Electrocatalysis DOI 10.1007/s12678-015-0250-5
ORIGINAL RESEARCH
Influence of CeO2 on Pt-Pd/CeO2-OMC Catalysts for Formic Acid Oxidation Ateeq ur Rehman 1 & Sk Safdar Hossain 3 & Sleem ur Rahman 1 & Shakeel Ahmed 2 & Mohammad M. Hossain 1
# Springer Science+Business Media New York 2015
Abstract This article deals with the promotional effects of CeO2 on PtPd/CeO2-OMC electrocatalysts. The synthesized catalysts are characterized using different physicochemical techniques and evaluated in a formic acid oxidation fuel cell. N2 adsorption/desorption analysis shows that CeO2 modification increases the surface area of OMC from 1005 to 1119 m2/ g. SEM, XRD, and TEM analysis reveal that the presence of CeO2 enhances the active metal(s) dispersion on the CeO2OMC surface. The average particle size of the dispersed metal decreases with the increase of Pt/Pd ratio on CeO2-OMC support. Cyclic voltametry measurement of Pd/CeO2-OMC gives 12 % higher anodic current activity with 83-mV negative shift of the peak E as compared to unmodified Pd/OMC. In bimetallic catalysts, the addition of Pt improves the activity and stability of the catalysts significantly. Among the bimetallic samples, Pd3Pt1/CeO2-OMC displays superior current density (74.6 mA/cm2), which is 28.3 times higher than that of Pt/ CeO2-OMC. It also shows higher stability (on 32.8 mA/cm2) for an extended period of time (30 min) with least indication of CO poisoning effects.
* Mohammad M. Hossain
[email protected] 1
KACST-TIC for CCS and Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia
2
Center for Refining and Petrochemicals-Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia
3
Department of Chemical Engineering, King Faisal University, Al Hasa, Saudi Arabia
Keywords CeO2 . Ordered mesoporous carbon (OMC) . Electrocatalyst . Formic acid fuel cell
Introduction Fuel cells are considered as efficient energy conversion technique to produce electricity with minimum environmental pollution. In this context, direct formic acid fuel cells (DFAFCs) receive growing attentions because of their easy handling, less toxicity, high theoretical open circuit potential (E) of 1.48 V which is higher than that of hydrogen (1.23 V) and methanol (1.21 V) and (3) requires lower crossover through membrane as compared to methanol and ethanol [1]. Although formic acid exhibits relatively lower energy density (2104 Wh/L) than methanol (4900 Wh/L), its lower membrane crossover allows the DFACFs to operate at high formic acid concentrations (5–12 M) compared to methanol concentration (1–2 M). Thus, the DFAFCs offer higher overall energy outputs than the other similar fuel cells [2–4]. In addition to the above technical advantages, mass scale applications of DFAFCs can also create opportunities of utilizing CO2 (from fossil fuel combustion) as a source of formic acid production via electrochemical conversion of carbon dioxide. An integrated approach not only offers DFAFCs as an efficient energy generator but also contribute to the global efforts on the CO2 utilization/sequestration, addressing the greenhouse gas effects [5–8]. In order to capitalize the above mentioned advantages and to address the practical problems of the present DFAFCs, significant research and developmental efforts have been undertaken. Possibly, the most leading issue with the present DFAF Cs is the excessive use of noble metal-based electrocatalysts to accelerate the slow kinetics of formic acid oxidation (FAO) reaction [1, 2, 9–11]. In addition to the high costs, the noble
Electrocatalysis
metal-based electrocatalysts also suffer from severe poisoning due to the strong adsorption of the carbon monoxide as produced during fuel oxidation [12]. Among the metals, Rh, Pt, and Pd are extensively studied as active components of the anode electrocatalysts. Although Pd-based electrocatalysts show higher catalytic activity for FAO reactions than Pt, it still lacks stability for the longer period of operations [13–16]. These difficulties warrant further research to develop highly durable and efficient Pd-based electrocatalysts for DFAFCs. In the open literature, many transition metals have been reported as promoter/modifier to enhance the catalytic activity and stability of Pd catalysts. The most common studied bimetallic catalysts include PdCo, PdNi, PdAu, PdPt, PtBi, PdSn, and PdFe [17–23]. In the open literature, Vulcan XC-72 carbon black is the most widely used support in electrocatalysts. The large surface area, large pore volume, and good electrical conductivity make Vulcan-XC-72 as an attractive support. The main drawback of Vulcan-XC-72 is the noncontribution (to the catalytic process) of the noble metal particles which are trapped in the deep cracks of the phase boundaries and micropores of Vulcan XC-72 [24, 25]. Carbon black also suffers with serious corrosion problems when used in acidic medium for fuel cell oxidation [26, 27]. In order to avoid the problems associated with carbon black, there are many other carbon materials investigated as electrocatalyst support, including carbon nanotubes (CNTs) [2, 4, 23, 25, 26], nanofibers (CNFs) [11], ordered mesoporous carbon (OMCs) [30–35], graphene [28], and metal carbides [29]. Among the above support materials, ordered mesoporous carbon (OMC) found wide ranges of potential applications due to their uniform pore structure, large pore volumes, high surface areas, superior electrical conductivity, and chemical stability [21, 30–35]. For example, hybrid Pt/WO3-OMC electrocatalyst showed