Homework 4 -ro /c-o rt Gas phase o Ti02/Si02 o o M transesterification -* ...... oh -2 MeOH 0 °, • Continuous flow fixed un (L,L)-LD bed reactor Temperature [°C] Conversion sample 1 (4.2 wt% Ti) [%] Conversion sample 2 (0.9 wt% Ti) [%] 220 ^^^^^^^^^ 230 48 9.6 240 59 13 250 82 17 260 93 21 Selectivity = 90 % in all cases; WHSV = 15.5 h"1 ACS Cotol. 2018, 8, 9, 8130-8139 Create Arrhenius plots for both catalysts (plot In k [mo|Lp Scat"1 h_1] vs-1000/T [K"1] Estimate apparent Ea and A from Arrhenius equation What makes the difference between these two catalysts? Heterogeneous catalysis (C9981) Lecture 5 Catalysts characterization styskalik@chemi.muni.cz styskalik.sci.muni.cz 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) Diffuse reflectance UV-Vis spectroscopy (DRUV) Ultraviolet Visible Near Infrared 200 400 800 2,500 —(nm) v(cnrr1)-«— 50,000 25,000 12,500 4,000 E(eV) -«— 6 3 1.5 0.5 LMCT and MLCT ^_ Vibration overtones MMCT anc' combinations (intervalence) d—d transitions • LMCT = ligand-to-metal charge transfer • MLCT = metal-to-ligand charge transfer • MMCT = metal-to-metal charge transfer Diffuse reflectance UV-Vis spectroscopy (DRUV) 1.2- 1.0- 245 o c 300 280 V content (V atoms / nm2) -2% (0.6) -4% (1.4) -6% (2.1) 8% (3.1) -14%1(13.7) 300 400 I (nm) v o 2 5 500 Isolated and polymerized VxOy? TABLE 4: Band Maxima and Edge Energies of V-Reference Compounds band max. 5 compounds (nm) (cV) molecular structure3 v2o5 236,334,481 2.3 polymerized VOJVOt, 250,370 2.8 polymerized VO* (mcta-vanadate) NaVO; 281,353 3.2 polymerized VO4 (mcta-vanadate) NH.VO. 288,363 32 polymerized VO., (mcta-vanadatc) Mg:V:0- 280 3.5 dimeric VO-. (pvro-vanadate) Mg.V:0, 260.303 3.5 isolated VO, (ortho-vanadate) Na.VOj 253,294 3.9 isolated VO4 (ortho-vanadate) Gao era/., J. Phys. Chem. B 102 (1998) 10842. i-iorrer monomer, hydroxylated di-iei O O 600 0 0 OH 0 OH v V V °° 0 0 ? 0 OH d rror oligomer 0 O O O v V V v 0 0 0 0 0 s s n 1 0 0 o ignnors undo- stnncy state conditions O o o I liv o M v V Ionization Spectroscopies Photoelectron spectroscopy: UV: valence shell ionizations X-ray: core electron ionizations (XPS, XANES, EXAFS): ^photon BE + kinetic energy (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 continuum virtual levels valence levels core levels X-ray photoelectron spectroscopy (XPS) We measure the number and the energy of photoelectrons emitted from the surface layer "-photon 11 v BE + kinetic energy (KE) • Number of photoelectrons gives access to quantity • Energy of photoelectrons gives access to quality • Depth? 5-20 nm. ^/) -1—• c o o CM O X-ray photoelectron spectroscopy (XPS) Survey 45j 40. 35. 30. 10j 5j Name Pos. At% Co 2p3 781.23 4.376 0 Is 531.23 21.759 C Is 284.23 73.866 Auger a 1000 800 CO p7 j Co 600 400 Binding Energy (eV) 200 Universitě catholique de Louvain (Belgium) - Surface Characterisationplattorm (SUCH) lei +51 (U)l0 47 36 64 - CasaXPS X-ray photoelectron spectroscopy (XPS) Surface vs. bulk composition ± < + 4- 3- 2- 1 - K. Bouchmella et al. /Journal of Catalysis 301 (2013) 233-241 Surface Bulk Calcination 5?* Figure 6. Surface Re/(Si + Al + Re) atomic ratios (XPS, gray bars) and bulk Re/(Si + Al + Re) atomic ratios (EDX, black squares) for xerogels and calcined catalysts. Reproduced with permission from [177]. Copyright Elsevier, 2013. X-ray photoelectron spectroscopy (XPS) Oxidation state C° c C" c"1 civ 1000 0 Binding energy [eV] Who will hold its electrons more powerfully? C° or CIV? X-ray photoelectron spectroscopy (XPS) • Oxidation state C° c C" c1" civ 1000 0 Binding energy [eV] • 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 292 290 288 286 284 282 280 binding energy (eV) X-ray