Carbonate species selective oxidants

In 1999, Binet et al.395 published a review on the response of adsorbed molecules to the oxidized/reduced states of ceria. In light of recent infrared studies on ceria, the assignments for OH groups, methoxy species, carbonate species, and formates are highly instructive. The OH and methoxy species have been briefly discussed. Characteristic band assignments of carbonate and formate species are provided below, the latter formed form the dissociative adsorption of formic acid, the reaction of CO with H2-reduced ceria surface, or via selective oxidation of methanol. Formate band intensities were a strong function of the extent of surface reduction of ceria. 

Infrared spectra of propene and isobutene on different catalysts were measured by Gorokhovatskii [143]. Copper oxide, which converts olefins to butadiene and aldehydes, shows adsorption complexes different from structures on a V2Os—P2Os catalyst which produces maleic acid anhydride. Differences also exist between selective oxidation catalysts and total oxidation catalysts. The latter show carbonate and formate bands, in contrast to selective oxides for which 7r-allylic species are indicated. A difficulty in this type of work is that only a few data are available under catalytic conditions most of them refer to a pre-catalysis situation. Therefore it is not certain that complexes observed are relevant for the catalytic action. 

The separation of the two sets of desorption products may indicate that they are from different sites. That is, branching of the selective and nonselec-tive oxidation takes place on adsorption of butene. This can be confirmed if the two sets of products can be varied independently. This is shown by two experiments. The first experiment makes use of the fact that butene and butadiene adsorb on the same sites. Butadiene is first adsorbed onto the catalyst (5). The catalyst is then heated to 210°C, desorbing all of the unreacted butadiene, but leaving on the surface the precursors of the combustion products. Since desorption of the unreacted butadiene does not involve a net chemical reaction, the adsorpton sites involved are not affected. The catalyst is then cooled to 22°C, and cis-2-butene is adsorbed. If selective oxidation and combustion take place on the same site, the adsorbed butene would undergo both reactions. If they take place on separate sites, and butene adsorbs only on the selective oxidation site (because the combustion site is covered by species from butadiene adsorption), the adsorbed butene would form only butadiene. Subsequent desorption yields a profile similar to that for a single adsorption of ds-2-butene (Fig.l, curve b). More importantly, within experimental errors, the amount of butadiene evolved is the same as in a ds-2-butene adsorption experiment, and the amount of C02 evolved is the same as in a butadiene adsorption experiment. Thus, the adsorbed butene forms only butadiene. These results show that under these experimental conditions (i.e., in the absence of gas-phase oxygen), the production of butadiene and carbon dioxide takes place on separate sites. 

In comparison to the bismuth molybdate and cuprous oxide catalyst systems, data on other catalyst systems are much more sparse. However, by the use of similar labeling techniques, the allylic species has been identified as an intermediate in the selective oxidation of propylene over uranium antimonate catalysts (20), tin oxide-antimony oxide catalysts (21), and supported rhodium, ruthenium (22), and gold (23) catalysts. A direct observation of the allylic species has been made on zinc oxide by means of infrared spectroscopy (24-26). In this system, however, only adsorbed acrolein is detected because the temperature cannot be raised sufficiently to cause desorption of acrolein without initiating reactions which yield primarily oxides of carbon and water. 

The high concentration of oxygen in the feed for the 20% oxygen system causes an over oxidation of the carbon species to CO, reducing the selectivity towards acetylene. This product flexibility could be useful in that it would allow for hydrogen to be produced... 

A major objective of the work employing infrared spectroscopy is the identification of the species involved in the reactions that gold is adept at catalysing ((selective) oxidation of carbon monoxide, water-gas shift, etc.),... 

There was already a considerable body of knowledge on catalysts of this type [29]. For those used for selective oxidations, there was much evidence to show that the active phase was a monolayer of oxovanadium species chemically bonded to the TiC>2 surface such a material would have about 1 wt% V2O5 for a TiC>2 area of 10m2g l, but technical catalysts usually contained substantially larger amounts. The excess appeared to be in the form of V2O5 microcrystals which neither helped nor hindered in selective oxidation it seemed to serve as a reserve supply to replenish the monolayer, should it become depleted. There was also evidence that uncovered TiC>2 surface was harmful, in that it could cause deep oxidation to carbon oxides. In these applications, the anatase form of TiC>2 was generally used, and unless the contrary is stated the formula TiC>2 will imply this form. 

As-received UD90 was oxidized for 2, 6,17, 26, and 42 h at 430°C, which is the temperature for slow ND oxidation as discussed in Sect. 5.3.3 [94]. Figure 12.18a shows the HRTEM images of two ND powders oxidized at 430°C for 2 and 42 h, respectively. The weight loss due to oxidation was 13% and 74% after 2 and 42 h, respectively. While oxidation for 2 h removes mainly amorphous carbon and other non-diamond species [95], longer oxidation times result in selective oxidation of smaller crystals, thus shifting the size distribution toward larger values. 

The catalytic performance of 03 to produce carbon oxides in MSR can, to some extent, be rationalized using a concept of Tatibouet et al. [35], which was originally applied to the selective oxidation of methanol and is based on the influence of the strength of acidic and basic oxide surface sites on product formation. According to this concept, the formation of carbon oxides (and preceding formate species) reqnires the presence of redox-active surface centers as well as both weak acidic and rather strong basic sites. 363 is a reducible/amphoteric oxide with... 

The addition of Cu or Ni into ceria has different effect on the sulfur selectivity of the catalyst under fuel-rich conditions. Cu promotes the complete oxidation and it is selective for Sj. Conversely, on Ni/ceria catalysts, side reactions favor HjS production over elemental sulfur. It is worth to note that both the catalysts suppress carbon formation. The high carbon resistance shown by the metal/ceria based catalysts may be attributed to a higher dispersion of metals into this kind of matrices. Moreover, the high mobility of oxygen ions in ceria allows a rapid supply of oxygen to the metal interfaces speeding up the surface oxidation of carbon species and thus inhibiting deposition of carbon on the catalyst surfaces [25,26]. 

Supported Pt and Pd were shown to yield some selectivity in batch conditions using air as oxidant, but the principle products were the unwanted single carbon species such as CO2, HCHO and HCOOH. Under these conditions supported Au catalysts were totally inactive. Using pure oxygen and 3 bar pressure, Au became active, and the formation of Cl by-products was eliminated when NaOH was added. Using 1 wt% Au supported on either graphite or activated carbon, 100% selectivity to glyceric acid was... 

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