Single crystal studies

A quite nice illustration of such a type of study has been published in 1985 by Bachman et al. on a 6 pm CaF crystal This was the smallest single crystal ever used for X-ray diffraction experiment. For an ideally imperfect crystal, the scattering power S may be defined as  

The dissociation of NO is a crucial elementary step in the mechanism of NO reduction by CO in automotive exhaust catalysis. The overall reaction here is  

Analyzing the rates of dissociation and desorption (as described in Chapter 2) yields kinetic parameters for the elementary steps (see Table 4.2). The attractive feature of static SIMS is that it provides a way to measure the kinetics of reactions on the surface, which would be very difficult to do with any other technique. Of course, such experiments require that SIMS operates under truly static conditions 

Another example of this type of investigation is the SIMS study of the H-D isotope exchange reaction in ethylidyne on platinum, as reported by Creighton et al. 

As a final example, we show how SIMS can be used to identify the ratedetermining step in a sequence of elementary reactions [41]. Imagine the following situation we have a Rh(lll) surface partially covered by N-atoms which we 

The reduction of chemisorbed O on Fe(lOO) by H2 has been studied by Augeer electron spectroscopy and LEED at 473 and 673 K [134]. The reaction proceeds via the formation of H [134] no reaction is detected if the surface is completely covered by O initially [134]. The apparent activation energy of the reaction is 59 + 4kJ/mole [134]. The transport of O to the surface is fast compared to the surface reaction [134]. 

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Other Inorganics. Inorganic species in solution have been studied very effectively by Raman spectroscopy. Work in this area includes the investigation of coordination compounds (qv) of fluorine (qv) (40), the characterization of low dimensional materials (41) and coordinated ligands (42), and single-crystal studies (43). Several compilations of characteristic vibrational frequencies of main-group elements have been pubflshed to aid in the identification of these species (44,45). 

Theories of the oxidation of tantalum in the presence of suboxide have been developed by Stringer. By means of single-crystal studies he has been able to show that a rate anisotropy stems from the orientation of the suboxide which is precipitated in the form of thin plates. Their influence on the oxidation rate is least when they lie parallel to the metal interface, since the stresses set up by their oxidation to the pentoxide are most easily accommodated. By contrast, when the plates are at 45° to the surface, complex stresses are established which create characteristic chevron markings and cracks in the oxide. The cracks in this case follow lines of pores generated by oxidation of the plates. This behaviour is also found with niobium, but surprisingly, these pores are not formed when Ta-Nb alloys are oxidised, and the rate anisotropy disappears. However, the rate remains linear it seems that this is another case in which molecular oxygen travels by sub-microscopic routes. 

R.B. Grant, and R.M. Lambert, A single crystal study of the silver-catalyzed selective oxidation and total oxidation of ethylene, 7. Catal. 92, 364-375 (1985). 

Table XXI shows the crystallographic data as far as they are known. The structures of the three sulfide iodides have been determined by single-crystal studies. Powder patterns of the 8n(IV) compounds not given here have been reported elsewhere (24).

Figure 2. A comparison of the rate (turn-over frequency) of methane synthesis over single crystal and supported ruthenium catalysts. Total reactant pressure for the single crystal studies was 120 Torr.

Mechanistic Details Revealed by Single-Crystal Studies... 

Dilute Au in Zn and Cd single crystals Study of the microscopic vibrational properties of the Au impurity in the host lattice... 

In their 1967 book, Atkins and Symons6a summarized much of the early ESR work on small inorganic radicals, many of which were produced by photolysis or radiolysis of single crystals of the precursor molecules. A good example of the application of the information that can be obtained from such single-crystal studies is the analysis of the spectrum of N03, produced by y-irradiation of KN03 crystals by Livingston and Zeldes.6b Table 4.1 summarizes the results. 

Le Pape, L., Lamotte, B., Mouesca, J.-M., and Rius, G. 1997. Paramagnetic states of four iron-four sulfur clusters. 1. EPR single-crystal study of 3+ and 1+ clusters of an asymmetric model compound and general model for the interpretation of the g-tensors of these two redox states. Journal of the American Chemical Society 119 9757-9770. 

Mono- or single-crystal materials are undoubtedly the most straightforward to handle conceptually, however, and we start our consideration of electrochemistry by examining some simple substances to show how the surface structure follows immediately from the bulk structure we will need this information in chapter 2, since modern single-crystal studies have shed considerable light on the mechanism of many prototypical electrochemical reactions. The great majority of electrode materials are either elemental metals or metal alloys, most of which have a face-centred or body-centred cubic structure, or one based on a hexagonal close-packed array of atoms. 

Magnetic measurements on oriented single crystals are the ideal technique for direct observation of the magnetic anisotropy. Therefore at a first glance, it may appear surprising that relatively few single-crystal studies on lanthanide... 

Both ionic forms have the same set of characteristic bands in their IR spectra and thus cannot be identified in solution. The 3IP NMR spectrum contains only one averaged signal. The IR study revealed tautomeric transformations in solution. The characteristic absorption band of the P—H bond and carbonyl group appeared in solution spectra that is possible only on dissociation of the a-hydroxyalkyl fragment, present only in the second tautomeric form. An X-ray single-crystal study showed the compound (128) to be in a cyclic form. 

A single-crystal study of oxidized plastocyanin demonstrated that the absence of Cu-S(Met) EXAFS was due to Debye-Waller damping (30). The analysis benefits from the three-fold enhancement of Cu-S(Met) scattering amplitude expected for... 

Yamaguchi K (1995) Phonons of indium selenide single crystals studied by inelastic electron tunneling spectroscopy. Phys Status Solidi B 190 409 113... 

I would also like to thank Dr. D. A. Dyk and Dr. S. L. Gupta for their skillful assistance for my experiments and us id discussions. Specific thanks go to Dr. I. Bae for his instruction for the in situ infrared spectroscopic measurement. I am thankful for Dr. S. Wasmus for the assistance in the use of the on-line mass spectrometer. I thank Dr. K. Sashikata for the instruction for the platinum single crystal studies. 

The same group used EMLRS technique to single crystal studies. - Although COad detected on all three low index surfaces. HCOad ss foxmd only on Ptdll) and Pt(lOO) as active intermediates. 

Recently, the steady-state reaction kinetics of CO oxidation at high pressure over Ru , Rh " , Pt, Pd, and Ir single crystals have been studied in our laboratory. These studies have convincingly demonstrated the applicability and advantages of model single crystal studies, which combine UHV surface analysis techniques with high pressure kinetic measurements, in the elucidation of reaction mechanisms over supported catalysts. 

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