Band carbon monoxide adsorption

Spectra of CO adsorbed on nickel and copper films obtained by Bailey and Richards (23) are shown in Figure 11. Carbon monoxide adsorption at 77°K resulted in the appearance of only a single band at 2097 cm l for nickel and 2109 cm l for copper. Further CO exposure of the sample at 1.5°K resulted in the observation of a second band near 2143 cm l on both metals. The low frequency band in each case was identified with chemisorbed CO while the high frequency band was associated with physisorbed CO. Bailey and Richards (23) have also used their apparatus to obtain spectra of N2 chemisorbed on nickel, and benzene and... 

Carbon monoxide adsorption on ZnO has been found to depend strongly on the pretreatment of the oxide. Kortiim and Knehr (94) detected a weakly bound CO with at least one rotational degree of freedom showing rotational-vibrational bands with maxima at 2188 and 2120 cm-1 and a strongly bound complex similar to bidentate carbonate. In addition, well-degassed ZnO specimens adsorbed CO with the IR frequency within the carbonyl... 

Evidence for ensemble effects in VIIIC/IB alloys has been obtained by examination of carbon monoxide adsorption by infrared spectroscopy. This technique has been applied to the systems Pd-Ag, Ni-Cu, and Pd-Au. It is generally accepted that carbon monoxide may chemisorb in bridged or linear forms, the former providing an absorption band in the region 1900-1950 cm and the other in the region 2000-2050 cm . There may be a distinguishable contribution to the former from CO bonded... 

The study of carbon monoxide adsorption by Angell and Schaffer (6) has been discussed elsewhere (7, 74, 75). An interesting observation not previously emphasized is the appearance of cation specific absorption bands in the spectra of the multivalent cation zeolites because of the CO interacting with the exchanged cation via the carbon atom. They showed that the band frequency was a function of the electrostatic field strength which polarized the CO molecules. 

The first case (a) discussed above is nicely demonstrated in a study of carbon monoxide adsorption on copper in contact with an aqueous solution of 0.1 M KCIO4 [204] only a single-sided band of the CO-stretching vibration is observed at 2110 cm-i (Fig. 5.51). 

Infrared spectroscopy can be used to obtain a great deal of information about zeolitic materials. As mentioned earlier, analysis of the resulting absorbance bands can be used to get information about the structure of the zeolite and other functional groups present due to the synthesis and subsequent treatments. In addition, infrared spectroscopy can be combined with adsorption of weak acid and base probe molecules to obtain information about the acidity and basicity of the material. Other probe molecules such as carbon monoxide and nitric oxide can be used to get information about the oxidation state, dispersion and location of metals on metal-loaded zeolites. 

Figure 2.16. Calculated dissociative nitrogen ( ), carbon monoxide ( ), and oxygen ( ) chemisorption energies over different 3d transition metals plotted as a function of the center of the transition metal rf-bands. A more negative adsorption energy indicates a stronger adsorbate-metal bond. Reproduced from [32].

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-... 

EMIRS studies of ethanol on platinum electrodes have demonstrated the presence of linearly bonded carbon monoxide on the surface [106]. An important problem in the use of EMIRS to study alcohol adsorption is the choice of a potential window where the modulation is appropriate without producing faradaic reactions involving soluble products. Ethanol is reduced to ethane and methane at potentials below 0.2 V [98, 107] and it is oxidized to acetaldehyde at c 0.35 V. Accordingly, a potential modulation would be possible only within these two limits. Outside these potential region, soluble products and their own adsorbed species complicate the interpretation of the spectra. The problem is more serious when the adsorbate band frequencies are almost independent of potential. In this case, the potential window (0.2-0.35 V) is too narrow to obtain an appropriate band shift and spectral features can be lost in the difference spectrum. 

Optical spectroscopy has been applied with a good deal of success to the identification of chemisorbed species and of the nature of the surface bond. Infrared spectra have been most useful in studies of simple molecules, such as carbon monoxide adsorbed on platinum or nickel, and ultraviolet spectra for the characterisation of more complex interipediates, such as carbonium ions and ion radicals. The frequency of the adsorption band (or bands) often serves to identify the adsorbed species by comparison with spectra of known compounds. Quantitative information may then in principle be obtained by measuring the area under the adsorp-... 

