Propylene Selective Oxidation as Studied by Oxygen-18 Labelling on Well-Denned MoO3 Catalysts

V. CortCs Corberan and S. Vic Bell6n (Editors), New Developments in Selective Oxidation II 0 1994 Elsevier Science B.V. All rights reserved.

67

Pro ylene Selective Oxidation as Studied by Oxygen-18 Labelling on WellDetned Moo3 Catalysts M. Abona, M. Roulleta, J. Massirdies, P. Delich&r$ and A. Guerrero-Ruizb aInstitut de Recherches sur la Catalyse, CNRS - 69626 Villeurbanne CMex France bDepartamento de Quimica Inorghica, Facultad de Ciencias, UNED, 28040 Madrid, Spain

ABSTRACT The propylene oxidation reaction has been studied using 1 8 0 on well defined Moo3 crystallites. Results show that the formation of products -acro?ein and carbon dioxideinvolves the lattice oxygen. The different 180-labelling, according to selective or total oxidation products, agrees with the structure sensitivity of this reaction. Post reaction analysis (by LRS, SIMS, LEIS) support a redox equilibrium which determines the 180 concentration in the surface layers. 1. INTRODUCTION

We have previously studied the structure-sensitivecharacter of the oxidation of propylene (1-4) on Moo3 crystallites. It has been shown that mild oxidation to acrolein occurs on (120) faces, i.e. the stepped-like lateral faces, whereas total oxidation to CO takes place on the basal (010) faces. This approach has been possible thanks to the p r e p i o n of [lo01 oriented MOO plates with crystallites characterized by a more important development of the exposed laiiral faces (3). The activation of dioxygen and the nature of oxygen species (lattice oxygen or adsorbed oxygen species) involved in the formation of products are important steps of the mechanism. In the present work, these processes have been investigated using 180 The l 8 0 labelling has been measured for the main products of the reaction: acrolein andEO This labelling has been also studied on the catalysts using Laser Raman Spectroscopy (LR$ and surface sensitive techniques: Secondary Ion Mass Spectroscopy (SIMS) and Low Energy Ion Scattering (LEIS). The relevance of oxygen-labelling techniques to study the propylene oxidation mechanism has been previously established by Keulks et al. (5-7) and other studies (8-11) on bismuth molybdate catalysts.

2. EXPERIMENTAL

-3

The propylene oxidation using 1802 (Eurisotop, purity = 98%) has been perform 420°C in a closed circulating reactor (volume: 158 cm3) in order to reduce the 1 0at 2 consumption. The gas phase was analysed with an on-line mass spectrometer connected to the reactor through a metering leak valve. In blank experiments, it has been checked that no reaction occurs in our experimental conditions, discarding then the homogeneous oxidation reaction. The initid gas phase composition was usually l80 /C3H6/He: 100/38/622, at atmospheric pressure. The reaction mixture was circulated for30 minutes prior to reaction to ensure mixing. The Moo3 catalysts were first pretreated at 450°C under oxygen ('602) for an hour. Oxygen was then evacuated after cooling down to room temperature. A dry ice/acetone cold trap (-77OC) placed just after the reactor in the circulating loop removed all condensible products, that is mainly acrolein and water. The presence of this cold trap

68

was necessary because, in preliminary experiments, it was observed that the rate of acrolein oxidation was about 3 times higher than the rate of mild oxidation of propylene. The analysis of the condensible products required to warm up the trap, the reactor being then by-passed. The circulation ensured a VVH of 360 OOO h - l for a mass of l . l g of catalyst. The high VVH and the presence of a cold trap allowed to discard the secondary reactions such as the formation of C02 by acrolein oxidation. It was also checked that no exchange between 1802 and 160-lattice oxygen occured under our experimental conditions. Such an exchange was only evidenced at much higher temperatures, near 600°C.Carbon dioxide (9, 12, 13) and even more water (8,14) are known to favour the oxygen isotopic scrambling on oxides but this phenomenon must be reduced since H20 is eliminated by trapping. Regarding the influence of C02, it was observed that the relative amounts of labelled and unlabelled C02 (amu 48, 46, 44) remained nearly constant for several hours at the end of the raction, suggesting that the isotopic scrambling can be neglected. The two types of [OlO] and [loo] oriented MOO catalysts compared in the present work have been previously characterized by XRD, S E d HREM, XPS analysis (3). Their main physical properties are recalled in Table I. Besides a different proportion of exposed faces, these catalysts allowed a preferential inspection, by surface sensitive techniques, of either the (010) faces or the stepped lateral (100) faces, i.e. the (120) faces (3, 4). Table 1 Physical characterization of the Moo3 catalysts A(BET) (m2.g-1)

