- Email: [email protected]
The influence of solid precursors nature on structural, textural and surface properties of framework zirconium phosphates synthesized via mechanochemical activation
Solid State Ionics 141–142 Ž2001. 683–688 www.elsevier.comrlocaterssi
The influence of solid precursors nature on structural, textural and surface properties of framework zirconium phosphates synthesized via mechanochemical activation S.N. Pavlova a,) , V.A. Sadykov a , G.V. Zabolotnaya a , R.I. Maximovskaya a , V.I. Zaikovskii a , S.V. Tsybulya a , E.B. Burgina a , M.V. Chaikina b, D. Agrawal c , R. Roy c a
BoreskoÕ Institute of Catalysis SB RAS, pr. LaÕrentieÕa, 5 NoÕosibirsk State UniÕersity, PirogoÕa, 1 630090, NoÕosibirsk, Russia b Institute of Solid State Chemistry, NoÕosibirsk, Russia c Materials Research Laboratory, The PennsylÕania State UniÕersity, UniÕersity Park, PA, USA
Abstract The framework zirconium phosphates with incorporated La3q and NHq 4 cations were synthesized via mechanical activation of solids followed by the hydrothermal treatment. Their phase composition, local bulk structure, surface properties and microstructure appear to be defined both by the structure of amorphous zirconium phosphates formed via mechanical activation of solids depending on precursors acidity and reactivity and by the pH-dependent mechanism of subsequent crystallization during hydrothermal treatment. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Framework zirconium phosphates; Mechanochemical activation; Hydrothermal treatment
1. Introduction Framework zirconium phosphates of NZP type have such attractive features as fast-ion conductivity Žincluding proton conductivity., near-zero thermal expansion and ion-exchange properties due to their structural peculiarities w1–4x. Recently, these systems were demonstrated to be promising as acid catalysts for skeletal isomerization and dehydroaromatization of hydrocarbons. A soft chemical route for synthesis of highly dispersed framework zirconium phosphates via the mechanical activation ŽMA. of solids fol-
)
Corresponding author. Fax: q7-3832343056. E-mail address: [email protected] ŽS.N. Pavlova..
lowed by the hydrothermal treatment ŽHTT. of MA products was elaborated w5,6x. The chemical reactions between hydrated solids are known to be greatly accelerated under mechanical stresses due to the solids transfer into so-called ‘atomic-ionic state’ and rapid migrations of ions including protons w7–9x. Evidently, for such reactions, the degree of interaction between starting solids and the yield of products must depend upon the nature of reagents. In this work, the effect of the solid ammonium phosphates acidity and the nature of starting zirconium compounds on these solids reactivity in the course of mechanical coactivation was studied. The structural, textural and surface properties of framework zirconium phosphates produced by subsequent hydrothermal treatment of MA
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 7 7 8 - 0
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
684
products was considered as well. Systems containing both lanthanum and ammonium cations were prepared to check the impact of heterovalent substitution in the NZP-type structure on stability and structural features of complex zirconium phosphates synthesized via this route.
2. Experimental Ten to twenty grams of the stoichiometric mixture of starting compounds ŽTable 1. were activated in the high power EI-2 = 150 mill as in Ref. w5x. The molar ratio of phosphate groups to zirconium cations in reaction systems always corresponded to stoichiometric one for basic framework zirconium phosŽ . phates composition Me1q 2 Zr4 P6 O 24 6r4 . However, the molar ratio of the sum of substituting cations 3q ŽNHq 4 and La , in our case, not considering protons bound with acid phosphate groups. to zirconium was in excess of the stoichiometric one Žless than 0.5., varying from 1.5 to 5 as dependent upon the nature of starting salts. Suspensions of MA mixture in
distilled water were loaded into teflon-lined Parr acid-digestion bombs and kept at 175–2008C for 5–7 days. To some portions of these precursors polyethylene oxide ŽPEO. at a ratio Zr:PEO; 10:1 and HNO 3 Žto decrease pH. were added. After HTT, solids were separated by centrifugation, washed with ethanol and distilled water to remove PEO and any soluble salts, dried at 1208C and calcined at 4008C. The phase composition of samples was examined by the X-ray diffraction method with Cu K a radiation ŽXRD, a URD-63 diffractometer. combined with Infrared spectroscopy ŽFTIRS, a Fourier transform BOMEM MB-102 IR spectrometer. using samples pressed as wafers with KBr. The microstructure of samples was studied using transmission electron microscopy ŽTEM, Jeol 200 C, 200 kV.. 31 P MAS NMR ŽCXP-300 Bruker spectrometer, 121.47 MHz, 3.2 kHz spinning rate. was used to characterize a local coordination of phosphate groups. Here, the isotropic chemical shift values Ž d iso . were calculated relative to 85% H 3 PO4 as an external standard with negative values corresponding to a shift into strong fields.
Table 1 Parameters of synthesis and some properties of the samples calcinated at 4008C Sample
Starting compounds
pH beforerafter hydr. treat.
Phase composition after 4008C
S Žm2 rg.
