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Article

Pd-M (M = Ni, Co) Bimetallic Catalysts with Tunable Composition for Highly Efficient Electrochemical Formic Acid Oxidation

by
Qingwei Ding
1,
Qing Luo
1,*,
Liang Lin
1,
Tianlun Yang
1,
Xingping Fu
2,
Laisen Wang
1 and
Caixia Lei
3,*
1
College of Materials, Xiamen University, Xiamen 361005, China
2
Fujian Provincial Key Laboratory of Eco-Inductrial Green Technology, Wuyi University, Wuyishan 354300, China
3
Xiamen Key Laboratory of Power Metallurgy Technology and Advanced Materials, School of Materials Science and Engineering, Xiamen University of Technology, Xiamen 361024, China
*
Authors to whom correspondence should be addressed.
Processes 2023, 11(6), 1789; https://doi.org/10.3390/pr11061789
Submission received: 22 May 2023 / Revised: 6 June 2023 / Accepted: 9 June 2023 / Published: 12 June 2023
(This article belongs to the Special Issue Design and Synthesis of Metal-Organic Framework Materials)
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Abstract

:
Bimetallic Pd-based catalysts for formic acid oxidation (FAO) are one of the most promising anode materials for the next generation of direct formic acid fuel cells (DFAFC). It is imperative to develop a simple strategy for preparing efficient, stable, and clean nanoparticle catalysts. Herein, we prepared a series of Pd, PdNi, and PdCo nanoparticle catalysts using the nanoparticle beam composite deposition system, which revealed good catalytic activity and stability in the process of FAO. The incorporation of Ni or Co prevents the adsorption of active intermediates and the accumulation of toxic intermediates in the process of FAO. Therefore, more Pd active centers can be used to decompose formic acid directly by dehydrogenation. The results indicate that PdNi-2 (Pd0.9Ni0.1) and PdCo-3 (Pd0.89Co0.11) catalysts exhibit the optimal catalytic performance, with the mass activity of 1491.5 A g−1Pd and 1401.7 A g−1Pd, respectively, which is 2.1 and 2 times that of the pure Pd sample. By optimizing the rate of Pd to transition metal M (Ni, Co), a high-performance Pd-based catalyst was obtained through their synergistic effect, which provides a new approach for designing efficient anode catalysts for DFAFCs.

Graphical Abstract

1. Introduction

Direct formic acid fuel cell (DFAFC) is known as an ideal handheld and mobile power source and attracts significant research attention due to its advantages of high energy density, simple power system integration, limited fuel crossover, and environmental friendliness [1,2]. The development of robust electrocatalysts for anodic formic acid oxidation (FAO) is critical to the manufacture of commercially viable DFAFCs.
Palladium (Pd) is an effective anodic catalyst due to the preferred direct dehydrogenation pathway, but it is expensive and needs to reduce Pd usage and further enhance its catalytic performance [3]. One benchmarking strategy is to introduce transition metals (Fe, Co, Ni, Cu, Au, Ru, etc.) into Pd to adjust their geometric and electronic structures [4,5,6,7]. The enhancement of the electrocatalytic performance of the alloy components is on account of the electronic effect of the electronic interaction between the Pd atoms and the doped metals, which causes the reduction of the surface binding energy between the adsorbed particles and the metal nanoparticles [8]. In addition to the approach of transition metal alloying, preparing hollow/ultra-thin/porous structures of Pd nanoparticles with high surface area is another effective method to enhance their catalytic activity [9,10]. For example, Liu et al. developed a hollow Pd-Ag bimetallic nanostructure with a large active surface area by incorporating silver atoms into the Pd lattice [11]. Zhang et al. synthesized highly perforated ultra-thin Pd nanosheets via a restricted growth method, and their unique structure revealed excellent catalytic activity and durability [12]. Zhang et al. prepared bidimensional curved nanoporous Pd-Cu catalysts using a simple one-pot reaction, which have efficient catalytic activity for FAO [13]. However, these preparation methods inevitably involve the use of surfactants and stabilizers. The introduction of surfactants and stabilizers can regulate the size and dispersion of Pd nanocatalysts, and these surfactant molecules tend to adsorb on the surface of the nanoparticles, greatly hindering the adsorption of reactants and thus reducing the catalytic activity [14]. Therefore, effectively regulating the size and dispersion of Pd nanoparticles without incorporating any stabilizer or surfactant is an ideal method to enhance the electrocatalytic performance.
In this work, we prepared PdCo and PdNi alloy catalysts with small particle sizes and uniform dispersion via the nanoparticle beam composite deposition system. The activity and durability of PdXNi100-X and PdYCo100-Y with different metal ratios for FAO were investigated. The results displayed that the nanocatalysts had the optimum activity and stability when the atomic ratios of Ni and Co were 10% and 11%, respectively. This paper provides a new approach to the design of high-performance anodic catalysts for DFAFCs.

