754
J. Phys. Chem. 1983, 87,754-761
makes it clear that the mass-velocity term is responsible for both the relativistic decrease in re and the increase in De, a fact first discovered by Ziegler, Snijders, and Baerends14 in their relativistic X a calculations utilizing the operator described in the Introduction, and confirmed by Snijders and Pykko15 via single-center expansion DHF calculations. As the molecule forms the bond, the kinetic energy initially decreases, but near equilibrium it is significantly larger than in the atoms, and the mass-velocity term is therefore stabilizing. The one-electron Darwin term is of opposite sign but smaller in magnitude, and so the relativistic decrease of the kinetic energy dominates the correction. The present work thus provides yet more confirmation for the view espoused by Ziegler et al. that the bond contraction is due to the p4 term, and not (in this series) dominated by an orbital contraction effect. (14)T.Ziegler, J. G. Snijders, and E. J. Baerends, Chen. Phys. Lett., 75,1 (1980). (15)J. G. Snijders and P. Pyykko, Chem. Phys. Lett., 75,5 (1980).
The results presented here suggest that the RPTl approach can provide a reasonably accurate, yet economical, alternative to DHF calculations. The fact that good agreement was obtained with the DHF results without the inclusion of the spin-orbit operator is probably related to the dominance of the H( ls)-Ag(5s) bonding interaction; those bonding situations where p or d shells of the heavy atom play a role will no doubt require the introduction of the spin-orbit operator as a perturbation. In order to refine the present approach for -the first two transition series, or to extend the method to the third row, one must find a way around the p4divergence. Contracting the inner shells does not prevent the collapse; it just makes it somewhat less dramatic. At present, it appears that the simple expedient utilized by Cowan and Griffin in their atomic work, i.e., forcing the wavefunction to behave properly near the nuclei, may be the simplest and most easily implemented approach. Registry No. Silver, 7440-22-4; silver hydride (AgH), 1396701-6.
Structures of Supported Vanadium Oxide Catalysts. 1. V,O,/TiO, (Rutile), and V,O,/TIO, (Mixture of Anatase with Rutile)
(Anatase), V,05/Ti0,
Makoto Inomata,+ Kenjl Marl,+ Aklra Mlyamoto, Toshlakl Ui, and Yulchl Murakaml Lbpafiment of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan (Received: February 23, 1982; In Final Form: September 9, 1982)
Structures of vanadium oxide supported on Ti02have been investigated by using the rectangular pulse technique coupled with various physicochemical measurements including X-ray diffraction and IR, ESR, and UV-visible spectroscopy. In situ IR spectra of adsorbed ammonia have also been measured to investigate the acid properties of the catalysts. It has been found that modification of the TiOzsupport-anatase, rutile, or a mixture of anatase with rutile-does not affect significantly the structure of the supported vanadium oxide catalyst. The V=O species or the (010) face of Vz05 has been selectively exposed on the surface irrespective of the kind of Ti02 support. When the Vz05 content increases to 5 mol %, the surface of Ti02is covered by 1-3 layers of Vz05 lamellae; however, the TiOz surface is not completely covered by V205but it is exposed to the catalyst surface. There are both Bronsted and Lewis acid sites on the Ti02surface, while only the Bronsted acid site is formed on the unsupported V205 catalyst. As the V205 content increases to 5 mol %, the amount of the Bronsted acid site increases and the amount of the Lewis acid site decreases. When the Vz05 content is 10 mol %, the TiOz surface is fully covered with 5-8 layers of V205 lamellae, and almost all of the catalyst surface (ca. 90%) is occupied by the (010) face of V205. When the V205content increases further, the layers of Vz05lamellae on TiOz become thicker, which results in a decreased fraction of the (010) face of V205on the catalyst surface. Correspondingly, there are only Bronsted acid sites on the V205/Ti02catalysts with the Vz05content greater than 10 mol %. According to the results of IR and UV-visible spectra of the catalysts, the coordination of oxygens around a vanadium ion in the supported catalysts is almost the same as that in the unsupported V205 catalyst; neither the wavenumber of the V=O stretching vibration nor the edge of the 02--V5+ charge-transfer band has shifted markedly by supporting Vz05on TiOz. On the basis of these results, structures for the V205/Ti02 catalysts have been determined. Introduction It is well-known that V205supported on TiOz is an active and selective catalyst for the oxidations of hydrocarbons.’ This catalyst has received attention for the reduction of nitric oxide with ammonia because of its high activity at low temperatures and its high resistance to poisoning by S0,.2-4 From the viewpoint of solid-state chemistry, attention has recently been given to the evolution of oxygen from V205/Ti02catalysts; this reaction for VZO5/TiOz (anatase) has been found to occur a t a much lower temPresent address: Kinu-ura Research Department, JGC Co., sunosaki-cho, Handa, Aichi 475, Japan.
