Temperature programmed reactions spectroscopy

Although reactions (2-8) through (2-11) explain the results satisfactorily, one needs to be aware that TP studies detect only those reactions in the catalyst that are accompanied by a net production or consumption of gases. Suppose, for instance, that reaction (2-11) is the result of two consecutive steps  

If reaction (2-13) follows reaction (2-12) instantaneously, the effect will not be noticeable in the H2 signal [12]. In spite of these limitations, we conclude that TPS with mass spectrometric detection is a highly useful technique for studying the sulfidation of hydrotreating catalysts. We shall return to the sulfidation of molybdenum oxides in the chapters on photoemission (Chapter 3), ion spectroscopy (Chapter 4), and in a case study on hydrodesulfurization catalysts in Chapter 9. 

This technique is a variant of thermal desorption in which products from a surface reaction are desorbed and separated out mass spectro-metrically. This field has been pioneered by Madix and good reviews of this type of work are available [326, 327]. A few examples of such data 

On the Ru 1010 surface, Larsen and Dickenson [329] found the chemistry somewhat different. The major differences were that large amounts of H20 and CO were evolved and a mechanism involving the formation of an anhydride intermediate (proposed earlier by Falconer and Madix 

A further system which Madix and co-workers [331] have studied in detail is methanol adsorbed on Cu 110. Isotopic labelling was used to ease the interpretation of the reaction mechanism on the surface by differentiating between C-bonded hydrogen and hydroxyl hydrogen and by using labelled 1802 to pre-dose the surface and distinguish it from the oxygen in methanol. The reactions observed in this case are shown in Fig. 43 and the mechanism is 

Steps 2C and 3C show the reaction of adsorbed alcohol with pre-dosed oxygen D280 is the only water product evolved and is desorbed at low temperatures leaving two methoxy species for every pre-dosed oxygen atom. The products evolved at 350 K are then desorbed in decomposition-limited peaks from the break-up of the methoxy. The work of Wachs and Madix [331] showed further reaction to produce C02 desorption from the surface at 480 K, but such a strongly bound species could not be observed by Bowker and Madix [331] or by Sexton [332] and so some impurity adsorption must be inferred in the earlier work. 

One further example, which indicates a very unusual surface reaction, is the work by Falconer and Madix [330], mentioned earlier, on the HCOOH/ Ni 110 system. The anhydride intermediate which was formed was observed to decompose autocatalytically (a surface explosion ) that is, once the decomposition begins, it accelerates rapidly until all material is used. Thus, if the surface, with anhydride present, was heated to just below the desorption peak and then held at that temperature, an exponential increase in C02 evolution with time was observed isothermally. These results were explained in terms of an island mechanism. The adsorbate is held on the surface in islands and cannot decompose except at vacant sites a good fit to the data was obtained with the relationship 

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Thenual desorption spectroscopy (TDS) or temperature progranuned desorption (TPD), as it is also called, is a simple and very popular teclmique in surface science. A sample covered with one or more adsorbate(s) is heated at a constant rate and the desorbing gases are detected with a mass spectrometer. If a reaction takes place diirmg the temperature ramp, one speaks of temperature programmed reaction spectroscopy (TPRS). 

Desorption is important both because it represents the last step in a catalytic cycle and because it is also the basis of temperature-programmed desorption (TPD), a powerful tool used to investigate the adsorption, decomposition and reaction of species on surfaces. This method is also called thermal desorption spectroscopy (TDS), or sometimes temperature programmed reaction spectroscopy, TPRS (although strictly speaking the method has nothing to do with spectroscopy). 

Temperature-programmed reaction spectroscopy offers a straightforward way to monitor the kinetics of elementary surface reactions, provided that the desorption itself is not rate limiting. Figure 7.14 shows the the reaction CO -f O CO2 + 2. ... 

The reactions of ethylene, water, and methanol with coadsorbed oxygen on Pdf 100) were studied with temperature programmed reaction spectroscopy (TPRS) and high resolution electron energy loss spectroscopy (EELS). 

Temperature programmed sulfidation or temperature programmed reaction spectroscopy usually deal with more than one reactant or product gas. In these cases a TCD detector is inadequate and one needs a mass spectrometer for the detection of all reaction products. With such equipment one obtains a much more complete picture of the reaction process, because one measures simultaneously the consumption of reactants and the formation of products. 

Important information on reaction mechanisms and on the influence of promoters can be deduced from temperature programmed reactions [2], Figure 2.8 illustrates how the reactivity of adsorbed surface species on a real catalyst can be measured with Temperature Programmed Reaction Spectroscopy (TTRS). This figure compares the reactivity of adsorbed CO towards H2 on a reduced Rh catalyst with that of CO on a vanadium-promoted Rh catalyst [13]. The reaction sequence, in a simplified form, is thought to be as follows ... 

Temperature programmed reaction spectroscopy is a very useful tool for investigating which reactions can take place when several species are present on a surface. If desorption follows instantaneously, its peak can be used to derive an activation energy for the rate-determining step that precedes it. 

The tools used for the experiments described below have been described in several books and review articles (1-3). Surface structure is determined by low energy electron diffraction (LEED), surface composition by Auger electron spectroscopy (AES), and reaction kinetics and mechanism by temperature programmed reaction spectroscopy (TPRS). Standard ultra-high vacuum technology is used to maintain the surface in a well-defined state. As this article is a consolidation of previously published work, details of the experiments are not discussed here. 

This reaction sequence was definitively shown by use of temperature programmed reaction spectroscopy ( 7) The key to the success of this method was that reaction (4) was the rate-limiting step, allowing positive identification of the CH30(a) intermediate by TPRS. Isotopic substitution with b0 and deuterium was used to identify steps (2) and (3). 

TPRS = temperature-programmed reaction spectroscopy XRD = X-ray diffraction BET = Brunauer-Emmett-Teller method (specific BET surface area) and BJH = Barrett-Joyner-Halenda method (determination of pore volume and diameter), both determined by nitrogen physisorption NMR= characterization by solid-state NMR. 

As NO dissociation produces two atoms from one molecule, the reaction can only proceed when the surface contains empty sites adjacent to the adsorbed NO molecule. In addition, the reactivity of the molecule is affected by lateral interactions with neighboring species on the surface. Figure 4.10 clearly illustrates all of these phenomena [38]. The experiment starts at low temperature (175 K) with a certain amount (expressed in fraction of a monolayer, ML) of NO on the Rh(100) surface. During temperature programming, the SIMS intensities of characteristic ions of adsorbed species are followed, along with the desorption of molecules into the gas phase, as in temperature-programmed desorption (TPD) or temperature-programmed reaction spectroscopy (TPRS) (see Chapter 2). 

The heterogenization of MAO-activated Nd(z 3-C3H5)3 dioxane on MAO-functionalized Si(>2 was reported by T. Riihmer et al. [307]. In situ DRIFT (= diffuse reflectance infrared Fourier transform) spectroscopy and TPRS (= temperature-programmed reaction spectroscopy) were employed... 

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