Mean size of crystallites Olm)

Exposed (120) faces (96)

Exposed (010) faces ( W )

~~

M&3[W

0.5

6x2~6

20

60

Moo3[0101

0.05

350x25~1000

7

90

3. RESULTS

3.1. Study of the 180-labelling of products In the static circulation reactor at 420°C, with a dry ice cold trap, the main products are acrolein and C02 in agreement with previous studies in a differential microreactor. After calibration of the mass spectrometer, the selectivity pattern is also comparable to that previously measured (4). The Moo3 [l,OO] catalysts gives an amount of acrolein twice larger than C02 whereas the reverse ratio of selectivity is measured for the Moo3 [OlO] catalyst. In the presence of 1802, the percentage of 180 in acrolein (a ) and in C02 (a ) can be compared as a function of the time of reaction for Moo3 [l& in Fig.1 and &r Moo3 [OlO] in Fig.2. a is defined by the following equations: a*( %) =

I(58) I(56)

+ I(58)

x 100

a@)=

+ I(46) x loo 2[1(48) + I(46) + I(44)] 21(48)

with I(x): intensity of the amu x peak measured by mass spectrometry (amu: atomic mass unit). As shown in Fig.1 and 2, aAand a first increase with time b t then come to a nearly on the surface of the constant value suggesting a quasi-equil%rium between 180 and

1$

69

ca yst C160 mu 44) is mainly detected at the beginning of the reaction but in eed C g O l i O and 61@02(amu 46 and 48 respectively) rapidly grow with time and C1 02 becomes then prevalent. Note that ac is always higher than aA. Moreover about the same aA is obtained on both Mo03 samples, the e remark is also valid for ac. These results correspond to the same relative pressures ofyP0 (100 Ton) and propylene (38 Ton). For an initial mixture with l802/C Hg: 11 Torr/4 t o n , aA = 40 and a - 50 have been obtained. On the other hand, wdh a much larger pressure of reac %62/C&H6 = 200 Torr/76 Ton, we have measured a - 70 and a - 85. The 1 0 iabelling en clearly depends on the initial pressure o f reactants an2 more generally on the experimental conditions. For mixtures richer in propylene with 180 /C3H6 = 100 Torr/77 Ton, QA and ac first increase with time as already observed but Aen reach a maximum and decrease as shown in Fig.3.

$

vb

Figure 1. l 8 0 labelling of the products vs reaction time on Moo3[ 100](1802/C3H6: 100/38)

5

10

15

time (h)

Figure 2. 180 labelling of the products vs reaction time on Md3[010](1802/C3H6: 100138)

20

40

60

time ( h )

Figure 3. 180 labelling of the products vs reaction time on M003[010](1802/C3Hg: 100/77)

70

3.2. Study of the

lb labelling of M a 3 catalysts

3.2.1. Post-reaction analysis by LRS Changes in LRS spectra have been recorded only when the Mo03 crystallites are first finely ground. Such a treatment indeed increases the ratio surfacidvolume of the samples characterized by a very small surface yea (Table I). Fig. 4 co pares the LRS spectra obtained after propylene oxidation with 6 0 2 (Fig. 4a) and with IF02 (Fig. 4b). The three main bands characteristics of MOO (15) have been observed in both cases at 996 cm-1 (Mo = 0), 820 cm-1 (Mo-0-Mo) and 666 cm-1 (three-bonded oxygen). After reaction with 1802 (Fig. 4b), two additional features have been recorded at lower frequencies as expected (16): a weak band at 948 cm-1 attributed to Mo = 180 and a shoulder in the Even after a careful shape analysis, no change medium band near 790 cm-l (Mo-~~O-MO). be detected in the third band at 666 cm-1. The relative intensity of the additional Mo = 'i"gb band with respect to the Mo = l60 band is less than 1%. The same ratio is found for the band near 790 cm-1 with respect to the band at 820 cm-1. 3.2.2. Post-reaction analysis by S M S One of us has performed a SIMS analysis on the Moo3 catalysts after reaction with 1802 (17). The more significant result is reported here in Fig. 5 showing the ratio (180/160) as a function of the sputtering depth on MoO [OlO] and Mo03 [ I F ] . This ratio has been a uall measured by the r lative in ensity C?I the secondary cluster ions [100M0180]+ and f3M0160]+ typical of l%O and 6O content, respectively. It may be observed that the 8 0 amount is larger on the (010) faces than on the (120 faces mainly exposed on the surface of [ 1001 oriented MOO? crystallites. Moreover the 1 0 concentration first decreases with depth down to a nearly co"nt&t value.

i

d

I

h

lo00

600 wave number (cm

Figure 4. LRS spectra on grounded MOO (a) after reaction with 1602, (b) ader reaction with 1802.

