J. Chem. SOC., Faraday Trans. I , 1987, 83 (lo), 3149-3159 Temperature-programmed Desorption Study of the Interactions of H,, CO and CO, with LaMnO, Luis G. Tejuca*Jf and Alexis T. Bell Department of Chemical Engineering, University of California, Berkeley, California 94720-9989, U S . A . Jose Luis G. Fierro and Juan M. D. Tascon Instituto de Catalisis y Petroleoquimica, C.S.I.C., Serrano 119, 28006 Madrid, Spain Surface interactions of H,, CO and CO, with the perovskite-type oxide LaMnO, have been studied by temperature-programmed desorption (t.p.d.) and i.r. spectroscopy. A t.p.d. desorption peak of H, at 355-360 K, which increases in intensity with increasing reduction temperature of the oxide ( Tr), is assigned to molecular adsorption of H, on reduced manganese sites (Mn"+, n z 2).CO adsorption yielded t.p.d. peaks of CO and CO,. A peak of CO at 473 K (for oxidized LaMnO,) associated with a carbonate group and peaks at 360-395 K, 540-550 K and 773-800 K (for reduced LaMnO,) associated with linear and bridged CO species adsorbed on Mnn+ ions were observed. A very wide CO, desorption peak at 473 K and tail centred at 773 K (oxidized LaMnO,) are associated with monodentate and bidentate carbonates interacting with Mn3+. CO, adsorption yielded t.p.d. peaks of CO, at 345-385 K and at 540-665 K whose intensity decreased and increased, respectively, with T,. These are associated with monodentate and bidentate carbonates, respectively, interacting with reduced sites of manganese or La3+. Detection of bands at ca. 2900 cm-l in the i.r. spectrum obtained after CO + H, adsorption, the appearance of new CO desorption features at 570 K and above 860 K, and the detection of a new H, desorption peak at 770-785 K in the t.p.d.spectra obtained after CO-H, or H,-CO adsorptions suggest decomposition of an oxygenated species formed by interaction of CO and H, adsorbed on the same adsorption centre. Increasing attention is being paid to CO hydrogenation because of its importance for obtaining oxygenated compounds from other sources than petroleum. As catalysts for this reaction, noble metals dispersed on different supports were La,O, has been found to be a suitable support for obtaining high yields of oxygenated Other compounds, such as LaRhO,, where the noble metal is in the B position of a perovskite structure, have also been employed.6 However, recent results have shown that this oxide undergoes reduction under the reaction conditions, giving place to metallic rhodium supported on La,O, and, presumably, on part of the perovskite that may remain unmodified.' Therefore, we have turned our attention to other oxides of a higher thermal stability in a reducing atmosphere, and in particular LaMO, perovskites, where M is a first-row transition metal.As a first step in the study of these oxides as catalysts for CO hydrogenation, a study has been conducted of the surface interactions of CO and H, with LaMnO,, using temperature-programmed desorption (t.p.d.) spectroscopy. Interactions of individual gases and interactions of CO-H, as a function of the reduction temperature of the oxide t On leave from Instituto de Catalisis y Petroleoquimica, C.S.I.C., Serrano 119, 28006 Madrid, Spain.3 1493150 Interactions of H,, CO and CO, with LaMnO, by T.P.D. are reported. To complement results on CO adsorption, CO, interactions with LaMnO, have been studied. Some i.r. spectroscopic data have also been included. Experimental Equipment The flow system used in the temperature-programmed desorption experiments has been described previously.* The LaMnO, sample (0.5 g) was placed in a quartz microreactor which could be heated at a programmed temperature up to 1 K s-l. The analysis of the effluent gases was made by means of a UTI model 100 C mass spectrometer. A data- acquisition system based in a microprocessor was used to record the signal intensity for a series of preselected masses and the temperature of the catalyst bed.Details of the i.r. spectrophotometer and the i.r. cell used are given el~ewhere.~ Materials The preparation (amorphous precursor decomposition) and characterization of LaMnO, samples similar to that used in this work have been described previo~sly.