2. Experimental Procedure

2.1. Synthesis of PdNi and PdCo Nanocatalysts

PdNi and PdCo catalysts with different atomic percentages were prepared using the nanoparticle beam composite deposition system (Figure S1), with two independent DC power sources controlling the output power of the upper and lower targets in the sputtering chamber, respectively. Before the experiment began, the Pd target (purity 99.99%), Ni target (purity 99.99%), Co target (purity 99.99%), and substrate materials (glass slide, silicon slice, Mo net, and glassy carbon electrode) should all be cleaned and dried. When the vacuum of the entire system was lower than 5.0 × 10−4 Pa, we injected argon gas at a certain flow rate of 248 standard cubic centimeters per minute (sccm), turned on the DC power supply, and pre-sputtered the target materials for 1–2 h. While preparing the samples, high-density Pd and transition metal atomic vapor were sputtered onto the target surface. These atoms then collided with argon molecules, cooled down, and nucleated to form alloy nanoclusters. Finally, due to the pressure difference, the alloy nanoclusters softly landed on the substrate of the deposition chamber, forming a catalyst film. The two targets were co-sputtered by controlling the DC output powers of the upper and lower targets respectively. Because the loss rate of the target was different under different sputtering power, the atomic percentages of PdNi or PdCo catalysts were different. During the experiment, the power of the Pd target was fixed at 150 W, and the sputtering power of the Ni target or Co target was set at 0, 120, 140, 160, 180, and 200 W respectively. A series of PdM (M = Ni, Co) bimetallic nanoparticle catalysts with different transition metal content were synthesized.

2.2. Characterizations of PdNi and PdCo Nanocatalysts

The crystal structures of the catalysts were characterized on X-ray diffraction (XRD, Rigaku SmartLab SE, Tokyo, Japan) using Cu Kα radiation (λ = 1.5406 Å). The working current and voltage are 30 mA and 40 kV respectively. The electronic states and surface elemental compositions of the samples were determined on an X-ray photoelectron spectrometer (XPS, Thermo Scientific K-Alpha, Waltham, MA, USA) with an Al Kα radiator (1486.6 eV). The structural morphology and elemental quantification of the catalysts were investigated via scanning electron microscopy (SEM, Zeiss Sigma300, Oberkochen, BW, Germany) and energy-dispersive spectroscopy (EDS, Smartedx, Oberkochen, BW, Germany) operating at 3 and 15 kV respectively. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images were analyzed via a field emission transmission electron microscope (FE-TEM, Talos F200s, Hillsboro, OR, USA) operating at 200 kV.

2.3. Electrocatalytic Performance Measurements

The electrochemical experiments proceeded on a traditional three-electrode cell consisting of a platinum counter electrode, a glassy carbon working electrode modified with the catalyst sample, and a saturated Ag/AgCl reference electrode via a CHI660E electrochemical instrument (Austin, TX, USA). In the CO stripping measurement, CO was bubbled into a 0.5 M H2SO4 solution for 20 min, during which CO molecules were completely adsorbed and occupied the active centers on the catalyst surface. Afterward, N2 was bubbled to remove excess CO and O2 from the solution for another 20 min. In the end, the CO stripping voltammetry curves were recorded at 50 mV s−1 from −0.22 to 1.0 V. To test the catalytic activity of the sample in formic acid solution, cyclic voltammetry (CV) was performed in the 0.5 M H2SO4 + 0.5 M HCOOH solution. N2 was bubbled to remove O2 and other impurities from the solution for 20 min, and then the CV was conducted at 50 mV s−1 from −0.22 to 1.0 V. All measurements proceeded at room temperature (25 ± 1 °C).

