Article pubs.acs.org/cm
Effect of Rhodium Distribution on Thermal Stability of Nanoporous Palladium−Rhodium Powders Markus D. Ong,† Benjamin W. Jacobs,† Joshua D. Sugar,† Michael E. Grass,‡ Zhi Liu,‡ George M. Buffleben,† W. Miles Clift,† Mary E. Langham,† Patrick J. Cappillino,† and David B. Robinson*,† †
Sandia National Laboratories, Livermore, California Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California
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S Supporting Information *
ABSTRACT: Powders of nanoporous palladium and palladium alloy particles are of potential value for storage of hydrogen isotopes, as long as the pores remain stable over a useful range of temperatures and chemical environments. Rhodium alloys are known to have enhanced hydrogen storage and improved thermal stability versus pure palladium. However, the distribution of rhodium on pore and particle surfaces is critical to this. Pores are more ordered and thermally stable in rhodium-rich regions. Treatment of particles at elevated temperature under reducing conditions can cause rearrangement of Rh and Pd at the surface, but not a major change in Rh distribution throughout the particle. Heating in the presence of hydrogen causes more rapid pore rearrangement than heating in vacuum subsequent to hydrogen exposure, suggesting a direct chemical influence of hydrogen on mobility of surface atoms. These results provide a clear path to future improvements in the stability of nanoporous metals in reducing atmospheres. KEYWORDS: mesoporous, pore collapse, core−shell, surface restructuring, segregation, EDSSI
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INTRODUCTION In nanoporous palladium and palladium-alloy particles, the pores provide high surface area and can improve surface-limited reaction rates in metal hydride batteries and gas storage beds. When tritium is stored, the pores can provide an escape path for the helium decay product.1−3 Nanoporous palladium and palladium alloys can be synthesized in a scalable fashion using surfactant templates.4,5 The resulting particles are several hundred nm to a few micrometers in diameter, with closely packed 3 nm pores. However, such nanostructures can be subject to thermally induced changes in structure and properties to a much greater extent than bulk materials.6−8 Bimetallic nanoparticles have been shown in many cases to confer enhanced chemical and physical properties, such as chemical reactivity9−13 or stability,14−16 in comparison to their component metals. This can include enhancement of hydrogen storage properties.17,18 These materials can form uniform alloys, core−shell structures, or graded compositions,19−22 and this variation can be key to the unique properties of these materials. The same is true for nanoporous materials. Understanding and controlling the effect of component distribution is of general importance in tailoring and optimizing properties. Incorporating other high-melting point metals into nanoporous palladium has been an effective means to increase the thermal stability of these nanoporous materials. In situ © 2012 American Chemical Society
transmission electron microscopy (TEM) results have shown that pores collapse at 150 °C in pure palladium,8 but in a 10% rhodium alloy, pores do not collapse until 400 °C.5 Initial results from Auger electron spectroscopy showed that the concentration of Rh was enhanced at the surface of the assynthesized alloy.5 This segregation is a particularly significant factor when the aging of the material is considered because the surface diffusion of atoms is expected to be the primary agent of this morphological evolution.23 In general, for fcc metals, the activation energy for surface diffusion scales with melting temperature, so a higher melting point metal should provide more stability against surface diffusion.24 If Rh is present at the surface of pores and faces a greater activation barrier to surface diffusion, it can be expected to stabilize pores at elevated temperatures. However, creating and confirming such a distribution presents significant synthetic and analytical challenges. In our previous work, samples were handled in air prior to analysis, and some surface oxide was known to be present. Metal atoms in an oxide have strong chemical bonds to neighboring atoms, so it can be expected that their diffusion rates are very slow. The oxide layer is unlikely to exist in a Received: September 8, 2011 Revised: December 12, 2011 Published: February 27, 2012 996
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absorption experiments. These were performed with 100 mg scale samples, placed into a small glass loading tube and dropped into the bulb of the porosimeter’s sample tube. The powder was gently shaken onto the sample bulb and loading tube surfaces for good thermal contact and rapid gas transport. The surface area was quantified using the BET method.27 The BJH method28 was used to determine the pore size distribution of the material. Prior to analysis, samples were degassed at 50 °C under vacuum for 15 h. The heating was done without taking the sample off the porosimeter analysis line in order to maintain a good seal on the sample holder. The gas manifold was operated under manual control to evacuate the system for the vacuum experiments. The time scale of the porosimetry experiment was about 5 h, ensuring that