33. Chemisorption Techniques 3.1 Introduction: Physisorption/Chemisorption3.2 Classical Models3.3 Active Metal Area Measurement3.4 Adsorption Thermodynamics3.5 Pulse vs. Static3.6 Temperature Programmed Analyses
53.1 Introduction 3.1 Introduction: Physisorption/Chemisorption 3.2 Classical Models3.3 Active Metal Area Measurement3.4 Adsorption Thermodynamics3.5 Pulse vs. Static3.6 Temperature Programmed Analyses
6The Nature of Gas Sorption at a Surface When the interaction between a surface and an adsorbate is relatively weak only physisorption takes place.However, surface atoms often possess electrons or electron pairs which are available for chemical bond formation.This irreversible adsorption, or chemisorption, is characterized by large interaction potentials which lead to high heats of adsorption.
8On The Nature of Chemisorption Chemisorption is often found to occur at temperatures far above the critical temperature of the adsorbate.As is true for most chemical reactions, chemisorption is usually associated with an activation energy, which means that adsorbate molecules attracted to a surface must go through an energy barrier before they become strongly bonded to the surface.
9Adsorption Potentials Potential energy curves for molecular (non-dissociative) adsorption
10Adsorption Potentials Potential energy curves for activated adsorption
11Adsorption Potentials Potential energy curves for non-activated adsorption
12Isobars(b)Quantity adsorbed(c)(a)TemperatureIsobaric variation in quantity adsorbed with temperature. Physisorption isobar (a) represents lower heat of adsorption than chemisorption isobar (b).
13On The Nature of Chemisorption Because chemisorption involves a chemical bond between adsorbate and adsorbent, unlike physisorption, only a single layer of chemisorbed species can be realized on localized active sites such as those found in heterogeneous catalysts.However, further physical adsorption on top of the chemisorbed layer and diffusion of the chemisorbed species into the bulk solid can obscure the fact that chemisorbed material can be only one layer in depth
14QuantachromeI N S T R U M E N T SClassical Models3.2
17Irving Langmuir ( )Graduated as a metallurgical engineer from the School of Mines at Columbia University in 1903M.A. and Ph.D. in 1906 from Göttingen.Instructor in Chemistry at Stevens Institute of Technology, Hoboken, New Jersey.1909 –1950 General Electric Company at Schenectady where he eventually became Associate Director1913 -Invented the gas filled, coiled tungsten filament incandescent lamp.1919 to 1921, his interest turned to an examination of atomic theory, and he published his "concentric theory of atomic structure" . In it he proposed that all atoms try to complete an outer electron shell of eight electrons
18Irving Langmuir ( )1927 Coined the use of the term "plasma" for an ionized gas.1932 The Nobel Prize in Chemistry "for his discoveries and investigations in surface chemistry"With Katherine Blodgett studied thin films.With Vincent Schaefer discovered that the introduction of dry ice and iodide into a sufficiently moist cloud of low temperature could induce precipitation.
193.2.1 Langmuir’s “Kinetic” Approach rate of adsorption = ka P(1-)where is the fraction of the surface already covered with adsorbate, i.e., = V/Vmrate of desorption = kd Suggests a dynamic equilibrium. Is it?
20Langmuir (continued…) At equilibrium (any pressure)ka P(1-) = kd from which = V/Vm = KP/(1+KP)where K = ka / kd.In its linear form, the above equation can be expressed as:1/V = 1/Vm + 1/(VmKP)
21Or, if you prefer…Confining adsorption to a monolayer, the Langmuir equation can be writtenwhere V is the volume of gas adsorbed at pressure P, Vm is the monolayer capacity (i.e. θ=1) expressed as the volume of gas at STP and K is a constant for any given gas-solid pair. Rearranging in the form of a straight line (y=ab+x) gives
23Temperature Dependent Models generallyK = Ko exp(q/RT)where Ko is a constant, R is the universal gas constant, T is the adsorption temperature and q is the heat of adsorptionLangmuir:K is constant;q is constant at all Temkin: assumed that q decreases linearly with increasing coverageFreundlich: assumed that q decreases exponentially with increasing coverage
24TemkinTemkin assumed that q decreases linearly with increasing coverage, that is,Q=qo(1- )Where qo is a constant equal to the heat of adsorption at zero coverage ( = 0) and is a proportionality constant.