perature than that for unsupported V205 or V2O5/TiOZ (rutile) catalysts.”’ As for the Vz05/Ti02(anatase), a (1)(a) D. J. Hucknall, “Selective Oxidation of Hydrocarbons”, Academic Press, New York, 1974; (b) M.S.Wainwright and N. R. Foster, Catal. Rev., 19, 211 (1979); (c) A. Bielanski and J. Haber, ibid., 19, 1 (1979);(d) R. Higgins and P. Hayden, “Catalysis”,Vol. 1, The Chemical Society, London, 1977,Chapter 5,p 168; (e) M. Blanchard, G. Louguet, J. Rivasseau, and J. C. Delgrance, Bull. SOC.Chem. Fr., 8,3071 (1972); (0 S. JivAs and S. T. Lundin, J. Appl. Chem. Biotechnol., 27,499(1977); (9) A. Anderson and S. T. Lundin, J. Catal., 65,9 (1980); (h) D. Kh. Sembaev. B. V. Suvorov. L. I. Saurambaeva. and Kh. T. Suleimanov. Kinet.Katal., 20,750(1979);(i) B.V. Suvorov, A. D. Kagarlistakii, D: Kh. Sembaev, and A. I. Loiko, Zbid., 19, 197 (1978);6) W.E.Slinkard and P. B. Degroot, J. Catal., 68,423 (1981).
0022-3654/83/2087-0754$01.50/00 1983 American Chemical Society
Structures of Supported V205 Catalysts
transformation of anatase into rutile has been observed to be associated with the evolution of oxygen. According to Vejw and Cowtine: the simultaneousreduction of Vz05 and transformation of anatase into rutile are topotactic reactions activated by the remarkable fit of the crystallographic patterns in contact at the V205-Ti02(anatase) interface. These findings stimulate investigations of the structures of Vz05supported on Ti02 with various modifications. Although valuable information has been obtained for the structure of supported vanadium oxide catalysts by using various physicochemical measurements,*1° further investigations are necessary in order to determine the structure of Vz05/Ti02catalysts. We have previously established the rectangular pulse technique which allows the determination of the number of surface V=O species and the number of layers of V205 lamellae on the s ~ p p o r t . ~ J ' -The ~ ~ purpose of this study was then to determine the structure of V205supported on TiOz with various modifications including anatase, rutile, and the mixture of anatase with rutile. This was done by using the rectangular pulse technique coupled with various physicochemical measurements including X-ray diffraction, IR, ESR, and UV-visible spectra. In situ IR spectra of adsorbed ammonia were also measured to investigate the acid properties of the catalysts.
Experimental Section Catalysts. Three kinds of Ti02, denoted by Ti02(a), TiOz(r), and TiO2(a--1), were used as supports. As described below, they consisted of anatase, rutile, and the mixture of anatase with rutile, respectively. Ti02(a)and Ti02(r)were prepared by hydrolysis of Ti(S04)2and TiC14, respectively, followed by calcination in air at 873 K for Ti02(a)and in O2 at 773 K for Ti02(r). Ti02(a-r) was prepared by calcination of TiOz (Nippon Aerosil) in O2at 773 K. The respective BET surface areas of TiOz(a), Ti02(r),and TiO,(a-r) were 48.2, 16.7, and 40.0 m2 g-'. Supported vanadium oxide catalyst was prepared by impregnation of the support with an oxalic acid solution of (2)(a) H. Matsuoka, H. Nakamura, T. Takeda, K. Ohsato, K. Mori, and Y. Ochiai, Symposium on Stack Gas Cleanup, ACS/CSJ Chemical Congress Preprints, Honolulu, 1979,p 541; (b) T. Shikada, K. Fujimoto, T. Kunugi, H. Tominaga, S. Kaneko, and Y. Kubo, Id.Eng. Chem. Prod. Res. Deu., 20,91 (1981);(c) S.Okazaki, M. Kumasaka, J. Yoshida, K. Kosaka, and K. Tanabe, ibid., 20, 301 (1981). (3)M. Inomata, A. Miyamoto, T. Ui, K. Kobayashi, and