changes to the sample were gradual and that it remained close to thermal and chemical equilibrium. For the hydrogen environment experiments, the manifold was loaded to 8 Torr H2 at room temperature, and all the valves were closed. The valve to the sample holder was then opened, and the hydrogen was allowed to expand into the sample volume, causing a decrease in pressure. Typically, the pressure would fall to about 1 Torr when the valve to the sample was opened, and would rise to about 3 Torr when the heat was applied. This procedure ensured that the Pd−Rh would remain in the alpha phase, because the transition to the hydrided beta phase typically occurs at pressures greater than 10 Torr at 25 °C and at even higher pressures at increased temperatures.5 The heater was ramped at 10 °C/min until reaching 150 °C, and the temperature was maintained for the specified duration. Subsequent surface area, pore size distributions, and hydrogen absorption isotherms were then measured before repeating the heating procedure. The cumulative time of aging at 150 °C could then be recorded, and the surface area could be described as a function of this time. Ambient-pressure X-ray photoelectron spectroscopy (XPS) was conducted at the ambient pressure XPS endstation32 at beamline 9.3.2 at the Advanced Light Source at Lawrence Berkeley National Laboratory. The endstation is equipped with molecular leak valves for dosing up to 1 Torr of gas. Temperature is controlled with a ceramic button heater. Pressures up to 100 mTorr H2 and 300 mTorr O2 were used in these experiments. Samples were prepared by pressing the palladium alloy powders into a soft gold foil and rolling a glass vial over the powder to embed the material in the foil. Survey and high-resolution spectra were obtained using two incident photon energies, 750 and 490 eV. These energies were selected to probe different sample depths and to avoid convolution of the Rh and Pd 3d photoelectron peaks with the Auger transitions of Pd, Rh and O. The kinetic energy of the electrons originating from the 3d orbitals of Pd and Rh are 350−400 eV and 150−200 eV at incident photon energies of 750 and 490 eV, respectively. The corresponding mean free paths are then approximately 0.9 and 0.6 nm, respectively.29 Transmission electron microscopy (TEM) was performed on a JEOL 2010F field-emission electron microscope operating at 200 kV in both TEM and scanning transmission electron microscopy (STEM) modes. In situ heating was done using a Gatan 652 heating holder and a Protochips Aduro heating system. The powder samples were prepared in one of two ways. For TEM analysis the particles were suspended in ethanol and sonicated. A small amount of the suspension was either drop cast on to copper TEM grids with lacey carbon support films and Protochips thermal devices, or the grids were dipped into the suspension and allowed to dry. STEM images in Figures 7 and 10 and Figures S4 and S7 in the Supporting Information were processed using the “unsharp mask” filter in Adobe Photoshop (100%, radius 5 pixels, zero threshold) to reduce blurring of pore edges. Elemental analysis was performed in the TEM using EDS (Oxford INCA x-sight) spectrum imaging (Digital Micrograph, Gatan Inc.). Line scans through particles were performed by identifying the centroid in the palladium image, and averaging lines through the particle over 180°. To obtain thin sections of larger particles, we mixed the powder with an epoxy resin (Epo-Tek 353ND, Epoxy Technology, Inc.). The epoxy/powder mixture was then loaded into a 3 mm outer diameter 2.3 mm inner diameter brass tube (Gatan, Inc.). Discs of material were then cut with a diamond blade saw (South Bay Technology, Inc. 660 low-speed diamond saw) from the tube, and they
hydrogen storage application, so pore stabilization by oxide could be a misleading effect. A further concern is that the rhodium may not stay at the surface. Prior studies on bimetallic RhPd nanoparticles have shown by ambient-pressure X-ray photoelectron spectroscopy that, under reducing conditions at elevated temperature, the surface Rh concentration decreases relative to the raw material.25,26 Understanding the degree and relevance of Rh rearrangement is essential to understanding the reliability of this metal as a pore stabilizer. In this work, we study the rhodium distribution in several individual particles that are exposed to hydrogen at various temperatures by electron microscopy, and observe a strong correlation between Rh concentration within a particle and the local pore stability, regardless of the surface oxidation state. We confirm that a room-temperature hydrogen treatment is effective at removing an oxide layer, and pores are stable to subsequent heating under vacuum, but if hydrogen is present at elevated temperature, that stability is lost. At extreme temperatures − above those normally used for hydrogen storage − some rearrangement of the rhodium distribution occurs at the surface, which may impact pore stability, but we do not observe a major change in Rh distribution throughout a particle. Microscopic observations correlate well with surface studies (X-ray photoelectron and Auger electron spectroscopy) and bulk measurements (porosimetry and hydrogen uptake). The results clearly show the advantage conferred by rhodium, and the need for synthetic procedures that can improve its distribution.