MA-4
ZrOŽNO 3 . 2 P 2H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O
7
50
MA-4.0
ZrOŽNO 3 . 2 P 2H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O
7a
MA-5.3 MA-5.3.0 MA-5.3 ) MA-5.2 MA-5.1 MA-2
ZrOCl 2 P 8H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOCl 2 P 8H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOCl 2 P 8H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOCl 2 P 8H 2 O, ŽNH 4 . 2 HPO4 ZrOCl 2 P 8H 2 O, NH 4 H 2 PO4 ZrOCl 2 P 8H 2 O, LaŽNO 3 . 3 P 6H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOŽNO 3 . 2 P 2H 2 O, LaŽNO 3. . 3 P 6H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOŽNO 3 . 2 P 2H 2 O, LaŽNO 3. . 3 P 6H 2 O, NH 4 H 2 PO4. ZrOCl 2 P 8H 2 O, LaŽNO 3 . 3 P 6H 2 O, NH 4 H 2 PO4 ZrOŽNO 3 . 2 P 2H 2 O, LaŽNO 3 . 3 P 6H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O q NH 4 H 2 PO4 ZrOCl 2 P 8H 2 O LaŽNO 3 . 3 P 6H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O q NH 4 H 2 PO4
7 7a 1r0.5 4.6r6.4 1.3r1.2 7
cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase, ZrP2 O 7 traces cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase, ZrP2 O 7 cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase rhombohedral NH 4 Zr2 ŽPO4 . 3 amorphous amorphous cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase
40 50 9 42 58 40
7
cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase
67
1.9r2
a-ZrPO4 OH, cubic NH 4 Zr2 ŽPO4 . 3 admixture
30
0.75r1
cubic NH 4 Zr2 ŽPO4 . 3 , LaPO4 admixture
30
2
rhombohedral NH 4 Zr2 ŽPO4 . 3 , LaPO4 admixture
16
1
rhombohedral NH 4 Zr2 ŽPO4 . 3 , LaPO4 admixture
40
MA-3 MA-7 MA-8 MA-9 MA-10 a
The samples were prepared without PEO.
34
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
3. Results and discussion Mechanical activation of the starting mixtures ŽTable 1. was found to be accompanied by decomposition of crystallohydrates releasing water, thus wetting the particles. Hence, during mechanical activation, conditions similar to those of HTT of Žsuper.saturated solutions are realized. After the products discharging from mills, such product phases as crystalline cubic NH 4 Cl or orthorhombic NH 4 NO 3 salts and amorphous zirconium phosphates were revealed proving pronounced reagents interaction. For mixtures containing ŽNH 4 . 3 PO4 , zirconium oxochloride was found to possess a higher reactivity as compared with zirconium oxonitrate. It is manifested in a bigger shift of the 31 P NMR lines into a strong field ŽFig. 1.. Such a behavior can be assigned to a higher water content in hydrated ZrOCl 2 ŽTable 1., which helps to create hydrothermal conditions in the course of MA. A specificity of interaction at the MA stage between ammonium phosphate and zirconium oxochlo-
Fig. 1. 31 P MAS NMR spectra of the samples MA-4 Ž1., MA-5.3 Ž2,5., MA-5.2 Ž3,6., MA-5.1 Ž4,7.: 1–4, after mechanical activation; 4–7, after hydrothermal treatment.
685
ride or oxonitrate affecting the structure of emerging primary products appears also to be manifested in the features of zirconium phosphates phases produced by subsequent HTT and calcination. After HTT, for samples obtained using both hydrated Zr salts, a cubic ammonium zirconium phosphate and orthorhombic zirconium orthophosphate with admixtures of layer phosphates are formed, narrower lines in the XRD pattern of MA-4.0 sample implying a higher crystallinity ŽFig. 2.. After subsequent calcination of both samples at 4008C, reflexes of layered phosphate disappear. For MA-4.0 sample, calcination only slightly affects the ratio of the cubic NH 4 Zr2 ŽPO4 . 3 and orthorhombic phase reflexes intensities, whereas for MA-5.3.0, calcination strongly decreases the intensity of XRD reflexes corresponding to the last phase Ž d h k l ; 2.65, 3.3 A.. According to IR and 31 P NMR data w5x, the MA-4 sample also contains an appreciable amount of zirconium pyrophosphate, while only its traces are found in MA5.3.0 sample. This may be due to the chlorine anions ability to retard the formation of condensed phosphates w10x. PEO addition to water suspension of MA products was found to favor the cubic phase formation at a neutral pH of HTT ŽFig. 2, sample MA-4.. The same effect causes the addition of La to the mixture of the starting compounds. For samples prepared from zirconium oxonitrate, orthorhombic phase formation appears to be favored by the effect of the crystalline product—orthorhombic ammonium nitrate on the structure of primary zirconium phosphate particles w5,6x. For the same zirconium precursor, the degree of interaction between zirconium cations and phosphate groups at the MA stage was found to depend on the acidity of ammonium phosphate. Thus, for MA-4 and MA-5.3 samples prepared from mixtures containing ŽNH 4 . 3 PO4 , in the 31 P MAS NMR spectra several equidistant lines situated in the range of q1 % y 15 ppm ŽFig. 1. and corresponding to a stepwise change of the phosphate group cationic environment testify the formation of zirconium phosphate fragments. The broad line at y15.5 ppm typical for acid layered zirconium phosphates w11x appears in the spectra of MA-5.2 sample ŽŽNH 4 . 2 HPO4 was used., whereas for MA-5.1 sample ŽNH 4 H 2 PO4 was used., a broad line at y25.7 ppm is observed
686
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
Fig. 2. X-ray diffractograms of the samples MA-4 Ž1., MA-4.0 Ž2,3., MA-5.3.0 Ž4,5.: 1,2,4-after hydrothermal treatment, 3,5-after calcination at 4008C. B, cubic NH 4 Zr2 ŽPO4 . 3 ; o, layer phosU phate, , orthorhombic b-ZrŽOH.PO4 .
