J. MATER. CHEM., 1991, 1(3), 423-427 Progress towards Preparation of High-surface-area Rare-earth Oxides Linda A. Bruce, Simon Hardin, Manh Hoang and Terence W. Turney CSIRO, Division of Materials Science and Technology, Locked Bag 33, Clayton, Victoria 3168, A ustra lia A series of high-surface-area rare-earth oxides has been prepared by precipitation from aqueous solution as either the hydroxide or the carbonate, followed by controlled dehydration, acetone washing, vacuum drying and calcination at ca. 600 "C. The materials have been characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR) and surface analysis. Precipitation by hydroxide produces a higher surface area for the lighter rare earths, whereas precipitation by carbonate is superior for the heavier members.By judicious choice of method, pure oxides (containing only trace amounts of surface carbonate) with surface areas >50 m2 g-' can be obtained for all rare earths studied. No microporosity was observed in any of the oxides. Keywords: Rare-earth oxide; B.E.T. surface area; Catalyst synthesis Rare-earth oxides are of considerable current interest, both as oxidation catalysts for reactions such as methane coup- ling'.2 and alcohol dehydr~genation,~ and as supports for transition metals in, for example, the hydrogenation of carbon However, commercially available rare-earth oxides possess very low surface areas, generally <10 m2 g-', which restricts their usefulness as catalysts or as catalyst supports.Published efforts to increase the surface area of rare-earth oxides fall into two categories, one commencing from a precursor rare-earth salt and the other from a pre- formed oxide. Each has generally led to only modest improve- ment in surface areas. Thus: (i) Work by several groups has shown that calcination of the voluminous hydrogel of rare- earth hydroxide obtained by reaction of rare-earth ions with hydroxide ions usually leads to an oxide surface area of <40 m2 g-1.8-'1 An exception is Yb203, which has been reported with B.E.T. surface areas of up to 54 m2 g-', after calcination at 600 0C.9 Very slow, isobaric thermal dehy- dration of cerium(1v) hydroxide has yielded CeO, with areas of 58 m2 g-' and 23 m2 g-' after calcination at 500 and 650 "C, respectively.'2 However, the preparation time of 24 days makes this method rather impractical.Other preparative routes to rare-earth oxides have been tried, including decomposition of salts, such as the trichloroacetate, carbonate, oxalate or nitrate, but the surface area is generally under 10 m2 g- '.I3-" (ii) The most commonly employed technique to increase the area of preformed rare-earth oxides has been repetitive hydrolysis and recalcination of commercially avail- able oxide sample^.^.'^*'^ Thus, increases in area from <1 to 28.5 m2 g-' have been reported for La203, and from 2.8 to 13 m2 g-' for Sm203. This paper reports two studies on the preparation of high-surface-area oxides in the series Y and La through Lu.One technique, hereafter termed the 'hydrox- ide method', involves dehydration of rare-earth hydroxide gels by washing with a water-miscible solvent, followed by calci- nation to the oxide. The other, termed the 'carbonate method', consists of a similar dehydration of rare-earth 'carbonate' gels, followed by decomposition to the oxide. In addition, some preliminary work is included on the effect of the temperature of precipitation and the order of mixing of the reactants. Controlled dehydration of hydrogels by employing water- miscible, non-aqueous solvents is a technique which has been reported to produce high-area alumina,,' silica2' and zir- conia-titaniat2 from their respective hydrogels. The method has also been used as a step in the production of high-density (i.e.low-area) rare-earth oxide ceramics from Gd, Er, Y, Eu, Hf, and Zr23 at temperatures in excess of 1800 "C. However, the nature of the products in the temperature region generally of interest for catalytic materials (<go0 "C) has not been studied before. Experimental Materials The rare-earth source materials were stated to be of at least 99.9% purity by the suppliers as follows: Sm203, Ho203, Yb203, y2°3, Nd203, Dy203, pr6011, Gd(N03)3*5H20 (Aldrich); Er2O3, Tb407 (Cerac); La203 (Strem); La(N03)3* 6H20 (Baker); Ce(N03)3 6H20, (NH4),[Ce(N03),] (99%), 'ammonium carbonate' (ca. 5 :1 mixture of ammonium bicarbonate and ammonium carba- mate) (Ajax). Hydrated nitrate salts of the rare earths were prepared in quantitative yields from the commercial oxides following the procedure of Marsh,24 by dissolution in excess nitric acid, evaporation, recrystallization from water and drying over phosphorus pentoxide in vacuo.Methods Hydroxide hydrogels were prepared by slow addition of ammonia (1 mol dm-3) via a dropping funnel (ca. 1 dm3 h-') to a stirred solution of metal nitrate (0.3-0.4 or 0.025-0.05 mol dmP3), under a nitrogen atmosphere. Gels based on carbonate-type precipitation were prepared by similar addition of 'ammonium carbonate' solution (0.3-1 mol dm- 3, to a stirred metal nitrate solution (0.025-0.05 mol dm-3) through which carbon dioxide was bubbled. In each case the pH was monitored during the precipitation.Preparative pro- cedures are typified by the following examples. Preparation of Sm203by the Hydroxide Method Sm(N03)3-6H20 (1 1.11 g), dissolved in distilled water (1 dm3), was titrated with ammonia (500 cm3, 1 mol dm-3) with vigor- ous stirring under a nitrogen atmosphere at room temperature, to a final pH of 10.5. The product gel was separated from the supernatant liquid by centrifugation, redispersed in water (2 dm3) and centrifuged five times to a final conductivity of <2.5 mS m- 'and then centrifuged with acetone (2 dm3) three times. Preliminary drying in a stream of dry nitrogen was followed by drying in vacuo at room temperature for 1 day to a residual pressure of <1 Pa, and then with a stepwise increase in temperature, as the residual vacuum improved, to a final temperature of 400 "C and a pressure of <0.5 Pa.Final calcination to 620 "C was in air in a muffle furnace for 4 h: yield 3.8 g (86%). Preparation of Sm203by the Carbonate Method Sm(N03)3*6H,0 (13.8 g) dissolved in water (3 dm3) was added dropwise over 30 min to a solution consisting of commercial 'ammonium carbonate' (24 g) in water (1 dm3). During the addition, carbon dioxide was bubbled vigorously through the mixture, which was also stirred with a paddle stirrer. Precipitation commenced at pH 4.3; at the end of the addition the pH was 7.3. After bubbling COz for a further 2 h, the pH was 6.7. The hydrogel was separated by centrifug- ation and redispersed and recentrifuged four times with water (1 dm3), and then three times with acetone (0.5 dm3).Prelimi- nary drying was in uucuo at room temperature, followed by heating in air at 120 "C overnight. The sample was then calcined at 400 "C for 2 h followed by 600 "C for 2 h: yield 4.9 g (90%). A number of the precipitations were made by adding the nitrate solution to the stirred base (referred to as 'reverse order') and, in others, the precipitation was carried out at 60 "C instead of at the ambient temperature (ca. 21 "C). Characterisation Adsorption-desorption isotherms of nitrogen at -196 "C were obtained using a Carlo Erba 1800 instrument. XRD data were obtained using a Siemens D-500 diffractometer (filtered Cu-Ka radiation, scan steps 0.04").Infrared spectra were obtained as KBr discs (ca.2% m/m oxide concentration) using a Perkin-Elmer 577 infrared spectrometer; KBr (Ajax AR) was recrystallised from water with addition of conc. HBr and dried at 300 "C before use. Selected area electron diffrac- tion (SAD) and TEM studies were made using a JEOL- 1OOCX electron microscope. Samples were dispersed ultrasonically in hexane, and examined on holey carbon films. X-Ray photo- electron spectra (XPS) were recorded on a Vacuum Gener- ators' ESCALAB with an aluminium anode at 150 W (pass energy 30 eV, 4 mm slits). Nitrate analysis was made using an ion-selective electrode from EDT Research. Results and Discussion Hydroxide Gel Synthesis The initial pH of rare-earth nitrate solutions was usually low (PH =3-4) owing to traces of free nitric acid from the prepara- tive procedure and hydrolysis of the hydrated rare-earth cations. On addition of 5-10cm3 of aqueous ammonia, the pH rose sharply to ca. 8 for La to Sm, or to ca.7 for Gd to