Preparation and Application of Palladium⁃Copper Nano Electrocatalysts

Chunyi Ye; Yang Yang; Xuexian Wu; Ping Ding; Jingli Luo; Xianzhu Fu

doi:10.7536/PC211028

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (9) : 1896-1910 DOI: 10.7536/PC211028

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Preparation and Application of Palladium⁃Copper Nano Electrocatalysts

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Abstract

Nanomaterials that contain noble metal palladium are kinds of electrocatalytic materials which have excellent properties. Among them, Pd⁃Cu binary materials have attracted much attention in recent years due to their low cost and high activity. The introduction of Cu not only reduces the amount of Pd, but also causes ligand effect, stress effect, aggregation effect and so on, which provides a variety of ways to optimize the electrocatalytic performance. Constructing special morphology and structure can make the catalyst expose more active sites to increase the electrochemical active surface area and improve the electrocatalytic performance. In addition, the adjustment of Pd⁃Cu component or the construction of composite structure can adjust the d⁃band center, thereby optimizing the electrode interface adsorption energy, and finally enhancing activity and improving stability. This review summarizes some preparation methods of Cu⁃Pd nano electrocatalysts with different structures, such as spherical particles, porous structure, branched structure, hollow nanocage, core⁃shell structure and single atom. In addition, the application of Cu⁃Pd nano electrocatalysts in organic small molecule oxidation (such as methanol, ethanol, formic acid oxidation), inorganic small molecule reduction (such as oxygen reduction, carbon dioxide reduction) and electroless copper plating is also summarized. Finally, the development prospect of Cu⁃Pd nano electrocatalyst is presented.

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Pd⁃Cu binary material / core⁃shell structure / single atom / nanoparticle / electrocatalysts

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Chunyi Ye, Yang Yang, Xuexian Wu, Ping Ding, Jingli Luo, Xianzhu Fu. Preparation and Application of Palladium⁃Copper Nano Electrocatalysts. Progress in Chemistry, 2022, 34(9): 1896-1910 DOI:10.7536/PC211028

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1 Introduction

2 Structure and preparation of Pd⁃Cu nano electrocatalyst

2.1 Porous structure

2.2 Branched structure

2.3 Hollow structure nanocages

2.4 Core⁃shell structure

2.5 Spherical nanoparticles

2.6 Pd⁃Cu single atom dispersion electrocatalysts

3 Application of Pd⁃Cu nano electrocatalysts in oxidation of small organic molecules

3.1 Methanol oxidation reaction(MOR)

3.2 Ethanol oxidation reaction(EOR)

3.3 Formic acid oxidation reaction(FAOR)

3.4 Glucose oxidation reaction

4 Application of Pd⁃Cu nano electrocatalyst in reduction of inorganic small molecule

4.1 Oxygen reduction reaction(ORR)

4.2 CO₂reduction reaction

4.3 Reduction of N₂to ammonia,reduction of N₂O and hydrogen evolution reaction(HER )

5 Application of Pd⁃Cu nano electrocatalyst in electroless copper plating

6 Conclusion and prospect

1 Introduction

The rapid development of human society has led to a rapid increase in energy demand, which has brought about two serious global problems: the energy crisis caused by the increasing depletion of non-renewable fossil energy, and the environmental crisis caused by pollutant emissions. It is urgent to develop renewable energy and efficient and clean energy technology. In this regard, China has put forward the great strategic goals of "carbon neutralization" and "carbon peak". Electrochemical reaction has the advantages of high energy conversion efficiency, clean and environmental protection, which is one of the important ways to achieve the goal of "double carbon"^[1]. Compared with traditional fossil energy combustion power generation, hydrogen and other fuels can directly convert chemical energy into electricity through electrochemical oxidation in fuel cells under mild conditions, which is not limited by Carnot cycle, so the energy conversion efficiency is high, clean and environmentally friendly^[2,3]. Renewable electricity such as photovoltaic and wind energy can also efficiently electrolyze water and CO₂ to produce fuel molecules (chemicals) such as hydrogen and alcohol by means of electrochemical reactions, thus storing and efficiently utilizing these renewable energy sources with obvious peak and valley effects. The electrochemical reaction between fuel molecules and electrical energy often requires the use of precious metal catalysts to proceed smoothly. At present, platinum (Pt) electrocatalysts with high activity are the most widely used, but the scarcity of Pt resources (only 37 ppb in the earth's crust) and its high price (its cost can account for more than 55% of the total cost of equipment) hinder the large-scale application of these technologies, and it is necessary to find suitable low-cost alternative materials^[4].

Pd and Pt are the same group elements, have similar physical and chemical properties, are face-centered cubic (fcc) structure, have similar lattice constants, and have more reserves of Pd, which is a very promising alternative to Pt-based catalysts^[5]. In addition, the addition of other non-noble metals in the preparation can not only reduce the amount of noble metals and thus reduce the cost, but also improve the electrochemical activity, selectivity and long-term stability of the catalyst by using the synergistic effect between them^[6]. Cu, as a 3D transition metal, has some interesting and unique physicochemical properties, and is a good choice as a second element because of its high natural abundance and low cost. Pd-Cu electrocatalysts have been widely studied in recent years^[7]^[8]. The Cu component introduced into the Pd catalyst can adjust the d-band center of Pd by means of ligand effect, strain effect, geometry effect, etc., and the appropriate d-band position can optimize the adsorption and desorption of reactants or intermediate products, thereby improving the catalytic activity and anti-poisoning ability^[9⇓~11]. The special structure can also affect the catalytic activity. The hollow and porous structure can increase the relative surface area of the catalyst and expose more active sites^[12,13]. The dendritic structure can increase the surface area, and the atoms in the corner tend to have higher activity^[14]. The core-shell structure and the single-atom dispersion structure are used to more effectively utilize the metal in the bimetallic catalyst which is more beneficial to the catalytic performance^[15,16]. Structurally ordered intermetallic compounds usually exhibit superior electrocatalytic activity and stability than disordered alloys. The so-called "ordering effect" means that the intermetallic compound alloy has a catalytic performance superior to that of a disordered alloy of the same crystal system and metal composition. Nanocrystals of intermetallic compounds have unique geometries and properties that allow precise control of their geometry. The ordering effect originates from geometric and/or electronic effects associated with the catalyst structure. Compared with the disordered alloy surface, the lattice strain of the intermetallic compound surface is stronger, which can cause the d-band center shift and change the electronic effect of the surface, so that the alloy shows higher structural stability and activity. Intermetallic compounds generally have higher durability than their disordered alloy counterparts because of their lower heat of formation and ability to resist dissolution to non-noble metals due to the increased strength of regular heteroatom bonds^[17,18].

