579
J. Phys. Chem. 1994,98, 579-584
Diffuse Reflectance Spectroscopy Study of the Thermal Genesis and Molecular Structure of Chromium-Supported Catalysts Bert M. Weckhuysen,' An A. Verberckmoes, An L. Buttiens, and Robert A. Schoonheydt Centrum vmr Oppervlaktechemie en Katalyse. K. U.Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee. Belgium Received: May 20, 1993; In Final Form: September 14, 1993'
The diffuse reflectance spectra of a series of chromium-supported silica aluminas, with varying Si02 content, have been investigated before and after drying and after calcination, successiveCO reductions, and recalcination. The molecular structure of chromium before drying is influenced by the isoelectric point (IEP)of the supports. It is concluded that the lower the IEP of the support, the higher the amount of surface dichromate, After calcination at 720 OC,Cr is anchored to the surface without changing the chr0mate:dichromate ratio. Reduction results in the formation of octahedral Cr3+,pseudooctahedral and pseudotetrahedral CrZ+. A higher silica content of the support, a higher reduction temperature, and a higher calcination temperature result in a deeper reduction of the supported Cr. Recalcination restores the initial chromate:dichromate ratio on each support; however, small quantities of Cr3+remain in the samples. This deeper reduction and reversibility can be explained by the low OH content of the supports calcined at 720 OC.
TABLE 1: Characteristics of the Supports
Introduction Chromium-supported catalysts are usually prepared by impregnation of a chromium salt onto the support,followed by drying and calcination. During the calcination procedure, the thermodynamically favoured reaction 'CrO3 l/zCrzO3 + 3 / 2 0 2 " does not take place (or only to a small extent) in the presence of an oxide support. This phenomenon is generally explained by the anchoring of Cr onto the support (=stabilization). However, the molecular structure of the anchored Cr6+species and the kind of interaction between Cr and the support are points of discussion in the literature. The surface complex is considered to be chromate and/or dichromate,'" but also the presence of polychromates9JO and highly distorted monochromate' I is claimed. Several types of interactions of the surfacecomplexesare proposed in the literature. Most investigations assume an esterification reaction between chromic acid and the hydroxyl groups of the supports, resulting in the formation of surface chromates and/or dichromates (and polychromates).1.4.*.12 Theexothermalpeak in differential thermal analysis (DTA) curves of Cr-supported catalysts around 250 O C is ascribed to this reaction.8 Additional evidence comes from infrared (IR) investigations, indicating a consumption of OH groups by depositing Cr on the upp port.^ This OH consumption is proportional to the quantity of deposited Cr. Recently, Turek et al. have shown that on A1203this OH consumption starts from the more basic OH groups to the more acidic groups, indicating an acid-base type reaction.13 McDaniel has followedthe reaction between Cr02C12 and silica, observing the release of HCl.1.5 This can be explained only by the reaction of surface hydroxyls with the Cr precursor. A similar reaction between CrO3 and the support is envisaged by Wittgen et al.," Ellison et aI.,l5Iwasawa et a1.,16and Hogan." In accordance with Turek et al,,I3 Ellison et al.15 and Fubini et a1.'* suggest a dkcondensation of the polyanions, followed by chemisorption of the monomeric species, while Bhutani et al.19 suggest a ligand-exchange reaction between Cr and Sb203. In a previous study we have reported the quantification of Cr2+,Cr3+,and Cr6+ by diffuse reflectance spectrocopy (DRS) after calcination at 550 OC, successive CO reductions and recalcination.* On the calcined catalysts chromate and dichroTo whom correspondence should be addressed. Abstract published in Aduunce ACS Absrrucrs, December 15, 1993.
