Exothermal Water Release from Pseudoboehmite Gels and Their

Sep 25, 2008 - Institute of Chemistry, Berlin Humboldt University, Germany. J. Phys. Chem. C , 2008, 112 (42), pp 16438–16444. DOI: 10.1021/jp803469...
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J. Phys. Chem. C 2008, 112, 16438–16444

Exothermal Water Release from Pseudoboehmite Gels and Their Mechanically Treated Analogs Caused by Activated Hydrogen Reinhard Sto¨βer and Michael Feist* Institute of Chemistry, Berlin Humboldt UniVersity, Germany ReceiVed: April 22, 2008; ReVised Manuscript ReceiVed: June 30, 2008

Exothermal differential thermal analysis (DTA) effects for water release have been observed both for mechanically activated alumina gels, which may contain residual nitrate, and for strongly amorphisized AlO(OH) gels at comparably low temperatures (150-350 °C) under hydrogen. The observations are based on an investigation combining thermal analysis (TA), on-line coupled to mass spectrometry (MS), with ESR spectroscopy. This allowed the identification and quantification of trapped NO2 molecules as well as to follow the mechanically and thermally induced changes of the Al-O-H matrix via the contained (or doped) Fe3+ impurities. The appearance of the exothermal DTA effects is related not only to a strong mechanical activation and the access of hydrogen during the thermal treatment but also to the interaction of hydrogen with metallic platinum, for exampel, as crucible material or as metal powder. This interaction is discussed to be the origin of the chain start of the radical reaction between H2 and NO2 yielding H2O and NO. An experimental application of the observed phenomena is comparably simple and might promote a new kind of characterization of mechanically activated and highly reactive solid materials. Introduction Because of its unique mechanical, optical, and chemical properties, corundum (R-Al2O3) as a functional material is widely applied either in crystalline or fibrous form or even as coating material for protective layers. The preparation of corundum-based materials, including the adaptation to specific requirements by the technological practice, comprises a multitude of well-mastered synthesis and analysis procedures. The sol-gel process,1 especially, opened entirely novel preparation routes for producing corundum materials. It is of great chemical interest that not only the properties of intermediates but the corundum formation process itself can be controlled both by doping with Fe3+ ions1-3 and by mechanical treatment (e.g., by high energy ball milling yielding nanoparticles4-6). Interaction with various inert or reactive gases (N2, O2, H2, etc.) during the thermal decomposition of a starting component, for example, such as AlO(OH), is possible as well. Evolved gas analysis (EGA) by thermal analysis (TA) simultaneously coupled to a mass spectrometer (MS)7,8 is a useful tool for understanding the processes when xerogels1 transform into R-Al2O3 matrices. TA-MS measurements enable both an energetic and a quantitative description (mass balance) of the transformation process, as well as an analysis of the gaseous products. The possibility of changing the gas atmosphere is of crucial importance here. In addition to the EGA, the investigation of the solids, that is, prior to heating, as well as the residues, by applying a spectroscopic method such as electron spin resonance (ESR), yields valuable information. The combination TA-ESR, hitherto rather seldomly applied in physical or inorganic chemistry, is especially promising for two reasons: (i) trapped intermediates in the thermally treated solids can be studied, and (ii) local structural changes (e.g., in the environment of doped or residually contained Fe3+ ions) * Corresponding author. E-mail: [email protected]. Present address: Institut fu¨r Chemie der Humboldt-Universita¨t zu Berlin, BrookTaylor-Strasse 2, D-12489 Berlin, Germany.

