Anodic electrocatalytic conversion of carboxylic acids on thin films of RuO2, IrO2, and Pt
Graphical abstract
Introduction
As demand for sustainable hydrocarbon fuels and products increases [1,2], abundant and renewable biomass resources are promising carbon sources for producing biocrude products through thermochemical conversion processes such as fast pyrolysis and hydrothermal liquefaction [[3], [4], [5], [6], [7]]. Downstream processing of biomass-derived feedstocks to liquid transportation-range fuels, biofuels, and value-added chemicals must include hydrogenation and deoxygenation steps to lower the oxygen content [8,9].
As an alternative to traditional thermochemical approaches, electrocatalytic upgrading can be accomplished under mild operating conditions (e.g., at ambient temperature and pressure), using water as the hydrogen source, and straightforward coupling with renewable electricity sources (e.g., solar, wind, and hydropower) [10,11]. Electrocatalytic hydrogenation (ECH) of biomass-derived molecules (e.g., aldehydes, ketones, phenolics, and carboxylic acids) is being actively explored to complement thermocatalytic hydrogenation. Aldehydes, ketones, and phenolics are reactive toward ECH, and some of these compounds may show higher reaction rates than thermocatalytic hydrogenation with hydrogen (H2) at ambient temperature [[12], [13], [14], [15], [16], [17], [18], [19], [20], [21]]. However, under typical ECH conditions, carboxylic acids are not reactive. For instance, Lopez-Ruiz et al. reported that common base and Pt-group metals (i.e., Pd and Ru) are not active for the reduction of benzoic acid, lactic acid, and acetic acid [19,22]. Similarly, Nilges et al. and Xin et al. reported that the carboxylic group of levulinic acid was not reduced even at -2.4 V vs. the reversible hydrogen electrode (RHE) on Pb [16,23]. Therefore, alternative electrocatalytic approaches need to be developed to valorize carboxylic acids.
(Non-)Kolbe electrolysis is an electrocatalytic decarboxylation (ECDX) reaction that enables carboxylic acid conversion to paraffins, olefins, and alcohols via carbon-carbon bond cleavage and CO2 elimination under positive applied potentials [[24], [25], [26]]. In (non-)Kolbe electrolysis (illustrated in Scheme 1 with valeric acid [VA]), a carboxyl-radical is generated through deprotonation and one-electron oxidation, and then followed by decarboxylation yielding an alkyl-radical and CO2. Two alkyl-radicals can be coupled to form an octane via dimerization (i.e., Kolbe electrolysis). Alternatively, the alkyl-radicals can be further oxidized to carbocations to yield butanol or butene via the reaction with OH− or deprotonation (i.e., non-Kolbe electrolysis). The generated alcohol can react further with carboxylic acid to form esters or can be oxidized to aldehydes, ketones, and carboxylic acids with shorter carbon chain (i.e., deep oxidation products). Consequently, paraffins, olefins, alcohols, esters, and other oxidation products are obtained through (non-)Kolbe electrolysis.
Multiple reaction parameters have been studied to advance our understanding of the reaction mechanisms of (non-)Kolbe electrolysis and control the selectivity to paraffins, olefins, and alcohols [23,[27], [28], [29], [30], [31], [32], [33]]. For example, Levy et al. demonstrated that increasing carboxylic acid concentration facilitates the generation of alkyl-radicals, thereby increasing selectivity to paraffin compounds [30]. Walling’s group and Schröder’s group reported that water promoted the formation of alcohols and esters from short carbon chain carboxylic acid through non-Kolbe electrolysis, whereas methanol favored generation of paraffin compounds via the Kolbe reaction [23,33]. In most of the previously reported work, chronopotentiometry (from 100 to 250 mA cm−2geo) was used with the goal of reaching the high anodic potentials needed for radical generation (>2.5 V vs. RHE) [30,34,35]. However, reaction control through applied potential on ECDX activity, product selectivity, and current efficiency (CE) still remains unexplored.
