Electro-catalysis of carbon black or titanium sub-oxide supported Pd–Gd towards formic acid electro-oxidation

Abstract

Carbon black supported Pd–Gd catalysts (Pd–xGd/C, x is weight percent in catalyst) with different amounts of Gd were prepared by a simultaneous reduction reaction with sodium borohydride in aqueous solution. The structure, morphology and element valence state of these catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. The electro-catalytic performance of these catalysts for formic acid oxidation was investigated using cyclic voltammetry (CV), chronoamperometry (CA) and CO stripping experiments. It is found that the Pd–2.5Gd/C catalyst has a better electro-catalytic activity than the Pd/C catalyst, which can be explained by a bi-functional mechanism. In addition, the higher content of metallic Pd caused by the addition of Gd also contributes to the better catalytic activity of Pd–2.5Gd/C. Based on the good electro-catalytic performance of the Pd–2.5Gd/C, Pd–xGd/Ti₄O₇ (x stands for weight percentage in whole catalyst) catalysts were also prepared and characterized in the same way. The Pd–2.5Gd/Ti₄O₇ and Pd–4Gd/Ti₄O₇ catalysts exhibited better catalytic activities than the Pd–2.5Gd/C catalyst, which mainly results from the further increase of metallic Pd caused by Ti₄O₇.

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Introduction

As humans attach much importance to energy and the environment, direct formic acid fuel cells (DFAFCs) have gained increasing attention as a promising clean energy conversion device, due to their merits, such as relatively low fuel toxicity, high energy density and low penetration efficiency through a Nafion membrane.^1–3 It is well known that Pt based catalysts are effective for the electro-oxidation of formic acid. However, with Pt as the anode catalyst, the surface is usually heavily poisoned by the strong adsorption of CO intermediates produced during the oxidation of formic acid, leading to the lowering of catalytic performance.⁴ Moreover, the high cost of Pt is also an obstacle to its use for commercial applications. As compared with Pt, Pd based catalysts not only possess good electrochemical activity,^5,6 but also have much better tolerance to CO poisoning and relatively cheaper price.

Although palladium has many advantages, the electro-oxidation of formic acid on Pd catalysts in general requires a substantially high overpotential (∼0.3 V), which significantly impedes the large-scale commercialization of DFAFCs.⁷ Additionally, the stability of Pd in acidic environment still needs to be further enhanced to meet practical application. A great many efforts have been made to improve the stability and catalytic activity of palladium by means of adding another metal into Pd based catalyst, such as Co,⁸ Fe,⁹ Ni,¹⁰ Bi,¹¹ Mn,¹² Cu,^13,14 Cr,¹⁵ Au,¹⁶ Pb,¹⁷ Pt¹⁸ and so on. The main reason for enhancement in Pd based catalyst can be attributed to a bi-functional mechanism.^19,20 In recent years, researches on Pt and Pd catalysts doped with rare-earth metals demonstrated that the rare-earth metal doping could also enhance the electro-catalytic activity of these Pt or Pd catalyst, which may not be only due to the bi-functional mechanism.^21,22 In our previous work, it was found that terbium²³ can promote the electro-oxidation of formic acid on Pt based catalyst. XPS analyses showed that the addition of Tb results in the increase of metallic Pt content, which is the other reason of the promotive effect besides the bi-functional mechanism. This research inspires our interest in the promotive effect of other rare earth elements. Gadolinium is adjacent to terbium in the periodic table of elements. Moreover, gadolinium has a more abundant terrestrial reserve than terbium. Thus, in this study, Pd/C electro-catalysts containing different amounts of Gd were prepared, and their activities towards the electro-oxidation of formic acid were evaluated.

It is well known that catalyst supports have a great impact on catalytic performance. Although carbon black is a common support material for Pt or Pd based catalysts due to its good electron conductivity and high surface area,²⁴ the corrosion of the carbon support would cause an adverse impact on working efficiency and the performance of fuel cells.²⁵ Therefore, the non-carbon materials have attracted much greater attention.^26,27 In our previous work,²⁸ it is found that Pt catalyst supported on titanium sub-oxide (Ti₄O₇) possesses better catalytic performance than Pt catalyst supported on carbon black for formic acid electro-oxidation. The performance improvement was mainly attributed to the higher content of metallic Pt caused by Ti₄O₇, as well as the high electrical conductivity of the Ti₄O₇ support material. To further enhance the performance of the Pd–Gd bimetallic catalysts, Ti₄O₇ was also used as a support material in this study, and the activities of these catalysts supported on Ti₄O₇ towards the electro-oxidation of formic acid were investigated.

