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YBa2Cu3O6 – A Selective Ammoxidation Catalyst
G. Centi and F. Trifiro' (Editors),New Developments in Selective Oxidation 0 1990 Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
275
YBa2Cu3Q - A SELEClWE AMMOXIDATION CATALYST J.C. OTAMIRII, A. ANDERSSON', S. HANSEN2 and J.-0. BOV& bepamnent of Chemical Technology,Chemical Center, University of Lund, P.O. Box 124, S-22100 Lund (Sweden) 2Department of Inorganic Chemistry 2, Chemical Center, University of Lund, P.O. Box 124, S-22100 Lund (Sweden) SUMMARY YBa2Cu30~,, OSxSl, was used as a catalyst for the oxidation of toluene in presence of oxygen and ammonia, The partial pressures of reactants were varied. It was observed that when x is above zero,the material is active for total combustion. The activity is highly dependent on the value of x. After reductive treatment of sample having x>O in reactant stream without molecular oxygen producing yBa2cu306, the partial pressure of oxygen was increased from low to high. At low oxygen pressure, the material was active and selective for formation of benzonitrile. A dramatic transition from selective to non-selective region was observed to occur at a distinct pressure, which according to X-ray diffraction analysis is due to incorporation of oxygen species into the lattice. Under selective condirions, Yh2Cu306 is more active for nitrile formation in comparison with vanadium oxide catalysts. Catalytic behaviours are discussed considering possible surface structures.
INTRODUCI'ION Earlier it was discovered that CuO can be used as a catalyst for oxidation of propene to acrolein. However, its usefulness is limited due to the difficulty of maintaining the surface coverage of oxygen in a suitable range. It was demonstrated that when clusters of more than 5 adjacent oxygens are present at the surface, total combustion is predominant. A hypothesis was advanced that for selective reaction to occur, it is necessary to have structurally isolated sites of appropriate metal-oxygen bond strength [l-31. Considering the recently discovered superconductormaterial YBa2Cuj07 [4], it is of potential interest for use in catalytic oxidation since it has structurally isolated Cu-layers containing mobile oxygen species, which can easily be abstracted [5]. When they are all removed, YBa2Cu306 is formed having characteristic layers of Cul+ [6], which hypothetically can adsorb oxygen giving Cu3+ and nucleophilic oxygen species. The latter are believed to be involved in selective oxidation and ammoxidation mechanisms [7-91. Therefore, in order to gain some insight into the catalytic behaviour of these new materials, they are in the present investigation used as catalysts for the amoxidation of toluene to produce benzonitrile, which is used e.g. in the synthesis of benzoguanamine [lo]. METHODS YBa~Cu306+~. with x equal to 1 and 0, Y2BaCuOg and SrnBa2Cu307 were prepared from stoichiometric mixtures of appropriate and pure, >99 %, chemicals of Y2O3, Sm2O3, BaC03 and CuO according to the procedure described elsewhere [111.
276
Catalytic activity investigations of prepared phases were carried out in a differential and isothermal plug flow reactor made of Pyrex glass and maintained at 400 OC. Reactants, oxygen, ammonia, and toluene, mixed with inert nitrogen, were introduced and controlled by HI-TEC mass flow controllers. Products, benzonimle, benzaldehyde, C02, and CO, were analyzed on a Varian Vista 6OOO gas chromatograph. Reactor and analysis setup has earlier been described in detail [9]. Freshly prepared YBa2Cu306+,, with x=l, and reduced sample, with x=O, were studied at a constant high and low partial pressure of oxygen, respectively. while varying the partial pressures of ammonia and toluene, and also at constant pressures of ammonia and toluene, while varying pressure of oxygen. Powder X-ray data were recorded at room temperature using a Guinier-Haggcamera with quartz monochromator, CuKal radiation and Si as internal standard. Accurate lattice constants were obtained by least-squares refinement. The smooth variation of lattice constants with lattice oxygen content observed for YBa2CU306+, phases [12] was used to estimate the value of x. RESULTS Catalysison YBa&&Q7 Figure 1 shows the dependency of rates over freshly charged YBa2Cu307 as partial pressure of oxygen (Po) is varied from high region towards low at constant pressures of ammonia (PA)and toluene (PT). The rates show partial order dependency, however, it is worthy to note that the xates hardly increase at high pressures of oxygen. At zero pressure of oxygen the rate for nitrile formation decreases slowly with time, whereas other rates rapidly go to zero. The x-value is dependent on the partial pressure of oxygen. After use at PO = 17.30 H a , the composition is YBa2Cu306.4. When the partial pressure of oxygen is set to zero, the x-value gradually approaches zero.
4'o
I
___ 0
10
20
30
v,
v
Fig. 1. Rates for formation of nimle 0 , aldehyde 0 , C02 and CO over YBa~Cu306+,,x>O, as a function of partial pressure of oxygen. PA = 2.58 kPa and PT = 0.77 kPa.
277
In Fig. 2 is given the dependency of rates on partial pressure of ammonia with pressures of oxygen (high) and toluene maintained constant. The rates for formation of benzaldehyde, C02, and CO decline with increase in pressure of ammonia while that of nitrile increases and remains constant at higher pressures.
2.0
0
4.0
a0
8.0
Fh3 (kPa)
Fig. 2. Rates for formation of products over Y B a 2 C ~ 3 0 6 +x>o, ~ , versus partial pressure of ammonia. PO = 17.30 kPa and PT = 0.77 kPa. Notations: cf. Fig. 1. The variation of rates with partial pressure of toluene, Fig. 3, shows also partial order dependency. In this figure, it could be seen, that the dependencies for benzaldehyde and C02 are strong, whereas for nimle and CO, the rates are virtually constant at high pressures of toluene.
"
-
0.5
n
n
Y
0
@
0
1.0
1.5
20
2.5
F O L (kPd
Fig. 3. Influence of partial pressure of toluene on rates for formation of products over Y B ~ ~ C U ~ O ~ + ~ , x>O. Po = 17.30 kPa and PA = 2.58 H a . Notations cf. Fig. 1.
