Heterogeneous catalysis Lecture 5 Catalysts characterization Catalyst characterization •Outline –DRUV –XPS –XANES & EXAFS –Chemisorption –ToF-SIMS Diffuse reflectance UV-Vis spectroscopy (DRUV) •Is there any link between reflectance and absorbance? •Yes, there is! •Kubelka-Munk model allows to obtain quantitatively the absorption spectrum of a solid from diffuse reflectance measurement (theory behind in textbooks) I J Diffuse reflectance UV-Vis spectroscopy (DRUV) •LMCT = ligand-to-metal charge transfer •MLCT = metal-to-ligand charge transfer •MMCT = metal-to-metal charge transfer Diffuse reflectance UV-Vis spectroscopy (DRUV) Photoelectron spectroscopy: UV: valence shell ionizations X-ray: core electron ionizations (XPS, XANES, EXAFS): valence levels core levels virtual levels ionization continuum Ephoton = hν = BE + kinetic energy (KE) 0 BE KE Photoelectron Spectroscopy analyzes the energies of the ionized electrons (XPS). X-ray Absorption Spectroscopy analyzes the absorption curve of the X-ray spectrum associated with ionization of a core electron (XANES and EXAFS) Ionization Spectroscopies X-ray photoelectron spectroscopy (XPS) •We measure the number and the energy of photoelectrons emitted from the surface layer Ephoton = hν = BE + kinetic energy (KE) •Number of photoelectrons gives access to quantity •Energy of photoelectrons gives access to quality •Depth? 5–20 nm. X-ray photoelectron spectroscopy (XPS) Auger X-ray photoelectron spectroscopy (XPS) •Surface vs. bulk composition Surface Bulk Calcination K. Bouchmella et al. /Journal of Catalysis 301 (2013) 233–241 X-ray photoelectron spectroscopy (XPS) •Oxidation state Binding energy [eV] 0 1000 C0 CI CII CIII CIV ? •Who will hold its electrons more powerfully? C0 or CIV? X-ray photoelectron spectroscopy (XPS) •Oxidation state Binding energy [eV] 0 1000 C0 CI CII CIII CIV •Why do we talk about carbon? •Surface of each sample contains some carbon impurities = Adventitious carbon = calibration of BE X-ray photoelectron spectroscopy (XPS) •Oxidation state Výsledek obrázku pro c 1s xps spectra X-ray photoelectron spectroscopy (XPS) •Similar consideration: SiO4 vs. CSiO3 Ta(OSi)x(OTa)6-x vs. Ta(OTa)6 Electronegativity (O) = Electronegativity (C) = Electronegativity (Si) = Electronegativity (Ta) = X-ray photoelectron spectroscopy (XPS) •Spin-orbital interaction and other electron interactions lead to additional features in XP spectra characteristic for –Oxidation states –Groups –… Characteristic for Mn2+ X-ray photoelectron spectroscopy (XPS) •Spin-orbital interaction and other electron interactions lead to additional features in XP spectra characteristic for –Oxidation states –Groups –… Characteristic for aromatic carbon Photoelectron spectroscopy: UV: valence shell ionizations X-ray: core electron ionizations (XPS, XANES, EXAFS): valence levels core levels virtual levels ionization continuum Ephoton = hν = BE + kinetic energy (KE) 0 BE KE Photoelectron Spectroscopy analyzes the energies of the ionized electrons (XPS). X-ray Absorption Spectroscopy analyzes the absorption curve of the X-ray spectrum associated with ionization of a core electron (XANES and EXAFS) Ionization Spectroscopies XANES and EXAFS •In XANES and EXAFS we measure number and energy of transmitted photons, which caused ionization of a core electron •(i.e. absorbance vs. energy) X-ray storage ring Iref Io It Ifluor Sample Monochromator Metal Reference Foil Synchrotron Energy needed is so much greater than that available from a typical X-ray tube found in most labs XANES and EXAFS •XAS: X-ray absorption spectroscopy; alternatively XAFS: X-ray absorption fine structure • –XANES: X-ray absorption near edge structure (pre-edge); alternatively NEXAFS (near edge x-ray absorption fine structure) – –EXAFS: Extended x-ray absorption fine structure (= behind edge) What is an edge? XANES and EXAFS valence levels core levels virtual levels Edge = 0 BE KE EXAFS XANES XANES and EXAFS XANES EXAFS XANES •Qualitative information on coordination environment (dipole selection rules apply) –Is there a pre-edge feature? •Yes – no inversion center •No – there is an inversion center •Quantitative info also possible •Figure from Farges, et. al. Physical Review B: Condens. Matter 1997, 56(4), 1809-1819. XANES •Coordination environment XANES •Position of the edge…binding energy…oxidation state EXAFS Eo XANES EXAFS •What type of information is hidden there??? Energy EXAFS •The scattering phenomena Scattering from Ti Back-scattering from “neighbors” EXAFS 0 5 10 15 k (Å-1) c(k) Eo EXAFS Step 1: The data Step 2: Extract c(k) Step 3: The FT Step 4: Develop structural model that fits that data d(M-X), CN(X), AtNum(X) Fourier Transform Math EXAFS — Ti(Si8O20)4 — FEFF 8 fit 5 0 10 0 1 2 3 4 5 6 Ti O O O O 0 1 2 3 4 5 6 R (Ǻ) 5 0 10 15 Ti O O Cl Cl Ti–O Ti∙ ∙ ∙ Si Ti–O Ti∙ ∙ ∙ Si Ti–Cl •Structure elucidation of an amorphous material! •If more Ti sites, than average; analysis more difficult! Chemisorption •Interactions of A,B, and P with catalyst