pubs.acs.org/Langmuir © 2010 American Chemical Society
Controlling the Adsorption Kinetics via Nanostructuring: Pd Nanoparticles on TiO2 Nanotubes Andrei Honciuc,*,† Mathias Laurin,† Sergiu Albu,‡ Marek Sobota,† Patrik Schmuki,‡ and Joerg Libuda†,§ †
Lehrstuhl f€ ur Physikalische Chemie II, Friedrich-Alexander-Universit€ at Erlangen-N€ urnberg, Egerlandstrasse 3, D-91058, Erlangen, Germany, ‡Lehrstuhl f€ ur Korrosion und Oberfl€ achentechnik, Friedrich-AlexanderUniversit€ at Erlangen-N€ urnberg, Martensstrasse 7, 91058 Erlangen, Germany, and §Erlangen Catalysis Resource Center, Friedrich-Alexander-Universit€ at Erlangen-N€ urnberg, Egerlandstrasse 3, D-91058, Erlangen, Germany Received May 27, 2010. Revised Manuscript Received July 8, 2010 Activity and selectivity of supported catalysts critically depend on transport and adsorption properties. Combining self-organized porous oxide films with different metal deposition techniques, we have prepared novel Pd/TiO2 catalysts with a new level of structural control. It is shown that these systems make it possible to tune adsorption kinetics via their nanostructure. Self-organized TiO2 nanotubular arrays (TiNTs) prepared by electrochemical methods are used as a support, on which Pd particles are deposited. Whereas physical vapor deposition (PVD) in ultrahigh vacuum (UHV) allows us to selectively grow Pd particles at the tube orifice, Pd/TiNT systems with homogeneously distributed Pd aggregates inside the tubes are available by particle precipitation (PP) from solution. Both methods also provide control over particle size and loading. Using in-situ infrared reflection absorption spectroscopy (IRAS) and molecular beam (MB) methods, we illustrate the relation between the nanostructure of the Pd/TiNT systems and their adsorption kinetics. Control over the metal nanoparticle distribution in the nanotubes leads to drastic differences in adsorption probability and saturation behavior. These differences are rationalized based on differences in surface and gas phase transport resulting from their nanostructure. The results suggest that using carefully designed metal/TiNT systems it may be possible to tailor transport processes in catalytically active materials.
1. Introduction Heterogeneous catalysts are a key component in 21st century technology. Not only most bulk and fine chemicals are produced using heterogeneously catalyzed reaction steps, but suitable catalysts may also be the key ingredients in future energy production and storage.1,2 The success of new catalysts critically depends on careful optimization of processes, with the aim at maximizing their activity and selectivity. There are numerous reports in the literature3,4 that address the very critical issue of improving catalyst’s performance via nanostructuring. Thus, we may reiterate that the use of a novel generation of nanostructured support materials may open up strategies toward the optimization of selectivities and activities of catalytic materials on a rational basis. From a materials science perspective, the self-organized titanium oxide nanotubes arrays, prepared by electrochemical methods, represent such an example of porous oxide structures with a high degree of structural control.5-7 In this work we focus on TiO2 nanotube (TiNT) arrays; their material properties and applica*To whom correspondence should be addressed. E-mail: andrei.honciuc@ chemie.uni-erlangen.de. (1) Ertl, G.; Kn€ozinger, H.; Weitkamp, J. Handbook of Heterogeneous Catalysis; VCH: Weinheim, 1997; p 5 v (xxiiv, 2479 pp). (2) Centi, G.; Perathoner, S. Top. Catal. 2009, 52(8), 948–961. (3) Astruc, D. Nanoparticles and Catalysis; Wiley-VCH: Weinheim, 2008; p xxiii, 640 pp. (4) Scott, S. L.; Crudden, C. M.; Jones, C. W. Nanostructured Catalysts; Kluwer Academic: New York, 2003; p xvi, 334 pp. (5) Albu, S. P.; Ghicov, A.; Aldabergenova, S.; Drechsel, P.; LeClere, D.; Thompson, G. E.; Macak, J. M.; Schmuki, P. Adv. Mater. (Weinheim, Ger.) 2008, 20(21), 4135–þ. (6) Ghicov, A.; Schmuki, P. Chem. Commun. (Cambridge, U. K.) 2009, 20, 2791– 2808. (7) Bauer, S.; Kleber, S.; Schmuki, P. Electrochem. Commun. 2006, 8(8), 1321– 1325.
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tions were discussed in detail in the recent literature.6,8-12 Highly ordered TiNT arrays consist of vertically oriented nanotubes, with tunable width/length aspect ratio, tube morphologies, and crystalline structure.13-15 Furthermore, their large surface area and welldefined oriented geometry are highly relevant for applications where surfaces and interfaces play an important role, such as heterogeneous catalysis and energy harnessing.16,17 Used as a catalyst support, such structures may provide pathways to an enhanced level of control over transport processes. These nanostructures are particularly attractive also due to their relatively low cost of fabrication on large surfaces and can potentially be integrated in catalytic systems on already available surfaces. Toward the preparation of TiNT arrays based catalysts, the second and equally critical step is the deposition of the active component.8,18,19 Typically, the active phase is the noble metal (8) Paramasivalm, I.; Macak, J. M.; Schmuki, P. Electrochem. Commun. 2008, 10(1), 71–75. (9) Balaur, E.; Macak, J. M.; Taveira, L.; Schmuki, P. Electrochem. Commun. 2005, 7(10), 1066–1070. (10) Oh, S.; Jin, S. Mater. Sci. Eng., C 2006, 26(8), 1301. (11) Macak, J. M.; Tsuchiya, H.; Ghicov, A.; Yasuda, K.; Hahn, R.; Bauer, S.; Schmuki, P. Curr. Opin. Solid State Mater. Sci. 2007, 11(1-2), 3–18. (12) Roy, P.; Kim, D.; Lee, K.; Spiecker, E.; Schmuki, P. Nanoscale 2010, 2(1), 45–59. (13) Ghicov, A.; Tsuchiya, H.; Macak, J. M.; Schmuki, P. Electrochem. Commun. 2005, 7(5), 505–509. (14) Albu, S. R.; Kim, D.; Schmuki, P. Angew. Chem., Int. Ed. 2008, 47(10), 1916–1919. (15) Macak, J. M.; Schmuki, P. Electrochim. Acta 2006, 52(3), 1258–1264. (16) Hervier, A.; Renzas, J. R.; Park, J. Y.; Somorjai, G. A. Nano Lett. 2009, 9 (11), 3930–3933. (17) Park, J. Y.; Lee, H.; Renzas, J. R.; Zhang, Y. W.; Somorjai, G. A. Nano Lett. 2008, 8(8), 2388–2392. (18) Mohapatra, S. K.; Kondamudi, N.; Banerjee, S.; Misra, M. Langmuir 2008, 24(19), 11276–11281. (19) Macak, J. M.; Schmidt-Stein, F.; Schmuki, P. Electrochem. Commun. 2007, 9(7), 1783–1787.