~ Its B.E.T. specific surface area as determined by N, adsorption (SN2 = 0.162 nm2) at 77 K was 11 .O m2 g-l. The gases used (He, H,, CO, CO, and a mixture of 21 YO 0,-79 YO He) were purified by standard methods. For calibration, mixtures of H,-He, CO-He and C0,-He were used. Methods T.p.d. experiments were performed after gas adsorption on oxidized and reduced (at different temperatures) samples. For oxidation, a mixture of 0,-He was passed for 1 h through the sample at 873 K.The reduced samples were prepared from oxidized (as above) LaMnO, by passing a H, flow for 1 h at the desired temperature. The samples so treated will be referred to hereafter as LaMnO, (ox 873) and LaMnO, (red T,.), where Tr is the reduction temperature in K (from 373 to 873 K). After the appropriate oxidation and reduction the sample was outgassed by passing a He flow for 1 h through the microreactor at 873 K. The adsorption of individual gases was effected by passing a flow of H,, CO or CO, for 0.5 h at room temperature (r.t.) and then a He flow for 15 min at r.t. for removal of the physisorbed part. The successive adsorption CO-H, was carried out by passing flows of CO (0.5 h, r.t.), He (1 5 min, r.t.), H, (0.5 h, r.t.) and He (15 min, r.t.).H,-CO adsorption was effected in the same manner passing first H, and then CO. After the oxidation-reduction, outgassing and adsorption steps, the reactor was repressurized with He, then heating of the catalyst was started at 0.5 K s-l and the data acquisition system activated. The maximum heating temperature was limited to 873 K (100 K below the preparation temperature) to avoid changes in specific surface area. Between two successive t.p.d. runs, the sample underwent treatments of oxidation, reduction and outgassing as indicated above. The flow rates used in oxidation (0,-He), reduction (H,), outgassing (He), adsorption (H,, CO or CO,) and t.p.d. experiments (He as carrier) were, in all cases, 50 cm3 min-l. The mass spectrometer was calibrated daily against the corresponding gas-He mixtures.For i.r. spectroscopic experiments, the sample was placed in an i.r. cell connected to a high vacuum system in which a pressure of Torr (1 Torr = 133.3 N mP2) could be maintained. The sample first underwent oxidation (100 Torr 0,, 1 h) or reduction (100 Torr H,, 1 h) at the indicated temperature. After this it was outgassed under high vacuum for 15 h at 773 K and then contacted with the gas or gas mixture (100 Torr, 1 h) under study.L. G. Tejuca et al. 3151 I I I I I I 300 500 7 00 900 TlK Fig. 1. T.p.d. of H, after H, adsorption at r.t. on LaMnO, reduced at 373 (a), 473 (b), 573 (c) and 723 K (d). Results and Discussion H, Adsorption T.p.d. spectra after adsorption of H, at r.t. on LaMnO, reduced at 373, 473, 573 and 723 K are shown in fig. 1.That corresponding to LaMnO, (red 873) is given in fig. 6. The spectra show a desorption peak at 355 K which shifts to 360 K and increases in intensity with T,. This intensity increase is more marked for T, 3 723 K. No desorption peak was observed after H, adsorption on the oxidized sample. Desorption above 773 K is due to residual H, from the previous reduction treatment. The observed t.p.d. peak is similar to that recorded at 363 K by Spinicci and Tofanaril' in the system H,- Ni-SiO, of weakly adsorbed hydrogen. A second peak found by these authors at 588 K was not observed in our case. However, desorption peaks at 350 and 615 K were observed in the system H,-LaNiO,.'l Heterolytic dissociative adsorption, as occurs in H, adsorption on COO - MgO solid solutions,12 can be ruled out since neither water nor hydroxyl groups were detected by mass spectrometry and i.r.spectroscopy, respectively. The intensity increase of t.p.d. peaks with T, suggests that reduced manganese ions (Mn"+) are directly involved in H, adsorption. The low desorption temperature points to molecular adsorption similar to that observed by Uchida and Bell1, with a desorption peak at 673 K in the system showed LaMnO, to be stable in an H, atmosphere up to 723 K. Its reduction starts at ca. 750 K and reaches a stable reduction of 1 e- per molecule ( i e . , Mn3+ --+ Mn2') at 1073 K. The intensity increase of the desorption peak of H, at T, > 373 K shows that the oxide surface undergoes substantial