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Sb2O4 Biphasic Catalyst by Oxygen spillover
Studies in Surface Science and Catalysis 138 A. Guerrero-Ruizand I. Rodriguez-Ramos(Editors) 9 2001 Elsevier Science B.V. All rights reserved.
363
Enhancement of the Catalytic Performance of NiMoO4 and Modification of the Kinetic parameters of Oxidative Dehydrogenation of Propane over NiMoO4/Sb204 Biphasic Catalyst by Oxygen spillover H. M. AbdelDayem and P. Ruiz Unit6 de catalyse et chimie des mat6riaux divis6s, Universit6 catholique de Louvain, Croix du Sud 2/17,1348 Louvain-la- Neuve, Belgium, Fax (3210) 473649. Partial oxidation of propane to propene was investigated over monophasic NiMoO4 and biphasic NiMoO4/ff-Sb204 in different proportions in heterogeneous reaction region. The existence of ff-Sb204 in the mixture with NiMoO4 produces a synergetic effect between the two phases. Selectivity towards propene was maximal with a mass ratio of NiMoO4/(NiMoO4+ c~-Sb204) close to 0.25. The presence of ~-Sb204 in the mixture with NiMoO4 modifies the kinetic parameters of the reaction namely, decreases the partial pressure propane order and activation energy of propene formation. XRD and XPS characterization of pure NiMoO4 and mechanical mixtures before and after reaction revealed neither formation of new oxide phases between NiMoO4 and ff-Sb204 nor contamination of one phase by an element of the other phase. Confocal LRS indicated that the addition of o;-Sb204 to NiMoO4 modifies the coordination state of Mo in the NiMoO4. These results could be explained as an action of oxygen spillover. 1. INTRODUCTION The selective oxidation of propane to propene is promising and important target in the field of partial oxidation of alkane. This reaction is the first step in the oxidation of propane to acrolein and in the ammoxidation of propane to acrylonitrile. Recently, metal molybdate based catalysts have been reported by different authors as effective catalysts for the selective partial oxidation of propane [1]. One important characteristic of metal molybdate catalysts is the presence of a synergetic effect between different phases. NiMoO4-MoO3 catalyst oxidize butane to maleic anhydride with high performance than pure phases [2a]. NiMoO4 with 15-40% "excess" MoO3 exhibited higher selectivity and activity compared to single compound phase. Similar results have obtained with MnMoO4-MoO3 [2b]. A cooperation between oxide phases via oxygen spillover (Oso) has been postulated to explain such effects [2b]. In recent years evidences to explain synergetic effect between oxide phases by oxygen spillover has been accumulated. About 40 mixtures of pure single or mixed oxides, doped or not, investigated in different laboratories, exhibited similar synergy in different oxidation reactions and this cooperation was explained as results of oxygen spillover [3]. In this work the oxidative dehydrogenation of propane has been used as reaction test. The objective of this contribution is to show that the cooperation between phases via Oso leads to a synergetic effect between the oxide phases and also to show that, for the first time, that
364 Oso can modify the kinetic parameters of the reaction. This is demonstrated by using stoichiometeric NiMoO4 as an Oso acceptor phase and ot-Sb204 as a typical Oso donor phase [3]. Aside from the oxygen spillover, a common explanation of synergy in such experiments is the formation of a surface contamination of elements of one phase on the surface of the other phase. Surface contamination was studied by photoelectron spectroscopy (XPS). Another explanation of the synergetic effects is the formation of a new phase associating two elements in a mixed compound in particular NiSb206. For this reason the catalytic activity of NiSb206 has been studied. X-ray diffraction measurements were performed to determine the crystalline phases and to detect phase modifications or formation of new compound. Mo coordination changes; were studied by using Raman spectroscopy analysis. 2. E X P E R I M E N T A L
Pure stoichiometeric NiMoO4 was prepared (adapting the procedure described by Ozkan et al. [2a]) by coprecipitation from aqueous solutions of 750 ml of 0.057M ammonium heptamolybdate (Merck 99+%) and 750ml of 0.4M of nickel nitrate (Adrich 99+%), at pH=6 and at temperature of 60~ The total addition period was 0.5 h. After filtration the precipitate was dried in air at 110~ for 12h; then calcined under a flow of oxygen at 500~ for 12h. The purity of the prepared NiMoO4 from excess MoO3 was checked by Raman Spectroscopy and thermal analysis (DTA/TGA). ot-Sb204 was prepared by oxidation of Sb203 (Aldrich 99+%) by HNO3 (65%) followed by calcination of the obtained solid at 500 ~ in air after it has been cleaned from HNO3 with water. NiSb206 (Ni:Sb 1:2) was prepared by coprecipitation from nickel chloride (Aldrich 99.95%) and antimony pentachloride SbC16 (Aldrich 99%); they were dissolved in an aqueous HC1 solution (Vel 37%). The catalyst precursor was dried overnight at 120~ and then calcined at 700~ for 16 h. The biphasic catalysts were prepared mixing mechanically both oxides in n-pentane (Aldrich 98+%) in different mass ratio (Rm= 1.0, 0.5, 0.25 and 0.1, Rm= 1.0 is pure 100%NiMoO4 and Rm= 0 for 100% (1-Sb204). Catalytic activity measurements were performed in a conventional fixed bed reactor system at atmospheric pressure by feeding 02, C3H8 and He as diluent. Total feedrate was 30 ml/min. The partial pressures of oxygen and propane were varied between 5.08kPa and 30.4kPa. Conversion was kept below 10% to operate reactor in differential mode. The reaction was studied at three temperatures: 400, 450 and 490~ No homogeneous gas phase for production of propene and no diffusion limitations were observed in this temperature range. The synergetic effect in selectivity are calculated according to a formula: Synergyeffect = SAB S(A+B)x100 -
S(A+B) Where SAB is selectivity of the mixture and S(A+m is the theoretical selectivity which would be observed in the absence of any synergetic effect, defined for mixture with a given Rm as: R m x Y A+ ( 1 - R m ) x Y B S(A+B) = R m x C A + ( 1 - R m ) x C B In which CA and CB are the propane conversion and YA and YB are the yield in propene of the NiMoO4 and ~-8b204 respectively.