The spectrum of carbon monoxide adsorbed on nickel oxide prepared at 200° may be divided into two regions (Table I, la) (60). The first includes two bands at 2060 and 1960-1970 cm i, the second three bands at 1620, 1575, and 1420-1440 cm-i. Bands at 2060 and 1960-1970 cm- are typical of carbonyl structures and are found in the spectrum of carbon monoxide on metallic nickel (61). It has been suggested by some authors (62) that, in our experiments, these bands were also produced by the adsorption on the metal, the oxide being supposed oxygen-deficient. Chemical analyses (30) have shown, however, that, NiO(200°) contains an excess of oxygen and magnetic susceptibility measurements (33) have demonstrated that the quantity of metal is very small. Since the intensity of these bands is strong, we believe that they are not produced exclusively by the chemisorption of carbon monoxide on the metal but mainly by the adsorption on exposed nickel ions. 

The band at 2060 cm-i disappears after evacuation of carbon monoxide at room temperature (Table I, lb). A fraction of the reversibly adsorbed gas is therefore located on cationic sites. Since the band at 1960-1970 cm- is observed after the evacuation, an irreversible fraction of carbon monoxide is also chemisorbed on cationic sites. A subsequent adsorption of oxygen produces, however, the disappearance of this band (Table I, Ic), demonstrating that oxygen interacts with carbon monoxide irreversibly adsorbed on cationic sites. No gas is evolved from the surface during the adsorption of oxygen. The interaction product therefore remains in the adsorbed state and its structure must be similar to the structure of species formed previously during the adsorption of carbon monoxide since no new band appears in the spectrum after the adsorption of oxygen. 

NiO(250°) contains more metallic nickel than NiO(200°). Magnetic susceptibility measurements have shown that carbon monoxide is adsorbed in part on the metal (33) and infrared absorption spectra have confirmed this result since the intensity of the bands at 2060 cm-i and 1960-1970 cm-1 is greater when carbon monoxide is adsorbed at room temperature on samples of nickel oxide prepared at temperatures higher than 200° and containing therefore more metallic nickel (60). Differences in the adsorption of carbon monoxide on both oxides are not explained entirely, however, by a different metal content in NiO(200°) and NiO(250°). Differences in the surface structures of the oxides are most probably responsible also for the modification of their reactivity toward carbon monoxide. In the surface of NiO(250°), anionic vacancies are formed by the removal of oxygen at 250° and cationic vacancies are created by the migration of nickel atoms to form metal crystallites. Carbon monoxide may be adsorbed in principle on both types of surface vacancies. Adsorption experiments on doped nickel oxides, which are reported in Section VI, B, have shown, however, that anionic vacancies present a very small affinity for carbon monoxide whereas cationic vacancies are very active sites. It appears, therefore, that a modification of the surface defect structure of nickel oxide influences the affinity of the surface for the adsorption of carbon monoxide. The same conclusion has already been proposed in the case of the adsorption of oxygen. 

The same is true for the heterogeneous system involving 80203 photoexcited in the absorption band of F-type hole eolour centres. The quantum ydeld of photobleaching of F-type colour centres is = 0.022 0.002 in vacuo, and 4>v = 0.046 0.003 in a reducing (H2 or CO) atmosphere. The quantum yield of the photostimulated adsorption of dihydrogen and carbon monoxide on coloured scandia is 4> = 0.023 0.002, so that the 4> of photobleaching of hole colour centres in H2 (or CO) reflects the sum of the quantum yields of photobleaching of colouration in vacuo and the surface photochemical processes. That is,... 

The adsorption of CO and CO2 on zirconia was also studied using infrared spectroscopy, which provides direct evidence for surface intermediates. The results are presented in Figure 3. Carbon monoxide formed the formate (bands at 2880, 1580, 1387, and 1360 cm ) after adsorption at 225 and 500 C, and possibly a small amount of bicarbonate (band at 1610 cm ) after adsorption at 225 C. Carbon dioxide formed the bicarbonate (bands at 1610, 1430, and 1220 cm ) and a carbonate (band at 1335 cm ) after adsorption at 225 and 500 C. 

Several types of cells have been constructed and are commercially available that allow an in situ measurement or have a movable holder to evacuate and heat the sample. With these cells it is also possible to admit gas or vapor to subsequently measure the frequency shift upon adsorption. This technique can reveal additional information about the surface properties of the material. Carbon monoxide, for instance, has been applied as a probe to detect hydroxyl groups at the silica surface. Upon adsorption of CO at 77 K, the band due to free hydroxyl groups shifts to a lower frequency by 78 or 93 cm-1 (35, 36, 37). 

---