ARTICLE pubs.acs.org/IECR
Catalytic Performance of Solution Combustion Synthesized Alumina- and Ceria-Supported Pt and Pd Nanoparticles for the Combustion of Propane and Dimethyl Ether (DME) Sara Colussi,*,† Arup Gayen,‡ Jordi Llorca,§ Carla de Leitenburg,† Giuliano Dolcetti,† and Alessandro Trovarelli† †
Dipartimento di Chimica, Fisica e Ambiente, Universita di Udine, 33100 Udine, Italy Department of Chemistry, Jadavpur University, 700032 Kolkata, India § Institut de Tecniques Energetiques and Centre for Research in Nanoengineering, Universitat Politecnica de Catalunya, 08028 Barcelona, Spain ‡
ABSTRACT: Pd- and Pt-based catalysts supported on CeO2 and Al2O3 have been prepared by traditional incipient wetness impregnation and by solution combustion synthesis, and their catalytic activities have been compared for propane and DME combustion. Kinetic measurements carried out in a laboratory-scale recycle reactor apparatus indicate that combustion synthesized Pd-based materials are much more active than the corresponding impregnated ones, whereas for Pt-based catalysts no significant difference is observed. On the basis of high-resolution transmission electron microscopy (HRTEM) results, the enhanced catalytic activity is tentatively ascribed to the coexistence of highly active palladium centers (PdII species) that form upon solution combustion synthesis and nanosized metallic Pd particles.
1. INTRODUCTION Catalytic combustion is a valid tool for both emissions control and energy production on both large scale and microscale.16 In recent years the increasing demand of green energy and the coming into force of more stringent regulations on exhaust emissions have been the driving force for the research on this technology. The benefits of catalytic combustion with respect to traditional flame combustion are well established and consist mainly in lower emissions of nitrogen oxides, CO, and unburned hydrocarbons due to lower operating temperatures and high efficiency.7 Noble metals-based catalysts are well-known for their high activity in the catalytic combustion of hydrocarbons. In particular, Pd-based materials are recognized as the most active for the catalytic combustion of methane, whereas for higher hydrocarbons supported platinum catalysts are the preferred choice.8,9 Many research works investigated the possibility of enhancing the activity of supported noble metal catalysts by adding suitable promoters and/or by finely tuning metal support interaction. For example, for the catalytic combustion of methane several attempts were made to stabilize the palladium oxide active phase.1013 Farrauto et al.14 first reported a beneficial effect of the presence of CeO2 on the stabilization of PdO, and the same improvement was observed also by other authors.1519 In a paper from our group it was demonstrated that, when ceria is added as a dopant onto alumina support, a physical contact between Pd and CeO2 particles is necessary to promote Pd reoxidation.20 Ceria was found to affect positively also the catalytic performances and stability of other metals2123 and of different catalyst formulations.24 In a recent paper we have reported a significant increase in reaction rates for methane catalytic combustion of Pd/CeO2 samples made by solution combustion synthesis with respect to their impregnated r 2012 American Chemical Society
counterpart.25 The solution combustion synthesized catalysts presented a peculiar PdOCe superstructure, analogous to that theoretically described by Mayernick et al.,26,27 who demonstrated by DFT calculations that on this structure the methane activation barrier is the lowest. Pd-substituted CeO2 was found to be highly active also for the reduction of NO by CO28 and for CO oxidation reaction.29 High activity for methane combustion of Pd-based catalysts prepared by solution combustion synthesis with different formulations was observed also by Specchia et al.3032 The possibility of obtaining a stronger metalsupport interaction by the insertion of the noble metal into the crystal structure of the support, or by creating a strong metaloxygen bond at the support surface, has been investigated in many research works. These new structures sometimes reveal unexpected properties that result in an improvement of catalytic performances. Fu et al.,33 for example, reported a superior activity for the water gas shift reaction of nonmetallic Au and Pt species embedded in ceria, whereas Nagai et al.34 observed a sintering inhibition of Pt on ceria-based support, due to the formation of a PtOCe bond that acted as an anchor for Pt particles. The strong Ptceria interaction was experimentally and theoretically investigated by Vayssilov et al.,35 who concluded that the oxygen reverse spillover from nanostructured ceria support to platinum is mainly responsible for enhanced catalytic activity. Special Issue: Russo Issue Received: July 29, 2011 Accepted: December 12, 2011 Revised: December 6, 2011 Published: January 10, 2012 7510
