X. LI et al.: PREPARATION OF Fe-Pd BASED NANOPOROUS AMORPHOUS ALLOYS AND THEIR ELECTROCATALYTIC ... 661–667 PREPARATION OF Fe-Pd BASED NANOPOROUS AMORPHOUS ALLOYS AND THEIR ELECTROCATALYTIC PROPERTIES DURING DECOMPOSITION OF FORMIC ACID PRIPRAVA AMORFNIH NANOPOROZNIH ZLITIN NA OSNOVI Fe-Pd IN NJIHOVE ELEKTROKATALITI^NE LASTNOSTI MED RAZKRAJANJEM Xue Li 1 , Jibo Zheng 1 , Gonghao Lu 2* ,Y uLiu 1 , Dechuan Yu 1 , Shengli Li 1 1 School of Materials and Metallurgy, University of Science and Technology, Liaoning, 185 Qianshan , Zhong Road, Anshan 114051, China 2 School of Chemical Engineering, University of Science and Technology Liaoning, 185 Qianshan Zhong Road, Anshan 114051, China Prejem rokopisa – received: 2019-11-02; sprejem za objavo – accepted for publication: 2020-05-10 doi:10.17222/mit.2019.266 In this study, Fe-Pd based amorphous alloys with a nanoporous structure were successfully prepared with dealloying treatment and their catalytic properties were investigated during the decomposition of formic acid (HCOOH). Fe60Pd20P20 amorphous-al- loy ribbons with 1 mm in width and 25 μm in thickness were prepared using vacuum melt-spinning. Then, nanoporous amor- phous alloys with a three-dimensional uniform network structure were prepared with electrochemical dealloying in a sulfuric acid (H2SO4) solution, using the Fe60Pd20P20 amorphous ribbons as the precursor alloy. Finally, the electrocatalytic properties of these nanoporous amorphous alloys were studied with cyclic voltammetry. In a mixed solution of H2SO4 (0.5 mol/L) and HCOOH (0.5 mol/L), the catalytic results showed that the nanoporous amorphous alloy dealloyed at a constant potential of 0.72V exhibited an obviously negative shift of 0.35 V in the oxidation-peak potential of HCOOH, while the oxidation-peak cur- rent density increased about 34 times. This means that the Fe-Pd based nanoporous amorphous alloy has an obvious catalytic ef- fect on the decomposition of HCOOH. Keywords: nanoporous, amorphous alloy, dealloying, formic acid, electrocatalysis Avtorji tega ~lanka opisujejo uspe{no pripravo nanoporoznih amorfnih zlitin na osnovi Fe-Pd s posebnim postopkom obdelave (razlegiranjem) in njihove kataliti~ne lastnosti pri razkroju v mravlji~ni kislini (HCOOH). Avtorji so pripravili amorfne trakove zlitine Fe60Pd20P20 {iroke 1 mm in 25 μm debele trakove s postopkom nalivanja trakov na hitro vrte~em se valju v vakuumu (angl.: vacuum melt-spinning). Z elektrokemijskim razlegiranjem so nato v raztopini `veplene kisline, (H2SO4), iz trakov izdelali amorfne tridimenzionalne strukture. Nazadnje so njihove elektrokataliti~ne lastnosti {tudirali s cikli~no voltametrijo. V me{ani raztopini H2SO4 (0.5 mol/L) in HCOOH (0.5 mol/L) so kataliti~ni rezultati pokazali, da so nanoporozne amorfne zlitine razlegirane pri konstantnem potencialu 0,72 V in ka`ejo o~iten premik maksimuma oksidacijskega potenciala HCOOH za 0,35 V. Oksidacijski maksimum tokovne gostote se je pove~al za okoli 34-krat, kar pomeni, da ima izbrana Fe-Pd nanoporozna amorfna zlitina pomemben kataliti~ni vpliv na razkroj HCOOH. Klju~ne besede: nanoporozne amorfne zlitine, razlegiranje, mravlji~na kislina, elektrokataliza 1 INTRODUCTION Nanoporous metal materials have attracted much at- tention in recent years because of their high specific sur- face area and special interface characteristics. 1 There- fore, they have been applied in many fields such as catalysis, 2 sensors, 3 filters, 4 optical detectors 5 and super- capacitors. 6 The preparation methods for nanoporous materials include the template method, 7 dealloying method, 8 layer self-assembly technology 9 a n ds oo n . Among these preparation methods, the dealloying method utilizes the large difference in the electrode po- tential between the alloy components. The active compo- nents rapidly dissolve in the electrolyte, while the inert components diffuse and recombine to form a three-di- mensional connective porous network structure. 