J. CHEM. SOC. FARADAY TRANS., 1995, 91(9), 1385-1390Characterisation and Activity of Praseodymium Oxide Catalystsprepared in Different Gases from Praseodymium Oxalate HydrateMicroscopic, Thermogravimetric and IR Spectroscopic Studies1385Gamal A. M. HusseinChemistry Department Faculty of Science, Minia University, El-Minia 61519, EgyptPro, .*, and Pr203 catalysts have been obtained as final decomposition products of Pr, (C204), * 10H,O indifferent gases (0, , N, and H,). The decomposition processes were characterized by thermogravimetric (TG)and differential thermal analysis (DTA), X-ray diffraction (XRD) and IR spectroscopy of the solid-phase products.The results indicate that the compound dehydrates in three steps at 100, 150 and 380°C and that the anhydrousoxalate decomposes at 445°C to form two phases of Pr,O,(CO,) depending upon the atmosphere used for thereaction.On further heating is formed at 550°C in 0, and at 650°C in N, . In contrast, Pr203 was formedat 650°C in H,. Surface area measurements and scanning electron microscopy (SEM) have shown thatproduced at 700°C has a different surface area depending on the gas used: 43 and 64 m2 g-' for 0, and N,,respectively. The surface area of Pr203 formed in H, is 59 m2 g-'. The texture of the catalyst has been found todepend upon the decomposition atmosphere.IR spectroscopy has been used to analyse (qualitatively and quantitatively) the gas-phase reaction productsbetween room temperature and 400 "C from the dehydrogenation and dehydration reactions of propan-2-01 overPro,.,,, and Pr203 catalysts.The results revealed that Pr,O, is a selective dehydration catalyst at 275"C,decomposing propan-2-01 into propene (ca. 80%). However, Pro, is a dual function dehydration/dehydrogenation catalyst.Praseodymium oxides comprise Pr203, Pro, and a range ofintermediate phases: Pro,.,,,, PrO,.,,,, PrO,.,,, ,PrOl.780, Pro,.,,, and Pr,O, possesses hexagonalstructure, Pro, the fluorite structure and PrO,.,soand Pr01.833 are oxygen-deficient modifications of the fluoritestructure. The production of these phases depends on theprecursor used and the atmosphere and temperature ofdecomposi tion.,Moosath et aL5 studied the decomposition ofPr2(C204), . 4H20 in air and reported that complete dehydrationoccurs at 370 "C, followed by rapid decomposition ofanhydrous oxalate to Pr,O,CO, at 480 "C.The latter decomposesat 560 "C to yield Pr,O, as a final product. Muraishi etaL6 studied the decomposition of Pr malonates in an N,atmosphere. They stated that Pr,O,CO, forms as an intermediateat 329°C and that Pr203 is obtained as the finalproduct at 617 "C.In contrast, Dassuncao et aL7 studied the decomposition ofhydrated praseodymium basic carbonate [Pr,(OH),(CO,), .H,0] in air, and stated that Pr,O,CO, was formed at 460°Cand decomposed to Pro,.,,, at 570 "C via a Pr202.6(C03)o.4intermediate. In a later study,, for the decomposition ofPr,(C,O,), - 10H,O and Pr(CH,COO), * H,O in air, it wasfound that both oxalate and acetate of praseodymium formedPr,O,CO, as an intermediate, which decomposes at 575 "Cto give Pro,.,,, as a final product. The main differencebetween the final products obtained from acetate and oxalateat 800°C for 1 h in air is in the surface area; Pro,.,,,obtained from Pr acetate had a surface area of 18 m2 g-'and Pr oxalate 8 m2 g-l.Metal-oxide catalysts with high surface area and reactivityare often obtained from the thermal decomposition of precursorc o m p ~ u n d s .~ ~ ' ~ The release of volatile components forcesgeneration of fast-transport pathways (pores) through thebulk material."*'2 It has been established that the surfacetexture and the performance of solid catalysts are criticallycontrolled by their preparation and pretreatment conditions.' 2-1 The decomposition of propan-2-01 has beenwidely used to examine the acid-base properties of metaloxidecatalysts.'6-'8 It is generally assumed that acidicoxides catalyse dehydration and basic oxides catalyse dehydrogenation.In the present study the effect of the reaction atmosphereon the thermal decomposition course and final decompositionproducts of Pr oxalate decahydrate was examined byTG and DTA.The solid products at intermediate temperatureswere subjected to IR and XRD. The final decompositionproducts, 700°C for 1 h in O,, H, and N,, weresubjected to surface area measurements and SEM and testedqualitatively and quantitatively for the decomposition ofpropan-2-01.ExperimentalPraseodymium oxalate decahydrate (PrOx), Pr,(C,O,), .10H,O, (Aldrich, 99.99%), was calcined at 200, 450 and700 "C for 1 h in different atmospheres: 0, , H, and N, .Thecalcination temperatures were chosen on the basis of thethermal analysis results (see below). Propan-2-01 and acetone(BDH, spectroscopic grade) were thoroughly degassed byfreeze-pumpthaw cycles under vacuum prior to use.Thermal AnalysisTG and DTA of the parent material (PrOx) were carried outusing heating rates of 2-20°C min-', up to 800"C, in adynamic atmosphere of 0, , H, or N, (20 ml min- l), using amodel 30H Shimadzu analyser. 10-15 mg of the test samplewere used for the TG measurements and highly sintered ct-A1,03 was the thermally inert reference material for theDTA.IR SpectroscopyIR spectra were obtained at a resolution of 5.3 cm-', overthe range 4000-400 cm-', using a model 580B Perkin-Elmerspectrophotometer, equipped with a 3700 PE data station forspectral acquisition and handling1386 J.CHEM. SOC. FARADAY TRANS., 1995, VOL. 91(I) IR spectra of PrOx and its solid calcination productswere obtained from thin ( ~ 2 0 mg rn-,), lightly loaded( < 1 YO) KBr-supported discs.