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 Heater T Controller Gases are adsorbed onto the cooled sample. Pressure in chamber recovers. Rotate sample to face mass spectrometer. Apply linear heating rate to sample. A plot showing partial pressure of gas species vs. temperature is obtained. N2(l) UHV Thermocouple Mass Spectrometer Precision leak valve Turbo molecular pump

What Does TPD do for us? Gives us information about: Rate of desorption Kinetic order of desorption Number of adsorption sites/packing structures Sticking probabilities/Surface Coverage Energy of desorption Surface reactions

TPD Gives us the Rate of Desorption 𝑑Ө( 𝑇 𝑠 ) 𝑑𝑡 = 𝑉 𝐴 𝑘 𝐵 𝑇 𝑔 𝑑𝑃 𝑑𝑡 + 𝑆𝑃 𝑉 CONSTANT 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).

What Does TPD do for us? Gives us information about: Rate of desorption Kinetic order of desorption Number of adsorption sites/packing structures Sticking probabilities/Surface coverage Energy of desorption Surface reactions a

TPD Theory Typically TPDs are run with a linear heating ramp. T= Temperature β= Heating rate t= Time The rate of desorption can be written as follows. Applying a linear heating ramp where Gives the equation Ө=Surface coverage kd= Desorption rate constant m= desorption order

TPD Theory Typically TPDs are run with a linear heating ramp. T= Temperature β= Heating rate t= Time The rate of desorption can be written as follows. Applying a linear heating ramp where Gives the equation Ө=Surface coverage kd= Desorption rate constant m= desorption order

TPD Theory Typically TPDs are run with a linear heating ramp. T= Temperature β= Heating rate t= Time The rate of desorption can be written as follows. Applying a linear heating ramp. Gives the equation Ө=Surface coverage kd= Desorption rate constant m= desorption order

TPD Theory Typically TPDs are run with a linear heating ramp. T= Temperature β= Heating rate t= Time The rate of desorption can be written as follows. Applying a linear heating ramp where Gives the equation Ө=Surface coverage kd= Desorption rate constant m= desorption order

TPD Theory Typically TPDs are run with a linear heating ramp. T= Temperature β= Heating rate t= Time The rate of desorption can be written as follows. Applying a linear heating ramp where Gives the equation Ө=Surface coverage kd= Desorption rate constant m= desorption order

TPD Theory Desorption is an activated process that obeys the Ahrrenius equation. A= Pre-exponential factor Ed= Activation energy for desorption R= Ideal gas constant Plugging in kd from the Arrhenius equation gives the Polyani Wigner equation.

TPD Theory Adsorption is an activated process that obeys the Ahrrenius equation. A= Pre-exponential factor Ed= Activation energy for desorption R= Ideal gas constant Plugging in kd from the Arrhenius equation gives the Polyani Wigner equation.

TPD Theory Adsorption is an activated process that obeys the Ahrrenius equation. A= Pre-exponential factor Ed= Activation energy for desorption R= Ideal gas constant Plugging in kd from the Arrhenius equation gives the Polyani Wigner 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 m=0 Shifts to higher temperature with increasing coverage. Exhibits a shared leading edge. Shift is due to intermolecular interactions. Seen in multilayer desorption.

First Order m=1 Desorption temperature is independent of coverage. Asymmetric peaks with an ascending leading edge. Occurs when a molecule adsorbs and then desorbs without dissociating. Ranke, Wolfgang “Thermal Analysis-TDS” Lecture. Fritz- Haber Institut

Second Order m=2 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. Ranke, Wolfgang “Thermal Analysis-TDS” Lecture. Fritz- Haber Institut

What Does TPD do for us? Gives us information about: Rate of desorption Kinetic order of desorption Number of adsorption sites/packing structures Sticking probabilities/Surface coverage Energy of desorption Surface reactions a a

Packing Structure/Sites Terrace Peaks (4X4) (√3 × √3) (7X7) Saturation dose of CO on Cu(111) Steps m/z=28

Packing Structure/Sites Terrace Peaks TPD is saturation dose of CO on 1% Pd/Cu(111) (4X4) (√3 × √3) (7X7) Steps Pd sites m/z=28 M. D. Marcinkowski Nat. Mater., 12, 523-528 (2013).

