CO Adsorption Properties

In a first step comparable TPD experiments of CO interaction on MgO(lOO) and Pt(111) as a function of coverage were recorded and are depicted in Fig. 4.6. 

Spectra on MgO(lOO) show the expected behavior of a weakly physisorbed molecule with one desorption peak a (maximum at 112 K), independent of the coverage. The observed peak is in good agreement with the literature [27], the low amount of detected CO is related to a very small sticking coefficient. 

This is not a first order characteristic, but an effect of the weak adsorption [26]. 

The integration of the peak areas from the TPD spectra (not shown), reveal an linear increase in area with respect to the presented coverage range, a saturation coverage is not reached. As the observed behavior correlate well with literature further discussion is omitted. 

From 0.1 CO/SA for the peak at 15.8 eV a shoulder at 15.0 eV begins to appear and becomes more pronounced at 0.5 COjSA. For coverages well over a ML (2 CO ISA) these features are clearly visible along with two smaller, broader peaks at 9.5 eV 

---

Abbet S, Riedo E, Brune H, et al. Identification of defect sites on MgO(lOO) thin films by decoration with Pd atoms and studying CO adsorption properties. J Am Chem Soc. 2001 123 6172-8. 

The results presented in the previous sections demonstrate the importance of point defects at the surface of oxide materials in determining the chemical activity of deposited metal atoms or clusters. A single Pd atom in fact is not a good catalyst of the cyclization reaction of acetylene to benzene except when it is deposited on a defect site of the MgO(lOO) surface. A detailed analysis of the reaction mechanism, based on the calculation of the activation barriers for the various steps of the reaction, and of a study of the preferred site for Pd binding, based on the MgO/Pd/CO adsorption properties, has shown that the defects which are most likely involved in the chemical activation of Pd are the oxygen vacancies, or F centers, located at the terraces of the MgO surface and populated by two (neutral F centers) or one (charged paramagnetic F centers) electrons. 

Magg N, Giorgi J, Frank M, Immaraprom B, Schroeder T, Baumer M, et al. (2004). Alumina-supported vanadium nanoparticles structural characterization and CO adsorption properties. 

Ahn. P.K. et al.. Effect of alkaline earth added to platinum-supporting oxides on platinum dispersion and dehydrogenation activity, Appl. Catal. A, 101. 207, 1993. Tamiu a. H.. Katayama. N.. and Furuichi. R., The Co+ adsorption properties of ALO,. Fe,O,. Fe,O4. TiO,. and MnOj evaluated by modeling with the Frumkin isotherm., 7 Colloid Interf. Sci.. 195. 192, 1997. 

Adsorption studies which have been conducted on the group VIII elements using other techniques will then be reviewed in a systematic sequence. This will be followed by the CO adsorption properties of the remainder of the transition metals. In all these sections, a comparison between the results being described and those previously discussed for the tungsten system will be made wherever possible. Finally, any overall conclusions that emerge for CO adsorption will be indicated. 

The similarities between the CO adsorption properties of cobalt and those of iron evaporated metal films have already been mentioned. The calorimetric heat of CO on cobalt films is very close to that on iron 80) as is the surface potential caused by this adsorption 123). The CO uptakes on catalysts of the three metals iron, cobalt, and nickel have also been reported to be very close in value 133), although the group Vlllb and c elements all crystallize normally in face center cubic lattices, unlike the group Villa metals. 

The similarity of the CO adsorption properties of palladium and nickel, as compared with the CO-platinum system, is perhaps surprising for the chemical and physical properties of palladium are far closer to platinum than they are to nickel. 

Although the CO adsorption studies carried out on these elements are not as extensive as on their nearest neighbors in the periodic table (Cr, Mo, W), there have been a number of important investigations on these systems. These studies indicate some close similarities between the CO adsorption properties of these elements and those of tungsten, but also a number of significant differences. Like the tungsten group of metals, these three elements crystallize in a body-centered cubic lattice. 

