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Monodisperse Pt/Pd Bimetallic Nanocrystals Demonstrate Platinum Effect on Palladium Methane Combustion Activity and Stability Emmett D. Goodman, Sheng Dai, An-Chih Yang, Cody Wrasman, Alessandro Gallo, Simon Russell Bare, Adam Scott Hoffman, Thomas F. Jaramillo, George W. Graham, Xiaoqing Pan, and Matteo Cargnello ACS Catal., Just Accepted Manuscript • Publication Date (Web): 18 May 2017 Downloaded from http://pubs.acs.org on May 18, 2017
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Uniform Pt/Pd Bimetallic Nanocrystals Demonstrate Platinum Effect on Palladium Methane Combustion Activity and Stability Emmett D. Goodman,a Sheng Dai,b An-Chih Yang,a Cody Wrasman,a Alessandro Gallo,a,c Simon R. Bare,c Adam S. Hoffman,c Thomas F. Jaramillo,a George W. Graham,b Xiaoqing Pan,b,d and Matteo Cargnello*a a
Department of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis,
Stanford University, Stanford, CA 94305, USA b
Department of Chemical Engineering and Material Science, University of California-Irvine,
Irvine, CA 92697, USA c
Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575
Sand Hill Rd., Menlo Park, CA 94025, USA d
Department of Physics and Astronomy, University of California-Irvine, Irvine, CA 92697, USA
*
Corresponding author; email:
[email protected]; phone: +1-650-7246422
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ABSTRACT
Bimetallic catalytic materials are in widespread use for numerous reactions, as properties of a monometallic catalyst are often improved upon addition of a second metal. When dealing with bimetallic catalysts, it remains challenging to establish clear structure-property relationships using traditional impregnation techniques, due to the presence of multiple coexisting active phases of different sizes, shapes, and compositions. In this work, a convenient approach to prepare small and uniform Pt/Pd bimetallic nanocrystals with tailorable composition is demonstrated, despite the metals being immiscible in the bulk. By depositing this set of controlled nanocrystals onto a high surface area alumina support, we systematically investigate the effect of adding platinum to palladium catalysts for methane combustion. At low temperatures and in the absence of steam, all bimetallic catalysts show activity near-identical to Pt/Al2O3, with much lower rates compared to the Pd/Al2O3 sample. However, unlike Pd/Al2O3, which experiences severe low-temperature steam poisoning, all Pt/Pd bimetallic catalysts maintain combustion activity when exposed to excess steam. These features are due to the influence of Pt on the Pd oxidation state, which prevents the formation of a bulk-type PdO phase. Despite lower initial combustion rates, hydrothermal aging of the Pd-rich bimetallic catalyst induces segregation of a PdO phase in close contact to a Pd/Pt alloy phase, forming more active and highly stable sites for methane combustion. Overall, this work unambiguously clarifies the
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activity and stability attributes of Pt/Pd phases which often coexist in traditionally synthesized bimetallic catalysts, and demonstrates how well-controlled bimetallic catalysts elucidate structure-property relationships.
KEYWORDS: Monodisperse nanocrystals, Pt/Pd bimetallic catalysts, structure-property relationships, methane combustion, steam-resistance, hydrothermal stability
Introduction Multimetallic nanomaterials are a growing class of materials finding many practical applications in catalysis,1,2 data storage,3 sensing,4 and medicine.5 The utility of these materials stems from unique properties of strength, durability, and activity which are often superior to those of the individual metal components. However, in the design of bimetallic materials, it is crucial to understand which structural features produce these advantageous qualities. Especially as we move towards the design of atomically precise materials, we are concerned with correlating complex nanostructure with desired macroscopic system properties. Finding such structure-property relationships is critically important for the directed synthesis of next generation nanomaterials, which strive to make the most effective use of rare and expensive elements. The field of thermal heterogeneous catalysis has long taken advantage of complex, multimetallic nanostructures.6,7 For efficient catalysis, a specific and controlled interaction between multiple catalytic phases is often essential for a desired activity, selectivity or stability.