(11) IR spectra of propan-2-01 and its gaseous catalyticdecompositionproducts were taken with the help of a speciallydesigned variable-temperature IR cell equipped withKBr windows. The following standard procedure wasadopted. The catalyst (100 mg in powder form) was heated ina stream of oxygen at 600°C for 30 min to clean the surfaceof carbonate ~ontamination'~ and cooled to room temperatureunder vacuum (10- Torr).Propan-2-01 (10 Torr)was allowed into the cell at various temperatures from 100 to4OO"C, and maintained in contact with the catalyst for 10min.Propan-2-01 and its gaseous decomposition products werequantitatively analysed using the standard QUANT softwarefrom Perkin-Elmer and the on-line data acquisition system.Accordingly, the amounts of the gaseous components(reactant and products) were determined from calibrationcurves relating the IR-absorption intensity at a certain frequencyto the calibrated gas pressure (Torr). The calibrationcurves were derived from IR data obtained from authenticsamples of each of the gas-phase components under identicalspectroscopic conditions. The absorption intensity wasmeasured at 3665 & 5 cm-' for propan-2-01, 1740 f 5 cm-'for acetone (the dehydrogenation product) and at 910 f 5cm- ' for propene (the dehydration product).16XRDXRD analyses of PrOx and its calcination products werecarried out by means of a model JSX-60 PA Jeol diffractometerusing Ni filtered Cu-Ka radiation.For identification purposesthe diffraction patterns (Z/Zo us. d-spacing) obtainedwere matched with ASTM standards.N,-adsorption MeasurementsN,-sorption isotherms were determined volumetrically at- 196 "C using microapparatus based on a design describedby Lippens et The test samples were outgassed at 200 "Cfor 2 h while evacuating at lo-' Torr.Electron MicroscopySamples of the final decomposition product (700 "C) wereexamined in a Jeol 35CF scanning electron microscope tocharacterize the texture.Before examination, the sampleswere rendered conducting by pre-coating with a thin film ofAu-Pd.Results and DiscussionTG and DTA curves for PrOX heated up to 800°C at a rateof 10 "C min- ' in O,, H, and N, atmospheres are shown inFig. 1. IR spectra and XRD powder patterns obtained forPrOx and its solid decomposition products at 250, 450 and700°C are shown in Fig. 2 and 3. Fig. 4A shows the N,-sorption isotherms for PrOx calcined at 700°C for 1 h indifferent gases. The corresponding pore-volume-distributioncurves are given in Fig. 4B. The SEM for the same samplesare shown in Fig. 5. Fig. 6A shows the IR gas-phase spectrafrom 10 Torr of propan-2-01 in contact with PrOx (700°C in0,) after consecutive 10 min periods at the temperaturesindicated. Quantitative analysis of the gas-phase composition,resulting from 10 Torr of propan-2-01 giving rise toacetone and propene, after consecutive 10 min intervals at thetemperatures indicated over the PrOx (700°C in O2 and H,)catalysts, is given in Fig.6B.10203040Fig. 148.3%', 53.1%0 100 200 300 400 500 600 700 800temperatu re/"CTG (a) and DTA (b) curves for PrOx measured in a dynamicatmosphere (20 ml min-') of A, 0,; B, N, and C, H,; at a heatingrate of 10°C min-'. DTA peak labels I-VII are discussed in the textand occur at temperatures (/"C for I-VII); A: 100, 150, 375, 390,415,425, 550; B: 110, 150, 370, 390, 415, 445, 675; C: 110, 150, 380, 420,(no V or VI), 650.Characterization of the Thermal EventsDehydration ProcessesEvents Z and IZ (100-150°C).The TG and DTA curves(Fig. 1) show that processes I and I1 are overlapping endothermicweight-loss (WL) processes. The WL observed(21.8%) from both processes is very close to that expected forrelease of 9 mol of water (22.3%).I(100"C)4.2%Pr,(C,O,), - 10H,OII( 1 so "C)Pr2(C204)3. 8H20 ,,% ' Pr2(C204)3 * H2° (l)As reported earlier,' the IR spectrum of the solid phase at200°C (Fig. 2) bears a great deal of similarity to that forunheated PrOx because both show absorption bands arisinJ. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91I . I . l r . . , l . , , r l I2000 1500 1000 500wavenumberlcm-'Fig. 2 IR spectra of (a) KBr-supported PrOX and its solid decompositionproducts taken at room temperature after heating for 1 h in(b) 0,-H,-N, at 200"C, (c) 0, at 450"C, (6) 0, at 700"C, (e) H, at450 "C, (f) H, at 700 "C, (9) N, at 450 "C and (h) N, at 700 "Cfrom oxalate.However, the corresponding XRD pattern (Fig.3) indicates that the products are amorphous and so water ofhydration is very important for the coherency of the PrOXcyrstal."Event ZZZ (375 "C). Process 111 largely overlaps with processIV and brings the total WL to 24.5%, close to that expected(24.8%) for the complete dehydration of PrOx. These resultsI I5 15 25 35 45 55281deg reesFig. 3 XRD powder patterns of (a) PrOx and its decompositionproducts taken at room temperature after heating for 1 h in (b)0,-H,-N, at 200"C, (c) 0, at 450°C to give Pr,O,CO, (II), (d) 0,at 700°C to give Pro,,,,,, (e) H, at 450°C to give Pr,O,CO, (I),(f) H, at 700°C to give Pr,O,, (9) N, at 450°C to give Pr,O,CO,(I) and (h) N, at 700°C to give Pro.1833PIP"0.2 t" n0 20 40 60 80 100Fig.4 A, BET N,-sorption isotherm at - 196°C for PrOx calcinedfor 1 h at 700 "C in (a) N,, (b) 0, and (c) H, . B, Pore-volume distributioncurves for the same system: (-) N,, (-.-.) 0, and(.*...) H,.r;/Aare in agreement with the earlier study,' for the dehydrationof PrOx in air, i.e. the dehydration of PrOx is not affected bythe gaseous atmosphere.Decomposition Processes in OxygenEvents IV, V and VZ (390-425 "C). Fig. 1A shows that processesIV, V and VI are overlapping exothermic WL processes.The TG curve shows that these processes areresponsible for the decomposition of anhydrous PrOx toPr202C03, as indicated by the observed WL (48.3%) whichis very close to that expected (48.6%) for the formation ofPr202C03 from unstable Pr2(C,04), as follows'IV( 3 90°C)37.5% ' pr2(c03)3V(4 1 5 "C)VI(42 5 "C)Pr20(C03)2 48.3% ' Pr2O2Co3 (2)The IR spectrum [Fig.2(a)] and XRD pattern [Fig. 3(a)]support reaction (2). The IR spectrum displays absorptions at1560, 1440, 880 and 840 cm-' assignable to oxycarbonatespecies.' The strong absorptions emerging at 650-450 cm -are related to Pr-0 vibration modes.21 XRD also indicatesthe presence of crystalline Pr202C03 (II)(ASTM No. 25-696).Event VZZ (550°C). On further