J. Phys. Chem. C 2008, 112, 19555–19559
19555
Catalytic and Mechanistic Investigation of Polyaniline Supported PtO2 Nanoparticles: A Combined in situ/operando EPR, DRIFTS, and EXAFS Study Florian Klasovsky,† Jens Hohmeyer,† Angelika Bru¨ckner,‡ Marcus Bonifer,† Ju¨rgen Arras,† Martin Steffan,† Martin Lucas,† Jo¨rg Radnik,‡ Christina Roth,§ and Peter Claus*,† Department of Chemistry, Ernst-Berl Institute/Chemical Technology II, Darmstadt UniVersity of Technology, Petersenstrasse 20, D-64287 Darmstadt, Germany, Institute for Materials Science, Darmstadt UniVersity of Technology, Petersenstrasse 23, D-64287 Darmstadt, Germany, and Leibniz Institute for Catalysis Berlin, Richard-Willsta¨tter-Strasse 12, D-12489 Berlin, Germany ReceiVed: July 7, 2008; ReVised Manuscript ReceiVed: September 26, 2008
Compounds containing electrically conducting macromolecules bear interesting properties as functional materials in various fields. As for catalytic applications, however, principles are scarcely, if at all, understood. Here we describe a systematic investigation of the relationship between the structure of a new type of conducting polymer supported metal catalyst and the catalyst’s oxidation activity. We have found that nanoclusters of unusual β-PtO2 (d ) (1.9 ( 0.5) nm) could be deposited on polyaniline, and the resulting catalyst exhibits exceptionally low light-off temperatures in carbon monoxide oxidation even in the presence of other environmental pollutants. Characterizing the working state of the active catalyst by combining in situ/operando techniques (EPR, DRIFTS, and EXAFS), we visualized a distinctly increased charge carrier density within the support. Our results suggest that the intensive contact between an electron-conducting polymer support and up-grown nanoparticles affords a charge exchange between redox centers, thereby boosting catalytic activity dramatically. 1. Introduction For new systems potentially exhibiting uncommon performance in heterogeneous catalysis, in situ and operando techniques offer strong tools for deciphering working principles. Among these promising but less understood materials, composites containing electron-conducting polymers are of interest in various fields like corrosion protection, sensing, and electronic devices.1 Whereas polyaniline (PANI) is often used in electrocatalysts,2,3 the use of PANI in chemical catalysis is hitherto rather unknown.2,4-6 In contrast, many mechanisms over conventional heterogeneous catalysts are exhaustively characterized as, e.g., CO oxidation by supported Pt catalysts, where type and behavior of oxidic phases crucially influence catalyst activity.7,8 As a general phenomenon, catalyst performance may be considerably sensitive to strong metal support interactions (SMSI) between supports and nanoparticulate active centers.9,10 In the present study we demonstrate substantial progress toward the understanding of a new type of heterogeneous (metal oxide on electron-conducting polymer) catalyst for carbon monoxide oxidation for all cases where low-temperature CO removal in the presence of pollutants is important. In particular, we demonstrate a very simple chemical method giving highly dispersed, immobilized PtO2 species, homogeneously distributed onto PANI. The catalyst’s high activity in CO oxidation at temperatures that are up to 70 K lower compared to a commercial reference catalyst (γ-alumina supported monometallic platinum; 1.5 wt % Pt) can be retained also in cases in which catalyst poisons, such as NO, SO2, or hydrocarbons, are added to the reactant. Especially by using operando EPR and * Email:
[email protected]. † Department of Chemistry, Darmstadt University of Technology. ‡ Leibniz Institute for Catalysis (LIKAT). § Institute for Materials Science, Darmstadt University of Technology.
