Pt Flecks on Colloidal Au (Pt∧Au) as Nanostructured Anode

J. Phys. Chem. C 2009, 113, 20903–20911

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Pt Flecks on Colloidal Au (Pt∧Au) as Nanostructured Anode Catalysts for Electrooxidation of Formic Acid Dan Zhao, Yuan-Hao Wang, and Bo-Qing Xu* InnoVatiVe Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua UniVersity, Beijing, 100084, China ReceiVed: May 1, 2009; ReVised Manuscript ReceiVed: September 8, 2009

Nanostructured Pt-on-Au electrocatalysts (coded as Ptm∧Au, m being the atomic Pt/Au ratio), prepared by Pt deposition on Au colloids in two size ranges (Au-I, 10.0 ( 1.2 nm; Au-II, 3.0 ( 0.6 nm) (Zhao and Xu, Phys. Chem. Chem. Phys. 2006, 8, 5106), were employed for the electrooxidation of formic acid (HCOOH) at concentrations of 0.2-3.2 M by cyclic voltammetry. The HCOOH electrooxidation over a Pt shell fully covering Au-I colloids (Ptm∧Au-I at m > 0.2, Pt dispersion < 20%) occurred mainly in the high potential range (0.6-1.0 V vs SCE). The lowering of m in Ptm∧Au-I samples resulted in a remarkable increase in the current of HCOOH electrooxidation in the lower potential range (-0.2 to 0.6 V) due to a continued enhancement in the Pt utilization associated with the changes in the Pt-dispersion state. The areal activity (intrinsic activity) of Pt flecks (Pt dispersion > 50%) was 5 times and their mass-specific activity 25 times higher than those of the conventional Pt/C and core@shell structured Pt∧Au-I catalysts. The use of Ptm∧Au-II for HCOOH electrooxidation produced qualitatively similar results, thus demonstrating a dramatic enhancement of the electrocatalytic activity of Ptm∧Au/C by reducing the domain size of Pt deposits on Au surfaces. Moreover, the intrinsic activity of Pt flecks in Ptm∧Au-II was found to be 4 times higher than those in Ptm∧Au-I, which uncovers that smaller Au particles can serve as a kind of “activity promoter” to their carrying Pt flecks. The current of HCOOH electrooxidation over the Ptm∧Au catalysts also varied significantly, according to the HCOOH concentration. The highest current was obtained only in an appropriate HCOOH concentration window for a given Pt∧Au/C catalyst. 1. Introduction The study of electrocatalytic oxidation of oxygenated C1 molecules, such as methanol (CH3OH) and formic acid (HCOOH), is of ongoing interest, not only for using them as the practica1 fuels for polymer electrolyte membrane fuel cell (PEMFC) technologies but also for providing mechanistic insight into the electrooxidation of small organic molecules.1-3 Platinum as well as their alloys represents up to now the most efficient catalyst materials for the electrooxidation of these C1 molecules. However, the accumulation of poisonous species, such as COad, on active Pt surfaces during the electrooxidation process of such molecules always produced a high overpotential, resulting in slow kinetics of the anodic reactions and a significant loss in the fuel cell efficiency.4,5 Since oxygenated surface species, such as OHad, were responsible for oxidative removal of poisons, the effort of introducing metal promoters in Pt-based electrocatalysts for facilitating formation of oxygenated species and removal of poisons at lower potential has been spotlighted for a long time.6-9 It was found that ruthenium sites in PtRu catalysts would favor the formation of oxygenated species at potentials being 200-300 mV lower than that on pure Pt, which dramatically decreased the onset potential for the electrooxidation of CH3OH on PtRu catalysts compared with other Ptbased electrocatalysts.10-13 However, PtRu catalyst was found to be not that effective for HCOOH electrooxidation.14,15 Unlike the electrooxidation of CH3OH, in which poisons are formed inevitably in the stepwise CH3OH dehydrogenation sequence and thus oxidative removal of poisons was necessary * To whom correspondence should be addressed. Phone: +86-1062792122. E-mail: [email protected].

for fully oxidizing CH3OH to CO2, the electrooxidation of HCOOH proceeds according to the so-called “dual pathways”:16-20 One is the direct pathway involving a fast oxidation mechanism to CO2 via reactive intermediates, such as adsorbed COOH or HCOO (formate) species, that form from a direct dehydrogenation of HCOOH, which is evident from the oxidation currents occurred below the onset potential of OHad formed on Pt (ca. 0.6 V vs NHE). The other is an indirect pathway including a step involving at least one poisonous intermediate identified mainly as COad, being produced from the dehydration of HCOOH; its further oxidation to CO2 would require a much higher potential. Although the presence of ruthenium in PtRu electrocatalysts could facilitate the oxidative removal of COad in the HCOOH electrooxidation, it would apparently suppress the direct pathway of HCOOH electrooxidation on Pt.15 Therefore, innovating Pt-based electrocatalysts to promote effectively the direct dehydrogenation of HCOOH would be more significant for speeding up the electrooxidation of HCOOH. It was reported by Weaver et al.21 that the particle size or dispersion of Pt could significantly influence the behavior of HCOOH electrooxidation. These authors observed that the currents associated with the electrooxidation of HCOOH via the direct dehydrogenation pathway, detected in the low potential range (0-0.8 V vs NHE), were enhanced by reducing the Pt particle size in the anode catalysts.21 High currents of HCOOH electrooxidation in the low potential range were also obtained recently over fairly dispersed Pt entities deposited on the surface of colloidal Au particles (Pt dispersion, 10-70%).22-24 These earlier results suggest that the dispersion state of Pt can be

