of Langmuir in the 1920s, use the concept of sites, which are atomic groupings on the surface involved in bonding to other atoms or molecules. These theories treat the sites as if they are stationary gas-phase species which participate in reactive collisions in a similar manner to gas-phase reactants. 

Eley-Rideal reaction, wherein a gas-phase species reacts directly with an adsorbed intermediate without having to be bound to the surface itself thus,... 

Irreversible adsorption The LH mechanisms assume that the adsorption of all gas-phase species is in equilibrium. Some mechanisms, however, occur by irreversible steps. In these cases, the intermediates are again treated in the same manner as reactive intermediates in homogeneous mechanisms. An example is the Mars-van Krevelen (1954) mechanism for oxidation, illustrated by the following two steps ... 

Eley-Rideal mechanisms If the mechanism involves a direct reaction between a gas-phase species and an adsorbed intermediate (Eley-Rideal step, reaction 8.4-5), the competition between the reactants for surface sites does not occur. From equations 8.4-6 and -21, since one reactant does not have to adsorb on a site in order to react,... 

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