s hi fibe am Spe ng w tu D mo two 1. Introduction origin h as perfect heat and electric re muc vantage rodyn structu signs[1 leache [3,4]. I e interc viscous, denser, harder and more refractory, the thermal-expansion coefficient of oxynitride glasses decreases [6–9]. methods. The fir Journal of Non-Crystalline Solids 357 (2011) 3338–3344 Contents lists available at ScienceDirect Journal of Non-Cr ev words, more dense silicate layers alternate with less dense interspaces filledwithH-bonded silanol groups. UnlikeAl-silicate glass, the Zr-silicate one is characterized by a framework structure where six coordinated Zr (IV) cations serve as connectors linked with a few SiO4 tetrahedra [2]. Itwasnoticed [5] the similarity between the structureof SiO4 units in silicates and SiN4 units in silicon nitride in which oxygen or nitrogen atoms are tetrahedrally oriented around the central silicon. Further- more, it was proposed that due to the similarity between the bond length of Si–N (0.174 nm), Si–O (0.170 nm) and Al–O (0.175 nm), these materials are zeolites [18–23], amorphous silica [24,25] and various mesoporous crystalline silicas [26–29]. The substitution of O for N results in acquisition of new properties like pronounced basicity of nitridated silicate materials which makes them as potential solid base catalysts. Regarding silicate glasses we didn't find any data on their low temperature nitridation with ammonia when the silicate framework structure is changed slightly. Taking into account the ability of leached silicate glasses to absorb a large amount of polar molecules like H2O nitrogen can enter the structure of alum Replacement of O by N leads to the formation and glasses. The nitrogen substitution for oxy ⁎ Corresponding author. Tel.: +7 383 3309770; fax: E-mail address:
[email protected] (B.S. Bal'zhinimae 0022-3093/$ – see front matter © 2011 Elsevier B.V. A doi:10.1016/j.jnoncrysol.2011.05.032 onnected randomly with etwork is not continuous, ridging oxygens. In other at high temperatures (TNTg) [15–17]. At relatively low temperatures (673–973 K) the only solid materials of silicate origin were nitridated with ammonia. Among each other, via bridge oxygens. However, this n it is alternated by protons bound with non-b on industrial scale and widely used insulators. However, these materials a area, although they have obvious ad stability (up to 1500 K), improved hyd possibility to create new types of catalytic reactors with new flexible de Although the molecular structure of clear yet, some models were proposed that SiO4 tetrahedra in the network ar h less known in catalysis s such as high thermal amic properties, and the red catalyst beds and ,2]. d Al-silicate glasses is not n particular, it was found constituent oxides (MgO, Y2O3, SiO2, Al2O3, Li2O, BeO etc) and nitride Si3N4 (source of nitrogen) under inert atmosphere at 1773–2073 K [7,10–13]. The secondway consists in synthesis of the base oxide glass or melt followed by treatment with ammonia at high temperatures [6,8,14]. Most interesting method is based on sol–gel technique. This way involves stages of porous gel preparation, its further ammono- lysis at TbTg and, finally, the converting of ammoniated gel into glass ino-silicate framework. of new nitrogen ceramics gen makes glasses more (NH3) [30] we at silanol groups by of fiber glass ma 2. Experimental The industria of glass melt at +7 383 3308056. v). ll rights reserved. st and widely used way is the direct melting of the The fiber glass materials of silicate ave long been produced Oxynitride glasses can be prepared by different high temperature Gas phase nitridation of silicate fiber glas Three case studies Yu.K. Gulyaeva, A.P. Suknev, E.A. Paukshtis, B.S. Bal'z Boreskov Institute of Catalysis, Pr.Akademika Lavrentieva, 5, Novosibirsk, 630090, Russia a b s t r a c ta r t i c l e i n f o Article history: Received 17 March 2011 Received in revised form 20 May 2011 Available online 25 June 2011 Keywords: Fiber glass; Silanol group; Nitridation; NHx species Three types of the leached (REM)were nitridated with Infrared Fourier-Transform the formed NHx species. Alo NHx species take place. In si not a diffusion of NH3 or H2O due to dehydroxylation of