Bulk Pt (e.g., foils, meshes, and wires) have been used extensively to study (non-)Kolbe electrolysis because of their practicality [[24], [25], [26], [27], [28],30,36,37]. Compared to bulk Pt, nanoparticles are more reactive and readily form PtO2 layers. Therefore, bulk Pt is more stable than nano-scaled Pt materials under the potential required for ECDX [38,39]. However, the high cost and low surface area of bulk Pt materials hinder the development of (non-)Kolbe electrolysis into a commercially relevant application.
In this work, we investigated RuO2, IrO2, and Pt thin films (RuO2-TF, IrO2-TF, Pt-TF) and compared their activities to that of Pt foil for ECDX of VA. RuO2-TF exhibited similar activity compared to Pt foil, while IrO2-TF and Pt-TF were the least active electrodes. We obtained similar distributions of Kolbe (i.e., octane) and non-Kolbe products (e.g., butene, butanol, butanoic acid) from RuO2-TF and Pt foil; however, Pt foil had a higher CE than RuO2-TF because of its lower activity for the oxygen evolution reaction (OER). IrO2-TF and Pt-TF were more active for OER than for ECDX. We investigated the role of applied potential on the ECDX reaction with both RuO2-TF and Pt foil, which will serve as guideline for the selection of catalyst and applied potential in the anodic electrocatalytic valorization of carboxylic acids.
Section snippets
Chemicals
RuCl3·xH2O (38.0–42.0 % Ru basis), H2IrCl6·xH2O (≥99.98 %), H2PtCl6·6H2O (∼38.0 % Pt basis), oxalic acid (98 %), iso-propanol alcohol (IPA, ≥99.9 %), K2CO3 (≥99.0 %), Na2SO4 (≥99.0 %), and VA (>99 %) were purchased from Sigma Aldrich. H2O2 solution (∼30 %) was purchased from Fisher scientific. Titanium (Ti) foil was purchased from Alfa Aesar. All chemicals were used as received without further purification. Nitrogen (N2, 99.9 %, Airgas) and ultrahigh-purity H2 (99.999 %, Airgas) were used for
Materials synthesis and physical characterizations
RuO2-TF, IrO2-TF, and Pt-TF were synthesized on pre-polished Ti foil through thermal decomposition or electrodeposition, followed by calcination [40,41]. The morphologies and crystalline structures of the as-prepared TF electrodes (RuO2-TF, IrO2-TF, and Pt-TF) were characterized by SEM and XRD analyses. As shown in Fig. 1a,d, and S5, a RuO2 TF formed an apparently smooth, continuous surface on the Ti foil. RuO2 nanorod-like microstructures 10–100 nm in diameter were observed on the RuO2-TF.
Conclusions
This work evaluates the use of thin films (TFs) as an alternative to bulk electrodes for the electrocatalytic decarboxylation (ECDX) of carboxylic acids through Kolbe and non-Kolbe electrolysis in an aqueous system at ambient temperature and pressure. The TFs of RuO2, IrO2, and Pt were composed of micro-scaled structures and were compared with bulk Pt (foil) for VA ECDX at constant current (50 mA cm−2geo) and as function of applied potential (3.0–5.0 V vs. RHE). While Pt foil was active for
CRediT authorship contribution statement
Yang Qiu: Methodology, Investigation, Writing - original draft. Juan A. Lopez-Ruiz: Data curation, Conceptualization, Writing - review & editing. Udishnu Sanyal: Data curation, Writing - review & editing. Evan Andrews: Data curation. Oliver Y. Gutiérrez: Supervision, Funding acquisition, Writing - review & editing. Jamie D. Holladay: Conceptualization, Supervision, Funding acquisition, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The research described in this paper was undertaken under the Chemical Transformation Initiative at Pacific Northwest National Laboratory (PNNL) conducted under PNNL’s Laboratory Directed Research and Development Program. PNNL is a U.S. Department of Energy multiprogram national laboratory located in Richland, Washington. We gratefully acknowledge Nathan Canfield, Teresa Lemmon, and Marie Swita at PNNL for their help with the sample analysis and catalyst characterization. We also acknowledge
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