proceeding prior to the final transformation into R-Al2O3 can be followed. In this concern, it was of special importance to find a good fit of the sol-gel systems to both the structure and the requested properties of the R-Al2O3 systems via a directed activation by mechano-chemical or chemical treatment. Applying the combination TA-ESR, we observed an interesting effect that, to the best of our knowledge, has not been predicted or described in literature until now. It concerns exothermal DTA effects of partly unusual signal shape that have been observed for mechanically activated doped and undoped xerogels during TA under N2/10%H2 in the presence of Pt. They will be elucidated in this paper, taking into account the properties of the activated matrix and activated surfaces as well as cavities for the stabilization of reactive species. This enhanced reactivity refers both to intermediates and to extremely activated OH groups causing the exothermal water release. We further refer to experiments in progress that are performed with the same matrices and additives in the form of spinprobes (TEMPO and others) at the comilling. These spinprobes were also remarkably stabilized at and in the activated matrix and were proven in the temperature range up to ∼200 °C. Experimental Methods Materials. The xerogels have been synthesized as described previously1 and mechanically activated by using a Pulverisette (Fritsch, Germany). The “high energy ball milling” procedure has been applied to samples of 2 g per milling beaker utilizing 5 milling balls (beaker and balls made from silicon nitride). The milling time was between 2 and 50 h at 600 rpm. Crystalline AlO(OH) (Nabaltec) and R-Al2O3 (Aldrich) were used as commercially available; all other chemicals, for example, the nitrates, were chemically pure (p.A.) and were taken from usual laboratory stocks. The Fe3+ doping was made by adding Fe(NO3)3 · 9H2O to the sol. The presence of nitrate ion traces in the nondoped samples is a consequence of the sol-gel preparation process,

10.1021/jp8034693 CCC: $40.75  2008 American Chemical Society Published on Web 09/25/2008

Water Release from Pseudoboehmite Gels where nitric acid is used for the precipitation.1 The gel properties prevent a complete removal of nitrate ions even after multiple careful washing procedures. The thermal treatment, which follows the xerogel formation, has hitherto been regarded to be sufficient for the nitrate removal. The present study, however, shows that these nitrate traces contribute to one kind of the exothermicity investigated here. On the other hand, there is no simple alternative to the use of HNO3, as other anions and ligands disturb, at least during the seed formation and activation steps. Thermal Analysis. Together with the conventional TA curves (T, DTA, TG, and DTG), the ion current (IC) curves have been recorded in the multiple ion detection (MID) mode8 utilizing a simultaneously coupled TA-MS device (Netzsch STA 409 C Skimmer equipped with a Balzers QMG 421 mass spectrometer).7 Various gas atmospheres have been applied (synthetic air, N2, N2/10%H2, H2). Further experimental details were as follows: DTA-TG sample carrier system with Pt/PtRh10 thermocouples; heating rate, 10 K/min; platinum or corundum crucibles (0.8 mL); sample mass, 40-80 mg, measured against empty reference crucible; carrier gas flow, 70-100 mL/min; data evaluation utilizing the manufacturer’s software Proteus (v. 4.1+) and Quadstar 422 (v. 6.02) without smoothing, etc. The temperature evaluation concerning onset (Ton), extrapolated onset (Tex on), and peak temperatures (TP) of the characteristic temperatures and their labeling follow international recommendations.9 To avoid the suppression of fine experimental details, the DTA curves are generally not baseline corrected, for example, by curve subtraction procedures. Various sample carrier systems have been applied. As a consequence, the thermal asymmetry is individual for each carrier system, especially in the lowtemperature range up to 500 °C, and causes differently shaped DTA baselines. To ensure an unambiguous assignment of exothermal reaction ranges, second heating runs have been recorded in cases where the DTA effects were flat. On the other hand, these second heating runs were not always recorded for the whole temperature range up to 1200 °C (see e.g., Figure 6), as the sintering processes reduce the thermal contact between sample and crucible, which hinders a suitable baseline comparison. ESR Spectroscopy. The ESR spectra have been obtained with an X-band spectrometer ERS 300 manufactured by ZWGMagnettech GmbH (Berlin-Adlershof, Germany). The chromium signal (*) of MgO:Cr3+ (g′ ) 1.9796) served as reference for intensity and g′ values. Results and Discussion Thermal Analysis Results. Usually, the liberation of H2O from hydrates or OH-containing compounds during TA runs causes endothermal effects both in inert and in reactive gas atmospheres. It is, therefore, rather surprising that during the TA of the system {Xerogel + 1 wt % Al(NO3)3 · 9H2O}, precedingly activated by high-energy ball milling, the DTA curve under N2/10% H2 shows distinct exothermal peaks between 170 and 400 °C, exhibiting a quite unusual signal shape (Figure 1a). Such effects have not been observed under N2 or N2/O2. These findings are accompanied by further interesting, but unexpected, phenomena: according to Figure 1b, a direct correspondence exists between the exothermal DTA peaks and both the DTG peaks and the IC curves for the mass numbers m/z ) 18 (H2O+) and 30 (NO+). As expected, the mechanically activated samples age, especially under humid air. Consequently, the distinct DTA effects disappear, and the DTA curve after three months aging shows

