In situ spectroscopic studies

In Situ Spectroscopic Studies of Oxygen Electrocatalysts Involving Transition Metal Macrocycles... 

Carefully determined conversion-time diagrams, in-situ spectroscopic studies and, if possible, kinetic time laws belong to the fundamentals of catalysis research and are prerequisites for a mechanistic understanding [8]. 

Whyman, R., In Situ Spectroscopic Studies in Homogeneous Catalysis. In Moser,... 

An added difficulty that arises in the in-situ spectroscopic study of electrocatalytic systems in solution is that the active species will be located in the vicinity of the electrode so that the material in solution will generally represent a large background signal making the detection and identification of related species difficult. Thus, it would be ideal to be able to probe only that region proximal to the electrode surface and furthermore to be able to obtain structural information of the species involved. 

Ford et al.60 also made a significant contribution to the metal carbonyl catalyzed shift reaction in acidic medium. A solution of Ru3(CO)i2 (0.006-0.024 M with 0.25-2.0 M H2S04 4.0-12.0 M H20) in 5 ml of diglyme had good catalytic activity at 100 °C. They used a batch reactor with Pqo = 0-9 atm. Typical H2 turnover activity was reported to be about 50 turnovers per day. Their in situ spectroscopic studies show that the principal component was HRu2(CO)8-. They found that, at low CO partial pressures (< 1 atm), the catalysis was first order in Ru. However, at high CO partial pressures, the rate was inhibited. On the basis of their studies, they proposed the catalytic cycle outlined in Scheme 15. 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

M. A. Habib and J. O M. Bockris, Potential-Dependent Water Orientation An In Situ Spectroscopic Study, l.anginuiil 388 (1986). 

Recently, it was proven by in situ spectroscopic studies [36] that the mechanism of the Friedel-Crafts acetylation is not altered by using ionic liquids. Experimental data... 

In situ spectroscopic studies have identified a variety of species, such as formate, dioxymethylene, carbonate, and methoxide, to coexist under methanol synthesis conditions on Cu/ZnO-based catalysts [22, 23], Fourier transform infrared spectroscopy studies of CuZn-based catalysts under H2/C02 identified the presence of formate bound to both Cu and ZnO, whereas methoxide was found on ZnO only. Carbonates were found to form via C02 adsorption on ZnO [24] and partially oxidized Cu [23], and were quickly converted into formate via Cu-activated hydrogen. Upon exposure to CO mixtures, only zinc-bound formate was observed [22], The hydrogenation of these formates to methoxide is thought to be rate determining in methanol synthesis. 

H. Baltruschat, R. BuBar, S. Ernst and F. Hernandez, in In-situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis Paul A. Christensen, Andrzej Wieckowski and Shi-Gang Sun (eds), Elsevier, 2007. 

Csihony, S., Mehdi, H., Homonnay, Z., Vertes, A., Farkas, O., Horvath, I. T. In situ spectroscopic studies related to the mechanism of the Friedel-Crafts acetylation of benzene in ionic liquids using AICI3 and FeCl3. J. Chem. Soc., Dalton Trans. 2002, 680-685. 

Grunwaldt, J.-D. Wandeler, R. Baiker, A. Supercritical fluids in catalysis opportunities of in situ spectroscopic studies and monitoring phase behavior. Catal. Rev. Sci. Eng. 2003, 45 (1), 1-96. 

The use of infrared spectroscopy in the Earth and environmental sciences has been widespread for decades however, until development of the attenuated total reflectance (ATR) technique, the primary use was ex situ material characterization (Chen and Gardella, 1998 Tejedor-Tejedor et al., 1998 Degenhardt and McQuillan, 1999 Peak et al., 1999 Wijnja and Schulthess, 1999 Aral and Sparks, 2001 Kirwan et al., 2003). For the study of environmental systems, the strength of the ATR-Fourier transform infrared (FTIR) technique lies in its intrinsic surface sensitivity. Spectra are collected only from absorptions of an evanescent wave with a maximum penetration depth of several micrometers from the internal reflection element into the solution phase (Harrick, 1967). This short optical path length allows one to overcome any absorption due to an aqueous phase associated with the sample while maintaining a high sensitivity to species at the mineral-water interface (McQuillan, 2001). Therefore, ATR—FTIR represents a technique capable of performing in situ spectroscopic studies in real time. 

Ex-Situ and In-Situ Spectroscopic Studies of the Passive Film on Lithium in Non-Aqueous Solvents", D. Scherson and... 

Ex-situ and in-situ spectroscopic studies of the passive film on lithium, Quarterly report (9/30/1991 -12/31/1991) to LBL, Uni. of California. Departments of Chemistry and Physics, Case Western Reserve University, 12/1991. 