EXAFS we can demonstrate that such activity boost is a direct effect of the interplay between active sites and the conducting polymer support. 2. Experimental Section 2.1. Catalyst Preparation. The support material polyaniline (9.5 g, Mw ) 65.000, Aldrich, SBET ) 37 m2 g–1 VPore ) 0.23 mL g–1) was suspended in 50 mL of diluted aqueous Na2CO3 (10 wt %, Merck). The suspension was combined with 20 mL of H2PtCl6 solution (Heraeus, containing 0.5 g Pt) and stirred for 15 min at 363 K with a pH maintained neutral by further addition of aqueous Na2CO3 (Merck). After addition of a mixture containing formaldehyde solution (Merck) and NaOH solution (10 wt %, Merck) in a volume ratio of 4.8/1, the suspensions were stirred for an additional 20 min, filtered, washed, and dried in air overnight. 2.2. High-Resolution Transmission Electron Microscopy (HRTEM). HRTEM was performed using a 300 kV JEOL JEM-3010 microscope equipped with a LaB6 cathode. Samples were prepared by suspending ground catalyst powder in methanol and dripping the suspension on a 3.2 mm copper mesh coated with a continuous carbon film. 2.3. Ex Situ and Operando X-ray Absorption Fine Structure (EXAFS) Spectroscopy. To elucidate the chemical nature of platinum and to obtain quantitative structural information, we carried out X-ray absorption fine structure spectroscopy. The X-ray absorption spectrum of the Pt L3-edge was obtained at the beamline X1 at Hasylab, Hamburg, with the synchrotron source operated at an energy of 4.45 GeV and an initial positron beam current of 120 mA. The measurements were carried out in transmission mode in an energy range from E ) 11 300 eV up to 12 800 eV (Pt L3-edge at 11 564 eV), using a thin Pt metal foil as reference. A Si(111) double-crystal monochromator was used and detuned to 50% intensity to avoid higher harmonics
10.1021/jp805970e CCC: $40.75 2008 American Chemical Society Published on Web 11/14/2008
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Figure 2. Dependence of CO conversion on temperature over each 100 mg of PtO2/PANI (5.0 wt % Pt) and a reference catalyst (Pt/γalumina, 1.5 wt % Pt) without any other feed additives (flow rates of 1, 11.5, and 43.5 mL min-1 for CO, O2, and N2, respectively).
Figure 1. PtO2 in the backbone of PANI.
present in the X-ray beam. The intensities of the focused beam and the transmitted beam were detected by three gas-filled ion chambers in series. Extraction of the EXAFS data from the measured raw data was performed either with the program package WinXAS11 (for qualitative comparison only) or with the XDAP code developed by Vaarkamp et al.12 The pre-edge was approximated by a modified Victoreen curve, and normalization was carried out by dividing the absorption spectrum by the height of the absorption edge at 50 eV above the edge.13,14 The background was approximated by a spline function defined by NPTS
(µi - BCKi)2
i)1
e-WEki
∑
2
e SM
(1)
with SM ) smoothing parameter. The data as finally shown in this paper were obtained with a multiple shell R-space fit with k1 weighting, ∆k ) 3.0-14 Å-1 and ∆R ) 1.4-3.2 Å. To obtain the final fit, all parameters were fully optimized in k0, k1, k2, and k3 weighting. Theoretical phase shifts and backscattering amplitudes for the Pt-Pt, Pt-Cl, and Pt-O absorber-scattering pairs were used in EXAFS data analysis,15 which were generated utilizing the FEFF7 code.13 Sample preparation was done by diluting 180 mg of the powder sample in 200 mg of boronitride and pressing it into a dense pellet (calculated to have an absorbance of 1.5). The pellet was then fixed onto the multiple sample holder, which was cooled down to liquid nitrogen temperature. Two spectra were recorded with recording times of approximately 45 min each and added to improve the spectra quality. Data analysis was carried out in two steps: first of all, the near-edge region is discussed qualitatively and compared to literature data. In a second step, an EXAFS analysis is performed fitting the Fourier-transformed spectrum to a reasonable model using the software package XDAP. The near-edge region, FT, and data of polyaniline supported platinum in k space are referred to a commercial carbonsupported platinum catalyst (Pt/C) that was purchased from E-TEK. Operando measurements were carried out in a tubular reactor equipped with Al and Be windows at the ends.16 The gas stream conditions (flow rates of 1, 11.5, and 43.5 mL min-1 for CO, O2, and N2, respectively) were introduced via several mass flow controllers. 