10.1021/jp904046h CCC: $40.75  2009 American Chemical Society Published on Web 11/05/2009

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SCHEME 1: Dispersion State of Pt Deposits in Ptm∧Au-I Nanostructures According to Atomic Pt/Au Ratio (m)

critical to HCOOH electrooxidation, but a possible influence of the substrate Au particles on the electrocatalysis of their carrying Pt entities was not taken into consideration. To better understand the relationship between the dispersion state of Pt and its catalytic activity for HCOOH electrooxidation, it is essential to investigate the HCOOH electrooxidation catalysis by systematically varying the dispersion state of Pt to include up to the totally dispersed Pt entities (approaching 100% Pt dispersion) in the Pt-on-Au system. It would be interesting to know the intrinsic activity and selectivity of small Pt flecks with the highest possible dispersions (ca. 90-100%).24-26 Also, a variation in the particle size of the underlying Au colloids could enable detection of a possible Au size effect on the electrocatalytic performance of and/or the reaction pathway for HCOOH oxidation over their carrying Pt entities. In our previous work,25,26 we obtained two series of nanostructured Pt-on-Au particles (coded as Ptm∧Au, m being the atomic Pt/Au ratio) by depositing Pt entities on colloidal Au particles of two size ranges (i.e., Au-I, 10.0 ( 1.2 nm; Au-II, 3.0 ( 0.6 nm). Comprehensive characterizations, including UV-vis, XPS, TEM, and XRD measurements,25,26 have shown that the dispersion state of Pt deposits in these nanostructured Ptm∧Au particles changed, according to the number of m, from shell-like Pt overlayers to two-dimensional rafts and/or very small flecks of Pt clusters at the surface of Au particles, as shown in Scheme 1. These Ptm∧Au particles with varied Au particle sizes and a broad spectrum of Pt dispersion states are employed in this work as the anode catalysts for HCOOH electrooxidation. We disclose that the performance of HCOOH electrooxidation depends not only on the dispersion state of Pt but also on the particle size of the underlying Au colloids. Moreover, the use of smaller Au particles is found, for the first time, to be advantageous for the generation of more active Pt catalysts, favoring the direct dehydrogenation reaction pathway for HCOOH electrooxidation. This study also shows that the current of HCOOH electrooxidation is significantly affected by the concentration of HCOOH in the electrolyte; maximum currents of HCOOH electrooxidation can be obtained only in an appropriate HCOOH concentration window. 2. Experimental Section As detailed in our previous work,25,26 two series of nanostructured Ptm∧Au particles were prepared by depositing Pt on colloidal Au particles of two average sizes (Au-I, 10.0 ( 1.2 nm; Au-II, 3.0 ( 0.6 nm) through hydrogen reduction of the desired amounts of PtCl62- in solution containing preformed Au colloids. The obtained Ptm∧Au particles were then loaded on the widely used Vulcan XC-72 carbon black (BET surface area ) 250 m2/g) to prepare the electrocatalysts. The measurements of metal loadings, the electrochemical active surface area (EAS), and the dispersion or utilization of Pt in the Ptm∧Au/C electrocatalysts were also described in detail in the previous work.25 A Pt/C having comparable electrocatalytic performance to the commercial E-TEK Pt/C catalyst27,28 was also employed as a reference catalyst in this work.