reduction at H,-Ru * A1,0,.Previous t.p.r.3152 Interactions of H,, CO and CO, with LaMnO, by T.P.D. r 1 I 1 I I I 1 300 500 700 900 T/K Fig. 2. T.p.d. of CO after CO adsorption at r.t. on LaMnO, oxidized at 873 K (a); reduced at 573 (b) and 873 K (c). temperatures much lower than those needed for reduction of the bulk. Taking into account these results it seems plausible to assume that molecular adsorption of H, takes place on Mnn+ (n 2) sites. La3+ is very stable and difficult to reduce. Assuming the lower Miller index planes (loo), (1 10) and (1 11) to be the most frequently exposed ones in LaMnO,, a concentration of 2.67 x lo'* manganese ions rn-, in the surface was calculated. On the basis of this value, the ratios between adsorbed H, molecules and manganese ions as a function of the reduction temperature were calculated. The coverages found are very low for Tr = 373 K and reach a value of 0.1 for the maximum reduction temperature.CO Adsorption In fig. 2 t.p.d. spectra of CO after CO adsorption on LaMnO, are shown. CO desorption from LaMnO, (ox 873) [fig. 2(a)] presents a wide peak centred at 473 K, a poorly resolved shoulder at 540 K and a tail at ca. 773 K. On LaMnO, (red 573) [fig. 2(b)] two well defined peaks appear at 360 and 550 K. CO desorption above 773 K is also observed. On LaMnO, (red 873) [fig. 2(c)], the three desorption peaks which appear at 395, 545 and 800 K increased substantially in intensity. In addition, a shoulder at 860 K can be seen.L. G. Tejuca et al. 3153 I I I I I 2200 2000 1900 Vcm-' Fig.3. 1.r. spectra obtained after contacting LaMnO, reduced at 773 K with 50 Torr CO at r.t. (a), 373 (b), 473 (c), 573 (d) and 773 K (e). Contact time 0.5 h at each temperature. I I I I I I 1 300 500 700 900 TIK Fig. 4. T.p.d. of CO, after CO adsorption at r.t. on LaMnO, oxidized at 873 K (a); reduced at 573 (6) and 873 K (c). Formation of hydrogen-containing species such as formate or hydrogencarbonate should not take place given the high initial outgassing temperature and the absence of hydroxyl groups (these were not detected by i.r. spectroscopy). On the other hand, any species arising from CO interactions with lattice oxygen, viz. carbonates, would show a decrease in concentration with the reduction temperature as occurs for the desorption peak at 473 K on the oxidized surface [fig.2(a)]. A CO, desorption peak after CO adsorption was observed at the same temperature [fig. 4(a)]. This suggests that at 473 K carbonate decomposition with formation of CO and CO, takes place. Therefore, the peak3154 Interactions of H,, CO and CO, with LaMnO, by T.P.D. at 473 K is associated with a carbonate species. The shoulder at 540 K and the tail centred at 773 K [fig. 2(a)], which become well resolved peaks on reduced surfaces [fig. 2(b) and (c)] and increase in intensity with the degree of reduction (i.e. with a decrease in surface oxygen), should be associated with CO adsorbed on Mn"+ ions. These reduced ions may be produced on the oxidized surface by the outgassing treatment at 873 K in a He flow. The peaks at 360-395, 540-550 and 773-800 K [fig.2(a), (b) and (c), respectively] which increase in intensity with increasing reduction temperature are, therefore, assigned to CO species adsorbed on reduced Mnn+ sites. Similarly to our observations, an increase in CO adsorption with increasing outgassing temperature (and therefore with increasing surface reduction) was found in the system CO-Fe,O,. l5 Watson and Somorjai' found only one desorption peak of CO at 498 K on LaRhO,. Jensen and Massothl' recorded a desorption peak below 373 K and a second peak at 693 K in the system CO-Fe-Mn oxide. Our t.p.d. spectra with three desorption peaks seem to indicate a rather heterogeneous surface of LaMnO,. Indeed, this and other LaMO, oxides have been reported to exhibit a non-stoichiometric character.l' After CO adsorption on LaMnO, (red 773) at r.t.an i.r. band at 2130 cm-l and a shoulder at 2080 cm-' (fig. 3) were observed. These bands disappeared after heating (in the presence of the gas) at 573 K (2130 cm-l) and 773 K (2080 cm-l). They should be due to CO species adsorbed on centres with back-donation capacity (i.e. on centres where the back-donation of electrons from the metal to the antibonding n* orbital of CO is favoured) since their frequency is lower than that of CO gas (2143 cm-l). This is consistent with CO adsorption on Mnn+, presumably Mn2+. 