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SiO2 catalysts: A CO-oxidation study
SOLID STATE
Solid State lonics 63-65 ( 1993 ) 736-742 North-Holland
IONICS The cobalt-molybdenum interaction in CoMo/SiO2 catalysts: A CO-oxidation study M. de Boer l, E.P.F.M. Koch, R.J. Blaauw, E.R. Stobbe, A.N.J.M. H o f f m a n n , L.A. Boot, A.J. van Dillen and J.W. Geus Department of Inorganic Chemistry, University of Utrecht, Sorbonnelaan 16, P.O. Box 80083, 3508 TB Utrecht, The Netherlands
The interaction among cobalt, molybdenum and silica in CoMo/SiO 2 catalysts has been investigated in a series of catalysts with the same overall composition and varying interaction among the components. X R D and DRIFTS give a rough indication of the phases present, and the formation of CoMoO4 can be easily monitored. The formation of CoMoO4 is observed in most of the catalysts due to the poor interaction between molybdenum oxide and silica. The presence of other minor, non-crystalline phases must be attested by other methods, such as, T P R and catalytic oxidation of carbon monoxide. The presence of"free" cobalt can be demonstrated by a low temperature peak in TPR. The catalytic oxidation of CO may function as a useful indicator for the interaction between cobalt and molybdenum, because the oxidation activity of cobalt species incorporated in a molybdate lattice (i.e. CoMoO 4) is considerably less than cobalt oxide.
1. Introduction
The interaction between cobalt oxide and molybdenum oxide in hydrotreatment catalysts is a subject of intense debate. Numerous reviews have covered this field [e.g. 1-4]. The discussion has focused on y-A1203 supported catalysts, because of the suitability of this support for industrial purposes. The preference of 7-A1203 is due to the ease of shaping support bodies (by extrusion) and its high affinity for MOO3, which ensures a high dispersion. Disadvantages of y - A 1 2 0 3 a r e the poor resistance to SO2 in the presence of O2 (giving A12(SO4)3) and the inclination of cobalt and nickel to form aluminates as spinels or inverse spinels [5-9 ]. The reactivity of the cobalt ions decreases considerably when incorporated in 7-A1203. For instance, the reduction temperature of Co304 and COA1204 in a TPR experiment may differ 600 K [5]. Formation of the aluminates takes place at temperatures beyond ca. 600 K [ 5,10 ], depending on the conditions: oxygen, and, especially, traces of moisture accelerate the formation of spinels. The cobalt (nickel) spinels formed 1 Present address: Akzo Chemicals B.V., Nieuwendammerkade 1-3, P.O. Box 26223, 1002 GE Amsterdam, The Netherlands.
during calculation are hardly sulfidable [11], and are therefore undesirable for a proper hydrotreatment performance. Due to the good interaction between Co and 7-A1203 (and Mo and y-A1203), the reaction between cobalt and molybdenum in the oxidic catalyst is highly unlikely [12]. Cobalt molybdate is rarely found in active CoMo/AI203 catalysts. It is assumed by most authors that this phase is unfavorable for the formation of a highly active HDS catalyst after sulfidation [13], though others claim that CoMoO4 is a good precursor [ 14 ]. The poor interaction between M O 6+ and SiO2 [ 15 ] is the cause of the unsuitability of CoMo/SiO2 catalysts for HDS applications, despite the high affinity of cobalt for SiO2. Precipitation of Co 2+ on SiO2 may give rise to the formation of cobalt (hydro)silicate [16,17]. The most appropriate techniques to examine the interaction between cobalt, molybdenum and a support are, depending on the dispersion of the phases, XRD, TEM/SAED, LRS, IR, MES, XPS, EXAFS, several temperature programmed reactions, and catalytic performance tests. The interaction between cobalt, molybdenum, and silica in oxide catalysts will be investigated with "straightforward" techniques, such as, XRD, DRIFTS, TPR, and a less conventional method, like catalytic oxidation of car-
0167-2738/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
M. De Boer et al. /Cobalt-molybdenum interaction in CoMo/Si02 catalysts
bon monoxide. The use of CO oxidation to probe the C o - M o interaction is based on the assumption that the high intrinsic oxidation activity of Co304 decays upon incorporation of cobalt in a CoUoO4 lattice.
2. Experimental
The cobalt-molybdenum interaction can be systematically varied by the use of different preparation procedures, each with its own merits. The resulting set of silica supported cobalt-molybdenum catalysts has (roughly) the same overall composition, but with a strongly varying interaction among the components. The intended composition of 4 wt% CoO, and 15 wt% MoO~ represents a typical commercial HDS catalyst. A matrix of preparation techniques (table 1 ) describes the combinations applied in this series. Deposition precipitation from a homogeneous solution (HDP) [18], wet impregnation (WI), and mechanical mixing (MM). The first row and first column of the table represent the deposition of either cobalt or molybdenum on silica by means of HDP. This method is used with the intention to prepare a "monolayer" type of catalyst, i.e. a catalyst with optimum dispersion of the active component. For this reason HDP of Co 2÷ and Mo 3÷ [ 15] are chosen because of the good interaction of the less acidic Mo 3+ with the acidic SiO2. The second row and column denote the impregnation of either cobalt (5) or molybdenum (3) on Mo/SiO2 or Co/SiO2, previously prepared by HDP. This procedure is applied to acquire a moderately good interaction between the aqueous precursor and the (previously deposited) counterpart. The third row and third column of table 1 reflect mechanical mixing of cobalt (6) or molybTable 1 The matrix of preparation techniques used to establish a constant overall composition and different Co-Mo-SiO2 interactions. The numbers correspondto the codes in table 2. Molybdenum --, J,Cobah
HDP
Wl
MM
HDP WI
4 5
3 7
2
MM
6
-
1
737
denum oxide (2) with Mo/SiO2 or Co/SiO2, previously prepared by HDP. Obviously, the mechanical mixing does not provide an intimate contact between the components. The bulk cobalt and molybdenum oxide samples used for the mechanical mixing are prepared by deposition precipitation of COC12.6H20 or MoC163from homogeneous solution in absence of SiO2. The bulk oxides are previously calcined at 723 K in air for 16 h before mechanical mixing. The same applies to the samples prepared by HDP. After addition of the second component the catalysts are calcined once more under the same conditions. The preparation sequence of all samples is reviewed in table 2 (vide infra), that incorporates the "one elements" systems, Co/SiO2 (A) and Mo/SiO2 (B) for reference purposes. SiO2 (Si in table 2) was "prepared" by suspending Aerosil 200 V (Degussa) in acidified (HCI, p H = 1 ) demiwater and raising the pH by decomposition of urea (363 K); after filtration, washing with demiwater, the support was dried at 393 K in air overnight and calcined at 723 K in air during 16 h. This procedure was followed to make a fair comparison between the silica used in the supported catalyst systems and the mechanical mixtures. All catalysts were calcined at 723 K in air during 16 h before characterization and catalytic activity measurements. 2.1. C a r b o n m o n o x i d e o x i d a t i o n
