Ammoxidation of toluene by YBa2Cu3O6+x and copper oxides

Applied Catalysis, 65 (1990) Elsevier Science

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Ammoxidation oxides

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Printed in The Netherlands

of toluene by YBa,Cu,O,

+ x and copper

Activity and XPS studies J.C. Otamiri*, S.L.T. Andersson and A. Andersson Department of Chemical Technology, Chemical Center, University of Lund, P.O. Box 124, S-221 00 Lund (Sweden), tel. (+46-46) 108284, fax. (+46-46) 146030 (Received 19 March 1990, revised manuscript received 30 May 1990)

Abstract The catalytic activities of YBa2Cu306+, and copper oxides have been studied in the ammoxidation of toluene. At low oxygen pressures, over YBa,Cu s 0 6+x selective ammoxidation to benzonitrile occurs, whereas at high pressures, only total combustion of toluene to carbon oxides is observed. X-ray photoelectron spectroscopy (XPS) analysis oKBa,Cu 3 0 6+r catalysts indicates predominance of Cu (I) states under partial ammoxidation conditions, while under total oxidation conditions, Cu (II) states are predominant. Comparison with the catalytic performance of copper oxide catalysts shows that differences do exist. The catalysis by YBa2C!u,06+, is greatly influenced by the oxygen content of the bulk material despite its surface composition.

Keywords: toluene ammoxidation, yttrium, barium, copper, catalyst characterization

(XPS),

selectivity

(benzonitrile ) .

INTRODUCTION

Oxidation and ammoxidation of alkylaromatics are usually performed over vanadium oxide catalysts [l-3]. However, we have recently found that the 0 3 6+r compounds [ 41 and its samarium analogue newly discovered YBa,Cu are active and selective for ammoxidation of toluene to benzonitrile on condition that their oxygen content is close to six [ 5,6]. At higher oxygen content, total combustion is predominant. The variation of the catalytic performance with oxygen content was explained by the presence of characteristic Cu( I) layers in YBa2Cu306 as compared to YBa,Cu,O,. Surface pairs of Cu(1) layer species, after adsorption of ammonia and oxygen can form Cu (III) 0x0 and imido species believed to play a vital role in partial (amm ) oxidation mechanisms [ 7-91. In this context, it is worth mentioning that at an early stage copper oxide was tried as catalyst in the ammoxidation of propene to acrylonitrile [ 10,111. It was however, found, that useful products were formed only

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in a narrow window of oxygen pressure due to the necessity of having a catalyst surface of a suitable reduction degree. Indeed, this finding is similar to the one observed using YBa, CU~O~+~for toluene ammoxidation [ 5,6]. In addition to ammoxidation, YBa, Cu, Osfz and its substituted analogues have been tested as catalysts for the oxidation of carbon monoxide and methane, and dehydrogenation of methanol [ 12-151. Unfortunately, there could be some complications using these new materials as catalysts, since it has been reported that their surfaces degrade to various products when exposed to water and carbon dioxide [ 16,171. In ammoxidation the latter compounds are formed. Therefore, the present investigation of ammoxidation of toluene over was carried out using X-ray photoelectron spectroscopy (XPS) YBa&uJ&+, to characterise the surface states of the catalyst after use at different reaction conditions. Also comparison of catalytic performances of YBa,Cu30s+, and copper oxides is presented. EXPERIMENTAL

