Desorption profile

It can be expected that the electronic structure changes would be reflected by the heats of adsorption of suitable chosen molecules. Indeed, Shek et al (17) report that one maximum in the thermal desorption profile of CO shifts to lower temperatures when the Cu content of alloys increases. If the variations in the entropy changes upon adsorption can be neglected (probably - they can) this would indicate a lower heat of adsorption of CO on alloys than on Pt from abt. 33 Kcal/mol on pure Pt,to 26 Kcal/mol for an alloy with abt. 20% Cu. 

Catalyst characterization - Characterization of mixed metal oxides was performed by atomic emission spectroscopy with inductively coupled plasma atomisation (ICP-AES) on a CE Instraments Sorptomatic 1990. NH3-TPD was nsed for the characterization of acid site distribntion. SZ (0.3 g) was heated up to 600°C using He (30 ml min ) to remove adsorbed components. Then, the sample was cooled at room temperatnre and satnrated for 2 h with 100 ml min of 8200 ppm NH3 in He as carrier gas. Snbseqnently, the system was flashed with He at a flowrate of 30 ml min for 2 h. The temperatnre was ramped np to 600°C at a rate of 10°C min. A TCD was used to measure the NH3 desorption profile. Textural properties were established from the N2 adsorption isotherm. Snrface area was calcnlated nsing the BET equation and the pore size was calcnlated nsing the BJH method. The resnlts given in Table 33.4 are in good agreement with varions literature data. 

Adsorption/desorption kinetics the time of the adsorption-regeneration cycle greatly depends on the kinetics of the C02 adsorption-desorption profile, which is measured in breakthrough experiments. Sorbents that adsorb and desorb C02 in a shorter time are preferred as these reduce the cycle time as well as the amount of sorbent required, and ultimately the cost of C02 separation. 

TPD of Cu-Al-MCM-41 (after NO adsorption under 0.8% NO in He) was eondueted (Table 14). NO and NO2 are the species detected coming off the surface as the temperature of the catalyst is increased. Two features were observed in the NO desorption profile a principal peak at 149 °C and a second NO desorption feature at higher temperature (440 °C). This indicates that there are at least two types of NO adsorption sites available. The presence of two types of adsorbed NO species over Cu catalysts has been reported earlier in the literature[45]. These have been proposed to be the desorption of NO from Cu ions and nitrate (NO3 ), nitrite (NO2 ) or N02 adsorbed species, respectively. Assuming the sensitivity factors of the peaks at low and high temperature are equivalent, the areas can be used to estimate the normalized desorption of NO. As listed in Table 14, the amount of NO desorbed at low temperature is close to the total amoimt of desorbed NO. This feature indicates that copper is mainly as isolated Cu in the catalyst. During NO desorption, a small amoimt of NO2 (8.2 pmol/g) desorbed at 80 °C. 

Figure 24. Temperature Programmed Desorption profile of NH3 on Cul-xZnxFe204. Broad peak was deconvoluted for x = 0.5 composition.

Fig. 29. S02 desorption profile of TPD measurement on the PtBa/Al203 with Fe-compounds exposed to a simulated oxidizing condition containing S02.

Fig. 1. The temperature programmed desorption profiles for a-Fe203 (a) Blank desorption without adsorbates (b) ris-2-butene adsorption (c) butadiene adsorption (d) cis-2-butene adsorption from a catalyst depleted of selective oxidation sites. From ref. 5, reprinted with permission, copyright 1979 by the American Chemical Society.

The temperature programmed desorption profile for the adsorption of butadiene in place of cis-2-butene is shown in Fig. 1, curve c. Two sets of products are observed. The product below 210°C is unreacted butadiene, and the products above 210°C are carbon dioxide and water. The similarity in the evolution of the combustion products of butene and butadiene is an indication that their combustion proceeds via similar reaction mechanisms. The similarity in the desorption of butadiene suggests that in butene adsorption, butadiene desorption is desorption limited. Indeed, that both butene and butadiene adsorb on the same type of sites has been confirmed by sequential adsorption experiments. The results are shown in Table III. It was found that if the C4 hydrocarbons are adsorbed sequentially without thermal desorption between adsorptions, the amounts of the final desorption products are the same as those in experiments where only the first hydrocarbon... 

The second experiment is to perform adsorption-desorption of butene on a catalyst that is depleted of selective oxidation sites. When 10 pulses of cis-2-butene are passed over a catalyst at 210°C (5), which is a temperature too low for the production of C02, the catalyst is reduced. The number of selective oxidation sites is substantially reduced as is evident by the much lower conversion to butadiene in the last pulse. Then the oxide is cooled to 22°C, and cis-2-butene is adsorbed. The resulting desorption profile is shown in Fig. 1, curve d. Clearly, there is no more butadiene production, while the combustion products are produced in a somewhat larger quantity. These results again support the conclusion that the selective oxidation and the combustion sites are independent. 

