ON THE ADSORPTION OF CARBON DIOXIDE BY METAL OXIDES

V. V. NAKHALOV, N. F. STAS’, G. G. SAVEL’EV,
A. S. GUZENBERG, A. M. RYABKIN, T. S. GORINA

(Presented by the scientific seminar of the Department of General and Inorganic Chemistry)

The number of materials used as adsorbents is continuously increasing in connection with the expansion of the areas of their application.

Nevertheless, the development of adsorbents with high capacity and specificity, suitable for repeated use with periodic regeneration, remains an urgent problem up to the present time. In this connection, metal oxides are promising for the purification of gases from $\mathrm{CO_2}$ and $\mathrm{H_2O}$.

The first studies on the adsorption of various gases on metal oxides were carried out by A. Benton [1]. Subsequently, adsorption on metal oxides was investigated by Garner and co-workers [2]. For the investigations, oxides were chosen mainly that were used as catalysts in various chemical processes. However, no systematic study of the adsorption properties of most oxides has been carried out.

The binding of $\mathrm{CO_2}$ by a metal oxide in chemical interaction according to the reaction:

\[ \mathrm{Me_nO_m + mCO_2 = Me_n(CO_3)_m} \]

can proceed over a fairly wide temperature interval. The possibility of chemical interaction of $\mathrm{CO_2}$ with metal oxides and of regeneration of adsorbents by thermal dissociation of the resulting carbonates is confirmed by thermodynamic calculations [3].

Chemisorption is characterized by high specificity, large values of the heats of adsorption, and a tendency toward irreversibility. The heats of chemisorption proceeding with the formation of chemical bonds considerably exceed the heats of physical adsorption and approach the heats of chemical reactions. Unlike carbon and metals, oxides possess two chemically different types of adsorption centers: these are metal ions and oxygen ions. Oxygen ions, upon adsorption of $\mathrm{CO_2}$, participate in the formation of a surface carbonate according to the equation [4]:

\[ \mathrm{CO_2 + O^{2-} \to CO_3^{2-}.} \]

Metal ions are also active adsorption centers, but the mechanism of the process has not been sufficiently studied.

Chemisorption of carbon dioxide by the surface of a metal oxide may be either reversible or irreversible. An example of reversible chemisorption is the chemisorption of $\mathrm{CO_2}$ on nickel oxide [5]. Carbon dioxide chemisorbed on metal oxides is always desorbed in

in the form of CO₂. Desorption of CO₂ often also occurs in the case where CO adsorption has been carried out.

The study of CO₂ adsorption on metal oxides has been undertaken in order to elucidate the mechanism of the process [6–10]. With the aid of IR spectroscopy it was possible to establish that adsorption on metal oxides produces several forms of chemisorbed CO₂, which under certain temperature conditions can transform into one another.

For the use of an adsorbent in a technological process, an important characteristic is its capacity, i.e., the amount of gas that 1 g of adsorbent is able to absorb at a given temperature and pressure. Such information in the literature is very sparse and pertains mainly to synthetic zeolites. The adsorption capacity of zeolites in CO₂ adsorption is in most cases small and amounts to 10 mg/g [11] or somewhat higher [12].

Calculations carried out from the equations of chemical reactions show that, by means of chemisorption of CO₂ on metal oxides, it would be possible to achieve an increase in adsorption capacity by a factor of 10–100. The results of the calculations are summarized in Table 1.

Table 1

The lightest oxides should have the greatest adsorption capacity. However, since the theory of adsorption is still insufficiently developed, it is not possible to predict the adsorption capacity under real conditions. It can be determined only experimentally.

We studied the adsorption of CO₂ on the 17 oxides indicated in the table. To measure the magnitude of adsorption, an automatic gravimetric apparatus based on McBain balances was used. The oxides were obtained by thermal decomposition in vacuum of the carbonates and hydroxides of the corresponding metals. The synthesis of the hydroxides and carbonates was carried out from chemically pure reagents. The hydroxides were obtained by precipitation with ammonia from salt solutions; the carbonates were precipitated with ammonium carbonate [13, 14]. The charge of hydroxide or carbonate was 25–100 mg. The metal oxide obtained in vacuum upon decomposition of the hydroxide or carbonate was cooled to room temperature and thermostated. Then carbon dioxide from a cylinder was admitted through a drying system into the reaction vessel containing the oxide up to atmospheric pressure, and the gas was passed through at a low rate (0.5 L/min). Recording of the change

the adsorbent weight was carried out automatically on the chart tape of an EPP-09 self-recorder. The change in weight was recorded with an accuracy of ±\(10^{-5}\) g.

The maximum amounts of carbon dioxide adsorbed on the individual oxides are presented in Table 1.

The adsorption capacity of metal oxides depends on many factors: the method of preparing the oxide, the stoichiometric composition, preliminary treatment, etc. We observed that freshly prepared oxides have a greater adsorption capacity than oxides that had been stored for a long time at room temperature. Oxides obtained at lower temperatures are, as a rule, more active. To lower the temperature, the decomposition of carbonates and hydroxides was carried out in a vacuum of \(10^{-3}\) mm Hg. Of the oxides investigated, magnesium oxide has the highest adsorption capacity—1000 mg/g—which is close to the calculated value. According to calculations, beryllium and aluminum oxides should have a higher adsorption capacity, but we were unable to activate them completely. Two oxides, SnO and Bi\(_2\)O\(_3\), adsorb dry CO\(_2\) in negligible amounts.

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