Synthesis and Comparative Study of Nanoporous Palladium-Based

Article pubs.acs.org/JPCC

Synthesis and Comparative Study of Nanoporous Palladium-Based Bimetallic Catalysts for Formic Acid Oxidation Brian D. Adams, Robert M. Asmussen, Cassandra K. Ostrom, and Aicheng Chen* Department of Chemistry, Lakehead University, 955 Oliver Road, Thunder Bay ON P7B 5E1, Canada ABSTRACT: In view of the inherent limitations of current portable technology energy sources, the implementation of micro fuel cells is becoming increasingly appealing. This has generated great interest in the development of direct formic acid micro fuel cells. In this study, nanoporous Pd and four nanoporous bimetallic Pd-M catalysts with an atomic ratio of 90:10, where M = Cd, Pb, Ir, and Pt, were synthesized via a facile hydrothermal method and examined for the electrochemical oxidation of formic acid. The electrocatalytic activity of these nanoporous electrode materials was studied with the use of linear sweep voltammetry and chronoamperometry. Our chronoamperometric measurements have shown that the initial electrochemical performance of the nanoporous Pd-M electrodes toward formic acid oxidation was almost independent of the alloying materials; however, the second incorporated metal strongly affected the stability of the Pd-based electrocatalysts. The mechanisms of the oxidation of formic acid were further examined with the aid of in situ electrochemical ATR-FTIR spectroscopy. For the nanoporous Pd, PdCd, and PdPb catalysts, oxidation proceeds through the direct mechanism, whereas the indirect mechanism, along with major CO poisoning, was observed in the case of the PdIr and PdPt catalysts. The incorporation of even small amounts of Pt and Ir to Pd was found to inhibit the oxidation of formic acid. On the other hand, the addition of Pb to Pd served to promote the direct mechanism, which in turn makes these Pd-based catalysts both cost and electrocatalytically more effective. Direct pathway: HCOOH → CO2 + 2H+ + 2e−

1. INTRODUCTION In view of the inherent limitations of current portable technology energy sources, the implementation of micro fuel cells is becoming increasingly appealing. Wherein, micro fuel cells are capable of delivering higher energy density power than the most competitive rechargeable batteries.1 Direct fuel cell development utilizing organic molecules, such as methanol and formic acid, has been a challenging topic of research. Both direct formic acid fuel cells (DFAFCs) and direct methanol fuel cells (DMFCs) have the potential for miniaturization.2,3 However, due to their reasonably high power output and low fuel proton exchange membrane permeation rate, DFAFCs have garnered more appeal than their DMFC counterparts.2,4 The development of an appropriate anode material for the oxidation of formic acid is necessary for the application of DFAFCs. Recently, a variety of Pt-based4−24 and Pdbased18,23,25−38 catalysts for the electrochemical oxidation of formic acid have been studied. The electrochemical oxidation of formic acid is thought to occur through the combination of two distinct pathways: direct and indirect.2 Direct oxidation occurs via a dehydrogenation reaction to form CO2 through one or more active intermediates, without the formation of CO as a reaction intermediate. The indirect reaction pathway involves the dehydration of formic acid to CO, which, depending on the applied potential, may poison the electrode or be further oxidized to produce CO2:2 © 2014 American Chemical Society

(1) +

Indirect pathway: HCOOH → COads + H 2O → CO2 + 2H + 2e

−

(2)

Formic acid oxidation involving Pt is commonly known to occur via dual direct and indirect reaction pathways.2 The second pathway occurs more predominately, which leads to CO poisoning of the pure Pt surface, thereby reducing its catalytic performance.12,39 Bimetallic Pt anode catalysts often exhibit better performance in comparison to pure Pt due to the incorporated second metal weakening CO adsorption.4,13 However, as effective as the Pt bimetallic approach may be, Pd has been shown to be superior over Pt to serve as the anode for DFAFCs.3,40 It has become widely accepted that the oxidation of formic acid on Pd occurs primarily through the direct pathway, which avoids the formation of the catalyst poisoning intermediate (carbon monoxide). In comparison, the oxidation of formic acid on Pd has been less studied than for Pt. Most of the work on this topic has focused on pure Pd nanomaterials,3,26,31,32,36,37,40 whereas few studies have systemically investigated bimetallic Pd-based materials as anodes for the electrooxidation of formic acid. A multistep reaction, similar to that for Pt, is the proposed mechanism of formic acid Received: September 20, 2014 Revised: November 22, 2014 Published: November 24, 2014 29903

dx.doi.org/10.1021/jp5095273 | J. Phys. Chem. C 2014, 118, 29903−29910

The Journal of Physical Chemistry C

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Figure 1. SEM images of the electrode surfaces at 15 000× magnification: (a) Pd, (b) PdCd, (c) PdIr, (d) PdPb, (e) PdPt. The EDX spectra of all the catalysts are shown in (f).