25Where A = RT/qo and B = A ln Ko + 1/ Temkin = A ln P + B or, since = V/VmV = Vm A lnP + VmBWhere A = RT/qo and B = A ln Ko + 1/
26Temkin PlotLn(P)VSlope = VmAIntercept = VmBV = Vm A lnP + VmB
27Multiple Temkin Plots to find experimentalextrapolatedV* denotes “temperature invariant” or “thermally irreversible” quantityLn(P)Temp HTemp MTemp L
28FreundlichTemkin assumed that q decreases exponentially with increasing coverage, that is,Q = -qm lnWhere qm is a constant equal to the heat of adsorption at =
29Where C=RT/ qm and D = C lnKo Freundlichln = C lnP + D or, since = V/Vmln(V/Vm) = C lnP + DWhere C=RT/ qm and D = C lnKo
30Freundlich (continued…) Ln(P)Ln(V)Slope = CIntercept = D + ln(Vm)Ln(V/Vm) = C lnP + D
31Multiple Temkin Plots to find experimentalextrapolatedLn(V)* denotes “temperature invariant” or “thermally irreversible” quantityLn(P)Temp HTemp MTemp L
32QuantachromeI N S T R U M E N T SActive Metal Area3.3
333.3 Active Metal Area 3.3.1 Principles of Calculation 3.3.2 Choice of Adsorbate3.3.3 Active Site Size Calculation3.3.4 Metal Dispersion3.3.5 Accessible vs non-accessible sites
34Active Site Quantification Because the formation of a chemical bond takes place between an adsorbate molecule and a localized, or specific, site on the surface of the adsorbent, the number of active sites on catalysts can be determined simply by measuring the quantity of chemisorbed gas
35Active Site on a Catalyst Metal on support.Island-like crystallitesNot all metal atoms exposed.Adsorption technique perfectly suited.(cf Chemical analysis of entire metal content )
363.3.1 Principles of Calculation Monolayer Volume, Vm = volume of gas chemisorbed in a monomolecular layer
37Methods to Determine Vm = volume of gas chemisorbed in a monomolecular layerExtrapolationBracketingLangmuirTemkinFreundlich
38Extrapolation method Vm First (only?)isotherm Volume Adsorbed Pressure (mm Hg)First (only?)isotherm
39The second isotherm combined Weak only Volume Adsorbed Pressure (mm Hg)
41Vm from Pulse Titration … will be covered in 3.5.2
42Metal Area Calculation To Calculate Metal Surface Area: A = (Vm) x (MXSA) x (S) x 6.03 x 10-3 (units m2/g)where MXSA = metal cross sectional area (Å2)and S = stoichiometry = metal atoms per gas moleculeTo calculate metal area per gram of metal, Am: Am = A x l00/Lwhere L = metal loading (%) = known value from chemical analysis
43StoichiometryThe gas-sorption stoichiometry is defined as the number of metal atoms with which each gas molecule reacts.Since, in the gas adsorption experiment to determine the quantity of active sites in a catalyst sample, it is the quantity of adsorbed gas which is actually measured, the knowledge of (or at least a reasonably sound assumption of) the stoichiometry involved is essential in meaningful active site determinations (area, size, dispersion).
443.3.2 Choice of Adsorbate Chemisorption CO or H2 on Pt, Pd at 40 oC CO or H2 on NiFor metal-only area (& dispersion etc)PhysisorptionN2 at 77KAr at 87KKr at 77KCO2 at 273KFor total surface area and pore size
453.3.3 Active Site Size Calculation To calculate average crystallite size:d = (L x 100 x f )/AD (units Å)where f = shape factor = 6ρ = density of metal (g/ml)
46Shape Factor & Crystallite Size The default shape factor of 6 is for assumed cubic geometry. Consider a cube of six sides (faces) each of length l. then the total surface area, A = 6l2.The volume of the cube is given by l3 or, in terms of total area, substitute A /6 for l2 to giveV= lA/6For a cube whose mass is unit mass, its volume is given by 1/ (where is the density of the material). V=1/
47Shape Factor & Crystallite Size For the same cube of unit mass, the area is then the area per unit mass A and l is rewritten d (crystallite size), the length required to give a cube whose mass is unity. Equating both terms for volume:dA/6=1/ or d=6/A For a supported metal, the loading, L, must be taken into consideration. d=L6/A Other geometries can be treated in a similar fashion. For example, a rectangular particle whose length is three times its width has a shape factor of 14/3.