365 XRD was performed on a Kristalloflex Siemens D5000 diffractometer using the K~l,2 radiation of Cu (X = 1.5418A) for 20 angles varying from 10 ~ to 80 ~ The fluorescence contribution was eliminated from the diffracted beam using a curved graphite monochromator. The scan rate was 0.4 deg. min-1 corresponding to a step size of 0.04 degree and a step time of 6s. The particle size (t(A))of NiMoO4 was calculated by using Scherrer formula: t(A) = 0.92 X / B cos0. where B is peak width at the half high of [330] peak of NiMoO4 at 20= 43.841 degree. Specific surface areas (SBET) were measured on a Micromeritics ASAP 2000 instrument (adsorption of Krypton). Theoretical SBET were calculated both for the fresh and the used mechanical mixtures of NiMoO4 and c~-Sb204 by using the formula: Theoretical SBET = (SBET)NiMoO4. Rm + (SBET)~-Sb2O4 (1-Rm) In which (SBET)NiMoO, and (SBET)a-Sb204 are the BET surface areas of nickel molybdate and antimony oxide respectively. XPS analyses were performed with an SSX-100 model 206 X-ray photoelectron spectrometer. The binding energies were calibrated against the C([C-(C,H)] (carbon involved in a single bond with carbon or hydrogen) line fixed at 284.8 eV. The analysis first consisted in a scan of the binding energies between 0.0 and 1100 eV. This aimed at detecting all the different elements present at the surface of the samples. Cls,Ni2p, Mo3d, Sb3d3/2 and Ols bands were thereafter recorded. The XPS peaks were decomposed by a least square routine provided by the manufacturer with an 85/15 Gaussian/ Lorentzian ratio and a non linear baseline. Confocal laser Raman spectroscopy (LRS) was performed with a Dilor Labram Spectrometer equipped with a computerized X-Y transition stage. The Raman spectra were excited using a He-Ne laser (632.8nm, 10mW laser power at the sample surface). The spectrometer resolution was 7cm q. The full spectra, hereafter shown for Raman shifts, between 200 cm ~ and 1200 cm -1, were built by merging three partial spectra successively, obtained by averaging 3 scans of 15 sec. It was checked that the spectra shown were not to be perturbed by any thermal degradation of the samples under the laser beam during the analysis. In addition, to have a representative picture and to check homogeneity of the surface, about 30 spectra were measured at different dispersed spots of a sample. 3. RESULTS
3.1. Catalytic activity and empirical kinetic parameters Both c~-Sb204 and NiSb206 are fully inactive. For similar conversion the addition of (x-Sb204 in the biphasic catalyst increased the propene selectivity of pure NiMoO4. Moreover the propene selectivity and synergetic effect in selectivity increased with increasing ~-Sb204 amount in the mixture and reached a maximum value for the 25%NiMoO4+75%Sb204 mixture (Rm= 0.25) followed by a decrease at higher ~-8b204 content namely Rm= 0.1 as shown in Figures 1 and 2 respectively (Catalytic test conditions were partial pressures of C3H8, O2 and He are 10.13, 5.07 and 86.1kPa respectively, temperature 450~ As shown in Table 1 the presence of ~-Sb204 in the mixture decreases the partial pressure propane order and the apparent activation energy of the reaction. The partial pressure oxygen order, for both 100% NiMoO4 and mixtures, was near zero.
366
1~176 14~176 90 i, i ................................................................................................................... 1200 - ..................................................................... I I S y n e r g e t i c in(S) i
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
=
80
~ ,~
o~ 70 60 o 50 >" 40 ~ 30 ~
,,i-,,i
~'~
'~ ~ ~ ~,
20
~
10 0 1
0,5 Rm 0,25
1000 800600-
4002000-~
50%Sb204 75%Sb204 90%Sb204 c~-Sb204 weight% in mixture
0,1
Figure 1. Variation of propene selectivity with the change of the composition of the mechanical mixture of ~-8b204 and NiMoO4 (Rm).
1l l
~
Figure 2. Variation of synergetic effect in propene selectivity with the change of the ~-8b204 amount in the mechanical mixture of ~-Sb204 and NiMoO4.
Table 1 Empirical oxygen (m) and propane (n) partial pressure orders at 450~ (E) calculated from power law equation Catalyst (n) (m) NiMoO4(100%) 1.0 Near zero NiMoO4(50% )+SbzO4(50% ) 0.6 Near zero NiMoO4(25%)+Sb204(75%) 0.5 Near zero
and activation energy E (kJ mol l) 61.3 63.0 51.7
3.2 C h a r a c t e r i z a t i o n r e s u l t s
3.2.1. Specific s u r f a c e a r e a (SBET) The SBET analysis of pure oxides and their mixture 25% NiMoO4+75% ~-Sb204 before and after reaction are reported in Table 2. Values in parentheses are the theoretical values calculated on the basis of the composition of the mixture and the observed values of the corresponding constituting pure oxides. Table 2 SBET values (m2g -1) of NiMoO4, ~-8b204 and mechanical mixture(Rm=0.25) before and after reaction (in parenthesis theoretical values after reaction). Catalyst SBET A (SBET)% 100%NiMoO4 before reaction 41.0 0.0 after reaction 35.6 -13.2 100%Sb204 1.1 0.0 25% NiMoO4+75%Sb204 before reaction 11.3(11.1 ) 0.0 after reaction 10.4(9.7) -8.0 Note: A(SBET) is the variation in surface area (SBET) in %.