In this paper, the preparation methods of Pd-Cu nanocatalysts with different morphologies and structures are reviewed, and their applications in electrooxidation of small organic molecules (alcohols, organic acids, glucose, etc.) And electroreduction of small inorganic molecules (O₂, N₂, CO₂, etc.) Are discussed.

2 Tructure and preparation method of palladium-copper nano electrocatalyst

2.1 Porous nanostructure

Porous nanostructures have the advantages of low density and high specific surface area, which can make the reactants fully contact with the active sites of the catalyst, increase the active surface area of the electrocatalyst and enhance the catalytic activity^[12]. Yan et al. Used ascorbic acid reductant to reduce copper chloride and chloropalladate by hydrothermal method at 200 ℃, and cetyltrimethylammonium bromide (CTAB) was used as surfactant^[19]. The prepared PdCu nanoalloy has a porous structure, and its SEM image and XRD pattern are shown in Fig. 1. Yang et al. Used ethylene glycol as a reducing agent and Pluronic P123 as a dispersant to prepare hierarchical porous PdCu nanoalloy networks with nanorods as basic units by the same hydrothermal method^[20]. Serov et al. Used silica with high specific surface area as a sacrificial template, attached palladium nitrate and copper nitrate to the template, and after reduction with hydrogen at 300 ° C, the silica support in the product was etched with KOH, and finally the porous PdCu nanoelectrocatalyst with high specific surface area was obtained^[21]. Lv et al. Synthesized single-crystal mesoporous PdCu nanocubes (s mesoPdCu) with the aid of cetyltrimethylammonium chloride (CTAC) and additional Cl^-^[22]. The ordered mesopores expose a large amount of surface area and active sites, and facilitate the rapid diffusion of reactants and products. The single crystallinity ensures the long-range structure and enhances the electron transport.

Dealloying is a common method to prepare porous structures, which allows the active components in the alloy to dissolve selectively, while the noble metal components remain to form porous structures^[23]. Song et al. And Jiang Kun et al. Prepared carbon-supported palladium-copper alloy (PdCu/C), and then dealloyed it by cyclic voltammetry to form a porous structure^[23]^[24]. Xie et al. First obtained PdCu alloy thin films by overpotential co-deposition, and then selectively removed Cu from the PdCu thin films by anodic linear sweep voltammetry (LSV) in a solution containing sulfuric acid and perchloric acid to prepare nanoporous PdCu films^[25]. Yang et al. Used a magnetron sputtering system to prepare Pd-Cu thin films, and also used cyclic voltammetry scanning for dealloying, so that a Pd coating layer with a thickness of about 1.5 nm was formed on the surface^[26]. Liu et al. First synthesized graphene oxide, and then prepared vertically oriented reduced graphene oxide (VrGO) supported dealloyed PdCu nanocatalyst (PdCu/VrGO) by simple cyclic voltammetric electrodeposition of graphene oxide in the presence of sodium chloropalladate and copper ethylenediaminetetraacetate.The schematic diagram is shown in Figure 2a. The rGO is almost upright on the surface of the electrode, completely exposing the PdCu nanoparticles on the surface^[27]. Dealloying makes the PdCu alloy particles form a porous structure and have a Pd-rich shell structure to improve the electrocatalytic performance. The dealloying process may also be carried out in acid. Park et al. Synthesized highly monodisperse 3 nm spherical PdCu alloy nanoparticles in high yield by co-reduction method using trioctylamine (TOA) as solvent and reducing agent, oleic acid as stabilizer, palladium diacetylacetonate and copper diacetylacetonate as precursors, and used acetic acid to chemically dealloy PdCu/C nanoparticles to form porous structures^[28]. Miyazawa et al. Synthesized a bimodal nanoporous PdCu electrocatalyst using nanoporous copper as a sacrificial template^[10]. The nanoporous copper was prepared by dealloying of Cu_0.3Mn_0.7 in hydrochloric acid. Xu et al. Obtained nanoporous copper (NPC) by dealloying CuAl, and then prepared nanotubular mesoporous PdCu bimetallic catalyst by replacement reaction. The XRD patterns and TEM images are shown in Fig. 2b and C^[29].

2.2 Dendritic nanostructure

Dendritic nanostructured catalysts can expose more active sites, have larger electrochemical active surface area, have higher atom utilization than nanospheres, and their step and corner sites often have higher catalytic activity^[30]^[14].

Chen et al. Used oleylamine to reduce copper diacetylacetonate at 180 ° C, and prepared spherical, concave cubic, and dendritic PdCu nanoalloys, respectively, by adding different amounts of palladium diacetylacetonate^[14]. Xiong et al. used benzyl alcohol as a solvent and tungsten hexacarbonyl as a reducing agent to reduce copper diacetylacetonate and palladium diacetylacetonate with the help of a dispersant, polyvinylpyrrolidone (PVP), to obtain a nano-dendritic PdCu alloy, whose XRD pattern and TEM image are shown in fig. 3^[31]. The generation of nano-dendrites can be attributed to the galvanic replacement between the initially formed PdCu alloy with high Cu content and the Pd²⁺ ions in the solution, which usually results in a branched structure. Yang et al. Synthesized dendritic PdCu nanoalloys using a one-step hydrothermal reduction method^[30]. The CTAB in the reactant can not only act as a capping agent, but also provide bromide ions to guide the growth of nanoparticles to form branched structures. Hydrochloric acid is used to control the reaction kinetics to promote the growth of dendritic structures.

2.3 Hollow structured nanocage

The hollow nanocage structure can increase the surface area of the electrocatalyst, so that the reactant can better contact with the catalyst and improve the catalytic activity. Liu et al. Obtained monodisperse cuprous oxide (Cu₂O) spherical particles by reducing copper acetate with glucose in N, N dimethylformamide (DMF) solvent with the help of surfactant PVP^[13]. Then the Cu₂O is disproportionated by palladium chloride acid to obtain a PdCu/Cu₂O precursor, and finally the inner Cu₂O is etched by sulfuric acid to obtain PdCu alloy hollow particles with different shapes (bowl-shaped, apple-shaped or spherical), wherein the shapes of the PdCu alloy hollow particles depend on the ratio of the Cu₂O to the palladium chloride acid. When the proportion of palladium chloride acid is high, the generated PdCu is compact and can support a spherical shape. With the decrease of the proportion of palladium chloride acid, the surface collapses after the corrosion of internal Cu₂O and becomes apple-shaped or bowl-shaped (Fig. 4 and 5A). Sheng et al. Used copper chloride as a precursor, added ascorbic acid in the presence of PVP to prepare octahedral Cu₂O, then added palladium chloride acid for disproportionation reaction, and finally added acetic acid to etch the remaining Cu₂O to obtain porous octahedral PdCu nanocages (Fig. 5B)^[32]. Chen et al. Prepared a solution containing both copper sulfate and palladium chloride. When glucose was added, palladium chloride was reduced to Pd, which then converted the Cu²⁺ to Cu₂O octahedra^[33]. Because of the high activity of Cu₂O, the tips of its particles will be etched by Pd, and eventually form corner-etched Cu₂O octahedra. The PdCu flower-like alloy nanocage was obtained by adding chloropalladous acid into the corner etched Cu₂O octahedron, and then adding ammonia water to etch off the remaining Cu₂O after the reaction (Figure 5C).