3. Results and Discussion

3.1. EDS and XRD Characterisation

The atomic percentages of PdNi and PdCo catalysts were obtained by EDS (Table 1). For convenience, the Pd97Co3, Pd94Co6, Pd89Co11, Pd82Co18, and Pd70Co30 catalysts were named PdCo-1, PdCo-2, PdCo-3, PdCo-4, and PdCo-5, respectively. The PdNi catalysts were also named this way. Figure 1a,b show the XRD spectra of Pd, PdNi, and PdCo catalysts. For the Pd sample, the observed four peaks at 40°, 46°, 68°, and 82° corresponded to the reflection faces of the Pd face-centered cubic crystalline (JCPDS No. 65-2867). The PdNi and PdCo catalysts also exhibited a characteristic Pd face-centered cubic crystalline. There is no separation peak observed for the metallic phases of Ni, Co, and their oxide phases, indicating alloying has occurred. As the Ni and Co content increases, the Pd diffraction peak positions of PdNi and PdCo catalysts move slightly towards higher 2θ angles. The phenomenon can be ascribed to lattice distortion [15]. The crystal structures of Ni and Co are face-centered cubic and body-centered cubic, respectively. When they enter the Pd matrix, the lattice distortion degree of the alloy varies due to the different atomic radius, solid solubility, and electron cloud density, resulting in different degrees of XRD diffraction peak angle deviation. Eleven catalysts were prepared, and the performance of each catalyst was introduced using Pd, PdNi-2, and PdCo-3 catalysts as examples. The average nanoparticle sizes of Pd, PdNi-2, and PdCo-3 samples were calculated using the Debye–Scheler formula, which were 5.4, 4.0, and 3.8 nm, respectively.

3.2. XPS Characterisation

The electronic states and surface elemental compositions of the prepared Pd, PdNi-2, and PdCo-3 samples were analyzed via XPS. Figure S2 shows that C, O, and Pd were detected in all samples, and Ni and Co were also detected in PdNi-2 and PdCo-3 catalysts, respectively. As shown in Figure 2, the Pd 3d spectrum of pure Pd sample contained two peaks, Pd 3d3/2 (high energy band) and Pd 3d5/2 (low energy band), which were further divided into two types of peaks [16]. The peaks at 340.9 and 335.6 eV separately belong to Pd 3d3/2 and Pd 3d5/2 of metal Pd. The weaker bimodal peaks at 342.8 and 336.9 eV separately correspond to the Pd 3d3/2 and Pd 3d5/2 of Pd Ⅱ. [17]. Notably, the binding energy of Pd 3d in PdNi-2 and PdCo-3 catalysts moved negatively by 0.2 and 0.3 eV, respectively, relative to that of the Pd sample. This indicates that electrons in PdNi and PdCo alloys transferred from Ni or Co to Pd, as Pd has a higher electronegativity (2.20) than Ni (1.91) and Co (1.88) [18,19]. In accordance with the d-band theory, the transfer of electrons to Pd will cause the d-band center to shift towards a more negative direction, leading to a bond weakening between Pd and the reaction intermediate [20]. The variation in binding energy is a result of the partial shift of electrons in the alloy, indicating that the alloy is formed between the metallic components [21].

3.3. SEM Characterisation

The surface structural morphology of Pd, PdNi-2, and PdCo-3 catalysts was observed by SEM. Figure 3 shows that all three catalysts are granular films formed by the accumulation of nanoclusters, exhibiting a three-dimensional porous structure. This highly porous structure and huge voids endow the catalysts with large specific surface areas and electrochemical activity [22]. By comparing the high-resolution scanning images (Figure 3b,d,f), it is concluded that the nanoclusters formed by co-sputtering Ni or Co with Pd have smaller particle sizes and more uniform distributions, which is attributed to the reduced aggregation of the catalysts. This may be due to the change in the alloy structure, which alters the electronic density and structure of Pd nanoclusters, thereby weakening their aggregation [15].