corresponding to PO43y group in a rather symmetric environment similar to that in the high-temperature NZP phases w5x. In agreement with this data, in the IR spectra of MA-5.3 sample prepared from ŽNH 4 . 3 PO4 ŽFig. 3., a doublet at 1000–1100 cmy1 corresponds to distorted tetrahedral phosphate groups in the nuclei of zirconium phosphates w5x, while for MA-5.1 and MA-5.2 samples obtained from the acid ammonium phosphates more symmetric IR band at 1020–1050 cmy1 is observed similar to that in amorphous zirconium phosphate samples obtained via sol–gel route w5,6x. For La-containing samples, ammonium phosphate acidity was found to affect the mode of the reagents interaction during MA as well ŽFig. 4.. Hence, increased acidity of phosphate groups helps to enhance their interaction with the lattice of zirconium salts during MA, probably, via acid hydrolysis of oligomerized zirconium oxocations. Suspensions of activated mixtures have a different pH depending on the ammonium phosphate acidity
ŽTable 1.. After HTT of the neutral suspensions Ži.e. samples MA-5.3 and MA-4., the crystalline cubic NH 4 Zr2 ŽPO4. 3 and orthorhombic b-ZrŽOH.PO4 phases with admixtures of layered zirconium phosphate are formed ŽFig. 2.. The MA-5.1 and MA-5.2 samples prepared from acid suspensions with pH ; 1 and ; 5, correspondingly, are X-ray amorphous, whereas the rhombohedral NH 4 Zr2 ŽPO4 . 3 phase crystallizes, if a suspension of the mechanically activated MA-5.3 sample is subjected to HTT at pH ; 1 ŽTable 1, sample MA-5 ) .. Such a difference in the structure of samples produced by HTT at low pH is due to peculiarities of the structure of zirconium phosphates nuclei formed during MA of reagents. Earlier w5,6x, crystallization of zirconium phosphates sols was found to depend on their pH being able to proceed only in strongly acidic solutions, thus implying operation of the dissolution– precipitation mechanism. For the products of MA, the same type of crystallization can be proposed in the case of suspensions with a low pH. Indeed, the nuclei of zirconium phosphates contained in the MA-
Fig. 3. IR spectra of the samples MA-5.3 Ž1., MA-5.2 Ž2. and MA-5.1 Ž3. after mechanical activation.
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
Fig. 4. 31 P MAS NMR spectra of the samples MA-7 Ž1–3. and MA-9 Ž4,5.: 1,4-after mechanical activation; 2-after hydrothermal treatment, 3,5-after calcination at 4008C.
5.3 sample are apparently easily dissolved at pH ; 1, yielding the rhombohedral NH 4 Zr2 ŽPO4 . 3 phase via such a route ŽTable 1.. However, this mechanism certainly does not operate in the case of HTT of amorphous samples prepared from acid ammonium phosphates. Though more detailed studies are required to elucidate this phenomenon, some preliminary ideas can be speculated. It seems that some requirements concerning the chemical composition of the reacting systems are to be satisfied for occurring the dissolution–precipitation mechanism. First, the surface layer of the amorphous primary particles is required to be positively charged for dissolving in the acidic media w12x. Since for reaction systems containing acid ammonium phosphates, the ratio of phosphate groups to a sum of zirconium and ammonium cations increases as compared with systems based on ŽNH 4 . 3 PO4 , the excess of phosphate groups in the system is expected to favor their concentration at the surface of primary particles making them negatively charged. It creates a charged barrier for
687
the transfer of zirconium cations into solution required for particles dissolution. From the other side, during HTT, crystallization into framework zirconium phosphate particles or any other acidic zirconium phosphate phase can proceed easily in the case of systems with a higher ratio of substituting cations to Zr w13x, and, hence, a higher driving force for crystallization. Certainly, when acidic ammonium phosphates are used, this ratio is lower, thus making generation of a critical size nucleus of a new crystalline phase less probable. All this reasoning is supported by the fact that in the case of La-containing systems prepared via MA stage from mixtures containing acid ammonium phosphates, HTT in acid solution does produce crystalline phases ŽTable 1., certainly via dissolution– precipitation mechanism. In this case, generation of critical size nucleus can also be facilitated by a higher strength of interaction between phosphate groups and highly charged La cations. For La-containing mixtures with acid ammonium phosphates, as for La-free samples, mechanical activation generates amorphous zirconium phosphates with a local phosphate group environment corresponding to that in the framework zirconium phosphates Ž31 P MAS NMR data, Fig. 4.. According to XRD, after activation, MA-7 sample contains the orthorhombic NH 4 NO 3 phase along with the partially hydrated initial zirconium oxonitrate ZrŽNO 3 . 