Zhao et al. We would use Pt utilization (UPt) instead of Pt dispersion to directly relate it with the ever increasing concern on saving the precious metal in electrocatalysis, although both the utilization and the dispersion of Pt refer to the same property, that is, percentage exposed for a given metallic Pt catalyst. In other words, UPt expresses more clearly the usability of Pt atoms for the electrochemical reaction;25,26 a thorough description of UPt and its measurement by cyclic voltammetry (CV) in 0.5 M H2SO4 was originally given in ref 25. The characterization procedures and results of TEM, UV-vis, XPS, and XRD measurements for the two series of Ptm∧Au samples can also be found in our previous work.26 As a supplement, surface-enhanced Raman spectra (SERS) of Rhodamine B on Au and Ptm∧Au samples were measured in this work. These SERS measurements were carried out on an area with a diameter of ca. 5 µm by a 20× long working distance objective on a RM2000 microscopic confocal Raman spectrometer (Renishaw PLC), employing an air-cooled He-Ne laser operating at 632.8 nm. The samples were made in the form of thin films by air-drying the colloidal solutions (containing an equal amount of Au) on glass flakes (1 × 1 cm2); before the spectra was recorded, 1 mL of 1.0 × 10-6 M Rhodamine B solution was dripped onto the samples and dried in air for 30 min. CV measurements were carried out at 298 K in a threeelectrode electrochemical cell. A saturated calomel electrode (SCE) was used as the reference electrode and a 1.0 cm × 1.0 cm Pt foil as the counter electrode. All potentials reported in this work are given with respect to SCE. HCOOH electrooxidation reactions were studied by recording CV curves at a scanning rate of 20 mV/s from -0.2 to 1.0 V in 0.5 M H2SO4 solutions containing varying HCOOH concentrations (0.23.2 M). 3. Results and Discussion 3.1. Pt Dispersion and SERS Behavior of Ptm∧Au Particles. As detailed in our previous TEM, UV-vis, XPS, XRD, and electrochemical characterizations,26 the change of dispersion state of Pt on Au-I nanoparticles (ca. 10.0 nm) with the atomic Pt/Au ratio (m) could be illustrated as Scheme 1. For this series of Ptm∧Au-I samples, the ones with m > 0.2 showed Au@Pt nanosuctures in which the Pt deposits fully covered the Au particles in a low Pt dispersion or utilization (UPt < 12%). With continuously lowering m in Ptm∧Au-I, the dispersion state of Pt deposits would gradually change from a monatomic Pt overlayer (m ) ca. 0.2) to two-dimensional rafts and then very small flecks of Pt clusters (m < 0.2) on the underlying Au particles, resulting in nano/subnano “Pt flecks” on Au nanoparticles with dramatically enhanced Pt utilization. For the two samples of m e 0.05, the size of Pt flecks would be no larger than 1.0 nm, leading to 100% Pt utilization for electrochemical reactions (UPt ) 99.2%, Table 1).25,26 For Ptm∧Au-II samples, the Pt deposits appeared as two-dimensional rafts or flecklike Pt clusters on Au-II (ca. 3.0 nm) nanoparticles as long as m was controlled to be less than 0.5; the domain size of Pt deposits would decrease and their Pt dispersion increase with decreasing m (Table 1), as demonstrated in our earlier paper.26 Considering that colloidal Au particles are efficient in inducing surface-enhanced Raman spectroscopy (SERS) and the enhancement factor is very sensitive to the physical and chemical nature of the Au surface,29 the SERS of Rhodamine B (a welldocumented indicator) on Au and Ptm∧Au samples were measured as a supplement to ref 26. As shown in Figure 1, Rhodamine B adsorbed on Au-I particles (ca. 10.0 nm) showed

(Pt∧Au) Anode Catalysts for Electrooxidation of HCOOH TABLE 1: Electrochemical Parameters and Activity for Formic Acid Electrooxidation on Ptm∧Au/C and Pt/C Catalysts catalytic activity for HCOOH electrooxidationa

catalysts

EAS (m2/g-Pt)

Pt1.0∧Au-I/C Pt0.5∧Au-I/C Pt0.2∧Au-I/C Pt0.1∧Au-I/C Pt0.05∧Au-I/C Pt0.5∧Au-II/C Pt0.2∧Au-II/C Pt0.1∧Au-II/C Pt/C

26.4 27.2 55.9 126.0 235.7 96.9 146.8 218.5 74.4

UPt (%)

intrinsic activity (A/m2-Pt)

mass specific activity (mA/mg-Pt)

11.2 11.5 23.7 53.4 99.2 41.4 62.1 90.4 31.4

0.4 0.7 2.7 4.3 3.9 13.8 15.6 14.1 0.3

11.3 20.3 151.6 546.2 912.4 1345.5 2290.1 3060.8 22.1

a At 0.40 V for formic electrooxidation in (0.5 M H2SO4 + 2.0 M HCOOH) electrolyte.

Figure 1. Surface-enhanced Raman spectra (SERS) of Rhodamine B on Au and Ptm∧Au samples: (a) Au-I, (b) Pt0.05∧Au-I, (c) Pt0.2∧AuI/C, (d) Pt1.0∧Au-I/C, (e) Au-II, and (f) Pt0.1∧Au-II.

the typical SERS bands (Figure 1, spectrum a): the band at around 1600 cm-1 can be assigned to the aromatic CdC stretching, between 1330 and 1560 cm-1 to the aromatic C-C stretching, those between 1000 and 1300 cm-1 to the aromatic C-H stretching and the bridged C-C stretching, and the band at around 610 cm-1 is attributed to an aromatic bending mode.30,31 In contrast, Rhodamine B adsorbed on Au-II (ca. 3.0 nm) showed much weaker SERS bands (Figure 1, spectrum e), which could be attributed to the relatively less aggregation of the indicator molecules on smaller Au nanoparticles.32,33 The intensity of SERS signals on Ptm∧Au-I decreased by a factor of 10 and 25, respectively, when m was increased from 0.05 to 0.2 (Figure 1, spectra b and c), and completely disappeared at m ) 1.0 (Pt1.0∧Au-I, Figure 1, spectrum d). Compared with the spectrum on Pt-free Au-I, such dramatic loss in SERS intensity of the Ptm∧Au-I samples with increasing m is similar to our earlier observations on the weakening of Au signals in UV-vis and XPS measurements,25,26 which further demonstrates a gradual covering of Pt deposits on the Au surface. However, for the Ptm∧Au-II samples, the signals cannot be resolved even when m was lowered to 0.1 (Figure 1, spectrum f), although the spectrum was recorded with many more scans at a slower scan rate. Similar results were also observed earlier when a small amount of Pt or Pd was deposited on the surfaces of SERS-active substrates, such as Au and Ag.29,34,35 It was