Experimental

The chemicals used in this work were carbon black (Vulcan XC-72), PdCl₂, Na₃C₆H₅O₇·2H₂O, NaBH₄, H₂SO₄, HCOOH, Gd(NO₃)₃, Nafion solution (5 wt% in isopropanol and water) and de-ionized water. Ti₄O₇ was purchased from ShanDong Lianmeng Chemical Group Co. Ltd.

The catalyst ink was prepared by ultrasonically mixing 4 mg electro-catalyst sample in 2 mL of ethanol for about 20 min. The electro-catalyst was made as follows. The carbon black or Ti₄O₇ was dispersed into 75 mL water under ultrasonication. Then 0.01 M PdCl₂ aqueous solution and Gd(NO₃)₃ were added into the above suspension under magnetic stirring. Subsequently, sodium citrate, whose amount of substance is 1.5 times of that of Pd and Gd, was added into the above suspension. Afterwards, NaBH₄ aqueous solution (freshly prepared) was added dropwise. About 6 hours later, the suspension was filtered, washed and dried under 60 °C overnight. Then the catalysts were obtained and they were denoted as Pd–1Gd/C, Pd–2Gd/C, Pd–2.5Gd/C, Pd–10Gd/C, Pd–2.5Gd/Ti₄O₇, Pd–4Gd/Ti₄O₇ and Pd–5Gd/Ti₄O₇, respectively. The theoretical content of Pd is 20 wt%.

Structure and morphology of the catalysts were investigated using X-ray diffraction (XRD, X'Pert-PRO MPD, Cu K_α) and transmission electron microscopy (TEM, JEM-2100, 200 kV). Element valences state analyses were performed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi (Thermo Fisher Scientific Inc.)).

The electrochemical measurements were conducted on an electrochemical workstation system (CHI760D, Chenhua, Shanghai) with a three-electrode cell using Pt foil and saturated calomel electrode (SCE) as the counter and reference electrodes, respectively. Cyclic voltammetry (CV) was performed in a 0.5 M H₂SO₄ + 0.5 M HCOOH solution, where oxygen was removed by purging N₂ for 15 min. The working electrode was prepared by dropping 5 μL of the electro-catalyst ink onto glassy carbon electrode (GCE). Then 1 μL of Nafion solution was dropped on top to fix the electro-catalysts. All potentials in this work were given versus SCE electrode (+0.241 V vs. NHE). The CV experiments were conducted at a sweep rate of 50 mV s⁻¹, with the potential ranging from −0.2 V to 1.0 V. CO striping was performed as follows. After purging the solution with N₂ 20 min, gaseous CO was bubbled for 15 min to form CO adlayer on catalysts while maintaining potential at 0.1 V. Excess CO in solution was purged with N₂ for 20 min, and CO stripping voltammetry was recorded in 0.5 M H₂SO₄ at 20 mV s⁻¹. To investigate the stabilities of electrodes, the chronoamperometry (CA) experiments were conducted at 0.2 V for 4000 s with the same potential range. All the measurements were performed three times in this study.

Results and discussion

Fig. 1 displays the XRD patterns of the as-prepared Pd–Gd/C catalyst as well as the Pd/C catalyst for comparison Pd/C. All of them were supported on the carbon black. Thus, the peak appeared at about 25° in all the XRD patterns is recognized as the (002) plane of support material Vulcan XC-72 carbon support. The other four peaks around 40°, 46°, 67° and 81° represent the diffraction of Pd crystal faces (111), (200), (220) and (311), respectively, which shows the characteristics of face-centered cubic (fcc) structure of palladium, but no Gd peaks can be found. However, when compared with the peak of Pd for Pd/C, the addition of Gd causes slight shifts of the Pd peaks with 0.2°, 0.3°, 0.3° and 0.4° for Pd–1Gd/C, Pd–2Gd/C, Pd–2.5Gd/C and Pd–10Gd/C, respectively, which indicates that the Gd had been alloyed with the Pd. In other words, a solid solution of Pd and Gd formed.

Fig. 1 XRD patterns of (a) Pd/C, (b) Pd–1Gd/C, (c) Pd–2Gd/C, (d) Pd–2.5Gd/C and (e) Pd–10Gd/C catalysts.