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Figure 1. (a) DTA curves of the system {xerogel + 1 wt % Al(NO3)3 · 9H2O}, 8 h milled, measured under N2/10% H2. (b) DTG curves referring to the measurements shown in panel a.

only a broad (but exothermal) signal. Quite surprisingly, the mechanical activation can be re-established, as can be demonstrated via the reproduction of the unusual DTA effects after the second and the third milling process (Figure 1). As a further consequence of the mechanical activation, TP of the crystallization peak, which indicates the formation of R-Al2O3, is shifted to lower values in the range of 950-1200 °C. Concerning the phenomenon of the distinct exothermal peaks, it must be stated that their appearance is not only related to the xerogel matrix precedingly milled together with an additive and the hydrogen-containing atmosphere, but is also restricted to performing the TA run in platinum crucibles. Figure 2a shows the influence of utilizing platinum or corundum crucibles for the same system as discussed above. No low-temperature exothermicity occurs in corundum crucibles. Figure 2b compares the DTA curves in platinum crucibles under N2 and N2/10% H2; without hydrogen, no exothermicity appears. Under N2, only the weak endothermal dehydration range around 200 °C occurs, which, as expected, is lacking in the second heating curve (dotted line); the same is true for the crystallization

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Figure 2. DTA curves of the system {xerogel + 1 wt % Al(NO3)3 · 9H2O}, 8 h milled, and the influence of the crucible material and the gas atmosphere: (a) Under N2/10% H2 in platinum or corundum crucibles; (b) under N2 or N2/10 %H2 in platinum crucibles (second heating run, dotted line); (c) Under N2/10% H2 in corundum crucible with added Pt powder. The exothermal reaction range is dashed.

peak of R-Al2O3. The baseline comparison for N2 reveals that no further thermal event except the endothermal dehydration occurs. Figure 2c, finally, shows that in corundum crucibles under N2/10% H2 exothermal DTA signals can be observed only after adding platinum powder to the sample. Figure 2 in toto unambiguously proves the combined effect of platinum and hydrogen. It seems justified to conclude that the dashed exothermal DTA signals are related to the action of activated hydrogen. Further observations, obtained with other, similarily composed samples, confirm the possibility of observing exothermal DTA effects in the mentioned temperature range. Thus, Figure 3 depicts the DTA curves for nonactivated xerogels and for those that have been prepared by comilling together with various Fe species. The nontreated xerogel gives exothermal but lessstructured effects at ∼300 or 413 °C. However, doping the system during the sol-gel preparation route with Fe(NO3)3 · 9H2O leads to a greater peak multitude that is, regarding both the curve shape and the corresponding temperatures, quite similar to the situation obtained by comilling the xerogels together with Al(NO3)3 · 9H2O (Figure 1). The other findings presented in Figure 3 correspond well to the results described here, but a certain specificity due to each individual dopant persists. To clarify the origin of the exothermal DTA peaks, at least four questions have to be answered: (i) what causes the exothermicity in the given temperature range? (ii) Where does the oxygen in the liberated water comes from? (iii) How and where the hydrogen gets activated? (iv) Can one expect a participation of other activated constituents of the matrix, for example OH groups or defects, in the reaction processes?

