Phosphoric Acid Adsorbed on Silica-Ceria Matrix Obtained by Sol-Gel Method : Studies of Local Structure , Texture and Acid Property

Óxidos mistos de SiO 2 /CeO 2 (designados como SC) contendo quantidades variáveis de céria, SC1 = 5,6, SC2 = 8,0 e SC3 = 13,0 (% em massa), foram preparados pelo método de processamento sol-gel e obtidos como sólidos amorfos possuindo áreas superficiais específicas de SC1 = 463, SC2 = 474 e SC3 = 460 mg. Íons fosfatos foram imobilizados na superfície destes sólidos, através da ligação química Ce-O-P, pela imersão de SC em uma solução de ácido fosfórico. Este procedimento produziu sólidos com as seguintes quantidades de P adsorvidas na superfície (em % atômica): SCP1 = 0,60, SCP2 = 0,71 e SCP3 = 1,63. A energia de ligação P 2p 3/2 , obtida por XPS, e o único pico observado em -10 ppm, por NMR de P, revelaram que o fosfato adsorvido na superfície é a espécie H 2 PO 4 . Utilizando-se piridina como sonda molecular, somente sítios ácidos de Brønsted foram detectados. A determinação das quantidades destes sítios ácidos, feita pela adsorção de NH 3 gasosa sobre a superfície, revelou os seguintes valores para cada sólido tratado com o íon fosfato: SCP1 = 0,37, SCP2 = 0,43 e SCP3 = 0,51 mmol g.


Introduction
Cerium(IV) phosphate can be prepared by reacting phosphoric acid with diammonium cerium(IV) nitrate, by an acid sol-gel process involving controlled precipitation, 1 or by using a chelating agent to obtain nanocrystalline mesoporous solids with controlled pore size. 2 It can also be obtained by hydrothermal synthesis from the reaction of CeO 2 and phosphoric acid. 3A recent interesting process described in the literature consists of a reproducible and controllable route to produce cerium phosphate nanotubes. 4erium phosphate can have many applications as ceramic material, 5 as a glass presenting optical 6 and luminescent 7 properties, as ion exchanger 8 and catalyst. 9t can also be prepared as phosphate particles dispersed in hybrid organic-inorganic matrices, aiming at their use in filtering process. 10,11rocesses to prepare metal oxides incorporated into a silica matrix have been described in recent years.3][14][15] SiO 2 /M x O y obtained by this method normally presents high thermal and mechanical stability provided by the silica framework. 13,16espite this rigid structure, the metal oxide M x O y presents reactive MOH groups on the surface which can easily react with organic or inorganic acids forming a stable linkage. 17n this work, the preparation of microporous SiO 2 /CeO 2 mixed oxide by the sol-gel process and the subsequent immobilization of phosphate groups onto the surface of this matrix are reported.The objective was to obtain cerium(IV) confined in a silica matrix and attached to phosphate species by Ce-O-P bonds.The textural, thermal stability, local structure and acidic properties of SiO 2 / CeO 2 /Phosphate were investigated by XRD, XPS, FTIR and SEM/EDS techniques.

Preparation of SiO 2 /CeO 2 by the sol-gel method
The silica/ceria mixed oxide, SiO 2 /CeO 2 , was prepared by the sol-gel method.In a reaction flask containing 150 mL of absolute ethanol (Synth), 175 mL of tetraethylorthosilicate (TEOS-Aldrich), 16 mL of twice distilled H 2 O and 2.3 mL of concentrated nitric acid were added and the resulting solution was heated with constant stirring at 353 K for 3 hours.After cooling the solution, different amounts of (NH 4 ) 2 Ce(NO 3 ) 6 (0.014, 0.027 and 0.040 mol), dissolved in mixtures containing 100 mL of ethanol and 25 mL of twice distilled water, were added.The mixtures were stirred for additional 3 hours at 298 K.Then, 1.3 mL of concentrated HNO 3 in 9.5 mL of distilled water were added and the solution was stirred for other 2 hours.The solvent was evaporated from the mixtures at 373 K, producing gelatinous materials.The gels obtained were heated at 473 K under constant air flow for two hours.The materials obtained with increasing amount of ceria will be hereafter designated as SC1, SC2 and SC3.
The thermal stabilities were investigated submitting SC1, SC2 and SC3 to heat treatment at 473, 673, 873, 1073 and 1273 K, with a plateau of 8h for each temperature.
Phosphate adsorption on the SiO 2 /CeO 2 mixed oxide About 1.5 g of each SC were immersed in 40 mL of a 0.1 mol L -1 H 3 PO 4 solution and allowed to stand, with occasional shaking, for 72 hours.The mixtures were filtered, washed with water and dried at room temperature.They were designated as SCP1, SCP2 and SCP3, respectively.

