Temperature Programmed Desorption TPD

TPAT technologies include temperature programmed desorption (TPD), temperature programmed reduction (TPR), temperature programmed oxidation (TPO), temperature programmed sulfide (TPS), temperature programmed surface reaction (TPSR) and so on. TPD is the most extensively studied, widely applied and the most mature method. Hence, TPD will be focused in the following section. 

The following phenomena may occur during the process of TPD Molecules desorption from the surface, re-adsorption into the surface from gaseous molecules diffusion from the surface to the subsurface, and diffusion from the subsurface to the surface and molecules diffusion in the inner of pore. 

The nature of adsorption site of catalyst surface has a direct impact on the molecular adsorption-desorption behaviors, and the interactions between the adsorbed molecules will also have some impact on the TPD performance. 

Equation (7.87) ignores the diffusion of molecule from the surface to the subsurface layer and molecule interaction. 

Under desorption conditions with constant heating rate, we get  

One can then use the standard Redhead analysis to the data of Fig. 5.24. To this end one uses the Redhead equation in its generalized form derived by Falconer and Madix 48 

The resulting Ed dependence on eUwR is shown in Fig. 5.26 for Pt, as well as for Ag and Au.44 47 The results are striking In all three cases Ed decreases linearly with increasing UWid 

In view of the potential-work function equivalence of solid state electrochemistry (Eq. 4.30 or 5.18) and of the fact that for non-activated adsorption, AEd Pt=0=A AHo,pt, where AHo.pt is the enthalpy of chemisorption of O on Pt, these equations can also be written as  

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Temperature-programmed desorption (TPD) is amenable to simple kinetic analysis. The rate of desorption of a molecular species from a uniform surface is given by Eq. XVII-4, which may be put in the form... 

Fig. 15. Temperature-programmed desorption (TPD) spectra SWCNTs, (b) activated carbon and (c) open-tip SWCNTs [52].

Figure 2.27. Temperature programmed desorption (TPD) spectra of carbon monoxide (measured by Ap) as a function of temperature from nickel surfaces (a) Ni(l 11), (b) Ni(l 11) when the initially dosed surface has been subjected to an electron beam (150 pA for 10 minutes over an area of 1 mm2) and (c) a cleaved nickel surface.85 Reprinted with permission from Elsevier Science.

As already noted the strength of chemisorptive bonds can be varied in situ via electrochemical promotion. This is the essence of the NEMCA effect. Following initial studies of oxygen chemisorption on Ag at atmospheric pressure, using isothermal titration, which showed that negative potentials causes up to a six-fold decrease in the rate of 02 desorption,11 temperature programmed desorption (TPD) was first used to investigate NEMCA.29... 

Temperature programmed desorption, TPD detection of backspillover species, 228 of oxygen, 228 Thermodynamics of adsorption, 306 of spillover, 104, 499 Three phase boundaries charge transfer at, 114 electrocatalysis at, 115 length, measurement of, 243 normalized length, 243 Time constants ofNEMCA analysis of, 198 and backspillover, 198 prediction of, 200... 

The opposite of adsorption, desorption, represents the end of the catalytic cycle. It is also the basis of temperature-programmed desorption (TPD), an important method of studying the heats of adsorption and reactions on a surface, so that the activation... 

Several spectroscopic, microscopic and diffraction techniques are used to investigate catalysts. As Fig. 4.2 illustrates, such techniques are based on some type of excitation (in-going arrows in Fig. 4.2) to which the catalyst responds (symbolized by the outgoing arrows). For example, irradiating a catalyst with X-ray photons generates photoelectrons, which are employed in X-ray photoelectron spectroscopy (XPS) -one of the most useful characterization tools. One can also heat a spent catalyst and look at what temperatures reaction intermediates and products desorb from the surface (temperature-programmed desorption, TPD). 

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). 