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Reductive treatment Fresh samples, YBa2Cu307, SmBa2Cu307 and YzBaCuOg which is a wellknown contaminant in superconductor materials [13] were heated to reaction temperature (400 OC) in presence of oxygen. Reduction of samples was canied out by performing the experiments in the absence of molecular oxygen for one hour at fixed conditions of temperature (400 OC) and pressures of toluene (0.77 H a ) and ammonia (2.58 kPa). Then, the pressure of oxygen was increased to the level of selective conditions for nimle formation, and activities were measured as a function of time. The results are given in Table 1. For comparison data are also included for a sample freshly prepared as YBa2Cu306 and heated to reaction temperature in nitrogen. TABU 1 Reaction ratesa at 400 as a function of the-on-stream for various samples after reductive treatment. Sample
Rate x I@ (moles m-2 min-1)
Ti
Niaiie
CO,
co
YBa2Cu306
1.97 2.05 2.14
0.36 0.27 0.23
0.02 0.02 0.01
10 25 40
YBa2cu3@
1.65 1.61 1.59
0.33 0.32 0.32
0.02 0.02 0.02
10 25 40
smBaZcu3%
1.77 2.08 2.10
0.33 0.34 0.33
0.02 0.03 0.03
10 25 40
Y2BaCuOg
0.27 0.42 0.5 1
0.89 0.63 0.52
0.03 0.03 0.03
10 25
~~
40
aPo = 2.16 kPa, PA = 2.58 Wa, and P, = 0.77 kPa. From the table it could be observed, that the behaviour of reduced YBa2Cu307, and SITIB~~CU~O, is similar to that of YBa2Cu306, which is active and selective for toluene ammoxidation under the conditions used in the experiments. The Y2BaCuOg compound is found to be less active and less selective. Catalvsis on Y B a D & Reduced YBa2Cu307 sample, with a composition close to YBazCug06, was then used for experiments in which the partial pressures of reactants were varied. The results are given in Figs. 4-6. In series where the partial pressure of ammonia or toluene was varied, the partial pressure of oxygen was kept low.
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In Fig. 4 are the rates obtained when the partial pressure of oxygen was varied gradually from low region towards high. This figure shows some features worth noting: i) There is a clear region of sharp transition in selectivity, ii) At low partial pressure of oxygen, the catalyst is selective for nitrile formation, iii) At higher pressures, the activity towards total combustion dramatically increases and is about ten times higher than before reduction, and iv) The passing of the rate for CO formation through a maximum. 40 r
Fig. 4. Effect of partial pressure of oxygen on rates for formation of products over YBa2Cu306x, x = 0. PA = 2.58 kPa and PT = 0.77 kPa. Notations: cf. Fig. 1.
..
h H g @pa)
Fig. 5. Rates for formation of products on YBa2Cu30bx, x = 0, versus partial pressure of ammonia. PO = 2.16 kPa and PT = 0.77 Ha. Notations: cf. Fig. 1.
280
Figure 5 (above) shows how the rates vary as the partial pressure of ammonia is varied from high region towards low at fixed partial pressures of oxygen and toluene. The rate for nitrile formation passes through a maximum and is higher than before reduction, cf. Fig. 2. Benzaldehydeis formed at low partial pressure of ammonia but not in its absence. A sharp increase in the rate of C02 formation occurs as low partial pressures are approached.The rate for CO formation also increases but declines at zero pressure of ammonia. The dependency of rates on partial pressure of toluene is given in Fig. 6. There is an almost first order dependency of rates on pressure of toluene. Comparison with Fig. 3 shows that the rates for formation of nitrile and C02 have reversed places. The rate for formation of C02 before reduction was higher than after reduction, whereas for nitrile formation the opposite is the case. Another feature is the fact that after reduction aldehyde is not formed when the partial pressure of oxygen is maintained low.
Fig. 6. Influence of partial pressure of toluene on rates for formation of products over YBa2Cu306tx, x = 0. PO = 2.16 P a and PA = 2.58 P a . Notations: cf. Fig. 1.
In Fig. 7 are reaction rates plotted as a function of reaction time for a YBa2CU306 sample which before use had been stored under ambient conditions for 10 days. Initially, the material though active was non-selective. After use for few hours, the rate for C02 formation dropped to a very low value, while that for nitrile formation increased more than twice. This behaviour was always observed when using YBa~Cu306samples which had been stored in an air atmosphere for several days. It is probably due to removal of some oxygen species which have been incorporated into the lattice during storage.
281
Time on stream
(mln)
Fig. 7. Reaction rates over YBa2Cu306 as a function of time. PO = 2.16 kPa, PA = 2.58 kPa, and PT = 0.77 kPa. Notations: cf Fig. 1.
TABLE 2 Lattice constants (A) and oxygen content (x) of catalysts. Fmh sample
orthorhombic a=3.8203(8) b=3.8853(7) ~=11.679(2) x= 1
tetragonal a=b=3.8582(2) ~=11.830(1) X=O
At high Po, before reductive eeatmentg
tetragonal a=b=3.858l(5) c=l1.764(1) x=0.4
At IOW Po, after reductive treatmentb
tetragonal tetragonal a=b=3.8572(4) a=b=3.8569(2) ~=11.830(1) x=o
At high Po, after reducuve treatmen@ ~~
~=11.834(1) X=O
orthorhombic a=3.855(1) b=3.892(1) c=l1.712(2)
orthorhombic a=12.177(2) b=5.6571(9) c=7.130(1)
tetragonal a=b=3.8841(5)
orthorhombic a=12.172(1) b=5.6590(5) c=7.1294(7)
x=l
c=l1.829(2)
x=o
tetragonal tetragonal a=b=3.8572(4) a=b=3.857l(4) c=l 1.818(2) ~=11.819(2) x=o
X=O ~~
aP0 = 17.30 kPa, PA = 2.58 kPa, and PT = 0.77 kPa bPo = 2.16 kPa, PA = 2.58 kPa, and PT = 0.77 kPa.
