Working catalysts monitoring

Bruckner, A. (2009). Electron Paramagnetic Resonance A Powerful Tool for Monitoring Working Catalysts, Adv. Catal, 52, pp. 265-308. 

Chrominm Oxides. - Supported chromium oxides are extremely important industrial catalysts widely used for the polymerisation of ethylene, as well as the generation of valuable alkenes via the dehydrogenation of low-cost alkane feedstocks. Despite the extensive uses of these catalysts, a great deal of controversy still remains on the nature of the chromia active sites present in the working catalyst, and many spectroscopic techniques have been used to characterise the catalysts. Among these techniques, EPR has played a vital role, as it is extremely sensitive to both the oxidation state of the ion and the phases of the supported chromia, and can also be used to monitor the catalyst under in situ conditions. ... 

The basic experimental setup is shown schematically in Figure 13.10a. The metal working catalyst electrode, usually in the form of a porous metal film 3 to 20 pm in thickness, is deposited on the surface of a ceramic solid electrolyte (e.g., YSZ, an conductor, or P"-AI2O3, a Na+ conductor). Catalyst, counter, and reference electrode preparation and characterization details have been presented in detail elsewhere, together with the analytical system for on-line monitoring of the rates of catalytic reactions by means of gas chromatography, mass spectrometry and IR spectroscopy. 

The size and location of metal clusters in working catalysts not only depend on the preparation techniques employed to obtain the fresh catalysts, but also on the stability of metal clusters. In the course of catalytic reactions or during regeneration treatments, sintering of metal clusters frequently happens as a result of complexation and transport processes favored by the presence of molecules such as O2, Clj, CO, and NHj. Sintering as well as redispersion processes are not well documented and require additional characterization studies, preferably with in situ measurements, to monitor the state of metal clusters as a function of the treatment conditions. 

Two thermocouples, Em at x = 0 and Ex at a distance x, permit the monitoring of the atomic hydrogen concentration change along the side-tube. The atoms recombining on the thermocouple tip covered by the active catalyst evolve the heat of reaction and thus the thermoelectric power becomes a relative measure of the concentration of atoms in the gas phase. Finally, one obtains for the direct use in an experimental work the following equation... 

Work with the objective of comparing oxo-ions with oxide particles in order to test the validity of this reasoning has been reported by Chen et al. who used a catalyst that initially contains Fe oxo-ions, [HO-Fe-0-Fe-OH] +. These sites were first converted to Fc203 particies by a simpie chemical treatment. This was followed by another treatment, which redispersed these Fc203 particies back to oxo-ions. The change in particle size was monitored by a spectroscopic method based on the observation that in zeolites metal ions and oxo-ions, that are attached to the wall of a cage, give rise to a typical IR band caused by the perturbation of the vibrations of the zeolite lattice. 

Reaction Monitoring - Heterogeneous Catalysts Under Working Conditions... 

Extended life for the zeolite Beta catalyst has been demonstrated in this work using the same, or similar, crude tBA feedstocks to those employed in Table 1. Isobutylene generation has been monitored over ca. 500 to 1000 hours of service, under steady state reactive distillation conditions, without significant losses in activity or changes in product compositions. 

Recent studies have shown that the reaction, as described by Eq. (11), also requires Pt(II) as a necessary catalyst (29, 84). A range of substituted benzenes has been examined, and by studying concurrent hydrogen-deuterium exchange, it was concluded that the two reactions had common intermediates (29). In this work aqueous acetic acid was used as the solvent, and reactions were followed by measuring the concentration of the chlorobenzene product. Of the several possible mechanisms for the oxidation that have been given (29), only one will be considered here this is the one that has received substantial support from the most recent work (84). In this study, the loss of reactant benzene, the formation of product chlorobenzene, and the formation of platinum(II) were monitored as the reaction proceeded. Also aqueous trifluoroacetic acid was used as the solvent, as it is known that acetic acid is oxidized to chloroacetic acid by platinum(IV) (18). 

In this section the use of amperometric techniques for the in-situ study of catalysts using solid state electrochemical cells is discussed. This requires that the potential of the cell is disturbed from its equilibrium value and a current passed. However, there is evidence that for a number of solid electrolyte cell systems the change in electrode potential results in a change in the electrode-catalyst work function.5 This effect is known as the non-faradaic electrochemical modification of catalytic activity (NEMCA). In a similar way it appears that the electrode potential can be used as a monitor of the catalyst work function. Much of the work on the closed-circuit behaviour of solid electrolyte electrochemical cells has been concerned with modifying the behaviour of the catalyst (reference 5 is an excellent review of this area). However, it is not the intention of this review to cover catalyst modification, rather the intention is to address information derived from closed-circuit work relevant to an unmodified catalyst surface. 

This latter interpretation would mean that with the approach depicted in Fig. 10, the catalyst itself could be monitored. The authors reported that the silica-supported Nafion could not be observed in the beginning of their experiments and appeared in the spectra only after the catalyst interacted with octanol. This observation may indicate that the octyl groups promote the sticking of the catalyst particles onto the ATR probe, within the evanescent field. However, the example also shows that this approach may not be without problems, because it depends on the adsorption of the particles from the slurry reactor onto the ATR element. This process is accompanied by the adsorption of molecules on the catalyst surface and complicates the analysis. More important, as also indicated by the work of Mul et al. (74). this adsorption depends on the surface properties of the catalyst particles and the ATR element. These properties are prone to change as a function of conversion in a batch process and are therefore hardly predictable. 

Copeland and Miller [56] at Boston College, MA have applied the classical fluorescent PET pH signaling systems to advance the field of catalyst discovery. In this powerful work, the pH sensor 10 monitors the surface regions of a... 

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