Carbon monoxide desorption

Carbon Monoxide Desorption. The details of CO and H adsorption on Ni as a function of titania coverage were examined through TPD to differentiate between long and short-range interactions. In the extremes, if long-range electronic interactions dominate, incremental addition of titania adspecies will effect all adsorption sites equally however, if local interactions are important, new binding states will be observed near titania while the remainder of the surface sites will remain unperturbed. 

A simplified flow diagram of a COSORB unit is shown in Figure 16-24. The overall pnKess generally includes (1) feed gas preparation, (2) carbon monoxide absorption (com-plexing), (3) carbon monoxide desorption (decomplexing), (4) aromatic solvent recovery from effluent gas streams, and (5) compression of CO product stream. Steps (1) and (5) are not shown on the flow sheet. 

Taylor TL, Weinberg WH. A method for assessing the coverage dependence of kinetic parameters application to carbon monoxide desorption from iridium (110). Surf Sci. 1978 78(2) 259. 

The adsorption of carbon monoxide retards the reduction reaction with the rate constant k, followed by the desorption reaction with a rate constant k in the overall rate equation... 

Figure 2.2. Thermal desorption spectra of carbon monoxide, measured mass spectrometically at mass 28 (atomic units, a.u.), on a platinum (100) surface upon which potassium has been pre-adsorbed to a surface coverage of 0K.7 Reprinted with permission from Elsevier Science.

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.

We have undertaken a series of experiments Involving thin film models of such powdered transition metal catalysts (13,14). In this paper we present a brief review of the results we have obtained to date Involving platinum and rhodium deposited on thin films of tltanla, the latter prepared by oxidation of a tltanliua single crystal. These systems are prepared and characterized under well-controlled conditions. We have used thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and static secondary Ion mass spectrometry (SSIMS). Our results Illustrate the power of SSIMS In understanding the processes that take place during thermal treatment of these thin films. Thermal desorption spectroscopy Is used to characterize the adsorption and desorption of small molecules, In particular, carbon monoxide. AES confirms the SSIMS results and was used to verify the surface cleanliness of the films as they were prepared. 

On a third sample, thermal desorption of carbon monoxide was carried out before and after annealing to 760 K and after sputtering into the region where the Ti" " was a minimum (l.e., about 180 seconds in Fig. 3). The TDS results are summarized in Table I. The last column gives the Integrated peak desorption area for carbon monoxide. 

The carbon monoxide reaction is well studied and the observed kinetics are well understood. Of particular interest is the so-called CO-inhibiting regime , characterized by carbon dioxide covering and blocking the surface, so that the reaction rate is governed by CO desorption rate (see original citations in [78]). 

Gilman S. 1963. The mechanism of electrochemical oxidation of carbon monoxide and methanol on platinum. I. Carbon monoxide adsorption and desorption and simultaneous oxidation of the platinum surface at constant potential. J Phys Chem 67 1989-1905. 

Thermal desorption spectra of carbon monoxide on polycrystalline and on single crystal platinum are well known from experiments in the gas phase [48,49], The system is therefore appropriate to test the experimental setup. 

Adsorbed carbon monoxide on platinum formed at 455 mV in H2S04 presents a thermal desorption spectrum as shown in Fig. 2.4b. As in the case of CO adsorption from the gas phase, the desorption curve for m/e = 28 exhibits two peaks, one near 450 K for the weakly adsorbed CO and the other at 530 K for the strongly adsorbed CO species. The H2 signal remains at the ground level. A slight increase in C02 concentration compared to the blank is observed, which could be due to a surface reaction with ions of the electrolyte. Small amounts of S02 (m/e = 64) are also observed. 

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). 

The purpose of this article is to review the results of transient low pressure studies of carbon monoxide oxidation over transition metal substrates. Particular emphasis is given to the use of in-situ electron spectroscopy, flash desorption, modulated beam and titration techniques. The strengths and weaknesses of these will be assessed with regard to kinetic insight and quantification. An attempt will be made to identify questions that are ripe for investigation. Although not limited to it, the presentation emphasizes our own work. A very recent review of the carbon monoxide oxidation reaction C l) will be useful to readers who are interested in a more comprehensive view. 

Figure 2.9 Thermal desorption of carbon monoxide from two rhodium surfaces in ultrahigh vacuum, as measured with the experimental set-up of Fig. 2,10. Each curve corresponds to a different surface coverage of CO. At low coverages CO desorbs in a single peak indicating that all CO molecules bind in a similar configuration to the surface. At higher coverages, an additional desorption peak appears, indicative of a different adsorption geometry (courtesy of M.J.P. Hopstaken and W.E. van Gennip [141).

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

AljOj catalysts, metal dispersion, 39 240 -amine complexes, COj reduction, 28 142 -carbon monoxide adsorption, 28 4, 6 energy, 28 15 structure, 28 10, 14 desorption, 28 23 as catalyst... 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

Kinetics of Adsorption and Desorption and the Elovich Equation C. Aharoni and F. C. Tompkins Carbon Monoxide Adsorption on the Transition Metals R. R. Ford... 

---