Heterogeneous chemistry

Heterogeneous reaction at the interface between a solid surface and the adjacent gas is central to many chemical processes. Examples include deposition or etching of materials, atmospheric corrosion, combustion of solids, and heterogeneous catalysis. 

For homogeneous gas-phase kinetics one may incorporate arbitrarily complex reaction mechanisms into the mass and energy conservation equations. Aside from questions of units, there is almost no disagreement in the formulation of the elementary rate law the rate of progress of each reaction proceeds according to the law of mass action. The CHEMKIN software [217] is widely used in the kinetics community to aid in the formulation and solution of gas-phase kinetics and transport problems. 

The Surface Chemkin formalism [73] was developed to provide a general, flexible framework for describing complex reactions between gas-phase, surface, and bulk phase species. The range of kinetic and transport processes that can take place at a reactive surface are shown schematically in Fig. 11.1. Heterogeneous reactions are fundamental in describing mass and energy balances that form boundary conditions in reacting flow calculations. 

Generally, there are three types of chemical species that we must account for in describing heterogeneous reactions species in the gas phase, species residing at the interface of the gas and the solid, and species residing within the bulk solid (i.e., below the gas-surface interface). Ultimately we must describe the production or destruction rates of all chemical species in the system. 

In gas-phase chemistry it is straightforward to specify the concentrations of all of the chemical species, such as by a single array of the species mole fractions, which sums to unity. The situation can be much more complex in heterogeneous reactions. For example, there may be multiple, distinct solid phases, or different types of surfaces or materials all present simultaneously. The formalism that we describe is a very general and systematic way to account for the different groupings and normalization constraints among many col- 

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D. Avnir, ed.. The Fractal Approach to Heterogeneous Chemistry, Wiley, New York, 1989. 

Avnir D (ed) 1989 The Fractal Approach to Heterogeneous Chemistry Surfaces, Colloids, Polymers (New York Wiley)... 

Heterogeneous chemistry occurring on polar stratospheric cloud particles of ice and nitric acid trihydrate has been estabUshed as a dorninant factor in the aggravated seasonal depletion of o2one observed to occur over Antarctica. Preliminary attempts have been made to parameterize this chemistry and incorporate it in models to study ozone depletion over the poles (91) as well as the potential role of sulfate particles throughout the stratosphere (92). 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

Atmospheric particulates (sea salt, carbonaceous soot, and sulfuric acid aerosols) are known to provide a condensed phase for conq>lex heterogeneous chemistry to occur. Although the presence of atmospheric particulates are known to alter trace gas concentrations, details of the specific chemical mechanisms for condensed phase chemistry have not been identified. 

When NMHC are significant in concentration, differences in their oxidation mechanisms such as how the NMHC chemistry was parameterized, details of R02-/R02 recombination (95), and heterogenous chemistry also contribute to differences in computed [HO ]. Recently, the sensitivity of [HO ] to non-methane hydrocarbon oxidation was studied in the context of the remote marine boundary-layer (156). It was concluded that differences in radical-radical recombination mechanisms (R02 /R02 ) can cause significant differences in computed [HO ] in regions of low NO and NMHC levels. The effect of cloud chemistry in the troposphere has also recently been studied (151,180). The rapid aqueous-phase breakdown of formaldehyde in the presence of clouds reduces the source of HOj due to RIO. In addition, the dissolution in clouds of a NO reservoir (N2O5) at night reduces the formation of HO and CH2O due to R6-RIO and R13. Predictions for HO and HO2 concentrations with cloud chemistry considered compared to predictions without cloud chemistry are 10-40% lower for HO and 10-45% lower for HO2. 

Atmospheric aerosols have a direct impact on earth s radiation balance, fog formation and cloud physics, and visibility degradation as well as human health effect[l]. Both natural and anthropogenic sources contribute to the formation of ambient aerosol, which are composed mostly of sulfates, nitrates and ammoniums in either pure or mixed forms[2]. These inorganic salt aerosols are hygroscopic by nature and exhibit the properties of deliquescence and efflorescence in humid air. That is, relative humidity(RH) history and chemical composition determine whether atmospheric aerosols are liquid or solid. Aerosol physical state affects climate and environmental phenomena such as radiative transfer, visibility, and heterogeneous chemistry. Here we present a mathematical model that considers the relative humidity history and chemical composition dependence of deliquescence and efflorescence for describing the dynamic and transport behavior of ambient aerosols[3]. 