investigated in this study. The surface species formed during the interactions between formic acid and Pd-based catalysts have been studied using in situ electrochemical ATR-FTIR spectroscopy. The objectives of this study were three-fold: (i) to synthesize five different Pd-based nanoporous materials by employing a facile hydrothermal method; (ii) to compare the activity of these Pd-based nanomaterials toward the electrocatalytic oxidation of formic acid; and (iii) to decipher the reaction pathway of formic acid oxidation on these prepared nanoporous electrocatalysts using the in situ ATR-FTIR technique.

oxidation on Pd. Besides the aforementioned direct and indirect oxidation mechanisms, the electrochemical oxidation of formic acid through absorbed formate species has also been reported:41,42 Formate pathway: HCOOH → −COOHads + H+ + e− → CO2 + 2H+ + 2e−

(3)

Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) is a powerful tool for the surface analysis of electrocatalytic reactions, specifically at Pt42−44 and Pd46−48 catalytic surfaces. This technique may be combined with standard electrochemical methods to compare catalytic systems and decipher mechanistic changes directly at the reaction surface.49,50 Considering that the incorporation of a second metal is known to affect the catalytic activity of anodes, the effects of a small amount of metal dopants that are alloyed with Pd for applications toward formic acid oxidation were

2. EXPERIMENTAL METHODS The Pd-based catalysts were fabricated using the hydrothermally assisted reduction of Pd, Pt, Ir, Pb, and Cd precursors. For electrochemical testing, Ti plates (1.25 cm × 0.80 cm × 0.5 mm) were utilized as the substrate. The Ti plates were initially degreased in an ultrasonic bath of acetone for 10 29904



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of ultrasonic treatment, and then coated onto the ZnSe window. The incidence angle of the infrared beam was controlled with an in-house produced variable angle accessory, which was set to 60° for all experiments. The spectra were recorded using a single potential step procedure in which the electrode potential was controlled by a potentiostat and increased in steps of +0.10 V beginning at −0.1 V. All interferograms were acquired at 4 cm−1 resolution, to which 400 scans were added and averaged. The interferograms were shown in terms of relative change of the electrode reflectivity, which is defined as51