2.4. X-ray Photoelectron Spectroscopy (XPS). XPS has been applied to obtain information on the chemical environment of surface atoms, in particular the oxidation state of platinum loaded to the PANI support and the chemical state of the PANI nitrogen. Surface analytical measurements were performed by
means of a VG ESCALAB 220 iXL spectrometer with a Mg KR radiation source. The C 1s-peak at 284.8 eV was used as a reference for the binding energy. After satellite and background subtraction, the peaks were fitted with Gaussian-Lorentzian curves. The peak positions could be determined with a precision of (0.1 eV. The quantitative compositions were elucidated from the peak areas, which were divided by the transmission function of the spectrometer and element specific Scofield factors. 2.5. In Situ Diffuse Reflectance-Infrared Fourier Transform Spectroscopy (DRIFTS). In situ DRIFTS experiments have been conducted in a patent-registered17 reaction cell, adapted in a Bruker Equinox 55 IR spectrometer. Special design of the cell allows eliminating almost all contributions of IR active gas phase molecules, and a KBr reference can be placed in a position equivalent to the sample under reaction conditions without opening the reaction chamber. The catalysts have been heated up in a flow of nitrogen to 483 K to remove water from the support before being exposed to a CO/O2/N2 mixture (9.6:4.8:85.6; total flow 66.3 mL min–1). 2.6. Operando EPR Experiments. EPR spectra in X-band (ν ≈ 9.5 GHz) were recorded using a homemade fixed-bed flow reactor directly implemented in the cavity of the spectrometer. Before the in situ measurements were started, the samples were pretreated for 2 h at 533 K in N2 flow and for another 2 h at 433 K in air flow. For measurements under reaction conditions, 10 mg of the catalysts was heated stepwise in a flow of 1.35% CO, 14.9% O2/N2 (total flow 7.4 mL min–1) to 453 K with simultaneous measurement of CO and CO2 concentrations (operando experiment) by nondispersive IR photometry. 2.7. Catalyst Activity. The catalytic runs were carried out in a fivefold parallel fixed-bed reactor system which was designed for the investigation of automotive exhaust gases under realistic conditions of feed composition, temperature, and heating ramps. The reaction was followed by on-line FTIR measurements. 3. Results and Discussion 3.1. Cluster Size and Oxidation State. The preparation concept (deposition-precipitation of H2PtCl6) is based on the assumption that hydrolyzing Pt-hydroxo species coordinatively anchor on the nitrogen atoms in the backbone of the conjugated polymer PANI, thereby serving as nucleation sites during further precursor addition (Figure 1). Direct interaction between Pt and N in the as-prepared material (5.0 wt % metal loading; nanocluster size (1.9 ( 0.5) nm) could be corroborated using recent XPS measurements.18 As to the nature of the supported nanoparticles, Pt signals with BE of 75.0 and 317.2 eV for the Pt 4f7/2 and 4d5/2 electrons, respectively, point to the presence
Polyaniline Supported PtO2 Nanoparticles
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TABLE 1: Catalytic Activities Given as Temperatures at 10% and 50% CO Conversion, T10 and T50 (Light-Off Temperature), Respectively, and Number of Cycles for Different Diesel Exhaust Gas Mimics of Increasing Complexitya entry
feed composition
PtO2/PANI, T10 (K)
PtO2/PANI, T50 (K)
Pt/γ-alumina, T50 (K)
cycles
1 2
1000 ppm CO, 500 ppm C3H8, 2% CO2, 4% O2 1000 ppm CO, 500 ppm C3H8, 2% CO2, 4% O2 + 400 ppm NO, + 30 ppm SO2 1000 ppm CO, 500 ppm C3H8, 2% CO2, 4% O2 + 400 ppm NO, + 30 ppm SO2 + 10% H2O
304 386
372 419
439 458
25 15
324
396
432
9
3 a
Gas mixtures balanced with N2/Ar obtaining a gas hourly space velocity (GHSV) of 150 000 h-1 with a catalyst mass of 70 mg.