What Does TPD do for us? Gives us information about: Rate of desorption Kinetic order of desorption Number of adsorption sites/packing structures Sticking probabilities/Surface coverage Energy of desorption Surface reactions a a a

Surface Coverage Methanol on Cu(111) Ө= 𝐴𝑟𝑒𝑎 𝑢𝑛𝑑𝑒𝑟 𝑇𝑃𝐷 𝑐𝑢𝑟𝑣𝑒 𝑓𝑜𝑟 𝑢𝑘𝑛𝑜𝑤𝑛 𝑐𝑜𝑣𝑒𝑟𝑎𝑔𝑒 𝐴𝑟𝑒𝑎 𝑢𝑛𝑑𝑒𝑟 𝑇𝑃𝐷 𝑐𝑢𝑟𝑣𝑒 𝑓𝑜𝑟 𝑘𝑛𝑜𝑤𝑛 𝑐𝑜𝑣𝑒𝑟𝑎𝑔𝑒 ∗𝑘𝑛𝑜𝑤𝑛 𝑐𝑜𝑣𝑒𝑟𝑎𝑔𝑒 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 me Methanol on Cu(111) M. B. Boucher ACS Nano, 7, 6181-6187 (2013).

What Does TPD do for us? Gives us information about: Rate of desorption Kinetic order of desorption Number of adsorption sites/packing structures Sticking probabilities/Surface coverage Energy of desorption Surface reactions a a a a

How do we find the Energy of Desorption? Complete Analysis Leading Edge Analysis Redhead Analysis Vary Heating Rate 𝑅 𝑑 =− 𝑑Ө 𝑑𝑇 = 𝐴 β Ө 𝑚 𝑒 −𝐸 𝑑 𝑅𝑇 Polyani Wigner

Complete Analysis 𝑅 𝑑 =− 𝑑Ө 𝑑𝑇 = 𝐴 β Ө 𝑚 𝑒 −𝐸 𝑑 𝑅𝑇 𝑅 𝑑 =− 𝑑Ө 𝑑𝑇 = 𝐴 β Ө 𝑚 𝑒 −𝐸 𝑑 𝑅𝑇 Polyani Wigner ln⁡(𝑅 𝑑 )=ln 𝐴 Ө 𝑚 β − 𝐸 𝑑 𝑅 1 𝑇 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 𝑅 𝑑 =− 𝑑Ө 𝑑𝑇 = 𝐴 β Ө 𝑚 𝑒 −𝐸 𝑑 𝑅𝑇 𝑅 𝑑 =− 𝑑Ө 𝑑𝑇 = 𝐴 β Ө 𝑚 𝑒 −𝐸 𝑑 𝑅𝑇 Polyani Wigner ln⁡(𝑅 𝑑 )=ln 𝐴 Ө 𝑚 β − 𝐸 𝑑 𝑅 1 𝑇 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. 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 ln⁡(𝑅 𝑑 )=ln 𝐴 Ө 𝑚 β − 𝐸 𝑑 𝑅 1 𝑇 A and Ed both depend on the coverage. This method takes a long time but gives accurate results. Ranke, Wolfgang “Thermal Analysis-TDS” Lecture. Fritz- Haber Institut