heating, process VII takesplace endothermally (Fig.1A) at 550°C. The maximum WLthus determined (53.1 %) agrees well with the theoreticalvalue (53.09%) expected for a complete conversion of PrOxinto PrO,.,,, , i.e. process VII is an oxidative decompositionprocess (Pr3 + -+ Pr3.66 +).The IR spectrum of PrOx treated at 700 "C (Fig. 2) declaresthe absence of detectable absorptions arising from oxycar1388 J. CHEM. SOC. FARADAY TRANS,, 1995, VOL. 91Fig. 5700 "C in (a) N, , (b) H, and (c) 0,SEM micrographs (2500 x ) for PrOx calcined for 1 h atonate species. However, the absorptions below 850 cm- ',which are related to lattice-vibration modes of Pr-0, areretained.,' The corresponding XRD for PrOx at 700 "C (Fig.3) shows only the pattern for crystalline PrO,.,,, (ASTMIn comparison with the earlier study in air,' it is clear thatthe decomposition of PrOx in 0, is nearly identical to thatobtained in air.NO.6-329).Decomposition Processes in HydrogenFig. 1B shows that the decomposition of PrOx in hydrogen isnearly similar to that obtained in oxygen (Fig. lA), especiallyconsidering the number and nature of the thermal eventsinvolved. The main differences are (i) reaction (2) in processesIV, V and VI probably takes place asIV( 3 90 "C) V(415"C)Pr2(C204)3 34.5% ' pr2(c204)0.5(c03)2.5 ,,% 'VII(44 5 "C) Pr2(C03)3 48.3% ' Pr202C03 (I) (3)3600 3100 2600 2100 1600 1100 600wavenumber/cm - '' 0 50 100 150 200 250 300 350 400reaction ternperature/"CFig. 6 A, IR spectra of (IP) propan-2-01 (10 Torr) and its decompositionproducts [(A) acetone, (P) propene, (M) methane, (I) isobutene]in contact with PrO,,,,, for 10 min at the indicated temperatures.B,IR quantitative analysis of the gas-phase composition resulting from10 Torr of (a) propan-2-01 decomposing over (-) PrO,,,,, and(---) Pr,O, to give (b) acetone and (c) propene. Measurements madeafter consecutive 10 min intervals at the temperatures indicated in A.The WL observed throughout process IV is 34.5% which isvery close to that calculated (34.4%) for the formation ofPr2(C204)o~5(C03)2~5 and so the H, atmosphere retards theformation of both Pr2(C03)3 and Pr,O,CO, . (ii) The structureof Pr,O,CO, (I) (ASTM No. 37-805) formed in H, isdifferent from that obtained in 0, at 450°C [Fig.3(e)]. (iii)Process VII is shifted to higher temperature (675 "C, Fig. lB),also the WL observed is 54.1%, close to that calculated(54.5%) for the formation of Pr203. XRD at 700°C [Fig.3(b)] reveals the formation of the hexagonal structure Pr203(ASTM No. 6-410) and so the oxidation state of Pr does notchange during the decomposition in hydrogen.Decomposition Processes in NitrogenThe decomposition of anhydrous PrOx in nitrogen (Fig. 1C)takes place through one large endothermic effect located at420°C. The WL observed by the end of process IV is 48.3%,which is very close to that (48.6%) calculated for the formationof Pr,02C0, . The XRD, Fig. 3(g), reveals the formationof Pr,0,C03 (I) and so in both N, and H, the Pr,O,CO,formed had the same structure.However, the Pr,02C0, thusformed in an N, atmosphere decomposed at 650°C (Fig. 1C)to give PrO,,,,, as the final product, as indicated by the WLobserved (53.1%) by the end of the decomposition course andby the XRD pattern of PrOx (N,) [Fig. 3(b)]. The finaJ. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 1389decomposition step is, therefore, oxidative as in the case of0, (Pr3 + -, Pr3.66 + ).Surface CharacterizationAll the isotherms shown in Fig. 4A are close to BET-typeIV,,, revealing a porous character. The hysteresis loopsexhibited in Fig. 4A have almost the shape of type (B) H3.These general features indicate that the majority of the poresare slit-shaped and/or non-parallel plates. 23 The presence ofmicropores and mesopores was indicated by the pore-sizedistribution (PSD) curves (Fig.4B) and texture data (Table 1).The PSD curve of PrOx (700°C in 0,) (SBE, = 43 m2 g-')exhibits mainly micropores and mesopores (three peaks at 19,26 and 35 A) which are probably the reason for the lowsurface area in comparison with the other samples. However,it has a higher area than that obtained (8 m2 g-') at 800°Cin air. The PSD curve of PrOx (700°C in N,), which has thesame crystal structure and composition (Pro1.,,,) (S,,, = 64m2 g-') (three peaks at 20,28 and 40 A), shows growth of themesopores which is probably responsible for the higher areain comparison to the 0, sample. On the other hand, the PSDcurve and texture data of the hexagonal Pr,O, obtained inan H, atmosphere (SBET = 59 m2 g-') which is different incomposition and structure, exhibits a wider spectrum ofmesoporosity (three peaks at 22, 30 and 45-60 A) and so theH, affected not only the composition and structure of thefinal product but also the surface area and texture.The similarityof the texture data for PrOx calcined in both 0, andN, at 700°C is most probably because they are formed fromthe same Pr,O,CO, (I) intermediate.The SEM of PrOx (Fig. 5) supports the above results andreveals the presence of well formed crystals of a wide range ofsizes with rough surface steps and non-parallel plate-shapedcrystals. The pores are randomly distributed over the surfacewhich was noted to be rough with extensive porosity andinterplating.Catalytic Activity for Propan-2-01 DecompositionIR spectra from the gas phase of propan-2-ol/PrOl~,,,(PrOx, 700°C in 0,) at different temperatures are shown inFig.6A. The room temperature and 150 "C spectra displaythe characteristic bands of propan-2-01., At 200°C a tiny butimportant absorption emerges at 1740 cm - ', which developedmarkedy in the 250°C spectrum together with anotherabsorption at 1250 cm-'. The two bands mark the formationof gas-phase acetone,, indicating that the dehydrogenation ofpropan-2-01 started at 200 "C. At 250 "C (Fig. 6A) additionalabsorptions emerge at 1650 (doublet) and 915 cm-' that aredue to propene. Hence, the dehydration of propan-2-01occurs at 250°C. Following the reaction at 300°C theacetone