~
-~
282
Lattice constants determined by X-ray diffraction are given in Table 2 for various samples. Also included are x-values as estimated using the published relationship between lattice constants and oxygen content of YBa2C~306+~ phases [12]. The composition of Sm-substituted samples was estimated by comparing cell parameters and catalytic activity with corresponding values for Y B ~ ~ C U ~phases. O , ~ +From ~ the table, it can be concluded that the x-value of used YBa2CugOhX sample not being subjected to reductive treatment is well above zero.After reductive treatment and further use at low and high partial pressure of oxygen, respectively, the oxygen content of catalysts is close to 6 oxygen atomdunit cell. However, the c axis repeat of catalysts used at high oxygen pressure, non-selective conditions. was always found to be slightly shorter than that measured after use at low oxygen pressure, selective conditions. This implies that the x-value for catalysts run under non-selective conditions is slightly above zero.The lattice constants determined for Y2BaCuOg are identical for freshly prepared and used samples, and they also agree with those reported in the original structure determination [141. DISCUSSION A drawing of the YBa2Cu307 structure is shown in Fig. 8. There are two structurallydifferent Cu positions, noted Cu(1) and Cu(2) [15]. The formers are connected via 0(4), thus, forming chains in the [OlO]direction between Ba-layers. Cu(1)-chains are connected to Cu(2)-layers by O(1). In the Cu-layers, Cu(2) is coordinated to five oxygen species, 2 x 0(2), 2 x O(3) and 1 x O(1). It has been shown that there are no distinct Cu2+ and Cu3+ sites. The valence of Cu in both sites is intermediate between +2 and +3 [6].Oxygen O(4) in the chains have been found to be mobile and can be totally abstracted [5].When this occurs, the structure changes from orthorhombic YBa2Cu307 to tetragonal YBa2Cu306. The latter structure can simply be derived from the former by removal of O(4) so that the coordination of Cu(1) is changed from square. planar to linear twofold [16]. As aresult, distinct Cu*+ at Cu(1) sites, and Cu2+ at Cu(2) sites are formed [6].
Fig. 8. Drawing of the YBa2Cu307 structure.
283
From the fact that the main difference between the structures of the orthorhombic and tetragonal phases is connected to the coordination of Cu( l), it follows that it is reasonable to compare their catalytic behaviom in terms of possible surface coordinationsof Cu( 1). At the surface of YBa~Cu307.undercoordinatedCu(1) and Cu(2) can exist, serving as possible adsorption sites for toluene and ammonia. The number of undercooniinated species depends on the partial pressure of oxygen. Molecular oxygen can adsorb in the form of diatomic species. As a consecutive step, when dissociation is possible, monoatomic oxygen species can also be formed. However, dissociation is probably not facile due to lack of oxygen vacancies in the bulk. A common feature of oxygen species pmjecting from the surface is that they are undercoordinated,which renders them electrophilic in character. It has been established that electrophilic oxygen participates in the degradation of hydrocarbons leading to total combustion [7,9,17]. Indeed, YBa2Cu306tx. with x well above zero, was found to be non-selective in catalytic (ammhxidation, cf. Figs. 1-3. In YBa2Cu306, Cu(1) is two-coordinated due to that O(4) positions are vacant. After adsorption of molecular oxygen, two options are possible depending on the partial pressure of oxygen. At low pressure of oxygen, adsorbed diatomic oxygen can react with co-adsorbed ammonia to give water under simultaneous oxidation of low valent Cu( 1) to Cu3+ and formation of nucleophilic Cu=Oand Cu=NH species. Substantial evidence exist for nucleophilic oxygen species and imido species to be involved in selective oxidation and ammoxidation mechanisms, respectively [7-9], which is vexified by the present investigation. Figures 4-6 show that YBa2Cu306 is selective for nitrile formation at low partial pressure of oxygen. Furthermore, the finding that the rate for formation of benzaldehyde is zero in absence of ammonia, and passes through a maximum as the partial pressure of ammonia is increased suggests that co-adsorption of ammonia is a prerequisite for formation of nucleophilic oxygen species. On the contrary, when the partial pressure of oxygen is high, the catalyst is nonselective, cf. Fig. 4. This can be seen as a result of the facile dissociation of adsorbed diatomic oxygen at YBa2Cu306 One of the oxygen species can migrate into a neighbowing oxygen vacancy situated between two Cu(1) sites. Consequently, the remaining monoatomic surface species will have electrophilic character due to that Cu has to share its availablevalence electrons between both oxygen species. The rate for formation of CO2 over YBa2Cu306tx at high pressure of oxygen depends on the value of x. When the value is small, the rate is much higher compared to when x is high, cf. Figs. 1 and 4. Several explanations are possible for this behaviour, of which a few will be mentioned briefly. One is that the electronic properties of surfaces must be influenced by the occupancy frequency of exterior O(4) positions, cf. Fig. 7, consequently affecting adsorption and reactivity properties. Another factor of importance is that the number of active sites increase when the value of x decrease. In case of YBa2Cu307, if extending the bulk structure to the surface, Cu(1) at (100) faces cannot adsorb pmjecting single coordinated oxygen species. When the composition is close to YBa2Cu306, such an adsorption is possible producing electrophilic oxygen species on the condition that neighbowing O(4) positions are only partly filled.
284
At low pressure of oxygen, 2.5-5 kPa, the rates for formation of nitrile and C02 over yBa2cu306 at 400 % are 16-19 and 2-4 pnole m2min-l, respectively. Over V205, under the same conditions. the corresponding rates are 2-4 and 0.3-0.5 pmole m-2 min-1, respectively [18,19]. In conclusion, it has been shown that YBa2Cu306 is an active and selective catalyst for ammoxidation of toluene at low partial pressures of oxygen. ACKNOWLEDGMENT Financial support from the National Swedish Board for Technical Development (STU) and the Swedish Natural Science Research Council (NFR) is gratefully acknowledged. REFERENCES 1 2
3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18 19
J.L. Callahan and R.K. Grasselli, AIChE J., 9 (1963) 755. R.K. Grasselli and J.D. Burrington, in D.D. Eley, H.Pines and P.B. Weisz (Eds.), Advances in Caralysis, Vol. 30, Academic Press, New York, 1981, pp. 133-163. F. Cavani, G.Centi, F. T n f m and R.K. Grasselli, Catal. Today, 3 (1988) 185. M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q Wang and C.W. Chu, Phys. Rev.Lea., 58 (1987) 908-911. A. Manthiram, J.S. Swinnea, Z.T. Sui, H. Steinfink and J.B. Goodenough, J. Am. Chem. SOC.,109 (1987) 6667-6669. M.OKeeffe and S. Hansen, J. Am. Chem. SOC., 110 (1988) 1506-1510. J. Haber, in J.P. Bonnelle, B. Delmon and E. Derouane (Eds.), Surface Properries and Catalysis by Non-Merals, Reidel, Dodrecht, 1983, Ch. 1, pp. 1-45. R.K. Grasselli, J.F. Brazdil, and J.D. Burrington, Proc. 8th Int. Congr. Catalysis, Berlin(West), July 2-6, 1984, Verlag Chemie, Weinheim, 1984, Vol. V, pp. 369-380. A. Andersson and S. Hansen, J. Catal., 114 (1988) 332-346. Kirk-Other, Encyclopedia of Chemical Technology, 3rd edn., Vol. 15, Wiley, New York, 1981, p. 906. S. Hansen, J. Otamiri, J.-0. Bovin and A. Andersson, Nature, 334 (1988) 143-145. C.N.R. Rao, J. Solid Stare Chem., 74 (1988) 147-162. H. Steinfink, J.S. Swinnea, Z.T. Sui, H.M. Hsu and J.B. Goodenough, J. Am. Chem. SOC., 109 (1987) 3348-3353. C. Michel and B. Raveau, J. Solid State Chem., 43 (1982) 73-80. F. Beech, S. Miraglia, A. Santoro and R.S. Roth,Phys. Rev.,B35 (1987) 8778-8781. J.S. Swinnea and H. Steinfink, J. Marer. Res., 2 (1987) 424-426. A.M. Gasymov, V.A. Shvets and V.B. Kazansky, Kinet. Karal., 23 (1982) 951-954. J.C. Otamiri and A. Andersson, Catal. Today, 3 (1988) 211-222. J.C. Otamiri and A. Andersson, Card. Today, 3 (1988) 223-234.
