In-situ detection of chemisorbed CO on a polycrystalline platinum. Optical infrared visible sum-frequency generation Ž SFG.

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1 21 August Chemical Physics Letters In-situ detection of chemisorbed CO on a polycrystalline platinum foil using infrared visible sum-frequency generation H. Harle, A. Lehnert, U. Metka, H.-R. Volpp ), L. Willms, J. Wolfrum Physikalisch-Chemisches Institut der UniÕersitat Heidelberg, Im Neuenheimer Feld 253, D Heidelberg, Germany Received 17 June 1998 Abstract Optical infrared visible sum-frequency generation SFG. surface vibrational spectroscopy was used for in-situ detection of CO adsorbed on a polycrystalline platinum foil in the range of K at a CO pressure of 1 mbar under flow conditions. From the SFG spectra the frequency of the CO vibrationally resonant contribution was determined as a function of temperature. At 300 K a center-frequency of 2096"4 cm y1 was obtained which is characteristic of the stretching vibration of CO on top -bound at Pt terrace sites. At 660 K, the highest temperature where adsorbed CO could be detected on the foil, the center-frequency was 2057"5 cm y1 which can be attributed to CO molecules bound to defect-site Pt atoms. Results obtained for the polycrystalline foil are compared with literature data for single crystal substrates. Results are presented where the SFG technique was applied for in-situ CO detection during heterogeneous CO oxidation on platinum over a pressure range of 10 y mbar. q 1998 Elsevier Science B.V. All rights reserved. 1. Introduction Among alternative combustion technologies, catalytic combustion is a promising method for burning fuel in lean mixtures resulting in a significant reduction of the formation of pollutants, improved ignition and an enhanced stability of the flames wx 1. Heterogeneous combustion processes are determined by the interaction of diffusion processes from and to the gas phase, by adsorption and desorption on the surface and by surface diffusion and reaction kinetics w2,3 x. Concentrations of reactants and products on the surface are connected with adsorption and desorption equilibria depending on the reactants and products ) Corresponding author. wolfrum@sun0.urz.uni-heidelberg.de gas-phase concentrations and by transport processes. Thus, depending on the operating conditions, different partial processes can become rate-determining leading to to a completely different behaviour of the global surface reaction wx 4. As a consequence, the development of appropriate mathematical models for the numerical simulation of surface reactions and their coupling to the surrounding gas phase is essential for the detailed understanding of heterogeneous catalysis under technically relevant conditions. Computational tools for the description of heterogenous reaction systems have recently been developed w5,6 x, which include detailed surface chemistry by extending the concept of elementary reactions originally introduced by Bodenstein for gas-phase reaction syswx 7 to catalytic combustion processes. as well as tems detailed models for molecular multi-species trans r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.. PII: S

2 27 port. In contrast to flame chemistry wx 8, only a few complete surface reaction mechanisms have been derived, which are mainly based on studies of elementary surface reaction steps carried out under ultra-high vacuum UHV. conditions and on well-dew9 11 x. The use of this kind of fined single surfaces surface kinetics data in the modelling of technical processes which usually take place at high pressure pressure gap. and on polycrystalline catalyst material materials gap. emphasizes the importance of developing in-situ diagnostics techniques for molecular level studies of adsorbed species under practical catalytic combustion conditions. While laser spectroscopic techniques are now a normal working tool in gas-phase combustion diagnostics, quantitative laser-based in-situ diagnostics for the investigation of heterogeneous combustion processes is