J. Phys. Chem. 1082, 86, 3263-3269
and third-shell water molecules than in bulk water and (ii) the effect of hydrophobic interactions between the water molecules and the resin matrix. The first explanation is not supported by quantum mechanical calculations (see above). Therefore, at present we support the second alternative. More experimental data are needed to confirm this. In conclusion, we would like to emphasize the fact that, as a consequence of the ionogenic group being osmotically inactive, the counterions in the resin matrix do not behave like ordinary electrolyte solutions. In the absence of strong interactions between the ionogenic groups and counterions, as in the present case, the resin phase closely resembles a solution containing only counterions, unperturbed to a large extent by the presence of ionogenic groups or resin matrix. As such the resin phase provides an opportunity to study single-ion-solvent interactions. It is for this reason that the results obtained in the present study could be interpreted only in terms of the theoretical predictions of quantum mechanical calculations on single-ion-solvent aggregates. To the best of our knowledge, this is the first time that such theoretical predictions of single-ion behavior have been experimentally proved. Therefore, this study provides additional experimental evidence for the existence
3263
of a second hydration shell for the lithium ion. More strikingly, it not only proves experimentally that the hydrogen bonds between the first and second hydration shell are stronger than in bulk water but also provides a quantitative measure for the same. This study further supports the theoretical prediction that the influence of lithium ions does not extend beyond the second hydration layer and indicates that the molecules in the third hydration layer form hydrogen bonds with neighboring water molecules (in the second hydration layer), as in the bulk solvent. It is to be emphasized that the term ”the third hydration shell/layer” is used in the present context merely to distinguish those water molecules which are not part of the first and second hydration shells. The present study of lithium ion-water interactions does not give any indication of the existence of a less structured solvent region between the cosphere of an ion and the bulk solvent and consequently lends support to Symons’s viewpoint that solvent molecules beyond the second hydration shell gradually merge with the bulk solvent.
Acknowledgment. The authors express their sincere thanks to Dr.K. N. Rao for his interest and encouragement during the course of this investigation.
Novel Nitrogen-Potassium Surface Complex on Alumina, Magnesia, and Calcium Oxide Ken-lchl Alka,‘ Hldeo Mldorlkawa, and Aisumu Orakl Research Laboratory of Resources Utlkatbn, Tokyo Institute of Technology, hkgatsuta 4259, M/&rI-ku, Yokohama 227, Japan (Received: Januaty 13, 1982; I n Flnal Form: Apr//6, 1982)
Novel nitrogen-potassium complex formation on A1203, MgO, and CaO surfaces is suggested. The complexes were formed when NH3was decomposed over A1203-K, MgO-K, or CaO-K at 350 OC, or when N2was introduced to Ru-AlZO3-K, Ru-MgO-K, or Ru-CaO-K catalysts at >200 “C. The rate of the surface complex formation from N2 was almost proportional to Ru content, while the formation occurred slowly on A1203or CaO even without the presence of Ru if any K was present. The N2 complex formation reaction was reversible with the presence of Ru; i.e., N2 was desorbed or readsorbed depending on the temperature and pressure. The surface complexes gave strong IR absorptions at 2020-2030 cm-’ on A1203,1950 cm-’ on MgO, and 1920-2000 cm-’ on CaO. The wavenumbers were not dependent upon the presence of Ru but depended upon the kind of oxide, suggestingthat the surface complex has a strong interaction with the oxide surface. The linear relation between the amounts of adsorbed K and N2 suggests the formation of a novel K-N2 surface complex, while the wavenumber or the stability of the complexes excludes the possibility of known nitrogen compounds of potassium such as azide or nitride.
