1. Catalyst characterization 0. General overview, and x-ray methods 1. BET, porosimetry, chemisorption 2. Temperature programmed methods Edd A. Blekkan, Dep. of Chemical Engineering Catalysis and Kinetics Group NTNU

2. Introduction Catalyst Characterization Heterogeneous catalysis: Transformation of molecules at the interface between a solid (catalyst) and the gaseous or liquid phase carrying these molecules We need to understand: –What is the composition of the catalyst bulk surface (catalysis is a surface phenomenon) –How does it change chemical reactions exchange of atoms between surface and bulk sintering and loss –How is the gas (liquid) phase changed (=kinetics) –What is the nature of the interface when reaction occurs adsorbed species, bonding with the surface, intermediates etc..

3. General scheme of characterization

4. General scheme: Techniques

5. X-ray diffraction X-rays –wavelengths in the Å range –high energy, can penetrate solids Diffraction (= elastic scattering of the photons) pattern can be used to study –identify phases in (crystalline) bulk solids –particles, particle size Bragg relation n =2dsin ,where n = 1,2,… (order) d = lattice spacing = wavelength  = angle between X-ray beam and the normal to the reflecting plane

6. Particle size measurement Diffraction line of a perfect, infinite crystal = narrow “spike” Smaller particles = line broadening Scherrer formula used to calculate particle size: whereL is the dimension of the particle is the wavelength  is the peak width  is the angle of reflection K is a constant, can be assumed to be 1 (This is a simplified analysis)

7. Example 1. Phase identification

8. Example 2. Supported metal particles

9. Contents part. 1: Techniques applied for studying surfaces Adsorption: general background and theory Total surface area –BET and other methods Pores and pore size distributions Specific surfaces (chemisorption methods) –background –dispersion –techniques Examples of applications

10. Fundamentals: adsorption Adsorption precedes catalysis Definition (Thomas): “the preferential accumulation of material - the adsorbate - at a surface” Adsorption is distinguished from absorption –adsorption: gas uptake (at fixed T and P) is proportional to the surface area –absorption: gas uptake (at fixed T and P) is proportional to the volume of the material: –not always a clear distinction: intercalation of species between layers can sometimes generate more internal area (e.g. clay minerals, graphite) highly micro-porous materials with cavities with molecular dimensions (e.g.zeolites)

11. General classification of adsorption Tabell fysikalsk vs. kjemisorpsjon Richardson tab 7.3 side146

12. Lennard-Jones diagram The figure depicts the energies associated with a molecule approaching a surface Due to physisorbed precursor state activation energy for chemisorption is low non-activated if crossover X is below potential energy zero Fig 2.2 T&T side 67

13. T&T fig 2.3. Side 68 Sticking Sticking coefficient: the probability of a collision with the surface leading to adsorption s can be very low (10 -15 )

14. Isotherms and isobars Equilibrium distribution of adsorbate molecules between surface and gas phase is –function of temperature –function of gas pressure –function of the nature and area of the adsorbent –nature of the adsorbate Isotherm: amount adsorbed at equilibrium as f(P) at constant T Isobar: amount adsorbed at equilibrium as f(T) at constant P Isostere: Relation between T and P at equilibrium for a given amount of adsorbate T&T Fig 2.20 s.79

15. Brunauer classification of adsorption isotherms Empirical observation: 5 types of isotherms Most systems are “Type I” T&T fig 2-21 s. 80 el. tilsv

16. Adsorption isotherms Equations describing isotherms are available Many can be derived theoretically (e.g. BET, Freundlich, Temkin) using assumptions about the heat of adsorption T&T Tab. 2-1 s. 80- klipp vekk eq. nr

18. Heat of adsorption from isotherms At true adsorption-desorption equilibrium the heat of adsorption -  H a at a given coverage  can be obtained from isotherms measured at different temperatures using the Clausius-Clapeyron equation: T&T fig2.22a side 82

19. Heat of adsorption can be a function of surface coverage Major effect: Strongest adsorption sites are filled first On single crystal faces –At high coverage dipole-dipole interactions comes into effect –Overlapping molecular orbitals contribute –long range interactions T&T fig 2.42 s.118

