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Microcalorimetric Investigation of the Interaction of Carbon Monoxide with Coprecipitated Cupric Oxide—Zinc Oxide Catalysts in Well-Defined Oxidation States
Applied Catalysis, 36 (1988) 287-298 Elsevier Science Publishers B.V., Amsterdam -
287 Printed in The Netherlands
Microcalorimetric Investigation of the Interaction of Carbon Monoxide with Coprecipitated Cupric Oxide-Zinc Oxide Catalysts in Well-Defined Oxidation States ELI0 GIAMELLO*, BICE FUBINI and VERA BOLIS Universith di Torino, Zstituto di Chimica Generale e Znorganica, Facohi di Farmacia, Via P. Giuria 9, 10125 Turin (Italy) (Received 6 May 1987, accepted 24 July 1987)
ABSTRACT Carbon monoxide is readily adsorbed onto copper-zinc systems of different copper content. The adsorptive capacity, the heat and the kinetics of the interaction depend upon the oxidation state of the sample. The prevailing interaction on fully reduced samples is adsorption of carbon monoxide on copper metal, whereas in the case of partially oxidized samples and in that of cupric clxide-zinc oxide obtained by calcination of the precursor, redox reactions occur involving reduction of the surface, in parallel with the coordination of carbon monoxide on copper centres in various oxidation states. The heat of coordination of carbon monoxide onto copper (0) and copper (I) centres is in the range 66-43 kJ/mol and 110-66 kJ/mol respectively. The assignments have been made on the basis of the correspondence between carbonyl IR frequencies and heat of adsorption. Quantitative data on adsorption are also discussed in terms of the effect of the dispersion of the active phase and the role of the zinc oxide matrix.
INTRODUCTION
The catalysts employed in modern processes for methanol synthesis and water-shift reactions are ternary systems where the active components are metallic copper and zinc oxide. The state of oxidation of the surface in the working catalyst, the role of the different components and the nature of the active sites have been the object of a great deal of active research in recent years [ 1,2]. Some controversies, however, still exist. In particular, there is controversy about the presence of partially oxidized copper (I) in the reduced catalyst, the role of the metal-support interaction, the nature of the active sites and the catalytic mechanism [ 3-81. In spite of all the work that has been performed on these systems in the last five years, there is still a lack of quantitative analysis data of the surface chemistry of these catalysts e.g. adsorptive capacity
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
and binding energies towards the gases used in the syntheses. We have recently investigated a series of Cu-ZnO catalysts with different copper loading by adsorption microcalorimetry and electron paramagnetic resonance (EPR) spectroscopy: in order to avoid the superposition of too many processes we have only considered biphasic solids obtained initially from well-characterized monophasic precursor [ 91. The aim of the investigation was twofold: (i) to give information about the surface activity of the solids and, (ii) to characterize the solids obtained in the different stages of catalyst preparation, a direct correlation between the “history” of the catalyst and its properties being very likely. Previous papers were devoted to the measurement of the metal surface area [ lo], to the oxidation of the metal particles [ 111, and to the state of copper in the catalysts [ 121. In the present one the surface interaction of carbon monoxide is considered in detail for two reasons: carbon monoxide is, on the one hand, a molecule directly involved in syngas reactions and, on the other hand, acts as an excellent, weakly basic probe for surface sites. Particular attention has been given to the energies of interaction and related quantities on solids with a well defined oxidation state. We have thus considered the adsorption of carbon monoxide on both calcined ( CuO-ZnO) and reduced ( Cu-ZnO) systems. Furthermore, partially oxidized samples obtained after contact of nitrous oxide or oxygen with the reduced samples at room temperature have also been considered. These treatments lead to the partial (N,O) or total (0,) formation of a surface layer whose stoichiometry can be roughly assumed to be Cup0 [lo]. Recent findings by Chinchen and Waugh on commercial CuO-ZnO/Alg03 indicate that, due to the effect of carbon dioxide in the feed gas, the surface of the catalyst works in a partially oxidized state [ 131 comparable to that obtained by nitrous oxide saturation. The results of carbon monoxide interaction with systems in different, well-defined oxidation states, may therefore afford important information on the surface reactivity of the working catalyst. EXPERIMENTAL
Cupric oxide-zinc oxide calcined samples ( C ) were obtained by calcination at 573 K for 6 h of a monophasic precursor of general formula (Cu, Zn, _,) ( C03) 2(OH) 6 as prepared by Petrini et al. [ 91 by coprecipitation from a solution of copper and zinc nitrates. The reduced copper-zinc oxide samples (R) were obtained by hydrogen reduction at 483 K [lo]. Samples with 3 and 30% copper content [ Cu: (Cu+ Zn) atomic percentage] respectively were employed: details of the chemical and morphological characterization of the solids are reported in ref. 12. A sample of pure cupric oxide with a surface area of 6.5 m2/g prepared by the same procedure was also used. The oxidized samples were obtained by saturation at room temperature of
the reduced samples either by nitrous oxide (N ) or by small doses of oxygen ( 0) followed by complete evacuation of the excess gas; the nitrous oxide reaction led to a partial oxidation of the copper surface up to a coverage (0:Cu) of about 0.3 [ lo]. Oxygen adsorption involved a deeper oxidation of the metal with formation of a few layers of cuprous oxide [ 111. The samples are referred to hereafter by a letter indicating the nature of the copper phase (C,R,N,O) followed by the numbers 3 or 30 indicating the copper loading e.g. N30 means a sample with 30% of copper which has undergone reduction followed by saturation with nitrous oxide. The heat of adsorption of carbon monoxide was measured at 300 K by means of a Tian-Calvet microcalorimeter connected to a volumetric apparatus. The technique described in ref. 14 was followed: this procedure permits the simultaneous determination of the adsorption heats and adsorbed amounts for small doses of gas. The gas pressure was monitored by means of a Baratron transducer gauge.