CO oxidation is done in a fully automated apparatus for the catalytic combustion of carbon monoxide and methane. A mixture of high purity CO (99.995%), 02 (99.999%) and He (99.999%) (1/ 1/98) is led through 200 mg of the catalyst with a flow of 100 ml m i n - i. The quartz reactor has an internal diameter of 8 ram. The LHSV amounts ca. 12000 h-~. The catalyst is previously crushed in a mortar and pelletted at 100 MPa for one minute. A sieve fraction of 400-625 pm is used for this test. The samples are in situ pretreated in O2/He (10/90) for four hours at 723 K. Samples of the gas stream before and after the reactor are taken by a Valco valve, installed in a Perkin Elmer (type 8700) gaschromatographs. The components, CO and CO2 are separated on a Porapak R (3 m) column and detected by a Flame Ionization Detector (FID) after
738
M. De Boer et al. / Cobalt-molybdenum interaction in CoMo/Si02 catalysts
Table 2 Characteristics of the fresh, calcined Co-Mo/SiO2 catalysts. Code
Si A B C D E 1 2 3 4 5 6 7 8
Preparation method
" H D P " of SiO2 (vide supra )* HDP of Co 2+ on SiO2 b> HDP of MoC163- on SiO2 b) HDP of CoCI/'6H20 b) HDP ofMoCI63- b) HDP of MoC163- and Co 2+ MM of Si, C, and D MM of D with A Wl of AHM on A HDP of Co 2+ + M o 3+ on SiO2 W1 of Co 2+ on B MM of C with B WI of Co 2+ and AHM on SiO2 MM of D, E, Si
[Co] a) (wt%)
3.6 100 63 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6
[Mo] ") Mo/Co (wt%) (tool/tool)
-8.6 100 37 14.0 14.0 14.0 11.2 8.3 8.3 14.0 14.0
-1 2.2 2.2 2.2 1.7 1.3 1.3 2.2 2.2
BET surf. area (m2g i)
XRD phases before CO-ox.
after CO-ox.
amorphous amorphous MoO3 c) Co304 MoO3 c) MoO3 c), CoMoO4e), Co304 MoO3 cj, CoMoO4 e), Co304 MoO3 c.d) MoOsc), CoMoO4 e) MoO3 ~), CoMoO4 e), Co2SiO4 MoO3 c), CoMoO4 ~), Co3SiO4 MoO3 ~), CoMoO4 ~), Co204 MoO3 c), CoMoO4 e), Co2SiO 4 MoO3 c), CoMoO4 ~), Co304
n.d. n.d. f) n.d. f) n.d. f~ n.d. r) n.d. f) idem g) idem ~) idem g) idem g) idem g) n o Co304h) idem ~) no Co304h)
186 202 145 3 2 2 171 173 156 154 144 158 162 175
~ The Co and Mo contents are determined by means oflCP and calculated on basis of Co304 and MOO3. b) Decomposition of urea at 368 K is used to raise the pH. c) ASTM file: 05-508; d) ASTM file: 21-569. e) ASTM file: 21-868, Co304 and Co2SiO4 correspond to ASTM files 9 - 4 1 8 and 1 5 - 4 9 7 respectively. r) n.d. = n o t determined; g) idem =equal to the analysis before CO-oxidation; ~) MoO3 and CoMoO4 remained.
methanization. The in-built integrator integrates the peaks of the FID. For each sample three complete conversion curves (ascending and descending temperature) are measured in the region between 323 and 773 K with temperature steps of 10 K.
3. Results and discussion
The characteristic properties of the CoMo/SiO2 catalysts are summarized in table 2. The catalysts containing only one of the two elements (Co or Mo, A - D ) are included in the table for reference purposes, since they form the "building unit" for the other catalysis (2,3,5,6). The cobalt and molybdenum contents reveal that the intention to keep the composition of all catalysts constant at 15 wt% M o O 3 and 4 w% CoO is not completely fulfilled; the M o / Co ratio is, however, higher than unity for all samples. The cobalt content shows no deviation in the series of catalysts because all cobalt from the cobalt precursor ends up in the catalyst, due to the preparation procedure. On the other hand, some molybdenum does not end up in the catalyst, especially when H D P is used as a preparation method.
As expected from earlier discussion all samples as contained in table 2 exhibit a crystalline MoO3 phase due to the poor interaction of Mo 6+ with SiO2. The use of Mo 3+ leads to considerable improvement of the interaction, but insufficient to achieve an X-ray amorphous catalysts with 8.6 wt% MOO3. Surprisingly, another M o O 3 phase than the orthorhombic m o l y b d i t e ( ~ - M o O 3 , ASTM: 05-508 [ 19] ) is formed in sample 2: the hexagonal M o O 3 structure (ASTM: 21-569), possibly due to a slightly lower calcination temperature. Datta et al. [20] observed the same feature for mechanical mixtures of M o O 3 and SIO2. From the powder diffraction files it appears that the hexagonal phase irreversibly transforms into the orthorhombic phase between 723 and 773 K. The transformation is apparently complete at the calcination temperature of 723 K for supported catalysts, but not for sample 2, a mechanical mixture. Two phases of monoclinic CoMoO4 are frequently met in (un)supported cobalt-molybdenum catalysts: a - C o M o O 4 ( o r (confusingly) 13-CoMoO4, ASTM: 21-868 [21]) and b - C o M o O 4 ( o r a-CoMOO4, ASTM: 25-1434 [22] ). The a-CoMoO4 is the most stable modification [22-24], and is formed out o f b - C o M o O 4 at ~673 K [24]. The b-phase is as-
M. De Boer et al. / Cobalt-molybdenum interaction in CoMo/Si02 catalysts
sumed to be always present in combination with the a-phase, but often XRD-amorphous. Only the amodification of CoMoO4 is encountered in these catalysts. All samples of the preparation matrix contain a-CoMoO4, except sample 2. This can be understood when one recalls that a reaction between the bulk M o O 3 phase and the cobalt (hydro)silicate is hardly feasible. The cobalt ions are entirely incorporated in the SiO2 matrix, and the contact between cobalt and molybdenum is poor due to the mechanical mixing. Nevertheless, a CoMoO4 phase evolves when molybdenum is offered to the same substrate in the form of aqueous Mo 6+ (sample 3). Free cobalt oxide (C0304) is present in samples 1, 6, and 8, as monitored by XRD. The observation 0fC0304 in 1 and 6 is no surprise since it was added. Unfortunately sample 8 contains C0304 as well, although it was designed to be composed of bulk C o M o O 4 solely. Crystalline cobalt silicate is found in 4, 5, and 7, the samples in which cobalt was impregnated or coprecipitated with molybdenum. As mentioned before, samples 2 and 3 do not reveal cobalt silicate, because an ( X R D ) amorphous (hydro)silicate has been formed [ 17 ]. DRIFTS experiments on the series of catalysts give the same picture: formation of C o M o O 4 in all cases, sample 2 excluded. The presence of CoMoO4 can be deduced from a band near 941-8 cm -], caused by the VMo-o in a CoMoO4 lattice [24,25]. These bulk characterization methods give a rough indication of the phases in the catalysts, and the extent of interaction between the components can be derived to a certain level. However, it is conceivable that some additional (non-crystalline) species are present that may seriously affect the properties of the catalysts. Because of the non-uniformity of the structure of the catalysts, the use of advanced spectroscopic techniques (e.g. XPS and EXAFS) is useless. It will be shown that TPR and a catalytic test reaction are useful tools to disclose additional facts on the interaction between cobalt, molybdenum and silica. The TPR profiles of the CoMo/SiO2 catalysts ( 18) are shown in fig. 1. The TPR profile of samples containing supported CoMoO4 (3-5,7) exhibits strong resemblance to that of supported M o O 3 [ 26 ]: a relatively sharp peak at 760-780 K and a broad peak at 900-1000 K. The first peak in CoMoO4 cat-
739
i
6 .........