Yb. At the same time, the onset of precipitation was evidenced by some opalescence of the solution. The bulk of the precipi- tation occurred within a narrow pH range. At [OH-]/ [RE3'] x 3, there was a rapid further rise in pH. On com- pletion of ammonia addition, the suspension gave no indi- cation of substantial settling, and was intractable to filtration. Centrifugation (2500 rpm for 30 min) gave a voluminous hydrogel with a slight opalescence of the supernatant liquid in some cases, indicating product loss.In subsequent water washes to remove soluble ions, it was sometimes necessary to add up to 10 vol% of conc. ammonia solution to facilitate isolation of the gel. After acetone washes, the gel settled more J. MATER. CHEM., 1991, VOL. 1 readily. Yields by this method were variable, and ranged from <50% for LazO3 to 90% for Smzo3. In preparing the rare-earth oxides, acetone was chosen as the solvent for controlled dehydration (uide infra). As removal of acetone by direct calcination frequently led to partial coking of the product, the gel was first dried in a stream of nitrogen; further acetone and less volatile condensation prod- ucts of acetone were removed in uacuo, first at ambient temperature and then at successively higher temperatures to 400 "C, prior to calcination.Water and nitrogen oxides were also condensed in the cold trap owing to the partial dehy- dration of hydroxide to oxide and decomposition of occluded nitrates, respectively. The evolution of nitrogen oxides in the course of vacuum decomposition of acetone-treated gels was not surprising. The occlusion of substantial nitrate-ion concentrations (0.06-0.25 mol residual nitrate per mole of oxide) in the rare-earth hydroxide gel has been noted in earlier st~dies.'~*'~ The conditions required to free the product from residual nitrate were determined. On dissolution of samples in 0.1 mol dm-3 sulphuric acid, analysis (using an ion-selective electrode) showed that occluded nitrate was present (ca. 3% m/m) after calcination at 420 "C, but was below detection limit (<0.06% m/m) after calcination at 620 "C.Surface nitrate was not detectable by ESCA analysis after calcination at 620 "C in air for 4 h. After heating to 620 "C, only Ce02 as prepared from the hydroxide was still amorphous to X-rays. However, SAD showed the material to be microcrystalline and no amorphous material was evident by TEM. All sesquioxides were obtained as the low-temperature C phase, except Laz03 and Nd203, which were A phase; ceria was f.c.c. These structures are in agreement with literature expectation^.'^ Terbium and praseo- dymium are known to form a range of non-stoichiometric oxides; those obtained here were Tb407 (JCPDS File #32- 1286), for which a defect fluorite structure has been sug- gested,'* and Pr6011 (JCPDS File #6-0329) with an f.c.c.structure. Infrared spectra of freshly prepared samples (as KBr discs) all displayed broad, weak absorption bands in the 3450 and 1630 cm-' regions, attributable to v(H-0-H) and 6(0-H) vibrations of adsorbed water molecules. Dehydration to the oxide was complete under the heating regime used; none of the preparations displayed the strong, sharp band at 3600 cm- which is characteristic of lattice hydr~xide.'~ How-ever, aged samples of oxides, especially Laz03 and Ndz03, displayed this band as a result of the well known hydroxyl- ation reaction of rare-earth oxides with atmospheric water vapo~r.~O The hydroxide preparations were performed in an inert atmosphere to minimise adsorption of COz by the gel.Even with this precaution, all of the samples displayed a weak band at 1060 cm-I and, with the exception of Ce02 and Tb407, weak bands also near 1500, 1460 and 860 cm-'.The bands at 1060, 1500 and 146Ocm-' arise from C-0 stretching modes of unidentate carbonate and that at 860 cm-' from the out-of-plane bending mode of the bidentate carbonate group.31 The interaction of atmospheric carbon dioxide with Tb407 and CeOz to form carbonate is evidently considerably weaker than that for the other rare earths studied. Samaria and La203 qualitatively gave the strongest bands. The interac- tion of rare-earth oxides with carbon dioxide has been exten- sively studied.30 Quantitative temperature-programmed adsorption-desorption measurements, examining the forma- tion and decomposition of the carbonate phases formed from the current oxide preparations, have been reported separ- at el^.'