2.4 Core-shell nanostructure

The core-shell structure nano-electrocatalyst with Pd as the shell layer can greatly increase the exposed surface of Pd atoms, improve the utilization rate of Pd atoms and the electrocatalytic activity, and reduce the amount of Pd^[34]. Guo Ying added PVP to ethylene glycol as a dispersant and used ascorbic acid to reduce copper chloride to obtain copper colloid^[15]. Then ascorbic acid and palladium chloride acid were added to continue the reaction, so that the surface of copper particles was covered with Pd, and the core-shell structure Cu @ Pd colloid with a particle size of about 11 nm was obtained. Takashima et al. dropped the ink made of Pd/C powder on the FTO substrate, dried it, and then immersed it in a solution containing copper sulfate to deposit Cu at an electrode potential of 0.35 V for 180 s, resulting in underpotential deposited Cu/Pd nanoparticles with a core-shell structure (UPD Cu/Pd), with an average size of 4.7 nm, and Cu formed a single atomic layer on the surface of Pd^[35]. Luo et al. Reacted copper diacetylacetonate and palladium diacetylacetonate with oleylamine and trioctylphosphine at 240 ° C, and the product was loaded on carbon and then heat-treated to obtain hierarchical CuPd nanospheres (^CGCu₁Pd₁NS/C⁃HT)^[1]. As a working electrode, it was electrochemically treated in a copper sulfate solution to coat a layer of Cu, and then electrochemically treated in a chloroplatinic acid solution to convert Cu into Pt to obtain ^CGCu₁Pd₁@Pt_MLNS/C⁃HT with a core-shell structure. Their TEM images and synthesis process are shown in Figure 6. Li et al. Dissolved nickel diacetylacetonate and copper chloride in oleylamine at 180 ℃ for 4 H to obtain fine copper nanowires^[36]. Then the copper nanowires were reacted with palladium diacetylacetonate in 1-octadecene to obtain Cu @ Pd nanowires with core-shell structure.

2.5 Spherical nanoalloy particle

The Cu in the PdCu alloy nanoparticles can regulate the electrocatalytic activity of Pd. Wang et al. And Kang et al. Prepared PdCu bimetallic alloy by co-impregnation method using Vulcan XC-72 as carbon support, palladium nitrate and copper nitrate as precursors, and hydrogen reduction, in which the size of PdCu prepared by Wang et al. Was about 20 nm^[37]^[38]. Zhang et al. Used oleylamine as solvent and reducing agent to reduce copper diacetylacetonate and palladium diacetylacetonate at 185 ℃ under the condition of triphenylphosphine as dispersant, and supported them on carbon black by ultrasound to obtain Pd₇₁Cu₂₉/C with an average particle size of about 4. 7 nm^[39]. Wang et al. Used palladium chloride and copper chloride as precursors, sodium citrate as a stabilizer, and ethylene glycol as both a solvent and a reducing agent to prepare a Pd-rich PdCu alloy on the surface, and supported it on carbon^[40]. The Pd-rich surface makes it have a strong resistance to Cu dissolution in acidic solution. Gong et al. Used PVP as a dispersant to change the commonly used heating conditions to microwave-assisted synthesis of PdCu₄/C^[41]. Ye et al. Mixed the treated graphene oxide (GO) with copper sulfate and used ascorbic acid and hydrazine hydrate for co-reduction to obtain the Cu/rGO intermediate^[42]. Then it was reacted with palladium chloride acid at 80 ° C for 12 H to obtain PdCu/rGO, and the PdCu alloy with a size of 12.73 nm was supported on reduced graphene oxide (rGO) (Fig. 7). Dong et al. Also used a similar method to synthesize PdCu/graphene hybrid materials, except that the reducing agent was replaced by potassium borohydride and hydrazine hydrate, and the Pd source was replaced by palladium chloride, and the particle size was similar, 13.67 nm^[43].

2.6 Palladium-copper single-atom dispersion electrocatalyst

Single-atom catalysts are a rapidly emerging direction in catalyst morphology design. Atomic dispersion can ensure the full exposure of each metal atom and maximize its use in catalytic reactions, which is of great significance to reduce the cost of precious metal catalysts and improve their catalytic activity^[45]. Han et al. Dissolved tetraamine dichloropalladium, copper chloride and hydroxylamine hydrochloride in deionized water and reduced them with glucose ethanol solution^[46]. The reduced material was heat-treated at 600 ℃ to obtain PdCu/NC sites on N-doped carbon. Due to the presence of Cu, this kind of bimetallic monoatomic dispersion catalyst has better performance than Pd monoatomic dispersion catalyst. In addition to such single-atom catalysts, PdCu single-atom alloy catalysts in which isolated Pd atoms are supported on Cu are more common. Cao et al. Prepared PdCu single-atom alloy catalysts by substitution reaction, co-impregnation and sequential impregnation, respectively, and Pd atoms were deposited on the (111) crystal plane of Cu^[47]. The weak binding of the intermediate to Cu (111) leads to strong selectivity in the hydrogenation of acetylene and styrene. The catalyst prepared by electrodisplacement method has the best catalytic activity in acetylene hydrogenation, while the catalyst prepared by sequential impregnation method has the best stability. Pei et al. Prepared PdCu monatomic alloy catalyst by impregnating silica gel into a mixed solution of copper nitrate and palladium nitrate^[16]. They believed that simply increasing the atomic ratio of Cu/Pd could not effectively reduce the cost, but would lead to the waste of Cu atoms. Xing et al. Used a urea deposition precipitation method to prepare a single-atom alloy catalyst^[48]. The γ⁃Al₂O₃ carrier is firstly added into a copper nitrate solution, copper hydroxide is precipitated by using urea, and then the mixture is calcined to obtain the CuO/Al₂O₃. The obtained CuO/Al₂O₃ is dipped into dinitrosodiammine palladium solution, and the steps are repeated to react to generate Pd⁃CuO/Al₂O₃. Finally, it was reduced under hydrogen atmosphere to obtain Cu₅Pd/Al₂O₃, as shown in Fig. 8.