i
i
273 373 473 573 673 773 873 973 1073
T e m p e r a t u r e (K) Fig. 1. TPR patterns of CoMo/SiO2 catalysts; the numbers correspond to the samples in table 2.
alysts is positioned at lower temperature than in MoO3 samples, due to the better reducibility of the cobalt ions that act as nuclei for the reduced phase. A high temperature XRD experiment on sample 7 disclosed that the reduction of the CoMoO4 phase proceeds according to eq. ( 1 ): CoMoO4
T~6SO K C o M o O 3
T~970 K C 0 2 M 0 3 .
T-823 K C o 3 M O ( 1)
The hexavalent molybdenum ions are reduced to MO 4÷ at approximately 680 K in the cobalt molybdite phase. This temperature is lower than Tma~, in the TPR profile of fig. 1 probably because the heating rates in both experiments deviate. A metallic phase with Co3Mo3 stoichiometry appears at a relatively low temperature. Due to the higher reducibility of cobalt oxide [ 5,16,27 ] with respect to MoO3 this metallic phase is enriched with cobalt despite the lower cobalt content of the catalyst. At ca. 970 K (the expected temperature of Mo o formation) a new me-
M. De Boer et al. ~Cobalt-molybdenum interaction in CoMo/Si02 catalysts
740
tallic phase is formed that is enriched of molybdenum. The reduction of bulk Co304 is claimed to occur in two steps via CoO to Co o in the temperature region 570-670 K. The (onset) reduction temperature and the total reducibility decrease strongly when (hydro)silicates have been formed [16]. It can be derived from fig. 1 that some' 'free cobalt" (i.e., cobalt oxide, cobalt (hydro)silicate, etc., not incorporated in molybdenum oxide as CoMoO4) is present in samples 1, 2, 6, and 8. Co304 was found in catalyst 1,6, and 8 by X R D of the fresh samples. The presence of free cobalt species in sample 2, according to T P R (and not X R D ) is due to the amorphous (hydro)silicate that does not react with bulk MOO3. The same (hydro)silicate (A in table 2) layer is, at least partially, converted into CoMoO4 in catalyst 3, which possesses a reduction profile characteristic of CoMoO4.
Cobalt oxide exhibits high activity for oxidation reactions with its position near the top of the classical vulcano plot [28]. Therefore, it is a good candidate for catalytic combustion or incineration of hydrocarbons. One of the simplest reactions is the oxidation of carbon monoxide. The intrinsic activity of MoO3 for catalytic oxidation is much lower due to the high M o - O binding energy, which makes, on the other hand, the high valent metal oxides suitable catalysts for mild oxidation reactions. The activity of bulk C0304 (C), C o / S i O 2 ( A ) , bulk MoO3 (D), Mo/SiO2 (B), and bulk CoMoO4 is shown in the conversion curves in fig. 2.
I{X)
/ i ?~" . . . . . . . . . . .
80
o
v
/
j
/
/
20
//
/
/ 273
373
// 473
d 573
673
Temperature (K) Fig. 2. Conversion curves for the catalytic oxidation of CO on (a) Co30, (up and down) (b) Co/SiO2; (c) MOO3; (d) Mo/ SiO/and (e) CoMoO4. [CO] = [Oz] = 1 vol.%; LHSV~ 12,000 h- '; 200 mg catalyst (Co304 diluted 1: 10).
Obviously, the activity of the cobalt catalysts is superior to that of the molybdenum containing samples. The lower activity of Co/SiO2 (higher surface area) compared to Co304 must be attributed to the presence of cobalt (hydro)silicate. The highest oxidation capacity is assessed on very small (supported) cobalt oxide crystallites that may result when the (hydro)silicate is reduced at a high temperature and reoxidized. The hysteresis in the conversion curve of Co(If, III)304 is caused by its transformation into the lower valent C o ( I I ) O at high temperature. The reversible reconstruction of the Co304 lattice in the descending conversion curve induces a higher activity than the ascending curve. This feature has been previously reported by Oku et al. [ 29 ] using XPS. Apparently, the C o 3 0 4 ~ C o O transition occurs at lower temperatures than predicted for bulk cobalt oxides by thermodynamics. The activity of bulk MoO3 is poor, and it hardly improves at higher dispersions (Mo/SiO2). As expected, the activity of bulk CoMoO4 is in between that of Co304 and MOO3, which leads us to the postulation that the incorpo-
ration of cobalt in a molybdate lattice deactivates the cobalt species ,for catalytic oxidation. This postulate will be used for the explanation of the activity of the CoMo/SiOz catalysts and the related cobalt-molybdenum interaction. The activity of the CoMo/SiO2 catalysts and the kinetic parameter derived from the Arrhenius plots are enlisted in table 3. Prior to the C O - o x i d a t i o n experiments the samples were in situ calcined at 723 K for fours hours under 10% Oz/He. Three conversion curves between 223 and 673 K were recorded. The third measurement was used to calculate the apparent activation energy and the preexponential factor in a linear part of the Arrhenius plot, using the same conversion interval for all samples. The reaction rates for the oxidation of CO are the highest for samples 1 and 2 as in accordance with the observation of free Co304 in the X R D pattern for sample 1 and the presence of free cobalt species in T P R for sample 2. Samples 6 and 8 that showed free cobalt in T P R as well, are somewhat less active in the test reaction. The different oxidation behavior is best expressed by the large difference in the value of the apparent Ea (compare 1 and 2 to 6 and 8). The explanation for this paradox (samples 1, 2, and 6 all revealed the presence of some free cobalt species in
M. De Boer et aL / Cobah-molybdenum interaction in CoMo/SiOe catalysts
741
Table 3 Activity and kinetic parameters of CoMo/SiO2 catalysts in the oxidation of CO at T = 673 K. The apparent Ea and ko were calculated for all samples in the same conversion interval in a linear part of the Arrhenius plot. Sample
Description
Log rate ( r / m m o l e s -I m -2)
Apparent Ea (kJ mole - I )
ko ( s - l m -2) ")
BET surface area (m 2)
C D E 1 2 3 4 5 6 7 8
Co304 MoO3 CoMoO4 M M D, C, Si MM D , A WI Mo6+/A HDP Mo s+, CO2+ - S i WI Co2+/B MM C, B WI Mo 6+, C o 2 - - Si MM D, E, Si
- 4 . 7 b) -6.3 - 5.3 -5.8 -5.8 -6.4 -6.0 -6.1 -6.3 -6.8 -8.2
92 56 63 92 82 69 63 63 62 61 -
2X 10 I1 4 4 X 103 3 × l0 T 2X 107 3 × 104 3X 104 2 × 104 1 × 104 2X 103 -
3 2 4 171 173 156 154 144 158 162 175
a) The/Co values are normalized on the BET-surface area. b) The figure cannot be compared to others, because 100% conversion is achieved at lower temperature.