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The presence of intimate contact between multiple active phases has been proven to be crucial for activity and/or selectivity in many important reactions, such as methanol synthesis,8 hydrogen peroxide synthesis,9 and water-gas shift;10,11 additionally, understanding the role of an atomically dispersed secondary phase, or a metal promoter, is critical in reactions such as ammonia synthesis.12–14 Frequently, such catalysts are optimized via loosely directed trial and error. As computation and experiment collaborate to better predict and design more effective catalysts,15 well-defined model systems of complex catalysts become increasingly crucial to connect experiment to theory16 and to fundamentally understand the properties of multi-phase systems. Rational catalyst design is especially critical in the synthesis of efficient automotive exhaust catalysts, which are composed of significant quantities of precious metals. Stringent regulations regarding the removal of unburned hydrocarbons and CO have led to drastic improvements in this technology, most of which takes advantage of Pt or both Pt and Pd as the active oxidation phase(s).17 These two metals are attractive because of their high activity and stability, especially when alloyed.18 As a recent additional challenge, regulations on methane emissions are being implemented, requiring the removal of this highly refractory compound under demanding conditions.19 Effectively using these valuable metals for such challenges means understanding how each element contributes to the total properties of the catalyst. Yet despite the demanding activity and stability requirements of these catalysts, which often operate in conditions of excess steam, there exists a lack of consensus regarding the synergistic aspects of these metals for combustion activity and hydrothermal stability. For example, many works find the addition of amounts of platinum to increase palladium activity due to an increased stability,20–24 or resistance to steam,21,25,26 while others find a decrease in activity due to platinum preventing the formation of an oxidized PdO phase,20,22,27
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which is known to be highly active for methane combustion.28–30 These seemingly contradictory conclusions are likely due to the fact that different preparative impregnation methods, followed by a variety of high-temperature thermal treatments, can result in a range of coexisting populations of Pd, PdO, Pt, and Pt/Pd.20,21,31 Such methods generally preclude the rational design of catalyst active phases, because an assortment of present phases makes it difficult to draw clear structure-property relationships. In this work, colloidally synthesized, monodisperse Pt/Pd bimetallic nanocrystals serve as well-defined catalyst active phases, to carefully study the influence of Pt on Pd activity and stability for methane combustion. As has previously been reported, we find a Pd/Al2O3 catalyst exhibits the highest methane oxidation rates in the absence of steam (dry conditions). Surprisingly, across all bimetallic Pt/Pd compositions, including a Pt-only sample, we find nearidentical low-temperature dry methane combustion rates. Furthermore, while the lowtemperature activity of the pure-Pd catalyst suffers greatly with steam introduction, Pt-containing catalysts suffer minimal deactivation upon addition of steam. We additionally observe that under accelerated hydrothermal aging treatments, palladium-rich bimetallics maintain activity much better than both platinum-rich bimetallics, as well as either single-metal catalyst. Finally, we correlate the increased activity of the 1/4 Pt/Pd bimetallic catalyst, post hydrothermal aging, to phase segregation of the more highly active PdO phase from sintered Pt/Pd bimetallic aggregates. These model catalysts, each with uniform composition, therefore help elucidate how the interaction between Pt and Pd affects activity in methane combustion and provides resistance to steam introduction and hydrothermal aging.
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Synthesis of Bimetallic Nanocrystals All syntheses were performed using standard air-free Schlenk techniques using a slightly modified procedure.32 Pt(acac)2 (98%, Acros Organics) and Pd(acac)2 (35% Pd, Acros Organics) were used as metal precursors; 1-octadecene (ODE, 90%, Acros Organics), 1-tetradecene (TDE, 94%, Alfa Aesar), and 1-dodecene, (DDE, 93-95%, Acros Organics) were used as solvents as received. All syntheses reacted at reflux; to achieve a desired reflux temperature, a mixture of solvents was used.33 Pure ODE (10 mL) was used for Pt-rich (Pt/Pd = 4/1, 3/2) bimetallics while 5 mL ODE and 5 mL TDE was used for Pd-rich bimetallics (Pt/Pd = 2/3, 1/4). These gave solutions that refluxed at 320oC and 280oC, respectively. 