1 Introduction

The world's growing energy demand and fossil fuel consumption have correspondingly increased greenhouse gas emissions, driving the development of alternative energy sources with no or less pollution. Direct ethylene glycol fuel cells (DAFCs) have attracted much attention due to their high theoretical energy density, low environmental hazard, sufficient supply, renewability, and low volatility. Its commercialization has been an important research focus [1,2,3]. Rational design and fabrication of low-cost materials, strong carbon monoxide (CO) anti-toxicity, and long-term stability of anode electrodes are the keys to liquid fuel oxidation. However, the low oxidation kinetics, C–C bond cleavage during oxidation reactions, and the formation of polarized carbon-containing intermediates such as –COO, –CO, and –COOH on the catalyst surface severely hinder the commercialization of DAFCs [4, 5].

Indium oxide (In2O3) is widely used in various industries because of its excellent optical and electrical properties, non-toxicity and large activity [6, 7]. For example, it is used in lithium-ion batteries, dye-sensitized solar cells, and photocatalytic and gas sensors [8,9,10,11,12]. However, there are few research on the application of In2O3 in the electrocatalytic oxidation of ethylene glycol. Metal–organic frameworks (MOFs) are used as template precursors to synthesize different functional materials to form ordered and regular pore structures [13,14,15]. In another case, MOFs act as precursors and are converted to oxides upon high-temperature calcination [16,17,18]. This open frame structure and strong interaction with guest oxide nanoparticles (NPs) have greatly enhanced catalytic performance in many fields [19]. However, the electrocatalytic performance of MOFs is limited because of their poor electrical conductivity [20, 21]. A facile way to improve electrical conductivity is to grow MOFs on conductive carbon materials such as graphene. For example, Yuan and co-workers [22] report a ZnSe–FeSe2/RGO nanocomposite which enhances sodium/potassium storage. Nevertheless, due to the strong π–π interaction of graphene and its two-dimensional (2D) structure, it is hard to effectively expose all open metal-based active sites [14].

In the past few years, researchers have been focused on increasing the electrocatalytic active sites by doping metal atoms to improve the electrocatalytic performance of ethylene glycol oxidation reaction (EGOR) [23,24,25]. During EGOR, more C–C cleavage is required for complete oxidation. Several partially oxidized by-products are produced during this process, including glyoxal, glycolaldehyde, glycolic acid, oxalic acid, and glyoxylic acid [2, 26]. Complete oxidation of one EG molecule to CO2 releases 10 electrons, producing a large amount of CO toxic intermediate [27]. This requires electrocatalysts with high activity and good CO resistance. Pd is cost-effective with high atomic efficiency and catalytic performance [28]. Recently, combining palladium with non-noble metal (Cu, Fe, Co, etc.) has been shown to be an effective strategy for the construction of bimetallic and even trimetallic palladium-based catalysts [29, 30]. In addition, the reported Pd alloys and compounds perform very well [31,32,33,34]. Besides, the introduction of oxophilic metals such as Pb can well promote the formation of surface-adsorbed OH substances ([OH]ads) [35]. It can effectively convert CO-toxic intermediates on Pd to CO2 at lower potentials, ultimately accelerating EGOR [36, 37]. At present, there are many kinds of electrocatalysts for metal and metal oxides, such as Cu-based bimetallic catalysts, TiO2/Ti3C2Tx MXene and MoSe2/Ti3C2Tx [38,39,40].

Herein, a three-dimensional (3D) columnar composite PdPbBi@rGO/In2O3 was first prepared by a simple hydrothermal, calcination and reduction method (Scheme 1). The obtained trimetallic catalyst (PdPbBi@rGO/In2O3) exhibits excellent electrochemical performance. The forward current density is 229.12 mA·cm−2, which is 8.46 times that of Pd/C (27.07 mA·cm−2). The electrochemical active surface area (ECSA) of PdPbBi@rGO/In2O3 is 85.87 m2·g−1Pd, which is 3.38 times that of Pd/C (25.41 m2·g−1Pd). In addition, PdPbBi@rGO/In2O3 has higher toxicity resistance and better stability to CO than commercial Pd/C. This new electrocatalyst offers inspiration for the commercial application of fuel cells.