3.4. TEM Characterisation

Figure S3a and Figure 4a,h show the TEM diagrams and nanoparticle size distributions of Pd, PdNi-2, and PdCo-3 catalysts. It is observed that the dispersion of PdNi and PdCo particles is better than that of Pd nanoparticles. By selecting the diameters of over 200 nanoparticles in the TEM diagrams, nanoparticle size distributions were obtained. The average nanoparticle sizes of Pd, PdNi-2, and PdCo-3 samples were 5.7, 4.4, and 4.1 nm, respectively. The consequences show that alloying Pd with Ni or Co can reduce the agglomeration phenomenon, resulting in more uniform dispersion of nanoparticles and smaller particle sizes. Figure S3b and Figure 4b,i display the high-resolution TEM (HRTEM) diagrams of Pd, PdNi, and PdCo samples. The obtained lattice spacings were 0.226, 0.224, and 0.225 nm, which are consistent with the Pd (111) planes. Figure S3c exhibits the SAED analysis of the Pd catalyst, which corresponds to crystal planes (111), (200), (220), and (311) of Pd from the inner ring to the outer ring, suggesting that the catalyst has a polycrystalline structure. From Figure 4c,j, it can be seen that the SAED patterns of PdNi-2 and PdCo-3 catalysts were similar to that of Pd catalyst, and no diffraction peaks of Ni or Co were detected, further confirming the formation of PdNi and PdCo alloys. Figure S3f and Figure 4d,k exhibit the Fast Fourier Transform (FFT) analyses of Pd, PdNi-2, and PdCo-3 catalysts, with bright and clear diffraction patterns exhibiting six-fold rotational symmetry, indicating good crystallinity of the samples and the dominant crystal plane being the Pd (111) plane [23]. Besides, the elemental distributions of Pd, Ni, and Co were studied via energy-dispersive X-ray (EDX) and high-angle annular dark-field scanning TEM (HAADF-STEM). The representative HAADF-STEM analyses of Pd, PdNi-2, and PdCo-3 catalysts and the corresponding element distribution diagrams (Figure S3d,e and Figure 4e–g,l–n) clearly reveal that the corresponding distributions of Pd (yellow), Ni (blue) and Co (green) elements were completely consistent. According to the previous report, the size and dispersion of nanocatalysts have a great influence on electrochemical performance [24]. This is attributed to the fact that the smaller the nanoparticle size, the higher its chemical potential, and the larger the rate of surface area to volume [25].