still under development w12 x. Only recently infrared visible sum-frequency generation SFG. was used for the detection of CO during heterogeneous CO oxidation at high pressures w12,13x and for the investigation of pressure w14x and temperature-dependent w15x changes in the chemisorption of CO over a Pt 111. single crystal model catalyst. The work described in this Letter focuses on the extension of these studies to assess the potential of SFG techniques for bridging the materials gap in the PtrCO system by investigating the possibility for in-situ CO coverage measurements on a polycrystalline Pt foil catalyst. 2. Experimental Experiments were carried out using the SFG specw15x in a reaction chamber trometer described in Ref. schematically depicted in Fig. 1, which allowed studies over a wide pressure range from UHV conditions y10 base pressure of 3 = 10 mbar. up to atmospheric pressure. In the high-pressure regime, the present experimental arrangement allows the study of adsorptionrdesorption and reactive processes of a well-defined stagnation point flow of reactant mixtures on the catalyst surface. The chamber was pumped by a turbo pump backed by a Roots pumping station. The cell pressure could continuously varied by a couple of gate valves and y10 y4 was measured by an ionization gauge y4 mbar, a thermocouple gauge mbar. and a Fig. 1. Schematic diagram of the reaction chamber used for the detection of adsorbed CO on Pt foil via the SFG technique. RFA: retarding field analyser for AES Auger electron spectroscopy. and LEED low-energy electron diffraction.. QMS: quadrupole mass-spectrometer for thermal desorption measurements. Baratron mbar.. The chamber was equipped with a quadrupole mass spectrometer, an q Ar sputter source, a retarding field analyser RFA. for Auger electron spectroscopy AES. and low-energy electron diffraction LEED. and CaF2 and quartz windows which served as entrance and exit ports for the laser beams and the sum-frequency signal, respectively. The sample was mounted on a copper block and could be translated, tilted and rotated by means of a manipulator fitted with a differentially pumped rotary feedthrough. The Pt foil Advent, purity ) 99.99%; height: 1 cm; length: 2.5 cm; thick: mm. and could be resistively heated. The mounting allowed work in the temperature range of K using direct sample heating with a proportional-integral-derivative PID. control unit. The substrate temperature was measured by an Ni NiCr thermocouple spot welded to the Pt foil. A clean platinum sample could be obtained by applying several cycles of Ar q ion sputtering followed by oxidation at 1000 K. After the last sputter cycle, at 300 K with 3 kev Ar q ions for 45 min, the sample was heated to 750 K for 5 min. Then the sample was cooled to 300 K. This procedure ensured reproducibility of the temperature-dependent CO

3 28 SFG spectra measurements. CO purity ) %. flow was dosed with a leak valve and SFG spectra were recorded at different substrate temperatures. For the optical measurements a 40 ps mode-locked Nd:YAG laser system was used. A part of its output was frequency doubled to 532 nm and used as the visible input frequency for the SFG process. The other part was used to pump an optical parametric system to generate infrared IR. radiation tunable in the frequency range of cm y1 with a pulse duration of 25 ps. The absolute frequency of the tunable IR laser radiation was calibrated by measurements of the gas-phase CO absorption bands around 2143 cm y1 in a reference cell filled with CO. Numerical modelling of the measured gas-phase absorption band shapes was used to determine the laser bandwidth to be 7"1 cm y1. Both the visible and IR laser beams were p- polarized and overlapped at the surface. The incident angles were 558 for the visible and 358 for the IR beam. The energy for the visible and IR beams were 400 and 30 mj per pulse, respectively. The spot size of the visible beam was 5 mm in diameter and the IR beam was slightly focussed to fall within the visible. The sum-frequency signal reflected from the Pt