features. j ourna l homepage: www.e ls materials with ammonia: nimaev ⁎ r glass materials of silicate origin modified with Zr, Al and rare earth metals monia at temperatures ranging from 673 to 973 K. DRIFTS (Diffuse Reflectance ctroscopy) and H/D exchange using ND3 were applied for characterization of ith nitridation of silanol groups their dehydroxilation as well as hydrolysis of RIFTS study showed that the limiting steps are slow chemical reactions, but lecules in the bulk of glasses. The concept on strained siloxane bridges formed adjacent silanol groups was used for explanation of the reaction kinetics © 2011 Elsevier B.V. All rights reserved. ystalline Solids i e r.com/ locate / jnoncryso l tempted to nitridate silicate glasses by substitution of NHx groups, whichwould open away for application terials in base catalysis. l production of fiber glass materials includes making high temperatures, manufacturing of elementary fibers with a typical diameter of 7–10 μm, twisting of these fibers into threads of 0.3–1.0 mm in diameter with final manufacturing of glass fiber fabric. The important step is leaching with inorganic acids resulting in complete removal of sodium. Three types of industrial leached glass fabric were used in the study. Chemical composition of these materials measured by atomic- emission spectroscopy is presented in Table 1. In the REM-Si glass the rare earth metals were represented mostly by Ce–La fraction. For nitridation, 3 g of a fabricwas unwoven into threads and densely loaded in a tubular quartz reactor (din=7 mm, length of a fabric layer The nitridation kinetics of fiber glass materials was studied by DRIFTS in situ using high-temperature cell with 80 cm3 in volume (Fig. 1). A pieceof fabric (1)was coiled on a sealedquartz tube (2)with a heating element (3) inside it. Thermocouple (4) was arranged directly in a sample bed. Then the tubewas fixed coaxially and hermetically in a largerflowtube (5) equippedwithCaF2windows (6), andfinally the cell was placed in the diffusion reflection device inside the focus of the optics. The IR-radiationwas focused on the single thread of fabric piece. In a flow of helium the sample was heated to 773 K, then helium was replaced by ammonia, and the sample was held in flowing ammonia at this temperature for 4 h. After the nitridation, ammonia was replaced by a helium flow saturated with water vapor (CH2O≈3.5%). The volume feed rate for all gases was 4 cm 3/s, so the characteristic time of purging the cell was 20 s. 3. Results Fig. 2 shows DRIFT spectra of hydroxyl groups in Zr–Si, Al–Si and REM-Si glasses nitridated at 773 K. The spectra are characterized by the bands at 3615, 3660 and 3677 cm−1 corresponding to Si–OH stretching vibrations. According to [29,33], these bands are assigned to isolated OH groups residing in microcavities, their position depending on the pore size: as the size decreases, the band shifts to the low-frequency region. Thus, when going from REM–Si to Zr–Si glasses, microcavities1 (less dense interspaces) formed upon leaching decrease in size, since the band shifts from 3677 to 3615 cm−1. In case of Al–Si material the size of less dense interspace takes an intermediate position. 1 The term “microcavity” is used for zeolites. In case of leached glasses we use term 3339Y.K. Gulyaeva et al. / Journal of Non-Crystalline Solids 357 (2011) 3338–3344 10 cm). In a flow of anhydrous ammonia (99.9%), the threads were heated and nitridated in the temperature range of 673–973 K for 4 h. The volume feed rate of ammonia was 1.2 cm3/s. Cooling to room temperature was also performed in flowing ammonia, then the sample was unloaded in air from the reactor. The single threadwasmounted on a holder of diffusion reflection device to record the DRIFT spectra, the area of sample radiation in focus being 1.5 mm2. Spectra were recorded in air in the range of 400–6000 cm−1 at a resolution of 2 cm−1 using a Shimadzu FTIR 8300 IR-Fourier Spectrometer with diffusion reflection device DRS-8000. In diffuse reflectance technique the intensity of observed absorption bands is represented as Kubelka–Munk function, F(R)=C·ε/S, where C and ε are concentration and extinction coefficient of absorbed species, S is scattering