J. Phys. Chem. B 1999, 103, 2431-2438
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Platinum Aluminophosphate Oxynitride (Pt-AlPON) Catalysts. Consequences of Surface Hydrolysis and Heating Processes on the Structure M. A. Centeno*,† and P. Grange Unite´ de Catalyse et Chimie des Mate´ riaux DiVise´ s, UniVersite´ catholique de LouVain, Place Croix du Sud 2/17, 1348 LouVain-la-NeuVe, Belgium ReceiVed: August 27, 1998
Consecutive heating/cooling cycles over a platinum aluminophosphate oxynitride (Pt-AlPON) catalyst with a high nitrogen content (17.8 wt %) have been studied by diffuse reflectance infrared spectroscopy (DRIFTS). Heating steps to 500 °C produces the desorption of part of the nitrogenous species of the catalyst (NH4+, NH3, -NH2, -NH-) as gaseous ammonia. During the treatment at room temperature, N2 saturated with water produces a hydrolysis reaction that transforms part of the bulk nitride ions into NH4+ surface species. These species are easily removed from the catalyst surface during the next heating step. As a result of combined successive hydrolysis/heating processes, the catalyst loses practically all its nitrogen content, reaching a structure similar to that of its AlPO4 precursor.
Introduction High surface area aluminophosphate oxynitride catalysts (AlPON), synthesized from nitridation with ammonia at high temperature of the amorphous aluminum phosphate (AlPO4) precursor,1-3 have been described as a new family of solid base catalysts.2,4-6 The basic properties of these solids increase with the nitrogen content. The N/O ratio in the solid can be adjusted by controlling the synthesis parameters (NH3 flow, temperature and time of nitridation). Thus, AlPON catalysts with tailored acid-base properties can be obtained. Besides this, the high surface area of AlPONs makes them promising catalysts and catalyst supports for basic or acidobasic reactions. In this sense, AlPONs have shown good activity in Knoevenagel condensation.7 The AlPON have also been used as supports to prepare polyfunctional catalysts with combined basic and metallic sites. Ni-AlPON8 and Pt-AlPON9-11 have been synthesized, showing good results for the low-pressure one-step methyl isobutyl ketone (MIBK) synthesis from acetone and the dehydrogenation of isobutane to isobutene, respectively. Besides the good catalytic activity, a suitable solid for catalytic purposes should be stable in working and storage conditions. In a recent paper12 in which we compare the structure and surface thermal stability of AlPON and fresh and reduced Pt-AlPON catalysts, we have shown that AlPON solids and Pt-AlPON catalysts with a low nitrogen content are very sensitive to the presence of water in the atmosphere, undergoing hydrolysis even when the concentration of water in the gas flow is as low as 3 ppm. Hydrolysis transforms part of the bulk N3ions into surface nitrogen species that are susceptible to easy removal from the surface by heating (NH, NH4+). Because of it, the total nitrogen content of the catalyst continuously diminishes after each storage and heating treatment. The aim of the present paper is to study “in situ” by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), * Corresponding author. Telephone: +32 10 473585. Fax: +32 10 473649. E-mail:
[email protected]. † On leave from Departamento de Quı´mica Inorga ´ nica e Instituto de Ciencia de Materiales de Sevilla, Centro de Investigaciones Cientı´ficas Isla de la Cartuja, Universidad de Sevilla-CSIC, Sevilla, Spain.