Tetragonal YBa,Cu,O, [ 181 and orthorhombic YBa&usOy [ 191 were prepared from stoichiometric amounts of BaCO, (Merck, p.A.), CuO (Merck, p.A.) and Y,O, (Ventron 99.9%) by heating in nitrogen or oxygen, respectively, at 950’ C. Multiple grinding and re-heating were adopted until pure phases according to X-ray diffraction analysis were obtained. YBa,C&O, was finally treated for 12 h in pure oxygen, at 650°C and then cooled slowly to room temperature. The sample obtained showed diamagnetic response when cooled in liquid nitrogen and placed in front of a magnet. YBa,Cu80, was finally heated in nitrogen at 780’ C for 12 h and rapidly quenched to room temperature. This sample did not show any diamagnetic response when cooled in liquid nitrogen. CuO (BdH, Analytical Reagent), and CuaO (Kebo, Purum) used as catalysts were formed into pellets, then crushed and sieved. For all samples, particle size fraction of 0.125-0.250 mm was used in the catalytic measurements carried out at 4OO”C, in a differential and isothermal plug flow reactor made of Pyrex glass. An amount of 350 mg of fresh sample diluted ten times with quartz was used in each experiment. The inlet partial pressures of toluene and ammonia were kept constant at 0.77 and 2.58 kPa, respectively, while that of oxygen was varied. The products, benzonitrile and carbon oxides were analyzed on a Varian Vista 6000 gas chromatograph [ 201. Samples for XPS studies were obtained by carrying out the following activity measurement. YBa,Cu,O, was heated to reaction temperature in oxygen and then subjected to the series of treatments (A-D) given in Table 1. Each treatment was carried out until steady state, except the treatment at zero pressure of oxygen which lasted for 30 min. After each treatment a sample was taken out for XPS analysis. YBa, Cu, OS was heated to reaction temperature in nitrogen and subjected to treatment A, Table 1, for XPS analysis. At other treat-

161 TABLE 1 Notations and results for YBazCu 30 6+x catalysts subjected to different reaction treatments at 400°C r-Value of fresh catalyst

x= 1 xx1 zx 1 TX 1 X%0 r-0 r= 0 x-0

Catalyst notation for XPS

Sample Sample Sample Sample Sample

1 2 3 4 5

Reaction treatment

;I $ $

Reaction rate (pm01 rn-’ min-‘) Benzonitrile

CO2

co

0.0 3.4 8.0 0.0 0.0 3.1 11.4 0.0

57.5 0.1 2.5 123.0 337.0 0.1 2.3 259.0

1.6 0.0 0.3 1.2 0.6 0.0 0.0 1.5

“Reaction at 42.5 kPa oxygen after heating to reaction temperature.pNH3= 2.58 kPa, PcTnn= 0.77 kPa. bReaction at zero pressure of oxygen after treatment A. J&na = 2.58 kPa, pc?ns =0.77 kPa. Reaction at 2.16 kPa oxygen after treatment B. pnus = 2.58 kPa, pcTns= 0.77 kPa. ‘Reaction at 42.5 kPa oxygen after treatment C. pNH3= 2.58 kPa, pc7us =0.77 kPa.

ments using this sample, only rates were measured for comparison purposes. Results obtained are included in Table 1. In the experiments to compare the catalytic behaviour of YBa,Cu, Ostx and copper oxides, YBa2Cu30s and Cu,O were heated to reaction temperature in nitrogen and partial pressure of oxygen was varied from 0 to 42.5 kPa. CuO was heated to reaction temperature in pure oxygen and the pressure varied from 42.5 kPa to 0. In both cases, at zero partial pressure of oxygen, reaction lasted for 30 min. XPS characterisations were performed in a Kratos XSAM 800 instrument. An Al-anode (1486.6 eV) was used. The slit width was set at 40” and the analyzer operated at 40 eV pass energy at high magnification. Charging effects were corrected for by adjusting the main C 1s peak to a position at 285.0 eV. Analysis of spectra was carried out with the DS800 system. Sensitivity factors used in the quantitative analysis were mean values obtained from measurements on several pure YBa2CuB07 samples and were for 0 1s 1.0, Y 3d 1.15, Ba 4d 1.54and Cu 2p3/2 2.65. Procedures were similar as reported elsewhere [ 171. The catalysts taken out from the reactor under nitrogen were mounted on the sample holder and inserted in the vacuum system under high purity argon. The vacuum was initially in the 10-8Torr range due to sample outgassing, but some hours later it was in the lo-” Torr range.