RESULTS Interaction of carbon monoxide with calcined samples The interaction of carbon monoxide with the calcined samples is better described in terms of chemical reactivity than in terms of simple adsorption. The initial doses of carbon monoxide were taken up without leaving a residual pressure which only appeared when considerable amounts of carbon monoxide had reacted, corresponding to the onset of slow adsorptive phenomena, The molar heats of the initial doses were typical of a chemical reaction: Fig. 1 shows the partial molar heat (d Q int/dna= evolved heat/amount taken up for each dose ) as a function of uptake per square metre of the sample, in the case of C30 and pure cupric oxide. The two diagrams show a very similar trend indicating the progressive decrease of the molar heat of interaction from values around 300 kJ/mol to 60-70 kJ/mol. A substantial amount of the carbon monoxide taken up was irreversibly held at the surface as indicated by the fact that only a fraction (X-30% ) of the emitted heat could be recovered by evacuation at room temperature. At the end of the two experiments small amounts of carbon dioxide were present in the gas phase. This fact, as well as those reported above, indicated that alongside the adsorption a redox process was occurring which involved the reduction of the surface by carbon monoxide. In spite of the different amount of copper present in the two samples the amount of taken up was approximately the same for both samples and was quite large (5.10-” mol/m2 corresponding to about 3 molecules/nm2). In the case of C3 the total uptake was 2.5.10-” mol/m’. The well-resolved EPR spectrum arising from both isolated and magnetically interacting cupric ions, reported in ref. 12, was not affected by carbon monoxide reaction at room temperature.
q”“/kJ
3
mol-’
r-7
I 2
II
---7
I”’ 1
L2
Fig. 1. Partial molar heat (evolved heat/adsorbed amount) of carbon monoxide and on pure CuO (dashed line) as a function of adsorbed amount.
Carbon monoxide
interaction
with reduced and reoxidized
on C30 (full line)
samples
The reduced (R) samples interact with carbon monoxide in a completely different way as indicated by the comparison between C30 and R30 reported in Fig. 2 where the plot of C30 is the same as that reported in Fig. 1. The two diagrams report the partial molar heat (Fig. 2a) and the half deviation time ( ti ) of the thermogram (Fig. 2b) as a function of uptake per m2 of sample. The latter parameter (time at which the deviation from the baseline has attained half the value of the maximum) indicates semiquantitatively the kinetics of the heat emission and is therefore proportional to the activation energy of the interaction. Adsorption on R3 (Fig. 3a and b) and R30 was, except for the initial doses in the two cases, fast, non-activated and pressure dependent, in agreement with the findings of other authors [ 151. The values of t+ were around 4-5 min and were typical of almost instantaneous phenomena. The heat of adsorption fell from an initial value of about 110-100 kJ/mol to a value which, in spite of the slight dispersion of the experimental points, could be considered nearly constant with further increase in coverage (around 45 kJ/mol) and typical of a weak chemisorptive interaction. The carbon monoxide uptake decreased by about 2.5 times upon reduction from C30 to R30 whereas in going from C3 to R3 a greater decrease (about 10 times ) was found. As already reported in ref. 12 a small fraction of the adsorbed carbon monoxide is irreversibly held at room temperature. This fraction ranged from 8 to 13% for R30 and from 15 to 18% for R3 of the total released heat: this is in
291
5
‘...
. ... .-
**w
1
R 30
2
3
4
5
Fig. 2. (a) Partial molar heat vs. adsorbed amount of carbon monoxide on C30 andR30. (b) Half deviation time of the thermograms ( rt ) vs. adsorbed amount for C30 (w ) and R30 ( 0 ) . agreement with the findings of Parris and Klier in similar experiments [ 161. A large fraction of carbon monoxide adsorption on R samples involved interaction with metallic copper. This is documented in Fig. 4a and b where the isotherms of adsorption for R3 and R30 in the range O-40 Torr (O-5.33 kPa) are reported and refer respectively to the total surface area of the samples (Fig. 4a) and to the free copper surface area S cU (Fig. 4b ) as measured by microcalorimetry following the decomposition of nitrous oxide at the surface [lo]. The adsorbed amount on R30 per m2 of the sample exceeded that on R3 by about 6 times, but, when the adsorption was computed as a function of ScU the two curves lay close one to the other. The effect of the reoxidation of the samples on the adsorption of carbon monoxide is reported in Fig. 3 in terms of partial molar heat and ti for the samples containing low amounts of copper (R3, N3, 03). In this experiment the 03 sample was obtained by a different procedure from that reported in the experimental section. Here the sample N3 was saturated with oxygen after the run of carbon monoxide adsorption (Fig. 3 plot N3) and the successive evacuation. The reactivity of the sample towards carbon monoxide increased with progressive oxidation. On N samples the oxygen coverage was about 0.3 (0:Cu)
292
au.
“+ i.-., c-1 1
,
loo-
.,.,
._.-?A._._._
66-
i
43-
0:4
0.6
0‘3
I
,
1.0
i
@
. n&ml 02
04
06
0.8
m? 1.0
Fig. 3. (a) Partial molar heat vs. adsorbed amount of carbon monoxide on R3, N3 and 03. (b) Half deviation time of the thermograms vs. adsorbed amount for R3 ( 0 ) , N3 ( A ) and 03 ( n )
Fig. 4. Adsorption isotherms of CO on R3 and R30. (a) Adsorption in ,umol per m* of sample. (b) Adsorption in pm01 per m2 of metal copper.