Leading Edge Analysis ln⁡(𝑅 𝑑 )=ln 𝐴 Ө 𝑚 β − 𝐸 𝑑 𝑅 1 𝑇 Natural log of Polyani Wigner 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 𝐸 𝑑 𝑅 𝑇𝑝 2 = 𝐴 β 𝑒 − 𝐸 𝑑 𝑅 𝑇 𝑝 For 1st order desorption there is a special case. Rearrange. Take natural log and solve for Ed. 𝐸 𝑑 𝑅 𝑇𝑝 2 = 𝐴 β 𝑒 − 𝐸 𝑑 𝑅 𝑇 𝑝 𝐸 𝑑 𝑅 𝑇 𝑝 = 𝐴 𝑇 𝑝 β 𝑒 − 𝐸 𝑑 𝑅 𝑇 𝑝 ln 𝐸 𝑑 𝑅 𝑇 𝑝 =ln 𝐴 𝑇 𝑝 β − 𝐸 𝑑 𝑅 𝑇 𝑝 𝐸 𝑑 = 𝑅 𝑇 𝑝 ln 𝐴 𝑇 𝑝 β −ln 𝐸 𝑑 𝑅 𝑇 𝑝

Redhead Analysis 𝐸 𝑑 = 𝑅 𝑇 𝑝 ln 𝐴 𝑇 𝑝 β −ln 𝐸 𝑑 𝑅 𝑇 𝑝 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. P. A. Redhead, Vacuum, 12, 203-211 (1962).

Redhead Analysis 𝐸 𝑑 = 𝑅𝑇 𝑝 𝑙𝑛 𝐴 𝑇 𝑝 β −3.64 𝐸 𝑑 = 𝑅𝑇 𝑝 𝑙𝑛 𝐴 𝑇 𝑝 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. P. A. Redhead, Vacuum, 12, 203-211 (1962).

Vary Heating Rate 𝐸 𝑑 𝑅 𝑇𝑝 2 = 𝐴 β 𝑒 − 𝐸 𝑑 𝑅 𝑇 𝑝 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. 𝐸 𝑑 𝑅 𝑇𝑝 2 = 𝐴 β 𝑒 − 𝐸 𝑑 𝑅 𝑇 𝑝 ln β 𝑇𝑝 2 = −𝐸 𝑑 𝑅 𝑇 𝑝 +ln 𝐴𝑅 𝐸 𝑑

Vary Heating Rate 𝐸 𝑑 𝑅 𝑇𝑝 2 = 2𝐴Ө 𝑇 𝑝 β 𝑒 − 𝐸 𝑑 𝑅 𝑇 𝑝 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. 𝐸 𝑑 𝑅 𝑇𝑝 2 = 2𝐴Ө 𝑇 𝑝 β 𝑒 − 𝐸 𝑑 𝑅 𝑇 𝑝 𝐸 𝑑 𝑅 𝑇𝑝 2 = 𝐴 Ө 0 β 𝑒 − 𝐸 𝑑 𝑅 𝑇 𝑝 ln β 𝑇𝑝 2 = − 𝐸 𝑑 𝑅 𝑇 𝑝 +ln 𝐴𝑅 Ө 0 𝐸 𝑑 Falconer and Madix, Surf. Sci., 48, 393-405 (1975).

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

What Does TPD do for us? Gives us information about: Rate of desorption Kinetic order of desorption Number of adsorption sites/packing structures Sticking probabilities/Surface coverage Energy of desorption Surface reactions a a a a a

Temperature Programmed Reaction (TPR) Hydrogenation m/z=2 m/z=2 m/z=28 m/z=106 m/z=26 m/z=104 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).

Temperature Programmed Reaction (TPR) Dehydrogenation D S T m/z=2 HCOOH CO2+H2 HCOOH(g)HCOO(a) + H(a) H(a) ½ H2(g) HCOO(a)  CO2(g) + ½ H2(g) S T m/z=44 m/z=29 Expose the surface to HCOOH and get CO2 and H2.

What Does TPD do for us? Gives us information about: Rate of desorption Kinetic order of desorption Number of adsorption sites/packing structures Sticking probabilities/Surface coverage Energy of desorption Surface reactions a a a a a a

Disadvantages of TPD Questions? 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?