162

RESULTS

Activity measurement over YBu2Cu306+,

catalysts

In Table 1 are the results obtained when fresh samples of YBa,Cu,O, and YBa, Cu3O6 were subjected to series of treatments A-D. Treatments A and D at high partial pressure of oxygen resulted only in total oxidation, while selective ammoxidation was noticed with treatments B and C at zero and low partial pressure of oxygen. At the latter conditions, carbon dioxide formation was low and carbon monoxide was not observed especially for YBa,Cu, OS. Benzonitrile formation over the two catalysts during treatment B was the same, but at higher oxygen pressure, treatment C, the activity over YBa,Cu, 0, was just a little lower. With treatment A, the activity for carbon dioxide formation over YBa, Cu30, was about six times higher than YBa, Cu, 07. During treatment D these samples showed less difference. Carbon dioxide formation over YBa,Cu,O, increased in comparison with that during treatment A. For YBa, Cu, 0,) the opposite was the case. Fig. 1 shows results when the activity over YBa, Cu, 0, and YBaz Cu3 O6 was followed as a function of time-on-stream during treatment A. We can notice that for YBa2Cu307 steady state was reached rather quickly, while over YBa,Cu,O,, it occurred later. A comparison of total activity shows that it is higher for YBa, Cu, OS. Catalysis over YBa, Cu, 06, C&O and CuO The rates for formation of products measured as a function of oxygen pressure over YBazCu,O,, Cu,O and CuO are shown in Figs. 2-4. These figures are characterised by some features which are worth noting. For example, in '-3.0 -2.5 g cz-2.0 z B -1.5 5 -1.0 g h -0.5 $

0;o.o

50

100 150 200 250 300 Time on stream (min)

350

400

Fig. 1. Rates for formation of carbon oxides at treatment A as a function of time on stream over YBa,Cu,Os (w CO,; A CO), and YBa2Cu,07 (0 COz; A CO). Conditions: cf. Table 1.

163

2.5 715 510 Partial pressure of 0, (kPa)

Partial pressure

OiO,

(kPa)

Fig. 2. Rates for formation of benzonitrile over YBaZ Cu3O6,CuO, and Cue0 as function of oxygen partial pressure, ( n YBa, CuQ0,; q CuO; 0 CueO). Arrows indicate the direction in which the oxygen pressure was varied. Fig. 3. Rates for formation of carbon dioxide over YBa,Cu,O,, CuO, and CuaO as function of oxygen partial pressure, (m YBazCu,O,, 0 CuO; 0 Cu,O). Arrows indicate the direction in which the oxygen pressure was varied.

2 .c

3.02.4-

% %i 2 j_

‘.a

g

1.2-

$ ;

0.6-

o.oP 0

-

I

10 Partial pf,“,sure

30 of Oz (kPay

50

Fig. 4. Rates for formation of carbon monoxide over YBaeCu,O,, CuO, and Cue0 as function of oxygen partial pressure, (H YBa,Cue06; 0 CuO; 0 CueO). Arrows indicate the direction in which the oxygen pressure was varied.

Fig. 2, the rate for formation of benzonitrile at zero pressure of oxygen was lowest for YBa,Cu,O,, but it increased and passed through a maximum as pressure was increased. At zero pressure, the rate over YBa,Cu3 OS compared with those for copper oxides only varied slowly with time. Over Cu,O, no product was observed initially and after 30 min the high rate shown in the figure was obtained. For this catalyst also, the rate rapidly diminished as oxygen pressure was increased. CuO showed a very narrow and sharp maximum as low

164

partial pressures were approached. At zero pressure of oxygen the rate was for the first 15 min low but had after 30 min increased. In Fig. 3 are shown the rates for formation of carbon dioxide. These rates show sharp transitions with oxygen pressure. However, the transition is quite different for each catalyst. CuO treated from high oxygen pressure (42.5 kPa) to zero showed transition at low oxygen pressures, whereas over CuzO, it occurred at comparatively higher pressures. In the case of YBazCu, 0, the transition occurred at intermediate pressures compared with those for copper oxides. Also, the activities for both copper oxides were almost the same at high oxygen pressure, while over YBa, Cu306, it was a little higher. The rates for formation of carbon monoxide with partial pressure of oxygen, Fig. 4, have common features. All rates passed through maxima which are associated with the transitions in the rates for formation of carbon dioxide, Fig. 3. After the maxima, the rates increased as partial pressure of oxygen was further raised.

