Adsorption-desorption profile

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. 

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. 

The Nz adsorption-desorption profiles and the porosimetry measurements confirmed the presence of mesopores in the nickelsilicate membranes obtained by immersion, into the gel, of the alumina support treated with a solution of TMAOH-l-HzOz+HzO. The average pore diameter was varied between 2.4 and 4.2 nm. Table 2 lists some permeation... 

Instrumentation. NO, adsorption/desorption profiles were obtained using a Seiko TG/DTA 320 coupled to a VG Micromass quadrapole MS. The two instruments were coupled a heated (ntf C ) fused silica capillary transfer line... 

TG-MS procedures. The TG conditions kept constant during the acquisition of adsorption/desorption profiles were sweep gas flow rate of 500 ml/min metered at room temperature and pressure, and a constant carbon sample volume weighing approximately 8 to 20 mg. The MS was scanned over a 0-100 amu range with a total measurement interval of approximately 30 s per 100 amu NO (mass 30) or NO2 (mass 30 and 46) were identified by comparing amu 30/46 ion ratios. 

Figure 1 shows two series of CV adsorption-desorption profiles for UPD H on Rh and Pt from 0.50 M aqueous H2SO4 solution. In the case of Rh there is only one adsorption-desorption peak where as in the case of Pt there are two peaks. Upon the temperature increase, the peaks shift towards less-positive potentials and there is a slight redistribution of the charge between the two peaks for Pt (18,19). These data allow calculation of AG (Hupo) based on equation 5 and the results are shown in Figure 2 as 3D plots of AG (Hupd) versus assumes the... 

The isotherms of all samples are shown in Fig. 4.7, indicating adsorption-desorption profiles of type fV, associated with mesoporous solids [6]. 

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. 

To probe the formation of coke we conducted TPO measurements on samples previously used in the butene TPD experiments. The TPO profiles corresponding to catalyst INiSZ(s) are shown in Fig. 6. Significant evolution of CO2 was detected, indicating the formation of coke during the adsorption/desorption of 1-butene. The INiSZ(s) catalyst exhibited almost twice as much CO2 as the unpromoted SZ sample. 

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 of acetic acid on Pt(lll) surface was studied the surface concentration data were correlated with voltammetric profiles of the Pt(lll) electrode in perchloric acid electrolyte containing 0.5 mM of CHoCOOH. It is concluded that acetic acid adsorption is associative and occurs without a significant charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(lll) than on polycrystalline platinum. A classification of adsorption processes on catalytic electrodes and atmospheric methods of preparation of single crystal electrodes are discussed. 

Other resolutions of the Poisson Nernst Planck equations (i.e. using various simplifying assumptions) have been proposed that couple the adsorption, desorption and permeation of ions through a membrane (e.g. [273,274]) as might be observed for a carrier-mediated transport. For example, for a symmetrical membrane (identical electrolyte on both sides of the membrane) and variation in the electrical potential profile given by i//m, /int can be estimated from ... 

Figure 14. Simple model demonstrating how adsorption and surface diffusion can co-Urnit overall reaction kinetics, as explained in the text, (a) A semi-infinite surface establishes a uniform surface coverage Cao of adsorbate A via equilibrium of surface diffusion and adsorption/desorption of A from/to the surrounding gas. (b) Concentration profile of adsorbed species following a step (drop) in surface coverage at the origin, (c) Surface flux of species at the origin (A 4i(t)) as a function of time. Points marked with a solid circle ( ) correspond to the concentration profiles in b. (d) Surface flux of species at the origin (A 4i(ft>)) resulting from a steady periodic sinusoidal oscillation at frequency 0) of the concentration at the origin.

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. 

The lumped pore model (often referred to as the POR model) was derived from the general rate model by ignoring two details of this model [5]. The first assumption made is that the adsorption-desorption process is very fast. The second assumption is that diffusion in the stagnant mobile phase is also very fast. This latter assumption leads to the consequence that there is no radial concentration gradient within a particle. Instead of the actual radial concentration profile across the porous particle, the model considers simply its average value. 

The nitrogen adsorption/desorption isotherms show typical type IV profiles (IUPAC classification [18]) for the pristine MCM-48 and the Co/Fe/O/MCM-48 silica materials A and B (figure 3). 

Both types of molecular sieves, MCM-36 and MCM-41, demonstrate large BET surface area and high static sorption capacity (see Table 2). Considerable qualitative differences are observed in N2 isotherms, which are shown in Figure 3. The nitrogen isotherm for MCM-41, prepared with cetyltrimethylammonium cation, is type IV [9] and shows the characteristic reversible steep capillary condensation at p/p0 = -0.4 corresponding to the pore opening -40 A [1]. MCM-36 also shows the type IV isotherm with almost linear and reversible uptake increase up to - p/p0 = 0.5, followed by a hysteresis loop. This profile of adsorption/desorption is typical for layered materials with slit-like porosity generated between layers [9],... 

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 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. 

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