Synergism of Multicomponent Catalysis: One-Dimensional Pt-Rh-Pd Nanochain Catalysts for Efficient Methanol Oxidation.

Designing Pt-based alloy catalysts with multicomponent composition and a controllable structure is important to improve the utilization efficiency of precious metals and catalytic activity, but it still face a lot of challenges for simple preparation. Herein, we used insulin amyloid fibrils as templates and their own one-dimensional spiral structure to synthesize Pt-Rh-Pd ternary alloy nanochains under mild conditions. The prepared Pt-Rh-Pd alloy nanochains (NCs) have uniform diameter, and the particle size is only 2 nm. This ultrafine structure increases the specific surface area of the catalyst to a certain extent, and the synergistic effect of the three metals improves the catalytic performance. Compared with commercial Pt/C and binary Pt-Rh NCs, the as-presented Pt-Rh-Pd NCs show better methanol oxidation activity ability and stability against CO poisoning. The peak current density of front sweep is 1.48 mA cm-2, which is 1.7 times higher than that of commercial Pt/C (0.89 mA cm-2) and 1.4 times higher than that of the Pt-Rh NCs (1.07 mA cm-2), indicating great application potential as high-performance electrocatalysts in fuel cells.

Introduction

Precious metals such as Pt and corresponding alloy nanomaterials are widely used as high-efficiency catalysts in many fields due to their excellent electrical conductivity, high catalytic activity, corrosion resistance, high temperature resistance, etc. such as, fuel cell reaction, gas sensors, emissions of electronic components, and automotive exhaust.[1−5] Based on the principle of cheap and readily available raw materials, more research has been conducted on methanol in fuel cells.[6−8] However, the high loading of platinum (Pt) and its instability limit commercialization in the field of methanol oxidation.[9,10]

As known, the structure and composition of alloys or heterogeneous catalysts are closely related to their catalytic performance.[11−14] The biological template has many advantages, such as a wide range of sources, various forms, adjustable size, and great repeatability, which is conducive to the controlled synthesis of catalysts.[15,16] In electrochemical tests, Pt dissolution or nanoparticle aggregation often occurs, which leads to the loss of active materials and the degradation of methanol oxidation performance.[17−19] Therefore, it is desirable to use a structure to assist the multimetal active center and work together to improve the stability of the catalyst.[20,21] Lately, a battery of studies has shown that Pt-M alloy catalysts have superior activity, stability, and higher resistance to CO poisoning compared to single metal Pt catalysts.[22,23] For example, Narayanamoorthy et al. prepared Pt-Rh nanoclusters by a formic acid reduction method, which presented high activity and durability in methanol oxidation and had higher anti-CO toxicity than traditional Pt-based catalysts.[24] Zhao et al. prepared porous Pt-Pd-Cu with a highly active and durable three-dimensional structure. Its mass specific activity and specific activity were 6.5 times and 7.2 times higher than those of commercially available Pt/C, respectively.[25] Ma et al. used Te nanowires as a sacrificial template to prepare a multistep Pt-Ru-Pd-Te catalyst with high stability and resistance to CO toxicity.[26] For electrocatalysts, the oxidation reaction may cause the surface-active components to dissolve. Au or Pd atoms play an important role in protecting low-coordination sites, active Pt sites, and certain facets from being oxidized due to their higher reduction potential.[27−29] Also, the addition of Rh can improve the anti-CO toxicity of the catalyst.[30,31] The prepared Pt-Rh-Pd ternary catalysts can make full use of the synergistic effect of the three metals, thereby improving the overall anti-CO poisoning ability and catalytic activity of the catalyst. However, there are not many studies on trimetal nanoalloys and a few reports on the preparation of one-dimensional ternary structures. For one-dimensional structured catalysts, there are electronic effects or a dual mechanism of action in the structure, which makes electron and proton transport easier and more efficient. Meanwhile, they usually show excellent characteristics in terms of thermodynamics and chemical stability. Furthermore, the unique structure can achieve self-supporting effects, anisotropy, and fewer lattice boundaries on the surface, thus showing better catalytic performance.[32,33] Therefore, it is especially important to find a simple and controllable method to achieve the ternary Pt-Rh-Pd one-dimensional nanocatalysts.

