J, Phys, Chcm, 1994,98, 9657-9664
9657
Formation, Characterization, and Catalytic Activity of Gadolinium Oxide, Inf'rared Spectroscopic Studies Gama1 A. M. Hussein Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt Received: January 25, 1994; In Final Form: June 30, 1994@
Gdz03 was obtained as a final product of the thermal decomposition of Gd(CH&00)34HzO. The decomposition processes up to 800 "C were characterized by TG, DTA, XRD, and IR spectroscopy of the gas and solid phase products. Activation energies were determined nonisothermally for thermal events during decomposition. Gd(CH3CO0)34HzO is completely decomposed to Gd2O3 at 650 "C through two different intermediates: GdO(CH3C00) at 325 "C and Gd20~C03(in two different crystalline phases) at 365 and 405 "C. Oxide samples obtained at 800 and 1000 "C were subjected to texture analysis , pyridine adsorption, and qualitative and quantitative activity tests for the decomposition of 2-propanol vapor at 100-400 "C. The results revealed that (i) Gd203 at 800 "C has higher surface area (27 m2/g) and is mainly mesoporous in comparison to that of Gd203 at 1000 "C (12 m2/g), which is mainly microporous in nature. (ii) Gd203 at 800 "C is a basic surface and contains two different Lewis acid sites. At I 100 "C, pyridine interacts with the surface, forming carboxylate and nitrite surface species, which decomposed at 300 "C, yielding a carbonate surface species. (iii) Gd2O3 catalyzes 2-propanol dehydrogenation (forming acetone) at I 1 5 0 "C and dehydration (forming propene) at I250 "C. At I 350 "C, acetone was involved in a secondary surface reaction, presumably with surface hydroxyl groups created from the water vapor (dehydration product), to give rise to C&, C02, and isobutene. Gd2O3 at 1000 "C shows more dehydrogenation activity than does the 800 "C material. 2-Propanol is irreversibly adsorbed at 25 "C on GdzO3 in the form of coordinated molecules and two different types of 2-propoxide ions (terminal and bridge bonded).
Introduction The decomposition of 2-propanol over metal oxide catalysts generally proceeds through two ways, dehydration and dehydrogenation.' The selectivity factor has been reported to correlate well with acid-base properties of the metal For instance, dehydrogenationis favored on basic oxides, while dehydration prevails on acidic oxides. Therefore, sometimes 2-propanol is used in order to characterize the acid-base properties of metal oxides4x6 High surface area and reactive metal oxide catalysts are often synthesized via thermal decomposition of the corresponding precursor compounds .7-9 The release of volatile components forces generation of fast transport pathways (pores) throughout the material bulkG7The chemistry of metal oxide surfaces is related in general to factors such as position of the metal in the periodic table,lOJ1predominant cationic states," and the degree of coordinative unsaturation of cations or anions. Additionally, the degree of surface hydroxylation and the nature of the surface hydroxyl groups retained on the surface (protonic or anionic)12 contribute, sometimes critically, in surface chemistry modifications. Also, variation of the methods of preparation and pretreatment conditions of metal oxide catalysts can lead to differences in the behavior, sometimes as great as the differences between catalysts of obviously different chemical composition. l3 Monoclinic14gadolinium sesquioxide, GdZ03,is of interest as a catalyst for the dimerization of methane,15 l-butene isomerization, hydrogenation of 1,3-butadiene, and acetone aldol addition.16 It is also used as a selective catalyst for the dehydration of 2-alkanols to form the thermodynamically unstable l-01efins.l~ Moreover,16 it is used as a support for metals that catalyze methanol formation from CO:! and Hz. GdzO3 is a basic metal oxide, and the catalytic properties are @
Abstract published in Advance ACS Abstracts, August 15, 1994.
dependent on the pretreatment conditions which may determine the basicity.16 Synthesis and characterization of Gd203 is seldom encountered in the literature. The present investigation aims to explore the thermal decomposition of Gd(CH3COO)y4H20,to examine the acidity of the surface, and to test the catalytic activity of Gd2O3 thus produced. To accomplish these objectives (i) thermal processes involved in the decomposition were monitored by TG and DTA, (ii) intermediates and final solid products were characterized by XRD and IR spectroscopy, (iii) surface and texture analyses of GdzO3 were characterizedby N2 sorption isotherms (at - 195 "C), and (iv) acidity of the Gd2O3 surface was measured by pyridine adsorption and catalytic activity for the decomposition of 2-propanol.
