Adsorption autocatalytic

It is considered in adsorption-autocatalytic theory that the interaction happens at the interface of gas-solid. In some cases, it is foimd that of the auto-catalytic phenomenon happens at the early stages of reduction. The value of this theory indicates the necessity of direct contact of the reductant with metal oxides. It is possible to evaluate the mechanism and kinetics of reduction by use of law of physical chemistry, physics and surface chemistry. The great effect of product (H2O) on the rate of reaction confirms the important role of adsorption. Because H2O is a very active adsorbent, it can occupy the most active areas of oxide, and thus greatly reduce the rate and degree of reduction. 

Whereas other metal salts, especially lead stearates and srdfates, or mixtures of Groups 2 and 12 carboxylates (Ba—Cd, Ba—Zn, Ca—Zn) ate also used to stabilize PVC, the tin mercaptides are some of the most efficient materials. This increased efficiency is largely owing to the mercaptans. The principal mechanism of stabilization of PVC, in which all types of stabilizers participate, is the adsorption of HCl, which is released by the PVC during degradation. This is important because the acid is a catalyst for the degradation, thus, without neutralization the process is autocatalytic. 

NO, the monomer C is CO, and the products are A2 = N2 and CB = CO2. The adsorption probability of C species (Fc) is the parameter of the model. The slow rate-determining step in this sequence is the dissociation of NO which requires a neighboring site to proceed. Since product formation liberates more vacant sites than those necessary for the dissociation of NO, an autocatalytic production of vacant sites takes place. 

The decomposition behavior of formic acid on the close-packed Ru(lOTO) surface parallels the reaction on nickel, except that the autocatalytic process was not observed (lOJ). Water was desorbed at 183 K by apparent second-order kinetics following adsorption of HCOOH at 100 K. Subsequent desorption of Hj, COj, and CO suggested the formation of the surface anhydride. The rate constant for decomposition was 2.6 x 10 sec exp —26.9 kcal/mol// r. ... 

Using the assumption about the equilibrium adsorption of oxygen, Eigenberger represented this mechanism by the still simpler autocatalytic scheme... 

The Wicke and Eigenberger models are models for an ideal adsorption layer. They have been examined at the Institute of Catalysis, Siberian Branch of the U.S.S.R. Academy of Sciences [93-104,108,109] independently of Wicke and Eigenberger (the first publications refer to 1974). It was shown [93-96] that, for the detailed mechanisms of catalytic reactions either with the steps that are linear with respect to intermediates or with non-linear steps (but containing no interactions between various intermediates), the steady state of the reaction is unique and stable (autocatalytic steps are assumed to be absent). Thus the necessary condition for the multiplicity of steady states is the presence of steps for the interaction between various intermediates in the detailed reaction mechanism [93-100]. Special attention in these studies was paid to the adsorption mechanism of the general type permitting the multiplicity of steady states [102-104]... 

Mechanistically, the transformation of the N-NDR into an HN-NDR can be explained by the fact that adsorbed halides inhibit the dissociative adsorption of H2O2. The decrease in the reaction current due to the loss of PtOH or the formation of upd-H upon a negative voltage shift is overcompensated by the increase in current density due to the desorption of halide ions. Sustained periodic oscillations appear under potentiostatic as well as galvanostatic conditions in the presence of halides [57] (Fig. 23). The oscillations that are associated with the NDR in the upd-H region were termed oscillations D, those connected to the autocatalytic adsorption of H2O2 oscillations C. 

The existence of bistability in the //under conditions under which chemical variable, on which the current depends, exhibits bistability as a function of DL. Thus, in S-NDR systems we have to require that the dynamic equations contain a chemical autocatalysis. As set forth below, m takes the role of the negative feedback variable. The positive feedback might be due to chemical autocatalytic reaction steps as is the case in Zn deposition [157, 158] or CO bulk oxidation on Pt [159], S-shaped current-potential characteristics may also arise in systems with potential-dependent surface phase transitions between a disordered (dilute) and an ordered (condensed) adsorption state due to attractive interactions among the adsorbed molecules. 