3.5. Electrochemical Performance

Pd-based nanocatalysts are usually deemed as excessively effective electrocatalysts for FAO [26]. The incorporation of transition metal into Pd can efficiently improve its electrocatalytic performance, including activity, durability, and anti-CO poisoning [27]. To estimate the electrocatalytic performance of Pd, PdNi, and PdCo samples for FAO in an acidic solution, the CV curves were studied in an N2-saturated sulfuric acid solution and a mixed solution of sulfuric acid and formic acid, respectively. Figures S4 and S5 show the 1–50 CV scans of pure Pd, PdNi, and PdCo catalysts scanned in N2-saturated sulfuric acid solution, respectively. All CV scans exhibit distinct hydrogen adsorption and desorption peaks from −0.22 to 0.1 V, with the 0.1–0.3 V region corresponding to the double-layer capacitance area. The Pd surface began to oxidize around 0.5 V, and the reduction of Pd oxides was seen from 0.4 to 0.6 V during negative scanning. Figure 5a,b display first CV scans of pure Pd, PdNi, and PdCo catalysts in 0.5 M H2SO4 solution. Compared with the pure Pd sample, the PdNi samples exhibited lower current density for hydrogen adsorption and desorption, suggesting that the hydrogen adsorption/desorption on the catalysts was inhibited [28]. The first anode peak current density of dehydrogenation reduced in the order of Pd, PdNi-1, PdNi-2, PdNi-3, PdNi4, and PdNi-5, indicating that the increase in Ni content causes the Pd d-band center to move down and the adsorption energy to weaken [29]. Moreover, with the increase of transition metal content, the first anodic dehydrogenation peak potentials of PdNi and PdCo catalysts became more negative, indicating that hydrogen adsorption and desorption occur at lower potentials. Besides, the oxidation onset potential of the Pd sample was significantly higher than those of the PdNi and PdCo catalysts, suggesting that the latter had higher electrochemical activity and a larger electrochemical active surface area (ECSA) [30]. The negative shift of the Pd oxides reduction peak potentials on PdNi and PdCo catalysts is beneficial for the catalysts to provide -OH at lower potentials, promoting electrocatalytic oxidation reactions [31].
Figure 5c,d present the maximum current density of Pd oxides reduction peaks using Pd, PdNi, and PdCo catalysts in sulfuric acid solution in the 1st and 50th circles, respectively. As the content of transition metal Ni or Co increased, different PdNi or PdCo catalysts exhibited similar “volcano” curves in the maximum peak current density in sulfuric acid solution, indicating that the optimal catalytic performance can be obtained by regulating the atomic percentage of transition metal [32]. The highest peak current density for PdNi-2 and PdCo-3 catalysts is at the top of the “volcano” curve, which was 2.16 and 2.51 mA cm−2, respectively, whereas that of the Pd catalyst was only 1.35 mA cm−2. The incorporation of transition metal Ni or Co enhances the possibility of Pd oxidation, contributing to the formation of more Pd oxides. The maximum peak current density of the Pd sample decreases to 0.40 mA cm−2 after 50 scans, while that of PdNi-2 and PdCo-3 catalysts was still maintained at 1.18 and 1.27 mA cm−2, respectively, which is 3.0 and 3.2 times higher than Pd catalyst, respectively, suggesting that the durability of catalysts doped with transition metal Ni or Co was greatly improved. The improved durability of the catalysts may be attributed to the stabilizing effect of transition metals, which helps to prevent undesired agglomeration of nanoparticles [33]. This is evident in the SEM images, where it can be observed that the agglomeration of the alloy catalysts was reduced, thereby enhancing their durability.
Figures S6 and S7 show the 1–50 CV scans of pure Pd, PdNi, and PdCo samples in an N2-saturated mixed solution of sulfuric acid and formic acid. During the forward scanning process, the two peaks of FAO were located at around 0.4 and 0.6 V, corresponding to the two pathways, namely the direct pathway (dehydrogenation pathway) and the indirect pathway (dehydration pathway) [34]. For the direct pathway, the absence of toxic intermediate COads prevented catalyst deactivation, allowing the catalyst to maintain high catalytic activity, leading to a higher peak current density of FAO. As the scan potential increases, the electrocatalytic process of a Pd-based catalyst for FAO is the same as that of a Pt-based catalyst. The COads on the catalyst surface can be promptly removed by oxidation, allowing the catalytic performance to maintain a higher peak current density. However, as the scanning potential arrives at the Pd oxidation potential, the chemical process of FAO weakens. In the process of potential reverse scanning, the oxide on the Pd surface is reduced, exposing the Pd active centers and causing a sharp increase in the current density of the FAO peak [35]. The peaks of the PdNi and PdCo samples were sharper than that of the Pd sample, suggesting the presence of more active centers and better catalytic performance for FAO. Figure 6a,b show the first CV scans of pure Pd, PdNi, and PdCo samples in the mixed solution of sulfuric acid and formic acid. The FAO peak current density of PdNi and PdCo samples during negative scanning was much higher than the peak current density of the Pd sample, suggesting that the incorporation of transition metal Ni or Co enhanced the possibility of Pd oxidation, generating more oxides of Pd, which further proves the view in sulfuric acid solution. In addition, the reverse scanning curve of the low potential region was nearly identical to the forward scanning curve, suggesting the Pd sample was not deactivated by the toxic intermediate.