excellent performance for methanol oxidation in a direct methanol fuel cell [30]. In many cases, the modification of the support is also beneficial to improve the activity of the supported catalysts. The commonly used support modifiers are TiO2, WO3, CeO2, ZrO2, NiO, and Fe2O3 [3, 9, 30, 36–38]. It is believed that partially filled d- or f-orbital of the transition metals allow them to switch between valences which are the key for catalytic activities. Among these metal oxides, CeO2 is widely used as reducible oxide due to its high oxygen carrying capacity. The Ce4+/Ce3+ redox couple release oxygen in different conditions, which enhance the electro-oxidation abilities of Pt/CeO2-C catalysts [39]. Wang et al. [40] demonstrated superior activity of Pd/CeO2-C electrocatalysts as compared to Pd/C in a formic acid fuel cell. It is suggested that the presence of CeO2 promotes the direct oxidation pathway (dehydrogenation) instead of the dehydration pathway. As a result, the overall performance of the catalyst is improved. Yang et al. [9] considered that the improved catalytic activity
was due to the oxygen vacancies provided by CeO2 for the further oxidation of CO-like intermediate species, which previously poisoned the active Pt sites by CO chemisorption. Feng et al. [10] reported 1.67 times higher peak current density (j) after modification Pd/C with CeO2. The CeO2-modified electrocatalysts also remained stable for longer (seven times) period of time. Feng also attributed the improved performance to the higher electrochemical surface area (ECSA), the electronic effect, and the presence of oxygen containing CeO2 composite. To the knowledge of the present authors, there is no report available in the open literature demonstrating the performance of CeO2-OMC composite as support material for Pt/Pd-based formic acid oxidation electrocatalyst. Considering the advantages of CeO2 as support modifier, the present research is focused on the CeO 2 -modified PtPd/OMC-CeO2 bimetallic electrocatalyst for oxidation of formic acid. The compositions of Pt and Pd in the Pt-Pd bimetallic catalysts are also varied to investigate their effects on the performance. The morphology, structural properties, and composition of PtPd/CeO2-OMC catalysts are studied by using various physical as well as electrochemical characterization techniques.
Experimental Preparation of PtPd/CeO2-OMC Electrocatalyst In this study, there are three major steps involved in preparation of electrocatalysts: (i) synthesis of CeO2-SBA-15, (ii) synthesis of CeO2-OMC, and (iii) preparation of PtPd/CeO2OMC. The SBA-15 silica sample was synthesized by TEOS polymerization method as reported by Zhao et al. [41] and Jun et al. [42] with slight changes. The prepared SBA-15 was then modified with CeO2 by wetness impregnation method using Ce(NO3)3·6H2O as Ce precursor. A Ce(NO3)3·6H2O solution was prepared in deionized water under stirring at room temperature for 30 min. The solution was added to desired amount of preheated SBA-15 at 110 °C. The resultant suspension was ultrasonicated for 24 h at room temperature and then dried at 110 °C to remove the water completely. Finally, the sample was reduced at 450 °C under argon flow for 4 h to decompose cerium nitrate salt to cerium oxide. CeO2-modified OMC support was prepared by carbonization of sucrose into mesopores of CeO2-SBA-15 as described by Wang et al. [43] with some modifications. In this method, 1.0 g of CeO2-SBA-15 was added to a solution containing 1.25 g of sucrose, 0.14 g of sulfuric acid in 5.0 g of deionized water. The mixture was then placed in an oven at 100 °C for 6 h; after that, the oven temperature was increased to 160 °C at a heating rate of 2 °C/min. The sample was kept at 160 °C for another 6 h. The above steps were repeated by adding 0.8 g of
Electrocatalysis
sucrose to ensure the complete filling of CeO2-SBA-15 silica pores. The resultant material was pyrolyzed at 800 °C under N2 flow for 6 h to obtain the carbon–silica composite. The composite was washed with 5 wt.% HF solution to remove the silica template. Finally, the sample was kept overnight, filtered, washed with deionized water, and dried at 110 °C for 4 h. Bimetallic PtPd/CeO2-OMC electrocatalysts were prepared by the borohydride reduction method using 0.1 M NaBH4 (metals, NaBH4 =1:1 w/w) solution as a reducing agent. The details of the catalyst preparation steps can be found elsewhere [44]. In all the catalyst samples, the nominal metal loading was 20 wt.%. Material Characterizations The XRD of the prepared support and catalyst samples were conducted using a Smart Lab (9 kW) Rigaku X-ray diffractometer, with a diffraction angle range 2θ=5–80° using Cu Kα radiation with a scan rate of 2°/min. An ultra-high resolution FETEM (JEOL, JEM-2100F), operated at an accelerating voltage of 200 kV, was employed to capture the images of the metal dispersed on support surface. Specific surface areas and pore volume of the synthesized samples were determined by N2 adsorption/desorption using a Micromeritics model ASAP 2010 analyzer. Physical adsorption of N2 was carried out in a liquid nitrogen bath maintaining at 77 K. The morphologies of the support and catalysts were studied by using a scanning electron microscope (JEOL JSM6460LV) operated at 20 kV equipped with energy dispersive X-ray. TGA was recorded using a Shimadzu TGA-60 between 25 and 800 °C at the default ramp rate of 10 °C/min under dry air to determine the thermal stability of the support and electrocatalysts.