In the face of these confusing results, Stanley and co-workers decided to initiate an extensive series of in situ spectroscopic studies to understand what was occurring under hydroformylation conditions with the neutral and dicationic precursor complexes rac-1 and rac-4, why they give such dramatically different catalytic results, in marked contrast to mononuclear hydroformylation catalyst. In a recent book on catalysis by di- and polynuclear metal complexes, edited by R. D. Adams and F. A. Cotton, George G. Stanley gives a detailed account of this fascinating story which reads as exciting as a detective novel. [12]... 

Scheme 4, Interpretation of the in situ spectroscopic studies on the reactivity of the dicationic complex rac-8 towards CO and H2.

Early studies at platinum electrodes have shown that the formation of adsorbed CO2 requires the presence of adsorbed hydrogen, thus, a reduction takes place leading to the surface species denoted as reduced CO2. According to recent in situ spectroscopic studies, both linearly bonded and multibonded CO molecules are the main adsorbates formed as a result of CO2 reduction at polycrystalline platinum electrodes. 

Grunwddt, J.D., Wandeler, R., Baiker, A. (2003) Supercritical fluids in catalysis opportunities of in situ spectroscopic studies and monitoring phase behavior, Catal. Rev. Sci. Eng. 45, Nl, 1-96 Minder, R, Mallat, T., Pickel, K.H., Steiner, K., Baiker, A. (1995) Enantioselective hydrogenation of ethyl pyruvate in supercritical fluids, Catal. Lett. 34, 1-9. 

In addition to simultaneous in situ spectroscopic studies, accompanying ex situ investigations also provide valuable information about the specific interaction of the substrates (imines and respective hydrogenation products) with both the chiral modifier (P-acid) and the solid catalyst. Thus, FTIR spectroscopic analysis of the catalyst after adsorption of the imine points to a strong interaction of the latter with the catalyst surface, in particular with the support, which is reflected by marked band shifts. It could be shown that the surface of the catalyst is mainly covered by the imine after use in the hydrogenation reaction, besides some small quantities of the product [11]. 

Two examples supported by detailed in situ spectroscopic studies will highlight the above problem. 

Liu G, Hakimifard M, Garland M (2001) An in situ spectroscopic study of the mtheniiun catalyzed carbonylatirm of piperidine starting with trimthcminm dodecacarbonyl the importance of path dependence in hmnogeneous catalysis. J Mol Catal A Chem 168 33-37... 

Generally, the ad-atoms cause positive catalytic effects with significant enhancement of the electrocatalytic activity of platinum in several cases. Several reviews have been published on this subject [26-28, 67]. Very recently Ross [68] and Jarvi and Stuve [29] have discussed the more recent advances in our understanding of the fundamentals of Ci electrocatalysis by ad-atoms. The different types of enhancement (third-body effect, bifunctional mechanism, poison destabilization, and electronic modification) are well documented. The new information obtained from the in situ spectroscopic studies about the nature of poisons and the dependence of their coverages on potential, as well as the use of single-crystal electrodes with defined surface structure and specific reactivity, enables a deeper insight in the electrocatalysis by ad-atoms. As a general rule, one can say that, except for methanol, the more susceptible... 

Precatalytic Reactions and Xpre. The catalyst precursor must transform under reaction conditions into intermediates to obtain an active system. This transformation may involve, in a small number of cases, only a single elementary step, for example, the dissociation of a ligand from a transition-metal complex. However, a series of elementary reaction steps are usually required to convert the catalyst precursor. Useful examples include (1) the degradation of a polynuclear precursor to mononuclear intermediates, (2) the modification of a precursor with a ligand L which is used to control selectivity, and (3) the transformation of finely divided metal. The characteristic time scale for the precatalytic reaction will be denoted tpre, and the instantaneous reaction rate will be denoted Ppre- Precatalytic phenomena and the associated induction periods have been directly monitored in a number of in situ spectroscopic studies using a variety of mononuclear, dinuclear, polynuclear, and metallic precursors (11). 

At low temperatures, the only products that form are N2O and N2. In situ spectroscopic studies of working Cu and Ag catalysts show that apart from adsorbed oxygen, there is a high surface coverage of nitrite and nitrate speciesl . Hence, on these metals at low temperature, N2 and N2O production is likely the result of consecutive reactions of NOj, the most abundant reaction intermediate (MARI), with NH3. N2 is formed by the reaction of nitrite with NH3, whereas N2O can also form via reaction of nitrate with ammonia (see also Section 6.4.1). 

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