10,11 Compared with the other methods, the dealloying method has the advantages of a low cost, simple and effi- cient process and easy industrialization. Since J. Erlebacher et al. 10 reported on a nanoporous metal structure with pore sizes of 2–50 nm obtained with the dealloying method in 2001 any nanoporous metal materi- als have been reported, including binary alloys of Cu-Pd, 12 Pd-Au, 13 Cu-Pt, 14 Ni-Pt, 15 Ag-Pd 16 and Ag-Cu, 17 and ternary or multi-component alloys of Ag-Au-Pt, 18 Al-Pd-Au, 19 Mg-Ni-Y 20 and Cu-based, 21 Pt-based, 22 Mg-based, 23 Pd-based 24 and Al-based alloy systems. 25 However, there are few reports on Fe-based ternary or multi-component nanoporous metal materials. The key to the preparation of nanoporous metal mate- rials by dealloying is the selection of suitable precursors. The precursor should have a homogeneous composition in the single-phase region. 26 Amorphous alloys have fea- ture structures of long-range disorder and short-range or- der, a homogeneous element composition, a wide range of the single-phase region and an active surface. There- Materiali in tehnologije / Materials and technology 54 (2020) 5, 661–667 661 UDK 544.022.6:661.741.2:544.47 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(5)661(2020) *Corresponding author's e-mail: ghlu@ustl.edu.cn (Gonghao Lu) fore, amorphous alloys are ideal precursors for nano- porous metal materials obtained by dealloying. They pro- vide a new idea for preparing ternary or multi-component nanoporous materials by dealloying. In this study, an Fe 60 Pd 20 P 20 amorphous alloy was chosen as the precursor for dealloying. The effect of the constant potentials on the preparations of nanoporous amorphous alloys was first investigated. And then, the catalytic properties of these Fe-based nanoporous amor- phous alloys with respect to formic acid (HCOOH) were also further studied with the electrochemical method. 2 MATERIALS AND METHODS 2.1 Materials Fe (99.9 w/%), Pd (99.9 w/%) and the prealloyed in- got of Fe 3 P (99.5 w/%) were purchased from Sigma- Aldrich (China). Analytical-grade sulfuric acid (H 2 SO 4 ) and HCOOH were purchased from Sinopharm Chemical Reagent Co. Ltd. 2.2 Methods Fe 60 Pd 20 P 20 alloy ingots were prepared by vacuum in- duction melting under a high purified-argon atmosphere using the elements of high purity including Fe, Pd and Fe 3 P. The Fe 60 Pd 20 P 20 amorphous-alloy ribbons with 1 mm in width and 25 μm in thickness were prepared by melt-spinning. The structure of the amorphous ribbons was examined with X-ray diffraction (XRD, X’Pert Pow- der, PANalytical B.V. Company) with a Cu-K radiation. The morphologies of the amorphous ribbon and dealloying samples were observed with a field-emission scanning electron microscope (FESEM, ZEISS sigma 500, Carl Zeiss AG Company) and the compositions were determined with energy-dispersive spectrometer (EDS, QUANTAX, Bruker Corporation). The electrochemical experiment and dealloying treat- ment were performed in a standard three-electrode cell with an electrochemical workstation (CHI660D, Shang- hai Chenhua Device Company). The Fe 60 Pd 20 P 20 amor- phous alloy ribbon was used as the working electrode and an Ag/AgCl electrode was used as the reference electrode with a platinum electrode as the counter elec- trode. The dealloying treatments at different constant po- tentials were carried out in an H 2 SO 4 electrolyte (1 mol/L) for 2 h. The morphologies and compositions of the samples were observed with XRD, FESEM and EDS. The prepared nanoporous sample was adhered to a glassy carbon electrode with a Nafion solution and this glassy carbon electrode was used as the working elec- trode. Hg/Hg 2 SO 4 was used as the reference electrode with a platinum wire