Introduction It has been shown that Ru catalysis is remarkably promoted by addition of K for catalytic activation of N2 such as in ammonia synthesis or isotopic equilibration of N2.1-3 During these studies it was found that the number of adsorbed N2molecules was greater than that of surface Ru atoms on Ru-AC-K (AC = active carbon) catalyst, where N2was presumed to be dissociated on Ru and to migrate to the K ~ u r f a c e . ~Meanwhile, adsorbed Nzon Ru-A1203-K was found to give an IR band at 2020 cm-l, which was assigned to molecular N2 chemisorbed on Ru in a (1)A. Ozaki, K.Aika, and H. Hori, Bull. Chem. SOC.Jpn., 44,3216 (1971). (2)K.Aika, H. Hori, and A. Ozaki, J. Catal., 27, 424 (1972). (3)K.Uyabe, K. Aika, and A. Ozaki, J. Catal., 32, 108 (1974). (4)K.Aika and A. Ozaki, J. Catal., 35, 61 (1974). 0022-3654/82/2086-3263$0 1.25/0
manner similar to the dinitrogen complex of R u . ~Finally, ~~ adsorbed N2 was found to be stoichiometrically correlated with K over Ru powder and was suggested to give a novel complex like (KNzRu), in several outer layers of Ru powder.lPa This was also deduced from the binder action of N2 between K and Ru powder.’,* Does the “supported” Ru-K catalyst also form such a N2 complex? If so, most of the Ru on AC-K4 or A1203-K9 would become a (KN2Ru), complex because the number (5)M.Oh-kita, K.Aika, K. Urabe, and A. Ozaki, Chem. Commun., 147 (1975). (6)M.Oh-kita, K.Aika, K. Urabe, and A. Ozaki, J. Catal., 44,460 (1979). (7)A. Ohya, K.Urabe, and A. Ozaki, Chem. Lett., 233 (1978). (8) A. Ohya, K. Urabe, K. Aika, and A. Ozaki, J. Catal., 58,313 (1979). (9)K. Urabe, K. Shiratori, and A. Ozaki, J. Catal., 55, 71 (1978).
0 1982 American Chemical Society
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The Journal of Physical Chemlstty, Vol. 86, No. 16, 7982
Aika et ai.
of Nz molecules adsorbed approaches the number of Ru atoms (N2/Ru = 0.68 for 2% Ru-A1203-K and 0.79 for 5% Ru-AC-K). Is metallic Ru so corrosive to K or N2? Although the linear relationship between K and N2 has been examined on AC4 or on A1203,6no such relationship has been obtained between Ru and Nz. The aim of the preesent work at the begining is to clarify the quantitative relationship between Ru and Nz adsorbed on a support with K. Finally, N2 was found to be adsorbed on a support without Ru if any K is present. Thus, the nature of K-N2 surface complexes was studied mainly by means of IR technique over A1203,MgO, or CaO. Experimental Section A sample (50-120 mg) of Alz03 (Alon C) from Japan Aerogel, MgO or CaO(s) both from Soekawa Rikagaku, or CaO(w) from Wako Chemicals was pressed into a wafer, evacuated at 350 "C, and treated with K vapor (K from Wako Pure Chemicals) in an in situ IR cell. Ru-A1203 (0.1%,LO%, 2.0%,and 5.0% Ru, RuCl3.3Hz0 from Koso Chemicals), 5.0% Ru-MgO, 5.0% Ru-CaO(s), and 4.2% Ru-CaO(w) were prepared by an impregnation method in aqueous or acetone solution. After pressed into wafers, these samples were evacuated at room temperature, reduced with H2 at 100, 150, and 250 OC for 2 h and then at 350 "C for 10 h, and treated with K vapor in an in situ IR cell. N2 (200 torr) or NH3 (100 torr) was introduced to a sample preheated to the proper temperature. The resultant IR absorption was recorded by a JASCO-A3 spectrometer. The practical apparatus and procedure were the same as those reported previously.6 The amount of K added was held at a level (below 10 wt %) similar to that reported previously! K (10 wt %) on Al2O3 was calculated to correspond to about a 70% monolayer (closed packing with the van der Waals distance) of dispersed K on Alz03 the BET area of which is said to be 100 m2/g. Adsorption measurements of Nz or Hz were made by using a conventional circulation system with volumes of 60 or 123 mL. The pressed Alz03 wafers for the same samples as used for IR measurements were crushed to a granular consistency, about 2.1 g of which was used for the adsorption measurements. Catalyst treatment and K addition were followed in the same way as in the case of the IR measurement. EPR samples were prepared in this system and transferred to EPR tubes. The EPR spectrum was recorded with a Varian E-12 at -196 "C. N2 and H2 were purified with copper-kieselguhr and Pd-A1203, respectively, and NH3 was purified by distillation in a vacuum system. Atomic percentages of isotopes used were 92.8 or 99.3 for 15N2,99.2 for 15NH3,and 99.5 for D2, respectively. Gases were analyzed mass-spectrometrically. Impurities according to emission spectrochemical analysis were Si and Mg in Alon C AZO3(assay above 97%, SOz, Fe20S,and TiOz as labeled impurities), Ca and A1 in MgO (assay 99.75%),Mg, Al, Si, and Ti in CaO(s) or CaO(w) (assay both 99.99%), and Na in K (assay 95%). Results N2 Adsorption on A120rK. Nz adsorption onto Alz03-K was measured at 325-353 "C. The amount adsorbed as a function of time is shown in Figure 1. The initial adsorption rate is roughly proportional to Ru content between 0.1% and 5% Ru samples, suggesting that Ru is necessary for Nz activation. It is surprising in this respect that A1203-K adsorbs Nz without the presence of Ru. No Nz adsorption was observed on A1203 without K. Adsorbed amounts of N2when the rate leveled off were at a similar level of 0.4-0.8 mmol of Nz (about 10 mol % of K) inde-
0.6
-
-
-
?/
325'C,
5%Ru ( 0.955mmol )
0.6 0
e-.
