Bands of Adsorbed CO

FIGURE 36. LPSIRS spectra of CO adsorbed on a palladium electrode in 1 M HCIO4 at different constant potentials. After Kunimatsu.  

FIGURE 37. EMIRS spectra of CO adsorbed on Pd in 1 M H2SO4 for different degrees of coverage 6 by adsorbed 

No doubt this results from the influence of solvation and of the electrode potential. 

The data given in this table are a very good example of the success of infrared reflectance spectroscopy in investigations of the solid/solution interface. 

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By increasing the hydrogen reduction temperature, the intensity of the 2100 cm-1 band of adsorbed CO decreased progressively, and at temperatures > 250°C, a new weak band at 2070 cm-1 appeared. This band was partially removed by evacuation at room temperature. 

Main IR bands of adsorbed CO 2060-80 cm-1 as on clean Pt Type I New band at 2120 cm-1 Type II... 

Observations that may support this mechanism have been reported specifically, these include the disappearance of IR bands of adsorbed CO during the Fischer-Tropsch reaction (17) and during alkene hydroformy-lation, consistent with CO insertion between metal atoms and alkyl ligands (18). It is important to realize that to insert CO into metal alkyl groups, the alkyl group has to be attached to surface metal sites with empty d-orbitals (19). 

For all the studied pretreatment temperatures three bands of adsorbed CO were observed at 2166, 2142 and 2126 cm". Changes in the spectra of the OFl groups were much less pronounced as compared with those observed for silicalite or Ti-silica gel, that is why the band at 2166 cm", whose maximum position does not coincide with that of molecules adsorbed on silanol groups, should rather be attributed to a new type of sites appearing due to the presence of titanium. 

The spectrum of this zeolite is more complicated, In the OH stretching region three bands were observed after pretreatment at 573 or 673 K, at 3750, 3620 and 3540 cm", the former two resist pumping at 973 K, when the last one disappears. After CO adsorption at 77 K (fig. 2.) these bands diminish while two bands of perturbed hydroxyls show up at 3660 and 3330 cm . That at 3660 cm disappears after pumping off the gaseous CO at 77 K together with the band of adsorbed CO at 2160 cm and the restoration of the 3750 cm band, however, to remove all the CO bands and to recover the initial OH group spectrum, evacuation at room temperature is needed. 

The infrared bands of adsorbed CO may be related to the activity in the electrochemical reduction. Ni and Fe electrodes reduce CO to methane, ethylene and ethane electrochemically with the current efficiency 3 % at a constant current density 2.5 mA / cm (the electrode potentials are around -1.5 V) in 0.1 M KHC03.[8,11] The Cu electrode effectively reduces CO to hydrocarbons with the current efficiency 50 % at 2.5 mA / cm (the potential is -1.4 V).[4,5,11] Thus the activity order in CO reduction is Cu > Ni Fe. The linear CO is more easily reduced than bridged one on the Ni electrode. The order of the electrochemical activity of metals in CO reduction roughly agrees with the reverse order of the adsorption strength of CO. 

On the Pd reduced samples, the CO adsorption carried out at room temperature shows the three infra- red bands of adsorbed CO linearly bonded CO at 2050 cm, bridged bonded CO near 1950 cm and multibonded CO near 1850 cm When increasing amounts of Mn are added to Pdioo, the CO species adsorbed on top (vCO at 2050 cm ) decrease and disappear quasi completely at higher Mn contents ( Mn at.% > 35 %). Moreover, the intensities of the two other infra-red bands at 1950 and 1850 cm decrease (Fig. 5). These IR bands are attributed to CO adsorbed either on Pd, Mn (11) or Pd-Mn dual sites. 

Recently, Fenelon and Rubalcava (18) studied the interaction of CO with Na and Ca A and X zeolites at pressures of about 10 torr. Using isotopic CO and by analyzing the absorption band contours of the adsorbed species, gaseous and liquid CO, they concluded that the CO molecules freely rotate in the Na zeolites until they collide with the cage walls. For Ca zeolites, the absorption bands of adsorbed CO indicate strongly hindered rotation. This is plausible since CO adsorbs preferentially on the multivalent ions and is held more strongly than on univalent ions. 

The isocyanate bands on Pt/Mn0ySi02 and Pt/CoOx/Si02 are not stable above 350°C. No bands of adsorbed CO or NO were found on manganese oxide, while linear and bent NO were clearly visible on cobalt oxide. The decoration with -NCO, resulting in an inhibition of the CO/NO reaction at lower temperatures, took place over Pt/CoOx/Si02. 

Similarly, IR investigation of CO adsorption on molecular sieves was used to characterize Lewis acidity of cations (C-sites) and true Lewis acidity (L-sites) [ 740]. The interaction of CO with cations (acid C-sites) was dealt with already in Sect. 5.5.2.2. In particular, Angell and Schaffer [595] have carried out a detailed study of CO adsorption on a series of X- and Y-type zeoHtes containing monovalent and divalent cations of alkali, alkaline earth and transition metals. A linear relationship was found between the position of the IR stretching band of adsorbed CO and the Coulomb field, q/r, of the respective cationic adsorption center. This is similar to the observation made by Ward in the case of pyridine attached to cations (vide supra). It should be noted, however, that CO, like pyridine, is not capable of entering the sodalite cages and the hexagonal prisms of the faujasite structure, so that the cations located there are not detected by these probes. 