Figure 3. In situ DRIFT spectra of 5Pt/γ-alumina (a) and PtO2/PANI (b) and under a mixture of O2/CO diluted in N2 (total flow ) 66.3 mL min-1; 9.6 vol % CO) at 473 K. The O2/CO ratio has been increased from 0.5 to 2.0 (a) and 0.0 to 1.0 (b).
Figure 4. Comparison of ex situ and operando EXAFS signals for PtO2/PANI obtained under reaction conditions (flow rates of 1, 11.5, and 43.5 mL min-1 for CO, O2, and N2, respectively) at 423 K.
of platinum oxides (PtO2: Pt 4f7/2 BE ) 74.1...75.6 eV, Pt 4d5/2 BE ) 318.1 eV; PtO: Pt 4f7/2 BE ) 72.4...74.6 eV, Pt 4d5/2 BE ) 317.1 eV),19 a fact that could be substantiated in an ex situ EXAFS measurement (Figure S3).
3.2. Catalyst Activity. In the oxidation of nitrogen-diluted carbon monoxide using a homemade fixed bed reactor system,20 the PANI catalyst features a very high activity: starting at temperatures below 353 K, the reaction exhibits a light-off temperature (the temperature at which 50% of CO is converted, T50) of 388 K and reaches full conversion around 393 K (Figure 2), while a commercial oxidation catalyst (Pt supported on γ-alumina, SBET ) 300 m2 g–1, VPore ) 0.95 mL g–1) shows no activity in this temperature range, a fact that must not be traced back necessarily to different metal loadings.21 In fact, CO turnover frequency (TOF) for PtO2/PANI surpasses that of Pt/ γ-alumina by 2 orders of magnitude (5 × 10-2 s-1 vs 7 × 10-4 s-1, respectively). However, typical exhaust streams of, e.g., diesel automotive catalysts22 normally contain environmental pollutants such as NO, SO2, hydrocarbons, and water, so further experiments (Table 1) were carried out applying appropriate model gas mixtures. When the reaction mixture contains 1000 ppm CO and 500 ppm C3H8 in a lean mixture with oxygen (entry 1), 10% of the CO is converted already near room temperature and T50 is still very low (372 K). The CO-light-off temperature of the catalyst was maintained constant over 25 cycles accordant with 125 min time-on-stream (Figure S1). In contrast, the commercial oxidation catalyst exhibits a T50 which is 67 K higher than that for the PANI supported catalyst. Adding 400 ppm NO and 30 ppm SO2 increases T50 as expected; however, the PtO2/PANI performance is still much better than that for the reference catalyst (∆T ) 39 K, entry 2). Most strikingly, our catalyst retains this outstanding performance also during additional feeding of H2O, where we observe 10% and 50% conversion of CO at 324 and 396 K, respectively (entry 3). Under the same conditions, the activity of the reference catalyst is again much lower (∆T ) 36 K). In this regard, PtO2/PANI also surpasses supported Au catalysts well-known for low-temperature CO oxidation, which deactivate heavily during automotive pollution abatement.23 The absence of any agglomeration after the catalytic run was proved by TEM, again underlining the strong interaction between active sites and PANI (Figure S2). Even after 117 cycles and temporary addition of NO, SO2, and water over several cycles representing a total time-on-stream of 585 min, the PANI supported Pt catalyst retains a CO-light-offtemperature of 415 K. 3.3. DRIFTS and EXAFS Studies. A first hint that the active sites in Pt/γ-alumina indeed consist of oxidized platinum could be obtained upon using the reactant CO as a probe molecule in in situ DRIFTS experiments. In order to make band intensities comparable with the PtO2/PANI catalyst, a 5Pt/γalumina with the same metal loading (5.0 wt %) was applied. 5Pt/γ-alumina exhibits an intense band around 2065 cm-1 under stoichiometric conditions, typical for CO linearly adsorbed on metallic Pt0 (Figure 3a). At a ratio of O2/CO ) 2.0, the aforementioned band is no longer present, revealing a band at ca. 2110 cm-1, which is consistent with an oxidized platinum surface under oxygen-rich conditions. PtO2/PANI (Figure 3b)
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Figure 5. EPR intensities (double integrals, left axes) as a function of temperature measured in air or CO/O2 feed flow (A, B, left axes) and CO/CO2 concentrations in the effluent stream (B, right axis, only for the PANI supported catalyst, pure PANI was not active). Open circles: CO was replaced by N2. Upon reintroduction of CO, the value returns to its previous position. (Further information regarding EPR signal intensity is detailed in the Supporting Information.)