Scheme 1
scheme 1

Schematic diagram of preparation process of rGO/In2O3 electrocatalyst

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2 Experimental

2.1 Synthesis of rGO/In2O3

Graphene oxide (GO) was prepared from graphite powder by modified Hummer’s method [41, 42]. 0.27 g indium nitrate (In(NO3)3·xH2O), 0.1980 g 1,3,5-benzentricarboxylic acid (H3BTC), and 0.0495 g GO were added to a mixture of 3 ml H2O and 12.4 ml N,N-dimethylformamide (DMF) and then stirred for 1 h. Then the mixed solution was poured into a 50 ml Teflon-lined autoclave and a hydrothermal reaction was performed at 120 °C for 5 days. After cooling, the obtained black product was filtered, washed three times alternately with distilled water, DMF and absolute ethanol, and dried under vacuum at 40 °C overnight. The resulting product was then calcined at 800 °C in a nitrogen atmosphere for 3 h. Finally, the precursor rGO/In2O3 was obtained.

2.2 Synthesis of PdPbBi@rGO/In2O3 electrocatalysts

50 mg rGO/In2O3 was disperses in EG (50 ml) by ultrasonication, then 5 ml palladium (II) chloride (PdCl2) (0.0189 mol·L−1) was added to the above solution and stirred evenly. After that, 126 μl lead nitrate (Pb(NO3)2) (0.0756 mol·L−1) and 375.2 μl bismuth nitrate (Bi(NO3)2·5H2O) (0.0189 mol·L−1) were simultaneously added to the above solution under magnetic agitation. And the mixture was stirred at room temperature for 3 h. 80 mg sodium borohydride (NaBH4) was dissolved in 20 ml distilled water, drop by drop into the mixed solution and stirred for 4 h. Then the product was collected by filtration, washed three times alternately with distilled water and ethanol, and dried overnight in 40 °C vacuum to obtain PdPbBi@rGO/In2O3. By the same procedure, PdPb@rGO/In2O3, PdBi@rGO/In2O3 and Pd@rGO/In2O3 electrocatalysts can also be synthesized without the addition of Bi(NO3)2·5H2O, Pb(NO3)2.

3 Results and discussion

3.1 Phase analysis

Structures of PdPbBi@rGO/In2O3, PdPb@rGO/In2O3, PdBi@rGO/In2O3, Pd@rGO/In2O3, rGO/In2O3 and In2O3 were analyzed by X-ray diffractometer (XRD) (Fig. 1a). The principal diffraction peaks of the support rGO/In2O3 at 2θ = 21.60°, 30.54°, 35.46°, 41.74°, 45.58°, 51.00°, 60.71° correspond to (211), (222), (400), (332), (134), (440), (622) planes of the In2O3 (ICDD No. 71-2195), indicating that In2O3 has been successfully synthesized [43, 44]. At 2θ = 25.0°, the peak of In2O3 overlaps with the broad peaks of graphene, so that the (002) plane of graphene cannot be observed [45]. In addition, the diffraction peaks around 39.9° and 46.10° of the four catalysts belong to the (111) and (200) crystal planes of the face-centered cube (fcc) crystalline of Pd, respectively [42]. Figure 1b is the magnified XRD pattern. Compared with the diffraction angle of Pd@rGO/In2O3, the Pd (111) planes of PdBi@rGO/In2O3, PdPb@rGO/In2O3 and PdPbBi@rGO/In2O3 are shifted. The Pd (111) planes are shifted from 39.6° to 39.4°, 39.1° and 38.8°, respectively. This indicates that alloys have been formed between Pd, Pb and Bi [46, 47]. Furthermore, in the bimetallic PdPb, PdBi and trimetallic PdPbBi@rGO/In2O3 catalysts, due to the content of Pb and Bi, there are thin or amorphous phases, and no obvious Pb or Bi diffraction peaks are observed.