min, followed by an additional 10 min immersion in pure water (18.2 MΩ cm), which was obtained from a NANOpure Diamond UV ultrapure water purification system. The substrates were then etched in 18% HCl at approximately 85 °C for 20 min. The etched Ti substrates were subsequently transferred into a Teflon-lined autoclave, where 10 mL of an aqueous mixture of inorganic metal precursors and an excess amount of the reducing agent were added. Ammonium formate (Aldrich, 99.995%) was used as the reducing agent, and the metal precursors were PdCl2 (Alfa Aesar, 99.9%), H2PtCl6 6H2O (Alfa Aesar, 99.9%), IrCl3 3H2O (Pressure Chemical Co.), Pb(NO3)2 (Sigma-Aldrich, 99+% A.C.S. reagent grade), and Cd(NO3)2 4H2O. In all cases, the added PdCl2 precursor and reducing agent were kept constant at 5.0 mM and 1.0 M, respectively. Pure water was used as the solvent in all cases, and a dilute HCl (1.0%) was added to the PdCl2 solution to enhance solubility. To grow the nanoporous Pd catalyst, only PdCl2 was added to the vessel, whereas for the bimetallic catalysts, stoichiometric amounts of the PdCl2 and other metal precursors were added to obtain the desired compositions of Pd:Pt, Pd:Ir, Pd:Pb, and Pd:Cd of 90:10. The autoclaves were sealed and heated to 180 °C for 8 h. Subsequent to cooling to room temperature, the Ti plates that were coated with the Pd, PdPt, PdIr, PdCd, and PdPb thin films were dried under argon and then rinsed with pure water. The coating loads of the catalysts were obtained by weighing the Ti plates prior to, and following, the application of the films. The morphologies of the prepared catalysts were examined using a JEOL JSM 5900LV scanning electron microscope (SEM). The surface compositions were investigated with an Oxford Links ISIS energy dispersive X-ray spectrometer (EDX) and verified to be equivalent to the input compositions of metal precursor solutions prior to reduction. The X-ray diffraction (XRD) patterns of the as-prepared samples were recorded using a Philips PW 1050-3710 diffractometer with Cu Kα radiation. The electrochemical performance of the fabricated nanoporous Pd-based electrodes toward the oxidation of formic acid was characterized using cyclic voltammetry (CV) and chronoamperometry (CA). A three-electrode cell system and a VoltaLab 80 PGZ402 potentiostat were employed in our electrochemical studies. The counter electrode utilized was a Pt wire coil (10 cm2), which was cleaned before each experiment by flame annealing and quenching with pure water. The reference electrode consisted of a saturated calomel electrode that was connected to the cell via a salt bridge. The working electrodes used were the prepared Ti/Pd, Ti/PdPt, Ti/PdIr, Ti/PdPb, and Ti/PdCd nanomaterials. The coated Ti plates were annealed at 250 °C for 2 h in a tube furnace under argon to ensure good surface adhesion of the Pd-based thin coatings. The supporting electrolyte employed for these electrochemical studies was 0.1 M H2SO4, and the solution was deaerated with ultrapure argon gas prior to measurements to remove any dissolved oxygen. Argon gas was continually passed over the solution throughout the experimentation. Data acquisition and analysis were performed using VoltMaster 4 software. All measurements were conducted at room temperature (22 ± 2 °C). All in situ ATR-FTIR experiments were performed using a Nicolet Fourier transform infrared spectrometer with a liquid N2-cooled MCT detector. The spectroelectrochemical cell consisted of a Teflon chamber with a ZnSe hemisphere window, which was sealed on the bottom. The synthesized nanoporous catalysts were suspended in pure water with the aid

ΔR /R = [R(E2) − R(E1)]/R(E1)

where R(E1) and R(E2) are the reflectivity from the electrode surface at potentials E1 and E2, respectively. The spectra recorded at E1 = −0.20 V was employed as a reference.

3. RESULTS AND DISCUSSION 3.1. Morphology and Composition Characterization. The morphologies of the Pd-based catalysts are displayed in Figure 1. The effect on the morphology of the Pd following the addition of 10 at. % of Pt, Ir, Cd, and Pb was negligible, though it can be observed that Cd initiated the formation of small dendritic structures, which is consistent with our previous studies with this bimetallic system.52,53 The other catalysts exhibited a random array of nanoporous structure with a particle size distribution ranging from ∼10 nm to ∼200 nm. From the EDX spectra (Figure 1f), the two primary peaks arising from palladium (2.99 and 3.15 keV) were visible in all catalysts. For the catalysts that were doped with the second metals, an additional peak was observed, which corresponded to that metal additive. These peaks are located at 2.16, 2.22, 3.31, and 2.52 keV for Ir, Pt, Cd, and Pb, respectively. Using an average of five random points taken from each surface, the composition was found to be 10 at. % of Ir, Pt, Cd, and Pb (Figures 1b−e). This is consistent with the compositions of the precursors which were added to the hydrothermal vessels. Inductively coupled plasma (ICP) was further employed to verify the composition of the formed catalysts. Liquid samples extracted from the hydrothermal vessels following the reaction were analyzed. On the basis of the variances in the concentrations of the metals prior to, and following the reactions (i.e., the amount of metal ions reduced), the compositions of the synthesized catalysts were determined. Table 1 presents the ICP results for the amount of precursors that remained in solution (i.e., not reduced) following the hydrothermal reductions, confirming that the ammonium formate reducing agent was potent enough to completely Table 1. Inductively Coupled Plasma Results for the Synthesis of the Pd-Based Ctalysts

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sample

initial [Pd] (ppm)

final [Pd] (ppm)

Pd PdCd

532.10 532.10

0.0057 0.0273

PdIr

532.10

0.0034

PdPb

532.10

0.0046

PdPt

532.10

0.0275

initial [M] (ppm)

final [M] (ppm)