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B. DELMON (Univ. Catholique de Louvain, Belgium): Due to the fact that catalyst surfaces are usually reduced in their steady state during catalytic oxidation it might seem doubtful that copper remains in the Cult o r Cus+ oxidation state, with no Cuo and, consequently, Cu crystallites being formed. The absence of new lines in X-ray diffraction cannot be a fully convincing proof since small crystallites might not be detectable. Did you find a change in the intensity ratio of Cu/Y or Cu/Ba XPS lines, or changeso of ISS signals after use of the catalyst? If really no Cu were formed, this would indicate a really exceptional strength of the chemical bonds involving Cu. In ammoxidation, ammonia is a very strong reducing agent, even in the presence of 0,. If this is so, this could give a clue to the very special electronic structure of superconductors. A. ANDERSSON (University of Lund, Sweden): For both freshly prepared samples and used samples only X-ray diffraction lines belonging to YBa,Cu,O,+x could be detected. Use in catalytic reaction did not cause any change in the intensity of the X-ray lines that cannot be explained as due to change in oxygen content. Also, XPS analysis did not show formation of Cuo. However, the ratio of Cu/Y and Cu/Ba XPS lines showed some dependence on reaction conditions. In this regard, it should be noted that YBa2Cu306+xfaces can expose both Cu-, Y-, and Ba-layers and that their distribution possibly depends on the composition of the reactant stream. O.V. KRYLOV (Acad. of Sciences, MOSCOW, USSR): In connection with an interesting observation of Dr. Andersson and his collaborators I should like to comment about many similarities between high temperature semiconductors and oxide catalysts of partial oxidation. Both of them have oxygen-deficient lattice. In the case of high temperature semiconductors, oxygen vacancies in the lattice must be stable and only motion of electron pairs must be observed. On the contrary, in oxide catalysts of partial oxidation such vacancies must move. It is very possible now to search new high temperature semiconductors from oxidative catalysis.
A. ANDERSSON: Thank you for your comment, we believe that such an approach may yield fruitful results. M. MISONO (The University of Tokyo, Japan): Very interesting results. I would like to know more about the chemical reactivity and the composition of the surface of YBa,Cu,O,+x. Is it stable at high temperatures against CO,, H,O, etc.? Is the surface composition the same as in the bulk? Segregation of certain elements (Ba, etc.) has often been indicated in the reported papers of electric conductivity. A. ANDERSSON: Our XPS results, that will be published elsewhere, clearly show the existence of carbonate species both in freshly prepared samples and in used samples. In fresh samples, the amount is highly dependent on the preparation method used. After use in catalytic reaction, only a minor variation of the amount of carbonate species in comparison with fresh samples was observed. Examination of the catalyst before and after use in the reactor by high-resolution transmission electron microscopy,
286
r e v e a l s a n i n c r e a s e i n t h e number o f c r y s t a l s t r u c t u r e d e f e c t s on t h e ( 0 0 1 ) p l a n e ( r e f . 1 ) . Such d e f e c t s a r e p o s s i b l y formed under t h e i n f l u e n c e o f H 0 ( r e f . 2 ) . However, once a s t e a d y s t a t e h a s been r e a c h e d , no change w i t h t i m e i n t h e f o r m a t i o n o f p r o d u c t s was detected f o r t h e p e r i o d it was used, which was up t o 3 d a y s . 1 2
S . Hansen, J . O t a m i r i , J.-0. Bovin and A . 3 3 4 (1988) 1 4 3 . B . G . Hyde e t a l . , N a t u r e , 327 (1987) 4 0 2 .
Andersson, N a t u r e ,
PAJONK (Univ. Claude B e r n a r d Lyon I , F r a n c e ) : I would l i k e t o know i f your c a t a l y s t i s s t a b l e w i t h t i m e on s t r e a m . Due t o t h e m o b i l i t y of oxygen i n s i d e t h e s t r u c t u r e o f your h i g h Tc s u p e r c o n d u c t o r , why d i d you n o t t r y t o o x i d i z e , e . g . , p r o p y l e n e which c o u l d have been l e s s complex t o i n t e r p r e t w i t h r e s p e c t t o t h e r e a c t i o n mechanism?
G.M.
