1989
Chemisorption of Oxygen on NiO
Study of Metal Oxide Catalysts by Temperature Programmed Desorption. 1. Chemisorption of Oxygen on Nickel Oxide Masakaru Iwamoto, Yuklhlro Yoda, Makoto Egashira, and Tetsuro Seiyama” Department of Materials Science and Technology,Faculty of Engineering, Kyushu University, Higashi-ku, Fukuoka. 8 12 Japan, and Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, Nagasaki, 852 Japan (Received September 10, 1975; Revised Manuscript Received March 22, 1976) Publication costs assisted by Kyushu University
The chemisorption of oxygen on nickel oxide was investigated over a wide range of temperature (0-600 “C) by means of the temperature programmed desorption (TPD) technique. At least four different states of adsorbed oxygen were indicated by the appearance of four TPD peaks, with peak maxima located at 30-40 (a), 320-360 (p), 420-450 (y), and 520-550 “C (6), respectively. Taking into account their adsorptive characteristics, the corresponding adsorbed species were tentatively assigned to 0 2 ( a ) ,0 2 - @),and 0- (y and 6). Experimental facts that none of the & y, and 6 oxygen retarded the chemisorption of the’others suggested that these chemisorptions took place on different surface sites, while the chemisorption site for a was estimated to be common to /3 oxygen. The activation energies of adsorption for /3 and y were 12.8 and 17.5 kcal/mol, respectively, and that of desorption for p was 23 kcal/mol. Adsorbed oxygen in various forms amounted altogether to ca. 3 X 1013molecules/(cm2 of NiO surface). The corresponding surface coverage was estimated to be ca. 4% with respect to surface nickel atoms, suggesting adsorptions on some sorts of surface defects. Discussions of these results are presented in relation to the oxidation power distribution of the surface excess oxygen, the oxygen isotopic exchange reaction, the binding energy of oxygen, etc.
Introduction Catalytic oxidation is one of the most interesting reactions in heterogeneous catalysis. For an understanding of it, it would be very important to know the adsorption state of oxygen. For example, Boreskov et a1.2 pointed out that the catalytic activities of metal oxides for the complete oxidation or for the homomolecular oxygen isotopic exchange are closely related to the energies of metal-oxygen bonds of the oxides. This indicates that the rate-determining step for these reactions includes the rupture of the metal-oxygen bond, and suggests as well the significance of the experimental examination of interactions between oxygen and the catalyst surface. In addition, knowledge on the nature of the adsorbed oxygen and its adsorption mechanism would provide valuable criteria for catalyst conditioning. Interactions between oxygen and metal oxides have been studied fairly extensively by a number of workers with techniques such as adsorption isotherm, adsorption kinetics, and ESR. Several types of adsorbed oxygen species have been proposed over various metal oxides, including 0 2 - and 0confirmed by ESR.3 The adsorption of oxygen, however, may in fact depend very sensitively on the nature and the preparation method of the oxides used. Therefore, we attempted to probe oxygen adsorption on various metal oxides systematically by means of the temperature programmed desorption (TPD) technique. The TPD technique developed by CvetanoviE and Amenomiya4 is a simple but very useful one to investigate interactions between a gas and a solid, and has been applied by several workers to studies of gas adsorption on metals or metal oxides.5@On the oxygen-metal oxide systems, however, few applications have been reported so far. In the present paper, the chemisorption of oxygen on nickel oxide was investigated. Nickel oxide was chosen because it is a typical oxidation catalyst. As for the oxygen-NiO system,
Charman et al.7 suggested three or possibly four modes of oxygen adsorption on the basis of adsorption kinetics. Recently the existence of several modes of oxygen adsorption was in fact observed by means of the temperature programmed desorption technique by Gay.8 However, Gay’s results were insufficient in terms of reproducibility and characterization for each mode of adsorption. From the viewpoint of nonstoichiometry, on the other hand, adsorbed oxygen on NiO may be regarded as surface excess oxygen, which is measurable with chemical techniques. Dered and Stohg measured, by means of modified Bunzen-Rupp and hydrazine methods, the excess oxygen content on NiO samples obtained by decomposition of nickel salts and showed that oxygen coverage was changeable with the method and condition of sample preparation. Uchijima et a1.I0 showed the distribution of oxidation power of excess oxygen on nickel oxide by the measurements using iodometric and hydrazine methods. These results obtained by chemical techniques are quite interesting, but the mode and nature of oxygen adsorption on NiO have remained almost unsolved due to the limitation of the methods. Considering that the various modes of oxygen adsorption are associated with different binding energies between oxygen and surface, it will be reasonably suggested that TPD is one of the best techniques for revealing the situation. The purpose of the present investigation is to establish the nature and occurrence of several modes of oxygen adsorption on NiO. Experimental Section In a preliminary experiment, we measured the temperature programmed desorption chromatograms of oxygen from commercial nickel oxide. However, it was revealed that (1) TPD chromatograms included water vapor as well as oxygen, and (2) the nickel oxide used was shown to contain trace amounts of Ni2Si04 by x-ray diffraction. The present work was performed to the complete exclusion of such flaws. The Journal of Physical Chemistry, Voi. 80, No. 16, 1976