(0.0204 mmol )
- e - e - * + Y
I
353'C.
O.l%Ru
/35OoC, O%Ru 10
15
( hr )
Time
Figure 1. Time course of N, adsorptlon onto 0%, 0.1% and 5.0% Ru-AI,O,-K at 325-353 "C. PN2= 600 torr for 0% and 0.1% Ru, 355 torr for 5.0% Ru catalyst. Catalyst weight before reduction is 2.07 f 0.01 g.
-s
-
U
9
90-
0
n N
Z
80 -
Time
(hr)
Figure 2. Time course of N2 desorption (0)and adsorption (0)onto 0.1 % Ru-AI,03-K at 353 "C. After the N, adsorption shown in F$we 1, the catalyst was cooled and evacuated at 25 "C, and then heated to 353 O C in a closed system in order to m&sure N, desorption; 600 torr of N2 was introduced onto the caulyst at 353 "C in order to measwe N, adsorptbn. Upper curve indicates N, uptake; lower curve indicates N2 pressure.
pendently of Ru content. Even in the case of 0.1% RuAl203-K, the amount of adsorbed N2 is much higher than the total number of Ru atoms (Nz/Ru = 27), confirming that Nz is not bound to Ru. The reversibility of adsorption and desorption of N2 was measured at 353 "C on 0.1% Ru-AlZO3-K following the run shown in Figure 1. Changes of Nzuptake when gasphase N2 was removed (open circle) and when gaseous N2 was added (filled circle) are shown in Figure 2. More than 20% of the N2 uptake, corresponding to about 5 times of total Ru, was proved to be desorbed and readsorbed repeatedly, suggesting reversible spillover of N2onto A1203-K from Ru. IR Absorption Due to N2 on A1203-K. IR spectra of 2.0%, 0.1%, and 0% Ru-A1203-K were measdred before and after N2 adsorption at 350 "C. The results are shown in Figures 3-5. A strong IR absorption, which already has been assigned to dinitrogen species6 (called type-A species'O), were obtained within 1 h at 350 "C on 2% RuA1203-K (Figure 3). A similar peak developed on 0.1% Ru-AlZO3-K (Figure 4), where the peak after 6 h ( c ) was thicker and a little bit deeper than that after 1h (b). In ~~~
~
~~
(10)M.Oh-kita, H.Midorikawa, K. Aika, and A. Ozaki, J. Catal., 70, 384 (1981).
The Journal of Physicel Chemistry, Vol. 86, No. 16, 1982 3285
Novel Nitrogen-Potassium Surface Complex
TABLE I: IR Data of Adsorbed N, on Al,03 System wavehalfI5N number, width, shift, sample sourcea cm-' cm-I cm-I
wa wb il v
I
I1
2% Ru-Al,O,-K
N,
2% Ru-Al,O,-Na
N,
0.1%Ru-Al,O,-K Al,O,-K
N, N,or NH3
Al,O,-Na
NH,
2020-2030 1950 2030-2050 1980 2025 2030
2000
1600
I000 Frequency (cm-')
1400
20-30 30
106
82 14
1950 2045 1980
2020 2400
" , OL
112
20 20
IO
350 "C.
Flgure 3. Variation of I R spectrum of 2 % Ru-AI,O,: (a) after H, reduction at 350 O C , (b) after K addition and We treatment at 350 OC, (c) after N, adsorption for 1 h at 350 OC.