recorded from −0.2 to 1.2 V (vs Ag/AgCl) at a scan rate of 20 mV/s in 0.5 M H2SO4 solution with and without 0.5 M HCOOH. Chronoamperometry (CA) at 0.3 V (vs Ag/AgCl) in N2-saturated 0.5 M H2SO4 with 0.5 M CHOOH was also recorded. Electrochemical active surface area (ECAS) and CO poisoning tolerance of catalyst samples were determined by CO stripping voltammetry. In stripping voltammogram, CO was bubbled through 0.5 M H2SO4 electrolyte solution for 30 min, keeping working electrode in the cell under constant applied electrode E of 0.2 V. The system was first purged with nitrogen, and then, the electrolyte solution was aerated with CO in order to dissolve CO into the solution. The CO stripping voltammograms were recorded from −0.2 to 1.2 V (vs Ag/AgCl) at a scan rate of 20 mV/s to ensure the complete oxidation of adsorbed CO (COads). Finally, ECAS were calculated using 0.42 mC/cm2 charge associated for COads monolayer [45–47].
Results and Discussion Physical Characterizations Figure 1 shows the XRD patterns of Pd/OMC, Pd/CeO2OMC, Pt/CeO 2-OMC, Pd3 Pt 1/CeO 2-OMC, and Pd1 Pt 3/ CeO2-OMC. In all samples, diffraction peak at 2θ=19.18° corresponds to the (002) plane of the carbon support [9]. Diffraction peaks at 40.4°, 47.0°, and 68.0° correspond to the (111), (200), and (220) planes of Pd, indicating the characteristics of face-centered cubic (fcc) crystalline structure of palladium nanoparticles (JCPDS, Card No. 65-6174). The XRD patterns of Pt/CeO2-OMC also show similar Pt diffraction
Electrochemical Measurements Bio-Logic VMP3 potentiostat was employed to measure the formic acid electro-oxidation at ambient temperature in a three electrode cell assembly. A 3-mm diameter glassy carbon (geometrical area, 0.076 cm2) covered with a thin layer of Nafionimpregnated catalyst was used as a working electrode. A Pt grid connected with Pt wire (8-cm length, 1.23-mm diameter) and an Ag/AgCl electrode (3.5 M KCl) were used as the counter and reference electrodes, respectively. At first, 5 mg of catalyst was dispersed in 1 mL of ethanol, 20 μL Nafion, and water solution (5 wt.% Nafion) by sonication for 30 min to form a catalyst ink. A total of 10 μL of this ink was transferred (by pipette) to the polished surface of the glassy carbon. For all experiments, the metal loading on the working electrode was maintained at 0.127 mg/cm 2 . CV data were
Fig. 1 Wide angle XRD pattern of CeO2-OMC support, Pd/OMC, and PtPd/CeO2-OMC catalysts with various Pt/Pd ratios
Electrocatalysis
peaks at 40.2°, 46.8°, and 68.2° angles corresponding to (111), (200), and (220), respectively. Diffraction patterns of both the Pd and Pt containing samples are quite similar, and these observations are in line with the results reported by Sun et al. [33]. Pd/Pt (111) plane has shown the largest intensity among all the planes. For fuel cell applications, the (111)-fcc plane is more desirable due to its less resistance to oxidation. Also, Pd/Pt (111) plane is used to calculate the crystalline size using Scherrer equation [34]. The XRD patterns of both the CeO 2 -OMC and OMC supported electrocatalysts suggest that the support modification and decreasing Pd content ratio has no prominent effect on diffraction peaks except making it broader and slightly moving the peaks to lower angle. Cerium oxide peaks appeared at 27.6°, 44.0°, 53.0°, and 69.7°, indexed to fcc-phase of cerium oxide (JCPDS no. 34-0394) [48], and their diffraction peak intensity are too weak to be useful for the calculation of cerium crystalline size. The crystalline sizes for Pd/OMC, Pd/CeO2-OMC, Pd3Pt 1/ CeO2-OMC, Pd1Pt3/CeO2-OMC, and Pt/CeO2-OMC were found to be 6.5, 6.2, 5.8, 4.8, and 4.3 nm, respectively. It was noticed that the crystalline size decreased substantially with the increase of Pt ratio. Particle size, shape, and their dispersion on support material have profound effect on electrocatalytic activity of catalysts [49], and TEM characterization is useful to investigate such properties. The HRTEM images (Fig. 2) of the CeO2-OMC supported catalysts show that 3:1 w/w ratio of stabilizing agent trisodium citrate/noble metal precursors and higher Pt/Pd ratio are favorable to obtain nano size particles