XRD and/or TPR) comes from characterization after the CO-oxidation experiments: neither of the samples 6 and 8 exhibits free cobalt after CO-oxidation. The cobalt oxides must have reacted with molybdena to cobalt molybdate. The apparent E,s give a more direct relationship between oxidation activity and cobalt-molybdenum interaction. A high apparent E, ( ~ 90 kJ mole -~) is obtained for the catalysts with free cobalt oxide (sample 1 and 2, prepared by mechanical mixture), whereas the low apparent Ea ( ~ 56 kJ mole- l ) over MoO3 is found for none of the catalysts. Catalysts 3-7 all possess apparent/?as with values in between that of MoO3 and Co304 (typically ~ 63 kJ m o l e - l ) , which coincides with the E : v a l u e connected with bulk C o M o O 4 . The strongly varying values for the apparent Ea are not reflected by large differences in reaction rates (table 2). The cause of this phenomenon is the existence of two reaction mechanisms for the oxidation of carbon monoxide: the associative mechanism (low E~) at low temperature and the redox mechanism (high E~) at high temperature. The temperature at which the transition from the associative to the redox mechanism occurs, depends on the chemical properties of the catalytically active material. At the same temperature the redox mechanism (high apparent E~ combined with a high ko) may already proceed on a cobalt oxide lattice, while the associative mechanism (low apparent Ea and a low ko) takes places on molybdenum oxide. Consequently,
the value of the apparent Ea is indicative of the species that causes the catalytic activity.
4. Concludingremarks It can be concluded that the kinetic parameter of the catalytic oxidation of CO are a useful tool to establish the presence of "free cobalt oxide species". With the combined information of XRD, DRIFTS, TPR, and CO-oxidation a schematic representation of the structure of the catalysts can be constructed. The results are shown in fig. 3. The structures in fig.
g ~
I
5
/ 2
~ []
[
I
6 ~
3
4
7
l__
.
MoO3 Co304
I
] SiO 2
I
I Co 2SiO4
18
Fig. 3. Schematic presentation of the structure of the CoMo/SiO2 as derived from the characterization methods; the numbers correspond to the catalysts in table 2.
742
M. De Boer et al. ~Cobalt-molybdenum interaction in CoMo/SiOe catalysts
3 correspond to the situation after CO-oxidation. Four phases are discerned: MoO3 in all samples due to the nonstoichiometric composition, C o M o O 4 in all samples except 2, C o 2 5 i O 4 in samples 4, 5, and 7 (the cobalt silicates in sample 2, and 3 are probably amorphous cobalt hydrosilicates), and C0304 only in samples 1, and 2. The installation of the interaction between the phases must be considered with care, because thermal treatments severely affect the solid state reactions and the structure of the oxidic phases. The stability of Co/SiO2 can be very high when a cobalt ( hydro )-silicate is formed (sample 2 and 3), but the mobility of hexavalent molybdenum oxide species on a SiO2 support induces a reaction between cobalt and molybdenum species. A catalytic test reaction that discriminates between a more active oxide (C0304) and a less active (MOO3 or CoMoO4) oxide is useful in assessing the interaction between cobalt and molybdenum oxide species, but the thermal instability of the mechanical mixtures obfuscates a fair comparison. The apparent activation energies for the CO-oxidation reaction give the best indication of the presence of "free" cobalt oxide because the overall activity of the samples do not show very explicit differences.
