Glycerol electro-oxidation to dihydroxyacetone on phosphorous-doped Pd/CNT nanoparticles in alkaline medium

Highlights

• Phosphorus doping facilitated the reduction of Pd²⁺ to Pd.

• ESA and current density increased 2.84 and 1.4 times, respectively for P-doped electrocatalysts compare to counterpart.

• A high DHA selectivity of 90.8% was achieved for P-doped electrocatalyst.

Abstract

In this communication report, a comparative study between P-doped Pd/CNT and bare Pd/CNT catalyst was carried out to decouple the effects of phosphorous-doping on electro-oxidation of glycerol. The initial characterization results suggested that Pd and its oxides were successfully incorporated within the pore channels of CNTs support for both catalysts by using hydrazine-assisted hydrothermal technique. The XPS results revealed that the amount of Pd²⁺ for bare Pd/CNT were 1.4 times higher than P-doped electrocatalysts (about 70.1% and 48.7%, respectively) which confirms that phosphorus facilitates the reduction of Pd²⁺ to metallic Pd (Pd⁰). The electrochemical results showed that the electrochemical surface area (392.22 m² g_Pd⁻¹) and current density (26 mA/cm²) for P-doped Pd/CNT catalyst were 2.84 and 1.6 times, respectively, higher than Pd/CNT catalyst. The P-doped catalyst was found to suppress the formation of carbonaceous intermediates; thus, improved the glycerol oxidation reaction. Small quantities of deep oxidation side products such as mesoxalic acid (<2%) and tartronic acid (<0.1%) were found along with the dihydroxyacetone (DHA), a major product of glycerol electro-oxidation. The best performing catalyst exhibited 1.4 folds higher DHA selectivity (90.8%) compared to the Pd/CNT.

Introduction

Recently, biodiesel has gained substantial interests due to the finite fossil fuel resources, leading to a simultaneous increase in the crude glycerol (a biodiesel waste that originates from transesterification of animal fat and vegetable oils) [1]. The projected production of biodiesel is 42 giga L by 2020. This will contribute to massive glycerol production as a by-product (100 g glycerol/1 kg of biodiesel produced) [2]. The electro-oxidation of glycerol has been touted as the most promising pathway for value-added chemicals generation from biodiesel waste [3]. This reaction has been mainly studied over Pt and Au-based catalysts due to their high activity for oxidation of alcohols such as methanol, ethanol, and formic acid [4]. For example, Zhao et al. [5] studied the methanol electro-oxidation over electrodeposited AuPt alloy nanocatalysts. These nanaocatalyst materials showed superior catalytic activity as compared to Au-core Pt shell and Pt-core Au-shell at the lowest working potential of 0.25 V. In addition, it was suggested that the alloy formations enhanced the catalytic activity of both Pt (low activity) and Au (no activity for methanol in alkaline media). Recently, Li et al. [6] reported an efficient trimetallic CoPtAu catalyst for electro-oxidation of formic acid, methanol, and ethanol. Their results further revealed that the high activity of ternary catalyst is due to stability of Co through L₁₀- structure, which facilitates the interaction among substrate molecules, and the active sites of the catalyst. Nevertheless, high cost, low availability, and rapid poisoning of active sites of existing catalysts are major impediments towards commercialization [7]. Some earlier studies found that the less expensive and more abundant Pd based catalysts showed better performance towards electro-oxidation of alcohols [8,9].

Various palladium-based alloy catalysts such as PdAg [10], PdNi [11], PdAu [12], PdCuPb/C [13], and PdAu/C [14], have been investigated for electro-oxidation of alcohols. However, it was reported that the electrocatalytic activity and stability of the catalyst can be further enhanced by introducing promoter (heteroatoms) to the catalyst [15]. In this respect, Kulesza et al. [16] reported that the substitution of lattice sites with heteroatoms (N, Fe or Co) can remarkably improve the stabilization and electrocatalytic activity of graphene based catalysts for both alkaline as well as in acidic media. Likewise, Dioati et al. [17] studied the interaction between Pt with Ir, Rh and Ni, and also interaction of Ni with Ir and Au over carbon nitride support for ORR performance. The results revealed that the incorporation of group VI metals significantly amplified the onset potential (from ca. 300 to ca.780 to ca.900 mV) vs RHE, respectively, for Au, Ir and Pt. Moreover, their work further elaborated the role of secondary metals in promoting the selectivity of ORR reactions via 4-electrons system. In addition, Sun et al. doped phosphorous into Pd based catalyst for formic acid electro-oxidation. Their results revealed that the phosphorous significantly enhanced the activity of catalyst by facilitating better dispersion of palladium nanoparticles due to electronic effect [18]. More recently, it was found that the doping of carbon support with palladium can enhance the oxygen vacancies on catalyst surface. It contributed to the oxidative conversion of intermediate during ethanol oxidation reaction [19]. Similarly, another study reported that the addition of phosphorous has accelerated the oxidation of CO_ads to CO₂ in ethanol electro-oxidation via bifunctional mechanism [20].

So far, there is no study exists which discusses adequately about the effects of phosphorus (P) doping on electro-oxidation of glycerol. Therefore, in the present work, phosphorous-doped and bare Pd nanoparticles on MWCNT support were prepared using hydrothermal method and investigated for electro-oxidation of glycerol. The as prepared catalysts were characterised by XRD, XPS, FESEM and TEM for crystallography, oxidation states and particle size measurements. A strong interaction of P with Pd was achieved by carefully monitoring the catalyst preparation and was further confirmed by XPS analysis. The effect of P-doping into Pd/CNT was studied for electrochemical performance and influence on GOR products distribution was compared with bare Pd/CNT.