with uniform distribution on the support surface [11]. It is clear in Fig. 2a that most of Pd particles are properly dispersed on the modified support and shows minimum agglomeration. Interestingly, gradual decrease in nano particle size and metal distribution are seemed more ordered in Fig. 2b, c as compared to other catalysts synthesized in this study. The average particle sizes of Pd/OMC, Pd/CeO2-OMC, Pd3Pt1/CeO2OMC, Pd1Pt3/CeO2-OMC, and Pt/CeO2-OMC samples are found to be 7.8, 4.8, 3.3, 3.1, and 2.3 nm, respectively. The measured sizes are in good agreement with the values calculated from Scherrer’s equation using XRD data. The morphology of as-synthesized OMC support and the electrocatalysts were further analyzed by using scanning electron microscopy (SEM) (Fig. 3). As clearly shown in Fig. 3a, the porous nature OMC retained aggregated rope-like morphology. Short distance between individual ropes results the broad surface to surface 3D interconnections between the twisted ropes like structures. The length of twisted structures is estimated to be around 4–8 μm [43]. SEM image of Pd3Pt1/ CeO2-OMC (Fig. 3b) shows the uniform loading of metal particles on the support. The metal crystal sizes also appeared
to be narrowly distributed. Within the resolution level of SEM, it is not possible to differentiate any significant changes in shape between support and catalyst material except length of rope reduced to 3–5 μm in Fig. 3b. The EDX images of Pd1Pt3/CeO2-OMC and Pt/CeO2OMC samples (Fig. 3c, d) reveal the presence and successful deposition of metal elements on OMC support. It can be seen in Fig. 3c that energy peaks for both Pt (2.37 kV) and Pd (2.71 kV) elements are much closer to each other, indicating that both metals exhibit same diffraction pattern, as already shown in XRD analysis. Composition of Pd, Pt, and Ce in all catalyst samples (by EDX) is presented in Table 1. The thermal stability of the CeO2-OMC and Pd3Pt1/CeO2OMC samples was tested by TGA analysis with a heating rate of 10 °C/min in the range of 25 to 800 °C [50]. Figure 4 displays the weight loss of various samples as function of temperature. For both the samples, initial weight loss indicates the removal of moisture from the samples. For CeO2-OMC sample, the sharp weight loss was observed between 470 and 560 °C while for the Pd3Pt1/CeO2-OMC sample, the weight loss was detected 420 to 565 °C. For both the samples, the weight loss was due to the rapid oxidation of OMC. The residual mass greater than 20 % in catalysts showed the oxide formation of PtPd catalyst which occurs at 800 °C [9]. These observations confirm that actual and expected compositions are quite consistent. Figure 5 shows the N2 adsorption/desorption isotherms and corresponding Barret-Joyner-Halenda (BJH) pore size distribution curves of the OMC, CeO2-OMC support, and the prepared catalyst samples. Monolayer–multilayer adsorption, a capillary condensation, and a multilayer adsorption on the outer particle surface are the three phases which can be distinguished from the figure in all samples. OMC and the CeO2-OMC samples exhibited type IV isotherm with a slightly sharp capillary condensation step between p/p0 = 0.42 and 0.95. This lower pressure capillary condensation indicates that OMC and CeO2-OMC support contains smaller average pore sizes. BET surface area, pore size, and total volume of OMC and PtPd-based catalysts were calculated from the nitrogen adsorption isotherm data and summarized in Table 1. The BET surface area of OMC (1005 m2/g) is 4 times larger than commonly used fuel cell support VulconXC-72 (254 m2/g) [24]. The measured area is also in agreement with material as reported by Zeng et al. [30]. The surface area of modified CeO2-OMC support is 1119 m2/g which is 114 m2/g higher than that of the unmodified support material. These results indicate that the addition of cerium oxide have significantly improved the porosity and other support properties of OMC material. The BJH pore size for OMC and CeO2-OMC is measured as 3.8 and 3.4 nm, respectively. From the BJH, pore size distribution curve, it is quite clear that pore size of all sample is very consistent and is between 3 and 4 nm.
