TPD—See Temperature programmed

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). 

Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

With the aim of suppressing the concurrent condensation reactions a number of doped MgO catalysts was prepared and their catalytic activity investigated (Table 2). Surface basicity of these catalysts was measured by means of temperature programmed desorption of irreversibly adsorbed C02 (see methods section). Desorption peaks in the TPD experiments are considered to appear at higher temperatures as the basic sites on the surface... 

The same set-up as used for TPD can be applied to study reactions between adsorbed molecules. In the experiment shown in Figure 2.14, O-atoms have been co-adsorbed with CO [17]. During temperature programming, C02 forms and desorbs instantaneously (see inset of Fig. 2.14). In order to derive the activation energy and pre-exponential factor, we start by assuming that the reaction exhibits first-order kinetics in the surface concentrations of CO and O hence, we can write the rate as ... 

The concentration of the relevant surface functional groups can be determined by the methods described in Chapter 2. Temperature-programmed desorption (TPD) seems to be the best method to determine the concentration of oxygen groups, especially in the case of porous carbons [21,37-41], and x-ray photoelectron spectroscopy (XPS) is the method used to determine the concentration of nitrogen functions (see Chapter 7). 

Transmission FTIR spectroscopy may be combined with a number of other experimental techniques such as, e.g., temperature-programmed reduction (TPR), oxidation (TPO) or desorption (TPD) of probes monitored by, e.g., frequency response (FR) spectroscopy (see also Sect. 4.2), electron spin resonance (ESR) spectroscopy, etc. 

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