367 The SBET surface areas of 25% NiMoO4+75% ~-5b204 mixture are the same as those calculated theoretically and no difference are observed between the value of SBET of fresh and used mixture after reaction. Similar results have been obtained for the other mixtures of Rm= 0.5 and 0.1. Contrary the surface area of pure NiMoO4 decreased by about 13% after reaction.
3.2.2. X-ray diffraction (XRD) The XRD analysis (Table 3) of 100%NiMoO4, 100%17,-Sb204 and their mixture 25% NiMoO4 + 75% ot-Sb204 showed the presence of the monoclinic cz-NiMoO4 phase of JCPDs file 31-902 and o~-Sb204 (Cervantite) of JCPDS standard 11-0964. No indication of the presence of MoO3 and NiO was observed in the spectrum of NiMoO4. The XRD diffraction patterns of the mechanical mixtures of NiMoO4 and ~-Sb204 in different proportions correspond to their pure oxide phases constituting the mixtures. After reaction, both the mixtures and pure NiMoO4 showed no disappearance of any peak and no appearance of a new peak namely, no impurity was detected such as the formation of a new phase (e. g., NiSb206, MoO3 and NiO). In Table 3 we also reported the calculated particle size of NiMoO4 in 100%NiMoO4 and in mixture of Rm = 0.25 before and after reaction (values in parentheses). In the case of the 100%NiMoO4, the particle size is increased after reaction. however in the case of mixture no strong modification in the particle size was observed after reaction. Table 3 Results of XRD analysis ((values in parenthesis after reaction).
Catalyst
XRD phases
100%NiMoO4 cz-NiMoO4 100%Sb204 c~-Sb204 25% NiMoO4+75% ~-Sb204 c~-NiMoO4+c~-Sb204 Note: At is the variation in particle size in %.
t(A) 681.0(792.0) 677.0(730.0)
At(A)% +16.3 +7.8
3.2.3. X-ray photoelectron spectroscopy (XPS) Tables 4 and 5 give the results of the XPS analysis obtained for NiMoO4 and NiMoO4+~-Sb204 sample (Rm= 0.25). For both pure NiMoO4 and mixture we can see that Table 4 XPS results" atomic ratios for Mo/M, Sb/M, Ni/M and C/M for fresh and used NiMoO4 and 25%NiMoO4+75%cz-Sb204 mixture (Rm=0.25). Catalyst 100%NiMoO4
(before) (after)
Mo/Mo+Ni
Mo/M
0.43 0.44
0.43 0.44
0.44 0.40
0.31 0.30
Ni/M
Sb/M
C/M
1.23 1.12
25%NiMoO4+75% c~-8b204 (before) (after)
0.38 0.38
0.32 0.32
1.30 1.10
368 the surface composition is not changed after reaction namely Mo/M, Ni/M and Sb/M ratios (M=Mo+Ni +Sb) are not modified after reaction. In addition no carbon deposition is observed after reaction i.e., the carbon to metals ratio C/M is not modified after reaction. On the other hand the binding energies (Table 5) of molybdenum (Mo3ds/2 = 232.5eV, Mo3d3/2 = 235.5eV), nickel (Ni2p3/2 = 855.6eV and Ni2pl/2 = 873.3eV), antimony (Sb3ds/2 = 530.7eV and Sb3d3/2 = 540.0eV) and oxygen (Ols = 530.4eV) were not modified after reaction. The significant differences between the binding energies of Ni2P3/2 and Ols of NiMoO4 and of NiO indicates that there is no NiO in the samples. Table 5 XPS results: binding energies (BE_+0.2 eV) for Mo3ds/2, Ni2p3/2, Sb3ds/2 and Ols peaks for fresh and used NiMoO4, 25%NiMoO4+75%Sb204 mixture (Rm=0.25) and ~-5b204. catalyst (Mo3d 5/2) (Ni2P3/2) (Sb3d5/2) (O 1s) 100% ot-Sb204 (before) 530.7 530.4 (after) 530.6 530.3 100%NiMoO4 (before) 232.5 855.6 530.4 (after) 232.4 855.5 530.4 25%NiMoO4 (before) 232.3 855.6 530.7 530.5 (after) 232.3 855.5 530.7 530.6 NiO (reference) 853.9 529.3
3.2.4. Laser Raman spectroscopy (LRS) Figure 3 shows the Raman spectra obtained for pure oxides NiMoO4, ~-Sb204 and their mixture (25% NiMoO4+75%ct-Sb204) before and after reaction. In the case of pure NiMoO4 the Laser Raman bands at 707, 911 and 961 cm ~ clearly indicated the presence of ot-NiMoO4 [5] in which the Ni +2 and Mo+6 species are in octahedral oxygen environment. In addition to these bands we also observed a Raman bands at 817, 887 and 937 cm 1 which have been assigned previously to deformed tetrahedral molybdenum species [5]. After reaction pure NiMoO4 showed a similar spectrum as that observed before reaction. In the case of the fresh (25%NiMoO4+75%~-Sb204) mixture the spectrum obtained is described by a mixture of ct-Sb204 and NiMoO4. A similar Raman bands of Mo octahedral and deformed tetrahedral of pure NiMoO4 have been observed except a small shift have been observed in the Raman bands at 940, 890 and 821 cm ~ which were attributed to deformed tetrahedral Mo species. The important observation is that after reaction the Raman bands attributed to deformed tetrahedral Mo species have disappeared and only the bands characteristic for octahedral Mo of NiMoO4 and the bands of ct-Sb204 have observed. 4. DISCUSSION XRD and XPS characterization of pure NiMoO4 and mechanical mixtures before and after reaction revealed that neither formation of a new phase between -NiMoO4 and -Sb204 nor contamination of one phase by an element of other phase occurred during reaction. On the other hand we have observed that, an artificial contamination of NiMoO4 by -Sb204 precursor (it consider tha.t -Sb204 covered NiMoO4 by monolayer) exhibited a strong tendency to disappear after long reaction time (48h)[4].