20. Some definitions Handboook tab 1 s 428 portrait

21. Physical adsorption IUPAC classification of isotherms Handbook fig 1 s 428

23. The BET isotherm Theoretical development based on several assumptions: –multimolecular adsorption –1st layer with fixed heat of adsorption H 1 –following layers with heat of adsorption constant (= latent heat of condensation) –constant surface (i.e. no capillary condensation) gives OT fig1.3

24. The BET isotherm, cont. Plot of left side vs. p/p 0 should give straight line with slope s and intercept I Reorganizing gives Knowledge of S 0 (specific area for a volume of gas then allows the calculation of the specific surface area Sg: where m p is the mass of the sample OT fig1.5

25. BET cont’d BET method useful, but has limitations –microporous materials: mono - multilayer adsorption cannot occur, (although BET surface areas are reported routinely) –assumption about constant packing of N 2 molecules not always correct? –theoretical development dubious (recent molecular simulation studies, statistical mechanics) - value of C is indication o f the shape of the isotherm, but not necessarily related to heat of adsorption

26. Simplified method 1-point method –simplefied BET assuming value of C  100 (usually the case), gives –usually choose p/p 0  0,15 –method underestimates the surface area by approx. 5%.

27. Adsorbates An adsorbate molecule covers an area , calculated assuming dense packing of the molecules in the multilayer. The corresponding area per volume gas is S 0 :

28. Porosity and pore size The pore structure (porosity, pore diameter, pore shape) is important for the catalytic properties –pore diffusion may influence rates –pores may be too small for large molecules to diffuse into Measurement techniques: –Hg penetration –interpretation of the adsorption - desorption isotherms –electron microscopy techniques

29. Hg penetration Based on measuring the volume of a non-wetting liquid forced into the pores by pressure (typically mercury) Surface tension will hinder the filling of the pores, at a given pressure an equilibrium between the force due to pressure and the surface tension is established: where P = pressure of Hg,  is surface tension and  is the angle of wetting Common values used:  = 480 dyn/cm and  = 140° give average pore radius valid in the range 50 - 50000Å

30. Pore size distribution If the Hg-volume is recorded as a function of pressure and this curve is differentiated we can find the pore size distribution function V(r)=dV/dr OT fig 2.3.

31. The Kelvin equation If adsorbent is mesoporous we get Type IV isotherm Deviation upwards is due to filling of mesopores by capillary condensation - curved liquid meniscus in narrow pores with radius r k : V is molar volume of the liquid, minus sign introduced since in the actual range of measurement 0 < p/p 0 <1

32. The Kelvin equation Since capillary condensation is preceeded by multilayer adsorption on the wall the value is corrected with t, the thickness of this layer: Cylindrical pores: r p = r k + t Parallell sided slits: d p = rk + 2t Value of t determined from measurements without capillary condensation Practical experience, typical values give for circular pores: Values for t have been found to be a function of r k, e.g. for r k > 20Å:

33. Adsorption-desorption hysteresis Hysteresis is classified by IUPAC (see fig.) Traditionally desorption branch used for calculation H1: narrow distribution of mesopores H2: complex pore structure, network effects, analysis of desorption loop misleading –H2: typical for activated carbons H3 & 4: no plateau, hence no well- defined mesopore structure, analysis difficult –H3: typical for clays Handbook fig 2 s 431

34. Chemisorption and dispersion Supported metals: metal particle size and dispersion are very important parameters A wide range of techniques available for assessment of particle sizes –electron microscopy (direct observation) –XRD (line broadening analysis) –SAXS (small angle x-ray scattering) –XPS (ratio between surface concentration of support component (e.g. Si in SiO 2 ) and active metal) –Magnetic methods –Chemisorption of probe molecules Methods have different strengths and drawbacks, combinations of 2 or more methods will give best understanding of a system

35. Dispersion - Particle size - Surface area Dispersion: Fraction of surface atoms of a metal in a catalyst: D = N S /N T Chemisorption can give direct measurement of N S, knowledge of N T allows direct calculation of D Assumptions about metal structure, particle shape and exposure of crystal planes allows the calculation of D from relationships with particle size