as the counter electrode. Electro- chemical activities were evaluated with a cyclic voltammetric curve obtained in the H 2 SO 4 solution (0.5 mol/L), while catalytic properties were evaluated with a cyclic voltammetric curve obtained in the mixed solution of H 2 SO 4 (0.5 mol/L) and HCOOH (0.5 mol/L). The scanning rate was 50 mV/s. 3 RESULTS AND DISCUSSION 3.1 Alloy design and amorphous characteristic In order to obtain homogeneous nanoporous materi- als, the amorphous alloy precursor should meet the re- quirements of the dealloying method as well as Inoue’s principles for the preparation of amorphous alloys. 26,27 The dealloying requirements include four points: 1) the alloy should have a wide single-phase region; 2) the elec- trode potential differences between the alloying elements should be large enough; 3) the highly active element should be the main component of the alloy; and 4) the diffusion rate of the inert elements at the interface of the alloy and electrolyte should be fast enough. The mixing enthalpies of the three elements in the Fe 60 Pd 20 P 20 alloy are –4 KJ/mol for Fe-Pd, –36.5 KJ/mol for Pd-P and –39.5 KJ/mol for Fe-P, respectively, 28 and all the values are negative. In addition, the atomic radiuses are 0.1241 nm for Fe, 0.1376 nm for Pd and 0.1000 nm for P. All the ratios of atomic radiuses are more than 12 %, meet- ing Inoue’s principles. These data ensure that a uniform amorphous state is easily obtained in the Fe 60 Pd 20 P 20 al- loy. On the other hand, the standard reduction potentials are –0.440 V for Fe and 0.987 V for Pd, with the stan- dard hydrogen electrode as the reference electrode. 29 The potential difference between Fe and Pd is large enough. Moreover, the Fe element is easy to corrode and dissolve in an acid solution, and Pd easily diffuses and recom- bines to form a three-dimensional connective porous net- work structure. 26,30 And then, the addition of the P ele- ment can promote the formation of the nanoporous structure and enhance the catalytic property and stability of the nanoporous material. 31 Finally, from an economic point of view, the active Fe element is cheap and easy to get, and Pd is a relatively cheap precious metal as its price is only half of the price for gold. Because of the X. LI et al.: PREPARATION OF Fe-Pd BASED NANOPOROUS AMORPHOUS ALLOYS AND THEIR ELECTROCATALYTIC ... 662 Materiali in tehnologije / Materials and technology 54 (2020) 5, 661–667 Figure 1: XRD pattern of Fe 60 Pd 20 P 20 amorphous-alloy ribbon above good features, the Fe 60 Pd 20 P 20 amorphous alloy was designed as the precursor alloy in this study. The XRD pattern of the Fe 60 Pd 20 P 20 amorphous-alloy ribbon is shown in Figure 1. The XRD pattern shows a broad peak around 2 = 40–48° and no distinguishable diffraction peaks of crystalline phases, which indicates that the prepared Fe 60 Pd 20 P 20 alloy ribbon is amorphous. 3.2 Analysis of the electrochemical property of the amorphous alloy Figure 2 shows a dynamic-potential polarization curve of Fe 60 Pd 20 P 20 in 1 mol/L H 2 SO 4 electrolyte ex- posed to air at 298 K. The interval form I to II on the po- larization curve is the electro-dissolution range. The cur- rent density increases rapidly to reach the maximum with the positive shift of the metal electrode potential and the active metal element dissolves, as shown with the equa- tion Fe Fe 2+ +2e – . When the potential exceeds point II, the metal electrode material is partially electrochemi- cally passivated, which reduces the dissolution rate. Thus, the current density rapidly drops from the highest value to point III, and the interval from II to III is the electrode transition zone. When the potential exceeds point III, the metal electrode is in a stable state and the current density hardly changes with the voltage. There- fore, the polarization curve is