Fig. 1
figure 1

a XRD patterns of PdPbBi@rGO/In2O3, PdPb@rGO/In2O3, PdBi@rGO/In2O3, Pd@rGO/In2O3, rGO/In2O3 and In2O3; b diffraction peaks of Pd in PdPbBi@rGO/In2O3, PdPb@rGO/In2O3, PdBi@rGO/In2O3 and Pd@rGO/In2O3

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3.2 Morphology and composition analysis

Surface morphology of PdPbBi@rGO/In2O3 was investigated by field emission scanning electron microscopy (FESEM). Compared with rGO/In2O3 (Fig. 2a), the morphology of PdPbBi@rGO/In2O3 (Fig. 2b) is a prismatic surface covered with wrinkled rGO and metal NPs. Pd-Pb-Bi NPs are mostly uniformly spread over the surface of rGO/In2O3. By analyzing the elemental mapping in Fig. 2c, we can further demonstrate that Pd, Pb, Bi, In and O are evenly distributed on the support rGO/In2O3. Energy dispersive X-ray spectrometer (EDS) spectrum (Fig. 2d) showed that the actual atomic loadings of Pd, Pb and Bi on the catalyst were 18.40%, 4.83% and 2.11%, respectively. The mass percentages of Pd, Pb and Bi measured by inductively coupled plasma optical emission spectrometer (ICP-OES) were 15.45%, 6.02% and 3.25%, respectively. The test results of both were close (Table S1), which was also in agreement with the initial addition ratio of the reactants. The above results indicate that PdPbBi@rGO/In2O3 has been successfully synthesized, and the PdPbBi NPs are uniformly dispersed on the surface of the support rGO/In2O3.

Fig. 2
figure 2

FESEM images of a rGO/In2O3 and b PdPbBi@rGO/In2O3; c elemental mappings and d EDS results of PdPbBi@rGO/In2O3

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High-resolution transmission electron microscope (HRTEM) was used to further observe the morphology of PdPbBi@rGO/In2O3 (Fig. 3). In2O3 is a columnar structure with a length of about 2 μm (Fig. S1), which is consistent with SEM image. Figure 3a, b shows the crystalline properties of PdPbBi@rGO/In2O3. The lattice fringe spacing of 0.272 nm corresponds to the (222) plane of In2O3 in Fig. 3a. From the continuous lattice spacing we know that the crystal phase of In2O3 is pretty good. Figure 3b shows the well-shaped lattice stripes with interplanar spacing of 0.228 nm, corresponding to the (111) plane of Pd. Compared with pure Pd (0.223 nm), the interplanar spacing of Pd in the catalyst is slightly larger. The lattice of Pd expands with the addition of Pb and Bi NPs, confirming the formation of PdPbBi nanoalloys [48]. It is evident that most of the PdPbBi NPs are uniformly distributed on rGO/In2O3. More than 100 particles in Fig. 3c were randomly selected for statistical analysis to determine the size of the NPs. The statistical results in Fig. 3d show that the particle size of Pd-Pb-Bi alloy ranges from 1.5 to 7.5 nm, with an average particle size of (4.50 ± 0.86) nm. Smaller metal particles can better adsorb oxygenated substances, which is beneficial for the electrocatalytic oxidation of EG.

Fig. 3
figure 3

HRTEM images of a–c PdPbBi@rGO/In2O3; d particle size distribution of Pd-Pb-Bi NPs

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3.3 Analysis of elements and valence states

