Interaction of NH3 with H2O in the Vanadium Pentaoxide Hydrate

Feb 15, 1997 - The adsorbed state of NH3 in the vanadium pentaoxide hydrate, V2O5‚nH2O, and its chemical reactivity with additionally introduced H2O...
1 downloads 0 Views 409KB Size
Download PDF
1352

Langmuir 1997, 13, 1352-1358

Interaction of NH3 with H2O in the Vanadium Pentaoxide Hydrate Interlayer Spaces: Topotactic Crystal Growth of Ammonium Vanadate Film† S. Kittaka,* H. Hamaguchi, T. Umezu, T. Endoh, and T. Takenaka Department of Chemistry, Faculty of Science, Okayama University of Science, Okayama 700, Japan Received November 21, 1995. In Final Form: August 14, 1996X The adsorbed state of NH3 in the vanadium pentaoxide hydrate, V2O5‚nH2O, and its chemical reactivity with additionally introduced H2O were studied by gravimetric adsorption measurements, Fourier transform infrared (FT-IR) spectroscopy, mass spectroscopy, and X-ray defraction (XRD) analyses. It was confirmed by FT-IR spectroscopy that NH3 molecules are adsorbed at first to form NH4+ by reacting with intrinsically present H2O and secondly as molecular NH3 or IR-inactive NH4+. These adsorbed states were also confirmed by mass spectroscopy. When H2O vapor is introduced in the NH3-presorbed sample, NH4VO3 is formed topotactically with its (001) plane parallel to that of V2O5‚nH2O. Here, H2O reacts with additional molecular NH3 giving locally the NH3 water between the layers. The NH3 water led to the dissolution of the hydrate and to crystallization of NH4VO3.

Introduction Vanadium pentaoxide hydrate, V2O5‚nH2O, has an orthorhombic layered structure.1 Its physicochemical properties are very similar to that of smectite-type clays2-5 except that this compound is chemically and physically active.6 The layer surface of the layered materials is promising to present good stages for the catalytic reactions and for making intercalation compounds which are used in various practical ways. NH3 adsorption in the montmorillonite has been studied by French groups.7,8 It has been found that preliminary present or introduced chemical species like H2O or organics play an important role in the adsorption mechanism, while the structures of substrate clay minerals are not affected. NH3 molecules interact strongly just with the presorbed H2O in the layer space of the clays by forming NH4+ ions but induce no changes in the layered structure. Casal et al. have found the formation of NH4+ between the solid layers by the adsorption NH3 on V2O5‚nH2O. However, they did not investigate this process further. The present authors have studied the ion exchange reactions of alkaline ions in an aqueous system and found that KV3O8, RbV3O8, and CsV3O8 were formed some time after the ion exchange reactions,9 which is in contrast * To whom correspondence may be addressed: tel, 086-252-3161; fax, 086-254-2891; e-mail, [email protected]. † Presented at the Second International Symposium on Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids, held in Poland/Slovakia, September 4-10, 1995. X Abstract published in Advance ACS Abstracts, February 15, 1997. (1) Kittaka, S.; Uchida, N.; Miyahara, H.; Yokota, Y. Mater. Res. Bull. 1991, 26, 391. (2) Aldebert, P.; Baffier, N.; Legendre, J.-J.; Livage, J. Rev. Chim. Miner. 1982, 19, 485. (3) Bouhaouss, A.; Aldebert, P.; Baffier, N.; Livage, J. Rev. Chim. Miner. 1985, 22, 417. (4) Mooney, R.; Keenan, A. G.; Wood, L. J. Am. Chem. Soc. 1952, 74, 1371. (5) Kittaka, S.; Uchida, N.; Kihara, T.; Suetsugi, T. Langmuir 1992, 8, 245. (6) Kittaka, S.; Sumida, M.; Kuroda, Y. Mater. Res. Soc. Symp. Proc. 1994, 346, 697. (7) Mortland, M. M.; Fripiat, J. J.; Chaussidon, J.; Uytterrhooeven, J. J. Phys. Chem. 1962, 67, 248. (8) Casal, B.; Ruiz-Hitzky, E.; Serratosa, J. M. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2225. (9) Kittaka, S.; Uchida, N.; Katayama, M.; Doi, A.; Fukuhara, M. Colloid Polym. Sci. 1991, 269, 835.