bands are slightly intensified, the propene absorptionsgrow stronger and absorptions in the vcH region (3100-2800 cm- ') are re-structured.Table 1in O,, N, and H,Surface-textural characteristics of PrOx calcined at 700 "C>T SBET v, atmosphere /m2 g-' C /ml g-'0, 43 72 0.059 20, 22, 40N, 64 44 0.070 20, 27, 50H, 59 60 0.55 20, 28, 50-60BET surface area (S,,,), constant (C), pore volume (V,) andmaximum pore radius (rmax).At 350 "C, absorptions due to alcohol are hardly detectableand acetone absorptions, while weakened, remain up to400°C. In contrast, absorptions due to propene are intensified.Moreover, at 350°C new absorptions emerge at 3010and 1310 cm-' (due to rnethane),l6 at 890 cm-' (due toisobutene) and at 2340 and 670 cm-' (due to CO2).I6 Thesenew absorptions are greater in the 400°C spectrum (Fig.6A).The formation of the CH,, isobutene and CO, by-productsis due to surface reactions of acetone via an aldol-type condensation.Such reactions involve adsorbed and gas-phaseacetone molecules, as well as nucleophilic surface OHg r o ~ p s . ' ~ . ~ ~ - ~ ~The IR gas-phase spectrum of propan-2-01 over Pr203 isnearly similar to that obtained for PrO,.,,, . The main differencesobserved with Pr203 are (i) acetone first appeared at200°C and its peak grew stronger at 250"C, but disappearedcompletely at 400 "C, (ii) propene first appeared at 200 "C, (iii)both CO, and CH, were also detected, while isobutene wasnot.The above results are presented on a quantitative basis inFig. 6B. It is clear that propan-2-01 over Pr,O, starts todecompose at 200 "C, acetone emerges simultaneously at200°C and then propene at 250°C.The rate of alcoholdecomposition appears to reach a maximum at 300"C, asdoes the production of propene (ca. 80%) at 350"C, and thatof acetone (ca. 20%) at 300 "C. At 400 "C, the acetone decomposescompletely. For propan-2-01 over Pro1.,,, , the productionof propene is CQ. 50% at 325°C. At 400°C theamount of acetone commences to decrease.These activity and selectivity results for both Pr,O, andPro,.,,, in the decomposition of propan-2-01 may indicatethat Pr203 (hexagonal structure) is more acidic (highly activedehydration catalyst)27 than Pr01.833. The high dehydrogenationactivity of Pro,.,,, also reveals the basicity.However,acetone formation can occur by a redox me~hanism.,~Conclusion(1) The thermal decomposition of PrOx in different atmospheresinvolves the following pathways :Pr,(C,04), - 10H,O!-+ Pr,(C,04), . 8H,O -!!+I11Pr2(C204)3 * H2° - (A) Inoxygenpr2(c204)3 - Pr2(C03)3 --+ Pr,O(CO,), - IV V VIVI1 Pr,O,CO,(II) - 2Pr01.,,,(B) In nitrogenIV VIIPr2(C204)3 - Pr202C03(1) 2Pr01.833(C) In hydrogenIVPr2(C204)3 - pr2(c204)0.5(c03)2.5VI vn Pr,(C03), - Pr,O,CO,(I) - Pr203(2) The dehydration processes are not affected by the natureof the atmosphere. The water of hydration is responsible forthe crystal coherency of PrOx. (3) Two different phases ofPr,O,CO, were detected as stable intermediates during thedecomposition of PrOx. These depend upon the atmosphere.(4) PrOx decomposes to Pro,.,,, in 0, and N,.However,the hexagonal Pr203 was formed as a final product in H, . (5)The decomposition processes and the final decompositionproducts are mainly controlled by the nature of the atmosphere.(6) Pr oxides obtained at 700°C have surface areas o1390 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 9132 (in O,), 64 (in N2) and 59 m2 g-' (in H,) and have twodifferent types of porosity (micropores and mesopores). (7)Pr,O, obtained at 700°C is a selective catalyst for the dehydrationof propan-2-01 (to form propene, ca. 80%, maximizedat 300"C), with a lesser tendency for dehydrogenation (toform acetone, ca. 10% maximized at 275 "C) indicating that itis an oxide catalyst. In contrast Pro,.,,, functions for dehydrationand dehydrogenation.(8) The decompositionpathway and the composition, structure, surface texture andcatalytic activity of the final products are mainly controlledby the atmosphere during the decomposition of PrOx.ReferencesY. Wilbert, A, Duquesnoy and F. Marion, C.R. Acad. Sci. Paris,1966,263,1539.B. G. Hyde, D. J. M. Bevan and L. Eyring, Philos. Trans. R. SOC.London, 1966,259, 583.L. Eyring and N. C. Baenziger, J. Appl. Phys. Suppl., 1962, 33,428.J. M. Hong, A. F. Clifford and P. A. Faeth, Znorg. Chem., 1962,2, 719.S. S. Moosath, J. Abraham and T. V. Swaminathan, 2. Anorg.Chem., 1963,324,90.K. Muraishi, H. Yokobayeshi and K. Nagase, Thermochim.Acta, 1991, 182, 209.L. M. Dassuncao, 1. Giolito and M.Ionashiro, Thermochim.Acta, 1989, 137, 319.G. A. M. Hussein, J. Anal. Appl. Pyrolysis, 1994, 29, 89.9101112131415161718192021222324252627S. A. Mansour, G. A. M. Hussein and M. I. Zaki, Thermochim.Acta, 1989, 150, 153.G. A. M. Hussein, J. Powder Tech., 1994,80,265.D. L. Trimm and A. Stanislaus, Appl. Catal., 1986,21,215.G. A. M. Hussein and H. M. Ismail, J. Colloid Su$, 1995, in thepress.K. Tanabe, K. Mismo, Y. Ono and H. Hattori, New Solid Acidsand Bases, Elsevier, NY, 1989, pp. 41-47.G. C. Bond and E. F. Thair, Appl. Catal., 1991,71, 1.H. Arakawa, Technol. Jpn., 1988,21,32.G. A. M. Hussein, N. Sheppard, M. I. Zaki and R. B. Fahim, J.Chem. SOC., Faraday Trans. I , 1989,85,1723.K. S. Kim and M.A. Barteau, J. Mol. Catal., 1990,63, 103.G. A. M. Hussein and H. M. Ismail, J. Bull. Chem. SOC. Jpn.,1994,67,2634.J. B. Peri and B. H. Hannan, J. Phys. Chem., 1960,64,1526.B. C. Lippens, B. G. Linsen and J. H. de Boer, J. Catal., 1964,3,32.J. A. Goldsmith and S. D. Ross, Spectrochim. Acta, Part A, 1967,23,1909.S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area andPorosity, Academic Press, London, 2nd edn., 1982.K. S. W. Sing, Pure Appl. Chem., 1982,54,220.G. A. M. Hussein, Thermochim. Acta, 1991,180, 187.A. V. Kiselev and A. V. Uvarov, Su$ Sci., 1967,6,399.M. I. Zaki and N. Sheppard, J. Catal., 1983,80, 114.C. Lahouse, A. Aboulayt, F. Mauge, J. Bachelier and J. C.Lavalley, J. Mol. Catal., 1993, 84, 284.Paper 4/05964F ; Received 29th September, 199J.CHEM. SOC. FARADAY TRANS., 1995, 91(9), 1385-1390Characterisation and Activity of Praseodymium Oxide Catalystsprepared in Different Gases from Praseodymium Oxalate HydrateMicroscopic, Thermogravimetric and IR Spectroscopic Studies1385Gamal A. M. HusseinChemistry Department Faculty of Science, Minia University, El-Minia 61519, EgyptPro, .