Valence state analysis of elements in PdPbBi@rGO/In2O3 was studied by X-ray photoelectron spectroscopy (XPS). The survey spectra show the photoelectron peaks of Pd, Pb, Bi, In, C and O in the trimetallic catalyst (Fig. S2). Pd 3d shows two asymmetric peaks, where binding energies (BEs) of Pd(0) 3d5/2 and Pd(0) 3d3/2 signals are detected at 335.3 and 340.8 eV (Fig. 4a) [49]. Figure 4b shows that the weak signal of Pb(II) is located at 138.3 eV (4f7/2) and 143.2 eV (4f5/2). Strong peaks of 137.0 and 141.9 eV correspond to Pb(0) 4f7/2 and Pb(0) 4f5/2, respectively, confirming that Pb exists in PdPbBi@rGO/In2O3 as elemental and divalent ions [50]. The BEs of 157.3 and 162.6 eV are assigned to Bi 4f7/2 and Bi 4f5/2 of Bi(0), respectively (Fig. 4c). Notably, the BE of Pd in PdPbBi@rGO/In2O3 shows slightly positive shifts compared to those of pure Pd (Pd 3d5/2 at 335.0 eV, Pd 3d3/2 at 340.3 eV). This may be due to the fact that the incorporation of Pb and Bi affects the electronic structure of Pd, showing remarkable changes in the alloying phase, thereby improving the catalytic performance [28, 51]. Figure 4d depicts XPS spectrum of In 3d, showing two peaks at 444.8 and 452.3 eV. This can be attributed to the characteristic spin–orbit splitting of 3d5/2 and 3d3/2, respectively [52]. This proves that the valence state of indium in In2O3 is mainly + 3 [53]. Compared with pure In (In 3d5/2 at 443.8 eV, In 3d3/2 at 451.4 eV), the BE of In also shows slight positive shifts, indicating the interaction between In2O3 and rGO or metal NPs. The BE of O 1s is 531.8 eV compared to O vacancies (533.2 eV) (Fig. 4e) [54, 55]. This indicates the formation of In–O–In bonds in PdPbBi@rGO/In2O3 [56]. XPS spectrum of C 1s in Fig. 4f shows that there are two peaks, the prominent peak at 285.1 eV belongs to the C–C bond of rGO, another peak at 289.1 eV corresponds to the O–C=O functional group of rGO [57]. The peaks of oxygenated functional groups are particularly small, indicating that GO is reduced to rGO in the electro-catalyst.

Fig. 4
figure 4

XPS spectra of a Pd 3d, b Pb 4f, c Bi 4f, d In 3d, e O 1s and f C 1s in PdPbBi@rGO/In2O3

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3.4 Electrochemical performance analysis

In Fig. 5a, the anodic peaks between −0.1 and 0.5 V show the formation of PdO monolayer, Bi3+ and Pb2+ [58]. The reduction of Pd2+ to Pd, Pb to Pb2+ and Bi to Bi3+ results in apparent cathode peaks between −0.1 and 0.8 V [27]. According to the integral area of reduction peak (Fig. 6), the ECSAs of PdPbBi@rGO/In2O3, PdPb@rGO/In2O3, Pd@rGO/In2O3, PdBi@rGO/In2O3 and Pd/C are 85.87, 72.52, 60.48, 38.54 and 25.41 m2·g−1Pd, respectively. The corresponding ECSAs data are listed in Table 1. The above results illustrate that the PdPbBi@rGO/In2O3 catalyst has more active centers and can absorb more oxygenated substances, thereby better promoting the electro-catalytic oxidation of EG. The EGOR performances of five catalysts were evaluated in 1.0 mol·L−1 KOH + 0.5 mol·L−1 EG (Fig. 5b). The cyclic voltammetry (CV) curve has two peaks, the peak with higher density at 0.5–0.7 V (forward scan) is designated as the ethylene glycol oxidation intermediate, and the other at 0–0.3 V (reverse scan) is the process by which intermediates such as CHx and CO are further oxidized to CO2 [1, 59]. The forward peak current densities (Ip,f) of PdPbBi@rGO/In2O3 (229.12 mA·cm−2) are 8.22, 1.73, 1.34 and 1.08 times higher than those of Pd/C (27.07 mA·cm−2), Pd@rGO/In2O3 (200.21 mA·cm−2), PdBi@rGO/In2O3 (172.27 mA·cm−2) and PdPb@rGO/In2O3 (162.61 mA·cm−2), respectively. The onset potential during electro-oxidation of EG (Eonset) of PdPbBi@rGO/In2O3 (−0.56 V) is smaller than those of PdPb@rGO/In2O3 (−0.51 V), PdBi@rGO/In2O3 (−0.50 V), Pd@rGO/In2O3 (−0.48 V) and Pd/C (−0.33 V). Meanwhile, the catalytic activity of the PdPbBi@rGO/In2O3 catalyst is significantly higher than that of the catalysts reported in other studies (Table S2).