483.3 Metal Dispersion Supported metals It is most likely that the catalyst exists as a collection of metal atoms distributed over an inert, often refractory, support material such as alumina.At the atomic level it is normal that these atoms are assembled into island-like crystallites on the surface of the support.
493.3 Metal DispersionIn the case of supported metal catalysts, it is important to know what fraction of the active metal atoms is exposed and available to catalyze a surface reaction.Those atoms that are located inside metal particles do not participate in surface reactions, and are therefore wasted.
50Exposed metal atomsSince these islands vary in size due to both the intrinsic nature of the metal and the support beneath, plus the method of manufacture more or less of the metal atoms in the whole sample are actually exposed at the surface. It is evident therefore that the method of gas adsorption is perfectly suited to the determination of exposed active sites.supportExposed active sitesAdsorbed gas
51Metal DispersionDispersion is defined as the percentage of all metal atoms in the sample that are exposed.The total amount of metal in the sample is termed the loading, χ , as a percentage of the total sample mass, and is known from chemical analysis of the sample.
52Metal Dispersion The dispersion, δ, is calculated from: Where M is the molecular weight of the metal, Na is the number of exposed metal atoms found by adsorption and WS is the mass of the sample.
533.3.5 Accessible vs. Non-accessible Sites Adventitious moistureReducing gas accessibilityDiffusionPurgePhysisorption blocksBulk hydrideSpilloverStoichiometryCharacterization gas vs. Process gas
54Spatial OrderingThere may exist a number of different adsorption sites that involve different numbers of metal atoms per adsorbate molecule.
55Adsorption Thermodynamics QuantachromeI N S T R U M E N T SAdsorption Thermodynamics3.4
563.4 Adsorption Thermodynamics Isosteric Heats from IsothermsSee also activation energy under 3.6.1
573.4.1 Heats of AdsorptionWhenever a gas molecule adsorbs on a surface, heat is (generally) released, i.e. the process of adsorption is exothermic.This heat comes mostly from the loss of molecular motion associated with the change from a 3-dimensional gas phase to a 2-dimensional adsorbed phase.Heats of adsorption provide information about the chemical affinity and the heterogeneity of a surface, with larger amounts of heat denoting stronger adsorbate-adsorbent bonds.There are at least two ways to quantify the amount of heat released upon adsorption: in terms of (i) differential heats, q, and (ii) integral heat, Q.
58Differential Heats of Adsorption q, is defined as the heat released upon adding a small increment of adsorbate to the surface.Its value depends on (i) the strength of the bonds formed and (ii) the degree to which surface is already covered.i.e a plot of q vs. θ provides a curve illustrating the energetic heterogeneity of the surface.Use it to fingerprint surface energetics and to test of the validity of any Vm evaluation method used (see earlier) since each method assumes a different relationship between q and θ.
59Differential Heats of Adsorption Since q can, and most often does, vary with θ, it is convenient to express it as an isosteric heat of adsorption, that is, at equal surface coverage for different temperatures.Thus, obtain two or more isotherms at different temperatures.Determine pressures corresponding to equal coverage at different temperatures.Construct an Arrhenius plot of (lnP) versus (1/T). Values for q at any given coverage, θ, can be calculated from the Arrhenius slopes, m.
60and R is the universal gas constant. Slopes of (lnP) vs. (1/T).wherem = d lnP/d(1/T)and R is the universal gas constant.
61Integral Heat of Adsorption This is simply defined as the total amount of heat released, Q, when one gram of adsorbent takes up X grams of adsorbate. It is equivalent to the sum, or integral, of q over the adsorption range considered, that is:where Vm is expressed in mL at STP, and θ ideally ranges from θmin = 0 to θmax = maximum coverage attained experimentally.
62Experimental Approaches QuantachromeI N S T R U M E N T SExperimental Approaches3.5
64Preparation Techniques • Sample is heated under inert flow to remove adsorbed moisture. Whilst reduction step creates moisture, we don’t ant the reducing gas to compete for diffusion to surface.• Reduce with H2: can be pure hydrogen or diluted with nitrogen or argon. Higher concentrations give higher space velocities for the same volumetric flow rate.