A good method is simple and highly controllable. However, at present, most methods need to add surfactants in the preparation of Pt-based alloys to control the morphology and size of the products, so it is difficult to completely remove the surfactants in subsequent processing and they are easy to adsorb on the surface of the alloy particles, which seriously affect the catalytic performance.[34−38] The bovine insulin fiber powder can be processed into long and thin insulin fibers with length up to several microns.[16,23,39] Also, the insulin fibers can exist stably under low-pH conditions and are good templates for preparing one-dimensional nanostructures.[39,40] In this work, we prepared Pt-Rh-Pd alloy particles with a small size of 2 nm and allowed them to grow along the biomolecular insulin fiber template to form uniform and branchless Pt-Rh-Pd nanochains. The in situ growth can overcome the disadvantage of easy agglomeration of nanoparticles and expose more active sites. In addition, the preparation process of the biological template method is mild and pollution-free and no additional surfactant needs to be added. In electrochemical tests, the Pt-Rh-Pd NC catalysts show higher catalytic activity and stability than the commercial Pt/C.

Results and Discussion

Characterization of Pt-Rh-Pd Nanochains

Preparation of nanomaterials by using biomolecules of various shapes and sizes as templates is a promising approach because of its high morphological controllability, simple operation, and mild conditions.[15] Insulin is a protein hormone with a relatively small molecular weight and a spiral structure. Studies have shown that high protein concentrations, low pH, and high temperatures can make insulin form long and thin insulin fibers.[40,41] Insulin fiber is a kind of starch-like fiber with a diameter of 5–10 nm and length up to micrometers (as shown in Figure S1), which has high mechanical strength, good resistance to protease, and can exist stably in a range of pH = 1–2. These properties make it a good biological template for assembling one-dimensional structure nanomaterials.

The morphology and composition of samples were investigated with TEM and EDX. A large number of longer spiral Pt-Rh-Pd nanoparticle chains can be seen in Figure a, and the bottom of the figure is clean and free of impurities. In the partially enlarged view in Figure a, it can be clearly seen that the alloy nanoparticles are uniformly distributed along the inner and outer surfaces of the insulin fiber. The selected area diffraction pattern in Figure b shows that the product is a good polycrystalline material with a lattice spacing of 0.225 nm corresponding to the (111) crystal plane, and the diffraction rings from the inside to the outside correspond to the crystal planes (111), (200), (220), and (311) of Pt. EDX spectrum (as shown in Figure c) analysis shows that the product is composed of Pt, Rh, and Pd, and its atomic ratio is about 4:2:1. Therefore, the molecular formula of Pt-Rh-Pd NCs is Pt4Rh2Pd.

Figure 1

(a) TEM images, (b) HRTEM image and SAED pattern, and (c) EDX spectrum of Pt-Rh-Pd NCs.

To further analyze the distribution of Pt, Rh, and Pd elements in the nanochain, STEM-EDS characterization was performed, as shown in Figure . Since the synthesized nanochains are extremely thin and difficult to distinguish, the invisible positions may also contain nanochains, which may result in different element distributions at non-nanochain positions. Elemental mapping analysis (Figure a,d) shows the uniform distribution of Pt, Rh, and Pd elements along the nanochains. The linear scan (Figure e,f) also further confirms the uniformity of the distribution of the three elements Pt, Rh, and Pd and also indicates that the ternary Pt-Rh-Pd is an alloy structure.

Figure 2

(a–d) STEM-EDS images with elemental analysis of Pt, Rh, and Pd. (e and f) Linear scanning curve of Pt-Rh-Pd nanochains.