Characterisation by XPS Table 2 shows binding energies (BE) and full widths at half maximum (FWHM) for various core levels measured for the YBa,Cu,O,+, samples. TABLE 2 Core line binding energies” (eV) and half width@ (eV) for reference samples and YBa,Cus06+r catalysts used at different reaction conditions Sample

0 1s

Y 3P,,,

Y 3d

Ba 3db,2

Ba 4d

cu 2p

Cu LMM”

Sample 1

531.4 (2.9) 531.8 (2.4) 531.4 (2.4) 531.8 (3.2) 531.7 (3.6) 530.9 (2.3) 530.0 (1.7)

300.2 (3.2) 300.4 (3.3) 300.5 (3.0) 300.4 (3.1) 300.6 (2.8)

157.2 (3.9) 157.3 (3.9) 157.3 (3.9) 157.5 (4.1) 158.6 (3.7)

779.8

89.3

933.4

(2.5) 780.0 (2.4) 780.0 (2.4) 780.0 (2.4) 780.1 (2.3) _

(4.4) 89.5 (4.5) 89.6 (4.4) 89.6 (4.3) 89.6 (4.2)

(3.2) 932.9d (1.9) 933.0d (2.0) 933.6 (3.4) 933.6 (3.3) 932.9d (1.6) 934.0 (3.4)

918.1 (3.4) 916.6 (3.4) 916.6 (3.6) 918.1 (3.2) 918.1 (3.2) 916.4 (2.9) 917.7 (3.2)

Sample 2 Sample 3 Sample 4 Sample 5 C&O cue

“Referenced to C 1s = 285. = eV. Positions at peaks maxima. ‘Given within brackets. CL2,3M4,5M4,5 kinetic energy. ‘No satellite. Otherwise a satellite at about 942 eV.

I

I

I

535

530

525

BINDING

ENERGY

(eV1

BINDING

ENERGY

(eV)

Fig. 5.0 1s spectra for YBa2CuB06+x used with treatments A-D. (a) Sample 1; (b) Sample 2; (c) Sample 3; (d) Sample 4; and (e) Sample 5. Notations as in Table 1. Fig. 6.0 1s difference spectra for YBazCu 30 6+Xusedwith treatments A-D. (a) Sample 1 -Sample 2; (b) Sample 4-Sample 3; and (c) Sample 5-Sample 2. Notations as in Table 1.

Large differences were observed in the Cu 2p and Cu LMM line after various treatments. For 0, Y and Ba levels, differences were rather small. The 0 1s spectra for the various catalysts are shown in Fig. 5. The spectra are dominated by a peak just below 532 eV. Upon use a clear effect at the low BE side of the main peak is observed. This was further investigated by calculating difference spectra between catalyst used at high and low oxygen pressures, cf. Fig. 6. The difference spectra show one 0 1s component at 529.6530.0 eV. The Cu 2p spectra are shown in Fig, 7. It is clearly seen that catalysts used at low oxygen pressures give narrow peaks without satellites (spectra b and c), while those used at high pressures have broad peaks at higher BE and with satellites (spectra a, d and e). Reference spectra for CuzO and CuO are also included. In general, Cu (II) is associated with the presence of a strong satellite structure [21,22], while Cu(1) does not have this structure. The weak structure seen in spectrum f is due to contamination by surface CuO. The resemblance with the reference spectra concerning the position, half width and absence or presence of satellite structure shows that during and after use at low and high oxygen pressures copper in YBazC~30B+z is in Cu( I) and Cu(II) states, respectively. Table 3 shows data for Cu 2~~~~lines and satellites. The

166

I

960 BINDING

I

I

950

940

ENERGY

930 ieV)

Fig. 7. Cu 2p spectra for YBa,Cu,O,,. used with treatments A-D. (a) Sample 1; (b) Sample 2; (c) Sample 3; (d) Sample 4; (e) Sample 5; (f) Cu,O; and (g) CuO. Notations as in Table 1.