surface was detected after filtering of scattered light by a dielectric filter and a monochromator by a photomultiplier and a gated integrator and transferred to a laboratory computer via an analogue-to-digital converter. Every point in the SFG spectra shown in Fig. 2 was obtained by averaging over 120 laser shots. At a laser repetition rate of 10 Hz, the time for the measurement of a complete SFG spectrum was typically 30 min. The principles of the SFG surface vibrational spectroscopy technique have been described in detail elsewhere w16 x. SFG is a second-order non-linear optical process where a tunable infrared v. IR and a visible v. VIS laser beam are mixed to generate a sum-frequency output v. SFG. In the electric dipole approximation this process is allowed only in a medium without centrosymmetry. The intensity ISFG of the generated SFG signal is proportional to the product of the non-linear surface susceptibility xs 2. and its complex conjugate < 2. < 2 < I Ax PConj x s x s x qx < 2 SFG S S. S NR R. 1. Fig. 2. SFG spectra of CO adsorbed on a polycrystalline Pt foil. The SFG signal intensity ISFG is plotted versus the frequency v. IR of the tunable IR laser. Solid lines represent the results of least-squares fits. Details of the fit procedure are described in the text. Spectra were recorded under CO adsorptionrdesorption equilibrium conditions at a CO pressure of 1 mbar. xs 2., which is a third-order tensor quantity, can be modelled in the lowest order as the sum of a non-resonant x NR 2. and resonant term x R 2. associated with a vibrational mode of the adsorbate w 17 x. xs 2. sx NR 2. qx R The resonant term x R 2. can be expressed, within the dipole approximation, as w18x NTXMX D r A 2. R x R s s " v IR yv X qi G. vir yv X qi G. 3. where N denotes the surface density of adsorbed molecules, TX is the IR transition moment and MX is a term proportional to the Raman transition moment of the vibrational mode of the adsorbed molecule X. v X is the corresponding molecular vibrational frequency, G is the homogeneous Lorentzian half-width of the mode and D r is the population difference between the ground and vibra-

4 29 tional excited states. If the adsorbate vibration is both Raman and IR active, the resonant contribution becomes significant as the IR laser is tuned through the vibrational mode. Therefore the SFG spectrum provides similar information to conventional vibrational spectroscopy with the major difference that the SFG signal originates predominately from the surface. In the following A, defined via Eq.. R 3, will be denoted as the amplitude of the resonant contribution. The non-resonant term x NR 2. can in principle consist of contributions originating from the adsorbate, the surface, or from cross-terms resulting from adsorbate surface interaction. In the present study we observed a noticeable non-linear response from the clean Pt surface, which was independent of the IR wavelength over the spectral region of the CO y1 stretching frequencies cm.. Followw x 2. ing Ref. 19, the non-resonant term x NR can be expressed as x 2. sa e if, 4. NR NR where ANR is the magnitude of the vibrationally non-resonant contribution due to electronic excitations of the Pt surface and the adsorbate and F is its phase, relative to the vibrational resonance. Inserting. 3 and. 4 into. 2 yields the following expression XsCO.: non-linear surface susceptibility xs 2.. The numerical least-squares fit procedure was used to determine the parameters v CO, A R, G, and ANR and F as a function of the substrate temperature. The SFG spectra were recorded by tuning the IR laser over the frequency region of cm y1 in which stretching vibrations of terminally adsorbed CO can be excited. Under the experimental conditions of the present study no bridge-bound CO were observed on the Pt foil in the frequency region of cm y1. Bridge-bound CO was previously observed via SFG under UHV conditions on a Pt 111. single crystal surface Ts150 K. at an IR frequency of y1 about 1854 cm w19 x. The resonant contribution to the SFG spectrum which originates from the adsorbed CO molecules can be characterized by the resonant amplitude A R, the CO vibrational frequency v and the Lorentzian width 2 G. of the resonant contribution. In Fig. 3, the values of these three parameters obtained in the CO A 2. R if S NR v IR yvco qi G. x s qa e, 5... Eq. 5 was used together with Eq. 1 for the numerical simulation of the measured SFG spectra. 