293 TABLE I Sample composition and quantitative data for carbon monoxide adsorption Cu loading Cu surface area BET surface area CO adsorption at 40 CO ads. molar CO/Cu,,,,,, Torr (5.33 kPa) heat range (wt.%) (mzcug-‘cu) Cm*g-‘1 (pm01 m-‘) (kJ mol-‘) Reduced samples Fu 2.08 R30 22.90
166 81
Partially oxidized samples N3 03 030 Calcined samples c3 c30 cue
-
52 54
0.25 1.60
llOloo-
43 43
0.20 0.23
_
0.37 1.22 2.46
190- 43 185- 80 200-100
-
-
2.20 4.90 6.40
ZBO- 80 310- 90 340- 80
-
formally equivalent to 60% of a cuprous oxide monolayer [lo]: in such conditions the uptake of carbon monoxide was larger than on the reduced sample. The three initial doses had an heat of interaction ranging from 185 to 155 kJ/mol, i.e. higher than in the case of the reduced samples but definitely lower than those observed for the calcined samples. The kinetics of the interaction in this range were slower than for the corresponding range for the R samples. From the 4th to the last dose on N3, heats and kinetics of carbon monoxide adsorption exhibited the values observed, in a wider range of coverage on R3, indicating that this part of the interaction concerned the fraction of the surface unaffected by oxidation. On 03, where extensive oxidation of the surface and subsurface layers had occurred, after an initial value of 190 kJ/mol the heat of adsorption decreased to a constant value of 80 kJ/mol. Evidence of the weak and fast interaction on metallic copper was no longer observed either in terms of heat of interaction (Fig. 3a) or in terms of kinetics-of heat emission (Fig. 3b): the tt values, in fact, remain higher than those recorded in the cases R3 and N3. When carbon monoxide was contacted with 03 directly obtained by reoxidation of R3 according to the procedure indicated in the experimental section, the carbon monoxide uptake was 1.22*10-6 mol/m’, i.e. close to the sum of the uptake of N3 and 03 in Fig. 3. All quantitative data are collected in Table 1. Comparison with quantitative data for carbon monoxide adsorption on zinc oxide [ 171 indicated that the zinc oxide contribution to the total adsorption on copper-zinc was negligible.
294
DISCUSSION Carbon monoxide uptake The trend of the differential heat for carbon monoxide adsorption onto the various copper-zinc samples examined indicated the presence of different phenomena, albeit occurring simultaneously. However, analysis of Table 1 indicates that the total carbon monoxide uptake and the interval of variation of the molar heat clearly change both with copper loading and with the state of the sample. The molar heat of interaction ranges in intervals depending on the degree of oxidation of the surface: the higher the oxidation state, the higher the average heat evolved per mole of carbon monoxide adsorbed. The heat of interaction of carbon monoxide could thus be used as an approximate and preliminary check on the state of the supported copper. When carbon monoxide is contacted with reduced (R) systems adsorption mainly involves interaction with the surface of metallic copper. This is well documented (Fig. 4) when the adsorbed amount is computed as a function of the metal area: the slight difference between the values attained by R3 and R30 probably depends on the morphological differences between the two samples: on the basis of a study by Okamoto et al. [ 41 high copper content samples are made by well-dispersed metal particles whereas in low content ones, twodimensional epitaxial layers of copper on zinc oxide are formed. The type and the relative fraction of different copper crystal faces exposed in the two cases would therefore be different and account for the differences in the carbon monoxide uptake per unit area. On both R3 and R30, however, the carbon monoxide adsorption attains a surface coverage ranging from 20 to 23% of the exposed copper atoms (Table 1, col. 6). This value is not too far removed from the oxygen coverage (30% ) obtained by nitrous oxide decomposition and used for copper area measurements [ lo]. Only a fraction of the sites involved in carbon monoxide coordination, however, are active in the decomposition of nitrous oxide. This is evidenced by adsorption on samples precontacted with nitrous oxide (N3, Fig. 3 ) where weak energy, fast carbon monoxide adsorption on copper was not eliminated by the previous nitrous oxide oxidation and showed up at high coverage, after saturation of the oxidized sites, The noncoincidence between the sites for nitrous oxide decomposition and for carbon monoxide coordination is not surprising and can be understood in terms of the well-known differences in the reactivity of the different copper crystal faces towards the two gases [ 18,191. The increasing oxidation of the samples caused an increase in the carbon monoxide uptake and in the molar heat of interaction: the uptake in the case of N3 (Fig. 3) was 50% higher than that on R3. It should be noted, however, that the oxygen coverage on N3 was similar to that observed by Chinchen and Waugh on Cu-Zn/AlzO, after catalytic reaction in an atmosphere of hydrogen,
carbon monoxide and carbon dioxide [ 131: the carbon samples than that on R ones) should thus be the reference working catalyst. further after O2 oxidation kJ/mol) were the low energy copper metal sites still observed on N3 were no longer detectable. surface oxidation cuprous oxide upon oxygen saturation 10,111; data on 0 can thus be considered typical of the carbon monoxide-cuprous samples was much higher in the case of R3-03 (5 times) than in that of R30-030 (1.6 times). The difference observed in this case can be ascribed to the morphology of the samples: the high dispersion of copper in R3 (the metallic fraction of the total surface was 6.5% in R3 and 37% in R30) caused an extended contact with the matrix allowing a more efficient stabilisation of the carbon monoxide oxidation products on the basic zinc oxide. This agrees with the findings of Boccuzzi et al. [ 201 on reoxidized copper-zinc oxide who assigned the formation of carbonates and other oxidized species to reactions occurring at the borderline between metallic and oxidic phases. Finally, the calcined samples showed the highest uptake with results between 1.5 and 2 times that of the reoxidized ones. The reactivity of C30 and pure cupric oxide (Fig. 1) were, quite surprisingly, rather close one to the other: also this result can be ascribed to the effect of the matrix basicity which facilitated the reduction of the supported cupric oxide particles. Heat of interaction
The reaction of carbon monoxide with C samples mainly involves redox processes. This is indicated by the high heat of interaction, the slow and decreasing rate of the process and the presence of carbon dioxide in the gas phase. It should be kept in mind that the reduction of cuprous and cupric oxides by carbon monoxide (leading to carbon dioxide) are exothermic, with standard enthalpies of reduction of - 116.4 and - 127.7 kJ/mol respectively: these data, however, are not directly comparable with those measured in the experiment illustrated in Fig. 1 where the oxidation products are mainly constituted of surface carbonates of various composition. IR investigations on pure cupric oxide [ 211 and on cupric oxide-zinc oxide samples [ 221 have shown that, besides carbon dioxide, surface carbonates are formed with parallel formation of reduced copper (I) and copper (0) which, in their turn, coordinate carbon monoxide. It is therefore very hard to distinguish between the different thermal contributions to the overall data reported in Fig. 1. The analogy between the behaviour of cupric oxide and C30 indicated that the reduction involved
the cupric oxide supported particles: in fact the intense EPR spectra assigned to both isolated and clustered copper (II) ions [ 121 were not affected all by carbon monoxide interaction up to 373 K. This means that isolated copper ( II) sites are less reducible than cupric oxide aggregates and, furthermore, the EPR parameters of isolated ions present at the surface, if any, are little influenced by carbon monoxide coordination. The redox interaction play a role also when carbon monoxide is adsorbed on N and 0 samples as confirmed by IR data on the same systems [ 20 J .