Experimental Section Materials. Gd(CH3C00)3*4H20(denoted GdAc) used was 99.99% pure (Aldrich product). From the thermal analyses (see below), the solid phase decomposition products were obtained by heating at various temperatures in the range 150-1000 "C for 1 hr in atmospheric air flowing at 20 mL/min. The calcination products are indicated in the text by the GdAc and the temperature applied. Thus, GdAc800 means the calcination products of GdAc at 800 "C for 1 hr. The 2-propanol, acetone, and pyridine were spectroscopic grade BDH (U.K.) products. Prior to use they were throroughly degassed by freeze-pump-thaw cycles performed under vacuum. Thermal Analysis. TG and DTA of the parent material (GdAc) were carried out by heating at different rates (0 = 2-20 "C) up to 800 "C, in atmospheric air (20 mLJmin), using a Model 30H Shimadzu analyzer (Japan). Samples of 10-15 mg were used for the TG measurements; highly sintered a-ALO3 was the thermally inert reference material for the DTA.
0022-3654/94/2098-9657$04.50/0 0 1994 American Chemical Society
9658 J. Phys. Chem., Vol. 98, No. 38, 1994
Hussein Gd(CH & 0 0 ) ~ . 4 H @
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Figure 1. TG and DTA curves for GdAc, at 20 "C/min in an atmosphere of air (20 mL/min).
DTA peak temperatures (Tmax)as a function of the heating rate (6) were incorporated into Flynn's equation18 to calculate the corresponding activation energy (AE, kJ/mol) from the linear plots of log I3 versus UTmax, according to the following relationship:
AE = -R/[b d log 8/d( llr)]
(1)
where R is the gas constant (8.31 J/(mol.K) and b is a constant (0.457). X-ray Diffractometry. XRD analyses of GdAc and its calcination products were carried out on a Model JSX-60 PA Jeol diffractometer (JAPAN) equipped with a source of Zr-filter Mo K a radiation. For identification purposes, the diffraction pattems (VIovs &spacing (A)) were matched with ASTM standards. Infrared Spectroscopy. All the IR spectra were measured on a Model 580B Perkin-Elmer spectrophotometer (U.K.) equipped with a 3500 PE data station for spectra acquisition and handling and had a resolution of 5.3 cm-' over the frequency range from 4000 to 400 cm-'. (I) Gas phase decomposition products of GdAc were identified by IR spectra taken from the atmosphere surrounding a 0.5 g portion of GdAc being heated at 20 "C/min to various temperatures (100-600 "C) for 5 min, in a specially designed IR cell.19 The cell was evacuated briefly to Torr prior to heating the sample. (11) IR spectra for solid phase decomposition products were from KBr-supported discs (IIwt %; 20-30 mg/cm2). (111) IR spectra of pyridine and/or 2-propanol decomposition were taken in a cell containing a fumace section, where the catalyst could be heated at various temperatures. A standard procedure was used for obtaining spectra with different catalysts. Powder (100 mg) of the test catalyst was pressed in a very thin wafer and placed in a quartz glass holder mounted in the heatable (upper) part of the cell. The cold (lower) part of the cell was equipped with NaCl windows. Thus, spectra were measured at room (beam) temperature. The following procedure was adopted. The catalyst was heated in a steam of oxygen at 650 "C for 1 h to clean the surface from carbonate contamination16 and cooled to room temperature under vacuum to Torr. After recording the background spectrum of the cellcatalyst in vacuum, 3 Torr of pyridine or 10 Torr of 2-propanol vapor were allowed into the cell. The catalyst disc was then raised to the fumace section of the cell and heated consecutively
in the resulting gaseous atmosphere at the specific temperature for 10 min in each case. After each temperature the catalyst disc in its holder was lowered into the IR beam for obtaining the spectrum of the catalyst-adsorbed species-gas phase at room temperature. By raising the catalyst disc, the spectrum of the gas phase could be measured separately. The spectrum of the catalyst-adsorbed species was obtained by ratioing the gas phase spectrum or by pumping out the gas phase at each temperature. The background spectrum of the initial catalyst in vacuum could also be ratioed to give the spectrum of adsorbed species alone. The 2-propanol and its decomposition gas products were quantitatively analyzed, using the standard PerkinElmer QUANT software and the on-line data acquisition system. The amounts of the gas phase components (reactants and products) were determined from calibration curves relating the IR absorption intensity at a certain analytical frequency to the calibrant gas phase pressure (Torr). The calibration curves were derived from IR spectra data obtained from authentic samples of each gas phase component under identical spectroscopic conditions. The absorption intensity3 was 3665 & 5 cm-' for 2-propanol, 1740 & 5 cm-' for acetone, and 913 f 5 cm-' for propene. N2 Adsorption Measurements. N2 sorption isotherms were determined volumetrically at - 196 "C using a microapparatus based on the design described by Lippens et al.*O Test samples were outgassed at 220 "C for 6 h under a vacuum of Torr. Pore size distribution analyses were also determined.