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EXPERIMENTAL METHODS
The synthesis procedure of nanoporous Pd−Rh alloys is similar to that published elsewhere,5 but scaled up to produce several grams. Ammonium tetrachloropalladate, ammonium tetrachloroplatinate, and anhydrous rhodium(III) chloride were purchased from Alfa. The surfactant Brij 56 [an oligo(ethylene glycol) hexadecyl ether], sodium chloride, and hydrochloric acid were purchased from Aldrich. A gas mixture of 5% hydrogen in nitrogen was obtained from Matheson Tri-Gas. All materials were used as received. Deionized water with a resistivity of 18 MΩ cm was prepared in the laboratory. A 1:19 molar Rh:Pd ratio was obtained by adding 21.2 g ammonium tetrachloropalladate (74.6 mmol), 0.88 g rhodium(III) chloride (4.2 mmol), and 1.72 g sodium chloride (29.4 mmol) to 96 mL water. The salts dissolved after sonicating and overnight heating at 90 °C. Brij 56 was melted in an 85 °C water bath, and 19.8 mL was added to the solution, which was then heated and vortexed three times, and then allowed to cool to form a paste. The paste was extruded through a 10 mL syringe with no needle into four 500 mL flasks, which were then capped with a septum and purged with nitrogen. After passing through a water bubbler to maintain humidity, the hydrogen gas mixture was streamed through the septum, and the hydrogen reacted to reduce the metal salts over the course of 11 days. To remove the surfactant and byproducts, the paste was dissolved in a 3:1 mixture of ethanol to water with intermittent vortexing and sonication. The dark solids suspended in the solution were separated by centrifugation at 6600 rpm in 50 mL tubes, and the supernatant was decanted. This step was repeated two more times with the same mixture. About 24 h elapsed during the solvent treatment. The effectiveness of the solvent wash is discussed in the Supporting Information (Figure S6). The product was dried under vacuum, and vented over the course of 1 h; rapid venting leads to oxidative heating of the sample. The resulting product was 7.8 g of a gray powder that had a density of 2.3 g/cm3. Surface area measurements, hydrogen absorption measurements and pore size distributions were performed using a Micromeritics ASAP 2020 porosimeter, using nitrogen as the analytic gas at 77 K for porosimetry experiments, and hydrogen gas at 298 K for hydrogen 997
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were mechanically thinned to approximately 100 μm using successively finer silicon carbide grit paper on a grinding wheel. The discs were then thinned to electron transparency using conventional mechanical dimpling (Gatan, Inc. 656 Dimple Grinder) and ion milling (Fischione Instruments 1010 ion mill). Auger electron spectroscopy was performed on a Physical Electronics 680 scanning Auger spectrometer. Auger scans were taken using a 5 keV 18 nA electron beam, averaged over a 100 × 100 μm area.
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RESULTS AND DISCUSSION 1. Surface Oxidation State. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy allow us to establish the chemical state and stability of surface atoms. Though they selectively probe only a few atomic layers at the surface, these techniques usually measure many square micrometers of sample area. Auger electron spectroscopy (Figure 1) informs us of the identity and relative amounts of
Figure 2. XPS survey scan of as-synthesized nanoporous Pd-5% Rh. The incident photon energy used here is 750 eV.