INTERACTIONS OF VIBRATION ALLY-EXCITED MOLECULES AT SURFACES A PROBE FOR ELECTRONICALLY NONADIABATIC EFFECTS IN HETEROGENEOUS CHEMISTRY... 

An important aspect of studying metastable dissociation of these clusters is that the measurements enable a determination of the surface composition of mixed systems. This is important in designing experiments to study the heterogeneous chemistry of aqueous systems. For example, the loss channel of H20 is found to be open to all (H20)n(CH30H)mH+ except (H20)(CH3OH)mH+ for which the water loss is relatively small. For the water-rich composition mixed clusters, the results show that water molecules have a tendency to build a cage structure in the cluster size region m + n = 21, with 0 < m < 8. 

B. Sapoval, M. Rosso, and J. -F. Gouyet, in The Fractal Approach to Heterogeneous Chemistry, Ed. by D. Avnir, Wiley, New York, 1989, p. 227. 

Daoud M, Martin JE (1989) In Avnir D (ed) The fractal approach to heterogeneous chemistry. WUey, New York... 

Feder, J., Fractals Physics of Solids and Liquids, Plenum Press, New York 1988 Avnir, D., Ed., The Fractal Approach to Heterogeneous Chemistry, John Wiley Sons, New York, 1989. 

Avnir, D. (1989) The fractal approach to heterogenous chemistry. J. Wiley, New York Avnir, D. Farin, D. Pfeifer, P. (1983) Chemistry in noninteger dimension between two and three. II. Fractal surfaces at adsorbens. J. Chem. Phys. 79 3566-3571 Avotins, P.V. (1975) Adsorption and coprecipitation studies of mercury on hydrous iron oxide. Ph.D. Thesis, Stanford University, California, 124 p. 

Farin, D. Avnir, D. (1989) The fractal nature of molecule-surface interactions and reactions. In Avnir, D. (ed.) The fractal approach to heterogenous chemistry. Wiley, New York, 271-294... 

De Haan, D. O., T. Brauers, K. Oum, J. Stutz, T. Nordmeyer, and B. J. Finlayson-Pitts, Heterogeneous Chemistry in the Troposphere Experimental Approaches and Applications to the Chemistry of Sea Salt Particles, Int. Rev. Phys. Chem., in press (1999). 

Stelson, A. W and J. H. Seinfeld, Chemical Mass Accounting of Uban Aerosol, Environ. Sci. Technol., 15, 671-679(1981). Stickel, R. E., J. M. Nicovich, S. Wang, Z. Zhao, and P. H. Wine, Kinetic and Mechanistic Study of the Reaction of Atomic Chlorine with Dimethyl Sulfide, J. Phys. Chem., 96, 9875-9883 (1992). Swartz, E J. Boniface, I. Tchertkov, O. V. Rattigan, D. V. Robinson, P. Davidovits, D. R. Worsnop, J. T. Jayne, and C. E. Kolb, Horizontal Bubble Train Apparatus for Heterogeneous Chemistry Studies Uptake of Gas-Phase Formaldehyde, Environ. Sci. Technol, 31, 2634-2641 (1997). 

In short, although relatively little is known about the possibility of heterogeneous chemistry of organics in the upper troposphere, the results of initial laboratory studies suggest that this may be important. 

Miller, T. M., and V. H. Grassian, "Heterogeneous Chemistry of N02 on Mineral Oxide Particles Spectroscopic Evidence for Oxide-Coordinated and Water-Solvated Surface Nitrate, Geophys. Res. Lett, 25, 3835-3838 (1998). 

Zhang, R., M.-T. Leu, and L. F. Keyser, Heterogeneous Chemistry of HONO on Liquid Sulfuric Acid A New Mechanism of Chlorine Activation on Stratospheric Sulfate Aerosols, . /. Phys. Chem., 100, 339-345 (1996). 

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