Surface vibrational spectroscopic catalysts

Relatively few vibrational spectroscopic investigations have been reported for acyclic alkanes, whether adsorbed on single-crystal or on finely divided metal surfaces. The spectra of the cyclic alkanes are more conveniently discussed later (Section V.A) because of their relationships to the spectra of aromatic species into which they are readily converted by metal catalysts. 

Comparisons with the vibrational spectroscopic studies of the adsorption and dehydrogenation of ethene on single-crystal Pt surfaces (Section X.B.l) show that the di-cr-C2H4 to ethylidyne conversion occurs on (111) facets of the Pt crystallites of the catalysts. It is considered that the di-cr -C2H4 species occur on metal sites on which this conversion is not allowed, perhaps on (100), (110), or (210) facets. It is not clear whether the labile it-C2H4 species is formed on amorphous areas of the clean Pt particles or whether it occurs on sites which are affected by proximity of the metal oxide support (408) we favor the former possibility. 

In this article (Part I) we have comprehensively reviewed the structural implications of the vibrational spectroscopic results from the adsorption of ethene and the higher alkenes on different metal surfaces. Alkenes were chosen for first review because the spectra of their adsorbed species have been investigated in most detail. It was to be expected that principles elucidated during their analysis would be applicable elsewhere. The emphasis has been on an exploration of the structures of the temperature-dependent chemisorbed species on different metal surfaces. Particular attention has been directed to the spectra obtained on finely divided (oxide-supported) metal catalysts as these have not been the subject of review for a long time. An opportunity has, however, also been taken to update an earlier review of the single-crystal results from adsorbed hydrocarbons by one of us (N.S.) (7 7). Similar reviews of the fewer spectra from other families of adsorbed hydrocarbons, i.e., the alkynes, the alkanes (acyclic and cyclic), and aromatic hydrocarbons, will be presented in Part II. 

There is a number of vibrational spectroscopic techniques not directly applicable to the study of real catalysts but which are used with model surfaces, such as single crystals. These include reflection-absorption infrared spectroscopy (RAIRS or IRAS) high-resolution electron energy loss spectroscopy (HREELS, EELS) infrared ellipsometric spectroscopy. 

Inelastic neutron scattering (INS) is a spectroscopic technique in which neutrons are used to probe the dynamics of atoms and molecules in solids and liquids. This book is the first, since the late 1960s. to cover the principles and applications of INS as a vibrational-.spectroscopic technique. It provides a hands-on account of the use of INS. concentrating on how neutron vibrational spectroscopy can be employed to obtain chemical information on a range of materials that are of interest to chemists, biologists, materials scientists, surface scientists and catalyst researchers. This is an accessible and comprehensive single-volume in imary text and reference source. 

A number of modern physical techniques are used to characterize heterogeneous catalysts. These methods range from techniques probing the interaction of catalysts with probe molecules, to in situ surface characterization techniques as well as structural elucidation under both in situ and ex situ conditions. In general, interaction of catalysts with probe molecules is followed using some spectroscopic property of the probe molecule itself and/or the changes induced by the heterogeneous catalyst. The spectroscopic techniques used include vibrational spectroscopies, NMR spectroscopy, UV-Vis spectroscopy and mass spectrometry to name a few examples. Similarly, in situ techniques tend to use properties of probe molecules but also combined with structural techniques such as X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). In recent years XAS has been widely used in the characterization of catalysts and catalyst surfaces. 

IR spectroscopy was mainly used to characterize the sorbed species. The zeolite powder was pressed into self supporting wafers and analyzed in situ during all treatments (i.e., activation, sorption, reaction) by means of transmission absorption IR spectroscopy using a BRUKER IPS 88 FTIR spectrometer (resolution 4 cm" ). For the sorption experiments, an IR cell equipped with IR transparent windows which could be evacuated to pressures below 10" mbar was used [11]. The activated zeolite wafer was contacted with a constant partial pressure (0.001 mbar) of the adsorbate at 308 K until adsorption-desorption equilibrium was reached (which was monitored by time resolved IR spectroscopy). For the coadsorption experiments, the catalysts were equilibrated with 0.001 mbar of both adsorbates admitted in sequentional order. The spectra were normalized for the sample thickness by comparing the intensities of the absorption bands of the adsorbate with the integral intensity of the lattice vibration bands of the zeolite between 2090 and 1740 cm". The surface coverage was quantified by calibration with gravimetric measurements (under conditions identical to the IR spectroscopic experiments). 

Hydride complexes of platinum have received considerable study since the preparation of PtHCl(PEt3)2- Spectroscopic studies by NMR techniques have been widely used because of the structural information which can be obtained from coupling constant data to Pt and other nuclei. Platinum is widely used as a heterogeneous catalyst, and vibrational studies on platinum hydride complexes have been useful for comparison of a hydrogen atom bonded to a single platinum with that bonded to a surface. Complexes of platinum have been used to catalyze hydrogenation, hydrosilylation and isomerization reactions with alkenes and alkynes, as well as H/D exchange reactions on alkanes. Hydride complexes are frequently proposed as intermediates in these reactions, and the pathways related to the known chemistry of hydride complexes. 

Spectroscopic investigations also showed that the introduction of the solid alkali did not lead to an anomalous population of rotational levels in NH but raised the vibrational temperature from = 3(XK) K to c= 4000 K. It was sugg ted that the recombination of ions, presumably N, on the catalyst surface leads to an increase in the number of nitrogen molecules at high levels of vibrational excitation. However, the location of NaOH catalyst at the cathode or anode does not influence significantly either the decomposition of ammonia or the synthesis of hydrazine (Fig. 25)... 

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