During the experiment, we found that the amount of precursor salt added and the choice of reducing agent will affect the morphology of the sample. First, the reducing agent NaBH4 was selected to explore the effect of different mixed salt dosages on morphology. As shown in Figure S2a, when 30 μL of mixed salt solution is added, there are few Pt-Rh-Pd nanoparticles growing along the insulin fibers, and more insulin fibers do not function as templates, resulting in a low amount of nanochains. With the addition amount increased to 60 μL, as shown in Figure S2b, the insulin fiber is almost completely covered, and the Pt-Rh-Pd nanoparticles uniformly dispersed along the axial direction of the insulin fibers to form the chain-like structure with length up to micrometers. When the salt solution was continually added to 90 μL, as shown in Figure S2c, the insulin fibers were supersaturated, and then a large number of Pt-Rh-Pd NCs are formed. At the same time, many particles continue to accumulate on the particle chain and form aggregates.

In addition, the effect of reducing agent on sample morphology under 60 μL of mixed salt conditions was also discussed. Before reduction, the color of the hatching system with insulin fibers in metal salt solution is all pale yellow. After adding a relatively weak reducing agent dimethylborane (DMAB), the color of the system became brown and black, while the solution with a strong reducing agent sodium borohydride (NaBH4) quickly turned dark black. It can also be seen from Figure S3a that although the nanoparticles grow on protein fibers, they are unevenly distributed with many breakpoints on particle chains. In contrast, the nanoparticle chains prepared with NaBH4, as shown in Figure S3b, grow uniformly along the fiber axis with uniform diameter and particle size and almost no breakpoints. The different phenomena may be caused by the different intensities of the reducing agent. The strong reducing NaBH4 can quickly reduce the precursor salt system so that the ions adsorbed on the inner and outer surfaces of the insulin fiber can grow rapidly during the nucleation process. The relatively weaker reducing ability of DMAB makes the nucleation and growth rate of the nanoparticles slow, and they eventually dispersed randomly around the insulin fibers.

In addition, we also performed X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and other physical characterizations to study the phase, composition, and crystal structure of the samples. For comparison, XRD patterns of Pt-Pd and Pt-Rh prepared under the same experimental condition are shown as curves b and c in Figure a, respectively, and there are four diffraction peaks for each sample. Taking the diffraction peak at the (111) crystal plane as an example, as shown in Figure b, the Pt-Rh-Pd alloy is located exactly between Pt-Pd (red line) and Pt-Rh (green line), indicating that the synthesized product is the Pt-Rh-Pd alloy (black line). In addition, there is no other impurity peak in curve a, and the diffraction peak of a single element (Pt, Rh, or Pd) does not exist, indicating that the synthesized product is in an alloy state and the purity is very high. XPS was used to analyze the composition and surface valence of Pt-Rh-Pd NCs. Figure d shows the Pt 4f XPS spectra, and the characteristic peaks in the figure show that the Pt element exists in the alloy in the valence states of Pt(0), Pt(II), and Pt(IV). Among them, the two characteristic peaks of 4f5/2 and 4f7/2 of Pt(0) are the strongest, indicating that Pt in the product mainly exists in the metal state. The presence of a large amount of metallic Pt provides additional sites for the deposition of methanol or ethanol molecules for subsequent testing. The existence of Pt(II) and Pt(IV) may be caused by the adsorption of oxygen on the surface of the product during preparation.

Figure 3

(a and b) XRD image of the Pt-Rh-Pd NCs. (c) XPS spectra and (d) Pt 4f spectra of Pt-Rh-Pd NCs.

