Zeolite carbon monoxide adsorption

XL30). Mossbauer spectroscopy (KFKIj was applied to follow the state of Fe species in the zeolites. Carbon monoxide and ammonia adsorption (monitored with FTIR) (EQUINOX 55) was used to determine the nature, concentration and acid strength of the active sites in the Fe-TON zeolites. 

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. 

Fig. 4J Langmuir plots (a) for propane on 5A zeolite (courtesy Ruthven) (fc) for carbon monoxide on zeolite CaY-54 (courtesy Stone). In (a) the adsorption is expressed in terms of number C of molecules of adsorbate per cavity in (b), as m (stp).

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

It is generally accepted that localization and coordination of monovalent Cu ions in different zeolites have significant influence on the catalytic activity. The localization and coordination of Cu ions was studied by means of adsorption of small probe molecules, in particular, carbon monoxide was used often due to its ability to form a stable mono-carbonyl complex with the Cu+ ion. The formation of this complex was investigated by the FTIR and by the microcalorimetry [1-3]. 

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. 

At the cold start of the engine the catalyst is not able to oxidize carbon monoxide and hydrocarbons present in the exhaust. Therefore, zeolites are added into y-Al203-based catalytic washcoat for HC adsorption at low temperatures, resulting in an integrated adsorber-reactor system (Jirat et al., 2001 Kryl et al., 2005). For optimum operation of such a system, the consecutive HC desorption induced by increasing temperature should not occur earlier than the catalyst light-off. 

By in situ MAS NMR spectroscopy, the Koch reaction was also observed upon co-adsorption of butyl alcohols (tert-butyl, isobutyl, and -butyl) and carbon monoxide or of olefins (Ao-butylene and 1-octene), carbon monoxide, and water on HZSM-5 (Ksi/ Ai — 49) under mild conditions (87,88). Under the same conditions, but in the absence of water (89), it was shown that ethylene, isobutylene, and 1-octene undergo the Friedel-Crafts acylation (90) to form unsaturated ketones and stable cyclic five-membered ring carboxonium ions instead of carboxylic acids. Carbonylation of benzene by the direct reaction of benzene and carbon monoxide on solid catalysts was reported by Clingenpeel et al. (91,92). By C MAS NMR spectroscopy, the formation of benzoic acid (178 ppm) and benzaldehyde (206 ppm) was observed on zeolite HY (91), AlC -doped HY (91), and sulfated zirconia (SZA) (92). 

The Cr A and several other zeolites containing transition metal ions, which may exist in two or more valence states, were also found to be oxidation catalysts. One such system of note is the copper containing Type Y zeolite, the redox chemistry of which was studied in several recent investigations (2, 3.4, 5). These studies established the range of conditions at which copper exists in divalent, monovalent, or zerovalent state and in particular determined the reduction conditions in hydrogen and carbon monoxide atmospheres for a complete conversion of Cu Y to Cu Y but no further to Cu°. The Cu ions in type Y zeolite were reported to be specific adsorption centers for carbon monoxide ( 6), ethylene ( 7), and to catalyze the oxidation of CO (8). In the present work the Cu ions were also found to be specific adsorption centers for oxygen. 

L. M. Kustov, V. B. Kazansky, S. Beran, L. Kubelkova, and P. Jiru, Adsorption of carbon monoxide on ZSM - 5 zeolites infrared spectroscopic study and quantum - chemical calculations, J. Phys. Chem. 91, 5247-5251 (1987). 

Palladium nanoparticles (nm-Pd) were synthesized by ship-in-a-bottle technique in supercages of NaA zeolite. The behaviors of electrodes of thin film of nm-Pd accommodated in NaA zeolite were characterized by cyclic voltammetry. The results illustrated that the nm-Pd possess particular properties for hydrogen reaction, i.e. in contrast to hydrogen absorption on massive palladium electrode, the surface processes of hydrogen adsorption-desorption become the dominant reaction on electrodes of thin film of nm-Pd. The processes of adsorption and desorption of carbon monoxide on the electrodes were studied using in situ electrochemical FTIR reflection spectroscopy. It has been revealed that in comparison with CO adsorbed on a massive Pd electrode, the IR absorption of CO adsorbed on nm-Pd particles accommodated in NaA zeolite has been enhanced to about 36 times. 

