TPD Analysis

The complete desorption analysis described above is rarely used in TPD. Many authors rely on simplified methods, which make use of easily accessible spectral features such as the temperature of the peak maximum, Trrl lx, and the peak width at half-maximum intensity, W. We describe these methods briefly and evaluate their merits. 

Particularly popular among surface scientists is the Redhead method [28], in which the activation energy of desorption is given by  

Equation (2-16) is approximately correct for first-order desorption and for values of v/p between 108 and 1013 K 1. It is very often applied to determine Edes from a single TPD spectrum. The critical point, however, is that one must choose a value for v, the general choice being 1013 s 1, independent of coverage. As we explain below, this choice is only valid when there is little change in entropy between the molecule in the ground state and the transition state [27, 29, 30]. The Redhead formula should only be used if a reliable value for the prefactor is available  

Another popular method has been developed by Chan, Aris and Weinberg [31]. These authors expressed E es (0) and v(6) in terms of the peak maximum temperature Tmax and the peak width, either at half or at three-quarters of the maximum intensity. Their expressions for first-order desorption are  

Lateral interactions between adsorbed species may make the kinetic parameters a function of coverage. In this case, it would be incorrect to rely on integral methods that depend on the properties of the entire TPD curve. Miller et al. [33] and Nieskens et al. [36] showed that doing so may induce artificial compensation effects in the results. Differential methods, which analyze a part of a trace are - in 

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Crystallites were greater than 25 nm after TPD analysis ... 

Iron on alumina as well as iron and iridium supported on the same carrier were suspected of exhibiting hydrogen spillover after treatment with hydrogen at 400 to 500°C. The TPD analysis is described by Paryjczak amd Zielinski (32,33). The high-temperature hydrogen desorption peak was attributed to the spiltover species. 

Representative TPD results for benzene, toluene, m-xylene and mesitylene adsorbed on H-ZSM-11 zeolite and naphthalene desorption from H-ZSM-11 and HY zeolites are shown in Fig. 1. The occurrence of one or two peaks depends on the adsorbate and zeolite. Using HZSM-11 zeolite, the TPD analysis for benzene and toluene (Fig. la), and naphthalene (N1 in Fig. Ic) show only a single peak, whereas, for m-xylene and mesitylene (Fig. 16) two peaks are observed. It is possible that the desorption of the aromatic hydrocarlmns from H-... 

Gas-phase synthesis of 2MN can be carried out efficiently over H-ZSM-5 and H-ZSM-11 type zeolites. The results are consistent with the Rideal type mechanism for alkylation of naphthalene with methanol. The first step in the alkylation reaction of naphthalene is the chemisorption of methanol on the Bronsted acid sites. Methoxy groups are formed on the surface and according to TPD analysis, naphthalene reacts with them impacting, directly from the gas phase. The reaction seems to occur on the external surface of the crystallites of the medium pore zeolites. Using large pores zeolites, the reaction also takes place also in the channel space, and the selectivity of B derivatives is suppressed. 

Several TPD analysis are carried out concerning carbon black, Pd containing catalyst and mixture of both of them. 

A few attempts have also berxi made to quantify the concenttalion of the electrochemically active groups by means of voltammetry and polarograply [188, 189], Kinoshita and co-workas attempted to quantify the quinone and hydroquinone groups detected by voltammetry on carbon blacks [190], An excellent agreement with data available fiom TPD analysis was reported, although the authors themselves acknowledge that such... 

Samples of the three calcined monoliths, after completing the campaign of DeNOx tests and being unloaded from the reactor, were reduced into fine powders and subject to TPD analysis (temperature programmed desorption) in a microreactor. Under a flow of He, the temperature of the catalyst bed was progressively increased up to 800°C and a mass spectrometer registered the nature and concentration of desorbed species. Sulfate... 

The TPD analysis demonstrated that ethanol strongly interacts with the heteropolyanion structure because it desorbs as CO/COj above 673 K. No molecular desorption is observed during the analysis (see the m/e = 31 signal), which suggests that the alcohol is chemisorbed within the structure. 

Fig. 12.18 TPD analysis of dihydrogen evolved from AC power charged at h at 50°C in 0.5 mol L NajSO [70]...

It is obvious that the Boehm titration method is the most popular one for the determination of various types of acidic (and basic) surface functionalities in carbon materials. From 1966 until 2002, when Boehm himself published a critical assessment of the analysis of surface oxides on carbon [201], an exhaustive utilization of this method has been desaibed by many authors. They underlined its simplicity, but pointed out also the need for using other complementary methods such as potentiometric titration, tanperature-programmed desorption (TPD), spectroscopic methods (mainly XPS and FTIR), and thermodynamic approaches such as calorimetry. The case of TPD is of special interest, to identify oxygenated functionalities. However, the CO and CO2 peaks must certainly be deconvo-luted before the surface composition can be estimated. Thus, a quantitative TPD analysis of surface functional groups is sensitive to the deconvolution method and to experimental conditions. The results are generally discussed in relation to those of DRIFTS and XPS analysis, as can be seen from the references listed in Table 3.1. 

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