2 ŽOH. 2 P 2H 2 O. As in the case of acid sol samples w5x, orthorhombic zirconium hydroxo-orthophosphate a-ZrPO4 ŽOH. is obtained via subsequent hydrothermal treatment of this sample suspension at pH ; 2. For MA-8 sample prepared using ZrOCl 2 , the product of the mechanical treatment is the cubic NH 4 Cl phase, and the cubic NH 4 Zr2 ŽPO4 . 3 phase with admixtures of LaPO4 are formed by HTT of its suspension with pH ; 1. It should be mentioned that for the same zirconium precursor, the change of NH 4rLa atomic ratio in the initial mixture affects the phase composition of zirconium phosphate crystallized at low pH Žcompare samples MA-7 and MA-9, MA-8 and MA-10., higher ammonium content being favorable to the rhombohedral phase formation. According to TEM data, the particles of the cubic NH 4 Zr2 ŽPO4 . 3 phase are a rather isotropic three-dimensional aggregates with typical sizes up to 1 mm
688
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
comprised of nearly coherently stacked smaller particles w5x. The particles of the b-ZrŽOH.PO4 orthorhombic phase have an another type of morphology. They are represented by thin Žca. 10 nm. transparent platelets with typical sizes up to several microns composed of smaller blocks. These platelets are stacked with a strong misorientation forming a texture, which is reflected in the typical microdiffraction. In the MA-5 ) sample, the rhombohedral NH 4 Zr2 ŽPO4 . 3 phase crystallizes as 200–500 nm individual hexagonal, rhombohedral or rectangular particles unlike the morphology of this phase in samples obtained via sol–gel method w5x. The specific surface of MA-7 sample, comprised mainly of a-ZrPO4 OH phase ŽTable 1., is an order of magnitude higher than that of samples obtained from the acid sols. This is explained by the different morphology of a-ZrPO4 OH phase formed using mechanical activation route: in this case, it presents ladder-like anisotropic particles consisting of thin platelets. The surface of zirconium phosphates obtained via mechanical treatment mainly contains weakly acidic P–OH groups Ž3670 cmy1 band., which are disturbed by CO adsorption w6x. Relatively strong Broensted acid centers are represented by Zr–OH groups Žband at 3740 cmy1 .. Lewis acid centers— unsaturated Zr 4q cations form complexes with CO Žband at 2180–2200 cmy1 .. For all types of relatively strong acid sites, their surface density is not higher than 10% of monolayer; hence, they can be termed as surface defects. In general, samples with a cubic structure prepared via MA route possess a lower density of acid sites as compared to rhombohedral samples prepared via sol–gel route, that is
explained by a higher Zr–O coordination number w6x, i.e. a higher degree of bulk and surface coordination saturation. However, MA samples are characterized by a higher share of the strongest Lewis centers probed by ERS w6x. Acknowledgements In Russia, this research has been supported by the University of Russia Program under Grant No. 3414. References w1x J.B. Goodenough, H.J. Hong, J.A. Kafalas, Mater. Res. Bull. 11 Ž1976. 173. w2x J. Alamo, R. Roy, J. Mater. Sci. 21 Ž1986. 444. w3x J. Alamo, Solid State Ionics 63–65 Ž1993. 547. w4x S. Komarneni, Int. J. High Technol. Ceram. 4 Ž1988. 31. w5x V.A. Sadykov, S.N. Pavlova, G.V. Zabolotnaya, R.I. Maximovskaya, S.V. Tsubulya, E.B. Burgina, V.I. Zaikovskii, G.S. Litvak, M.V. Chaikina, V.V. Lunun, N.N. Kuztetsova, R. Roy, D.K. Agraval, Mater. Res. Innovations 2 Ž1999. 328. w6x V.A. Sadykov, S.N. Pavlova, G.V. Zabolotnaya, D.I. Kochubei, R.I. Maximovskaya, V.I. Zaikovskii, V.V. Kriventsov, S.V. Tsubulya, E.B. Burgina, E.A. Paukshtis, A.M. Volodin, V.N. Kolomiichuk, M.V. Chaikina, V.V. Lunun, N.N. Kuztetsova, R. Roy, D.K. Agraval, Phosphorus Res. Bull. 10 Ž1999. 400. w7x Y.P. Butyagin, Usp. Khim. 53 Ž1984. 1719 win Russianx. w8x M. Senna, Solid State Ionics 63–65 Ž1993. 3. w9x E.G. Avvakumov, Chem. Sustainable Dev. 2 Ž1994. 476 win Russianx. w10x J. Livage, Catal. Today 41 Ž1998. 3. w11x N.J. Clayden, J. Chem. Soc., Dalton Trans. Ž1987. 1877. w12x J.W. Murray, J. Colloid Interface Sci. 46 Ž1974. 357. w13x V.I. Petkov, A.I. Orlova, D.A. Kapranov, Zh. Neorg. Khim. 43 Ž1998. 1534 win Russianx.
