Application of Binary PdSb/C as an Anode in a Polymeric Electrolyte Reactor-Fuel Cell Type for Electrosynthesis of Methanol from Methane

Abstract

PdSb catalyst prepared in different compositions were applied as an anode in a polymeric electrolyte reactor - fuel cell type (PER-FC) to convert methane into oxygenated products and energy in mild conditions. The PER-FC polarization curves for Pd₉₀Sb₁₀/C presented maximum current density about 0.92 mW cm^-2 about 15% higher than PdSb materials. However, the material Pd₅₀Sb₅₀/C showed higher reaction rate for methanol generation than the other materials occurring close to the OCV (r ~ 7 mol L^-1 h^-1). The qualitative analyses of PER-FC effluent by FT-IR identified products as methanol, carbonate and formate ions from the partial oxidation of methane for all materials.

Keywords:
Methane into methanol; polymeric electrolyte reactor - fuel cell type; chemical and energy co-generation

1. Introduction

Recently, approximately a quarter of the world's energy produced is used to supply the demand of the chemical transformation industry^1,2. This energy is applied in thermal processes that, although effective, they do not reach maximum efficiency due to the waste of heat caused by the lack of suitable materials and/or reactors that can avoid losses during chemical transformation reactions³. In addition, economic, social, legal and environmental organizations put pressure on the research and development area to use the best technological options available so as not to incur high costs so that this energy becomes a resource with low pollution processing.

Therefore, electrochemical processes play an important role, as they act directly on the electrostatic interactions between electron and atomic nucleus through the direct potential application on an atom or molecule, reducing energy losses to the environment⁴. Based on countries willingness to change their primary energy sources to less polluting energy sources, electrochemical processes can gain notoriety and occupy a large space in industrial processes².

Nowadays, the industrial processes of partial oxidation of methane occur by converting CH₄ into syngas to be applied in Fischer-Tropsch synthesis, as well a to obtain products such as methanol, formaldehyde, formic acid and other chemicals that are applied as industrial inputs^5,6. These processes need high temperatures and pressure conditions due to stability CH₄ and it difficult polarization that make it hard to break the C-H bond^3,5,7-9. However, by electrochemical processes, for example, need to be processed under mild conditions to favor the electrostatic interactions of the molecule of interest^10-12. The application of electrosynthesis processes increases the yield per unit of energy spent and selectivity¹³ unlike traditional thermochemical processes^10-12.

Electrochemical reactors can be built in different ways, one of the most interesting being the electrolytic reactor that uses a solid membrane, capable of reconciling anodic and cathodic reactions, in different chambers of the same device, independently. This makes it possible to associate reactions with energy cogeneration while converting to chemicals^14-17. In recent years, some studies dealt with the partial oxidation of methane in reactors that use polymeric electrolyte reactor - fuel cell type (PER-FC)^9,18-24, which is still an expanding area. It is known that the application of palladium as a catalyst in these devices favors the production of methanol due to its affinity with methane^18,20,25 having the ability to activate water as the key factor in the transformation of hydrocarbons into oxygenated products. However, the Pd is an expensive and rare metal^26,27.

A strategy to reduce the cost and increase the chemical conversion would be the production of binary materials combining noble metal with other metals or semi-metals with capacity for chemical interaction with methane and/or water activation. The combination of Pd with Sb intensifies the effects of Pd sorption, since the addition of Sb can suppress the CO formation under working conditions in fuel cells^28,29. Sb-based material have been used in oxidative reaction of methane, as due to the fact that this metal facilitates thermo oxidative coupling^30,31, and presents selective oxidation of methane to formaldehyde that is promoted by the redox cycle of α-Sb₂O₄ and Sb₂O₄−x³².

Antimony oxides are widely applied as co-catalysts for various reactions in several applications such as in a fuel cell^30,33,34, due to their ability to activate water, as they are good electrical conductors, interesting for electrocatalysis^35-39.

This work investigates for the first time the partial oxidation of methane using a PER-FC with binary catalysts based on Pd combined with Sb with different atomic ratios, in alkaline medium, for products and energy co-generation. Therefore this research proposes to make new contributions to the theme.

