Modulation excitation spectroscopy (MES) allows sensitive and selective detection and monitoring of the dynamic behavior of species directly involved in a reaction. The method, combined with proper in situ spectroscopy, is powerful for elucidating complex systems and noisy data as often encountered in heterogeneous catalytic reactions at solid–liquid and solid–gas interfaces under working conditions. The theoretical principle and actual data processing of MES are explained in detail. Periodic perturbation of the system by an external parameter, such as concentration and temperature, is utilized as stimulation in MES. The influence of stimulation shape upon response analysis is explained. Furthermore, an illustrative example of MES, enantioselective hydrogenation at a solid-liquid interface, is presented.

A method for in situ monitoring of surface and gas species utilizing separately the difference and sum reflectivity of two polarizations, normal and parallel to the surface, measured by polarization-modulation infrared reflection-absorption spectroscopy is presented. Surface and gas-phase spectra were separately but simultaneously obtained from the reflectivities. The technique is combined with modulation excitation spectroscopy to further enhance the sensitivity, and a small-volume cell was designed for this purpose. CO oxidation over a 40 nm Pt film on aluminum was investigated under moderate pressure (atmospheric pressure, 5% CO, and 5%–40% O₂) at 373–433 K. The surface species involved in the oxidation process and the gas-phase species, both reactant (CO) and product (CO₂), could be simultaneously monitored and analyzed quantitatively. In addition, the reflectivity change of the sample during the reaction was assigned to a near-surface bulk property change, that is, surface reconstruction to the oxide phase. Under an O₂-rich atmosphere, two reactive phases, denoted as low- and high-activity phases, were identified. A large amount of atop CO was observed during the low-activity phase, while the adsorbed CO completely disappeared during the high-activity phase. The presence of an infrared-inactive CO₂ precursor formed by the reaction between surface oxide and gaseous CO during the high-activity phase was inferred. The desorption of the CO₂ precursor is facilitated under a CO-rich atmosphere, most likely, by surface reconstruction to metallic Pt and a competitive adsorption of CO on the surface.

Square-wave stimulation used in modulation excitation spectroscopy [D. Baurecht, U.P. Fringeli, Rev. Sci. Instrum. 72 (2001) 3782] can have significant advantages over a simple sinusoidal-wave due to the high odd-frequency terms contained in square-wave, particularly when a system response is close to linear. Phase-sensitive detection (PSD) affords separating the signals of the different frequency terms with a high signal-to-noise ratio by averaging a number of modulation cycles. A modulation excitation experiment applying square-wave stimulation provides the same information as several experiments applying sinusoidal-wave stimulations at the same frequency as the square-wave stimulation and at higher frequencies. The amplitude and the phase lag of a response obtained by PSD at fundamental and higher frequencies using square-wave stimulation are related to the ones obtained by sinusoidal-wave stimulation using transfer function of a general system. Mixing property of a PM-IRRAS (polarization–modulation infrared reflection–absorption spectroscopy) flow-through cell was studied by a simple mixing tank model using square-wave concentration stimulation. The advantages of square-wave stimulation are shown by the characterization of the mixing property.

Polarization-modulation infrared reflection-absorption spectroscopy (Pm-irras) is a sensitive tool for the analysis of species residing at gas-solid and gas-liquid interfaces. the polarization-modulation allows excellent back-ground compensation and the analysis of surface/interface species under moderate pressure (e.g. atmospheric pressure of ir-absorbing gases) is possible. we demonstrate a new possibility to extract simultaneously information of gas and solid phases in addition to surface species from the Pm-irras experiments, using co oxidation over Pt film as an example. modulation excitation spectroscopy (mes) has been combined with this technique to enhance the sensitivity and to analyze the kinetic behavior of species. the surface species involved in the oxidation process, the state of Pt, and the gas phase species (co and co₂) could be simultaneously monitored in situ and analyzed quantitatively. the technique can serve as a valuable tool for investigations of various dynamic phenomena occurring at gas-solid interfaces.

Tetrakis(trimethylsiloxy)titanium (TTMST, Ti(OSiMe₃)₄) possesses an isolated Ti center and is a highly active homogeneous catalyst in epoxidation of various olefins. The structure of TTMST resembles that of the active sites in some heterogeneous Ti−Si epoxidation catalysts, especially silylated titania−silica mixed oxides. Water cleaves the Ti−O−Si bond and deactivates the catalyst. An alkyl hydroperoxide, TBHP (tert-butyl hydroperoxide), does not cleave the Ti−O−Si bond, but interacts via weak hydrogen-bonding as supported by NMR, DOSY, IR, and computational studies. ATR−IR spectroscopy combined with computational investigations shows that more than one, that is, up to four, TBHP can undergo hydrogen-bonding with TTMST, leading to the activation of the O−O bond of TBHP. The greater the number of TBHP molecules that form hydrogen bonds to TTMST, the more electrophilic the O−O bond becomes, and the more active the complex is for epoxidation. An allylic alcohol, 2-cyclohexen-1-ol, does not interact strongly with TTMST, but the interaction is prominent when it interacts with the TTMST−TBHP complex. On the basis of the experimental and theoretical findings, a hydrogen-bond-assisted epoxidation mechanism of TTMST is suggested.

The comparison between experimental and calculated VCD spectra allowed the unequivocal assignment of the absolute configuration of heptahelicene C₃₀H₁₈ as P(+).

A new ATR-IR cell was designed, and its performance was characterized by modulation excitation spectroscopy (MES). The new cell allows concentration modulation at relatively high frequency without unnecessary phase delay in the response. The response delay due to convection and diffusion was studied at different flow rates and modulation frequencies by experiments and simulations. The diffusion behavior of a small relatively fast-diffusing molecule, acetonitrile, was compared with that of a large slow-diffusing molecule, hemoglobin, in water. Experimentally, significant differences in their diffusion behavior were observed. The flow and diffusion behavior of the probe molecules was described using two different models, the diffusion layer model and the convection−diffusion model, and the theoretical results were compared with the experiments. The diffusion layer model allows estimating an effective diffusion layer thickness near the surface of the internal reflection element. However, the simulated response is significantly different from the experimental one. On the other hand, the convection−diffusion model describes the flow and diffusion behavior of the solute molecules with high accuracy. This work forms the basis for the investigation of chemical and physical kinetics such as surface reaction and diffusion by MES. It also suggests criteria for appropriate experimental conditions in ATR-IR MES experiments.