^ Calcination near 800 "C is necessary to ensure com- plete removal of surface carbonate from La203.However, J. MATER. CHEM., 1991, VOL. 1 425 Table 1 TG of carbonate gels weight loss (YO) observed calculated to M203 rare earth T< 600 "C 600 cT/"C <800 M,(CO3)3 MOHC03 M2°(C03)2 M202C03 La 18.6 8.6 28.8 24.5 21.3 11.9 Sm 21.4 2.4 27.5 23.3 20.1 11.2 Tb 20.7 2.9" 26.5 22.4 19.4 10.7 Er 16.2 2.5 25.5 21.6 18.7 10.2 Ce 19.0b 0 - 26.5' - 20.4' ~~ ~ ~ T< 700 "C; Tc450 "C; 'calculated weight losses for 'Ce(OH),CO3' and 'CeOCO,' converting to CeO,.this step results in a considerable lowering of surface area (to 27 m2 g-', vide infra). Carbonate Gel Synthesis As in the hydroxide method, the addition of the first 5-10 cm3 of base often led to an initial sharp rise in pH due to neutralisation of residual free nitric acid. Precipitation occurred at pH 4.5-5.5, and a further sharp rise in pH was observed at [ammonium carbonate]/[M3+] ratio of ca. 3. In contrast to the hydroxide gel, carbonate gels settle spon- taneously to a significant extent, such that ca. 80% of the supernatant liquid may be removed by decantation, and a short centrifugation suffices to give good separation of the gel. No addition of ammonia is necessary in subsequent washing steps.In contrast to the hydroxides, the carbonate precipitates were readily dried in uucuo at room temperature only. This difference may partly be due to formation of more of the less volatile acetone condensation products in the ide route. The carbonate route did not produce La203 and Nd203 as pure products; carbonate phases were observed as substantial components. Infrared spectra confirmed that both La and Nd gels underwent only partial transformation to oxide, even at 700 "C, with strong carbonate bands persisting. All other samples appeared to convert almost completely to oxide, leaving only weak bands in similar positions to those observed from the hydroxide route. Retention of carbonate in the lighter rare earths is in accordance with their higher ba~icity.~.'~Hence, lighter rare-earth oxides are better pre- pared by the hydroxide method.Surface Areas Adsorption-desorption isotherms at -196 "C were measured for rare-earth oxides obtained by each route under a variety of preparative and calcination conditions. They were found to be either type I1 or type IV (both of which are amenable to B.E.T. analysis), depending on the particular method and hydroxide method, by a base-catalysed aldol c~ndensation.~~ member of the series. B.E.T. parameters were determined by Final oxide yields from the carbonate method were typically >90%. After the oxide samples had been dried to 120 "C, XRD traces were featureless, with the exception of that from the Pr preparation which corresponded to * 8H20 (JCPDS File #3 1-1143).Infrared spectra from all specimens showed broad intense carbonate bands. Possible compositions of the solid at this stage might be M,(CO3)3, MOHC03, M20(C03),, M202C03, or a mixed hydroxycarbonate such as M2(OH)6 -2x(co3),.34 Thermogravimetric analysis (TG), using N2 as flow gas, was carried out on samples predried at 120 "C, for Sm, Er, Tb, CeIV and La. Table 1 shows the experimental weight losses observed to 600 "C, and any additional weight loss to 800 "C. Also shown in Table 1 are calculated weight losses from the various carbonate species, assuming conversion to the appropriate oxide. If the original precipitate were M2(C03)3, the observed weight loss would be substantially larger than that found.The figures for Sm and Tb are very close to those for hydroxycarbonate. How- ever, the analysis for Tb is complicated by the variable oxidation states of the precipitate and decomposition product. For Er, the losses are too low for hydroxycarbonate, yet too high for oxycarbonate. Thus, either a mixed hydroxy-oxycarbonate formulation or one corresponding to M2(OH)6-2x(C03)xappears likely.34 For Ce, on the other hand, oxycarbonate losses come closest to the