The influence of solid precursors nature on structural, textural and surface properties of framework zirconium phosphates synthesized via mechanochemical activation S.N. Pavlova a,) , V.A. Sadykov a , G.V. Zabolotnaya a , R.I. Maximovskaya a , V.I. Zaikovskii a , S.V. Tsybulya a , E.B. Burgina a , M.V. Chaikina b, D. Agrawal c , R. Roy c a
BoreskoÕ Institute of Catalysis SB RAS, pr. LaÕrentieÕa, 5 NoÕosibirsk State UniÕersity, PirogoÕa, 1 630090, NoÕosibirsk, Russia b Institute of Solid State Chemistry, NoÕosibirsk, Russia c Materials Research Laboratory, The PennsylÕania State UniÕersity, UniÕersity Park, PA, USA
Abstract The framework zirconium phosphates with incorporated La3q and NHq 4 cations were synthesized via mechanical activation of solids followed by the hydrothermal treatment. Their phase composition, local bulk structure, surface properties and microstructure appear to be defined both by the structure of amorphous zirconium phosphates formed via mechanical activation of solids depending on precursors acidity and reactivity and by the pH-dependent mechanism of subsequent crystallization during hydrothermal treatment. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Framework zirconium phosphates; Mechanochemical activation; Hydrothermal treatment
1. Introduction Framework zirconium phosphates of NZP type have such attractive features as fast-ion conductivity Žincluding proton conductivity., near-zero thermal expansion and ion-exchange properties due to their structural peculiarities w1–4x. Recently, these systems were demonstrated to be promising as acid catalysts for skeletal isomerization and dehydroaromatization of hydrocarbons. A soft chemical route for synthesis of highly dispersed framework zirconium phosphates via the mechanical activation ŽMA. of solids fol-
)
Corresponding author. Fax: q7-3832343056. E-mail address: [email protected] ŽS.N. Pavlova..
lowed by the hydrothermal treatment ŽHTT. of MA products was elaborated w5,6x. The chemical reactions between hydrated solids are known to be greatly accelerated under mechanical stresses due to the solids transfer into so-called ‘atomic-ionic state’ and rapid migrations of ions including protons w7–9x. Evidently, for such reactions, the degree of interaction between starting solids and the yield of products must depend upon the nature of reagents. In this work, the effect of the solid ammonium phosphates acidity and the nature of starting zirconium compounds on these solids reactivity in the course of mechanical coactivation was studied. The structural, textural and surface properties of framework zirconium phosphates produced by subsequent hydrothermal treatment of MA
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 7 7 8 - 0
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
684
products was considered as well. Systems containing both lanthanum and ammonium cations were prepared to check the impact of heterovalent substitution in the NZP-type structure on stability and structural features of complex zirconium phosphates synthesized via this route.
2. Experimental Ten to twenty grams of the stoichiometric mixture of starting compounds ŽTable 1. were activated in the high power EI-2 = 150 mill as in Ref. w5x. The molar ratio of phosphate groups to zirconium cations in reaction systems always corresponded to stoichiometric one for basic framework zirconium phosŽ . phates composition Me1q 2 Zr4 P6 O 24 6r4 . However, the molar ratio of the sum of substituting cations 3q ŽNHq 4 and La , in our case, not considering protons bound with acid phosphate groups. to zirconium was in excess of the stoichiometric one Žless than 0.5., varying from 1.5 to 5 as dependent upon the nature of starting salts. Suspensions of MA mixture in
distilled water were loaded into teflon-lined Parr acid-digestion bombs and kept at 175–2008C for 5–7 days. To some portions of these precursors polyethylene oxide ŽPEO. at a ratio Zr:PEO; 10:1 and HNO 3 Žto decrease pH. were added. After HTT, solids were separated by centrifugation, washed with ethanol and distilled water to remove PEO and any soluble salts, dried at 1208C and calcined at 4008C. The phase composition of samples was examined by the X-ray diffraction method with Cu K a radiation ŽXRD, a URD-63 diffractometer. combined with Infrared spectroscopy ŽFTIRS, a Fourier transform BOMEM MB-102 IR spectrometer. using samples pressed as wafers with KBr. The microstructure of samples was studied using transmission electron microscopy ŽTEM, Jeol 200 C, 200 kV.. 31 P MAS NMR ŽCXP-300 Bruker spectrometer, 121.47 MHz, 3.2 kHz spinning rate. was used to characterize a local coordination of phosphate groups. Here, the isotropic chemical shift values Ž d iso . were calculated relative to 85% H 3 PO4 as an external standard with negative values corresponding to a shift into strong fields.
Table 1 Parameters of synthesis and some properties of the samples calcinated at 4008C Sample
Starting compounds
pH beforerafter hydr. treat.
Phase composition after 4008C
S Žm2 rg.