reacts in a comparable manner while the O2/CO-ratio is increased, but in this system no band remains above 2100 cm-1. A similar phenomenon, explainable by low residual CO concentrations on the surface of a highly active catalyst, has been described elsewhere.24 Variation of the O2/CO-ratio is a completely reversible process for both PtO2/PANI and 5Pt/γalumina. Accordingly, the observed concurrent decreases of both 2110 and 2065 cm-1 bands are not helpful to decipher the Pt oxidation state in PtO2/PANI during CO oxidation, so EXAFS was now performed under operando conditions. Upon applying the same reaction conditions as in Figure 2 at temperatures from 298 to 423 K, nearly identical white lines and near-edge regions resulted for the fresh and working catalyst, respectively, so one may conclude that PtO2 in the catalyst remains unchanged under the conditions applied (Figure 4). Likewise, comparison of ex situ and operando EXAFS results for Pt/γ-alumina confirms PtO2 to prevail in this catalyst as well with only minor contributions from metallic Pt. 3.4. EPR Studies. As both Pt/γ-alumina and PtO2/PANI hence contain PtO2 nanoparticles under working conditions, the PANI support must play an active role in the catalytic cycle, a notion which we could prove during operando EPR experiments while heating pure and Pt doped PANI in air as well as in CO/ O2 flow. Plots of EPR signal intensity multiplied by temperature I(T)•T for pure PANI and the PANI supported catalyst in air run almost parallel indicating similar Pauli susceptibility values (χP, Supporting Information Figure S4). This means that the concentration of polarons in PANI is not considerably influenced by deposition of Pt on its surface. Moreover, the same slope (indicating unchanged χP values) is obtained when heating pure PANI in CO/O2 flow instead of air (Figure 5A). In contrast, the PtO2/PANI catalyst under the same conditions starts to oxidize CO to CO2 above 393 K, and concurrently the I(T)•T values also start rising (circle in Figure 5B). In fact, the EPR intensity values parallel almost exactly the activity trend. This is a clear indication that charge carriers originating from the CO oxidation reaction are transferred to the conduction band of the PANI support mediated by strongly bound PtO2 species. Interestingly, the increase of the number of polarons is reversible when CO in the feed stream is replaced by N2 at 453 K (open circle in Figure 5B). Upon reintroduction of CO, the value returns to its previous position, illustrating the temporal character of the charge transfer. Therefore, it cannot be ruled out that charge storage within PANI or PANI mediated charge transfer between active sites is crucial for obtaining low light-off temperatures with PtO2/PANI; further kinetic measurements will follow in order to depict a concise reaction mechanism.
