불포화탄화수소에의한 팔라디움 멤브레인의 수소투과 저하에 관한 연구

정순화, 草壁克己*, 諸岡成治*, 김상돈

한국과학기술원 화학공학과, 일본 큐슈대학 대학원 재료물성공학전공

Deactivation of a hydrogen-permeating palladium membrane by

coexisiting unsaturated hydrocarbons

SoonHwa Jung, Katsuki Kusakabe*, Shigeharu Morooka* and SangDone Kim

Deaprtment of Chemical Engineering, KAIST, Taejon, 305-701 Korea

*Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka, 812-8581 Japan

Introduction

Palladium and palladium-alloy membranes permeate hydrogen with a high selectivity. When palladium-base membranes are combined with chemical reactors, the selective removal of hydrogen from reactants shifts the reaction equilibria in the direction of hydrogen formation. Membrane reactors such as these have been used for steam reforming, aromatization of propane, and dehydrogenation of hydrocarbons. The permeation rate of hydrogen through a palladium-based membrane is inversely proportional to the thickness of the membrane. Thus, attempts have been made to form thin and dense palladium membranes on porous substrates by electroless plating,  chemical vapor deposition and sputtering.

However, coexisting species often damage palladium membranes and decrease the hydrogen permeation rate. Contaminants such as sulfur, chlorine and arsenic chemically react with palladium, leading to a collapse of the membrane. Unsaturated hydrocarbons, which were mixed in hydrogen, also deactivated the surface sites [Kusakabe et al., 1996]. Collins et al. [1996] observed that a palladium membrane was deactivated during the dehydrogenation of propylene, resulting in a significant reduction in hydrogen permeance. This decrease was attributed to the deactivation of both membrane and catalyst, but the specific contribution by each of these was not determined.

Carbonaceous matters serve as precursors of carbon, which is dissolved in palladium and may suppress the hydrogen permeation rate. Ziemecki et al. [1985] examined the phase transformation from palladium to palladium carbide by X-ray diffraction techniques. A layer of carbon was formed on the palladium surface, and carbon atoms were then transported into the metal lattice by an activated diffusion mechanism. In order to prevent the deactivation of membrane and catalyst by coking, steam is usually mixed in the feed of a dehydrogenation reactor. However, the major part of the data collected to date are related to the effect of steam on supported palladium catalysts. The present study reports an investigation of the effects  of coexisting hydrocarbons and steam on the hydrogen permeation of palladium membranes. Depth profiles of carbon concentration in the palladium membranes were determined by secondary ion mass spectroscopy, and the relationship between hydrogen permeation rates and carbon concentrations was discussed. Palladium membranes deactivated by coexisting unsaturated hydrocarbons were regenerated using hydrogen as well as oxygen.

Experimental

A palladium disk of 50□m in thickness and 5.0mm in diameter was used as a membrane, which clamped between two soft Cu gaskets. Before the permeation test, palladium disk was heat-treated at 600oC in H2 atmosphere for 6h and adjusted to the experiment temperature. Hydrogen gas permeation test was carried out in the mixed gases containing H2, N2, steam and hydrocarbons at the permeation temperature of 350-600oC. The gas composition was determined with a gas chromatography with equipped with a thermal conductivity detector (Shimadzu, GC-8A).

Hydrogen flux, JH2, through a palladium membrane is defined as follows

JH = (pfeedn - ppermeaten)

When the rate-determining step is the diffusion in the membrane, the flux is proportional to square roots of partial pressures (n=0.5). However, the deactivation changes the value of n in the range of  n=0.5-1, which depends on the degree of the deactivation. The permeance is defined by the unit of mol.m-2.s-1.Pa-1 in this study. Palladium membranes, which had been deactivated were treated at 600 oC in a flow of oxygen-free hydrogen or a mixture of O2 (20%) and N2 (80%). Depth profiles of carbon in palladium membranes were determined by secondary ion mass spectroscopy (SIMS, Atomica SIMS-4000M) with a 200 □m diameter beam size at an acceleration energy of 150 keV.

Results and discussions

Figure 1 shows Arrhenius plots of hydrogen permeance for the  feed of equimolar mixtures for H2-N2, H2-CH4 and H2-C3H8 systems. Coexisting CH4 and C3H8 had only a negligible effect on hydrogen permeance, which remained unchanged with time. The apparent activation energy for hydrogen permeation was approximately the same for all these mixtures, 11.4 kJ.mol-1, and was in agreement with a value reported by DeRosset [1960], 11.9 kJ.mol-1.