Published on Web 08/10/2010
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Figure 1. (A) TR/IRAS spectra of CO adsorbed on Pd (PVD, 0.8 nm)/TiNT acquired at 90 K after UHV deposition of Pd at room temperature and cleaning with O2/CO. (B) Evolution of the CO peak area with gas exposure. (C) Top view SEM image of the TiNTs decorated with Pd nanoparticles of 4-5 nm average size. (D) SEM image cross-section view showing the nanoparticles present mostly distributed within ∼100 nm of from the top of the TiNTs.
nanoparticles. Size, shape, and distribution of these particles inside the tubes will control the catalytic properties of the material. Thus, it is a major challenge to achieve control over these parameters. Here, we focus on the loading of TiNT arrays with Pd nanoparticles. We have explored a variety of preparation methods including impregnation techniques, particle precipitation, e-beam deposition in high vacuum, and physical vapor deposition (PVD) in ultrahigh vacuum (UHV). Highlighting the potential of these systems, we focus in the present article on the most promising preparation techniques: particle precipitation (PP) from solution and PVD in UHV. These methods allow the preparation of very different particle distributions inside the tubes, i.e., selective deposition of Pd particles at the tube orifices or, alternatively, deposition of a homogeneous distribution of Pd particles inside the inner tube surface. For both methods, the average Pd particle size can be varied over a large range of values. A more complete overview including all methods as well as cleaning and characterization procedures will be given in a forthcoming publication. In order to probe differences in the adsorption behavior on both types of samples, we investigate the kinetics of CO adsorption Langmuir 2010, 26(17), 14014–14023
using molecular beam (MB) methods under UHV conditions.2021 In combination with in-situ IR reflection absorption spectroscopy (IRAS), this technique allowed us a direct comparison of adsorption and reaction probabilities on the specific surface sites.21 Clear differences in the adsorption kinetics are observed, which can be straightforwardly related to gas phase transport inside the tubes and surface transport on the TiO2 support. This illustrates that it should indeed be possible to tune adsorption and reaction properties of well-defined porous nanostructures on a rational basis.
2. Experimental Section TiO2 nanotube arrays are prepared by electrochemical anodization of a 2 1.5 cm 3 mm Ti pieces in a glycerol:water (6:4, (v) electrolyte containing 0.27 M NH4F. The tubes were grown for 1 h at a voltage of 25 V; the initial voltage was achieved by increasing the potential in steps of 0.25 V/s in order to maintain reproducibility (20) Scoles, G. Atomic and Molecular Beam Methods; Oxford University Press: New York, 1988. (21) Libuda, J.; Freund, H. J. Surf. Sci. Rep. 2005, 57(7-8), 157–298.
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Figure 2. (A) TR/IRAS spectra of CO adsorbed on Pd(PVD, 3 nm)/TiNT acquired at 90 K after preparation at room temperature 300 K and O2/CO treatment at 500 K. (B) Evolution of the CO peak area with the gas exposure. (C) Top-view SEM image of TiNT array decorated with Pd nanoparticles deposited by e-beam evaporation of 3 nm Pd in UHV condition. (D) SEM cross-section view of TiNT array with Pd nanoparticles distributed mostly within 100 nm depth from the top of the nanotubes (see inset). among TiNT samples. Pt was used as counter electrode. Details of the tube growth mechanisms were described elsewhere.6 The dimensions of the TiO2 nanotubes obtained with the anodization parameters chosen were nanotube length ∼1.2-1.5 μm, nanotube average diameter at the top 92 ( 12 nm, and the average nanotube interspace 35 ( 7 nm. Scanning electron microscopy (SEM, Hitachi FE-SEM S4800) was employed for morphological characterization of the TiNT and Pd/TiNT samples. The cross-sectional TiNT images were obtained after gently scratching of the sample surface and detaching small oxide microlayers from the surface. Following anodization, TiNT samples were immediately cleaned with water, blown dry by a nitrogen gas stream. After drying, the TiNTs were annealed in a furnace, in air at 500 °C, following a slow heating ramp of 8 °C min-1. After preparation, the TiNT samples were used in the subsequent steps for loading with metal nanoparticles. Two methods were employed for nanoparticle deposition: (i) physical vapor deposition (PVD) in UHV and (ii) particle precipitation (PP) in solution: 14016 DOI: 10.1021/la102163a
(i) TiNT arrays were introduced into the UHV chamber via the sample transfer system. Prior to metal deposition, the TiNTs were degassed by flashing to 500 K and recooling to room temperature. Subsequently, 0.8 and 3 nm Pd were deposited under UHV conditions using a commercial e-beam evaporator (Focus EFM3). The deposition rate was 0.5 A˚/min as calibrated by a quartz microbalance, and the angle of the sample surface normal with respect to the Pd beam was ∼40°. The nanoparticles resulted from the e-beam evaporation of 0.8 nm Pd in the UHV have an average diameter of 4-5 nm after undergoing the cleaning procedure and are mostly distributed on top of the nanotube (Pd(PVD, 0.8 nm)/TiNT). Similarly, after e-beam evaporation of 3 nm Pd, nanoparticles of 17 nm average diameter are obtained (Pd(PVD, 3 nm)/TiNT) (see Figure S1a,b). The SEM images (Figures 1 and 2) were acquired after the O2/CO cleaning procedure at 500 K. (ii) The precipitation method consisted in the deposition of the nanoparticles from a reduction reaction of palladium(II) acetylacetonate (Sigma-Aldrich, 99%) precursor with formic acid (Sigma-Aldrich, 96%); the reaction conditions can be adjusted such that different size nanoparticles are Langmuir 2010, 26(17), 14014–14023
