Exothermal Water Release from Pseudoboehmite Gels and Their Mechanically Treated Analogs Caused by Activated Hydrogen

Abstract

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 NO 2 molecules as well as to follow the mechanically and thermally induced changes of the Al-O-H matrix via the contained (or doped) Fe 3+ 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 H 2 and NO 2 yielding H 2 O 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.

Fe3+ Ions Acting as Probes and Agents in Aggregation Processes and Solid‐State Reactions in AlO(OH)/Al2O3 Matrices

“…The exothermal peak at ∼267°C in curve c (N 2 /H 2 ) is somewhat unusual and represents an exothermal water release. This peak is related to the reaction of “NO 2 precursors” present in the sample 13 . HNO 3 was used as a peptizing agent in the sol–gel process, which leads to remaining traces of NO 3 − in the xerogel 7 .…”

“…It is noteworthy in this context that the DTA curves 2c and 2f exhibit a comparably stronger exothermicity above 600°C under N 2 /H 2 , which may indicate the participation of activated hydrogen H 2 (Pt) in the reaction of the transforming matrix. As discussed comprehensively in, 13 the formation of activated hydrogen H 2 (Pt) takes place due to the utilization of platinum crucibles for the TA measurements in an N 2 /H 2 atmosphere. When working with corundum crucibles, the effect disappears, but can be restored by adding Pt powder to the sample.…”

“…The present study focused on the T P shift obtainable by thermal analysis (TA) runs and on specific chemical reactions in order to establish the activation. This approach was developed previously 13 and is based (i) on exothermal water release due to a reactive interaction of activated hydrogen (H 2 /Pt) with trapped NO 2 molecules and/or activated surfacially bound OH groups and (ii) on the surface reaction of Fe species (e.g., Fe 2 O 3 , etc.) that can be followed precisely by applying, e.g., electron spin resonance (ESR) and Mössbauer spectroscopies 10 .…”

“…A combination of the methods of TA with Mössbauer and ESR spectroscopy is expected to provide an insight into the mechanochemically (comilling) and thermally induced aggregation and solid‐state reactions in the iron subsystem. Based on previously obtained results 7–9,11,13 , the following problems will be dealt with:…”

The fate of Fe3+ ions in the system {AlO(OH)-xerogel/Fe-compounds} after mechanical activation and different thermal treatments studied by Mössbauer, ESR spectroscopy and thermal analysis

“…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) without smoothing etc. For details of the temperature evaluation concerning the onset, T on , extrapolated onset, T ex on , and peak temperatures, T P , of the characteristic temperatures, as well as for discussing DTA curve shapes and base line variations see [4] and the literature cited there.…”