S0743-7463(95)01063-8 CCC: $14.00

with the systems with clays. Next, we studied the chemical activity of NH3 in V2O5‚nH2O with intrinsically present and/or added H2O molecules between the layers.10 The formation of thin single crystal films not only of the inorganic but also of the organic materials on solid surfaces is an important subject in the materials sciences.11,12 There have been various methods to obtain such films, e.g., molecular beam epitaxy, chemical vapor deposition, spin coating, etc., which usually require some after-treatments such as heating and/or chemical reactions. It has been confirmed that V2O5‚nH2O with orthorhombic structure forms a well-defined film by coating over the flat substrate with its a-b plane parallel to the substrate and b-axis parallel to the coating or dragging direction.13 Therefore, the present authors examined the possibility of taking advantage of both the crystallographical orientation property and intercalation property of V2O5‚nH2O to form a new single crystal film of topotactic NH4VO3. Experimental Section Materials. A sample used was formed as a wet gel by ionexchange polymerization from aqueous NH4VO3 solution (sample A).12 The wet gel was freeze-dried for adsorption measurements and mass spectroscopy examinations. The H2O content at 25 °C under a vacuum is ca. 0.38 mol mol-1 of V2O5. Dehydrated sample B was prepared by heating sample A at 200 °C for 2 h in a vacuum to examine the effect of intrinsic H2O, where the H2O content was 0.214 mol mol-1 of V2O5. To specify the chemical state of NH3 in V2O5‚nH2O, NH4+ was introduced in the sample through ion-exchange reaction, by contacting the wet gel with 0.05 mol dm-3 NH4NO3 solution repeatedly (sample C). The sediment thus obtained was purified by decanting with distilled H2O several times and freeze-dried. The amount of introduced NH4+ was analyzed to be 0.26 mol mol-1 of V2O5: NH3 ejected by dissolving the sample in a concentrated NaOH was trapped by H2SO4 solution and titrated with NaOH solution. The H2O content in the ion-exchanged sample remaining after evacuation was determined to be 0.54 mol mol-1 V2O5 by combining the thermal weight loss and the NH3 content. Adsorption. Adsorption measurements of gases NH3 and H2O were conducted gravimetrically on the freeze-dried sample at 25 °C using a Cahn 2000 electrobalance together with a (10) Kittaka, S.; Suetsugi, T.; Kuroki, R.; Nagao, M. J. Colloid Interface Sci. 1992, 154, 216. (11) MRS Bull. 1995, 20 No. 5. (12) MRS Bull. 1995, 20 No. 6. (13) Kittaka, S.; Hamaguchi, H.; Shinno, T.; Takenaka, T. Langmuir, submitted.

© 1997 American Chemical Society

Interaction of NH3 with H2O

Figure 1. Adsorption isotherms of sequential adsorptions of H2O (0), NH3 (O adsorption; b desorption), and then H2O (4) (a) on sample A (V2O5‚nH2O) and (b) on sample C (V2O5‚nH2O intercalated with NH4+). Baratron capacitance manometer 390H, which was controlled by a personal computer system (NEC 9801VX). Prior to each adsorption measurement, the sample was evacuated under a high vacuum using a turbomolecular pump. The time required for equilibrating the NH3 adsorption was less than 1 h, as was the case for the H2O adsorption on the NH3-presorbed sample below 8 Torr but above which the adsorption was gradual and required more than 1 h. FT-IR Measurements. The wet gel was loaded on a stainless steel gauze (50#) and dried in air in the dark. The specimens thus prepared were mounted in a vacuum with ZnSe windows and Thermo-Rikoh GI-1000 heating system which was equipped with an IR irradiation apparatus. IR measurements were performed by using a JEOL JIR-100 spectrophotometer at the resolution of 4 cm-1 and accumulating 100 times. Mass Spectroscopy. Interaction of NH3 molecules with layer surfaces was studied by thermal desorption spectroscopy using a mass spectrometer. The freeze-dried samples of 20 mg were evacuated at 25 °C and contacted with 10 Torr NH3 overnight and heated in vacuum using a turbomolecular pump at increasing temperatures at the rate of 5 °C min-. To study the interaction between NH3 and H2O, saturated H2O vapor was contacted with the NH3-presorbed sample for 10 h. The analyzer used was an ULVAC MASSMATE-100. XRD Measurements. To study the structural changes of the material during chemical processes, X-ray diffraction (XRD) analyses were applied in two modes, i.e., reflection and transmission modes. The reflection mode was used for the layered structure analysis, in which the sample gel was coated on a flat glass plate and dried in air in the dark. For the analysis of the a-b plane, the wet gel was coated over the thin mica flake followed by drying and studied by the transmission mode experiments, in which the narrow divergence slit (0.05 mm) was used. The X-ray diffractometer used was a RIGAKU RAD-2R equipped with a Cu X-ray tube.