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Figure 3. (A) TR/IRAS spectra of CO adsorbed on Pd(pp, 4 nm)/TiNT acquired at 90 K after the first O2/CO treatment cycle at 500 K. (B) Peak integral area as a function of CO exposure. The solid lines between the data points were drawn as guides to the eye. (C) Top-view SEM image of the TiNT array decorated with Pd nanoparticles obtained by precipitation, showing nanoparticles of narrow size distribution and ∼4 nm average diameter. (D) Cross-section SEM image of the Pd(pp, 4 nm)/TiNT showing an uniform particle distribution outside and inside of the nanotubes and complex rugged morphology in between the nanotube arrays (see insets). obtained: nanoparticles of ∼14 nm average diameter (Pd(pp, 14 nm)/TiNT) and nanoparticles of ∼4 nm average diameter (Pd(pp, 4 nm)/TiNT). For the Pd(pp, 14 nm)/TiNT) sample, the preparation method involved precipitation/deposition of the Pd metal from palladium(II) acetylacetonate, Pd(AcAc)2 (SigmaAldrich Chemicals, 99%), after reduction with formic acid (Aldrich, 98%). A previously prepared TiNT sample was suspended through one of the ports of a three-neck flask in a solution of 3.5 mg of palladium(II) acetylacetonate with 50 mL of diethyl ether (Sigma-Aldrich, 99% ACS reagent). The solution was continuously stirred and heated to the boiling temperature of 34 °C under refluxing conditions. 20 mL of formic acid (SigmaAldrich, 99%) was added to the initial solution; the temperature of the new solution increased rapidly to 43 °C and slower Langmuir 2010, 26(17), 14014–14023
thereafter up to the boiling temperature of the mixture of 55 °C. Under continuous stirring and heating for ∼30 min in the temperature interval 43-55 °C, the solution slowly changed color from initially yellow to brown and dark-brown, suggesting a slow reduction of the Pd complex to the Pd metal; the heating was stopped after the completion of the reaction, i.e., when the solution became clear. For the preparation of the Pd(pp, 4 nm)/ TiNT) sample the procedure was similar to the one previously described except that heating of the reaction mixture was stopped at ∼51 °C under continuous stirring, quenching this way the formation of larger particles. At the end the color of the mixture was still dark brown and remained so even after reheating and stirring, suggesting that the reagents were consumed and small Pd nanoparticles were present in solution. DOI: 10.1021/la102163a
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Figure 4. (A) TR/IRAS spectra of CO adsorbed on Pd(pp, 14 nm)/TiNT, with 7.85 wt % Pd loading and an average particle size of 14 ( 2, acquired at 90 K after cleaning with O2/CO at 500 K. (B) Peak integral area as a function of CO exposure. The solid lines between the data points were drawn as guides to the eye. (C) Top-view SEM image of the TiNT array decorated with Pd nanoparticles obtained by precipitation, showing nanoparticles of narrow size distribution and ∼14 nm average diameter. (D) Cross-section SEM image of the Pd(pp, 14 nm)/TiNT showing an uniform particle distribution outside and inside of the nanotubes (see insets).
The above amount of Pd precursor used corresponds to 7.85 wt % Pd loading on the TiNT for Pd(pp, 14 nm)/TiNT) and 7.2 wt % Pd loading for the Pd(pp, 4 nm)/TiNT), as determined from EDX analysis. From the SEM morphological analysis in the case of Pd(pp, 14 nm)/TiNT) (see Figure 3) the Pd nanoparticles obtained have a density of 900 nanoparticles/μm2 and average size of 14 nm (see Figure S1c), distributed homogeneously inside and outside of the tube walls. Similarly, for the Pd(pp, 4 nm)/ TiNT) case, the SEM analysis shows that the nanoparticles obtained (see Figure 4) have a 4 nm average diameter (see Figure S1d) and a particle density of 6700 nanoparticles/μm2, distributed homogeneously inside and outside of the nanotube walls. SEM images were acquired before and after the O2/CO cleaning 14018 DOI: 10.1021/la102163a
procedure at 500 K, and no visible morphological changes appear to the nanoparticles. After preparation and before the transfer of the samples in the UHV chamber, all Pd/TiNT samples were subject to an ultraviolet/ozone (UVO) precleaning procedure for 30 min by exposure of the samples to a 50 mW He-Cd laser (325 nm). The UVO cleaning procedure ensures that major organic contaminants from the preparation are removed from the surface of nanotubes. MB/TR-IRAS experiments were performed in an UHV molecular beam apparatus with a base pressure of 2 10-10 mbar. The set up allows up to four effusive beams and one supersonic beam to be superimposed on the sample surface. Additionally, the system is equipped with an FTIR spectrometer (Bruker IFS66/v), Langmuir 2010, 26(17), 14014–14023
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a beam monitor for intensity calibration and alignment of the beams, two quadrupole mass spectrometers, a vacuum transfer system, a high-pressure cell, and a Pd e-beam evaporator (Focus EFM3); the details and capabilities of a similar molecular beam apparatus were described elsewhere.22 The CO beam (Linde, 99.97%) was generated from an effusive beam and modulated by a valve system. The MB/TR-IRAS experiments followed a fixed protocol used for each sample including the following steps: (a) Initial degassing procedure: brief heating of the sample to 500 K and immediate cooling to 90 K. (b) O2/CO cleaning procedure: in order to activate the catalyst’s surface, the sample was heated to a constant temperature of 500 K and exposed to O2 (beam intensity 3.2 1015 cm-2 s-1) from an effusive beam source for 20 min and subsequently to CO (beam intensity 3 1015 cm-2 s-1) also from an effusive beam source for 15 min. The procedure was repeated a second time with exactly the same parameters. Exposing the catalyst’s surface to a flux of O2 at high temperature should remove carbonaceous contaminants, and the exposure to CO gas should remove the adsorbed O2 and oxide formed at the Pd surface. At the end of the second CO treatment the sample was kept for 5 min at 500 K in UHV in order to remove any CO adsorbed on the sample. (c) MB/TR-IRAS was performed in a remote-controlled sequence, by exposing the sample to pulses of CO at beam intensities of 1.8 1014 cm-2 s-1 (equivalent pressure: 6.3 10-7 mbar) and 3.5 1014 cm-2 s-1 (equivalent pressure: 1.2 10-6 mbar) at 90 K, followed by immediate acquisition of an IRAS spectrum at the end of each pulse. The exposure time of the MB pulses was adjusted such that the Pd/TiNT sample exposed to incremental CO doses divided in 5 pulses of 1 langmuir (1 langmuir corresponds to a gas dose of 1.33 10-6 mbar 3 s), 5 pulses of 3 langmuirs, and 12 pulses of 5 langmuirs. Thus, the CO exposure was 80 langmuirs. Each spectrum consisted of 256 scans and was acquired at a resolution of 2 cm-1 with a typical acquisition time of 40 s.
