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Preparation and Adsorption Properties of Pd Nanoparticles Supported on TiO2 Nanotubes Andrei Honciuc,*,† Mathias Laurin,† Sergiu Albu,‡ Max Amende,† Marek Sobota,† Robert Lynch,‡,| Patrik Schmuki,‡ and Joerg Libuda†,§ Lehrstuhl fu¨r Physikalische Chemie II, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058, Erlangen, Germany, Lehrstuhl fu¨r Korrosion und Oberfla¨chentechnik, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Martensstraβe 7, 91058 Erlangen, Germany, and Erlangen Catalysis Resource Center, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058, Erlangen, Germany ReceiVed: August 17, 2010; ReVised Manuscript ReceiVed: October 7, 2010
Here we present preparation methods for a series of novel model heterogeneous catalysts consisting of Pd nanoparticles dispersed on self-organized TiO2 nanotube arrays (TiNTs). For a comprehensive comparative study of TiNTs’ surface adsorption properties, different chemical and physical methods were employed for the deposition of the Pd nanoparticles: (i) e-beam deposition in high vacuum (HV) and (ii) in ultrahigh vacuum (UHV), (iii) impregnation, and (iv) metal precipitation. Following preparation, the Pd/TiNTs were exposed to several O2 (oxidative) and CO (reductive) cleaning cycles at 500 K in UHV. After each cleaning cycle, the CO adsorption properties were systematically probed with time-resolved infrared reflection-adsorption spectroscopy to monitor the surface state of the catalyst. It is shown that clean and well-defined Pd/TiNT catalysts can be obtained, which are well suited for future adsorption and reactivity studies. 1. Introduction Heterogeneous catalysts are involved in many chemical processes1,2 enabling transformation of fossil and natural resources into valuable products.3 Further improvement of the catalysts’ performance is a key step toward improved sustainability of the related processes. Toward this aim, one strategy involves refining of the catalysts’ material structure at the nanometer scale. Recent reports3-5 show clear benefits of employing materials with nanodesigned shapes, surfaces, structures, pores, etc., to enhance both the catalyst’s activity and selectivity. Thus, cheap and flexible methods to tailor novel nanostructured materials hold a high potential in heterogeneous catalysis. The preparation of TiO2 nanotube arrays (TiNT)6 represents such a route to nanomaterials with tunable morphology and structure. Arrays of highly oriented TiNTs can be fabricated electrochemically at relatively low costs, even on large surface areas or on existing catalyst supports. TiNTs have already been shown to hold a unique potential with respect to a variety of other applications, e.g. in sensor technology,7-11 for biological implants for bone growth,12 in solar cells,13-17 photocatalysis,18,19 membranes,20 for smart self-cleaning materials,21 and others.6 The growing interest in TiNTs is driven by their large surface area, their adjustable structure and ease of fabrication, good chemical stability, nontoxicity, eco-friendliness, and biocompatibility.22 * To whom correspondence should be addressed. † Lehrstuhl fu¨r Physikalische Chemie II, Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg. ‡ Lehrstuhl fu¨r Korrosion und Oberfla¨chentechnik, Friedrich-AlexanderUniversita¨t Erlangen-Nu¨rnberg. § Erlangen Catalysis Resource Center, Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg. | Present address: Zakład Procesoˇw Elektrodowych, Instytut Chemii Fizycznej (IChF), Polskiej Akademii Nauk (PAN), ul. Kasprzaka 44/52, 01-224 Warszawa, Poland.
