Adsorption/reaction, simultaneous

Also, to measure properly the kinetics of a reaction, the technique should not alter the reactant concentration significantly (Zasoski and Burau, 1978). Thus, the sample and the suspension should have a similar solid to solution ratio at all times. Unfortunately, this has not been the case in most batch studies (Barrow, 1983). Most kinetic batch studies involving soil constituents have used large solution soil ratios where the concentration in the solution and the quantity of adsorption vary simultaneously. 

In the following analysis, adsorption will be discussed first, with emphasis on ehemisorption. Adsorption can occur without reaction, and the mathe-matieal treatment of nomeactive adsorption will lead naturally to the analysis of simultaneous adsorption, reaction, and desorption. 

Catalyst deactivation in the form of coking, during the hydrodechlorination of 1,2-dichloropropane (12DCP) over Pt-Cu/C catalysts, has been investigated with a novel microbalance-TEOM. The amount of coke deposition on both fresh and spent catalysts has been continuously measured. The activity and selectivity to propene decreases with an increase in the coke formation. The catalyst deactivation is mainly attributed to coke deposition. The TEOM technique with a well-defined gas-phase system, a good mass resolution, and a long-term stability is uniquely suitable to study simultaneous adsorption, reaction, and coke deposition. 

Since Rupprecht and Patashnick developed the TEOM 1500 series analyzer, some work associated with this technique has been reported worldwide. For instance, simultaneous measurements of adsorption, reaction, and coke formation were investigated by Hershkowitz and Madiera [5], Chen et al. [6-9], and Liu et al. [10]. Rekoske and Barteau [11] and Petkovic and Larsen [12] used this technique to investigate catalytic kinetics. 

The objective of this paper is to measure, with the TEOM technique, simultaneous adsorption, reaction, and coke deposition during the hydrodechlorination of 12DCP over Pt-Cu/C catalysts and to investigate whether catalyst deactivation is due to coke deposition. 

The TEOM is a unique technique that is suitable for simultaneous measurements of adsorption, reaction, and coke formation. The adsorption isotherms of 12DCP on activated carbon and catalysts have been accurately determined. The isotherm data can be described by the Toth model. Only minor differences exist between the adsorption on activated carbon and fresh catalysts. The reduction of catalysts can lead to the chemisorption of 12DCP on metallic species, which enhances the adsorption capacity. 

On Pt-based electrocatalysts, some evidences suggest that the first electron transfer composed of O2 adsorption with simultaneous electron transfer and proton addition is the rate determining step at the overall reaction processes. This may be represented by [7]... 

The last two steps, steps 6 and 7, of the sequence involve the analogous transport of products back to the bulk fluid. Steps 1 and 7 are external physical rate processes which are in series with chemical steps 3-4-5 of adsorption-reaction-desorption in other words, they are separated from the chemical steps. Steps 2 and 6, on the other hand, are internal physical rate processes which are concurrent with chemical steps and require simultaneous treatment of chemical and physical rate processes. Heat transfer between bulk fluid and outer catalyst surfece and within the porous particle is also treated by the same reasoning. 

In the adsorption of a humic molecule to a mineral surface, several reactions may occur simultaneously formation of inner-sphere complexes between carboxylate groups and the mineral through ligand-exchange reactions, formation of outer-sphere complexes with carboxylate (RCOO ) and hydroxyl (RCOH) groups, and protonation-deprotonation reactions. The overall adsorption reaction for an HS—for example a fulvic acid molecule—is written as (Filius et al. 2001, 2003)... 

Hershkowitz, F., and Madiara, P.D., Simultaneous measurement of adsorption, reaction, and coke using a pulsed microbalance reactor, Ind. Eng. Chem. Res., 32(12), 2969-2974 (1993). 

Work in the area of simultaneous heat and mass transfer has centered on the solution of equations such as 1—18 for cases where the stmcture and properties of a soHd phase must also be considered, as in drying (qv) or adsorption (qv), or where a chemical reaction takes place. Drying simulation (45—47) and drying of foods (48,49) have been particularly active subjects. In the adsorption area the separation of multicomponent fluid mixtures is influenced by comparative rates of diffusion and by interface temperatures (50,51). In the area of reactor studies there has been much interest in monolithic and honeycomb catalytic reactions (52,53) (see Exhaust control, industrial). Eor these kinds of appHcations psychrometric charts for systems other than air—water would be useful. The constmction of such has been considered (54). 