Sto¨βer and Feist

Figure 3. DTA curves of various xerogels under N2/10% H2: (a) untreated xerogel; (b) sol-gel-prepared xerogel doped with Fe(NO3)3 · 9H2O; (c) xerogel doped with 1 wt % Fe(NO3)3 · 9H2O and subsequent milling; (d) xerogel doped with 1 wt % Fe(0) and subsequent milling; (e) xerogel doped with 1 wt % K3[Fe(CN)6] and subsequent milling; (f) xerogel doped with 1 wt % Fe2O3 and subsequent milling. The milling conditions were always identical (cf. Experimental Methods). Note that DTA scaling and temperature axis are identical for all partial figures.

To answer these questions, similar systems have been investigated for comparison: (i) crystalline R-Al2O3 was milled alone and together with various dopants for 50 h maximum. Here, only samples doped with 1 wt % Al(NO3)3 · 9H2O or Fe(NO3)3 · 9H2O gave exothermal DTA peaks in the mentioned temperature range. (ii) Originally crystalline AlO(OH) yields similar findings (Figure 4). It has to be noted here that the milling process reduces the particle size almost to the nanometer range.3,4 In the temperature range of 320-410 °C under N2/ 10% H2 (Pt), exothermal effects clearly appear, and TP decreases to 1055 °C. Via the fine structure10 of the Fe3+ impurity traces, the corresponding ESR spectra (see below) indicate the local formation of R-Al2O3. Prior to the TA run, trapped NO2 molecules (Figure 4b) as well as Fe3+ species can be detected. This means that the phenomena described above are related to a sufficient amorphization of a crystalline matrix2,3 and the existence of NO2 or NO2 precursors. These NO2 species adopt a probe function in a double aspect: on the one hand, they indicate spatial possibilities of stabilization and, implicitely, the amorphization degree of the matrix; on the other hand, NO2 can be regarded as a “sleeping probe” for the TA that yields the well-expressed exothermal effects under N2/10% H2 (Pt) when the required temperature interval (∼250 °C) is reached. One can deduce from these results that a strong amorphization of the {AlO} matrix is a crucial but is not a sufficient

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Figure 4. (a) DTA curve under N2/10% H2 of {AlO(OH)cryst + 1 wt % Al(NO3)3 · 9H2O}, 50 h milled; (b) ESR spectrum of the milled sample prior to the TA run, measured in the range 0-500 mT; (c) Like panel b but in the range of g′ ≈ 2.

prerequisite for the observed effects. Because the effects could be detected only after having added nitrates before milling, pure Al(NO3)3 · 9H2O has been investigated as well. Other than in the case of the doped and, therefore, strongly diluted systems, the pure compound already yields the expected effect without mechanical activation (Figure 5); it is remarkably strengthened, however, after a milling process of only 2 h. For a deeper understanding of the nature of the exothermal DTA effects, it is essential to note that the intensity of the ESR signals of NO2 has been much more decreased by the thermal reaction of the carrier gas N2/10% H2 (Pt) than in pure N2 (Pt). This means that residual amounts of NO2 are retained in deeper traps up to approximately 600 °C.1 The corresponding results obtained by TA-MS measurements are presented in Figure 5, panels a and b: In the temperature range ∼200 to 300 °C, exothermal DTA effects were observed as well; a direct correspondence can be found in the IC curves, that is, the release of H2O, NO, and partly O2 (probably from the thermal decomposition of residual NO2). It is noteworthy that above 200 °C no further NO2 release can be detected. H2O and NO are released until ∼250 °C after desorption or as reaction products of H2 and NO2, whereby the latter can no longer be detected by MS. A part of the NO2 is trapped by the activated matrix and will react with H2 when the temperature interval around 250 °C is reached. During the first intensive NO2 release from the nitrate ions around 150 °C, the reaction to form H2O and NO takes place after the start of the chain reaction (see Figure 5). But a part of NO2 is also liberated without further reaction. In the DTA, however, the endothermal water release both from the xerogel

Figure 5. TA-MS curves of pure Al(NO3)3 · 9H2O, 2 h milled, with the IC curves for the mass numbers m/z ) 18 (H2O+), 30 (NO+), 32 (O2+), and 46 (NO2+): (a) under N2/10 %H2 with exothermal effects above 200 °C and (b) under N2.