62.39 [Cd] 106.68 [Ir] 115.00 [Pb] 108.27 [Pt]

0.0056 [Cd] 2.0030 [Ir] 0.0422 [Pb] 0.0136 [Pt]

calcd composition (at. % M) 0 9.99 [Cd] 9.82 [Ir] 9.99 [Pb] 9.99 [Pt]



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PdPt, and PdIr catalysts. The peaks of the PdPb and PdCd catalysts, as well as the corresponding oxidation peaks, possessed a much higher current. The charge for hydrogen desorption was integrated to be 26.6, 120.1, 26.5, 54.1, and 32.1 mC for the nanoporous Pd, PdCd, PdIr, PdPb, and PdPt, respectively. This was due to the expansion of the Pd lattice by the larger atomic radii of the Pb and Cd atoms, which allowed for enhanced absorption kinetics and the diffusion of H atoms within the crystal lattice.53 For this reason, the desorption of hydrogen from these two catalysts (especially for PdCd) extended into the double layer region. From an examination of the oxygen region, a well-defined oxide reduction peak was visible for each nanoporous Pd-based catalyst. The PdPb catalyst underwent oxidation at the lowest potential, followed by PdIr, and formed more surface oxide than the other catalysts. The activity of the catalysts in the oxidation of formic acid was studied by linear sweep voltammetry (Figure 4). The peak

reduce all of the metal precursors under the experimental conditions, with the exception of a negligible concentration of IrCl3 that remained following the formation of the Pd−Ir catalyst. 3.2. Structural Characterization. X-ray diffraction was carried out with the five Ti substrate supported Pd-based catalysts (Figure 2). In all cases, the face-centered cubic crystal

Figure 2. XRD patterns of the electrode surfaces. Peaks identified with * arise from the titanium substrates.

structure of palladium was observed with no additional phase present, with the exception of those peaks derived from the Ti substrate. This indicated that all of the mixed metal catalysts formed alloys with palladium. For PdCd and PdPb, a shift in the diffraction peaks to lower angles and peak broadening occurred. The larger atomic radii of both Cd and Pd resulted in an expansion of the crystal lattice relative to pure Pd. 3.3. Electrochemical Performance. The fundamental electrochemical properties of the catalysts were examined via cyclic voltammetry in the supporting electrolyte (Figure 3). Three major regions were observed in these CVs: the hydrogen region (−225 mV to 50 mV vs SCE); the double layer region (50 mV to 250 mV vs SCE); and the oxygen region (250 mV to 1000 mV vs SCE). The cathodic peak in the hydrogen region, due to hydrogen adsorption and absorption at the surface and into the palladium lattice, was virtually identical for the Pd,

Figure 4. Anodic portion of cyclic voltammograms in 0.1 M H2SO4 + 0.1 M HCOOH at a scan rate of 50 mV/s.

potential increased in the following order: PdPt (0.055 V) < Pd (0.110 V) < PdPb (0.125 V) < PdIr (0.145 V) < PdCd (0.160 V vs SCE); and the peak current increased in the order of: PdPt (17.90 mA cm−2) < PdPb (21.87 mA cm−2) < Pd (22.72 mA cm−2) < PdIr (24.70 mA cm−2) < PdCd (28.47 mA cm−2). The peak of the formic acid oxidation at the nanoporous PdIr was much broader than that at the other Pd-based nanoporous electrode surface. As seen in Figure 3, hydrogen adsorption and absorption may occur at the electrode potential lower than 0.0 V vs SCE, forming Pd(H)-M. Hydrogen sorption into both Pd and PdCd(10%) was quantified at various potentials in our previous study,53 showing that the amount of sorbed hydrogen was slowly increased with the decrease of the potential from 0.0 to −0.2 V vs SCE, attaining PdH0.0084 and (PdCd)H0.086 at −0.2 V. To avoid the possible influence of the hydrogen sorption on the electrochemical oxidation of formic acid, chronoamperometry was carried out at 0.1 V vs SCE, where there is no hydrogen sorption. As shown in Figure 5a, for all the assynthesized Pd-based nanoporous electrodes, the initial current responses at 100 mV linearly increased and reached the highest values within 0.8 s, giving the following order: PdPt (25.57 mA cm−2 @ 0.5 s) ≈ PdPb (25.59 mA cm−2 @ 0.8 s) < Pd (26.48 mA cm−2 @ 0.7 s) < PdCd (27.74 mA cm−2 @ 0.8 s) < PdIr (28.97 mA cm−2 @ 0.7 s). It is interesting to note that the