almost a straight horizontal line and the interval from III to IV is a stable zone. In the study, the constant potentials of 0.72 V (disso- lution zone), 0.77 V (transition zone) and 0.97 V (stabil- ity zone) were selected for dealloying experiments. The effect of the constant potentials on the preparation of the Fe-Pd based nanoporous amorphous alloys was investi- gated. Figure 3 shows the changes in the current densi- ties of the Fe 60 Pd 20 P 20 amorphous ribbons with the dealloying times. When the constant potential is 0.72 V, the electrochemical reaction on the metal electrode is Fe Fe 2+ +2e –32 and the current density increases with the dealloying time. When the constant potential is 0.77 V, the current density increases slowly with the dealloying time, and the current density is lower than that of the constant potential of 0.72 V. When the con- stant potential is 0.97 V in the stable zone of the metal electrode, the current density remains relatively stable after a period of dealloying of 15 minutes. 3.3 Effect of dealloying constant potentials on nano- porous structures In order to confirm the effect of dealloying constant potentials on the nanoporous structures of the prepared samples, the sample morphologies were observed with FESEM (Figure 4). Figure 4a shows the SEM morphol- ogy of an original Fe 60 Pd 20 P 20 amorphous alloy ribbon (as-quenched). No crystalline phase was observed on the surface of the sample, which is consistent with the XRD result. Figure 4b shows the morphology of the sample prepared by dealloying at a constant potential of 0.72 V for 2 h (NP-Pd1). A large number of pores with diame- ters of 10–25 nm appeared on the surface of the sample and nanoporous structures were observed. When the dealloying constant potential was 0.72 V in the dissolu- tion zone of the polarization curve, the corrosion rate of Fe was greatly increased. The EDS results showed that the Fe content in the sample significantly decreased to 42.05 % (Table 1), indicating that Fe was corroded and dissolved in the electrolyte (Fe Fe 2+ +2e – ). In the pro- cess of Fe corrosion, Pd diffused and recombined to form a nanoporous structure. However, as the ligament was relatively thick, no three-dimensionally connected pore structure was formed. Figure 4c shows the morphology of the sample pre- pared by dealloying at a constant potential of 0.77 V for 2 h (NP-Pd2). A lot of pores with diameters of 50–90 nm appeared on the surface of the sample. The EDS results showed that the content of Fe was reduced because Fe in the sample was corroded and dissolved in the electrolyte (Table 1). Compared with the sample prepared at the constant potential of 0.72 V, the Fe and Pd content changed less because the dealloying constant potential X. LI et al.: PREPARATION OF Fe-Pd BASED NANOPOROUS AMORPHOUS ALLOYS AND THEIR ELECTROCATALYTIC ... Materiali in tehnologije / Materials and technology 54 (2020) 5, 661–667 663 Figure 3: Changes of the current densities of the Fe 60 Pd 20 P 20 amor- phous ribbons with the dealloying times in 1 mol/L H 2 SO 4 solution at 25 °C Figure 2: Polarization curve of Fe 60 Pd 20 P 20 amorphous-alloy ribbon in 1 mol/L H 2 SO 4 solution at 25 °C was 0.77 V in the transition region of the polarization curve. Parts of the reaction should thus be Fe + H 2 O (FeOH) ad +H + +e – and (FeOH) ad +H 2 O [Fe(OH) 2 ad + H + +e – . 