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[9] H. Schmalrzried, Z. Phys. Chem. 28 ( 1961 ) 203; Ber. Deutsch, Keram. Ges. 42 (1965) 11- and Z. Elecktrochem. 67 (1963) 93. [10] R.L. Chin and D.M. Hercules, J. Phys. Chem. 86 (1982) 360. [11] K. lnamura, T. Takyu, Y. Okamoto, K. Nagata and T. Imanaka, J. Catal. 133 (1992) 498. [ 12] L.E. Makovsky, J.M. Stencel, F.R. Brown, R.E. Tischer and S.S. Pollack, J. Catal. 89 (1984) 334. [13] V.H.J. de Beer, M.J.M. van der Aalst, C.J. Machiels and C.G.A. Schuit, J. Catal. 43 (1976) 78. [ 14] V.I. Yerofeyev and I.V. Kaletchits, J. Catal. 86 (1984) 55. [ 15 ] M. de Boer, A.J. van Dillen, J.W. Geus, M.A. Vuurman and I.E. Wachs, Catal. Letters 11 ( 1991 ) 227. [ 16] M.P. Rosynek and C.A. Polansky, Appl. Catal. 73 ( 1991 ) 97. [17] P. Gajardo, P. Grange and B. Delmon, J. Phys. Chem. 83 (13) (1979) 1771. [ 18 ] J.W. Geus, in: Studies in surface Science and Catalysis Vol. 16, eds. G. Poncelet, P. Grange and P.A. Jacobs (Elsevier, Amsterdam, 1983)p. 1. [ 19 ] Powder Diffraction File, Inorganic Phases, Search Manual ( Hanawalt ) ( Int. Centre for Diffraction Data, Swarthmore PA, 1989). [20] A.K. Datta, J.-W. Ha and J.R. Regalbuto, J. Catal. 133 (1992) 55. [21 ] P. Courline, P.P. Cord, G. Pannetier and R. Montarnal, Bull. Soc. Chim. Fr. 12 (1968) 4816. [22] G.W. Smith and J.A. lbers, Acta Cryst. 19 (1965) 269. [23] P. Gajardo, D. Pirotte, C. Defosse, P. Grange and B. Delmon, J. Elect. Spectr. Relat. Phenom. 17 (1979) 121. [ 24 ] U. Ozkan and G.L. Schrader, Appl. Catal. 23 ( 1987 ) 327. [25] F.R. Brown and L.E. Makovsky, Appl. Spectrosc. 31 (1) (1977) 44. [26] K.C. Pratt, J.V. Sanders and V. Christov, J. Catal. 124 (1990) 416. [ 27 ] J.H.A. Martens, H.F.J. van "t Blik and R. Prins, J. Catal. 97 (1986) 200. [28]G.I. Golodets, in: Heterogeneous Catalytic reactions involving molecular oxygen Studies in Surface Science and Catalysis, ed. J.R. Ross (Elsevier, Amsterdam, 1983) p. 312. [29] M. Oku and Y. Sato, Appl. Surface Sci. 55 (1992) 37. [ 30 ] J. van den Berg, A.J. van Dillen, J. van der Meyden and J.W. Geus in: Surface Properties and Catalysis by NonMetals, ed. J.P. Bonnelle (Reidel, Dordrecht, 1983 ) p. 493.
Solid State lonics 63-65 ( 1993 ) 736-742 North-Holland
IONICS The cobalt-molybdenum interaction in CoMo/SiO2 catalysts: A CO-oxidation study M. de Boer l, E.P.F.M. Koch, R.J. Blaauw, E.R. Stobbe, A.N.J.M. H o f f m a n n , L.A. Boot, A.J. van Dillen and J.W. Geus Department of Inorganic Chemistry, University of Utrecht, Sorbonnelaan 16, P.O. Box 80083, 3508 TB Utrecht, The Netherlands
The interaction among cobalt, molybdenum and silica in CoMo/SiO 2 catalysts has been investigated in a series of catalysts with the same overall composition and varying interaction among the components. X R D and DRIFTS give a rough indication of the phases present, and the formation of CoMoO4 can be easily monitored. The formation of CoMoO4 is observed in most of the catalysts due to the poor interaction between molybdenum oxide and silica. The presence of other minor, non-crystalline phases must be attested by other methods, such as, T P R and catalytic oxidation of carbon monoxide. The presence of"free" cobalt can be demonstrated by a low temperature peak in TPR. The catalytic oxidation of CO may function as a useful indicator for the interaction between cobalt and molybdenum, because the oxidation activity of cobalt species incorporated in a molybdate lattice (i.e. CoMoO 4) is considerably less than cobalt oxide.
1. Introduction
The interaction between cobalt oxide and molybdenum oxide in hydrotreatment catalysts is a subject of intense debate. Numerous reviews have covered this field [e.g. 1-4]. The discussion has focused on y-A1203 supported catalysts, because of the suitability of this support for industrial purposes. The preference of 7-A1203 is due to the ease of shaping support bodies (by extrusion) and its high affinity for MOO3, which ensures a high dispersion. Disadvantages of y - A 1 2 0 3 a r e the poor resistance to SO2 in the presence of O2 (giving A12(SO4)3) and the inclination of cobalt and nickel to form aluminates as spinels or inverse spinels [5-9 ]. The reactivity of the cobalt ions decreases considerably when incorporated in 7-A1203. For instance, the reduction temperature of Co304 and COA1204 in a TPR experiment may differ 600 K [5]. Formation of the aluminates takes place at temperatures beyond ca. 600 K [ 5,10 ], depending on the conditions: oxygen, and, especially, traces of moisture accelerate the formation of spinels. The cobalt (nickel) spinels formed 1 Present address: Akzo Chemicals B.V., Nieuwendammerkade 1-3, P.O. Box 26223, 1002 GE Amsterdam, The Netherlands.
during calculation are hardly sulfidable [11], and are therefore undesirable for a proper hydrotreatment performance. Due to the good interaction between Co and 7-A1203 (and Mo and y-A1203), the reaction between cobalt and molybdenum in the oxidic catalyst is highly unlikely [12]. Cobalt molybdate is rarely found in active CoMo/AI203 catalysts. It is assumed by most authors that this phase is unfavorable for the formation of a highly active HDS catalyst after sulfidation [13], though others claim that CoMoO4 is a good precursor [ 14 ]. The poor interaction between M O 6+ and SiO2 [ 15 ] is the cause of the unsuitability of CoMo/SiO2 catalysts for HDS applications, despite the high affinity of cobalt for SiO2. Precipitation of Co 2+ on SiO2 may give rise to the formation of cobalt (hydro)silicate [16,17]. The most appropriate techniques to examine the interaction between cobalt, molybdenum and a support are, depending on the dispersion of the phases, XRD, TEM/SAED, LRS, IR, MES, XPS, EXAFS, several temperature programmed reactions, and catalytic performance tests. The interaction between cobalt, molybdenum, and silica in oxide catalysts will be investigated with "straightforward" techniques, such as, XRD, DRIFTS, TPR, and a less conventional method, like catalytic oxidation of car-
0167-2738/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
M. De Boer et al. /Cobalt-molybdenum interaction in CoMo/Si02 catalysts
bon monoxide. The use of CO oxidation to probe the C o - M o interaction is based on the assumption that the high intrinsic oxidation activity of Co304 decays upon incorporation of cobalt in a CoUoO4 lattice.
