© 2004 quantachrome instruments this presentation is the property of quantachrome corporation and...

© 2004 Quantachrome Instruments

This presentation is the property of Quantachrome Corporation and has been provided to Johnson-Matthey for its internal use by its bona-fide

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QuantachromeI N S T R U M E N T S


ChemisorptionChemisorption


3


3. Chemisorption Techniques

3.1 Introduction: Physisorption/Chemisorption

3.2 Classical Models

3.3 Active Metal Area Measurement

3.4 Adsorption Thermodynamics

3.5 Pulse vs. Static

3.6 Temperature Programmed Analyses


IntroductionIntroduction


3.1


3.1 Introduction

3.1 Introduction: Physisorption/Chemisorption


3.3 Active Metal Area Measurement


3.5 Pulse vs. Static

3.6 Temperature Programmed Analyses


The 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.


Physisorption vs Chemisorption


On 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.


Adsorption PotentialsPo

tent

ial E

nerg

yP

C

ΔHc

ΔHp

A

Potential energy curves for molecular (non-dissociative) adsorption


Po

ten

tial E

ne

rgy

X + X

ΔHact.

X2

P

C

ΔHdissoc.

A

Adsorption Potentials

Potential energy curves for activated adsorption


Adsorption Potentials

Potential energy curves for non-activated adsorption


Isobars

Isobaric variation in quantity adsorbed with temperature. Physisorption isobar (a) represents lower heat of adsorption than chemisorption isobar (b).

Temperature

Qua

ntity

ad

sorb

ed

(a)

(b)

(c)


On 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


Classical ModelsClassical Models


3.2



3.2.1 Langmuir

3.2.2 Freundlich

3.2.3 Temkin


Adsorption Process

Active Sites (Adsorbent)

AdsorbateAdsorptive


Graduated as a metallurgical engineer from the School of Mines at Columbia University in 1903

1903-1906 M.A. and Ph.D. in 1906 from Göttingen.

1906-1909 Instructor in Chemistry at Stevens Institute of Technology, Hoboken, New Jersey.

1909 –1950 General Electric Company at Schenectady where he eventually became Associate Director

1913 -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

Irving Langmuir (1881-1957)


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"

1935-1937 With Katherine Blodgett studied thin films.

1948-1953 With Vincent Schaefer discovered that the introduction of dry ice and iodide into a sufficiently moist cloud of low temperature could induce precipitation.

Irving Langmuir (1881-1957)


3.2.1 Langmuir’s “Kinetic” Approach

rate of adsorption = ka P(1-)

where is the fraction of the surface already covered with adsorbate, i.e., = V/Vm

rate of desorption = kd

Suggests a dynamic equilibrium. Is it?


Langmuir (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)


Confining adsorption to a monolayer, the Langmuir equation can be written

where 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

KPKP

VV

m

1

mm V

P

KVV

P

1

Or, if you prefer…


Langmuir Plot

1/P

1/V

Slope = 1/(VmK)

Intercept = 1/Vm

1/V = 1/Vm + 1/(VmKcP1/s)


Temperature Dependent Models

generally

K = 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 adsorption

• Langmuir:K is constant;q is constant at all • Temkin: assumed that q decreases linearly with

increasing coverage• Freundlich: assumed that q decreases

exponentially with increasing coverage


TemkinTemkin 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.


Temkin = A ln P + B

or, since = V/Vm

V = Vm A lnP + VmB

Where A = RT/qo and B = A ln Ko + 1/


Temkin Plot

Ln(P)

V

Slope = VmA

Intercept = VmB

V = Vm A lnP + VmB


Multiple Temkin Plots to find

Ln(P)

V

Temp H Temp M Temp L

*mV

* denotes “temperature invariant” or “thermally irreversible” quantity

experimental extrapolated


FreundlichTemkin assumed that q decreases

exponentially with increasing coverage, that is,

Q = -qm ln

Where qm is a constant equal to the heat

of adsorption at = 0.3679


Freundlich

ln = C lnP + D

or, since = V/Vm

ln(V/Vm) = C lnP + D

Where C=RT/ qm and D = C lnKo


Freundlich (continued…)

Ln(P)

Ln(V

)

Slope = C

Intercept = D + ln(Vm)

Ln(V/Vm) = C lnP + D


Multiple Temkin Plots to find

Ln(P)

Ln(V

)

Temp H Temp M Temp L

*mV

* denotes “temperature invariant” or “thermally irreversible” quantity

experimental extrapolated


Active Metal AreaActive Metal Area


3.3


3.3 Active Metal Area

3.3.1 Principles of Calculation

3.3.2 Choice of Adsorbate

3.3.3 Active Site Size Calculation

3.3.4 Metal Dispersion

3.3.5 Accessible vs non-accessible sites


Active 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


Active Site on a Catalyst

• Metal on support.