*, and Pr203 catalysts have been obtained as final decomposition products of Pr, (C204), * 10H,O indifferent gases (0, , N, and H,). The decomposition processes were characterized by thermogravimetric (TG)and differential thermal analysis (DTA), X-ray diffraction (XRD) and IR spectroscopy of the solid-phase products.The results indicate that the compound dehydrates in three steps at 100, 150 and 380°C and that the anhydrousoxalate decomposes at 445°C to form two phases of Pr,O,(CO,) depending upon the atmosphere used for thereaction.On further heating is formed at 550°C in 0, and at 650°C in N, . In contrast, Pr203 was formedat 650°C in H,. Surface area measurements and scanning electron microscopy (SEM) have shown thatproduced at 700°C has a different surface area depending on the gas used: 43 and 64 m2 g-' for 0, and N,,respectively. The surface area of Pr203 formed in H, is 59 m2 g-'. The texture of the catalyst has been found todepend upon the decomposition atmosphere.IR spectroscopy has been used to analyse (qualitatively and quantitatively) the gas-phase reaction productsbetween room temperature and 400 "C from the dehydrogenation and dehydration reactions of propan-2-01 overPro,.,,, and Pr203 catalysts.The results revealed that Pr,O, is a selective dehydration catalyst at 275"C,decomposing propan-2-01 into propene (ca. 80%). However, Pro, is a dual function dehydration/dehydrogenation catalyst.Praseodymium oxides comprise Pr203, Pro, and a range ofintermediate phases: Pro,.,,,, PrO,.,,,, PrO,.,,, ,PrOl.780, Pro,.,,, and Pr,O, possesses hexagonalstructure, Pro, the fluorite structure and PrO,.,soand Pr01.833 are oxygen-deficient modifications of the fluoritestructure. The production of these phases depends on theprecursor used and the atmosphere and temperature ofdecomposi tion.,Moosath et aL5 studied the decomposition ofPr2(C204), . 4H20 in air and reported that complete dehydrationoccurs at 370 "C, followed by rapid decomposition ofanhydrous oxalate to Pr,O,CO, at 480 "C.The latter decomposesat 560 "C to yield Pr,O, as a final product. Muraishi etaL6 studied the decomposition of Pr malonates in an N,atmosphere. They stated that Pr,O,CO, forms as an intermediateat 329°C and that Pr203 is obtained as the finalproduct at 617 "C.In contrast, Dassuncao et aL7 studied the decomposition ofhydrated praseodymium basic carbonate [Pr,(OH),(CO,), .H,0] in air, and stated that Pr,O,CO, was formed at 460°Cand decomposed to Pro,.,,, at 570 "C via a Pr202.6(C03)o.4intermediate. In a later study,, for the decomposition ofPr,(C,O,), - 10H,O and Pr(CH,COO), * H,O in air, it wasfound that both oxalate and acetate of praseodymium formedPr,O,CO, as an intermediate, which decomposes at 575 "Cto give Pro,.,,, as a final product.The main differencebetween the final products obtained from acetate and oxalateat 800°C for 1 h in air is in the surface area; Pro,.,,,obtained from Pr acetate had a surface area of 18 m2 g-'and Pr oxalate 8 m2 g-l.Metal-oxide catalysts with high surface area and reactivityare often obtained from the thermal decomposition of precursorc o m p ~ u n d s . ~ ~ ' ~ The release of volatile components forcesgeneration of fast-transport pathways (pores) through thebulk material."*'2 It has been established that the surfacetexture and the performance of solid catalysts are criticallycontrolled by their preparation and pretreatment conditions.' 2-1 The decomposition of propan-2-01 has beenwidely used to examine the acid-base properties of metaloxidecatalysts.'6-'8 It is generally assumed that acidicoxides catalyse dehydration and basic oxides catalyse dehydrogenation.In the present study the effect of the reaction atmosphereon the thermal decomposition course and final decompositionproducts of Pr oxalate decahydrate was examined byTG and DTA.The solid products at intermediate temperatureswere subjected to IR and XRD. The final decompositionproducts, 700°C for 1 h in O,, H, and N,, weresubjected to surface area measurements and SEM and testedqualitatively and quantitatively for the decomposition ofpropan-2-01.ExperimentalPraseodymium oxalate decahydrate (PrOx), Pr,(C,O,), .10H,O, (Aldrich, 99.99%), was calcined at 200, 450 and700 "C for 1 h in different atmospheres: 0, , H, and N, .Thecalcination temperatures were chosen on the basis of thethermal analysis results (see below). Propan-2-01 and acetone(BDH, spectroscopic grade) were thoroughly degassed byfreeze-pumpthaw cycles under vacuum prior to use.Thermal AnalysisTG and DTA of the parent material (PrOx) were carried outusing heating rates of 2-20°C min-', up to 800"C, in adynamic atmosphere of 0, , H, or N, (20 ml min- l), using amodel 30H Shimadzu analyser. 10-15 mg of the test samplewere used for the TG measurements and highly sintered ct-A1,03 was the thermally inert reference material for theDTA.IR SpectroscopyIR spectra were obtained at a resolution of 5.3 cm-', overthe range 4000-400 cm-', using a model 580B Perkin-Elmerspectrophotometer, equipped with a 3700 PE data station forspectral acquisition and handling1386 J.CHEM. SOC. FARADAY TRANS., 1995, VOL. 91(I) IR spectra of PrOx and its solid calcination productswere obtained from thin ( ~ 2 0 mg rn-,), lightly loaded( < 1 YO) KBr-supported discs.