In the mid 1970s, Falconer and Madix observed a surface- kinetic explosion for the decomposition of formic acid (HCOOH) [23] and acetic acid (CH COOH) [24] on the Ni(llO) surface, characterized by very narrow product desorption peaks in TPRS. Such autocatalytic reactions have also been observed in the decomposition of acetic acid on Pd(llO), Rh(llO), Rh(lll), and even supported Rh catalyst by Bowker et al. [70-75]. In general, these reactions exhibit accelerations in rate as the reaction proceeds to completion. Earlier work hypothesized that decomposition of the carboxylate species formed following adsorption of the acids on the surface was initiated at vacancies (i.e. bare metal sites) and propagated by the further creation of vacancies as the products desorbed from the surface [23, 24]. The rate of decomposition was well described by the rate equation r = -k(C / Cj )(Cj - c+/Cj), in which C is the instantaneous surface concentration of carboxylate, C, is the initial surface concentration, and/is the density of initiation sites. Since the decomposition produced an ever-increasing concentration of vacant sites, a kinetic explosion occurred. 

Aas N, Bowker M (1993) Adsorption and autocatalytic decomposition of acetic-acid on Pd(llO). J Chem Soc Faraday Trans 89 1249... 

Autocatalytic deposition can easily be utilized to completely coat a nonconductive surface starting from few catalytic nuclei usually consisting of palladium or similar. It can also start on a glass surface by simple adsorption of hydrolyzed nickel ions on the surface. 

Autocatalytic deposition of nickel and other metals was investigated by means of the electrochemical techniques.1 3 For this purpose, both steady-state and transient methods were applied. The results of these studies give important information about the reactions occurring during the autocatalytic deposition and about the surface where the intermediate species of reaction are stabilized by adsorption. 

A careful consideration of all the above points suggests that the mechanism based on intermediate hydrolyzed species is clearly more heuristic.1,5,6,15 This approach explains the importance of the adsorption of colloids at the surface at which the autocatalytic deposition takes place. 

Figure 10 Photographic development mechanism. The reduction potential, E"(Ag+/Agn), ofthe latent image clusters, when in contact with a solution. Increases with the number of atoms n. Therefore a nuclearity threshold for developmen t is created by the redox poten tial of the developer E°(CP/D). Above the critical nuclearity n, the potential E°(Ag yAg ) is higher than E°(CA/D), and alternate electron transfer toward A g, and Ag adsorption on Err allows the cluster to grow autocatalytically. On the contrary, when l"(Ag, /Agg is lower than E°(E>-/D), corrosion ofsubcritical clusters takes place by oxidizing molecules, such as D or Ox [7],...

Because amino groups act autocatalytically (15-17) in the presence of water, for acid catalysis an excess of HC1 was used to overcompensate the formation of -NH3+C1 . In these cases, the gels were washed with methanol and water until no Cl" could be detected in the filtrate. How far the incorporation of amino groups into silica could affect the adsorption of acid components was of interest. Lactic acid and a sulfonic acid (a commercially available dye named Telon Light Yellow) were chosen as test components (18). In Figure 7 the adsorption isotherm of lactic acid is shown. Unmodified Si02 does not have remarkable adsorption in aqueous solution under these circumstances. The result shows the effect of the amino modification quite clearly, because the lactic acid load of the adsorbent is remarkable, and it is difficult to adsorb small water-soluble molecules in an aqueous environment. 

Even though the Ca is there it does not poison acetate formation, which appears to occur with a similar adsorption probability, but it does stabilise it towards decomposition. In fact the desorption at 455K, occurring in the presence of Ca, is what is known as a surface explosion [8], an autocatalytic decomposition, showing a very narrow half-width for the peak and anomalous desorption kinetics. 

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