MA-4
ZrOŽNO 3 . 2 P 2H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O
7
50
MA-4.0
ZrOŽNO 3 . 2 P 2H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O
7a
MA-5.3 MA-5.3.0 MA-5.3 ) MA-5.2 MA-5.1 MA-2
ZrOCl 2 P 8H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOCl 2 P 8H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOCl 2 P 8H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOCl 2 P 8H 2 O, ŽNH 4 . 2 HPO4 ZrOCl 2 P 8H 2 O, NH 4 H 2 PO4 ZrOCl 2 P 8H 2 O, LaŽNO 3 . 3 P 6H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOŽNO 3 . 2 P 2H 2 O, LaŽNO 3. . 3 P 6H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O ZrOŽNO 3 . 2 P 2H 2 O, LaŽNO 3. . 3 P 6H 2 O, NH 4 H 2 PO4. ZrOCl 2 P 8H 2 O, LaŽNO 3 . 3 P 6H 2 O, NH 4 H 2 PO4 ZrOŽNO 3 . 2 P 2H 2 O, LaŽNO 3 . 3 P 6H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O q NH 4 H 2 PO4 ZrOCl 2 P 8H 2 O LaŽNO 3 . 3 P 6H 2 O, ŽNH 4 . 3 PO4 P 3H 2 O q NH 4 H 2 PO4
7 7a 1r0.5 4.6r6.4 1.3r1.2 7
cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase, ZrP2 O 7 traces cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase, ZrP2 O 7 cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase rhombohedral NH 4 Zr2 ŽPO4 . 3 amorphous amorphous cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase
40 50 9 42 58 40
7
cubic NH 4 Zr2 ŽPO4 . 3 , orthorhombic phase
67
1.9r2
a-ZrPO4 OH, cubic NH 4 Zr2 ŽPO4 . 3 admixture
30
0.75r1
cubic NH 4 Zr2 ŽPO4 . 3 , LaPO4 admixture
30
2
rhombohedral NH 4 Zr2 ŽPO4 . 3 , LaPO4 admixture
16
1
rhombohedral NH 4 Zr2 ŽPO4 . 3 , LaPO4 admixture
40
MA-3 MA-7 MA-8 MA-9 MA-10 a
The samples were prepared without PEO.
34
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
3. Results and discussion Mechanical activation of the starting mixtures ŽTable 1. was found to be accompanied by decomposition of crystallohydrates releasing water, thus wetting the particles. Hence, during mechanical activation, conditions similar to those of HTT of Žsuper.saturated solutions are realized. After the products discharging from mills, such product phases as crystalline cubic NH 4 Cl or orthorhombic NH 4 NO 3 salts and amorphous zirconium phosphates were revealed proving pronounced reagents interaction. For mixtures containing ŽNH 4 . 3 PO4 , zirconium oxochloride was found to possess a higher reactivity as compared with zirconium oxonitrate. It is manifested in a bigger shift of the 31 P NMR lines into a strong field ŽFig. 1.. Such a behavior can be assigned to a higher water content in hydrated ZrOCl 2 ŽTable 1., which helps to create hydrothermal conditions in the course of MA. A specificity of interaction at the MA stage between ammonium phosphate and zirconium oxochlo-
Fig. 1. 31 P MAS NMR spectra of the samples MA-4 Ž1., MA-5.3 Ž2,5., MA-5.2 Ž3,6., MA-5.1 Ž4,7.: 1–4, after mechanical activation; 4–7, after hydrothermal treatment.
685
ride or oxonitrate affecting the structure of emerging primary products appears also to be manifested in the features of zirconium phosphates phases produced by subsequent HTT and calcination. After HTT, for samples obtained using both hydrated Zr salts, a cubic ammonium zirconium phosphate and orthorhombic zirconium orthophosphate with admixtures of layer phosphates are formed, narrower lines in the XRD pattern of MA-4.0 sample implying a higher crystallinity ŽFig. 2.. After subsequent calcination of both samples at 4008C, reflexes of layered phosphate disappear. For MA-4.0 sample, calcination only slightly affects the ratio of the cubic NH 4 Zr2 ŽPO4 . 3 and orthorhombic phase reflexes intensities, whereas for MA-5.3.0, calcination strongly decreases the intensity of XRD reflexes corresponding to the last phase Ž d h k l ; 2.65, 3.3 A.. According to IR and 31 P NMR data w5x, the MA-4 sample also contains an appreciable amount of zirconium pyrophosphate, while only its traces are found in MA5.3.0 sample. This may be due to the chlorine anions ability to retard the formation of condensed phosphates w10x. PEO addition to water suspension of MA products was found to favor the cubic phase formation at a neutral pH of HTT ŽFig. 2, sample MA-4.. The same effect causes the addition of La to the mixture of the starting compounds. For samples prepared from zirconium oxonitrate, orthorhombic phase formation appears to be favored by the effect of the crystalline product—orthorhombic ammonium nitrate on the structure of primary zirconium phosphate particles w5,6x. For the same zirconium precursor, the degree of interaction between zirconium cations and phosphate groups at the MA stage was found to depend on the acidity of ammonium phosphate. Thus, for MA-4 and MA-5.3 samples prepared from mixtures containing ŽNH 4 . 3 PO4 , in the 31 P MAS NMR spectra several equidistant lines situated in the range of q1 % y 15 ppm ŽFig. 1. and corresponding to a stepwise change of the phosphate group cationic environment testify the formation of zirconium phosphate fragments. The broad line at y15.5 ppm typical for acid layered zirconium phosphates w11x appears in the spectra of MA-5.2 sample ŽŽNH 4 . 2 HPO4 was used., whereas for MA-5.1 sample ŽNH 4 H 2 PO4 was used., a broad line at y25.7 ppm is observed
686
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
Fig. 2. X-ray diffractograms of the samples MA-4 Ž1., MA-4.0 Ž2,3., MA-5.3.0 Ž4,5.: 1,2,4-after hydrothermal treatment, 3,5-after calcination at 4008C. B, cubic NH 4 Zr2 ŽPO4 . 3 ; o, layer phosU phate, , orthorhombic b-ZrŽOH.PO4 .
