Temperature Programmed Reaction Mass

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). 

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. 

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).

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. 

Temperature programmed reaction (TPR) experiments were carried out by adsorbing allyl alcohol and allyl iodide on a 9.0 wt% Mo03/Si02 sample and monitoring the evolved products by mass spectrometry. The Raman spectra of the pure liquid reference compounds are shown in Fig. 2. They agree well with those reported earlier (18-20). 

A new experimental setup has recently been designed to study the chemical properties of size-selected metal clusters deposited on oxide substrates [210,211], Pd clusters have been produced by a laser evaporation source, ionized, then guided by ion optics through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer [210-212], The monodispersed clusters have been deposited with low kinetic energy (0,l-2eV) onto an MgO thin-film surface. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [224], Figure 6 shows the temperature-programmed reaction (TPR) spectra for the cyclotrimerization of acetylene on supported Pd (1 30)... 

Two sets of experiments were also carried out by means of a previously described temperature-programmed reaction (TPR) apparatus, equipped with a mass spectrometric (MS) detector [13] and operated isothermally (200°C) in the pulse mode. In the first set some 2 pi pulses of HEP were injected in the flowing carrier gas (ultrapirre helium, > 99.9999 vol%) just before the catalyst bed (50 mg of Y84). The second set of experiments was carried out on another fi esh batch of the same catalyst under exactly the same conditions, but after poisoning the catalyst surface by some pulses of CO2. 

The apparatus which has been described in detail previously (ref 2), allows both temperature programmed desorption (TFD) and temperature programmed reaction (TPRn) studies to be effected. The quantitative analysis of the gas stream eluting from the reactor was accomplished by mass spectrometry. 

Figure B 1.25.11 shows the temperature programmed reaction between 0.15 ML of CO and 0.24 ML of NO adsorbed at 150 K on a Rh(l 11) single crystal [21]. The spectra show the desorption of species with masses 28, 29, 30, 44 and 45, corresponding to N, CO, NO, N.,0 and CO., respectively, as functions of the...

---