coefficient. In contrast to the transmissionmode the diffuse reflectance one doesn't allow to measure the concentrations quantita- tively due to uncertainty of scattering coefficient. In our case it is known that the scattering coefficient S in the range of NH2-groups absorbance 3300 cm−1 is less on 30% only than that in the range of OH-groups absorbance 3600 cm−1[31]. Moreover, the extinction coefficient of NH2-groups is also less on 30% than that of OH-groups [32]. So, the concentration of NH2-groups can be estimated using the following ratio: FNH Rð Þ= FOH Rð Þ = CNH⋅εNH⋅SOH COH⋅εOH⋅SNH Thus knowing the concentration of OH-groups obtained from TPD study [3] the concentration of NH-groups can be evaluated from this ratio to accuracy of not more than 30%. For a more detailed identification of NHx species, the hydrogen/ deuterium exchangewas performed on a Zr–Si sample using ND3with 99.9% enrichment. 1 g of the glass was nitridated as threads in a tubular quartz reactor for 4 hours at a temperature of 773 K. Heating to 773 K and cooling to room temperature were carried out also in flowing ammonia. After cooling to room temperature, ammoniawas replaced by dry argon to blow reactor for 10 min. Then the reactor was cut off by pinching a silastic pipes located at the both ends of reactor, the reactor was placed in a dry box. In an argonmedium, the nitridated sample was transferred to a gas IR cell of 250 cm3 in volume. The sample was evacuated (residual pressure of 10−2 Pa) at room temperature for 30 min, then the cell was filled with 800 μmol of ND3 and heated to 723 K at a 35–50 K ramp. After each 35–50 K, a spectrum of the gas phase was recorded. At 723 K the sample was held for 30 min until spectra of the gas phase stopped to change, deuterated ammonia was then evacuated. The sample was taken out of the cell andwas placed on Table 1 Chemical composition of the starting fiber glass materials. Glass fabric Chemical compositiona, wt.% Si O Al Zr REM Na K Ca Fe Zr–Si 38.2 48.6 0.56 11.8 – 0.05 0.06 0.07 0.06 Al–Si 45.1 52.3 1.6 – – 0.07 0.04 0.05 0.08 REM–Si 38.9 46.6 0.32 – 13.1 0.01 0.05 0.08 0.1 a Accuracy of element measuring was 2% for Si, O, Zr, 10% for Al, REM and 20% for the rest. holder of diffuse reflectancedevice to recordDRIFT spectra in air at room temperature. Fig. 1. An outline of the high-temperature cell: (1) glass fabric; (2) inner tube; (3) heating element; (4) measuring thermocouple; (5) outer tube; (6) CaF2 windows. “less dense interspace” [2] where hydroxyls are located. ammonia gas phase are observed (Fig. 4) [36,37]. Note, that NH3 molecules didn't yield duringH/D exchange. In approximately 30 min of the H/D exchange at 723 K, changes in the spectrum of deuterated ammonia species virtually ceased. The concentration of ND3 (bands at 2420 cm−1 and 2556 cm−1) decreases in about 2.5 times. Fig. 5 shows the DRIFT spectrum of nitridated Zr–Si glass fiber after treatment with deuterated ammonia. As a result of H/D exchange, the bands at 2385, 2411 and 2503 cm−1 appear in the region typical of stretching N–D vibrations [38,39]. The first two bands correspond to N–D vibrations in deuterated Si–ND2 amines. The latter, more intense band at 2503 cm−1 is caused by N–D stretching vibrations in Si–NDH species. Therewith, in the region of NH vibrations, the asymmetric 3330 cm−1 band with a shoulder at 3290 cm−1 is observed instead of two bands at 3280 and 3364 cm−1 corresponding to stretching vibrations in Si-NH2. In silica the isolated Si-NDH groups are characterized by 2 bands at 3494 and 2578 cm−1 belonging to NH and ND strecthing vibrations [40]. In case of glass the most NH2 groups are H-bonded, therefore the band at 3494 cm−1 should shift to low freduency region. That's why the intensive band at Fig. 2. DRIFT spectra of different fiber glasses nitridated at 773 K for 4 h. 3340 Y.K. Gulyaeva et al. / Journal of Non-Crystalline Solids 357 (2011) 3338–3344 In nitridated REM-Si glass new absorption bands at 3282, 3373, 3424 and 3500 cm−1 appeared in the spectra. The first two bands correspond to stretching vibrations of the hydrogen-bonded Si–NH2 groups, whereas the low-intensive bands 3424 and 3500 cm−1 are typical of isolated Si–NH2 groups [22,23,35]. As