Results and Discussion TG and DTA curves recorded for GdAc at 20 " C h i n , in an atmosphere of air, are shown in Figure 1. GdAc decomposes via six weight loss (WL) processes: three endothermic (I (140 "C), I1 (190 "C), and VI (640 "C)), and three overlapped exothermic (111 (325 "C), IV (365 "C), and V (405 "C)). Figure 2 exhibits IR spectra taken from the gas phase surrounding the same sample of GdAc after it has been heated successively to the various temperatures specified (100-600 "C) for 5 min in each case and then cooled to room temperature. The IR spectra and X-ray powder diffractograms obtained for GdAc and its solid phase decomposition products at different temperatures, 200-1000 "C for 1 h, are likewise presented in Figures 3 and 4, respectively. The IR spectra given in Figure 5 are due to pyridine adsorbed on GdAc800 and evacuated at different temperatures (25-300 "C). Figure 6A, shows the IR gas phase
IR Studies of Gadolinium Oxide
J. Phys, Chem., Vol. 98, No, 38, 1994 9659 TABLE 1: Energies of Activation for the Processes Occurrlng through the Decomposition of GdAc process I I1 I11 IV VI AE, kJ/mol 57.0 63,O 148.0 160.0 258.5 Event I11 (Figure 1) is exothermic,with a maximum at 325 OC. The total WL determined on completion of process I11 is 42.5%, which is identical to that (42.5%) calculated for the decomposition of Gd-acetate to Gd-oxyacetate as follows:
GdOCH3CO0
+ CH3COCH3+ CO,
(3)
Reaction 3 is corroborated by the IR gas phase at 300 "C (Figure 2), in which absorptions characteristic of acetone (at 1740, 1370, 1220 cm-l) 23 and C02 (at 2345 and 670 cm-1)23are observed. These results are in agreement with the earlier study for the decomposition of Pr(CH3C00)3*H20, in which Pr-oxyacetate was formed as an unstable intermediate. Events IV and V (Figure 1) are largely overlapped exothermic processes, which maximized at 365 and 405 "C, respectively. These two events bring the WL to 49.0%, which is close to that (49.9%) expected for conversion of Gd(CH3C00)3*4H20 into Gd202C03; that is, GdOCH3COO is decomposed to Gd202COS as follows; b
Wave numbe r (crn-1) -* Figure 2. IR spectra from the gas phase surrounding a 0.5 g portion of GdAc heated at 10 Wmin to the temperatures indicated (a = acetone; m = methane; i = isobutene).