Electrocatalysis Fig. 2 TEM images and corresponding histograms of (a) Pd/CeO2-OMC, (b) Pd3Pt1/ CeO2-OMC, (c) Pd1Pt3/CeO2OMC, and (d) Pt/CeO2-OMC
Electrochemical Characterization CO Stripping Analysis The COads stripping voltammograms for the bulk Pd/ OMC, Pd/CeO2-OMC, Pt/CeO2-OMC, and PdPt/CeO2OMC films are recorded to evaluate the COads poisoning resistance of the electrocatalyst samples (Fig 6a). Second voltammogram cycle (dotted lines) after COads stripping confirmed the completion of COads oxidation. It showed that hydrogen adsorption/desorption peaks of Pt and Pd were regained and no further oxidation process occurs in Pd/Pt oxide potential region. With the unmodified Pd/ OMC electrocatalyst, the peak E and onset E appeared at 580 and 505 mV, respectively. For CeO2-modified Pd/ CeO2-OMC sample, 30 and 62 mV negative shift (as compared to Pd/OMC) in peak E and onset E were observed. This lower onset values indicated that CeO2 promoted the CO oxidation activity of Pd-based electrocatalysts. On the other hand, for Pt containing samples, both peak and onset E showed a positive shift toward a higher E depending upon Pt/Pd ratios. The Pd 3 Pt 1 /CeO 2 -OMC catalyst exhibited the onset E at
340 mV which was 1.3, 1.2, and 2.2 times lower than Pd/CeO2-OMC, Pd1Pt3/CeO2-OMC, and Pt/CeO2-OMC electrocatalysts, respectively. The relatively higher values of peak and onset E of Pt/CeO2-OMC were possibly due to the poisoning of Pt surface in which no active sites were available for hydrogen oxidation. Table 2 summarizes the CO stripping intensity, peak, and onset E for all the studied catalysts. It is clear from this table that the intensity of CO oxidation peaks was varied with the variation of Pt contents. For Pd3Pt1/CeO2-OMC catalyst, intensity of CO peak was about 13.6 mA/cm 2 which was remarkably higher than that of Pd/CeO 2 -OMC (7.8 mA/cm2) but was smaller than Pd1Pt3/CeO2-OMC (15.5 mA/cm2). For this observation, one can conclude that Pd3Pt1/CeO2-OMC is more CO tolerant than the other electrocatalysts with higher Pt contents. Table 2 also lists ECAS of Pt and Pd calculated by using 0.42 mC/cm2 as the charge associated to the monolayer on Pt and Pd nano particles. The Pd3Pt1/CeO2-OMC catalyst also shows highest ECAS values (48.7-m2/g metal) as compared to the all other catalysts. The high ECAS value further indicates that the bimetallic Pd3Pt1/CeO2OMC catalyst is a potential CO tolerance catalyst.
Electrocatalysis Fig. 3 Low magnification SEM and EDX images of (a) CeO2OMC, (b) Pd3Pt1/CeO2-OMC, (c) Pd1Pt3/CeO2-OMC, and (d) Pt/CeO2-OMC
Cyclic Voltammetry Analysis The formic acid electro-oxidation activity of the synthesized electrocatalysts was studied by cyclic voltammetry (CV) measurements in 0.5 M HCOOH and 0.5 M H2SO4 solution at a scan rate of 20 mV/s. Figure 7a reports typical current peaks associated to all studied electrocatalysts during formic acid oxidation while Fig. 7b compares the maximum current for each sample.