-.xu -

- 3

Sputtering Depth (am.)

')

Figure 5. 180/160 ratio measured by SIMS after reaction on (a) Mo03[010], (b) Mo03[100l

71

3.2.3. Post reaction LEIS analysis

This technique is the most sensitive one to the topmost surface lay r md the experimental conditions -4He+ (1 kev) with low current density (3O.SpA.rnm-?)- strongly reduce the sputtering damage. Post-reaction LEIS spectra are shown in Fig.6 for MoO [loo] and in Fig.7 for Mo03 [OlO]. In Fig. 6a and 7a, the analysis was directly performd whereas in Fig. 6b and 7b the spectra were recorded after heating the sam les at about is clearly 400°C for 5 minutes under ultra-high vacuum (UHV). The presence of detected n the urface of both samples with a ratio 180/l60 = 3/2 on MOO [lo01 (F. 6a) and psO/Of6 = 2/3 on Moo3 [OlO] (Fig. 7a). After heating under U aV , the 18gd signal vanished on Mo03 [loo] (Fig. 6b) and is sharply reduced on Mo03 [OIO] (Fig.%).

Ib

4. DISCUSSION

As shown in Fig. 1 and Fig.2, the oxygen-18 labelling of acrolein is very similar on Moo3 [loo] and Moo3 [OlO]. The same behaviour is observed for the C02 labelling a . This observation is in agreement with the structure-sensitivityof C H6 oxidation on M d 3 (1, 2, 4), phenomenon further confirmed in the present work: (12Offaces active in selective oxidation and (010) faces active in combustion are both exposed on the two samples. Fig. 1 and Fig. 2 also point to the participation of surface lattice oxygen (16&-) to the formation of both acrolein and carbon dioxide. Indeed aA and 01 increase in the course of the run, especially in the be inning, but never reach lOO%.%ote that the reaction conditions labelling of the products owing to the very small surface area of strongly favour the the samples. Takin as usual 10x10-6 mol. of 1602/m2, one could only obtain a few percents of C3H41 0 (less than 3%) from the initial surface la tice oxygen in the best case, that is on the catalyst with the larger surface area (0.5 m2! /g) and assuming the sole formation of acrolein. Note also th t the 180-labelling is significantly lower (aA = 40, a = 50) for low initial pressure of * 180 /C3H6 = 11 Torr/4 Ton. The pmcipation lattice oxygen is also evidenced in Ag.3 wiere a decreases with time when the reactant mixture becomes poor in 180 . The participation of lattice oxygen has been already well established on bismuth m o l y d t e (5-11) oxides at least for acrolein formation whereas the que ti n remained debated for C02 production. In this respect the formation of C1602 and c1&?80 is informative. The nearly constant a value reached by the 18 labelling of products would be a kind of fingerprints of the quasi-equilibrium of the 180/ 6 0 ratio on the catalyst surface. The fact that 'YA< ac strongly suggests that the 180 concentration is lower on (120) faces active in acrolein formation. This seems to be in line with the fact that the combustion reaction requires more surface oxygens that the selective reaction to acrolein. The nearly constant a values further suggest a quasi equilibrium in agreement with a redox mechanism as also evidenced by a post-reaction XPS analysis showing Mov species (Fig.8). The replenishment of surface oxygen vacancies created by the formation of products would occur via two different sources:

A0

8

5

1$

P

- 1802 activation leading to surface lattice oxygen 1802- with a possible diffusion from the surface towards the bulk. - Bulk lattice oxygen 1602- diffusing towards the surface, thus explaining that the 180 labelling do not quickly reach about 100%. Owing to the lamellar crystal structure, the diffusion of oxygen ions could well be anisotropic, favoured in directions normal to the [OlO] axes and therefore towards the (120) faces. Such an equilibrium will indeed depend on the experimental conditions such as the relative 1802 pressure as illustrated in Fig.3. Furthermore a has been shown to depend on the absolute pressure of reactants as already reported. We will now discuss the post-reaction analysis of the catalysts. The additional LRS ands associated with lattice 180 are so weak that it can be infered that the amount of 1 0 is

k

72

-

-

250-

_ _ _ _ _ _ _ ~ _ _ _ _7

---

-

--

150

340

. -~~ ~

400

520

460

340

400

E(eW

E(eW

Figure 6. LEIS anal sis on MoO-[lOO

520

460

(a) after reaction, (b) after reaction and heating at

140JK A

400°C under UHV J60 near 4&V, -

,

J80 near 45OeV).