Studies on the application of TiNT materials in heterogeneous catalysis are pending. While TiNTs themselves are only moderately active materials for gas-phase reactions, they could be the ideal supports for catalytically active noble-metal nanoparticles. Better insight into the interaction of gases with pure and metal-loaded TiNTs could be obtained by suitable model catalyst studies. First studies on pure TiNTs have indeed indicated pronounced structure dependencies.23-25 Large effects are also expected for supported nanoparticles on TiNTs, as the support structure may have a great influence on the adsorption properties and reactivity.26-28 In this work, we explore the preparation and characterization of noble-metal-loaded TiNTs using an integrated surface-science approach. The systems prepared may offer unique opportunities, for example: (i) the metal nanoparticle distribution in the TiNTs can be controlled, and its influence on the adsorption and reaction kinetics could be studied (and utilized toward the optimization of catalytic properties), and (ii) the influence of the TiNTs’ morphology and structure on the adsorption and catalytic properties could be investigated in great detail. Toward such studies, the development of suitable preparation methods for metal-loaded TiNTs is a key step. Metal-supported heterogeneous catalysts are extensively used in the petro-chemical industry and refinery.29-32 Their activity strongly depends on the physical and chemical properties of the support. The support plays an important role with respect to the accessibility of reactants to and from the active sites, its chemical role in the reaction, metal loading capacity, and prevention of catalyst deactivation due to particle agglomeration and sintering,33 etc. The use of novel TiNT materials as supports for Pd nanoparticles may provide some advantages such as enhanced chemical stability of the substrate, high-loading capacity due to the pore structure, a rough surface morphology which may prevent agglomeration and sintering, and improved control of the reactant fluxes to and from reaction sites. In this work we explore different
10.1021/jp107791r 2010 American Chemical Society Published on Web 11/04/2010
Pd Nanoparticles Supported on TiO2 Nanotubes characterization methods for Pd-loaded TiNTs on planar Ti samples. The adsorption properties of these samples are monitored in ultrahigh vacuum (UHV) using CO as a probe molecule.34,35 From the IR spectra of adsorbed CO, detailed information on the local structure and cleanliness can be obtained. This also allows us to test the stability of the Pd/ TiNT system, and the effectiveness of surface cleaning procedures, as a necessary precondition for future applications of these materials in catalysis. 2. Experimental Section Preparation of the TiNT Arrays. TiO2 nanotube array are prepared by electrochemical anodization in an electrolyte solution of glycerol with 40% vol water and 0.27 M NH4F. The tubes were grown for 1 h at a voltage of 20 V; the final voltage was achieved by increasing the potential with a step rate of 0.25 V/s. Pt was used as counter electrode. TiNT growth mechanisms were described elsewhere.6 The 3 mm thick titanium plate (Alfa Aesar, 99.5%) was cut into pieces of 2 × 1.5 cm2, and the samples’ surface was mirror polished using diamond paste. The TiNT with a length of 1.2-1.5 µm and diameters of 92 ( 12 nm were obtained on the surface. The average intertube distance is 35 ( 7 nm. After anodization TiNT samples were annealed in a furnace in air at 500 °C, following a slow heating ramp of 8 °C min-1. Subsequently, the annealed TiNTs were used for Pd nanoparticle deposition using different methods; these methods are described in the next section. Scanning electron microscopy (SEM, Hitachi FE-SEM S4800) was employed for morphological characterization of the TiNT and Pd/TiNT samples. After preparation and before any UHV molecular beam studies, all Pd/TiNT samples were subject to an ultraviolet/ozone (UVO) cleaning procedure involving exposure of the samples to a 50 mW He-Cd laser (325 nm) for 30 min. This precleaning procedure ensures that major organic contaminants from the preparation are removed. Molecular beam time-resolved infrared reflection-absorption spectrum (MB/TR-IRAS) experiments were performed in an UHV molecular beam apparatus with a base pressure below 2 × 10-10 mbar. The setup 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), 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.35 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: (1) Degassing: brief heating of the sample to 500 K and immediate cooling to 90 K. (2) Initial probing of the catalyst’s surface: 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 Pd/TiNT sample exposed to incremental CO doses divided in 5 pulses of 1 L (1 Langmuir corresponds to a gas dose of 1.33 × 10-6 mbar · s), 5 pulses of 3 L, and 2 pulses of 5 L. Thus, the total CO exposure was 30 L. Each