seen from Fig. 2 the intensity of absorption band at 3424 cm−1 decreases when going from REM-Si to Al–Si and Zr–Si glasses. On the other hand the overlapping of this bandwith intensive bands at 3340 and 3364 cm−1 takes place. As a result the pronounced peak at 3424 cm−1 (REM–Si) the shoulder at 3416 cm−1 (Al–Si glass) and disappearance of band at 3424 cm−1(Zr–Si glass) are observed. It is interesting to compare spectra of OH-groups in calcined and nitridated glasses at the same temperature and time of treatment (Fig. 3). It is seen that the concentration of isolated silanol groups in calcined sample is noticeably higher than in nitridated one, and for REM–Si glass it is expressed more pronounced than for Zr–Si glass. This means that nitridation process in REM-Si glass is more favorable to dehydroxylation than in Zr-Si glass. As for absorption bands of Si-NH-Si species, the identification is difficult, because they absorb in range of 3200–3400 cm−1 and may overlap with intense bands of Si–NH2 groups. Thus, to identify Si–NH–Si groups, hydrogen/deuterium exchange between ND3 and amine groups of Zr–Si glass was carried out. The H/D exchange starts at 673 K and high substitution degree is attained at 723 K, when the noticeable intensities of N–D vibrations bands in NHD2 −1 −1 (2430 and 2557 cm ) and NH2D (2478 cm ) in the spectra of Fig. 3. DRIFT spectra of fiber glasses calcined (1) and nitridat 3330 cm−1 can be assigned to NH stretching vibrations in H-bonded Si–NDH. Regarding the shoulder at 3290 cm−1 this band can be attributed to NH vibrations in the hydrogen-bonded Si-NH-Si species [35]. An intensity ratio of the bands at 3330 and 3290 cm−1 was used to estimate the fraction of bridge Si–NH–Si species, which constitutes no more than 15% of the total content of NHx groups. The total amount of deuterium atoms transferred into the glass bulk (CD glass) as a Si-NHD (CNHD), Si-ND2 (CND2) and Si-OD (COD) can be expressed as: CNHD + 2CND2 + COD = 3C o ND3− 3CND3 + 2CNHD2 + CNH2Dð Þ; where CND3, CNHD2 and CNH2D— the concentrations of different ammonia species in gas phase. Assuming the extinction coefficients of isotope-mixed ammonia molecules are close to each other, the amount of deuterium atoms in gas phase can be estimated. Taking into account that CND3 o =800 μmol/g this value corresponds 1450 μmol D/g. Thus, the value of CD glass evaluates as 950 μmol D/g. At elevated temperatures, the nitridation reactions are accompanied by dehydroxylation, two neighboring silanol groups interacting with each other to formwater and bridge oxygen of Si–O–Si. Thus, raising of the temperature decreases the intensity not only ofNH2 groups, but also of hydroxyl groups (Fig. 6). At that, the concentration of OH groups decreases more rapidly as compared to the concentration of amine groups, so a relatively high concentration of Si–NH2 is retained up to 973 K. Note, that at lower temperatures (b673 K) the nitridation of glasses results in mostly ammonia ions. ed (2) at 773 K for 4 h: (a) Zr–Si glass; (b) REM–Si glass. Fig. 4. IR spectra of gaseous ammonia during H/D exchange at different temperatures: Fig. 6. DRIFT spectra of nitridated Zr–Si fiber glass at different temperatures: 1–673 K; 2–773 K; 3–873 K; 4–973 K. 3341Y.K. Gulyaeva et al. / Journal of Non-Crystalline Solids 357 (2011) 3338–3344 Intensity of the absorption band at 3050–3200 cm−1, which is typical of ammonium ions, decreases with temperature, ammonium ions vanishing at 773 K and higher temperatures. This is confirmed also bydisappearanceof thebandat 1440 cm−1 in the low-frequency region (it was not shown on Fig. 6). Ammonium ions in REM-Si glass are even less stable, disappearing completely at 673 K (Fig. 7). This may be relatedwith a lower acidity of thismaterial in comparisonwith the Zr–Si one. To investigate the amination kinetics of zirconium-silicatefiber glass material at 773 K, an in situ DRIFTS study was performed. In this case, spectra were recorded at high temperature, but not at room temperature; this explains slight shifting of the bands toward lower frequencies. As seen from Fig. 8a, the formation of amine groups (absorption bands at 3275, 3355, 3470 cm−1) was observed already in the first minutes