Many research studies have been focused on the biological template methods. As previously reported, the insulin fibers are excellent biological templates for deposition of metal element Ag on the surface by an electrodeposition method.[42] Meanwhile, they could be used as templates to bond with metal salt ions, then rinsed, and reduced, finally preparing a spiral Ag nanochain.[43] Based on the above experiments, we have summarized the possible formation mechanism of Pt-Rh-Pd NCs, as shown in Figure a. The pretreated bovine insulin powder forms insulin fibers in the form of DNA molecules, and it shows only one absorption peak at 279 nm in the UV–vis test, as shown by the d line in Figure b. The formation of INSAFs (insulin amyloid fibrils) indicates that they can stably exist in subsequent chemical reactions. In addition, the amide sites on the surface of INSAFs could cooperate with the metal salt solution to form metal ion nucleation centers.[15,39] It can be seen from the UV–vis test that for pure PtCl4 and RhCl3 solution, each has an absorption peak at 258 and 222 nm, respectively, and two appeared for K2PdCl6 solution at 208 and 236 nm. When the INSAFs and the [PtCl]4-, [RhCl]3-, and [PdCl]2- mixture were incubated under acidic conditions, due to the electrostatic interaction between the positively charged functional groups on the inner and outer surfaces of INSAFs and the negative charges of the complex ions in solution, the complexes of [PtCl]4-, [RhCl]3-, and [PdCl]2- were adsorbed on the insulin fibers, which caused the absorption peaks of curves a, b, and c to significantly decrease or even disappear, as shown by curve e. After NaBH4 was added dropwise, the ions in the solution were reduced to form Pt-Rh-Pd alloy nanoparticles along INSAFs. The nanoparticles connected with each other and formed nanochains. It can be seen in the UV–vis test curve e that the absorption peak is missing at 260 nm, and the characteristic peak of insulin fiber at 279 nm appears again, which indicates that [PtCl]4-, [RhCl]3-, and [PdCl]2- in the solution are almost all reduced.

Figure 4

(a) Schematic illustration of the formation of Pt-Rh-Pd NCs. (b) UV–vis absorption spectra during the formation of Pt-Rh-Pd NCs.

Electrocatalytic Performance of Pt-Rh-Pd Nanochains

The electrochemical properties of Pt-Rh-Pd NCs were studied and analyzed using a three-electrode system. Cyclic voltammetry (CV) is a test method for measuring MOR (methanol oxidation reaction) performance in the electrochemically active region of Pt-Rh-Pd NCs. For comparison, the commercial Pt/C and Pt-Rh NCs (Figure S4) prepared under the same experimental condition were also tested. The curves in the two CV plots all have three distinct potential ranges: the hydrogen suction and desorption zone (−0.23 V to 0 V), electrical double layer zone (0–0.075 V), and oxide formation and reduction zone of Pt (0.075–1.15 V). As can be seen from Figure a, the hydrogen adsorption and stripping current of the ternary Pt-Rh-Pd NCs are the strongest. In evaluating the performance of electrocatalysts, electrochemical surface area (ECSA) is one of the important indicators for evaluating catalyst activity and usually obtained using a CV curve in N2 saturated solution.[44−46] By integrating the desorption peak region of hydrogen, the ECSA values of Pt-Rh-Pd NCs, Pt-Rh NCs, and commercial Pt/C catalysts are 67.21, 37.87, and 43.85 m2/g respectively. As shown in Figure b, the higher ECSA value for the ternary Pt-Rh-Pd NCs indicates that they have a higher specific surface area at the same mass and may contain the most active sites.

Figure 5

(a) CV curves and (b) ECSAs in 0.1 M HClO4 at a scan rate of 50 mV/s. (c) CV of methanol oxidation in 0.1 M HClO4 + 1 M CH3OH at a scan rate of 50 mV/s. (d) Bar graph with peak current density emphasizing MOR activity. (e) Chronoamperometry curve with a fixed potential of 0.6 V for different samples.