Besoukhanova et al. [29] prepared 5 wt.% Pt/L zeolites by impregnation of L zeohtes exchanged with Li, Na, K, Rb and Cs with Pt(NH3)4Cl2. A TEM study indicated that the Pt-dispersion was inhomogeneous with large particles on the external surface, 1-2.5 nm particles both inside and outside the zeolite, and cyhnders (length 4-7 nm, width 1.5 nm) inside the zeolite. It was assumed that IR bands of adsorbed CO near 2000 cm corresponded to Pt-carbonyls formed by the reaction of CO on very small nuclearity Pt-clusters (e.g., to 0.6-0.8 nm) not visible on TEM micrographs. The band near 2060 cm attributed to linear-... 

In the present work low temperature adsoi ption of fluoroform and CO, were used to characterize surface basicity of silica, both pure and exposed to bases. It was found that adsorption of deuterated ammonia results in appearance of a new CH stretching vibration band of adsorbed CHF, with the position typical of strong basic sites, absent on the surface of pure silica. Low-frequency shift of mode of adsorbed CO, supports the conclusion about such basicity induced by the presence of H-bonded bases. 

The strain or stress will either lead to narrower or broader d bands that are shifted up or down in energy, respectively. An upward shift leads to a stronger interaction with the 2jt orbital of adsorbed CO and thus to a stronger chemisorption bond. Stress has the opposite effect. 

Another very interesting result obtained from these FURS measurements is the difference between adsorbed CO obtained from dissolved CO and that from the dissociation of adsorbed methanol. The shift in wave number is more important with dissolved CO. These shifts may also be correlated with the superficial composition of the alloys, and it was observed that the optimized composition for the oxidation of CO (about 50 at.% Ru) is different from that for the oxidation of methanol (about 15 at.% Ru). FTIR spectra also revealed that the amount of adsorbed CO formed from methanol dissociation is considerably higher on R than on Pt-Ru. For a Ptog-Ru-o i alloy, the amount of linearly adsorbed CO is very small (Fig. 8), suggesting a low coverage in the poisoning species. Moreover, by observing the potentials at which the COj IR absorption band appears, it is possible to conclude that the oxidation of both (CHO)ads and (CO)acis species occurs at much lower potentials on a R-Ru alloy electrode than on pure Pt. 

The nature of surface sites on the reduced 0.5 wt.% RU/AI2O3 and 0.5 wt.% Ru/Ti02 catalysts was probed by FTIR spectroscopy of adsorbed CO. Four adsorbed CO bands were... 

The surface state of the spent catalysts was also studied by FTIR of adsorbed CO following Ar purge at 550°C and cooling to room temperature. Two strong and broad bands were observed at ca. 2130 and 2072 cm" over the RU/AI2O3 catalyst, assigned as Ru°-CO and Ru (CO)2, respectively. 

These two-step features, which will be further proved by the FTIR spectra of adsorbed CO, can be summarized as follows. The adsorption of CO, being accompanied by the increase of the coordination munber due to the formation of mono- and dicarbonyl species, causes a shift of the d - d transitions toward the values more typical of the octahedral coordination. Furthermore, in the presence of CO (electron donor molecule) more energy is required to transfer electrons from O to Cr as a consequence, the O Cr(II) CT transition shifts at higher frequencies (from 28000-30000 to 33 700cm ). At increasing CO pressure the CO Cr(II) CT transition also becomes visible (band at 33400 cm ). Analogous features have been reported in the past for NO adsorption on the reduced Cr/Si02 system [48,82]. 

A very different picture comes from the IR spectra of sepiolite. The different spectra of adsorbed CO over sepiolite are represented in Figure 9.2. Introduction of CO resulted in the appearance of a series of absorption bands with v... 

Figure 1 shows the series of infrared spectra collected during 02 pulse studies into flowing He/propylene at 250 °C. The initial exposure of the catalyst to the He/propylene flow produced bands at 1982 cm 1 and 1810 cm 1. The band at 1982 cm"1, in the range of adsorbed CO, is a result of interaction of propylene with surface OH. This is evidenced by a decrease in the OH intensity which is accompanied by an increase in the intensity of the 1982 cm"1 band. This 1982 cm"1 band can also be produced from adsorption of PO. The band at 1810 cm"1 is due to CH2 wagging of propylene the band at 1590 and 1465 cm"1 can be... 

The top curve shows the spectrum of adsorbed CO that is observed when no nitrile compound is added to the electrolyte. The C-0 stretching frequency occurs at 2085 cm, which is characteristic of a saturated CO adlayer at this potential. The next three spectra were recorded in solutions which contain 1.0 M CH.CN, 0.2 M C-H.CN, and 0.1 M HOOCCH.CN, respectively. The intensity of the vfcO) band is reduced about 50% in each case. This indicates that the amount of CO adsorbed on the electrode is reduced by the... 

The lower two spectra were recorded in CO saturated solution which contained 0.1 M C H,.CN. The adsorbate layers were produced by cycling the potential with the electrode pulled away from the window as described above, except that a different final potential was chosen to end each cycle. Spectrum c was recorded at a final potential of 0.05 V. At this point no v(C0) band is observed. Spectrum d was recorded at a final potential of 0.55 V (c.f. Figure 1), and shows the band at v(C0) - 2078 cm" that we assigned to a partial coverage of adsorbed CO. We can show that this change in the spectrum is irreversible by returning the potential to 0.05 V. The v(C0) band is still observed with the same peak area, and the frequency is shifted by only the amount predicted by the known potential dependence. 

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