observed fig- ures. The shape of the TG curve for lanthanum shows a two- stage decomposition. The overall weight loss is close to that of La2(C03)3, decomposing via the oxycarbonate to the oxide. After they had been heated to 600 "C, the oxides of Ce, Tb, Er, Ho and Yb prepared by the carbonate route were X-ray amorphous, but SAD showed that they were microcrystalline; again, no amorphous material was detected by TEM.The pure-oxide phases obtained were the same as from the hydrox- least-squares analysis of plots in the range 0.05 <p/po<0.35; 'c' values were typically between 100 and 150, justifying application of the B.E.T. method. Confirmation that the preparations were not microporous was obtained from both a, and t plot^,^',^^ which invariably gave zero intercept on the adsorption axis. A separate study of the relationship of particle size and morphology to surface area and mesoporosity is published elsewhere.37 Tables 2 and 3 list the highest areas obtained by acetone dehydration and calcination of the hydrogels precipitated at Table2 B.E.T.surface area of rare-earth oxides prepared by the hydroxide route oxide y203 Ce02 Pr,O 11 Nd203 Sm203 Gd203 Tb40, Dy2°3 Ho203 Er*O3 Yb203 surface area/m2 g- [MI" B/Mb B.E.T.' ref? 0.44 56.4 63 0.385 13.0 69 0.050 10.0 55 0.025 20.0 86 0.30 13.5 60 0.025 20.0 53 0.025 20.0 60 0.025 20.0 34 0.025 20.0 50 0.025 20.0 50 0.025 20.0 25 0.025 13.9 26 " Initial molar concentration of rare-earth ion, M3+. B/M = ammonia/metal molar ratio; ammonia added dropwise as 1 mol dm-' solution to stirred solution of M3+ at ambient temperature. 'After calcination to 620 "C for 4 h, following acetone dehydration pro- cedure.d 600 "C<calcination temp. <650 "C; highest area found in literature. From ref. 11. From ref. 9. From ref. 10. From ref. 6. Table 3 B.E.T. surface area of rare-earth oxides prepared by carbonate route oxide [MI" [Bib B/M' B.E.T. area/m2 g-' y2°3CeO, 0.010 0.016 1.oo 0.303 16.7 12.Od 98 180 Pr,O 11 Sm203 0.020 0.010 1.oo 0.303 6.3 10.0 <10 52 0.010 0.303 10.0 53 Hoz03 0.0 16 0.303 6.3 56 Er203e 0.010 0.303 10.0 58 Yb203e 0.010 0.303 12.0 59 L'203 0.010 0.303 13.1 59 "Initial molar concentration of rare earth ion, M3+ or 'Molar concentration in titrant of commercial (NH,),[Ce(NO,),]. 'ammonium carbonate', B. Final molar ratio of 'ammonium carbon- ate': metal; carbonate solution added dropwise to stirred solution of metal salt at ambient temperature.Reverse order of addition, with Ce" solution added dropwise to stirred solution of carbonate. 'Precipitation at 60 "C. ambient temperature through the series, by the hydroxide and carbonate routes, respectively. It should be emphasised that not all of the preparative variables have been optimised, and so further improvement in areas can confidently be expected. These tables also give for comparison the previous best reported value under similar final calcination temperature. As with lanthanum, the low-area (22 m2g-') product from neo- dymium by the carbonate method was shown to be impure by TEM and IR studies, and these are therefore omitted from Table 3.The B.E.T. surface area of samples of La203 prepared by the hydroxide method is given in Table4. The results show that the area obtained was increased from 37 to 69 m2 g-' by the introduction of acetone dehydration to otherwise identical drying treatments. The value of 37 m2 g-' alone is comparable to the highest literature value we have found (Table 2). Substitution of methanol for acetone was found to give a product of lower surface area, possibly due to polar interactions with the gel in a similar manner to water. Washing with higher alcohols, such as propan-2-01, gave results similar to acetone, as shown in Table 4, but greater solvent retention made drying in uucuo more difficult.The use of chemical dehydrating agents, such as 2,2-dimethoxypropane (Table 4) or trimethyl orthoformate, offered no advantages over acetone. It appears that removal of water at low temperatures with organic solvents discourages agglomeration of the primary rare-earth hydroxide or carbonate particles, ultimately resulting in high-surface-area material. The effectiveness of gradual removal of water at low