2415
C
.-0m
3600
.-m
UI C
+E $
1
20301 I1
2400
2000
I
1900
1800
cm-' Flgure 4. Variation of I R spectrum of 0.1 % Ru-A1,03-K (a) after H, reduction at 350 O C , (b) after K addMon and N, adsorption at 350 OC for 1 h, (c) after N, adsorption ELt 350 O C for 6 h.
::/-..'
:Ik E2z
10%
2030 4000 i
"
%
O
2800
2400
I
1
2000
1900
,
cm-'
I
I
1500
1400
1
1300
1200
I100
Flgure 6. Variation of I R spectrum of KBr-K: (a) after treatment with N, at 325 OC; (b) after ND, (114 torr) treatment at 325 OC for 2 h, followed by evacuation at 25 OC for 1 h; (c) after further K addffion (In N, at 313 OC 2 h), followed by ND, treatment at 313 'C.
E
s
3200
0
Frequency 2600 (cm-') '
1800 '
'
1600 '
'
1400 '
Flgure 5. Variation of I R spectrum of Al,03: (a) after evacuation at 350 OC, (b) after K addition and He treatment at 350 OC, (c) after N, adsorption at 350 OC for 1 h, (d) for 3 h, (e) for 7 h, (f) after ND, treatment at 350 O C for 1 h.
the case of Alz03-K, the peak corresponding to N2 uptake developed slowly (Figure 5). These developing rates of IR absorption (Figures 3-5) correspond well to the volumetric Nz adsorption rates (Figure 1). A smaller peak at 1950
cm-' in Figure 3 has been assigned elsewherelo to dinitrogen species (type B) appearing on introduction of hydrogen. NH3 Decomposition over A1203-K. Though NH3 was not decomposed on A1203 at 350 OC,it was decomposed on A1203-K above 300 O C . The products of decomposed ND3 were rich in Dz (D2/N2= 30/1), confirming that N2 was adsorbed. IR spectra after ND3decomposition at 350 "C for 1 h gave a strong peak due to N2 at 2030 cm-' as is shown in Figure 5f. Weak peaks at 2530,2460, and 1154 cm-' were assigned to be adsorbed ND3 on A1203 because ND3 on A1203gave peaks at 2520 (v), 2380 (v), and 1180 cm-' (6) at room temperature. A peak due to KND2 (see next section) was not observed. The IR data of dinitrogen species are summarized in Table I. The peak position is practically unaffected by both the N2 source (Nz or NH3) and the Ru content. However, Ru content does affect the half-width. Na gives higher wave numbers. Another feature is the small isotope shift, as has been pointed out elsehwere.lOJ1 IR Spectra of KBr-K Treated with ND3 A KBr wafer was treated with K and aged in He at 350 "C,and then Nz was introduced at 325 O C ; however, no peak was observed, as is shown in Figure 6a. Subsequent treatment with ND3 (114 torr) at 325 "Cgave three peaks at 2415 (v), 2330 (v), and 1122 cm-l(6), which were assigned to KNDz because ND3 was proved not to be adsorbed on KBr. the second peak position (2330 cm-l) may not be correct because of C02interference (2349 cm-') outside of the IR cell. Additional peaks at 3200 and 1384 cm-' are assigned to KNHF It is to be noted that no peak due to adsorbed N2 was observed around 2000 cm-l. (11) K. Aika, H. Midorikawa, and A. Ozaki, to be submitted for publication. (12) M. Oh-kita, K. Urabe, and A. Ozaki, J. Catal., 52, 432 (1978).
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The Journal of phvsical Chemistry, Vol. 86, No. 16, 1982
Aika et ai.
TABLE 11: I R Data of Adsorbed N, on MgO System sample
source
wavenumber,a cm-'
reactivity with HZb
- (350 "C)
MgO-K 5% Ru-MgO-K
a
He-NH, (350 "C) 1945 (1925) N, (200 "C) 2170 (2150) N, (300 "C) 2080 (2040) N, (175-350 "C) 1950 (1930) N, (350 "C) 1890 (1860) 5% Ru-MgO-Na N, (350 "C) 2090 (2060) N, (200-350 "C) 1982 (1960) Data for "N, are given in parentheses. t + = disappeared; - = unchanged.