Figure 3. Cyclic voltammograms of the catalysts in 0.1 M H2SO4 at a scan rate of 50 mV/s. 29906



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Figure 6. Ratios of the current density of the nanoporous Pd-M electrodes over the polycrystalline Pd wire at their highest current (red) and steady state current at 500 s (blue) measured at 0.10 V in 0.1 M H2SO4 + 0.1 M HCOOH.

(5.11), PdCd (5.35), and PdIr (5.59)], indicating that the initial electrode performance toward formic acid oxidation is independent of the alloying materials. In contrast, the ratio of the steady state current measured at 500 s gave rise to the following order: PdIr (1.97) < PdPt (2.44) < PdCd (3.91) < Pd (4.62) < PdPb (5.26), showing that the second incorporated metal strongly affected the stability of the Pd-based electrocatalysts. To understand the origin of the degradation of electrocatalytic activity for particular catalysts and the mechanisms of electrochemical oxidation of formic acid at the Pd-based nanoporous electrodes, in situ ATR-FTIR studies were carried out. 3.4. In Situ Electrochemical ATR-FTIR Study. In situ electrochemical ATR-FTIR spectra were recorded during the electrochemical oxidation of formic acid at different applied electrode potentials on the Pd-M nanoporous catalysts in a 0.1 M H2SO4 + 0.1 M HCOOH solution. Figure 7a depicts the spectra that resulted from the electrooxidation of formic acid on the nanoporous Pd electrode. Starting at 0.0 V, the generation of a peak at 2340 cm−1 was observed, which corresponds to the complete oxidation of formic acid to the final CO2 product. The intensity of this peak grows through the potential range, as formic acid may be oxidized at a higher rate at increased anodic biases. It is known that the penetration of the IR evanescent wave into solution is restricted to a few nanometers. The appearance of the strong peak indicates that the nanoporous structure of Pd may help capturing the formed CO2. The unexpected bipolar feature of this peak might be attributed to the surface roughness of the deposited thin film on the ATR-IR window, resulting in the anomalous effects in the FTIR spectra.54,55 Two smaller bands appeared at 1749 and 1700 cm−1, corresponding to the CO stretching mode of formic acid in solution as it is brought in close proximity to the catalyst surface.56 Figure 7b resulted from the oxidation of formic acid on the PdCd, showing the complete oxidation of formic acid with a peak at 2340 cm−1, initially appearing at 0 V. Along with the CO stretch at 1749 cm−1, the peak at 1610 cm−1 might be due to water IR absorption.57 The oxidation of formic acid underwent a stark change at the PdIr catalyst (Figure 7c). Here, there is only a small peak present at 2340 cm−1, evidence of CO2 generation at the surface with the spectra being dominated by a bipolar peak centered between 2026 and 1990 cm−1, showing the formation of linearly bonded CO (COL) at the nanoporous PdIr surface. The bipolar feature seen arose from a potential dependent shift in the infrared

Figure 5. Chronoamperometric curves of the catalysts in 0.1 M H2SO4 + 0.1 M HCOOH at 0.10 V.