369
20~.1 \
402.3 100%Sb204
:3
,,961.0
v
(/) (-
rn ixture (after)
J
__=
82 1.0
909.0
j962.0
/94(
m ix tu re (before) 937 341.0
961.0
81 7 396.0
817 100%NiMoO4
i
100
300
i
500
:
i
700
900
,
=
1100
1300
Raman Shift (cm-1)
Figure 3. Confocal laser Raman spectra of 100%NiMoO4 and 100%(~-8b204 and of their mixture (25%NiMoO4+75%Sb204) before and after reaction. This allowed to suggest that the role of o;-Sb204 in the enhancement of propene selectivity and modification of kinetic parameters of the reaction is interpreted as a cooperation between two separate phases according to a remote control mechanism [3]. (x8b204 is Oso donor and NiMoO4 is a oxygen spillover acceptor [3]. Even though, if NiSb206 is formed as a new phase between NiMoO4 and o;-8b204 and it has not been detected by XRD. NiSb206 is considered as an effective oxygen spillover donor which increasing selectively the formation of acrylic acid from acrolein [6]. This suggestion is consistent with the the previous works done in our laboratory for the oxidative dehydrogenation of propane and ethane over MgVO/ot-Sb204 and MoVO or NiVO/ot-Sb204 respectively [7]. The principle consequence of this cooperation is the increase in the selectivity. Previous studies for propane and ethane oxidation over NiCoMoO4 and NiMoO4 respectively [lb,8] demonstrated that the homolytic C-H bond break is the rate determining step (RDS), involving most probably an electron rich oxygen of molybdenum (octahedral) located adjacent to catalyst cation (Ni-O-Mo). It has also be concluded that the NiMoO4 possessing octahedral co-ordination is not only more active but also more selective. In this study, the high dependence of the formation rate of propene on the partial pressure of propane confirm that the rate determining step is the activation of propane. Our LRS results showed that in the case of pure NiMoO4 the Mo species detected is not only MoO6 octahedral but also MoO4 tetrahedral. In the case of the mixture, only high electron rich octahedral MoO6 sites have been detected. These results indicate that (~-Sb204
370 inhibits the presence of tetrahedral molybdenum species on c~-NiMoO4. This led us to suggest that the activation of propane (RDS) is easier over NiMoO4 in mixture than in the case of pure NiMoO4, which reflects the low dependence on propane pressure (low propane order), low activation energy and enhancement of propene selectivity when NiMoO4 is mixed with c~Sb204. In addition, where tetrahedral Mo (MOO4) species are believed to be more effective for chemisorption of propene than propane namely in the case of pure NiMoO4, the adsorption of formed propene is very strong and easily decomposed to COx. However in the case of the mixture, propene is easily desorbed from the NiMoO4 surface and the possibility to complete the oxidation to COx is decreased. It can be suggested that ~-Sb204 facilitates the reoxidation (lower oxygen order) of the Mo sites to more active MoO6 during reaction by Oso. On the other hand SBET and XRD characterization indicated that the addition of ~-Sb204 to NiMoO4 stabilized the surface of NiMoO4 (no modification of SBET) after reaction. This might be due to ~-8b204 inhibiting the sintering of NiMoO4 particles (no change in particle size observed after reaction in the case of the mixture). ~-Sb204 inhibits the presence of Mo tetrahedral species on NiMoO4 which might be it easily sintered. REFERENCES
1. (a) C. Mazzochia, C. Aboumrad, C. Diagne, E. Tempesti, J. M. Hermann and G.Thomas, Catal. Lett., 10 (1991) 181. (b) D. L. Stem and R. K. Grasselli, J. Catal. 167 (1997) 560. 2. (a) U. S. Ozkan. and G. L. Schrader, J. Catal., 95(1985)126. (b) U. S. Ozkan, R. C. Gill and M. R. Smith, Appl. Catal .62 (1990) 105. 3. L. T. Weng and B. Delmon, Appl. Catal. A, 81 (1992) 141. 4. (a) H. M. AbdelDayem, F. Christiaens, P. Ruiz and B. Delmon, submitted to 4th Int.Congr.on Oxidation Catalysis, September 16-21,2001, Berlin Germany. (b) H. M. AbdelDayem and P. Ruiz, submitted to 4 th Int. Symp. on Catalysts Deactivation, October 7-10, 2001, Lexington, Kentucky, USA. 5. J. Grimblot, E. Payen and J. P. Bonnelle, proc. 4 Th Int. Conf. on the Chem.and Uses of Molybdenum (H. F. Barry, P. C.H. Mitchell, Eds.) Climax Molybdenum Cy, 1982, pp.261. 6. S. Briet, M. Estenfelder, H.-G. Lintz, A. Tenten and H. Hibst, Appl. Catal. A., 134 (1996) 81. 7. (a) S.R.G. Carraz~m, C. Peres, J. P. Bernard, M. Ruwet, P. Ruiz and B. Delmon, J. Catal. 158(1996)452. (b) T. Osawa, P. Ruiz and B. Delmon, Catal. Today 61 (2000) 317. 8. A. Kaddouri, R. Anouchinsky, C. Mazzocchia, L. Maderia and M. F. Portela, Catal. Today, 40 (1998) 20.
