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2009, 113, 20152–20155 Published on Web 10/29/2009
Temperature Programmed Desorption - An Application to Kinetic Studies of CO Desorption at Electrochemical Interfaces Bin Geng,† Jun Cai,† Shao-Xiong Liu,† Pu Zhang,† Zhi-Qiang Tang,† Dong Chen,† Qian Tao,† Yan-Xia Chen,*,† and Shou-Zhong Zou‡ Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, 230026, China, and Department of Chemistry and Biochemistry, Miami UniVersity, Oxford, Ohio 45056 ReceiVed: September 2, 2009; ReVised Manuscript ReceiVed: October 22, 2009
We describe a novel and simple method for kinetic studies of unimolecular desorption in electrochemical environment based on the reformulated first-order Polanyi-Wigner equation. With the knowledge of adsorbate coverage, kinetic parameters such as activation energy and rate constant of desorption can be obtained. The viability of this approach is demonstrated by temperature programmed COad desorption from a Pt electrode in 0.5 M H2SO4 at 10 to 80 °C. The coverage of COad at each temperature is determined by CO stripping charge. The activation energy for CO desorption is estimated to be ca. 101 kJ/mol at saturation coverage and increases with decreasing COad coverage. The rate constant for CO desorption is estimated to be on the order of 10-4 s-1 and displays an exponential increase with the temperature at T > 30 °C. The general applicability of this approach for the kinetic studies of adsorption-desorption processes is briefly discussed. 1. Introduction Adsorption/desorption processes are the critical steps in heterogeneous catalysis including electrocatalysis. The strong adsorption of reactants, reactive intermediates, products, and spectators at catalyst surfaces will greatly affect the reaction kinetics. Thus, fundamental understanding of the kinetics of adsorption/desorption processes is of great importance for improving the efficiency of related catalytic processes.1 At solid/ gas interface in vacuum, temperature programmed desorption (TPD) has been widely used to study the kinetics of such processes, from which information on heat of adsorption, adsorbate coverage, adsorbate interactions, adsorption sites as well as phase transitions may be obtained.2 In electrochemical systems, however, due to the limited temperature windows (i.e., between ca. 0-100 °C), the complexities in controlling the cell temperature, as well as the interference of aqueous electrolyte in the double layer, no kinetic measurement by TPD has been reported so far. In this communication, we describe a novel and straightforward method for the kinetic studies of unimolecular desorption in electrochemical environment based on the reformulated firstorder Polanyi-Wigner equation. The applicability of this method is demonstrated by using the CO desorption from Pt electrodes as an example, since CO is arguably the most commonly found adsorbate/intermediate on the surface of transition metal catalysts for diverse applications such as pollution controls for the automobile industry, Fischer-Tropsch synthesis, and polymer electrolyte membrane fuel cells (PEMFCs).1,3 The activation energy and rate constant for COad * To whom correspondence should be addressed. E-mail: yachen@ ustc.edu.cn. † University of Science and Technology of China. ‡ Miami University.
10.1021/jp908481y CCC: $40.75
desorption in aqueous electrolyte are derived. The effect of COad coverage on such parameters is discussed. 2. Theory Taken that at transition metal surfaces the unimolecular desorption processes follows the first order Polanyi-Wigner equation4
ν)-
dθ ) k · θ ) θ · A(θ, T) · exp-Edes(θ,T)/kT dt
(1)
where k is the rate constant for first order CO desorption, θ is the instantaneous coverage, Edes(θ,T) is the activation energy for the desorption process and A(θ,T) is the frequency factor. During the measurement, a linear increase in the cell temperature (T) can be programmed, thus
T ) T0 + βt
(2)
where β is the heating rate used for the experiment; substituting eq 2 into eq 1 and rearranging the equation yields
[
]
Edes(θ, T) 1 A(θ, T) dT dθ ) exp θ β kT
(3)
By assuming that A is independent of temperature and coverage, we can integrate the above equation using the initial experimental conditions, that is, a coverage value of θ0 at T0. This yields 2009 American Chemical Society
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J. Phys. Chem. C, Vol. 113, No. 47, 2009 20153
[
∫θθ θ1 dθ ) - Aβ · ∫TT exp 0
0
]
Edes(θ, T) dT kT
(4)
Rearranging the above equation, we have
[
∫TT exp 0
]
Edes(θ, T) β θ dT ) - · ln kT A θ0
(5)
If the heating rate β is a fixed constant in the experiment, at the initial temperature T0 the surface coverage of the adsorbate (θ0) is known, and the coverage of the adsorbate (θ) at any desired temperature can be determined by measuring the oxidative charge from the CO stripping experiment (see below), the desorption energy Edes(θ,T) can be calculated numerically by commercial software according to eq 5.5
Figure 1. Cyclic voltammograms of COad stripping from a Pt electrode at different temperatures in 0.5 M H2SO4. Black dotted trace represents the blank CV of the clean Pt electrode recorded at 10 °C. Scan rate: 20 mV · s-1.
