(Preeti Aghalayam, Sep 2011)

Transcription

1 (Preeti Aghalayam, Sep 2011)

2 Adsorbed States As A approaches the catalyst surface, the PE of the system goes through a minimum. Potential Energy (system) ΔH c ΔH p Physisorption vs. Chemisorption A + Catalyst This is adsorption The shallow minimum is the physisorption and the deeper one is the chemisorption Physisorption might be pre- cursor to chemisorption Distance from catalyst: Reaction co- ordinate Energy released during adsorption is indicated on the figure as enthalpies

3 Adsorbed States Adsorbed States 2A Potential Energy Α 2 * E act A 2 Potential Energy Α 2 * A 2 2Α 2Α Distance from catalyst Distance from catalyst It is typically assumed that dissociative adsorption goes through a molecularly adsorbed pre- cursor state Note how the two potential energy curves cross each other above If the cross- over occurs above the PE zero (figure on left), we encounter activated adsorption Typically activation energy for adsorption is assumed to be zero. So that the heat of adsorption = activation energy for desorption

4 Desorption is the opposite of adsorption: Move up from the well in the PE diagram, away from the catalyst We have already encountered desorption while discussing adsorption isotherms Desorption follows an Arrhenius rate form typically: Potential Energy Adsorbed States 2 Α Α 2 * Distance from catalyst 2 A A 2 r des = Aexp(! E d RT )! m (this is the Polanyi- Wigner formula) Desorption is an important phenomena in particular we use desorption studies to understand adsorption energetics

5 Desorption is activated (& endothermic) Typically we need to heat a surface to desorb the stuff adsorbed on it We do this under controlled conditions, measuring: Rate of heating (try to maintain a linear temperature promile, for example) Amount of desorbed stuff (using mass spectrocopy, perhaps; expressed in terms of coverage, perhaps) Rate of desorption (this is just the time variation of coverage) Then, we use this information to back- track out details of the adsorption/desorption Stuff like order of desorption (in coverage), activation energy of desorption (or heat of adsorption), pre- exponential factor, etc. r des = Aexp(! E d RT )! m

6 The oxygen TPD spectrum demonstrates one peak whereas the NO2 demonstrates several peaks Comparing with previous slide, the 800K peak is indicative of E des ~50kcal/ mol The complex shapes of TPD curves indicate effects such as coverage- dependent kinetic parameters, multiple adsorbed states, etc. (Han et al., 2006)

7 (Preeti Aghalayam, Sep 2011)

8 TPD spectra are obtained by heating pre- adsorbed catalyst samples at a known rate, and measuring the gaseous species coming off, as a function of time kcal/mol The rate of desorption demonstrates a peak temperature Rate of desorption kcal/mol 50 kcal/mol First order desorption is asymmetric around the peak temperature Temperature (K) The peak temperature increases with increase in the activation energy Second order desorption is symmetric around the peak temperature

9 This is Redhead s plot for a Mirst order desorption reaction with pre- exponential factor of s - 1 For your heating rate, Mind the peak temperature, Migure out activation energy from the plot Notice that sensitivity to heating rate is not very strong (From King, 1975) This is an experimental TPD curve for the adsorption of CO on Pt(111). Activation energy of ~100kJ/mol can be estimated (from

10 Start with the Polanyi- Wigner equation Differentiate wrt temperature, set to zero You end up with the following equations: ln( T 2 p! ) = E d + ln E d RT p "R First Order Desorption ln( T 2 p! ) = E d + ln RT p E d 2"R# p Second Order Desorption Using experimental data at different heating rates, obtain peak temperature (T p ) vs. heating rate (β) data Make straight line plot, get E d from the slope Notice that second order reactions have dependence on the peak (and therefore initial) coverage

11 This is a simulated TPD spectrum The peak temperature data is picked out from the graph β Tp ln(tp2/b) 1/tp With TPD data from at least three heating rates, the activation energy can be easily determined y = 17012x R² = This is the relevant straight line plot E/R E kcal/mol The predicted activation energy is correct

12 Redhead s Plot Only for Mirst order reactions Pre- exponential factor assumed s - 1 Needs one TPD spectrum No calculations! Peak Temperature method Can be used for other reaction orders (1 st, 2 nd, etc.) but different plots have to be made for each order Pre- exponential factor can also be estimated Needs several TPD spectra, at same initial coverage but different heating rates Some (simple, minimal) calculations & plotting Assumes coverage independent kinetics Assumes coverage independent kinetics These methods have their advantages & disadvantages. Newer methods that can deal with much more complex TPD spectra are also found in the literature

13 Rules: Based on the hint given, guess at the answer The questions are: ID the song ID the adsorption/desorption characteristics

14 Redhead s plot method => E~150kJ/mol ln(tp^2/b) vs. 1/Tp 10 9 y = 12603x R² = (From Masuda et al., 1997) Peak temperature method => E~105 kj/mol Why are the predicted activation energies from the two methods so different?

15 A single heating rate ~10 K/min Can t use Redhead plot! Don t have data at different heating rates, can t use Peak Temperature method Let us just guess at the Eact based on the following Redheadlike plot Eact(kJ/mol) vs Tp (K) The spectrum has three distinct peaks. These correspond to three states of H 2 adsorption on the catalyst In this exercise, lets focus on the first (low temperature) state (From Smeds et al., 1996) E~100kJ/mol

16 (From Johanek t al., 2001) Unsymmetric shape! Two distinct (but overlapping) peaks are visible T p for first peak is not changing as the initial coverage changes However, it changes a fair bit for the second one The Pd foil spectrum is simpler; occurs at higher temperatures First order reaction; followed by higher order reaction Molecular adsorption, followed by dissociation Weakening of the adsorption bond because of the alloying

17 Desorption is a very important catalytic reaction Arrhenius type equations describe the rate of desorption Parameters of the desorption reaction are usually evaluated using Temperature Programmed Desorption (TPD) experiments Lot of mathematical and graphical analyses are used to understand TPD data Redhead Plot Method Peak Temperature Method The TPD spectra exhibit several interesting features including heating rate & initial coverage dependence

18 D. A. King, Thermal desorption from metal surfaces: A review, Surface Sciences 47 (1975) 384 T. Masuda, Y. Fujikata, H. Ikeda, S. Matsushita, K. Hashimoto, A method for calculating the activation energy distribution for desorption of ammonia using a TPD spectrum obtained under desorption control conditions, Applied Catalysis A: General 162 (1997) 2940 P. A. Redhead, Thermal desorption of gases, Vacuum 12 (1962) 203 S. Smeds, T. Salmi, L. P. Lindfors, 0.Krause, Chemisorption and TPD studies of hydrogen on Ni/AI 2 0 3, Applied Catalysis A: General 144 (1996) V. Johanek, N. Tsud, V. Matolin, I. Stara, TPD and XPS study of the CO adsorption on transition- SP metal systems: Pd and Al, Vacuum 63 (2001) 15-22