Fig. 5
figure 5

CV curves of electrocatalysts in a 1.0 mol·L−1 KOH, b 1.0 mol·L−1 KOH + 0.5 mol·L−1 EG (scan rate: 50 mV·s−1); c EGOR curves of PdPbBi@rGO/In2O3 at various scan rates; d curves of peak current density (jP) of four different catalysts and square root of scanning rate (ν1/2); e CA curves of five catalysts for 3600 s; f Nyquist plots of five catalysts

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Fig. 6
figure 6

a Integral area and b ECSA normalized area of five catalysts

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Table 1 Results of CV measurements of all catalysts modified electrodes
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We further investigated the kinetics of PdPbBi@rGO/In2O3 on catalytic EG oxidation (Fig. 5c). The peak current density (jP) increases as the potential scanning rate increases from 50 to 250 mV·s−1, and the corresponding peak potential (V) shows a continuous positive shift. The square root of the scan rate (ν1/2) for four catalysts is linear with jP (Fig. 5d). The diffusion correlation factor (R2) of all lines is 0.99, indicating that the electro-oxidation reaction of each catalyst is an irreversible electrode process controlled by diffusion [60, 61]. The corresponding order of slope values is as follows: PdPbBi@rGO/In2O3 (3.99) > PdPb@rGO/In2O3 (3.39) > PdBi@rGO/In2O3 (2.33) > Pd@rGO/In2O3 (1.67). Higher slope values indicate better electrooxidation kinetics. The above results indicate that among all the catalysts, PdPbBi@rGO/In2O3 has the best kinetics for EGOR and the highest electrocatalytic activity.

To evaluate the EGOR stability of these five catalysts, we tested them in a solution containing 0.5 mol·L−1 EG + 1.0 mol·L−1 KOH at −0.1 V for 3600 s by chrono-amperometry (CA). The current densities of all five catalysts show a significant decrease in the initial stage, and then decrease slowly (Fig. 5e). In the initial stage, the rapid decline in the density of the current is caused by the accumulation of toxic intermediate species on the Pd surface, resulting in slow oxidation kinetics and rapid decrease of catalytic activities. After that, the current densities slowly decline until they reach a relatively steady state [2]. Compared with PdPb@rGO/In2O3 (33.79 mA·cm−2), PdBi@rGO/In2O3 (23.79 mA·cm−2), Pd@rGO/In2O3 (19.43 mA·cm−2) and Pd/C (4.03 mA·cm−2), PdPbBi@rGO/In2O3 maintained improved electrooxidative activity towards EG, and the retained current density (40.47 mA·cm−2) still reaches 51.38% of the initial value. The normalized currents of the five catalysts also show the same pattern after 3600 s (Fig. S3). From above results, it is proved that doped alloy NPs can improve the catalytic activity of the catalyst.

Diffusion kinetics and charge transfer of different electrocatalysts on electrode/electrolyte interface were evaluated by measuring Nyquist plot of electrochemical impedance spectroscopy (EIS) (Fig. 5f). Nyquist’s arcs are displayed in the following order: PdPbBi@rGO/In2O3 (12.68 Ω) < PdPb@rGO/In2O3 (13.19 Ω) < PdBi@rGO/In2O3 (17.17 Ω) < Pd@rGO/In2O3 (18.88 Ω) < Pd/C (35.19 Ω). The smaller the impedance is, the more favorable the transfer of charge is from electrode to fuel, reducing the fuel oxidation activation barrier and the electrode reaction overpotential [62]. EIS results confirm that the trimetallic catalyst PdPbBi@rGO/In2O3 exhibits superior charge transfer properties, faster electrochemical kinetics and better catalytic performance than other catalysts.

Figure 6a is the integral area data of PdPbBi@rGO/In2O3, PdPb@rGO/In2O3, PdBi@rGO/In2O3, and Pd@rGO/In2O3 catalysts. The integral area (X) was obtained from the reduction peak of oxides appearing from −0.1 to −0.8 V in CV curve in 1.0 mol·L−1 KOH electrolyte, and ECSA values were calculated from X. The integral area of the PdPbBi@rGO/In2O3 is larger than that of PdPb@rGO/In2O3, PdBi@rGO/In2O3, and Pd@rGO/In2O3. The normalized area is obtained by comparing the ECSA value to the mass of Pd (Fig. 6b).