3. Results and discussion SFG spectra of CO terminally adsorbed on the polycrystalline Pt foil were recorded in the temperature range of Ts K at a CO pressure of 1 mbar under adsorptionrdesorption equilibrium conditions. Several of the measured spectra are depicted in Fig. 2 at T s 700 K a SFG signal from chemisorbed CO could no longer be detected.. Solid lines represent results obtained in the numerical least-squares fit of the experimental points using Eq.. 1 for the SFG intensity I and Eq.. 5 for the SFG Fig. 3.. a The amplitude of the resonant contribution A.. R b CO vibrational resonance frequency v.. c Width FWHMs2 G. CO of the resonant contribution of the SFG spectra as defined in the text. Values are plotted against the Pt foil temperature. Solid lines are drawn just to guide the eye. The open triangle at T s700 K in. a corresponds to A s0. R

5 30 present study are plotted against the temperature of the Pt foil. The plot of the resonant amplitude AR versus substrate temperature Fig. 3a. clearly indicates that the equilibrium concentration of chemisorbed CO decreases with increasing surface temperature. However, care has to be taken in using AR as a measure of the CO surface coverage. Independent calibration experiments under UHV conditions, in which SFG studies and temperature programmed desorption TPD. measurements for the direct determination of the relative CO surface coverage were combined, showed that in the COrPt foil system the SFG intensity at the resonance frequency I v. SFG CO is the spectroscopic quantity which correlates linearly with CO surface coverage w20 x. Therefore the SFG spectra measured in the present study were also evaluated in order to determine I v. SFG CO and hence the relative CO coverage as a function of the substrate temperature see Fig. 4.. The relative CO coverage versus substrate temperature plot shown in Fig. 4 indicates the presence of two different adsorptionrdesorption regimes. One in the temperature range 300-T-580 K, in which the equilibrium CO coverage decrease almost linearly with substrate temperature and a second one at T)600 K in which the equilibrium CO coverage decreases more rapidly with increasing substrate temperature. This behaviour could be ex- Fig. 4. Relative CO coverage as a function of the Pt foil substrate temperature measured under CO adsorptionrdesorption equilibrium conditions at a CO pressure of 1 mbar. The open square at T s700 K corresponds to zero coverage. The solid line is the result of a numerical simulation assuming two different CO adsorption sites and a stagnation point flow of CO onto the Pt foil corresponding to the conditions of the experiment w20 x. plained by the presence of two distinct adsorption sites on the Pt foil. The solid line depicted in Fig. 4 represents the result of a numerical modelling calculation of a stagnation point flow of CO onto the Pt foil corresponding to the conditions of the experiw20 x. The simulations were based on a mean ment field model w21,22x assuming two different CO adsorption sites. For the CO species correlated to the first kind of adsorption sites, a coverage-dependent activation energy for desorption increasing from 55 kjrmol at saturation coverage. to 110 kjrmol with decreasing CO coverage was used. For the second adsorption site, the activation energy for desorption was 118 kjrmol. More detailed information about the nature of the different adsorption sites in the COrPt foil system can be obtained from the analysis of the temperature dependence of the resonance frequency vco depicted in Fig. 3b and a comparison with the results of SFG and IR reflection-absorption spectroscopy IRAS. experiments on different