J. Phys. Chem. C, Vol. 113, No. 49, 2009 20905 proposed that electron migration from the d states of non-SERSactive metal (Pt or Pd) to those of SERS-active substrates (Au or Ag) could be responsible for the quenching of the surface plasmon excitations, which are necessary for sensitivity enhancements in Raman spectroscopy. Although our earlier characterizations with UV-vis, XPS, and XRD failed to detect an obvious electronic effect between Pt and Au in Ptm∧Au samples,25,26 the remarkable weakening of SERS signals by Pt deposition on the surface of colloidal Au particles, especially for Ptm∧Au-II samples, uncovers a subtle electronic interaction between Pt deposits and their underlying Au surface even at a very small amount of Pt. 3.2. Formic Acid Electrooxidation. Figure 2 shows the CV curves of HCOOH electrooxidation on Au/C, Ptm∧Au/C, and Pt/C catalysts in the solution containing 0.5 M H2SO4 and 2.0 M HCOOH. Considering that the anodic or positive-going scans (scanning form low potential to high potential) allow poisons formed during the electrochemical reaction to accumulate on the catalyst surface,4,5 which is also more close to the real working situation than the negative-going scans, the evolution of oxidation current during the positive-going scans of the CV measurement was taken to evaluate the catalyst performance in this work. No significant oxidation current was observed on both Au-I/C and Au-II/C catalysts (Au-II/C was shown as an example in Figure 2f), indicating that the Pt-free Au nanoparticles were totally inactive for the HCOOH electrooxidation reaction.36-38 On Ptm∧Au-I/C and Pt/C catalysts (Figure 2a-e, j), two oxidation current peaks were detected, respectively, in low (peak-I, -0.2 to 0.6 V) and high (peak-II, 0.6-1.0 V) potential ranges, which indicate that HCOOH was electrooxidized by both the direct and the indirect pathways on these catalysts.18,21 The change of m in Ptm∧Au-I/C catalysts significantly affects the relative current of the two oxidation peaks. For Ptm∧AuI/C samples with m > 0.2 (Figure 2d,e), electrooxidation of HCOOH mainly occurred in the high potential range featuring a peak at ca. 0.75 V, which resembles the behavior of the reference Pt/C catalyst (Figure 2j); when Pt was lowered to m e 0.2 (Figure 2a-c), the main oxidation currents were detected in the lower potential range, featuring an intensive and broad peak at ca. 0.40 V. These results confirm that the behavior of HCOOH electrooxidation is very sensitive to the dispersion state of Pt on Au particles.22-24 The main currents for HCOOH electrooxidation in the high potential range (0.6-1.0 V) suggest that the indirect pathway via COad oxidation18,19 would be prevailing over those Ptm∧Au-I/C samples in which Pt fully covered the surface of Au particles (m > 0.2). However, the currents in the low potential range (-0.2 to 0.6 V) from the direct dehydrogenation pathway21-23 were insistently intensified when the dispersion state of Pt deposits was changed from a monatomic Pt overlayer (m = 0.2) to two-dimensional rafts or very small flecks of Pt clusters on the same Au particles (m < 0.2). The present findings of the effect of decreasing the domain size of Pt deposits (i.e., increasing the dispersion of Pt) in Ptm∧Au-I/C catalysts (Scheme 1) on HCOOH electrooxidation resemble the observation in an earlier report of Park et al.21 in which a series of monometallic Pt/C catalysts with varied Pt particle sizes from 10.0 to 2.0 nm were employed for HCOOH electrooxidation. The so-called “ensemble effect” of Pt sites, that is, continuous neighboring Pt sites are required for the indirect pathway to form poisonous COad but such a requirement for the neighboring sites would not be necessary for the direct dehydrogenation pathway of HCOOH electrooxidation, was

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Figure 2. CV curves of HCOOH electrooxidation on Ptm∧Au/C, Pt/C, and Au/C catalysts in the solution containing 0.5 M H2SO4 and 2.0 M HCOOH (scanning rate ) 20 mV/s). The solid lines are for positive-going and dotted lines for negative-going scans: (a) Pt0.05∧Au-I/C, (b) Pt0.1∧AuI/C, (c) Pt0.2∧Au-I/C, (d) Pt0.5∧Au-I/C, (e) Pt1.0∧Au-I/C, (f) Au-II/C, (g) Pt0.1∧Au-II/C, (h) Pt0.2∧Au-II/C, (i) Pt0.5∧Au-II/C, and (j) Pt/C.