363
Enhancement of the Catalytic Performance of NiMoO4 and Modification of the Kinetic parameters of Oxidative Dehydrogenation of Propane over NiMoO4/Sb204 Biphasic Catalyst by Oxygen spillover H. M. AbdelDayem and P. Ruiz Unit6 de catalyse et chimie des mat6riaux divis6s, Universit6 catholique de Louvain, Croix du Sud 2/17,1348 Louvain-la- Neuve, Belgium, Fax (3210) 473649. Partial oxidation of propane to propene was investigated over monophasic NiMoO4 and biphasic NiMoO4/ff-Sb204 in different proportions in heterogeneous reaction region. The existence of ff-Sb204 in the mixture with NiMoO4 produces a synergetic effect between the two phases. Selectivity towards propene was maximal with a mass ratio of NiMoO4/(NiMoO4+ c~-Sb204) close to 0.25. The presence of ~-Sb204 in the mixture with NiMoO4 modifies the kinetic parameters of the reaction namely, decreases the partial pressure propane order and activation energy of propene formation. XRD and XPS characterization of pure NiMoO4 and mechanical mixtures before and after reaction revealed neither formation of new oxide phases between NiMoO4 and ff-Sb204 nor contamination of one phase by an element of the other phase. Confocal LRS indicated that the addition of o;-Sb204 to NiMoO4 modifies the coordination state of Mo in the NiMoO4. These results could be explained as an action of oxygen spillover. 1. INTRODUCTION The selective oxidation of propane to propene is promising and important target in the field of partial oxidation of alkane. This reaction is the first step in the oxidation of propane to acrolein and in the ammoxidation of propane to acrylonitrile. Recently, metal molybdate based catalysts have been reported by different authors as effective catalysts for the selective partial oxidation of propane [1]. One important characteristic of metal molybdate catalysts is the presence of a synergetic effect between different phases. NiMoO4-MoO3 catalyst oxidize butane to maleic anhydride with high performance than pure phases [2a]. NiMoO4 with 15-40% "excess" MoO3 exhibited higher selectivity and activity compared to single compound phase. Similar results have obtained with MnMoO4-MoO3 [2b]. A cooperation between oxide phases via oxygen spillover (Oso) has been postulated to explain such effects [2b]. In recent years evidences to explain synergetic effect between oxide phases by oxygen spillover has been accumulated. About 40 mixtures of pure single or mixed oxides, doped or not, investigated in different laboratories, exhibited similar synergy in different oxidation reactions and this cooperation was explained as results of oxygen spillover [3]. In this work the oxidative dehydrogenation of propane has been used as reaction test. The objective of this contribution is to show that the cooperation between phases via Oso leads to a synergetic effect between the oxide phases and also to show that, for the first time, that
364 Oso can modify the kinetic parameters of the reaction. This is demonstrated by using stoichiometeric NiMoO4 as an Oso acceptor phase and ot-Sb204 as a typical Oso donor phase [3]. Aside from the oxygen spillover, a common explanation of synergy in such experiments is the formation of a surface contamination of elements of one phase on the surface of the other phase. Surface contamination was studied by photoelectron spectroscopy (XPS). Another explanation of the synergetic effects is the formation of a new phase associating two elements in a mixed compound in particular NiSb206. For this reason the catalytic activity of NiSb206 has been studied. X-ray diffraction measurements were performed to determine the crystalline phases and to detect phase modifications or formation of new compound. Mo coordination changes; were studied by using Raman spectroscopy analysis. 2. E X P E R I M E N T A L
Pure stoichiometeric NiMoO4 was prepared (adapting the procedure described by Ozkan et al. [2a]) by coprecipitation from aqueous solutions of 750 ml of 0.057M ammonium heptamolybdate (Merck 99+%) and 750ml of 0.4M of nickel nitrate (Adrich 99+%), at pH=6 and at temperature of 60~ The total addition period was 0.5 h. After filtration the precipitate was dried in air at 110~ for 12h; then calcined under a flow of oxygen at 500~ for 12h. The purity of the prepared NiMoO4 from excess MoO3 was checked by Raman Spectroscopy and thermal analysis (DTA/TGA). ot-Sb204 was prepared by oxidation of Sb203 (Aldrich 99+%) by HNO3 (65%) followed by calcination of the obtained solid at 500 ~ in air after it has been cleaned from HNO3 with water. NiSb206 (Ni:Sb 1:2) was prepared by coprecipitation from nickel chloride (Aldrich 99.95%) and antimony pentachloride SbC16 (Aldrich 99%); they were dissolved in an aqueous HC1 solution (Vel 37%). The catalyst precursor was dried overnight at 120~ and then calcined at 700~ for 16 h. The biphasic catalysts were prepared mixing mechanically both oxides in n-pentane (Aldrich 98+%) in different mass ratio (Rm= 1.0, 0.5, 0.25 and 0.1, Rm= 1.0 is pure 100%NiMoO4 and Rm= 0 for 100% (1-Sb204). Catalytic activity measurements were performed in a conventional fixed bed reactor system at atmospheric pressure by feeding 02, C3H8 and He as diluent. Total feedrate was 30 ml/min. The partial pressures of oxygen and propane were varied between 5.08kPa and 30.4kPa. Conversion was kept below 10% to operate reactor in differential mode. The reaction was studied at three temperatures: 400, 450 and 490~ No homogeneous gas phase for production of propene and no diffusion limitations were observed in this temperature range. The synergetic effect in selectivity are calculated according to a formula: Synergyeffect = SAB S(A+B)x100 -
S(A+B) Where SAB is selectivity of the mixture and S(A+m is the theoretical selectivity which would be observed in the absence of any synergetic effect, defined for mixture with a given Rm as: R m x Y A+ ( 1 - R m ) x Y B S(A+B) = R m x C A + ( 1 - R m ) x C B In which CA and CB are the propane conversion and YA and YB are the yield in propene of the NiMoO4 and ~-8b204 respectively.