To investigate further the change in electron distribution within the metal cluster we used CO as a probe molecule to monitor the modifications induced in the Ni6 clusters by the interaction with the support. We found that when CO is adsorbed on the one-layer Ni6 cluster deposited on alumina the CO vibrational frequency is substantially blue shifted compared with free Ni6. This indicates that charge transfer from the Ni overlayer to the oxide occurs, with a consequent reduction of back-donation into the Ni-CO bond. When, on the other hand, CO is adsorbed on the two-layer Nig cluster deposited on alumina we found little change in the CO adsorption properties compared with the situation without substrate. The metal layer in direct contact with the substrate is partially oxidized whereas the second Ni layer is almost unperturbed by the substrate. The change in electronic structure at the interface is rapidly screened for the upper metal layers thus, Ni atoms of the second layer where CO is adsorbed behave similarly in supported and unsupported clusters. 

In order to further elucidate the CO adsorption properties on Pt(lll) a series of temperature dependent MIE spectra was recorded and is depicted in Fig. 4.9. For these spectra 0.4 COjSA, equivalent to approximately one ML, was dosed. The temperature was increased stepwise, and MIE spectra were always taken at T < lOOK, to exclude possible temperature effects. After dosage the features of the 7t/5a MOs at 16.0eV an the 4a MO at 17.8eV are visible—showing a comparable relaxation shift and a slightly higher chemisorption shift than presented above. With increasing temperature the features start to vanish at 330 K and disappear completely above 360 K. At even higher temperatures the spectra resemble the Ft(111) bulk spectrum. 

Hydrated metal sulphates have long been used to study water removal processes, and characteristic kinetic behaviour is conveniently illustrated by reference to these substances. Frost and co-workers [602,603] have investigated the structures, stabilities and adsorption properties of various intermediate amorphous phases, the immediate reaction products which can later undergo reorganization to yield crystalline phase. 

Results for other metals of the Pt-group are due to Frumkin and co-workers8,10,11,14 (Table 22). However, an electrode with the surface renewed in closed circuit has been used by Lazarova767 to study depend linearly on pH with a slope ofca. 55 mV. This has been explained by the adsorption properties of Rh toward H and O, which shift <7-0 to more negative values. Anions have been observed to specifically adsorb on Rh more strongly than on Pt in the sequence... 

Besides the effect of the presence of alkali on CO adsorption, there is also a stabilizing effect of adsorbed CO on the adsorption state of alkali. Within the high alkali coverage range the number of CO molecules adsorbed on promoted surface sites becomes practically equal to the number of alkali metal species and their properties are not dependent on the CO coverage. In this region CO adsorption causes also stabilization of the adsorbed alkali, as indicated by the observed high temperature shift of the onset of alkali desorption. 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

The acidic properties of the bare supports were studied by IRS method using CO adsorption at 77 K. The IR spectra were measured on a Shimadzu FTIR-8300 spectrometer over a range of 700-6000 cm with a resolution of 4 cm. Before spectra registration, sample of the supports powder was pressed in wafer (p = 0.007-0.016 g/cm ) and treated in vacuum (450°C, 1 hr., < 10 Torr). 

Morales F., de Smit E., de Groot F.M.F., Visser T., and Weckhuysen B.M. 2007. Effects of manganese oxide promoter on the CO and H2 adsorption properties of titania-supported cobalt Fischer-Tropsch catalysts. J. Catal. 246 91-99. 

Ming, J., Koizumi, N., Ozaki, T., and Yamada, M. 2001. Adsorption properties of cobalt and cobalt-manganese catalysts studied by in-situ diffuse reflectance FTIR using CO and CO+H2 as probes. Appl. Catal. A Gen. 209 59-70. 

At the end of the seventies, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering. Oxidation, followed by reduction at moderate temperatures restored the adsorption properties of the metal in full. The suppression of adsorption after high temperature reduction was attributed to a strong metal-support interaction, abbreviated as SMSI [2]. 

---