Cu(II)/Cu ratios in used catalysts were determined from the Cu Zp,,, satellite/main line intensity ratio, using the ratio of 0.65 for CuO as a reference for 100% Cu (II). Calculated ratios are included in Table 3. Cu L2,3M4,5M4,5 spectra show clear differences for Cu (0)) Cu (I) and Cu (II) species [18,21-231 and are shown in Fig. 8 for the catalysts (spectra a-e). Cup0 and CuO (spectra f and g) give kinetic energies of 916.4 and 917.7 eV, respectively. The spectra for the catalysts are quite close to either one of these. No Cu(0) at 919.0 eV [21] was observed. For samples used at low oxygen pressures, the peak is at about 916.5 eV (spectra b and c) identifying Cu(1). After use at high oxygen pressure, the samples show peaks at about 918.0 eV (spectra a and d) suggesting the presence of Cu (II). The Ba 4d,,, spectra and the difference spectrum for Samples 3 and 5 used at low and high oxygen pressure, respectively, are shown in Fig. 9. The spectrum for Sample 5, is reminiscent of mainly one component with a position at 89.7 eV. The spectrum for Sample 3, clearly deviates, which is best noticed in the height of the valley between the two 46 components. The difference spectrum, Sample 3-Sample 5, gives a BE of 88.4 eV. The differences observed are caused probably by different bulk contributions to spectra. Similar effects were observed in the Y 3d spectra. In this case however, the small splitting between

167 TABLE 3 Data for the Cu 2p,,, core line with satellite” for reference samples and YBaa CU~O~+~catalysts used at different reaction conditions Sample

BE and FWHM (eV) Satellite

Sample 1 942.5 Sample 2 Sample3

Cu (II)b/Cu

Cu(II)‘/

(%)

(%)

(Y +Ba + Cu)

Total rated

@llol rrz

933.9

0.50

77

26

59.0

(3.5) 932.9

0.00

0

0’

3.5

-

(1.9) 933.1

0.00

0

Of

11.0

(2.0) 933.8

0.60

92

47

124.0

(3.5) 933.8

0.60

92

58

338.0

0.00

0

0.65

100

942.2

Sample 5

(5.0) 942.1 (4.8)

cuso 942.4 (5.1)

min-r

)

Main line

(4.6) -

Sample 4

cue

Intensity ratio satellite/main line

(3.7) 932.9 (1.6) 934.2 (3.6)

*Fitted with Gaussian components. *Cu (II ) /Cu ratio calculated using CuO as reference for 100%. ‘Cu (II) concentration obtained by multiplying the Cu (II) /C u ratio by percent Cu according to XPS analysis, Table 4. ‘Reaction conditions as in Table 1. ‘47.6% Cu(1) according to XPS. ‘57.3% Cu(1) according to XPS.

the 3d components resulted in broad rather featureless peaks and attempts to subtract spectra failed, but Sample 5 gave more of the high BE component than Sample 3. The results of the quantification of the XPS spectra are given in Table 4. Significant changes in the surface composition of the catalysts with different treatments are observed. Comparison of the oxygen content of samples used at low or high oxygen pressure, shows that for the latter it is higher. The data indicate, that use of catalysts first at high and then at low oxygen pressure increases the copper concentration as for Samples 1, 2 and 3. Upon reoxidation, the copper concentration decreases slightly, cf. Sample 4. DISCUSSION

Surface composition The XPS studies of the used YBazC~B06+x catalysts showed major effects in the Cu 2p and Cu LMM lines, whereas for the 0, Y and Ba levels, small differences were noticed as seen in Table 2. The variation in BE, measured at

168

I 100 KINETIC

ENERGY

ie’.‘)

90 BINDING

ENERGY

80 (eV)

