Synthesis and Electrocatalytical Application of Hybrid Pd/Metal Oxides/MWCNTs

The performance of Pd electrocatalysts for formic acid electrooxidation was improved by application of metal oxide-multiwall carbon nanotubes composites as a catalyst support. Hybrid oxides/MWCNTs were synthesized by two different methods: chemical reduction method and impregnation method. Pd based catalysts were synthesized by polyol method on the MWCNTs or oxide/MWCNTs composites. The In2O3 was deposited on MWCNTs by impregnation method (In2O3/MWCNTs-IM support) and in the presence of NaBH4 (In2O3/MWCNTs-NaBH4 support). The physical properties of the Pd/In2O3/MWCNTs-IM, Pd/In2O3/MWCNTs-NaBH4, Pd/SnO2/MWCNTs, and Pd/MWCNTs catalysts were characterized and their electrocatalytical performance in formic acid oxidation was compared. During Pd deposition on In2O3/MWCNTs-NaBH4 support, InPd2 structure was formed as observed by XRD.The electrochemical tests indicate that the two Pd/ In2O3/MWCNTs electrocatalysts have higher electrocatalytic activity than those of Pd/SnO2/MWCNTs and Pd/MWCNTs. The best performance was observed for the catalyst obtained by In2O3 impregnation of MWCNTs denoted by Pd/In2O3/MWCNTs-IM.


Introduction
Growing environmental pollution and the fact that crude oil resources are going to be exhausted stimulate an intensive development of work on replacing petroleum fuels by biofuels and by application of fuel cells for production of electricity.The application of biofuels results in reducing the emission of CO 2 and other pollutants.The content of CO 2 emitted in conversion of biofuels is equivalent to the respective content absorbed from the atmosphere by green plants in a process of biosynthesis; thus the application of biofuels as energy sources leads to overall CO 2 zero emission, what is very important for decreasing of the effects of global warming.Fuel cells are the most efficient electrical energy producing devices, operating using fuels and oxygen as reactants.Fuel cells directly transform chemical energy to electric energy with high efficiency.The most common fuels are hydrogen in proton exchange membrane fuel cells (PEMFCs), methanol in direct methanol fuel cells (DMFC), and formic acid in direct formic acid fuel cells (DFAFC).All these fuels can be produced from biomass.Fuel cells operating on liquid fuels for small and medium scale applications have prospects of commercialisation, and low temperature direct formic acid fuel cells (DFAFC) appear among the most promising in this respect [1].DFAFC has a number of advantages over direct methanol fuel cells [1]: (i) it has higher power density and higher energy efficiency, (ii) crossover flux of formic acid through Nafion5 membrane is several times smaller than that of methanol [2], and (iii) formic acid is less toxic than methanol and does not have the risk of producing hazardous by-products during oxidation (e.g., formaldehyde).A number of reviews on DFAFC [1,[3][4][5][6][7][8] have been published.
Palladium is a very effective electrooxidation catalyst formic acid; however, it works perfectly in the very pure, and so, very expensive formic acid.In a formic acid of lower purity, palladium becomes gradually poisoned, mainly, by -CH 3 group containing impurities [9][10][11][12], which are oxidized via adsorbed CO, blocking further reaction.
