Effect of the hydrogen concentration on the adsorption and catalytic properties of a PdRu alloy

Effect of the hydrogen concentration on the adsorption and catalytic properties of a PdRu alloy

Journal of the Less-Common Metals, 89 (1983) 585 585 - 586 EFFECTOFTHEHYDROGENCONCENTRATIONONTHE ADSORPTION AND CATALYTIC PROPERTIES OF A Pd-Ru V...

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Journal of the Less-Common

Metals, 89 (1983)

585

585 - 586

EFFECTOFTHEHYDROGENCONCENTRATIONONTHE ADSORPTION AND CATALYTIC PROPERTIES OF A Pd-Ru V. G. DOBROKHOTOV

ALLOY*

and V. M. GRYAZNOV

A. V. Topchiev Institute of Petrochemical

Synthesis, Moscow (U.S.S.R)

L. F. PAVLOVA

P. Lumumba (Received

People’s Friendship

University, Moscow (U.S.S.R.)

June 2, 1982)

Summary An increase in the hydrogen concentration in the Pd-Ru alloy increases the adsorption of cyclohexane on the hydrogen-poor QCphase and decreases it on the hydrogen-rich fl phase. The kinetics of the hydrogenation of benzene to cyclohexane on the (Yand fl phases are different. These phenomena indicate that the adsorbed hydrogen interacts with the hydrocarbon species.

The isotherms of cyclohexane vapour adsorption on the walls of a Pd-6 wt.%Ru capillary tube 10 m long were determined by a pulsed gas chromatography technique. Figure 1, curve A, shows the increase in the Henry coefficient of cyclohexane adsorption on the hydrogen-poor (x phase at a temperature of 303 K with increasing hydrogen pressure. This behaviour may be due to the attraction between differently polarized adsorbed species of hydrogen and cyclohexane. In contrast, the Henry coefficient decreases with increasing hydrogen pressure for the hydrogen-rich fl phase (Fig. 1, curve B). Hydrogen adsorption on this phase is mainly molecular and competes with cyclohexane adsorption. Benzene hydrogenation into cyclohexane was studied by introducing benzene vapour pulses into the HZ-N, flow. The chromatograms of the substances obtained on the (Yphase are shown in Fig. 2(a). Methane was used as the reference gas. Cyclohexane formed on the (Yphase flows out of the capillary tube 3 min later than the unconverted benzene. An increase in the amount of benzene added by a factor of 2 or even 4 does not have a significant effect on the position of the cyclohexane peak (compare the three C6Hi2(2) curves in Fig. 2(a)) or on the amount of cyclohexane produced which is equal to the amount of cyclohexane formed during the hydrogen treatment of strongly sorbed benzene at the same temperature (303 K). All these facts suggest that the strongly sorbed benzene is hydrogenated on the cxphase of the Pd-Ru alloy. *Paper presented at the International of Metal Hydrides, Toba, Japan, May 30 0022-5088/83/0000-0000/$02.75

Symposium

on the Properties

and Applications

- June 4, 1982. 0 Elsevier

Sequoia/Printed

in The Netherlands

586

Fig. 1. Henry coefficient of cyclohexane vapour adsorption as a function of the hydrogen partial pressure for the CYphase (curve A) and the p phase (curve B) of the Pd-Ru alloy.

1r m’i

!

(a)

2 ml

TlME (b)

Fig. 2. The retention times of methane (reference gas) and of the cyclohexane introduced (CbHr2 (1) peaks) and obtained during benzene hydrogenation (CeHrz(2) peaks) on (a) o-Pd-Ru and (b) fi-Pd-Ru.

In contrast, Fig. 2(b) shows that the maximum of the cyclohexane peak for benzene hydrogenation on the fl phase is near the maximum of the benzene peak. However, the cyclohexane peak is highly asymmetric. This may be due to the hydrogenation of loosely adsorbed benzene together with the hydrogenation of strongly adsorbed benzene. The kinetics of benzene hydrogenation on the p phase are not zero order as in the case on the (Yphase; the order of the reaction on benzene is a proper fraction. The adsorption strength of benzene on the /3 phase is less than that on the (Yphase because the retention time of benzene is much shorter (Fig. 2(b)). It is easy to see from Fig. 1 that this effect is more pronounced for cyclohexane. These data can be explained by competition between the adsorption of hydrocarbon molecules and of molecular hydrogen. Conversely the adsorption of atomic hydrogen on the cr phase increases the cyclohexane adsorption as shown in Fig. 1, curve A.