2. Materials and Methods

For the synthesis of PdSb binary catalytic systems in different atomic compositions, Palladium II Nitrate (Pd(NO₃)₂.2H₂O – Aldrich) and Antimony III Acetate((CH₃CO₂)₃Sb - Aldrich) with 20 wt% of metal loading, were used in this study as metal sources. Sodium Borohydride (NaBH₄ – Aldrich) was used as a reducing agent and Vulcan Carbon (XC72) was the support. In the reduction step, the support and the metal sources were dissolved in a mixture of water/2-propanol 50/50 (v/v), later the solution content Sodium Borohydride in excess with 10 mL 0.01 mol. L^-1 of KOH was added. The obtained catalysts were washed with water and dried at 70°C for 2 hours with vacuum filtered.

The synthetized catalysts were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The micrographs were obtained using a JEOL JEM-2100 electron microscope, operated at 200 Kv. For histogram construction and average size calculation, 300 nanoparticles of each catalyst were averaged digitally. Information on crystalline structures of all catalysts was investigated using the X-ray diffractometer model Miniflex II, with Cukα radiation source of 0.15406Å. Analyses conditions were set at 2θ range 20–90º, with 2 min ^-1 scan speed.

The electrochemical measures via technique of Rotating Ring Disk Electrode (RRDE) were performed using a Parstat 3000A bipotentiostat/galvanostat and an RRDE (Pine Instruments) composed of a gold ring (area = 0.19 cm²) and a glassy carbon disc (area = 0.25 cm²) with collection factor of 0.37. In addition, in the conventional electrochemical cell the reference electrode used was Ag/AgCl (3mol L^-1 KCl) and Pt was used as counter electrode (area_g = 2 cm²). The working electrodes was prepared with 15 µL aliquot of each previously prepared paint composed of a mixture of 8 mg of each catalyst + 750 µl of H₂O, 250 µl of isopropyl alcohol and 15 µl of 5% Nafion D-520. All experiments with different catalysts were performed in 1mol L^-1 KOH medium. The curves were obtained at different speeds ranging from 100, 400, 600, 900, 1600 and 2500 rpm with potential range of 0.2 V - -0.85 V, and v = 10 mV s^-1. Ring was polarized at 0.2 V.

For perform in PER-FC tests, the membrane electrode assembly (MEA) was made with a Nafion® 117 membrane treated with KOH per 48 hours, Pt/C BASF catalyst (20% by weight) with 1 mg cm^-2 as cathode in gas diffusion electrodes; and catalysts with different PdSb ratios (20 wt%) as anode. All electrodes were prepared by painting the Nafion soaked catalyst ink, over a PTFE treated carbon cloth. The ink was previously prepared using the dry electrocatalyst and a 5% in mass of Nafion D-520 solution (Aldrich). The reactor, a cell with ElectroChem unit-type serpentine distribution, was supplied with CH₄ at a flow rate of 100 mL min^-1 and 1.0 molL^-1 KOH at a flow rate of 1 mL min^-1 at room temperature at the anode and external O₂ with the temperature controlled humidifier bottle at 85°C with a flow of 400 ml min^-1 in the cathode.

The Fourier Transform Infrared (FTIR) spectroscopy technique was used to identify the different species formed during the electrochemical oxidation of methane in alkaline medium at different potentials. Anodic reaction products were collected by 300s increments of 50 mV and analyzed by ATR-FTIR performed on an ATR accessory (MIRacle with a ZnSe Crystal Plate Pike®) installed on a Nicolet® 6700 FT-IR spectrometer equipped with a cooled MCT detector with N₂ liquid. The quantification of methanol was developed by Boyaci’s method with Raman spectroscopy^20,40, using Horiba Scientific MacroRam Raman spectroscopy equipment, set at 785 nm wavelength.

3. Results and Discussion

Figure 1 shows the x-ray diffractograms of PdSb materials supported on carbon. The peak near to 25^o was attributed to carbon (JCPDF # 50-926) for all materials. Analyzing the materials containing Pd it is possible to observe peaks about 2θ ≈ 40º, 47º, 68º, 82º and 87º associated, respectively, to (111), (200), (220), (311) and (222) planes of Pd (JCPDS # 89-4897) with face-centered cubic (FCC) structure. With the increase in the amount of antimony in the catalyst, the peaks will lose resolution. Narrower and more defined peaks appear, indicating an improvement in the crystallographic structure of the structure present by the noble metal.

Figure 1
X-ray diffractograms of PdSb/C catalysts and different compositions.