photoelectron spectroscopy (XPS) Similar consideration: Si04 vs. CSi03 110 106 104 102 100 Binding energy (eV) Electronegativity (O) = Electronegativity (C) = Ta(OSi)x(OTa)6.xvs.Ta(OTa)6 CO ecira ( XANES) obtained From M>lid& containing vanadium. (]>] Schematic energy level diagram flu vanadium, The transitions are from the K-slieJ] to empty valence smiw. XANES involves transition? ir> vjil*nce otitis and trtti thus he used Lo probe chemical bonding. XANES 49G9 4970 4971 4972 ABSOLUTE POSITION (eV) FIG. 2. Normalized pre-edge height vs energy position for Ti AT-pre-edge features in model compounds listed on Table I showing three domains for fourfold, fivefold, and sixfold coordinated Ti. 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 aieis uo!iep!xo'"Á§J8U8 §U!pu!q'"a§pa aqip uoiijsod • S3NVX EXAFS Energy EXAFS • The scattering phenomena 1 Back-scattering from "neighbors" Step 1: The data Step 4: Develop structural model that fits that data d(M-X), CN(X), AtNum(X) EXAFS Step 2: Extract %(k) Math ro I ° -t—' u < cj ' cd H-' CO u Optimal binding Desorption limited strong "Bond strength" weak Sabatier's principle; Volcano plot@Lecture 1 (a) (b) dibenzothiophene 2 c o CD c o V— o 03 CD MoS, Periodic group RS! H-h Heat of adsorption Figure 2.15 Examples of volcano plots, describing the reaction rate as a function of the heat of adsorption (left), and the activity of the second-row and third-row transition metal sulfides in the hydrodesulfurization of dibenzothiophene (right). 10 Chemisorption Different sites = different probes - Acid sites • NH3, alkylamines, pyridine, 2,6-dimethylpyridine, CO - Basic sites • C02 - Redox • H2, 02, N20 - Reactants can be used as probes • e.g. ethanol dehydration = I can study chemisorption 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 60 □_ h-C/) 50 I 40 "O CD _q 30 Ö C/) "O CO 20 CD E "5 10 > 0 Uptake physisorption + chemisorptii physisorption only (chemisorbed NH3 already there) 0 0.02 0.04 0.06 0.08 NH3 pressure / MPa Temperature? Ist isotherm evaci jation 2nd isotherm Difference ]_st _ 2nc| 0.1 chemisorbed NH Chemisorption • Thermally programmed desorption/oxidation/reduction Furnace Chemisorption • Thermally programmed desorption/oxidation/reduction Time Chemisorption • Thermally programmed ... Temperature / "C Chemisorption • Pulse titration VENT KYDROGEN Fl«. 1. Schematic «>f the tt»n» *yMcni. V - "Moore" llnw controller, N.V - iirt-illr vhIv»\ I) = ih«*inuti fioiiUut-tivity detevtur, B m glc baefcrhl»h »r»1v*, I » ft-jw»rt **V»r»An" *Amplr-injrrtnr S — MmpU loop, 1( — rotameter. The t«etup for hydrogen adsorption is* »hown, Prereduction vu carried mil with the valve Ii in il* alternate position Oxvgttu rri*-int-»*rplioii wm> -IiiJu«! hy rrpUcing the ri\ o CQ c o 0) cr Cu Ref. unaged ex-LDH binary conv. I conv. II 0 0.000 250 °C 210 °C 0.005 0.010 0.015 Stacking fault probability 0.020 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 cata 3 CD CO c > TO GC 1700 1600 1500 1400 1300 Wavenumber [cm 1] 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 + H20 - 2 CH4 + 02-> C2H6 + 2 H20 H6--C6+H, -1-1--- Mg O Mg O Mg O Angew. Chem. int. €d. 2015, 54, 3465-3520 Chemisorption • Example: CO adsorption on MgO Chemisorption Example: CO adsorption on MgO (02-2) {00-2) 2 nm 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 - AI02" is a function of Al concentration in top 1 nm - AISi03" is a mixed Al-Si cluster (good Al dispersion) - Al204" signifies badly dispersed Al species AISi03/AIQ2_ _AI2Q47AIQ2 -.-1-«- - AI-8SiPhSi-Et AI-8SiPhSi-Ac AI-8SiPhSi-Et AI-8SiPhSi-Ac Catalyst characterization • Outline - DRUV -XPS - XANES & EXAFS - Chemisorption -ToF-SIMS