Intermediate surface complexes

Enantioselectivity as a function of the bond strength in intermediate surface complexes... 

It has been shown that the TEA process leads to high-quality films [43—45]. The mechanism involving the CBD of CdS thin films from the ammonia-thiourea system have been studied in situ by means of the quartz crystal microbalance technique (QCM) [25]. The formation of CdS was assumed to result from the decomposition of adsorbed thiourea molecules via the formation of an intermediate surface complex with cadmium hydroxide. This mechanism is different from the dissociation mechanism involving the formation of free sulfide ions in solution, and which had previously been reported [46-49]. Thus, the influence of growth parameters such as bath temperature, deposition rate, bath composition, etc., on various film properties has been studied [37, 39, 41, 50, 51], and the main parameters which determine the quality of the films were deduced. The chemical deposition of CdS thin films generally consisted of the decomposition of thiourea in an alkaline solution containing a cadmium salt The deposition process was based on the slow release of Cd and S ions in solution which then condensed on an ion-by-ion basis on the substrate. The reaction process for the formation of CdS may be described by the following steps [25, 35, 36, 43, 52-54]. 

In this figure, A denotes the substrate, Mj and M2, two different modifiers, R and S are the R) and (S) product enantiomers respectively, Z is the surface site and ZM1AH2 and ZM2AH2 are intermediate surface complexes. Racemic cycles including steps 9, 10 or 9, 11 lead to a racemic mixture of (R)- and (S)-product enantiomers, while routes involving modifier Mj (steps 1,3,4 or 1,3,5) or modifier M2 (2, 6, 7 or 2, 6, 8) are enantioselective. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

A particular feature of the whole process is the trade-off between the key intermediates of both mechanistic cycles. While the N—N bond formation (controlled by thermal stability of the mononitrosyl intermediate) is favored by lower temperatures, the 0-0 bond formation step (constrained by endothermic decomposition of the nitrate intermediate) is favored by higher temperatures. Indeed, as revealed by operando IR studies (Figure 2.24), at low temperatures nitrates accumulate on the surface, whereas at high temperatures the surfaces is essentially depleted of the mononitrosyl complexes. The optimal reaction temperature corresponds, therefore, to a subtle balance between the rate of formation of the Cu NO Z surface complex in the early stages, and the rate of decomposition of the CuN03 Z complex in the late stages of the reaction. 

The concept of intact emission of adsorbed molecular species for identifying reaction intermediates is also well illustrated in several recent studies. Benninghoven and coworkers (2-4,12) used SIMS to study the reactions of H2 with O2, C2H4 an< 2H2 on P°ly polycrystalline Ni. For the C2H /Ni interaction, for example, direct relationships could be established between characteristic secondary ions and the presence of specific surface complexes (12). In another study, Drechsler et al. (13) used SIMS to identify NH(ads) as the active intermediate during temperature-programmed decomposition of NH3 on Fe(110). 

One of the oldest mechanisms of interaction between adsorbed reactant and adsorbed TA has been proposed by Klabunovskii and Petrov [212], They suggested that the reactant adsorbs stere-oselectively onto the modified catalyst surface. The subsequent surface reaction is itself nonstere-ospecific. Therefore, the optically active product is a result of the initial stereoselective adsorption of the reactant, which in turn, is a consequence of the interactions between reactant, modifier, and catalyst. The entities form an intermediate chelate complex where reactant and modifier are bound to the same surface atom (Scheme 14.4). The orientation of the reactant in such a complex is determined by the most stable configuration of the overall complex intermediate. The mechanism predicts that OY only depends on the relative concentrations of keto and enol forms of the reactant,... 

A strongly coupled surface complex between a formate ion and adsorbed formic acid is the decomposition intermediate on silver (58). 

The surface complex shown in Scheme V may be identical with the common C intermediate of C5 cyclization and C5 cyclic isomerization 15a). Its degree of dissociation may not be too high (57), and the bond between it and the metal may have some sort of fluxional character. 

Many of the characterization techniques described in this chapter require ambient or vacuum conditions, which may or may not be translatable to operational conditions. In situ or in opemndo characterization avoids such issues and can provide insight and information under more realistic conditions. Such approaches are becoming more common in X-ray adsorption spectroscopy (XAS) methods ofXANES and EXAFS, in NMR and in transmission electron microscopy where environmental instruments and cells are becoming common. In situ MAS NMR has been used to characterize reaction intermediates, organic deposits, surface complexes and the nature of transition state and reaction pathways. The formation of alkoxy species on zeolites upon adsorption of olefins or alcohols have been observed by C in situ and ex situ NMR [253]. Sensitivity enhancement techniques play an important role in the progress of this area. In operando infrared and RAMAN is becoming more widely used. In situ RAMAN spectroscopy has been used to online monitor synthesis of zeolites in pressurized reactors [254]. Such techniques will become commonplace. 

This pentacoordinated surface intermediate undergoes rapid hydrogenolysis of the Sn-R bonds (Scheme 2.36). The primary surface complex [MsSnRs] is formed [121]. 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

The Ea for the dissolution of hematite by mercapto carboxylic acids in acid media in the presence of UV radiation was lower (64 5 kj mol ) than that for dissolution in the absence of radiation (94 8 kJ mol ) (Waite et al. 1986). The reaction in both cases was considered to involve formation of an intermediate organic-Fe surface complex which decomposed as a result of intramolecular electron transfer to release Fe". UV irradiation enhanced the decomposition of the surface complex either through excitation of the ligand field states associated with the free electrons on the S atoms, or through high energy charge transfer states. 

The similarity between the measured activation energies for the reaction-limited production of acrolein over Cu20(100) from allyl alcohol in UHV or propene following a 1 atm. exposure gives a clear indication that these reactions involve the same surface intermediate, an allyloxy. This similarity also suggests that the surface intermediates formed by these two routes behave in a chemically similar fashion. For the (100) surface, the Cu -alkoxide surface complex is similar regardless of whether oxygen from... 

The key requirement for a SET step in the photocatalytic process seems to be the surface complexation of the substrate, according to an exponential dependence of the probability of electronic tunneling from the distance between the two redox centers [66]. However, as was pointed out in the preceding section on the key role of back reactions, the presence of a SET mechanism could be a disadvantage from an applicative point of view. If the formed SET intermediate (e.g., a radical cation) strongly adsorbs and/or does not transform irreversibly [e.g., by loss of CO from a carboxylic acid or fast reaction with other species (e.g., superoxide or oxygen)], it can act as a recombination center, lowering the overall photon efficiency of the photocatalytic process. 

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