Vapor formation, catalyst deactivation

The nitration of toluene in the presence of sohd acid catalysts (ZSM5, Mordenite, Beta, L, zeolites and MCM-41 and sulfated zirconia) has been studied in the vapor and Uquid phases. The data indicate that the catalysts deactivate by two different modes, coke formation and pore plugging by adsorbed toluene. 

Au/y-Al203 deactivates in CO oxidation, but it exhibits stable activity in selective CO oxidation (SCO) in the presence of H2. The activity for CO oxidation could be suppressed significantly by thermal treatment at 100°C, and the lost activity could be recovered by exposing the catalyst to water vapor at room temperature. A catalyst deactivated in CO oxidation could be regenerated by exposing it to H2 at room temperature or by running the SCO reaction over it. The results can be explained with a reaction mechanism for CO oxidation involving an active site that consists of an ensemble of metallic Au atoms and Au-hydroxyl. Deactivation in CO oxidation is due to the formation of an inactive carbonate, and deactivation by thermal treatment is due to dehydroxylation of the Au-hydroxyl. 

Ozone acts as the precursor of the key oxidants in the toluene decomposition. The presence of water vapor is also very important, as has been demonstrated for the catalytic oxidation of benzene with ozone on supported manganese Oxides catalysts [68]. It suppresses the catalyst deactivation by inhibiting the buildup of organic by-products on the catalyst surface, including formic acid and strongly bound surface formates. Scheme 18.2 proposes a general pattern mechanism for the plasma-driven total oxidation of the hydrocarbons. 

The use of Pd-MRs for MDR helps to reduce the operating temperature, leading to suppression of catalyst deactivation due to coke deposition. By adding H2O vapor in the reactant feed, MSR will take place together with MDR. As a result, carbon formation can be reduced due to the oxidation of the carbon precursors - such as partially hydrogenated CH species - leading to improved catalyst stability. A desirable H2/CO ratio can be obtained conveniently via adjustment of the CH4/H2O ratio in the feed [67]. Furthermore, methane conversion and hydrogen yield can be increased, whereas CO2 conversion and CO yield will be decreased. 

Alumina spheres polluted by carbon residues have been also reactivated by use of microwaves [33]. Their regeneration has been performed in a stream of air and in the presence of silicon carbide as an auxiliary microwave absorber. Microwave heat treatment led to full recovery of the catalyst in times varying from a half to a quarter of the conventional treatments. Regeneration of a commercial Ni catalyst (Ni/Al203) deactivated, presumably, by coke formation, by means of a flow of hydrogen or oxygen and water vapor under the action of microwave irradiation was, however, unsuccessful [34]. 

All NOx-containing combustion gases also contain a significant amount of water vapor (2-15%), therefore resistance to water poisoning is important for practical application of a catalyst. Iwamoto, et al and Li and Hall (9) reported that the catalytic activity of Cu-ZSM-5 for NO conversion to N2 was decreased in the presence of 2% water vapor, but it could be recovered after removal of water vapor. However, Kharas and coworkers (10) found that Cu-ZSM-5 was severely deactivated under a typical automotive fuel lean exhaust gas (10% H2O, GHSV = 127,(X)0 h" ) over the temperature range of 600 to SOO C. The authors attributed the catalytic deactivation of Cu-ZSM-5 to the formation of CuO and the disruption of zeolitic crystallinity and porosity. 

The presence of water vapor plays an important role in the gas-phase photocatalysis. For example, moderate levels of water vapor promote the photocatalytic degradation of toluene and m-xylene [66, 74], whereas they inhibit the degradation of ethylene [75]. In some cases, such as TCE [62], deactivation of the photocatalyst was observed in the absence of water vapor, which probably is due to the exhaustion of surface hydroxyls or to the formation of an intermediate species that blocks the active catalyst sites. However,... 

Depending on the H2S/CH4 ratio, hydrogen sulfide reforming occurs at a temperature higher than 850°C. At these temperatures, the elemental sulfur produced is in the vapor phase and cannot cause deactivation of metal sulfide-based catalysts. However, the formation of solid carbon can be harmful for the catalysts. To avoid coking effect, the molar ratio of H2S/CH4 must be greater than 4. As indicated by thermodynamic analyses [21], a higher H2S/CH4 ratio can reduce carbon formation to zero at lower temperatures. 

Causes of deactivation are basically three-fold chemical, mechanical or thermal— hereby six different routes of deactivation of catalyst material are described (some have been introduced before, without further explanation) poisoning (i.e. CO on Pt), fouling (i.e. coke formation during ethene hydrogenation on Pt), thermal degradation, vapor compound formation accompanied by transport, vapor-solid and/or solid-solid reactions, and attrition/crushing [162, 163]. 

Vapor-phase carbonylation catalyzed by copper-MOR (Cu-mordenite) has been reported. However, a successful process will require a better catalyst system. The Cu-MOR catalyst suffers from deactivation, and there are substanbal side reactions of hydrocarbon formation over the Brpnsted acid sites. 

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