The structure and composition effects of the Pt-Rh-Pd NC and Pt-Rh NC catalysts on MOR were studied in N2-saturated 0.1 M HClO4 and 1.0 M methanol solutions by linear sweep voltammograms and compared with those of the commercial Pt/C catalysts. The scanning range is −0.21 to 1 V, and the sweep speed is 50 mV/s. The ordinate is unified with ECSA calibration. As can be seen from Figure c, the Pt-Rh-Pd NCs exhibit a higher current density than commercial Pt/C over the entire scanning range. The peak forward current density of the three samples in methanol oxidation is Pt-Rh-Pd NCs > Pt-Rh NCs > commercial Pt/C. In addition, compared with commercial Pt/C, the positive scanning peaks of Pt-Rh-Pd NCs and Pt-Rh NCs in methanol perchlorate are shifted by 110 and 50 mV, respectively. The results show that their ability to catalyze the oxidation of methanol is better than commercially available Pt/C. Figure d is a histogram of the current density corresponding to the front sweep of each sample. From these data, we measured a specific activity of 1.48 mA cm–2 for our best MOR catalyst tested, namely, Pt-Rh-Pd NCs, which is 1.4- and 1.7-fold higher than those of Pt-Rh NCs (1.07 mA cm–2) and commercial Pt/C (0.89 mA cm–2).

To further study the catalytic performance of the catalyst, we have done a chronoamperometry curve test to explore the stability. The fixed potential is 0.6 V, and the measurement time is 50 min. As shown in Figure e, the three curves present different current decay behaviors with the extension of test time. Among them, the current density of the ternary Pt-Rh-Pd catalyst and binary Pt-Rh catalyst decreased less than that of commercial Pt/C. In particular, the ternary Pt-Rh-Pd NC catalyst shows the smallest decrease in current density. At 50 min, the current density of commercial Pt/C is almost zero, whereas that of the Pt-Rh-Pd catalyst still maintains at 0.5 mA cm–2, indicating that the ternary Pt-Rh-Pd catalyst has a more stable persistence of methanol oxidation.

Usually, there will be toxic small molecules such as CO generated in the process of methanol catalysis, which easily adhere to the catalyst surface and affect the activity of the catalyst. The ratio of forward sweep peak current density (Jf) to anti-sweep peak current density (Jb) could be used to estimate the magnitude of anti-poisoning ability. The higher the ratio, the greater the ability of the catalyst surface to remove CO. In this paper, Pt-Rh-Pd NCs have higher resistance to CO poisoning and catalytic activity than some reported Pt-based ternary alloy nanoparticles compared to catalysts.[47−50] From Table S1, we can see that the Jf/Jb value of Pt-Rh-Pd nanochains is the highest among the three catalysts, indicating that it can completely oxidize the methanol molecules to CO2 and, to a certain extent, also explaining the superiority of its catalytic activity. Present research work demonstrates a new clue for the preparation of multicomponent metal nanostructures and catalytic composite materials with broad application prospects.[51−63]

Conclusions

In summary, Pt-Rh-Pd NCs with uniform particle size were successfully prepared by using insulin fibers as templates. TEM, SAED, HRTEM, XRD, and other characterization methods were used to characterize the morphology and structure, which proved that a polycrystalline Pt-Rh-Pd alloy was obtained. The catalytic performance of methanol oxidation was studied by an electrochemical test and compared with commercially available Pt/C. Tests show that the peak current density of Pt-Rh-Pd NCs during the oxidation of methanol is 1.48 mA cm–2, which is much higher than those of Pt-Rh NCs (1.07 mA cm–2) and commercial Pt/C (0.89 mA cm–2). Also, it has good stability. In short, the prepared Pt-Rh-Pd NCs by using insulin fibers as templates under mild and non-polluting conditions make full use of the advantages of these three metals and greatly improve catalytic performance. This method may be applied to the preparation of other multicomponent catalysts with excellent performance.

Experimental Section

Materials

All the chemicals used in this work are of analytical grade and used without further purification. Bovine insulin (99%) was purchased from Sigma Aldrich. PtCl4 (AR), RhCl3·3H2O (AR), and K2PdCl4 (AR) were purchased from Beijing Research Institute for Nonferrous Metals. Sodium borohydride (NaBH4) was purchased from Tianjin Chemical Reagent Factory. The ultrapure DNase/RNase-free distilled water (UPD water) was used in all synthesis processes.