(11) IR spectra of propan-2-01 and its gaseous catalyticdecompositionproducts were taken with the help of a speciallydesigned variable-temperature IR cell equipped withKBr windows. The following standard procedure wasadopted. The catalyst (100 mg in powder form) was heated ina stream of oxygen at 600°C for 30 min to clean the surfaceof carbonate ~ontamination'~ and cooled to room temperatureunder vacuum (10- Torr).Propan-2-01 (10 Torr)was allowed into the cell at various temperatures from 100 to4OO"C, and maintained in contact with the catalyst for 10min.Propan-2-01 and its gaseous decomposition products werequantitatively analysed using the standard QUANT softwarefrom Perkin-Elmer and the on-line data acquisition system.Accordingly, the amounts of the gaseous components(reactant and products) were determined from calibrationcurves relating the IR-absorption intensity at a certain frequencyto the calibrated gas pressure (Torr). The calibrationcurves were derived from IR data obtained from authenticsamples of each of the gas-phase components under identicalspectroscopic conditions.The absorption intensity wasmeasured at 3665 & 5 cm-' for propan-2-01, 1740 f 5 cm-'for acetone (the dehydrogenation product) and at 910 f 5cm- ' for propene (the dehydration product).16XRDXRD analyses of PrOx and its calcination products werecarried out by means of a model JSX-60 PA Jeol diffractometerusing Ni filtered Cu-Ka radiation. For identification purposesthe diffraction patterns (Z/Zo us. d-spacing) obtainedwere matched with ASTM standards.N,-adsorption MeasurementsN,-sorption isotherms were determined volumetrically at- 196 "C using microapparatus based on a design describedby Lippens et The test samples were outgassed at 200 "Cfor 2 h while evacuating at lo-' Torr.Electron MicroscopySamples of the final decomposition product (700 "C) wereexamined in a Jeol 35CF scanning electron microscope tocharacterize the texture.Before examination, the sampleswere rendered conducting by pre-coating with a thin film ofAu-Pd.Results and DiscussionTG and DTA curves for PrOX heated up to 800°C at a rateof 10 "C min- ' in O,, H, and N, atmospheres are shown inFig. 1. IR spectra and XRD powder patterns obtained forPrOx and its solid decomposition products at 250, 450 and700°C are shown in Fig. 2 and 3. Fig. 4A shows the N,-sorption isotherms for PrOx calcined at 700°C for 1 h indifferent gases. The corresponding pore-volume-distributioncurves are given in Fig. 4B. The SEM for the same samplesare shown in Fig. 5. Fig. 6A shows the IR gas-phase spectrafrom 10 Torr of propan-2-01 in contact with PrOx (700°C in0,) after consecutive 10 min periods at the temperaturesindicated.Quantitative analysis of the gas-phase composition,resulting from 10 Torr of propan-2-01 giving rise toacetone and propene, after consecutive 10 min intervals at thetemperatures indicated over the PrOx (700°C in O2 and H,)catalysts, is given in Fig. 6B.10203040Fig. 148.3%', 53.1%0 100 200 300 400 500 600 700 800temperatu re/"CTG (a) and DTA (b) curves for PrOx measured in a dynamicatmosphere (20 ml min-') of A, 0,; B, N, and C, H,; at a heatingrate of 10°C min-'. DTA peak labels I-VII are discussed in the textand occur at temperatures (/"C for I-VII); A: 100, 150, 375, 390,415,425, 550; B: 110, 150, 370, 390, 415, 445, 675; C: 110, 150, 380, 420,(no V or VI), 650.Characterization of the Thermal EventsDehydration ProcessesEvents Z and IZ (100-150°C).The TG and DTA curves(Fig. 1) show that processes I and I1 are overlapping endothermicweight-loss (WL) processes. The WL observed(21.8%) from both processes is very close to that expected forrelease of 9 mol of water (22.3%).I(100"C)4.2%Pr,(C,O,), - 10H,OII( 1 so "C)Pr2(C204)3. 8H20 ,,% ' Pr2(C204)3 * H2° (l)As reported earlier,' the IR spectrum of the solid phase at200°C (Fig. 2) bears a great deal of similarity to that forunheated PrOx because both show absorption bands arisinJ. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91I . I . l r . . , l . , , r l I2000 1500 1000 500wavenumberlcm-'Fig. 2 IR spectra of (a) KBr-supported PrOX and its solid decompositionproducts taken at room temperature after heating for 1 h in(b) 0,-H,-N, at 200"C, (c) 0, at 450"C, (6) 0, at 700"C, (e) H, at450 "C, (f) H, at 700 "C, (9) N, at 450 "C and (h) N, at 700 "Cfrom oxalate.However, the corresponding XRD pattern (Fig.3) indicates that the products are amorphous and so water ofhydration is very important for the coherency of the PrOXcyrstal."Event ZZZ (375 "C). Process 111 largely overlaps with processIV and brings the total WL to 24.5%, close to that expected(24.8%) for the complete dehydration of PrOx. These resultsI I5 15 25 35 45 55281deg reesFig. 3 XRD powder patterns of (a) PrOx and its decompositionproducts taken at room temperature after heating for 1 h in (b)0,-H,-N, at 200"C, (c) 0, at 450°C to give Pr,O,CO, (II), (d) 0,at 700°C to give Pro,,,,,, (e) H, at 450°C to give Pr,O,CO, (I),(f) H, at 700°C to give Pr,O,, (9) N, at 450°C to give Pr,O,CO,(I) and (h) N, at 700°C to give Pro.1833PIP"0.2 t" n0 20 40 60 80 100Fig.4 A, BET N,-sorption isotherm at - 196°C for PrOx calcinedfor 1 h at 700 "C in (a) N,, (b) 0, and (c) H, . B, Pore-volume distributioncurves for the same system: (-) N,, (-.-.) 0, and(.*...) H,.r;/Aare in agreement with the earlier study,' for the dehydrationof PrOx in air, i.e. the dehydration of PrOx is not affected bythe gaseous atmosphere.Decomposition Processes in OxygenEvents IV, V and VZ (390-425 "C). Fig. 1A shows that processesIV, V and VI are overlapping exothermic WL processes.The TG curve shows that these processes areresponsible for the decomposition of anhydrous PrOx toPr202C03, as indicated by the observed WL (48.3%) whichis very close to that expected (48.6%) for the formation ofPr202C03 from unstable Pr2(C,04), as follows'IV( 3 90°C)37.5% ' pr2(c03)3V(4 1 5 "C)VI(42 5 "C)Pr20(C03)2 48.3% ' Pr2O2Co3 (2)The IR spectrum [Fig.2(a)] and XRD pattern [Fig. 3(a)]support reaction (2). The IR spectrum displays absorptions at1560, 1440, 880 and 840 cm-' assignable to oxycarbonatespecies.' The strong absorptions emerging at 650-450 cm -are related to Pr-0 vibration modes.21 XRD also indicatesthe presence of crystalline Pr202C03 (II)(ASTM No. 25-696).Event VZZ (550°C).On further heating, process