Temperature programmed reaction with

A differential flow microreactor was used for the preparation of the nitrided catalyst and the TPD, TPR, and NH3-TPD measurements. Nitriding of the molybdena-alumina and alumina was carried out by temperature-programed reaction with NH3 (NH3-TPR).1719 The MoCV A1203 precursor was oxidized at 723 K for 24 h, cooled to 573 K, reacted with NH3 at 49.6 (xmols-1 from 573 to 773, 973 or 1173 K at a rate of 0.0167 Ks-1, held at the nitriding temperature for 3 h, and then cooled to room temperature (RT) in flowing NH3. The catalysts were characterized by TPD, TPR, and NH3-TPD under in situ conditions, while BET and diffuse reflectance FTIR measurements were carried out after passivation. For the diffuse reflectance FTIR study, the catalysts after NH3 treatment... 

Marecot et al. studied the regeneration of the metallic function by temperature-programmed reaction with hydrogen up to 1073 K. The only gaseous product was methane produced at a well-defined temperature. This temperature decreases from Pt to Re and Ir and the... 

Table 2. Zeolite Y products obtained from temperature programmed reaction with...

Sequences such as the above allow the formulation of rate laws but do not reveal molecular details such as the nature of the transition states involved. Molecular orbital analyses can help, as in Ref. 270 it is expected, for example, that increased strength of the metal—CO bond means decreased C=0 bond strength, which should facilitate process XVIII-55. The complexity of the situation is indicated in Fig. XVIII-24, however, which shows catalytic activity to go through a maximum with increasing heat of chemisorption of CO. Temperature-programmed reaction studies show the presence of more than one kind of site [99,1(K),283], and ESDIAD data show both the location and the orientation of adsorbed CO (on Pt) to vary with coverage [284]. 

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). 

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 reaction spectra depicting the reaction of H2O and OH groups with oxygen on Pd(lOO) are shown in fig. 3. Curve (a) was obtained for H2O adsorption on the clean surface and contains two peaks, the state at 167 K of multilayer H2O, and the <>2 state at 182 K due to H2O bound directly to the surface /7/. An additional state at 255 K, labelled y, is observed following coadsorption of 1 0 and 0 (fig. 3b). This state represents the reaction of OH groups III... 

Figure 4, Temperature programmed reaction spectra of CH3OH, H2CO and HpO following adsorption of multilayers of CH3OH on Pd(lOO) at 130 K precovered with 0.25 ML atomic oxygen. The oxygen was adsorbed at 300 K. All traces are on the same scale.

The evolution of methylchlorosilanes between 450 and 600 K is consistent with the 550 - 600 K typical for the catalytic Rochow Process [3]. It is also reasonably consistent with the evolution of methylchlorosilanes at 500 - 750 K reported by Frank and Falconer for a temperature programmed reaction study of the monolayer remaining on a CuaSi surface after catalytic formation of methylchlorosilanes from CHaCl at higher pressures [5]. Both of these observations suggest that the monolayer formed by methyl and chlorine adsorption on pure CuaSi is similar to that present on active catalysts. For reference, methylchlorosilanes bond quite weakly to tiie surface and desorb at 180 - 220 K. It can thus be concluded that the rate-determining step in the evolution of methylchlorosilanes at 450 - 600 K is a surface reaction rather an product desorption. 

Figures. Temperature-programmed reaction of methane with FeZSM-5 surface before a-oxygen loading (a) and after ot-oxygen loading (b). A - time moment of opening the microreactor B - time moment of switching on the programmed heating (6 K/s).

Figure 9.7 Temperature-programmed reaction (TPR) spectra for CO oxidation at a series of model catalysts prepared by the soft landing of mass-selected Aun and AunSr cluster ions on MgO(lOO) thin films which are vacancy free (typically 1 % of a monolayer), (a) MgO (b) Au3Sr (c) Au4 (d) Au8. Also shown is the chemical reactivity R of pure Aun and AunSr clusters with 1 < n < 9. (Reproduced from Ref. 21).

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

Temperature programmed reaction methods form a class of techniques in which a chemical reaction is monitored while the temperature increases linearly with time [1,2]. Several forms are in use. All these techniques are applicable to real catalysts and single crystals and have the advantage that they are experimentally simple and inexpensive in comparison to many other spectroscopies. Interpretation on a qualitative basis is fairly straightforward. However, obtaining reaction parameters such as activation energies or preexponential factors from temperature programmed methods is a complicated matter. 

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 ... 

Figure 2.16 Temperature programmed reaction between O atoms and ethylene adsorbed on Rh(l 11). The majority of the adsorbed ethylene decomposes in several steps to H and C atoms, which react with the adsorbed O atoms to form H2, H20, CO and C02. Because there is insufficient oxygen, the surface still contains carbon at the end of the experiment (adapted from [36],...

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

Spectroscopic developments have accelerated advances in the field of catalysis. This volume analyzes the impact on catalyst structure and reactivity of EXAFS, SIMS, MSssbauer, magic-angle spinning NMR (MASNMR), and electron-energy-loss vibrational spectroscopy. Many of these techniques are combined with other analytical tools such as thermal decomposition and temperature-programmed reactions. 

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). 

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