3. Results and Discussion The CO adsorption kinetics on Pd/TiNT is investigated for the samples prepared by (i) PVD in UHV (section3.1) and (ii) PP from solution (section 3.2). In order to differentiate between the nanoparticle size and distribution effects, small and big nanoparticles were prepared with each method. The particle size and particle distribution for all samples was investigated by SEM subsequently to the CO adsorption experiments. For this purpose, the samples were removed from the UHV system via a load-lock system and inserted into an SEM apparatus after gently scratching the oxide surface to dislodge microflakes of TiNTs for cross-section analysis. Top and side views of the Pd loaded TiNTs are shown in bottom parts of Figures 1-4. CO adsorption was probed by pulsing CO from a MB source and detection of IR spectra according to the procedure described in section 2. Two pieces of information are available from IRAS: First, CO IR spectra can be used as a structural probe, providing information on the types of adsorption sites available on the surface. Second, the adsorption kinetics on the Pd and TiO2 sites can be monitored. 3.1. Pd Particles Prepared by PVD in UHV. We start by considering Pd particles, deposited by PVD on TiNT arrays at room temperature and under UHV conditions. Two Pd loadings were investigated in order to obtain different particles sizes. The corresponding SEM and IRAS data for nominal Pd thicknesses of 0.8 and 3 nm are displayed in Figures 1 and 2, respectively. For the lower metal loading (0.8 nm, Figure 1), we observe a high density of small Pd nanoparticles, mostly decorating the orifices (22) Libuda, J.; Meusel, I.; Hartmann, J.; Freund, H. J. Rev. Sci. Instrum. 2000, 71(12), 4395–4408.
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of the nanotube walls. The average diameter of the Pd particles is around 5 nm. At higher metal loading (3 nm, Figure 2) much larger Pd particles are formed, which are characterized by a broad size distribution (most particles are between 10 and 60 nm in size). The broad size distribution is a typical consequence of particle coalescence occurring at higher loading. Most particles exhibit an elongated shape along the tube orifice, and it may be explained by coalescence effects. Previous STM23 and HR-TEM24 work showed that Pd nanoparticles preferentially expose (111) and a smaller fraction of (100) facets. The most important observation is derived from the crosssection view of the nanotubes, shown in Figures 1D and 2D. We found that nearly all Pd particles are located within a depth of 100 nm from outer nanotube orifices. This is the result of the deposition geometry with a large tilt angle, 40°, between metal evaporator and surface normal. a. CO Adsorption on Pd(PVD, 0.8 nm)/TiNT. The IR spectra taken during exposure of Pd(UHV, 0.8 nm)/TiNT to CO at a sample temperature of 90 K are displayed in Figure 1A. Based on CO adsorption studies on Pd single crystals25-31 and on Pd nanoparticles,32-36 the interpretation of these spectra is straightforward. The CO spectra are often used as a probe for the surface sites available and the types of facets that Pd nanoparticles expose. Therefore, it is important here to briefly overview of the typical CO adsorption behavior onto the Pd(111) and Pd(100) facets from single-crystal studies. On Pd(111), CO adsorbs preferentially on 3-fold hollow sites in the low coverage limit, on bridge and hollow at intermediate coverage, while only in the high coverage limit the on-top sites are occupied. On Pd(111), the CO on 3-fold hollow sites shows vibrational frequencies between 1830 cm-1 at low coverage and ∼1920 cm-1 at θ = 0.5 (θ is the coverage defined as the ratio of adsorbate species to surface adsorption sites). At θ = 0.6-0.7, CO on bridge sites gives rise to a band at ∼1960 cm-1. At saturation coverage θ = 0.75, CO adopts a characteristic high coverage structure with 3-fold hollow (∼1893 cm-1) and on-top (∼2097 cm-1) CO. The CO adsorption behavior on Pd(100) is quite different, with bridgebonded CO appearing at ∼1895 cm-1 in the zero coverage limit, shifting continuously with increasing coverage to ∼1997 cm-1 at saturation (θ = ∼0.8).37 Bridging sites, located mainly at particle edges, give rise to bands in the 1960 and 2000 cm-1 range. In addition, the fraction of on-top sites increases with decreasing particle size.38,39 (23) Schauermann, S.; Hoffmann, J.; Johanek, V.; Hartmann, J.; Libuda, J.; Freund, H. J. Angew. Chem., Int. Ed. 2002, 41(14), 2532–þ. (24) Henry, C. R.; Chapon, C.; Penisson, J. M.; Nihoul, G. Z. Phys. D: At., Mol. Clusters 1989, 12(1), 145. (25) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3(2-3), 107. (26) Szanyi, J.; Kuhn, W. K.; Goodman, D. W. J. Vac. Sci. Technol., A 1993, 11, 1969–1974. (27) Kuhn, W. K.; Szanyi, J.; Goodman, D. W. Surf. Sci. 1992, 274(3), L611– L618. (28) T€ushaus, M.; Berndt, W.; Conrad, H.; Bradshaw, A. M.; Persson, B. Appl. Phys. A: Mater. Sci. Process 1990, 51(2), 91. (29) Goodman, D. W. Chem. Rev. (Washington, DC, U. S.) 