Fig. 2 displays the TEM images and particle size distribution of the Pd/C and Pd–2.5Gd/C catalysts. It can be observed that the catalyst particles in Pd/C are well dispersed on carbon supports. After the addition of Gd, the Pd–Gd particles have a little aggregation. The average sizes of particles in Pd/C and Pd–2.5Gd/C could be estimated to be about 5.1 nm and 3.45 nm from Fig. 2(c) and (d), respectively.

Fig. 2 TEM images and particle size distribution of the Pd/C (a and c) and Pd–2.5Gd/C (b and d) catalysts.

Fig. 3(a) presents the CVs of Pd/C, Pd–1Gd/C, Pd–2Gd/C, Pd–2.5Gd/C and Pd–10Gd/C catalysts in 0.5 M H₂SO₄ solution. The peaks ranging from −0.2 to 0.1 is in accordance with the absorption/desorption of hydrogen, while those above 0.2 V are ascribed to the oxidation and reduction of surface metal. The electrochemical active surface areas (ECSAs) can be calculated using oxide reduction voltammetry by following equation:

where S is the Pd oxide reduction peak area in voltammetry; v is the scan rate; Q is the charge needed for the reduction of single layer of oxide on Pd, which is 0.000405 C cm⁻² and m is the loading amount of catalyst.²⁹ The values are 22.1 m² g⁻¹ for Pd/C and 47.2 m² g⁻¹ for Pd–2.5Gd/C, respectively. It is apparently seen that the ECSAs of Pd–Gd/C catalysts mainly increase with the addition of Gd. Fig. 3(b) shows the CVs of formic acid oxidation on the prepared catalysts, and the results were normalized by the ECSA. It can be found that the current density on the Pd–2.5Gd/C catalyst is much higher than that on the Pd/C catalyst, indicating that formic acid electro-oxidation is more active on the former than on the latter. This suggests that the addition of an appropriate amount of Gd has a significantly promotive effect on the electro-catalytic activity of Pd/C catalyst for formic acid oxidation. Therefore, in the remaining session, attention is focused on the best-performing catalyst, namely, Pd–2.5Gd/C.

Fig. 3 CVs of Pd/C, Pd–1Gd/C, Pd–2Gd/C, Pd–2.5Gd/C and Pd–10Gd/C catalysts in (a) 0.5 M H₂SO₄ and (b) 0.5 M HCOOH + 0.5 M H₂SO₄ solutions at a scan rate of 50 mV s⁻¹.

It is well known that the most commonly accepted mechanism of formic acid oxidation is the so-called parallel or dual pathway mechanism.³⁰ Direct oxidation on Pd catalysts occurs via a dehydrogenation reaction without forming CO as a reaction intermediate:

Pathway 1

Pd + HCOOH → Pd + CO₂ + 2H⁺ + 2e⁻ (1)

The second reaction pathway forms adsorbed CO as a reaction intermediate by dehydration:

Pathway 2

Pd + HCOOH → Pd–CO + H₂O (2)

Pd + H₂O → Pd–OH + H⁺ + e⁻ (3)

Pd–CO + Pd–OH → Pd + CO₂ + H⁺ + e⁻ (4)

Pathway 2 is similar to the well-known bi-functional catalysis of methanol electro-oxidation on Pt-alloy catalysts,¹⁹ which involves the successive oxidation of the functional group (–OH group) with adsorbed CO on Pd surface. Although pathway 1 is dominant for formic acid oxidation on Pd, a partial formic acid still oxidizes on Pd via pathway 2. Thus, a high resistance to CO poisoning is also important for Pd based catalysts towards formic acid electro-oxidation. To evaluate the resistance to the CO poisoning of the Pd/C and Pd–Gd/C catalysts, CO stripping were recorded, as shown in Fig. 4. Although Pd–2.5Gd/C and Pd/C have similar peak potentials, the onset potential of CO oxidation on Pd–2.5Gd/C at 0.45 V is slight more negative than that on Pd/C at 0.55 V, indicating that the addition of Gd contributes to the removal of CO poisoning intermediate out of the surface of the Pd–2.5Gd/C catalyst. In other words, according to the bi-functional mechanism,^19,20 the Gd activated water at lower potentials than Pd and the activated water could oxidize the adsorbed CO and therefore liberated Pd active sites. This result helps to explain the higher activity of the Pd–2.5Gd/C catalyst for the oxidation of formic acid.

Fig. 4 CO stripping curves on the Pd/C (a) and Pd–2.5Gd/C (b) catalysts recorded in 0.5 M H₂SO₄ solution.

