J. Phys. Chem. C 2008, 112, 3789-3793
3789
Particle Size Effects in Pd-Catalyzed Electrooxidation of Formic Acid Weijiang Zhou and Jim Yang Lee* The Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed: September 3, 2007; In Final Form: December 28, 2007
Small carbon-supported Pd nanoparticles with controllable size (2.7-9.0 nm) were prepared by a solution chemistry method, characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopies, and evaluated as anode catalysts for the electrooxidation of formic acid at room temperature. Measurements of catalytic activity by electrochemical methods (cyclic voltammetry and chronoamperometry) revealed a strong potential-dependent particle size effect. Classical volcano plots of catalytic activity vs particle size were obtained from which the optimal Pd particle size for the respective potential region was determined.
I. Introduction
II. Experimental Section
The direct formic acid fuel cell (DFAFC) is another variant of the polymer electrolyte membrane (PEM) fuel cells well suited for mobile use.1-5 Formic acid is an avirulent nonflammable liquid fuel with a low crossover rate through the Nafion membrane; hence, a relatively high power density is possible with aqueous solutions. For the electrooxidation of formic acid at room temperature, Pd-based catalysts have been found to be more active than Pt-based catalysts commonly used in hydrogen and direct methanol fuel cells.6 A complete mechanistic understanding of formic acid electrooxidation is needed for the rational design of catalysts incorporating various beneficial geometric and electronic effects in catalysis. Presently, it is not entirely clear whether particle size affects formic acid electrooxidation. No size effect was found in the study of formic acid electrooxidation on Pt catalysts covering a wide range of sizes.7 On the other hand, Zhou et al. reported particle size effects in the electrooxidation of formic acid over Pd. Binding energy shifts and d-band vacancy changes were used to rationalize the changes in specific activity with particle size.8 However, the relevance of the latter study was undermined by the use of relatively large and unsupported Pd particles which are not useful in practical situations. The particle size effects should be more prevalent in supported catalysts where a high dispersion of small metal nanoparticles is maintained by a catalyst support with large surface area. Supported catalysts are the only practical means to achieve good utilization of expensive noble metals in electrocatalysis. The particle size effects in formic acid electrooxidation were investigated in this study using a series of carbon-supported Pd nanoparticles with sizes most relevant to industrial catalysis (2.7-9.0 nm). There has not been any prior study with the same scope and emphasis despite the current interest in direct liquid fuel cells. The catalysts were prepared by the borohydride (NaBH4) reduction of Pd precursor salts, where the particle size was controlled by varying the citrate/Pd ratio.9 No postsynthesis heat treatment was applied.
All chemicals, PdCl2, trisodium citrate, sodium borohydride (NaBH4), formic acid (HCOOH), perchloric acid (HClO4), and HCl, were purchased from Sigma-Aldrich and used as received. Carbon XC-72R (SBET ) 240 m2/g) was supplied by Cabot and treated by reflux in 5 M HNO3. The Pd/C catalysts were prepared using a slight modification of a published procedure:9 A calculated amount of trisodium citrate solution was added dropwise to a stirred carbon slurry in water, and stirring was continued for another hour. Under stirring a stoichiometric amount of PdCl2 solution (in 0.1 M HCl) was introduced dropwise followed by an excess of freshly prepared ice-cold NaBH4 solution. The reduction temperature was maintained by an ice-water bath. The solid Pd/C (containing 10 wt % Pd) product was recovered by centrifugation and vacuum dried at 70 °C overnight. All electrochemical measurements were carried out on an Autolab potentiostat/galvanostat (with GPES software v4.9) using a standard three-electrode cell. The electrolyte was 0.1 M HClO4 with or without 3 M formic acid. The working electrode was a thin layer of Nafion-impregnated catalyst cast on a glassy carbon disc (Φ ) 5 mm) held in a Teflon cylinder prepared according to a previous procedure.10 A Pt gauze and an Ag|AgCl (in saturated KCl) electrode were used as the counter electrode and reference electrode, respectively. All potentials were referred to the RHE in this paper. CO-stripping voltammetry began with bubbling CO over the working electrode at 0.04 V for 15 min.11 The electrolyte was then degassed in Ar. The electrode potential was scanned from 0.04 to 1.2 V. The scan direction was then reversed to end the scan at 0.02 V. Cyclic voltammetry (CV) was also carried out after 2 h of potentiostatic polarization at different potentials (0.2 and 0.4 V). The electrode after copious washing with deionized (DI) water was transferred to a supporting electrolyte without formic acid. The potential scan began at the polarization potential and proceeded cathodically to 0.02 V and then anodically to 1.2 V. X-ray diffraction (XRD) characterization was carried out on a Bruker GADDS diffractometer using a Cu KR source (λ ) 1.54056 Å) operating at 40 kV and 40 mA. Transmission electron microscopy (TEM) was performed on a JEOL JEM 2010 microscope. X-ray photoelectron spectroscopic (XPS)
* To whom correspondence should be addressed. Phone: 65-65162899. Fax: 65-67791936. E-mail:
[email protected].