Rh is observed: the ratio of Rh to Pd (observed with a 0.9 nm escape depth) is approximately 2:1 despite the fact that the ratio of Rh to Pd salts used in synthesis was 1 to 19 and the reduction reaction proceeded to high yield. The presence of an oxygen peak suggests the presence of oxidized metal on the surface of the material. Auger lines from Pd and Rh are visible between the O peak and the Pd peak. The two peaks near 85 eV are from the gold foil used as the substrate to mount the powder for the experiment, and carbon at ∼286 eV can also be observed. High-resolution scans of the metal peaks show the oxidation state of each, which is a function of the gas environment in the chamber. Figure 3a shows reduction of Rh at room temperature in 0.1 mTorr of hydrogen. The chemical shift of Rh 3d5/2 for the oxide is +1.1 eV relative to metallic Rh. The off-the-shelf material has rhodium oxide on the surface, but it is reduced easily in the presence of small amounts of hydrogen. The time gap between each curve in Figure 3a is approximately 5 min. In 100 mTorr of oxygen, the oxide does not return at this time scale and temperature, but does at temperatures near 160 °C, consistent with previous work.33 The time gap between each curve in Figure 3b is approximately 8 min, and the temperature is ramped at approximately 10 °C per minute between the first and third curve and then held at 160 °C for the remainder of the experiment. The oxidation/reduction chemistry of palladium is observed more clearly with a more Pd-rich sample (49:1 Pd:Rh) that did not exhibit as much Rh at the surface as the 19:1 Pd:Rh material. The high resolution Pd 3d peaks are shown in Figure 4a for the 19:1 Pd:Rh oxidized, reduced, and as prepared. For each scan, the curves were shifted to align the top of the C 1s peak, and the background-subtracted areas were normalized. There is little change in any of the scans, and no evidence of an oxide layer. This suggests that nearly all of the Pd measured here is below the surface, and covered by Rh. If only surface oxide can form, this Pd would not be oxidized, so no oxide peak should be present. In contrast, the Pd in the 49:1 Pd:Rh sample is prone to oxidation because the surface contains mostly Pd and very little Rh. As Figure 4b shows, very little oxide is present on the fresh sample, but after heating at 150 °C in 300 mTorr O2, oxide is clearly observed. The oxide on this sample was then removed with 0.1 mTorr H2 during a temperature ramp from 0 to 50 °C over 30 min. The differing oxidation behavior of Pd for the 19:1 and 49:1 sample indicates a complete skin layer of Rh covered the surface for the 19:1 sample, but not for the 49:1 sample. 2. Overall Pore Stability. To determine whether oxide reduction influences pore collapse in nanoporous Pd alloys, we
Figure 1. Auger electron spectra of palladium-5% rhodium alloy particles after exposure to several environments. Top: a sample after storage for several days in air. Middle: The same sample exposed in a glovebag to 1% H2 in N2 for 10 min before insertion into the chamber. Bottom: the same sample removed from the chamber, exposed to air for 10 min, and then returned to the chamber.
surface elements. It shows that samples stored in air have an oxide layer, and that this can be stripped at room temperature by exposure to 1% hydrogen in nitrogen using a glovebag, validating a convenient method to achieve this chemical change. This hydrogen pressure is not high enough to transform palladium to the hydrogen-loaded beta phase at room temperature, but the gas can still react with the surface and absorb as a dilute solid solution. Upon subsequent air exposure of similar duration, the oxide is still mostly absent. Surface chloride is present under all conditions, a remnant of the synthesis. These spectra also confirm that rhodium is present at high concentrations at the particle surfaces. At the approximately 1 nm escape depth of Auger electron spectroscopy, a 3:1 ratio of rhodium to palladium is observed for the palladium5% rhodium sample. Ambient-pressure XPS can provide information about the chemical state of the metals at the surface, and show the kinetics of oxidation and reduction as it occurs.25,30−32 With a synchrotron, tuning of the incident energy changes the photoelectron escape depth, providing a nondestructive depth profile. A survey scan of the as-synthesized Pd−Rh is shown in Figure 2. Pd and Rh are the strongest peaks in the spectrum, and are of primary interest. Significant surface enrichment of 998
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Figure 3. High-resolution scan of the Rh 3d5/2 peak showing (a) reduction of rhodium oxide under 0.1 mTorr H2 at room temperature and (b) oxidation of rhodium oxide under 100 mTorr O2. The initial temperature is 16 °C, and the temperatures of the second and third scan are 40 and 100 °C, respectively. The temperatures of the last two scans are 160 °C. The oxide peak is at 308.2 eV, and the metal peak is at 307.2 eV. The time sequence progresses from the bottom spectrum to the top. The incident photon energy used here was 490 eV.