36. Particle size Particles usually have a range of sizes - particle size distribution –can be narrow (e.g. metals in zeolite cages) –can be broad with one or more maxima Particles also have a range of shapes - not necessarily nice geometries A collection of n i spherical particles of have mean particle sizes based on length or volume (or weight):

37. Relationships

38. Plotted relationships Pt Pd Ni (spherical particles) Handbook fig. 2 & 3 side 441

39. Gas Chemisorption Selective chemisorption of a gas: –formation of (or estimate of the amount of gas in) a monolayer of adsorbed gas –array of experimental techniques available, including commercial equipment Static methods: volumetric or gravimetric Dynamic methods: –Flow technique (frontal chromatography) –Pulse technique Desorption method (TPD) –A range of possible adsorbate gases available H 2,CO,O 2, commonly used N 2 O,NO,N 2,H 2 S,CS 2,hydrocarbons used for special applications

40. Handbook fig 4 side 443 Example: CO on EuroPt-1 pt/SiO2 Monolayer amount v m found by extrapolation of flat part of isotherm Specific metal surface A and dispersion can be calculated: where v m is in cm 3 (STP), n is the chemisorption stoichiometry, m is the sample mass (g) and wt% is the meal loading

41. Not always straight forward Hydrogen adsorption on Pt/Al 2 O 3 at 333 K (Top) –no flat part of isotherm –can be fitted to Langmuir isotherm(dissociative) to obtain v m CO on Fe (bottom) at 90 K –a) Total adsorption –b) Second isotherm after evacuation = physical adsorption –c) Difference is chemisorbed CO –But: all adsorption is in principle reversible: pumping efficiency and evauation time can generate similar differences –Common practice to distinguish between “weak” and “strong” adsorption

42. Hydrogen chemisorption Hydrogen adsorbs dissociatively on metals: H 2 + 2M  2M-H Stoichiometry: 1 H-atom per metal surface atom valid for a number of transition metals Pt much studied, recommended value now (?) 1,1 H atoms per metal surface atom (Boudart & Benson) Standardized methods available (ASTM) –evacuation, oxidation, reduction, evacuation –followed by adsorption at 298 K, equilibrium times of 30 - 60 min.

43. H 2 -O 2 titration Sensitive and simple method for supported Pt: –Pt + ½H 2  Pt-HHC; hydrogen chemisorption –Pt + ½O 2  Pt-OOC; oxygen chemisorption –Pt-O + 3/2H 2  Pt-H+ H 2 OHT; hydrogen titration –2Pt-H + 3/2O 2  2Pt-O+ H 2 OOT; oxygen titration –Stoichiometries: HC : OC : HT : OT = 1 : 1 : 3 : 3 Sensitivity 3-fold enhanced, but care must be taken, accepted procedures followed

44. Hydrogen chemisorption: Sources of error Spillover of H-atoms to the support - can give H : M > 1 SMSI-effect (decoration of metal particles by reduced support species) reduces hydrogen uptake (TiO 2 ) Absorption of hydrogen, hydride formation (Pd, usually avoided by keeping T low < 373 K, titration method) Presence of impurities like Cl, S, C, water, metals can alter uptake General concern about stoichiometry “All chemisorption is a research project in its own right”

45. General guidelines for choice of adsorbate Tabell gammel bok

46. Dynamic method: Flow method (frontal chromatography) Quick method, but isotherm not easily available. Here performed in transient kinetic apparatus Fig. Fra Bariås

47. Pulse technique Simple experiment Can be combined with desorption experiment Pulse time (exposure to adsorbate) is short - kinetics of adsorption can influence the results –cobalt: adsorption slow - pulse technique with hydrogen unsuited Time between pulses important parameter: desorption kinetics can also influence the result Purity of carrier gas important (e.g. small trace of oxygen will titrate surface)

48. Effect of time between pulses Chromatograms of pulsed hydrogen adsorption on Pt/Al 2 O 3. From Gervasini and Flego, Appl. Catal., 1991, 72, 153.

