In-situ IR measurements

In-situ IR measurements overcome the problems mentioned for in situ NMR spectroscopy. The information that we obtain from vibrational spectroscopy is far less detailed, however, than that from NMR. The concentration of the catalyst may be equal to the one used in practical catalytic systems. Secondly, autoclaves have been equipped with IR cells, either as flow cells or via real in-situ monitoring in the Moser cell (see below), which allows one to work with gaseous reactants. In the following we will mention a (very) few examples of complexes that may be intermediates in the hydroformylation reaction observed with these two techniques. 

More recently, Dalla Betta and Shelef (51) performed in situ IR measurements with AljOj-supported ruthenium, exposed to 1 bar total pressure mixtures of H2 CO He = 0.075 0.025 0.9 at temperatures from 250°C upward. Up to 250°C adsorbed CO was present at almost complete monolayer coverage. At higher temperatures the coverage decreased. As the phenomenon is irreversible with respect to a lowering of the reaction temperature, the authors conclude that it reflects surface blocking by a reaction residue rather than a temperature dependence of the carbon monoxide adsorption-desorption equilibrium. 

Suzuki S, Nakashima S (1999) In situ IR measurements of OH species in quartz at high temperatures. Phys Chem Minerals 26 217-225... 

Addition of lithiated trimethylsilyldiazomethane to aldehydes provided adduct 103 which was shown through in situ IR measurements and quenching experiments to be in equilibrium with the 1,3-Brook-rearranged carbanion 104. Addition of methanol or methyl iodide provided 105 and 106, respectively. Stereocontrolled hydride shift upon formation of the rhodium-stabilized carbenoid provided (2)-selective formation of silyl enol ethers 107 and 108. ... 

Fig. 1.6. Thin layer cell for in situ IR absorption-desorption measurements based in a model proposed by Seki et al. [32]. WE = working electrode, W = window, L=lead, M = micrometer screw, S = steel spring, SM = simmer gasket.

The effect of CO2 poisoning has been well understood. Gut et al. showed that CO is produced by a Pt anode when fed with CO2/H2 mixtures, indicating that the reverse water-gas shift reaction is active at cell operating temperatures of 70°C. ° As previously observed, this has low performance over and above that expected for dilution. Gut et al.. Ball et al., and de Bruijn et al. showed that PtRu anodes were less poisoned by CO2. Using in situ IR and mass spectroscopy measurements, Smolinka et al. showed that CO2... 

The ATR technique is now routinely used for IR spectroscopy as it allows measurement of spectra for a variety of sample types with minimal preparation. The crystals employed are generally prismatic in shape, allowing contact of a flat surface with the sample. The ATR method was first adapted for HP IR spectroscopy by Moser [29-33], who realised that a conventional autoclave could easily be adapted for in situ IR spectroscopy by fitting an ATR crystal of cylindrical cross section. The technique developed by Moser is thus known as cylindrical internal reflectance (CIR) spectroscopy and high pressure cells based upon the CIR method have been commercialised by Spectra-Tech. A typical CIR cell is illustrated in Figure 3.8. 

The above examples illustrate clearly the impact of HP-NMR and HP-IR on our insight into the mechanism of the rhodium catalyzed hydroformylation. In situ IR can be applied on a routine basis to probe the resting state of the system, provided that a cell that is not prone to diffusion limitations is used. The majority of ligands lead to rhodium hydrides as the resting state and only a few systems, viz those rich in carbonyls or very electron poor, show acyl species as the resting state. A combination of IR and NMR measurements is needed to obtain full characterization of all species. 

Infrared spectroscopy can provide a great deal of information on molecular identity and orientation at the electrode surface [51-53]. Molecular vibrational modes can also be sensitive to the presence of ionic species and variations in electrode potential [51,52]. In situ reflectance measurements in the infrared spectrum engender the same considerations of polarization and incident angles as in UV/visible reflectance. However, since water and other solvents employed in electrochemistry are strong IR absorbers, there is the additional problem of reduced throughput. This problem is alleviated with thin-layer spectroelectro-chemical cells [53]. 

The increasing application of spectroscopic methods in electrochemistry has characterized the last decade and marked the beginning of new developments in electrochemical science [1]. Among these methods, in-situ infrared spectroscopy provides a very useful tool for characterizing the electrode-solution interface at a molecular level. First in-situ infrared (IR) electrochemical measurements were performed in 1966 [2] using the internal reflection form [3]. However, problems in obtaining very thin metal layers on the surface of the prisms used as IR windows, delayed the extensive application of in-situ IR spectroscopy until 1980, when the method was applied in the external reflection form [4]. The importance of this step does not need to be emphasized today. 

Unfortunately the tendency to create expressions just highlighting some particular experimental approach have led to an unnecessary multiplicity of names in the literature for the in-situ IR method. They give the erroneous impression that a large number of different mehtods exist, while in fact these are only different experimental approaches all using the same basic principle the subtraction of two spectra measured either at two potentials or under two conditions of light polarization. 

We report an in situ IR spectroscopic study performed during sorption and kinetic measurements to show that these adsorbate structures consequently predetermine the chemical selectivity. The importance of the individual surface species (and their relative concentrations) on the activity and selectivity of the catalyst for the alkylation of aromatic molecules is investigated. 

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