Chemisorption of Gases on Ammonium Salts

V. F. Komarov, V. S. Varlamova

(Presented by the scientific seminar of the sector of radiation-chemical analysis, Institute of Nuclear Physics)

The study of the kinetics of the reaction of thermal decomposition of solids, especially in the presence of a gaseous atmosphere, is accompanied by the need to determine the type of interaction between the gas and the substance [1]. One such method is infrared spectroscopy of molecules sorbed on a solid surface.

The possibility of this kind of determination follows from the presence of electron exchange between the solid substance and the chemisorbed gas molecule [2–3], which depends on the nature of the adsorbent and the adsorbate. Calculations by Nagaev [4] have shown the existence of a dependence of the strength of chemisorption bonds, and consequently also of the degree of electron exchange, on the ionicity of the bond in the crystalline adsorbent. As the latter increases, the bond energy of the chemisorbed molecule with the crystal increases, while the activation energy of chemisorption decreases. Redistribution of electron densities in the molecule during chemisorption leads to a change in the frequencies of the valence vibrations of the principal bonds of the molecule. By the MO method, using nitrogen oxide and carbon monoxide as examples, it has been shown that, in the case of formation of chemisorption bonds at the expense of an electron of the gas molecule, the frequency of the valence vibrations of the nitrogen—oxygen and carbon—oxygen bonds should increase, whereas in the case of formation of a bond at the expense of an electron of the adsorbent it should decrease [5].

In the present communication, results are given on the study of the absorption spectra of NO and CO chemisorbed on ammonium perchlorate, chlorate, and bicarbonate.

Experimental Part

The absorption spectra of chemisorbed NO and CO molecules were studied with an IKS-14 spectrophotometer in the range 400–4000 cm\(^{-1}\). All measurements were carried out according to a differential scheme in gas cuvettes with potassium bromide windows, in a stream of the gas under investigation at atmospheric pressure. A thin film of the substance under investigation was deposited on the window of the working cuvette by sublimation in vacuum; the structure of its surface was monitored by observation under a microscope. The results are given in Table 1.

The natural frequencies of the valence vibrations of the gases studied and of the products of their oxidation are, respectively, \(\nu_{\mathrm{NO}} = 1876\ \text{cm}^{-1}\),

$\nu_{\mathrm{CO}} = 2170\ \mathrm{cm}^{-1}$, $\nu_{\mathrm{NO_2}} = 1360\ \mathrm{cm}^{-1}$, and $\nu_{\mathrm{CO_2}} = 2349\ \mathrm{cm}^{-1}$ and do not overlap with the vibrational frequencies of the sorbed molecules. The deviations in the vibrational frequencies of sorbed molecules according to parallel experiments do not overlap with the effect of frequency changes upon adsorption on different adsorbents. From the data of Table 1 on the adsorption of nitrogen oxide it is evident that the strength of the coordination bonds of the adsorbed gas with the substance increases from ammonium bicarbonate to perchlorate (a shift of vibrations into the high-frequency region).

Table 1

Vibrational frequencies of the nitrogen—oxygen and carbon—oxygen bonds of chemisorbed CO and NO molecules*

* In the table, the square brackets indicate the sources used for interpretation of the spectra.

In the same direction, the ionicity of the bonds between the cation and anion of the adsorbent increases, as follows from the values of the dissociation constants of the corresponding acids: $K_{\mathrm{H_2CO_3}} = 6.4$; $K_{\mathrm{HClO_3}} = -1$; $K_{\mathrm{HClO_4}} = -8$.

In adsorption accompanied by electron capture by the nitrogen oxide molecule, strengthening of the bond is observed in going from bicarbonate to ammonium chlorate and from chlorate to ammonium perchlorate in the case of adsorption of the oxidation product—nitrogen dioxide. The strengthening of the chemisorption bonds in the series from bicarbonate to perchlorate is also evidenced by the fact of formation on the surface of the latter of positively

charged nitrogen oxide molecules, whereas on the other two adsorbents such forms of adsorption could not be detected.

In the chemisorption of carbon monoxide, in all three cases the formation of linear structures of the type—$\mathrm{C}\equiv\mathrm{O}^{+}$ [5, 8] is observed; in this case, strengthening of the chemisorption bonds is manifested in the transition from ammonium chlorate to ammonium perchlorate, while in the case of bicarbonate and chlorate no difference is discernible.

Thus, the data obtained on the chemisorption of nitrogen oxide on solid ammonium salts indicate that the strength of the chemisorption bonds depends on the degree of ionicity of the bond in the adsorbent and increases as the latter increases, which is consistent with the general theoretical calculations of Nagaev [3, 4]. In the case of adsorption of carbon monoxide this dependence is weaker. The literature data on the adsorption of nitrogen oxide on chromium oxides (ionicity of the bond in the molecule 43%) and iron oxides (ionicity 38%) also indicate strengthening of the bond in the transition from $\mathrm{Fe}_{2}\mathrm{O}_{3}$ to $\mathrm{Cr}_{2}\mathrm{O}_{3}$ [8].

REFERENCES

1. V. F. Komarov, V. V. Boldyrev, V. T. Shipatov, V. V. Nakhalov, R. I. Akimova. In the present collection.

2. F. F. Vol’kenshtein. Electronic Theory of Catalysis on Semiconductors. Fizmatgiz, 1960.

3. E. L. Nagaev, ZhFKh, 35, 2376, 1961.

4. E. L. Nagaev. Kinetics and Catalysis, 3, 907, 1962.

5. E. M. Shustorovich. ZhSKh, 3, 103, 1962.

6. L. Bellamy. Infrared Spectra of Complex Molecules. IL, 1963.

7. A. Cross. Introduction to Practical Infrared Spectroscopy. IL, 1961.

8. L. M. Roev, A. N. Terenin. DAN SSSR, 125, 588, 1959.

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