0022-3654/94/2098-0S79$04.50/0
SA-n
with n= 0 20
40 60 100
BET surface area (m2/g)
pore vol (mL/g)
365 331 329 253 135
2.0 1.7 1.4 1.o
0.8
pore size
(nm) 3-9 1S-8
1.5-7 1.5-10 1-3
mate are present with traces of Cr5+. The dichromate:chromate ratio is the highest for silica, zero for alumina, and in between for silica alumina. After reduction three new species were formed: octahedral Cr3+, pseudooctahedral Cr2+ and pseudotetrahedral CrZ+. On alumina only octahedral Cr3+ and small portions of octahedral Cr2+ were observed, while on silica and silica alumina the CrZ+:Cr3+ratio increases with the silica content. In this work three subjects will be studied systematically: (1) the interaction between Cr and the support beforeand after drying, after calcination and after rehydration; (2) the influence of the Si02:A1203ratio of the amorphous supports on the molecular structure of supported Cr after calcination; (3) the influence of a high-temperature calcination (720 "C) and the SiO2:AI2O3 ratio of the amorphous supports on the speciation of reduced and recalcined Cr.
ExperimentaI Section Sample Preparation and Characterization. Preparation. A series of Si02eA1203 was prepared following a modified method of Chen et al.21 They are indicated as SA-n, where n is the weight percentage of Si02. The appropriate amounts of tetraethylorthosilicate (TEOS) and aluminum triisopropoxide (Al(iP)3) were mixed in 128 mL of ethanol during 30 min at room temperature. After addition of 35 mL of 1 M HCI, the acid hydrolysis started and the suspension was mixed for 1 h. The resulting gel was dried at 60 OC and 100 OC for 8 h and calcined at 550 OC for 16 h. The obtained samples were crushed. Si02 was prepared following a procedure described previously.2 The Cr catalysts were prepared by the incipient wetness method with chromium(V1) oxide (Cr03). The chromium loading was 0.2 wt %.
Characterization. The characteristics of the supports were measured by dynamic N2 adsorption on an Omnisorp 100 (Coulter), after pretreatment in vacuum at 200 OC for 8 h. These characteristics are given in Table 1. 0 1994 American Chemical Society
Weckhuysen et al.
580 The Journal of Physical Chemistry, Vol. 98, No. 2, 1994
TABLE 2
Color of the Samples after Mfferent Pretreatments
pretreatment before drying after drying after caiciiation, 720 OC after reduction, 200 OC after reduction, 300 OC after reduction, 400 OC after reduction, 600 O C after recalcination, 550 OC after rehydration
Cr/SAO strong yellow strong yellow yellow yellow yellow-green light blue white-blue yellow strong yellow
CrfSA20 yellow yellow-orange yellow yellow yellow-green green blue-grey yellow yellow
Cr/SA40 yellow-orange yellow-orange yellow-orange yellow-orange yellow-green light green blue-grey yellow-orange yellow-orange
TABLE 3 Survey of the Absorption Bands of the Cr Catalysts before Drying, after Drying, and after Calcination pretreatment before drying
Cr catalysts CrfSAO Cr/SA20 Cr/SA40 CrfSA60 CrfSA100 after drying at 90 OC Cr/SAO Cr/SA20 Cr/SA40 CrfSA60 Cr/SA100 after calcination at 720 OC
a
absorption bands' (cm-I) 22 700 (sh,w); 27 OOO, 36 800 22 800 (sh); 26 900,36 600 22 700 (sh); 27 OOO, 36 700 22 700 (sh,s); 27 400,37 000 22 900 (fb); 28 600; 37 OOO 22 700 (sh,w); 27 600; 37 700 22 700 (sh); 27 500; 37 800 22 700 (sh); 27 300; 36 900 22 700 (sh,s); 27 500,37 900 16 500 (vw); 23 000 (sh,s); 28 500; 37 700 (sh); 41 300 Cr/SAO 27 300; 41 200 Cr/SA20 21 700 (sh,w); 27 300,40 500 CrfSA40 21 700 (sh); 27 200,40 500 Cr/SA60 21 800 (fb); 28 100,31 100 (sh); 40 500 Cr/SA100 IS 600 (vw); 21 700; 27 200 (sh); 30 500,41 000
sh, shoulder; fb, full band; s, strong; w, weak; vw, very weak.