• Island-like crystallites

• Not all metal atoms exposed.

• Adsorption technique perfectly suited.

(cf Chemical analysis of entire metal content )


3.3.1 Principles of Calculation

Monolayer Volume, Vm= volume of gas chemisorbed in a monomolecular layer


Methods to Determine Vm

•Extrapolation

• Bracketing

• Langmuir

• Temkin

• Freundlich

= volume of gas chemisorbed in a monomolecular layer


Vm

Vol

ume

Ads

orbe

d

Pressure (mm Hg)

Extrapolation method

First (only?)isotherm


Vol

ume

Ads

orbe

d

Pressure (mm Hg)

The second isotherm

combined

Weak only


Vol

ume

Ads

orbe

d

Pressure (mm Hg)

The difference isotherm

combined

Weak only

Strong


Vm from Pulse Titration

… will be covered in 3.5.2


Metal 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 molecule

To calculate metal area per gram of metal, Am:

Am = A x l00/L

where L = metal loading (%) = known value from chemical analysis


Stoichiometry

The 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).


3.3.2 Choice of Adsorbate

Chemisorption

• CO or H2 on Pt, Pd

at 40 oC

• CO or H2 on Ni

For metal-only area

(& dispersion etc)

Physisorption

• N2 at 77K

• Ar at 87K

• Kr at 77K

• CO2 at 273K

For total surface area

and pore size


3.3.3 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)


Shape 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 give

V= lA/6

For a cube whose mass is unit mass, its volume is given by 1/ (where is the density of the material).

V=1/


Shape 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.


Supported metalsIt 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.

3.3 Metal Dispersion


3.3 Metal Dispersion• In 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.


Exposed 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.

support

Exposed active sitesAdsorbed gas


Metal Dispersion

• Dispersion 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.


Metal 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.

%WL

NM

SAv

a 100100


3.3.5 Accessible vs. Non-accessible Sites1. Adventitious moisture2. Reducing gas accessibility3. Diffusion4. Purge5. Physisorption blocks6. Bulk hydride7. Spillover8. Stoichiometry9. Characterization gas vs. Process gas


Spatial Ordering

There may exist a number of different adsorption sites that involve different numbers of metal atoms per adsorbate molecule.


Adsorption Adsorption ThermodynamicsThermodynamics


3.4



3.4.1 Isosteric Heats from Isotherms

See also activation energy under 3.6.1


3.4.1 Heats of Adsorption

• Whenever 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.


Differential 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.


Differential 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.


Slopes of (lnP) vs. (1/T).

mRq where

m = d lnP/d(1/T) and R is the universal gas constant.


Integral 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.

max

min

qV

Q m d22414


Experimental ApproachesExperimental Approaches


3.5


3.5 Experimental Approaches

3.5.1 Pulse

3.5.2 Static


Preparation 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.


Preparation 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.


Chemisorption Techniques

• Vacuum method

• Flow methods


Vacuum Technique

• Sample is heated under inert flow

• Reduced with H2

• Purged with inert, cooled under vacuum/flow

• Adsorbate dosed to obtain isotherm

• Calculate the amount adsorbed


Static (volumetric) Setup

furnace

manifold

adsorptives

vent

diaphragm pump

Turbo-molecular

(drag) pump

Flow “U” cell


Setup

Filler rod goes here

Quartz wool

sample capillary


3.5.2Flow (Pulse) Chemisorption


Flow Types of Analysis

TPR TPO TPD Monolayer by Titration BET

support

active sites

A flow system permits multi-functional catalyst characterization :


OverviewAnalysis is done by detecting changes in gas

composition downstream of sample.

• Detector senses – abstraction of reactive species during

adsorption – evolution of previously adsorbed species during

desorption– decomposition products

• Signal detection– Standard: thermal conductivity detector– Optional: mass spectrometer


ChemBET™ 3000 TPR


Flow Diagram

AB

1

2

3

4

A

IN

OUT

CLICK FOR BYPASS & LONGPATH




Flow/Static (FloStat™) Flow Diagram

12

3

4

5

to mass spec

(optional)

to vent

B

A

oil-free high vacuum

vapor source

(optional)

heater

Schematic representation only. Some vacuum volumetric components omitted for clarity.

heated zone (vapor option)


TPRWin™ Software

Data Acquisition


Overview• Quartz flow-through

cell allows – high-temperature (up

to 1100 degC) – in-cell temperature

monitoring– Two t/c’s if necessary,

one to DAQ, one to MassSpec.

– mass spectrometer sampling port.