To further investigate the function of Gd, Pd 3d core level spectra of the Pd/C and Pd–2.5Gd/C catalysts were recorded in Fig. 5. Both of the profiles were fitted by two pairs of overlapping Lorentzian curves. For Pd/C, the more intensive peaks (336.3 and 341.6 eV) are attributed to metallic palladium, Pd(0). The other pair of peaks (337.5 and 342.8 eV) are ascribed to the Pd(II) chemical state on Pd oxides or hydroxides. For Pd–2.5Gd/C, they are similar to those of Pd/C. However, the relative intensity of metallic Pd(0) in Pd–2.5Gd/C with 53.5% is higher than that in Pd/C with 49.4%. Table 1 lists the relative intensity of Pd(0) and Pd(II) of these two catalysts evaluated from XPS analyses. This suggests that the addition of Gd can enhance the content of Pd(0) while reduces the content of Pd(II). That is to say that Gd has a “metallization” effect on Pd. This may ascribe from the difference of valence electron configuration between Gd and Pd. The valence electron configuration of Gd is 4f⁷5d¹6s², electrons outside which are easy to lose. On the contrary, the Pd has a valence electron configuration of 4d¹⁰, which contains unoccupied orbits. Therefore, some electrons of Gd could have a trend to transfer to Pd, resulting in an increase of metallic palladium. As is well known, the higher the content of metallic state in catalysts, the better the catalytic performance.^31,32 Hence, the higher content of Pd(0) caused by the addition of Gd also contributes to the better catalytic activity of the Pd–2.5Gd/C catalyst.

Fig. 5 XPS spectra of Pd (3d) for (a) Pd/C and (b) Pd–2.5Gd/C catalysts.

Table 1 Binding energy (B.E.) and relative intensity of species from curve-fitted XPS spectra of Pd 3d

Samples	B.E. of Pd 3d5/2 (eV)	B.E. of Pd 3d3/2 (eV)	Species	Relative intensity (%)
Pd/C	336.3	341.6	Pd(0)	49.4
	337.5	342.8	Pd(II)	50.6
Pd–2.5Gd/C	335.6	340.8	Pd(0)	53.5
	336.7	342.1	Pd(II)	46.5

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Additionally, the durability of different catalysts towards formic acid electro-oxidation was examined by CA experiments. Fig. 6 shows the CA plots of Pd–2.5Gd/C and Pd/C at 0.2 V vs. SCE in 0.5 M HCOOH + 0.5 M H₂SO₄ solution. The currents on both catalysts drop rapidly at first, and then become relatively stable. The initial surge of current is caused by a charging current.³³ The current for formic acid oxidation on the Pd–2.5Gd/C catalyst is significantly larger than that on the Pd/C catalyst over the whole time range. This indicates that the stability of Pd–2.5Gd/C is fair besides better catalytic activity.

Fig. 6 CA curves of the Pd/C and Pd–2.5Gd/C catalysts in 0.5 M HCOOH + 0.5 M H₂SO₄ solution at 0.2 V.

Based on the good electro-catalytic performance of the Pd–2.5Gd/C catalyst, Pd–Gd catalysts supported on Ti₄O₇ were prepared and their catalytic performance for formic acid electro-oxidation was investigated.

Fig. 7 shows the XRD patterns of Pd–xGd/Ti₄O₇ (x: 2.5, 4 and 5 at percentage) catalysts. All of them have a similar pattern due to approximate content of Gd. The peaks around 40°, 46°, 67° and 82° represent the (111), (200), (220) and (311) planes of Pd, respectively. Other peaks except Pd peaks were well matched with the characteristic peaks of Ti₄O₇ (PDF (the powder diffraction file): 50-0787). Although no peak of Gd can be seen, it is still observed that the diffraction peaks at ca. 40° shift a little to lower 2θ value with the increase of Gd content, which indicates that the alloying of Gd and Pd had occurred.

Fig. 7 XRD patterns of Pd–xGd/Ti₄O₇ (x: 2.5, 4 and 5 at percentage).

Fig. 8 presents the TEM images of the Pd–2.5Gd/Ti₄O₇ and Pd–4Gd/Ti₄O₇ catalysts. It can be seen that Pd–Gd nanoparticles gather together in these two catalysts, which is similar to the dispersion state of Pt nanoparticles on Ti₄O₇.²⁸ This results from the hydrophobicity and higher mass density of Ti₄O₇ as compared with carbon black according to our previous analyses.²⁸

Fig. 8 TEM images of Pd–2.5Gd/Ti₄O₇ (a) and Pd–4Gd/Ti₄O₇ (b).