+t (300°C) t t (25OOC)
+t
(room temperature)
t + (350°C)
++ ( 3 5 0 ° C )
TABLE 111: I R Data of Adsorbed N, on CaO System sample
source
CaO( s)-K 4.2% Ru-CaO(s)-K
NH, (3 50 "C) N, (350 "C)
CaO(w)-K
N, then Ha (350 "C) N H , ( 3 5 0 C)
~I
>~
N, ( 3 5 0 "C) Data for 15N, are given in parentheses. t + = disappeared. 5% Ru-CaO(w)-K
IR Absorption Due to NH, Decomposition on MgO-K. Though no IR absorption was observed by Nz (200 torr) treatment at 350 "C for 1h, NH, (100 Torr) treatment at 350 "C for 1h gave a strong peak at 1945 cm-' as shown in Figure 7. Since 15NH3gave a different wavenumber (1925 cm-l), it was assigned to the dinitrogen species. The peak was hardly changed by Hz treatment (200 torr) at 350 "C for 6-10 h. IR Absorption Due to N2 on Ru-MgO-K. 5% RuMgO-K gave four IR peaks after N2 (200 torr) treatment as shown in Figure 8. All of the peaks have a 15N isotope shift, as shown in Table 11, suggesting their assignment to adsorbed N2. The peak at 2170 cm-l, which disappeared above 300 "C, was identical with one obtained from NH3 decomposition on MgO," suggesting that this species was adsorbed on MgO. Since the peaks at 2090 and 1982 cm-' for 5% Ru-MgO-Na (Figure 8B) are considered to correspond to the peaks at 2080 and 1950 cm-' for the K system, these dinitrogen species must be related to alkali metal. The reactivity with H2 was examined and is shown in Table 11. The species with lower wavenumber has higher reactivity as in the case of Ru-A1203-K.'0 It is to be noted that the peak at 1890 cm-' disappears a t room temperature on introduction of HP. The main peak (1950 cm-l) must correspond to one obtained from NH3 decomposition on MgO-K (1945 cm-'). While the main peak on Ru-MgO-K (1950 cm-') disappeared on introduction of H2 at 250 "C, that on MgO-K (1945 cm-l) did not at 350 "C, suggesting that Ru catalyzes the surface reaction of adsorbed N2 with HP. Indeed, NH3 was detected after Hz treatment of N2-treated Ru-MgO-K. IR Absorption Due to N2 on CaO(s)-K. Though N2 (200 torr) treatment of CaO(s)-K at 350 "C gave no new IR peak,NH3 (100 torr) decomposition over CaO-K at 350 "C brought a broad peak from 1980 to 1920 cm-' as is shown in Figure 9c. In the case of Ru added sample (4.2% Ru-CaO-K), N2 (200 torr) treatment gave a similar broad peak at 2000-1920 cm-' and sharp peaks in the 600-cm-' region as shown in Figure 9b. The 15Nisotope shift (see Table 111) confirmed that both the broad band and the 660-cm-l band involve N-containing species. The broad one is considered to be due to K-bound N2 species as on the Al2O3-K system; however, the peak at 660 cm-l mitght be due to a N2 species bound to Ru. The latter disappeared, and the former remained as a sharp peak at 1955 cm-' (1930 cm-*for 15Nz)following H2 (100 torr) treatment at 350 "C for 1 h.
reactivity with H Z b
wavenumber,a cm" 1920-1990 1920-2000(1900-1980) 660 (650) 1955 (1930) 2170 1930-2000 1980
+ (350°C) + t (350 "C)
+ = decreased.
I 2LOO
1945
2000
1900
Frequency (cm-' ) Figure 7. Variation of I R spectrum of MgO-K: (a) after K treatment in He at 350 "C,(b) after NH3 treatment at 350 "C for 1 h.
1982
0
2000
1900
1800
Frequency (cm? )
Figure 8. Variation of I R spectrum of adsorbed N2 with temperature. N,, 200 "C,1 (A) for 5 % Ru-MgO-K: (-) after K addition; h; (.-) N,, 350 "C,2 h. (B) For 5 % Ru-MgO-Na: (-) after Na addition: (-.-) N,, 200 O C , 1 h; (.-) N,, 350 'C, 3 h. (-e-)
The Journal of Physical Chemistty, Vol. 86, No. 16, 7982 3267
Novel Nitrogen-Potassium Surface Complex
2000 1920
00-
I
-
2400
2000
1900
Frequency (cm4 )
Flgure 10. Variation of IR spectrum of CaO(w): ( I after K addition in He at 350 OC, (b) after N, treatment at 350 OC for h, (c) after NH, treatment at 350 OC for 1 h.
