Heterogeneous catalysis Lecture 6 Catalysts characterization - continuation Catalyst characterization •Outline –Special MAS NMR techniques –Hydrophilicity vs. Hydrophobicity meas •Water sorption •Dynamic water sorption •Inverse gas chromatography •Microcallorimetry •Quartz crystal microbalance –In situ and operando techniques Special MAS NMR techniques •MQMAS •DNP Hydrophilicity vs. Hydrophobicity •Why? –Remember: Adsorption/desorption step in cata as important as cata rxn itself (@Lecture 1) –Moreover: strong efforts nowadays put forward bio-sources instead of fossil fuels –Compare •Oil – long hydrocarbon chains (hydrophobic) •Wood – cellulose, lignin = sugar based materials = a lot of oxygen, OH groups (hydrophilic) Hydrophilicity vs. Hydrophobicity •Possibilities: –Water sorption –Dynamic vapor sorption/Quartz crystal microbalance –Inverse gas chromatography –Microcallorimetry Hydrophilicity vs. Hydrophobicity •Problems: –What is a measure of hydrophobicity? •A material can have high/low affinity to both water and organic molecules (i.e. if a sample is hydrophilic, it does not necessarily mean it is hydrophobic!) –% of pore volume filled with water at certain p/p0 –Hydrophobic index (water/toluene competitive sorption) –Heat of adsorption, heat of immersion –… –Chemisorption: Do chemisorbed molecules account for hydrophobicity/philicity? Hydrophilicity vs. Hydrophobicity •Water sorption –Similar to N2 physisorption –Known volume and pressure in the cell, known mass of the sample Hydrophilicity vs. Hydrophobicity •Water sorption Very strong interaction water-adsorbent Very weak interaction water-adsorbent Water clustering = water-water interaction (liquefaction, condensation) stronger than water-adsorbent Hydrophilicity vs. Hydrophobicity •Water sorption –Plotting ln p against 1/T at constant adsorption uptake gives a straight line with a slope equal to Hiso/R –Hiso = isosteric heat of adsorption –Isotherms at multiple temperatures needed! Hydrophilicity vs. Hydrophobicity •Water sorption –Hiso = isosteric heat of adsorption Hydrophobic material = very low Hiso = negligible interaction water-adsorbent Hydrophilic material = very high Hiso = strong interaction water-adsorbent Hiso more or less constant = water-water interaction (condensation) Hydrophilicity vs. Hydrophobicity •Dynamic vapor sorption/Quartz crystal microbalance –You deposit your material on an accurate microbalance –You expose it to vapors of different gases/liquids (water, alcohols, hydrocarbons,…) –You follow the uptake by changes of mass –If we can control/follow pressure, then isotherms can be obtained similar to a classic physisorption • Hydrophilicity vs. Hydrophobicity •Inverse gas chromatography –You pack column (≈ 50 cm) with the material you want to test (≈ 0.5 g) –You inject series of gases/liquids (e.g. methane, ethane,…hexane, heptane; methanol, ethanol,…; benzene, toluene, xylene…) –You follow retention time (you directly see „affinity“ of your material to selected liquids) –Models (math) can give surface energy,… Hydrophilicity vs. Hydrophobicity •Immersion microcalorimetry –Evacuated sample sealed in a bulb with brittle end –Bulb immersed in a testing liquid, sealed –Bulb broken (rod pushed down) –Liquid gets into the bulb, adsorbs, heat of immersion released and measured Hydrophilicity vs. Hydrophobicity •Immersion microcalorimetry –Ti-MCM-41, pure inorganic vs. increasing degree of surface silylation (increasing carbon content) J. Silvestre-Alberó et al. / Applied Catalysis A: General 507 (2015) 14–25 In situ and operando techniques •In situ = online analysis of a working catalyst •Operando = online analysis of a working catalyst at relevant conditions (p, T, WHSV) In situ and operando techniques •We already know these techniques, let‘s look at the examples! –Low energy electron diffraction (LEED, gives similar results to x-ray diffraction = analysis of crystal structures) –X-ray absorption near edge structure (XANES) –Diffusive reflectance infrared Fourier transform spectroscopy (DRIFTS) In situ and operando techniques •Example: LEED – low energy electron diffraction •Ethylbenzene dehydrogenation to styrene –600 °C, 1 atm, 10-fold excess water vapor –Over Fe2O3 epitaxially grown on Pt(111) •Flow reactor located in a high pressure cell –Heated by lasers –GC-MS analysis of catalytic products –LEED analysis enabled by gate valve (high pressure/ultra high vacuum) and sapphire window – – • In situ and operando techniques •Example: LEED – low energy electron diffraction – • •Before catalytic reaction • After catalytic reaction Initial period, no catalytic activity Working catalyst In situ and operando techniques •Example: LEED – low energy electron diffraction – • Active catalyst Non-crystalline Fe2O3 Metastable (Reduction!) Carbon deposition (Styrene!) Loss of catalytic activity 0.5 eq O2 addition In situ and operando techniques •Example: XANES – x-ray absorption near edge structure •Methanol oxidation to formaldehyde over Cu –25–450 °C, 1 mbar –Cu in the form of polycrystalline foil – –2 CH3OH + O2 → 2 CH2O + 2 H2O – • In situ and operando techniques •Example: XANES – x-ray absorption near edge structure – –2 CH3OH + O2 → 2 CH2O + 2 H2O – • In situ and operando techniques •Example: XANES – x-ray absorption near edge structure • 2 oxide and 1 suboxide species In situ and operando techniques •Example: XANES – x-ray absorption near edge structure • •Positive correlation between catalytic activity and Cu suboxide species •Explanation/idea/description of active species: Subsurface oxygen cover by a strained layer of copper atoms •Results confirmed by near ambient pressure XPS (NAP XPS) In situ and operando techniques •Example: DRIFTS – diffuse reflectance infrared Fourier transform spectroscopy •Coupling of ethanol and acetaldehyde to 1,3- butadiene over Ta doped zeolite –300 °C, 1 atm –Well dispersed (virtually isolated) Ta sites – –2 C2H5OH + CH3CHO → C4H6 + 2 H2O – • In situ and operando techniques •Example: DRIFTS – diffuse reflectance infrared Fourier transform spectroscopy • In situ and operando techniques •Example: DRIFTS In situ and operando techniques •Example: DRIFTS – diffuse reflectance infrared Fourier transform spectroscopy •