The minute quantity of adsorbate remaining on the column after weakly bound probe has desorbed is chemisorbed to strongly acidic or basic sites on the substrate. The desorption profile obtained by ramping the column temperature is an index of the range of effective bond strength between the solid and adsorbed vapor. The flame ionization detector also registers desorption of adventitious organic contaminants polytherms with no probe on the column must be obtained separately so that sorbate and contaminant desorption can be deconvolved. 

Figure 5 CH4 and CH3 are observed during heating the used, basic Pt/HT catalyst to 450°C in (ramp 5°C/min) in H2, indicating the desorption of CH4. The same desorption profile was observed for the other basic catalysts.

Figure 13. Quantitation of fluoranthene in SRC using standard addition method by monitoring the fragmentation process 202 - 187 while sample was desorbed from the solids probe. Inset is a typical SRC desorption profile (containing 9 fig...

Fig. 1. (a) A chemical structure of a 2.5th generation carboxylic acid-terminated poly(amido amine) (PAMAM) dendrimer. (b) Transmission surface enhanced infrared absorption spectra (SEIRAS) of dendrimer adlayers prepared at 30 min adsorption from aqueous solutions (0.01 wt.%) of a dendrimer at different pHs. Numerical values are pHs of the solutions, (c) Adsorption-desorption profiles as a function of time at different pHs and adlayer thicknesses at adsorption and desorption equilibrium as a function of pH for aqueous solutions (0.1 wt.%) of the dendrimer. The symbols, j and J, in the top figure denote start of adsorption and desorption, respectively. In the bottom figure, filled circle and opened square denote adlayer thicknesses at adsorption and desorption equilibrium, respectively. The dark tie denotes the calculated dendrimer size width. A solid curve is drawn to be visual, (d) Schematic illustration of dendrimers adsorbed at different pHs. Reprinted with permission from Ref. [69], 2006, American Scientific Publishers. 

Fig. 8 Desorption profiles for trichloroethene (TCE) from Santa Clara sediment...

Despite these principal ambiguities the thermal desorption method is a standard characterization technique in carbon surface chemistry. Various examples and data about desorption profiles for a selection of carbon treatments can be found in the literature [88, 90, 155, 182, 183]. 

The characterization technique of CO Temperature-Programmed Desorption has been studied with Pt reforming catalysts. Critical factors in the experimental procedure and the catalyst pretreatment conditions were examined. The CO desorption spectrum consists mainly of two peaks which are probably combinations of other peaks and the result of various binding energy states of CO to Pt. These in turn could be due either to the interaction between Pt and the alumina support or the results of high and low coordination sites on the Pt crystallites. No significant relationship between the character of the CO desorption profile and the activity of commercial catalysts was observed. 

Changes in the catalytic activity of activated alumina surfaces have also been probed by Temperature Programmed Desorption (TPD) and FTIR photoacoustic measurements [74]. Ammonia TPD allows a fast and convenient determination of the overall acidity of a solid surface. The desorption profiles provide information on the distribution, the amount and strengths of the acid surface sites, since molecules adsorbed at weaker sites desorb at lower temperatures than those adsorbed at stronger acidic sites. The activation of y-alumina with CHC1F2 resulted in a sig-... 

The adsoqjtion of NO on metal loaded ceria has been examined for Pt, and Pd, As known from work on single crystals, NO dissociates to some extent on each of these metals. The amount of dissociation is dependent upon the structure of the metal surface. Gorte considered Pt and Pd particles deposited on rough, poly crystal line ceria films grown by spray pyrolysis.For Pt they found variation in the TPD results (amount of NO uptake and shape of N2 desorption profile) that varied with the size of the Pt particles. However, the results were comparable to NO TPD results from Pt grown on sapphire. It was concluded that no unusual interaction existed between Pt and the (oxidized) ceria. For Pd it was found that a pronounced difference in the TPD product ratio, NO/N2, occurred for Pd on ceria compared to Pd on sapphire. They attributed the difference to NO adsorption on reduced ceria. 

The metallic Ni and CaO sites adjacent to reduced nickel metal sites on KNiCa/ZSl catalyst were considered as major sites of CO2 chemisorption. The chemisorbed amounts of CO2 on KNiCa/ZSl catalyst were increased to a value 40% higher than that of Ni/ZSl catalyst. As shown in the CO2 desorption profiles in Figure 2, the integrated amounts of CO2... 

The effect of particle size the results of the TPR experiments on the catalysts that do form hydride are shown in Fig. 2. The H2-desorption profiles of chlorine-containing catalysts have a second peak at a higher temperature. 

Temperature programmed desorption profiles of CO in Ar were obtained for the same catalysts of figure 4. The most significant result is that the peak corresponding to CO is shifted to higher temperatures than in the case of the TPSR experinents indicating a promotional effect of H2 on the CO disproportionation. This effect has been observed by others (ref. 11). 

Figure 7. Temperature Programmed Desorption profiles obtained by heating the spent H-ZSM5 catalyst in He flow and detecting the evolution of adsorbed species after reaction at various temperarnres.

---