Pt single crystal surfaces w19,23,24 x. In Ref. w19 x, Daum et al. determined in their SFG studies on a Pt 111. single crystal at Ts300 K a value of vco s2093.3"1.6 cm y1 at saturation coverage. A comparable value of vco s2097 cm y1 was reported by Hayden et al. w23x in the high coverage limit on a Pt-sw = 100.x surface. In both cases the frequency was assigned to CO adsorbed on-top on 111. terrace sites. Within the experimental uncertainty vco s 2096"4 cm y1 observed in the present study at Ts300 K is in good agreement with these values, indicating the presence of terraces on the Pt foil. In the low coverage regime -40% of saturation coverage. resonance frequencies in the range vco s cm y1 were determined in the IRAS studies on the Pt-sw = 100.x surface an assigned to CO linearly bound on step-sites w23 x. These values, as well as their dependence on the CO coverage, are in agreement with the results of the present studies, suggesting a similarity in topology of the Pt-sw = 100.x and the polycrystalline Pt foil surface. This interpretation is also in agreement with results of TPD studies, which revealed close similarity in the terracerstep site structure be = 100.x and a polycrystalline tween the Pt-sw4 Pt surface w23,25 x. A detailed discussion of experimental data obtained in UHV studies on recrys-

6 31 tallised 1114-oriented Pt foils w26x can be found in Ref. w27 x. The temperature dependence of the width FWHMs 2 G. of the resonant contribution of the SFG spectra is depicted in Fig. 3c. Deconvolution of the measured spectra with the laser bandwidth was not performed. Eq.. 5 was directly used together with Eq.. 1 for the numerical simulation to obtain the necessary parameters. The same method for analysis of the room temperature SFG spectra of CO terminally adsorbed on a Pt 111. surface was used in w x y1 Ref. 19 where a value of 2 G s 10 cm was reported at Ts300 K for the saturation coverage. A slightly higher value of 2 Gs14"1 cm y1 was determined in the present work at the same substrate temperature. A comparable value of 16"1 cm y1 was obtained previously on a disordered Pt 111. surface, which was sputtered in order to obtain a high density of step-sites w15 x. The difference in the width of the SFG spectra observed on the ordered and disordered Pt 111. surface was attributed to an inhomogeneous broadening effect w28x originating from the high density of step-sites on the disordered Pt 111. surface. Comparison with the present result suggests a similar inhomogeneous broadening mechanism for the Pt foil in the temperature range 300- T-600 K. The increase of the width observed at T)600 K which coincides with the change in the adsorptionrdesorption kinetics, see Fig. 4. could be explained by the onset of an additional inhomogeneous broadening effect due to the loss of local order of the strongly adsorbed step-site CO molecules at low surfaces coverages w29 x. According to Eq.. 4, the non-resonant part of the SFG spectra can be described by A NR, the magnitude of the vibrationally non-resonant contribution and F which represents the phase between the non-resonant and the vibrationally resonant contribution. In the present study for both quantities only a weak dependence on the substrate temperature was observed in the analysis of the measured SFG spectra. Both quantities depend on the electronic properties of the Pt surface which seem to change only slightly in the substrate temperature range K. Future studies will focus on the application of the SFG technique for in-situ detection of surface species during heterogeneous reactions on polycrystalline substrates over a wide pressure range. This repre- Fig. 5. SFG spectra of chemisorbed CO measured during the high-pressure reaction of COrO rn mixtures % CO, 0.5% O, 99% N. on a Pt catalyst surface. Relative gas-phase concentrations of CO and O2 were close to typical concentrations present in the exhaust gas of an SI engine. sents an extension of our ongoing work on the high-pressure CO oxidation on Pt 111., in which SFG is used to monitor the concentration