4. Conclusion To summarize, we demonstrated a new, hitherto unknown, unusual electron-conducting polymer (PANI) supported PtO2 catalyst in which a vital interplay between support and supported metal is believed to account for unusually low light-off temperatures and high stability in CO oxidation, even in the presence of catalyst poisons coexisting in practical feedstocks. The physicochemical characterization of the material by EPR, DRIFTS, and EXAFS highlighted the importance of in situ/ operando techniques for a better understanding of the working oxidation catalyst. From the clear interplay between EPR intensity and catalyst activity (Figure 5), which has been found for the first time, it can be concluded that charge carriers originating from the oxidation of carbon monoxide are transferred to the conduction band of the electron-conducting polymer mediated by strongly bound PtO2 species. The simple synthesis protocol for the PANI supported catalyst system of the present work opens up the possibility of designing a broad range of nanosized precious metal oxides on conducting polymers which can be used as a new class of materials for environmental and chemical catalysis. Supporting Information Available: Figures S1-S4 showing additional details of catalyst activity and catalyst characterization (TEM, EXAFS, EPR). This material is available free of charge via the Internet at http://pubs.acs.org References and Notes (1) Mallick, K.; Witcomb, M. J.; Scurrell, M. S. Gold Bull. 2006, 39, 166. (2) Rimbu, G. A.; Jackson, C. L.; Scott, K. J. Optoelectron. AdV. Mater. 2006, 8, 611. (3) Mallick, K.; Witcomb, M. J.; Scurrell, M. S. Platinum Met. ReV. 2007, 51, 3. (4) Punniyamurthy, T.; Reddy, R. S.; Das, S. Tetrahedron Lett. 2004, 45, 3561. (5) Chattopadhyay, A.; Majumdar, G.; Goswami, M.; Sarma, T. K.; Paul, A. Langmuir 2005, 21, 1663. (6) (a) Gallon, B. J.; Kojima, R. W.; Kaner, R. B.; Diaconescu, P. L. Angew. Chem. 2007, 119, 7389–7392. (b) Gallon, B. J.; Kojima, R. W.; Kaner, R. B.; Diaconescu, P. L. Angew. Chem., Int. Ed. 2007, 46, 7251. (7) Seriani, N.; Pompe, W.; Ciacchi, L. C. J. Phys. Chem. B 2006, 110, 14860. (8) Tsotsis, T. T.; Lindstrom, T. H. Surf. Sci. 1985, 150, 487. (9) Tauster, S. J.; Fung, S. C. J. Catal. 1978, 55, 29. (10) Narayanan, S. J. Sci. Ind. Res. 1985, 44, 580. (11) Ressler, T. J. Phys. IV 1996, 7, 269. (12) http://www.xsi.nl. XDAP software, code and licensing, last updated August 2005, last accessed September 2005. (13) v. Dorssen, G. E.; Koningsberger, D. C.; Ramaker, D. E. J. Phys.: Conden. Matter 2002, 13529. (14) Koningsberger, D. C.; Mojet, B. L.; van Dorssen, G. E.; Ramaker, D. E. Top. Catal. 2000, 10, 143.
Polyaniline Supported PtO2 Nanoparticles (15) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. ReV. B 1995, 52, 2995. (16) Haass, F.; Bron, M.; Fuess, H.; Claus, P. Appl. Catal., A 2007, 318, 9. (17) Fehlings, M.;et al. DE19910291, 1999. (18) Steffan, M.; Klasovksy, F.; Arras, J.; Roth, C.; Radnik, J.; Hofmeister, H.; Claus, P. AdV. Synth. Catal. 2008, 350, 1337. (19) NIST X-ray photoelectron spectroscopy database 20, version 3.4. (20) Bonifer, M.; Lucas, M.; Claus P. TU Darmstadt, WO 2007104290, 2007.
J. Phys. Chem. C, Vol. 112, No. 49, 2008 19559 (21) Arnby, K.; To¨rncrona, A.; Andersson, B.; Skoglundh, M. J. Catal. 2004, 221, 252. (22) Kaspar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77, 419. (23) (a) Bond, G. C.; Louis, C.; Thompson, D. T. In Catalysis by Gold, Catalytic Science Series, Vol. 6; Imperial College Press: London, 2006. (b) Pattrick, G.; van der Lingen, E.; Corti, C. W.; Holliday, R. J.; Thompson, D. T. Top. Catal. 2004, 30/31, 273. (24) Chen, M. S.; Cai, Y.; Yan, Z.; Gath, K. K.; Axnanda, S.; Goodman, D. W. Surf. Sci. 2007, 601, 5326.
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