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Industrial & Engineering Chemistry Research Also in the field of nonprecious metal catalysts there are examples of metalceria interactions that modify the properties of both metal and ceria surfaces. Li et al. observed a change in Cu reducibility promoted or inhibited by different dopants on CeO2; the transition metals that promote Cu reducibility, namely, Mn and Fe, lead to a beneficial effect in selective CO oxidation due to the formation of Cu+ ions and oxygen vacancies.36 Caputo et al. reported a strong metalceria interaction with copper via the formation of a CuOCe phase very interesting for the preferential oxidation of CO due to its singular redox properties.37 These peculiar MOCe structures seem to be promising for their application in microscale combustion, for which highly active materials are required. Moreover, solution combustion synthesis allows for one-step preparation of structured catalysts that are commonly employed in microscale applications, thus avoiding the multistep preparation required, for example, when using dip coating or impregnation of ceramic monoliths. For microscale and portable applications, high energy density liquid hydrocarbon fuels are the preferred choice as convenient energy carriers, and among these propane and dimethyl ether (DME) have been recently considered. Their catalytic combustion on metal-based catalysts has been studied by different authors.24,3841 In this study the catalytic performances of Pd- and Pt-based catalysts supported on Al2O3 and CeO2 for the combustion of propane and DME are investigated. HRTEM studies are carried out to analyze the structures of the samples. Catalytic activities of solution combustion synthesized samples are compared with the ones of the correspondent impregnated materials by means of reaction rate measurements. To perform a consistent kinetic analysis, a laboratory-scale recycle reactor apparatus has been developed to overcome the issues due to operation under differential conditions (i.e., the intrinsic difficulty of measuring very small differences in composition). A large recycle ratio allows the approach to a perfectly mixed flow, comparable to that of a continuous stirred tank reactor (CSTR), for which the reaction rate measurement is simpler and integration is not necessary.42
2. EXPERIMENTAL METHODS 2.1. Catalyst Preparation. Catalysts consisting of nominal 2% wt Pd and Pt were prepared by incipient wetness impregnation (IW) on commercial Al2O3 and CeO2 supports (Grace Davison), starting from a solution of Pd(NO3)2 (10% wt, Aldrich 99.999%) and (NH3)4Pt(NO3)2 dissolved in an adequate amount of distilled water. Prior to impregnation, the supports were calcined in air at 1073 K. Samples so obtained were dried overnight at 393 K and calcined in air at 1073 K. Solution combustion synthesized samples with nominal 2% wt Pd or Pt were prepared by one-step solution combustion synthesis (SCS), using (NH4)2Ce(NO3)6 (Treibacher A.G., Austria) for ceriasupported catalysts or Al(NO3)3 3 9H2O (Aldrich 98%) for alumina-supported ones, and Pd(NO3)2 (Johnson Matthey Chemicals, U.K.) or (NH3)4Pt(NO3)2 as oxidizers, and oxalyldihydrazide (C2H6N4O2, ODH) as the reducer. The solution obtained by the dissolution of support and metal precursors and of the fuel in distilled water was put into a preheated furnace in static air at 623 K, where the combustion took place. After combustion, the powder so obtained was removed from the furnace and left to cool at room temperature. To obtain the solution combustion synthesized samples, stoichiometric mole ratios were used: 0.0098 (NH3)4Pt(NO3)2:1.9902 Al(NO3)3:2.9833 ODH
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Figure 1. Scheme of the recycle reactor apparatus.