M. Iwamoto, Y.
1990
Apparatus. The apparatus used for the TPD experiments is shown in Figure 1. It is in principle the same as that described in the l i t e r a t ~ r eThe . ~ reactor (R) was devised to minimize the dead volume and consisted of a quarz tube of 9.6 mm i.d. with a protective quarz tube of 8.0 mm 0.d. with a thermocouple (TC1) inserted inside. The detector was a conventional thermal conductivity cell with four tungsten filaments, G80-TCD of Yanagimoto Mfg. Co. Ltd. The temperature of catalyst was raised a t uniform heating rates (from 5 to 30 OC/min) using a programming controller with a thyrister voltage regulator, and measured by a chromel-alumel thermocouple located on top of the catalyst bed. Uniform temperature increase was very important; otherwise the distortion of the chromatogram or even the appearance of false desorption peaks would occur. The temperature lag between the sample and the thermocouple was corrected using the results of preliminary heat transfer experiments. The correction value was, for example, 4 "C at a heating rate 20 "C/min under a gas flow rate 30 ml/min. Materials. Commercial helium gas (above 99.99596, Air Product and Chemical) was used as a carrier. To remove trace amounts of moisture, it was passed through molecular sieve 5A at liquid nitrogen temperature. Commercial oxygen (above 99.8%) was also dried before use by liquefaction a t liquid nitrogen temperature. NiO samples were prepared by calcining nickel carbonate precipitated from a nickel nitrate solution with ammonium carbonate. Surface area and color of the samples obtained are summarized in Table I together with the calcination (or decomposition) conditions. The surface area was determined by the BET method. For TPD experiments, the powder was sieved and a fraction of 20-48 mesh was used. Procedure. A fixed amount of NiO of ca. 1ml (2.00 g for NiO-I-IV or 0.60 g for NiO-V) was loaded into the reactor. Before use, the oxide was evacuated for more than 2 h at the calcination (cgdecomposition) temperature (see Table I) until no more wa.ter was condensed in a liquid nitrogen trap connected to the reactor. Oxygen adsorption prior to the TPD experiment was usually processed in two ways. In both ways, the NiO sample, after evacuation (520 "C, which are designated here as peaks p, y, and 6, respectively. Peak a, to be shown later, denotes an additional peak observed around 30 "C in certain cases when oxygen is preadsorbed below 100 OC. As shown in the figure, peak /3 is commonly observed over all the samples, though the peak area (amount of desorbed oxygen) differs largely with samples. The difference was mainly due to the variation in surface area, as indicated by the fact that the amounts of &oxygen per surface area were 0.005, 0.008, 0.009, 0.006, and 0.008 ml/m2 for NiO-I, -11, -111, -IV, and -V, respectively. On the other hand, peaks y and 6 are not always observed but are remarkable on the samples NiO-I, -11, and -V. This suggests that the adsorption sites on the NiO
1991
Chemisorption of Oxygen on NiO
/
r
7:lO'C
O
r
b : ,::
. ':',
NiO-N NiO-V
1
5 0
TEMPERATURE ' 0 1
,
200 TEMPERATURE
1
400 ("C)
1
1
600
Figure 3. TPD chromatograms of oxygen from NiO-V after oxygen adsorption at various temperatures for 1 h at 120 Torr by treatment A. Dotted line (curve 16 of Figure 4) is also included for comparison.
Figure 2. TPD chromatograms of oxygen from various NiO samples. Preadsorptionof oxygen was carried out for 1 h at 300 "C and 100 Torr by treatment A. Curves 1-5 are chromatograms for NiO-I, -11, -111, -IV, and -V, respectively, obtained in the first run of TPD experiments. The fifth run on NiO-Ill gave curve 6 (see text).