Results and Discussion Adsorption of NH3 and Its Interaction with H2O in the V2O5‚nH2O. Figure 1a shows the isotherms of sequential adsorption of NH3 and H2O on sample A. The

Langmuir, Vol. 13, No. 5, 1997 1353

adsorption isotherm of H2O is stepwise and reversible, indicating sequential monolayer and double layer adsorption.14 The adsorption mode of NH3 can be assigned to the Langmuir type. Even when the system was maintained in 10 Torr NH3 for a few days, the adsorbed amount did not change. Under high vacuum, the adsorbed NH3 was not desorbed at all, i.e., adsorption was irreversible. Adsorption of H2O on the NH3-presorbed sample was stepwise. Above 8 Torr, adsorption of H2O was quite slow to reach the equilibrium state. The plotted values are those determined 1 h after setting the system at indicated increasing vapor pressures. Of the sample evacuated at 200 °C (sample B, not shown here), adsorption modes of NH3 and successive H2O were similar to the previous case of the untreated sample, although full adsorption of NH3 decreased from 1.05 to 0.78 mol mol-1 of V2O5. Figure 1b shows the isotherms of the sequential adsorption of H2O, NH3, and then H2O on sample C. The adsorption of H2O is reversible and similar to those found for the samples intercalated with alkaline ions Na+, K+, Rb+, and Cs+.5 It is to be noted that preliminary introduction of NH3 (NH4+) did not stimulate the chemical reactions in the interlayer spaces. The adsorption is of the Langmuir type and the adsorbed amount is smaller than that for sample A. Adsorbed NH3 was partly irreversibly desorbed by evacuation. The adsorption of H2O in the NH3-presorbed sample increased significantly above 8 Torr, as in the case of sample A. The adsorbed amount of H2O for the first step is ca. 0.95 mol mol-1 of V2O5, which is smaller than that for sample A. The adsorbed amount above the vapor pressure P ) 8 Torr is quite large again. Figure 2a shows the IR spectra of NH3 adsorbed on sample A at increasing pressures. The spectral pattern of the evacuated sample is typical of V2O5‚nH2O, giving the OH stretching bands at 3573, 3401, and 3200 cm-1, together with the H2O scissoring band at 1616 cm-1 and a broad band at around 1700 cm-1. The 1700 cm-1 band has been considered to be due to H3O+, which is strongly dependent on sample preparation. Introduction of NH3 affected both the OH stretching and scissoring bands slightly and gives rise to the new bands at 3236, 3048, 2840 and 1411 cm-1. These new bands are the same as that reported by Ruiz-Hitzky and Casal.15 They have assigned the first three bands to the NH stretching vibrations of NH4+ adsorbed on the layer surfaces through one of the NH bonds. Therefore, adsorbed NH4+ has C3v symmetry. The 1411 cm-1 band is ascribed to the scissoring vibration of NH4+. The chemical processes that are anticipated through these facts are as follows. The sample is partly hydroxylated and ionized

V-O-V + 2H2O f 2VOH + H2O f 2VO‚H3O+

(1)

By introducing NH3

VO-H3O+ + NH3(ads) f VO-NH4+ + 2H2O(ads) (2) The positions of the NH stretching bands were unchanged at vapor pressures from 1 to 10 Torr and the spectrum obtained at 10 Torr did not change upon evacuation. Here, it may be noted that at least ion-exchangeable protons in the V2O5‚nH2O (ca. 0.3 mol mol-1 of V2O5) should react completely with the same amount of NH3 to give VO-NH4+, (14) Kittaka, S.; Ayatsuka, Y.; Ohtani, K.; Uchida, N. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3825. (15) Ruiz-Hitzky, E.; Casal, B. J. Chem. Soc., Faraday Trans. 1 1986, 82, 1597.