Y. Murakami, Ind. Eng. Chem. Prod. Res. Deu., 21, 424 (1982). (4)Y. Murakami, M. Inomata, A. Miyamoto, and K. Mori, Proc. Znt. Congr. Catal., 7th, 1344 (1980). (5)G. C. Bond, A. J. Sirkiny, and G. D. Parfitt, J. Catal., 57, 476 (1979). (6)D. J. Cole, C. F. Cullis, and D. J. Hucknall, J . Chem. SOC.,Faraday Trans. I, 72, 2185 (1976). (7)A. Vejux and P. Courtine, J . Solid State Chem., 23, 93 (1978). (8)(a) B.M. Fabuss, Actes Congr. Int. Catal., Znd, 2561 (1961);(b) F. Roozeboom, M. C. Mittelmeijer-Hazeleger, J. A. Moulijn, J. Medema, V. H. J. de Beer, and P. J. Gellings, J. Phys. Chem., 84,2783(1980);(c) S.Yoshida, T. Murakami, and K. Tarama, Bull. Inst. Chem.Res.,Kyoto Uniu., 51,195 (1973);(d) D. J. Cole, C. F. Cullis, and D. J. Hucknall, J. Chem. SOC.,Faraday Trans. I , 72, 2744 (1976); (e) H. Takahashi, M. Shiotani, H. Kobayashi, and J. Sohma, J . Catal., 14,134(1969);(0 M. Nakamura, K. Kawai, and Y. Fujiwara, ibid., 34, 345 (1974); (g) K. Tarama, S. Yoshida, S. Ishida, and H. Kakioka, Bull. Chem. SOC.Jpn., 41,2840 (1969); (h) J. Haber and J. Stoch, React. Kinet. Catal. Lett., 9,319 (1978);(i) K. Dyrek, E. Serwicka, and B. Grzybowska, ibid., 10, 93 (1979); (j) S. A. Surin, A. D. Shuklov, B. N. Shelimov, and V. B. Kazanskii, Kinet. Katal., 19,435 (1978). (9)M. Akimoto, M. Usami, and E. Echigoya, Bull. Chem. SOC.Jpn., 51,2195 (1978). (10)S.Yoshida, T. Iguchi, S. Ishida, and K. Tarama, Bull. Chem. SOC. Jpn., 45,376 (1972). (11)A. Miyamoto, Y.Yamazaki, M. Inomata, and Y. Murakami, J. Phys. Chem., 85,2366 (1981);Chem. Lett., 1355 (1978). (12)M. Inomata, A. Miyamoto, and Y. Murakami, J. Chem. SOC., Chem. Commun., 1009 (1979). (13)M.Inomata, A. Miyamoto, and Y. Murakami, J.Phys. Chem., 85, 2372 (1981).
The Journal of Physical Chemktty, Vol. 87, No.
1
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130
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Figure 1. X-ray diffraction diagrams of V,O,/TiO,(a) catalysts with various V205 content: (-) V,05; (-0) anatase.
ammonium metavanadate followed by calcination at 773 K in a stream of O2 for 3 h. V205/Ti02(a)(10 mol 70 V205),Vz05/TiOz(r)(10 mol % Vz05),and V205/Ti02(a-r) (10 mol 3'% VzO5) treated with an ammoniacal solution were prepared in a manner similar to that of Yoshida et al.;1° these catalysts are hereafter referred to as monolayer ~ata1ysts.l~ Characterizations of the Catalysts. X-ray diffraction diagrams of the catalysts were obtained with a Rigaku GF 2035 X-ray diffractometer with Cu target. IR spectra of catalysts were recorded on Jasco EDR-31 emissionless infrared diffuse reflectance spectrometer with KBr as a di1~ent.l~ UV-visible reflectance spectra of the catalysts were observed in 250-800-nm range by Jasco UNIDEC-505 spectrophotometer. ESR absorption measurements were made at X-band on JEOL ME 1X spectrometer at room temperature. Infrared measurements of ammonia adsorbed on the catalysts were carried out in situ on Jasco IR-G spectrometer. Before measurement, a disk of the catalyst was heated in situ under vacuum for 1 h at 673 K followed by the adsorption of ammonia at a room temperature for 30 min and subsequent evacuation at a room temperature for 30 min. The number of surface V=O species and the number of layers of V205 lamellae on support were determined by using the rectangular pulse technique described p r e v i ~ u s l y . ~ J ~The - ' ~ BET surface area of catalysts was determined by using a conventional flow-type apparatus with Nz as an adsorbate.
