Catalyst deactivation volatilization

Besides the three main deactivation mechanisms described above, there are also some other ways of catalyst deactivation. At elevated temperatures, a loss of catalytic activity may be the result of volatilization. Direct metal loss through volatilization is rather... 

Other workers (165) used X-ray photoelectron spectroscopy (XPS) to examine the influence of ammonia oxidation on the surface composition of alloy gauzes. After several months on stream, the surface was covered by the same types of highly faceted structures noted by others. As illustrated in Fig. 14, XPS analysis provides evidence that the top microns, and in particular the top 100 A of the surface, were enriched in rhodium. This enrichment was attributed to the preferential volatilization of platinum oxide. The rhodium in the surface layers was present in the oxide form. Other probes confirm the enrichment of the surface in rhodium after ammonia oxidation (166). Rhodium enrichment has been noted by others (164, 167), and it has been postulated that in some cases it leads to catalyst deactivation (168). 

Catalyst deactivation in the oxidation of volatile organic compounds by some metal oxides... 

In the mixed-phase CD reaction system, propylene concentration in the liquid phase is kept extremely low (<0.1 wt%) due to the higher volatility of propylene to benzene. This minimizes propylene oligomerization, the primary cause of catalyst deactivation and results in catalyst run lengths of 3 to 6 years. The vapor-liquid equilibrium effect provides propylene dilution unachievable in fixed-bed systems, even with expensive reactor pumparound and/or benzene recycle arrangements. 

The polymer is then discharged in a receiver recovering the resultant gas (6) and to a proprietary unit for monomer stripping and catalyst deactivation in the polymer spheres (7). Residual hydrocarbons are stripped out and recycled to reaction, while the polymer is dried by a close-loop nitrogen system (8) and, free from volatile substances, sent to liquid and/or solid additives incorporation step (9). 

Quantitative aspects. Figures 1 and 2 compare the CP and SPE spectra of the highly aromatic refinery coke concentrates and Figure 3 presents the decays of the aromatic peak intensities in the CP and SPE-DD experiments on the residue-derived coke. The carbon skeletal parameters obtained from the SPE and CP C NMR experiments for both samples are summarised in Table 1. The first coke concentrate obtained from the catalyst deactivated with the residue feedstock gave a broad aromatic band, possibly due to the incomplete removal of the rare-earth during the HCl wash. Indeed, it was found that, after the final HCl wash, much narrower spectral bands were obtained and the quality of the spectra shown are comparable to those obtained for low-volatile coals and anthracites (with similar aromaticities as the cokes, see following). 

There are four principal ways in which catalysts undergo deactivation (1) poisoning, (2) fouling, (3) sintering, and (4) volatilization. Mechanistically these processes can be classified as chemical, mechanical, or thermal. These mechanisms of catalyst deactivation are described and discussed in detail in several recent reviews and books. This review focuses on some impOTtant scientific facets of one of these important mechanisms, namely sintering. 

The catalyst deactivation rate decreased when CO2 was introduced into the system with a very law p ial pressure. These results indicate that the carbonate species were formed instead of the volatile species. The carbonate species are more stable but less catalytically active. 

After each steam experiment, it was noticed that the reactor walls had deteriorated. Potentially this is from the volatile lithium diftlising into the quartz to form an inert lithium silicate phase. This lithium depletion also probably contributes to catalyst deactivation. The lithium content and the surface area of the fresh and used catalysts are listed in Table 3. 

Catalyst may be useful for either activity or selectivity, or both. Another important issue is the catalyst stability. A catalyst with good stability will change very slowly over the course of time under the conditions of use. Indeed, it is only in theory that the catalyst remains unaltered during the reaction. Actual practice is far from this ideal, as the progressive loss of activity could be associated with coke formation, attack of poisons, loss of volatile agents, changes of crystalline structure, which causes a loss of mechanical strength. Due to the extreme importance of catalyst deactivation, the kinetic aspects of this phenomenon will be treated in a separate chapter. 

The very last step in filamentons carbon growth has been much less studied than the nucleation itself and at present is still poorly understood. A proposed reason for catalyst deactivation consists of the reconstruction of the surface at the front side of the metal particle, on which hydrocarbon decomposition occurs during the process of carbon deposition [40] that causes particle encapsulation. Another possibility conld stem from loss of active metal during the process, due to either (1) particle disintegration (dusting), (2) particle encapsulation inside the tube via capillarity forces, (3) formation of volatile metal carbonyls when CO is used as carbon source [73], or (4) metal sublimation in the case of small nanoparticles at high temperatures. 

The effect of a wide range of feed concentration of PCE from 30 to 10,000 ppm on the stability of chromium oxide supported on Ti02 and AI2O3 for the removal of chlorinated volatile organic compounds (CVOCs) has been investigated over a fixed bed flow reactor. Both chromium oxide catalysts exhibited stable PCE removal activity up to 100 h of reaction time without any catalyst deactivation when 30 ppm was introduced into the reactor. 

Gas-phase analysis reveals the evolution of CO2, H2O, and formic acid with recalcitrant benzoate spedes (1604, 1517, 1497, 1454, 1419, 1280, and 1180cm ) found to accumulate at the catalyst surface, which are expected to be the source of catalyst deactivation by blocking active sites (Figure 4.5). Conclusively, the aromaticity of toluene plays a key role in the deactivation. The high stabihty of benzyl radicals favors the photocatalytic oxidation of this volatile organic compound (VOC) and the formation of recaldtrant-oxygenated aromatic molecules that accumulate on the photoactive surface. 

In contrast, the volatilization of noble metals does not appear to be the most probable deactivation pathway. Corella etal. performed a deactivation study over Pt- and Pd-based commercial catalysts for the oxidation of dichloromethane for a period of 120 h( 1,000 ppm of DCM, 10,000h , 1 vol% steam, 400 C). The analysis by ICP-MS of the condensates obtained in these tests showed the absence of Pt and Pd, and consequently catalyst deactivation could not be assigned to the volatilization of the active phase. More recently Miranda et al. also confirmed no volatiUzation of the active phase when using different noble metals such as Ru, Pd, Rh and Pt supported over alumina for TCE oxidation. 

Catalyst deactivation is a great impediment to industrial applications. In general, for Cl-VOC oxidation there are several factors that lead to catalyst deactivation, such as volatilization of the active phase, poisoning by strong adsorption of CI2/HCI, formation of a coke deposit on the porous structure, and metal particle sintering caused by the temperature. Transition metal oxides, particularly chromium and vanadia, are deactivated by the volatilization of metal oxychlorides formed on the catalyst... 

Volatile catalyst Non- volatile catalyst Without catalyst deactivating agent With catalyst deactivating agent... 

---