considered responsible for their observation.21 On our Ptm∧Au samples, the availability of adjacent Pt sites on Au particles would apparently decrease when the dispersion state of Pt deposits was changed from continuous (shell-like) overlayers to discontinuous (flecklike) clusters, which would then result in continued restriction of HCOOH dehydration and, therefore, promote the direct dehydrogenation pathway for the electrooxidation reaction of HCOOH. A recent report by Wang et al.23 also demonstrated that the direct dehydrogenation pathway was the predominant reaction channel in HCOOH electrooxidation over submonolayer Pt deposits on Au colloids. For the three Ptm∧Au-II/C samples (m ) 0.1, 0.2, and 0.5), which contain only Pt flecks of high dispersions (UPt ) 4190%, Table 1) on Au-II (ca. 3.0 nm) particles,26 the electrooxidation of HCOOH was detected only in the low potential range (-0.2 to 0.6 V) of the positive-going scan curve, showing a peak at ca. 0.45 V (Figure 2g-i). In contrast to Pt/C and “core@shell” structured Ptm∧Au-I/C catalysts, the absence of the oxidation current peak in the high potential range on these Ptm∧Au-II/C samples further demonstrates that the direct dehydrogenation pathway was the main reaction channel for HCOOH electrooxidation on the highly dispersed Pt flecks (UPt > 50%) of Pt∧Au catalysts. In addition, the peak potential of HCOOH electrooxidation was ca. 50 mV higher on Ptm∧AuII/C catalysts (Figure 2 g-i) compared with the corresponding peak-I on Ptm∧Au-I/C of m e 0.2 (Figure 2a-c). The Ptm∧AuII/C catalysts showed a spikelike oxidation current peak on the negative-going scan curve, which is also different from the Ptm∧Au-I/C catalysts containing Pt flecks of similar domain

sizes (m e 0.2 or UPt > 50%, Table 1). The negative-going scan curves of these Ptm∧Au-I/C catalysts (m e 0.2) featured instead a plateau-like current peak. These electrocatalysis data suggest that the behavior of HCOOH oxidation on Ptm∧Au is influenced not only by the dispersion state of Pt but also by the size of the underlying Au nanoparticles. Actually, our observation that Pt flecks on the smaller Au-II particles caused a much faster attenuation of the SERS signal than those on the bigger Au-I particles (Figure 1) offers a piece of evidence for a relatively stronger electronic interaction between Pt and Au in Ptm∧Au-II than in Ptm∧Au-I samples, which could be related with the size-dependent facet or twine structure property of the Au particles.39 The relatively stronger Pt-Au interaction in Ptm∧Au-II samples might be a reason for the special shape of HCOOH electrooxidation CV curves observed on these samples as the electronic effect in bimetallic catalysts always plays an important role in chemisorption and surface catalysis.40-42 In our earlier electrochemical measurement of UPt,26 the oxidative desorption peaks of adsorbed hydrogen on the CV curves in 0.5 H2SO4 were found extended to higher potentials on Ptm∧Au-II/C than on Ptm∧AuI/C with similar UPt, which evidenced that the adsorption strength of H on Pt of the Ptm∧Au-II catalysts was stronger than on their Ptm∧Au-I counterparts.43 Figure 3 shows the positive scan curves of HCOOH electrooxidation on both series of Ptm∧Au/C catalysts. The current density (J ) A/m2-Pt) was obtained by current normalization according to the electrochemical active surface area (EAS) of Pt in each catalyst. It is seen that the value of J in the low

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Figure 3. Positive-going curves of CV tests for formic acid electrooxidation on (A) Ptm∧Au-I/C, Pt/C, and (B) Ptm∧Au-II/C catalysts. A: (a) Pt0.05∧Au-I/C, (b) Pt0.1∧Au-I/C, (c) Pt0.2∧Au-I/C, (d) Pt0.5∧AuI/C, (e) Pt1.0∧Au-I/C, and (f) Pt/C. B: (a) Pt0.1∧Au-II/C, (b) Pt0.2∧AuII/C, and (c) Pt0.5∧Au-II/C.

potential range (peak-I, -0.2 to 0.6 V) increased remarkably with decreasing m down to m e 0.1 for the Ptm∧Au-I/C catalysts (Figure 3A). In contrast, significantly larger and comparable J values were obtained for the three Ptm∧Au-II/C catalysts (Figure 3B). Table 1 reports the utilization (UPt), electrochemical active surface area (EAS), and activity data of Pt in Ptm∧Au/C and Pt/C catalysts. The values of J at 0.40 V on the CV curves of Figure 3 were used to show the areal activity (or intrinsic activity) of Pt,44 and the mass-specific activity (MSA) was defined as the current at 0.40 V normalized to a milligram of Pt in the electrode catalysts.25,26 The MSA data on both series of Ptm∧Au/C catalysts generally increased with decreasing m or increasing UPt. However, the number of MSA was not always proportional to the increase of UPt among all of the Ptm∧Au/C catalysts, suggesting that a contribution to MSA from an enhancement of the intrinsic activity of Pt should be taken into consideration. It could be found from Table 1 that, when UPt was increased from ca.11% of Pt overlayers in the “core@shell” structures (i.e., m > 0.2) to above 50% of Pt flecks in Ptm∧AuI/C of lower m, the intrinsic activity was enhanced by more than 5 times, and the MSA was increased at least by a factor of 25. Therefore, the dispersion state change of Pt deposits from the shell-like overlayers to flecklike clusters in Ptm∧Au-I/C catalysts is effective not only for enhancing UPt25,26 but also for increasing the intrinsic activity for HCOOH electrooxidation. It would be interesting to correlate the intrinsic activity data of the highly dispersed Pt flecks with their UPt or dispersion