spectra from a 10 Torr dose of 2-propanol in contact with GdAc800 after consecutive 10 min heatings at the temperatures indicated (150-400 "C). Figure 6B, shows the quantitative analysis of the gas phase composition, resulting from an initial 10 Torr dose of 2-propanol (A), giving rise to the acetone (B) and propene (C) products, after consecutive intervals within the temperatures indicated over the GdAc8OO and -1000 samples. IR spectra of surface species resulting from adsorption of 2-propanol on GdAc8OO at the temperatures indicated (25-400 "C), followed by cooling to room temperature under vacuum, are given in Figure 7. I and 11. Formation and Characterization of Gdz03. 1. Characterization of Decomposition of GdAc. Events I and I1 are shown by the TG and DTA curves (Figure 1) to be overlapping, endothermic, WL processes maximized respectively at 140 and 190 "C. The IR spectra of the gas phase released at 150 and 200 "C (Figure 2) displayed nothing but absorptions at 3440 and 1630 cm-l due respectively to VOH and &+OH vibrations of water molecules.21 Thus, H2O is released in the gas phase due to events I and II. The total WL effected (17.5%) through both events is fairly close to that expected (17.7%) for the release of four moles of water, i.e.,
Gd(CH3C00)3'H20
190 "C
II(33.3%) Gd(CH,COO), (2)
The corresponding activation energies (Table 1) of 57 (I) and 65 kJ/mol (11) are within the range characteristic of dehydration processes." Consistently, the IR spectrum obtained for GdAc200 (Figure 3) exhibits bands at 1700-500 cm-' assigned for the vibration modes of CH3COO- species.21
2Gd0CH3C00
365 and 405 OC IV and "(49.0%;
Gd,02C0,
+ CH3COCH3 (4)
The IR spectrum of the gas phase at 400 "C (Figure 2) monitored the intensity of acetone bands (decomposition gas product from reaction 4). In support, the IR spectrum obtained for GdAc350 (Figure 3) displayed absorptions at 1540, 1485, 1430, 1390, 1340, and 1060 and a doublet at 860 cm-' due to vibration modes 24 of co32-;also, the strong absorptions emerging at 600-400 cm-' are mostly related to Gd-0 vibration modes.24 Moreover, the XRD diffractogram for GdAc350 (Figure 4) detects only Gd202C03, but in two different phases (*Gd,02COS(ASTM No. 25-339) and Gd202C03 (ASTM No. 28-680)). This may give the reason for the two exothermic effects located at 365 and 405 "C. On a further heating, event VI takes place endothermally (Figure 1) at 640 "C. The maximum WL thus determined (55.0%) agrees well with that expected (55.3%) for an overall conversion of GdAc to GdzO3. Accordingly, the intermediate, Gd202C03, must have decomposed at 640 "C, with an energy of activation (Table 1) of 258 kJ/mol to form Gd2O3. The XRD scans for GdAc800 (Figure 4) show nothing but a pattern of peaks due to Gd203 (ASTM No. 12-797). The peaks intensify with rising temperature up to 1000 "C, i.e. sintering may occur. The corresponding IR spectrum (Figure 3) declares the absence of absorptions (at 1600-800 cm-') due to C032- species. However, it maintains displaying the absorptions below 600 cm-', which are related to lattice vibration modes of Gd203.24 The IR spectrum of the gas phase at 600 "C (Figure 2) display more bands due to CO (at 2140 cm-'), methane (at 3010 and 1310 cm-'), and isobutene (at 890 ~ m - l ) .The ~ ~ formation of these gases is expected as a result of the involvement of acetone in surface-mediated bimolecular r e a ~ t i o n . , ~ , ~ ~ 2. Sulface Texture (Sulface Area and Pore Size). Nitrogen adsorption studies at -196 "C on GdAc800 and -1000 gave, with the two samples, type I1 isotherms27with type H3 (B) hysteresis loops occurring at p/po 25.5. These general features
Hussein
9660 J. Phys. Chem., Vol. 98, No. 35, 1994 IR-Solid Phase
2,000 1,800 1,600 1,400
1,200 1,000 800
600
400
Wavenumber (cm-')--+
Figure 3. IR spectra from KBr-supported samples of GdAc and its solid decomposition products obtained by calcining for 1 h in air.