In the diffractogram of the Sb/C is possible to observe peaks at ≈ 27^o, 31^o, 45^o and 55^o relative to the Sb₂O₃ phase (JCPDS # 5-534), the peak 27^o being apparent in the diffractograms of materials Pd₈₀Sb₂₀, Pd₇₀Sb₃₀, Pd₆₀Sb₄₀ and Pd₅₀Sb₅₀. The peak at ≈ 28^o observed in Sb/C and Pd₅₀Sb₅₀/C can be associated to Sb (JCPDS # 85-1322). The Pd₆₀Sb₄₀/C and Pd₅₀Sb₅₀/C catalysts still show a peak at 41^o that can be attributed to the Pd₈Sb₃ phase (JCPDS # 30-95) with rhombohedra structure, which does not interfere with peak 220 of Pd at 68^o, observed also by Yu and Pickup⁴¹. However, the materials Pd₈₀Sb₂₀/C and Pd₇₀Sb₃₀/C presents the peak ≈ 68 shifted to less positive values, indicating an expansion in the Pd network parameter by inserting Sb atoms.

TEM images of the PdSb catalysts (Figure 2) were observed, in materials synthesized with a high amount of Pd, a good dispersion with some regions of agglomerations in the Vulcan XC72 support, in catalysts with increased Sb precursor, the average particle size and the amount of agglomerates show increasing behavior, which refers to the Sb₂O₃ phase⁴². The mean particle sizes are 6.5 nm for Pd/C, 6.7 nm for Pd₉₀Sb₁₀/C, 8.6 nm for Pd₈₀Sb₂₀/C, 8.3 nm for Pd₇₀Sb₃₀/C, 11 nm for Pd₆₀Sb₄₀/C, 10.7 nm for Pd₅₀Sb₅₀/C and for Sb/C does not possible to account due the high agglomeration, which may also be due to the ripening effect⁴³, where smaller particles with greater surface energy favour the dissolution in larger particles.This result explains the low resolution of Pd peak observed in XRD measures, and is in agreement of the literature^35,44.

Figure 2
Micrographs obtained by TEM and distribution histograms of PdSb/C materials in different compositions.

Figure 3 present the cyclic voltammetry pattern obtained for PdSb electrocatalysts in alkaline media, is possible to observe the region of adsorption/desorption of hydrogen on Pd (- 0.85V to -0.45V).The increase of Sb amount showed an increase in the current values in the double layer (-0.45 – 0.15 V) in relation to Pd/C, which may be attributed to the formation of antimony species^35,45. For Pd₇₀Sb₃₀/C, Pd₆₀Sb₄₀/C, Pd₅₀Sb₅₀/C and Sb/C materials with current close to -0.18V is possible to see a peak, may be attributed to the change of oxidation state of Sb⁴⁶, similar peak is observed for Pd₈₀Sb₂₀, however shifted for 70 mV more negative (-0.25V) probably due to the phase difference observed by XRD measurements.

Figure 3
Cyclic voltammetry of PdSb materials in KOH 1 mol.L^-1 (v= 10 mV.s^-1)

The fundamental step for the oxidation of methane by electrochemical ways is the activation of water that generates hydroxyl radicals which in turn promote the breaking of the C-H bond^26,27,47,48, in agreement with Equations 1 and 2.

$H_{2}O+MO\to MOH+OH•$ (1)

$C{H}_4+OH•\to C{H}_{3}OH$ (2)

Garcia et al.⁴⁹ to detect indirectly the these oxygenated radicals use the rotation ring disk experiments to measure the H₂O₂ or in its deprotonated form due to the alkaline medium HO₂^-1 in alkaline media, as an Equation 3 ⁵⁰:

$•OH+•OH\to H{O}_2^{-1}$ (3)

To evaluate the ability of PdSb/C materials to perform this step, rotating ring disk measurements were made. In Figure 4 the results of RRDE curves for HO₂^-1 generation and detection process reveals that the highest currents measured for HO₂^-1 detection correspond to materials with the highest amounts of Sb (Pd₅₀Sb₅₀/C, Pd₆₀Sb₄₀/C and Sb/C), being that the material Sb/C presented a current three times greater than the second most active material during the whole potential range. The richest materials in Pd (Pd/C and Pd₉₀Sb₁₀/C) compose the second group formed by the most active materials. However the materials Pd₈₀Sb₂₀/C and Pd₇₀Sb₃₀/C, that presented greater distortions in the Pd FCC structure few amount of HO₂^-1, indicating that these materials can operate in a 4 electron mechanism to reduce oxygen in accordance with the literature⁴⁶.

Figure 4
RRDE voltammograms at 1600 r.p.m. in O₂saturated electrolyte with the disc current with v = 10 mV s^-1, ring current (polarized at 0.2V) corresponding to hydrogen peroxide detection.