Figure 6c,d show the maximum peak current density in the 1st and 50th circles of FAO over Pd, PdNi, and PdCo catalysts in the mixed solution of sulfuric acid and formic acid. The consequences are similar to those in sulfuric acid solution, and “volcano” curves are also observed. According to the maximum peak current density during the first forward scanning of FAO, the activity of FAO on PdNi catalysts followed an order of PdNi-2 > PdNi-3 > PdNi-1 > PdNi-4 > PdNi-5 > Pd. The maximum current density of PdNi-2 was 80.67 mA cm−2, which is around 1.92 times that of the pure Pd sample. Similarly, the activity of FAO on PdCo catalysts followed an order of PdCo-3 > PdCo-4 > PdCo-2 > PdCo-1 > PdCo-5 > Pd, and the maximum current density of the PdCo-3 catalyst was 74.97 mA cm−2, which is around 1.79 times that of pure Pd sample. The incorporation of Ni or Co suppresses the adsorption of reactive intermediates during the FAO process, directly oxidizing them to CO2, and preventing the accumulation of toxic intermediates [28,36]. It is noteworthy that as the enhance of Ni content, the initial potentials of PdNi catalysts gradually decreased, especially for PdNi-5, whose initial potential dropped to −0.143 V, much lower than that of the Pd sample (−0.109 V), suggesting PdNi catalysts are more prone to FAO reaction than pure Pd catalyst. These improvements may be due to Pd altering its electronic structure by forming an alloy with Ni or Co, which helps to remove intermediate species on Pd, thereby preventing the adsorption of formats [22]. The maximum peak current density of the Pd sample decreased to 15.51 mA cm−2 after 50 scans, while that of PdNi-2 and PdCo-3 catalysts was still maintained at 40.81 and 52.26 mA cm−2, which is 2.6 and 3.4 times that of Pd sample. The decrease of the stability of the samples in sulfuric acid solution is mostly due to the Pd dissolution in an acidic medium, while in the mixed solution of sulfuric acid and formic acid, there is another important reason: the active centers of the Pd surface are slowly deactivated by the toxic intermediate COads [37]. The improvement in the catalytic activity of PdNi/PdCo samples may be due to the incorporation of Ni/Co, which results in the generation of Ni/Co hydroxides on the sample surface, promoting the oxidation and desorption of intermediate species, thereby reproducing the Pd active centers for FAO [38]. Excessive Ni or Co content on the sample surface can hinder the formation of Pd active centers [39]. Therefore, adding a suitable amount of Ni or Co can significantly increase the electrocatalytic activity of the sample for FAO.
The ECSA directly affects the intrinsic catalytic performance of the sample. As is well known, Pd is prone to adsorb hydrogen, which leads to a higher ECSA value, so using CO stripping voltammetry to calculate the ECSAs of Pd samples is necessary [40]. Figure 7 reveals the CO stripping voltammetry curves of Pd, PdNi, and PdCo samples in sulfuric acid solution. The CO stripping peak intensity of PdNi and PdCo samples was obviously higher than that of pure Pd samples, suggesting PdNi and PdCo samples have larger ECSAs and higher catalytic activity. In comparison to the Pd sample, PdNi and PdCo catalysts had lower CO oxidation potentials, and the oxidation peak potential of COads decreased with increasing Ni or Co content. Ni and Co are both aerobic and provide oxygen species for CO oxidation through a bifunctional catalytic mechanism. The presence of Ni or Co enhances FAO by providing oxophilic sites for the adsorption of -OH, which reflects the role of these transition metals in weakening the adsorption bond of CO on Pd active centers [41,42].
Figure 8a,d show the ECSAs of Pd, PdNi, and PdCo samples. It can be observed that the ECSAs of alloy samples improve with the enhancement of transition metal Ni or Co content. The ECSA value of the PdCo-5 catalyst reached 38.8 m2 g−1Pd, which is around 2.3 times that of the pure Pd sample. The specific activity and mass activity were normalized by the ECSAs and the Pd quality, respectively, which reveals the intrinsic activity of the electrocatalysts and the practical utilization of Pd atoms. By improving the utilization efficiency of precious metal Pd, the cost of catalysts can be lowered. The mass activity and specific activity of the PdNi-2 sample were the highest among all PdNi catalysts, which were 1491.5 A g−1Pd and 60.0 A m−2Pd, respectively, which are 2.1 and 1.5 times that of pure Pd sample (Figure 8b,c). Similarly, the mass activity and specific activity of the PdCo-3 sample were the highest among all PdCo catalysts, reaching 1401.7 A g−1Pd and 46.5 A m−2Pd, respectively, which are 2 and 1.1 times that of pure Pd sample (Figure 8e,f). The results suggest that the appropriate addition of Ni or Co can promote the activity of Pd-based samples, similar to the structure of a “volcano”. The improvement of the activity of PdNi and PdCo samples can be ascribed to several factors. Firstly, the larger ECSAs of PdNi and PdCo catalysts and higher utilization of Pd atoms due to their smaller particle sizes and more uniform distributions increases the contact area between the active centers on the Pd surface and the solution. Secondly, the incorporation of Ni or Co prevents the adsorption of active intermediates in the process of FAO, avoiding the accumulation of toxic intermediates, and thereby allowing more Pd active sites to decompose formic acid by the direct pathway. Finally, the synergistic effect of PdNi or PdCo will appropriately lower the Pd d-band center and adsorb less active intermediates, thus improving the efficiency of catalytic reactions [43,44].

