Reaction temperature-programmed

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). 

E.M. Stuve, and R.J. Madix, Bonding and dehydrogenation of ethylene on palladium metal. Vibrational spectra and temperature-programmed reaction studies on Pd(100), J. Phys. Chem. 89, 105-112 (1985). 

The carbon removal reaction supposedly takes place at two-phase boundary of a solid catalyst, a solid reactantfcarbon particulate) and gaseous reactants(02, NO). Because of the experimental difficulty to supply a solid carbon continuously to reaction system, the reaction have been exclusively investigated by the temperature programmed reaction(TPR) technique in which the mixture of a catalyst and a soot is heated in gaseous reactants. 

Fig. 4. Temperature programmed reaction on combustion of carbon particulates over LaojCso.sMnOs catalyst heating rate=...

Temperature-programmed reaction methods form a class of techniques in which a chemical reaction is monitored while the temperature increases linearly in time. Several forms are in use temperature-programmed reduction, oxidation and sulfidation. 

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. ... 

Figure 7.14. The temperature-programmed reaction and corresponding Arrhenius plot based on rate expression (21) enables the calculation of kinetic parameters for the elementary surface reaction between CO and O atoms on a Rh(lOO) surface.

Temperature-programmed reactions between small amounts of adsorbed species are an excellent way to study the intrinsic reactivity of catalytic surfaces. Such experiments on rhodium (100) and (111) surfaces covered by small amounts of CO and... 

Figure 10.8. Temperature-programmed reaction of NO and CO on two surfaces of rhodium. The initially molecularly adsorbed NO dissociates entirely at relatively low temperatures, but NO does not desorb. Note the difference in selectivity and reactivity between the surfaces on Rh(lOO) most of the CO oxidizes to CO2 and the reaction already...

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). 

Figure 1. Temperature programmed reaction spectra of C2H4 and H2 following saturation coverage of C2H4 on the clean and oxygen covered Pd(lOO) surface at 100 K. Atomic oxygen was generated on the surface by exposure to O2 at 300 K.

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.

Thermal desorption spectroscopy and temperature programmed reaction experiments have provided significant insight into the chemistry of a wide variety of reactions on well characterized surfaces. In such experiments, characterized, adsorbate covered, surfaces are heated at rates of 10-100 K/sec and molecular species which desorb are monitored by mass spectrometry. Typically, several masses are monitored in each experiment by computer multiplexing techniques. Often, in such experiments, the species desorbed are the result of a surface reaction during the temperature ramp. 

Catalytic Reduction of SO3 Stored in SOx Transfer Catalysts -A Temperature-Programmed Reaction Study... 

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. 

Figure 3. Temperature-programmed reaction spectra morutoring m/e = 73 (a cracking fragment of trimethylsilane) after adsorbing a saturation coverage of methyl groups on CusSi surfaces prepared by ion bombardment at 160 K (dotted curve) and 330 K (solid curve).

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 5. n-Butane conversion to isobutane as a function of temperature in a temperature programmed reaction experiment conducted over 0.4 g of INiSZ(s) catalyst under an n-butane/ hydrogen mixture (n-C4 molar fraction = 0.34) at a constant heating rate of 2C/min... 

The catalytic conversion of NO was investigated Grst in absence of catalyst (blank). The results reported in Fig. 1 show that the homogeneous gas phase oxidation of the alkane starts at 650 K. No reduction of NO is observed in the homogeneous process, then the production of N2 can be ascribed to the catalytic reduction. The catalytic properties were determined by temperature programmed reaction (ramp 2 K min-l). The temperature was increased from 523 to 673 K and back. 

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).

Theses compounds are separately delivered in the full range of temperature, at the very beginning of the temperature-programmed reaction. This is quite different from... 

Figure 8.8. Temperature-programmed reaction of CO (l%) + NO (1%) over a pre-oxidized 0.2% Rh/Ce02—A1203 catalyst (Pirault and Marecot, unpublished results).

When the ceria-containing catalyst is pre-reduced before reaction, a typical temperature-programmed reaction profile can be observed (Figure 8.9). While... 

Eliason, S.A., and Bartholomew, C.H. 1997. Temperature-programmed reaction study of carbon transformations on iron Fischer-Tropsch catalysts during steady-state synthesis. Stud. Surf. Sci. Catal. 111 517-26. 

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