consecutive heating and hydrolysis processes over a Pt-AlPON catalyst with a high nitrogen content, to determine if these processes can finally destroy the structure of the AlPON catalyst and probably modify its catalytic properties. Experimental Section High surface area amorphous AlPO4 precursor was synthesized by mixing for 1 h at room temperature (RT) two aqueous solutions of Al(NO3)3‚9H2O (Merck) and (NH4)H2PO4 (Merck) of the same molarity (0.66 M) to obtain an Al/P ratio of 1. Then, an excess of citric acid (Merck) was added and the mixture was kept overnight under continuous stirring. After water evaporation under reduced pressure, the gel obtained was dried for 10 h at 100 °C in a vacuum oven (50 mbar). Finally, the solid was calcined for 16 h at 550 °C. Aluminophosphate oxynitride (AlPON) used as support was obtained by nitridation of the AlPO4 precursor in dry NH3 stream at 750 °C. The Pt-AlPON catalyst was prepared by impregnation of Pt(acac)2 dissolved in 20 mL of acetone. The AlPON support, previously dehydrated for 48 h at 100 °C, was added to the Pt(acac)2 solution and the slurry carried into dryness by continuous stirring at RT. The solid obtained was finally ovendried at 100 °C overnight. For obtaining the metallic platinum, Pt-AlPON catalysts were treated under N2 flow by increasing the temperature from RT to 500 °C at 2.5 °C/min, maintaining the solid at this temperature for 1 h and then reducing in pure H2 flow for 2 h at that temperature. The total nitrogen content of the AlPON and Pt-AlPON solids was determined by titration with H2SO4 of the NH3 liberated in alkaline digestion with molten KOH at 400 °C. The specific surface area was measured after 2 h degasification at 200 °C by the single-point BET method in a Micromeritics Flowsorb II 2300 apparatus. The amount of platinum was determined in an ICP-AES Philips PV8250 spectrometer. Platinum dispersion was determined by H2 chemisorption in a Micromeritics ASAP 2000 apparatus. H2 uptake was used to
10.1021/jp9835418 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/12/1999
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Figure 1. Schematic presentation of experimental procedure. Number of the hydrolysis/heating cycles and time is illustrated for each process.
TABLE 1: Physical Properties of the Catalysts
catalyst
SBET (m2 g-1)
total N content (% w/w)
Pt content (% w/w)
Pt dispersion (%)
AlPON support fresh Pt-AlPON reduced Pt-AlPON
55 55 55
17.8 17.8 14.4
0.45 0.45
2.2
determine the metal dispersion by assuming that the hydrogen/ platinum stoichiometry is 1. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were collected in a Brucker IFS88 infrared spectrometer with KBr optics and using a deuterated triglycine sulfate (DTGS) detector. The Pt-AlPON catalyst was placed without dilution inside a controlled environmental chamber (Spectra-Tech 0030-103) attached to a diffuse reflectance accessory (Spectra-Tech collector). The solid was heated from room temperature (RT) to 500 °C under a 30 mL min-1 N2 flow (l’Air Liquide, 99.999%), obtaining spectra (200 scans, 4 cm-1 resolution) every 100 °C after 1 h of stabilization. Then, N2 flow was switched to H2 and kept 2 h under such conditions to reduce the catalyst, following the same procedure as that used for the preparation of the activated Pt-AlPON. After that, the catalyst was cooled to RT under N2 flow and made contact with 30 mL min-1 of water-saturated N2 for 13 h to allow the hydrolysis reaction to take place. Then, the solid was heated again to 500 °C under N2 flow and cooled to RT, and a new hydrolysis reaction was carried out. Nine hydrolysis/heating cycles, with different times on stream, were run. The experimental procedure is shown schematically in Figure 1. DRIFTS spectra were obtained after each treatment. An aluminum mirror was used as background, and data are presented in absorbance mode without any treatment. A commercial software (Brucker OPUS/IR 2.2) was used to calculate the area of the DRIFTS bands after baseline correction with a polynomial function. Results Catalysts. The physical properties of fresh and reduced PtAlPON catalysts, as well as of the AlPON support, are shown in Table 1. Although the specific surface area of the AlPON support is not affected by the impregnation and the reduction procedure, the reduction step induces a decrease in the total nitrogen content of the solid (around 15%). Drifts Analysis. Initial Heating Step. Figure 2 shows “in situ” DRIFTS spectra of the Pt-AlPON catalyst obtained at the
Figure 2. “In situ” DRIFTS spectra of Pt-AlPON catalyst as a function of temperature obtained during the initial heating step.