for 2PtAl SCS; 0.02 (NH3)4Pt(NO3)2:0.98 (NH4)2Ce(NO3)6: 1.98 ODH for 2PtCe SCS; 0.02 Pd(NO3)2:1.98 Al(NO3)3:2.99 ODH for 2PdAl SCS; and 0.03 Pd(NO3)2:0.097 (NH4)2Ce(NO3)6:2.358 ODH for 2PdCe SCS. In the following, impregnated samples will be referred to as 2MAl IW and 2MCe IW (2%M/ Al2O3 IW and 2%M/CeO2 IW with M = Pd, Pt) and solution combustion synthesized samples will be referred to as 2MAl SCS and 2MCe SCS (2%M/Al2O3 SCS and 2%M/CeO2 SCS with M = Pd, Pt). 2.2. Catalyst Characterization. Catalysts were characterized by elemental analysis, BET surface area measurements, X-ray diffraction analysis (XRD), and high-resolution transmission electron microscopy (HRTEM). Surface area measurements were carried out with a Micromeritics Tristar porosimeter by measuring N2 adsorption at 77 K. XRD spectra were collected with a Philips X0 Pert diffractometer using Cu Kα radiation, with a step size of 0.02° and a counting time of 80 s per step. HRTEM was performed with a JEOL 2010F microscope equipped with a field emission gun. The point-to-point resolution of the instrument was 0.19 nm, and the resolution between lines was 0.14 nm. The sample was deposited from an alcohol suspension over a grid with holey-carbon film. Elemental analysis was carried out using an ICP mass technique. 2.3. Catalytic Tests. Catalytic tests for propane and DME combustion were carried out in a quartz microreactor (i.d. = 6 mm, length = 400 mm) by loading catalyst powder pressed and sieved (100 < L < 200 μm) on a quartz wool bed. For propane combustion 60 mg of catalyst was loaded, whereas for DME combustion 30 mg was used. In both cases the catalytic powders were diluted with an equal amount of quartz beads. The total flow rate was 40 mL/min (0.5% C3H8, 5% O2 in He for propane; 0.5% DME, 3% O2 in He for DME). The reactor was put into a tubular furnace, heated to 973 K for propane and to 723 K for DME combustion at a rate of 5 K/min. The reaction products were monitored by an online Agilent CP4900 microgas chromatograph equipped with two columns: a molecular sieve and a Poraplot Q. 2.4. Kinetic Measurements. Kinetic measurements were carried out in the same testing apparatus used for catalytic tests by recirculating the gas mixture through an NMP 830 KNDC micropump (KNF Italia), as shown in Figure 1. When the micropump was switched on, the whole system behaved as a mixed-flow reactor, allowing for consistent reaction rate measurements. The chosen recycle ratio, defined as R/F, was 35, and the flow rate on the recycle branch (R) was measured with a Bronkhorst EL-Flow flowmeter. Reaction rate measurements were carried out after a first heating/cooling cycle in the reaction mixture (40 mL/min; 0.5% C3H8, 5% O2 in He for propane; 0.5% DME, 3% O2 in 7511
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Table 1. Noble Metal Loading and Surface Area of Pt and Pd Samples NM (wt %)
SA (m2/g)
2PtAl IW
1.96
97.9
2PtAl SCS 2PtCe IW
1.69 1.90
31.6 12.3
2PtCe SCS
2.40
6.6
2PdAl IW
2.13
93.5
2PdAl SCS
1.73
43.8
2PdCe IW
1.74
12.2
2PdCe SCS
1.72
5.9
sample
Figure 2. Experimental and theoretical residence time distributions for the recycle system.
He for DME) to stabilize the catalyst behavior. Due to the intrinsic higher activity of platinum with respect to palladium, it was not possible to measure the reaction rates at the same temperature for both metals. For propane combustion the temperature was raised to 503 K for Pt-based samples and to 573 K for Pd-based ones; for DME the temperature was raised to 403 and 533 K for Pt- and Pd-based catalysts, respectively. The reaction rate was then calculated considering the conversion measured after 2 h at constant temperature.
3. REACTOR CHARACTERIZATION: RESIDENCE TIME DISTRIBUTION MEASUREMENTS Prior to kinetic measurements, the recycle reactor was characterized to ensure its behavior was close to that of a CSTR. This was carried out by measuring the residence time distribution (RTD) in the system after a step perturbation (stimulus response experiment).42 At t = 0 Ar tracer was fed to the reactor (total flow rate = 40 mL/min) with a recycle ratio of 35, and its concentration at the exit of the recycle loop was recorded by mass spectrometer. In Figure 2 the residence time distribution of the recycle reactor system is shown. The RTD function is calculated as EðtÞ ¼
dFðtÞ dt
ð1Þ
CðtÞ C0
ð2Þ
with FðtÞ ¼
where C(t) is the concentration of the tracer measured at the reactor outlet at time t and C0 is the inlet tracer concentration. The RTD calculated on the basis of tracer measurement was compared with the RTD obtained for a CSTR with the same volume of the experimental setup, which is also reported in Figure 2. For the sake of comparison the RTD curve of the model is shifted of 45 s, which is the time necessary in the real system for the tracer to reach the reactor outlet. For a CSTR of a given volume V the RTD is given by 1 EðtÞ ¼ et=τ τ
ð3Þ
where τ = V/v is the mean residence time (V = reactor volume, v = flow rate).
