Desorption product

TPD is frequently used to detenuine (relative) surface coverages. The area below a TPD spectrum of a certain species is proportional to the total amount that desorbs. In this way one can detennine uptake curves that correlate gas exposure to surface coverage. If tire pumping rate of the UHV system is sufiBciently high, the mass spectrometer signal for a particular desorption product is linearly proportional to the desorption rate of the adsorbate [20, 21] ... 

Chemisorption of simple diatomic molecules has usually been the object of thermal desorption studies. Recently, there has been a growing interest in the application of this method to the investigation of surface phenomena produced by more complex molecules which yield either fragment desorption products or catalytically formed species (35, 46a 46h). Also, physisorp-... 

TPSR results are presented in Fig. 4. Propene is produced when the sample temperature is above 350 TC on both samples, which means converting of propane over CNF catalysts could occur without oxygm. The desorption products amounts are 0.35 and 0.26 mmol/g for CNF-RA and CNF-HA respectively while the percentages of propene in llie desorption substances over these two sample are 51.4% and 87.7%. These results imply that the propene selectivity may increase, at least partly, due to restriction of oxidation of propane to COx by heat treatment at the cost of catalytic activity. 

The hydroxyalkyl radicals CH3CHCH2OH and CH3CH(OH)CH2 formed by reactions of OH with C3H6 can also be adsorbed on the soot. However, the resulting desorption products are not further useful for NO removal, and therefore, the reactivity of these radicals is decreased. 

One of the most significant recent insights in surface chemical dynamics is the idea that the principle of detailed balance may be used to infer the properties of a dissociative adsorption reaction from measurements on an associative desorption reaction.51,52 This means, for example, that the observation of vibrationally-excited desorption products is an indicator that the dissociative adsorption reaction must be vibrationally activated, or vice versa the observation of vibrationally-cold desorption products indicates little vibrational promotion of dissociative adsorption. In this spirit, it is... 

The resulting radicals are not usually observed, but thermal desorption products indicate the nature of the surface intermediates. Molybdenum(V) dispersed on silica also gives rise to 0 and O2 ions when exposed to N2O and O29 respectively. The 0 ion on this surface may be used to activate methane and ethane in a catalytic cycle which leads to their partial oxidation. 

The extra oxygen adsorbed on the Mo(l 12)-(1 X2)-0 surface drastically changes the selectivity of the reaction. TPR spectra of methanol from the (lX2)-0 surface with 0.20 ML of preadsorbed extra oxygen after exposure to 4 L of methanol at 200 K are different from the spectra for the surface without the extra oxygen on the following points (1) considerable reduction of the peaks of CH4 and H2 at 560 K, the second is (2) disappearance of the peak of recombinative desorption of CO at 800 K, and (3) appearance of the peak of H20 at 580 K. The amounts of desorption products are summarized in Table 8.2. Selectivity to CH20 increased to 88%. Particularly, reduction of re-combinative desorption of CO at 800 K indicates that complete decomposition of methoxy to C(a) and O(a) is considerably suppressed by the presence of extra oxygen. Detection of H20 and... 

The desorption peak of 16 amu around 200 K is due to methane, which is produced in the titanium sublimation pump during methanol exposure, and adsorbed on the sample holder. The main desorption products are H2 around 400 K and CO above 800 K. The H2 desorption peak is much larger than that observed upon the H2 adsorption up to saturation on the clean surface. The H2 desorption peaks seem to consist of three components two relatively small components at 350-400 K and around 500 K and a sharp peak at 410 K. For the HD desorption trace, only the peak at 350-400 K is seen. These results suggests that the methanol decomposition reaction on the clean Mo(l 12) surface proceeds as follows. 

The most significant observation is the large differences in reactivities of the three forms of oxygen ions, with O" Oj OJ. This is well illustrated by the reaction with ethylene where O- ions react readily at — 60°C, Oj ions react at 25°C with a half-life of ca. 5 min, whereas only one-third of the Oj ions react after 2 hr at 175°C. The authors propose a number of surface intermediates (Table XIV) in the oxidation reactions based on analysis of the desorption products and IR studies. 

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 separation of the two sets of desorption products may indicate that they are from different sites. That is, branching of the selective and nonselec-tive oxidation takes place on adsorption of butene. This can be confirmed if the two sets of products can be varied independently. This is shown by two experiments. The first experiment makes use of the fact that butene and butadiene adsorb on the same sites. Butadiene is first adsorbed onto the catalyst (5). The catalyst is then heated to 210°C, desorbing all of the unreacted butadiene, but leaving on the surface the precursors of the combustion products. Since desorption of the unreacted butadiene does not involve a net chemical reaction, the adsorpton sites involved are not affected. The catalyst is then cooled to 22°C, and cis-2-butene is adsorbed. If selective oxidation and combustion take place on the same site, the adsorbed butene would undergo both reactions. If they take place on separate sites, and butene adsorbs only on the selective oxidation site (because the combustion site is covered by species from butadiene adsorption), the adsorbed butene would form only butadiene. Subsequent desorption yields a profile similar to that for a single adsorption of ds-2-butene (Fig.l, curve b). More importantly, within experimental errors, the amount of butadiene evolved is the same as in a ds-2-butene adsorption experiment, and the amount of C02 evolved is the same as in a butadiene adsorption experiment. Thus, the adsorbed butene forms only butadiene. These results show that under these experimental conditions (i.e., in the absence of gas-phase oxygen), the production of butadiene and carbon dioxide takes place on separate sites. 

Fig. 2. Thermal desorption product distribution in cis-2-butene adsorption and desorption on a-Fe203 as a function of equilibration time. C4H6 purged butadiene purged out at 22°C C02 /4 C02 thermally desorbed divided by four C4H6 des butadiene thermally desorbed total C4H6 total butadiene produced (equals the sum of that purged out and that thermally desorbed). From ref. 5, reprinted with permission, copyright 1979 by the American Chemical Society.

