Catalyst stability analysis

Catalyst stability studies were conducted using a variety of model feeds. The results using 0.7 wt % propionic acid in aqueous HBr demonstrate the effectiveness of operating at partial conversion to monitor deactivation. Figure 6 shows that at 292 °C, the propionic acid-contaminated feed caused rapid deactivation. Subsequent analysis of the catalyst showed carbon deposits on the catalyst. 

Of the various methods of weighted residuals, the collocation method and, in particular, the orthogonal collocation technique have proved to be quite effective in the solution of complex, nonlinear problems of the type typically encountered in chemical reactors. The basic procedure was used by Stewart and Villadsen (1969) for the prediction of multiple steady states in catalyst particles, by Ferguson and Finlayson (1970) for the study of the transient heat and mass transfer in a catalyst pellet, and by McGowin and Perlmutter (1971) for local stability analysis of a nonadiabatic tubular reactor with axial mixing. Finlayson (1971, 1972, 1974) showed the importance of the orthogonal collocation technique for packed bed reactors. 

Villadsen, J. and Michelsen, M. L. (1972) Diffusion and reaction in spherical catalyst pellets steady state and local stability analysis. Chem. Engng Sci. 27, 751. 

The field of chemical kinetics and reaction engineering has grown over the years. New experimental techniques have been developed to follow the progress of chemical reactions and these have aided study of the fundamentals and mechanisms of chemical reactions. The availability of personal computers has enhanced the simulation of complex chemical reactions and reactor stability analysis. These activities have resulted in improved designs of industrial reactors. An increased number of industrial patents now relate to new catalysts and catalytic processes, synthetic polymers, and novel reactor designs. Lin [1] has given a comprehensive review of chemical reactions involving kinetics and mechanisms. 

A recapitulation of the catalyst stability data reported indicates that within the time scale of the hydrotreating runs, the UOP-filtered SRC filter feed gave relatively stable performance. SRC itself caused substantial catalyst deactivation Synthoil gave stable performance after an initial deactivation. Since dissolved metals and particulate matter are known to have an adverse effect on catalysts, a correlation was sought based on an analysis for these components. 

These findings seemed to point to catalyst degradation induced by the reagents, and indeed further NMR analysis showed extensive catalyst degradation in the presence of acrolein, less degradation with croton-aldehyde, and essentially no degradation with cinnamaldehyde (nitrone did not exert any effect on the catalyst stability). In agreement with the results of these experiments, we observed also that the non-supported catalyst showed a marked instability and decrease in chemical efficiency when recycled. 

They treated the system much like a CSTR, with the balance for the gas-phase concentration substituted by the coverage equation for the catalyst. Ray and Hastings then applied the analytical treatment that they had developed for the CSTR in this same publication. Stability analysis revealed that the critical Lewis numbers for oscillations were in a range that did not allow for oscillations on normal nonporous catalytic surfaces. However, as Jensen and Ray 243) showed, a certain model for catalytic surfaces, the fuzzy wire model, with the assumption of a very rough surface with protrusions is able to produce Lewis numbers in the proper range for the occurrence of oscillations. This model, however, included both mass and heat balances as well as coverage equations, thus combining the two classes of reactor-reaction models discussed above. 

A stability analysis for such a system was performed by Wicke et al. (98), who modeled the H2/O2 reaction on Pt catalysts. The reaction was simplified to two differential equations that are easily treated analytically ... 

Titanium tetrachloride and aluminium triethyl form a hydrocarbon soluble complex at low temperatures which decomposes at —30°C to give the trichloride as a major product [32]. Complexes containing tetravalent titanium stabilized by adsorption on titanium trichloride apparently persist in catalysts prepared at Al/Ti ratios below 1.0 [33], but at higher ratios there are some Ti(II) sites present in the catalyst [34]. Analysis shows that at Al/Ti ratios above 1.0 the solid precipitate contains divalent titanium or even lower valency states of the metal [35]. Reduction of TiCl4 with AlEt2 Cl is less rapid and extensive than with AlEts and even at high Al/Ti ratios [36] reduction does not proceed much below the trivalent state. Aluminium alkyl dihalides are still less reactive and reduction to TiClj is slow and incomplete except at high Al/Ti ratios or elevated temperatures [37]. 

Before the experiment the catalyst sample was activated in air flow in situ at 500 C for 2h, and then was blown by N2 at the same temperature for 0.5 h. During the experiments, i.e., a period of 4 h with hourly collection of probes for gas chromatographic analysis changes in catalyst stability and selectivity were not observed. 

