Mesoporous boron nitride in contact with water - chemical stability and adsorption properties
Description
In this research project we synthesized two different porous boron nitrides - a mesoporous one (BN-meso) and a reference micro-mesoporous one (BN-ref). Our goal was to identify the influence of different pore sizes towards the chemical stability of these materials. We focussed mainly on water, as it is present in a significant amount of industrial adsorption applications. The samples were exposed to humid air, water vapor and liquid water for multiple weeks. Prior to and after this exposure we characterized the materials with XRD, XPS, SEM, mercury intrusion, ATR-IR, TG and nitrogen sorption. Additionally, we calculated pore size distributions using QSDFT for the adsorption branch of nitrogen sorption at 77 K on carbon with slit and cylindrical pores. Based on the water vapor adsorption isotherms and pore size distributions we conclude that mesoporous BN ist stable against water in the vapor phase. Micro-mesoporous BN is stable neither towards vapor nor liquid water.
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Synthesis of mesoporous BN: Urea (Alfa Aesar, 98+%) and boric acid (VWR Chemicals, 99+%) in a molar ratio of boric acid to urea 1:3, both precursors homogeneously ground in ball mill (PM 100, Retsch, 450) for 5 minutes (450 rpm). Mixture transferred to an alumina combustion boat crucible and heated in a tube furnace (ROC 50/610/14, Thermconcept). Two heating levels: 200 °C for 2 h followed by 1300 °C for 4 h with a heating rate of 5 K min -1. Nitrogen atmosphere (purity 5.0) at a constant flow rate of 80 ml min -1. Synthesis of micro-mesoporous BN: Urea (Alfa Aesar, 98+%) and boric acid (VWR Chemicals, 99+%) in a molar ratio of boric acid to urea 1:3, both precursors homogeneously ground in ball mill (PM 100, Retsch, 450) for 5 minutes (450 rpm). Mixture transferred to an alumina combustion boat crucible and heated in a tube furnace (ROC 50/610/14, Thermconcept). 1300 °C for 4 h with a heating rate of 5 K min -1. Nitrogen atmosphere (purity 5.0) at a constant flow rate of 80 ml min -1. Air stability: Sample was transferred to a 50 mL polypropylene (PP) beaker, stored at room temperature in a fuming hood (humidity ~ 30%). Water stability: approximately 0.16 g of the material transferred in a 50 mL PP beaker at room temperature. 40 mL of deionized water and a stirrer added. After closing the beaker, the mixture was stirred at 500 rpm. Filtrate washed with 50 mL deionized water and dried in an oven at 80 °C for 24 h (DRY-Line, VWR). XRD: STOE STADIP (STOE & Cie GmbH), Mythen 1K detector (DECTRIS), Cu Kα (40 kV, 40 mA), step-width of 0.2 °2θ s-1, spectra evaluated by Match! (Version 3.3.0, Crystalimpact). ATR-IR: Alpha II FT-IR spectrometer from Bruker, platinum ATR module with diamond crystal, 5 to 10 mg powdered sample on the crystal, measuring between 500 to 4000 cm-1 (resolution of 4 cm-1), scan replied 20 times. XPS: VG ESCALAB 220i-XL measurement device (Thermo Scientific), Al-anode (12 kV, 20 mA, 240 W), Al Kα radiation, vacuum (10-8 mbar), electron binding energy range of - 5 to 1205 eV, step size of 0.5 eV (survey scan) and 0.1 eV (detail spectra) at pass energy of 50 eV, scans repeated 4 times. Samples mounted on conductive carbon tape, Unifit 2022 used to evaluate spectra. Peaks fitted by Voigt profiles. Excitation satellites and a suitable background have been subtracted. Mercury Intrusion: Pascal 140 and Pascal 440 by Porotec, 50 to 100 mg sample, intrusion up to 400 MPA at room temperature. Contact angle set to 140° and surface tension to 0.48 N m-1. Water vapor adsorption: autosorb iQ3 (Quantachrome Instruments), 25 °C, pressure range of 0.0 < p p0 -1 < 0.9, samples outgassed for at least 4 h up to 6 h under vacuum at 150 °C. Nitrogen adsorption: autosorb iQ3 (Quantachrome Instruments), 77 K. Samples outgassed in the same manner as for water vapor adsorption.