Preparation of the Insulin Fibers (INSAFs)

In a typical process, 1.1 mg of bovine insulin powder was dissolved in 1 mL of hydrochloric acid solution with a pH of 1.7 and a concentration of 25 mM and heated in a water bath at 70 °C for 9 h to obtain insulin fibers.

Preparation of the Pt-Rh Nanochains (Pt-Rh NCs)

PtCl4 (5 mM, 40 μL) and RhCl3 (5 mM, 40 μL) were added to the INSAF solution (1 mg/mL, 200 μL), and the mixtures were gently co-incubated overnight. The solution was followed by reduction by putting a drop (30 μL) of NaBH4 (10 mM, 120 μL) solution with an interval of 5 min. The mixtures were further gently stirred at room temperature for 10 h. Finally, the Pt-Rh NCs were obtained. This method is simple and reproducible.

Preparation of the Pt-Rh-Pd Nanochains (Pt-Rh-Pd NCs)

PtCl4 (5 mM, 20 μL), RhCl3 (5 Mm, 20 μL), and K2PdCl6 (5 Mm, 20 μL) were added to the INSAF solution (1 mg/mL, 200 μL), and the mixtures were gently co-incubated at room temperature for 2 h. At this time, the color of the incubation system with insulin fibers in the metal salt solution is light yellow. Subsequently, the solution was followed by reduction by putting a drop (30 μL) of NaBH4 (10 mM, 90 μL) solution with an interval of 5 min. The color of the solution gradually darkened and finally became black. The mixtures were further gently stirred for 2 h. Finally, the Pt-Rh-Pd NCs were obtained.

Characterization

In this experiment, the shape and size of the product were observed by transmission electron microscopy using an HT-7700 (Hitachi, Japan; acceleration voltage of 100 kV). HRTEM (high-resolution transmission electron microscopy) and SAED (selected area electron diffraction) (model JEM-2010) were conducted to analyze the crystal structure of the sample with an acceleration voltage setting of 200 kV. The EDX analyzer (model no.: Apollo XLT 4067) on the HT-7700 TEM monitors the composition of the sample. The X-ray powder diffraction (XRD) pattern was obtained on a D-max-2500/PC instrument to characterize the material composition of the product. The incident wavelength (λ) of the test was 0.15410 nm in steps of 2°/min from 10° to 90°. Ultraviolet–visible absorption spectra were measured with a UV-2550 double monochromator UV spectrophotometer (Shimadzu Corporation). The optical path was set to 10 nm, and 200–500 nm was set as the absorption spectrum of the test range. In this paper, QUANTERA-II SXM (Japan Ulvac-Phi Company) X-ray photoelectron spectrometer (XPS) was used to test the product surface atomic valence and elemental composition and get the relevant metal photoelectron spectra. STEM is a multipurpose microscope that contains both transmission electron microscopy and scanning electron microscopy. The electron beam was focused on the product surface, and a quick scan was performed on the product surface to obtain a product image.

Electrochemical Characterization

The electrochemical experiments were performed with a CHI-760E electrochemical workstation (Chenhua, Shanghai). A conventional three-electrode cell was used. A glassy carbon (GC) disk electrode (3 mm in diameter) served as the substrate for the support. Nine microliters of catalyst ink was pipetted out onto a mirror finish surface of GC for a working electrode after evaporation of solvent under an IR lamp. A saturated calomel electrode (SCE) and a large-area Pt plate were used as the reference and counter electrodes, respectively. The cyclic voltammograms (CVs) were recorded in N2-saturated 0.1 M HClO4 at a scan rate of 50 mV/s between −0.23 and 1.15 V. The electrochemically active surface area (ECSA) of each sample was estimated using the hydrogen desorption region in CV measurements carried out in fresh 0.1 M HClO4 solutions with a sweep rate of 50 mV/s. The methanol oxidation reaction and chronoamperometric experiments of each sample were investigated by CV studies in N2-saturated 0.1 M HClO4 + 1 M CH3OH solution, and the potential was scanned from −0.21 to 1 V at a scan rate of 50 mV/s.