Frequency (cm-')
Flgve 9. Variation of IR spectrwn of CaO(s)based catalyst: (a) 4.2% Ru-Caqs) after reduction with H, at 350 OC over 10 h, (b) after K treatment in N, at 350 OC for 3 h, (c) CaO(s)-K after NH, treatment at 350 OC for 1 h.
IR Absorption Due t o N2on CaO(w)-K. For CaO obtained from another source, which has the same impurities by emission spectrochemical analysis, N2 (200 torr) treatment at 350 OC after K addition gave a peak at 1980 cm-' as is shown in Figure lob. NH3 (100 torr) treatment at 350 OC gave a strong peak at 2000-1930 cm-' and a sharp peak at 2170 cm-l; the latter has been assigned to be Nz adsorbed on a CaO surface without K." EPR Measurements. Potassium addition to A1203 gave a strong EPR absorption a t g = 2.004 probably due to electrons on a surface donated from K atoms (K = K+ e-). The signal almost disappeared after NH3 (100 torr) treatment a t 350 "C, as is shown in Figure 11. Similar results were obtained on MgO, for which it was also proved that MgO contains impurities such as Mnz+(g = 2.008 with six hyperfine lines) or Cr3+ (g = 1.980), as is shown in Figure 12. The disappearance of unpaired electrons (g = 2.004 on A1203, g = 2.002 on MgO) following NH3 treatment is considered to be due to the reaction with hydrogen, the decomposition product from N H , which has been discussed for active carbon (e+ + H = H-).13 The signal for reduced Ru-MgO was quite different from MgO, as is shown in Figure 13. No EPR signal due to Mn2+nor CP+ was observed. This is probably caused by an addition of spillover hydrogen, which has been incorporated during Ru red~cti0n.l~A signal caused by K addition (g = 2.003 in Figure 13) is not so strong as that on MgO-K without Ru. The small signal may also be due to the reaction of unpaired electrons with spillover hydrogen. N2-treated Ru-MgO-K gave a much smaller signal (Figure 13), which might suggest a reaction of adsorbed N2 with K or unpaired electrons. Any peaks due to N2-, which have been
-1 50G
I
y
g.2.OoL
+
Flgure 11. EPR spectra of A1203,AI,O,-K. NH.3.
reported on a Ba(N3)2matrix,ls were not observed by the EPR measurement at 77 K. ~
(13)M.Ishizuka, K. Aika, and A. Ozaki, J. Catal., 38, 189 (1975). (14)L. M.Webber, J. Phys. Chem., 76,2694 (1972).
and AI,03-K treated with
~~
~
~~~
(15)P.L.Marinkas and R. H. Bartram, J. Chem. Phys., 48,927(1968). (16)J. Turkevich and T. Sato, Proc. Znt. Congr. Catal., 5th, 1972, 1, 587 (1973).
3288
The Journal of physical Chemktry, Vol. 86,No. 16, 1982
1 IlY
I It/
50 G
Aika et al.
stabilized on A1203,MgO, or CaO. A part of K must be consumed by the reaction with surface hydroxide (3680 cm-' shown in Figure 3a) giving H2, which was confirmed by a small amount of NH, formation on Ru-A120,-K after N2 treatmentlo and the decrease of a peak at 3680 cm-l. However, at least a part of K must remain reactive, which was confirmed both by EPR results (Figure 11)and by the reaction with O2 or H 2 0 after N2 treatment. EPR results suggest that K atoms partially transfer their electrons to the support, as is shown in Figure 11 or 13. However, the extent of EPR signal intensity seems to be independent of the ease of N2 complex formation, because Ru-MgO-K in which the number of free electrons is smaller (Figure 12) gives IR absorption similar to that on /--MgO-K (Figures 7 and 8). K seems to be dispersed atomically over supports, because g values are different from those of Na film (1.97)16or colloidal K (1.998).17 Formation of K-N2 Surface Complex from N2 It is demonstrated that dinitrogen reacts with K dispersed on a support with the following results: (1)The amount of adsorbed N2is linearly related to the amount of K (2) N2 adsorption occurs without the presence of Ru on A1203 or CaO if any K is present, which refutes the idea that Ru corrosibly adsorbs N2 by the aid of K, at least on those supports. (3) The wavenumber of the IR peak due to N2 depends on the kind of alkali metal. The complex formation (Figures 1and 5 ) , decomposition (Figure 2), or reaction with Hz (Tables I1 and 111) is catalyzed by Ru, which has been known to be active for the dissociation of N21-3
Flmue 12. EPR spectra of Mgo, MgO-K, and h4gO-K treated Wtth NH,. SensitMties are the same as those in Figure 11.