Commentary on Studies of Cations in Zeolites Adsorption of Carbon Monoxide Formation of Ni Ions and Na/ Centres, J. A. Rabo, C. L. Angell, P. H. Kasai and V. Schomaker, Discuss. Faraday Soc, 1966, 41, 328-349. 

Studies of Cations in Zeolites Adsorption of Carbon Monoxide Formation of Ni ions and Na centres... 

Table 2.—Adsorption of carbon monoxide on several bivalent-cation zeolites, SAMPLE compositions, LANGMUIR COMBINING CONSTANTS K AND NUMBERS o OF EFFECTIVE SITES, NON-SPECIFIC ADSORPTION COEFFICIENTS C, AND RELATIVE MOLECULAR OPTICAL DENSITIES Dq...

As has been reviewed above, the surface cations in anhydrous X and Y zeolites are incompletely co-ordinated with oxide ions, so that for molecules in the zeolite cavities the (enormous) electric field of a surface cation is not effectively shielded out by the fields of all the other ions. Essential consequences of this special accessibility of the cations are the carboniogenic activities and the specific complex-ing of, e.g., carbon monoxide. The shielding is still expected to be appreciable, however, and different for X and Y, and the differences between X and Y in catalysis and gas adsorption seem to correspond. 

J. M. Thomas comments on Studies of cations in zeolites adsorption of carbon monoxide formation of Ni ions and Na4 centres ... 

According to the data of analysis of many adsorption systems, the first term in Equation 9 corresponding to the second order appears only v hen considering adsorption of relatively small molecules. They include molecules of linear shape, such as the diatomic gases, carbon dioxide, carbon monoxide, etc. Experimentally realizable orders, n, are integers from 3 to 6 in the general case. With larger polyatomic molecules, no adsorption space remains in the zeolite voids for final adsorption under the effect of dispersion forces. Then Equation 9 retains only the second term, and Uon is expressed by Equation 12. 

Catalyst Characterization. Chemical analyses, x-ray diifraction analyses, and gas adsorption procedures were used to characterize the composition, crystallographic character, and surface structure of the nickel and cobalt zeolite catalyst preparations. The chemical and x-ray procedures were standard methods with the latter described elsewhere 11). Carbon monoxide chemisorption measurements provide useful estimates of the surface covered by nickel atoms from the zeolite substrate 10). 

A high degree of hydrophobic character is an almost unique characteristic of silicon-rich or pure-silica-type microporous crystals. In contrast to the surface of crystalline or amorphous oxides decorated with coordinatively unsaturated atoms (in activated form), the silicon-rich zeolites offer a well-defined, coordinatively saturated sur ce. Such surfrces, based on the strong covalent character of the silicon-oxygen bond and the absence of hydrophilic centers, display a strong hydrophobic character unmatched by the coordinativeiy unsaturated, imperfect surfaces. Also, hydrophobic zeolite crystals have been reported to suppress the water affinity of transition metal cations contained in the zeolite pores. This property permits the adsorption of reactants such as carbon monoxide or hydrocarbons in the presence of water. 

Iwamoto et al. (54) studied the activity of a series of metalion exchanged zeolites for the water-gas shift reaction. The lower water-gas shift activity of the acidic cations was explained in terms of hard-soft acid/base properties. In this model, carbon monoxide, which is a soft base, interacts more strongly with soft acid sites. The adsorption of CO is generally considered to be the rate controlling step in the water-gas shift reaction. Cations of lower acidity are generally softer acids and as such adsorb CO more readily. This would lead to higher surface concentrations of CO, thereby increasing the water-gas shift acitivity of the sample. 

---