Competitive co-adsorption

The bottom spectrum was obtained by cycling the electrode in CO-free SnCl /HjSO solution to ensure formation of a partial Sn adlayer and then replacing the cell contents with CO-saturated solution. The v(C0) band is still observed, which shows that the Sn adatoms do not saturate the surface even in the absence of competitive CO adsorption. The intensity and frequency of the v(C0) band have both decreased, which confirms that the CO adlayer is only partially complete. There is no evidence for a change in v(C0) beyond that expected for the coverage dependence expected in acid solution. This shows that there is no strong interaction between adsorbed CO molecules and neighboring Sn adatoms, in support of the assumptions used in the adatom oxidation model discussed above. 

Very often the liquids to be processed may be contaminated with substances detrimental to some types of zeolites consequently a complete knowledge of the process stream composition and physical properties must be available before preliminary sieve selection can be made. In the absence of prior knowledge of separation factors, competitive co-adsorption, environmental stability, regeneration techniques, or irreversible zeolite contamination, zeoli te contamination, zeolite specification must be proceded by time-con-... 

Table 2 lists some of the inorganic substances that have been determined based on the reduction of the ligand in the complex. In general, the achievable sensitivity and detection limit for this approach are usually not as good as those obtained with the direct reduction of the inorganic substances due to competitive co-adsorption of the ligand. Nevertheless, detection limits in the low nanomolar range are often achieved by this approach. 

Horner et al. have studied the mechanisms by which onium compounds inhibit steel corrosion. Kichigin et al. studied tetra-n-butylammonium cation (TBA ) and other quaternary nitrogen compounds in the presence of iodide, and Aramaki et al. studied TBA+ in the presence of halides, SOj", SCN , SH , NOJ, and N3. In all these cases, halides were shown to be necessary for good inhibitor performance and the observed behavior was consistent with competitive co-adsorption of ion pairs (side by side). Sulfonium derivatives also require the presence of halide ions and, likewise, apparently co-adsorb to form ion pairs on the surface. As discussed below, unsaturated oxygen compounds, such as aldehydes and acetylenic alcohols, also require a halide ion for good performance. 

It is important to note that reaction of COad occurs only at sufficiently high coverages, equivalent to a reduced reaction barrier (see the discussion of CO oxidation on Ru(0001) above). The high coverage is maintained by continuous OHad formation, in competition with re-adsorption of CO. The Pt islands help in maintaining the high coverage via (14.8). Finally, additional CO adsorption on the Pt monolayer islands and reaction with OHad on the Ru(0001) areas may be possible as well, and this would further increase the overall reaction rate. At these potentials, however,... 

All the catalysts contained 1 wt.% platinum, introduced into the zeolite through ion exchange by [Pt(NH3)4)]2+ in competition with NH4+ (NH4+/Pt=100), followed by calcination under dry air flow at 450°C for 4h. The dispersion of platinum, measured using CO adsorption followed by infrared spectroscopy, was about 60%. Whatever the crystallite size, the catalysts were first pelletized, then crushed and sieved to obtain 0.2 - 0.4 mm particles. 

The CsHe desorption was essentially inhibited in the presence of SO2 because sulfur species can react with Fe O radical to form a relatively stable Fe SOs Fe (see Eq. 23), resulting in a significant decline in the density of available adsorption sites for CsH . Simultaneously, the scarcity of a-02 surface species (Fe 02") due to a competitive SO2 adsorption (Eq. 22) leads to a decrease in both rates of propene oxidation and carbonaceous species (CO and CO2) formation. 

Practical considerations and implementation. Most investigations involve the use of distilled/deionised water with KNO3 as the nitrate ion source thereby avoiding any potential impact of water hardness and dissolved salts on the catalytic removal of nitrates. It has been pointed out that in the presence of anions such as S04 and bicarbonates, which may be present in tap-water at concentrations of above 90 ppm, reduced nitrate reduction rates are to be expected as a result of competitive anion adsorption. Pintar and co-workers have indicated that nitrate removal rates are reduced when using drinking water as opposed to distilled water. Chloride ion is known to reduce the rate of nitrate removal while the choice of cation as counter ion influences the rate in the order, < Na < Ca < Mg + <... 

Co-adsorption and mutual interactions between the reactants on the surface form the basis for understanding the microscopic steps of the reaction. Since product formation takes place rather rapidly above room temperature, this information mainly became available from low-temperature studies. As a result, these processes are much more complicated than can be described by a Langmuir-type adsorption model (i.e., simple competition for free adsorption sites) and, moreover, an asymmetric behavior is found which means that pre-adsorbed CO inhibits the adsorption of oxygen, whereas the reverse is not the case. At very low surface concentrations of CO and Oad these will be randomly distributed over the surface as illustrated schematically by Fig. 32a (88). 

It is interesting also to compare the results of the present experiment, which shows directly that a competitive mechanism occurs in the co-adsorption of NO and CO, with previous studies on several surfaces of the platinum group metals. On Pt(lll) and Pt(110), Lambert and Comrie (65) have inferred from thermal desorption data that gaseous CO displaces molecular NO from the surface and causes also a conversion between two thermal desorption states of molecular NO. Similarly, Campbell and White (55) report that adsorbed CO inhibits the oxidation of CO by NO at low temperature on polycrystalline Rh. They attribute this to the occupation of sites by CO which are required for NO adsorption and dissociation. Conrad et al. (66) have used UV-photoelectron spectroscopy to observe directly the displacement of molecular NO by gaseous CO from Pd(110) and polycrystalline Pd surfaces. Thus, it appears that adsorption of molecular CO and NO is competitive on these... 

Methanol strongly dissociatively chemisorbs on platinum surfaces, but the extent of adsorption depends critically on the surface morphology, the crystal face exposed, and the presence of other anions in solution that themselves can competitively co-adsorb. 

The sum of the exponents is close to unity. This equation now resembles eqn. (101) and so supports the idea of an electrochemical exchange mechanism at the platinum surface. However, quite different exponents (0.24 and 0.76, respectively) are predicted by electrode kinetic data [231]. As the concentration of Co(en)3+ rose further, the catalytic rate passed through a maximum because of the effect of competitive reactant adsorption. 

Although there have been several studies of chemisorption of certain molecules of interest, there are fewer studies of co-adsorption. This possibly arises from the increasing complexities of controlling coverage in situations where competitive adsorption may exclude or alter the coverage of the reactants. The goal of co-adsorption studies is to learn about interactions between surface coadsorbates, a subject of obvious importance to their catalytic reaction. It is especially of interest from the view of emission control catalysis to study coadsorption of an oxidant and a reductant. 

Mullins et al. have performed studies of the interaction of CO and NO using a model surface consisting of a low coverage of Rh and a ceria film of controlled oxidation state. Although the co-adsorption of these species is competitive, coadsorption by sequential exposures was performed using controlled sub-saturation doses of CO and NO. Since the adsorption of either CO or NO is strongly affected by the oxidation state of the ceria, it is not surprising that their interaction with each... 

Co-adsorption and competitive interactions within a given mechanism may be accounted for by a linear combination of terms, although higher order and cross-term dependences may also be applicable. 

The TPD data discussed above establish that H and CO compete for adsorption sites on a pel ycrys toil line nickel surface. Impertantly, the presence of titania adspecies leads to more competitive hydrogen adsorption in the presence of coadsorbed CO. This behavior had been suggested earlier by Vannice (18). 

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