Temperature-Programmed Reaction Spectroscopy in UHV

In principle, TPD can also be applied to high-surface area catalysts in a reactor, and this may yield useful qualitative information. Deriving quantitative information from TPD on supported catalysts is also possible, but requires that mass transfer properties such as intraparticle diffusion are properly taken into account. For details of this approach, the reader is referred to an interesting discussion by Kanervo et al. [38], 

As on every point of the TPRS curve the corresponding coverages of CO and O that are left on the surface can be calculated, we can thus divide the rate by the actual coverages at every temperature. Then, if we make the usual Arrhenius plot of (In r/d0dCo) against 1/T, a straight line evolves (Fig. 2.14), indicating that our assumption of first-order kinetics in the reactants is justified and that the reaction COa(js + Oaas can be considered an elementary step that is followed 

Note the wide temperature range over which the Arrhenius plot forms a straight line. (Adapted from [17]). 

TPRS 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. 

Barin and O. Knacke, Thermochemical Properties of Inorganic Substances. 

We conclude that prefactors on the order of 1013 s 1 correspond to transition states that resemble the ground state. 

In conclusion, TDS of adsorbates on single crystal surfaces measured in ultrahigh vacuum systems with sufficiently high pumping speeds provides information on adsorbate coverage, the adsorption energy, the existence of lateral interactions between the adsorbates, and the preexponential factor of desorption, which in turn depends on the desorption mechanism. Analysis of spectra should be done with care, as simplified analysis procedures may easily give erroneous results. 

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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. 

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