3. Experimental Section The thermostatic electrochemical cell contains a conventional three electrode glass cell with a jacket around the cell body, which allows water circulation from a thermostatic bath to control the cell temperature.6,7 A heating rate of 1.08 °C/min is used to ensure the temperature in the cell is well equilibrated with that of the circulating water in the glass jacket. The working electrode was a 50 nm thick Pt film chemically deposited on a Si-Prism (roughness factor ∼7).6,8 Cyclic voltammogram (CV) of the Pt electrode in 0.5 M H2SO4 at 10 °C (Figure 1, dotted line) shows the typical behavior of a polycrystalline Pt electrode. A Pt foil and a reversible hydrogen electrode (RHE) were used as the counter and reference electrode, respectively. All potentials are quoted against the RHE, which was maintained at 25 °C during the measurements. Saturated CO adlayer was obtained by purging CO (99.95%) into deaerated 0.5 M H2SO4 solution at 10 °C for ca. 20 min at 0.1 V. This potential was chosen to ensure that CO oxidation is negligible even when the cell temperature is as high as 80 °C. After that, the solution was thoroughly purged with N2 to remove dissolved CO in the solution. Then a CV of COstripping is recorded between 0.05 and 1.3 V at a scan rate of 20 mV · s-1. To measure the CO coverage at elevated temperatures, the cyclic voltammetric stripping of COad was started at ca. 0.5 °C below the desired temperature in the course of heating the cell from initial adsorption temperature (10 °C). Since the heating rate is only 1.08 °C/min, and the CO oxidation current appears from 0.4 to 0.9 V, the error of the temperature (∆T) at which the CO stripping charge is determined is within e (0.25 °C. After CO stripping experiment at a given temperature, CO adlayer was formed again on the same electrode at 0.1 V and 10 °C, and the CO stripping voltammogram was recorded at the next higher temperature. 4. Results and Discussion Figure 1 shows a representative set of CO stripping CVs at some selected temperatures between 10 and 80 °C after heating the saturated CO adlayer at Pt surface preformed at 0.1 V and 10 °C. No current associated with the deposition or oxidation of upd-H atoms is observed in the CV for CO stripping at 10 °C, confirming that the surface of the Pt film is fully saturated with CO at 0.1 V and 10 °C. The CO oxidation current starts at ca. 0.4 V with a preoxidation wave centered at ca. 0.5 V. The main CO oxidation peak appears at 0.72 V accompanied with a small shoulder at ca. 0.79 V. At E g 0.8 V, the oxidation
Figure 2. COad coverage and desorption rate constant at 0.1 V as a function of cell temperature.
of Pt film takes place, while the Pt oxide reduction peak appears at ca. 0.8 V in the negative going scan. When the cell temperature is increased, the current waves for the COad oxidation shift toward less positive potentials, while the corresponding peaks for the Pt oxide reduction and the H deposition shift to the positive and negative directions, respectively. These temperature-dependent features are in good agreement with previous literature reports,9-11 which reveal an enhanced reaction kinetics of CO oxidation and Pt oxide reduction with the increasing temperature. The prepeak before the main CO oxidation peak is likely from the oxidation of more weakly adsorbed CO.12 The negative shift of the H deposition peaks suggests the destabilization of upd-H atoms at Pt surfaces. Furthermore, at elevated temperatures a small current from upd-H oxidation was observed before CO removal. This current increases with the temperature, indicating the decrease of COad coverage at higher temperatures, as is clearly demonstrated in the θCO-temperature plot in Figure 2. The CO coverage was calculated by using the double layer charge-corrected COad stripping charge, taken that oxidizing a monolayer CO adlayer consumes 420 µC cm-2 of charge.8 The CO coverages given in Figure 2 are results of averaging over three sets of data measured at each temperature and are normalized to the CO coverage at 10 °C. The experimental error is within ca. (5%. From Figure 2, it is seen that COad coverage is roughly constant from 10 to 40 °C, followed by a faster decay toward higher temperatures. However, even by heating up to 80 °C, the coverage of COad at Pt film electrodes only drops by ca. 25% of its saturation value. The present observation is in well
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Figure 3. Activation energy of COad desorption from a Pt electrode at 0.1 V as a function of the COad coverage with different prefactors.