1995, 95(3), 523–536. (30) Ohtani, H.; Hove, M. A. V.; Somorjai, G. A. Surf. Sci. 1987, 187(2-3), 372. (31) Surnev, S.; Sock, M.; Ramsey, M. G.; Netzer, F. P.; Wiklund, M.; Borg, M.; Andersen, J. N. Surf. Sci. 2000, 470(1-2), 171. (32) Rupprechter, G. Phys. Chem. Chem. Phys. 2001, 3(21), 4621–4632. (33) Evans, J.; Hayden, B. E.; Lu, G. Surf. Sci. 1996, 360(1-3), 61–73. (34) Bertarione, S.; Scarano, D.; Zecchina, A.; Johanek, V.; Hoffmann, J.; Schauermann, S.; Frank, M. M.; Libuda, J.; Rupprechter, G.; Freund, H. J. J. Phys. Chem. B 2004, 108(11), 3603–3613. (35) Goyhenex, C.; Croci, M.; Claeys, C.; Henry, C. R. Surf. Sci. 1996, 352-354, 475. (36) Rainer, D. R.; Wu, M. C.; Mahon, D. I.; Goodman, D. W. J. Vac. Sci. Technol., A 1996, 14(3), 1184–1188. (37) Ortega, A.; Huffman, F. M.; Bradshaw, A. M. Surf. Sci. 1982, 119(1), 79. (38) Frank, M.; Baumer, M. Phys. Chem. Chem. Phys. 2000, 2(17), 3723–3737. (39) B€aumer, M.; Freund, H.-J. Prog. Surf. Sci. 1999, 61(7-8), 127.
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On the basis of the above information, the peak around 1915 cm-1 (Figure 1A) can be associated with CO adsorbed on Pd hollow sites, most probably on (111) facets, while the peak around 1980 cm-1 contains contributions from bridge-bonded CO on (100) facets26 and from defect sites like particles edges and steps.40 The blue shift of the bridge-bonded CO to 1996 cm-1 with the increasing surface coverage is due to adsorbate-adsorbate interactions;34 the vibrational frequencies at saturation coverage are typical for the bridge-bonded CO onto Pd.33 The absorption band around 2100 cm-1 is associated with on-top CO. The intensity ratio of on-top to bridge CO is typical for particle sizes in the range of 5 nm (compare ref 34). The peak at ∼2184 cm-1 appears in the IR spectra in Figure 1A, already at the lowest exposures, and grows slowly up to the highest CO exposures used in the present experiment. Absorption features in this frequency range are characteristic to CO adsorbed onto the TiO2-anatase surface and formation of surface carbonyls.41-44 Adsorption of CO occurs onto coordinatively unsaturated Ti4þ centers at defect sites or onto oxygen vacancies. In a previous study,42 CO adsorption was used as a probe for Lewis acids sites on titania surfaces. It was found that there may be two types of oxygen defect sites, denoted as R and β, which originate from four-coordinated Ti4þ and five-coordinated Ti4þ cations, respectively. Characteristic vibrational frequencies for RTi4þ-CO at 2208 cm-1 and βTi4þ-CO at 2185 cm-1 were found. Compared with the present data, we conclude that on the Pd(UHV, 0.8 nm)/TiNT only βTi4þ sites are available. The fact that no coverage-dependent shift is observed shows that adsorbate interactions are negligible for this species, which is consistent with adsorption on highly diluted defect sites. Finally, the fact that the βTi4þ-CO band is observed from lowest exposures indicates that the surface mobility of CO bound to the defect sites of the support is low on the time scale of the experiment. In case of rapid surface diffusion, the more favorable Pd sites would be occupied, before the CO adsorption at the energetically less favorable Ti4þ centers could be observed. Finally, the last peak observed at 2342 cm-1 slowly grows in intensity as a function of CO exposure. The feature can be associated with a CO2 species adsorbed on the titania. The formation of CO2 on the support is a somewhat surprising phenomenon, as it occurs at low temperatures (90 K), i.e., with a very low activation barrier. It should be noted, however, that in previous experiments on TiNT arrays similar phenomena were reported on bare TiNT arrays.45 The information on the adsorption kinetics extracted from the IRAS is summarized in Figure 1B. It is observed that the bridging CO peak (around 2000 cm-1) most rapidly increases in intensity and reaches saturation at CO exposures around 10-15 langmuirs. It is closely followed by the on-top CO peak (2100 cm-1) saturating at around 15 langmuirs. This behavior is similar to what is observed for CO adsorption on Pd nanoparticles on planar supports.39 A completely different behavior is observed for the CO adsorbed at TiO2 defect sites (2184 cm-1). The intensity of the latter band continuously increases without saturation even for the largest CO exposures used in the present experiment (40) Yudanov, I. V.; Sahnoun, R.; Neyman, K. M.; Rosch, N.; Hoffmann, J.; Schauermann, S.; Johanek, V.; Unterhalt, H.; Rupprechter, G.; Libuda, J.; Freund, H. J. J. Phys. Chem. B 2003, 107(1), 255–264. (41) Derrouiche, S.; Gravejat, P.; Bianchi, D. J. Am. Chem. Soc. 2004, 126(40), 13010–13015. (42) Hadjiivanov, K.; Lamotte, J.; Lavalley, J. C. Langmuir 1997, 13(13), 3374– 3381. (43) Linsebigler, A.; Lu, G. Q.; Yates, J. T. J. Chem. Phys. 1995, 103(21), 9438–9443. (44) Spoto, G.; Morterra, C.; Marchese, L.; Orio, L.; Zecchina, A. Vacuum 1990, 41(1-3), 37. (45) Funk, S.; Burghaus, U. Catal. Lett. 2007, 118(1-2), 118–122.