[ 211 in a recent paper on the interaction of carbon monoxide with pure cupric oxide. The authors, on the basis of the time needed to decrease the intensity of two different carbonylic bands of a given factor, were able to estimate the activation energy of desorption and consequently derive the heat of adsorption belonging the two species at 2114 and 2148 cm-’ assigned to carbon monoxide on two different Cu+ ions. The values obtained so far are 79.5 and 108 kJ/mol respectively. It is known that for carbon monoxide complexes on low charge ions or zerovalent metal atoms the same mechanism involving crdonation and 0-n synergism can be hypothesized, though acting to a different extent [ 241. We can therefore tentatively extrapolate the values given in ref. 21 to the IR frequencies monitored on copper-zinc oxide and assigned to carbon monoxide adsorbed on both copper (I) and copper ( 0 ) . To our knowledge the only data available are those obtained by Ghiotti et al. for samples similar to those employed in the present study [ 22,231. It turns out that the frequencies of 2098 and 2070 cm-’ found on reduced copper-zinc oxide and assigned to carbon monoxide on different copper (0 ) sites correspond to heats of adsorption of 66 and 43 kJ/mol respectively. Despite the arbitrariness of the extrapolation the two heats of adsorption so far calculated fit the experimental values found in the present study on similar samples quite well (see Figs. 2 and 3 with the values of 43 and 66 kJ/mol
297
TABLE 2 IR frequenciesand heat adsorption of carbon monoxide adsorbedon copper in various oxidation states Copper oxidation St&?
Carbonyl stretchingfreq. Heat of interaction from IR data (desorption rate) u (cm-‘) (kJ mol-‘)
Copper (I)
2114* 2148*
Copper (0)
2098** 2070**
Heat of interaction from direct calorimetric measurement(present work) (kJ mol-‘)
79.5* 108 * 66 *** 43 ***
*On cupric oxide samples,ref. 21. **Ref. 23. ***Derivedby extrapolationof the valuesin first and second row. on the Y-axis). The coupling between heat of interaction and C-O stretching frequency obtained by the method described above are collected in Table 2. On the basis of this result we can formulate the assignments given above more precisely: ( i) Carbon monoxide adsorbed with molar heat ranging from 66 to 43 kJ/mol and representing 85% of the whole adsorption on R samples, is related to the carbon monoxide-copper metal interaction and can be associated with two I.R. bands assigned to the carbonyl stretchings observed at 2098 and 2070 cm-’ marked
[231.
(ii) The remaining adsorbed carbon monoxide (15% ) with heat of adsorption between 110-100 and 66 kJ/mol, partially non-desorbable under evacuation, has to be related to interaction with partially oxidized forms of copper, very likely coordination onto surface unreduced copper (I) ions dissolved in the zinc oxide matrix; the range of frequencies typical of Cu+CO complexes corresponds in fact, according to the analysis of Lokhov et al. [ 211, to the heat range 118-84 kJ/mol, again in excellent agreement with our results on both R3 and R30. Although several other authors claim the presence of copper (I) dissolved in zinc oxide [ 3-51, it has however to be pointed out that Ghiotti et al. do not observe any IR band in the Cu+CO region [ 231. A small discrepancy therefore
remains between the present data and the IR data for the same samples which can probably be ascribed to the different reducing procedure adopted in the two techniques. The analysis of the heat of interaction on the basis of the correspondence with Lokhov’s results can be extended to the reoxidized and calcined systems: in the case of sample 0, in particular, coordination of carbon monoxide onto
298
surface Cu+ ions occurring at high coverage is observed (120-80 kJ/mol, Fig. 3 ) . This is also observed in the case of C samples (Fig. 1) where the coordination Cu + + CO is practically absent due to the large extent of the surface reduction. However the interaction Cu++CO, if any, cannot be analysed by extrapolation of Lokhov’s values owing to the different mechanism of carbon monoxide coordination. On high-charge ions, in fact, the coordination mechanism is based exclusively on cr donation (whose extent depends on the cationic charge density) without backdonation from the metal to the carbon monoxide orbitals [ 241.