2. Experimental
The cobalt-molybdenum interaction can be systematically varied by the use of different preparation procedures, each with its own merits. The resulting set of silica supported cobalt-molybdenum catalysts has (roughly) the same overall composition, but with a strongly varying interaction among the components. The intended composition of 4 wt% CoO, and 15 wt% MoO~ represents a typical commercial HDS catalyst. A matrix of preparation techniques (table 1 ) describes the combinations applied in this series. Deposition precipitation from a homogeneous solution (HDP) [18], wet impregnation (WI), and mechanical mixing (MM). The first row and first column of the table represent the deposition of either cobalt or molybdenum on silica by means of HDP. This method is used with the intention to prepare a "monolayer" type of catalyst, i.e. a catalyst with optimum dispersion of the active component. For this reason HDP of Co 2÷ and Mo 3÷ [ 15] are chosen because of the good interaction of the less acidic Mo 3+ with the acidic SiO2. The second row and column denote the impregnation of either cobalt (5) or molybdenum (3) on Mo/SiO2 or Co/SiO2, previously prepared by HDP. This procedure is applied to acquire a moderately good interaction between the aqueous precursor and the (previously deposited) counterpart. The third row and third column of table 1 reflect mechanical mixing of cobalt (6) or molybTable 1 The matrix of preparation techniques used to establish a constant overall composition and different Co-Mo-SiO2 interactions. The numbers correspondto the codes in table 2. Molybdenum --, J,Cobah
HDP
Wl
MM
HDP WI
4 5
3 7
2
MM
6
-
1
737
denum oxide (2) with Mo/SiO2 or Co/SiO2, previously prepared by HDP. Obviously, the mechanical mixing does not provide an intimate contact between the components. The bulk cobalt and molybdenum oxide samples used for the mechanical mixing are prepared by deposition precipitation of COC12.6H20 or MoC163from homogeneous solution in absence of SiO2. The bulk oxides are previously calcined at 723 K in air for 16 h before mechanical mixing. The same applies to the samples prepared by HDP. After addition of the second component the catalysts are calcined once more under the same conditions. The preparation sequence of all samples is reviewed in table 2 (vide infra), that incorporates the "one elements" systems, Co/SiO2 (A) and Mo/SiO2 (B) for reference purposes. SiO2 (Si in table 2) was "prepared" by suspending Aerosil 200 V (Degussa) in acidified (HCI, p H = 1 ) demiwater and raising the pH by decomposition of urea (363 K); after filtration, washing with demiwater, the support was dried at 393 K in air overnight and calcined at 723 K in air during 16 h. This procedure was followed to make a fair comparison between the silica used in the supported catalyst systems and the mechanical mixtures. All catalysts were calcined at 723 K in air during 16 h before characterization and catalytic activity measurements. 2.1. C a r b o n m o n o x i d e o x i d a t i o n
CO oxidation is done in a fully automated apparatus for the catalytic combustion of carbon monoxide and methane. A mixture of high purity CO (99.995%), 02 (99.999%) and He (99.999%) (1/ 1/98) is led through 200 mg of the catalyst with a flow of 100 ml m i n - i. The quartz reactor has an internal diameter of 8 ram. The LHSV amounts ca. 12000 h-~. The catalyst is previously crushed in a mortar and pelletted at 100 MPa for one minute. A sieve fraction of 400-625 pm is used for this test. The samples are in situ pretreated in O2/He (10/90) for four hours at 723 K. Samples of the gas stream before and after the reactor are taken by a Valco valve, installed in a Perkin Elmer (type 8700) gaschromatographs. The components, CO and CO2 are separated on a Porapak R (3 m) column and detected by a Flame Ionization Detector (FID) after
738
M. De Boer et al. / Cobalt-molybdenum interaction in CoMo/Si02 catalysts
Table 2 Characteristics of the fresh, calcined Co-Mo/SiO2 catalysts. Code
Si A B C D E 1 2 3 4 5 6 7 8
Preparation method
" H D P " of SiO2 (vide supra )* HDP of Co 2+ on SiO2 b> HDP of MoC163- on SiO2 b) HDP of CoCI/'6H20 b) HDP ofMoCI63- b) HDP of MoC163- and Co 2+ MM of Si, C, and D MM of D with A Wl of AHM on A HDP of Co 2+ + M o 3+ on SiO2 W1 of Co 2+ on B MM of C with B WI of Co 2+ and AHM on SiO2 MM of D, E, Si
[Co] a) (wt%)
3.6 100 63 3.6 3.6 3.6 3.6 3.6 3.6 3.6 3.6
[Mo] ") Mo/Co (wt%) (tool/tool)
-8.6 100 37 14.0 14.0 14.0 11.2 8.3 8.3 14.0 14.0
-1 2.2 2.2 2.2 1.7 1.3 1.3 2.2 2.2
BET surf. area (m2g i)
XRD phases before CO-ox.
after CO-ox.
amorphous amorphous MoO3 c) Co304 MoO3 c) MoO3 c), CoMoO4e), Co304 MoO3 cj, CoMoO4 e), Co304 MoO3 c.d) MoOsc), CoMoO4 e) MoO3 ~), CoMoO4 e), Co2SiO4 MoO3 c), CoMoO4 ~), Co3SiO4 MoO3 ~), CoMoO4 ~), Co204 MoO3 c), CoMoO4 e), Co2SiO 4 MoO3 c), CoMoO4 ~), Co304
n.d. n.d. f) n.d. f) n.d. f~ n.d. r) n.d. f) idem g) idem ~) idem g) idem g) idem g) n o Co304h) idem ~) no Co304h)
186 202 145 3 2 2 171 173 156 154 144 158 162 175
~ The Co and Mo contents are determined by means oflCP and calculated on basis of Co304 and MOO3. b) Decomposition of urea at 368 K is used to raise the pH. c) ASTM file: 05-508; d) ASTM file: 21-569. e) ASTM file: 21-868, Co304 and Co2SiO4 correspond to ASTM files 9 - 4 1 8 and 1 5 - 4 9 7 respectively. r) n.d. = n o t determined; g) idem =equal to the analysis before CO-oxidation; ~) MoO3 and CoMoO4 remained.
methanization. The in-built integrator integrates the peaks of the FID. For each sample three complete conversion curves (ascending and descending temperature) are measured in the region between 323 and 773 K with temperature steps of 10 K.
3. Results and discussion
The characteristic properties of the CoMo/SiO2 catalysts are summarized in table 2. The catalysts containing only one of the two elements (Co or Mo, A - D ) are included in the table for reference purposes, since they form the "building unit" for the other catalysis (2,3,5,6). The cobalt and molybdenum contents reveal that the intention to keep the composition of all catalysts constant at 15 wt% M o O 3 and 4 w% CoO is not completely fulfilled; the M o / Co ratio is, however, higher than unity for all samples. The cobalt content shows no deviation in the series of catalysts because all cobalt from the cobalt precursor ends up in the catalyst, due to the preparation procedure. On the other hand, some molybdenum does not end up in the catalyst, especially when H D P is used as a preparation method.
