Spectroscopy in-situ

Second, catalytic reactions do not necessarily proceed via the most stable adsorbates. In the ethylene case, hydrogenation of the weakly bound Jt-C2H4 proceeds much faster than that of the more stable di-cr bonded C2H4. In fact, on many metals, ethylene dehydrogenates to the highly stable ethylidyne species, =C-CH3, bound to three metal atoms. This species dominates at low coverages, but is not reactive in hydrogenation. It is therefore sometimes referred to as a spectator species. Hence, weakly bound adsorbates may dominate in catalytic reactions, and to observe them experimentally in situ spectroscopy is necessary. 

P. Canton, C. Meneghini, P. Riello, A. Benedetti, in B. M. Weckhuysen (ed.) In-Situ Spectroscopy of Catalysts, American Scientific Publishers, CA, USA, 2004. 

Pietrzyk, P., Gil, B. and Sojka, Z. (2007) Combining computational and in situ spectroscopies joint with molecular modeling for determination of reaction intermediates of deNOx process -CuZSM-5 catalyst case study, Catal. Today. doi 10.1016/j.cattod.2006.09.033. 

Banares, M.A. (2005) Operando methodology combination of in situ spectroscopy and simultaneous activity measurements under catalytic reaction conditions, Catal. Today, 100, 71. 

Chen, P. and Somorjai, G.A (2002) In situ catalysis and surface science methods, In-Situ Spectroscopy in Heterogeneous Catalysis (ed. J.F. Haw), Wiley-VCH Verlag, Weinheim, pp. 15-52. 

Phosphites have been used as ligands in Rh-catalyzed hydroformylation from the early days since their introduction in 1969.205 206 Identification of complexes occurred more recently. Ziolkowski and Trzeciak have studied extensively the use of phosphite ligands in the Rh-catalyzed hydroformylation of alkenes.207-210 The ligand tris(2-/er/-butyl-4-methylphenyl) phosphite (65) leads to extremely fast catalysis and in situ spectroscopy showed that under the reaction conditions only a mono-ligated complex [Rh(H)(CO)3(65)], (66), is formed due the bulkiness of the ligand.211-213... 

Ultrahigh vacuum surface spectroscopies can provide far greater breadth and depth of information about surface properties than can yet be achieved using in situ spectroscopies at the aqueous/metaI interface. Application of the vacuum techniques to electrochemical interfaces is thus desirable, but has been plagued by questions of the relevance of the emersed, evacuated surfaces examined to the real electrochemical interfaces. This concern is accentuated by surface scientists observations that in UHV no molecular water remains on well-defined surfaces at room temperature and above (1). Emersion and evacuation at room temperature may or may not produce significant changes in electrochemical interfaces, depending.on whether or not water plays a major role in the surface chemistry. 

J.F. Haw (Ed.), In-Situ Spectroscopy in Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2002. 

Further theoretical studies supported by in situ spectroscopy and high-resolution microscopy are needed to be able to understand this unusually strong bonding between Cu and Ce. To apply such first-principles quantum chemical MD approach, new computational methods accelerating computational time by several orders of magnitude must be developed. 

In Situ Spectroscopy—Electronic Structure and Redox Behavior of Nickel... 

S.M. Bennici, B.M. Vogelaar, T.A. Nijhuis and B.M. Weckhuysen, Real-time control of a catalytic solid in a fixed-bed reactor based on in situ spectroscopy, Angew. Chem., Int. Ed., 46, 5412-5416 (2007). 

Returning to the general liquid phase catalytic system, assume that you have chosen an appropriate spectroscopy to investigate the system under reaction conditions. The spectroscopy provides spectra, i. e. absorbance A(t), at specific intervals in time. If S denotes the complete set of all species that exist at any time in the physical system, then Sjo s is the subset of all observable species obtained using the in situ spectroscopy. This requires that the pure component spectra aj..as obs are obtainable from the multi-component solution spectra A t) without separation of constituents, and without recourse to spectral libraries or any other type of a priori information. Once reliable spectroscopic information concerning the species present under reaction are available, down to very low concentrations, further issues such as the concentrations of species present, the reactions present, and reaction kinetics can be addressed. In other words, more detailed aspects of mechanistic enquiry can be posed. 

The experienced catalytic chemist or chemical reaction engineer will immediately recognize that the study of a new catalytic reaction system using an in situ spectroscopy, has a great deal in common with the concepts of inverse problems and system identification. First, there is a physical system which cannot be physically disassembled, and the researcher seeks to identify a model for the chemistry involved. The inverse in situ spectroscopic problem can be denoted by Eq. (2). Secondly, the physical system evolves in time and spectroscopic measurements as a function of time are a must. There are realistic limitations to the spectroscopic measurements performed. For this reason as well as for various other reasons, the inverse problem is ill-posed (see Section 4.3.6). Third, signal processing will be needed to filter and correct the raw data, and to obtain a model of the system. The ability to have the individual pure component spectra of the species present in... 