1.r. bands at similar frequencies were found for CO interactions with Ni - A1203,18 COO - MgO solid ~olutions,~~ CuO,,O and MnO, on several supports.21v22 The desorption peaks at 360-395 K and 540-550 K (fig.2) can be correlated with the bands at 2130 and 2080 cm-l (fig. 3), which are attributed to linear CO adsorbed on Mn2+, probably with different coordination states. The species desorbing at 773-800 K is ascribed to bridge-bonded CO. T.p.d. spectra of CO, observed after CO adsorption are shown in fig. 4. This CO, should arise from CO oxidation via formation and decomposition of Carbonates. On the oxidized surface [LaMnO, (ox 873)] a wide peak at 473 K and a tail centred at 773 K were observed. The former correspond to monodentate carbonates. Its high intensity and its total disappearance on the reduced surface at 573 K point to interaction of these carbonates with Mn3+. The tail at 773 K is associated with carbonates of higher thermal stability, i.e.bidentate carbonates. l5 The concentration of both types of carbonate decreases as the availability of surface oxygen decreases, i.e. with increasing T,. Both monodentate and bidentate carbonates were detected by i.r. spectroscopy after CO adsorption at r.t. on LaMnO, (ox 873).,, However, no noticeable changes in the spectrum were observed after CO adsorption on samples with different degrees of reduction. This will be due to the low transmission of this oxide to i.r. radiation and, also, to its rather low specific surface area. The dissimilarity of t.p.d. spectra of CO, obtained after CO (fig. 4) and CO, (fig. 5 ) adsorption rules out any mechanism involving direct oxidation of CO to CO, by surface oxygen of the oxide, and formation of carbonates from this CO,.On the oxidized surface, almost all the adsorbed CO desorbs as CO,. On the contrary, on LaMnO, (red 873) less than 1 % of adsorbed CO desorbs as CO,. CO, Adsorption T.p.d. spectra of CO, obtained after CO, adsorption are shown in fig. 5. A peak at 385 K [fig. 5(a)] decreases in intensity and shifts towards lower desorption temperatures for increasing degrees of reduction [at 345 K on LaMnO, (red 873)]. A second desorption peak situated at 540 K on the oxidized surface undergoes a remarkable increase in intensity and shifts towards higher desorption temperatures as the reductionL. G. Tejuca et al. 3155 9 8 7 6 r( I N ti s 5 ; W 0” u 4 - E 2 -. 0 OI I \ 3 - 2 - 1 - I I I I I I 1 I I I I I I I I I I I I I I 1 I I I I I I I I I I I I I I I I I 1 I I I I I I I I I I I I I I I I I I I I I I I I I I 300 500 700 900 T/K Fig.5. T.p.d. of CO, (-) and CO (---) after CO, adsorption at r.t. on LaMnO, oxidized at 873 K (a); reduced at 573 (b), 723 (c) and 873 K (d). temperature increases [at 665 K on LaMnO, (red 873)]. A less intense peak centred at ca. 773 K observed on oxidized and, also, on reduced samples up to Tr < 573 K [fig. 5(a) and (b)] disappears for higher degrees of reduction. The two main desorption peaks are situated at temperatures which are near to those of desorption of /3 and 6 species reported by Klis~urski~~ for the system C0,-Co,O,. Note that an exothermic effect upon CO, adsorption (which becomes higher for higher degrees of reduction) was observed. A similar effect was reported for CO, adsorption on Mn0,.Ce0,.22 The peak at lower temperatures is assigned to a monodentate carbonate interacting with Mn3+.The opposite evolution of the peak situated at 540-665 K indicates the desorption of a carbonate of a different nature, most likely of bidentate character. Formation of this type of carbonate from CO, takes place on pairs of surface sites composed of a lattice oxygen and an anionic FAR I I 043156 Interactions of H,, CO and CO, with LaMnO, 0 by T.P.D. 0 s 0. Consistent with this interpretation, the concentration of carbonate species increases with increasing degree of reduction. This bidentate carbonate may interact with Mn2+ or La3+ ions. Monodentate and bidentate carbonates, the latter being in higher concentration than the former, were also found in the system