3 Application of Palladium-Copper Nanocatalysts in Oxidation of Small Organic Molecules

3.1 Methanol oxidation reaction (MOR)

Compared with other fuel cells, direct methanol fuel cell (DMFC) has attracted much attention because of its high volumetric power density and convenient storage and transportation of liquid fuel. Yan et al. Synthesized Pd₆₅Cu₃₅ electrocatalyst by adjusting the ratio of metal precursors, which exhibited higher MOR current density, electrochemical active surface area (ECSA) than commercial Pd/C under alkaline conditions, in addition to better long-term stability^[19]. They believe that there are two reasons, one is that the introduction of Cu brings a unique porous morphology, which improves ECSA and provides more active sites for catalytic reactions. Second, the introduction of Cu changes the electronic configuration of the PdCu bimetallic catalyst, resulting in a synergistic effect, which is beneficial to the electrocatalytic reaction. The dendritic nanostructured catalyst has porous structure, high specific surface area, interconnected nanostructure and excellent electronic conduction characteristics. Xiong et al. Synthesized PdCu nanodendrites with dendritic structure, and found that the specific activity and mass activity of Pd₃₅Cu₆₅ alloy nanodendrites for methanol oxidation were the highest by adjusting the proportion of metal atoms in them, which were 9.3 and 7.6 times higher than those of commercial Pd/C, respectively^[31]. Zhang et al. First synthesized Mg₅₀Cu₄₅Pd₅ alloy and then dealloyed it in hydrochloric acid to obtain porous PdCu nanoalloy, which showed excellent electrocatalytic activity for the oxidation of methanol and ethanol in alkaline medium^[11]. In addition, the dissolution of Cu and Mg during potential cycling may lead to surface reconstruction, producing nanoscale PdCu alloy core with Pd-rich surface. Cu atoms in the sublayer may modify the top Pd layer through the strain effect of lattice mismatch and the alloying effect of forming heteroatom bonds to enhance the catalytic activity. Yang et al. Obtained rGO standing on the electrode by adjusting the metal loading, on which PdCu nanoparticles were loaded^[27]. This structure fully exposes the active sites on rGO, and through dealloying, the nanoparticles on rGO also form a porous structure. The catalytic activity of Pd/C for methanol was 7. 1 times higher than that of commercial Pd/C under the combined action of multiple factors.

Noble metal aerogels are a unique class of free-standing nanowire networks with extremely high porosity, ultra-low density, and extremely large internal surface area, which host a large number of active centers that can eliminate the corrosion problem of carbon supports and enhance electron transfer and mass transfer. Wang et al. Greatly shortened the synthesis time of PdCu aerogel with the help of amide-functionalized ionic liquid (IL)^[9]. The power density of the DMFC with the synthesized IL/Pd₃Cu₁ as the anode catalyst is higher than that of the DMFC with the commercial Pd catalyst, the mass activity is 8. 5 times higher than that of the commercial Pd catalyst, the specific activity is 3. 2 times higher, and the anode onset potential is more negative. The three-dimensional porous structure of aerogel exposes more active centers, and the ligand effect between PdCu improves the catalytic activity. In addition, the interaction between Pd and Cu also reduces the d-band energy of Pd, and reduces the adsorption and poisoning of CO reaction intermediates on Pd. The combined effect of many aspects improved the methanol oxidation electrocatalytic performance and stability of IL/Pd₃Cu₁ (Fig. 9).

The ECSA of bowl-shaped PdCu hollow particles prepared by Liu et al. Is higher than that of commercial Pd/C, and the MOR mass activity and specific surface area activity are 3. 73 times and 2 times higher than those of commercial Pd/C, respectively^[13]. Chen et al. Prepared flower-like PdCu alloy nanocages, which exhibited 2.7 times higher catalytic activity for MOR than commercial Pd/C catalyst^[33]. They all have excellent MOR catalytic activity and durability, which can be attributed to the high surface area of their structure, which can enhance charge transport and provide sufficient active centers. The lattice constant of Cu is smaller than that of Pd, so the addition of Cu will lead to the contraction of Pd lattice, and the electron transfer effect will lead to the decrease of the d-band center of Pd, which will weaken the chemisorption energy of reaction intermediates (such as carbon monoxide) on the surface of Pd and improve the durability. Sheng et al. Prepared polyhedral porous PdCu nanocages, which are less likely to aggregate than nanoparticles, can improve the durability in the long-term catalytic process, and their activity is 2.9 times that of commercial Pd/C^[32].

3.2 Ethanol oxidation reaction (EOR)

Ethanol is a non-toxic and renewable green clean fuel, and its permeation efficiency through Nafion membrane in fuel cell is much lower than that of methanol^[50]. Direct ethanol fuel cell (DEFC) has high efficiency and high energy density, which is considered as a potential power source for future electric vehicles and portable devices. Sun Limei et al. Used sodium borohydride to reduce palladium chloride and copper chloride solutions with different ratios to obtain Pd-Cu alloys with different atomic ratios, and found that the catalytic performance of Pd-Cu/C catalyst increased first and then decreased with the increase of Pd ∶ Cu atomic ratio, and when Pd ∶ Cu = 8 ∶ 1, the Pd₈⁃Cu/C catalyst showed the best catalytic activity for ethanol electrochemical oxidation^[51]. Kang et al. Prepared PdCu/C by co-impregnation method, and considered that the addition of Cu could modify the d-band center of Pd, promote the adsorption of ethanol intermediate on Pd-based catalyst, and enhance the catalytic activity^[38]. They continued to heat treat the catalyst under CO atmosphere, and found that the current density of the heat-treated catalyst increased by 190% at -0.2 V, and had better resistance to poisoning. This is due to the fine-tuning of the oxidation state of the catalyst after heat treatment, which improves the catalytic activity and poisoning resistance. Guo et al. Prepared oxide-rich Pd_0.9Cu_0.1/C, and also believed that the CuO_x in Pd/Cu bimetallic nanoparticles played a role in improving the catalytic performance of EOR, and their morphology and performance are shown in Figure 10^[50].