Analysis of the above test results showed that the synergistic effect of PdPbBi NPs and rGO/In2O3 significantly enhanced its electrocatalytic activity. The excellent electrocatalytic activity of PdPbBi@rGO/In2O3 also contributes to the porous structure of rGO/In2O3. This enables the Pd-Pb-Bi alloy NPs to be uniformly dispersed with more active sites, thereby accelerating the Pd-Pb-Bi electron transfer from NPs to rGO/In2O3. To elucidate the excellent electrocatalytic activity of PdPbBi@rGO/In2O3, the electrocatalytic oxidation mechanism of EG is proposed (Scheme 2).

Scheme 2
scheme 2

Electrooxidation mechanism of EG and removal process of [CO]ads on Pd sites

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We infer that the removal of [CO]ads facilitated EGOR through several processes [63]:

$${\text{C}}_{{2}} {\text{H}}_{{6}} {\text{O}}_{{2}} \to [{\text{C}}_{{2}} {\text{H}}_{{6}} {\text{O}}_{{2}} ]_{{{\text{ads}}}}$$
(1)
$${\text{OH}}^{ - } \to [{\text{OH}}^{ - } ]_{{{\text{ads}}}}$$
(2)
$$[{\text{C}}_{{2}} {\text{H}}_{{6}} {\text{O}}_{{2}} ]_{{{\text{ads}}}} + {4}[{\text{OH}}^{ - } ]_{{{\text{ads}}}} \to \left( {{\text{CHO}}} \right)\left( {{\text{CHO}}} \right) + {\text{4H}}_{{2}} {\text{O}} + {\text{4e}}^{ - }$$
(3)
$$\left( {{\text{CHO}}} \right)\left( {{\text{CHO}}} \right) + {6}[{\text{OH}}^{ - } ]_{{{\text{ads}}}} \to \left( {{\text{COO}}^{ - } } \right)_{{2}} + {\text{4H}}_{{2}} {\text{O}} + {\text{4e}}^{ - }$$
(4)
$$\left( {{\text{COO}}^{ - } } \right)_{{2}} + {4}[{\text{OH}}^{ - } ]_{{{\text{ads}}}} \to {\text{2CO}}_{3}^{2 - } + {\text{2H}}_{{2}} {\text{O}} + {\text{2e}}^{ - }$$
(5)

As shown in the above steps, the oxidation mechanism of EG is described as Reactions (1–5). Completely EG oxidation into carbonate ions will produce 10 electrons through a series of C–C and C–H bond breaks [60]. Intermediates such as glyoxal (Reaction (3)) and oxalate ions (Reaction (4)) are produced on the surface of catalysts. This results in partial occupancy of Pd active sites, which slows down electro-oxidative kinetics and reduces electrocatalytic activity [61]. The trimetallic composite PdPbBi@rGO/In2O3 exhibits excellent electrocatalytic performance for EGOR due to the columnar structure and abundant [OH]ads obtained from Bi and Pb/PbO NPs [64].