J. Phys. Chem. C, Vol. 114, No. 47, 2010 20147 spectrum consisted of 256 scans and was acquired at a resolution of 2 cm-1 with a typical acquisition time of 40 s. (3) Cycle 1 (O2/CO cleaning procedure): to activate the catalyst’s surface the sample was heated up 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. At the end of the last CO pulse, the procedure was repeated 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 final CO treatment step, the sample was kept for 5 min at 500 K in UHV in order to remove any CO adsorbed on the sample. (4) Second surface probing: MB/TR-IRAS of CO surface adsorption was performed according to step 2. (5) Cycle 2 (O2/CO cleaning procedure): the O2/CO proceeded according to the parameters described in step 3. (6) The third surface probing: MB-TR-IRAS spectra of CO adsorption at 90 K were recorded under experimental conditions described in step 2. 3. Methods for the Pd Nanoparticle Deposition The aim of the present study is to establish preparation procedures for Pd nanoparticles on TiNTs and to achieve control over their properties such as surface cleanness, particle size and distribution on the nanotubes walls. For this, different chemical and physical methods were employed: (i) e-beam deposition in high vacuum (HV) and (ii) in ultrahigh vacuum (UHV), (iii) wet impregnation, and (iv) metal precipitation from solution. 3.1. Pd/TiNT Preparation in UHV and HV by e-Beam Deposition. For physical vapor deposition (PVD) of Pd onto the TiNT arrays, the TiNT samples were introduced into the UHV chamber via a load-lock sample transfer system. After the initial degassing of the bare TiNTs by heating to 500 K, the sample was cooled to room temperature and Pd films with a nominal thickness of 0.8 nm were deposited using a commercial e-beam evaporator (Focus EFM3). The deposition rate was 0.5 Å per minute, as calibrated by a quartz microbalance, and the angle of the sample surface normal with respect to the Pd beam was 40°. We will further refer to these samples as Pd(UHV, 0.8 nm)/TiNT. In a similar fashion, Pd was deposited in HV onto TiNT samples using a commercial vacuum coating system (BalzersPfeiffer PLS 570) with a base-pressure of 5 × 10-6 mbar. The corresponding samples are denoted as Pd(HV, 0.8 nm)/ TiNT. Here, the e-beam deposition of Pd was performed at an evaporation rate of ∼0.5 Å/s, with an angle of 30° between the sample normal and the Pd beam. After deposition, the Pd/TiNT samples underwent the O2/CO cleaning cycles at 500 K, according to the procedures described in section 2. A SEM analysis was performed after these cleaning cycles. The SEM results appear similar for the both cases, and the corresponding images are presented in Figure 1. The Pd nanoparticles obtained by e-beam evaporation of 0.8 nm Pd, both in UHV and HV, have an average diameter of 4-5 nm after cleaning. It should be noted that most particles are located close to the top end of the nanotubes, e.g., at the orifices of the tubes (see parts c and d of Figure 1). Energydispersive X-ray elemental analysis (EDX) yielded a Pd loading of 0.9 wt % for both samples.
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Figure 1. (a) TR-IR spectra of CO adsorption onto the Pd/TiNT sample prepared by evaporation of 0.8 nm Pd in UHV, function of gas exposure before and after cleaning with O2/CO at 500 K. (b) TR-IR spectra of CO adsorption onto the Pd/TiNT prepared by evaporation of 0.8 nm Pd in HV, function of gas exposure before and after cleaning with O2/CO at 500 K. (c) Top view and (d) cross-section view SEM images of the Pd/TiNT arrays prepared by e-beam deposition of Pd in HV, showing ∼4 nm average diameter Pd nanoparticles mainly distributed on top of the nanotubes.
3.2. Pd/TiNT Prepared by Chemical Reduction of Organometallic Pd Precursors. For the deposition of Pd via precipitation, we used palladium(II) acetylacetonate Pd(AcAc)2 (Sigma-Aldrich Chemicals, 99%) that was reduced 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 palladium(II) acetylacetonate with 50 mL of diethyl ether (Sigma-Aldrich, 99% ACS reagent). The solution was continuously stirred and heated up to the boiling temperature of 34 °C under refluxing conditions. Under these conditions 10 mL of formic acid (Sigma-Aldrich, 99%) were added; the temperature of the new solution increased rapidly to 43 °C and slower thereafter up to the boiling temperature of the new mixture of 55 °C. During the continuous stirring and heating for approximately 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 completion of the reaction, i.e., when the solution became clear. The above amount of Pd precursor used corresponds to 7.85 wt % Pd loading on the TiNT. From the SEM analysis, see parts b and c of Figure 2, the Pd nanoparticles obtained have a density of 900 nanoparticles/µm2 and average size of 14 ( 2 nm distributed homogeneously inside and outside of the tube walls. We will refer to these samples as Pd(pp, 14 nm)/ TiNT.