49. Chemisorption - summary Attractive method - gives catalytically relevant data Several possibilities of making errors or introducing artifacts –choice of technique –choice of adsorbate –choice of conditions –assumptions made for calculations Should be combined with other methods available –several physical measurement principles applied reduces the danger of errors

50. Catalyst characterisation 2. Temperature programmed methods Edd A. Blekkan, Dep. of Chemical Engineering Catalysis and Kinetics Group NTNU

51. Temperature Programmed methods Thermal analysis (TGA, DSC, DTA etc.) –standard techniques in solid state chemistry, used for characterisation of properties and reactivities of solid materials –involves the measurement of the response (e.g. mass change, energy exchange etc. with change (usually a linear ramp) in the temperature) –also applicable for studies of catalyst preparation - decomposition of salts and precursors –not a topic today TP-methods in catalysis –TPx, where x can be Reduction TPR Oxidation TPO Desorption TPD Sulfidation TPS Reaction Spectroscopy TPRS (or TPR or TPRx) For model systems in vacuum: TDS: Thermal Desorption Spectrsocopy allows study of adsorption -desorption processes, kinetic steps, energetics etc.

52. TPR Metal catalysts are prepared via precursors and must be reduced: MOn + nH 2  M + nH 2 O Reduction can only occur if thermodynamically allowed: The more “noble” the metal - the easier the reduction (from a thermodynamical point of view), higher ratio water : hydrogen allowed Gas composition becomes important: hydrogen purity, water removal Base metals: Study thermodynamics and kinetics “Noble” metals: Study reduction kinetics Temperature ramping –allows a more rapid investigation –may resolve different processes

53. Handbook fig 1%2 side 677 Experiments Gradients unwanted - use differential conditions –but must ensure sufficient analytical precision Gas must be pure, without traces of O 2 or poisons Analysis of hydrogen consumption –TCD –MS –can also use TGA/EGA type apparatus Usually one of several functions in “multi-purpose” characterisation instrument (TPx, pulse adsorption)

54. Interpretation Qualitative interpretation –temperature of reduction onset, reduction completion –comparison of samples, fingerprinting –simple or multistep reduction –slow or fast reduction –effect of promoter, support, metal loading etc. Simple quantitative interpretation –calculation of degree of reduction from H 2 consumption –potential problem: stoichiometry of oxide e.g. supported cobalt: Co 3 O 4 or CoO? Quantitative interpretation of kinetic parameters –possible if the process is uniform and clear (no overlapping, interference form other processes) particles uniform in size and composition no diffusion limitations, heat transfer effects on rates –usually not suited for practical supported metal catalysts

55. Example 1: Bimetallic catalyst Prestvik (NTNU, Thesis 1995) studied Pt-Re/Al 2 O 3 reforming catalysts using TPR after different drying:

56. Hydrogen consumption Differences in peak temperatures and hydrogen consumption due to changes in reduction mechanism

57. Pt-Re reduction mechanism

58. Example 2: Reduction promoter Interaction with the support leads to poor reducibility of supported cobalt catalysts Addition of easily reducible metal like Pt promotes the reduction, as seen from TPR profiles

59. Conventional (isothermal) reduction process can be checked: The degree of reduction after a “normal” isothermal reduction can be checked by subsequent TPR - reducible cobalt in TPR indicates incomplete reduction

60. Summary TPR Simple, cheap routinely applied technique Suitable for rapid assessment of –reducibility –interaction in bimetallic systems –support effects, promoters Caution: –Data from practical supported catalysts usually not suitable for evaluation of kinetic processes (influence of various other processes like mass and heat transfer) –Profiles strong function of conditions –Only gas phase composition monitored - solid state reactions without H 2 consumption are not detected (sintering and particle growth, structural changes)

61. References and background literature Handbook of Heterogeneous Catalysis, ed. By G. Ertl, H. Knözinger and J. Weitkamp, VCH, Weinheim 1997. J.M. Thomas and W.J. Thomas, “Principles and Practice of Heterogeneous Catalysis”, VCH, Weinheim 1997. J.W. Niemantsverdriet, “Spectroscopy in Catalysis”, VCH, Weinheim 1993. O. Tronstad, “Overflate og porefordelingsmålinger, Inst. For industriell kjemi, NTH 1992. F. Dellanay (Ed.), “Characterization of Heterogeneous Catalysts”, Dekker, New York 1984.