Surface species, concentrations

We imply that the activities of the surface species are proportional to their concentrations. Surface species concentrations can be expressed in moles per liter solution, per g (or kg) solid, per cm (or m ) of solid surface, or per mole of solid. Applying the mass law of equation 5 we find... 

Adsorptive stripping analysis involves pre-concentration of the analyte, or a derivative of it, by adsorption onto the working electrode, followed by voltanmietric iiieasurement of the surface species. Many species with surface-active properties are measurable at Hg electrodes down to nanoniolar levels and below, with detection limits comparable to those for trace metal detemiination with ASV. 

Only those components which are gases contribute to powers of RT. More fundamentally, the equiUbrium constant should be defined only after standard states are specified, the factors in the equiUbrium constant should be ratios of concentrations or pressures to those of the standard states, the equiUbrium constant should be dimensionless, and all references to pressures or concentrations should really be references to fugacities or activities. Eor reactions involving moderately concentrated ionic species (>1 mM) or moderately large molecules at high pressures (- 1—10 MPa), the activity and fugacity corrections become important in those instances, kineticists do use the proper relations. In some other situations, eg, reactions on a surface, measures of chemical activity must be introduced. Such cases may often be treated by straightforward modifications of the basic approach covered herein. 

The situation illustrated in Figure 4 allows both species to coexist. Either of the two sets of curves can be considered the oxidized species the other is the reduced species. The choice depends on whether oxidation or reduction is occurring at the surface. Assume the upper curve is the reduced species and the lower curve is its oxidized form. An appHed voltage has maintained fixed surface concentrations for some period of time including and The concentration profile of the oxidized species decreases at the electrode surface (0 distance) as it is being reduced. Electrolysis therefore results in an increase in the concentration of reduced species at the surface. The concentration profiles approach bulk values far from the surface of the electrode because electrolysis for short times at small electrodes cannot significantly affect the concentrations of species in large volumes of solution. 

The effect of Oj, SOj, NOj, HjS, CI2, CO and NH3 on Sn/50%Pb in atmospheres of different relative humidity were investigated but only SO2 and NO2 were active at low concentrations (<100 ppm). An XPS study of Sn/50 7oPb solder exposed to O2, HjO and NO2 was conducted to establish both the surface species formed and the ratio of the concentration of each metal in the surface. Previous XPS studies had only considered the interaction of tin/lead solder with the air . 

Irreversible Unimolecular Reactions. Consider the irreversible catalytic reaction A P of Example 10.1. There are three kinetic steps adsorption of A, the surface reaction, and desorption of P. All three of these steps must occur at exactly the same rate, but the relative magnitudes of the three rate constants, ka, and kd, determine the concentration of surface species. Suppose that ka is much smaller than the other two rate constants. Then the surface sites will be mostly unoccupied so that [S] Sq. Adsorption is the rate-controlling step. As soon as a molecule of A is absorbed it reacts to P, which is then quickly desorbed. If, on the other hand, the reaction step is slow, the entire surface wiU be saturated with A waiting to react, [ASJ Sq, and the surface reaction is rate-controlling. Finally, it may be that k is small. Then the surface will be saturated with P waiting to desorb, [PS] Sq, and desorption is rate-controlling. The corresponding forms for the overall rate are ... 

A typical result for DPV In Fig. 4a shows the presence of two redox couples with peak potentials of 0.25 V and 0.19 V ( lOmV). Similar results have also been obtained with SWV. The relative Intensities of the two peaks vary from sample to sample but are always present with activated electrodes. The similarities between the potentials found for the surface species and for the oxidation of ascorbic acid suggest that an ec catalytic mechanism may be operative. The surface coverage of the o-qulnone Is estimated to be the order of 10 mol cm . It Is currently not possible to control the surface concentration of the o-qulnone-llke species or the oxygen content of the GCE surface. 

Because the second harmonic response is sensitive to the polarizability of the interface, it is sensitive to the adsorption and desorption of surface species and is capable of quantifying surface species concentrations. Furthermore, SHG can be used to quantify surface order and determine surface symmetry by measuring the anisotropic polarization dependence of the second harmonic response. SHG can also be used to determine important molecular-level and electrochemical quantities such as molecular orientation and surface charge density. 

For catalytic reactions carried out in the presence of a heterogeneous catalyst, the observed reaction rate could be determined by the rate of mass transfer from the bulk of the reaction mixture and the outer surface of the catalyst particles or the rate of diffusion of reactants within the catalyst pores. Consider a simple first order reaction its rate must be related to the concentration of species S at the outer surface of the catalyst as follows ... 

Figure 2.6 Variation of the concentration of surface species calculated from the differential equations describing the model for ammonia oxidation.45 Efficient low-energy pathways to products are available through the participation of surface transients present at immeasurably low concentrations under reaction conditions. The NH3 surface concentration is 10 6 ML. (Reproduced from Ref. 45).

A second equation is needed to determine the surface tension as a function of axial position. We adopt the quasistatic assumption that a is a unique equilibrium function of the surface excess concentration, T, even during dynamic events (17). A surface species continuity balance dictates how T varies along the interface. Upon neglect of surface diffusion and for h <1, the steady state form of this balance is... 

The industrial application of Plasma Induced Chemical Vapour Deposition (PICVD) of amorphous and microcrystalline silicon films has led to extensive studies of gas phase and surface processes connected with the deposition process. We are investigating the time response of the concentration of species involved in the deposition process, namely SiH4, Si2H6, and H2 by relaxation mass spectroscopy and SiH2 by laser induced fluorescence. 

At high methanol concentration, such as those employed in an actual fuel cell, the chronocoulometric and ECTDMS results agree with the IR data in that the predominant adsorbed species is C-O. However, the IR duta were obtained at methanol concentrations ranging from 10 2 M to 1 M. At methanol concentrations of 10 2 M the predominant surface species detected by the chronocoulometry and ECTDMS experiments is COH and it is evident that the energy difference between COH,ds and C=Oads must be slight. 

Figure 14.8 shows how the concentrations of the surface species vary with pH in the calculation results. 

Fig. 14.10. Concentrations (mmolal) of surface species on hydrous ferric oxide exposed at 25 °C to a solution containing Ca, SO4, Hg, Cr, As, and Zn, calculated using a sliding pH path.

The promoting and inhibiting species Aj are most commonly aqueous species, but may also be mineral, gas, or surface species. For aqueous, mineral, and surface species, mj is formally the volumetric concentration, in units such as mol cm-3 or mol l-1, but in geochemical modeling we commonly carry this variable as the species molality. Sometimes, especially when Aj is H+ or OH-, mj in this equation is understood to stand for the species activity rather than its molality. For a gas species, mj represents partial pressure or fugacity. 

The example of uranyl reduction shows the utility of this approach. The concentrations of the two surface complexes vary strongly with pH, and this variation explains the observed effect of pH on reaction rate, using a single value for the rate constant k+. If we had chosen to let the catalytic rate vary with surface area, according to 17.12, we could not reproduce the pH effect, even using H+ and OH-as promoting and inhibiting species (since the concentration of a surface species depends not only on fluid composition, but the number of surface sites available). We would in this case need to set a separate value for the rate constant at each pH considered, which would be inconvenient. 

Infrared Measurements. Elementary-step models can be fit to transient gas-phase data (5) which are obtained by the methods described above. The models will then predict the surface intermediate concentration during transients and at steady state. It is clear that it is also important to observe these surface species experimentally, during transients as well as at steady state. Infrared spectroscopy can be used during catalysis in the presence of the gas phase, so it plays an important role in transient studies. 

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