K(a) + xN2
K(N2),(a)
K(N2),(a) + 3xH2% K(a) + 2xNH,
RU-MgO
I
-
9=1.975 9~1.975 50 G
L(
9= 2.003
Figure 13. EPR spectra of Ru-MgO, Ru-MgO-K, and Ru-MgO-K treated with N,. Sensklvites are the same as those in Figures 11 and 12.
Discussion Adsorbed State of K . Na is known to be stabilized on Na2C03, kieselguhr, or aromatic hydrocarbons with a charge transferred partially.18 K is also considered to be (17) R. C. McMillian, J . Phys. Chem. Solids, 25,773 (1964).
(1) (2)
where (a) denotes an adsorbed state. The complex formation from N2is only possible on a few active sites (A1203, CaO(w))with K when Ru is not present. In order to check the possible role of transition-metal impurities such as Fe in A1203,we included an impurity level of Fe(N03), in MgO-K which does not form K-N2 complex from N2. However, the addition of Fe did not help the N2 adsorption, suggesting that the active center on A1203or CaO(w) with K was not a transition-metal impurity. Formation of K-N2 Surface Complex from NH,. K-N2 surfaces complex easily formed from NH3 decomposition compared with N2 adsorption, for example, on A1203-K. In order to be incorporated as a surface complex, Nz must be activated or its bond weakened by a surface. Since NH, decomposition gives atomic N initially on a surface, it is reasonably understood that activated N2 is formed easily from NH3 if it can be decomposed. It is to be noted that A1203, MgO, or CaO with K prefers K-N2 surface complex when contacted with NH,, whereas KBr-K prefers KNHz from NH,. The former can catalyze NH, decomposition, while the latter cannot. A part of K on A1203 etc. must be turned to hydride after NH, decomposition; however, no quantitative measurement was made. Form of K-N2 Surface Complex. The form of nitrogen in the complex is considered to be molecular because of (1)the IR wavenumber region, (2) partial formation of N2H4upon decomposition with water,78 (3) and Nz uptake on A1203-K which cannot dissociate Nz to form ammonia. The stoichiometry of Nz/K = 1/1 might be suggested if (18) F. A. Cotton and G. Willkinaon, "Advanced Inorganic Chemistry, A Comprehensive Text", Wiley, New York, 1966.
J. Phys. Chem. 1982, 86,3269-3271
the results on Ru powder7l8could be extended to the system with support. The main IR peaks-2030 cm-l on A1203,1950 cm-’ on MgO, and 2000-1920 cm-’ on CaO-must represent the prevailing structure of the complex. The difference in wavenumber suggests that the complex has a strong interaction with these oxide surfaces. The reactivity and the stability are also different from each other. The adsorbed N2 on Ru-MgO-K is the most reactive to Hz. Neither azide, KN3, nor nitride, K3N, is stable at 350 OC.l9 KN, decomposes under these condition^.^ Alkali metal supernitride (KN,) is not known to be stable; however, IR absorption due to Li(N2) was observed by a matrix isolation technique at 1800 cm-l at 15 K.20 The N2- anion radical has been reported as an impurity in an azide matrix at lower temperatures.16 We could not detect any IR or EPR absorption due to Nz-. Thus, the simple KN2 is not considered to be formed; however, a KNz-like molecule might be stabilized on an oxide surface. In the case of Ru powder, KNz is considered to be bound to Ru instead of oxide to form (KN2R~),.78On Ru-support-K, Ru-bound Nz complex could also exist as a minor species, as was pointed out in the case of Ru-CaO(s)-K (Figure 9b). Further study is necessary to clarify the structure of the surface complex on oxides. The possibility of the formation of nitrogen compounds of Al, Mg, or Ca should be discarded. A part of AZO3may (19)J. C. Bailar et al., Eds., ‘Comprehensive Inorganic Chemistry”, Vol. 1, Pergamon Press, Elmeford, NY, 1973,pp 435. (20)R. C. Spiker, Jr., L. Andrews, and C. Trindle, J.Am. Chem. SOC., 94,2401 (1972).