REFERENCES 1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 19 20 21 22
23 24
K. Klier, Advances in Catalysis, Vol. 31, Academic Press, New York, 1982, p.243. H.H. Kung, Catal. Rev. Sci. Eng., 22, (1980) 235. R.G. Herman, K.Klier, G.W. Simmons, B.P. Finn, J.B. Bulko and T.P. Kobylinski, J. Catal., 56 (1979) 407. Y. Okamoto, K. Fukino, T. Imanaka and S. Teranishi, J. Phys. Chem., 87 (1983) 3747. T.M. Yurieva and T.P. Minyukova, Rect. Kinet. Catal. Lett., 29 (1) (1985) 55. S.P.S. Andrew, Post Congress Symposium, 7th International Congress on Catalysis, Osaka, 1980 paper 12. T.H. Fleish and R.L. Melville, J. Catal., 90 (1984) 165. G. Liu, D. Willcox, M. Garlandand H.H. Kung, J. Catal., 90 (1984) 139. G. Petrini, F. Montino, A. Bossi and F. Garbassi, in G. Poncelet, P. Grange and P.A. Jacobs (Eds.) , Preparation of Catalysts III, Studies in Surface Science and Catalysis, Vol. 16, Elsevier, Amsterdam, 1983, p. 735. E. Giamello, B. Fubini, P. Lauro and A. Bossi, J. Catal., 87 (1984) 443. B. Fubini and E. Giamello, J. Therm. Anal., 29 (1984) 655. E. Giamello, B. Fubini and P. Lauro, Appl. Catal., 21 (1986) 133. G.C. Chinchen and K.C. Waugh, J. Catal., 97 (1986) 280. B. Fubini, Rev. Gen. Therm., (1979) 297. D.L. Roberts and G.L. Griffin, Appl. Sur. Sci., 19 (1984) 298. G.E. Parrisand K. Klier, J. Catal., 97 (1986) 374. E. Giamello and B. Fubini, J. Chem. Sot. Faraday Trans. 1,79 (1983) 1995. J. Pritchard, T. Catterick and R.K. Gupta, Surf. Sci., 53 (1975) 1. G. Ertl, Surf. Sci., 6 (1967) 208. F. Boccuzzi, G. Ghiotti and A. Chiorino, Surf. Sci., 162 (1985) 361. Yu.A. Lokhov, V.A. Sadykov, SF. Tikhov and V.V. Popovskii, Kinet. Katal., 26 (I) (1985) 177. G. Ghiotti, F. Boccuzzi and A. Chiorino, in M. Che and G.C. Bond (Eds.) , Adsorption and Catalysis on Oxide Surfaces, Studies in Surface Science and Catalysis, Vol. 21, Elsevier, Amsterdam, 1985, p. 235. F. Boccuzzi, G. Ghiotti and A. Chiorino, Surf. Sci., 156 (1985) 933. Yu.A. Lokhov and A.A. Davidov, Kinet. Katal., 21(6) (1980) 1523.
287 Printed in The Netherlands
Microcalorimetric Investigation of the Interaction of Carbon Monoxide with Coprecipitated Cupric Oxide-Zinc Oxide Catalysts in Well-Defined Oxidation States ELI0 GIAMELLO*, BICE FUBINI and VERA BOLIS Universith di Torino, Zstituto di Chimica Generale e Znorganica, Facohi di Farmacia, Via P. Giuria 9, 10125 Turin (Italy) (Received 6 May 1987, accepted 24 July 1987)
ABSTRACT Carbon monoxide is readily adsorbed onto copper-zinc systems of different copper content. The adsorptive capacity, the heat and the kinetics of the interaction depend upon the oxidation state of the sample. The prevailing interaction on fully reduced samples is adsorption of carbon monoxide on copper metal, whereas in the case of partially oxidized samples and in that of cupric clxide-zinc oxide obtained by calcination of the precursor, redox reactions occur involving reduction of the surface, in parallel with the coordination of carbon monoxide on copper centres in various oxidation states. The heat of coordination of carbon monoxide onto copper (0) and copper (I) centres is in the range 66-43 kJ/mol and 110-66 kJ/mol respectively. The assignments have been made on the basis of the correspondence between carbonyl IR frequencies and heat of adsorption. Quantitative data on adsorption are also discussed in terms of the effect of the dispersion of the active phase and the role of the zinc oxide matrix.
INTRODUCTION
The catalysts employed in modern processes for methanol synthesis and water-shift reactions are ternary systems where the active components are metallic copper and zinc oxide. The state of oxidation of the surface in the working catalyst, the role of the different components and the nature of the active sites have been the object of a great deal of active research in recent years [ 1,2]. Some controversies, however, still exist. In particular, there is controversy about the presence of partially oxidized copper (I) in the reduced catalyst, the role of the metal-support interaction, the nature of the active sites and the catalytic mechanism [ 3-81. In spite of all the work that has been performed on these systems in the last five years, there is still a lack of quantitative analysis data of the surface chemistry of these catalysts e.g. adsorptive capacity
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
and binding energies towards the gases used in the syntheses. We have recently investigated a series of Cu-ZnO catalysts with different copper loading by adsorption microcalorimetry and electron paramagnetic resonance (EPR) spectroscopy: in order to avoid the superposition of too many processes we have only considered biphasic solids obtained initially from well-characterized monophasic precursor [ 91. The aim of the investigation was twofold: (i) to give information about the surface activity of the solids and, (ii) to characterize the solids obtained in the different stages of catalyst preparation, a direct correlation between the “history” of the catalyst and its properties being very likely. Previous papers were devoted to the measurement of the metal surface area [ lo], to the oxidation of the metal particles [ 111, and to the state of copper in the catalysts [ 121. In the present one the surface interaction of carbon monoxide is considered in detail for two reasons: carbon monoxide is, on the one hand, a molecule directly involved in syngas reactions and, on the other hand, acts as an excellent, weakly basic probe for surface sites. Particular attention has been given to the energies of interaction and related quantities on solids with a well defined oxidation state. We have thus considered the adsorption of carbon monoxide on both calcined ( CuO-ZnO) and reduced ( Cu-ZnO) systems. Furthermore, partially oxidized samples obtained after contact of nitrous oxide or oxygen with the reduced samples at room temperature have also been considered. These treatments lead to the partial (N,O) or total (0,) formation of a surface layer whose stoichiometry can be roughly assumed to be Cup0 [lo]. Recent findings by Chinchen and Waugh on commercial CuO-ZnO/Alg03 indicate that, due to the effect of carbon dioxide in the feed gas, the surface of the catalyst works in a partially oxidized state [ 131 comparable to that obtained by nitrous oxide saturation. The results of carbon monoxide interaction with systems in different, well-defined oxidation states, may therefore afford important information on the surface reactivity of the working catalyst. EXPERIMENTAL
Cupric oxide-zinc oxide calcined samples ( C ) were obtained by calcination at 573 K for 6 h of a monophasic precursor of general formula (Cu, Zn, _,) ( C03) 2(OH) 6 as prepared by Petrini et al. [ 91 by coprecipitation from a solution of copper and zinc nitrates. The reduced copper-zinc oxide samples (R) were obtained by hydrogen reduction at 483 K [lo]. Samples with 3 and 30% copper content [ Cu: (Cu+ Zn) atomic percentage] respectively were employed: details of the chemical and morphological characterization of the solids are reported in ref. 12. A sample of pure cupric oxide with a surface area of 6.5 m2/g prepared by the same procedure was also used. The oxidized samples were obtained by saturation at room temperature of
the reduced samples either by nitrous oxide (N ) or by small doses of oxygen ( 0) followed by complete evacuation of the excess gas; the nitrous oxide reaction led to a partial oxidation of the copper surface up to a coverage (0:Cu) of about 0.3 [ lo]. Oxygen adsorption involved a deeper oxidation of the metal with formation of a few layers of cuprous oxide [ 111. The samples are referred to hereafter by a letter indicating the nature of the copper phase (C,R,N,O) followed by the numbers 3 or 30 indicating the copper loading e.g. N30 means a sample with 30% of copper which has undergone reduction followed by saturation with nitrous oxide. The heat of adsorption of carbon monoxide was measured at 300 K by means of a Tian-Calvet microcalorimeter connected to a volumetric apparatus. The technique described in ref. 14 was followed: this procedure permits the simultaneous determination of the adsorption heats and adsorbed amounts for small doses of gas. The gas pressure was monitored by means of a Baratron transducer gauge.