Figure 4. Influence of HCOOH concentration on formic acid electrooxidation over (a) Pt0.05∧Au-I/C, (b) Pt0.2∧Au-I/C, (c) Pt0.5∧Au-I/ C, and (d) Pt/C catalysts.

data in Table 1. Pt flecks in Pt0.1∧Au-I/C (UPt ) 53.4%) and Pt0.05∧Au-I/C (UPt ) 99.2%) showed comparable intrinsic activities (ca. 4.0 A/m2-Pt), although Pt dispersions in these two catalysts were different by a factor of 2. Correspondingly, Pt flecks in the three Ptm∧Au-II/C catalysts also exhibited similar intrinsic activities (14-15 A/m2-Pt), although their UPt varied widely from 41.4% (m ) 0.5) to 90.4% (m ) 0.1). These results indicate that the intrinsic activity of Pt flecks in Ptm∧Au/C was not significantly affected by their domain size. In other words, the electrooxidation reaction of HCOOH over Ptm∧Au/C catalysts would become structure-insensitive when the Pt dispersion was high enough (UPt g 40%). This structure insensitivity is straightforward for a dominance of the direct dehydrogenation pathway, which is not demanding for Pt ensembles,21-24 in the electrooxidation of HCOOH over such highly dispersed Pt catalysts.

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Figure 5. Dependences of peak potential and peak current on formic acid concentration over (9) Pt0.05∧Au-I/C, (O) Pt0.2∧Au-I/C, (2) Pt0.5∧Au-I/C, and ()) Pt/C catalysts.

In addition, it is noticeable that the intrinsic activity of Ptm∧Au-II/C was always significantly higher than its Ptm∧AuI/C counterpart with similar UPt; for example, the intrinsic activity of Pt0.1∧Au-II/C (UPt ) 90.4%) was almost 4 times higher than that of Pt0.05∧Au-I/C (UPt ) 99.2%). In comparison with the conventional Pt/C catalyst containing 2-5 nm Pt particles, Pt flecks in the present Ptm∧Au-II/C catalysts were 30-50 times higher in intrinsic activity and 60-150 times higher in MSA. Thus, Pt flecks on smaller Au particles (Ptm∧AuII) were more active than those on bigger Au particles (Ptm∧AuI) at similar UPt levels, suggesting that smaller Au nanoparticles could serve as a kind of “activity-promoter” for the deposited Pt flecks in HCOOH electrooxidation. Sung et al.22 showed recently that fully alloyed PtAu nanoparticles (ca. 3.8 nm, UPt ) 12.6% or EAS ) 30.5 m2/g-Pt) containing equal Pt and Au atoms were highly active for HCOOH electrooxidation in the low potential range, the intrinsic activity of Pt in these alloyed particles being 8 times higher than that of a reference Pt/C catalyst. In a report of Zhong et al.,45 the alloyed PtAu catalysts

also showed unique activity for methanol electrooxidation. These authors believed that an electronic promotion effect between Au and Pt would be responsible for their observed enhanced catalytic activity.22,45 Although the electronic interaction in our Ptm∧Au samples was not that obvious in comparison with those alloyed PtAu particles, the relatively strong electronic interaction between Pt flecks and small Au nanoparticles, as uncovered by our SERS measurements (Figure 1), could be responsible for the higher intrinsic activity of Pt in Ptm∧Au-II than in Ptm∧Au-I samples. 3.3. Effect of HCOOH Concentration on Its Electrooxidation. We further investigated the effect of HCOOH concentration on its electrooxidation over Pt/C and typical Ptm∧Au/C catalysts with stepwise varying the concentration by 0.2 M HCOOH in the range of 0.2 to 3.2 M. Figure 4 shows the positive-going scan CV curves under five representative HCOOH concentrations on Pt0.05∧Au-I/C, Pt0.2∧Au-I/C, Pt0.5∧Au-I/C, and Pt/C catalysts. On every increment of the HCOOH concentration, the current and potential of peak-II on the Pt0.05∧Au-I/C

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Figure 6. A: Influence of HCOOH concentration on formic acid electrooxidation over (a) Pt0.1∧Au-II/C, (b) Pt0.2∧Au-II/C, and (c) Pt0.5∧Au-II/C catalysts. B: Dependences of peak potential and peak current on formic acid concentration over (9) Pt0.1∧Au-II/C, (O) Pt0.2∧Au-II/C, and (2) Pt0.5∧Au-II/C catalysts.