The in situ assembly of colloidal nanocrystals provides a new way to obtain functional micro-nanoscale structures. Gong et al. Used this method to obtain PdCu nanowire assemblies, which exhibited a high current density of 1598 mA/mg or 2120 mA/mg_Pd in EOR, much higher than that of commercial Pd/C and some reported PdCu catalyst samples^[52]. Abdel et al. Supported core-shell Cu @ Pd on tin dioxide and rGO composite support to obtain Cu@Pd/SnO₂⁃Gr^[53]. Different SnO₂ contents lead to different catalytic activities, and when the SnO₂ content is 20 wt%, its current density is 6.21 times higher than that of Pd⁃SnO₂/C. Many previous studies have focused on the modification of bulk materials by functionalized alloys or by electrodeposition processes, few studies have focused on the loading of Pd/Cu bimetals, and even fewer studies have used carbon nanomaterials as loading media. Dong et al. Believed that the surface of graphene oxide nanosheets contained a large number of oxygen functional groups, which could be used as an ideal anchor point for the deposition of other nanostructures, so they synthesized Pd/Cu/graphene composites^[43]. Pd is reduced on the Cu surface to form a bimetallic interface region, which shifts the center of the d-band and improves the electrocatalytic performance. The improvement of catalyst poisoning tolerance can be attributed to the uniform loading of bimetallic nanostructures on graphene nanosheets.

3.3 Formic acid oxidation (FAOR)

Direct formic acid fuel cell (DFAFC) is a promising power source because of its high theoretical open circuit potential, low operating temperature, non-flammability of formic acid, and safety in storage and transportation. Compared with direct alcohol fuel cells, these advantages make them more suitable for portable electronic devices^[20]. PdCu alloy has a unique advantage that Cu can reduce the d-band center of Pd, thus enhancing its resistance to CO poisoning and contributing to the direct two-electron oxidation of formic acid. The dendritic PdCu nanoalloy prepared by Yang et al. Benefits from the multi-branched structure, which provides a larger surface area and a synergistic effect between the alloy metals. Its catalytic performance for formic acid oxidation is 2. 4 times that of commercial palladium black, and its morphology is basically unchanged after the chronoamperometric (CA) stability test^[30]. In addition, the initial oxidation potential and oxidation peak potential of PdCu branched nanoalloy are lower than those of commercial palladium black in the CO stripping test, indicating that it has better anti-poisoning performance.

Li et al. Prepared Cu @ Pd core-shell nanowires, which have rough surface and porous structure, greatly increasing the surface area of Pd, thus having excellent electrochemical catalytic activity. The catalytic activity for formic acid oxidation is about 4 times that of commercial Pd/C, showing better stability in the early stage of the catalytic process^[36]. Through first-principles calculations, they showed that the synergistic effect of Cu and Pd made the d-band center of Pd atoms on the surface move down, so that PdCu (111) showed good catalytic activity for formic acid oxidation. The Pd₆₅Cu₃₅ prepared by Yan et al., mentioned above, also has good catalytic activity for formic acid, as shown in fig. 11^[19]. They also believe that the introduction of Cu changes the electronic structure of Pd/Cu bimetallic catalyst, resulting in a synergistic effect, which is beneficial to the electrocatalytic reaction. The use of polyols for the synthesis of metal nanoparticles can significantly improve their dispersion on carbon supports. Wang et al. Synthesized Pd₁Cu₁/C with the help of ethylene glycol, which has a unique alloy structure, Pd enrichment on the surface, and strong resistance to Cu dissolution in acidic solution^[40]. Its current density for formic acid oxidation is 2.2 times higher than that of Pd/C. According to Chen et al., the improved electrochemical catalytic performance of the prepared PdCu alloy polycube with rough surface is attributed to its lower d-band center and unique polycube structure with rough surface (with abundant active edge, corner and step atoms), which enhances its resistance to CO poisoning^[54]. Both Wang et al. And Chen et al. Recognize that the charge transfer effect and lattice contraction help to reduce the d-band center and improve the catalytic performance.

Zhao et al. Used carbon nanotubes (CNTs) loaded with PdCu alloy to obtain Pd₁Cu₃/CNT, and found that the functionalized CNTs embedded the nanocrystals of the alloy particles and promoted their nanoscale dispersion during calcination and reduction treatment^[5]. The surface analysis shows that there is a certain amount of Pd remaining on the surface of PdCu alloy nanoparticles, which is the reason for the improved electrocatalytic performance. Yang et al. Synthesized a hierarchical porous PdCu alloy network with nanorods as the basic unit, and the mass activity in FAOR was 5.5 times that of palladium black^[20]. Compared with monodisperse nanoparticles, the porous framework has a great inhibitory effect on particle migration, Oswald ripening, and aggregation, which is of great significance for the stability and durability of electrocatalysts.

Park et al. Observed that the addition of Cu could improve the thermal stability of nanoparticles, thus successfully preparing monodisperse PdCu nanoparticles at high temperature^[28]. They synthesized highly monodisperse 3 nm spherical PdCu alloy nanoparticles with high yield by co-reduction method, and used acetic acid to chemically dealloy the carbon-supported PdCu/C nanoparticles to form a porous structure. As shown in Fig. 12, the activity of the prepared PdCu/C catalyst for formic acid oxidation in acidic medium was at least more than three times higher than that of the commercial Pd/C catalyst, and it exhibited excellent electrocatalytic stability. The enhanced electrocatalytic performance is attributed to its small particle size and chemically dealloyed porous structure with a large number of active sites on its surface. Jiang Kun et al. Carried out cyclic voltammetric scan dealloying of PdCu in acidic solution^[24]. They believe that Cu alloying can significantly improve the stability of Pd electrocatalytic oxidation of formate, while dealloying can improve the electrocatalytic oxidation activity, but reduce the stability. Song et al. Hold the opposite view^[23]. They found that the dissolution of Cu in PdCu/C by cyclic voltammetry scanning would promote the rearrangement of Pd on the surface to form a Pd-rich surface, and the resulting porous structure could prolong the existence time of the active surface and improve its stability.

3.4 Glucose oxidation

Compared with fuels such as methanol and ethanol, glucose is safe, non-toxic and renewable, and can be obtained from a large number of plants and industrial production. However, the oxidation of glucose is still facing difficulties. Enzymes such as glucose oxidase and glucose dehydrogenase are effective catalysts for glucose oxidation due to their high activity and selectivity. However, the stability of the enzyme is poor, the service life of the system is short, and the cost of the enzyme is very high under the current production conditions, so it can not be industrialized^[55,56]. Therefore, the development of metal catalysts for glucose oxidation is very urgent. Chen et al. First synthesized Cu(OH)₂⁃Pd nanorods, and then annealed to form CuO: Pd nanorods^[57]. Compared with CuO and Cu(OH)₂⁃Pd, CuO and Pd have higher catalytic activity for the electrochemical oxidation of glucose. After 500 cycles, the current density of CuO/Pd decreased slightly, but it still had high catalytic activity. Ji et al. Deposited a three-dimensional porous copper film on copper foil, then anodized it to obtain a porous Cu@Cu₂O film, and finally deposited Pd particles in palladium chloride acid^[55]. The porous Cu@Cu₂O film and the supported Pd nanoparticles can produce a synergistic effect in the catalytic process, and its unique hierarchical porous structure is also beneficial to improve the electrocatalytic performance. The current density of porous Cu@Cu₂O⁃Pd is 2.7 times that of porous Cu@Cu₂O, 5.4 times that of porous Cu, and 13.9 times that of Pd.