X-ray diffraction and X-ray fluorescence
X-ray diffraction patterns were obtained on a Shimadzu XRD-6000 diffractometer by using Cu K α radiation (λ = 0.154 nm, 40kV per 30 mA) and a sweeping velocity of 2 degree min -1 (units of 2θ).
The amount of cerium in the silica matrix was determined by X-ray fluorescence analysis.Mechanical mixtures of finely powdered SiO 2 (Merck) and CeO 2 (Carlo Erba), containing different ratios of Si/Ce, were used to obtain the calibration curve.The measurements were carried out on a SHIMADZU EDX-700 using a rhodium X-ray anode (50 kV) as source.

Specific surface area (S BET ) and average pore size
Samples of the SC and SCP materials were previously heated at 343 K under vacuum (1.3 × 10 -3 Pa) for 1 h.The specific surface areas were determined by the BET method, while the average pore size was determined by nitrogen adsorption-desorption isotherms (using the BJH method) on an ASAP 2010 apparatus from Micromeritics.

Scanning electron microscopy (SEM) and X-ray emission analyses (EDS)
Samples were fixed onto a double faced tape adhered to an aluminum support and coated with a layer of gold (ca.15 nm) by a BalTec SCD 050 Sputter Coater apparatus (60 mA current for 60 s).The scanning electron microscopy analysis (SEM) was carried out using low vacuum microscopy on a JSM 5900LV equipment operating at an accelerating voltage of 25 kV.Images were obtained by using secondary electrons.X-ray emission spectroscopy (EDS) was used for elemental mapping with a Noram Voyager instrument.The micrographs and the elemental maps were obtained for all contents of CeO 2 oxide supported by silica.

High power decoupling-magic angle spinning 31 P NMR
The high power decoupling magic angle spinning (HPDEC-MAS) 31 P NMR spectra of all solids were obtained at room temperature on a Bruker AC300/P spectrometer operating at 121 MHz.Conditions used were a sequential pulse with an acquisition time of 41 ms and a relaxation time of 4 seconds.Phosphoric acid (85 wt%; δ = 0 ppm) and sodium hydrogen phosphate (δ = 0.9 ppm) were used as primary and secondary references for the chemical shifts, with a spectral width of 50 × 10 3 Hz.J. Braz.Chem.Soc.

X-ray Photoelectron Spectroscopy (XPS)
Spectra were recorded on a VSW HA-100 Spherical Analyzer operating with pass energy of 44 eV and an Al-K α radiation source (1486.6 eV).Samples were prepared as powder pressed into pellets, fixed to a stainless steel sample holder with double-faced tape and analyzed as prepared.The binding energies were referenced to the C 1s line at 284.6 eV and the photoemission lines were simulated with Gaussian line shapes.The relative surface compositions were calculated from the photoelectron peak areas after correction for the photo-ionization cross section and the difference in the electron escape depth. 18The atomic compositions were estimated from the C 1s, O 1s, Si 2p, P 2p and Ce 3d 5/2 integrated peak areas.

Infrared spectrometry for determining acidic properties
The Brønsted acid sites on the surface of all SC and SCP samples were studied by analyzing the infrared spectra of pyridine as a probe molecule.About 100 mg of the ground sample were wetted with liquid pyridine at room temperature and the samples were then submitted to vacuum (1.3 × 10 -5 Pa), initially at room temperature, to eliminate physically adsorbed pyridine.The self-supported pressed disks were heated under vacuum at 373 and 473 K and, for each temperature, spectra were obtained at room temperature on a Bomen Hartmann & Braun (MB-Series) spectrophotometer.FTIR spectra of samples as prepared and of the heated materials were obtained on the same spectrophotometer, using pressed KBr pellets containing 1 wt% of the material.

Ammonia gas adsorption
A 250 mL erlenmeyer flask with about 1 g of each SCP material was filled with NH 3 gas and closed for 30 minutes at room temperature.The excess of gas was pumped off at 1.3 × 10 -3 Pa at room temperature.The amount of ammonia adsorbed in the material was determined by the Kjeldhal method.