The good agreement between the theoretical and the experimental curves allowed the recycle loop to be considered as a perfectly mixed-flow reactor (CSTR), apart from the 45 s delay that is likely due to the part of the apparatus upstream of the recycle loop.
4. RESULTS AND DISCUSSION In Table 1 noble metal loading and BET surface area of each sample are reported. It can be observed that the solution combustion synthesized samples have a surface area which is about half that of impregnated catalysts, irrespective of the support. This is due to the high temperature reached in the combustion chamber during the preparation of the samples. The highly exothermic reaction is also likely responsible for the appearance of signals due to α-alumina on 2PtAl SCS, as observed in Figure 3a, which shows the X-ray profiles for alumina-supported catalysts. The predominant phases on alumina supports are different transitional aluminas, with some peaks belonging to the more crystalline α-alumina on IW samples and on 2PtAl SCS. The different features observed for alumina on 2PdAl SCS and 2PtAl SCS are due to the nature of the combustion synthesis, because the presence of local hot spots may influence the degree of crystallization and the formation of different alumina phases. Platinum is present in metallic form on both SCS and IW samples. By comparison of the X-ray patterns of 2PtAl IW and 2PtAl SCS, it is possible to observe that the SCS sample presents broader Pt peaks, indicating smaller particle size with respect to the IW sample. According to the Scherrer equation the mean particle sizes calculated for 2PtAl IW and 2PtAl SCS are 37 and 5 nm, respectively. On 2PdAl IW palladium is present as PdO, whereas on 2PdAl SCS a small peak belonging to Pd metal at 2θ = 40.2° can be detected (see inset). The peak at 2θ = 33.9° on 2PdAl SCS could belong to a transitional alumina (JCPDS file 02-1421) or to PdO. The attribution to alumina is made by taking into account the results from HRTEM analysis, where only Pd metal particles of about 34 nm are identified together with Pd particles of 1 nm or less and where no lattice fringe analysis is possible, which could likely not be detected by XRD. The crystal size calculated from XRD spectra for PdO on 2PdAl IW is 15 nm, whereas the calculation of the particle size of Pd metal on 2PdAl SCS was not possible due to the overlapping with alumina peak and the extremely low intensity of Pd peak. XRD profiles of ceria-supported samples are reported in Figure 3b. The overlapping of the main peak of CeO2 at 2θ = 33.0° with the one belonging to PdO at 2θ = 33.9° prevents 7512
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Figure 4. HRTEM image of 2PtCe SCS sample. (Inset) Fourier transform image of the CeO2 crystallite oriented along the [110] direction showing (111) planes at 3.13 Å.
Figure 3. (a) XRD of alumina-supported samples: [, θ-alumina; 4, α-alumina; 0, δ-alumina; 9, Pt; b, Pd; O, PdO. (Inset) 2PdAl SCS sample. (b) XRD of ceria-supported samples: X, CeO2; 9, Pt; b, Pd. (Inset) 2PtCe SCS sample.
the detection of palladium oxide on both 2PdCe IW and 2PdCe SCS. On the latter, a small peak belonging to metallic Pd can be observed, with a corresponding crystal size of 6 nm. Platinum is present only in metallic form on both 2PtCe IW and 2PtCe SCS (very small peak at 2θ = 39.7°). The calculated particle size is of 41 nm for the impregnated sample and 7 nm for the SCS one. X-ray patterns of spent catalysts were collected after combustion tests and kinetic measurements. For Pt-based catalysts the calculated crystal sizes of the spent catalysts were very close to those of the fresh ones, being 36 and 6 nm for 2PtAl IW and 2PtAl SCS, respectively, and 42 and 7 nm for 2PtCe IW and 2PtCe SCS, respectively. This excludes significant redispersion or sinterization of the metal particles. On Pd-based SCS samples the peak at 2θ = 40.2° corresponding to metallic Pd disappears, indicating that palladium oxidizes during combustion, but no peak belonging to PdO could be identified. This can be due to an overlapping of alumina or ceria peaks at 2θ = 33.9°, as observed on fresh catalysts, or to a redispersion of Pd metal, because the presence of very small Pd particles after combustion that escape XRD detection cannot be completely excluded.
Figure 5. HRTEM image of 2PdAl SCS sample: white arrows, Pd particles of