Guo et al. Synthesized a Pd⁃Cu₂O nanoheterostructure with a lattice mismatch of up to 8.68%, which possessed excellent glucose electrocatalytic oxidation activity^[58]. The research shows that the dislocation and defect of metal atoms can lead to the accumulation of electrons at the interface, and make the d-band center of Cu and Pd move to the Fermi level, and finally enhance the activity of the Pd⁃Cu₂O nano-heterostructure and accelerate the electrochemical oxidation of glucose. In addition, they also deposited the Cu₂O shell on the surface of Pd nanocube seed crystal, and continued to grow to form a core-shell structure Pd@Cu₂O truncated octahedron, and finally obtained the urchin-like Pd @ CuO Pd yolk eggshell nanomaterial after heating treatment in the environment with sodium chloropalladate as an adjuvant, as shown in Fig. 13^[59]. This nanomaterial has a better oxidation current in the electrocatalytic oxidation of 3 mmol/L glucose than Pd nanomaterial, Pd@Cu₂O truncated octahedron and other catalysts. Urchin-like nanomaterials have a high specific surface area, and their scattering structure can effectively reduce the contact area between nanomaterials and prevent the agglomeration of particles, thus having long-term stability. In addition, the scattering structure with high porosity can expose more catalytic active sites more fully, providing shorter diffusion channels and effective mass diffusion for electrons or ions in the catalytic reaction, which is also beneficial to the improvement of the electrocatalytic performance of the material.

4 Application of Palladium-Copper Nanocatalysts in the Reduction of Inorganic Small Molecules

4.1 Oxygen reduction reaction (ORR)

Oxygen reduction reaction is one of the important electrocatalytic reactions, which plays an important role in fuel cells, metal-air batteries and hydrogen peroxide synthesis^[60]. However, the reaction kinetics of ORR is relatively slow, which makes it necessary to develop electrocatalysts with high activity and low cost^[61]. The addition of metals with strong oxygen binding ability (Co, Ni, Cu, etc.) can reduce the binding of oxygen with noble metals and improve the ORR catalytic activity^[62]. As a transition metal, Cu has abundant redox valence States and is an excellent candidate for ORR electrocatalyst^[4]. Gobal et al. Used palladium chloride and copper chloride as metal sources to electrodeposit PdCu alloy on nickel cylindrical electrode, and its ORR activity was 4 ~ 5 times higher than that of monometallic Pd catalyst after heat treatment at 600 ℃^[60]. They believe that in addition to the adsorption of oxygen on the surface of Pd-Cu alloy, an intermediate is formed. Cu is a better O — O bond cleavage center, which stabilizes the intermediate and enhances the ORR rate. An et al. Prepared a hollow spherical Pd₁Cu₁/C with good catalytic activity, and the current loss was only 26.4% after 3000 CV cycles, while the activity of Pd/C and Pt/C catalysts decreased by nearly 80%^[61]. The hollow porous structure promotes the transfer of oxygen, and together with the addition of Cu, it enhances the dispersion of Pd and provides more Pd active centers. Xu et al. Synthesized a nanotubular mesoporous PdCu bimetallic catalyst, which showed excellent catalytic activity for oxygen reduction reaction. Its half-wave potential was 0.840 V, which was nearly 60 mV higher than that of the commercial Pd/C catalyst, indicating that its ORR activity was greatly improved and was also significantly better than that of the commercial Pt/C catalyst^[29]. Gong et al. Synthesized PdCu/C with mainly fcc phase by microwave-assisted method, and the ORR half-wave potential in alkaline environment was comparable to that of commercial Pt/C, which was almost unchanged after 1000 accelerated durability tests (ADT)^[41]. However, after testing in HClO₄, the limiting current density decreased by 12.7% and the electrochemical surface area was lost by 58.5%, which may be caused by the PdCu dealloying structure (fig. 14). Wang et al. explored the effects of different heat treatment temperatures on the morphology and catalytic performance of PdCu bimetallic catalysts, and also found that acid treatment of PdCu catalysts improved the ORR activity, which was due to the removal of undesirable phases (such as unalloyed copper)^[37].

The lower the position of the d-band center relative to the Fermi level, the smaller the binding energy of the oxygen-containing adsorbate on the metal catalyst. Achieving the optimal position design of the d-band center is the key to enhance the ORR activity of bimetallic electrocatalysts. The O metal bond must be strong enough to achieve a high oxygen species coverage; However, it should not be too strong, otherwise it will inhibit the next reaction of oxygen species^[63]. Tang et al. Performed density functional theory (DFT) calculations on the behavior of PdCu disordered alloy nanoparticles in ORR, and found that the enhancement of their activity was due to the different responses of the two metals to the d-band center shift^[62]. The charge transfer between Cu and Pd increases the d-band of Cu and decreases the d-band of Pd, resulting in an increase in the bonding strength between Cu and oxygen and a decrease in the bonding strength between Pd and oxygen, as shown in fig. 15. Bampos et al. Suggested that the enhanced ORR activity of their synthesized 7.5 Pd/2.5Cu was mainly related to the lattice strain of Pd, which could adjust the position of the d-band center^[63]. Yang et al. Prepared PdCu thin films by magnetron sputtering system and carried out dealloying treatment^[26]. After dealloying, a Pd-rich surface layer is formed, in which compressive lattice strain exists, enhancing the ORR activity of the film, which is very close to that of commercial Pt. After heat treatment of the prepared PdCu nanoparticles by Jiang et al., the disordered PdCu solid solution particles were transformed into ordered PdCu intermetallic compounds, which showed better activity and stability in ORR and EOR^[64]. DFT calculations show that the enhancement of catalytic activity is attributed to the existence of active empty sites and the surface compressive strain provided by small atoms such as Cu and Co. Coincidentally, Gunji et al. Prepared ordered intermetallic compound PdCu₃NPs/CB by polyol method and heat treatment, and obtained Cu-free surface structure by electrochemical dealloying, with lower d-band center and higher ORR activity^[65]. Miyazawa et al. Used nanoporous copper as a sacrificial template to synthesize bimodal nanoporous palladium copper, and increased the electrochemical specific surface area of the catalyst by dissolving and removing the nanoporous copper^[10]. Moreover, even if Cu is dissolved, there is still a part of Cu remaining on the Pd surface, and the ligand effect and strain effect provided by this part of Cu enhance the ORR activity of the catalyst. Nanoporous PdCu prepared by Xie et al. By dealloying method benefits from the unique porous structure, large Pd surface area and the modification of Pd electronic structure by residual Cu. When the potential is 0. 9 V vs RHE, the catalyst shows excellent catalytic performance, which is 6. 2 times that of commercial Pd/C catalyst^[25]. The half-wave potential only shifts negatively by 4 mV after 10 000 potential cycles in alkaline medium, indicating that the stability is also excellent. Liu et al. Used first-principles calculations to study the catalytic performance and reaction mechanism of intermetallic PdCu alloy as ORR electrocatalyst, and found that the improvement of catalytic performance was indeed due to the strain effect caused by lattice mismatch and charge transfer on the substrate^[66].