agreement with previous results that in CO free solution the COad desorption from Pt electrodes is negligibly slow at temperatures below 45 °C.9,11,13,14 Assuming that the frequency factor (A) is approximately equal to 1013∼1015 s-1,15,16 the coverage-dependent activation energies for COad desorption from Pt electrode are derived according to eq (5) and given in Figure 3. For the case when A ≈ 1014 s-1, at 0.1 V the activation energy is about 101 kJ/mol at saturated coverage, which increases sharply to ca. 109 kJ/mol with ca. 3% decrease in COad coverage. It follows a slower increase with further decrease in the COad coverage and reaches ca. 116 kJ/mol when the coverage is dropped to 75% of the saturated coverage (note if A is 1013 or 1015, the Edes changes only (15 kJ/mol, while the coverage-dependent changes in Edes remains constant). At solid/gas interface, the heat of CO adsorption on Pt surface has been studied with a variety of experimental techniques such as temperature-programmed desorption,15 low-energy electron diffraction (LEED),15 work function measurements,15 microcalorimetry,17 surface X-ray scattering (SXS),18 and so on. In studies under vacuum conditions, Ertl et al. observed a decrease in COad adsorption energy at Pt(111) from 138 kJ mol-1 at near zero coverage to 62 kJ mol-1 at the saturation coverage of 0.67 ML.15 Microcalorimetric experiments demonstrated that at 0.05 ML the heat of adsorption is near 173 kJ/mol and it decreases to 80 kJ mol-1 at 0.75 ML.17 Obviously, the activation energy for CO desorption from Pt film electrodes determined in the present study is ca. 38-58 kJ/mol higher than the heat of CO adsorption at Pt surfaces with similar coverage as determined in the solid/gas interface. From a recent DFT study on the adsorption of CO to a CO precovered Pt(111) with various initial CO coverages,19 it is found that when the COad coverage is smaller than 0.5 ML, there is no barrier for further CO adsorption. However, when initial COad coverage is higher than 0.5 ML, a barrier of 10 to 50 kJ/mol is estimated for the further uptake of CO to the Pt(111) surface. It is suggested that the barrier comes from the repulsion between the neighboring COad and the incoming CO molecules. Such a barrier should also exist in the desorption process according to the principle of microscopic reversibility. The presence of this barrier explains why the activation energy determined from the present data in the high coverage regime (>0.5 ML) is larger than the heat of CO adsorption. It also partly compensates the decrease of adsorption energy with increasing coverage, rendering a mild coverage-dependence of CO desorption activation energy and desorption rate (Figure 3). The activation energy for COad desorption estimated in the present study agrees well with the results from a recent online mass spectrometric study of CO desorption from Pt nanoparticle/
Letters gas interface,13,20 and SXS measurements at Pt(111)/gas interface with the CO pressure of 0.8 atm,18 where the CO desorption activation energy was found in the range of 99 to 110 kJ/mol. In addition, these numbers are slightly higher than (ca. 10 kJ/ mol) the 12CO/13COad isotope exchange experiments performed either at solid/gas13 or at electrode/aqueous electrolyte interfaces.21,22 It should be noticed that the experiments in electrochemical environment were done at different energy levels (presumably higher) from that of the solid/gas interface in the vacuum.23,24 This notion suggests that the applied electrode potential and the existence of aqueous electrolyte in the double layer do not have a significant effect on the stabilization of COad at Pt electrodes, which is consistent with our recent finding that the potential-induced changes in CO binding energies (∆Eb/∆E) are below 30 kJ/mol per volt.25 Besides obtaining the coverage-dependent activation energy for COad desorption, from the present data and eq (1) we can also derive the temperature-dependent rate constant for CO desorption. The results under the assumption that A is 1014 s-1 are also given in Figure 2. The sharp increase in the COad desorption rate constant at elevated temperature is in good agreement with the first order Polanyi-Wigner equation. The rate constant is of the same order as that measured by Korzeniewski and Huang in acetonitrile at 35 °C.26 Furthermore, we found that the rate constant for CO desorption is ca. 1 to 2 orders smaller than that obtained from 12CO/13COad isotope exchange done either at Pt/electrolyte21 or Pt/gas interfaces20 at room temperature, which suggests that the latter process are adsorption assisted.22 The high activation energy as well as the small rate constant for COad desorption from Pt electrode at 10 °C