3.3. Pd/TiNT Preparation by Impregnation Method. Pd nanoparticles were prepared by immersing a TiNT sample in an aqueous solution of 1.5% Pd(NH3)4Cl2 (Sigma-Aldrich, 99.8%) at pH ) 10, adjusted by NH4OH addition. During immersion, the nanotubes and the solution were sonicated for 15 min. The impregnation method is essentially based on bonding of the cation complex, Pd(NH3)42+, to the hydroxyl groups on the TiO2 nanotube walls, followed by an initial in situ self-reduction and deposition of the Pd metal.36 After ultrasonication we have dried the samples overnight in a vacuum desiccator and then introduced it into a furnace under continuous flow of a reducing gas mixture of 10% H2 in Argon (Varigon gas, Linde, 99%) at 500 °C for 90 min to accelerate complete reduction of the precursor salt; the gas flow in the oven was regulated to a flow of ∼200 mL/min. The resulting Pd nanoparticles as observed in SEM (see Figure 3) are heterogeneously distributed, predominantly on the outer surface of the tubes. The agglomeration at the outer surface of the nanotube can be explained by the effect of the capillary forces that act on the precursor salt solution. They cause the liquid to migrate from the larger capillaries (the inner tube diameter is 92 ( 12 nm) to the smaller capillaries (nanotube interspace is on the order of 35 ( 7 nm). Presumably, the more rugged outer-wall surface morphology of the nanotube than that of the inner walls may also favor trapping of the precursor salt. A third effect
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Figure 2. (a) TR-IR spectra of CO adsorption onto the Pd/TiNT sample prepared by precipitation (see text for details, Pd(pp, 14 nm)/TiNT), function of gas exposure before and after O2/CO cleaning cycles at 500 K. (b) Top view and (c) cross-section view SEM images of the TiNT arrays, showing ∼14 nm average diameter Pd nanoparticles distributed homogeneously inside and outside of the nanotubes.
Figure 3. (a) TR-IR spectra of CO adsorption onto the Pd/TiNT sample prepared by impregnation and reduction of Pd(NH3)4Cl2 in the nanopores (see text for details, Pd(imp, 11 nm)/TiNT), function of gas exposure before and after O2/CO cleaning cycles at 500 K. (b) Top-view and (c) cross-section view SEM image of the TiNT arrays, showing ∼11 nm average diameter Pd nanoparticles, distributed homogeneously outside of the nanotube walls and no internal deposition inside the nanotubes.
that may affect the interaction with the solution is the density of surface hydroxyl groups, which could differ between the inner and the outer walls. The final average Pd nanoparticle size was 11 ( 5 nm. It should be noted that this particle size and distribution is comparable to previous result reported from the
same procedure.37 The metal loading determined by EDX was 2.6% wt Pd. The particles are distributed mostly on the outside of the nanotube walls in patches that alternate with bare TiO2 surface. The particle density in the patches, which are located exclusively on the outside of the tubes, is of the order of 4000
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TABLE 1: Summary of the Literature CO Stretching Frequency on Pd(111) and Pd(100) Single Crystals as a Function of Coveragea Pd site assignment (cm-1) system
CO coverage (θ)
Pd(111)
0f0.5 0.6-0.7 0.75 0f0.8
Pd(100) a
on-top
bridge
3-fold hollow 1830f1920
2090 2107, 2110
1960 1893, 1895 1895f1997
comment
ref
continuous shift of the band with coverage bridge/on-top hollow/on-top continuous shift of the band with coverage
40,41,43,46,47,57 40,41,43,46,47,57 40,41,43,46,47,57 40,59
θ is the coverage defined as the ratio of the number of adsorbed species to the number of Pd surface atoms.
TABLE 2: Summary of the Evolution of CO Stretching Frequencies for Pd/TiNT Systems before Cleaning (BC) and after First (C1) and Second Cleaning Cycles (C2) Pd site assignment (cm-1) system
cleaning cycle
on-top
bridge
Pd(UHV, 0.8 nm)/TiNT
BC C1 C2 BC C1 C2 BC C1 C2 BC C1 C2
2087f2103 2107b 2107b
1986a,cf1996b,c 1986a,cf1996b,c
1915a 1915a
1984a,cf1995b,c 1984c,af1995b,c 1943d 1983a,cf1999b,c 1983a,cf1999b,c
1908a, 1892b 1908a, 1892b
Pd(HV, 0.8 nm)/TiNT Pd(pp, 14 nm)/TiNT Pd(imp, 11 nm)/TiNT
a
2104 2104 2102 2107 2107 2091 2104 2104
1992c 1992c
3-fold hollow
Pd2+ (cm-1)
2144e 2141e
TiO2 (cm-1) 2181 2181 2181 2181 2181 2181 2189 2189 2189 2181 2181 2181
Low CO coverage. b High CO coverage. c Bridge CO at edges, defects, and on (100) facets. d Isolated bridge carbonyls. e PdO phase.