1354 Langmuir, Vol. 13, No. 5, 1997

Kittara et al.

a

a

b

b

c

Figure 2. (a) FT-IR spectra of adsorbed NH3 on sample A at pressures (Torr): (1) 0, (2) 1.0, (3) 4.0, (4) 7.0, (5) 10.0, and (6) after 2 days under vacuum. Here a sharp stretching vibration band (3334 cm-1) for gaseous peak NH3 in the atmosphere was subtracted for clarity. (b) Those of adsorbed H2O on the NH3presorbed sample A at pressures (Torr): (1) 0, (2) 2.4, (3) 7.1, (4) 14.2, (5) 19.1, (6) 21.4. (c) Effect of thermal desorption under vacuum: (1) 21.4 Torr H2O vapor (sample 6 in Figure 2b), (2) evacuated at 25.0 °C, (3) 100 °C, (4) 200 °C, (5) 300 °C. The broken curve represents the NH4VO3 recrystallized through concentration-precipitation from its aqueous solution. The sample was suspended in an optical grade KBr powder and pelleted.

but IR spectra show the result that NH3 reacts with some portion of H2O and/or OH and a non-negligible amount of the latter remains after adsorption in 10 Torr NH3. Allowing for the irreversible amount of NH3 (1.05 mol mol-1 of V2O5) shown in Figure 1a, a relatively large

Figure 3. FT-IR spectra for sample C which was ion-exchanged with NH4+. (a) Adsorption of NH3 at pressures (Torr): (1) 0, (2) 1.0, (3) 4.0, (4) 5.0, (5) 8.0, (6) 10.0. (b) Those of H2O adsorbed on the NH3-presorbed sample C at pressures of (1) 0, (2) 2.4, (3) 7.1, (4) 14.3, (5) 19.1, and (6) 21.3 Torr.

proportion of NH3 remains between the layers without giving C3v-type NH4+. This may partly be explained by the finding that leaving the system for 2 days in a vacuum has increased the intensities of 3236 and 1411 cm-1 bands in place of the intensity decrease of the OH bands; i.e., the rather large decrease in the background level seems to hide the changes of the 3048 and 2840 cm-1 bands. The authors understand that a slow reaction has occurred between the layers to form adsorbed NH4+ with the C3v symmetry. This makes us suppose the presence of IR inactive species, including NH3 in addition to those just after equilibrating 10 Torr NH3. One possibility is the formation of IR-inactive Td-type NH4+ and the reaction

VO-H3O+ + NH4+(Td) + OH- f VO-NH4+ + 2H2Ov (3) Adsorption of H2O in the NH3-presorbed sample A below 7.2 Torr increased strongly the 3048 cm-1 band, together with the 2840 and 1411 cm-1 bands, the latter of which shifted slightly (Figure 2b). The appearance of the broad band at 3396 cm-1 may be explained by the adsorption of molecular H2O in combination with the appearance of the scissoring vibration of OH bonds at 1608 cm-1. The disappearance of the 3236 cm-1 band which signifies the decomposition of C3v type NH4+ is important. Further introduction of H2O molecules shifted slowly the 3049 cm-1 band to 2910 cm-1 (Figure 2b). It is interesting to find the parallel marked increase of the 1413 cm-1 band, repre-

Interaction of NH3 with H2O

a

Langmuir, Vol. 13, No. 5, 1997 1355

a

b b

Figure 4. Changes of the 001 XRD pattern of sample A upon adsorption of (a) NH3 and (b) H2O on the NH3-presorbed sample. Measurement was repeated after 95 s of each scanning from 2θ ) 5° to 11° at the scanning speed 10° (2θ min-1). The numbers in the figure signify the sequential measurements.

