Ultrafast synthesis of flower-like ordered Pd3Pb nanocrystals with superior electrocatalytic activities towards oxidation of formic acid and ethanol
Graphical abstract
A facile and ultrafast method was employed for the synthesis of ordered Pd3Pb. The material showed outstanding catalytic activity towards ethanol and formic acid oxidation compared to the commercial Pd on carbon support.
Introduction
Direct liquid fuel cells such as direct formic acid fuel cell (DFAFCs), direct ethanol fuel cell (DEFCs) have attracted a considerable attention as new generation power sources [1], [2], [3]. The design of efficient anode catalyst for the oxidation of small organic molecules (SOMs), like formic acid (HCOOH), ethanol (EtOH) and methanol (MeOH) has become an active area of research [4], [5], [6], [7]. In general, Pt [8] and Pt based alloys and intermetallics (PtPb [9], [10], Pt3Pb [11], [12], PtSn [13], PtBi [9], PtCo [14], PtZn [15], AuPt [16] PtM [M = Ni, Cu, Mn, Cr, V, Co] [17], [18], [19], [20]) are mostly considered as efficient anode materials in the fuel cells. But, one of the major problems for Pt based catalysts is carbon monoxide (CO) poisoning [21]. Alternatively, Pd nanoparticles (NPs) are found to be more active than the commonly used Pt catalysts in catalyzing formic acid oxidation (FAO) reaction in polymer electrolyte membrane fuel cells [22]. The enhancement in the adsorption/dissociation of HCOOH on Pd surface is believed to be the limiting step for the improvement of FAO activity [23]. The desired enhancement of adsorption/dissociation properties can be achieved through alloying Pd with oxophilic metals (M) like Co and Cu through so called ligand effect, which basically needs manipulating Pd-M bonding [21]. Alloys [24], bimetallics [25] and intermetallics [26], [27] based on Pd have exhibited excellent catalytic activity for formic acid oxidation as compared to Pt due to their ability to oxidize HCOOH to carbon dioxide (CO2) in direct pathway by avoiding the formation of CO [21].
Along with HCOOH, EtOH is also considered as a promising alternative fuel. Alkaline membrane DEFCs have already been considered as promising power source for different electronics and automobiles due to enhanced kinetics of electro-oxidation in alkaline medium [11]. Although Pt based NPs are considered as the best catalysts for Ethanol Oxidation Reaction (EOR) [11], [28], Pd based catalysts have also been widely studied in recent times as an alternative to Pt because of its low cost, greater abundance, good resistance to CO poisoning and high catalytic activity [29], [30]. Many alloys and oxide supported Pd-M (M = Ru, Cu, Au, Ni, Ag, Sn, Ir, Co and Pb) binary electrocatalysts have exhibited improved electrocatalytic activity for ethanol oxidation [14], [31], [32], [33], [34], [35], [36], [37], [38]. Shape tailored Pd and Pd based alloy nanocrystals are also found to exhibit enhanced electrocatalytic activity [39], [40]. Although alloys and bimetallics are promising materials, they suffer from surface segregation of metal atoms and surface poisoning by CO due to insufficient quantities of bimetallic elements on the surface [11]. In contrast to disordered alloy/bimetallic, ordered intermetallic compounds, such as PtPb, PtBi, Pt3Ti, Pt3V have shown excellent electrocatalytic performance towards HCOOH, EtOH, MeOH oxidation in terms of current density and CO tolerance [41], [42]. The advantage of ordered structure comes from its uniform surroundings of active sites. The number and distance between active sites play an important role in the catalytic activity. However it is very difficult to synthesize ordered intermetallic nanomaterial by low temperature solution based synthesis. Usually it requires post synthetic high temperature heat treatment to achieve ordered structure [42].
Inspired by the good catalytic activity of Pd, promoting effect of Pb [10], [11], [38], [43] and effect of ordered structure on the electrooxidation of alcohols and other organic fuels, herein, for the first time we report a facile ultrafast synthesis of flower-like and interconnected network type ordered Pd3Pb intermetallic nanocrystals at low temperature by solution based methods (hydrothermal, polyol). To the best of our knowledge, this is the first report on the surfactant free synthesis of flower-like ordered nanomaterials within 10 s. Interestingly, Pd3Pb has formed within 10 s by polyol method, which could presumably be due to ultrafast diffusion of constituent elements leading to the formation of an ordered intermetallic compound. We have studied the electrochemical oxidation of HCOOH in acidic medium and EOR in alkaline medium using the Pd3Pb catalysts synthesized at different conditions. The increase in catalytic activity for the compound formed by polyol method in ultrafast condition clearly indicates that the morphology of the nanoparticles plays a crucial role in the electrocatalysis. The electrocatalytic activity of all the different shaped Pd3Pb nanomaterials exhibited superior performance to the commercial Pd/C.
Section snippets
Chemicals
Potassium tetrachloropalladate (K2PdCl4), palladium acetylacetonate (Pd(acac)2), sodium borohydride (NaBH4) and nafion binder (5 wt%) were purchased from Sigma–Aldrich, lead acetate trihydrate (Pb(OAc)2·3H2O) and polyvinyl pyrrollidone (PVP) were purchased from SDFCL and tetra ethylene glycol (TEG) was purchased from Alfa Aesar. All the chemicals (more than 99% purity) were used as purchased without further purification. Millipore water of resistivity 18.2 MΩcm was used for the synthesis and
Structure, synthesis, shape and morphology
The compound Pd3Pb is a primitive cubic system having Cu3Au structure type with Pmm space group [44]. A typical representation of the unit cell of the Pd3Pb crystal structure in comparison with Pd structure is shown in Fig. 1d. Pure palladium metal has a face centered cubic crystal structure (Fmm) where all the corners and faces are occupied by the Pd atoms (Wyckoff no. 4a). Whereas, in Pd3Pb the Pb atoms occupy the corner positions (Wyckoff no. 1a) and the Pd atoms occupy half of the
Conclusion
A facile and ultrafast synthetic method has been employed for the preparation of Pd3Pb intermetallic nanoparticles in different morphologies for the first time. The synthesis methods can be extended for the formation of other Pd and group IV metal based intermetallics by choosing appropriate precursor salts. The electrochemical oxidation of formic acid and ethanol were studied on Pd3Pb nanocrystals of different shapes and morphologies in different supporting electrolytes (KOH, HClO4). Among all
Acknowledgments
We thank Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Sheikh Saqr Laboratory and Department of Science and Technology, India (DST) for financial support. R. K. J. thanks Council of Scientific and Industrial Research (CSIR), JNCASR and DST for research fellowship, U. S. thanks CSIR for research fellowship, and S. C. P thanks DST for Ramanujan Fellowship (Grant SR/S2/RJN-24/2010). We are grateful to Prof. C. N. R. Rao, F.R.S for his constant support and encouragement.
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