365 XRD was performed on a Kristalloflex Siemens D5000 diffractometer using the K~l,2 radiation of Cu (X = 1.5418A) for 20 angles varying from 10 ~ to 80 ~ The fluorescence contribution was eliminated from the diffracted beam using a curved graphite monochromator. The scan rate was 0.4 deg. min-1 corresponding to a step size of 0.04 degree and a step time of 6s. The particle size (t(A))of NiMoO4 was calculated by using Scherrer formula: t(A) = 0.92 X / B cos0. where B is peak width at the half high of [330] peak of NiMoO4 at 20= 43.841 degree. Specific surface areas (SBET) were measured on a Micromeritics ASAP 2000 instrument (adsorption of Krypton). Theoretical SBET were calculated both for the fresh and the used mechanical mixtures of NiMoO4 and c~-Sb204 by using the formula: Theoretical SBET = (SBET)NiMoO4. Rm + (SBET)~-Sb2O4 (1-Rm) In which (SBET)NiMoO, and (SBET)a-Sb204 are the BET surface areas of nickel molybdate and antimony oxide respectively. XPS analyses were performed with an SSX-100 model 206 X-ray photoelectron spectrometer. The binding energies were calibrated against the C([C-(C,H)] (carbon involved in a single bond with carbon or hydrogen) line fixed at 284.8 eV. The analysis first consisted in a scan of the binding energies between 0.0 and 1100 eV. This aimed at detecting all the different elements present at the surface of the samples. Cls,Ni2p, Mo3d, Sb3d3/2 and Ols bands were thereafter recorded. The XPS peaks were decomposed by a least square routine provided by the manufacturer with an 85/15 Gaussian/ Lorentzian ratio and a non linear baseline. Confocal laser Raman spectroscopy (LRS) was performed with a Dilor Labram Spectrometer equipped with a computerized X-Y transition stage. The Raman spectra were excited using a He-Ne laser (632.8nm, 10mW laser power at the sample surface). The spectrometer resolution was 7cm q. The full spectra, hereafter shown for Raman shifts, between 200 cm ~ and 1200 cm -1, were built by merging three partial spectra successively, obtained by averaging 3 scans of 15 sec. It was checked that the spectra shown were not to be perturbed by any thermal degradation of the samples under the laser beam during the analysis. In addition, to have a representative picture and to check homogeneity of the surface, about 30 spectra were measured at different dispersed spots of a sample. 3. RESULTS
3.1. Catalytic activity and empirical kinetic parameters Both c~-Sb204 and NiSb206 are fully inactive. For similar conversion the addition of (x-Sb204 in the biphasic catalyst increased the propene selectivity of pure NiMoO4. Moreover the propene selectivity and synergetic effect in selectivity increased with increasing ~-Sb204 amount in the mixture and reached a maximum value for the 25%NiMoO4+75%Sb204 mixture (Rm= 0.25) followed by a decrease at higher ~-8b204 content namely Rm= 0.1 as shown in Figures 1 and 2 respectively (Catalytic test conditions were partial pressures of C3H8, O2 and He are 10.13, 5.07 and 86.1kPa respectively, temperature 450~ As shown in Table 1 the presence of ~-Sb204 in the mixture decreases the partial pressure propane order and the apparent activation energy of the reaction. The partial pressure oxygen order, for both 100% NiMoO4 and mixtures, was near zero.
366
1~176 14~176 90 i, i ................................................................................................................... 1200 - ..................................................................... I I S y n e r g e t i c in(S) i
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
=
80
~ ,~
o~ 70 60 o 50 >" 40 ~ 30 ~
,,i-,,i
~'~
'~ ~ ~ ~,
20
~
10 0 1
0,5 Rm 0,25
1000 800600-
4002000-~
50%Sb204 75%Sb204 90%Sb204 c~-Sb204 weight% in mixture
0,1
Figure 1. Variation of propene selectivity with the change of the composition of the mechanical mixture of ~-8b204 and NiMoO4 (Rm).
1l l
~
Figure 2. Variation of synergetic effect in propene selectivity with the change of the ~-8b204 amount in the mechanical mixture of ~-Sb204 and NiMoO4.
Table 1 Empirical oxygen (m) and propane (n) partial pressure orders at 450~ (E) calculated from power law equation Catalyst (n) (m) NiMoO4(100%) 1.0 Near zero NiMoO4(50% )+SbzO4(50% ) 0.6 Near zero NiMoO4(25%)+Sb204(75%) 0.5 Near zero
and activation energy E (kJ mol l) 61.3 63.0 51.7
3.2 C h a r a c t e r i z a t i o n r e s u l t s
3.2.1. Specific s u r f a c e a r e a (SBET) The SBET analysis of pure oxides and their mixture 25% NiMoO4+75% ~-Sb204 before and after reaction are reported in Table 2. Values in parentheses are the theoretical values calculated on the basis of the composition of the mixture and the observed values of the corresponding constituting pure oxides. Table 2 SBET values (m2g -1) of NiMoO4, ~-8b204 and mechanical mixture(Rm=0.25) before and after reaction (in parenthesis theoretical values after reaction). Catalyst SBET A (SBET)% 100%NiMoO4 before reaction 41.0 0.0 after reaction 35.6 -13.2 100%Sb204 1.1 0.0 25% NiMoO4+75%Sb204 before reaction 11.3(11.1 ) 0.0 after reaction 10.4(9.7) -8.0 Note: A(SBET) is the variation in surface area (SBET) in %.