XRD A
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A
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2 0 ----> Figure 4. X-ray powder diffractograms of GdAc and its decomposition products obtained by calcining for 1 h in air. primarily indicate the ~ l i t - s h a p e dpore ~ ~ nature of the surface. The measured surface areas (SIET)indicate that GdAc8OO has a surface area of 27 m2/g; GdAclOOO has a surface area of 12 m2/g. Probably, the internal surfaces are inaccessible for N2 molecules in the case of GdAc1000. Moreover, the sintering process27 appears to be more extensive during calcination at 1000 "C than at 800 O C (see Figure 4). 3. Pyridine Adsorption and Sugace Acidity. The spectrum of GdAc800 was recorded over the frequency range 4000-800 cm-'. Two overlapping bands with maxima at 3660 and 3690 cm-' were assigned to stretching vibrations of two different types of surface hydroxyl groups. Low-frequency bands ('1560 cm-') are due to surface carbonate contaminants,16 which completely disappeared when the catalyst was treated in oxygen for 1 h at 650 "C. IR spectrum obtained from pyridine adsorbed at room temperature on GdAc800 (higher surface area) (Figure 5) displayed bands (at 1620, 1600, 1480, and 1445 cm-'), which
indicate Lewis acid sites28*29 on the surface of GdAc800. The occurrence of the 8, mode at two different frequencies (1620 and 1600 cm-') may indicate that Lewis acid sites have two different acid strength^.^^ Thermal evacuation at 100-300 "C (Figure 5 ) decreased the pyridine bands. The growth of the bands at 1560, 1460 cm-' are due to carboxylate surface species;30those at 1290, 1180, and 1070 cm-' are assigned to N-0 surface species;31and those at 1460, 1380, 1330, 1040, and 850 cm-I are due to carbonate surface These results suggest that the carboxylate and nitrite species are formed as cracking products of 100 and 200 "C adsorbed pyridine. At 300 "C (Figure 5 ) formation of the carbonate species seems to take place at the expense of decomposing carboxylates and nitrites. In an earlier study Zaki et al.32 found that, after exposing CeOz to pyridine at room temperature followed by evacuation at 100-400 "C, cracking of pyridine ligands occurred, leading to the formation of carboxylate, nitrite, and
IR Studies of Gadolinium Oxide
J. Phys. Chem., Vol. 98, No. 38, 1994 9661
t
C
.-0 fn .-fn
6 sc' a9 300°C
0
m
6 2 2000
1800
1600
(0
1400
1200
1000
800
Wavenumber ( cm-') + Figure 5. IR spectra of the surface species from the adsorption and surface reactions of 3 Torr of pyridine at the temperatures indicated, on GdAc800. carbonate surface species. This is attributed32 to the highly reactive basic sites associated with hydroxide and/or oxide ions. Also Zecchina and Stone33noticed the cracking of pyridine on alkaline earth oxides at 150 "C. They presumed the involvement of surface species other than OH- groups, namely, reactive oxygen species. Nevertheless, a contribution from the coexistence of OH- groups could not be excluded. Accordingly, pyridine cracking as detected by IR indicates the existence of highly reactive basic sites associated with the OH- and/or oxide ions. 111. Catalytic Activity of GdzO3 for 2-Propanol Decomposition. 1. IR Analysis of the Gas Phase Decomposition Products. IR spectra taken from the gas phase of 2-propanoy GdAc800 at different temperatures (room temperature to 400 "C) for 10 min are given in Figure 6A. The room temperature spectrum displays the characteristic bands of 2-propan01.~~ At 150 "C a tiny but significant absorption emerges at 1740 cm-', which developed markedly in the 200 "C spectrum together with another absorption at 1250 cm-'. The two bands mark the formation of gas phase acetone,25and accordingly, the dehydrogenation of 2-propanol started at 150 "C. At 250 "C (Figure 6A), additional absorptions emerge at 1650 (doublet) and 915 cm-l which are due to propene. Hence the dehydration of 2-propanol occurs at e 2 5 0 "C. Following reaction at 300 "C, the spectrum obtained (Figure 6A) indicates that the acetone is slightly intensified; propene absorptions grow stronger; and absorptions in the YCH region (3100-2800 cm-') are restructured. At 350 "C absorptions due to alcohol are hardly detectable and those due to acetone are weaker. In contrast, absorptions due to propene are intensified. Also at 350 OC, new absorptions emerge at 3010 and 1310 cm-' (due to methane), at 2340 and 670 cm-l (due to COz), and at 890 cm-' (due to i ~ o b u t e n e ) .These ~ ~ new absorptions are intensified at 400 "C. Since the disappearance of acetone and the appearance of C b , COz, and isobutene occur at the same temperature regime (350400 "C), they might be consistent. Similar spectra were recorded for the gas phase 2-propanol/ GdAc1000. The results obtained are similar to that for GdAc800 (Figure 6A). The main differences observed with