Luo et al. Believed that with the increase of specific surface area activity of ORR catalyst, the noble metal loading in the fuel cell decreases, which will lead to the increase of local O₂ transport resistance, which will seriously affect the performance of the fuel cell at high current density^[1]. Concentrating highly active atoms on the shell of the electrocatalyst is the most effective way to combat this problem. They prepared ^CGCu₁Pd₁NS/C⁃HT with compositional hierarchical structure for ORR catalysis in alkaline environment. The mass specific activity of ^CGCu₁Pd₁NS/C⁃HT was 13. 7 times higher than that of commercial Pd/C, and the stability was excellent, which was still 86. 4% of the initial value after 10 000 ADT cycles. They also covered the surface of ^CGCu₁Pd₁NS/C⁃HT with a Pt layer to obtain ^CGCu₁Pd₁@Pt_MLNS/C⁃HT for ORR catalysis under acidic conditions, and the mass activity was 13.3 times higher than that of commercial Pt. Moreover, its catalytic activity increased during the ADT cycle and remained higher than the initial value after 10,000 cycles (fig. 16). According to Nunes et al., in addition to the structure of the catalyst affecting its activity, its performance is also affected by the support material^[4]. The support material should have large porosity and high specific surface area to promote uniform distribution of metal particles, high electrical conductivity, and sufficient mechanical stability, resulting in a synergistic effect to improve the electrocatalytic performance. They loaded Cu and Pd on high specific surface area graphite (HSAG) and found that the electrode modified by HSAG had better electrocatalytic performance. The main role of the HSAG carrier should be to stabilize the metal nanoparticles and assist the electron conduction.

4.2 CO₂ electrochemical reduction reaction

Since the industrial revolution, the extensive use of fossil fuels, CO₂ greenhouse gas emissions, has brought serious environmental crisis. CO₂ can also be used as a carbon resource to convert into fuels and chemicals, such as methanol, formic acid, methane, ethanol and so on, through thermal catalysis, photocatalysis and electrocatalytic reduction, which play an important role in many fields^[67⇓~69]. Among these CO₂ conversion methods, electrocatalytic reduction of CO₂ in aqueous solution has been widely studied due to its mild reaction conditions, high conversion, and clean reaction process^[70]. However, the electrocatalytic reduction of CO₂ is confronted with high overpotential, low current efficiency due to hydrogen evolution reaction (HER), and poor catalytic selectivity. Therefore, it is imperative to develop catalysts that can both inhibit HER and improve catalytic activity for CO₂ reduction at low overpotential. Chen et al. Synthesized spherical, concave cubic and dendritic bimetallic CuPd nanoalloys, respectively^[14]. As shown in Fig. 17 (a, B), the spherical Cu: Pd: 0.3 alloy sample has the highest Faradaic efficiency for reduction to CO, reaching a maximum of 93% at -0.87 V. Compared with other catalysts, Cu _ Pd _ 0.3 also showed the highest specific activity and mass activity. Takashima et al. Deposited a Cu atomic layer on the surface of Pd particles by electrodeposition to form a core-shell structure UPD Cu/Pd^[35]. The faradaic efficiency of UPD Cu/Pd for the reduction of CO₂ to formic acid is 84%, which is higher than that of Pd electrode (69%). After 10 H of reduction, the faradaic efficiency of formic acid formation decreased to 65%, and the current density was significantly higher and more stable than that of Pd electrode. The s _ mesoPdCu synthesized by Lv et al. Has more excellent CO₂ reduction performance^[22]. When the cathode potential is between -0.1 and -0.5 V, the faradaic efficiency of s _ mesoPdCu is kept above 90%. In the 15 000 s stability test, the reduction current of s _ mesoPdCu only decreased from 7 mA/cm² to 5.7 mA/cm², and the average selectivity for formic acid formation was 100%. They believe that in the prepared catalyst, Cu can promote the positive shift of the 3D binding energy of Pd, reduce the d-band center of the catalyst, thereby reducing the adsorption strength of CO, improving the stability of CO poisoning, and improving the catalytic activity.

The deactivation of Cu electrode during the electrochemical reduction of CO₂ is closely related to the carbon species accumulated on the catalyst surface. Weng et al. Modified the Cu surface with Pd to induce the morphological and compositional structural reorganization of the catalyst, thus endowing the catalyst electrode with self-cleaning ability against carbon deposition and effectively stabilizing the catalytic selectivity of CO₂ reduction^[71]. Xie et al. Prepared a new type of Cu-Pd heterostructure based on CuCl⁃PdO_x hexagonal microplate, which was used to regulate the formation of selective monohydrocarbons by electrochemical reduction of CO₂^[72]. Compared with pure Cu or Pd, the Cu/Pd heterostructure exhibits a higher selectivity of CO₂ to CH₄.

4.3 Ammonia production by N₂ reduction, nitrous oxide reduction and hydrogen evolution reaction by water electrolysis (HER)

Ammonia (NH₃) is one of the most produced inorganic chemicals in the world. It is in great demand in fertilizer production, pharmaceutical production and many other industrial processes. Its high hydrogen content and energy density can also be used to store hydrogen as a fuel^[73]. Industrial production NH₃ rely on the Haber Bosch process, which is produced under harsh conditions and has relatively high energy consumption and CO₂ emissions. Electrochemical nitrogen reduction reaction (NRR) is a mild, environmentally friendly and low energy consumption process, which is considered to be a promising alternative to the Haber Bosch process, while renewable electricity can be used for energy storage through co-electrolysis of N₂ and H₂O. Single-atom catalysts are expected to greatly improve NRR performance with their minimal center size, nearly 100% atom efficiency, and unique low-coordination characteristics. Han et al. synthesized a diatomic Pd-Cu catalyst (as shown in Fig. 18) by modulating the single-atom Pd site through the introduction of Cu, and realized the catalysis of diatomic Pd-Cu on nitrogen-doped carbon^[46]. The introduction of Cu not only shifts the projected density of States of Pd to the Fermi level, but also promotes the d⁃2π^* coupling between Pd and adsorbed N₂, enhances the chemisorption and active protonation of N₂, and suppresses hydrogen evolution. The catalyst achieved a high faradaic efficiency of 24.8% ± 0.8% with a NH₃ yield up to 69.2±2.5μg·h^-1·

mg c a t - 1

, which is 14.6- and 4.2-fold higher than that of the single-atom Pd catalyst.