nanoparticles/µm2. We will refer to the samples prepared by this method as Pd(imp, 11 nm)/TiNT. 4. Properties and Cleaning of Pd/TiNT Monitored by TR-IRAS of Adsorbed CO The surface properties of the Pd/TiNTs, prepared with the above-described methods, were studied using TR-IRAS. To probe the catalysts’ surface as a function of CO exposure, a series of spectra was acquired after each O2/CO cleaning cycle. By employing TR-IR spectroscopy, important chemical and structural information can be extracted from the CO spectra, such as the availability of different types of adsorption sites and their coverage dependence, surface cleanliness, and the morphology of the particles. The TiNTs of ∼1 µm lengths are largely transparent to infrared and visible radiation,16,38,39 thus it is expected that gas adsorption will be probed not only at the surface, but also deep within the nanotubes. The sensitivity distribution will depend on the precise dielectric properties of the TiNT material. 4.1. TiNT-Supported Pd Nanoparticles Prepared under UHV Conditions. The TR-IR spectra of adsorbed CO on Pd (UHV, 0.8 nm)/TiNT are displayed in Figure 1a as a function of gas exposure and O2/CO cleaning cycle. For the analysis of the spectra the two most relevant regions are: (i) region characteristic to CO adsorbed on Pd, in the range of 1800 to 2100 cm-1 and (ii) region characteristic to CO adsorbed on TiO2, in the range 2160 and 2400 cm-1. The assignment of the bands can be made in the context of the previous CO adsorption studies on Pd single-crystals40-47 and TiO248-50 single crystals on Pd nanoparticles.51-57 CO is often used as a structural probe for Pd surfaces, providing information about the surface sites available and the types of facets exposed. On Pd(111), CO preferentially adsorbs on 3-fold hollow sites in the low coverage limit for energetic58 reasons. At intermediate coverage, hollow and bridge sites are
occupied, while in the high coverage limit, close to saturation, 3-fold hollow and on-top sites are populated. A summary of the CO stretching frequencies on Pd and TiO2 single crystals as a function of gas coverage is given in Table 1. Pd nanoparticles typically expose (111) facets and a minor fraction of (100) facets. Depending on the nanoparticles’ morphology, CO adsorption on additional sites is observed, the most prominent of which are bridging ones, mainly at particle edges and defects.49,53 Bridge-bonded CO at edges and defects appear in the range of 1960-2000 cm-1 in the high-coverage limit. In addition, the fraction of on-top CO increases with decreasing particle size.60,61 Thus, the dependence of the CO spectra on coverage for Pd nanoparticles characteristically differs from the behavior observed for the single crystal surfaces. For further details on such examples, we refer to the literature.49,53,57,60,61 In the TR-IRAS before cleaning (see Figure 1a and Table 2), we observe a single broad absorption band at 2087 cm-1 at lowest CO exposure, which continuously blue-shifts to 2103 cm-1 after exposure to 5 L CO. On the basis of the literature data (see Table 1), this peak corresponds to CO linearly bound to on-top sites. Note that on clean (111) facets this latter peak should appear only in the limit of large CO coverage, i.e., at coverages of θ > 0.6. On clean and regular Pd particles, it should follow the development of the characteristic 2-fold bridge feature around 2000 cm-1. The fact that this band evolves alone and appears already at the lowest CO exposure suggests that only on-top sites are available on the Pd particles before cleaning. This implies the presence of very small Pd clusters with illdefined facets (average diameter e2 nm, at the limit of SEM detection, see Figure SI1a of Supporting Information). It has been shown previously, that on small facets with high concentrations of steps and defects CO is predominately adsorbed in on-top configuration.57 A second effect that may contribute to the dominant on-top CO feature may be the presence of coadsorbed species on the Pd nanoparticles. For example,