32 Fe(OH) 2 ad . The formation from the corrosion reaction adheres to the surface of the sample, thus limit- ing further corrosion. Therefore, it is possible to form uniform nanoporous structures. Table 1: EDS analysis results for different nanoporous alloys Nanoporous alloy Element content (a/%) Fe Pd P NP-Pd1 42.05 42.32 15.63 NP-Pd2 53.77 25.80 20.43 NP-Pd3 55.72 23.31 20.97 Figure 4d shows the morphology of the sample pre- pared by dealloying at a constant potential of 0.97 V for 2 h (NP-Pd3). A lot of pores with diameters of 30–50 nm appeared on the surface of the sample and a three-dimen- sional porous network structure was observed. The size of the ligament was about 10 nm. The EDS results for each elements showed that the changes were small (Ta- ble 1) as the dealloying constant potential was 0.97 V in the stable region of the polarization curve. The dealloying rate was low enough, resulting in the forma- tion of three-dimensional uniform nanoporous structures. In a word, using an Fe 60 Pd 20 P 20 amorphous alloy ribbon as the precursor, nanoporous amorphous alloys with a three-dimensional uniform network structure can be pre- pared by electrochemical dealloying. Meanwhile, the constant potentials of the dealloying process have an im- portant influence on the compositions and morphologies of the nanoporous amorphous alloys. 3.4 Electrochemical activities of nanoporous amor- phous alloys Figure 5 shows cyclic voltammetric curves of the al- loy samples from 0.5 mol/L H 2 SO 4 solution. The curves clearly reflect the hydrogen-evolution and oxygen-evolu- tion processes of the electrodes as well as the adsorption and desorption behaviours of hydrogen and oxygen. Hc is the reduction peak of hydrogen, while Ha is the oxida- tion peak of hydrogen. Based on the adsorption/desorp- tion peak area of hydrogen, the electrochemical active surface area (ECSA) was obtained. The larger the elec- trochemical active surface area, the better is the catalytic activity of the amorphous alloy. 33 As shown in Figure 5, the redox peak of the as-quenched sample was very small. The NP-Pd1 sample showed the largest adsorp- tion/desorption peak, and the NP-Pd2 sample showed the second largest adsorption/desorption peak, while sample NP-Pd3 showed a small adsorption/desorption peak, close to that of the original amorphous-alloy ribbon. The ECSAs (in Figure 6) of the samples were calculated with Equation (1). 34,35 The ECSA value of the X. LI et al.: PREPARATION OF Fe-Pd BASED NANOPOROUS AMORPHOUS ALLOYS AND THEIR ELECTROCATALYTIC ... 664 Materiali in tehnologije / Materials and technology 54 (2020) 5, 661–667 Figure 4: SEM images of Fe 60 Pd 20 P 20 alloy and samples after potentiostatic dealloying in 0.5 mol/L H 2 SO 4 solution at 25 °C: a) as-quenched, b) NP-Pd1, c) NP-Pd2, d) NP-Pd3 as-quenched sample was 0.31×10 –3 cm 2 /g and the ECSA values of samples NP-Pd1, NP-Pd2 and NP-Pd3 were 90, 22 and 5 times larger than that of the as-quenched sample, respectively. These results indicate that the ECSA values of the nanoporous amorphous alloys were increased and the NP-Pd1 sample had the maximum ECSA value. ECSA (cm g) Charge (mC cm ) Catalyst loading (g c 2 2 / / / = = m)2 1 0( m Cc m) 22 × / (1) The charge is the total amount of the catalyst surface charge obtained by integrating the adsorption/desorption peak areas of hydrogen on the CV curve. 3.5 Electrocatalytic properties of nanoporous amor- phous alloys for the decomposition of HCOOH Figure 7 shows the cyclic voltammetric curves of the alloy samples in 0.5 mol/L H 2 SO 4 + 0.5 mol/L HCOOH. The results showed the nanoporous samples exhibited oxidation peaks in the positive and negative sweeps of HCOOH. In the positive sweep of HCOOH, CO 2 formed during the oxidation reaction. The reaction equation was HCOOH CO 2 +2H + +2e – . 