N₂O is one of the main components of greenhouse gases, and its global warming potential is 268 times that of CO₂. Baek et al. Prepared Pd/Cu catalysts with optimized composition by displacement method for the electrochemical reduction of N₂O in alkaline solution^[74]. Among the Pd/Cu catalysts with different compositions, Pd₆₀Cu₄₀ has the highest electrocatalytic activity, and its mass activity is significantly higher than that of commercial Pd/C when the reduction potential is less than-0. 3 V. The appropriate decrease of the d-band center of the catalyst promotes the adsorption and desorption equilibrium of N₂O reduction, thus significantly improving the catalytic reduction activity.

Hydrogen is an environmentally friendly energy carrier, which is clean, pollution-free and renewable. Hydrogen production by water electrolysis is an important strategy for the development of sustainable clean energy^[75]. In order to achieve higher energy efficiency of hydrogen evolution reaction, efficient catalysts that can reduce the overpotential to a minimum are needed. Zhang et al. Prepared monodisperse bimetallic Pd_xCu_100-_x nanoparticles with an average particle size of about 4.7 nm by one-pot method in oleylamine medium^[39]. Among them, mV vs RHE exhibited the best electrocatalytic activity with HER onset potential (75 mV vs RHE) much lower than that of commercial Pd/C (182 mV), and the exchange current density and Tafel slope were close to those of commercial Pt/C. Even after 1000 cycles in an acidic environment, almost no decrease in HER activity was observed, showing Pd₇₁Cu₂₉/C excellent durability. According to the calculation, the loss of ECSA after 1000 cycles is only one tenth of that of commercial Pt/C.

5 Application of Palladium-Copper Electrocatalyst in Electroless Copper Plating

Electroless copper plating can deposit copper metal layer on insulating substrate, which has important applications in conductivity, shielding and so on. Electroless copper plating usually includes pretreatment, roughening, presoaking, activation, acceleration, electroless copper plating and post-plating treatment. The activation step is to use a catalyst to promote the reduction of metal copper ions to obtain a metal copper layer. At present, Pd/Sn colloid is the most widely used catalyst, but its cost is high and its stability is general, so it is necessary to develop a suitable activation solution to replace it^[44]. Guo Ying prepared the core-shell structure nano-Cu @ Pd colloidal electrocatalyst by two-step reduction method^[15]. It can be used to activate the roughened epoxy resin substrate, and has good catalytic performance of electroless copper plating. As shown in Fig. 19, using SEM to observe the copper layer, it can be clearly seen that the copper layer is uniformly and densely deposited on the epoxy substrate, and the area not covered by the copper film can hardly be observed. The PdCu/rGO prepared by Ye et al. Can not only be used for MOR catalysis, but also be used as an electrocatalyst for electroless copper plating. Electroless copper plating addition experiments were carried out on two substrates, epoxy board and ordinary paper, to prepare conductive circuits^[42]. The electroless copper coating on the epoxy board is flat, and the copper particles are deposited uniformly. The copper coating on the A4 paper has good conductivity. After further bending test, it is found that the resistance value is basically unchanged after 100 times of bending, and it has good stability, as shown in Figure 20 (a). The ₅Pd⁃₈Cu/C QDs prepared by Biemolt et al. Can also be used for electroless copper plating on silicone rubber plates, and the average deposition rate can reach 35μg_Cu·mg_Pd^-1·s^-1^[76].

6 Conclusion and prospect

The common method for preparing PdCu alloy nano-electrocatalysts is to co-reduce the precursors of Pd and Cu, and the reducing agents are usually ascorbic acid, hydrazine hydrate, sodium borohydride, etc. However, if organic solvents such as ethylene glycol or oleylamine are used, the reducing agent may not be added because they have reducing properties. If the nano-alloy is loaded on the carbon carrier, the alloy can be prepared first, and then the carrier is added for ultrasound, so that the alloy is loaded on the carbon; Or the carrier is added before the reduction, the co-impregnation method is adopted, and the reduction is carried out by hydrogen after drying. The nanoalloy with specific morphology can improve the catalytic activity. In order to control the morphology of the alloy, the influence of surfactants, stabilizers and reducing agents in the synthesis process must be considered, because they are closely related to the dispersion and formation rate of the alloy, and these parameters will affect the final morphology. In addition, stepwise reduction is a good choice to control the morphology, which can not only synthesize hollow, cage-like alloys, but also prepare non-alloy nanoparticles with core-shell structure. Dealloying can selectively dissolve Cu in PdCu alloy from the alloy, which can not only be used to form nanoporous structure, but also adjust the morphology of nanoparticles and enhance the strain effect. Dealloying can be performed either by acid treatment or with the aid of cyclic voltammetry scans.

The improvement of catalytic activity mainly depends on two aspects, one is the exposure of active sites, the purpose of porous structure, hollow structure, core-shell structure, monatomic and diatomic catalysts is to expose active sites as much as possible, increase the electrochemical active surface area, and promote the contact between reactants and active sites; Small-sized catalysts possess a larger specific surface area, which can better expose active sites, such as nanoclusters. Taking the small size to the extreme is the single-atom catalyst, which is a very promising development direction, and its nearly 100% atom utilization rate can greatly reduce the cost. The design of special structures (such as dendritic and hollow nanocages) can also expose more active sites and increase the electrochemical surface area of the catalyst. If we can design the structure of nano-electrocatalysts with small size, it will be possible to greatly improve the catalytic activity. The other is to adjust the d-band center. In Pd-Cu alloy, the d-band center of Pd can be adjusted by the ligand effect and strain effect through the introduction of Cu, and the appropriate d-band position can optimize the adsorption and desorption of reactants or intermediate products, thus improving the catalytic activity and anti-poisoning ability.

For the future development, we believe that we should pay attention to the large-scale, low-cost and controllable preparation of high-performance PdCu electrocatalysts, and further study the structure-activity relationship of PdCu electrocatalysts.The PdCu electrocatalyst is applied to the electrochemical oxidation of organic small molecules such as methanol, ethanol, formic acid and the like, the electrochemical reduction of inorganic small molecules such as oxygen, nitrogen, carbon dioxide and the like, and the electroless copper plating in the field of electronic industry.

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Funding

Shenzhen Science and Technology Program(JCYJ20200109110416441)

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