RESULTS Interaction of carbon monoxide with calcined samples The interaction of carbon monoxide with the calcined samples is better described in terms of chemical reactivity than in terms of simple adsorption. The initial doses of carbon monoxide were taken up without leaving a residual pressure which only appeared when considerable amounts of carbon monoxide had reacted, corresponding to the onset of slow adsorptive phenomena, The molar heats of the initial doses were typical of a chemical reaction: Fig. 1 shows the partial molar heat (d Q int/dna= evolved heat/amount taken up for each dose ) as a function of uptake per square metre of the sample, in the case of C30 and pure cupric oxide. The two diagrams show a very similar trend indicating the progressive decrease of the molar heat of interaction from values around 300 kJ/mol to 60-70 kJ/mol. A substantial amount of the carbon monoxide taken up was irreversibly held at the surface as indicated by the fact that only a fraction (X-30% ) of the emitted heat could be recovered by evacuation at room temperature. At the end of the two experiments small amounts of carbon dioxide were present in the gas phase. This fact, as well as those reported above, indicated that alongside the adsorption a redox process was occurring which involved the reduction of the surface by carbon monoxide. In spite of the different amount of copper present in the two samples the amount of taken up was approximately the same for both samples and was quite large (5.10-” mol/m2 corresponding to about 3 molecules/nm2). In the case of C3 the total uptake was 2.5.10-” mol/m’. The well-resolved EPR spectrum arising from both isolated and magnetically interacting cupric ions, reported in ref. 12, was not affected by carbon monoxide reaction at room temperature.
q”“/kJ
3
mol-’
r-7
I 2
II
---7
I”’ 1
L2
Fig. 1. Partial molar heat (evolved heat/adsorbed amount) of carbon monoxide and on pure CuO (dashed line) as a function of adsorbed amount.
Carbon monoxide
interaction
with reduced and reoxidized
on C30 (full line)
samples
The reduced (R) samples interact with carbon monoxide in a completely different way as indicated by the comparison between C30 and R30 reported in Fig. 2 where the plot of C30 is the same as that reported in Fig. 1. The two diagrams report the partial molar heat (Fig. 2a) and the half deviation time ( ti ) of the thermogram (Fig. 2b) as a function of uptake per m2 of sample. The latter parameter (time at which the deviation from the baseline has attained half the value of the maximum) indicates semiquantitatively the kinetics of the heat emission and is therefore proportional to the activation energy of the interaction. Adsorption on R3 (Fig. 3a and b) and R30 was, except for the initial doses in the two cases, fast, non-activated and pressure dependent, in agreement with the findings of other authors [ 151. The values of t+ were around 4-5 min and were typical of almost instantaneous phenomena. The heat of adsorption fell from an initial value of about 110-100 kJ/mol to a value which, in spite of the slight dispersion of the experimental points, could be considered nearly constant with further increase in coverage (around 45 kJ/mol) and typical of a weak chemisorptive interaction. The carbon monoxide uptake decreased by about 2.5 times upon reduction from C30 to R30 whereas in going from C3 to R3 a greater decrease (about 10 times ) was found. As already reported in ref. 12 a small fraction of the adsorbed carbon monoxide is irreversibly held at room temperature. This fraction ranged from 8 to 13% for R30 and from 15 to 18% for R3 of the total released heat: this is in
291
5
‘...
. ... .-
**w
1
R 30
2
3
4
5
Fig. 2. (a) Partial molar heat vs. adsorbed amount of carbon monoxide on C30 andR30. (b) Half deviation time of the thermograms ( rt ) vs. adsorbed amount for C30 (w ) and R30 ( 0 ) . agreement with the findings of Parris and Klier in similar experiments [ 161. A large fraction of carbon monoxide adsorption on R samples involved interaction with metallic copper. This is documented in Fig. 4a and b where the isotherms of adsorption for R3 and R30 in the range O-40 Torr (O-5.33 kPa) are reported and refer respectively to the total surface area of the samples (Fig. 4a) and to the free copper surface area S cU (Fig. 4b ) as measured by microcalorimetry following the decomposition of nitrous oxide at the surface [lo]. The adsorbed amount on R30 per m2 of the sample exceeded that on R3 by about 6 times, but, when the adsorption was computed as a function of ScU the two curves lay close one to the other. The effect of the reoxidation of the samples on the adsorption of carbon monoxide is reported in Fig. 3 in terms of partial molar heat and ti for the samples containing low amounts of copper (R3, N3, 03). In this experiment the 03 sample was obtained by a different procedure from that reported in the experimental section. Here the sample N3 was saturated with oxygen after the run of carbon monoxide adsorption (Fig. 3 plot N3) and the successive evacuation. The reactivity of the sample towards carbon monoxide increased with progressive oxidation. On N samples the oxygen coverage was about 0.3 (0:Cu)
292
au.