65Preparation Techniques (continued…) Purging with inert gas (normally helium) strips excess reducing gas quickly. Can shorten prep time and/or give more reproducible data since hydrogen is difficult to pump.Cooling is done under vacuum/flow to ensure continued removal of residual reducing gas… though it is the hot removal step (above) which is critical. That is, don’t cool before removing as much reducing gas as possible.
71Flow Types of Analysis TPR TPO TPD Monolayer by Titration BET A flow system permits multi-functional catalyst characterization :active sitessupport
72OverviewAnalysis is done by detecting changes in gas composition downstream of sample.Detector sensesabstraction of reactive species during adsorptionevolution of previously adsorbed species during desorptiondecomposition productsSignal detectionStandard: thermal conductivity detectorOptional: mass spectrometer
74Flow Diagram A 1 B A 2 3 4 OUT IN CLICK FOR BYPASS & LONGPATH
75Flow/Static (FloStat™) Flow Diagram to ventheated zone (vapor option)Aheatervapor source (optional)B12to mass spec (optional)3oil-free high vacuum45Schematic representation only. Some vacuum volumetric components omitted for clarity.
77Overview Quartz flow-through cell allows T/C #1T/C #2Modified cell holderCapillary to mass spec.Gas flowQuartz flow-through cell allowshigh-temperature (up to 1100 degC)in-cell temperature monitoringTwo t/c’s if necessary, one to DAQ, one to MassSpec.mass spectrometer sampling port.
78Pulse TitrationMetal area, dispersion and crystallite size are calculated from the amount of analysis (reactive) gas adsorbed.Variable volumes of analysis gas are injected into the inert carrier gas stream, which continuously flows over the sample.Detector measures the volume of gas that remains unadsorbed by the sample. Subtraction from the total amount injected gives the total amount adsorbed to within 1uL accuracy.
79Titration Pulse Titration of Active Sites H2 or CO titration N2 and He carrier respectivelyConstant temperature (room temp?)Multiple injections until saturationMHH2COHeN2
82Titration Calculations 1. Calculate total nominal volume of reactive gas adsorbed by comparison with calibration injection or average of last n (three) peaks(note: peak area represents gas not adsorbed!)Total vol adsorbed =(Peak Avg - Peak1) + (Peak Avg - Peak2) + (Peak Avg - Peak3) etcx nominal injection volume = Vnom (units ml)
83Titration Calculations 2. Convert to STP: (Vnom) x (273/rt) x (Pamb/760) = Vstp (units ml) 3. Convert to specific volume adsorbed: Vstp /sample wt = Vsv (units ml/g) 4. Convert to micromoles per gram (weight as supplied ): Vsv / 22.4 = Vm (units mmole/g)
84Requirements for Different Analysis Types Long cellShort cellStd. cell5% H2100% H25% O2100% N2100% He30% N2Inj.FurnaceMantleDewarLong pathTPR()TPOTPDMetal Area**BETMixed Gases: the gases to be mixed should have significantly different thermal conductivities. Hydrogen (for TPR for example) should be blended with nitrogen or argon (inert carrier gases) and not be blended with helium (inert but too similar to hydrogen with respect to thermal conductivity). Similarly, when performing pulse titration, hydrogen should be injected into nitrogen or argon carrier, not into helium. Carbon monoxide should be injected into helium, not nitrogen! Choose one gas from family “1” and the other gas from family “2”…1) Helium, hydrogen2) Nitrogen, argon, carbon monoxide, carbon dioxideL* Using H2 active gas. If using CO, substitute 100% CO for 100% H2 & 100% He for 100% N2.
85Temperature Programmed (TP) Experiments QuantachromeI N S T R U M E N T STemperature Programmed (TP) Experiments3.5
863.6 Temperature Programmed (TP) Experiments TP-ReductionTP-OxidationTP-DesorptionTP-Reaction
873.6.1 TP-ReductionMetal oxides are readily characterized by their ease of reduction.CeO2 CeO2-x + x/2O2TPR profiles represent that ease of reduction as reduction rate as a function of increasing temperature.2CeO2 + H2 Ce2O3 + H2O
88Temperature Programmed Reduction A low concentration of pre-mixed hydrogen (e.g.5%) in nitrogen or argon (or other reducing gas for custom research applications) flows over the sample as it is heated during a linear increase (ramp) in temperature.Peak reduction temperature is also a function of heating rate and may be used to calculate activation energy for the reduction process.