-

.

' 7 7' 3 =m 250:

i

180*

I

l00C

150t

-

60

50

20

340

400

460

E(eV)

520

340

400

460

E(eW

520

Figure 7. LEIS analysis on Mo03[010], (a) after reaction, @) after reaction and heating at

400°C under UHV.

238

236

234

232

Binding Energy / ev

230

238

236 234 232 Binding Energy / cV

230

Figure 8. Post reaction XPS analysis of the Mo 3d levels on (a) Mo03[010], (b) Mo03[100]. The appearance of an additional doublet attributed to Mov is indicated by arrows.

73

small and restricted to a few surface layers, in agreement with SIMS analysis. However, it is interesting to note that only terminal and bridging oxygens appear to be involved in the formation of products. In bismuth molybdate catalysts, the preferential insertion of terminal (18, 19) or bridging oxygens (7) in acrolein is still debated. The SIMS experiments support the conclusion that 180 is present on the surface after reaction. $" amount of 180 would be higher on the basal (010) face 'n agreement with the isotopic 0 labelling of C 0 2 9 alr dy discussed. The decreasing I d 0 concentration with depth agrees with an anisotropic diffusion determined by the lamellar structure of MOO . L d S experiments clearly show th presence of 180 on both post reaction samples. However, the relative concentration 180,160 appears to be higher on MOO [lo01 than on Moo3 [OlO], at least prior to heating at 4o0"t under UHV conditions. Af?er this thermal treatment at the temperature of reaction, 1 0 is only detected on Mo03 [OlO]. As hydrocarbon residues including oxygen specie have been detected by XPS analysis of the C level, the sharp decrease of the surface 1$ signal after heating can be attributed very Ii&y to a combustion surface reaction. At a less extent other phenomenons can be invoked: dehydroxylation, desorption of weakly bonded adsorbed oxygens and anisotropic diffusion towards the bulk. The conflicting results gained by LEIS with respect SIMS concerning the 180/160 ratio show that comparative quantitative measurements by these surface sensitive techniques are difficult. Note that owing to their small size (table I), the analysis on [lo01 oriented crystallites is not defined as well as on a large (010) face of a single crystal. According to the precise geometry of the experiment, the analysis will include more or less various crystal faces in addition with the expected (120) faces. Other reasons inherent to the techniques can also be invoked: sputtering and definition of the initial surface state (mainly in SIMS experiments), depth of analysis and shadowing effects depending on the crystal orientation, different level of surface contamination by carbon species.. . 5. CONCLUSIONS

We have investigated the 1 8 0 activation and its insertion into the products (acrolein and CO in the oxidation reaction 0% propylene on [lo01 and [OlO] oriented Mo03 crystallites. The 80 labelling is in agreement with the structure sensitivity of this reaction confirmed in the present work: the labelling of acrolein and C02, respectively, is nearly the same on the two Moo3 catalysts as expected if each product is formed on a specific crystal face, (120) for acrolein and (010) for C02 (1-4). The participation of lattice o x y p in both reaction products has been clearly evidenced: preferential formation of C3H4 0 and C1602 in the beginning of the run and further establishment of a quasi constant 180 labelling. This quasi equilibrium implies a redox mechanism with replenishment of oxygen surface vacancies by two sources: activation of gaseous 1802 and diffusion of bulk lattice 160 ions towards the surface. Moreover the Raman spectra suggest that the active oxygens would be both terminal and bridging oxygens. As terminal Mo=O groups are typical of the basal (010) face, it can be tentatively proposed that such oxygens are active for total oxidation. About the question relative to a preferential oxygen activation according to the crystal faces, the higher 180 labelling for CO compared to that of acrolein supports that gaseous oxygen is preferent'ally activated on (OfO) faces in agreement with SIMS analysis on Moo3 samples. Surface 180 has been also clearly evidenced by LEIS on post reaction catalysts. The sharp decrease of the 180 signal after heating has been explained by a surface reaction with carbonaceous residues detected by XPS. Thanks to the uses of 1802 combined with complementary analysis of the products labelling and of the 1 8 0 concentration on the surface of well defined Moo3 crystallites, it has been possible to get a significant insight into important steps of the propylene oxidation reaction.

'1

74

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ACKNOWLEDGMENTS The authors are indebted to Prof. R. Olier (ECL) for the Raman study of the catalysts. They also thank Drs. J.C. Volta and J.C. Varine and Mr. V. Ducarme for fruitful discussions.