“+ i.-., c-1 1
,
loo-
.,.,
._.-?A._._._
66-
i
43-
0:4
0.6
0‘3
I
,
1.0
i
@
. n&ml 02
04
06
0.8
m? 1.0
Fig. 3. (a) Partial molar heat vs. adsorbed amount of carbon monoxide on R3, N3 and 03. (b) Half deviation time of the thermograms vs. adsorbed amount for R3 ( 0 ) , N3 ( A ) and 03 ( n )
Fig. 4. Adsorption isotherms of CO on R3 and R30. (a) Adsorption in ,umol per m* of sample. (b) Adsorption in pm01 per m2 of metal copper.
293 TABLE I Sample composition and quantitative data for carbon monoxide adsorption Cu loading Cu surface area BET surface area CO adsorption at 40 CO ads. molar CO/Cu,,,,,, Torr (5.33 kPa) heat range (wt.%) (mzcug-‘cu) Cm*g-‘1 (pm01 m-‘) (kJ mol-‘) Reduced samples Fu 2.08 R30 22.90
166 81
Partially oxidized samples N3 03 030 Calcined samples c3 c30 cue
-
52 54
0.25 1.60
llOloo-
43 43
0.20 0.23
_
0.37 1.22 2.46
190- 43 185- 80 200-100
-
-
2.20 4.90 6.40
ZBO- 80 310- 90 340- 80
-
formally equivalent to 60% of a cuprous oxide monolayer [lo]: in such conditions the uptake of carbon monoxide was larger than on the reduced sample. The three initial doses had an heat of interaction ranging from 185 to 155 kJ/mol, i.e. higher than in the case of the reduced samples but definitely lower than those observed for the calcined samples. The kinetics of the interaction in this range were slower than for the corresponding range for the R samples. From the 4th to the last dose on N3, heats and kinetics of carbon monoxide adsorption exhibited the values observed, in a wider range of coverage on R3, indicating that this part of the interaction concerned the fraction of the surface unaffected by oxidation. On 03, where extensive oxidation of the surface and subsurface layers had occurred, after an initial value of 190 kJ/mol the heat of adsorption decreased to a constant value of 80 kJ/mol. Evidence of the weak and fast interaction on metallic copper was no longer observed either in terms of heat of interaction (Fig. 3a) or in terms of kinetics-of heat emission (Fig. 3b): the tt values, in fact, remain higher than those recorded in the cases R3 and N3. When carbon monoxide was contacted with 03 directly obtained by reoxidation of R3 according to the procedure indicated in the experimental section, the carbon monoxide uptake was 1.22*10-6 mol/m’, i.e. close to the sum of the uptake of N3 and 03 in Fig. 3. All quantitative data are collected in Table 1. Comparison with quantitative data for carbon monoxide adsorption on zinc oxide [ 171 indicated that the zinc oxide contribution to the total adsorption on copper-zinc was negligible.
294
DISCUSSION Carbon monoxide uptake The trend of the differential heat for carbon monoxide adsorption onto the various copper-zinc samples examined indicated the presence of different phenomena, albeit occurring simultaneously. However, analysis of Table 1 indicates that the total carbon monoxide uptake and the interval of variation of the molar heat clearly change both with copper loading and with the state of the sample. The molar heat of interaction ranges in intervals depending on the degree of oxidation of the surface: the higher the oxidation state, the higher the average heat evolved per mole of carbon monoxide adsorbed. The heat of interaction of carbon monoxide could thus be used as an approximate and preliminary check on the state of the supported copper. When carbon monoxide is contacted with reduced (R) systems adsorption mainly involves interaction with the surface of metallic copper. This is well documented (Fig. 4) when the adsorbed amount is computed as a function of the metal area: the slight difference between the values attained by R3 and R30 probably depends on the morphological differences between the two samples: on the basis of a study by Okamoto et al. [ 41 high copper content samples are made by well-dispersed metal particles whereas in low content ones, twodimensional epitaxial layers of copper on zinc oxide are formed. The type and the relative fraction of different copper crystal faces exposed in the two cases would therefore be different and account for the differences in the carbon monoxide uptake per unit area. On both R3 and R30, however, the carbon monoxide adsorption attains a surface coverage ranging from 20 to 23% of the exposed copper atoms (Table 1, col. 6). This value is not too far removed from the oxygen coverage (30% ) obtained by nitrous oxide decomposition and used for copper area measurements [ lo]. Only a fraction of the sites involved in carbon monoxide coordination, however, are active in the decomposition of nitrous oxide. This is evidenced by adsorption on samples precontacted with nitrous oxide (N3, Fig. 3 ) where weak energy, fast carbon monoxide adsorption on copper was not eliminated by the previous nitrous oxide oxidation and showed up at high coverage, after saturation of the oxidized sites, The noncoincidence between the sites for nitrous oxide decomposition and for carbon monoxide coordination is not surprising and can be understood in terms of the well-known differences in the reactivity of the different copper crystal faces towards the two gases [ 18,191. The increasing oxidation of the samples caused an increase in the carbon monoxide uptake and in the molar heat of interaction: the uptake in the case of N3 (Fig. 3) was 50% higher than that on R3. It should be noted, however, that the oxygen coverage on N3 was similar to that observed by Chinchen and Waugh on Cu-Zn/AlzO, after catalytic reaction in an atmosphere of hydrogen,
carbon monoxide and carbon dioxide [ 131: the carbon samples than that on R ones) should thus be the reference working catalyst. further after O2 oxidation kJ/mol) were the low energy copper metal sites still observed on N3 were no longer detectable. surface oxidation cuprous oxide upon oxygen saturation 10,111; data on 0 can thus be considered typical of the carbon monoxide-cuprous samples was much higher in the case of R3-03 (5 times) than in that of R30-030 (1.6 times). The difference observed in this case can be ascribed to the morphology of the samples: the high dispersion of copper in R3 (the metallic fraction of the total surface was 6.5% in R3 and 37% in R30) caused an extended contact with the matrix allowing a more efficient stabilisation of the carbon monoxide oxidation products on the basic zinc oxide. This agrees with the findings of Boccuzzi et al. [ 201 on reoxidized copper-zinc oxide who assigned the formation of carbonates and other oxidized species to reactions occurring at the borderline between metallic and oxidic phases. Finally, the calcined samples showed the highest uptake with results between 1.5 and 2 times that of the reoxidized ones. The reactivity of C30 and pure cupric oxide (Fig. 1) were, quite surprisingly, rather close one to the other: also this result can be ascribed to the effect of the matrix basicity which facilitated the reduction of the supported cupric oxide particles. Heat of interaction