13:16O'C-O0C
--
14:22O"C-O0C
..
surface responsible to these peaks are strongly affected by the sample quality or its preparation. In addition, it should be mentioned that, when NiO samples freshly prepared by calcination in air were used, the TPD chromatograms underwent considerable changes with the repetition of the oxygen adsorption-desorption cycles. In the case of NiO-I11 the chromatogram changed gradually from curve 3 in Figure 2 with the repetition of exactly the same experiments and was finally "stabilized" after the fourth cycle as curve 6, which resembles curve 5 for NiO-V very well. Similar changes of the chromatograms were also observed for NiO-I and -11when the same cycles, i.e., evacuation at 600 "C, adsorption at 300 "C, and desorption up to 600 "C, were repeated. No such examination was made for NiO-IV because of the small surface area. In contrast, chromatograms for sample NiO-V prepared by decomposition in vacuo remained unchanged from curve 5 irrespective of the repetition of the cycles. These results are interesting, indicating that the adsorption sites for y oxygen are created by evacuation at higher temperatures. The change or the stabilization of the samples with the repetitious treatments mentioned above is probably caused by evacuation at 600 "C. The analyses regarding its implications, especially in terms of the NiO surface character, are difficult at the present stage, however. With these facts in mind, only the samples NiO-V and "stabilized" NiO-I11 were used in the experiments hereafter. Characterization of Desorption Peaks:In order to characterize the desorption peaks described above, the influence of the experimental conditions of the oxygen preadsorption on T P D chromatogram was examined. Figures 3 and 4 show the effect of the adsorption temperature when processed with treatments A and B, respectively. The dependence on the adsorption pressure is shown in Figure 5. Careful comparison of these chromatograms with each other affords the following
('C)
~15:305%-0'C I
tJl
z
0 n ul
0.6- 16:53O0C-OoC
-
W
a 0.4-
w 0 a
s
-
P 0.2-
0
200 TEMPERATURE
400
I 0
("C)
Figure 4. TPD chromatogramsof oxygen from NiO-V after oxygen adsorption at various temperatures for 1 h at 40 Torr by treatment 6.
information about the character of each desorption peak and the adsorbate involved. (1)A new peak, a , appeared a t 30-40 "C when oxygen was preadsorbed below 100 "C, as shown by curves 7,12, and 13. Though a oxygen was removed by brief pumping at 120 "C, it is likely to be a chemisorbed species since no desorption resulted by evacuation for 1h at 10 "C. (2) Peak a is absent in curves 14-16 in spite of the exposure to oxygen at lower temperatures. Since these chromatograms show large desorption peaks of 0 and/or y compared with curves 7,12, and 13, it is suggested that the adsorption in a form is inhibited by the presence of the others. To further confirm this, it was examined whether NiO samples which had already preadsorbed 0,y, or 6 oxygen could adsorb a oxygen additionally at 10 "C. As a result, a oxygen was adsorbed independently of y or 6, but inhibited exclusively by 0.Thus, The Journal of Physical Chemistry, Vol. 80, No. 18, 1976
M. Iwamoto, Y. Yoda, M. Egashira, and T. Seiyama
1992 in
"I
I 1 0
I
I
I
,
200 400 TEMPERATURE (TC)
600
Flgure 5. TPD chromatograms of oxygen from NiO-V after oxygen adsorption at various pressures for 1 h at 240 "C.
