temperature programmed desorption march 31, 2015

Temperature Programmed Desorption

March 31, 2015

Adsorption and Desorption

Eads

Eads

Non-activated, non-dissociative adsorption Activated, dissociative process.

Adsorption and Desorption

Eads

Eads

Non-activated, non-dissociative adsorption Activated, dissociative adsorption.

Temperature Programmed Desorption (TPD) Experimental Setup

UHV

Precision leak valve

Mass Spectrometer

Turbo molecular pump

N2(l)

T ControllerHeater

Thermocouple

1. Gases are adsorbed onto the cooled sample.

2. Pressure in chamber recovers.3. Rotate sample to face mass

spectrometer.4. Apply linear heating rate to

sample.5. A plot showing partial

pressure of gas species vs. temperature is obtained.

What Does TPD do for us?

Gives us information about:1. Rate of desorption2. Kinetic order of desorption3. Number of adsorption

sites/packing structures4. Sticking probabilities/Surface

Coverage5. Energy of desorption6. Surface reactions

=

TPD Gives us the Rate of Desorption

• Ө is the surface coverage• dӨ/dt is the desorption rate per unit area• Ts is the sample temperature• V is the chamber volume• A is the adsorbent area• Kb is the Boltzmann constant• Tg is the gas phase temperature• P is the pressure increase over the background• S is the pumping speed of the chamber

D. A. King, Surf. Sci., 47, 384-402 (1975).

CONSTANT




coverage5. Energy of desorption6. Surface reactions

a

TPD Theory • Typically TPDs are run with a linear heating ramp.

T= Temperatureβ= Heating ratet= Time

• The rate of desorption can be written as follows.

• Applying a linear heating ramp where

• Gives the equation

Ө=Surface coveragekd= Desorption rate constantm= desorption order




• Applying a linear heating ramp.






• Applying a linear heating ramp where



TPD Theory

A= Pre-exponential factorEd= Activation energy for desorptionR= Ideal gas constant

• Desorption is an activated process that obeys the Ahrrenius equation.

• Plugging in kd from the Arrhenius equation gives the Polyani Wigner equation.

TPD Theory


• Adsorption is an activated process that obeys the Ahrrenius equation.


TPD Theory


• Adsorption is an activated process that obeys the Ahrrenius equation.


Attard and Barnes. Surfaces. 1998

Why is there a peak maximum?

TPD Theory

• TPD peaks are a convolution of surface coverage and rate of desorption.

• A point of maximum desorption (TP) occurs because although kd increases exponentially with T surface coverage drops as T increases.

TPD Peak orders

What happens when we change m?

Zero Order

•Shifts to higher temperature with increasing coverage.•Exhibits a shared leading edge. •Shift is due to intermolecular interactions. •Seen in multilayer desorption.

m=0

First Order

• Desorption temperature is independent of coverage.• Asymmetric peaks with an ascending leading edge.• Occurs when a molecule adsorbs and then desorbs without dissociating.

m=1

Ranke, Wolfgang “Thermal Analysis-TDS” Lecture. Fritz- Haber Institut

Second Order


• Symmetric peak with shared trailing edge shifts to a lower temperature with increasing coverage.• Occurs when molecule adsorbs and in doing so dissociates on the surface, and then desorbs.• At higher coverage, probability of recombination is greater.

m=2





aa

(7X7)

(4X4)(√3 × √3)

Terrace Peaks

Steps

Packing Structure/Sites

Saturation dose of CO on Cu(111)

m/z=28

Packing Structure/SitesTPD is saturation dose of CO on 1% Pd/Cu(111)

(7X7)

(4X4)(√3 × √3)

Terrace Peaks

Steps Pd sites

M. D. Marcinkowski Nat. Mater., 12, 523-528 (2013).

m/z=28





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a

Surface Coverage

M

If the mass spec ionized all adsorbates area under the curve would be equal to coverage, but realistically it is only proportional to coverage.