In sulphur dioxide linear kinetics are generally observed due to control by phase boundary reactions, i.e. adsorption of SOj. RahmeF suggested that this is one of the conditions which favours simultaneous nucleation of sulphide and oxide at the gas/scale interface. The main reaction products are NiO, NijSj, Ni-S,j, and NiS04, depending on the temperature and gas pressure for example, according to the following reaction ... 

The effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

A number of metals have the ability to absorb hydrogen, which may be taken into solid solution or form a metallic hydride, and this absorption can provide an alternative reaction path to the desorption of H,. as gas. In the case of iron and iron alloys, both hydrogen adsorption and absorption occur simultaneously, and the latter thus gives rise to another equilibrium involving the transfer of H,<,s across the interface to form interstitial H atoms just beneath the surface ... 

The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

Fig. 4. Dependence of relative concentrationa nj/nt of reaction components A, B, and C on time variable r (arbitrary units) in the case of consecutive (— — ) reactions according to scheme (Ha) or parallel (C ) reactions according to scheme (lib). Ads X, Ads A, Des Y denotes that the rate determining step in the overall transformation is adsorption or desorption of the respective substance Des (B + C) denotes that the overall rate is determined by simultaneous desorption of the substance B and C. Ki/Ki = 0.5 for consecutive, and Ki /Ki — 0.5 for parallel reactions, b nxVn. 0 = 2.5 for consecutive reactions Kt = 0.5, and for parallel reactions Ki/Ki — 0.5. c nxVnA0 = 2.5 fcdesBKi Ky/fcdesoXj Kx = 10 [cf. (53)]. d Ki = 1.75 for consecutive, and Ki/Ki = 1.75 for parallel reactions.

The quantitative solution of the problem, i.e. simultaneous determination of both the sequence of surface chemical steps and the ratios of the rate constants of adsorption-desorption processes to the rate constants of surface reactions from experimental kinetic data, is extraordinarily difficult. The attempt made by Smith and Prater 82) in a study of cyclohexane-cyclohexene-benzene interconversion, using elegant mathematic procedures based on the previous theoretical treatment 28), has met with only partial success. Nevertheless, their work is an example of how a sophisticated approach to the quantitative solution of a coupled heterogeneous catalytic system should be employed if the system is studied as a whole. 

If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S, determined by the method of competitive reactions (Table II). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 50) have already been discussed (see p. 12). 

His researches and those of his pupils led to his formulation in the twenties of the concept of active catalytic centers and the heterogeneity of catalytic and adsorptive surfaces. His catalytic studies were supplemented by researches carried out simultaneously on kinetics of homogeneous gas reactions and photochemistry. The thirties saw Hugh Taylor utilizing more and more of the techniques developed by physicists. Thermal conductivity for ortho-para hydrogen analysis resulted in his use of these species for surface characterization. The discovery of deuterium prompted him to set up production of this isotope by electrolysis on a large scale of several cubic centimeters. This gave him and others a supply of this valuable tracer for catalytic studies. For analysis he invoked not only thermal conductivity, but infrared spectroscopy and mass spectrometry. To ex-... 

It is unlikely in real tribological events that adsorbed mono-layers work solely to provide lubrication. Instead, adsorption and chemical reactions may occur simultaneously in most cases of boundary lubrication. For example, fatty acid is usually regarded as a friction modiher due to good adsorp-tivity, meanwhile its molecules can react with metal or a metal oxide surface to form metallic soap which provides protection to the surface at the temperature that is higher than its own melting point. 

A plot of In(l-x.A) hi(l-XB) should restUt in a straight line if the assumption is true. Figure 6 shows the resvilt of simultaneous reactions of DHQ and CHE. The curvature of the In(l-XDHq) In(l-xcHE) plot confirms that the adsorption sites for CHE and DHQ over the NiMo(P)/Al203 catalysts are not the same. 

Anodic dissolution reactions of metals typically have rates that depend strongly on solution composition, particularly on the anion type and concentration (Kolotyrkin, 1959). The rates increase upon addition of surface-active anions. It follows that the first step in anodic metal dissolution reactions is that of adsorption of an anion and chemical bond formation with a metal atom. This bonding facilitates subsequent steps in which the metal atom (ion) is tom from the lattice and solvated. The adsorption step may be associated with simultaneous surface migration of the dissolving atom to a more favorable position (e.g., from position 3 to position 1 in Fig. 14.1 la), where the formation of adsorption and solvation bonds is facilitated. 

First H2 is physically adsorbed on the surface. The forces are weak and the adsorption enthalpy is only slightly negative. Subsequently, chemical bonding between Ni atoms and the hydrogen atoms of H2 molecule takes place while simultaneous dissociation of the H2 molecule takes place. The chemisorption enthalpy is strongly negative (exothermic reaction). 

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