10.1021/jp077068m CCC: $40.75 © 2008 American Chemical Society Published on Web 02/15/2008
3790 J. Phys. Chem. C, Vol. 112, No. 10, 2008
Zhou and Lee TABLE 1: Results of TEM and Voltammetric Characterization of Catalysts specific particle size mass area ESCAH/ ESCACO/ STEM STEM (nm) from (m2/g Pd, ESCAH ESCACO STEM)a TEM (m2/g Pd)b (m2/g Pd)b (%) (%) 8.67 ( 1.25 7.14 ( 1.40 5.83 ( 1.20 5.05 ( 0.75 3.75 ( 0.95 2.72 ( 0.51
57.6 69.9 85.6 98.8 133.1 183.5
16.8 21.4 47.7 56.7 72.4 79.5
41.6 47.6 64.9 69.7 92.9 132.5
29.2 30.6 55.7 57.4 54.4 43.3
72.2 68.1 75.8 70.5 69.8 72.2
a Calculated from the formula S ) 6000/(Fd), where F is density of Pd, 12.02 g/cm3, and d is the average particle size (nm, TEM results). b The calculation method was presented in refs 7 and 11.
TABLE 2: Binding Energy Pd3d(5/2) and Relative Intensities of Surface Pd Species from the Pd3d X-ray Photoelectron Spectra of Pd/C Catalysts Pd particle size (nm) 2.72 ( 0.51 3.75 ( 0.95 5.05 ( 0.75 5.83 ( 1.20 7.14 ( 1.40 Figure 1. TEM image of a Pd/C sample (10 wt %, citrate/Pd ) 8), and the corresponding histogram of particle size distribution.
analysis of the samples was carried out on a VG ESCALAB MKII, and the narrow scan XPS spectra of Pd3d were deconvoluted by XPSPEAK (version 4.1). III. Results Very small particles with uniform size distribution could be obtained by this method of preparation (see Figure 1 for the TEM image and histogram of the particle size distribution of a catalyst prepared with a citrate/Pd ratio of 8). Generally a decrease in the citrate/Pd ratio led to not only larger Pd particles but also less uniform size distributions. Some agglomeration of Pd particles was also found in samples with larger particles. Table 1 is a summary of the most important physical features of the catalysts. With this method carbon-supported Pd nanoparticles can be prepared with high metal loadings and a relatively clean metal surface (the residual citrate could be easily removed after washing with DI water). XPS detected very minor increases in the Pd3d binding energy (BE) with decreasing particle size, as shown in Table 2, in which the relative intensities of the different palladium species are also given. A decrease is expected from the reports of others and indicates a state of electron deficiency in small Pd nanoparticles,8,12 although the deficiency was not as apparent as in the case of Pd nanoparticles smaller than 2 nm.13 The extent of surface oxidation was found to increase as the particle size decreased from XPS results, even though no bulk oxide formation was detected by XRD in any of the catalysts. Oxygen chemisorption easily occurs at step and kink sites present on
8.67 ( 1.25
Pd surface species
Binding energy (eV)
Relative intensities (%)
Pd0 Pd2+ Pd4+ Pd0 Pd2+ Pd4+ Pd0 Pd2+ Pd4+ Pd0 Pd2+ Pd4+ Pd0 Pd2+ Pd4+ Pd0 Pd2+ Pd4+
335.8 336.8 338.1 335.6 336.9 337.9 335.5 336.6
45.5 50.8 3.7 55.5 29.5 15.0 69.5 30.5
335.6 336.6
76.7 23.3
335.5 336.4
80.2 19.8
335.4 336.3
81.1 18.9
the surface of the smaller Pd clusters resulting in the stronger affinity for oxygen. The strong affinity of small Pd nanoparticles for oxygen could facilitate the dissociative adsorption of water as OHads, which is a key surface species for removal of reactioninhibiting intermediates in formic acid oxidation. The XRD patterns of large Pd particles (9 nm) displayed all the characteristics of fcc Pd. A slight increase in the lattice parameter (evaluated from the shifts of the Pd(111) diffraction peaks to lower 2θ values) indicating expanded interatomic distances was found in the smaller particles.14,15 For the catalyst with the smallest Pd particles (2.7 nm), only the (111) diffraction peak was identifiable and the mean particle size could not be determined reliably from the Scherrer equation.16 CO-stripping voltammetry in 0.1 M HClO4 was used to evaluate CO tolerance and electrochemically active surface areas (ECSAs) of the catalysts. It was found (see Table 1) that the surface areas measured by CO adsorption and TEM were well correlated (ECSACO/STEM ) 0.7). The ECSACO-normalized catalytic activities could therefore be used as a consistent indicator of the intrinsic activity of small Pd nanoparticles in formic acid electrooxidation. Voltammetry in the absence (Figure 2) and presence (Figure 3) of preadsorbed CO revealed the following changes caused by decreasing particle size: A different distribution of exposed crystallographic planes on the particle surface inferable from the hydrogen adsorption/desorption region (0.0-0.3 V in Figure 2), negative shifts in the onset of -OHads formation, stronger -OHads adsorption, and positive shifts in the CO oxidation peak. These trends are very similar to the behavior of small Pt particles.17 The shifts in the onset
Pd-Catalyzed Electrooxidation of Formic Acid
J. Phys. Chem. C, Vol. 112, No. 10, 2008 3791
Figure 2. Steady-state CV of Pd/C catalysts in 0.1 M HClO4 aqueous solution. Scan rate was 5 mV/s. For brevity, only three samples with different Pd particle sizes are shown. Figure 4. CV of formic acid oxidation on Pd/C catalysts showing in 3.0 M formic acid + 0.1 M HClO4 only the positive-going potential scans.