Characteristics of the samples
The X-ray fluorescence analyses of the samples showed the following amounts of CeO 2 for the three SC mixed oxides (in wt%): 5.6, 8.9 and 13.0, which correspond to 0.33, 0.53 and 0.76 mmol of Ce per gram, respectively.The specific surface areas, S BET , and the average pore diameters, r -, are presented in Table 1.
The specific surface areas of SC1, SC2 and SC3 remained practically constant when compared with the corresponding modified solids SCP1, SCP2 and SCP3, except for SCP3, where a slight increase was observed.These results indicate that reaction of phosphoric acid with CeO 2 , resulting in formation of phosphate species on the surface, is not blocking the finest pores.Otherwise, a decrease of the specific surface areas by the species formed should be observed. 19ontents of P, Ce, Si and O on the surface of the SCP solids were determined by the XPS technique.Table 2 lists the results obtained.
As the amount of surface Ce increases on the surface, the quantity of P also increases, evidencing that phosphoric acid is retained by the reaction with the metal oxide sites on the surface.
Figure 1 shows the IR spectra of SC and pure SiO 2 treated at 473 K and compared with SC3 calcined at 1273 K.For pure SiO 2 , the band observed at 975 cm -1 (Figure 1d) is assigned to the Si-O stretching mode (νSiO) of free silanol, ≡Si-OH, group. 20In the mixed oxide this frequency is shifted to 948 cm -1 (Figures 1a-c), due to the Si-O-Ce linkage formation. 21,22This band disappears on heating SC3 (taken as a model material) at 1273 K, presumably due to the breaking of the Si-O-Ce bonding as a consequence of the increase in SiO 2 and CeO 2 particles size.

X-ray Diffraction patterns
Figure 2 shows the XRD patterns for SC1, SC2 and SC3 samples submitted to thermal treatments at 473, 673,

Scanning Electron Microscopy (SEM) and X-ray emission analysis (EDS)
Figure 3 shows the SEM micrographs and the corresponding cerium analyses by EDS (white points) for SCP3 (taken as the representative sample) thermally treated at 473 and 1273 K.The elemental mappings as a function of the thermal treatment temperature reveal that the cerium oxide is, within the magnification used, well dispersed in the silica matrix. P NMR analysis 31 P MAS NMR spectroscopy is a powerful technique for studying the environment of metal phosphates, since the chemical shift of the phosphate group is very sensitive to its local environment.In the present case, the spectra recorded for SCP1, SCP2 and SCP3 showed a single peak at ca. -10 ppm (Figure 4). Incomparison with crystalline pure cerium phosphate, the observed chemical shift can be assigned to the dihydrogen phosphate species bonded to the ceria particle, Ce-O-P(O)(OH) 2 .24 This assignment  Tansmittance / % is also consistent with the presence of dihydrogen phosphate species in various M-phosphates (M= Nb, Ti, Zr and Al).17,25-27

X-ray Photoelectron Spectroscopy (XPS)
Table 3 lists the XPS binding energy values (BE) for SCP1, SCP2 and SCP3 thermally treated at 473 K.
The phosphate-adsorbed mixed oxide presented an average binding energy value for the Si 2p peak at ca. 103.5 eV, with a width at half-maximum of 2.4 eV, similar to results from the literature for pure silica. 23,28,29All the samples had the XPS spectrum of the O 1s level at 530.6 and 532.4 eV, with full width at half-maximum of 2.2 and 2.3 eV, attributed to the oxygens present in SiO 2 28,29   and in CeO 2 , 30 respectively.
The XPS of the Ce 3d level is complex, since photoemission provokes the re-arrangement of the electrons from the valence level and the resulting final states contain different hybridizations.In Table 3, ν'' and ν''' for CeO 2 BE peaks refer to the bonding and antibonding states due to the Ce 4f final state hybridized with the O 2p orbital [the 3d 9 4f 1 (O 2p 5 )] electronic configurations and the [3d 9 4f 0 (O 2p 6 )] final state (ν'''), respectively.][32][33][34] Phosphorus presented only one photoemission peak, which is assigned to the P 2p 3/2 binding energy at ca. 134.0 eV (average value).This binding value is assigned to the H 2 PO 4 _ species on the surfaces of the three mixed oxides.These values are very close to that observed for phosphate ions adsorbed as H 2 PO 4 -species on the surfaces of SiO 2 /Nb 2 O 5 17 and SiO 2 /ZrO 2 (134.5 eV) 16 sol gel matrices.It is interesting to observe that P/ Ce atomic ratios obtained were: SCP1 = 2.8, SCP2 = 2.3 and SCP3 = 2.7.These values are not far from P/Ce = 2 ratio expected, if we consider that almost all ceria centers reacted and the species formed is the dihydrogen phosphate species.
The Ce/Si atomic ratios calculated for SCP1, SCP2 and SCP3 (Table 3) increased proportionally, taking into account an error of 8% in the calculation, as the ceria amount increased in these three matrices.Chemical Shift / ppm