In the forward scans, both the anodic current and peak E of PtPd/CeO2-OMC electrocatalysts were changed with the variation of Pd to Pt ratios. In the reverse scan, cathodic oxidation of formic acid and reduction of oxidized metals were recorded in around 370 mV [51]. At peak potential
Table 1 Surface properties of OMC, CeO 2 -OMC and Pt/Pd containing catalyst samples Sample
Vulcan-XC-72 OMC CeO2-OMC Pd/CeO2-OMC Pd3Pt1/CeO2-OMC Pd1Pt3/CeO2-OMC Pt/CeO2-OMC
Weight %
SBET
dBJH
Vtotal
Pd%
Pt%
Ce%
(m2/g)
(nm)
(cm3/g)
– – – 6.49 13.27 7.81 –
– – – – 9.34 7.93 11.81
– – 8.31 8.07 8.49 7.26 8.12
254 1005 1119 595 592 646 697
2.95 3.82 3.44 3.49 3.54 3.51 3.48
0.55 1.23 1.24 0.68 0.66 0.68 0.67
Fig. 4 TGA curves of CeO2-OMC and Pd3Pt1/CeO2-OMC samples with heating rate of 10 °C/min
Electrocatalysis
Fig. 5 N2 adsorption–desorption isotherm and BJH pore size distribution of OMC, CeO2-OMC, Pd/CeO2-OMC, Pd3Pt1/CeO2-OMC, Pd1Pt3/ CeO2-OMC, and Pt/CeO2-OMC samples
of 70 mV, the oxidation j of the Pd/CeO2-OMC sample was around 46.32 mA/cm2 (3.24 A/mg) which was about 5 mA/cm2 higher than that of the Pd/OMC (41.1 mA/cm2) electrocatalyst. This improved catalytic performance of the Pd/CeO2-OMC was due to better dispersion of Pt/Pd metals on CeO2-OMC as observed in SEM and TEM analysis. Furthermore, the observed current (46.32 mA/ cm2) with Pd/CeO2-OMC electrocatalyst was much higher than that of previously reported carbon supported Pd catalysts [9, 10, 13, 14, 52, 53]. As shown in Fig. 7b, the j of the Pd3Pt1/CeO2-OMC electrocatalyst for anodic scan was 74.6 mA/cm2. This j was 1.8, 1.61, 1.67, and 28.3 times higher than that of obtained with Pd/OMC, Pd/ CeO2-OMC, Pd1Pt3/CeO2-OMC, and Pt/CeO2-OMC, respectively. The peak E of Pd3Pt1/CeO2-OMC was also shifted positively as compared to the Pd/OMC, Pd/CeO2OMC, and Pd1Pt3/CeO2-OMC electrocatalysts. One can thus generally interpret that the formic acid electrooxidation on the two Pd/OMC and Pd/CeO2-OMC catalysts are much easier than those of the other electrocatalysts. However, due to large ECAS (reported in Table 2) and smaller particle size of Pt, electrochemical oxidation of formic acid was enhanced with Pd3Pt1/CeO2OMC electrocatalyst [9]. Moreover, the oxidation peak of the Pt/CeO2-OMC at 790 mV was ascribed to go through a multiple steps or indirect oxidation pathway. CO forms as an intermediate during the electrochemical reaction and strongly adsorbs on the surface of the catalysts which poison the active sites of the electrocatalyst. In the context of present study, due to the CO poisoning effects on Pt surface oxidation
Fig. 6 a: First and second cycles of CO stripping measurements for PtPd/CeO2-supported electrocatalysts in 0.5 M H2SO4 at a scan rate of 20 mV/s and b: enlargement of the CO oxidation peaks: (a) Pd/OMC, (b) Pd/CeO2-OMC, (c) Pd3Pt1/CeO2-OMC, (d) Pd1Pt3/CeO2-OMC, and (e) Pt/CeO2-OMC
current intensity decreased to 2.6 mA/cm2 [12], while in Pd/OMC, formic acid oxidation goes through a direct oxidation pathway [54]. Chronoamperometry Analysis Figure 8 displays the deactivation response for the five types of electrocatalysts which recorded at 0.3 V fixed E using 0.5 M HCOOH and 0.5 M H2SO4 solution at 25 °C for 1800 s (30 min). For all the catalysts, initially, a rapid fall in j were observed, after that the current decreased smoothly and finally reached to the pseudo steady state. As expected from the catalyst characterization, the activity of the Pd3Pt1/ CeO2-OMC electrocatalyst was significantly higher than the activities using Pd/OMC and PtPd/CeO2-OMC electrocatalysts. The steady-state j recorded with Pd3Pt1/ CeO2-OMC catalyst was about 32.8 mA/cm2. This j was 5.2 times higher than the steady-state value of Pd/OMC
Electrocatalysis Table 2
Electrochemical properties of catalysts on CO oxidation
Catalyst
Pd/OMC Pd/CeO2-OMC Pd3Pt1/CeO2-OMC Pd1Pt3/CeO2-OMC Pt/CeO2-OMC a
Area of desorbed CO peak (mC/cm2)
8.7 8.6 25.93 21.7 16.24
CO stripping Eonset (mV)
Epeak (mV)
505 440 340 430 750
580 550 585 620 860
ECASa (m2/g metal)
Peak intensity (mA/cm2)
16.3 16.1 48.7 40.6 30.4
11 7.8 13.6 15.5 8.2
θc ECASCO = w*0:42 , θc=mC/cm2 , w=0.127 mg/cm2 , 0.42 mC/cm2
(6.2 mA/cm2) catalyst. The j vales for Pd/CeO2-OMC, Pd1Pt3/CeO2-OMC, and Pt/CeO2-OMC catalyst samples were recorded as 11, 20.3, and 0.46 mA/cm2, respectively.