temperatures indicated during the initial heating step. The evolution of the catalyst surface with temperature is similar to that already reported for other AlPON and Pt-AlPON solids.12 At low temperatures, the presence of broad and intense bands due to adsorbed water (ca. 3500 cm-1, ν(OH); 1630 cm-1, δ(HOH)) and medium ones of metal-coordinated acetylacetonate (1531 and 1606 cm-1) that remain on the catalyst surface after Pt impregnation12-14 overlaps and/or masks the vibration bands corresponding to the surface species characteristic of the AlPON solids. Once adsorbed water and acetylacetonate species disappear by heating at 200-300 °C, bands due to hydroxyl groups bonded to tetrahedral aluminum (3785 cm-1, ν(AlOH)), hydroxyl groups bonded to tetrahedral phosphorus (3670 cm-1, ν(POH)), bridging OH groups (3580 cm-1), structural -NHspecies (∼3350 cm-1, ν(NH)), and -NH2 groups (1558 cm-1, δas(NH2); 3460 cm-1, νs (NH)) become visible.6,12,15 -NH2 groups can be associated with P or Al atoms, since P-NH2 and Al-NH2 species present an intense asymmetric bending band at the same position, about 1550 cm-1.6,16-18 The presence of ammonia species on the surface at low temperature is proved by the observation of a broad band at around 1456 cm-1 due to the asymmetric deformation vibration of ammonium ions (δas(NH4+)) and a shoulder at 1700 cm-1 assigned to δs(NH4+).12 The presence of NH3 coordinatively adsorbed on Lewis acid sites of the catalyst cannot be discarded, since it is characterized by a band at the same position as water adsorbed (ca. 1620 cm-1).17 These adsorbed ammonia species are also desorbed from the surface by the heating treatment at about 300 °C. In the framework region, the main feature is an intense and wide absorption located between 1500 and 1250 cm-1, which shifts to higher wavenumbers upon heating (from 1295 to 1318 cm-1). This band is due to the overlapped PdO, -PO2, and -PdN (-NdP-Nd) stretching vibration bands of the solid,12,19 and the shift is explained by the loss of nitrogen species during heating. The same behavior was observed when the nitrogen content in AlPON solids was modified, and this was ascribed
Pt-AlPON Catalysts to the influence of the lower electronegativity of nitrogen replacing oxygen linked to P.15,20 The loss of nitrogen species as gaseous NH3 from the catalyst surface during heating is also proved by the presence at high temperatures of weak bands at 3144, 3030, 2818, and 1400 cm-1 due to adsorbed NH4+ ions on the ZnSe windows of the DRIFTS chamber.12 Another important feature is the appearance at high temperature of bands at 2160 and 2270 cm-1. These bands have been widely observed in AlN, even thin films, SiN, and different oxynitride solids such as aluminophosphate (AlPON) and vanadium-aluminum (VAlON). Two different assignations have been given in the literature for these bands: metal hydride (M-H, MdAl, P) bond stretching20-22 and ν(NtN) stretching vibration of metal dinitrogen species (M-NtN, MdAl, P).16,23-26 In principle, we cannot discard any one, since both species can be formed from the gaseous NH3 liberated as a consequence of the nitrogen loss in the catalyst. Besides this, similar bands are also present in the precursor AlPO4 DRIFTS spectrum (not shown). In this case, these bands must be assigned to the combination of the intense ones observed in the framework region (PdO, Al-O, and P-O). A broad and weak band around 2675 cm-1 is also observed in the spectra. Its intensity passes through a maximum at 200 °C and then decreases until it practically disappears at 500 °C. It has been associated with hydroxy groups linked to phosphorus in P-N-P bonds of cyclic phosphonitrilates.27 Its decrease at high temperatures is associated with the nitrogen loss in the catalysts, since this band has only been observed in AlPON solids when the nitrogen content is higher than 17.5 wt %.19 Finally, broad bands at around 757, 1000, and 1800 cm-1 are developed with increasing temperature. The first two have been assigned to νas and νs of P-O-P bonds, respectively,19,28 although they can be also assigned to Al-O or a combination of P-O and Al-O stretching modes.29 The band at 1800 cm-1 is due to the combination of the 757 and 1000 cm-1 bands. Again, this observation can be related to the nitrogen loss in the coordination sphere of phosphorus, which causes an increase of the number of P-O-P and Al-O-P linkages. Reduction Step. Figure 3 shows the DRIFTS spectra of the catalyst surface at 500 °C just before and after the in situ reduction process in H2 atmosphere. The difference spectrum between them is also shown for clarity. Negative