The influence of the Pt crystalline surface orientation on the glycerol electro-oxidation in acidic media

doi:10.1016/j.electacta.2012.04.144

Electrochimica Acta

Volume 76, 1 August 2012, Pages 88-93

https://doi.org/10.1016/j.electacta.2012.04.144 Get rights and content

Abstract

We investigated the electrochemical oxidation of glycerol on low-index Pt single crystals in acidic media (H₂SO₄ and HClO₄) by cyclic voltammetry and Fourier Transform Infrared (FTIR) spectroscopy and we verified that this is a surface sensitive reaction. Pt(1 0 0) and Pt(1 1 0) surface structures favor the breaking of the C single bond CC bond at low potentials (say 0.05 V), as seen by the formation of CO, one of the adsorbed residues of the glycerol dissociation, which poisons these surfaces even at high potentials. Pt(1 1 1) surface structure does not favor the CCC bond breaking at potentials as low as 0.05 V. However, Pt(1 1 1) is less poisoned by residues of glycerol dissociation and, for this reason, it is more active for glycerol oxidation than Pt(1 0 0) and Pt(1 1 0) at low potentials. Carbonyl containing compounds and CO₂ were detected as reaction products of the glycerol oxidation on all investigated single-crystal Pt surfaces. The ratio between CO₂ and carbonyl containing compounds is clearly much higher for Pt(1 0 0) and Pt(1 1 0) than for Pt(1 1 1).

Introduction

Nowadays, glycerol is abundantly produced as a co-product of the biodiesel fabrication and its application is mainly restricted to raw material for pharmaceutical and cosmetics industries. As a result, the offer of this alcohol surpasses its actual demand and the accumulation of big volumes of glycerol can become an environmental problem in a short timescale. For this reason, there has been an effort in finding new applications for glycerol. One of the possibilities is the application of glycerol in direct glycerol fuel cells for electric energy generation and co-generation of commercially interesting chemicals, such as dihydroxyacetone [CH₂OH single bond (C double bond O)CH₂OH], which is used as an ingredient in sunless tanning products.

At present, some difficulties related to practical catalysts (usually metallic particles dispersed in carbon powder) make unfeasible the application of glycerol in direct glycerol fuel cells. One of the difficulties is to control the selectivity of the glycerol oxidation reaction towards the desired chemicals. Another difficulty is the formation of poisoning adsorbed intermediates on the catalyst surface at low overpotentials. In order to overcome these difficulties, studies on the electro-oxidation of glycerol on model surfaces, for example Au and Pt single crystals, are fundamental to unravel the mechanism of this process. These studies supply us with information about the role of the surface crystallographic orientation on the catalytic electro-oxidation of glycerol. Based on this knowledge, practical catalysts with specific sites can be designed to improve the electrochemical process for fuel cell application with co-generation of chemicals.

Glycerol reacts on metal surfaces through a complex mechanism that leads to the formation of a large variety of final reaction products, including compounds with one, two, and three carbon atoms [1], [2], [3], [4], [5], [6]. In particular, glyceraldehyde (CH₂OH single bond CHOHCHO), dihydroxyacetone [CH₂OH(C double bond O)CH₂OH], glyceric acid (CH₂OHCHOHCOOH), tartronic acid (COOHCHOHCOOH), glycolic acid (COOHCH₂OH), glyoxylic acid (COOHCHO), formic acid (HCOOH) and carbon dioxide (CO₂) were detected as solution phase reaction products of the glycerol oxidation on Pt in acidic medium [7], [8], [9], [10], [11]. The variety of incomplete oxidation products with the C single bond C bonds remaining intact or only partly broken points to pronounced kinetic limitations for the dissociative adsorption and subsequent complete oxidation of glycerol to CO₂. Details of the reaction mechanism of glycerol oxidation on Pt surfaces, however, are still largely unknown. In order to gain a better insight into the electrochemical reactivity of glycerol, detailed studies about glycerol adsorption and oxidation are necessary with the aim of identifying the main species involved in the different steps.