36,37 and the adsorption of CO 2 was very weak on the surface of Pd. According to the potentials and current densities of the oxidation peaks for different alloy samples, we can determine the electrocatalytic properties of nanoporous amorphous al- loys. 38 The potentials and current densities of the oxida- tion peaks were 0.15 V and 4.5 A/m 2 for the as-quenched sample, –0.2V and 155 A/m 2 for NP-Pd1, –0.2V and 115 A/m 2 for NP-Pd2 and –0.2V and 14 A/m 2 for NP-Pd3 (Figure 7). Compared with the as-quenched sample, the potential of NP-Pd1 was negatively shifted by 0.35 V and the cur- rent density of NP-Pd1 was enhanced 34 times, the po- tential of NP-Pd2 was negatively shifted by 0.35 V and the current density of NP-Pd2 was enhanced 26 times, and the potential of NP-Pd3 was negatively shifted by 0.40 V and the current density of NP-Pd3 was enhanced 3 times. Anyway, compared with the Fe 60 Pd 20 P 20 amor- phous ribbon, the potentials of the oxidation peaks for the nanoporous samples were negatively shifted by 0.35–0.4 V and all the current densities of the oxidation peaks were obviously improved. Therefore, the prepared nanoporous amorphous alloys showed catalytic proper- ties on the decomposition of HCOOH. In the dealloying process, the Fe element corroded and dissolved, while the Pd element diffused and recom- bined to form nanoporous structures. As a result, the ac- tive surface areas were significantly enlarged. Fe atoms can affect the electron structure on d orbital of Pd atoms due to a stress-induction effect, 39 thus increasing the cat- alytic activities of Pd. In addition, Pd atoms have a spe- cial affinity with hydrogen, 40 which causes the HCOOH molecules to be quickly dehydrogenated on the surface of Pd, and the reaction formula is HCOOH ad CO 2 + 2H + +2e – . Pd absorbs the reactants to activate the mole- cules by chemical adsorption bonding. Therefore, the ac- tivation energy of the reaction is reduced and thus the catalytic reaction rate of HCOOH is increased. Sample X. LI et al.: PREPARATION OF Fe-Pd BASED NANOPOROUS AMORPHOUS ALLOYS AND THEIR ELECTROCATALYTIC ... Materiali in tehnologije / Materials and technology 54 (2020) 5, 661–667 665 Figure 7: CV curves of alloy samples in 0.5 mol/L H 2 SO 4 and 0.5 mol/L HCOOH solution at 25 °C Figure 6: ECSA values of nanoporous alloy and as-quenched sample in 0.5 mol/L H 2 SO 4 solution Figure 5: CV curves of Fe 60 Pd 20 P 20 alloy and nanoporous alloy in 0.5 mol/L H 2 SO 4 solution NP-Pd1 has the highest Pd atomic content (42.32 %), which greatly improves the catalytic effect of sample NP-Pd1 on HCOOH. In contrast to the Fe 60 Pd 20 P 20 amor- phous-alloy ribbon, all the nanoporous amorphous alloys showed improvements in the catalytic properties on the decomposition of HCOOH. 4 CONCLUSIONS In summary, Fe 60 Pd 20 P 20 amorphous-alloy ribbons with 1 mm in width and 25 μm in thickness were pre- pared with vacuum induction melting and vacuum melt-spinning. The dealloying constant potentials have an important influence on the morphologies of nanoporous amorphous alloys. Three-dimensional nanoporous amorphous alloys with diameters of 20–40 nm were prepared by dealloying at a constant po- tential of 0.72 V for 2 h in 1 mol/L H 2 SO 4 solution using the Fe 60 Pd 20 P 20 amorphous ribbons as the precursor alloy. The nanoporous amorphous alloys had more surface ac- tive area and their electrocatalytic properties for HCOOH were enhanced. The nanoporous amorphous al- loy obtained at 0.72 V (NP-Pd1) exhibited the maximum surface active area and the best electrocatalytic effect among all the alloys. In the decomposition of HCOOH, compared with the Fe 60 Pd 20 P 20 amorphous ribbon, all the nanoporous amorphous alloys showed an increase in the catalytic properties. Among them, NP-Pd1 showed the best performance. The oxidation-peak potential was neg- atively shifted by 0.35 V and the oxidation-peak current density increased about 34 times in 0.5 mol/L H 2 SO 4 + 0.5mol/L HCOOH solution. Acknowledgment This work was sponsored by the National Natural Science Foundation of China (Grant No. 51501084), Natural Science Foundation of the Liaoning Province (Grant No. 201602398) and Foundation of the University of Science and Technology Liaoning (Grant No. 2015RC01). 5 REFERENCES 1 F. L. Jia, C. F. Yu, K. J. Deng, L. Z. 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