Pretreatment and Spectroscopy. Pretreatment. The samples were dried at 50 OC for 8 h and granulated. The size fraction 0.25-0.40 mm was loaded in a quartz flow cell with suprasil window for DRS. The samples were subsequently dried at 90 OC during 8 h followed by calcination at 720 OC during 8 h in an oxygen stream. DRS spectra were recorded of the samples as such, after drying and after calcination. The sampleswere reduced with CO at 200,300,400, and 600 OC during 30 min. After each reduction step DRS spectra were taken. Then, the samples were recalcined in oxygen during 8 h a t 550 OC, and DRS spectra were taken again. Calcined samples were rehydrated by flushing with a water-saturated 0 2 stream at room temperature during 30 min. and DRS spectra were recorded. An oxygen flow of 3600 mL/h and a CO flow of 1800 mL/h were used for all the pretreatments. Diffuse Reflectance Spectroscopy. DRS spectra were taken on a Varian Cary 5 UV-vis-near IR spectrophotometerat room temperature. The spectra were recorded against a halon white reflectance standard in the range 2200-200 nm. The computer processing of the spectra consisted of the following steps: (1) subtraction of the baseline; (2) conversion to wavenumber and calculation of the Kubelka-Munk (KM) function. The spectra were deconvoluted with a Spectra Calc program of Galactic Ind. Corp. into Gaussian bands. Results The samples have typical colors, depending on the pretreatment and the chemical composition of the supports. The colors are given in Table 2. 1. Interaction between Cr and the Support before and after Drying, after Calcination, and after Rehydration. The DRS spectra of the samples as such, after drying and after calcination are shown in Figures 1-5, and the positions of the band maxima are summarized in Table 3. All the spectra of the catalysts before drying are dominated by two intense bands around 27 500 and 37 000 cm-1, the latter being relatively broad. These bands are usually assigned as 0
CrfSA60 yellow-orange yellow-orange orange-yellow orange-yellow light green white-blue blue orange-yellow yellow-orange
CrfSA100 light orange light orange light orange light yellow green-blue white-violet light blue light orange light orange
1500
A
5
1000
i
P
E 500
C 0
40000
20000 cm-1
Figure 1. DRS spectra of Cr/SAO before drying (A), after drying (e), and after calcination (C).
-
Cfl+ charge transfers.2l A shoulder at around 22 500 cm-1, mostly ascribed to the presence of d i ~ h r o m a t e , lis~also * ~ present. Its intensity increases with increasing Si02 content and it becomes a full band for Cr/SA100. The more S i 0 2 in the support, the more orange the sample. After drying at 90 OC, the positions of the band maxima and the relative intensities of the bands are almost the same as before drying: only the color of the samples is intensified. After calcination the color was yellow for Cr/SAO and Cr/ SA20; yellow-orange for Cr/SA40, orange-yellow for Cr/SA60, and light orange for Cr/SA100, and the spectra have changed significantly. The spectrum of Cr/SAO is dominated by two bands at 27 300 and 41 200 cm-I, the latter being shifted by more than 4000 cm-I with respect to the 37 OOO-cm-l band before calcination. Similar observations are made for Cr/SA20 and Cr/SA40, only the low-frequency band shifts to 21 700-cm-I band and becomes more intense with increasing Si02 content of the supports. For Cr/SA60 and Cr/SA100 after calcination a new band grows in intensity around 30 500 cm-1, thus giving a four band spectrum: 21 700-21 800,27 300 (sh), 30 500, and 40 500 cm-', the latter being sharper than before calcination. Furthermore, a small band appears at around 15 500 cm-', indicativefor octahedral Cr3+.2 After rehydration of the calcined samples the same spectra were obtained as before drying (spectra A in Figures 1-5). When the spectra of the calcined Cr/SA-n catalysts are considered as a function of the Si02:A1203 ratio (spectra C in Figures l-s), the followingobservationsare made: ( 1) an increase of the intensity of the shoulder around 2 1 700 cm-1 with increasing Si02 content to a well-resolvedband for Cr/SA60 and Cr/SA100; (2) a decrease of the intensity of the band around 27 0oO cm-I with increasing Si02 content and a concomittant increase of the 30 500-cm-I band and (3) the width of the 40 000-41 0oO-cm-1 band increaseswith increasingA1203content of the supports. To our knowledge, this is the first time that the band at 21 700 cm-1 can be generated by varying the composition of the support. The deconvolution of the spectra in Gaussian bands is similar to our previous data.2 Two sets of four bands, typically for
The Journal of Physical Chemistry, Vol. 98, No. 2, 1994 581
Chromium-Supported Catalysts
1000 I
I
P. 4
I
L
500
-
-
-
C
0-
I
20000
40000
cm-'
Figure 2. DRS spectra of Cr/SA20 before drying (A), after drying (B), and after calcination (C).