Fig. 7 a: CV patterns of Pd/OMC, Pd/CeO2-OMC, and PtPd/CeO2OMC electrocatalysts with various Pt/Pd ratios in 0.5 M H2SO4 +0.5 M HCOOH solution. b: Maximum currents during CV patterns of Pd/OMC, Pd/CeO2-OMC, and PtPd/CeO2-OMC electrocatalysts with various Pt/Pd ratios in 0.5 M H2SO4 +0.5 M HCOOH solution
Conclusions A series of CeO2-OMC supported Pd/Pt-based electrocatalysts were synthesized, characterized, and evaluated using different electrochemical measurement techniques. The addition of CeO2 improved the porosity of OMC and specific surface area (1005 to 1119 m2/g). TEM measurement revealed that the presence of CeO2 improved the Pd and Pt metal dispersion on the support surface. The average particle size decreased with the increase of Pt mass fraction in a bimetallic PtPd/CeO2-OMC sample. Cyclic voltammetry and chronoamperometry measurements showed that Pd/CeO2-OMC catalyst exhibited higher electrocatalytic activity and long run stability for formic acid oxidation as compared to Pd/OMC catalyst. CeO2 modified support provided free oxygen vacancy during CO stripping oxidation which enhanced the CO poisoning tolerance effect of the PtPd-based bimetallic electrocatalysts as compared to the Pd/ OMC catalyst. The availability of higher surface area (48.7 m2/ g) and smaller particle size, Pd3Pt1/CeO2-OMC catalyst sample gives superior activity and stability among all catalysts for oxidation reactions.
Fig. 8 CA at 0.3 V (vs Ag/AgCl) for Pd/OMC and PtPd/CeO2-OMC electrocatalysts in the N2-saturated 0.5 M H2SO4 and 0.5 M HCOOH solution
Electrocatalysis Acknowledgments The research team acknowledges the financial support provided by King Abdul Aziz City for Science and Technology (KACST) to this research under KACST-TIC for CCS project no 03. The team also acknowledges the facilities and support provided by KFUPM.
References 1. 2. 3. 4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
O. Winjobi, Z. Zhang, C. Liang, W. Li, Electrochim. Acta 55(13), 4217 (2011) A. R. Cynthia, B. Akshay, G. P. Peter, 9th edn. (Springer, London, 2013), pp. 69–87. doi:10.1007/978-1-4471-4911-8_4 W.L. Qu, Z.B. Wang, X.L. Sui, D.M. Gu, G.P. Yin, Fuel Cells 13(2), 149 (2013) Z. Cui, C. Gong, C.X. Guo, C.M. Li, J. Mater. Chem. A 1, 1179 (2013) C. Delacourt, P.L. Ridgway, J.B. Kerr, J. Newman, Electrochem. Soc. 155(1), 42 (2008) H. Li, C. Oloman, Continuous co-current clectrochemical reduction of carbon dioxide. WO2007041872 B1 (2007) S. Yotsuhashi, H. Hashiba, M. Deguchi, Y. Zenitani, R. Hinogami, Y. Yamada, M. Deura, K. Ohkawa, AIP Advances 2, 042160 (2012). doi:10.1063/1.4769356 R. Reithmeier, C. Bruckmeier, B. Rieger, Catalysts 2(4), 544 (2012) Y.L. Jun, S.H. Neng, S. Ting, L.S. Jun, Sci. China Chem. 55, 391 (2012) L. Feng, J. Yang, Y. Hu, J. Zhu, C. Liu, W. Xing, Int. J. Hydrog. Energy 37, 4812 (2012) Y.H. Qin, Y. Jiang, H.H. Yang, X.S. Zhang, X.G. Zhou, L. Niu, W.K. Yuan, J. Power Sources 196(10), 4609 (2011) J.L. Haan, R.I. Masel, Electrochim. Acta 54, 4073 (2009) Y. Zhu, Y. Kang, Z. Zou, Q. Zhou, J. Zheng, B. Xia, H. Yang, Electrochem. Commun. 10, 802 (2008) X.M. Wang, Y.Y. Xia, Electrochim. Acta 54(7525) (2009) B.K. Sathe, B.K. Balan, V.K. Pillia, Energy Environ. Sci. 4, 1029 (2011) S. Mahima, R. Kannan, I. Komath, M. Aslam, V.K. Pillia, Chem. Mater. 20, 601 (2008) L. Zhang, L. Wan, Y. Ma, Y. Chen, Y. Zhou, Y. Tang, T. Lu, Appl. Catal. B Environ. 138-139, 229 (2013) R. Wang, H. Wang, X. Wang, S. Liao, V. Linkov, S. Ji, Int. J. Hydrog. Energy 38, 13125 (2013) M.D. Obradovic, S.L. Gojkovic, Electrochim. Acta 88, 384 (2013) X. Zhao, J. Zhu, L. Liang, C. Li, C. Liu, W. Xing, Appl. Catal. B Environ. 129, 146 (2013) B.W. Zhang, C.L. He, Y.X. Jiang, M.H. Chen, Y.Y. Li, L. Rao, S.G. Sun, Electrochem. Commun. 25, 105 (2012) T. Dandan, B. Wu, B. Wang, C. Deng, Y. Gao, Appl. Catal. B Environ. 103, 163 (2011) Y. Jin, C. Ma, M. Shi, Y. Chu, Y. Xu, T. Huang, Q. Huang, Y. Miao, Int. J. Electrochem. Sci. 7, 3399 (2012) E. Antolini, Appl. Catal. B Environ. 88, 1 (2009)
25.