As expected from earlier discussion all samples as contained in table 2 exhibit a crystalline MoO3 phase due to the poor interaction of Mo 6+ with SiO2. The use of Mo 3+ leads to considerable improvement of the interaction, but insufficient to achieve an X-ray amorphous catalysts with 8.6 wt% MOO3. Surprisingly, another M o O 3 phase than the orthorhombic m o l y b d i t e ( ~ - M o O 3 , ASTM: 05-508 [ 19] ) is formed in sample 2: the hexagonal M o O 3 structure (ASTM: 21-569), possibly due to a slightly lower calcination temperature. Datta et al. [20] observed the same feature for mechanical mixtures of M o O 3 and SIO2. From the powder diffraction files it appears that the hexagonal phase irreversibly transforms into the orthorhombic phase between 723 and 773 K. The transformation is apparently complete at the calcination temperature of 723 K for supported catalysts, but not for sample 2, a mechanical mixture. Two phases of monoclinic CoMoO4 are frequently met in (un)supported cobalt-molybdenum catalysts: a - C o M o O 4 ( o r (confusingly) 13-CoMoO4, ASTM: 21-868 [21]) and b - C o M o O 4 ( o r a-CoMOO4, ASTM: 25-1434 [22] ). The a-CoMoO4 is the most stable modification [22-24], and is formed out o f b - C o M o O 4 at ~673 K [24]. The b-phase is as-
M. De Boer et al. / Cobalt-molybdenum interaction in CoMo/Si02 catalysts
sumed to be always present in combination with the a-phase, but often XRD-amorphous. Only the amodification of CoMoO4 is encountered in these catalysts. All samples of the preparation matrix contain a-CoMoO4, except sample 2. This can be understood when one recalls that a reaction between the bulk M o O 3 phase and the cobalt (hydro)silicate is hardly feasible. The cobalt ions are entirely incorporated in the SiO2 matrix, and the contact between cobalt and molybdenum is poor due to the mechanical mixing. Nevertheless, a CoMoO4 phase evolves when molybdenum is offered to the same substrate in the form of aqueous Mo 6+ (sample 3). Free cobalt oxide (C0304) is present in samples 1, 6, and 8, as monitored by XRD. The observation 0fC0304 in 1 and 6 is no surprise since it was added. Unfortunately sample 8 contains C0304 as well, although it was designed to be composed of bulk C o M o O 4 solely. Crystalline cobalt silicate is found in 4, 5, and 7, the samples in which cobalt was impregnated or coprecipitated with molybdenum. As mentioned before, samples 2 and 3 do not reveal cobalt silicate, because an ( X R D ) amorphous (hydro)silicate has been formed [ 17 ]. DRIFTS experiments on the series of catalysts give the same picture: formation of C o M o O 4 in all cases, sample 2 excluded. The presence of CoMoO4 can be deduced from a band near 941-8 cm -], caused by the VMo-o in a CoMoO4 lattice [24,25]. These bulk characterization methods give a rough indication of the phases in the catalysts, and the extent of interaction between the components can be derived to a certain level. However, it is conceivable that some additional (non-crystalline) species are present that may seriously affect the properties of the catalysts. Because of the non-uniformity of the structure of the catalysts, the use of advanced spectroscopic techniques (e.g. XPS and EXAFS) is useless. It will be shown that TPR and a catalytic test reaction are useful tools to disclose additional facts on the interaction between cobalt, molybdenum and silica. The TPR profiles of the CoMo/SiO2 catalysts ( 18) are shown in fig. 1. The TPR profile of samples containing supported CoMoO4 (3-5,7) exhibits strong resemblance to that of supported M o O 3 [ 26 ]: a relatively sharp peak at 760-780 K and a broad peak at 900-1000 K. The first peak in CoMoO4 cat-
739
i
6 .........
i
i
273 373 473 573 673 773 873 973 1073
T e m p e r a t u r e (K) Fig. 1. TPR patterns of CoMo/SiO2 catalysts; the numbers correspond to the samples in table 2.
alysts is positioned at lower temperature than in MoO3 samples, due to the better reducibility of the cobalt ions that act as nuclei for the reduced phase. A high temperature XRD experiment on sample 7 disclosed that the reduction of the CoMoO4 phase proceeds according to eq. ( 1 ): CoMoO4
T~6SO K C o M o O 3
T~970 K C 0 2 M 0 3 .
T-823 K C o 3 M O ( 1)
The hexavalent molybdenum ions are reduced to MO 4÷ at approximately 680 K in the cobalt molybdite phase. This temperature is lower than Tma~, in the TPR profile of fig. 1 probably because the heating rates in both experiments deviate. A metallic phase with Co3Mo3 stoichiometry appears at a relatively low temperature. Due to the higher reducibility of cobalt oxide [ 5,16,27 ] with respect to MoO3 this metallic phase is enriched with cobalt despite the lower cobalt content of the catalyst. At ca. 970 K (the expected temperature of Mo o formation) a new me-
M. De Boer et al. ~Cobalt-molybdenum interaction in CoMo/Si02 catalysts
740
tallic phase is formed that is enriched of molybdenum. The reduction of bulk Co304 is claimed to occur in two steps via CoO to Co o in the temperature region 570-670 K. The (onset) reduction temperature and the total reducibility decrease strongly when (hydro)silicates have been formed [16]. It can be derived from fig. 1 that some' 'free cobalt" (i.e., cobalt oxide, cobalt (hydro)silicate, etc., not incorporated in molybdenum oxide as CoMoO4) is present in samples 1, 2, 6, and 8. Co304 was found in catalyst 1,6, and 8 by X R D of the fresh samples. The presence of free cobalt species in sample 2, according to T P R (and not X R D ) is due to the amorphous (hydro)silicate that does not react with bulk MOO3. The same (hydro)silicate (A in table 2) layer is, at least partially, converted into CoMoO4 in catalyst 3, which possesses a reduction profile characteristic of CoMoO4.
Cobalt oxide exhibits high activity for oxidation reactions with its position near the top of the classical vulcano plot [28]. Therefore, it is a good candidate for catalytic combustion or incineration of hydrocarbons. One of the simplest reactions is the oxidation of carbon monoxide. The intrinsic activity of MoO3 for catalytic oxidation is much lower due to the high M o - O binding energy, which makes, on the other hand, the high valent metal oxides suitable catalysts for mild oxidation reactions. The activity of bulk C0304 (C), C o / S i O 2 ( A ) , bulk MoO3 (D), Mo/SiO2 (B), and bulk CoMoO4 is shown in the conversion curves in fig. 2.
I{X)
/ i ?~" . . . . . . . . . . .
80
o
v
/
j
/
/
20
//
/
/ 273
373
// 473
d 573
673
Temperature (K) Fig. 2. Conversion curves for the catalytic oxidation of CO on (a) Co30, (up and down) (b) Co/SiO2; (c) MOO3; (d) Mo/ SiO/and (e) CoMoO4. [CO] = [Oz] = 1 vol.%; LHSV~ 12,000 h- '; 200 mg catalyst (Co304 diluted 1: 10).