of nitridation (curve 3 min). It is seen that the concentration of amine groups increases in the course of reaction, whereas that of hydroxyl groups (3600 cm−1) decreases (Fig. 8b). In about 100 min, the concentration of amine groups flattens out, but its slow growth still proceeds, while the concentration of hydroxyl groups is decreasing monotonically. This indicates that process of dehydroxylation proceeds rather independently from the nitridation one. As follows from Fig. 9, contacting with water results in hydrolysis of NHx species, i.e., a decrease in the amount of amines (band at 1–348 K, 2–723 K, 3–difference spectrum 2–1. 3275 cm−1) is observed, which is accompanied by a growth of silanol groups (band at 3600 cm−1). One may see that amines in the glass are Fig. 5. DRIFT spectra of Zr–Si fiber glasses nitridated at 773 K before (1) and after H/D exchange with ND3 (2). quite stable, as their hydrolysis is noticeable only at TN623 K. Similar to the formation of amines, their hydrolysis is slow: even after an hour a considerable amount of NHx species is still observed in the glass. 4. Discussion As follows from experimental data, the gas-phase nitridation of silicate glasses with ammonia includes the following catalytic reactions: Si� OH + NH3 ⇌ Si� NH2 + H2O ð1Þ Si� NH2 + SiOH ⇌ Si� NH � Si + H2O ð2Þ 2 Si� OH ⇌ Si� O� Si + H2O ð3Þ The nitridation reactions (1) and (2) as well as dehydroxylation of glasses (3) are reversible. As shown by the in situ DRIFTS studies, the reverse reactions of hydrolysis of NHx groups similar to the forward reactions are quite slow. According to Figs. 8 and 9, equilibrium is not established even after several hours of nitridation and hydrolysis. Note, that the hydrogen/deuterium exchange with ND3 also proceeds slowly. This suggests that reactions (1)–(3) are limited by diffusion of ammonia or water molecules in the bulk of glass matrix. Let us estimate the characteristic time of diffusion τD=L2/D, where L is the radius of fiber Fig. 7. DRIFT spectra of nitridated REM-Si fiber glass at different temperatures: 1–673 K and 2–773 K shoulder at 3424 cm−1 is shown by arrow. Fig. 8. DRIFT spectra recorded at 773 K for different time of Zr-Si glass nitridation (a); time d 3342 Y.K. Gulyaeva et al. / Journal of Non-Crystalline Solids 357 (2011) 3338–3344 glass and D is the diffusion coefficient of ammonia or water in the fiber glass bulk. As L≈5 μm and the diffusion coefficient of water measured by SSITKA at 673 K DH2O≈10−8 cm2/s [40], τD≈20 s, i.e., an equilib- rium of hydrolysis reactions of NHx species in the case of diffusion limitations should be attained in ten seconds. As seen from Fig. 9 the process of hydrolysis is not completed even in one hour. Obviously, the observed process of hydrolysis is not limited by diffusion of water in the fiber glass bulk, rather it is determined by the chemical reaction betweenH2O andNHx groups. Unfortunately, thediffusion coefficient of ammonia in glass is unknown. However, a similarity of electrophysical properties of ammonia and water suggests DNH3≈DH2O, thus allowing us to make the same conclusion for the nitridation reactions. The nitridation (Fig. 8) and hydrolysis (Fig. 9) do not rectify in the coordinates concentration vs. ffiffi t p ; this confirms additionally that diffu- sion is not a limiting step. Therefore, the rate of the nitridation reaction is determined by the rate of chemical interaction of ammonia molecules with silanol groups (reaction (1)). It is unexpected fact because despite high temperatures (773–973 K) the reaction between silanol groups with relatively high concentration and fastly diffusing ammonia molecules proceeds very slowly. This can be explained kinetically by the reaction running over active sites with extremely low concentration. Moreover, these centers should be characterized by excess of energy in order to be capable to dissociate ammoniamolecule. Unfortunately, there is nomuch research on the nitridationmechanismof glassmaterials. Most likely, the centers are so called highly strained siloxane bridges which are formed due to Fig. 9. Time dependence of absorption bands intensity at 3275 cm−1 (NHx) and at 3600 cm−1 (OH) during hydrolysis of nitridated at 773 