bands at 3350, 2153, 1560, and ∼1280 cm-1 and positive ones at 3785, 3670, 3575, 1010, 740, and 595 cm-1 are observed. Negative bands are due to species disappearing during the reduction step. As stated above, the 3350 and 1560 cm-1 bands are assigned to NH and NH2 species, respectively. The strong negative band at about 1280 cm-1 must be related to nitrogen loss, so we can tentatively assign it to -PdN vibrations. Finally, the observed decrease of the band at 2153 cm-1 allows us to assign it to metal dinitrogen rather than to metal hydride species. The M-NtN species can react with gaseous H2 and desorb from the catalyst surface, while the amount of M-H species is expected to increase in H2 atmosphere. In that sense, Mazur et al.26 observed a correlation between the presence of N2H+ ions in the gas phase and the IR band at 2130 cm-1 in ion-beamsputtered films of AlN in Ar, N2, and H2 mixtures. Positive bands are due to the appearance of Al-OH (3786 cm-1), POH (3670 cm-1), bridging OH groups (3575 cm-1), and P-O and Al-O stretching vibrations (1010, 740, and 575 cm-1). All these changes can be explained by thermal effects. In fact, as previously described,12 heating the AlPON and Pt-AlPON solids leads to a decrease in the M-NH2, NH, and N3concentration and, consequently, to a increase in the Al-O and
J. Phys. Chem. B, Vol. 103, No. 13, 1999 2433
Figure 3. “In situ” DRIFTS spectra of Pt-AlPON catalyst obtained during the reduction step at 500 °C: (A) in N2, before reduction; (B) after 2 h in H2; (C) difference spectrum (curve B - curve A).
Figure 4. “In situ” DRIFTS spectra of Pt-AlPON catalyst obtained at the end of every hydrolysis step at room temperature.
P-O one. The increase observed in the POH concentration has been explained assuming the hydrolysis reaction of PNH2 with water coming from Al-OH hydroxyls condensation:12,15,20
PNH2 + H2O f POH + NH3v Hydrolysis/Heating Cycles. Figures 4 and 5 show the in situ DRIFTS spectra of the catalyst surface obtained at the end of
2434 J. Phys. Chem. B, Vol. 103, No. 13, 1999
Centeno and Grange
Figure 6. Evolution of the area of the 1456 cm-1 δas(NH4+) DRIFTS band as a function of the hydrolysis/heating cycle number.
Figure 5. “In situ” DRIFTS spectra of Pt-AlPON catalyst obtained at the end of every heating step at 500 °C.
each hydrolysis (at RT) and heating step (at 500 °C), respectively, of the indicated cycle. After reaction with gas-phase water at RT, as for the initial spectrum of the catalysts, the spectra show intense bands due to adsorbed water (3500 cm-1, ν(OH); 1630 cm-1, δ(HOH)). These bands make the detection of NH and OH stretching vibrations of the solid difficult. However, some interesting considerations can be made concerning the other vibrations implied. First of all, the band at 1456 cm-1 assigned to NH4+ ions is observed. This band had disappeared after heating at 300 °C (Figure 2). The appearance after the hydrolysis process implies its regeneration during the reaction of the solid with the gas-phase water. Between the fourth and the eighth hydrolysis/heating cycle this band is not observed anymore, and it is observed again in the ninth cycle. It is important to note that when NH4+ ions are regenerated, the covering of the catalyst surface by adsorbed water is higher than that observed when there is no regeneration, in good agreement with the proposed relationship between the surface hydrolysis reaction and the ammonium formation. Figure 6 shows the area of this band as a function of the cycle number. Another important observation is the shift of the main band to higher wavenumbers with the hydrolysis/heating cycle (Figure 7). As stated above, this shift can be related to a nitrogen loss in the cycle. On the other hand, a better definition of the 1200-500 cm-1 region is produced after each cycle (Figure 4), until a structure with bands at 1070, 1030, 950, 750, and 578 cm-1 is reached, ascribed to P-O and Al-O stretching vibrations and characteristic of an AlPO4 solid.29 Besides this, broad bands at 1970, 1840, and 1617 cm-1 due to the combination of these P-O and Al-O bands are also observed after a considerable number of cycles. Finally, the bands at 1556 and 3450 cm-1 due to M-NH2 groups decrease until they disappear after the seventh cycle. Similar observations are made in the evolution of the PtAlPON DRIFTS spectra at 500 °C at the end of the heating steps (Figure 5). At that temperature, the catalyst surface is clean
Figure 7. Position of the DRIFTS main band in the framework region of the Pt-AlPON catalysts at room temperature (O) and 500 °C (b) as a function of the hydrolysis/heating cycle number.