highest currents were pretty close for all the five nanoporous electrodes; however, the decay of the current was at different rates. The current of the PdPt electrode decreased much quicker than that at the other electrodes. After 5 s, the current changed to the following order: PdPt (10.81 mA cm−2) < Pd (15.48 mA cm−2) < PdCd (17.55 mA cm−2) ≈ PdPb (17.80 mA cm−2) < PdIr (20.75 mA cm−2). Although the PdIr electrode still exhibited the highest current, it continued to decrease rapidly as seen in Figure 5b. After 200 s, it changed from the highest to the lowest. At 500 s, the steady-state current of the Pd-based nanoporous electrodes changed to the following order: PdIr (4.24 mA cm−2) < PdPt (6.39 mA cm−2) < PdCd (8.44 mA cm−2) < Pd (9.95 mA cm−2) < PdPb (11.34 mA cm−2). For comparison, we have also carried out the chronoamperometric measurement of the electrochemical oxidation of formic acid at a smooth polycrystalline Pd wire with a geometric surface area of 1.0 cm2. The initial current response at 100 mV increased to 5.18 mA cm−2 after 0.8 s, then decreased to 4.42 mA cm−2 after 5 s, and dropped to 2.15 mA cm−2 after 500 s. Taking into account the overall stability of the Pd wire, a ratio comparison of the current densities of nanopourous PdM samples over the polycrystalline Pd wire was performed. Both the ratios of the maximum peak current (red) within 0.8 s and the steady state current response at 500 s (blue) are plotted in Figure 6 for each nanoporous catalyst. The ratio of the maximum peak current for all the five nanoporous electrodes was relatively close [PdPt (4.94), PdPb (4.94), Pd 29907



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Figure 7. FTIR spectra of the formic acid oxidation in 0.1 M H2SO4 + 0.1 M HCOOH, on (a) Pd, (b) PdCd, (c) PdIr, (d) PdPb, (e) PdPt from −0.1 V (top) to 0.6 V (bottom) taken at 0.1 V intervals and (f) of all catalysts at +0.3 V.

that the large current response observed in the voltammograms was actually due to the formation of large amount of CO, which quickly deactivated the catalytic PdIr surface (Figure 5b). The PdPt catalyst also exhibited a linearly absorbed CO band along with a bridged CO band. CO2 was observed at low potentials but was absent with the increased dominance of COL and COB. It was apparent that the formic acid was being oxidized through an indirect mechanism; quite possibly via a competing mechanism between the Pd and Pt sites, as previously observed with the oxidation of methanol.59

frequency due to the Stark effect, as COL was present in both the measured and reference spectra. Figure 7d displays the electrooxidation of formic acid at the PdPb catalyst. With behavior akin to the PdCd catalyst in Figure 7b, a strong CO2 signal was observed at 2340 cm−1 along with the CO stretch at 1749 and 1710 cm−1. The PdPt catalyst produced much more complex spectra, as displayed in Figure 7e. CO2 was detected at low potentials (−0.10 and 0.0 V); however, it was absent at higher electrode potentials. A bipolar COL stretch was present at 2030 and 2002 cm−1. A bipolar bridged COB was also detected at 1890 and 1830 cm−1 along with a bipolar peak centered at 1680 and 1610 cm−1. In comparing the spectra of the five Pd-M catalysts, clear behavioral changes were observed. Figure 7f compares the spectra of the electrochemical oxidation of formic acid on the five nanoporous catalysts recorded at 0.30 V. The Pd (black), PdCd (red), and PdPb (blue) showed similar features with a large CO2 band present due to the complete electrooxidation of formic acid. As no CO was present on these three catalysts, it was evident that the formic acid was being oxidized via a direct mechanism. On the nanoporous PdIr electrode, a large bipolar band at 2026/1990 cm−1 could be clearly seen due to linearly absorbed CO at the surface that was produced during the indirect oxidation of formic acid. In the previous study,58 it was proposed that the addition of Ir to Pd catalysts promoted the direct mechanism according to the linear sweep voltammetric study. However, the ATR-FTIR spectra (Figure 7c) indicate

4. CONCLUSIONS Five Pd-based nanoporous catalysts (Pd, PdCd, PdPb, PdIr, and PdPt) with a controlled composition were synthesized and examined for their electrocatalytic activity in the oxidation of formic acid. For each of the bimetallic catalysts with 10 at. % additive metal, the hydrothermal synthesis method resulted in the formation of alloys between Pd and the additional metal as determined by XRD. Several electrochemical techniques, including linear sweep voltammetry and chronoamperometry, were utilized to compare the activity of the catalysts toward the oxidation of formic acid. The chronoamperometric measurements showed that the current responses at 0.10 V reached the highest value within 0.8 s, which were pretty close for all the five nanoporous Pd-M electrodes, indicating that the initial electrode performance toward formic acid oxidation is independent of the alloying materials (Cd, Ir, Pd, and Pt). In 29908



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contrast, the ratio of the steady state current measured at 500 s gave rise to the following order: PdIr (1.97)

Synthesis and Comparative Study of Nanoporous Palladium-Based

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