senting bending vibration. These facts suggest that hydration provokes the formation of the new chemical species from C3v-type NH4+. The 3396 cm-1 band became apparently less significant under the background increase, while the 1608 cm-1 band was clearer. Figure 2c shows that evacuation of sample A at 25 °C gives the bands at 3403, 3020, 2900, and 2831 cm-1. Here, the 1608 cm-1 band disappears completely; i.e., molecular H2O has been desorbed. These bands are quite similar to those of NH4VO3 recrystallized from commercially available chemicals in concentrated solution (broken curve) but differ from the spectrum of NH4VO3 reported by Onodera and Ikegami16 which is very similar to that shown in Figure 2a. According to Shibahara,17 the material (NH4VO3) recrystallized by a conventional concentration method is partly polymerized. Then, we can attribute the broad fashion of the spectrum at 3300-3000 cm-1 to the H2O molecules adsorbed on the sample surface in a high humidity (Figure 2c-1). The 3403, 2900, and 2831 cm-1 bands disappear by heating between 100 and 200 °C at which H2O and some NH4+ are demonstrated to be desorbed by mass spectroscopy. The 3020 cm-1 band due to the specified stretching (16) Onodera, S.; Ikegami, Y. Inorg. Chem. 1980, 19, 615. (17) Shibahara, T. Private communication.

Figure 5. Changes of the 001 XRD pattern of sample A upon adsorption of H2O observed by means of two modes after being adsorbed with NH3. (a) reflection mode: (1) V2O5‚nH2O under vacuum, (2) 10 Torr NH3, (3) sequentially contacted with saturated H2O vapor for 1 day, (4) 2 days, (5) 3 days, (6) 4 days, (7) 7 days, (8) 8 days, (9) 11 days, (10) 14 days, (11) NH4VO3. (b) Transmission mode: (1) under vacuum, (2) 10 Torr, (3) sequentially contacted with saturated H2O vapor for 3 days, (4) 4 days, (5) 5 days, (6) 7 days, and (7) 26 days.

vibration of NH3 remains almost unchanged up to 200 °C but apparently disappears at 300 °C. The latter observation may be explained by the fact that the sample has lost transparency by heating, since the mass analysis indicates that the species including NH3 derivatives remains at 300 °C. When the V2O5‚nH2O is dehydrated down to n ) 0.214 (sample B), spectral changes due to adsorption of NH3 were less significant than in the case of sample A (not shown here). The main reason for this might be that the sample becomes hard against intercalation of gases, NH3 and H2O, as observed previously in the rehydration of the heated sample.10 However, the sequential changes of adsorption behaviors of NH3 and H2O were basically the same. The spectral change of the sample C upon introduction of NH3 gives important information about the interactions between the substrate, NH3 and H2O (Figure 3). Adsorption of NH3 distorts the shape of the NH3 band at 3246 cm-1 and eventually erases it. At the same time, a broad 3008 cm-1 band grows with the slight intensity increase of the 1412 cm-1 band. A similar change is observed when a small amount of H2O vapor is introduced in the NH3presorbed sample A (Figure 2b, curve 2). This reflects the fact that the previously present H2O molecules, whose amount (0.54 mol mol-1 of V2O5) is larger than that of sample A (0.38 mol mol-1 of V2O5), is already active in the NH3 molecules to form some amount of NH4VO3 phase.

1356 Langmuir, Vol. 13, No. 5, 1997

Kittara et al.

Figure 6. Thermal desorption spectra of the sample A (V2O5‚nH2O) observed by means of mass spectroscopically, (a) intrinsic material, (b) NH3-presorbed in (a), (c) H2O-presorbed in (b). (d) NH4VO3. Numbers written in the figure indicate the mass numbers measured. The mass numbers 1 and 2 (omitted here) were parallel with the curves for the sum of the 17 and 18 and thus were simply identified as the fragments of NH3 and H2O.