4. Conclusions

In summary, we prepared PdNi and PdCo nanoparticles catalysts with different atomic percentages for FAO using the nanoparticle beam composite deposition system, aiming at reducing the content of precious metal Pd and improving the catalytic performance. Adding the transition metal Ni or Co to Pd not only improves the utilization of Pd atoms, but also avoids the accumulation of toxic intermediates. More importantly, the synergistic effect of PdNi or PdCo reduces the Pd d-band center and adsorbs less active intermediates, thereby improving the efficiency of the catalytic reactions. PdNi-2 and PdCo-3 catalysts had the best catalytic performance, whose mass activity was 2.1 and 2 times that of pure Pd catalysts, respectively. The maximum peak current density of the pure Pd sample been dropped to 15.51 mA cm−2 after 50 scans, while PdNi-2 and PdCo-3 catalysts were maintained at 40.81 and 52.26 mA cm−2. This work contributes to developing efficient Pd-based catalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr11061789/s1, Figure S1: Schematic diagram of the nanoparticle beam composite deposition system; Figure S2: XPS survey spectra of (a) Pd, (b) PdNi-2 and (c) PdCo-3 catalysts; Figure S3: (a) TEM analysis and particle size histogram, (b) HRTEM analysis and local magnification, (c) SAED analysis, (d) HAADF-STEM analysis, (e) elemental mapping image, (f) FFT analysis of Pd catalyst; Figure S4: Repeated 50 cycles CV curves of (a) Pd, (b) PdNi-1, (c) PdNi-2, (d) PdNi-3, (e) PdNi-4 and (f) PdNi-5 electrocatalysts tested in 0.5 M H2SO4 solution (scan rate = 50 mV s−1); Figure S5: Repeated 50 cycles CV curves of (a) Pd, (b) PdCo-1, (c) PdCo-2, (d) PdCo-3, (e) PdCo-4 and (f) PdCo-5 electrocatalysts tested in 0.5 M H2SO4 solution (scan rate = 50 mV s−1); Figure S6: Repeated 50 cycles CV curves of (a) Pd, (b) PdNi-1, (c) PdNi-2, (d) PdNi-3, (e) PdNi-4 and (f) PdNi-5 electrocatalysts tested in 0.5 M H2SO4 + 0.5 M HCOOH solution (scan rate = 50 mV s−1); Figure S7: Repeated 50 cycles CV curves of (a) Pd, (b) PdCo-1, (c) PdCo-2, (d) PdCo-3, (e) PdCo-4 and (f) PdCo-5 electrocatalysts tested in 0.5 M H2SO4 + 0.5 M HCOOH solution (scan rate = 50 mV s−1).