I
500
and after calcination (C).
400
J
A
0-
Figure 5. DRS spectra of Cr/SA100 before drying (A), after drying (B),
I
A
. 1
3
N
5
f-I
h
8
200
E
I
0
C
I 40000
20090
40000
20000
40000
an-'
20000
"1
an-'
Figure 3. DRS spectra of Cr/SA40 before drying (A), after drying (B), and after calcination (C).
Figure 6. DRS spectra of Cr/SAO after reduction at 200 (A), 300 (B), 400 (C), and 600 OC (D).
TABLE 4 Estimation of the Chromate:Dichromate Ratio after Calcination on the Different Supports 0.00 Ichromatc:Idichmmatc" ~chromatc:Cdichmma$
m .a
0.25 3.1 3.36
0.67 2.6 2.36
1.50 1.74 1.58
0.79 0.72
Determined as theratiooftheintensitiesofthedeconvolutedGaussian band at around 27 000 cm-1 for chromate and the deconvoluted Gaussian band at around 31 000 cm-I of dichromate. Determined as the Idmrmtc: Idichromtc ratio X K-I with K = 1.1, as obtained from the ratio of the extinctioncoefficientsof chromateand dichromateobtained from solution spectra.2 C
0
20000
40000 em.'
Figure 4. DRS spectra of Cr/SA60 before drying (A), after drying (B), and after calcination (C).
chromate and dichromate are obtained. The ratio of the intensity of the 27 OOO-cm-' band of chromate and the 3 1 000-cm-' band of dichromate is a measure of the chr0mate:dichromate ratio on amorphoussupports. These calculated values are shown in Table 4. It is concluded that the higher the Si02 content of the amorphous support, the higher the dichromate:chromate ratio. 2. Influence of a High-Temperature Calcination Step and the Si02:AI& Ratio on the Speciation of Reduced and Recalcined Cr. The reduction of Cr starts above 200 OC and is accompanied with the typical color changes of Table 2. The spectra of the reduced samples are shown in Figures 7-11. They can be devonvoluted as described previously.2 2.1. Influence on the Speciation of Reduced Cr. The decrease
of the chromate and dichromate bands upon reduction is accompaniedby the appearanceof a broad, weak, and asymmetric band between 10 000 and 20 000 cm-l for all samples. The maximum of this band shifts to the red with increasing silica contentofthesupports. After COreductionat 600OCthemaxima are as follows: SAO, 15 400 cm-1; SA20, 14 600 cm-l; SA40, 14 000 cm-l; SA60, 13 000 cm-1; SA100, 13 000 cm-I. Such a shift suggests at least two components. In the case of SA100 a second weak band in the range 7500-8000 cm-1 becomes apparent at the highest reduction temperatures. In agreement with our previousinterpretation,these bands are ascribed to three species: octahedral Cr3+ (1 5 000-17 000 cm-1); pseudooctahedral Cr2+ (12 500-13 000 cm-l) and pseudotetrahedral Cr2+(7500-8000 cm-1). The band at 15 000-17 000 cm-1 is the 4Aq lTzS transition of Cr3+0h,the band at 12 500-13 000 cm-1 is the 5T20of Cr2+ob,while the band at 7500-8000 cm-I is similar to band maxima reported for distorted Cr2+T,complexes. The relative concentrations of the last two species are higher than obtained
