Temperature-programmed desorption coverage

TPD Temperature programmed desorption After pre-adsorption of gases on a surface, the desorption and/or reaction products are measured while the temperature Increases linearly with time. Coverages, kinetic parameters, reaction mechanism... 

Finally, although both temperature-programmed desorption and reaction are indispensable techniques in catalysis and surface chemistry, they do have limitations. First, TPD experiments are not performed at equilibrium, since the temperature increases constantly. Secondly, the kinetic parameters change during TPD, due to changes in both temperature and coverage. Thirdly, temperature-dependent surface processes such as diffusion or surface reconstruction may accompany desorption and exert an influence. Hence, the technique should be used judiciously and the derived kinetic data should be treated with care ... 

NO is now chemisorbed on the Rh particles at a temperature where it does not adsorb on the AI2O3. The saturation coverage of NO on Rh(lOO) corresponds to one NO molecule per two rhodium surface atoms, with NO sitting in a c(2x2) surface structure. After having saturated the catalyst with NO, a temperature-programmed desorption experiment (TPD) is performed with a heating rate of 2 K min". NO is seen to desorb with a maximal rate at 460 K. The total NO gas that desorbs amounts to 18.5 mL per gram catalyst (P = 1 bar and T = 300 K). It can be assumed that NO does not dissociate on the Rh(lOO) surface. 

Figure 10.20 (A) Temperature-programmed desorption (TPD) and (B) Infrared reflection absorption spectroscopy (IRAS) spectra of CO adsorbed on Au/FeO(111) as a function of Au coverage. (Reprinted from Lemire, C. et al., Angew. Chem. Int. Ed., 43, 118-121,2004. With permission from Wiley-VCH.)...

Fig. 11. The temperature programmed desorption spectra for CO from Fe(lOO) following adsorption at 180 K. Curves (a)-(e) are arranged in order of increasing initial coverage by CO 45).

Figure 3 Sketch of an example of the evolution of a system during a temperature-programmed desorption experiment in the system s phase diagram. The fat line indicates the change of the temperature and coverage during the experiment, and the thin lines indicate the phase transitions (see text). The snapshots below the order-disorder transition line are taken during a simulation of the experiment. The coverages are 0.3, 0.5, and 0.7 ML. The snapshots above the order-disorder transition line show adlayers of 0.3 and 0.7 ML at high temperatures...

At intermediate coverages the dissociation starts between 200 and 450 K, then stops when the overall coverage becomes too high, and then restarts when new vacancies are formed due to NO desorption. The temperature-programmed desorption spectra also show a first-order-like N2 desorption peak... 

CO/Rh(100). - This system forms an example where we have determined lateral interactions by fitting temperature-programmed desorption spectra that were simulated using kinetic Monte Carlo to experimental spectra. For coverages below 5ML CO adsorbs at top sites, which form a square grid. CO desorption has a rate constant... 

Figure 13 Experimental (left) and simulated (right) temperature-programmed desorption spectra for COjRh( 100). The values on the right of each set of curves indicate initial coverages. The thin curves on the right are simulated spectra with the lateral interactions switched off. The heating rate is 5 Kjs ...

Figure 16 Simulated and experimental temperature-programmed desorption spectra for OlPt(lll). The solid lines are experimental spectra. The crosses indicate simulated spectra for a model of the lateral interactions with nearest and next-nearest pair interactions, and also a linear 3-particle interaction. The O2 is formed from two atoms at next-nearest-neighbor positions. The kinetic parameters are — 206.4 kj/mol, v = 2.5 x 10 s a = 0.773, cpxN — 19.9 kjjmol, tp NN = 5.5 kjjmol, and (punear = 6.1 kJImol. In each plot the curves from top to bottom are for initial oxygen coverage of 0.194, 0.164, 0.093, and 0.073 ML, respectively. The heating rate is 8 Kjs ...

The term 1 or h indicates low or high coverage of adsorbed ethene, as inferred from ethene exposures.h TPD, temperature-programmed desorption LITD, laser-induced thermal desorption 1 FT-MS, Fourier-transform mass spectrometry SIMS, secondary-ion mass spectrometry MS, mass spectrometry T-NEXAFS, transient near-edge X-ray absorption fine structure spectroscopy RAIRS, reflection-absorption infrared spectroscopy. d Data for perdeut-erio species.1 Estimated value. 

Temperature programmed desorption (TPD) is an experimental technique to measure surface kinetic parameters. The most straightforward analysis of TPD is due to Redhead [331], Assuming that the surface has some fractional coverage 0 of adsorbed A molecules, the desorption rate of A from the surface r(j (1/s) is taken to be... 

Fig. 2.9 Examples of temperature-programmed desorption following zeroth-, first- and second-order kinetics. Each curve corresponds to a different initial coverage of the adsorbate. Ag/Ru(001) Silver forms islands on the ruthenium substrate. Desorption of Ag from the edges of these islands gives rise to zeroth-order kinetics note the exponential increase of the low-temperature sides of the peak, as expected from Eq. (2-15). Desorption from the second layer of silver occurs at lower temperatures, indicating that Ag-Ag bonds are weaker than Ag-Ru bonds [20]. CO/Rh(111) At coverages...

Additionally, there is some experimental evidence for the crucial role subsurface oxygen is thought to play during the oscillations. Under conditions in which subsurface oxygen is assumed to be formed, complex LEED patterns evolve, temperature-programmed desorption (TPD) experiments yield oxygen coverages that exceed one monolayer, and two different reactivities with CO are observed (248). 

Fig. 1.12 Temperature-programmed desorption spectra of ethylene (0.5 L) on hydrogen-covered Pd(lll) as a function of hydrogen coverage. (Reprinted from [101] with permission from Elsevier.)...

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