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(80 langmuirs). There are two possible explanations for this scenario. It may either be due to an intrinsically low sticking probability of CO on TiO2 sites or due to hindered transport through the nanotubes. In section 3.2 it will be shown that by changing of the particle distribution inside the tubes it is possible to distinguish between these two hypotheses. b. CO Adsorption on Pd(PVD, 3 nm)/TiNT. Before discussing the influence of the particle distribution, however, we briefly consider effect of the particle size. In order to discriminate between the effect of nanoparticles distribution and potential size effect contributions to the adsorption kinetics, larger size nanoparticles were also studied. The IR data for CO adsorption on the sample prepared by PVD with larger Pd particles (Pd (PVD, 3 nm)/TiNT) are shown in Figure 2A. According to the previous discussion, these first features appearing at 1908 and ∼1986 cm-1 can be primarily attributed to CO adsorbed onto 3-fold hollow sites on Pd(111) facets and onto bridge-bonded CO at particle edges, steps, and (100) facets. This bridging CO band blue-shifts to values around 2003 cm-1 with increasing coverage, which is typical for bridge-bonded CO on Pd(100).27 The small coveragedependent shift of only 17 cm-1 (in contrast to ∼100 cm-1 observed for Pd(100)37) suggests, however, that this peak is not entirely due to (100) facets but contains a substantial contribution from defect and particle edges (compare ref 40). A high coverage CO phase on Pd(111),32 with CO occupying both on-top and hollow sites, is revealed by nearly simultaneous appearance of the bands at 1892 and 2107 cm-1. The clear appearance of these peaks indicates the presence of large well-ordered Pd(111) facets. The CO adsorption kinetics for Pd (PVD, 3 nm)/TiNT is displayed in Figure 2B. The intensities of the bridging CO band (∼2000 cm-1) and the on-top CO band (∼2100 cm-1) show a more rapid saturation behavior than the band at 2184 cm-1, representing CO at TiO2 defect sites. A closer comparison of the kinetic data in Figure 2B to the data for the smaller Pd particles in Figure 1B reveals an interesting difference. Whereas for the smaller particles saturation was achieved at exposures of around 15 langmuirs, much larger CO exposures are required for the present sample. In fact, the bridging CO peak saturates around 30 langmuirs of CO, and even 50 langmuirs is required for the on-top CO species. This particle size dependence appearing in the adsorption kinetics can be rationalized on the basis of the so-called “capture zone effect”, which has been observed for adsorption of various gases on oxide supported noble-metal catalysts.46-49,21,50 It is based on the fact that two channels contribute to adsorption on these systems. In addition to the CO molecules impinging and adsorbing onto the Pd particles themselves, the majority of CO molecules impinge on the oxide support. Part of the latter molecules is trapped in a weakly bound and mobile physisorbed state on the support. Via surface diffusion, these molecules may reach more strongly adsorbing sites on the metal particles (or also TiO2 defect sites in the present case). This additional adsorption channel may significantly enhance the adsorbate flux to the particles and thus substantially accelerate the adsorption rate (acceleration factors of 4-5 have been observed in the literature, see e.g. ref 21 and references therein). It should be noted that the diameter of the “capture zone” from which adsorbates are collected is determined by the activation energies for adsorption and diffusion (46) Gillet, E.; Channakhone, S.; Matolin, V.; Gillet, M. Surf. Sci. 1985, 152-153(Part 1), 603. (47) Matolin, V.; Gillet, E. Surf. Sci. 1986, 166(1), L115. (48) Becker, C.; Henry, C. R. Surf. Sci. 1996, 352-354, 457. (49) Becker, C.; Henry, C. R. Catal. Lett. 1997, 43(1-2), 55–57. (50) Harding, C. J.; Kunz, S.; Habibpour, V.; Heiz, U. Phys. Chem. Chem. Phys. 2008, 10(38), 5875–5881.
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on the support. At constant temperature, the relative contribution of the effect is, therefore, largest for small particles and negligible in the limit of large particles (see e.g. refs 21, 48, and 50 for a more detailed discussion). This directly explains the different saturation behavior for the PVD samples as a function of particle size: Whereas the capture zone effect substantially accelerates CO adsorption for the Pd (PVD, 0.8 nm)/TiNT sample with particle sizes around 5 nm, it plays only a minor role for the (PVD, 3 nm)/TiNT sample with particles, which are about 1 order of magnitude larger in diameter. Nevertheless, two different trends can be clearly observed in Figures 1B and 2B: one describing the CO adsorption kinetics onto the surface of the Pd nanoparticles and the other onto TiO2 surface. 3.2. Pd Particles Prepared by Particle Precipitation from Solution. As a second type of Pd/TiNT systems we investigated were samples prepared by precipitation from solution (see section 2 for details on the preparation). Again we investigate two samples corresponding to two different average particle sizes. The corresponding IRAS and SEM data are displayed in Figures 3 and 4. All samples were subject to O2/CO cleaning procedures, as described in section 2. For the sample shown in Figure 3, an average particle size of ∼4 nm is derived from SEM (Pd(pp, 4 nm)/ TiNT). The second sample, displayed in Figure 4, corresponds to a larger average particle size of about 14 nm (Pd(pp, 14 nm)/TiNT). SEM shows some indications for the formation of well-defined crystallites, in the latter case. From the Pd/TiNT cross-section images, presented in Figures 3D and 4D, it can be observed that the Pd particle in the present case are well-distributed over the complete interior wall surface of the nanotubes, in contrast to the samples prepared by PVD (see section 3.1). a. CO Adsorption on Pd(pp, 4 nm)/TiNT. TR-IRAS spectra in Figure 3A correspond to CO adsorption on Pd(pp, 4 nm)/ TiNT, i.e., nanoparticles with a smaller average size of 4 nm. A markedly different behavior is observed here, with a strong CO adsorption peak at ∼2096 cm-1 completely dominating the spectrum. This feature is attributed to on-top CO. We explain the absence of the characteristic bridge and hollow CO peaks by the presence of contaminations present as a consequence of the wet-chemical deposition method.51 Typically, atomic adsorbates block bridge and hollow site, displacing CO to more weakly bonding on-top sites.23,52 It should be pointed out that the contaminations present on the small Pd particles cannot be removed even by repeated O2/CO cleaning procedures described in the Experimental Section. Nevertheless, the CO adsorption kinetics on these particles is noteworthy (see Figure 3D). The CO peak attributed to on-top CO on Pd continuously grows as a function of exposure in parallel with the bands associated with CO on the TiO2 defects. This observation strongly suggests that the differences in CO adsorption kinetics on the PVD sample are a consequence of the Pd particle distribution rather than the result of different intrinsic sticking coefficients. In order to further clarify this point, CO adsorption is investigated on large Pd particles prepared by the particle precipitation method. b. CO Adsorption on Pd(pp, 14 nm)/TiNT. The TR-IRAS spectra of CO adsorbed on the large Pd particles prepared by particle precipitation from solution (Pd(pp, 14 nm)/TiNT) are displayed in Figure 3A. CO spectra are comparable to those observed for the samples prepared by PVD in UHV. This immediately shows that for the larger Pd particles contaminations play a minor role only. The CO adsorption proceeds according to (51) Beebe, T. P., Jr.; Yates, J. T., Jr. Surf. Sci. 1986, 173(2-3), L606. (52) Shaikhutdinov, S. K.; Frank, M.; Baumer, M.; Jackson, S. D.; Oldman, R. J.; Hemminger, J. C.; Freund, H. J. Catal. Lett. 2002, 80(3-4), 115–122.