Fig. 8. Cu L2,3M4,5M4,5 Auger spectra for YBa, CU~O,,, used with treatments A-D. (a) Sample 1; (b) Sample 2; (c) Sample 3; (d) Sample 4; (e) Sample 5; (f) Cu,O and (g) CuO. Notations as in Table 1. Fig. 9. Ba 4d representative spectra for YBa,Cu 30 6+x catalysts used with treatments A-D. (a) Sample 3; (b) Sample 5; and (c) Difference spectrum, Sample 3 -Sample 5. Notations as in Table 1.

peak maxima for the latter are caused by differences in the surface composition, X-ray diffraction patterns of used and fresh samples were identical, so decomposition of the bulk phase or formation of new phases can not be concluded [ 5,241. We have shown elsewhere [ 171 that interaction with atmospheric water and carbon dioxide produces a degraded surface layer which is represented by shifted peaks for 0, Y and Ba. For this surface, a strong 0 Is peak is at about 531.5 eV, Y at 157.5 eV, Ba 3d,,, at 779.9 and Ba 4d5,* at 89.4 eV and is in good agreement with the data measured for the used catalysts. The surface degradation products may constitute Ba (OH),, BaCO,, Y,BaCuO,, CuO, Ba&u( OH), and Y (OH), [25-291, and indeed, the main 0 Is peak at about 532 eV is caused by surface hydroxide and carbonate compounds [ 171. As the catalysts were handled under inert conditions during the entire process from the reactor to the XPS instrument, the surface compositions measured are representative for the state of active material in the reactor. The major effects observed in the Cu 2p line, Fig. 7, clearly show that use of the catalysts at high partial pressure of oxygen results in a surface composed almost exclusively of Cu (II) as is seen by broad peaks and strong satellites. At

169 TABLE 4 Composition’in atom-% calculated from XPS data for reference samples and YBa, Cus 06+r catalysts used at different reaction conditions Sample

Y

Ba

cu

0

Sample 1

&

$1

z,

53

Sample 2

(8:)

(Z,

cz,

45

Sample 3

(1351

$1

$1

41

Sample 4

(1:)

$1

(it,

50

Sample 5

(1k

(ii,

&

48

YBa, Cu, OGb YBazCu30,b C&O cue

8

17

25

50

(17) 8

(33) 15

(50) 23

54

66 56

34 45

“Composition excluding oxygen is given within parenthesis. bNominal composition.

low pressures however, the surface is composed of Cu(1) as indicated by narrow lines without satellites. The shifts in the Cu L,,SM,,SM,, Auger line, Fig. 8, confirm the above conclusions and further show that Cu (0) is not formed. The formation of 100% of Cu (I) is not concievable with the original structure of YBa,Cu,O,+, retained at the surface, and this is an evidence of surface degradation. However, 100% Cu (II) can be formed at r= 0.5. The Ba 4d photoelectron has an almost three times higher kinetic energy than the Cu 2p photoelectron. Consequently, the Ba 4d line must be collected from a rather thicker layer than the Cu 2p line. This therefore explains why only a surface layer of 100% Cu(I) is seen in the Cu 2p spectra, whereas a small bulk contribution appears in the Ba 4d spectra for samples used at low oxygen pressure, see Fig. 9. Thus, considering these reasonings and the XRD results [ 5,241, it is appropriate to suggest that decomposition or degradation is only limited to a thin overlayer. In this overlayer, finely dispersed Cu,O and CuO are probably involved as can be noticed from the 0 1s difference spectra between catalysts used at high and low pressures of oxygen which show one component at 529.6530.0 eV typical of CuO, see Table 2 and Fig, 6. The 0 1s line for CuBO is expected close to the main line, and hence can not be separated. Also, the surface degradation suggests formation of other copper compounds as well. The significant changes in the surface composition of the catalysts after use at different treatments as seen in Table 4, are probably due to heterogeneously