corresponding to PO43y group in a rather symmetric environment similar to that in the high-temperature NZP phases w5x. In agreement with this data, in the IR spectra of MA-5.3 sample prepared from ŽNH 4 . 3 PO4 ŽFig. 3., a doublet at 1000–1100 cmy1 corresponds to distorted tetrahedral phosphate groups in the nuclei of zirconium phosphates w5x, while for MA-5.1 and MA-5.2 samples obtained from the acid ammonium phosphates more symmetric IR band at 1020–1050 cmy1 is observed similar to that in amorphous zirconium phosphate samples obtained via sol–gel route w5,6x. For La-containing samples, ammonium phosphate acidity was found to affect the mode of the reagents interaction during MA as well ŽFig. 4.. Hence, increased acidity of phosphate groups helps to enhance their interaction with the lattice of zirconium salts during MA, probably, via acid hydrolysis of oligomerized zirconium oxocations. Suspensions of activated mixtures have a different pH depending on the ammonium phosphate acidity
ŽTable 1.. After HTT of the neutral suspensions Ži.e. samples MA-5.3 and MA-4., the crystalline cubic NH 4 Zr2 ŽPO4. 3 and orthorhombic b-ZrŽOH.PO4 phases with admixtures of layered zirconium phosphate are formed ŽFig. 2.. The MA-5.1 and MA-5.2 samples prepared from acid suspensions with pH ; 1 and ; 5, correspondingly, are X-ray amorphous, whereas the rhombohedral NH 4 Zr2 ŽPO4 . 3 phase crystallizes, if a suspension of the mechanically activated MA-5.3 sample is subjected to HTT at pH ; 1 ŽTable 1, sample MA-5 ) .. Such a difference in the structure of samples produced by HTT at low pH is due to peculiarities of the structure of zirconium phosphates nuclei formed during MA of reagents. Earlier w5,6x, crystallization of zirconium phosphates sols was found to depend on their pH being able to proceed only in strongly acidic solutions, thus implying operation of the dissolution– precipitation mechanism. For the products of MA, the same type of crystallization can be proposed in the case of suspensions with a low pH. Indeed, the nuclei of zirconium phosphates contained in the MA-
Fig. 3. IR spectra of the samples MA-5.3 Ž1., MA-5.2 Ž2. and MA-5.1 Ž3. after mechanical activation.
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
Fig. 4. 31 P MAS NMR spectra of the samples MA-7 Ž1–3. and MA-9 Ž4,5.: 1,4-after mechanical activation; 2-after hydrothermal treatment, 3,5-after calcination at 4008C.
5.3 sample are apparently easily dissolved at pH ; 1, yielding the rhombohedral NH 4 Zr2 ŽPO4 . 3 phase via such a route ŽTable 1.. However, this mechanism certainly does not operate in the case of HTT of amorphous samples prepared from acid ammonium phosphates. Though more detailed studies are required to elucidate this phenomenon, some preliminary ideas can be speculated. It seems that some requirements concerning the chemical composition of the reacting systems are to be satisfied for occurring the dissolution–precipitation mechanism. First, the surface layer of the amorphous primary particles is required to be positively charged for dissolving in the acidic media w12x. Since for reaction systems containing acid ammonium phosphates, the ratio of phosphate groups to a sum of zirconium and ammonium cations increases as compared with systems based on ŽNH 4 . 3 PO4 , the excess of phosphate groups in the system is expected to favor their concentration at the surface of primary particles making them negatively charged. It creates a charged barrier for
687
the transfer of zirconium cations into solution required for particles dissolution. From the other side, during HTT, crystallization into framework zirconium phosphate particles or any other acidic zirconium phosphate phase can proceed easily in the case of systems with a higher ratio of substituting cations to Zr w13x, and, hence, a higher driving force for crystallization. Certainly, when acidic ammonium phosphates are used, this ratio is lower, thus making generation of a critical size nucleus of a new crystalline phase less probable. All this reasoning is supported by the fact that in the case of La-containing systems prepared via MA stage from mixtures containing acid ammonium phosphates, HTT in acid solution does produce crystalline phases ŽTable 1., certainly via dissolution– precipitation mechanism. In this case, generation of critical size nucleus can also be facilitated by a higher strength of interaction between phosphate groups and highly charged La cations. For La-containing mixtures with acid ammonium phosphates, as for La-free samples, mechanical activation generates amorphous zirconium phosphates with a local phosphate group environment corresponding to that in the framework zirconium phosphates Ž31 P MAS NMR data, Fig. 4.. According to XRD, after activation, MA-7 sample contains the orthorhombic NH 4 NO 3 phase along with the partially hydrated initial zirconium oxonitrate ZrŽNO 3 . 