CO oxidation in O2 excess on the fresh model catalyst. XPS analysis indicates stabilization of Pt oxide in the case of Pt/ceria [98]. In contrast, similar measurements on Pt/alumina show that these Pt particles are metallic and the chemical state does not change significantly after several CO oxidation cycles in O2 excess [98]. The long-term lean CO oxidation experiments on Pt/ceria indicate that the catalyst activity gradually changes as a function of CO oxidation experiment time. Extensive CO reduction causes an up-shift of both T50 and E a, which is attributed to C deposition on the catalyst [9], probably via CO disproportionation at CO excess. A lower value of T50 and E can, however, be restored by running a H2 oxidation over the catalyst (cycles 5-7) around stoichiometric conditions a = 0.67). The influ-... 

HelUnckx, L., Grootjans, J., Van den Bosch, B., Stability analysis of catalyst particle through orthogonal collocation. Chem. Engng. Sci. 1972 27, 644-647. 

The effects of complex mechanisms with serial or parallel shifts in rate limiting steps must also be considered in the analysis of the temperature dependence of isotope effects [28]. tn addition, tedious attention to details of temperature effects on pH, acidity constants, reaction volumes, and substrate or catalyst stabilities may be needed in some cases to avoid problematic interpretations. For these reasons, temperature studies of isotope effects may not be as convincing as the observation of very large isotope effects in providing evidence for tunneling. 

The steady states which are unstable using the static analysis discussed above are always unstable. However, steady states that are stable from a static point of view may prove to be unstable when the full dynamic analysis is performed. That is to say simply that branch 2 in Figure 4.8 is always unstable, while branches 1,3 in Figure 4.8 and branch 4 in Figure 4.8 can be stable or unstable depending upon the dynamic stability analysis of the system. As mentioned earlier, the analysis for the CSTR presented here is mathematically equivalent to that of a catalyst pellet using lumped parameter models or a distributed parameter model made discrete by a technique such as the orthogonal collocation technique. However, in the latter case, the system dimensionality will increase considerably, with n dimensions for each state variable, where n is the number of internal collocation points. 

Now we are prepared to illustrate these experimental protocols of reaction progress kinetic analysis using data from reaction calorimetric monitoring of the aldol reaction shown in Scheme 27.1. We turn hrst to the issue of catalyst stability using our same excess protocol. In these aldol reactions, it was noted that the active catalyst concentration can be effectively decreased by the formation of oxazolidinones between proline and aldehydes or ketones, and that addition of water can suppress this catalyst deactivation. Same excess reactions carried out in the absence of water and in the presence of water are shown in Figure 27.3a and Figure 27.3b, respectively. The plots do not overlay in the absence of water, but they do when water is present. The overlay in these same [e] experiments in Figure 27.3b means that the total concentration of active catalyst within the cycle is constant and is the same in the two experiments where water is present. 

Metallosilicates containing Cr, Cu or Mo have been prepared by an acid-catalyzed sol-gel process. Structural information about the silicates were obtained by elemental analysis, TGA, XRD, XRF, N2-physisorption, EPR, UVA IS and FTIR spectroscopy. The silicates are active catalysts for the oxidation of hydrocarbons with tert-butyl hydroperoxide. Catalyst stabilities with regard to metal leaching during catalytic oxidations were investigated. 

The influence of different non-noble alloy component (e.g., Cu, Co, Ni, Fe) on the catalyst stability has not been well addressed until now. In a recent study, the specific activity of dealloyed PtCus decreased 24 % after 10,000 cycles between 0.5 and 1.0 V while that decreased 42 % for dealloyed PtCo3 [48]. The loss of Cu/Co in the catalysts was confirmed by compositional analysis using EDX. In comparison, dealloyed PtNia showed 50 % decrease in the specific activity after the same test protocol [50]. It therefore seems that the stability of the different catalysts is in the order of dealloyed PtCua > dealloyed PtCos > dealloyed PtNis [50]. This difference is likely due to the different redox chemistry or different Pt-M interactions of different alloy elements, but further systematic studies are needed to verify this hypothesis. 

Catalyst stability is very important for the long-term operation of the catalyst in practical applications. Thermal analysis has been widely applied as a useful method to characterize the stability of catalysts. This section describes some representative examples of the application of thermal analysis to fuel cell... 

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