Oxides hydrocarbon oxidation activities

Any substance capable of reacting with free radicals to form products that do not reinitiate the oxidation reaction could be considered to function as free-radical traps. The quinones are known to scavenge alkyl free radicals. Many polynuclear hydrocarbons show activity as inhibitors of oxidation and are thought to function by trapping free radicals [25]. Addition of R to quinone or to a polynuclear compound on either the oxygen or nitrogen atoms produces adduct radicals that can undergo subsequent dimerization, disproportionation, or reaction with a second R to form stable products. 

Platinum serves as the catalyst for the oxidation of CO and hydrocarbons. It is relatively insensitive to contamination by lead or sulfur. At high temperatures it is not known to dissolve in the washcoat, but sintering into larger particles may lead to a substantial loss of platinum surface area with dramatic consequences for the overall oxidation activity. 

Work has been concentrated on hydrocarbons containing activating groups, such as phenyl or /err-butyl, in order to achieve convenient rates of reaction. The relative rates of oxidation of two series of hydrocarbons with chromic acid in 95 % acetic acid are ... 

The catalyst composition has a role in the control of selectivity. The rutile-type V/Sb/(Nb) mixed oxide activates the hydrocarbon and ammonia. However, most of the ammonia is burnt to N2, rather than being inserted on the hydrocarbons this likely occurs because the catalyst is not veiy efficient in the generation of the selective Me=NH species when reaction temperatures lower than 400°C are used (11). In fact, with all catalysts the selectivity to A -containing compounds increased when the reaction temperature was increased, and the selectivity to N2 correspondingly decreased (Figure 40.6). The dilution of the active phase with tin... 

Oxidation is intimately linked to the activation of polycyclic aromatic hydrocarbons (PAH) to carcinogens (1-3). Oxidation of PAH in animals and man is enzyme-catalyzed and is a response to the introduction of foreign compounds into the cellular environment. The most intensively studied enzyme of PAH oxidation is cytochrome P-450, which is a mixed-function oxidase that receives its electrons from NADPH via a one or two component electron transport chain (10. Some forms of this enzyme play a major role in systemic metabolism of PAH (4 ). However, there are numerous examples of carcinogens that require metabolic activation, including PAH, that induce cancer in tissues with low mixed-function oxidase activity ( 5). In order to comprehensively evaluate the metabolic activation of PAH, one must consider all cellular pathways for their oxidative activation. 

In recent years, research on catalysts for the ATR of hydrocarbons has paid considerable attention to perovskite systems of general formula ABO3. In the perovskite stmcture, both A and B ions can be partially substituted, leading to a wide variety of mixed oxides, characterized by structural and electronic defects. The oxidation activity of perovskites has been ascribed to ionic conductivity, oxygen mobility within the lattice [64], reducibility and oxygen sorption properties [65, 66]. 

Nitrogen oxides (NOx) are removed by reaction with the reducing species present, viz. CO, H2 and hydrocarbons. The activity for NOx reduction increases in the order hydrocarbon < CO [Pg.63]

All the surface processes on automotive catalysts which have been tested for the effects of lead poisoning are affected by the access of lead to the catalyst surface. The effect will differ, though, for different surface processes. Oxidation of hydrocarbons has been found repeatedly to be more vulnerable than oxidation of carbon monoxide to lead poisoning (10, 19, 25). The initial oxidation activity of noble metal catalysts, never exposed to poisons, is higher for CO than for hydrocarbons (54). Therefore, it is best to use the effect of lead on hydrocarbon oxidation for assessing the susceptibility of a given oxidation catalyst to this type of poisoning. 

Figure 16 shows that hydrocarbon oxidation activity decreases logarithmically with the lead content of noble metal catalysts. Activity of these... 

Fig. 15. Comparison of the deterioration of methane oxidation activity with total hydrocarbon oxidation activity at 500°C. Lead level in fuel is 0.05 g/gal. [From Shelef et al. (10).]...

The evidence that phosphorus in fuel is detrimental to the oxidation activity of noble metal catalysts is quite convincing. Data from engine dynamometer runs with pelleted catalysts (28) show that raising the level of phosphorus from 60 to 130 mg/gal (added as cresyl diphenylphosphate, CDP) at a low concentration of lead, markedly suppresses the oxidation of hydrocarbons and of carbon monoxide. The effect on the oxidation of... 

Alkanes and Strong Solid Acids. Since the early reports by Nenizetscu and Dragan67 on alkane isomerization on wet aluminum chloride in 1933, all mechanistic studies have led to a general agreement on the carbenium-ion-type nature of the reaction intermediates involved in acid-catalyzed hydrocarbon conversions. In contrast with this statement, the nature of the initial step is still under discussion and a variety of suggestions can be found in the literature among which direct protolysis of C—H and C—C bonds, protonation of alkenes present as traces, and oxidative activation are the most often quoted.54,55... 

The multifunctionality is achieved through either the combination of two different compounds (phase-cooperation) or the presence of different elements inside a single crystalline structure. In antimonates-based systems, cooperation between the metal antimonate (having a rutile crystalline structure), employed for propane oxidative dehydrogenation and propene activation, and the dispersed antimony oxide, active in allylic ammoxidation, is made more efficient through the dispersion of the latter compound over the former. In metal molybdates, one single crystalline structure contains both the element active in the oxidative dehydrogenation of the hydrocarbon (vanadium) and those active in the transformation of the olefin and in the allylic insertion of the N H2 species (tellurium and molybdenum). 

Wang YX, Vazquez-Duhalt R, Pickard MA (2003) Manganese-lignin peroxidase hybrid from Bjerkandera adusta oxidizes polycyclic aromatic hydrocarbons more actively in the absence of manganese. Can J Microbiol 49 675-682... 

Together with the findings of EPR spectroscopy (Fig. 15), these results show clearly that in the presence of non-redox main-group oxide catalysts such as Bi203, hydrocarbons are activated by homolytic C—H bond cleavage leading to radicals that form stable products by subsequent C—C coupling in the gas phase. Lunsford et al were the first to have directly detected the radical intermediates in these reactions by their MIESR technique. 

New regulations in the United States and Europe mandate that automotive emissions must decrease substantially from current levels. As a result, there is a strong incentive to develop improved TWC with better oxidation activity at low temperatures since most of the hydrocarbons and CO are emitted immediately following cold starts of engines. As previously mentioned, the addition of transition metal oxides can have a beneficial effect on the performance of Au catalysts in CO oxidation. Combinations of Pt or Pd with transition metal oxides are also active in CO oxidation at low temperatures 50). Figure 11 shows examples of the reaction over Pt/MO/ Si02 catalysts. 

Rhodium-alumina catalysts with only 0.002 wt % Rh had a good activity for converting NO to nitrogen using simulated exhaust but inadequate oxidation activity. The latter situation can be improved by the addition of Pt or Pd, which have a well known ability for CO and hydrocarbon oxidation. However, more ammonia was formed under reducing conditions using R-Rh or Pd-Rh and also the delicate balance of reactions (CO + NO, CO + H2, etc.), which determines NO removal on the lean-side of the stoicheiometric point (i.e., excess of oxygen), was upset. Deposition of Pt... 

Several mechanisms have been proposed for glyphosate, such as uncoupling of mitochondrial oxidative phosphorylation, inhibition of aryl hydrocarbon hydroxylase activity, and inhibition of cytochrome P450 activity. However, surfactants present in many... 

---