Figure 4. High-resolution scans of the Pd 3d5/2 and 3d3/2 peaks. The green curve is the original sample stored several days under air. The black curve is the same sample after treatment with 300 mTorr O2 at 150 °C. The red curve is the same sample with subsequent treatment with 0.1 mTorr H2 after a 30-min temperature ramp from 0 to 50 °C. The incident photon energy used here was 490 eV.
Figure 5. Surface area of Pd-5% Rh as a function of cumulative heating time at 150 °C in vacuum and two separately measured samples in low pressures of hydrogen.
measured the surface area of the Pd-5% Rh sample by N2 porosimetry after heating at 150 °C in both vacuum and low pressures of hydrogen gas. The pressure of hydrogen was kept low to prevent the phase transition of the alloy from alpha to the hydrided beta phase in order to allow us to focus our study on surface effects. We use the specific surface area as a quantitative measure of material degradation from aging. Most of the surface area is inside the pores, and this is lost when the pores collapse. Sintering of particles can also occur; this effect is distinguishable through consideration of internal and external surface areas of a particle, along with analysis of the pore size distribution. Scanning electron microscopy of this material shows that most of the mass consists of particles in the 1−5 μm range. The theoretical specific surface area of 1 μm Pd spheres is 0.5 m2/g, so external surface area is a small fraction of the total observed, and the overall surface area loss can only be accounted for if it includes pore collapse. The results of the aging experiment are shown in Figure 5. Surface area decreases were observed for both conditions, but the loss of surface area was accelerated in the presence of hydrogen. This is indicative of some combination of accelerated pore collapse and particle sintering. The pore size distributions derived from the porosimetry experiments allow us to distinguish surface area associated with pores, which appears at pore diameters below the 10 nm range, from that of the interparticle surfaces, which appears at larger length scales. Figure 6 shows the evolution of the pore size distribution as the heating time progresses for both vacuum and
Figure 6. Pore size distribution of Pd-5% Rh after aging at 150 °C in (a) vacuum and (b) hydrogen.
hydrogen. The area under the curve represents the total pore volume, and most of this area is associated with pores. The total area decreases as the aging progresses, which follows the same trend as the loss in total surface area shown in Figure 5. The loss in porosity occurs mostly for the smallest pores, while the volume of larger pores increases. Smaller pores are transforming into larger pores, which could occur by fusion of pores, perhaps initiated at internal junctions or the particle surface, followed by diffusion of atoms at the pore surface. It could also occur by transport of vacancies through the bulk or along grain boundaries in an Ostwald ripening process.8 Larger pores are inherently more stable than the smaller pores, and are less likely to be destabilized during the heating. The overall loss of pore volume can result from the closing of pore entrances or channels to create inaccessible internal bubbles, vacancy transport to the particle surface, or collapse of the particle through larger-scale motion. Figure 6 shows that both the total 999
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Figure 7. STEM and EDSSI images of palladium-5% rhodium alloy particles that were exposed to 1% H2 in N2 for 10 min before insertion into the microscope, and then heated in the microscope to 400 °C for 30 min. Yellow represents rhodium, and cyan represents palladium.
they overlap, so multivariate statistical analysis and multiple linear least-squares fitting were used to denoise the data and quantify the intensities of the Pd and Rh L X-ray lines at each pixel based on pure rhodium and palladium reference spectra.34−41 The reference spectra each contained >3 × 105 counts above background, which results in Poisson noise (= √N/N) that is