a oxygen interacts with @ oxygen presumably by "sharing a common site". (3) On the other hand, comparison of curve 16 with curves 9 and 10 suggests that the adsorption sites for /3 oxygen are different from those for y and 6. This was supported by a series of TPD experiments. For instance, NiO samples which had preadsorbed 6 oxygen by treatment A a t 440 "C could adsorb p oxygen a t 240 "C to the same amount as attained in the absence of 6. Similar result was also obtained between @ and y oxygen or y and 6 oxygen. Therefore, it may be stated that each of @,y, and 6 oxygen has specific adsorption sites of its own. (4) The strength of the adsorption bonding should be in the order of a < @ < y < 6 oxygen in accordance with the sequence of desorption temperatures. ( 5 ) Figure 5 shows that oxygen adsorption in 6 form occurs easily and is saturated at the lowest oxygen pressure applied, while those in /3 and possibly y forms are dependent upon oxygen pressure. It is also noted that the peak temperature of @ peak is independent of the amount of @ oxygen. This indicates the first-order desorption for the @ peak as mentioned later. (6) When NiO samples preevacuated a t 600 "C for 1h were treated in vacuo at 240 "C for 1 h, no desorption peak appeared in the TPD chromatogram. This excludes the possibility that the strongly bound oxygen species such as the excess lattice oxygen or any adsorbed surface oxygen unable to be desorbed below 600 OC transforms thermally into species of weaker adsorption bonds during the treatments. ( 7 )After 6 oxygen was preadsorbed by treatment A at 440 "C, the sample was kept under evacuation for 1h at 240 "C. Subsequent TPD chromatogram showed only the 6 peak, indicating that no such transformation as 6 y or 6 @ occurred during the treatment. Similarly, it was assured that y oxygen was inactive for the transformation to @ a t 240 "C. Kinetics of Adsorption and Desorption. Kinetic data on oxygen adsorption were obtained under the condition of 240 "C and 120 Torr of oxygen pressure. After the adsorption treatment for a desired time the sample was briefly evacuated for 10 min at the same temperature, cooled to 0 O C in vacuo, and submitted to TPD measurement to determine the amount
- -
The Journal of Physical Chemisfry, Vol. 80, No. 18, 1976
of adsorbed oxygen vs. the adsorption time. It was confirmed that the desorption of oxygen during evacuation or cooling in this experiment was negligible. The results are shown in Figure 6. The amount of 6 oxygen, which was approximately estimated from the peak area below 560 "C, is obviously underestimated to a considerable extent so that only qualitative discussions about it are permissible. The amounts of 6 and y oxygen increase with time, attaining saturation value in 1h, while 6 becomes nearly saturated in the initial short period. This indicates that 6 adsorption occurs far more rapidly than @ or y adsorption. For @ and y adsorptions, the activation energies of adsorption (E,) were estimated from the temperature dependence of the adsorption rates (Figure 7). Here, the adsorption rates were calculated approximately from the amounts of adsorption during an initial short period, 6 min. The values obtained were E,(@)= 12.8 and E,(y) = 17.5 kcal/mol, respectively, for ,8 and y adsorptions. The latter coincides well with E , = 18 kcal/mol reported by Winterll for oxygen adsorption on nickel oxide a t 250-420 "C, though he made no distinction between /3 and y oxygen. The activation energy of desorption ( E d ) was also determined. Essentially TPD chromatogram shows the progress of desorption as a function of temperature. When the sample temperature increases linearly with time and the desorption takes place from a homogeneous surface without appreciable readsorption, peak temperature (TM)of a desorption peak is correlated with E d and the heating rate ( b ) by the following equation.4J2 For first-order desorption
For second-order desorption
+
Ed dUm2 2 In TM- In b = In E-RTM 2Rko
In U,%M
In these equations, u, is the amount of the adsorbate a t the full surface coverage, %Mthe surface coverage at peak maximum, and ko and R are constants. Thus, for first-order desorption we can obtain E d from the plots of (2 In TM- In b ) vs. ~ / T MSuch . an analysis was applied to the /3 peak. As shown in Table 11, TMincreased with an increase in heating rate, while the amount of desorbed oxygen remained unchanged. The plotting gave a straight line in agreement with the above equation as shown in Figure 8. From the slope and the intercept, &(@) and ko/um were determined to be 23.7 kcal/mol and 2.09 X los min-l, respectively, for sample NiO-V. The corresponding values for stabilized NiO-I11 were 23.0 kcal/mol and 1.98 X 10s min-l. The agreement of the values between the two samples was excellent. The value for Ed@) is close to that obtained by Gay.8 The peak width calculated with these rate parameters by the method of CvetaniviC et al.13 was 98 "C, which was consistent with the experimental value (ca. 100 "C). For the a, y, or 6 peaks no such analysis was made because the peak temperatures could not be measured with sufficient accuracy. Discussion Oxygen adsorbing (or desorbing) on NiO in the temperature range of 0-600 "C exhibited four characteristic peaks in the TPD chromatograms. Of these peaks, two (/3 and y) are clearly evident whereas the remaining two ( a and 6) are less evident being indicated by weak responses. Next concern would then
1993
Chemisorption of Oxygen on NiO
TABLE 11: Temperature Programmed Desorption of 6 Oxygena b, OC/min
TM, "C
10 15
320 324 338 335 340 346
20 20 25
30
0.0872 0.0922
0.0911 0.0909
0.0931 0.0910
Adsorptions of oxygen were carried out by treatment B at 530 "C and 40 Torr. v d is the amount of desorbed oxygen. a
ADSORPTION TIME (rnin)
Figure 6. Time course of
p, y, and 6 adsorptions. n 544
F
-m
N
146:42
40
lOOOlT Figure 7. The Arrhenius plots of the rate for p and y adsorptions (see text).