The 3008 cm-1 band may be assigned to the vibration mode due to the intermediate substance to NH4VO3, whose state is defined by the previous H2O content. Here, too, it is expected that IR-inactive NH3 should be present as in sample A. This was substantiated by the fact that when NH3-presorbed V2O5‚nH2O was contacted with H2O vapor, a spectral change quite similar to that for sample A appeared (Figure 3b). Therefore, these facts clearly indicate that NH3 molecules in excess of 0.26 mol mol-11 of V2O5 act to produce a new substance. Structural Changes upon Adsorption of NH3 and H2O. Figure 4 shows the displacement of 001 XRD peaks of sample A under NH3 adsorption (10 Torr) and successive H2O adsorption (23.8 Torr). The NH3 adsorption gives slight but detectable narrowing of interlayer distances. This cannot be understood by the molecular size of NH3, which is larger than that of H2O. The material becomes ionic by forming NH4+ and VO- through the reaction of VOH with adsorbed NH3 and then contracts the distance between the layers. This type of contraction is common to the samples intercalated with cations.5 Interestingly, contact of the sample with H2O vapor initially increases the interlayer distance of the sample and gradually decreases with time. The initial increase may be explained by the intercalation of H2O molecules between the layers. The following decrease of interlayer distances can be explained by the increase in the ionic character of the system through the reaction with NH3.

NH3 + H2O f NH4+ + OHV-O-V + NH4+ + OH- f VO-NH4+ + VOH

(4) (5)

Figure 5a shows changes of the XRD pattern of the sample after the rapid changes shown in Figure 4b. It is seen

that the layered structure is gradually destroyed and disappears almost completely after a week. Instead, new peaks appear at 18.12° and 36.68° after 1 day by contacting saturated H2O vapor. In addition, new weak peaks at 15.08° and 30.70° grow slowly with time. These peaks are assigned to the diffractions of 020, 001, 200, and 002 for orthorhombic NH4VO3; refer to the powder diffraction peaks of NH4VO3 (pattern 11). Here, it is evident that the intensities of the 001 and 002 diffraction peaks increased markedly as compared to the others. This indicates that the (001) plane of NH4VO3 grows parallel to the substrate glass plate. In other words, NH4VO3 crystallizes topotactically with the structure of V2O5‚nH2O by orienting the (001) plane parallel to the substrate. This was further substantiated by the transmission mode XRD analysis (Figure 5b). In accordance with the reflection mode experiments, where the 00l diffractions were observed predominantly, new peaks 020, 120, 200, and 140 appear a few hours after contacting the sample with H2O and indicate again that NH4VO3 crystallizes the expanding a-b plane on the substrate plane. In the case of sample C (ion exchanged with 0.26 mol of NH4+ per V2O5), adsorption of H2O on the just evacuated sample increases the interlayer distance from 1.0 to 1.1 nm and is in a rather simple mode in accordance with Langmuir-type adsorption isotherm and expected from IR study. Furthermore, sequential adsorptions of NH3 and H2O produced topotactically NH4VO3 as in the case of sample A. This fact indicates that initially present NH4+ does not react with H2O, but additionally introduced NH3 does react with H2O to produce NH4VO3. Here, the chemical processes found in these systems may be anticipated to be as follows:

NH3 + H2O f NH3OH(NH3 water)

(6)

Interaction of NH3 with H2O

Langmuir, Vol. 13, No. 5, 1997 1357

Produced NH3 water dissolves the V2O5 phase to give NH4VO3 through the reaction

NH4OH + V2O5 f NH4VO3

(7)

The process can be understood to be reconstruction from orthorhombic V2O5‚nH2O into orthorhombic NH4VO3 with their respective a-b planes coinciding with each other. Thermal Desorption of Adsorbed Species. Figure 6 shows the mass spectroscopically obtained thermal desorption spectra of samples A (a) intrinsic, (b) NH3presorbed, and (c) sequentially H2O-adsorbed. The gases detected from V2O5‚nH2O are due to H2O (18), and its fragments OH (17), O (16), and H(1) produced by electron shower in the ionization chamber. When NH3 is adsorbed, the peak of mass number 18, which suggests H2O desorption, increases markedly in intensity in two temperature ranges of 170 and 430 °C. The intensity increase at 170 °C signifies that adsorbed NH3 reacts with the substrate to evolve H2O. The mass number 16 can reasonably be assigned to NH2 rather than to fragmental O of H2O, since the latter O signal should be much smaller than the observed value. Then the mass number 17 should include NH3. In accordance with this, the mass number 15 can be assigned also to NH, which is a fragment of NH3 or of NH2. The signals for mass numbers 17 and 16 appeared at temperatures somewhat lower (110-150 °C) than that for the H2O desorption. It will be reasonable to understand that the H2O molecules formed are held on the layer surface up to 170 °C. The reactions during heating may be written as follows:

VO-NH4+ f VOH + NH3

(8)

V2O5‚NH3(ad) f V2O5 + NH3 (gas)

(9)

V2O5 + 2NH3(ad) f V2O4 + 2NH2 + H2O(ad) (10) With an increase in temperature above 200 °C, a new peak 28 appeared and could be assigned to N2. Allowing for negligibly small signals for them at the lower temperatures around 150 °C, at which desorption of NH3 is the main process, N2 ejection is led from NH3 adsorbed differently from that desorbed at lower temperatures. That is, the solid works as a catalyst to give N2:

3V2O5 + 2NH3(ad) f 3V2O4 + N2 + 3H2O (11) When H2O is additionally adsorbed over presorbed NH3, desorption of NH3 and its fragments NH2 and NH were markedly increased at lower temperatures, 100-220 °C. It is clear that the NH4VO3 phase, which has been produced by sequential adsorption of NH3 and H2O, is decomposed. This was anticipated by the composition of the evolved gases detected in this temperature region which is similar to the thermal desorption spectra of NH4VO3 (Figure 6d). The weak N2 peak in Figure 6d indicates that N2 ejection does not come from the decomposition of the NH4VO3 phase and supports the idea of catalytic formation through reaction 11. Figure 7 shows that NH3 is desorbed at 70, 220, and 320 °C from sample C (a). The apparently higher temperature of the main peak due to the NH3 ejection (220 °C) as compared to the case of sample A indicates that stronger interaction of NH4+ with the layer surface. In this sample, it is to be noted that N2 ejection is delayed after NH3 desorption. This indicates that some kind of nitrides are formed when NH3 is desorbed. When NH3 gas is presorbed on the sample C (b), the desorption of gases below 100 °C occurs in addition to that shown in

Figure 7. Thermal desorption spectra of sample C (V2O5‚nH2O ion-exchanged with NH4+) determined mass spectroscopically, (a) intrinsic material, (b) NH3-presorbed in (a), (c) H2Opresorbed in (b).

Figure 7a, although a significant increase of the H2O peak is not observed. Thus, we can conclude that desorbed NH3 molecules are from species adsorbed as molecules. The reaction of NH3 with the substrate to produce H2O at lower temperatures was not detected in contrast to sample A. This is probably due to the larger preliminary H2O content than that of sample A. The effect of H2O adsorption on the NH3-presorbed sample C on the desorption spectra was very similar to that on sample A (Figure 7c). This result agrees with that of the XRD analyses that the NH4VO3 phase is produced upon sequential contact of the sample with NH3 and H2O. Conclusions Here, we can summarize the adsorbed state of NH3 and H2O as follows. 1. NH3 molecules are adsorbed on V2O5‚nH2O to form NH4+ with the C3v symmetry when trace H2O is present. The NH4+ brings in the ionic character in the sample to

1358 Langmuir, Vol. 13, No. 5, 1997

contract the layered structure. The adsorption of NH3 proceeds further as IR-inactive species above ∼0.3 mol mol-1 of V2O5. 2. Excessive NH3, which strongly interacts with the layered surface, but does not react chemically, becomes active upon adsorption of H2O to produce NH4VO3. This process is acclerated at the surface at pressures above 8 Torr (relative pressure P/P0 ≈ 0.3). 3. The NH3 molecule, which was adsorbed as NH4+, was decomposed into nitride by reacting with V2O5 phase

Kittara et al.

during desorption at increasing temperatures and gave N2 catalytically at higher temperatures. Acknowledgment. This work was partly supported by a Grant-in-Aid for Scientific Research, No. 0645306, from the Ministry of Education, Science, Sports and Culture, Japan. The authors are indebted to Mrs. Atsushi Tanmoto and Hirobumi Maegawa for their help with the experiments. LA951063B