C0,-Fe,0,.15 On samples reduced at 723 and 873 K, desorption peaks of CO at 755 K [fig.5(c)] and 735 K [fig. 5(6)] in different positions from those of CO, peaks were observed. CO desorption is of the same order of magnitude as CO, desorption in the first case but substantially higher in the second where the molar ratio C0,-CO is 0.56. This supports the view that some of the carbonates mentioned before interact with reduced centres of manganese. CO-H, Coadsorption In fig. 6 t.p.d. spectra of H, recorded after H, adsorption on a clean surface of LaMnO, (red 873) and after successive adsorption of CO-H, or H,-CO are shown. In the CO-H, and H,-CO sequences the peak at ca. 360K (dashed line) is strongly inhibited and a new desorption peak appears at 770-785 K.A similar effect, viz. the perturbation of H, adsorption by CO, has been been reported for Zn0.26.27 The t.p.d. spectrum of CO after successive adsorption H,-CO on LaMnO, (red 873) is given in fig. 7 (the spectrum obtained after successive adsorption CO-H, is similar to that in fig. 7). The positions of the main peaks (395, 530 and 810 K) are similar to those of the desorption peaks of CO after adsorption of this gas on a clean surface of LaMnO, (red 873) [fig. 2(c)]. In addition to the three peaks just noted, a shoulder at 570 K was observed, which is related to interactions of CO with H,. Also, pronounced CO desorption occurs above 860 K which corresponds, in part, to the shoulder at the same temperature observed in the t.p.d. of CO-LaMnO, (red 873) [fig. 2(c)].From the above results it is observed that CO or H, desorption after adsorption of these molecules on a clean surface of LaMnO, (red 873) is equal to CO or H, desorption after sequential adsorptions CO-H, or H,-CO. In other words, the number of adsorbed molecules of CO or H, is not affected by the preadsorption of the other molecule (H, and CO, respectively). Mutual enhancement of CO and H, adsorption has been reported on ZnO-containing catalysts. 27, However, enhanced CO adsorption by H, preadsorption and slightly suppressed H, adsorption by CO preadsorption was observed on cobalt- thoria-Kieselguhr at room temperat~re.~~ The above results could indicate that CO adsorption occupies [fig. 6(a)] or displaces [fig. 6(b)] adsorbed H, from the sites involved in the desorption peak at ca.360 K to another type of site (viz. those associated to the desorption peak at 770-785 K). The data presented in fig. 1 and 2 suggest that both H, and CO adsorb on reduced manganese ions (Mn"+). On the other hand, the ratio of desorbed CO molecules to the estimated number of transition-metal ions was found to be equal to 0.5, i.e. adsorbed CO occupies half of the exposed manganese sites (coverages for H, are equal to 0.1 in all cases and therefore are much lower than those of CO). This would allow the displacement and adsorption of hydrogen to other manganese sites. However, a more plausible explanation for these experimental results would be the formation of an oxygenated surface species after the sequential adsorption CO-H, and H,-CO, at room temperature.Such a situation would involve the adsorption of CO and H, on the same Mn"+ site. McKee3' has observed coverages higher than 1 of interacting CO and H, adsorbed species on Ru powder.L. G. Tejuca et al. 3157 300 500 700 900 T/K Fig. 6. T.p.d. of H, after CO-H, (a) and H,-CO (b) adsorption at r.t. on LaMnO, reduced at 873 K (--) and t.p.d. of H, after H, adsorption at r.t. on LaMnO, reduced at 873 K (---). 7 6 5 * 'm f 4 H 8 c1 0 : 3 I 2 \ b 2 1 0 I J I I I I 1 I 300 500 700 900 T/K Fig. 7. T.p.d. of CO after H,-CO adsorption at r.t. on LaMnO, reduced at 873 K. 104-23158 Interactions of H,, CO and CO, with LaMnO, by T.P.D. Formation of an oxygenated species is supported by the following observations : (a) the appearance of a new H, desorption peak at 770-785 K and, also, the appearance of new CO desorption features at 570 K and above 860 K after H,-CO or CO-H, adsorptions (fig.6 and 7); (b) after successive adsorption of 50 Torr H, and 50 Torr CO on LaMnO, (red 773) at room temperature i.r. bands at 2935 and 2860 cm-l, which can be assigned to antisymmetric and symmetric stretching modes of C-H vibrations, appeared in the spectrum. These