Acidic properties
An investigation of the acidic properties of the SC and SCP materials was made by using pyridine as a probe molecule.Figure 5 presents the FTIR spectra obtained for the SC3 and SCP3 samples, taken as representatives of the other materials, which did not present any significant difference.The bands at 1599 and 1444 cm -1 are assigned to the 8a and 19b vibrational modes, respectively, of the pyridine molecule bonded to the surface by hydrogen bonds, possibly to the free silanol (≡SiOH) groups. 35,36e observe in Figures 5(I) and 5(II) that the intensities of these bands decrease on heat treatment under vacuum at 473 K.The band at 1490 cm -1 is assigned to the 19a vibrational mode and it is always present for all kinds of pyridine adsorption.
The band observed at 1546 cm -1 in Figure 5(II) is assigned to the 19a vibrational mode of the pyridine molecule adsorbed on Brφnsted acid sites. 37,38These Brønsted acid sites are due to the H 2 PO 4 -species bonded to the matrix surface by Ce-O-P linkages.The result agrees with the conclusion from X-ray photoelectron spectroscopy, where the spectra of phosphorus presented only one photoemission peak due to the dihydrogen phosphate species on the surface of the mixed oxides.We also observe that they are very stable, since they are present on the surface even after heating the sample at 473 K.In Figure 5(I), for the SC3 sample, the vibrational mode due to pyridine molecules adsorbed on the Brønsted acid sites is not observed.The broadened feature of the band in this region is due to the OH deformation mode of free silanol (≡SiOH) and the attached residual CeOH groups.

Ammonia gas adsorption
The infrared spectra of SCP1/NH 3 , SCP2/NH 3 and SCP3/NH 3 (solids with adsorbed NH 3 ) show a band observed at ≈1454 cm -1 , as seen in Figure 6.This vibration mode is assigned to the δNH 4 + deformation mode (F 2 mode) of the ion having a T d symmetry. 39The three SCP pure samples do not show this band.Chemical analyses carried out on SCP/NH 3 showed the following amounts of ammonia adsorbed on the surfaces: SCP1/NH 3 = 0.37, SCP2/NH 3 = 0.43 and SCP3/NH 3 = 0.51 mmol g -1 .Transmittance / %

Conclusions
SiO 2 /CeO 2 mixed oxide is obtained as a porous solid in which CeO 2 particles interact, through Si-O-Ce bonds, with the SiO 2 environment.This assumption is based on the IR absorption band observed at 948 cm -1 , assigned to the ν(Si-O) mode of the Si-O-Ce bond.The SEM and EDS images suggest that, within the magnification used, CeO 2 particles inside the silica matrix are homogeneously dispersed.
Phosphoric acid reacted with the CeOH group, resulting in dihydrogen phosphate species attached to the matrix surface, according to the following equation: The average pore diameters of SC and SCP are the same, i.e. 1.7 nm, except in the case of SCP2 where a small increase is observed, i.e. 1.9 nm.This indicates that the formation of cerium phosphate attached to the surface does not block the finest pores of the matrices.

Figure 3 .
Figure 3. a) Images obtained by MEV, b) the respective cerium mapping by EDS (white points), for SCP3 thermally treated at 473 K and c) MEV and d) the respective cerium mapping for SCP3 thermally treated at 1273 K.

Figure 5 .
Figure 5. Infrared spectra of pyridine adsorbed on (I) SC3 and (II) SCP3: a) room temperature, b) heated at 373 K and c) heated at 473 K.

Table 1 .
Values of specific surface area, S BET , and average pore diameters, r

Table 2 .
Elemental contents on SCP surface (in atom %), determined by XPS

Table 3 .
Ce/Si and P/Ce atomic ratios (error of 8%) and binding energy (BE) values obtained for SCP samples thermally treated at 473 K a in parenthesis: full widths at half-maximum; b standard CeO 2 .