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be reduced with K and might form some aluminum compounds of nitrogen. However, a great deal of N2 has been reported to be adsorbed on Ru-AC-K where no alkaliearth metal is p r e ~ e n t . ~ The smaller isotope shift (20-30 cm-’, see in Table I) than that calculated from a harmonic oscillator model (60-70 cm-’) suggests the N2 is not bound in a simple end-on comp1ex.l’ The smaller shift may suggest that the other side of N2 is bound to oxide, as has been discussed in the case of an 0-Ag frequency which has been explained as a [C2H40]-Ag vibration.21 Conclusions Novel K-N2 surface complex formation on A1203,MgO, or CaO is suggested. The study of the surface complex may open the way for novel methods of synthesizing N2H4, Nz complexes, or nitrogen-containing organic compounds. Regarding the catalytic synthesis of ammonia, the complex has been proved not to be a dynamic intermediate of ammonia synthesis.’O However, K which promotes the synthesis reaction must be stabilized in the form of a K-N2 surface complex resisting vaporization under NH, synthesis conditions.
Acknowledgment. This investigation was supported by a Grant-in-Aid for Scientific Research (No. 411108) from the Ministry of Education, Science, and Culture. We also acknowledge the contribution of Dr. M. Oh-kita. (21)W. (1973).
M. H.Sachtler, h o c . Int. Congr. Catal., 5th, 1972,2, 943
Theoretlcal Investigations of the Interaction of Isobutene with Sodium Ions Th. Weller,‘t R. Lochmann,t W. Meller,+and H.J. Kohlere S&bn phvslk and OR.?, Karl-Marx Unlversitaet, DDR-7010 Le@@, German Democratic Republlc (Received: December 14, 1981; In Flnal Fonn: March 9, 1982)
The investigation of the interaction of isobutene with sodium ions was carried out by semiempirical and ab initio methods. The calculation of the molecular electrostatic potential using PCILO wave functions revealed an area of the interaction with sodium which differed enormously with earlier papers. From these potential calculations a slightly dominant attractive interaction was supposed with respect to the location of the ion above the final atom of the double bond. The result was confirmed by extensive ab initio calculations using a STO-3G basis set, the area in the molecular plane and above the double bond being investigated especially. The influence of optimizing the bond length of the double bond is discussed.
Introduction Porous solids like zeolites (molecular sieves) play an important role in many industrial processes, especially in the chemical industry, where they are mainly used as catalysts or as adsorbents. Exchangeable metal cations may act as adsorption centers for hydrocarbons if they are able to undergo a specific interaction with metal cations, as with olefins or aromatics. This follows from thermodynamic from the results Of infrared measurementa,, and also from an analysis of proton4* and car‘Sektion Physik.
* Slchsische Akademie der Wissenschaften, DDR-7010 Leipzig,
German Democratic Republic. 5 ORZ.
0022-3654/82/2086-3269$01.25/0
bon-13 NMR measurement^.^-^ Especially it has been shown that butene and isobutene molecules in Na(X,Y) type zeolites interact predominantly with the exchangeable Na+ ions. (1)D. W.Breck, “Zeolite Molecular Sieves”,Wiley-Interscience,New York. 1972. (2) R. M. Barer, ‘Zeolites and Clay Minerals as Sorbents and Molecular Academic 1978. (3)A. V. Kiselev and V. I. Lygin, “IR Spectra of Surface Compounds and Adsorbed Species”, M ~1972.~ ~ ~ ~ , (4)H. Pfeifer, “NMR-Basic Principles and Progress”, Vol. 7,Berlin, 1972,pp 53-153. (5)H.Pfeifer, Phys. Rep., 26, 293 (1976). (6)J. Kiirger and D. Michel, Z . Phys. Chem. (Leipzig),257,983(1976). (7)D. Michel, Surf. Sci., 42,453 (1974). (8)D. Michel, W. Meiler, and H. Pfeifer, J.Mol. Catal., 1, 85 (1975). (9)H.Pfeifer, W.Meiler, and D. Deininger, Bull. Magn. Reson., submitted for publication.
0 1982 American Chemical Society