Palladium catalysts are very active in the oxidation of high purity formic acid, which is expensive.In low purity formic acid, the Pd catalysts become poisoned by CO originating from oxidation of impurities [13][14][15].While some of the catalysts are poisoning resistant in the long term, their activity is still too low and developing novel electrocatalysts is still crucial for their practical applications in DFAFC.For this reason, recently, intensive research is being carried out on developing more advanced Pd based nanocatalysts in order to improve the catalyst performance.
One of the most promising approaches, improving activity and stability of the catalysts, was modification of carbon support (carbon black, MWCNTs) with metal oxides.Among the oxides, TiO 2 [33,34], MoO  [35], ITO [36], ZrO 2 [37], SnO 2 -TiO 2 [38], and CeO x [39] were employed.It is expected that metal oxides will promote the oxidation of reaction poison species adsorbed on the electrode, change the electronic properties by metal-support interaction, and prevent Pd nanoparticles from sintering.
The other very efficient support modifiers employed were Ni 2 P [40,41], Co 2 P [42], and conducting polymers [43].It was also shown that the catalyst morphology optimization may lead to increasing electrochemically available surface, specific activity of the metal nanoparticles, and power density [44,45].
In this study, nano-hybrid MOS/MWCNTs, tin oxide/ MWCNTs, indium oxide/MWCNTs, and indium tin oxide/ MWCNTs were prepared by impregnation method as supports for electrocatalysts.Then, Pd nanoparticles were synthesized by polyol method to deposit on the prepared MOS/MWCNTs supports.The purpose of indium oxide or tin oxide modification is to supply oxidizing centers and change the electronic properties by metal-support interaction.
Multiwalled carbon nanotubes (MWCNTs) as a substrate can make fuel cells more efficient.The main purpose of using a support is to achieve an optimal dispersion of the catalytic active component and to stabilize it against sintering.Carbon nanotubes (CNTs) are of interest as catalyst support for applying in fuel cells due to their unique electrical and structural properties [46,47].CNTs can both improve the effectiveness of the catalyst by altering the electron conductivity of the catalyst layer and prevent sintering of metal nanoparticles.The authors in the recent researches used MWCNTs as the support of catalysts and obtained very promising results [26,48,49].However, tin and indium oxides are semiconductors with band gaps of 2.7-4.3 eV and 2.6-3.5 eV, respectively, and may brake electrical path between Pd nanoparticles and MWCNTs [49].Indium tin oxide (ITO, or tin-doped indium oxide) is a solid solution of indium (III) oxide (In 2 O 3 ) and tin (IV) oxide (SnO 2 ), typically In 2 O 3 :SnO 2 = 9:1 by weight%.It is one of the most widely used transparent conducting oxides because of its two chief properties, its electrical conductivity and optical transparency [50].The adding of Sn can replace In element in the In 2 O 3 lattice and form SnO 2 .The forming of SnO 2 will donate one electron to the conducting band and create oxygen vacancy increasing the carrier density and so, the oxide conductivity [51].Javier Parrondo et al. studied catalytic activities of Pt/In 2 O 3 /C composite catalysts with different compositions of In 2 O 3 as anode catalyst for direct ethanol fuel cells (DEFCs).It was demonstrated that Pt/In 2 O 3 /C has higher activity towards ethanol oxidation than that of Pt/C, and the enhancement of activity could be attributed to the effects of In 2 O 3 adjacent to Pt (bifunctional effect) [52].

