Oxidation state, changes under reaction conditions

An example where changes in pretreatment steps are very drastic is in hydrotreatment. The most applied catalysts are based on a combination of molybdenum and cobalt sulphides. They are manufactured in the oxidic state, and under reaction conditions these oxides are not stable and are transformed into the sulphidic state. In practice, the oxidic catalyst is presulphided, either in the reactor (by adding H2S or a compound that is easily converted into H2S), or ex situ by the manufacturer. These presulphiding procedures often involve temperature-programmed reactions. 

The mechanism will vary in precise detail according to the metal. In the case of ruthenium complexes, it is quite common to observe a conproportionation and the formation of a ruthenium(iv) intermediate. In other cases, the unavailability of the metal oxidation states precludes reaction. For example, cobalt(m) complexes of cyclam cannot be oxidised to imine species because although a cobalt(ii)/cobalt(m) couple is possible, the cobalt(n) oxidation state is not accessible under oxidative conditions. In the case of metal ions which can undergo two oxidation state changes, alternative mechanisms which do not involve radical species have been suggested. 

A considerable collection of data exists that describe the state of catalysts under reaction conditions. The cases presented here show that typical processes observed under reaction conditions include changes of the oxidation states (of transition metals), changes of particle size and shape (of metal clusters), or formation of coke. However, without the corresponding catalytic performance, relevant and spectator species cannot be distinguished. 

Small differences in the chemistry of the lanthanides are observed due to the gradually changing radial size of the elements, which decreases from 1.061 A for La to 0.848 A for Lu (77). Differences in chemistry also occur for the four elements in the series which have nontrivalent oxidation states accessible under normal reaction conditions Ce (4/°), Eu " (4/ ), Yb " (4/ and Sm (4 f ). Since Ce is a strong oxidizing agent and the... 

Ferrisilicates are considered for catalytic applications, primarily in their H-form, due to the acidic function on the Si-OH-Fe groups. In addition, the extra-framework ions may also play a catalytic role (e.g. exhibiting Lewis acidity). Further, in the case of comparable size of diameter of channels ancJ reaction components, shape selectivity is imposed by the zeolite structure. Thus, the overall catalytic performance is influenced by various sources. It should also be considered that in general case the structure, the distribution of iron components among the possible oxidation and coordination states may also change under catalytic conditions in a ferrisilicate. 

Non-Steady-State Reactors for Testing Fixed-Bed Catalysts In non-steady-state reactors, reaction conditions such as temperature or reactant concentrations are changed temporarily [103-105]. Temperatnre-programmed snrface reaction (TPSR) experiments, temperatnre-programmed desorption (TPD), and temperature-programmed reduction and oxidation (TPR, TPO) [106,107] are established methods dealing with non-steady-state reactor operation. Among these methods, TPSR is a technique that can be applied directly under reaction conditions relevant for catalytic processes. 

The oxidation of toluene under periodic conditions permits to obtain 1,2-diphenylethane at a selectivity of 0.9 and to avoid the production ofbenzaldehyde and other oxygenated products. This reaction is an example which illustrates how a process under periodic conditions may change the selectivities and yields of an heterogeneously catalysed reactions compared to steady-state conditions. 

One of the greatest advances that theory has made over the past decade has been its ability to examine the sensitivity of the state of the working surface to changes in reaction conditions and surface structure. This has required the ability to integrate ab initio-derived thermodynamic and kinetic results into phase equilibrium as well as atomistic kinetic simulations. The oxide and sulfide surfaces are sufficiently stable that useful studies can be carried out. CO oxidation catalyzed by Ru02 demonstrated that the maximum turnover rate occurs under conditions where the surface is in a disordered state at the boundaries of two phases, one of which is completely covered with oxygen adatoms and a second which is partially covered with CO. 

We have already noted that the standard free energy change for a reaction, AG°, does not reflect the actual conditions in a ceil, where reactants and products are not at standard-state concentrations (1 M). Equation 3.12 was introduced to permit calculations of actual free energy changes under non-standard-state conditions. Similarly, standard reduction potentials for redox couples must be modified to account for the actual concentrations of the oxidized and reduced species. For any redox couple. 

Oxidation-reduction reactions may affect the mobility of metal ions by changing the oxidation state. The environmental factors of pH and Eh (oxidation-reduction potential) strongly affect all the processes discussed above. For example, the type and number of molecular and ionic species of metals change with a change in pH (see Figures 20.5-20.7). A number of metals and nonmetals (As, Be, Cr, Cu, Fe, Ni, Se, V, Zn) are more mobile under anaerobic conditions than aerobic conditions, all other factors being equal.104 Additionally, the high salinity of deep-well injection zones increases the complexity of the equilibrium chemistry of heavy metals.106... 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

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