VII takesplace endothermally (Fig. 1A) at 550°C. The maximum WLthus determined (53.1 %) agrees well with the theoreticalvalue (53.09%) expected for a complete conversion of PrOxinto PrO,.,,, , i.e. process VII is an oxidative decompositionprocess (Pr3 + -+ Pr3.66 +).The IR spectrum of PrOx treated at 700 "C (Fig. 2) declaresthe absence of detectable absorptions arising from oxycar1388 J. CHEM. SOC. FARADAY TRANS,, 1995, VOL. 91Fig. 5700 "C in (a) N, , (b) H, and (c) 0,SEM micrographs (2500 x ) for PrOx calcined for 1 h atonate species. However, the absorptions below 850 cm- ',which are related to lattice-vibration modes of Pr-0, areretained.,' The corresponding XRD for PrOx at 700 "C (Fig.3) shows only the pattern for crystalline PrO,.,,, (ASTMIn comparison with the earlier study in air,' it is clear thatthe decomposition of PrOx in 0, is nearly identical to thatobtained in air.NO.6-329).Decomposition Processes in HydrogenFig. 1B shows that the decomposition of PrOx in hydrogen isnearly similar to that obtained in oxygen (Fig. lA), especiallyconsidering the number and nature of the thermal eventsinvolved. The main differences are (i) reaction (2) in processesIV, V and VI probably takes place asIV( 3 90 "C) V(415"C)Pr2(C204)3 34.5% ' pr2(c204)0.5(c03)2.5 ,,% 'VII(44 5 "C) Pr2(C03)3 48.3% ' Pr202C03 (I) (3)3600 3100 2600 2100 1600 1100 600wavenumber/cm - '' 0 50 100 150 200 250 300 350 400reaction ternperature/"CFig.6 A, IR spectra of (IP) propan-2-01 (10 Torr) and its decompositionproducts [(A) acetone, (P) propene, (M) methane, (I) isobutene]in contact with PrO,,,,, for 10 min at the indicated temperatures. B,IR quantitative analysis of the gas-phase composition resulting from10 Torr of (a) propan-2-01 decomposing over (-) PrO,,,,, and(---) Pr,O, to give (b) acetone and (c) propene. Measurements madeafter consecutive 10 min intervals at the temperatures indicated in A.The WL observed throughout process IV is 34.5% which isvery close to that calculated (34.4%) for the formation ofPr2(C204)o~5(C03)2~5 and so the H, atmosphere retards theformation of both Pr2(C03)3 and Pr,O,CO, . (ii) The structureof Pr,O,CO, (I) (ASTM No. 37-805) formed in H, isdifferent from that obtained in 0, at 450°C [Fig.3(e)]. (iii)Process VII is shifted to higher temperature (675 "C, Fig. lB),also the WL observed is 54.1%, close to that calculated(54.5%) for the formation of Pr203. XRD at 700°C [Fig.3(b)] reveals the formation of the hexagonal structure Pr203(ASTM No. 6-410) and so the oxidation state of Pr does notchange during the decomposition in hydrogen.Decomposition Processes in NitrogenThe decomposition of anhydrous PrOx in nitrogen (Fig. 1C)takes place through one large endothermic effect located at420°C. The WL observed by the end of process IV is 48.3%,which is very close to that (48.6%) calculated for the formationof Pr,02C0, . The XRD, Fig. 3(g), reveals the formationof Pr,0,C03 (I) and so in both N, and H, the Pr,O,CO,formed had the same structure.However, the Pr,02C0, thusformed in an N, atmosphere decomposed at 650°C (Fig. 1C)to give PrO,,,,, as the final product, as indicated by the WLobserved (53.1%) by the end of the decomposition course andby the XRD pattern of PrOx (N,) [Fig. 3(b)]. The finaJ. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 1389decomposition step is, therefore, oxidative as in the case of0, (Pr3 + -, Pr3.66 + ).Surface CharacterizationAll the isotherms shown in Fig. 4A are close to BET-typeIV,,, revealing a porous character. The hysteresis loopsexhibited in Fig. 4A have almost the shape of type (B) H3.These general features indicate that the majority of the poresare slit-shaped and/or non-parallel plates.23 The presence ofmicropores and mesopores was indicated by the pore-sizedistribution (PSD) curves (Fig. 4B) and texture data (Table 1).The PSD curve of PrOx (700°C in 0,) (SBE, = 43 m2 g-')exhibits mainly micropores and mesopores (three peaks at 19,26 and 35 A) which are probably the reason for the lowsurface area in comparison with the other samples. However,it has a higher area than that obtained (8 m2 g-') at 800°Cin air. The PSD curve of PrOx (700°C in N,), which has thesame crystal structure and composition (Pro1.,,,) (S,,, = 64m2 g-') (three peaks at 20,28 and 40 A), shows growth of themesopores which is probably responsible for the higher areain comparison to the 0, sample. On the other hand, the PSDcurve and texture data of the hexagonal Pr,O, obtained inan H, atmosphere (SBET = 59 m2 g-') which is different incomposition and structure, exhibits a wider spectrum ofmesoporosity (three peaks at 22, 30 and 45-60 A) and so theH, affected not only the composition and structure of thefinal product but also the surface area and texture.The similarityof the texture data for PrOx calcined in both 0, andN, at 700°C is most probably because they are formed fromthe same Pr,O,CO, (I) intermediate.The SEM of PrOx (Fig. 5) supports the above results andreveals the presence of well formed crystals of a wide range ofsizes with rough surface steps and non-parallel plate-shapedcrystals. The pores are randomly distributed over the surfacewhich was noted to be rough with extensive porosity andinterplating.Catalytic Activity for Propan-2-01 DecompositionIR spectra from the gas phase of propan-2-ol/PrOl~,,,(PrOx, 700°C in 0,) at different temperatures are shown inFig.6A. The room temperature and 150 "C spectra displaythe characteristic bands of propan-2-01., At 200°C a tiny butimportant absorption emerges at 1740 cm - ', which developedmarkedy in the 250°C spectrum together with anotherabsorption at 1250 cm-'. The two bands mark the formationof gas-phase acetone,, indicating that the dehydrogenation ofpropan-2-01 started at 200 "C. At 250 "C (Fig. 6A) additionalabsorptions emerge at 1650 (doublet) and 915 cm-' that aredue to propene. Hence, the dehydration of propan-2-01occurs at 250°C. Following the reaction at 300°C theacetone