The in situ spectroscopies and the signal processing have limitations. Therefore, the set of observable species is a proper subset of all liquid phase species S. The validity of Eq. (4), namely, that the number of observable species is less than the number of species, is easily verified. Regardless of the instrument, the sensitivity is finite, and some dilute and most trace species must be lost in the experimental noise. In addition, numerous experimental design shortcomings further contribute to the validity of Eq. (4). 

The automatic procedure for reference spectra generation was first demonstrated for the start-up of a homogeneous catalyzed rhodium hydroformylation of cyclo-octene using Rh4(CO)i2 as precursor, n-hexane as solvent and FTIR as the in situ spectroscopy at 298 K [63]. The first n spectra were (i) empty spectrometer compartment (background), (ii) n-hexane at 0.2 MPa in a high pressure thermostatically controlled cell fitted with Cap2 windows (iii) system equilibrated with 2.0 MPa CO, (iv) system upon addition of cyclo-octene, and (v) system upon addition of Rh4(CO)i2. The n=l reference spectrum, which contained atmospheric... 

This chapter has focused on formulating the context for Eq. (2) and demonstrating that a solution exists. The results to date show that a genuine and robust solution to Eq. (2) is available in the absence of a priori information. The solution, BTEM, is a very powerful tool in the investigation of homogeneous catalytic systems using in situ spectroscopies. 

Significant attention has once again been devoted to the in-situ or operando EPR characterisation of the supported VOx catalysts under working conditions.99 100 The term operando spectroscopy has been recently introduced115116 to distinguish it from in-situ spectroscopy which is commonly used in a broader sense not necessarily implying the simultaneous product analysis and spectroscopic characterisation of the catalysts under working conditions. Bruckner et... 

In our experience, the principal challenges in the application of ATR IR spectroscopy for investigations of functioning solid catalysts are associated with the sensitivity of the measurement and the complexity of the samples. The former is an issue common to most surface spectroscopies. The latter has to do with the simultaneous presence of many species at a catalytic solid-liquid interface these species include dissolved reactants, adsorbed intermediates, spectators, and products. The spectra are a superposition of the spectra of the individual species. The question of whether a species is a spectator or instead involved in the catalytic cycle is not easily answered and represents a challenge for in situ spectroscopy in general. Thus, there is a need for specialized techniques to be used in combination with ATR spectroscopy to enhance sensitivity and introduce selectivity. 

The growing interest in physical characterization of solid catalysts as they function has stimulated a new series of congresses, the first held in Lunteren (The Netherlands) in 2003 and the second in Toledo in 2006. The subject has been documented in recent books (B. M. Weckhuysen, Ed., In situ Spectroscopy of Catalysts, American Scientific Publishers, 2004, and J. F. Haw, Ed., In situ Spectroscopy in Heterogeneous Catalysis, Wiley-VCH, 2002) and in topical issues of journals Top. Catal. 15 (2001) Phys. Chem. Chem. Phys. 5, issue 20 (2003) and Catal. Today 113 (2006). It is our intention that our set of volumes be more nearly comprehensive than these publications, as well as providing many newer results. 

If high temperatures eventually lead to an almost equal population of the ground and excited states of spectroscopically active structure elements, their absorption and emission may be quite weak, particularly if relaxation processes between these states are slow. The spectroscopic methods covered in Table 16-1 are numerous and not equally suited for the study of solid state kinetics. The number of methods increases considerably if we include particle radiation (electrons, neutrons, protons, atoms, or ions). We note that the output radiation is not necessarily of the same type as the input radiation (e.g., in photoelectron spectroscopy). Therefore, we have to restrict this discussion to some relevant methods and examples which demonstrate the applicability of in-situ spectroscopy to kinetic investigations at high temperature. Let us begin with nuclear spectroscopies in which nuclear energy levels are probed. Later we will turn to those methods in which electronic states are involved (e.g., UV, VIS, and IR spectroscopies). 

Elsevier, C.J. (1994) NMR at elevated gas pressures and its application to homogeneous catalysis./. Mol. Catal., 92, 285. Weckhuysen, B.M. (2002) Snapshots of a working catalyst possibilities and limitations of in situ spectroscopy in the field of heterogeneous catalysis. Chem. Commun., 97. 

Hunger, M. (2005) Applications of in situ spectroscopy in zeolite catalysis. Micropor. Mesopor. Mater., 82, 241. 

---