A . ANDERSSON: R e f e r r i n g t o t h e a n s w e r s g i v e n t o p r o f e s s o r s Delmon
and Misono, some s t r u c t u r a l changes were o b s e r v e d a s a r e s u l t of c a t a l y t i c r e a c t i o n . Once a s t e a d y s t a t e was r e a c h e d , t h e p e r f o r m a n c e o f t h e c a t a l y s t was s t a b l e a s l o n g a s i t was u s e d ( u p t o 3 days). I n comparison w i t h t o l u e n e o x i d a t i o n , w e do n o t t h i n k t h a t t h e mechanism o f p r o p y l e n e o x i d a t i o n i s less complex. the
275
YBa2Cu3Q - A SELEClWE AMMOXIDATION CATALYST J.C. OTAMIRII, A. ANDERSSON', S. HANSEN2 and J.-0. BOV& bepamnent of Chemical Technology,Chemical Center, University of Lund, P.O. Box 124, S-22100 Lund (Sweden) 2Department of Inorganic Chemistry 2, Chemical Center, University of Lund, P.O. Box 124, S-22100 Lund (Sweden) SUMMARY YBa2Cu30~,, OSxSl, was used as a catalyst for the oxidation of toluene in presence of oxygen and ammonia, The partial pressures of reactants were varied. It was observed that when x is above zero,the material is active for total combustion. The activity is highly dependent on the value of x. After reductive treatment of sample having x>O in reactant stream without molecular oxygen producing yBa2cu306, the partial pressure of oxygen was increased from low to high. At low oxygen pressure, the material was active and selective for formation of benzonitrile. A dramatic transition from selective to non-selective region was observed to occur at a distinct pressure, which according to X-ray diffraction analysis is due to incorporation of oxygen species into the lattice. Under selective condirions, Yh2Cu306 is more active for nitrile formation in comparison with vanadium oxide catalysts. Catalytic behaviours are discussed considering possible surface structures.
INTRODUCI'ION Earlier it was discovered that CuO can be used as a catalyst for oxidation of propene to acrolein. However, its usefulness is limited due to the difficulty of maintaining the surface coverage of oxygen in a suitable range. It was demonstrated that when clusters of more than 5 adjacent oxygens are present at the surface, total combustion is predominant. A hypothesis was advanced that for selective reaction to occur, it is necessary to have structurally isolated sites of appropriate metal-oxygen bond strength [l-31. Considering the recently discovered superconductormaterial YBa2Cuj07 [4], it is of potential interest for use in catalytic oxidation since it has structurally isolated Cu-layers containing mobile oxygen species, which can easily be abstracted [5]. When they are all removed, YBa2Cu306 is formed having characteristic layers of Cul+ [6], which hypothetically can adsorb oxygen giving Cu3+ and nucleophilic oxygen species. The latter are believed to be involved in selective oxidation and ammoxidation mechanisms [7-91. Therefore, in order to gain some insight into the catalytic behaviour of these new materials, they are in the present investigation used as catalysts for the amoxidation of toluene to produce benzonitrile, which is used e.g. in the synthesis of benzoguanamine [lo]. METHODS YBa~Cu306+~. with x equal to 1 and 0, Y2BaCuOg and SrnBa2Cu307 were prepared from stoichiometric mixtures of appropriate and pure, >99 %, chemicals of Y2O3, Sm2O3, BaC03 and CuO according to the procedure described elsewhere [111.
276
Catalytic activity investigations of prepared phases were carried out in a differential and isothermal plug flow reactor made of Pyrex glass and maintained at 400 OC. Reactants, oxygen, ammonia, and toluene, mixed with inert nitrogen, were introduced and controlled by HI-TEC mass flow controllers. Products, benzonimle, benzaldehyde, C02, and CO, were analyzed on a Varian Vista 6OOO gas chromatograph. Reactor and analysis setup has earlier been described in detail [9]. Freshly prepared YBa2Cu306+,, with x=l, and reduced sample, with x=O, were studied at a constant high and low partial pressure of oxygen, respectively. while varying the partial pressures of ammonia and toluene, and also at constant pressures of ammonia and toluene, while varying pressure of oxygen. Powder X-ray data were recorded at room temperature using a Guinier-Haggcamera with quartz monochromator, CuKal radiation and Si as internal standard. Accurate lattice constants were obtained by least-squares refinement. The smooth variation of lattice constants with lattice oxygen content observed for YBa2CU306+, phases [12] was used to estimate the value of x. RESULTS Catalysison YBa&&Q7 Figure 1 shows the dependency of rates over freshly charged YBa2Cu307 as partial pressure of oxygen (Po) is varied from high region towards low at constant pressures of ammonia (PA)and toluene (PT). The rates show partial order dependency, however, it is worthy to note that the xates hardly increase at high pressures of oxygen. At zero pressure of oxygen the rate for nitrile formation decreases slowly with time, whereas other rates rapidly go to zero. The x-value is dependent on the partial pressure of oxygen. After use at PO = 17.30 H a , the composition is YBa2Cu306.4. When the partial pressure of oxygen is set to zero, the x-value gradually approaches zero.
4'o
I
___ 0
10
20
30
v,
v
Fig. 1. Rates for formation of nimle 0 , aldehyde 0 , C02 and CO over YBa~Cu306+,,x>O, as a function of partial pressure of oxygen. PA = 2.58 kPa and PT = 0.77 kPa.
277
In Fig. 2 is given the dependency of rates on partial pressure of ammonia with pressures of oxygen (high) and toluene maintained constant. The rates for formation of benzaldehyde, C02, and CO decline with increase in pressure of ammonia while that of nitrile increases and remains constant at higher pressures.
2.0
0
4.0
a0
8.0
Fh3 (kPa)
Fig. 2. Rates for formation of products over Y B a 2 C ~ 3 0 6 +x>o, ~ , versus partial pressure of ammonia. PO = 17.30 kPa and PT = 0.77 kPa. Notations: cf. Fig. 1. The variation of rates with partial pressure of toluene, Fig. 3, shows also partial order dependency. In this figure, it could be seen, that the dependencies for benzaldehyde and C02 are strong, whereas for nimle and CO, the rates are virtually constant at high pressures of toluene.
"
-
0.5
n
n
Y
0
@
0
1.0
1.5
20
2.5
F O L (kPd
Fig. 3. Influence of partial pressure of toluene on rates for formation of products over Y B ~ ~ C U ~ O ~ + ~ , x>O. Po = 17.30 kPa and PA = 2.58 H a . Notations cf. Fig. 1.
