Changes in the Catalyst Composition

A more complex phenomenon can be noticed when the catalyst is placed in contact with a solution containing both AlEt3 and MPT. In this case, as contact time increases, both MPT and the Al-alkyl are progressively fixed to the catalyst, again paralleled by a decrease in EB content, although more limited than in the previous case (Fig. 25). The extent of these phenomena depends basically on the MPT/AlEt3 

Interaction between the catalyst TiC /EB/MgCLj and TEA/MPT effect of the MPT/TEA molar ratio on the catalyst composition. (Reaction conditions T = 50 °C, TEA = 50 mmol/1, Ti = 7 mmol/1, time = 4 hours) 

Ageing conditions at MPT/TEA 0.3 mol Catalyst composition (mmol/mol MgCl2) EB MPT  

These results may be, at least qualitatively, interpreted on the basis of simple, simultaneous complexation and exchange equilibria between the various catalyst and cocatalyst components. A rough model could, for example, be made up of the following equilibria (where Cat-D indicates a free site on the catalytic surface)  

In fact, equilibria 1 and 2 account for the displacement of EB and its replacement with A1 which can be observed in Fig. 25, while equilibrium 3, with the subsequent degradation reaction, by removing AlEtj from equilibrium 1, explains the fact that the addition of MPT to the system attenuates the EB extraction. Finally, the symmetrical and opposite change of MPT and A1 content at increasing MPT/TEA ratios (Fig. 26) may be well accounted for by competition between equilibria 4 and 2. 

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When they occur over the same catalyst, stepwise catalytic transformations are usually conjugated through common catalytic intermediates, and, therefore, the methods of thermodynamics of nonequilibrium processes appear helpful in controlling the process selectivity even without any changes in the catalyst composition. It is important that these control methods are based on variations in the thermodynamic driving forces for the stepwise catalytic transformations and, therefore, do not need detailed knowledge of the mechanisms of these catalytic processes. 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

The reaction rate varies with the change in the solvent composition. The catalysis of pyridine-Cu in DMSO-benzene mixed solvent is summarized in Fig. 4 (a). The rate constant of the catalyst reoxidation (k0) and the overall rate increase although the rate constant of electron-transfer (ke) decreases with the benzene content. Instead of the benzene solvent, the copolymer of vinylpyridine with styrene (PSP) was used as a polymer ligand, as shown in Fig. 4 (b). The overall rate and k0 increase with the styrene content in the PSP ligand, just as the solvent effect of benzene. Only several times amount of styrene unit to Cu ion (as polymer concentration ca. 0.1 wt% of the solvent) affects... 

The partial structural decomposition of the POM, as evidenced by (i) the formation of small amounts of crystalline M0O3, and (ii) the change in the cationic composition. In regard to the latter point, the ammonium content in the catalyst decreased, and its place in the cationic position of the POM was occupied by Mo ions. The formation of Mo dimeric species was shown in the downloaded catalysts, which were made of neighboring Mo ions located in the anion and in the counteranion cationic position." ... 

First of all, it is clear that the reaction medium can affect a catalyst thus altering its properties. One must pay attention primarily to the studies performed by Boreskov and his school, who suggested a concept of the effect of the reaction mixture on the catalyst. The concept implies that this effect can also be outside the scope of complex reaction steps on the surface. A large number of experimental facts testifying to the changes in the catalyst properties as a result of varying the reaction mixture composition can be found in refs. 68 and 69. 

Figure 4.7 shows TOF as a function of the particle size of a monodisperse supported platinum catalyst Pt/Al203 in the reaction of deep methane oxida tion. One can see that equation (4.91) is sufficient to describe the experimental data when the Pt particles are more than 2 nm in size. When they are smaller, TOF deviates from the monotonous dependence (4.91). The reason may be a considerable change in the chemical composition of the anchored active com ponent due to the strong interaction of this component with the carrier surface. 