Figure 3. COads stripping voltammograms of different Pd/C catalysts in 0.1 M HClO4 solution. Only the oxidative CO removal region is shown here. Scan rate was 5 mV/s.
and peak potentials of CO stripping and the broadening of the CO-stripping peak could also be caused by CO adsorption as linear, bridged, and multiply bonded species on small nanoparticles.18 The observed particle-size dependence of hydrogen adsorption/desorption and CO oxidative removal implies that the Pd surface structure is a function of particle size and that CO oxidative removal is a structure-sensitive reaction on the Pd surface. CV and chronoamperometry (CA) of formic acid oxidation were carried out in 3 M formic acid solutions. Figure 4 shows the positive-going scans of typical voltammograms. Expectedly both CV (Figure 4) and CA (data not shown) measurements showed an increase in specific mass activity (i.e., current per unit mass of Pd) with decreasing Pd particle size. However, this was not caused simply by the larger surface area of small nanoparticles because current per unit ECSA also varied. Very distinct oxidation peaks were found in the negative-going scans of small Pd nanoparticles which could be attributed to the reaction between surface Pd oxides (PdOH) and adsorbed C1 intermediates or formic acid.19,20 The peaks were more noticeable in the more extensively oxidized small Pd nanoparticles. CV was also carried out after 2 h of potentiostatic polarization at different potentials. A broad potential plateau was found immediately following the small but distinct oxidation peak,
Figure 5. Steady-state CV of Pd/C (5.05 nm) in 0.1 M HClO4 solution after CA in 3.0 M formic acid + 0.1 M HClO4 for 2 h. Solid and dotted lines are first and second scans after CA. The scan began at the CA polarization potential and proceeded cathodically to 0.02 V before it was reversed and proceeded anodically to 1.20 V. The starting potential was returned in the backward scan. Scan rate was 5 mV/s.
where peak height was dependent on the polarization potential (Figure 5). The peak and plateau disappeared in the second cycle. The location of the peak coincided well with CO oxidation in anodic stripping voltammetry, indicating that there was COlike intermediate formed in formic acid electrooxidation. This implicates the involvement of the poisoning-intermediate (PI) pathway of formic acid electrooxidation. The currents in Figure 4 at 0.2 and 0.4 V were re-normalized by ECSACO and plotted against Pd particle size in the left panel of Figure 6. A classical volcano plot was obtained. It is interesting to note that the optimal particle size varied with the sampling potential and was 5.2 nm at 0.2 V and 6.5 nm at 0.4 V. A nearly identical trend was found using the CA currents at
3792 J. Phys. Chem. C, Vol. 112, No. 10, 2008
Zhou and Lee
Figure 6. Volcano plots of catalytic activities showing the particle size effects: (left) data from CV and (right) data from CA. Electrolyte was 3.0 M formic acid in 0.1 M HClO4.
2 h (near steady-state values) polarized at the same potentials (right panel of Figure 6). IV. Discussion The volcano plots in Figure 6 suggest that formic acid oxidation is a structure-sensitive reaction on Pd catalysts, similar to Pt catalysts.19,21 There are two generally accepted parallel pathways for formic acid electrooxidation, differentiated by the nature of the intermediates involved.21,22 In the active-intermediate (AI) pathway, formic acid oxidizes directly to CO2.
Pd + HCOOH f X f Pd + CO2 + 2H+ + 2e-
(1)
In the PI pathway, formic acid is progressively dehydrogenated to form a tenaciously held CO-like intermediate, which can only be oxidatively removed from the active site by oxygencontaining species on a neighboring surface site.
Pd + HCOOH f Y f Pd-CO + H2O
(2)
Pd + H2O f Pd-OH + H+ + e-
(3)
Pd-CO + Pd-OH f Pd + CO2 + H+ + e-
(4)
The PI pathway is therefore similar to the well-known bifunctional catalysis of methanol electrooxidation on Pt-alloy catalysts23 where water dissociation on the hydrophilic metal into surface -OHads plays an important role. The AI pathway, on the other hand, takes place without the participation of surface -OHads. A recent publication reported that formic acid electrooxidation on single-crystal Pd is affected by the crystallographic orientation of the surface.19 The onset potential for formic acid oxidation in the positive-going scan was found to increase in the order Pd(110) < Pd(111) < Pd(100), i.e., formic acid oxidation occurred the earliest (the most negative onset potential)
on Pd(110). The oxidation peak potential increased in the same order and was 0.4 V for Pd(110), 0.5 V for Pd(111), and 0.8 V for Pd(100), respectively. In addition, the Pd(110) plane also demonstrated the highest specific area activity followed by the Pd(111) plane in the low potential regions (