and the nitrate hydrate predominates so that the experimental curves represent a “net” effect. On the basis of these results, obtained from the examination of nitrate-containing matrices, attention will now be payed both to the mechanical activation of the main component of the matrix and to their possible indication via the system H2/Pt under the conditions of a TA heating run. Further experiments showed that the matrix activation via TA under N2/10%H2 is not restricted to the presence of NO2 precursors. Figure 6 presents two essential TA results that have been obtained from strongly activated, originally crystalline AlO(OH). The most important here is that exothermal effects appear during the water liberation from the matrix in the temperature range of 240-500 °C. They originate from an exothermal water release caused by the interaction of active gaseous species with highly activated OH groups. These OH groups have an activation degree that clearly

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Figure 6. DTA curves of the first and second (dotted line) heating run (Pt crucible) of the originally crystalline AlO(OH) after 50 h milling: (a) under N2/10% H2; (b) after adding 30 wt % Pt powder under 100% H2. (First and second heating curves shifted for better legibility; the exothermal reaction range is dashed).

differs from that of other OH-containing systems (see e.g., refs 11 and 12). The active species are formed by H2/Pt activation, that is, the production of activated H or OH, or by reaction of H2 with defect sites of the activated matrix (cf. Figures 4 and 7). Choosing pure H2 as carrier gas or adding 30 wt % Pt powder when using a corundum crucible confirm the appearance of exothermal DTA signals. There intensity, however, was not considerably increased (Figure 6). This is a further indication that we describe here a catalytically started chain reaction where the actual catalyst concentration is less important. ESR Spectroscopy. The selectivity and sensitivity of the ESR spectroscopy is a special advantage when combining it with TA information. This has been illustrated by discussing the findings in Figure 4 where we dealt, on the one hand, with proving the existence of trapped NO2 and, on the other hand, with the Fe3+ probes in the amorphous AlO(OH). Namely, the Fe3+ ions, being contained as impurities, demonstrate their probe character via a sensitive spectral indication of the thermal transformation of {AlO} matrices.13 As an example, Figure 7a depicts the ESR spectrum (77 K) of the originally crystalline sample of AlO(OH) after 50 h (!) milling prior to the TA heating run. The Fe3+ ions remain localized in a rhombohedrally distorted, 6-fold oxygencoordinated environment, as indicated by the signal at g′ ≈ 4.3 (B0 ≈ 150 mT). Only in the low-field region (g′ ≈ 13 (B0 ≈ 50 mT, see * in Figure 7a) can one detect weak characteristics of a beginning local corundum formation.5 At g′ ≈ 2 (B0 ≈ 334 mT), neighbored by the narrow reference signal, one observes ESR signals of defects that might be able to react with H2 in the temperature range near ∼250 °C. In that range under N2 atmosphere, one usually observes the recombination of the defects. Principally, Figure 7a is comparable with Figures 4b and 4c, where the latter ones represent a preceding milling process with Al(NO3)3 · 9H2O as an additive. Compared with Figure 7a, Figure 7b represents the same system after the TA heating run under N2/10% H2 with a resulting ESR overview spectrum being typical for this treatment. The broad background is caused by reduced Fe species (small amounts of Fe(0), see ref 14). At lower field (∼50 mT), a very narrow signal appears (g′ ≈ 13), which indicates the local corundum formation. In the X band (ν ≈ 9-10 GHz) and for not too high Fe3+ concentrations, the mechanically lessdistorted powder system R-Al2O3:Fe3+ yields practically “fin-