Characterization and Electrocatalytical
Performance.The acid-oxidizing degree of MWCNTs was determined by Raman spectra.The structures of the prepared samples were determined by XRD, FESEM, and HRTEM.In order to evaluate the composition of the hybrid nanoparticles, about 5mg sample was dissolved in aqua regia for 100h, and then the solutions were diluted in volumetric flask and filtered.The samples were analyzed by ICP (Perkin Elmer Optima-2000 DV) in Precise Instrument Center of Tatung University.Electrochemical activities of catalysts were characterized by C-V (cyclic voltammetry) measurement using a three-electrode cell and CHI Instrument Model 600-D potentiostat/galvanostat instrument, located in Department of Materials Engineering, Tatung University.Three-electrode cell system is composed of working electrode, Pt plate counter electrode, and a Ag/AgCl reference electrode.The electrolyte solution is a mixture of 1 M H 2 SO 4 and 3 M HCOOH in equal volume at room temperature.The rotating velocity of RDE working electrode is 1600 rpm.Scan rate is 10 mV/s with scan potential range from -0.2 V to 1 V.During each C-V experiment, the dissolved gas, oxygen, CO, or CO 2 is removed from the solution by purging Argon before and during testing the cell.

Structure Characterization. The Raman spectra of raw
MWCNTs and acid oxidized MWCNTs (AO-MWCNTs) are shown in Figure 2. Two features are observed in Raman spectra that include the disorder induced mode, the so-called D band (centered at ∼1350 cm -1 ), and the graphite mode, G band (centered at ∼1589 cm -1 ).For AO-MWCNTs, both functional groups and defects on the surface formed by the acid oxidation lead to an increase of D/G intensity ratio [20].
The XRD patterns of metal oxides modified MWCNTs and Pd electrocatalysts on various substrates are shown in Figure 3. Firstly, for In 2 O 3 modified MWCNTs via impregnation and NaBH 4 methods, both have same structure as the commercial In 2 O 3 powder.For the patterns of the prepared ITO in different SnO 2 and In 2 O 3 ratios, the large peak at 26 ∘ can be identified as carbon from MWCNTs, and the other SnO 2 characteristic peaks cannot be found.The basic structure and composition analysis (Table 1) show that the prepared ITOs may not perform as expected.
For the two In The FESEM and HRTEM morphology images of the hybrid nanoparticles are presented in Figure 4.By analyzing the fringe patterns of some particles in the HRTEM images, the nanoparticles can be recognized as Pd, In 2 O 3 , or SnO 2 .All the designed compositions were in a qualitative agreement with EDS analysis.According to the FESEM images, the SnO 2 particles have the smallest particles (around 2-10 nm) comparing to other MOS particles.The MOS particles are uniformly dispersed on the surface of MWCNTs with the exception of ITO, where some large conglomerates can be also visible.The fine Pd nanoparticles (around 2-10 nm) are deposited on the surface of MWCNTs.The Pd nanoparticles formed on MOS/MWCNT composites are much larger.This is confirmed by XRD data where using Sherrer's equation, the Pd average size can be estimated at 36.8 nm and 40.3 nm for Pd/In 2 O 3 /MWCNTs and Pd/SnO 2 /MWCNTs, respectively.
The composition of every sample, evaluated by ICP-OES analysis, is shown in Table 1.All the Pd based catalysts have

Conclusions
The MOS modified MWCNTs, SnO 2 /MWCNTs, and In 2 O 3 / MWCNTs were successfully synthesized by the impregnation method.In 2 O 3 /MWCNTs was also prepared by NaBH 4 method and during the Pd deposition on this support by polyol method, the InPd 2 was formed.From FESEM and HRTEM images, it is seen that the fine Pd nanoparticles (around 2-10 nm) were deposited on the surface of MWCNTs and MOS/MWCNTs.The SnO 2 has the smallest particles (around 2-10 nm) comparing to other MOS particles and it uniformly dispersed on the surface of MWCNTs. In

Figure 1 structure
Figure 1

2 O 3 /
MWCNTs prepared by two different methods, XRD patterns in (b) show the same structure as commercial indium oxide powder.After Pd depositing onto the substrates, AO-MWCNTs and the prepared MOS/MWCNTs, in (c) and (d), Pd particles were successfully synthesized.Pd/In 2 O 3 /MWCNTs-Impregnation, which is supported on In 2 O 3 /MWCNTs prepared by impregnation, still retains cubic indium oxide structure, while in In 2 O 3 /MWCNTs-NaBH 4 the In 2 O 3 was partly converted to orthorhombic InPd 2 in reaction with Pd.There are some researches who synthesized InPd 2 , In 3 Pd 5 , or InPd 3 by iodine catalyzed reaction from elements at 850 K [21, 22].

Figure 3 : 6 International
Figure 3: XRD patterns of the hybrid nanoparticles.

Figure 5 :Figure 6 :
Figure 5: ECSA plots of the catalysts normalized by Pd weight.

Table 2 :
The maximum current density and the corresponding potential of the prepared Pd based electrocatalysts.Pd amount about 18-19 wt%.It reveals that Pd can be deposited on the surfaces of MWCNTs or metal oxides modified MWCNTs.For the two ITO mentioned previously, SnO 2 amounts are considerably smaller than the theoretical values and, for this reason, in this study Pd has not been deposited on the two ITO/MWCNTs.Pd/SnO 2 /MWCNTs also have smaller SnO 2 amount than the theoretical value.The effect of SnO 2 in Pd/SnO 2 /MWCNTs may not be as pronounced as expected. similar

Table 2 .
The tendency of the electrochemical area of the catalysts is quite consistent with that of the particle size.That is, the smaller Pd particles of Pd/MWCNTs have larger surface area, and so, more active sites than the other two catalysts.The electrocatalytical performance of the catalysts in the mixture of formic acid and sulfuric acid is shown in Figure6 electrochemical analysis of Pd-series catalysts, Pd/ In 2 O 3 /MWCNTs catalyst, with In 2 O 3 deposited by impregnation method, has the highest electrooxidizing current density in cyclic voltammetry experiments.Both ECSA and CV results indicate the Pd/In 2 O 3 /MWCNTs have the highest electrocatalytic activity.In the present results, In 2 O 3 nanoparticle is the best electrocatalysts supporter in this study.