Results X-ray Diffraction. It was found that Ti02(a),TiOz(r), and TiOz(a-r) were composed of anatase, rutile, and mixture of anatase with rutile, respectively. Figure 1shows X-ray diffraction diagrams of VzO5/TiO2(a)catalysts. As shown, all of the diffraction peaks of V20,/Ti02(a) catalysts were assigned to either V205 or anatase: When the ~
~~
(14)Since it was difficult to filtrate the VzO6/TiOz(r) (10mol % ' VzOd treated with an ammoniacal solution, we could not precisely characterize the V,O,/TiO,(r) monolayer catalyst. However, some preliminary experiments suggested that the structure of the VzO6/TiOz(r) monolayer catalyst is not significantly different from that of the VzOS/TiOz(a)or V,06/Ti02(a-r) monolayer catalyst. (15)(a) M. Inomata, A. Miyamoto, and Y. Murakami, J. Catal., 62, 140 (1980); (b) Chem. Lett., 799 (1978).
756
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Inomata et al.
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TABLE I : Number of Surface V=O Species (L), S(,,,), and Number of Layers of V,O, Lamellae ( N ) for V , O , / T i O , ( a ) and V , O , / T i O , ( r ) with Various V,O, Content m ol
%
v,o,
1 2 5
f
10 25 50 monolayer
1 2 5 10 25 50 1oo=
L,
vmol g-'
S(,,,), m 2 g-'
56 120 184 135 60 65 126 77 109
118 105 91
60 22
SBET, m 2g-'
V ,O /"io,( a 1 6.9 14.7 22.6 16.6 7.4 7.9 15.4
47.2 45.2 26.3 22.8 10.3 11.4 32.1
V,O,/TiO,(r) 9.5 16.8 13.5 14.6
18.4 15.9 14.8 17.2 12.7 5.4
13.0 11.2 7.4 2.7
N exptl 1-2 1-2 2-3 5-8 30-40 50-60
calcd 2.2 2.1 3.2
8.3 39.6 59.8
1 1-2 1-2 3 -6 5-10 20-30 60-70
>50
1.6 2.2 5.0 10.6 26.2 63.8 503.8
Unsupported V,O,.
--
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ItA E NilIlEET
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Flgure 2. Infrared spectra of V205/Ti02(a)catalysts with various V205 content. The number in parentheses represents the V205 content of the catalyst.
Vz05 content was less than 5 mol % , only anatase peaks were observed. When the V205 content was more than 10 mol % ,the Vz05phase appeared in addition to the anatase phase. No peaks assignable to rutile or a compound of Vz05and TiOz were observed in the diffraction diagrams of the Vz05/TiOz(a)catalyst. This means that transformation of anatase into rutile does not take place under the condition of catalyst preparation used in this study. Similarly, the V205/Ti02(r)catalyst consisted of only VzO5 and rutile phases, while the Vz05/TiO(a-r) catalyst was composed of V2O5 and a mixture of anatase with rutile. I R Spectra of the Catalysts. Figure 2 shows infrared spectra of the VZO5/TiOz(a)catalysts with various VzO5 contents. As shown, an absorption band was observed at 1020 cm-l for the V205/TiOz(a)catalysts, which has been assigned to the V5+=0 stretching v i b r a t i ~ n . ~This J ~ absorption band became stronger with increasing Vz05content of the V2O5/TiOz(a)catalyst. No significant shift of the peak by the support was observed. Similarly, IR spectra of Vz05/TiOz(r)and Vz05/TiOz(a-r) were almost the same as those of the Vz05/TiOz(a)at any V205content. ESR Spectra of the Catalysts. Figure 3 shows ESR spectra of V205/TiOz(a),V205/TiOz(r),and VzO5/TiO2(a-r) with various VzO5 contents, which can be assigned to the V4+ i0n.~9~Hyperfine structures due to electron spin-nuclear spin coupling were observed in the spectra, though it was difficult to determine the hyperfine constants from the spectra. As the content of V205in the Vz05/TiOz(a)catalyst increased, the signal intensity increased slightly and the hyperfine structure gradually became diffuse. As shown for example in parts b and c of Figure 3, ESR spectra for the Vz05/Ti02(r)and Vz05/TiOz(a-r) catalysts were similar to those for the Vz05/Ti02(a)catalysts, while the signal intensity increased in the order, Vz05/TiOz(a)< Vz05/TiOz(a-r) < V205/ TiOz(r). The g values of the observed spectra were almost constant to within 1.971 f 0.003 and were independent of