The CVs of these Pd–Gd catalysts supported on Ti₄O₇ in 0.5 M HCOOH + 0.5 M H₂SO₄ solution are shown in Fig. 9. For comparison, the CVs of Pd/C and Pd–2.5Gd/C are also shown in this figure. It can be seen that with the same amount of Gd, the peak current density on Pd–2.5Gd/Ti₄O₇ is higher than that on Pd–2.5Gd/C. Furthermore, with the increase of Gd content, the peak current density firstly increases and then decreases. The Pd–4Gd/Ti₄O₇ catalyst exhibits best catalytic activity.

Fig. 9 CVs of formic acid electro-oxidation on the Pd/C, Pd–2.5Gd/C, Pd–2.5Gd/Ti₄O₇, Pd–4Gd/Ti₄O₇ and Pd–5Gd/Ti₄O₇ catalysts in 0.5 M HCOOH + 0.5 M H₂SO₄ solution at a scan rate of 50 mV s⁻¹.

To investigate the effect of the supports on electronic properties of catalysts, Pd 3d core level spectra of the Pd–2.5Gd/Ti₄O₇ and Pd–4Gd/Ti₄O₇ catalysts were recorded in Fig. 10. An analysis similar to Fig. 5 was carried out, and the results were summarized in Table 2. It can also be found that the relative intensity of metallic Pd(0) in Pd–2.5Gd/Ti₄O₇ and Pd–4Gd/Ti₄O₇ is 56.4 and 60.3%, respectively, higher than 53.5% obtained for Pd–2.5Gd/C. In addition, the Gd 3d core level spectra of Pd–4Gd/Ti₄O₇ were also recorded in Fig. 10(c), which suggested the existence of Gd³⁺ and thus prove its trend to lose electrons. The higher metallic content of Pd on Ti₄O₇ compared to that on carbon black suggests that the Ti₄O₇ support also has a “metallization” effect for the Pd–Gd catalyst, which is similar to the Pt/Ti₄O₇ catalyst in our previous work.²⁶ Specifically, the effect may be due to the electrostatic interaction between metal particles and supports. Because the valence electron configuration of Ti is 3d²4s², Ti₄O₇ is an oxygen deficit species, namely betatopic configuration. Therefore, some electrons of Ti₄O₇ may have a tendency to transfer to Pd, resulting in an increase of metallic palladium content. As mentioned before, a higher content of metallic palladium in catalysts leads to a better catalytic activity. This is the main reason for the better performance of the Pd–2.5Gd/Ti₄O₇ and Pd–4Gd/Ti₄O₇ catalysts.

Fig. 10 XPS spectra of Pd (3d) for Pd–2.5Gd/Ti₄O₇ (a) and Pd (3d) and Gd (3d) for Pd–4Gd/Ti₄O₇ (b and c).

Table 2 Binding energy (B.E.) and relative intensity of species from curve-fitted XPS spectra of Pd 3d

Samples	B.E. of Pd 3d5/2 (eV)	B.E. of Pd 3d3/2 (eV)	Species	Relative intensity (%)
Pd–2.5Gd/Ti4O7	335.3	340.7	Pd(0)	56.4
	336.5	342.3	Pd(II)	43.6
Pd–4Gd/Ti4O7	335.4	340.6	Pd(0)	60.3
	336.5	342.6	Pd(II)	39.7

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Conclusions

The carbon supported Pd–Gd alloy catalysts were prepared, and the catalytic performance towards formic acid electro-oxidation was investigated. As compared with the Pd/C catalyst, the Pd–2.5Gd/C catalyst has a better electro-catalytic activity and higher resistance to CO poisoning, which can be explained by a bi-functional mechanism. The analyses for XPS spectra indicate that the higher content of metallic Pd caused by the addition of Gd also contributes to the better catalytic activity of Pd–2.5Gd/C. Based on the good electro-catalytic performance of the Pd–2.5Gd/C, the Ti₄O₇-supported Pd–Gd catalysts with different amounts of Gd were synthesized, and the Pd–2.5Gd/Ti₄O₇ and Pd–4Gd/Ti₄O₇ catalysts displayed better electro-catalytic activities. The improvement is mainly attributed to the further increase of metallic Pd caused by the Ti₄O₇ support.

Acknowledgements

The authors are grateful for the financial support by One Hundred Talent Program of Chinese Academy of Sciences, as well as by the 973 Program (Grant No. 2015CB251303).

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