Sto¨βer and Feist gerprint spectra” with characteristic signals at g′ ≈ 13, 5.2, and 2.15 The small line width is consistent with the remaining low Fe3+ concentration. At B0 ≈ 150 mT with g′ ≈ 4.3, however, a signal is detected that, at best, indicates parts of the matrix containing a strongly distorted R-Al2O3.6 Figure 7c shows the expanded spectral range around g′ ≈ 2 with the narrow reference signal and a broad contribution of the Fe3+ fine structure. The spectra in Figures 7d and 7e refer to the system AlO(OH) milled together with 1 wt % Fe(NO3)3 · 9H2O for 50 h and subjected to TA under N2/10% H2. Two observations can be made: (i) the major part of the Fe3+ component has been reduced to give Fe3O4 or Fe(0) (see ref 14) and yields a broad intensive line. It is overlapped by the signals of Fe3+ in the distorted corundum matrix (cf. Figure 7b); and (ii) proving the existence of trapped NO2 was impossible. As a consequence, exothermal DTA signals can be expected only from matrix sites that were not in contact with the Fe component. The thermal decomposition of the nitrates under N2/10% H2 proceeds differently for the Al and Fe compounds. In the case of Fe3+, after reduction by H2, both a NO trapping by Fe2+ species (not detected by ESR here) and a deactivation of the AlO(OH) surface by formation of the prephases (FexAl1-x)2O314 have to be expected. The findings presented above demonstrate that the ESR of Fe3+ yields valuable information about local sites of the amorphous matrix prior to and after the thermal treatment by TA. For an almost comprehensive understanding of the results presented here, the trapped NO2(tr) is an essential intermediate that has already been attributed to spectral characteristics in Figure 4a. Figure 8 shows two spectra with the typical signal shape for NO2(tr) that reflects the immobilization of the paramagnetic species at 77 K. A thermal treatment up to 500 °C reduces the intensity of the NO2(tr) signal to ∼1/7 if the TA is performed under N2/10% H2. This means that after the thermal liberation of NO2 from the traps and starting the chain reaction via H2/Pt activation, the overall reaction produces exothermal DTA signals. This is obviously not the case for TA under N2. The simulation of the spectrum in Figure 8b shows an acceptable coincidence (cf. Figure 8c), and the obtained values correspond well to literature data.16 The NO2 molecules are sufficiently mobile at 293 K. The anisotropy of g and A is almost negligible due to the molecular reorientation, and three broad 14N-hfs lines remain (together with a broad background). This yields a hfs constant of A(14N) ) 5.38 mT, that is, a value that agrees fairly well with the mean value of the hf components obtained for the spectrum of the almost immobilized species at 77 K. The obviously chaotic movement of the NO2 molecules favors stabilization in “cages” of the amorphous matrix rather than by adsorption at the activated matrix surface. This interpretation is supported by the rather high ”thermal stability” of these trapped NO2 molecules, which are observable after thermal treatment up to ∼600 °C. On the Reaction Mechanism Causing the Sharp Exothermal DTA Peaks. As in the experiments presented here, hydrogen causes an exothermal water release only in the presence of Pt, it can be assumed that the mechanism of the reaction of H2 + NO2 yielding H2O + NO in contact with the system H2/Pt is of radical character. The chain start (eq 8) proceeds via the reactions according to eqs 1-4. It is interesting in this concern to compare the rate constants of the reactions reported in ref 17 (see below). It turns out that the reaction of activated Hads with NO2 (eq 6) is much faster than eq 8 and can start at comparably low temperatures, that is, remarkably

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Figure 7. ESR spectra (77 K) of the originally crystalline sample of AlO(OH), 50 h milled, showing the influence of the TA heating run under N2/10%H2 and the spectral characteristics of locally formed corundum, (R-Al2O3:Fe3+, marked +): (a) prior to the TA heating run; (b) after the heating run; (c) like panel b but in the range of g′ ≈ 2, showing only the reference signal (*) together with a broad fine structure of Fe3+ in corundum; (d) overview spectrum for {AlO(OH)cryst + 1 wt % Fe(NO3)3 · 9H2O}, 50 h milled, after the heating run; (e) like panel d but in the range of g′ ≈ 2 with information similar to panel c.

lower than 560 °C. It is noteworthy that the exponential NO2(g) consumption with time, measured in ref 17, corresponds quite well to the formation rates of NO and H2O. The same features could be observed in the study presented here during the TA of the nitrate-containing systems under N2/10% H2 (see, e.g., Figure 5). Therefore, both kinds of experimental findings confirm the almost stoichiometric character of the reaction of H2 with NO2. The H2/Pt Interaction and the Chain Reaction Start H2 + NO2. For a better understanding of a possible mechanism as discussed before, it seems helpful to comment on the reactive interaction of H2 with Pt. Even if the oxidation of H2 at Pt is a matter of fact since the time of Do¨bereiner, essential steps of the mechanism, such as the nature of the intermediates or the temperature dependence of the activation energy, are not completely clear.18 If an oxygen-loaded Pt surface, for instance, is in contact with a H2 atmosphere, the following reactions are reported.19

H2 S 2Hads 2Hads + Oads S H2Oads

(1) (2)

With increasing temperature, H2O will be desorbed and can no longer participate in subsequent autocatalytical reactions. Adsorbed OH is formed via the following reactions.