278
Reductive treatment Fresh samples, YBa2Cu307, SmBa2Cu307 and YzBaCuOg which is a wellknown contaminant in superconductor materials [13] were heated to reaction temperature (400 OC) in presence of oxygen. Reduction of samples was canied out by performing the experiments in the absence of molecular oxygen for one hour at fixed conditions of temperature (400 OC) and pressures of toluene (0.77 H a ) and ammonia (2.58 kPa). Then, the pressure of oxygen was increased to the level of selective conditions for nimle formation, and activities were measured as a function of time. The results are given in Table 1. For comparison data are also included for a sample freshly prepared as YBa2Cu306 and heated to reaction temperature in nitrogen. TABU 1 Reaction ratesa at 400 as a function of the-on-stream for various samples after reductive treatment. Sample
Rate x I@ (moles m-2 min-1)
Ti
Niaiie
CO,
co
YBa2Cu306
1.97 2.05 2.14
0.36 0.27 0.23
0.02 0.02 0.01
10 25 40
YBa2cu3@
1.65 1.61 1.59
0.33 0.32 0.32
0.02 0.02 0.02
10 25 40
smBaZcu3%
1.77 2.08 2.10
0.33 0.34 0.33
0.02 0.03 0.03
10 25 40
Y2BaCuOg
0.27 0.42 0.5 1
0.89 0.63 0.52
0.03 0.03 0.03
10 25
~~
40
aPo = 2.16 kPa, PA = 2.58 Wa, and P, = 0.77 kPa. From the table it could be observed, that the behaviour of reduced YBa2Cu307, and SITIB~~CU~O, is similar to that of YBa2Cu306, which is active and selective for toluene ammoxidation under the conditions used in the experiments. The Y2BaCuOg compound is found to be less active and less selective. Catalvsis on Y B a D & Reduced YBa2Cu307 sample, with a composition close to YBazCug06, was then used for experiments in which the partial pressures of reactants were varied. The results are given in Figs. 4-6. In series where the partial pressure of ammonia or toluene was varied, the partial pressure of oxygen was kept low.
279
In Fig. 4 are the rates obtained when the partial pressure of oxygen was varied gradually from low region towards high. This figure shows some features worth noting: i) There is a clear region of sharp transition in selectivity, ii) At low partial pressure of oxygen, the catalyst is selective for nitrile formation, iii) At higher pressures, the activity towards total combustion dramatically increases and is about ten times higher than before reduction, and iv) The passing of the rate for CO formation through a maximum. 40 r
Fig. 4. Effect of partial pressure of oxygen on rates for formation of products over YBa2Cu306x, x = 0. PA = 2.58 kPa and PT = 0.77 kPa. Notations: cf. Fig. 1.
..
h H g @pa)
Fig. 5. Rates for formation of products on YBa2Cu30bx, x = 0, versus partial pressure of ammonia. PO = 2.16 kPa and PT = 0.77 Ha. Notations: cf. Fig. 1.
280
Figure 5 (above) shows how the rates vary as the partial pressure of ammonia is varied from high region towards low at fixed partial pressures of oxygen and toluene. The rate for nitrile formation passes through a maximum and is higher than before reduction, cf. Fig. 2. Benzaldehydeis formed at low partial pressure of ammonia but not in its absence. A sharp increase in the rate of C02 formation occurs as low partial pressures are approached.The rate for CO formation also increases but declines at zero pressure of ammonia. The dependency of rates on partial pressure of toluene is given in Fig. 6. There is an almost first order dependency of rates on pressure of toluene. Comparison with Fig. 3 shows that the rates for formation of nitrile and C02 have reversed places. The rate for formation of C02 before reduction was higher than after reduction, whereas for nitrile formation the opposite is the case. Another feature is the fact that after reduction aldehyde is not formed when the partial pressure of oxygen is maintained low.
Fig. 6. Influence of partial pressure of toluene on rates for formation of products over YBa2Cu306tx, x = 0. PO = 2.16 P a and PA = 2.58 P a . Notations: cf. Fig. 1.
In Fig. 7 are reaction rates plotted as a function of reaction time for a YBa2CU306 sample which before use had been stored under ambient conditions for 10 days. Initially, the material though active was non-selective. After use for few hours, the rate for C02 formation dropped to a very low value, while that for nitrile formation increased more than twice. This behaviour was always observed when using YBa~Cu306samples which had been stored in an air atmosphere for several days. It is probably due to removal of some oxygen species which have been incorporated into the lattice during storage.
281
Time on stream
(mln)
Fig. 7. Reaction rates over YBa2Cu306 as a function of time. PO = 2.16 kPa, PA = 2.58 kPa, and PT = 0.77 kPa. Notations: cf Fig. 1.
TABLE 2 Lattice constants (A) and oxygen content (x) of catalysts. Fmh sample
orthorhombic a=3.8203(8) b=3.8853(7) ~=11.679(2) x= 1
tetragonal a=b=3.8582(2) ~=11.830(1) X=O
At high Po, before reductive eeatmentg
tetragonal a=b=3.858l(5) c=l1.764(1) x=0.4
At IOW Po, after reductive treatmentb
tetragonal tetragonal a=b=3.8572(4) a=b=3.8569(2) ~=11.830(1) x=o
At high Po, after reducuve treatmen@ ~~
~=11.834(1) X=O
orthorhombic a=3.855(1) b=3.892(1) c=l1.712(2)
orthorhombic a=12.177(2) b=5.6571(9) c=7.130(1)
tetragonal a=b=3.8841(5)
orthorhombic a=12.172(1) b=5.6590(5) c=7.1294(7)
x=l
c=l1.829(2)
x=o
tetragonal tetragonal a=b=3.8572(4) a=b=3.857l(4) c=l 1.818(2) ~=11.819(2) x=o
X=O ~~
aP0 = 17.30 kPa, PA = 2.58 kPa, and PT = 0.77 kPa bPo = 2.16 kPa, PA = 2.58 kPa, and PT = 0.77 kPa.