2 ŽOH. 2 P 2H 2 O. As in the case of acid sol samples w5x, orthorhombic zirconium hydroxo-orthophosphate a-ZrPO4 ŽOH. is obtained via subsequent hydrothermal treatment of this sample suspension at pH ; 2. For MA-8 sample prepared using ZrOCl 2 , the product of the mechanical treatment is the cubic NH 4 Cl phase, and the cubic NH 4 Zr2 ŽPO4 . 3 phase with admixtures of LaPO4 are formed by HTT of its suspension with pH ; 1. It should be mentioned that for the same zirconium precursor, the change of NH 4rLa atomic ratio in the initial mixture affects the phase composition of zirconium phosphate crystallized at low pH Žcompare samples MA-7 and MA-9, MA-8 and MA-10., higher ammonium content being favorable to the rhombohedral phase formation. According to TEM data, the particles of the cubic NH 4 Zr2 ŽPO4 . 3 phase are a rather isotropic three-dimensional aggregates with typical sizes up to 1 mm
688
S.N. PaÕloÕa et al.r Solid State Ionics 141–142 (2001) 683–688
comprised of nearly coherently stacked smaller particles w5x. The particles of the b-ZrŽOH.PO4 orthorhombic phase have an another type of morphology. They are represented by thin Žca. 10 nm. transparent platelets with typical sizes up to several microns composed of smaller blocks. These platelets are stacked with a strong misorientation forming a texture, which is reflected in the typical microdiffraction. In the MA-5 ) sample, the rhombohedral NH 4 Zr2 ŽPO4 . 3 phase crystallizes as 200–500 nm individual hexagonal, rhombohedral or rectangular particles unlike the morphology of this phase in samples obtained via sol–gel method w5x. The specific surface of MA-7 sample, comprised mainly of a-ZrPO4 OH phase ŽTable 1., is an order of magnitude higher than that of samples obtained from the acid sols. This is explained by the different morphology of a-ZrPO4 OH phase formed using mechanical activation route: in this case, it presents ladder-like anisotropic particles consisting of thin platelets. The surface of zirconium phosphates obtained via mechanical treatment mainly contains weakly acidic P–OH groups Ž3670 cmy1 band., which are disturbed by CO adsorption w6x. Relatively strong Broensted acid centers are represented by Zr–OH groups Žband at 3740 cmy1 .. Lewis acid centers— unsaturated Zr 4q cations form complexes with CO Žband at 2180–2200 cmy1 .. For all types of relatively strong acid sites, their surface density is not higher than 10% of monolayer; hence, they can be termed as surface defects. In general, samples with a cubic structure prepared via MA route possess a lower density of acid sites as compared to rhombohedral samples prepared via sol–gel route, that is
explained by a higher Zr–O coordination number w6x, i.e. a higher degree of bulk and surface coordination saturation. However, MA samples are characterized by a higher share of the strongest Lewis centers probed by ERS w6x. Acknowledgements In Russia, this research has been supported by the University of Russia Program under Grant No. 3414. References w1x J.B. Goodenough, H.J. Hong, J.A. Kafalas, Mater. Res. Bull. 11 Ž1976. 173. w2x J. Alamo, R. Roy, J. Mater. Sci. 21 Ž1986. 444. w3x J. Alamo, Solid State Ionics 63–65 Ž1993. 547. w4x S. Komarneni, Int. J. High Technol. Ceram. 4 Ž1988. 31. w5x V.A. Sadykov, S.N. Pavlova, G.V. Zabolotnaya, R.I. Maximovskaya, S.V. Tsubulya, E.B. Burgina, V.I. Zaikovskii, G.S. Litvak, M.V. Chaikina, V.V. Lunun, N.N. Kuztetsova, R. Roy, D.K. Agraval, Mater. Res. Innovations 2 Ž1999. 328. w6x V.A. Sadykov, S.N. Pavlova, G.V. Zabolotnaya, D.I. Kochubei, R.I. Maximovskaya, V.I. Zaikovskii, V.V. Kriventsov, S.V. Tsubulya, E.B. Burgina, E.A. Paukshtis, A.M. Volodin, V.N. Kolomiichuk, M.V. Chaikina, V.V. Lunun, N.N. Kuztetsova, R. Roy, D.K. Agraval, Phosphorus Res. Bull. 10 Ž1999. 400. w7x Y.P. Butyagin, Usp. Khim. 53 Ž1984. 1719 win Russianx. w8x M. Senna, Solid State Ionics 63–65 Ž1993. 3. w9x E.G. Avvakumov, Chem. Sustainable Dev. 2 Ž1994. 476 win Russianx. w10x J. Livage, Catal. Today 41 Ž1998. 3. w11x N.J. Clayden, J. Chem. Soc., Dalton Trans. Ž1987. 1877. w12x J.W. Murray, J. Colloid Interface Sci. 46 Ž1974. 357. w13x V.I. Petkov, A.I. Orlova, D.A. Kapranov, Zh. Neorg. Khim. 43 Ž1998. 1534 win Russianx.