The reaction of carbon monoxide with C samples mainly involves redox processes. This is indicated by the high heat of interaction, the slow and decreasing rate of the process and the presence of carbon dioxide in the gas phase. It should be kept in mind that the reduction of cuprous and cupric oxides by carbon monoxide (leading to carbon dioxide) are exothermic, with standard enthalpies of reduction of - 116.4 and - 127.7 kJ/mol respectively: these data, however, are not directly comparable with those measured in the experiment illustrated in Fig. 1 where the oxidation products are mainly constituted of surface carbonates of various composition. IR investigations on pure cupric oxide [ 211 and on cupric oxide-zinc oxide samples [ 221 have shown that, besides carbon dioxide, surface carbonates are formed with parallel formation of reduced copper (I) and copper (0) which, in their turn, coordinate carbon monoxide. It is therefore very hard to distinguish between the different thermal contributions to the overall data reported in Fig. 1. The analogy between the behaviour of cupric oxide and C30 indicated that the reduction involved
the cupric oxide supported particles: in fact the intense EPR spectra assigned to both isolated and clustered copper (II) ions [ 121 were not affected all by carbon monoxide interaction up to 373 K. This means that isolated copper ( II) sites are less reducible than cupric oxide aggregates and, furthermore, the EPR parameters of isolated ions present at the surface, if any, are little influenced by carbon monoxide coordination. The redox interaction play a role also when carbon monoxide is adsorbed on N and 0 samples as confirmed by IR data on the same systems [ 20 J .
[ 211 in a recent paper on the interaction of carbon monoxide with pure cupric oxide. The authors, on the basis of the time needed to decrease the intensity of two different carbonylic bands of a given factor, were able to estimate the activation energy of desorption and consequently derive the heat of adsorption belonging the two species at 2114 and 2148 cm-’ assigned to carbon monoxide on two different Cu+ ions. The values obtained so far are 79.5 and 108 kJ/mol respectively. It is known that for carbon monoxide complexes on low charge ions or zerovalent metal atoms the same mechanism involving crdonation and 0-n synergism can be hypothesized, though acting to a different extent [ 241. We can therefore tentatively extrapolate the values given in ref. 21 to the IR frequencies monitored on copper-zinc oxide and assigned to carbon monoxide adsorbed on both copper (I) and copper ( 0 ) . To our knowledge the only data available are those obtained by Ghiotti et al. for samples similar to those employed in the present study [ 22,231. It turns out that the frequencies of 2098 and 2070 cm-’ found on reduced copper-zinc oxide and assigned to carbon monoxide on different copper (0 ) sites correspond to heats of adsorption of 66 and 43 kJ/mol respectively. Despite the arbitrariness of the extrapolation the two heats of adsorption so far calculated fit the experimental values found in the present study on similar samples quite well (see Figs. 2 and 3 with the values of 43 and 66 kJ/mol
297
TABLE 2 IR frequenciesand heat adsorption of carbon monoxide adsorbedon copper in various oxidation states Copper oxidation St&?
Carbonyl stretchingfreq. Heat of interaction from IR data (desorption rate) u (cm-‘) (kJ mol-‘)
Copper (I)
2114* 2148*
Copper (0)
2098** 2070**
Heat of interaction from direct calorimetric measurement(present work) (kJ mol-‘)
79.5* 108 * 66 *** 43 ***
*On cupric oxide samples,ref. 21. **Ref. 23. ***Derivedby extrapolationof the valuesin first and second row. on the Y-axis). The coupling between heat of interaction and C-O stretching frequency obtained by the method described above are collected in Table 2. On the basis of this result we can formulate the assignments given above more precisely: ( i) Carbon monoxide adsorbed with molar heat ranging from 66 to 43 kJ/mol and representing 85% of the whole adsorption on R samples, is related to the carbon monoxide-copper metal interaction and can be associated with two I.R. bands assigned to the carbonyl stretchings observed at 2098 and 2070 cm-’ marked
[231.
(ii) The remaining adsorbed carbon monoxide (15% ) with heat of adsorption between 110-100 and 66 kJ/mol, partially non-desorbable under evacuation, has to be related to interaction with partially oxidized forms of copper, very likely coordination onto surface unreduced copper (I) ions dissolved in the zinc oxide matrix; the range of frequencies typical of Cu+CO complexes corresponds in fact, according to the analysis of Lokhov et al. [ 211, to the heat range 118-84 kJ/mol, again in excellent agreement with our results on both R3 and R30. Although several other authors claim the presence of copper (I) dissolved in zinc oxide [ 3-51, it has however to be pointed out that Ghiotti et al. do not observe any IR band in the Cu+CO region [ 231. A small discrepancy therefore
remains between the present data and the IR data for the same samples which can probably be ascribed to the different reducing procedure adopted in the two techniques. The analysis of the heat of interaction on the basis of the correspondence with Lokhov’s results can be extended to the reoxidized and calcined systems: in the case of sample 0, in particular, coordination of carbon monoxide onto
298
surface Cu+ ions occurring at high coverage is observed (120-80 kJ/mol, Fig. 3 ) . This is also observed in the case of C samples (Fig. 1) where the coordination Cu + + CO is practically absent due to the large extent of the surface reduction. However the interaction Cu++CO, if any, cannot be analysed by extrapolation of Lokhov’s values owing to the different mechanism of carbon monoxide coordination. On high-charge ions, in fact, the coordination mechanism is based exclusively on cr donation (whose extent depends on the cationic charge density) without backdonation from the metal to the carbon monoxide orbitals [ 241.
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