Oads + Hads S OHads Oads + 2Hads S H2Oads

(3) (4)

On the basis of these mechanistic considerations, various active species, especially Hads at the Pt surface, should also be able to

start chain reactions in the surrounding sample at temperatures of (250 ( 100) °C. Concerning the reaction of H2 with NO2, the chain reaction sequence proposed in ref 17 yields a description as follows: H2 + NO2 S H2O + NO

(5)

k ) 1.32 × 10 exp(-3.62 × 10 ⁄ RT)

NO2 + H S NO + OH

14

2

(6) H2 + OH S H2O + H

k ) 2.16 × 108 T1.51 exp(-3.43 × 103 ⁄ RT)

(7) The start reaction, according to eq 8, H2 + NO2 S HONO + H

k ) 7.32 × 1011 exp(-2.68 × 104 ⁄ RT)

(8) proceeds without a catalyst sufficiently fast at ∼560 °C and is considered to be the primary source of the needed radicals, that is, it directly produces the necessary radicals and indirectly produces the OH radicals via the decomposition of (HONO + M) where M represents a further reaction partner for the removal of the energy. Much larger differences of the rate constants and an activation energy of zero for the reaction with H have been reported in ref 20; for reaction 6: k ) 8.4 × 1013 T0 exp(0/RT). for reaction (8): k ) 1.3 × 104 T2.8 exp(-2.98 × 103/RT).

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exothermal signals as well, but they are not related to trapped NO2 and have to be attributed to the reaction of activated OH groups with activated H2. Accompanying the TA measurements, ESR spectroscopic investigations focusing on the Fe3+ impurities provided an interesting local picture of the matrix changes, for example, the mechanically induced local formation of R-Al2O3. The observed exothermal DTA effects are related to: (i) strong mechanical activation, including the formation of nanoparticles; (ii) stabilization by an amorphisized matrix and subsequent liberation of NO2; and (iii) access of H2 to the solid sample and its interaction with Pt (as crucible material or as finely powdered additive to the solid). The exothermicity results from a radicalic water formation where activated matrix constituents participate, too. Direct experimental proof of the existence or of the direct reaction of H · or OH · at the matrix sites could not be found but could not be excluded as well. On the basis of literature data concerning the reaction both of H2 with Pt and of H2 with NO2, we suggest that the start of the radical chain reaction H2 + NO2 proceeds cooperatively with activated H2/Pt. The experiments were comparably easy to perform. It seems to be promising to extend the described experimental approach to other systems as it allows the formation of strongly activated or especially structured (e.g., nano-) systems. Further attempts for finding a direct experimental proof of the hypothesis presented here are on-going and will be the subject of a following paper.

Figure 8. ESR spectra of Al(NO3)3 · 9H2O, 2 h milled, after the TA heating run (in Pt): (a) under N2/10% H2; (b) under N2; and (c) simulation of spectrum in panel b using the program SimFonia (Bruker) and the following parameters: ga ) 2,0062; gb ) 2,0019; gc ) 1,9910; Aa ) 5,23 mT; Ab ) 6,72 mT; Ac ) 4,84 mT; ∆Ba ) 0,45 mT; ∆Bb ) 0,45 mT; and ∆Bc ) 0,55 mT.