M. B. Boucher ACS Nano, 7, 6181-6187 (2013).

meMethanol on Cu(111)





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a

How do we find the Energy of Desorption?

• Complete Analysis

• Leading Edge Analysis

• Redhead Analysis

• Vary Heating Rate

= Polyani Wigner

Complete Analysis

= Polyani Wigner

=ln Natural log of Polyani Wigner

If we plot ln(Rd) vs. 1/T then the slope is related to Ed and the intercept is related to A

D. A. King, Surf. Sci., 47, 384-402 (1975).

Complete Analysis

= Polyani Wigner


If we plot ln(Rd) vs. 1/T then the slope is related to Ed and the intercept is related to A.

The expression is a function of coverage so it helps to fix the coverage.

D. A. King, Surf. Sci., 47, 384-402 (1975).

Complete Analysis

A and Ed both depend on the coverage. This method takes a long time but gives accurate results.

=ln


Leading Edge Analysis


• Using one curve plot the rate of desorption vs. 1/T for the leading edge of the TPD.

• At the leading edge coverage changes very little so an Arrhenius plot can be obtained.

• Signal to noise at the leading edge must be very good for this method to work well.

H2O from Cu(111)m/z=18

Redhead Analysis• Polyani Wigner.

•T is at a maximum Tp when:

• Using this derivative relationship the Polyani Wigner can be used to relate Tp, β, and Ed.

P. A. Redhead, Vacuum, 12, 203-211 (1962).

Redhead Analysis• For 1st order desorption there is a special case.

• Rearrange.

• Take natural log and solve for Ed.

=

=

ln =

=

Redhead Analysis

=

The second natural log is relatively small and for first order desorption Ed is related linearly to Tp as shown in the graph on the left. Therefore an estimate of its value can be made.


Redhead Analysis

𝐸𝑑=𝑅𝑇𝑝 [𝑙𝑛( 𝐴𝑇𝑝

β )−3.64 ]• Special case for first order desorption.• Useful for obtaining and estimate of Ed for first order peaks with

just a single TPD spectra.• Assumes Ed and A are coverage independent.• Typically assumes a value of A of 1013 s-1.• Assumes desorption occurs in a single step.• Error in Ed can be huge due to all these assumptions.


Vary Heating Rate• Start with general Redhead equation.

• For first order:

• Take natural log and rearrange.

• Plotting β/Tp vs 1/Tp gives a plot where Ed can be calculated from the slope and A can be calculated from the intercept.

=

= +ln

• For second order:

• Since for second order peaks are symmetrical the coverage at Tp is half the initial coverage. Therefore:

• Take natural log and rearrange.

Vary Heating Rate

=

=

= +ln

Falconer and Madix, Surf. Sci., 48, 393-405 (1975).

Vary Heating Rate

Falconer and Madix, Surf. Sci., 48, 393-405 (1975).





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Temperature Programmed Reaction (TPR)Hydrogenation

Expose the surface to hydrogen and styrene and get ethylbenzene. Expose the surface to acetylene and hydrogen and get ethene.

G. Kyriakou, Science, 335, 1209-1212 (2012).

m/z=2

m/z=2

m/z=26

m/z=28

m/z=104m/z=106

Temperature Programmed Reaction (TPR)Dehydrogenation

Expose the surface to HCOOH and get CO2 and H2.

HCOOH CO2+H2

m/z=2

m/z=44m/z=29

HCOOH(g)HCOO(a) + H(a) H(a) ½ H2(g) HCOO(a) CO2(g) + ½ H2(g)

D ST

S

T





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Disadvantages of TPD

• Destructive technique.• No way to see what is on the surface just what comes off.• Cannot identify binding sites, packing structures, or absolute

coverage by itself.• Data treatment can be complex and it is easy to make mistakes

when applying the different methods.• Temperatures of reactions are hard to determine.• Cross talk between masses can make spectra difficult to

determine.

Questions?

temperature programmed desorption march 31, 2015

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