of adsorbed water, so the hydroxyl and NH stretching structure is visible. The most important feature in this region is the progressive decrease in the intensity of the NH stretching vibration band. To better evaluate the qualitative changes observed during the different hydrolysis/heating cycles, an integration of the DRIFTS bands corresponding to P-OH, -NH-, and M-NH2 groups, at 500 °C, as a function of the cycle number has been carried out (Figure 8). The low intensities of the Al-OH DRIFTS band make the differences in their integration values insignificant, so we omitted them. P-OH DRIFTS band areas decrease from the first to the third cycle, then increase until the eighth one, and again decrease. Nitrogen species (NH and NH2) continuously decrease with the cycle number. However, we can note that while the PNH2 groups disappear after the seventh cycle, -NH- concentration reaches a constant value different from zero. We have shown above the evolution of the surface nitrogenous species of the Pt-AlPON solid with the cycle number. Owing to the high total nitrogen content of our solid (17.8 wt %), nitrogen atoms are mainly N3, since we are close to the maximum nitrogen content described for aluminophosphate oxynitrides, AlPON (27.5 mass %), which corresponds to an AlPON2 composition.1 It is then very important to know what the evolution of the nitride ions in the Pt-AlPON solid is under such conditions. As explained earlier, -PdN overlapped with PdO and -PO2 stretching vibrations to form the main broad and intense feature in the framework region. To follow the evolution of the -PdN bonds during the cycles, a decomposi-
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Figure 9. Decomposition of the main DRIFTS band in the framework region of the Pt-AlPON catalyst at 500 °C: (A) fresh catalyst; (B) after second cycle; (C) after fourth cycle; (D) after sixth cycle; (E) after seventh cycle; (F) after ninth cycle. For comparative purpose, the results for the precursor AlPO4 solid is also included (G).
structure, giving results between 1424 and 1356 cm-1 and showing, for AlP2O8H3 clusters, two different PdO stretching vibrations at 1350 and 1370 cm-1. Besides this, it has been shown that ν(PdO) in AlPO4 solids shifts to higher wavenumbers when the P/Al ratio increases.29,34 These last observations allow us to tentatively assign the doublet to two PdO groups with a different environment associated with the presence (component at 1342 cm-1) or not (component at 1401 cm-1) of the aluminum atoms in the second coordination sphere of phosphorus.
Figure 8. Evolution of the area of the POH, -NH-, and M-NH2 DRIFTS bands at 500 °C as a function of the heating step number.