367 The SBET surface areas of 25% NiMoO4+75% ~-5b204 mixture are the same as those calculated theoretically and no difference are observed between the value of SBET of fresh and used mixture after reaction. Similar results have been obtained for the other mixtures of Rm= 0.5 and 0.1. Contrary the surface area of pure NiMoO4 decreased by about 13% after reaction.
3.2.2. X-ray diffraction (XRD) The XRD analysis (Table 3) of 100%NiMoO4, 100%17,-Sb204 and their mixture 25% NiMoO4 + 75% ot-Sb204 showed the presence of the monoclinic cz-NiMoO4 phase of JCPDs file 31-902 and o~-Sb204 (Cervantite) of JCPDS standard 11-0964. No indication of the presence of MoO3 and NiO was observed in the spectrum of NiMoO4. The XRD diffraction patterns of the mechanical mixtures of NiMoO4 and ~-Sb204 in different proportions correspond to their pure oxide phases constituting the mixtures. After reaction, both the mixtures and pure NiMoO4 showed no disappearance of any peak and no appearance of a new peak namely, no impurity was detected such as the formation of a new phase (e. g., NiSb206, MoO3 and NiO). In Table 3 we also reported the calculated particle size of NiMoO4 in 100%NiMoO4 and in mixture of Rm = 0.25 before and after reaction (values in parentheses). In the case of the 100%NiMoO4, the particle size is increased after reaction. however in the case of mixture no strong modification in the particle size was observed after reaction. Table 3 Results of XRD analysis ((values in parenthesis after reaction).
Catalyst
XRD phases
100%NiMoO4 cz-NiMoO4 100%Sb204 c~-Sb204 25% NiMoO4+75% ~-Sb204 c~-NiMoO4+c~-Sb204 Note: At is the variation in particle size in %.
t(A) 681.0(792.0) 677.0(730.0)
At(A)% +16.3 +7.8
3.2.3. X-ray photoelectron spectroscopy (XPS) Tables 4 and 5 give the results of the XPS analysis obtained for NiMoO4 and NiMoO4+~-Sb204 sample (Rm= 0.25). For both pure NiMoO4 and mixture we can see that Table 4 XPS results" atomic ratios for Mo/M, Sb/M, Ni/M and C/M for fresh and used NiMoO4 and 25%NiMoO4+75%cz-Sb204 mixture (Rm=0.25). Catalyst 100%NiMoO4
(before) (after)
Mo/Mo+Ni
Mo/M
0.43 0.44
0.43 0.44
0.44 0.40
0.31 0.30
Ni/M
Sb/M
C/M
1.23 1.12
25%NiMoO4+75% c~-8b204 (before) (after)
0.38 0.38
0.32 0.32
1.30 1.10
368 the surface composition is not changed after reaction namely Mo/M, Ni/M and Sb/M ratios (M=Mo+Ni +Sb) are not modified after reaction. In addition no carbon deposition is observed after reaction i.e., the carbon to metals ratio C/M is not modified after reaction. On the other hand the binding energies (Table 5) of molybdenum (Mo3ds/2 = 232.5eV, Mo3d3/2 = 235.5eV), nickel (Ni2p3/2 = 855.6eV and Ni2pl/2 = 873.3eV), antimony (Sb3ds/2 = 530.7eV and Sb3d3/2 = 540.0eV) and oxygen (Ols = 530.4eV) were not modified after reaction. The significant differences between the binding energies of Ni2P3/2 and Ols of NiMoO4 and of NiO indicates that there is no NiO in the samples. Table 5 XPS results: binding energies (BE_+0.2 eV) for Mo3ds/2, Ni2p3/2, Sb3ds/2 and Ols peaks for fresh and used NiMoO4, 25%NiMoO4+75%Sb204 mixture (Rm=0.25) and ~-5b204. catalyst (Mo3d 5/2) (Ni2P3/2) (Sb3d5/2) (O 1s) 100% ot-Sb204 (before) 530.7 530.4 (after) 530.6 530.3 100%NiMoO4 (before) 232.5 855.6 530.4 (after) 232.4 855.5 530.4 25%NiMoO4 (before) 232.3 855.6 530.7 530.5 (after) 232.3 855.5 530.7 530.6 NiO (reference) 853.9 529.3
3.2.4. Laser Raman spectroscopy (LRS) Figure 3 shows the Raman spectra obtained for pure oxides NiMoO4, ~-Sb204 and their mixture (25% NiMoO4+75%ct-Sb204) before and after reaction. In the case of pure NiMoO4 the Laser Raman bands at 707, 911 and 961 cm ~ clearly indicated the presence of ot-NiMoO4 [5] in which the Ni +2 and Mo+6 species are in octahedral oxygen environment. In addition to these bands we also observed a Raman bands at 817, 887 and 937 cm 1 which have been assigned previously to deformed tetrahedral molybdenum species [5]. After reaction pure NiMoO4 showed a similar spectrum as that observed before reaction. In the case of the fresh (25%NiMoO4+75%~-Sb204) mixture the spectrum obtained is described by a mixture of ct-Sb204 and NiMoO4. A similar Raman bands of Mo octahedral and deformed tetrahedral of pure NiMoO4 have been observed except a small shift have been observed in the Raman bands at 940, 890 and 821 cm ~ which were attributed to deformed tetrahedral Mo species. The important observation is that after reaction the Raman bands attributed to deformed tetrahedral Mo species have disappeared and only the bands characteristic for octahedral Mo of NiMoO4 and the bands of ct-Sb204 have observed. 4. DISCUSSION XRD and XPS characterization of pure NiMoO4 and mechanical mixtures before and after reaction revealed that neither formation of a new phase between -NiMoO4 and -Sb204 nor contamination of one phase by an element of other phase occurred during reaction. On the other hand we have observed that, an artificial contamination of NiMoO4 by -Sb204 precursor (it consider tha.t -Sb204 covered NiMoO4 by monolayer) exhibited a strong tendency to disappear after long reaction time (48h)[4].