not too weak, not too strong (= physi/chemisorption on catalyst surface) Sabatier‘s principle; Volcano plot@Lecture 1 sabatier_principle Chemisorption •Different sites = different probes –Acid sites •NH3, alkylamines, pyridine, 2,6-dimethylpyridine, CO –Basic sites •CO2 –Redox •H2, O2, N2O –Reactants can be used as probes •e.g. ethanol dehydration = I can study chemisorption of ethanol • Chemisorption •Different ways how to do it –Volumetric –Thermally programmed desorption/oxidation/reduction –Pulse titration –Chemisorption of IR (NMR) active molecules and their analysis by IR (NMR) Chemisorption •Volumetric –Similar to N2 physisorption (glass tube of known volume, addition of known volume of gas, pressure measurement) –NH3 adsorption isotherms measured twice, sample evacuated between the two measurements (physisorption vs. chemisorption) –High temperatures in contrary to N2 physisorption (e.g. 50 °C) Chemisorption •Volumetric 1st isotherm 2nd isotherm Difference 1st – 2nd = chemisorbed NH3 Uptake = physisorption + chemisorption Uptake = physisorption only (chemisorbed NH3 already there) evacuation Temperature? Chemisorption •Thermally programmed desorption/oxidation/reduction Výsledek obrázku pro catlab reactor Detector (MS, TCD) Furnace Chemisorption •Thermally programmed desorption/oxidation/reduction Time NH3 Ar Ar 150 500 700 Dehydration Pretreatment Desorption Analysis Chemisorption •Thermally programmed … Výsledek obrázku pro tpd nh3 temperature profile NH3 TPD Chemisorption •Pulse titration Výsledek obrázku pro pulse chemisorption scheme Chemisorption •Pulse titration Výsledek obrázku pro hydrogen pulse titration dispersion Chemisorption •Pulse titration –Correct choice of gas (H2, CO, O2, N2O) –Correct choice of temperature •= Monolayer •= Dispersion, particle size, „active metal surface area“ Number of surface atoms Total number of atoms Chemisorption •Pulse titration –Very good correlation can be observed between active surface area by H2 chemisorption and activity in hydrogenation of … for classical hydrogenation catalyts (Pt, Pd) –H2 chemisorption is dissociative! –H–H bond breaks at the time of chemisorption, this is the key to high reactivity (adsorbed molecular hydrogen) –This is Langmuir‘s chemistry Pt Pt Pt H H H Chemisorption •X2 dissociative chemisorption (detour) Pt Pt Pt H H H 2A, ad A2, ad A2 Ediss, noncatalyzed Ea O2 adsorption on Pt(111) surface @165 K O–O in O2 = 1.2Å, here much longer „Hot“ O atoms Lifetime 10–13 s Chemisorption •Pulse titration –Very good correlation can be observed between active surface area by H2 chemisorption and activity in hydrogenation of … for classical hydrogenation catalyts (Pt, Pd) –BUT •Example: methanol synthesis over Cu NPs –CO2 + 2 H2 → CH3OH Pt Pt Pt H H H Science 18 May 2012: Vol. 336, Issue 6083, pp. 893-897 Chemisorption •Example: methanol synthesis over Cu NPs Chemisorption •Example: methanol synthesis over Cu NPs Chemisorption •Example: methanol synthesis over Cu NPs Chemisorption •Pulse titration –Homework –Silver (2.2 wt% on silica) –Surface reaction: 2Agsurf + O2 → 2 AgOsurf @T = 170 °C (no bulk oxidation!) –msample = 0.5021 g Vpulse = 0.00925 cm3 O2@RT@1atm Max TCD signal = 0.022 –D = ? Pulse TCD signal #1 0 #2 0 #3 0 #4 0 #5 0 … … #28 0 #29 0 #30 0.002 #31 0.005 #32 0.011 #33 0.016 #34 0.019 #35 0.021 #36 0.022 Chemisorption •Adsorption of IR (NMR) active molecules –Pyridine and its derivatives –CO –Trialkylphosphine oxides –… Chemisorption •Adsorption of IR (NMR) active molecules –Number of sites • Lambert-Beer law –Strength of sites •Desorption under vacuum at different temperatures –Nature of sites •Different vibration modes Chemisorption •Adsorption of IR (NMR) active molecules •Example: pyridine adsorption on an acid catalyst Chemisorption •Adsorption of IR (NMR) active molecules •Example: CO adsorption on MgO, a catalyst that activates methane for oxidation, ideal products are ethane and ethylene –2 CH4 + 0.5 02 → C2H6 + H2O –2 CH4 + 02 → C2H6 + 2 H2O – Mg O Mg O Mg O Hδ-∙∙∙C δ+H3 Chemisorption •Example: CO adsorption on MgO Chemisorption •Example: CO adsorption on MgO Chemisorption •Example: CO adsorption on MgO •Single step edge is the active site –Good correlation between integral of absorption band at 2147 cm–1 and catalytic activity –Band at 2147 cm–1 represents single step edge –Au atoms selectively deposits on single step edges (observed by HR-TEM) = catalyst poisoning, active site „titration“ ToF-SIMS •Time of flight secondary ion mass spectrometry •Catalyst surface bombarded by „primary“ ions (e.g. Bi5+) •Surface atoms (top ≈1 nm) expelled by this bombardment forming charged clusters = secondary ions created •Analysis of secondary ions gives us information about catalyst surface ToF-SIMS •Surface composition –AlO2– is a function of Al concentration in top 1 nm –AlSiO3– is a mixed Al-Si cluster (good Al dispersion) –Al2O4– signifies badly dispersed Al species Catalyst characterization •Outline –DRUV –XPS –XANES & EXAFS –Chemisorption –ToF-SIMS