of chemisorbed CO see Fig. 5. in the pressure range y mbar.. The detailed discussion of the experimental and theoretical results of the high pressure CO oxidation studies on a Pt 111. single-crystal surface will be the subject of a forthcoming publicaw20 x. tion 4. Conclusion After it has been demonstrated that optical infrared visible SFG surface vibrational spectroscopy can be used to bridge the pressure gap see Fig. 5 w20x and Ref. w13 x., in the present study it could be demonstrated that SFG surface vibrational spectroscopy can also be used for bridging the materials gap. By means of the SFG method in-situ CO detection and determination of CO coverages on a polycrystalline platinum foil in the substrate temperature range Ts K at a CO pressure of 1 mbar was possible. Furthermore the present study

7 32 indicates the possibility of the use of SFG to monitor different locations of CO chemisorbed on polycrystalline Pt foil by analysing the corresponding CO vibrational frequency. Acknowledgements The work was supported by the Deutsche Forschungsgemeinschaft DFG. via the Sonderforschungsbereich 359 Reaktive Stromung, Diffusion und Transport at the University of Heidelberg. Partial support by the TECFLAM research association is gratefully acknowledged. Thanks are due to F. Behrendt, R. Kissel-Osterrieder and J. Warnatz In- terdisziplinares Zentrum fur Wissenschaftliches Rechnen, University Heidelberg. for stimulating discussions and H.H. Rothermund Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin. is thanked for helpful communications and for sending a copy of Ref. w11 x. References wx 1 W.C. Pfefferle, L. Pfefferle, Progr. Energy Comb. Sci wx 2 R.A. van Santen, J.W. Niemantsverdriet, Chemical Kinetics and Catalysis, Plenum, New York, wx 3 J. Warnatz, U. Maas, R.W. Dibble, Combustion: Physical and Chemical Fundamentals, Modeling and Simulation, Experiments, Pollutant Formation, Springer, Heidelberg, wx 4 P.S. Cremer, X. Su, Y.R. Shen, G.A. Somorjai, J. Am. Chem. Soc wx 5 O. Deutschmann, F. Behrendt, J. Warnatz, Catalysis Today wx 6 F. Behrendt, O. Deutschmann, U. Maas, J. Warnatz, J. Vac. Sci. Technol. A wx 7 J. Wolfrum, H.-R. Volpp, R. Rannacher, J. Warnatz Eds.., Gas Phase Chemical Reaction Systems: Experiments and Models 100 Years after Max Bodenstein, Springer Series in Chemical Physics, Vol. 61, Springer, Heidelberg, wx 8 D.L. Baulch, C.J. Cobos, R.A. Cox, C. Esser, P. Frank, Th. Just, J.A. Kerr, M.J. Pilling, J. Troe, R.W. Walker, J. Warnatz, J. Phys. Chem. Ref. Data wx 9 H.-J. Freund, Ber. Bunsenges. Phys. Chem w10x G. Ertl, Ber. Bunsenges. Phys. Chem w11x H.H. Rotermund, Surf. Sci. Rep w12x see e.g. J. Wolfrum, Hottel Plenary Lecture: Lasers in Combustion-from Basic Theory to Practical Devices, 27th Symposium International. on CombustionrThe Combustion Institute, 1998 accepted for publication.. w13x G.A. Somorjai, Appl. Surf. Sci w14x X. Su, P.S. Cremer, Y.R. Shen, G.A. Somorjai, Phys. Rev. Lett w15x H. Harle, K. Mendel, U. Metka, H.-R. Volpp, L. Willms, J. Wolfrum, Chem. Phys. Lett w16x Y.R. Shen, The Principles of Nonlinear Optics, Wiley, New York, w17x J. Miragliotta, R.S. Polizotti, P. Rabinowitz, S.D. Cameron, R.B. Hall, Appl. Phys. A w18x J.H. Hunt, P. Guyot-Sionnest, Y.R. Shen, Chem. Phys. Lett w19x C. Klunker, M. Balden, S. Lehwald, W. Daum, Surf. Sci w20x H. Harle, A. Lehnert, U. Metka, H.-R. Volpp, J. Wolfrum, F. Behrendt, R. Kissel-Osterrieder, J. Warnatz, to be published. w21x J. Warnatz, M.D. Allendorf, R.J. Kee, M.E. Coltrin, Comb. Flame w22x O. Deutschmann, R. Schmidt, F. Behrendt, J. Warnatz, 26th Symposium International. on CombustionrThe Combustion Institute, 1996, p w23x B.E. Hayden, K. Kretzschmar, A.M. Bradshaw, R.G. Greenler, Surf. Sci w24x J. Xu, J.T. Yates Jr., Surf. Sci w25x C.L. Levoguer, R.M. Nix, Surf. Sci w26x D.A. King, in: R.F. Willis Ed.., Vibrational Spectroscopy of Adsorbates, Springer, Berlin, 1980, and references therein. w27x P. Hollins, Surf. Sci. Rep w28x R.G. Tobin, Surf. Sci w29x E. Schweizer, B.N.J. Persson, M. Tushaus, D. Hodge, A.M. Bradshaw, Surf. Sci