be on the corresponding adsorbed oxygen species and the adsorption sites for them. To begin with, /3 type oxygen will be considered. It is well known6J3 that the peak temperature T Mof a desorption peak in the TPD chromatogram shifts to the higher side with a decrease in the amount of adsorbed gas in the following four cases: (1)the desorption rate is of second order, (2) the surface is energetically heterogeneous, (3) readsorption occurs significantly in the desorption process, (4)diffusion controls the TPD process. For oxygen, none is the case since 2'~ is almost independent of adsorbed amounts, as shown in Figure 5. This suggests first-order desorption from an energetically homogeneous surface. In addition, the amount of @ calculated from Figure 5 was found to follow the Langmuir isotherm of molecular adsorption u KP -=.--u, 1+KP
where u is the adsorbed amount (ml STP/g) a t an equilibrium partial pressure of oxygen P(Torr), u, is the maximum adsorbed amount, and K is an equilibrium adsorption constant. u, and K were determined to be 0.0918 ml (STP)/g of catalyst and 0.183 Torr-l, respectively. From these two facts we tentatively assign p oxygen to be of a molecular type (probably 02-1.
The conclusion leads us to the estimation that a oxygen
li
160
165
170
should also be a molecular type (probably 0 2 ) , since a oxygen interacts with ,8 oxygen probably by sharing common sites. 6 oxygen, the strongly and rapidly chemisorbed species in Figures 3-6, is the predominant species formed in the adsorption at room temperature. From the heat of formation of oxygen anions in gas phase, it is anticipated that 0 2 - and 0species would be rapidly formed while the formation of 02would be slower.14J5 In fact, several investigators have reported that oxygen is adsorbed as 0- on nickel oxide in the first place. Bieradski and Najbar16 found, by determination of the mean electric charges acquired by an oxygen atom on the nickel oxide surface, that 0- was the predominant species in the irreversible adsorption a t room temperature, and that a t 150 "C only 0- was observed after a short period of adsorption. Robert et al.17 showed by means of XPS that a strongly chemisorbed oxygen (probably 0-, 531.4 eV(0 Is)), different from the lattice oxygen ( 0 2 - , 529.2 eV(0 ls)), was formed by exposing nickel oxide to desiccated air. Taking these results into account, 6 oxygen would be appropriately assigned to 0-. For y adsorption, its occurrence is caused by the pretreatment of NiO in vacuo a t higher temperatures as mentioned before. In this connection, very interesting is the fact reported by WinterlBthat the activation energy of the isotopic exchange reaction of oxygen was smaller on nickel oxide outgassed at 520 O C than on one exposed to oxygen at 620-320 "C. On the basis of our observation, this fact is well interpreted as follows: on the latter sample the isotopic exchange proceeds with the participation of 6 oxygen, while, on the former, y oxygen which is the more labile species produced after the evacuation takes part in the reaction. Thus, we tentatively assign y oxygen to 0- which is the same species as 6 oxygen but is adsorbed on different surface sites. This assignment accounts for the fact The Journal of Physical Chemistry, Vol. 80,No. 18, 1976
M. Iwamoto, Y.
1994
500 "C
- ,
0
50
100
150
-AG (kcallmole) Figure 9. The amounts of the excess oxygen of NiO as a function of the free energy of adsorption AG. Numerical values indicated In the figure
show the calcination temperatures of the samples.
that y oxygen has a stronger adsorption bond than p, probably 02-.