~
-~
282
Lattice constants determined by X-ray diffraction are given in Table 2 for various samples. Also included are x-values as estimated using the published relationship between lattice constants and oxygen content of YBa2C~306+~ phases [12]. The composition of Sm-substituted samples was estimated by comparing cell parameters and catalytic activity with corresponding values for Y B ~ ~ C U ~phases. O , ~ +From ~ the table, it can be concluded that the x-value of used YBa2CugOhX sample not being subjected to reductive treatment is well above zero.After reductive treatment and further use at low and high partial pressure of oxygen, respectively, the oxygen content of catalysts is close to 6 oxygen atomdunit cell. However, the c axis repeat of catalysts used at high oxygen pressure, non-selective conditions. was always found to be slightly shorter than that measured after use at low oxygen pressure, selective conditions. This implies that the x-value for catalysts run under non-selective conditions is slightly above zero.The lattice constants determined for Y2BaCuOg are identical for freshly prepared and used samples, and they also agree with those reported in the original structure determination [141. DISCUSSION A drawing of the YBa2Cu307 structure is shown in Fig. 8. There are two structurallydifferent Cu positions, noted Cu(1) and Cu(2) [15]. The formers are connected via 0(4), thus, forming chains in the [OlO]direction between Ba-layers. Cu(1)-chains are connected to Cu(2)-layers by O(1). In the Cu-layers, Cu(2) is coordinated to five oxygen species, 2 x 0(2), 2 x O(3) and 1 x O(1). It has been shown that there are no distinct Cu2+ and Cu3+ sites. The valence of Cu in both sites is intermediate between +2 and +3 [6].Oxygen O(4) in the chains have been found to be mobile and can be totally abstracted [5].When this occurs, the structure changes from orthorhombic YBa2Cu307 to tetragonal YBa2Cu306. The latter structure can simply be derived from the former by removal of O(4) so that the coordination of Cu(1) is changed from square. planar to linear twofold [16]. As aresult, distinct Cu*+ at Cu(1) sites, and Cu2+ at Cu(2) sites are formed [6].
Fig. 8. Drawing of the YBa2Cu307 structure.
283
From the fact that the main difference between the structures of the orthorhombic and tetragonal phases is connected to the coordination of Cu( l), it follows that it is reasonable to compare their catalytic behaviom in terms of possible surface coordinationsof Cu( 1). At the surface of YBa~Cu307.undercoordinatedCu(1) and Cu(2) can exist, serving as possible adsorption sites for toluene and ammonia. The number of undercooniinated species depends on the partial pressure of oxygen. Molecular oxygen can adsorb in the form of diatomic species. As a consecutive step, when dissociation is possible, monoatomic oxygen species can also be formed. However, dissociation is probably not facile due to lack of oxygen vacancies in the bulk. A common feature of oxygen species pmjecting from the surface is that they are undercoordinated,which renders them electrophilic in character. It has been established that electrophilic oxygen participates in the degradation of hydrocarbons leading to total combustion [7,9,17]. Indeed, YBa2Cu306tx. with x well above zero, was found to be non-selective in catalytic (ammhxidation, cf. Figs. 1-3. In YBa2Cu306, Cu(1) is two-coordinated due to that O(4) positions are vacant. After adsorption of molecular oxygen, two options are possible depending on the partial pressure of oxygen. At low pressure of oxygen, adsorbed diatomic oxygen can react with co-adsorbed ammonia to give water under simultaneous oxidation of low valent Cu( 1) to Cu3+ and formation of nucleophilic Cu=Oand Cu=NH species. Substantial evidence exist for nucleophilic oxygen species and imido species to be involved in selective oxidation and ammoxidation mechanisms, respectively [7-9], which is vexified by the present investigation. Figures 4-6 show that YBa2Cu306 is selective for nitrile formation at low partial pressure of oxygen. Furthermore, the finding that the rate for formation of benzaldehyde is zero in absence of ammonia, and passes through a maximum as the partial pressure of ammonia is increased suggests that co-adsorption of ammonia is a prerequisite for formation of nucleophilic oxygen species. On the contrary, when the partial pressure of oxygen is high, the catalyst is nonselective, cf. Fig. 4. This can be seen as a result of the facile dissociation of adsorbed diatomic oxygen at YBa2Cu306 One of the oxygen species can migrate into a neighbowing oxygen vacancy situated between two Cu(1) sites. Consequently, the remaining monoatomic surface species will have electrophilic character due to that Cu has to share its availablevalence electrons between both oxygen species. The rate for formation of CO2 over YBa2Cu306tx at high pressure of oxygen depends on the value of x. When the value is small, the rate is much higher compared to when x is high, cf. Figs. 1 and 4. Several explanations are possible for this behaviour, of which a few will be mentioned briefly. One is that the electronic properties of surfaces must be influenced by the occupancy frequency of exterior O(4) positions, cf. Fig. 7, consequently affecting adsorption and reactivity properties. Another factor of importance is that the number of active sites increase when the value of x decrease. In case of YBa2Cu307, if extending the bulk structure to the surface, Cu(1) at (100) faces cannot adsorb pmjecting single coordinated oxygen species. When the composition is close to YBa2Cu306, such an adsorption is possible producing electrophilic oxygen species on the condition that neighbowing O(4) positions are only partly filled.
284
At low pressure of oxygen, 2.5-5 kPa, the rates for formation of nitrile and C02 over yBa2cu306 at 400 % are 16-19 and 2-4 pnole m2min-l, respectively. Over V205, under the same conditions. the corresponding rates are 2-4 and 0.3-0.5 pmole m-2 min-1, respectively [18,19]. In conclusion, it has been shown that YBa2Cu306 is an active and selective catalyst for ammoxidation of toluene at low partial pressures of oxygen. ACKNOWLEDGMENT Financial support from the National Swedish Board for Technical Development (STU) and the Swedish Natural Science Research Council (NFR) is gratefully acknowledged. REFERENCES 1 2
3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18 19
J.L. Callahan and R.K. Grasselli, AIChE J., 9 (1963) 755. R.K. Grasselli and J.D. Burrington, in D.D. Eley, H.Pines and P.B. Weisz (Eds.), Advances in Caralysis, Vol. 30, Academic Press, New York, 1981, pp. 133-163. F. Cavani, G.Centi, F. T n f m and R.K. Grasselli, Catal. Today, 3 (1988) 185. M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q Wang and C.W. Chu, Phys. Rev.Lea., 58 (1987) 908-911. A. Manthiram, J.S. Swinnea, Z.T. Sui, H. Steinfink and J.B. Goodenough, J. Am. Chem. SOC.,109 (1987) 6667-6669. M.OKeeffe and S. Hansen, J. Am. Chem. SOC., 110 (1988) 1506-1510. J. Haber, in J.P. Bonnelle, B. Delmon and E. Derouane (Eds.), Surface Properries and Catalysis by Non-Merals, Reidel, Dodrecht, 1983, Ch. 1, pp. 1-45. R.K. Grasselli, J.F. Brazdil, and J.D. Burrington, Proc. 8th Int. Congr. Catalysis, Berlin(West), July 2-6, 1984, Verlag Chemie, Weinheim, 1984, Vol. V, pp. 369-380. A. Andersson and S. Hansen, J. Catal., 114 (1988) 332-346. Kirk-Other, Encyclopedia of Chemical Technology, 3rd edn., Vol. 15, Wiley, New York, 1981, p. 906. S. Hansen, J. Otamiri, J.-0. Bovin and A. Andersson, Nature, 334 (1988) 143-145. C.N.R. Rao, J. Solid Stare Chem., 74 (1988) 147-162. H. Steinfink, J.S. Swinnea, Z.T. Sui, H.M. Hsu and J.B. Goodenough, J. Am. Chem. SOC., 109 (1987) 3348-3353. C. Michel and B. Raveau, J. Solid State Chem., 43 (1982) 73-80. F. Beech, S. Miraglia, A. Santoro and R.S. Roth,Phys. Rev.,B35 (1987) 8778-8781. J.S. Swinnea and H. Steinfink, J. Marer. Res., 2 (1987) 424-426. A.M. Gasymov, V.A. Shvets and V.B. Kazansky, Kinet. Karal., 23 (1982) 951-954. J.C. Otamiri and A. Andersson, Catal. Today, 3 (1988) 211-222. J.C. Otamiri and A. Andersson, Card. Today, 3 (1988) 223-234.