Figure 2 shows the hydrogen permeance for an equimolar mixture of H2 and C3H6. The hydrogen permeance gradually decreased with increasing time. After 10 min, the higher the permeation temperature, the lower was the permeance. It was found that carbonaceous matter was deposited at the mouth of the injection nozzle and on the palladium surface, as a result of the decomposition of propylene, and that deposition proceeded faster at higher temperatures. The carbonaceous matter damages the active sites for hydrogen dissociation, thus, decreases the hydrogen permeation rate. Propylene is dissociatively chemisorbed on palladium and that carbon-carbon bonds were cleaved at temperatures above 199oC. Thus coking occurs simultaneously with hydrogen permeation, and the resulting carbon atoms are dissolved in the palladium lattice.

Figures 3 (a)-(c) show depth profiles of carbon in the palladium membranes which were deactivated by mixtures of H2-C3H6, H2-CH4 and H2-C3H8 at 600oC for 3 h, respectively. Figure 3(d) shows the depth profile in an unused palladium membrane as the reference. The strong change in the carbon signal at an etching time of 85-90 min is assigned to the epoxy bond, which was used to fix the sample in place, suggesting that the etching hole penetrated the membrane. Carbon was distributed in a relatively homogeneous manner in the membranes. The carbon level of (a) and (b) was higher than that of (c). The latter is close to that in the original palladium membrane, (d). The hydrogen permeances for the mixtures of H2-N2, H2-CH4 and H2-C3H8 were the same. This suggests that the carbon level in the palladium membranes had a negligible effect on hydrogen permeance. Carbonaceous matter, which was formed by dehydrogenation of propylene was the major origin of the deactivation.

It would be expected that the coexistence of steam would be effective in preventing coking, even after hydrogen is selectively removed from the reactor. Contrary to this expectation, the introduction of steam decreased the hydrogen flux especially at lower temperatures as shown in Fig. 4. When the feed of steam was stopped after measuring the permeation rate, the permeance was recovered to the initial value of the palladium membrane. Thus the decrease in permeance by steam may be caused by the competitive adsorption of steam and hydrogen on the active sites of the palladium surface. In contrast to the cases of the H2-CH4-H2O-N2 and H2-C3H8-H2O-N2 mixtures, the presence of steam .increased the permeance of H2-C3H6-H2O-N2 mixture as shown in Fig. 5. The role of steam in preserving the hydrogen permeance is less evident at lower temperatures. The carbon levels in the membrane were low compared to those in the membranes which had been deactivated without steam. These results suggest that steam volatilizes a portion of the carbonaceous matter, which is formed from propylene on the surface of the palladium membrane.

A palladium membrane, which had been deactivated by feeding the H2-C3H6 mixture at 600oC for 3 h, was exposed to hydrogen at 600oC. The hydrogen permeance was restored to 80% of the initial value after a week period. This suggests that hydrogen is not an effective reagent for the regeneration of the membrane. Regeneration of the deactivated palladium membranes in the mixture of oxygen and nitrogen at 600oC. The hydrogen permeance was recovered within 15 min for all membranes, irrespective of deactivation periods.

Conclusions

Methane and propane showed only a negligible effect on hydrogen permeance, but propylene strongly decreased the hydrogen permeance especially at higher temperatures. In terms of the deactivation mechanism, the occupation of active sites by carbonaceous matter is more dominant than is the hindrance of diffusion in the membrane by dissolved carbon atoms. Steam played two roles; i.e., by decreasing and increasing the permeation rate by competitive adsorption to active sites for hydrogen dissociation and by volatilization of the carbonaceous matter deposited on the membrane, respectively. Exposure to diluted oxygen at 600oC regenerated the deactivated membranes in a period of time less than an hour, while exposure to hydrogen required that the treatment time should be extended for longer than a week.

References

Collins, J. P., Schwartz, R. W., Shegal, R., Ward, T. L., Brinker, C. J., Hagen, G. P. and Udovich, C. A., "Catalytic dehydrogenation of propane in Hydrogen permselective membrane reactors", Ind. Eng. Chem. Res., 35, 4398 (1996).

DeRosset, A. J., "Diffusion of hydrogen through palladium membranes", Ind. Eng. Chem., 52, 525 (1960).

Kusakabe, K., Yokoyama, S., Morooka, S., Hayashi, J.-i. and Nagata, H.,  "Development of supported thin palladium membrane and application to enhancement of propane aromatization on Ga-Silicate catalyst", Chem. Eng. Sci., 51, 3027 (1996).

Ziemecki, S. B., Jones, G. A., Swartzfager, D. G., Harlow, R. S. and Faber, J.,  "Formation of interstitial Pd-C phase by interaction of ethylene, acetylene and carbon monoxide with palladium", J. Am. Chem. Soc., 107, 4547 (1985).