C. After purging out the gas-phase butene, the catalyst was heated to various temperatures indicated, and the desorbed species were purged out of the reactor by He. Then a pulse of oxygen (or NzO) was passed over the catalyst at various temperatures. After the pulse had completely left the reactor, thermal desorption was resumed, and the desorption products were collected and analyzed. 

Catalyst desorption temperature (°C) Gas pulse Pulsing temperature (°C) Pulse size (IO17 molecules) Thermal desorption products (1017 molecules m 2) ... 

If the bond-breaking step is slow compared with the rate of desorption products, that is kz fc6 and <3C kt, Eq. (26) simplifies to... 

Due to the multiple desorption products, the surface mechanism of adsorption and desorption of Cl etching seems to be quite complex. More theoretical studies are expected. 

Due to the multiple desorption products, the etching of the surface with halogen appears to be quite complex. A multi-step reaction mechanism has been suggested to account for the SiCl2 desorption species. In the case of fluorine atom adsorption, F atom abstraction and dissociative chemisorption mechanisms have been suggested. In order to account for the complex surface reactions, more studies are needed. 

The distribution of surface acidity strength has been studied by measuring the differential adsorption heat of ammonia. The differential scanning calorimetry (Setaram DSC 111) and the FTIR spectrophotometry (Nicolet 740) have been simultaneously used in order to measure the heat associated with the neutralization of the acidic sites and the amount of chemisorbed base, respectively. Once the sample is saturated at 250 °C and the total acidity measurement is obtained as the amount of base used in the titration, the measurement of the total acidity has been verified by desorption of the base at programmed temperature (TPD). A ramp of 5 °C/min between 250 °C and 500 °C has been followed, with a He flow of 20 cm3/min and using the FTIR spectrophotometry for the measurement of the desorption products. 

Figure 28. Thermal desorption (heating in flowing nitrogen, heating rate 3Ks 1) of CO2 and carbon monoxide from heavily oxidized fullerene black (treated in 10% ozone, oxygen at 333 K. in water). An IMR-MS detector was used for unperturbed gas analysis and simultaneous detection of other desorption products (see text and Fig. 29). The value for the bum-off temperature is defined as the temperature where a weight loss of 3% had occurred.

Unique absorption-desorption product recovery systems. 

Uranium oxides have been investigated as catalysts and catalyst components for selective oxidation. They are more commonly used as catalyst components, but there are also reports of uranium oxide alone as a selective oxidation catalyst The oxidation of ethylene over UO3 has been studied by Idriss and Madhavaram [40] using the technique of temperature programmed desorption (TPD). Table 13.3 shows the desorption products formed during TPD after ethylene adsorption at room temperature on UO3. The production of acetaldehyde from ethylene indicates... 

The sputtered (001) surface produced methanol as the primary high-temperature desorption product - more than half of the methoxy species present on the surface at 400 K recombined to form methanol at 580 K. The balance of surface methoxides decomposed into methane and CO in a 2 1 ratio between 590 and 600 K. X-ray photoelectron spectroscopy measurements for methanol adsorbed on the sputtered (001) surface at 300 K demonstrated that two types of carbon-containing species were present on the surface subsequent flashing to... 

On a sample pre-calcined at 673 K, further flash-heated under vacuum at 800 K, and finally treated with CO at 200 K, the subsequent TPD run showed that 63% of the preadsorbed CO was actually desorbed as CO2 (149). This fraction was much larger than that observed in a parallel study on the bare support. On repeating the experiment in a cyclic manner, successive but decreasing amounts of CO2 were observed. Conversely, on the catalyst reduced with H2 (Pic J0 Torr) at 773 K (5 min), prior CO adsorption, CO was practically the only desorption product The initial behaviour of the catalyst could be restored by subsequent oxygen treatment at 373 iC (149). Figure 4.11 summarizes the TPD-MS diagrams recorded after the two latter experiments. 

Alloyed Sn completely changed the selectivity of NO decomposition reactions, primarily forming N O as the primary desorption product in TPD. Based on HREELS spectra, NO bonds in the same bent-atop configuration on both Pt(lOO) and the Sn/Pt(100) alloys at low NO coverages. At monolayer coverage, two molecules of NO bond to one Pt atom (dinitrosyl species), which is an intermediate in... 

Table I shows that the differential heat of adsorption of n-alkane on "as received" carbon fibers Is low and closely approximates Its heat of liquefaction. This Indicates a low concentration of high energy sites on the "as received" fibers. The differential heat of adsorption on "cleaned fibers, especially T-300, Is greater than on "as received" fibers, suggesting that some of the high energy sites on the carbon fiber surfaces were occupied by physically adsorbed species. GC analysis of desorption products, collected In a liquid nitrogen trap, showed the presence of water and carbon dioxide.

Figure 3 shows a reaction profile obtained by bleeding the desorption products directly into the source of a mass spectrometer. (The detector signal is uncorrected for differential sensitivity, so the relative concentrations are not represented.)... 

The samples were gradually heated to 300°C while desorption products were pumped out through a liquid nitrogen cooled trap. The ratio of isomers was evaluated by Hl-N.M.R. Mass spectra included very weak signals at m/e 60 and 61 which were tentatively identified with HOAc arid DOAc, respectively. A control experiment established that no detectable amount of cis-trans isomerization occurred when vinyl acetate vapor was exposed to the same conditions in the absence of the carbon sample. 

Metallic Product desorption Product precipitation Interstitial diffusion PdH, Fe4N NijC Intermetallic compounds 10 6... 

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