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the characteristic sequence, starting with the population of the 3-fold hollow adsorption sites on (111) facets (∼1908 cm-1) and followed by the population of the bridge sites on (100) facets and at edges and defects (1986 cm-1); this latter peak shifts to a value of 1999 cm-1 at saturation. The on-top sites (∼2107 cm-1) start to become populated at exposures of 4-5 langmuirs. It is noteworthy that the peak at 1892 cm-1 is clearly observed at exposure exceeding ∼5 langmuirs as well; it reveals the presence of welldeveloped and largely contamination-free (111) facets (see discussion above and refs 26 and 32). Concerning the adsorption of CO on titania sites, it should be pointed out that the absorption band for βTi4þ-CO (∼2189 cm-1) is also present on the Pd(pp, 14 nm)/TiNT, already starting from exposures as low as 1 langmuir of CO. For the CO adsorption kinetics, the corresponding intensity development of the CO bands as a function of exposure is plotted in Figure 4B. We observe that the characteristic bands for CO adsorbed on Pd sites (∼1999 and ∼2107 cm-1) follow the same behavior as the CO band associated with adsorption on TiO2 defect sites (2189 cm-1). No clear saturation is observed, even up to CO exposures around 80 langmuirs. This behavior is similar to the one found for the Pd(pp, 4 nm)/TiNT sample; i.e., the small particles prepared by particle precipitation (in spite of the contaminations present in the latter case). It is in sharp contrast, however, to the adsorption kinetics on the samples prepared by PVD (Pd(PVD, 0.8 nm)/TiNT and Pd(PVD, 3 nm)). From this observation, we immediately conclude that the differences found for the adsorption kinetics on Pd and on titania sites for the samples prepared by PVD are not due to different intrinsic sticking probabilities on both types of sites. In fact, they must be but due to the hindrance of gas transport through the nanotubes. In other words, the Pd particles at the tube orifice experience the highest CO impingement rate. The CO impingement rate, and therefore the CO adsorption rate, drops rapidly, however, when leaving the surface region of the TiNT array and going to particle located deeper inside the tubes. This observation may appear trivial at a first glance. In fact, it is not and deserves a more in-depth discussion. First, let us consider a geometrically perfect nanotube without any active adsorption sites, which is exposed to a CO molecular beam. A brief estimate based on kinetic gas theory reveals that the pressure inside the tube will equilibrate to the outside (beam) pressure on a very short time scale (typically on the order of microseconds). In other words, the CO pressure inside the tubes would be constant on the time scale of the present experiments. This implies that in the limit of a very low density of adsorption sites being exposed we would not expect any delayed adsorption or a reduced effective sticking coefficient as a result of the presence of the nanotubes; however, this is not the case in the present experiment. The observed effect of reduced effective adsorption probability arises only if the density of adsorption sites inside the tubes is sufficiently large in comparison to the rate at which CO enters the tubes. More precisely speaking, the product of the number of adsorption sites inside the tube and the molecular flux into this tube must be large compared to the temporal resolution of the experiment. At this point, a rough estimate of the density of adsorption sites on the present samples is required. Here, the following estimate may be helpful: For the Pd(PVD, 0.8 nm)/TiNT sample the deposited Pd atom density corresponds to 6.8 1015 Pd atoms cm-2. For an average particle size of around 5 nm the dispersion, i.e., the fraction of Pd surface atoms, is around 0.2.53 (53) Meusel, I.; Hoffmann, J.; Hartmann, J.; Heemeier, M.; Baumer, M.; Libuda, J.; Freund, H. J. Catal. Lett. 2001, 71(1-2), 5–13.
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Figure 5. Schematic representation of the transport processes controlling the CO adsorption kinetics of Pd/TiNT samples studied in this work (see text for details).