This article therefore seeks to examine in depth just one mixed oxide catalyst, tin-antimony oxide, which has been commercially developed (2-5) for the oxidation of propylene to acrolein as well as for the ammoxidation of propylene to acrylonitrile and the oxidative dehydrogenation of butenes to 1,3-butadiene. A recent book (6) and a subsequent review (7) have shown how little unanimity has been established about the fundamental properties of the material. In particular there seems to be much confusion as to the phase composition, the nature of the cationic oxidation states, the chemical environment of the cations, the charge compensation mechanism, the nature of the active sites, the distortion of the host tin(IV) oxide lattice by the dopant antimony atoms and whether any changes in the catalyst result from the adsorption and catalytic processes. 

Depending on the possible interaction between substrate species and redox centers of the solid, one can consider (1) outer sphere electron transfer processes in the electrolyte-particle interface and (2) inner-sphere mechanism, involving composition changes in the catalyst and the substrate. 

Variation in the catalyst composition does not change the type of the dependencies shown in Fig. 3 but influences the activity and contribution of the parallel stages. 

Equations 12-16 permitted one also to rationalize11 the large inverse D KIE in the epoxidation of ethylene and their dependence on the inlet gas mixture. Since in the epoxyethane formation no C—H bond rupture takes place, the observed D KIEs are caused by the changes in the chemical composition of the surface layer of the catalyst controlling the catalytic reaction. In the presence of C2D4 the deep oxidation is slowed down, the concentration of [Ag2Os] centres and subsequently also the [Ag + (0 d)] centres increases and this results in the increase of the observed production of C2D40. 

Typical resistance responses to a step change in the gas composition are illustrated in Figure 1. The experimental results (open circles, labels 1, 2, 3) and the theoretical calculations (dash-dot lines, labels 1, 2, 3 ) are plotted in this figure. The resistance response to gas was measured in several sensors in which the rate parameters of the surface chemical reaction were made different by modification with the metallic catalyst. The temperature of the sensors was fixed at about 600 K long before the tests. 

The reactant mixture containing hydrocarbons of various structure and oxygen induces changes in the phase composition of the catalyst during the reaction. Simard et al. (64) suggested that the active surface of a vanadium catalyst represented a dynamic system involving V6+, V4+ and 02 ions. 

Differential thermal analyses (DTA) may also be employed to detect subtle changes in the catalyst structure. The phase transformations of the solid are a fingerprint of the catalyst composition and the changes may be detected in this manner. 

For an Al/Co ratio of about 1.5, ethane was also the main decomposition product at room temperature But the amount of methane increased significantly with the temperature and represented 50% of the hydrocarbons formed at 180°C in the presence of hydrogen This result could indicate a change of the catalyst composition even if we cannot explain clearly the origin of the methane ethane hydrogenolysis, AlEt acac decomposition,.. . ... 

Iron catalysts present special problems for the determination of the adsorbed species. The iron may be carburized during the reaction and this leads to a change in the bulk composition of iron catalyst with time on stream. Bianchi et al. determined that iron was partly converted into a mixture of e -Fe2 2C and x Fe2 5C during the reaction using Mossbauer spectroscopy. The adsorbed carbon surface species were determined to be present in three forms small amounts of reactive CH species which produced the bulk of the hydrocarbon products during reaction, a carbidic species with some associated hydrogen, and inactive graphitic carbon species. 

Specular reflection Fourier transform infrared spectroscopy, performed with the beam striking the sample at 45- from the surface normal, was also used for in situ studies of the catalytic reaction. This technique makes it possible to observe changes in the gasphase composition [4-5]. The broad band reflectivity of the overlayer can be used to obtain the composition of a thin oxide film [6], Infrared spectra have been obtained from polycrystalline samples and from Cu(llO). The former samples are akin to the ones used for mass spectrometric studies. Distinguished from those studies our FTIR work was performed with a reaction vessel at room temperature. The sample was in this case resistively heated by tantalum wires. This combination separates the gas temperature from the temperature of the catalyst. The temperature of the impinging molecules is a... 

The above observations in combination with unavoidable changes in the gas composition, due to the rapid rates and a limited vessel volume, give that the optimum conditions are limited to a narrow temperature and gas-composition window. Nevertheless, the high yields observed imder the present optimal conditions and the likelihood of stable operation at lower temperatures are most encouraging. These yields are not out of range with conventional catalysts based on rare metals and compare favourably to complex oxides and zeolites. 

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