170

distributed surface degradation products. The oxygen concentration of the catalysts decreases when used at low oxygen pressure, and increases at subsequent use at high oxygen pressure. The only element for which significant changes in valence state are observed is copper, thus, the different oxygen concentrations may primarily be associated to copper. Contribution to the 0 1sspectra is also made from carbonate and hydroxyl compounds. The accuracy in determination of the absolute oxygen concentration is rather limited as seen by comparing with the result for CuO in Table 4. The changes in concentration of Y, Ba and Cu are more easily noticed from the compositions excluding oxygen, which indicate large variations in surface compositions of Y, Ba and Cu compounds. Also, use of catalysts at low oxygen pressure seems to increase surface copper concentration considerably as a result of effective surface dispersion of Cu(1) compounds. Reoxidation of such catalysts results only in a small decrease in the surface copper content, now present as Cu (II). An initial heating in nitrogen followed by use at high oxygen pressure, Sample 5, gave the highest copper concentration. It seems tempting to suggest that reduction to Cu (I) brings about a high surface enrichment and dispersion of Cu (I) compounds, possibly copper oxide. Subsequent reoxidation converts these to Cu (II), possibly present as copper oxide. In this context, it is worth while to consider any relative variations in the Y and Ba concentration (Table 4). It is seen by taking the Y /Ba ratio that large increases in Cu is accompanied by a decrease in this ratio. From these results one may conclude that over YBazCu,O,+, the overlayer formed from reaction with the atmosphere is fully developed during catalytic reaction and protects the material from further decomposition. This protective overlayer is thin since effects from bulk states could still be observed, and in this layer finely dispersed Cu (I) and Cu (II) states, probably copper oxides, are present depending on the reaction condition. The role of bulk and surface states The starting material for Samples 1 to 4 was the oxygen-rich orthorhombic with xw 1, while for Sample 5, it was the tetragonal phase of YBa,Cu,O,+, phase with x z 0. From the activity results it is obvious, that the reactivity of these catalysts at high oxygen pressures differs, and at a first approach can be argued in terms of the oxygen stoichiometry [ 61. The results of the XPS studies show that the surfaces of these catalysts are complicated and different depending on reaction condition. However, there are several similarities. The results of the experiments carried out with treatment A, Table 1 and Fig. 1, clearly show that the activity over YBa, Cu, OS for carbon dioxide formation is several times higher than that over YBa,Cu,O,. This observation can easily be correlated to the higher concentration of Cu (II) states as identified by XPS analysis for the former catalyst, Table 4. However, the estima-

171

tion of surface Cu (II) species based on intensity ratio of satellite/main peak of Cu 2p,,, and percentage Cu by XPS, Table 3, shows that such correlation is not adequate as the total rates differ about six times whereas Cu (II) concentration differ only twice. Furthermore, Fig. 1 shows, that steady state was attained more easily over YBaz Cu, 0, than over YBaz Cu306, and this is probably a consequence of the different bulk oxygen compositions involved. In the case of YBa, Cu3 0,) bulk oxygen concentration is high, and defect concentration low. These will lead to fast establishment of equilibrium and low rate of oxygen dissociation [ 61. Over YBaz Cu, OS on the other hand, bulk oxygen concentration is low and there will be the tendency for oxygen diffusion into the bulk. But prior to diffusion, molecular oxygen must dissociate at the surface consequently creating active O- species [30] which are responsible for the high combustion rate observed [ 61. The YBaz Cu, OS after undergoing sequence of treatments A-D, showed slightly decreased reactivity for combustion as compared to when treated only in A. This is probably due to gradual reoxidation of the near surface states and decrease in copper since fully oxidized YBa,CuQ07 catalyst showed lower combustion rate, Table 1. YBa2Cu,0, on the opposite showed increased reactivity, which could be due to increased copper concentration, Table 4.It is of interest to mention that the reactivity of the Y-Ba-Cu-0 catalysts could be affected by the ordering of oxygen species [ 311. Oxygen species migrating into the bulk, especially in the case of YBa, Cu3 OS , can occupy vacant chain and/or interstitial positions [ 32 1, and this may influence the properties of both near surface and surface states. Such conclusion becomes even more evident if we consider our earlier findings that after use under non-selective conditions of a sample freshly charged as YBa,Cu,O,, the oxygen content had slightly increased [6]. In the case of a sample charged as YBazCu,O, and only subjected to high oxygen pressures, the x-value was 0.45, well above zero and the reactivity was low, which is in agreement with results in Fig. 1 and Table 1. Furthermore, results in Table 1 show that the catalysts during treatments B and C gave essentially the same activity level, and this means that their surface states are basically the same. It is important to point out that in an earlier study [6] similar results have been observed for YBaz Cu3Os+z and its samarium analogue treated under similar conditions. This shows that these results are quite reproducible. It is appropriate therefore to suggest, that the Cu-states in these systems are responsible for both selective and combustion reactions, while the bulk oxygen content, its mobility and ordering in the bulk and near surface region influence considerably the level of activity. Activity of YBa,Cu, O6 compared to copper oxides As Cu (I) and Cu (II) states are implicated in the catalysis involving YBa2CusOs+z, it is of great interest to compare its reactivity with those of