369
20~.1 \
402.3 100%Sb204
:3
,,961.0
v
(/) (-
rn ixture (after)
J
__=
82 1.0
909.0
j962.0
/94(
m ix tu re (before) 937 341.0
961.0
81 7 396.0
817 100%NiMoO4
i
100
300
i
500
:
i
700
900
,
=
1100
1300
Raman Shift (cm-1)
Figure 3. Confocal laser Raman spectra of 100%NiMoO4 and 100%(~-8b204 and of their mixture (25%NiMoO4+75%Sb204) before and after reaction. This allowed to suggest that the role of o;-Sb204 in the enhancement of propene selectivity and modification of kinetic parameters of the reaction is interpreted as a cooperation between two separate phases according to a remote control mechanism [3]. (x8b204 is Oso donor and NiMoO4 is a oxygen spillover acceptor [3]. Even though, if NiSb206 is formed as a new phase between NiMoO4 and o;-8b204 and it has not been detected by XRD. NiSb206 is considered as an effective oxygen spillover donor which increasing selectively the formation of acrylic acid from acrolein [6]. This suggestion is consistent with the the previous works done in our laboratory for the oxidative dehydrogenation of propane and ethane over MgVO/ot-Sb204 and MoVO or NiVO/ot-Sb204 respectively [7]. The principle consequence of this cooperation is the increase in the selectivity. Previous studies for propane and ethane oxidation over NiCoMoO4 and NiMoO4 respectively [lb,8] demonstrated that the homolytic C-H bond break is the rate determining step (RDS), involving most probably an electron rich oxygen of molybdenum (octahedral) located adjacent to catalyst cation (Ni-O-Mo). It has also be concluded that the NiMoO4 possessing octahedral co-ordination is not only more active but also more selective. In this study, the high dependence of the formation rate of propene on the partial pressure of propane confirm that the rate determining step is the activation of propane. Our LRS results showed that in the case of pure NiMoO4 the Mo species detected is not only MoO6 octahedral but also MoO4 tetrahedral. In the case of the mixture, only high electron rich octahedral MoO6 sites have been detected. These results indicate that (~-Sb204
370 inhibits the presence of tetrahedral molybdenum species on c~-NiMoO4. This led us to suggest that the activation of propane (RDS) is easier over NiMoO4 in mixture than in the case of pure NiMoO4, which reflects the low dependence on propane pressure (low propane order), low activation energy and enhancement of propene selectivity when NiMoO4 is mixed with c~Sb204. In addition, where tetrahedral Mo (MOO4) species are believed to be more effective for chemisorption of propene than propane namely in the case of pure NiMoO4, the adsorption of formed propene is very strong and easily decomposed to COx. However in the case of the mixture, propene is easily desorbed from the NiMoO4 surface and the possibility to complete the oxidation to COx is decreased. It can be suggested that ~-Sb204 facilitates the reoxidation (lower oxygen order) of the Mo sites to more active MoO6 during reaction by Oso. On the other hand SBET and XRD characterization indicated that the addition of ~-Sb204 to NiMoO4 stabilized the surface of NiMoO4 (no modification of SBET) after reaction. This might be due to ~-8b204 inhibiting the sintering of NiMoO4 particles (no change in particle size observed after reaction in the case of the mixture). ~-Sb204 inhibits the presence of Mo tetrahedral species on NiMoO4 which might be it easily sintered. REFERENCES
1. (a) C. Mazzochia, C. Aboumrad, C. Diagne, E. Tempesti, J. M. Hermann and G.Thomas, Catal. Lett., 10 (1991) 181. (b) D. L. Stem and R. K. Grasselli, J. Catal. 167 (1997) 560. 2. (a) U. S. Ozkan. and G. L. Schrader, J. Catal., 95(1985)126. (b) U. S. Ozkan, R. C. Gill and M. R. Smith, Appl. Catal .62 (1990) 105. 3. L. T. Weng and B. Delmon, Appl. Catal. A, 81 (1992) 141. 4. (a) H. M. AbdelDayem, F. Christiaens, P. Ruiz and B. Delmon, submitted to 4th Int.Congr.on Oxidation Catalysis, September 16-21,2001, Berlin Germany. (b) H. M. AbdelDayem and P. Ruiz, submitted to 4 th Int. Symp. on Catalysts Deactivation, October 7-10, 2001, Lexington, Kentucky, USA. 5. J. Grimblot, E. Payen and J. P. Bonnelle, proc. 4 Th Int. Conf. on the Chem.and Uses of Molybdenum (H. F. Barry, P. C.H. Mitchell, Eds.) Climax Molybdenum Cy, 1982, pp.261. 6. S. Briet, M. Estenfelder, H.-G. Lintz, A. Tenten and H. Hibst, Appl. Catal. A., 134 (1996) 81. 7. (a) S.R.G. Carraz~m, C. Peres, J. P. Bernard, M. Ruwet, P. Ruiz and B. Delmon, J. Catal. 158(1996)452. (b) T. Osawa, P. Ruiz and B. Delmon, Catal. Today 61 (2000) 317. 8. A. Kaddouri, R. Anouchinsky, C. Mazzocchia, L. Maderia and M. F. Portela, Catal. Today, 40 (1998) 20.