bands underwent an intensity decrease after heating at 373 K. Although CO dissociation was reported to be favoured by the presence of H2,31.32 in our case, dissociation does not seem likely to occur given the low adsorption temperature used and the not too dissimilar t.p.d. spectra of CO after CO (fig. 2) and H,-CO (fig. 7) adsorptions. Conclusions H,, CO and CO, interactions with LaMnO, at room temperature, as a function of the reduction temperature, have been studied by temperature programmed desorption and i.r.spectroscopy. The main conclusions drawn can be summarized as follows. H, adsorbs molecularly on reduced Mnn+ ( n z 2) sites. The oxide surface appears to undergo substantial reduction at temperatures much lower than those needed for reduction of the bulk. CO adsorption yielded CO and CO, desorption peaks which can be assigned to monodentate and bidentate carbonates interacting with Mn3+. The concentration of these carbonates decreases as the availability of surface oxygen decreases. Desorption peaks attributed to linear and bridged CO species adsorbed on Mn2+ were also detected . CO, adsorption produced monodentate and bidentate carbonates interacting with Mn3+ and Mn2+ (or La3+), respectively.A fraction of the carbonates formed on the reduced surface of the adsorbent desorbs as CO. H,-CO and CO-H, successive coadsorption results suggest the formation of an oxygenated surface species by adsorption of CO and H, on the same reduced Mnn+ (n z 2) site. We are indebted to the Spanish-North American Joint Committee for Scientific and Technological Cooperation for financial support (project no. CCB8409-003). One of us (L.G.T.) would like to thank the technical staff of the Department of Chemical Engineering, University of California, Berkeley, for their help during the time in which the experimental part of this work was carried out. References 1 Yu. A. Ryndin, R.F. Hicks, A. T. Bell and Yu. I. Yermakov, J . Catal., 1981, 70, 287. 2 R. F. Hicks and A. T. Bell, J . Catal., 1984, 90, 205. 3 R. F. Hicks and A. T. Bell, J . Catal., 1985, 91, 104. 4 J. S. Rieck and A. T. Bell, J. Catal., 1986, 99, 278. 5 R. P. Underwood and A. T. Bell, Appl. Catal., 1986, 21, 157. 6 P. R. Watson and G. A. Somorjai, J . Catal., 1982, 74, 282. 7 R. P. Underwood, A. T. Bell and L. G. Tejuca, unpublished results. 8 G. G. Low and A. T. Bell, J . Catal., 1979, 57, 397. 9 J. M. D. Tascon, L. Gonzalez Tejuca and C. H. Rochester, J . Catal., 1985, 95, 558. 10 R. Spinicci and A. Tofanari, React. Kinet. Catal. Lett., 1985, 27, 65. 1 1 L. Gonza'lez Tejuca and A. T. Bell, unpublished results. 12 A. Zecchina and G. Spoto, Z . Phys. Chem. (N.F.), 1983, 137, 173. 13 M. Uchida and A. T. Bell, J . Catal., 1979, 60, 204. 14 J. L. G. Fierro, J. M. D. Tascon and L. Gonzalez Tejuca, J . Catal., 1984, 89, 209. 15 T. J. Udovic and J. A. Dumesic, J . Catal., 1984, 89, 314. 16 K. B. Jensen and F. E. Massoth, J . Catal., 1985, 92, 98.L. G. Tejuca et al. 3159 17 R. J. H. Voorhoeve, J. P. Remeika and L. E. Trimble, Ann. N. Y. Acad. Sci., 1976, 272, 3. 18 J. B. Peri, J. Catal., 1984, 86, 84. 19 A. Zecchina, G. Spoto, S. Coluccia and E. Guglielminotti, J. fhys. Chem., 1984, 88, 2575. 20 A. A. Davydov and A. A. Budneva, React. Kinet. Catal. Lett., 1984, 25, 121. 21 M. A. Baltanas, A. B. Stiles and J. R. Katzer, J . Catal., 1984, 88, 516. 22 M. A. Baltanas, A. B. Stiles and J. R. Katzer, Appl. Catal., 1986, 20, 31. 23 L. Gonzalez Tejuca, C. H. Rochester, J. L. G. Fierro and J. M. D. Tascon, J. Chem. Soc., Furuday 24 D. G. Klissurski, J. Catal., 1974, 33, 149. 25 M. P. Rosynek and D. T. Magnuson, J. Catal., 1977, 48, 417. 26 J. C. Lavalley, J. Saussey and T. Rais, J. Mol. Catal., 1982, 17, 289. 27 E. Giamello and B. Fubini, in Adsorption at the Gas-Solid and Liquid-Solid interface, ed. J. Rouquerol 28 H. H. Kung, Catal. Rev.-Sci. Eng., 1980, 22, 235. 29 R. B. Gupta, B. Viswanathan and M. V. C. Sastri. J. Catal., 1972, 26, 212. 30 D. W. Mckee, J. Catal., 1967, 8, 240. 31 F. Solymosi, I. Tombacz and M. Kocsis, J. Catal., 1982, 75, 78. 32 J. S. Rieck and A. T. Bell, J. Catal., 1986, 99, 262. Trans. I , 1984, 80, 1089. and K. S. W. Sing (Elsevier, Amsterdam, 1982), p. 389. Paper 612099; Received 29th October, 1986