GdAclOOO are (i) acetone first appeared at 200 "C, (ii) 2-propanol is still present up to 350 "C, while acetone remained up to 400 "C, and (iii) no bands due to isobutene were detected. These results are presented quantitatively for both catalysts in Figure 6B. It is clear that the rate of alcohol decomposition appears to maximize at ca. 270 "C. GdAclOOO has a higher selectivity (but still limited activity) for acetone formation between 250 and 300 "C. At higher temperature GdAc800 was more selective for propene formation. 2. IR Analysis of the Surface Species Resulting from 2-Propanol. Adsorption and Decomposition over GdAc8OO. Spectra of surface species obtained from adsorption and surface reaction of 2-propanol (10 Torr) for 10 min on GdAc800 at various temperatures (room temperature to 400 "C) are shown in Figure 7. The room temperature spectrum had many features in common with the 150 "C spectrum. It showed the disappearance of the hydrogen-bonded YOH (at 3500-300 cm-') absorption after 5 min evacuation of the gas phase 2-propanol at room temperature. This may suggest that a small portion of the alcohol molecules were weakly held through the hydrogen bonding with surface OH groups. This gives an indication that Gd203 surface hydroxyls are anionic in nature;34that is, they are weak hydrogen-bond donors. The absorptions in the YCH region and below 1500 cm-' are listed, assigned, and compared with the corresponding absorptions of the Gd(C3H70)3 compound35 in Table 2. The observation above indicates, as expected, the presence of adsorbed alcohol and corresponding 2-propoxide specie^.^ Since the spectra in Figure 7 were obtained after evacuation of the gas phase at the temperature indicated, the absorption at 1260 cm-' of the BOH mode must have originated from freely bending hydroxyls of alcohol molecules held on the surface by bonds that are stronger than hydrogen bonding.36 Similar absorption in the 6 0 region ~ has been assigned37to coordinated (undissociated) alcohol molecules. These species can be observed at temperatures up to ~ 3 2 "C. 5 The absorptions of Y(C-O)IY(C-C) at 1165 and 1130 cm-l (Figure 7 and Table 2) have previously been3*observed on Ti02 and assigned to the presence of two different alkoxide species.
Hussein
9662 J. Phys. Chem., Vol. 98, No. 38, 1994
Room Temp,
co2
(MI IP=2propanol
4000
A-ecotone
3100
3600
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1600
1100
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50
100
150
300 Reaction TemperaturePC -----> 200
250
350
400
Figure 6. (A) IR spectra from 10 Torr of 2-propanol after being in contact with GdAc800 for 10 min at the temperatures indicated. (B) IR quantitative analysis of the gas phase composition showing the relation between the reaction temperature and the pressure (Torr), resulting from an initial dose of 10 Torr of 2-propanol (A), giving products of acetone (B) and propene (C) after consecutive 10 min intervals at the temperatures indicated over the catalysts GdAc800 and GdAc1000.
S p e ~ i f i c a l l y ,the ~ ~ higher frequency is due to an alkoxide coordinated to a single cation, and the lower one is due to an alkoxide bridge-bonded to two cations. The spectrum at 150 "C (Figure 7) displayed additional bands at 1730, 1585, and 1470 cm-', which were enhanced by increasing the temperature to 250 "C. The band at 1730 cm-' was due to adsorbed acetone molecules. The two bands at 1585 and 1470 cm-' were previously30 assigned to acetate surface species. It should be recalled at this stage that acetone first appeared in the gas phase at 150 "C (Figure 6A). Beyond 250 OC carbonate surface species (bands at 1560, 1440, 1340, 1300, and 1050 cm-') were also detected.
The intensities of absorptions (at 1165 and 1130 cm-') characteristic of 2-propoxide species changed as the temperature increased. The band at 1165 cm-' weakened first; that is, the bridge-bonded 2-propoxide is thermally more stable than the terminal one. This is in contrast with earlier studies by Zaki et al. for 2-propanol over Ce02,25,37TiO2, ZrO2, and HfO2. We found that the bridged 2-propoxide species are less stable in comparison with the terminal one. The spectrum obtained at 400 O C (Figure 7) indicates the disappearance of absorption bands of acetone and acetate and a further weakening of the 2-propoxide absorptions. In contrast, there is a gradual increase in the carbonate absorptions.
IR Studies of Gadolinium Oxide
J. Phys. Chem., Vol. 98, No. 38, 1994 9663
tI
3,600
3,100
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2,100
1,600
1,100
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Wavenumber