In conclusion, we tentatively assign a and oxygen to 0 2 and 0 2 - , respectively, probably on common surface sites, and y and 6 oxygen to 0- on different surface sites. What kind of sites are then responsible to these adsorbed state? Dereh and Stohg have measured and reviewed the excess oxygen on the surface of and in the bulk nickel oxide. According to them, nickel oxide prepared by thermal decomposition in vacuo of nickel salts such as carbonate shows the exact stoichiometry (Ni/O = l/l),l6J9 while the oxide obtained by decomposition in air retains a significant amount of excess oxygen unable to be removed readily.7,9,20It may then be suggested that surface nickel atoms uncovered by the excess oxygen act as the chemisorptive sites for oxygen. This, however, cannot give a quantitative explanation for our experimental results. The number of surface nickel atoms on the (loo), (110), or (111)plane of NiO is shown to be 1.15 X 1015,0.81 X or 1.32 X 1015atoms/cm2, respectively. On the other hand, the amounts of p, y, and 6 oxygen obtained from curve 16, for example, are 1.82 X 1013,0.48 X 1013,and 0.72 X 1013molecules/cm2 of NiO surface, respectively. The total number of the sites occupied by them is estimated to be 4.21 X 1013 sites/cm2 on the basis of the above assignment, which corre-
The Journal of Physical Chemistry, Vol. 80,No. 18, 1976
Yoda, M. Egashira, and T. Seiyama
sponds to surface coverage of ca. 4% with respect to surface nickel atoms. Consequently, it seems more reasonable to assume that oxygen is adsorbed onto surface defects such as oxygen vacancies, dislocations, edges, and corners. Finally, it would be noteworthy that the TPD chromatogram of oxygen obtained in the present study is well correlated to the distribution of oxidation power of the surface excess oxygen on nickel oxide measured by Uchijima et al.1° Using their data on the samples calcined at 500-600 "C, the amount of surface excess oxygen is depicted vs. the free energy change of adsorption, -AG, in Figure 9.Clearly, the distribution of the surface excess oxygen shows two maxima which correspond well to the desorption peaks of the TPD chromatograms measured after similar sample conditioning (Figure 2, curve 2 and 3). Taking into account that Ed is usually a monotonic function of T Min TPD experiments, probably such a correlation suggests a sort of parallelism between AG and E d for the adsorbed species involved.
Acknowledgment. The authors wish to express their gratitude to Dr. Noboru Yamazoe for helpful discussions. References and Notes (1)Author to whom correspondence should be addressed at Kyushu University.
(2)B. A. Sazonov, V. V. Popovskii, and G. K. Boreskov, Kinet. Katal., 9,312 (1968); F Basolo and R. L.Burwell, Jr., "Catalysis Progress in Research", Plenum Press, New York, N.Y., 1973,p 91. (3)J. H. Lunsford, Catal. Rev., 8, 135 (1973). (4)Y. Amenomiya and R. J. CvetanoviC, J. Phys. Chem., 67,144,2046,2705 (1963). (5) Y. Amenomiya, J. H.B. Chenier, and R. J. CvetanoviC, Proc. lnt. Congr. Catal., 3rd, 1964, 2 , 1135 (1965). (B)R. J. CvetanoviC and Y. Amenomiya, Catal. Rev., 6,21 (1972). (7.)H. 8 . Charman, R. M. Dell, and S. S. Teale, Trans. Faraday SOC., 59,453 (1963). (8)I. D. Gay, J. Catal., 17,245 (1970). (9)J. Dereh and J. Stoh, J. Catal., 18,249 (1970). (la)Y. Uchijima, M. Takahashi, and Y. Yoneda, Bull. Chem. SOC.Jpn., 40, 2767 (1967);J. Catal., 9,403 (1967). (11) E. R. S. Winter, J. Catal., 6,35 (1965). (12)J. L. Stakebake, J. Phys. Chem., 77,581 (1973). (13)R. J. CvetanoviC and Y. Amenomiya, Adv. Catal., 17,103 (1967). (14)E. R. S.Winter, Adv. Catal., IO, 196 (1958). (1.5)M. M. Taqui Khan and A. E. Marteil, "Homogeneous Catalysis by Metal Complexes", Academic Press, New York, N.Y., 1974,p 83. (16)A. Bierahski and M. Najbar, J. Catai., 25,398 (1972). (17)T. Robert, M. Bartel, and G. Offergeld, Surface Sci., 33, 123 (1972). (18)E. R. S.Winter, J. Chem. Soc., 2889 (1968). (19)A. Bieranski, J. Derek J. Haver, and J. Skiczynski, Trans. Faraday SOC., 58, 166 (1962). (29)S.J. Teichner et al., Bull. SOC.Chim. Fr., 3244,3251 (1967).