Y. Shao, G. Yin, J. Wang, Y. Gao, P. Shi, J. Power Sources 161(1), 47 (2006) 26. S. Yang, X. Zhang, H. Mi, X. Ye, J. Power Sources 175, 26 (2008) 27. B.R. Rauhe, R. Frank, E. McLarnon, J. Cairns, J. Electrochem. Soc. 142, 1073 (1995) 28. B. Seger, V.K. Prashant, J. Phys. Chem. C 113(19), 7990 (2009) 29. J.H. Dong, S.L. Jae, Energies 2(4), 873 (2009) 30. J. Zeng, C. Francia, C. Gerbaldi, V. Baglio, S. Specchia, A.S. Arico, P. Spinelli, Electrochim. Acta 94, 80 (2013) 31. S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412, 169 (2001) 32. S.H. Joo, C. Pak, D.J. You, S.A. Lee, H.I. Lee, J.M. Kim, H. Chang, D. Seung, Electrochim. Acta 521, 1618 (2006) 33. Z.P. Sun, X.G. Zhang, H. Tong, Y.Y. Liang, H.L. Li, J. Colloid Interface Sci. 337, 614 (2009) 34. J. Zeng, C. Francia, A.M. Dumitrescu, A.H.A.M. Videla, V.S. Ijeri, S. Specchia, Ind. Eng. Chem. Res. 51, 7500 (2012) 35. C. Liu, M. Chen, C. Du, J. Zhang, G. Yin, P. Shi, Y. Sun, Int. J. Electrochem. Sci. 7, 10592 (2012) 36. M.K. Jeon, J.Y. Won, K.R. Lee, S.I. Woo, Electrochem. Commun. 9(9), 2163 (2007) 37. G.E. Nagar, M.M. Ahmad, El-Deab, El-Anadouli, Electrochim. Acta 94, 62 (2013) 38. H. An, C. Zhou, D. Tao, Electrochim. Acta 92, 176 (2013) 39. R. Lin, C. Cao, H. Zhang, H. Huang, J. Ma, Int. J. Hydrog. Energy 37, 4648 (2012) 40. Y. Wang, S. Wang, X. Wang, Electrochem. Solid-State Lett. 12(5), B73 (2009) 41. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279, 548 (1998) 42. S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122, 10712 (2000) 43. L. Wang, S. Lin, K. Lin, C. Yin, D. Liang, Y. Di, P. Fan, D. Jiang, F.S. Xiao, Microporous Mesoporous Mater. 85, 136 (2005) 44. A. U. Rehman, S. S. Hossain, S. Rahman, S. Ahmed, M. M. Hossain, Appl. Catal. A. 482, 309 (2014). doi:10.1016/j.apcata. 2014.06.008 45. A. Czerwiński, Electroanal. Chem. 379, 487 (1994) 46. H.J. Kim, W.L. Kim, T.J. Park, H.S. Park, D.J. Suh, Carbon 46, 1393 (2008) 47. I. Habil, K. Sundmacher, M. Christov, H. Weiß, Kinetics of methanol electrooxidation on PtRu catalysts in a membrane electrode assembly, (pp. 27, 2005), http://diglib.uni-magdeburg.de/ Dissertationen/2005/tanvidakovic.pdf 48. C. Sun, Z. Xie, C. Xia, H. Li, L. Chen, Electrochem. Commun. 8, 833 (2006) 49. M. Subhramannia, V.K. Pillai, J. Mater. Chem. 18, 5858 (2008) 50. Z.H. Teng, G. Wang, B. Wu, Y. Gao, J. Power Sources 164, 105 (2007) 51. W. Chen, J. Kim, S. Sun, S. Chen, Langmuir 23(22), 11303 (2007) 52. J. Yang, C. Tian, L. Wang, H. Fu, J. Mater. Chem. 21, 3384 (2011) 53. Y. Lu, W. Chen, J. Phys. Chem. C 114, 21190 (2010) 54. J. Wang, G. Yin, Y. Chen, R. Li, X. Sun, Int. J. Hydrog. Energy 34, 8270 (2009)