The measurement of hydrogen peroxide detection current is relative to the tested electrodes, however the relationship between the disc and ring currents may indicate which material is preferred for the H₂O₂ generation mechanism. The selectivity for the formation of peroxide (% H₂O₂) can be calculated by Equation 4 ⁵¹:

$H_2{O}_2\%=\frac{200I_r/N}{I_d+I_r/N}$ (4)

Where I_d is the disk current, I_r is the ring current, and N is the RRDE collection efficiency (0.37). The calculated results are showed in Figure 5, where the H₂O₂% lines in the potential range from -0.8 V to -0.0 V, and the mean H₂O₂ production~ 20, ~5%, ~4%, ~4%, ~3%, ~1% and ~0.6% for Sb/C, Pd₆₀Sb₄₀/C, Pd₅₀Sb₅₀/C,Pd/C, Pd₉₀Sb₁₀/C, Pd₈₀Sb₂₀/C, and Pd₇₀Sb₃₀/C respectively.

Figure 5
H₂O₂% selectivity as a function of the applied potential.

Figure 6 presents the polarization curves of polymeric electrolyte reactor - fuel cell type for PdSb/C materials with KOH and methane fed on the anode. The maximum current density measured was 0.22 mA cm^-2, 0.16 mA cm^-2, 0.154 mA cm^-2, 0.149 mA cm^-2, 0.12 mA cm^-2, 0.11mA cm^-2, 0.09 mA cm^-2 respectively forPd₉₀Sb₁₀/C, Pd₇₀Sb₃₀/C, Pd/C, Pd₈₀Sb₂₀/C, Pd₅₀Sb₅₀/C, Pd₆₀Sb₄₀/C, and Sb/C. The OCV obtained is close than reported in the literature for methane oxidation at mild conditions using SER-FC (about 0.3 -0.4V)^{9,18,20,23,52}.

Figure 6
Polarization curves of a 5 cm² SER-FC at room temperature using PdSb/C catalysts anodes (1mg_metalcm^-2 catalyst loading) and Pt/C Basf for the cathode in all experiments (1mg_Pt cm^-2 catalyst loading with 20 wt% Pt loading on carbon), Nafion 117 membrane KOH treated KOH 1.0 molL^-1 + CH₄ 50 mL.min^-1, and O₂ flux of 400 mL min^{- 1}.

The SER-FC effluent was collected for qualitative and quantitative measures. Figure 7 shows IR spectra of partial oxidation products. The bands at 1482 cm^-1 for CH₃ d-deform⁵³, 1080 cm^-1 and 1030 cm^-119,20 confirm the methanol production over PdSb materials.

Figure 7
FTIR spectra were taken from effluent of SER-FC at several potentials in 1.0 mol.L^-1 KOH and the methane flow was set to 50 mL.min^-1.

The band at 1030 cm^-1, observed in all materials, increases with decreasing reactor potential. However, this band is not as selective as the band-centered ≈ 1080 cm^-1 that appears differently for each material. For Pd/C and Pd₈₀Sb₂₀/C this band was already observed in the OCV values and presents few changes in intensity up to zero V. For Pd₉₀Sb₁₀/C this same band increases in intensity with the reduction of the reactor potential up to 0.05 V, then, after this potential, the band decreases. For Pd₇₀Sb₃₀/C, this band is very close to the signal/noise and it was not possible describe its behavior. Pd₆₀Sb₄₀/C and Pd₅₀Sb₅₀/C shows an increasing behavior due to the potential decrease. Sb/C is observed only between 0.15 V and 0.05 V. The band at 1478 cm^-1 is only observed for Sb/C due to the absence of interference from the band related to carbonate ions (~1376 cm^-1), products of complete oxidation of the methane observed for all materials containing Pd, indicating the complete methane oxidation also occur^54,55. Complete oxidation can occur in stages, as confirmed by the presence of methanol during the potential variation. If this alcohol is present in greater quantity, when it is oxidized, the OCV increases. This fact may justify the OVC value higher than that observed in the literature.

The band centered at 1345 cm^-1that correspond to ν(COO) of formate solution⁵⁶,it is present only for Pd₇₀Sb₃₀/C. The formate is probably due to methanol oxidation reaction, more evidently observed in Pd₆₀Sb₄₀/C and Pd₅₀Sb₅₀/C, indicating that those compositions can promote higher methane oxidation than others. The band centered at ~ 1302 cm^-1 correspond to deg deform of methane^52,57 and it is observed in all materials. This signal increase may be due to the increase of methane solubility in solution²⁰.