Using this number and a CO saturation coverage of θ = 0.75,28 we obtain CO density of 8.2 1014 CO molecules cm-2 at saturation. Because of the nonquantitative character of the IRAS experiment25 it is not straightforward to transfer these numbers to the other samples. The similar intensities for CO adsorbed to Pd for all samples suggest, however, that the CO densities are at least in the same order of magnitude. The signals associated with CO adsorbed onto the titania defect sites are rather low, suggesting that these sites are likely to be even less abundant than the Pd sites. There are two possible scenarios which could limit the CO flux into deeper regions of the TiNT array: (i) The first would be that rapid adsorption at Pd or titania sites close to the orifice leads to rapid CO consumption and prevents CO from reaching deeper regions. (ii) The second explanation is that the morphology of the tubes and their entrances themselves reduces the probability of CO to enter deeper regions of the tubes. At this point we reconsider the CO flux in the experiment. Typical effective beam pressures in the experiment were around 1.2 10-6 mbar, corresponding fluxes of around 3.5 1014 CO molecules cm-2 (1 langmuir of CO corresponds to 3.8 1014 CO molecules cm-2). On the basis of these numbers and the adsorption site densities (see above), the first explanation can be straightforwardly ruled out. The density of adsorption sites is simply insufficient to completely capture a large fraction of the incoming CO. Therefore, it appears reasonable to consider intrinsic transport barriers related to the tube morphology. For the TiNTs used in the present experiment, it can be observed from the SEM images (see Figure 3C,D) that the inner nanotube walls are rather smooth. Here, no difficulty would be expected for the CO gas molecules to migrate rapidly through the tube. On the other hand, considerable ruggedness can be observed on the outer nanotube walls (Figure 3C,D). In addition, we observe exterior ripples similar to the bamboo-shaped nanotubes,14 leading to the formation of additional transport barriers. We therefore suggest that the reduced adsorption probability in deeper region of the TiNT array is mainly caused by transport hindrances giving rise to “reduced gas mobility” or reduced “nanotube conductance” toward the bottom of the TiNT array system. 14022 DOI: 10.1021/la102163a
Combining these findings with the information from section 3.1, we conclude that the adsorption probabilities on active metals site of the Pd/TiNT systems are strongly influenced by transport effects, both via the surface and via the gas phase (see Figure 5 for a summary of the discussed phenomena). Surface transport comes into play via the capture zones, where adsorbate molecules trapped on the titania support can reach the Pd particles via surface diffusion. The capture zone effect enhances the adsorbate flux to small particles, provided that they are located in the vicinity of the tube orifices. Gas phase transport becomes important as a result of “conductivity limitations” for the gas inside the TiNT arrays, most likely a direct consequence of the nanotube wall morphology. For the present Pd/TiNT systems gas phase transport limitations lead to a drastically reduced effective sticking coefficient for CO on Pd particles located in the bottom regions of the TiNT array. The exact mechanisms that lead to “conductivity limitations” deep inside the nanotubes array remain a conjecture at this point. Previous studies,54-56 which deal with gas diffusion in the nanotubes, pointed out that roughness and deviations from perfect nanotubular structures may lead to reduced gas diffusion in nanotubes. The ability to change the effective sticking coefficient via nanostructuring provides means of controlling the adsorption kinetics onto the Pd/TiNT catalysts via nanostructuring. In summary, both effects observed provide means to control reactant transport to the active centers of porous supported metal catalysts. It has been demonstrated that for catalytic reactions for which the adsorption step has a sufficient degree of rate control the reaction rate is directly dependent on the reactant flux to the active center.21,57-59 Thus, the phenomena discussed in the present paper provide a versatile tool for tuning reactivity and selectivity of catalytic material via their nanostructure. (54) Malek, K.; Coppens, M. O. J. Chem. Phys. 2003, 119(5), 2801–2811. (55) Gruener, S.; Huber, P. Phys. Rev. Lett. 2008, 100 (6). (56) Coppens, M. O.; Malek, K. Chem. Eng. Sci. 2003, 58(21), 4787–4795. (57) Ladas, S.; Poppa, H.; Boudart, M. Surf. Sci. 1981, 102(1), 151. (58) Harding, C. J.; Kunz, S.; Habibpour, V.; Teslenko, V.; Arenz, M.; Heiz, U. J. Catal. 2008, 255(2), 234–240. (59) Harding, C. J.; Kunz, S.; Habibpour, V.; Heiz, U. Chem. Phys. Lett. 2008, 461(4-6), 235–237.
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4. Conclusion We have prepared novel supported catalyst systems, based on the deposition of Pd nanoparticles on well-defined titania nanotube arrays. CO adsorption on these multinanostructured systems is probed by IR reflection absorption spectroscopy (IRAS) in combination with molecular beam techniques. 1. Pd nanoparticles have been prepared by two complementary techniques, i.e., (a) physical vapor deposition (PVD) under ultrahigh-vacuum conditions and (b) particle precipitation (PP) from solution. For both methods, the particle sizes can be varied over a large range. Whereas PVD leads to a system with Pd particles located close to the orifice of the nanotubes, a homogeneous distribution of particles inside the tubes can be obtained for Pd deposition via PP. 2. Pd nanoparticles’ surface is characterized by CO adsorption. The adsorption properties are largely consistent with the development of crystalline and well-shaped nanoparticles. In addition, titania defect sites in the interior of the nanotubes are probed by CO adsorption. 3. The kinetics of CO adsorption on the Pd particles is investigated via time-resolved IRAS. It is shown that two types of transport effects, related to the nanostructure of the systems, control the adsorption kinetics: (a) Surface transport strongly influences the adsorption rate on Pd particles located at the orifice of the nanotubes. Surface trapping and diffusion lead to a pronounced capture zone effect, which strongly enhances the (60) Hartmann, M. Angew. Chem., Int. Ed. 2004, 43, 5880–5882.
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adsorbate flux to small particles. (b) Gas phase transport is strongly dependent on the position of the particle inside the tube. Conductivity limitations as a result of the TiNT array morphology give rise to a suppressed adsorbate transport to particles located deeper inside the TiNT array system. These results demonstrate that it is possible to control the adsorption kinetics of the well-defined nanoporous oxide-supported metal systems via their structural properties. It is the coupling between reaction and transport processes that leads to unexpected benefits and provides guides to materials design.60 The class of nanostructured Pd/TiNT model catalysts allows modifying of the adsorption kinetic on rational basis providing simple nanostructural pathways toward controlling activity and selectivity of the catalyst. Acknowledgment. A.H. gratefully acknowledges financial support by a research grant of the Alexander von Humboldt Foundation. M.S. gratefully acknowledges support by the Fonds der Chemischen Industrie via a Kekule grant. The authors acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. Furthermore, the present work was partly supported by the DFG, Fonds der Chemischen Industrie, and the European Union (COST D-41). Supporting Information Available: Figure S1. This material is available free of charge via the Internet at http://pubs. acs.org.
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