Adsorption of carbon monoxide

Carbon monoxide is the molecule whose adsorptive properties have certainly been investigated in most detail. A review of the work prior to 1970 was published by Ford (2) in this series, but in the interim so much additional information has been obtained that a complete compilation of the data would be far beyond the scope of this article. Instead, the main emphasis will be given to a discussion of essential features that fortunately have much in common with different platinum group metals as well as with various crystal planes. 

Adsorption normally takes place in molecular form, although sometimes 

It is commonly accepted that chemisorption of CO on transition metals takes place in a way that is quite similar to bond formation in metal carbonyls (4). First experimental evidence for this assumption was obtained from a comparison of the C—O stretching frequencies (5) and was later confirmed by data on the bond strength (6) as well as by valence and core level ionization potentials obtained by photoelectron spectroscopy (7). Recent investigations have in fact shown that polynuclear carbonyl compounds with more than about 3-4 metal atoms exhibit electronic properties that are practically identical to those of corresponding CO chemisorption systems (8, 9), thus supporting the idea that the bond is relatively strongly localized to a small number of metal atoms forming the chemisorption site. 

(a) Enhancement factor R(e) as a function of the energy relative to the Fermi level for field emission from a CO/Ir(100). (b) Schematic representation of the CO levels that give rise to the enhanced emission just below the Fermi level (51). 

CO shows a general tendency for the formation of close-packed arrangements near saturation as will be outlined in the next section. 

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Of the four commercial processes for the purification of carbon monoxide two processes are based on the absorption of carbon monoxide by salt solutions, the third uses either low temperature condensation or fractionation, and the fourth method utilizes the adsorption of carbon monoxide on a soHd adsorbent material. AH four processes use similar techniques to remove minor impurities. Particulates are removed in cyclones or by scmbbing. Scmbbing also removes any tars or heavy hydrocarbon fractions. Acid gases are removed by absorption in monoethanolamine, hot potassium carbonate, or by other patented removal processes. The purified gas stream is then sent to a carbon monoxide recovery section for final purification and by-product recovery. 

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... 

Much work has been undertaken to understand the steps and intermediates by which the reaction occurs on the heterogeneous catalyst surface. However, the exact mechanism is not fully established. One approach assumes a first-step adsorption of carbon monoxide on the catalyst surface followed by a transfer of an adsorbed hydrogen atom from an adjacent site to the metal carbonyl (M-CO) ... 

Alternatively, an intermediate formation of an adsorbed methylene on the catalyst surface through the dissociative adsorption of carbon monoxide has been considered ... 

Y. Matsumoto, T. Onishi, and K. Tamam, Effects of Sulphur on a Palladium Surface on the Adsorption of Carbon Monoxide and the Adsorption and decomposition of nitric oxide, J.C.S. Faraday I 76, 1116-1121 (1980). 

Figure 7. Total internal reflection sum frequency generation (TIR-SFG) vibrational spectroscopy of high-pressure room temperature adsorption of carbon monoxide on PVP-protected Pt cube monolayers and calcined (373 K, 3h) monolayers [27], The infrared spectra demonstrate CO is adsorbed at atop sites, but is considerably red-shifted on the PVP-protected Pt cubes. After calcination, the atop frequency blueshifts to 2085 cm in good agreement with CO adsorption on Pt(l 0 0) at high coverages [28], (Reprinted from Ref [27], 2006, with permission from American Chemical Society.)...

Large platinum carbonyl clusters have been investigated as models for the adsorption of carbon monoxide on platinum surfaces and on platinum electrodes. An issue is how large the clusters must be before they adopt the properties of the bulk metal. Teo et al. have investigated the magnetic properties of the clusters [Pt6(CO)12]2+, [Pt9(CO)18]2+, [Pt y(CO)22f+, and... 

Adsorption of carbon monoxide takes place all over the surface and there is distinct evidence that, at least on nickel, the CO stretching frequency depends upon the coordination number of the nickel atom to which it is attached. Hence, the adsorption of carbon monoxide yields information about the relative numbers of surface atoms with different coordination numbers. This information, howrever, is at best merely of a semiquantitative nature. Steric effects also play a role, as is evidenced by the fact that the subcarbonyl species can be formed only on nickel atoms with a lowr coordination number. 

Fig. 25. Differential heats of adsorption of carbon monoxide at 30°C on fresh (A) or oxygenated (B) samples of a gallium-doped nickel oxide. Reprinted from (63) with permission J. Chim. Phys.

All preparations were structurally characterized by means of XRD (Siemens 5005). TEM imaging was performed with a Philips CM200 instrument. 27A1 and 29Si MAS NMR (Broker 500 MFlz and 360 MFlz respectively) was used to study the microporous phase and the kinetic of its formation. The relaxation delays were 0.2s and 200s respectively. Acidity was determined by the adsorption of carbon monoxide after activating the samples in vacuum (10 6 mbar) at 450°C for 1 h. The spectra were recorded on a Equinox 55 Broker spectrometer with a resolution of 2 cm 1 and normalized to 10 mg of sample. 

Hooker, M. P., and Grant, J. T. 1977. The use of Auger electron spectroscopy to characterize the adsorption of carbon monoxide transition metals. Surf. Sci. 62 21-30. 

Figure A.16 Energy diagram for the adsorption of carbon monoxide on the (100) plane of rhodium (from de Koster et al. [21]).