temperatures has been shown Table 4 Effect of dehydration medium on surface area of La203" dehydration medium [La]' B/Lac B.E.T. area/m2 g-water 0.385 13.0 36 acetone 0.385 13.0 69 acetoned 0.025 20.0 65 acetone 0.020 5.0' <10 propan-2-01 0.040 12.5 67 2,2-dimethoxypropan& 0.385 7.6 63 " After calcination to 620 "C for 4 h, following dehydration procedure. Initial molar concentration of La3+.'B/La =ammonia/La3+ molar ratio; ammonia added dropwise as 1 mol dm-' solution to stirred solution of La3+ at ambient temperature. Reverse order of addition, with La3+ added dropwise to the stirred solution of I mol dm-j ammonia at ambient temperat~re.~~ Using 'ammonium carbonate' instead of ammonia. Refluxing with 2,2-dimethoxypropane before calcination. J. MATER. CHEM., 1991, VOL. 1 previously for Ce02 by isobaric dehydration. l2 However, the procedure is time consuming. Non-aqueous solvents presum- ably decrease Ostwald ripening by lowering hydroxide solu- bility as well as lowering surface tension, leading to reduced agglomeration of primary crystallites during drying.Similar solvent effects have been observed in the formation of con- trolled-porosity aluminas.20 The addition of La3+ to the ammonia solution instead of vice versa caused an insignificant change in area (65 m2 g- '). On the other hand, attempted preparation of La203 by the carbonate method, rather than the hydroxide method, gave a product (after heating to 600 "C) of <10 m2 g-'; TG, SAD and IR studies showed this material to be still heavily contami- nated with carbonate. Thus, preparation of high-area La203 by the carbonate method is not considered feasible. In Table 5, a comparison is given of the areas obtained for Yb203 and Er203 by the hydroxide and carbonate routes. For these later rare earths, precipitation via the carbonate route gave larger areas than via the hydroxide. Examination of the table also shows that further gains were made by carrying out the precipitation at 60 "C instead of ambient, from 49 to 59 m2 g-' in the case of Yb2O3 and from 36 to 58 m2 g- ' in the case of Er2O3. The results also suggest that a slightly higher-area product was obtained if a lower B/M ratio was used.Optimal reagent concentrations and tempera- tures have yet to be established. Cerium differs from the other rare earths in giving a dioxide on heating. It is found that, with carbonate precipitations, the final surface area of the oxide obtained is strongly dependent on the initial concentration of ammonium ceric nitrate.38 Conclusion Rare-earth oxide powders with relatively high surface area have been prepared by two methods.The two routes are complementary in that the hydroxide method is more effective for the preparation of the lighter (earlier) rare-earth oxides and the carbonate method for the heavier (later) members of the series. Further improvement in the areas obtained so far should be possible for at least some of the series. Catalytic applications of these materials are currently under study. In particular, they have proven to be efficient catalyst supports in carbon monoxide hydrogenation, and active catalysts in methane coupling. Table 5 Effect of preparative conditions on surface area" of rare-earth oxides B.E.T. oxide methodb [MIc initial [Bid B/Me area/m2 g-' 0.010 1.o 16.7 39 0.016 0.303 6.7 48 0.010 0.303 12.0 49 0.010 0.303 12.0 59 0.025 0.016 1.o 0.303 13.9 6.3 26g36 0.010 0.303 10.0 58 0.025 1.o 20.0 2Y " After calcination at 600 "C for 2 h, following acetone dehydration.'C =carbonate method; H =hydroxide method. 'Initial molar con- centration of rare-earth ion, M3+. Molar Concentration of commer- cial 'ammonium carbonate', B. Molar ratio of 'ammonium carbonate'/metal; carbonate solution added dropwise to stirred solu- tion of metal salt at ambient temperature. f Precipitation at 60 "C. * Calcination at 620 "C. J. MATER. CHEM., 1991, VOL. 1 427 References 1 2 K. Otsuka, K. Jinno and A. Morikawa, J. Catal., 1986, 100, 353. A. Ekstrom, R. A. Regtop and S.K. Bhargava, Appl. Catal., 1990, 62, 253. 21 22 23 24 R. K. Iler, The Chemistry of Silica, Wiley, New York, 1979. R. A. Dombro and W. Kirch, Eur. Pat. 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