172

CuO and CuzO under conditions stated above. Copper oxides can be used for ammonolysis and ammoxidation [33] after reduction of the bulk oxide with hydrogen or when supported. In Fig. 2 benzonitrile formation over CuaO rapidly falls with introduction of small amount of oxygen. For CuO a sharp maximum was observed as low oxygen pressure was approached. The activity for YBa, Cu, OS, however, increases with introduction of oxygen and passes through an optimum. The differences in reactivity can be explained if we consider that adsorbed ammonia and oxygen can react at Cu (I) sites with the formation of Cu = 0 and Cu = NH species. The participation of such species have been implicated in partial oxidation and ammoxidation processes [ 7-91. These reactions will require transition of copper states from Cu (I) to Cu (III) and forth. In Y-Ba-Cu-0 compounds such transitions, though not fully established, can occur at the surface. Furthermore, these states will be stabilized as a result of migration of oxygen into the bulk material. Also, we might recall that the formal charge balance for Y-Ba-Cu-0 actually requires Cu (I ) transition to Cu (III) and back in order to obtain either YBa, Cu, OS or YBa, Cu, 0,. Over CuO such transition is less feasible except at high degree of reduction. However, in the presence of oxygen and at elevated temperatures the reduced phase could easily be reoxidized, thus it is not surprising to notice the rapid decline in the rate for benzonitrile formation. For YBa2 Cu3 OS, the broad optimum suggests that no rapid structural transition occurs. An alternative explanation could be that of cooperation between phases [ 341, which at this point is highly speculative as Y-Ba-Cu-0 and its ‘multiphase surface’ do not necessarily exit as separated phases or mixtures. The data presented in Fig. 3 on carbon dioxide formation show some similarities between the catalysts. The reactivity of YBa,Cu,O, catalyst is slightly different, it shows a sharp transition and a final activity level which exceeds those of copper oxides. This transition for YBa,Cu, O6 is not accompanied with phase transition, i.e. the catalyst remains tetragonal [ 6,241. In the case of copper oxides, phase transition must be anticipated [ 351. After the transitions, the rates over the copper oxides are basically the same indicating not only identical surface compositions, but also transformation of Cu,O to CuO. The pattern of activity for formation of carbon monoxide over these catalysts, Fig. 4, is slightly different. Over CuO we notice a very sharp maximum at low oxygen pressure which is associated with gradual reduction of the oxide to Cu,O as low pressures are approached. The shape of the curves for YBazC!u,Os and Cue0 is very similar, but the activity levels differ indicating that apart from the similarity in state of copper, other factors such as phase transitions Cu,O/CuO and oxygen content for YBa,Cu, 06, influence their reactivity. Consequently, a quantitative correlation to copper concentration as measured by XPS for YBazCua06+r did not fully account for the reactivity pattern observed. In conclusion therefore, it is reasonable to suggest, that Cu(I) and Cu(I1)

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surface states are active for partial ammoxidation and combustion, respectively. However, for copper oxides, structural transitions, i.e., CuzO-CuO, and vice versa influence reactivity, whereas for YBa, Cu, O6+r, the oxygen content of the bulk material strongly influences the surface states, which in turn determines its catalytic properties. ACKNOWLEDGEMENT

Financial support from the National Swedish Board for Technical Development (STU) and National Energy Administration is gratefully acknowledged.

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