Data for the adsorption of carbon monoxide on charcoal at 273 K analyzed with three equations in linearized form, are... 

Measurements of the adsorption of carbon monoxide on mica were made at two temperatures (Bawn, JACS 54 72, 1932). A slight correction should be made for adsorption on the glass vessel wall but is not made here. The pressure units are Torr/1000, and the adsorption is (ml STP)/100. 

Based on the experimental data and some speculations on detailed elementary steps taking place over the catalyst, one can propose the dynamic model. The model discriminates between adsorption of carbon monoxide on catalyst inert sites as well as on oxidized and reduced catalyst active sites. Apart from that, the diffusion of the subsurface species in the catalyst and the reoxidation of reduced catalyst sites by subsurface lattice oxygen species is considered in the model. The model allows us to calculate activation energies of all elementary steps considered, as well as the bulk... 

The result obtained for Af//°[Cr(CO)6, cr)] is some 50 kJ mol-1 more positive than the recommended value, -980.0 2.0 kJ mol-1 [149], a weighted mean of experimental results determined with several types of calorimeter. The large discrepancy is not due to an ill-assigned thermal decomposition reaction but to a slow adsorption of carbon monoxide by the chromium mirror that covered the vessel wall. This is an exothermic process and lowered the measured Ar//°(9.13). 

The advantages of electron spectroscopy for the study of adsorbed diatomic molecules are illustrated by reference to the adsorption of carbon monoxide, nitrogen, nitric oxide, and oxygen on different metal surfaces. 

In the investigation of adsorbed species using XAS, it is not always possible to probe adsorbate—substrate bonding by changing to the adsorption edge of the adsorbate, for example, the adsorption of carbon monoxide on carbon supported Pt particles. Carbon... 

Key (46) and Strickland-Constable (47) also support mechanism B for the carbon-carbon dioxide reaction. Strickland-Constable concludes from earlier measurements (46) that the rate of adsorption of carbon monoxide on carbon is too low to account for the retardation. 

Fig. 26. Resistance increase of a transparent nickel film (90 X 10 atoms/sq. cm.) on the adsorption of carbon monoxide at T = 90.4°K. [according to (18)].

Chemisorption of Carbon Monoxide. Chemisorption of carbon monoxide on NiO(250) does not change the electrical conductivity of the sample. The same result was obtained for NiO(200) (23). The curve of differential heats of adsorption of carbon monoxide on NiO(250) presents many similarities with the curve recorded in the case of NiO(200) (20). However, a few differences are noted. The heat of adsorption of the first dose (0.08 cc. per gram) of carbon monoxide on NiO(200) is high (42 kcal. per mole) (Table II). The adsorption of the next dose on the same oxide releases only 29 kcal. per mole. The initial high value of the heat adsorption was explained by the interaction of CO with excess surface oxygen (Table I), giving C02(ainitial heat of adsorption amounts to 29 kcal. per mole (Table II). It seems, therefore, that the surface excess oxygen... 

A Adsorption of oxygen on NiO(250) containing preadsorbed carbon monoxide B Adsorption of carbon monoxide on NiO(250) containing preadsorbed carbon monoxide and oxygen... 

Figure 3. Adsorption of carbon monoxide on nickel oxide containing preadsorbed oxygen...

Reaction Mechanism. The rate of production of heat as a function of time gives indications of the velocity of the process taking place on the catalyst surface (Figure 4B). For instance, it has been shown (20) that, on NiO(200), the adsorption of oxygen and the formation of CO.r(adfo ions are fast processes compared with the adsorption of carbon monoxide or the reaction between CO and CCV ds)- From calorimetric results and a kinetic study of the reaction, it has been concluded (8) that the decomposition of COa uds) ions by adsorbed carbon monoxide to yield carbon dioxide is the slowest step of the reaction mechanism on NiO(200) (Mechanism I). 

Thus, again the limiting step of both Interactions 2 and 4 cannot be the adsorption of carbon monoxide (Equation 3) since the rates of adsorption of CO, without interaction with preadsorbed species, on nickel oxides containing nearly the same quantity of 0"(ads) ions or C03"(ads) ions should be the same for the same coverage of the surface and the rate of production of heat would be the same. 

At temperatures between 750° and 1,050° abs., however, the adsorption of carbon monoxide being evidently much less, a simpler reaction took place. This proceeded with a velocity proportional to the pressure of oxygen when the carbon monoxide was in excess, and to the pressure of the carbon monoxide when the oxygen was in excess. At... 

The introduction of hydrogen at 100 torr on solid C produced an increase of the oh bands, which are now well resolved (3640-3540 cm-1) (Figure 5). The intensity of these bands increased slowly with the time the maximum value was reached after 6 hours at the same time, the water formation was detected by its 5h2o band at 1640 cm 1. After evacuation of hydrogen at room temperature, the adsorption of carbon monoxide generated bands at 2135, 2110, 2100, 1935, and 1895 cm 1. The last three bands were pressure dependent. Evacuation at 25 °C produced a partial removal of the 2100 cm 1 band, and the 1935-1895 cm-1 bands dis-... 

Non-dissociative, dissociative. If a molecule is adsorbed without fragmentation, the adsorption process is non-dissociative. Adsorption of carbon monoxide is frequently of this type. If a molecule is adsorbed with dissociation into two or more fragments both or all of which are bound to the surface of the adsorbent, the process is dissociative. Chemisorption of hydrogen is commonly of this type. 

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