Pd Nanoparticles Supported on TiO2 Nanotubes coadsorbed O, which could originate from oxygen reverse spillover is known to block hollow sites and forces CO onto on-top positions.62-64 In the large oxygen high-coverage limit the infrared band was observed at 2133 cm-1, and it can be argued that the band frequency blue-shifted value is coverage dependent. In comparison to the situation before cleaning, the pronounced spectral changes after cleaning (see Figure 1a/cycle 1 and Table 2) indicate significant changes in surface morphology. The peak at 1915 cm-1 corresponds to CO adsorbed on the 3-fold hollow sites, while the feature at 1983 cm-1 contains contributions from bridge bonded CO on (100)42 facets and from defect sites, particles’ edges, and steps.61 The latter peak shifts with increasing coverage to 1996 cm-1 (at 5 L CO), mainly due to static and dynamic adsorbate interactions.53 The prominent feature at 2107 cm-1 corresponds to linearly bound CO, both on defects and on (111) facets. Thus, the effect of the O2/CO cycles is 2-fold: (a) the removal of impurities and coadsorbates previously blocking the bridge sites and hollow sites; (b) morphological changes (see SEM, Figure SI1a of Supporting Information and parts c and d of Figure 1), resulting in larger Pd nanoparticles due to sintering and development of larger (111) and (100) facets. The last cleaning cycle (see Figure 1 a/cycle 2 and Table 2) leaves the TR-IR spectra unchanged. This can be taken as evidence that the Pd nanoparticles undergo no further morphological modifications after the initial cleaning procedure. In the second region of interest in the TR-IR spectra (see Figure 1a and Table 2) the absorption peak at ∼2181 cm-1 corresponds to CO adsorbed on the TiO2-anatase65 surface; in other words, the formation of oxide surface carbonyl species.48-50 Adsorption of CO occurs mainly at coordinatively unsaturated Ti4+ centers, i.e., at defect sites and oxygen vacancies. In a previous study,49 CO adsorption was used as a probe for Lewis acid sites on titania-anatase surfaces, and it was found that two types of oxygen defect sites exist. These sites, denoted as R and β, originate from the four-coordinated Ti4+ and fivecoordinated Ti4+cations, respectively. Characteristic vibrational frequencies for RTi4+-CO at 2208 cm-1 and βTi4+-CO at 2185 cm-1 were found. In comparison with the present data, we conclude that on the Pd(UHV, 0.8 nm)/TiNT only βTi4+sites are available, giving rise to the appearance of the βTi4+-CO band at 2181 cm-1. The more energetic RTi4+sites are, however, absent. It should also be noted that CO adsorption is very weak on defect-free TiO2. On the basis of the qualitative observation that the intensity of the βTi4+-CO band increases in intensity from before cleaning to cycle 2, therefore, it could be concluded that the vacancy sites are partially generated as a consequence of the O2/CO cleaning procedure at 500 K. The fact that no coverage-dependent shift is observed shows that adsorbateinteraction is negligible for this species, which is consistent with the adsorption on highly diluted defect sites. Finally, the fact that the βTi4+-CO band is already observed from lowest CO exposures indicates that the surface mobility of CO on the support is low on the time scale of the experiment. In case of rapid surface diffusion, i.e., reverse spillover, the more favorable Pd sites would be preferentially occupied, before the CO adsorption on the Ti4+ centers could be observed. The final peak appearing at 2341 cm-1, which slowly grows in intensity as a function of CO exposure, can be associated with a CO2 species adsorbed on the titania support. The formation of CO2 on the support appears surprising, particularly in view of the low adsorption temperature (90 K). This observation suggests a pathway for CO2 formation with low
J. Phys. Chem. C, Vol. 114, No. 47, 2010 20151 activation energy barrier.66,67 It should be noted however that in previous experiments on TiNT arrays similar phenomena were reported, both on bare TiNT arrays23 and on Au-covered titania surfaces.68 Furthermore, it was pointed out69 that the CO2 adsorption capacity of titania nanotubes may be enhanced in the presence of metal nanoparticles. We have, however, observed CO2 formation on TiNT samples even in the absence of Pd nanoparticles, see Figure SI2 of Supporting Information. We may invoke two possible explanations for the lowtemperature formation of CO2: (i) the reaction of CO with highly reactive oxygen species on the surface67 or (ii) via decomposition of CO on oxygen vacancy sites on TiO2,66,67,70-72 followed by reaction of CO with the resulting oxygen species. It should be noted that CO2 formation appears to be strongly influenced by the pretreatment of the support, i.e., the O2/CO cleaning cycles (compare before cleaning with cycles 1 and 2 in Figure 1a). 4.2. TiNT-Supported Pd Nanoparticles Prepared in HV. CO TR-IR spectra for the Pd(HV, 0.8 nm)/TiNT are displayed in Figure 1b and summarized in Table 2. It should be pointed out that, similar to the Pd nanoparticles prepared in UHV, SEM reveals sintering of the Pd particles (initially