K Zr–Si glass. The first defect is created by the dehydroxilation of two single Si–OH groupswhereas the second site is formed from single Si–OH and adjacent geminal Si(OH)2 groups. The strained oxygen in these paired centers is very reactive and chemisorbs such molecules as H2O, NH3, CH3OH [39,41–43]. In case of ammonia the breaking of N-H bond in adsorbed ammonia molecules takes place with formation of silanol and amine groups. Unfortunately, there is no quantitative data regarding the concentration of strained siloxane bridges excepting for [42]. Here, for dehydroxylation of two adjacent silanol groups at high temperature only (TN673 K) [39,41–43]. It well agrees with our data on that NHx species are formed namely at the temperature above 673 K. The nature of this oxygen bridge is still discussable. The most authors believe that strained bridge is characterized by more acute angles Si–O–Si in comparison with perfect tetrahedral angles [42,43]. At the same time these defects are interpreted as radicals formed due to cleavage of siloxane bridge: Si–O–Si→SiO·+O [27]. In our mind it is doubtful taking into account the very strong Si–Obond in silicatematerials. More feasible models of strained siloxane bridge were proposed in [42]: ependence of absorption bands intensity of 3275 cm−1 (NHx) and 3600 cm−1 (OH) (b). fumed silica (CAB-O-SIL S-17) the surface density of strained bridges was estimated with FTIR as Ns=1017/m2. The strained siloxane bridges of glass are mostly located in its bulk. Assuming that the surface densities of these bridges for silica and glass are equal, the density of these defects in the bulk of glass can be expressed asNv=Ns·4/d,where d is a diameter of fiber glass. Taking into account that density of fiber glass ρ=2.3 g/cm3, thevalue ofNv canbe estimated as 0.04 μmol/g. This value is four orders of magnitude less than concentration of OH- or NH2-groups in nitridated glass. Obviously, it is very hard to detect strained siloxane bridges by means of IR spectroscopy especially when their absorbance bands are overlappedwith stretching Si–O vibrations band. Thus, we believe that in spite of high temperatures extremely low concentration of Si(OH)–O–Si or Si–O–Si sites can explain the low rate of nitridation of leached silicate glasses. The formation of bridge Si–NH–Si species proceeds in a similar way with the involvement of neighboring Si–NH2 and Si–OH groups. Most likely, such pairs are even less in number. As follows from data on the hydrogen/deuterium exchange, the concentration of bridge species forming by the reaction (2) does not exceed 15% of the total amount of amines. Thus, the nitridation reactions proceed on the paired defect centers formed during dehydroxylation of the glass matrix. It can be 3343Y.K. Gulyaeva et al. / Journal of Non-Crystalline Solids 357 (2011) 3338–3344 expected that the concentration of NHx species will bemuch lower than the initial concentration of silanol groups COHo . By means of TPD study the COHo value in Al–Si glass was shown to be 3300 μmol/g [3]. Approximately the same concentration of hydroxyl groups was obtained for Zr- and REM-silicate glasses. Knowing COHo it is easy to evaluate the COH after calcination and nitridation at 773 K as a 500 μmol/g and200 μmol/g, respectively (see Fig. 3 a). As seen fromFig. 5 the amount of Si-OH groups decreases due toH/D exchange in 4 times. In other words the amount of deuterium in Si-OD is 150 μmol D/g. Moreover, there are practically no Si–NH2 groups in the nitridated glass after H/D exchange, so, taking into account that CD glass evaluates as 950 μmol D/g (see Section 3), 2CND2+CNHD is 800 μmol D/g. Unfortu- nately, the concentrations of deuterated amines can't be determined separately. If 2CND2NNCNHD the number of NH2-groups can be estimated as 400 μmol/g. In contrast, if 2CND2bbCNHD thevalue ofCNH2=800 μmol/g which is very close to CNH2=700 μmol/g estimated directly from DRIFTS data according procedure described in Section 2. This means that the fraction of NHD species is prevailing. So, the number of NH2 groups in the glass bulk can be estimated as 700±200 μmol/g, which corresponds to 20% of the COHo value. According to the data on H/D exchange, the concentration