The quantitative measures was based according to Boyaci’s method⁴⁰. To calculate the methanol production by the reaction rate (r) expressed in Equation 5. The analytical curve was constructed in the concentration range of 0.005-1.000 mol.L^-1 of methanol. For the following analytical curve, an intensity = 3.3509 + 3.983 [methanol] is obtained with the correlation coefficient being 0.97.

$r=\frac{Methano{l}_{amount}}{VolumexTime}$ (5)

The calculated r for each potential are shown in Figure 8. The materialsPd/C, Pd₉₀Sb₁₀/C, Pd₆₀Sb₄₀/C and Pd₅₀Sb₅₀/C produce methanol in appreciable quantities at all potentials. The higher r was obtained with Pd₅₀Sb₅₀/C (r ~ 7 mol L^-1 h^-1), occurring close to the OCV, and with Pd/C (~ 6.75 mol L^-1 h^-1 at 0.2V). These materials showed selective generation of H₂O₂ around 4 and 5% and convert methane into methanol in all potentials, selectivity similar to reported by Garcia⁴⁹ for more active materials for conversion of methane into methanol using copper complex. Indicating that reaction cam be depend heavily on an optimal amount of reactive oxygenated species to maximize conversion of methane into methanol.

Figure 8
Methanol concentration obtained for PdSb / C materials and proportions.

The materials Pd₈₀Sb₂₀/C and Pd₇₀Sb₃₀/C present relevant quantities only between 0.3V to 0.1V and in lesser quantities than the others. This result is in accordance with the selectivity results to H₂O₂ generation (Figure 4). Although Sb/C presents more selective for H₂O₂, practically no methanol was detected. This can be explained by i) the loading and unloading process due to the reversible reactions of the alloy characteristic of elementary Sb which are further impaired by the superficial passivation of Sb⁵⁸, ii) due to the high population of oxygenated reactive species, produced by the activation of water, the reaction are leading to more oxidized products, and/or iii) even with the high activity for generation of reactive species, the lack of methane adsorption sites hinder methanol conversion. While Pd catalysts were more selective for methanol, mainly those that presented higher selectivity values for H₂O₂. Results previously published by several groups on the study of kinetics and energetic interactions in PdO in relation to methane show that the formation of a thin layer of PdO on the electrocatalyst surface to obtain the necessary binding effect is responsible for decreasing the initial C-H bond in methane^20,25,59,60. The addition of antimony to Pd enhances the effect described with greater abundance of oxygenated reactive species.

4. Conclusion

PdSb materials synthesized by the NaBH₄ method were active for the conversion of methane into products with energy co-generation on a PER-FC, the main products being potassium formate and methanol, however, the selectivity and efficiency depending of the composition and the phases present in the material that can change the way the reaction proceeds. For Pd/Cand Pd₉₀Sb₁₀/C the Pd FCC is practically not altered by the addition of Sb, presents a selectivity of 3 to 4% for H₂O₂ and conversion of methane to methanol in all potentials. Pd₈₀Sb₂₀ and Pd₇₀Sb₃₀ the lattice parameter of Pd is expanded by the insertion of Sb atoms in the FCC structure, practically not selective to generating H₂O₂ (0.6% to 1%) and has limited conversion of methanol to the potential range of 0.6 to 0.2V. Pd₆₀Sb₄₀ and Pd₅₀Sb₅₀ present in addition to the FCC structure for Pd, also present in dominant form the structure rhombohedra phase Pd₈Sb3. These materials showed selective generation of H₂O₂ around 4 and 5% and convert methane into methanol in all potentials. These binary catalysts have proven to be more efficient than the Pd/C catalyst itself, in addition to a higher power density, due to the production of more oxidized derivatives, such as potassium formate. Sb/C, despite being more selective for the generation of H₂O₂, there is a lack of an interaction system between methane and the catalyst. This behavior increases the conversion of hydrocarbon to alcohol. In other words, the activation of water, although essential for the partial oxidation of methane, is not the only characteristic necessary for the conversion of this hydrocarbon into oxygenated products by means of a PER-FC.

5. Acknowledgments

We are grateful to CAPES (88882.315566/2019), CNPq (302709/2020-7), FAPESP (2017/11937-4), and CINE-SHELL (ANP)/FAPESP grants 2017/11937-4 for financial supports.

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Publication Dates

• Publication in this collection
  13 May 2022
• Date of issue
  2022

History

• Received
  19 Oct 2021
• Reviewed
  18 Mar 2022
• Accepted
  10 Apr 2022