Simple estimations of the reaction enthalpies for reactions 6 and 8 reveal that both reactions are exothermal, but reaction 6 is favored by a factor of 4. From the discussion presented above, one can deduce the following. If the reactants H2 + NO2 have access to activated H (i.e., Hads/Pt), the chain reaction 6 is clearly favored compared to reaction 8. Otherwise, the observed exothermal DTA effects should have been detected in H2 also in the absence of Pt. A direct indication of the transport of reactive species such as H · , OH · , etc. into the sample bulk has not been found until now. Possibly, the exothermal formation of H2O proceeds at strongly activated sites of the originally crystalline AlO(OH) without nitrate additives via a direct activation of H2 at matrix defects. Conclusions One can summarize that the narrow exothermal DTA peaks, observed during the TA of mechanically activated xerogels, indeed represent an exothermal water release. This is globally the result of a “net effect” based on the reaction of activated H2 with NO2 to yield H2O and NO. The global effect has been studied and clearly proved by the hitherto seldomly applied combination TA-MS/ESR. Very strongly mechanically activated Al-OH phases, interacting with the system H2/Pt, show these

Acknowledgment. The authors are grateful for the kind experimental and apparative support by Dr. Marianne Nofz (Bundesanstalt fu¨r Materialforschung and -pru¨fung, Berlin) and by Dr. Werner Hermann (Freie Universita¨t Berlin) for the possibility to discuss our results. References and Notes (1) Nofz, M.; Sto¨βer, R.; Scholz, G.; Do¨rfel, I.; Schulze, D. J. Eur. Ceramic Soc. 2005, 25, 1095–1107. (2) Sto¨βer, R.; Scholz, G.; Buzare´, J.-Y.; Silly, G.; Nofz, M.; Schulze, D. J. Am. Ceram. Soc. 2005, 88, 2913–2922. (3) Sto¨βer, R.; Nofz, M.; Feist, M.; Scholz, G. J. Solid State Chem. 2006, 179, 645–657. (4) Scholz, G.; Do¨rfel, I.; Heidemann, D.; Feist, M.; Sto¨βer, R. J. Solid State Chem. 2006, 179, 1119–1128. (5) Sto¨βer, R.; Scholz, G. Appl. Magn. Reson. 1997, 12, 167–181. (6) Scholz, G.; Sto¨βer, R.; Silly, G.; Buzare´, J.-Y.; Lallignant, Y.; Ziemer, B. J. Phys.: Condens. Matter 2002, 14, 2101–2117. (7) Emmerich, W.-D.; Post, E. J. Therm. Anal. 1997, 49, 1007. (8) Kaisersberger, E.; Post, E. Thermochim. Acta 1997, 295, 73. (9) Hill, J. O. For Better Thermal Analysis III, Special ed.; International Confederation for Thermal Analysis (ICTA): 1991. (10) Weill, J. A.; Bolton, J. R. In Electron Paramagnetic Resonance, 2nd ed.; Wiley & Sons: New York, 2007. (11) Kno¨zinger, H.; Ratnasamy, P. Catalysis ReViews 1978, 17, 31–70. (12) (a) Schrader, R. et al. Z. Anorg. Allg. Chem. 1967, 350, 120–129. (b) 130-136. (c) 137-142. (13) Sto¨βer, R.; Nofz, M.; Geβner, W.; Schro¨ter, Ch.; Kranz, G. J. Solid State Chem. 1989, 81, 152–164. (14) R. Sto¨βer, M. Feist, M. Menzel, G. Scholz, M. Nofz, in preparation. (15) Scholz, G.; Sto¨βer, R.; Krossner, M.; Klein, J. Appl. Magn. Reson. 2001, 21, 105–123. (16) Jomack, M.; Baberschke, K. Surf. Sci. 1986, 178, 618–624. (17) Mueller, A.; Gatto, J. L.; Yetter, R. A.; Dryer, F. L. Combust. Flame 2000, 120, 589–594. (18) Wintterlin, J. AdV. Catal. 2000, 45, 131–206. (19) Vo¨lkening, S.; Bedu¨rftig, K.; Jacobi, K.; Wintterlin, J.; Ertl, G. Phys. ReV. Lett. 1999, 83, 2672–2675. (20) Park, J.; Giles, N. D.; Moore, J.; Lin, M. C. J. Phys. Chem. A 1998, 102, 10099–10108.

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