Author Contributions

Conceptualization, Q.D. and Q.L.; methodology, Q.D. and Q.L.; software, Q.D.; validation, Q.L.; formal analysis, Q.D.; investigation, Q.D.; resources, Q.L.; data curation, Q.D.; writing—original draft preparation, Q.D.; writing—review and editing, Q.D., Q.L. and L.L.; visualization, T.Y.; supervision, X.F.; project administration, Q.L.; funding acquisition, Q.L., L.W. and C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was sponsored by the Natural Science Foundation of Fujian Province (Grant No. 2022J011265, No.2022J01042), the Natural Science Foundation of Guangxi Zhuang Autonomous Region (Grant Number: 2020GXNSFAA238018), the Open Fund of Fujian Provincial Key Laboratory of Eco-Inductrial Green Technology in Wuyi University (WYKF-EIGT2021-6).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All the data has been included in the text.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns of (a) Pd, PdNi-1, PdNi-2, PdNi-3, PdNi-4, PdNi-5 catalysts and (b) Pd, PdCo-1, PdCo-2, PdCo-3, PdCo-4, PdCo-5 catalysts.
Figure 1. XRD patterns of (a) Pd, PdNi-1, PdNi-2, PdNi-3, PdNi-4, PdNi-5 catalysts and (b) Pd, PdCo-1, PdCo-2, PdCo-3, PdCo-4, PdCo-5 catalysts.
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Figure 2. Pd 3d XPS spectra of (a) Pd, (b) PdNi-2, and (c) PdCo-3 catalysts.
Figure 2. Pd 3d XPS spectra of (a) Pd, (b) PdNi-2, and (c) PdCo-3 catalysts.
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Figure 3. Low- and high-magnification SEM images of (a,b) Pd, (c,d) PdNi-2, (e,f) PdCo-3 catalysts.
Figure 3. Low- and high-magnification SEM images of (a,b) Pd, (c,d) PdNi-2, (e,f) PdCo-3 catalysts.
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Figure 4. (a) TEM analysis and particle size histogram, (b) HRTEM analysis and local magnification, (c) SAED analysis, (d) FFT analysis, (e) HAADF-STEM analysis, (f,g) elemental mapping images of PdNi-2 catalyst; (h) TEM analysis and particle size histogram, (i) HRTEM analysis and local magnification, (j) SAED analysis, (k) FFT analysis, (l) HAADF-STEM analysis, (m,n) elemental mapping images of PdCo-3 catalyst.
Figure 4. (a) TEM analysis and particle size histogram, (b) HRTEM analysis and local magnification, (c) SAED analysis, (d) FFT analysis, (e) HAADF-STEM analysis, (f,g) elemental mapping images of PdNi-2 catalyst; (h) TEM analysis and particle size histogram, (i) HRTEM analysis and local magnification, (j) SAED analysis, (k) FFT analysis, (l) HAADF-STEM analysis, (m,n) elemental mapping images of PdCo-3 catalyst.
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Figure 5. First cycle CV scans of Pd, (a) PdNi, and (b) PdCo electrocatalysts tested in 0.5 M H2SO4 solution; maximum peak current density of Pd, (c) PdNi and (d) PdCo catalysts for the reduction of Pd oxides in 0.5 M H2SO4 solution (scan rate = 50 mV s−1).
Figure 5. First cycle CV scans of Pd, (a) PdNi, and (b) PdCo electrocatalysts tested in 0.5 M H2SO4 solution; maximum peak current density of Pd, (c) PdNi and (d) PdCo catalysts for the reduction of Pd oxides in 0.5 M H2SO4 solution (scan rate = 50 mV s−1).
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Figure 6. First cycle CV scans of Pd, (a) PdNi, and (b) PdCo electrocatalysts tested in 0.5 M H2SO4 + 0.5 M HCOOH solution; maximum peak current density of Pd, (c) PdNi and (d) PdCo catalysts for the electrooxidation of formic acid in 0.5 M H2SO4 + 0.5 M HCOOH solution (scan rate = 50 mV s−1).
Figure 6. First cycle CV scans of Pd, (a) PdNi, and (b) PdCo electrocatalysts tested in 0.5 M H2SO4 + 0.5 M HCOOH solution; maximum peak current density of Pd, (c) PdNi and (d) PdCo catalysts for the electrooxidation of formic acid in 0.5 M H2SO4 + 0.5 M HCOOH solution (scan rate = 50 mV s−1).
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Figure 7. CO stripping voltammogram curves of Pd, (A) PdNi, and (B) PdCo catalysts in 0.5 M H2SO4 solution (scan rate = 50 mV s−1).
Figure 7. CO stripping voltammogram curves of Pd, (A) PdNi, and (B) PdCo catalysts in 0.5 M H2SO4 solution (scan rate = 50 mV s−1).
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Figure 8. ECSA, mass activity, and specific activity of Pd, (a–c) PdNi, and (d–f) PdCo samples in 0.5 M H2SO4 solution or 0.5 M H2SO4 + 0.5 M HCOOH solution (scan rate = 50 mV s−1).
Figure 8. ECSA, mass activity, and specific activity of Pd, (a–c) PdNi, and (d–f) PdCo samples in 0.5 M H2SO4 solution or 0.5 M H2SO4 + 0.5 M HCOOH solution (scan rate = 50 mV s−1).
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Table
Table 1. EDS results of PdNi and PdCo catalysts.
Table 1. EDS results of PdNi and PdCo catalysts.
CatalystElementat%CatalystElementat%
PdNi-1Pd94.92PdCo-1Pd96.7
Ni5.08Co3.3
PdNi-2Pd90.3PdCo-2Pd93.94
Ni9.7Co6.06
PdNi-3Pd86.74PdCo-3Pd89.02
Ni13.26Co10.98
PdNi-4Pd79.03PdCo-4Pd82.12
Ni20.97Co17.88
PdNi-5Pd68.86PdCo-5Pd70.4
Ni31.14Co29.6
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Ding, Q.; Luo, Q.; Lin, L.; Yang, T.; Fu, X.; Wang, L.; Lei, C. Pd-M (M = Ni, Co) Bimetallic Catalysts with Tunable Composition for Highly Efficient Electrochemical Formic Acid Oxidation. Processes 2023, 11, 1789. https://doi.org/10.3390/pr11061789

AMA Style

Ding Q, Luo Q, Lin L, Yang T, Fu X, Wang L, Lei C. Pd-M (M = Ni, Co) Bimetallic Catalysts with Tunable Composition for Highly Efficient Electrochemical Formic Acid Oxidation. Processes. 2023; 11(6):1789. https://doi.org/10.3390/pr11061789

Chicago/Turabian Style

Ding, Qingwei, Qing Luo, Liang Lin, Tianlun Yang, Xingping Fu, Laisen Wang, and Caixia Lei. 2023. "Pd-M (M = Ni, Co) Bimetallic Catalysts with Tunable Composition for Highly Efficient Electrochemical Formic Acid Oxidation" Processes 11, no. 6: 1789. https://doi.org/10.3390/pr11061789

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