The electro-oxidation of alcohols, such as methanol and ethanol, on metallic single crystals has been extensively investigated and the effect of the surface crystallographic orientation on these reactions is well-established [12], [13], [14], [15], [16], [17], [18], [19], [20]. Differently, the electro-oxidation of glycerol over these model catalysts has been scarcely investigated and not much is known about the influence of surface structure on the oxidation of glycerol [2], [21]. In the present work, we investigate the electrochemical oxidation of glycerol on low-index Pt single crystals in acidic media (H₂SO₄ and HClO₄) by cyclic voltammetry and Fourier Transform Infrared (FTIR) spectroscopy. This study provides us with additional information on the structural sensitivity of the electro-oxidation of glycerol.

Section snippets

Experimental conditions, reactants and instrumentation

All the experiments were performed at room temperature (25 ± 1 °C). The chemicals used for solution preparations were high-purity sulfuric acid (Merck suprapur^®), perchloric acid (70%, Sigma–Aldrich), glycerol (J.T. Baker), and ultrapure water from Millipore system. The counter electrode was a 1 cm × 1 cm Pt foil. The reference electrode was a reversible hydrogen electrode (RHE) and all potentials in this work are referred to it. The working electrodes were Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) discs (10 mm

A general mechanism of glycerol electro-oxidation on Pt in acidic medium

With basis on the current knowledge of the electro-oxidation of glycerol on Pt in acidic media [7], [9], [10], [11], a general reaction mechanism can be formulated as follows: $\underset{glycerol}{H_{2} C (OH) HC (OH) H_{2} C (OH)} \to \underset{glyceraldehyde}{H_{2} C (OH) HC (OH) CHO + 2 H^{+} + 2 e^{-}}$ $\underset{glycerol}{H_{2} C (OH) HC (OH) H_{2} C (OH)} \to \underset{dihydroxyacetone}{H_{2} C (OH) CO H_{2} C (OH) + 2 H^{+} + 2 e^{-}}$ $\underset{glyceraldehyde}{H_{2} C (OH) HC (OH) CHO} \overset{H_{2} O}{\to} \underset{carboxylic acids}{R-COOH}$ $\underset{dihydroxyacetone}{H_{2} C (OH) CO H_{2} C (OH)} \overset{H_{2} O}{\to} \underset{carboxylic acids}{R^{'} -COOH}$ $\underset{carboxylic acids}{R-COOH} \to \underset{carbon dioxide}{{CO}_{2}}$ where R represents 0, 1 and 2

Conclusions

We investigated the electrochemical oxidation of glycerol on Pt single crystals of low index in acidic media (H₂SO₄ and HClO₄) by cyclic voltammetry and Fourier Transform Infrared (FTIR) spectroscopy and we verified this reaction to be surface sensitive. Linear bonded CO is a common adsorbed intermediate of glycerol oxidation on Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) in acidic media. This species remains attached to Pt(1 0 0) and Pt(1 1 0) up to higher potentials than on Pt(1 1 1). We believe that this is the

Acknowledgment

The authors gratefully acknowledge CNPq, Capes and FAPESP the financial support of this work.

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Citation Excerpt :
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The influence of the Pt crystalline surface orientation on the glycerol electro-oxidation in acidic media

Abstract

Introduction

Section snippets

Experimental conditions, reactants and instrumentation

A general mechanism of glycerol electro-oxidation on Pt in acidic medium

Conclusions

Acknowledgment

Applied Catalysis B: Environment

Journal of Electroanalytical Chemistry

Electrochemistry Communications

Applied Catalysis B: Environment

Journal of Electroanalytical Chemistry

Electrochimica Acta

Electrochimica Acta

Electrochimica Acta

Electrochimica Acta

Journal of Electroanalytical Chemistry

Journal of Electroanalytical Chemistry

Journal of Electroanalytical Chemistry

Electrochimica Acta

Journal of Electroanalytical Chemistry

Journal of Electroanalytical Chemistry and Interfacial Electrochemistry

Journal of Electroanalytical Chemistry and Interfacial Electrochemistry

Journal of Electroanalytical Chemistry