Obviously, the activity of the cobalt catalysts is superior to that of the molybdenum containing samples. The lower activity of Co/SiO2 (higher surface area) compared to Co304 must be attributed to the presence of cobalt (hydro)silicate. The highest oxidation capacity is assessed on very small (supported) cobalt oxide crystallites that may result when the (hydro)silicate is reduced at a high temperature and reoxidized. The hysteresis in the conversion curve of Co(If, III)304 is caused by its transformation into the lower valent C o ( I I ) O at high temperature. The reversible reconstruction of the Co304 lattice in the descending conversion curve induces a higher activity than the ascending curve. This feature has been previously reported by Oku et al. [ 29 ] using XPS. Apparently, the C o 3 0 4 ~ C o O transition occurs at lower temperatures than predicted for bulk cobalt oxides by thermodynamics. The activity of bulk MoO3 is poor, and it hardly improves at higher dispersions (Mo/SiO2). As expected, the activity of bulk CoMoO4 is in between that of Co304 and MOO3, which leads us to the postulation that the incorpo-
ration of cobalt in a molybdate lattice deactivates the cobalt species ,for catalytic oxidation. This postulate will be used for the explanation of the activity of the CoMo/SiOz catalysts and the related cobalt-molybdenum interaction. The activity of the CoMo/SiO2 catalysts and the kinetic parameter derived from the Arrhenius plots are enlisted in table 3. Prior to the C O - o x i d a t i o n experiments the samples were in situ calcined at 723 K for fours hours under 10% Oz/He. Three conversion curves between 223 and 673 K were recorded. The third measurement was used to calculate the apparent activation energy and the preexponential factor in a linear part of the Arrhenius plot, using the same conversion interval for all samples. The reaction rates for the oxidation of CO are the highest for samples 1 and 2 as in accordance with the observation of free Co304 in the X R D pattern for sample 1 and the presence of free cobalt species in T P R for sample 2. Samples 6 and 8 that showed free cobalt in T P R as well, are somewhat less active in the test reaction. The different oxidation behavior is best expressed by the large difference in the value of the apparent Ea (compare 1 and 2 to 6 and 8). The explanation for this paradox (samples 1, 2, and 6 all revealed the presence of some free cobalt species in
M. De Boer et aL / Cobah-molybdenum interaction in CoMo/SiOe catalysts
741
Table 3 Activity and kinetic parameters of CoMo/SiO2 catalysts in the oxidation of CO at T = 673 K. The apparent Ea and ko were calculated for all samples in the same conversion interval in a linear part of the Arrhenius plot. Sample
Description
Log rate ( r / m m o l e s -I m -2)
Apparent Ea (kJ mole - I )
ko ( s - l m -2) ")
BET surface area (m 2)
C D E 1 2 3 4 5 6 7 8
Co304 MoO3 CoMoO4 M M D, C, Si MM D , A WI Mo6+/A HDP Mo s+, CO2+ - S i WI Co2+/B MM C, B WI Mo 6+, C o 2 - - Si MM D, E, Si
- 4 . 7 b) -6.3 - 5.3 -5.8 -5.8 -6.4 -6.0 -6.1 -6.3 -6.8 -8.2
92 56 63 92 82 69 63 63 62 61 -
2X 10 I1 4 4 X 103 3 × l0 T 2X 107 3 × 104 3X 104 2 × 104 1 × 104 2X 103 -
3 2 4 171 173 156 154 144 158 162 175
a) The/Co values are normalized on the BET-surface area. b) The figure cannot be compared to others, because 100% conversion is achieved at lower temperature.
XRD and/or TPR) comes from characterization after the CO-oxidation experiments: neither of the samples 6 and 8 exhibits free cobalt after CO-oxidation. The cobalt oxides must have reacted with molybdena to cobalt molybdate. The apparent E,s give a more direct relationship between oxidation activity and cobalt-molybdenum interaction. A high apparent E, ( ~ 90 kJ mole -~) is obtained for the catalysts with free cobalt oxide (sample 1 and 2, prepared by mechanical mixture), whereas the low apparent Ea ( ~ 56 kJ mole- l ) over MoO3 is found for none of the catalysts. Catalysts 3-7 all possess apparent/?as with values in between that of MoO3 and Co304 (typically ~ 63 kJ m o l e - l ) , which coincides with the E : v a l u e connected with bulk C o M o O 4 . The strongly varying values for the apparent Ea are not reflected by large differences in reaction rates (table 2). The cause of this phenomenon is the existence of two reaction mechanisms for the oxidation of carbon monoxide: the associative mechanism (low E~) at low temperature and the redox mechanism (high E~) at high temperature. The temperature at which the transition from the associative to the redox mechanism occurs, depends on the chemical properties of the catalytically active material. At the same temperature the redox mechanism (high apparent E~ combined with a high ko) may already proceed on a cobalt oxide lattice, while the associative mechanism (low apparent Ea and a low ko) takes places on molybdenum oxide. Consequently,
the value of the apparent Ea is indicative of the species that causes the catalytic activity.
4. Concludingremarks It can be concluded that the kinetic parameter of the catalytic oxidation of CO are a useful tool to establish the presence of "free cobalt oxide species". With the combined information of XRD, DRIFTS, TPR, and CO-oxidation a schematic representation of the structure of the catalysts can be constructed. The results are shown in fig. 3. The structures in fig.
g ~
I
5
/ 2
~ []
[
I
6 ~
3
4
7
l__
.
MoO3 Co304
I
] SiO 2
I
I Co 2SiO4
18
Fig. 3. Schematic presentation of the structure of the CoMo/SiO2 as derived from the characterization methods; the numbers correspond to the catalysts in table 2.
742
M. De Boer et al. ~Cobalt-molybdenum interaction in CoMo/SiOe catalysts
3 correspond to the situation after CO-oxidation. Four phases are discerned: MoO3 in all samples due to the nonstoichiometric composition, C o M o O 4 in all samples except 2, C o 2 5 i O 4 in samples 4, 5, and 7 (the cobalt silicates in sample 2, and 3 are probably amorphous cobalt hydrosilicates), and C0304 only in samples 1, and 2. The installation of the interaction between the phases must be considered with care, because thermal treatments severely affect the solid state reactions and the structure of the oxidic phases. The stability of Co/SiO2 can be very high when a cobalt ( hydro )-silicate is formed (sample 2 and 3), but the mobility of hexavalent molybdenum oxide species on a SiO2 support induces a reaction between cobalt and molybdenum species. A catalytic test reaction that discriminates between a more active oxide (C0304) and a less active (MOO3 or CoMoO4) oxide is useful in assessing the interaction between cobalt and molybdenum oxide species, but the thermal instability of the mechanical mixtures obfuscates a fair comparison. The apparent activation energies for the CO-oxidation reaction give the best indication of the presence of "free" cobalt oxide because the overall activity of the samples do not show very explicit differences.
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