$${\text{2Pd}} + {\text{C}}_{{2}} {\text{H}}_{{6}} {\text{O}}_{{2}} + {\text{6OH}}^{ - } \to {2}[{\text{Pd}} - {\text{CO}}]_{{{\text{ads}}}} + {\text{6H}}_{{2}} {\text{O}} + {\text{6e}}^{ - }$$
(6)
$${\text{PbO}} + {\text{OH}}^{ - } \to [{\text{PbOOH}}]_{{{\text{ads}}}} + {\text{e}}^{ - }$$
(7)
$${\text{Pb}} + {\text{OH}}^{ - } \to [{\text{PbOH}}]_{{{\text{ads}}}} + {\text{e}}^{ - }$$
(8)
$${\text{Bi}} + {\text{OH}}^{ - } \to \left[ {{\text{BiOH}}} \right]_{{{\text{ads}}}} + {\text{e}}^{ - }$$
(9)
$$[{\text{PdCO}}]_{{{\text{ads}}}} + [{\text{PbOOH}}]_{{{\text{ads}}}} + {\text{3OH}}^{ - } \to {\text{Pd}} + {\text{PbO}} + {\text{CO}}_{{3}}^{{{2} - }} + {\text{2H}}_{{2}} {\text{O}} + {\text{e}}^{ - }$$
(10)
$$[{\text{PdCO}}]_{{{\text{ads}}}} + [{\text{PbOH}}]_{{{\text{ads}}}} + {\text{3OH}}^{ - } \to {\text{Pd}} + {\text{Pb}} + {\text{CO}}_{{3}}^{{{2} - }} + {\text{2H}}_{{2}} {\text{O}} + {\text{e}}^{ - }$$
(11)
$$[{\text{PdCO}}]_{{{\text{ads}}}} + \left[ {{\text{BiOH}}} \right]_{{{\text{ads}}}} + {\text{3OH}}^{ - } \to {\text{Pd}} + {\text{Bi}} + {\text{CO}}_{{3}}^{{{2} - }} + {\text{2H}}_{{2}} {\text{O}} + {\text{e}}^{ - }$$
(12)

The modified Pd is the catalytically active site. EG adsorbs on the Pd site (Reaction (6)) and dehydrogenates to the [CO]ads intermediate, resulting in deactivation of the Pd catalyst. Because of the higher affinity of Pb/PbO for oxygenated substances, oxidized surfaces such as OH− species are generated at Pb-Bi site than at Pd site (Reactions (7–9)), forming [PbOH]ads and [PbOOH]ads. This would facilitate the conversion of the adsorbed [CO]ads to CO32− (Reactions (10–12)) in alkaline media, thus making more Pd active sites available for EG oxidation. Furthermore, due to the different electronegativities of Pd, Pb and Bi, charges are transferred from Pb and Bi to the Pd surface. The reduction of the d-band center of Pd is due to this electronic effect between Pd–Pb–Bi. The combination of Pd with Pb and Bi alloys effectively modifies the electronic structure of Pd. This will significantly accelerate the electron transfer rate due to the synergistic effect of Pd–Pb–Bi, thereby increasing the catalytic activity [65]. On the other hand, Pb and Bi co-doping can promote the adsorption and oxidation of intermediate carbon-containing species, accelerate the oxidation elimination of intermediate poisons (especially CO), and enhance the electrochemical stability of EGOR [48]. Apart from that, the support with porous columnar structure and excellent electrical conductivity is favorable to provide larger surface area and loading of alloy NPs. As a result, the binding energy of the active site for CO adsorption and the activation energy for ethylene glycol adsorption oxidation decrease. These results provide important value for the development of high efficiency alcohol-based fuel cell electrocatalysts.

4 Conclusion

The Pd-Pb-Bi nanoalloys were supported on the hydrothermally and calcined precursor rGO/In2O3 by reduction method. Then a trimetallic columnar catalyst PdPbBi@rGO/In2O3 was obtained, which exhibits excellent electrochemical performance for EGOR. PdPbBi@rGO/In2O3 has higher forward current density (229.12 mA·cm−2), larger ECSA (85.87 m2·g−1Pd), smaller impedance (12.68 Ω) and smaller Eonset (−0.56 V) than commercial Pd/C, Pd@rGO/In2O3, PdBi@rGO/In2O3 and PdPb@rGO/In2O3. Specifically, the current density and ECSA are 8.46 and 3.38 times higher than those of Pd/C (27.07 mA·cm−2, 25.41 m2·g−1Pd), respectively. The oxidation mechanism of glycol and the process of CO removal are also discussed. The columnar support structure wrapped by reduced graphene oxide provides a huge active surface area for catalysis. Moreover, the electronic effect among Pd-Pb-Bi nanoalloy particles can accelerate the removal of CO intermediate species, resulting in more Pd active sites and improved electrocatalytic performance. Our first synthesis of this highly electrocatalytic composite material provides a good idea for further commercial application of direct fuel cells.