and Pt0.2∧Au-I/C catalysts changed synchronously with those of peak-I. The current and potential of peak-II varied, however, more pronouncedly than that of peak-I on the Pt0.5 ∧Au-I/C and Pt/C catalysts. These results indicate that the current and potential for HCOOH electrooxidation were also significantly influenced by the working concentration of HCOOH. The dependences of the current and potential of both peak-I and peak-II on HCOOH concentration are shown in Figure 5. In general, the currents of both peaks (i.e., Ipeak-I and Ipeak-II) on Pt0.05∧Au-I/C and Pt0.2∧Au-I/C increased fast up to 1.0 M HCOOH and then increased at a slow rate to reach a maximum when the HCOOH concentration was further increased to a certain value (e.g., 2.0 M for Pt0.05∧Au-I/C and 1.6 M for Pt0.2∧Au-I/C). A further increase in HCOOH concentration would then lead to current reduction on both peaks. Assuming a 10% uncertainty, both Ipeak-I and Ipeak-II reached their highest level within a HCOOH concentration range of 1.0-2.6 M on Ptm∧Au-I/C catalysts of m e 0.2. On Pt/C and Ptm∧Au-I/C catalysts of m g 0.5, the variation in HCOOH concentration showed little effect on Ipeak-I (Figure 5a), but its effect on Ipeak-II

(Figure 5c) was qualitatively similar to that on Pt0.05∧Au-I/C and Pt0.2∧Au-I/C catalysts. Ipeak-II reached its maximum at ca. 1.0 M HCOOH on Pt0.5∧Au-I/C and at ca. 2.2 M HCOOH on Pt/C catalysts. Therefore, an appropriate HCOOH concentration window would have to be determined in order to produce the highest current (or power) in HCOOH electrooxidation with Ptbased electrodes like Ptm∧Au-I/C and Pt/C. Considering that the dehydrogenation pathway in the low potential range and the dehydration pathway in the high potential range would compete in the electrooxidation of HCOOH, the increase in HCOOH concentration could benefit both pathways. However, it should be noted that the dehydrogenation of HCOOH is favorable for the kinetics of HCOOH electrooxidation, and the dehydration of HCOOH with the formation of poisonous surface intermediates can severely reduce the overall rate of HCOOH electrooxidation. It is, therefore, not surprising that the counterwork effects from the two pathways would play an important role in the current development for HCOOH electrooxidation with increasing HCOOH concentrations. The shift of peak potential toward higher potentials for both peak-I

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and peak-II (i.e., Epeak-I and Epeak-II), except Epeak-I on Pt0.5∧AuI/C (Figure 5b), with increasing the HCOOH concentration, was strong evidence for a continued accumulation of poisonous intermediates on the catalyst surface.46,47 Such an accumulation of poisons was supported by a more pronounced shift of Epeak-II (Figure 5d)10-13 since this peak-II is associated with the oxidative removal of surface poisons, such as COad.47 The insignificant change of Epeak-I in the low HCOOH concentration side and its stabilization at higher HCOOH concentrations on Pt0.05∧AuI/C and Pt0.2∧Au-I/C catalysts (Figure 5b) implies that HCOOH electrooxidation behavior was not obviously affected by surface poisons with increasing HCOOH concentrations, which, again, agrees with that the direct dehydrogenation pathway for HCOOH electrooxidation was the dominant reaction channel on these catalysts containing highly dispersed Pt flecks. Being independent of HCOOH concentration, the data of Ipeak-II on Pt0.5∧Au-I/C and Pt/C catalysts were always remarkably higher than those of Ipeak-I, further indicating that the indirect (dehydration) pathway of HCOOH electrooxidation18,19 was prevailing over the “core@shell” structured Pt0.5∧Au-I/C and conventional Pt/C catalysts. The small Ipeak-I data and their insignificant variation over these catalysts with varying HCOOH concentration were the direct consequences of severe blocking by intermediate poisons of the Pt surface from the indirect pathway. Moreover, the window of HCOOH concentration for producing the highest level of current on Pt0.5∧Au-I/C and Pt/C catalysts was obviously narrower than that on Ptm∧Au-I/C of m e 0.2. Thus, the decrease in domain size of Pt deposits on Au nanoparticles was a benefit for not only speeding up the electrooxidation of HCOOH via the direct dehydrogenation pathway but also producing higher currents in a wider HCOOH concentration window. Figure 6 presents the positive-going scan CV curves at five representative HCOOH concentrations (Figure 6A) and shows the dependences of the peak current and potential on HCOOH concentration over the Ptm∧Au-II/C catalysts (Figure 6B). The peak current (Ipeak) increased with the increase in HCOOH concentration up to its maximum and then decreased with further increments of HCOOH concentration; the peak potential (Epeak) also increased initially with HCOOH concentration up to ca. 0.45 V vs SCE and then became stabilized at around 0.48 V at high HCOOH concentrations (>2.0 M). The highest level of Ipeak, allowing a 10% variation, on the Ptm∧Au-II/C catalysts appeared at a HCOOH concentration range of 1.0-1.8 M, 1.0-2.4 M, and 1.6-2.4 M when m ) 0.1, 0.2, and 0.5, respectively. These results further indicate that a proper HCOOH concentration range was a key to producing the highest current for HCOOH electrooxidation on Pt-based electrocatalysts. Moreover, in contrast to Ptm∧Au-I/C catalysts, the absence of peak-II on Ptm∧Au-II/C catalysts in the entire HCOOH concentration range tested further demonstrate that the use of smaller Au nanoparticles to deposit Pt flecks is more of a benefit to improving the HCOOH electrooxidation on Ptm∧Au catalysts. Further understanding of the size-dependent Pt-Au interaction and structural details of Pt deposits in Ptm∧Au nanostructures would require comprehensive use of surface-specific and yet sensitive physical methods, such as atom resolved STEM and photoemission and X-ray absorption spectroscopy, which certainly deserves separate studies in the future. Also, in situ measurement of the poisonous intermediates in HCOOH electrooxidation at different HCOOH concentrations would be very helpful for gaining insight into the molecular processes of HCOOH electrooxidation over the highly dispersed Pt flecks in Ptm∧Au/C catalysts.