1-oleylamine (OLAM 70%, Aldrich), oleic acid (OLAC 90%, Aldrich), and trioctylphosphine (TOP, 97%, Aldrich) were used as received. The general synthetic methodology is as follows: metal salts (total .25 mmol) were added to a three-neck flask together with solvents (10 mL total), oleylamine (825 µL) and oleic acid (400 µL) at room temperature. The mixture was evacuated for 15 min (100 nanoparticle diameters using ImageJ software. Metallic dispersions were quantified by volumetric CO chemisorption (Table S1) measurements at room temperature following appropriate pretreatments. After ligand removal, the samples were placed in a U-shaped quartz reactor in the adsorption apparatus (Micromeritics Triflex), heated under 760 Torr of O2(5%)/Ar at 300 oC for 30 min, and then reduced under 760 Torr of H2(5%)/Ar at 300 oC for 30 min. The samples were evacuated for 4 h at the same temperature and cooled to 35 oC, and then CO adsorption was measured in the interval 100-400 Torr using the double isotherm method, with 1 h evacuation between the two isotherms. CO/Pt and CO/Pd stoichiometry was considered 1 in both cases, given the small size of the particles. X-ray absorption spectroscopy (XAS) spectra were collected at the Stanford Synchrotron Radiation Lightsource (SSRL) of SLAC National Accelerator Laboratory (Menlo Park, CA) on the unfocused 20-pole 2-T wiggler side-station beam line 7-3 under standard ring conditions of 3 GeV and approximately 500 mA. A Si(220) double-crystal monochromator was used for energy selection. A Rh-coated harmonic rejection mirror was used to reject the components of higher harmonics. The spectra were collected at room temperature using fluorescence detection, with (or without) deadtime correction. A Pd reference foil was scanned simultaneously for energy calibration, with the first inflection point of the spectrum set to 24350.0 eV. The samples were
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ground to a fine powder, packed into sample holders, and positioned at 45o to the incident beam with fluorescence spectra collected perpendicular to the X-ray beam. Extended X-ray absorption fine structure (EXAFS) data processing and fitting were conducted using the programs Athena and Artemis37 software in the Demeter XAS package with FEFF6.038 used to generate the absorber-backscatter paths used in the fitting procedure from crystallographic data of reference compounds. The Pd-O and Pd-Pd scattering paths were obtained from bulk Pd-O and Pd-Pd scattering paths, respectively. The Pd-Pt scattering path was simulated by modification of the bulk Pd structure to include Pt atoms. The EXAFS models only include single scattering paths. The value of S02 (0.8±0.1) was determined from the Pd foil. EXAFS data was fit over a range from approximately k = 3.0 to 13.1 Å-1 with a Fourier transform range of approximately R = 0.6 to 3.2 Å. Fitting was optimized using k-weightings of 1, 2, and 3. Catalytic Characterization Techniques All experiments were conducted at a total pressure of one atmosphere. Kinetic measurements, ignition/extinction curves, and hydrothermal aging experiments were performed in a U-shaped quartz microreactor with an internal diameter of 10 mm. The fast-treated catalyst was sieved below 180 µm grain size, mixed with enough diluent (same alumina as nanoparticle support) to avoid thermal gradients as suggested by literature studies,29 and loaded into the reactor to give a bed length of about 1.0 cm, between two layers of granular quartz used for preventing displacement of the catalyst powder and for preheating the reagent gases. For kinetic and ignition experiments, ~20 mg catalyst and ~200 mg diluent Al2O3 were used, while for aging experiments, ~80 mg catalyst and ~300 mg sieved quartz were used, so that catalyst could be recovered after catalytic tests for TEM analysis. In kinetic and aging experiments, gas-hourly
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space-velocity (GHSV) values were chosen for a desired conversion at a given temperature. The reactor was heated by a square furnace from micromeritics and the temperature of the catalyst was measured with a K-type thermocouple inserted inside the reactor, touching the catalytic bed. In blank experiments, quartz and alumina gave no appreciable methane conversion up to 550 oC. The reactant mixture composition was controlled by varying the flow rates of CH4(5%)/Ar, O2(5%)/Ar, and Ar (all certified mixtures with purity >99.999 from Airgas). Water was introduced into the reactor using a saturator by flowing the reaction mixture through milliQ water maintained at an appropriate temperature (30 oC) to provide the desired vapor pressure. In all catalyst tests, the catalyst was oxidatively cleaned of residual organics and carbonates within the reactor bed under O2(5%)/Ar flow at 45 mL min-1 at 300 oC for 30 min. The reactor was then cooled to the desired temperature, at which point the reactant mixture was introduced. For kinetic studies, the composition of effluent gases was monitored online using a Gas Chromatograph (GC, SRI Instruments) equipped with a thermal conductivity detector (TCD) and a Flame Ionization Detector (FID) with Ar as the carrier gas; GHSV was changed between samples to ensure similar rates at similar temperatures for low conversion kinetic data. For ignition and aging experiments, effluent was measured using an online mass spectrometer (Hiden HPR-20).
Results and Discussion Synthesis of Pt/Pd/Al2O3 catalysts Bimetallic nanocrystals were made in a one-pot synthesis by thermal reduction of Pt(II) and/or Pd(II) metal acetylacetonate precursors, which produced small and uniform nanocrystals of a desired composition. Figure 1 shows the synthetic control in the series of pure Pd, Pt/Pd with
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different atomic ratios, and pure Pt nanocrystals through representative TEM images at low magnification and corresponding histograms of particle size distributions.
Figure 1. From left to right: representative TEM images (top) and corresponding particle size distributions (N>100, bottom) of as-synthesized Pd, Pt/Pd = (1/4, 2/3, 3/2, 4/1) and Pt nanocrystals supported on TEM grids. Scale bars are 10 nm.
Small and highly uniform nanocrystals (