tion of the main band was made (Figure 9). First of all, we decomposed the main band present in the AlPO4 precursor into two bands, centered at 1342 and 1401 cm-1. Although we could tentatively assign one to PdO and the other one to -PO2 stretching vibrations, this assignation is not very clear. PdO bands often appear as a doublet.30-33 This doublet has been explained by the existence of two conformers with different phosphoryl vibration frequencies,31,32 although in this case, the splitting between them is quite small. In other cases, with a much larger splitting observed (up to 50 cm-1), one component of the doublet has been assigned to a molecular vibration not connected with the PdO band,33 or, if it is not possible, to the Fermi resonance of the PdO band with another vibration at approximately the same frequency.30 On the other hand, recent ab initio theoretical studies34 have calculated the frequencies associated with PdO stretching modes of amorphous aluminophosphates in cluster models based on metaphosphate-like
After that, we fixed the parameters of these two bands (position and half-height width) and used them to decompose the main band present in the Pt-AlPON catalyst. For a good fitting, a third band must be introduced at 1316 cm-1. This band must be related to the presence of nitrogen. The position of this band falls in the range attributed to -PdN (-NdP-Nd) bonds in cyclic ring structures (1315-1218 cm-1).35 Scnick and Lucke36 also reported the asymmetric stretching of PdN-P groups in phosphorus nitride amide (HPN2) at 1330 cm-1, and Benı´tez et al.5 have assigned a position of 1320 cm-1 to such groups in the AlPON framework. Figure 10 shows the integrated area values of the three components of the main peak at 500 °C as a function of the cyclic number. As can be seen, the intensity of the bands assigned to PdO bonds increases sharply while those of the PdN band decrease until they disappear with the number of heating steps. This is in good agreement with the nitrogen loss
2436 J. Phys. Chem. B, Vol. 103, No. 13, 1999
Centeno and Grange Merle-Mejean et al.21 in AlN when heating under inert atmosphere. They assign this band to Al-H bonds and explain the shift by the diffusion of the hydrogen atom into the AlN network at high temperature. The number, complexity, and behavior with temperature of the groups of bands in the 24002000 cm-1 region make difficult their assignment to metal hydride or metal dinitrogen species only. Besides this, the possibility of an intermediate species H-M-NtN, which also appears in the same region,16 and the participation of combination bands of the intense Al-O, P-O, PdO, and -PdNstretching modes complicate the situation even more. Thus, although in the reduction step the existence of M-NdN species is clearly indicated, we cannot fully discard the presence of M-H and H-M-NtN species.
Figure 10. Evolution of the area of the bands resulting from the decomposition of the main one in the framework region of the PtAlPON DRIFTS spectra at 500 °C as a function of the heating step number.
observed in the catalyst and also explains the shift of the main band to lower wavenumbers. It is also important to note that when the nitrogen content of the catalyst is very high (Figure 9A), the component at 1342 cm-1 is almost absent, pointing to the preferential and complete nitridation of PdO groups associated with aluminum atoms. Another interesting observation must be done concerning the group of bands between 2400 and 2000 cm-1. At RT, after the hydrolysis step, the intensity of such bands increases with the cycle number until it reaches a structure with three defined bands at 2349, 2286, and 2218 cm-1 (Figure 4). However, at 500 °C (Figure 5), the structure and intensity of these bands do not change. Moreover, a shift of these bands from 2218 and 2286 cm-1 to 2157 and 2270 cm-1 is observed from RT to 500 °C. A similar shift, from 2260 to 2160 cm-1, has been observed by
Discussion The thermal stability of the high nitrogen (17.8 wt %) PtAlPON catalyst presented in this study is similar to that previously described for a low nitrogen content Pt-AlPON catalyst.12 During the reduction of the platinum atoms of the catalyst (necessary for catalytic purposes) the solid is usually submitted to a heating process at 500 °C under inert gas (N2, He) and then under H2. The results presented in this paper show that the heating process under N2 flow produces the desorption from the catalyst surface of adsorbed ammonia species (NH3, NH4+) and adsorbed water and the decomposition of acetylacetonates from the platinum precursor. Also, a decrease of the amount of M-NH2, structural -NH-, and -PdN bonds is detected. All the nitrogenous species are desorbed from the catalyst surface as gaseous NH3, as proved by the detection of NH4+ species adsorbed onto the DRIFTS cell windows. The loss of nitride ions that form the catalyst bulk is also proved by the shift to high wavenumbers of the main peak in the framework region of