285
B. DELMON (Univ. Catholique de Louvain, Belgium): Due to the fact that catalyst surfaces are usually reduced in their steady state during catalytic oxidation it might seem doubtful that copper remains in the Cult o r Cus+ oxidation state, with no Cuo and, consequently, Cu crystallites being formed. The absence of new lines in X-ray diffraction cannot be a fully convincing proof since small crystallites might not be detectable. Did you find a change in the intensity ratio of Cu/Y or Cu/Ba XPS lines, or changeso of ISS signals after use of the catalyst? If really no Cu were formed, this would indicate a really exceptional strength of the chemical bonds involving Cu. In ammoxidation, ammonia is a very strong reducing agent, even in the presence of 0,. If this is so, this could give a clue to the very special electronic structure of superconductors. A. ANDERSSON (University of Lund, Sweden): For both freshly prepared samples and used samples only X-ray diffraction lines belonging to YBa,Cu,O,+x could be detected. Use in catalytic reaction did not cause any change in the intensity of the X-ray lines that cannot be explained as due to change in oxygen content. Also, XPS analysis did not show formation of Cuo. However, the ratio of Cu/Y and Cu/Ba XPS lines showed some dependence on reaction conditions. In this regard, it should be noted that YBa2Cu306+xfaces can expose both Cu-, Y-, and Ba-layers and that their distribution possibly depends on the composition of the reactant stream. O.V. KRYLOV (Acad. of Sciences, MOSCOW, USSR): In connection with an interesting observation of Dr. Andersson and his collaborators I should like to comment about many similarities between high temperature semiconductors and oxide catalysts of partial oxidation. Both of them have oxygen-deficient lattice. In the case of high temperature semiconductors, oxygen vacancies in the lattice must be stable and only motion of electron pairs must be observed. On the contrary, in oxide catalysts of partial oxidation such vacancies must move. It is very possible now to search new high temperature semiconductors from oxidative catalysis.
A. ANDERSSON: Thank you for your comment, we believe that such an approach may yield fruitful results. M. MISONO (The University of Tokyo, Japan): Very interesting results. I would like to know more about the chemical reactivity and the composition of the surface of YBa,Cu,O,+x. Is it stable at high temperatures against CO,, H,O, etc.? Is the surface composition the same as in the bulk? Segregation of certain elements (Ba, etc.) has often been indicated in the reported papers of electric conductivity. A. ANDERSSON: Our XPS results, that will be published elsewhere, clearly show the existence of carbonate species both in freshly prepared samples and in used samples. In fresh samples, the amount is highly dependent on the preparation method used. After use in catalytic reaction, only a minor variation of the amount of carbonate species in comparison with fresh samples was observed. Examination of the catalyst before and after use in the reactor by high-resolution transmission electron microscopy,
286
r e v e a l s a n i n c r e a s e i n t h e number o f c r y s t a l s t r u c t u r e d e f e c t s on t h e ( 0 0 1 ) p l a n e ( r e f . 1 ) . Such d e f e c t s a r e p o s s i b l y formed under t h e i n f l u e n c e o f H 0 ( r e f . 2 ) . However, once a s t e a d y s t a t e h a s been r e a c h e d , no change w i t h t i m e i n t h e f o r m a t i o n o f p r o d u c t s was detected f o r t h e p e r i o d it was used, which was up t o 3 d a y s . 1 2
S . Hansen, J . O t a m i r i , J.-0. Bovin and A . 3 3 4 (1988) 1 4 3 . B . G . Hyde e t a l . , N a t u r e , 327 (1987) 4 0 2 .
Andersson, N a t u r e ,
PAJONK (Univ. Claude B e r n a r d Lyon I , F r a n c e ) : I would l i k e t o know i f your c a t a l y s t i s s t a b l e w i t h t i m e on s t r e a m . Due t o t h e m o b i l i t y of oxygen i n s i d e t h e s t r u c t u r e o f your h i g h Tc s u p e r c o n d u c t o r , why d i d you n o t t r y t o o x i d i z e , e . g . , p r o p y l e n e which c o u l d have been l e s s complex t o i n t e r p r e t w i t h r e s p e c t t o t h e r e a c t i o n mechanism?
G.M.
A . ANDERSSON: R e f e r r i n g t o t h e a n s w e r s g i v e n t o p r o f e s s o r s Delmon
and Misono, some s t r u c t u r a l changes were o b s e r v e d a s a r e s u l t of c a t a l y t i c r e a c t i o n . Once a s t e a d y s t a t e was r e a c h e d , t h e p e r f o r m a n c e o f t h e c a t a l y s t was s t a b l e a s l o n g a s i t was u s e d ( u p t o 3 days). I n comparison w i t h t o l u e n e o x i d a t i o n , w e do n o t t h i n k t h a t t h e mechanism o f p r o p y l e n e o x i d a t i o n i s less complex. the