the kind of Ti02 support. The amount of the V4+ ion as determined by double integration of the ESR signal was at maximum a few percent of vanadium ions supported. Figure 3d shows an ESR spectrum of the Vz05/TiOz(a)(10 mol % ) which has been reduced by treatment with a mixture of NO (1000 ppm) and NH, (1000 ppm) at 573 K for 80 h.3J5 A singlet broad spectrum was observed and its intensity was much stronger than that of untreated V,05/Ti02(a) (10 mol % VZO5). UV-Visible Reflectance Spectra of Catalysts. As shown in Figure 4,the Vz05/Ti02(a)catalysts exhibited absorption in the wavelength region below 500-600 nm. The absorption in the 500-600-nm region is assigned to a charge-transfer transition from 02-to V5+.16J7 As the V205 content in the Vz05/TiOz(a)catalyst increased, the absorption in the 500-600-nm region increased but the position of its red edge (ca. 600 nm) did not change significantly. Results similar to those for the V205/TiOz(a) catalysts were obtained for the V205/Ti02(r)and VzO5/ TiOz(a-r) catalysts. The absence of absorption bands for one-electron d-d transitions in 600-800-nm region17 indicates that the m o u n t of V4+ions is small for all catalysts shown. T h e Number of Surface V=O Species and the Number of Layers of Vz05Lamellae. The number of surface V=O species and the number of layers of Vz05lamellae for the VZO5/TiO2(a-r) catalyst have previously been determined.13 Figure 5 shows examples of concentration profiles of N2 produced by the reaction of the rectangular pulse of the NO and NH, mixture with the Vz05/TiOz(a)catalyst, i.e., reaction 1. Using the method described in the NO + NH, + V-0 N2 + HzO + V-OH (1) previous paper, we determined the amount of the initial sharp N2 signal from the concentration profiles of N2. Similar to the case for the Vz05/TiOz(a-r)catalyst,13the amount of the initial sharp Nz signal was constant and independent of the reaction temperature. From the constant value, the number of surface V=O species on the catalyst, L, was determined, and the results are shown in -+
(16) (a) C.K.Jorgensen, "AbsorptionSpectra and Chemical Bonding in Complexes", Pergamon Press, Oxford, 1962; (b) J. Demuynk and G. Kaufmann, Bull. SOC. Chem. Fr., 11, 3840 (1969); (c) A. M.Gritskov, V. A. Shveta, and V. B. Kazanskii, Kinet. Katal., 14, 1062 (1973). (17)I. P.Mukhlenov, V. N. Pak, I. V. Shvedova, and E. I. Dobkina, Kinet. Katai., 19, 259 (1978).
The Journal of phvslcel Chemlstty, Vol. 87, No. 5, 1983 757
Structures of Supported V205 Catalysts
ZXlOOC . h
x 1
c _
Flgure 3. ESR spectra of V20s/Ti0&a), V,06/TiO&r), and V,O,/TiO&a-r) with various V205 content: (a) V206/Ti0,(a), (b) V,O,/TiO&), (c) V2051Ti02(a-r), (d) V206/Ti02(a) (10 moi % V206)reduced by treatment with a mixture of NO (1000 ppm) and NH, (1000 ppm) at 573 K for 80 h. The number of parentheses represents the Vp05 content of the catalyst.
130
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Flgve 4. W-visible ~Rectancespectra of V205/no&) cetalysts wtth various V,05 content. The number in parentheses represents the V2O5 content of the catalyst.
Table I. Since the V=O species is located on the (010) face of V206crystal and the surface density of the V=O on the (010) face is known as 4.872 mi2,the specific area of the (010) face of V206,S(olo), can be calculated from L. This is also shown in Table I with the results of the BET specific surface area of the catalyst, SBmFigure 6 shows to SBmwhich indicates the fraction of the ratio of S(olo)
(010) face of V20s on the surface of the catalyst. When was equal to 0, indithe V206content was 0,S(olo)/SBm cating the surface of an uncovered Ti02 support. As the V206 content increased up to 5 mol % (Figure 6), the fraction of the (010)face of V205 increased almost linearly. This means that the V206spreads over the surface of Ti02 with increasing V206content. When the V206content was 5 or 10 mol 3'% ,the maximum fraction of the (010) face of V206 (ca. 90%) was attained. When the V206 content increased further, the fraction of the (010) face decreased to the value of unsupported V206(50%at 100 mol % V206 content) where various crystal faces are considered to be exposed in addition to the (010) face.l' It is evident from Figure 6 that the relationship between S(oloJS, and the Vz06content does not change markedly with the kind of Ti02 support. Figure 7 shows the dispersion of V2O5, D, which is defined as the ratio of the number of surface V E O species, L, divided by the number of V205in the catalyst. This is given by eq 2 on the basis of the content of V206,
D=L
Mv,06X
+ MqyoJlOO - X) X
(2)
X (in percent), the molecular weights of Vz06,Mv206,and Ti02, MTio2.As shown in Figure 7, the dispersion was 5 0 4 0 % when the V206content was low (1or 2 mol %). This means that almost half of the V205in the catalyst
758
The Journal of Physical Chemistry, Vol. 87, No. 5, 1983
Inomata et ai.
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