bands are slightly intensified, the propene absorptionsgrow stronger and absorptions in the vcH region (3100-2800 cm- ') are re-structured.Table 1in O,, N, and H,Surface-textural characteristics of PrOx calcined at 700 "C>T SBET v, atmosphere /m2 g-' C /ml g-'0, 43 72 0.059 20, 22, 40N, 64 44 0.070 20, 27, 50H, 59 60 0.55 20, 28, 50-60BET surface area (S,,,), constant (C), pore volume (V,) andmaximum pore radius (rmax).At 350 "C, absorptions due to alcohol are hardly detectableand acetone absorptions, while weakened, remain up to400°C.In contrast, absorptions due to propene are intensified.Moreover, at 350°C new absorptions emerge at 3010and 1310 cm-' (due to rnethane),l6 at 890 cm-' (due toisobutene) and at 2340 and 670 cm-' (due to CO2).I6 Thesenew absorptions are greater in the 400°C spectrum (Fig.6A).The formation of the CH,, isobutene and CO, by-productsis due to surface reactions of acetone via an aldol-type condensation.Such reactions involve adsorbed and gas-phaseacetone molecules, as well as nucleophilic surface OHg r o ~ p s . ' ~ . ~ ~ - ~ ~The IR gas-phase spectrum of propan-2-01 over Pr203 isnearly similar to that obtained for PrO,.,,, . The main differencesobserved with Pr203 are (i) acetone first appeared at200°C and its peak grew stronger at 250"C, but disappearedcompletely at 400 "C, (ii) propene first appeared at 200 "C, (iii)both CO, and CH, were also detected, while isobutene wasnot.The above results are presented on a quantitative basis inFig.6B. It is clear that propan-2-01 over Pr,O, starts todecompose at 200 "C, acetone emerges simultaneously at200°C and then propene at 250°C. The rate of alcoholdecomposition appears to reach a maximum at 300"C, asdoes the production of propene (ca. 80%) at 350"C, and thatof acetone (ca. 20%) at 300 "C. At 400 "C, the acetone decomposescompletely. For propan-2-01 over Pro1.,,, , the productionof propene is CQ. 50% at 325°C. At 400°C theamount of acetone commences to decrease.These activity and selectivity results for both Pr,O, andPro,.,,, in the decomposition of propan-2-01 may indicatethat Pr203 (hexagonal structure) is more acidic (highly activedehydration catalyst)27 than Pr01.833. The high dehydrogenationactivity of Pro,.,,, also reveals the basicity.However,acetone formation can occur by a redox me~hanism.,~Conclusion(1) The thermal decomposition of PrOx in different atmospheresinvolves the following pathways :Pr,(C,04), - 10H,O!-+ Pr,(C,04), . 8H,O -!!+I11Pr2(C204)3 * H2° - (A) Inoxygenpr2(c204)3 - Pr2(C03)3 --+ Pr,O(CO,), - IV V VIVI1 Pr,O,CO,(II) - 2Pr01.,,,(B) In nitrogenIV VIIPr2(C204)3 - Pr202C03(1) 2Pr01.833(C) In hydrogenIVPr2(C204)3 - pr2(c204)0.5(c03)2.5VI vn Pr,(C03), - Pr,O,CO,(I) - Pr203(2) The dehydration processes are not affected by the natureof the atmosphere. The water of hydration is responsible forthe crystal coherency of PrOx. (3) Two different phases ofPr,O,CO, were detected as stable intermediates during thedecomposition of PrOx.These depend upon the atmosphere.(4) PrOx decomposes to Pro,.,,, in 0, and N,. However,the hexagonal Pr203 was formed as a final product in H, . (5)The decomposition processes and the final decompositionproducts are mainly controlled by the nature of the atmosphere.(6) Pr oxides obtained at 700°C have surface areas o1390 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 9132 (in O,), 64 (in N2) and 59 m2 g-' (in H,) and have twodifferent types of porosity (micropores and mesopores). (7)Pr,O, obtained at 700°C is a selective catalyst for the dehydrationof propan-2-01 (to form propene, ca. 80%, maximizedat 300"C), with a lesser tendency for dehydrogenation (toform acetone, ca. 10% maximized at 275 "C) indicating that itis an oxide catalyst. In contrast Pro,.,,, functions for dehydrationand dehydrogenation. (8) The decompositionpathway and the composition, structure, surface texture andcatalytic activity of the final products are mainly controlledby the atmosphere during the decomposition of PrOx.ReferencesY. Wilbert, A, Duquesnoy and F. Marion, C.R. Acad. Sci. Paris,1966,263,1539.B. G. Hyde, D. J. M. Bevan and L. Eyring, Philos. Trans. R. SOC.London, 1966,259, 583.L. Eyring and N. C. Baenziger, J. Appl. Phys. Suppl., 1962, 33,428.J. M. Hong, A. F. Clifford and P. A. Faeth, Znorg. Chem., 1962,2, 719.S. S. Moosath, J. Abraham and T. V. Swaminathan, 2. Anorg.Chem., 1963,324,90.K. Muraishi, H. Yokobayeshi and K. Nagase, Thermochim.Acta, 1991, 182, 209.L. M. Dassuncao, 1. Giolito and M. Ionashiro, Thermochim.Acta, 1989, 137, 319.G. A. M. Hussein, J. Anal. Appl. Pyrolysis, 1994, 29, 89.9101112131415161718192021222324252627S. A. Mansour, G. A. M. Hussein and M. I. Zaki, Thermochim.Acta, 1989, 150, 153.G. A. M. Hussein, J. Powder Tech., 1994,80,265.D. L. Trimm and A. Stanislaus, Appl. Catal., 1986,21,215.G. A. M. Hussein and H. M. Ismail, J. Colloid Su$, 1995, in thepress.K. Tanabe, K. Mismo, Y. Ono and H. Hattori, New Solid Acidsand Bases, Elsevier, NY, 1989, pp. 41-47.G. C. Bond and E. F. Thair, Appl. Catal., 1991,71, 1.H. Arakawa, Technol. Jpn., 1988,21,32.G. A. M. Hussein, N. Sheppard, M. I. Zaki and R. B. Fahim, J.Chem. SOC., Faraday Trans. I , 1989,85,1723.K. S. Kim and M. A. Barteau, J. Mol. Catal., 1990,63, 103.G. A. M. Hussein and H. M. Ismail, J. Bull. Chem. SOC. Jpn.,1994,67,2634.J. B. Peri and B. H. Hannan, J. Phys. Chem., 1960,64,1526.B. C. Lippens, B. G. Linsen and J. H. de Boer, J. Catal., 1964,3,32.J. A. Goldsmith and S. D. Ross, Spectrochim. Acta, Part A, 1967,23,1909.S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area andPorosity, Academic Press, London, 2nd edn., 1982.K. S. W. Sing, Pure Appl. Chem., 1982,54,220.G. A. M. Hussein, Thermochim. Acta, 1991,180, 187.A. V. Kiselev and A. V. Uvarov, Su$ Sci., 1967,6,399.M. I. Zaki and N. Sheppard, J. Catal., 1983,80, 114.C. Lahouse, A. Aboulayt, F. Mauge, J. Bachelier and J. C.Lavalley, J. Mol. Catal., 1993, 84, 284.Paper 4/05964F ; Received 29th September, 199