the spectra (Figure 7), produced by the decrease in the intensity of the contribution of the -PdN vibration band and the increase of the -PO2 and PdO bonds contributions (Figure 10). The increase of these two last vibrations is also observed by the better definition of the P-O and Al-O vibrations bands in the 1200-500 cm-1 region. Most of the differences observed in the reduction process itself in H2 atmosphere can be explained in terms of time of heating at 500 °C. The decrease of the band at 2153 cm-1 is the only variation in the IR spectra of the solid that can be related to the presence of hydrogen (Figure 3). The low platinum loading (0.46%) and dispersion (2.2%) can explain that no changes related to the platinum reduction are observed. After reduction, the catalyst is usually stored in a desiccator until the catalytic test. Although the concentration of gaseous water inside the desiccator is very low, it is sufficient to produce the hydrolysis of the catalyst surface, since it has been reported that these solids undergo hydrolysis in the presence of gas-phase water concentrations as low as 3 ppm.12 We reproduced the situation of the catalyst surface at the end of this process by quickening the hydrolysis reaction by putting the surface into contact with the water-saturated N2 flow. The most characteristic event of the hydrolysis reaction is the regeneration of NH4+ ions, which had been desorbed from the catalyst surface in the previous heating treatment and which will be desorbed in the next heating step. This formation of ammonium ions in the hydrolysis step and desorption of such species in the heating treatment is produced in every cycle and contributes to the decrease in the nitrogen content of the solid observed in each cycle. It is important to note that the amount of NH4+ ions generated by hydrolysis for a similar time period (13-15 h), followed by
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the area of the 1456 cm-1 DRIFTS band, decreases with the cycle number (Figure 6). This confirms a direct relationship between the rate of the hydrolysis reaction and the nitrogen content of the catalyst, as previously proposed.12 It is wellknown that phosphate glasses are chemically attacked in atmospheres containing gaseous water, breaking the bonds between the phosphate chains and forming a gelatinous material of high water content.37 Phosphate glasses can be made resistant to hydrolysis by adding cross-linking agents to the structure. Al3+ 38,39 and nitrogen37 have been shown to be effective. In AlPO4 solids, trivalent Al3+ cations create silica-like structural units by replacing nonbridging PdO bonds with bridging AlO-P units. In PON solids, the introduction of nitrogen into the glass structure replaces the non-cross-linked species PdO + -O-P with a PdN-P bond. From the results shown above, it is clear that the resistance of Pt-AlPON solids to hydrolysis increases when the nitrogen content decreases. This can be explained by assuming a higher hydrolysis resistance of the Al-O-P bonds compared to Pd N-P bonds, since we have shown that the decrease in the nitrogen content of the catalyst results in an increase in the number of P-O-P and Al-O-P linkages. Thus, AlPON solids are less resistant to hydrolysis than their AlPO4 precursors. However, the catalytic advantages arising from the presence of nitrogen justify their use as catalysts.9-11 Another point of interest is to determine how NH4+ ions are produced on the catalyst surface and which is the nitrogenous species responsible for its formation. After the initial heating step, -NH-, M-NH2, and N3- nitrogenous species are present on the catalyst surface. However, after the eighth cycle, M-NH2 species have disappeared and structural -NH- groups are present in very low concentrations (Figure 8), but the ammonium ions are formed after 86 h of hydrolysis (Figure 6). Thus, although we cannot fully discard the participation of -NHand M-NH2 species in the NH4+ regeneration, we propose nitride ions forming the bulk structure of the oxynitride as the main nitrogenous species responsible for the NH4+ surface ions produced by hydrolysis. At this stage of the discussion, we can recall that the synthesis of aluminophosphate oxynitrides, AlPON, proceeds by the substitution of the oxygen atoms of the AlPO4 precursor by nitrogen atoms provided by the dry, reactive NH3:
AlPO4 + xNH3 f AlPO4-3x/2Nx + 3x/2H2O
(1)
This process produces water. So the reverse reaction can describe the AlPON hydrolysis process.12 In this sense, PON hydrolysis has been explained by the following reaction sequence:37 (2)
(3)
The formation of NH4+ ions on the catalyst surface can be explained by the reaction of the ammonia liberated with another water molecule,
NH3 + H2O f NH4+ + OHor with hydroxyl groups on the surface,
(4)
M-OH + NH3 f M-O-NH4+
(M ) P, Al)
(5)