T/C #1T/C #2

Modified cell holder

Capillary to mass spec.

Gas flow


Pulse Titration • Metal 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.


Titration Pulse Titration of Active Sites

H2 or CO titration

N2 and He carrier respectively Constant temperature (room temp?) Multiple injections until saturation

M M MMH

H H H H

H2 CO

CO CO CO

N2He


Titration

Data Acquisition


Titration

injections

sig

nal

LOAD INJECT


Titration 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) etc

x nominal injection volume = Vnom (units l)


Titration Calculations

2. Convert to STP:

(Vnom) x (273/rt) x (Pamb/760) = Vstp (units l)

3. Convert to specific volume adsorbed:

Vstp /sample wt = Vsv (units l/g)

4. Convert to micromoles per gram (weight as supplied ):

Vsv / 22.4 = Vm (units mole/g)


Requirements for Different Analysis Types

Long cell

Short cell

Std. cell

5% H2

100% H2

5% O2

100% N2

100% He

30% N2

Inj. Furnace

Mantle Dewar Long path

TPR ()

TPO TPD Metal Area* () () * *

BET ()

* Using H2 active gas. If using CO, substitute 100% CO for 100% H2 & 100% He for 100% N2.

L


Temperature Programmed (TP) Temperature Programmed (TP) ExperimentsExperiments


3.5


3.6 Temperature Programmed (TP) Experiments

3.6.1 TP-Reduction

3.6.2 TP-Oxidation

3.6.3 TP-Desorption

3.6.4 TP-Reaction


3.6.1 TP-Reduction

• Metal oxides are readily characterized by their ease of reduction.

CeO2 CeO2-x + x/2O2

• TPR profiles represent that ease of reduction as reduction rate as a function of increasing temperature.

2CeO2 + H2 Ce2O3 + H2O


Temperature 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.


TPR Temperature Programmed Reduction

Metal oxide to metal 5% hydrogen reactive gas Balance N2 or Ar (not He ! ...unless MS) Ramp rate Activation Energy

H2

MO MO MOMO

H2O

M M MM


TPR

temperature

sig

nal

tmax


TPRLinearly ramped

furnace is essential for standard TP

profiles


time

sig

nal

tmax

tem

pera

ture

TPR Profiles for Different Heating Rates

1

2

3


TPR Profiles for Different Heating Rates

800 1000

0 20 40 60 80

100 120 140 160 180 3

1

2

Sig

nal

Temperature / K


TPR Profile

Heating Rate (K-1)

Peak Temperature (Tmax)

1 10 874

2 15 902

3 20 928

Heating Rate & Peak Temperature


Kissinger (Redhead) Equation

1.08 1.10 1.12 1.14

-11.2

-11.1

-11.0

-10.9

-10.8

-10.7

s lope = -8.6

Ea = 72 kJ mol -1

ln(

Tm

ax-2

)

1000 /Tmax

(K -1)

max

a2max T

1

R

EK

Tln


3.6.2 TP-Oxidation

• Temperature 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 coking– Carbides


TPO Temperature Programmed Oxidation

Metals and carbon to oxides 2-5% oxygen reactive gas balance He (not N2 !) Ramp rate Activation Energy

O2

C C CC

CO + CO2

M M MM

carbon


TPO: Signal vs. Temperature


TPO: Signal & Temp. vs. Time


Temperature 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 21 589-595 (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 161 626-640 (1996).


3.6.3 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.


TPD Temperature Programmed Desorption

Remove previously adsorbed species Helium/Nitrogen purge Ramp rate Activation Energy

NH3MO MO MOMO

NH3


Ammonia TPD


Pyridine TPD

Physisorbed pyridine is clearly evident in the first sample (low temp.), but absent in the second.

Multiple acid sites revealed by peak deconvolution


TPD

temperature

sig

nal

tmaxIncreasing mass


Overview• Quartz flow-through

cell allows – high-temperature (up

to 1100 degC) – in-cell temperature

monitoring– Two t/c’s if necessary,

one to DAQ, one to MassSpec.

– mass spectrometer sampling port.

T/C #1T/C #2



Gas flow


With Mass Spectrometer

Capillary or capillary connector to mass spectrometer

Tube endsjust below port connection

In-situ thermocouple

¼” swagelok® compression fitting

T/C #1T/C #2



Gas flow


3.6.4 TP-Reaction

• Essentially 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 surface

e.g. OHCHNiCONi Hn 242

2

© 2004 quantachrome instruments this presentation is the property of quantachrome corporation and...

Documents

temkin slide

depth slide

e n t s slide

nonactivated adsorption

adsorption thermodynamics

physical adsorption

irreversible adsorption

b c slide