The scanning transmission electron microscope (STEM) was used to directly observe nm size crystallites of supported platinum, palladium and first row transition metals. The objective of these studies was to determine the uniformity of size and mass of these crystallites and when feasible structural features. STEM analysis and temperature programmed desorption (TPD) of hydrogen Indicate that the 2 nm platinum crystallites supported on alumina are uniform In size and mass while platinum crystallites 3 to 4 nm in size vary by a factor of three-fold In mass. Analysis by STEM of platinum-palladium dn alumina established the segregation of platinum and palladium for the majority of crystallites analyzed even after exposure to elevated temperatures. Direct observation of nickel, cobalt, or iron crystallites on alumina was very difficult, however, the use of direct elemental analysis of 4-6 nm areas and real time Imaging capabilities of up to 20 Mx enabled direct analyses of these transition metals to be made. Additional analyses by TPD of hydrogen and photoacoustic spectroscopy (PAS) were made to support the STEM observations. 

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing BET method via Sorptomatic 1900 (Carlo-Erba). X-ray difiraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (0 / 20) dififactometer with Cu K monochromatic radiation. For the temperature-programmed desorption (TPD) experiments the catalyst (0.3 g) was pre-treated at diflferent temperatures (100-700 °C) under helium flow (5-20 Nml min ) in a micro-catalytic tubular reactor for 3 hours. The treated sample was exposed to methanol vapor (0.01-0.10 kPa) for 2 hours at 260 °C. The system was cooled at room temperature under helium for 30 minutes and then heated at the rate of 4 °C min . Effluents were continuously analyzed using a quadruple mass spectrometer (type QMG420, Balzers AG). 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

For illustration we will present some commonly used techniques for chemisorption measurements. Chemisorption can be measured gravimetrically, volumetrically, or spectroscopically. Also, pulse techniques, and Temperature Programmed Desorption (TPD) can be used. 

Temperature Programmed Desorption (TPD). Chemisorbed molecules are bonded to the surface by forces dependent on the nature of the sites. For instance, ammonia will be strongly adsorbed on acid sites, whereas it is only weakly adsorbed on basic sites. Consequently, the adsorbate complex formed with the basic sites will decompose at lower temperatures than that formed with the acid sites. The following example regarding the NH.i-zeolite H-ZSM-5 system will illustrate this. 

The Pt dispersion of the fresh samples was measured by dynamic hydrogen chemisorption by using a temperature-programmed desorption (TPD)/R/0 1100 ThermoFisher... 

Figure 9.15. Comparison of the total ammonia adsorption of coated and extruded V2O5/WO3—Ti02 catalysts. Catalyst volume = 7 cm3. Model gas for loading 10% 02, 5% H20, NH3 = 1000ppm, and balance N2. GHSV = 52000h 1. Model gas for temperature-programmed desorption (TPD) experiment 10% 02, 5% H20, NO = 1000 ppm, NH3 = 1000 ppm, and balance N2. NH3 desorbed is calculated as the sum of thermally desorbed NH3, directly measured at the catalyst outlet, and chemically desorbed NH3, measured by the reduction of the NO concentration due to the SCR reaction.

Temperature programmed desorption (TPD) of NH3 adsorbed on the samples was carried out on an Altamira TPD apparatus. NH3 adsorption was performed at 50°C on the sample that had been heat-treated at 120°C in a helium flow. After flushing with helium, the sample was subjected to TPD from 50 to 600°C (AT = 10°C min 1). The evolved NH3, H20 and N2 were monitored by mass spectroscopy by recording the mass signals of m/e = 16, 18 and 28, respectively using a VG Trio-1 mass spectrometer. 

After the catalyst was saturated with carbon dioxide, a temperature programmed desorption (TPD) was carried out by heating the sample in helium (40 cm3min 1) from room temperature to 873 K (10 Kmin 1). The mass spectrometer was used to follow water (mass 18), carbon monoxide (mass 28), carbon dioxide (mass 44) and oxygen (mass 32). 

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