of bridge Si–NH–Si groups is even lower, not exceeding 2–3% of COHo . This agrees well with the suggested mechanism of nitridation reaction running on the paired (defect) centers. As the temperature increases, the number of such paired centers is likely to decrease due to diminishing concentra- tion of silanol groups caused by their dehydroxylation and annealing of the defect centers in the silicate framework, which are bound with the strained bridge oxygens [27]. So, the concentration of amine groups should decline rapidly. Indeed, as the nitridation temperature grows from 693 to 973 K, the concentration of NH2 groups drops nearly by an order of magnitude (Figs. 6 and 7). All three types of silicate materials are nitridated nearly in a similar way, although, as seen from Fig. 2, the DRIFT spectra of NHx species, especially in fiber glasses modified with rare earth elements, are quite distinct. First, the absorption bands corresponding to stretching vibrations of NH2 group in the REM–Si sample are shifted toward higher frequencies. Besides, intensity of the band at 3500 cm−1 assigned to isolated SiNH2 is much higher as compared to Al–Si and particularly Zr–Si glasses. In addition, the band of silanol groups is gradually shifting to the low-frequency region, from 3677 to 3615 cm−1, when going from REM-Si to Zr–Si fiber glasses (Fig. 2). By analogy with zeolites, this could be related with the diminishing size of microcavitieswhere hydroxyl groups are located [29,33–35]. In the case of glass, there are not microcavities, but rather the regions with a less dense phase located between the layers of silicon-oxygen SiO4 tetrahedra (a dense phase) [2,3]. Nevertheless, it can be stated that the size of a less dense phase in Zr–Si glass is smaller as compared to REM–Si, i.e., hydroxyl groups in zirconium-silicate fiber glasses are localized in a more narrow space than in the case of silicate glass modified with rare earth elements. The Al–Si fiber glass takes an intermediate position. As the total content of silanol groups in all three glasses is nearly equal, density of OH groups in Zr–Si glass may exceed that in Al–Si and REM–Si glasses. Therefore, the concentration of isolated OH groups in REM–Si will be higher than that in Zr–Si glass, what follows fromFig. 2 (see the band at 3500 cm−1). This fact explains also higher stability of silanol groups in REM–Si glass against dehydroxylation (Fig. 3). Besides, a greater size of microcavities with amine groups assumes a smaller extent of their interaction with each other by hydrogen bonds. As a result, REM–Si has not only higher concentration of isolatedNH2groups, but bandsofH-bondedamines are less broadened and shifted to the high-frequency region. 5. Conclusion Three types of the leached fiber glass materials of silicate origin modified with Zr, Al and rare earth metals were nitridated with ammonia at elevated temperatures ranging from 673 to 973 K. By means of the DRIFTS technique the formation of NH2 groups from silanol ones was shown. At 773 K only 20% of initial silanol groups are nitridated to amine ones. With H/D exchange using deuterated ammonia, the amine and bridge NH groups were identified. The content of NH groups did not exceed 15% of the NH2 group content. As NHx species are formed from silanol ones, the density of hydroxyls inside of glass (which decreases when going from Zr–Si to REM–Si glasses) predetermines a relationship between H-bonded amine groups and isolated ones. Indeed, the DRIFT spectrum of NH2 groups in REM–Si glass is broadened to a smaller extent in comparison with other glasses. The kinetics of Zr–Si glass nitridation and further hydrolysis of the formed NHx species was studied in situ by DRIFTS using a flow high- temperature cell. In spite of rather high temperatures, the both processes are running very slow. The limiting step is a chemical reaction between ammonia (water) and silanol (amine) groups, but not a diffusion of NH3 or H2O molecules in the bulk of glass. To explain the observed kinetic data and a sharp decline in theNHx concentrationwith temperature, the paired active centers including strained siloxane bridge were proposed. 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