Zhao et al. 4. Conclusions This work demonstrates the structure-sensitive nature of HCOOH electrooxidation over nanostructured Pt-on-Au (Ptm∧Au) catalysts by a systematic manipulation of Pt deposits from shell-like overlayers to flecklike rafts or clusters. The direct dehydrogenation pathway for HCOOH electrooxidation occurring in the low potential range (-0.2 to 0.6 V vs SCE) dominated over the highly dispersed Pt flecks (UPt > 50%) in Ptm∧Au-I catalysts. In contrast, the indirect dehydration pathway occurring in the high potential range (0.6-1.0 V vs SCE) prevailed over conventional Pt particles and “core@shell” structured Ptm∧Au-I catalysts of low Pt utilization (UPt < 20%). The intrinsic activity of Ptm∧Au-I increased with UPt at low dispersion and became maximized when the Pt entities were made as highly dispersed Pt flecks (UPt > 50%). Due to the relatively stronger electronic interaction between Pt and small Au nanoparticles, Pt flecks in Ptm∧Au-II (m e 0.5) exhibited 5 times higher intrinsic activity than those of similar Pt dispersions in Ptm∧Au-I (m < 0.2), suggesting that smaller Au particles could serve as a kind of “activity promoter” for the highly dispersed Pt flecks. Moreover, in comparison with conventional nanosized Pt catalysts, the Pt flecks in Ptm∧Au-II showed an enhancement by 30-50 fold in the intrinsic activity and 60-150 fold in the mass specific activity. Investigation on the effect of HCOOH concentration on HCOOH electrooxidation over Pt and Ptm∧Au catalysts revealed that an appropriate HCOOH concentration window would exist for the generation of highest currents or power densities over Pt-based electrocatalysts. Therefore, the determination of such concentration windows would be a key to maximizing the power densities of future direct formic acid fuel cells. Acknowledgment. We thank Mrs. Feng-En Chen and Prof. Gao-Quan Shi (Department of Chemistry, Tsinghua University) for their help in SERS measurements. This work is supported bytheNSF(20773074and20590362)andMOST(2006AA03Z225) of China. References and Notes (1) Lu, G. Q.; Chrzanowski, W.; Wieckowski, A. J. Phys. Chem. B 2000, 104, 5566. (2) Okamoto, H.; Kon, W.; Mukouyama, Y. J. Phys. Chem. B 2005, 109, 15659. (3) Lemos, S. G.; Oliveira, R. T. S.; Santos, M. C.; Nascente, P. A. P.; Bulhoes, L. O. S.; Pereira, E. C. J. Power Sources 2007, 163, 695. (4) Adzic, R. R. Modern Aspects of Electrochemistry; Plenum Press: New York, 1990; Chapter 3. (5) Jarvi, T. D.; Stuve, E. M. Electrocatalysis; Wiley-VCH Press: New York, 1998; Chapter 3. (6) Mukerjee, S.; Lee, S. J.; Ticianelli, E. A. Electrochem. Solid-State Lett. 1999, 2, 12. (7) Shen, P. K.; Tseung, A. C. C. J. Electrochem. Soc. 1994, 141, 3082. (8) Gasteiger, H. A.; Markovic´, N. M.; Ross, P. N. J. Phys. Chem. 1995, 99, 8945. (9) Wang, J. S.; Xi, J. Y.; Bai, Y. X.; Qiu, X. P. J. Power Sources 2007, 164, 555. (10) Watanabe, M.; Uchida, M.; Motoo, S. J. Electroanal. Chem. 1987, 229, 395. (11) Gasteiger, H. A.; Markovic´, N. M.; Ross, P. N. J. Phys. Chem. 1994, 98, 617. (12) Arico`, A. S.; Antonucci, P. L.; Modica, E. Electrochim. Acta 2002, 47, 3723. (13) Kuk, S. T.; Wieckowski, A. J. Power Sources 2005, 141, 1. (14) Markovic´, N. M.; Gasteiger, H. A.; Ross, P. N.; Jiang, X. D.; Villegas, I.; Weaver, M. J. Electrochim. Acta 1995, 40, 91. (15) Rice, C.; Ha, S.; Masel, R. I.; Wieckowski, A. J. Power Sources 2003, 115, 229. (16) Capon, A.; Parsons, R. J. Electroanal. Chem. 1973, 45, 205.

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