89TPR Temperature Programmed Reduction Metal oxide to metal 5% hydrogen reactive gasBalance N2 or Ar (not He ! ...unless MS)Ramp rateActivation EnergyH2OMH2It is usual to react the unreduced catalyst, typically a metal oxide which may be supported or not, with a reducing gas, typically hydrogen diluted in an inert carrier gas – typically nitrogen. The change in hydrogen concentration is monitored as a function of increasing sample temperature.MO
90TPR tmax signal temperature The resulting temperature programmed reduction (also known as TPR) profile represents the relative ease with which the sample reacts with the hydrogen, or reduces. The peak in the profile represents the maximum reaction rate, and its temperature is related to the activation energy of the reduction process. Two or more well separated peaks are an indication that two or more distinct unreduced phases may be present in the sample.temperature
91Linearly ramped furnace is essential for standard TP profiles TPRLinearly ramped furnace is essential for standard TP profilesThe resulting temperature programmed reduction (also known as TPR) profile represents the relative ease with which the sample reacts with the hydrogen, or reduces. The peak in the profile represents the maximum reaction rate, and its temperature is related to the activation energy of the reduction process. Two or more well separated peaks are an indication that two or more distinct unreduced phases may be present in the sample.
92TPR Profiles for Different Heating Rates 3tmax2signaltemperatureThe resulting temperature programmed reduction (also known as TPR) profile represents the relative ease with which the sample reacts with the hydrogen, or reduces. The peak in the profile represents the maximum reaction rate, and its temperature is related to the activation energy of the reduction process. Two or more well separated peaks are an indication that two or more distinct unreduced phases may be present in the sample.1time
963.6.2 TP-OxidationTemperature programmed oxidation (using 2%-5% O2 in He for example) is performed in a manner analogous to TPR.TPO can be particularly useful for looking at carbons:Carbon supports (graphite vs. amorphous)Carbon deposits from cokingCarbides
97TPO carbon Temperature Programmed Oxidation Metals and carbon to oxides2-5% oxygen reactive gasbalance He (not N2 !)Ramp rateActivation EnergyCO + CO2O2Temperature Programmed Oxidation is used to evaluate a sample’s ease of oxidation. This analysis results in a fingerprint profile of oxidation rate (reduction in concentration of oxidizing species) as a function of increasing time and temperature. It is usual to react carbon samples or a reduced catalyst, ie a metal (which may be supported or not), with an oxidizing gas, typically oxygen diluted in an inert carrier gas – typically helium. The change in oxygen concentration is monitored as a function of increasing sample temperature. Carbon dioxide is mildly oxidizing and is occasionally used according to the following scheme: C + CO2 = 2CO.MCcarbon
100Temperature Programmed Oxidation Zhang and Verykios reported that three types of carbonaceous species designated as C, C, and C were found over Ni/Al2O3 and Ni/CaO±Al2O3 catalysts in the TPO experiments.Zhang ZL and Verykios XE,. Catal. Today (1994).Goula et al identified two kinds of carbon species on Ni/CaO Al2O3 catalysts from TPO experiments. The high-temperature peak was assigned to amorphous and/or graphite forms of carbon. The lower temperature peak suggested a filamentous form.Goula MA, Lemonidou AA and Efstathiou AM, J Catal (1996).
1013.6.3 Temperature Programmed Desorption The monitoring of desorption processes is equally easy.A pure unreactive carrier gas carries evolved species from the sample to the detector as the user-programmable furnace heats the sample.This technique is commonly employed to determine the relative-strength distribution of acidic sites by means of ammonia desorption.
106Overview Quartz flow-through cell allows T/C #1T/C #2Modified cell holderCapillary to mass spec.Gas flowQuartz flow-through cell allowshigh-temperature (up to 1100 degC)in-cell temperature monitoringTwo t/c’s if necessary, one to DAQ, one to MassSpec.mass spectrometer sampling port.
107With Mass Spectrometer T/C #1T/C #2Modified cell holderCapillary to mass spec.Gas flowCapillary or capillary connector tomass spectrometerTube endsjust below port connectionIn-situ thermocouple¼” swagelok® compression fitting
1083.6.4 TP-ReactionEssentially everything that is not standard TPR or TPO!!Can be a single reactive gas, or a mixture of reactants… akin to microreactor work.Need not be done over a bare metal surface… might have one reactive species preadsorbed on the surfacee.g.