A hydrolysis mechanism as proposed above explains the observed regeneration of ammonium ions on the catalyst surface, as well as the decrease in the nitrogen content of the catalyst (mainly -PdN- species) producing gaseous ammonia, and the increase in the number of P-O-P and Al-O-P linkages. Another point of interest is the evolution of the P-OH concentration during the hydrolysis/heating cycles. During heating, there may be a P-OH condensation, leading to a P-OH concentration decrease:
POH + POH f P-O-P + H2O
(6)
However, the P-NH2 hydrolysis and NH4+ removing reactions can result in a P-OH concentration increase in this heating step:
PNH2 + H2O f P-OH + NH3v
(7)
PO-NH4+ f P-OH + NH3v
(8)
The removal of ammonium ions from the catalyst surface can also proceed by condensation and formation of P-O-P bonds:
2PO-NH4+ f P-O-P + 2NH3v + H2O
(9)
This reaction can also provide the water necessary for the M-NH2 hydrolysis [eq 7]. During the initial heating step, it is widely proved that from 300 °C the number of POH groups increase with temperature.12 So reactions 7 and 8 are dominant compared to reaction 6. This is due to the high extension of the PNH2 hydrolysis reaction, since the amount of PNH2 groups is very high. However, in the following heating steps, the situation changes. The amount of PNH2 groups is lower, and reaction 7 becomes less important. In this case, the final amount of POH groups also depends on the relative proportion of reactions 6, 8, and 9. When NH4+ is regenerated (cycle numbers 1,2, 3, and 9), the POH concentration decreases. This points to a participation of the POH groups in the process of regeneration of the ammonium ions, as proposed in eq 5, and to the NH4+ removal process proceeding mainly by eq 9. The final equation of these two reactions means the condensation of two POH groups, eq 6, resulting in a decrease of the amount of POH. If there is no regeneration of NH4+ ions, only direct POH condensation and the PNH2 hydrolysis reaction are produced in the heating step. Under such conditions an increase of the POH concentration is always observed. Since PNH2 concentration is very low in the last hydrolysis/heating cycles, we must conclude that direct POH condensation is almost inexistent, and the POH groups present good thermal stability at 500 °C. The final result of each hydrolysis/heating cycle is a loss of nitrogen atoms from the catalyst as gaseous NH3. The amount of PNH2, -NH-, and -PdN bonds diminishes continuously with the successive cycles until practically reaching zero. If a sufficient number of cycles are carried out, this loss of nitrogen from the oxynitride can be important enough to transform it into an AlPO4 solid. Figure 11 compares the structure of the Pt-AlPON catalyst at the end of the treatment with the structure of its AlPO4 precursor. Both DRIFTS spectra are similar, the only difference being the presence of a small band at 3350 cm-1 due to remaining -NH- structural groups in the Pt-AlPON solid.
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Centeno and Grange References and Notes
Figure 11. “In situ” DRIFTS spectra at 500 °C of Pt-AlPON catalyst after the ninth hydrolysis/heating step and of the AlPO4 precursor.
Pt-AlPON catalysts have been presented as suitable catalysts for the isobutane to isobutene dehydrogenation,9-11 the activity, for a constant platinum content and platinum dispersion, being a function of their nitrogen content. On the other hand, these solids are very sensitive to the presence of water, undergoing hydrolysis under normal storage conditions. This process of hydrolysis, combined with thermal treatments, leads to a nitrogen loss in the catalyst, which can transform the active Pt-AlPON catalyst to an inactive Pt-AlPO one. So for a real application of these solids in catalytic processes, it is mandatory to avoid, as much as possible, the number of storage/heating steps before their catalytic use. Conclusions The use of AlPON solids as catalysts or catalyst supports must take into account the results presented in this paper. Heating processes of these oxynitrides produce the desorption of part of the nitrogenous species of the catalysts as gaseous ammonia. On the other hand, these catalysts undergo hydrolysis in the presence of gaseous water at room temperature, resulting in a transformation of part of their bulk nitrogen species into surface ammonium ions that can be easily removed from the catalyst surface by the next heating. Thus, after each combined hydrolysis/heating process, the catalyst loses an important amount of its nitrogen, increasing the number of P-O-P and Al-O-P linkages. These linkages are more resistant to hydrolysis than -PdN-P ones. If a sufficient number of hydrolysis/heating cycles is carried out, the AlPON solid loses all its nitrogen content and presents a structure almost similar to that of its AlPO4 precursor. Acknowledgment. M. A. Centeno thanks the European Union for a Training and Mobility of Researchers (TMR) postdoctoral grant. The authors also thank the “Re´gion Wallonne” and FNRS, Belgium, for financial support.
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