Electrocatalytic Hydrogenation Of Phenol Research Articles

Modulating the Coverage of Adsorbed Hydrogen via Hydrogen Spillover Enables Selective Electrocatalytic Hydrogenation of Phenol to Cyclohexanone.

Selective electrocatalytic hydrogenation (ECH) of phenol is a sustainable route to produce cyclohexanone, an industrially important feedstock for polymer synthesis. However, attaining high selectivity and faradaic efficiency (FE) for cyclohexanone remain challenging, owning to over-hydrogenation of phenol to cyclohexanol and competition of hydrogen evolution reaction (HER). Herein, by employing hydrogen spillover effect, we modulate adsorbed hydrogen species (Hads) coverage on Pt surface via migration to TiO2 in an anatase TiO2-supported Pt catalyst. In ECH of phenol, a high selectivity (94 %) and good FE (63 %) for cyclohexanone are obtained, showing more advantageous performance compared with previous reports. Cyclic voltammetry (CV) tests and electrochemical Raman spectroscopy reveal that Hads migrated from Pt to TiO2. We propose that TiO2-induced hydrogen spillover contributes to low Hads coverage over Pt, which effectively hinders over-hydrogenation of cyclohexanone and HER. We establish a scaling relationship between the intensity of hydrogen spillover and cyclohexanone selectivity by varying the types of anatase TiO2, and show the universality of the strategy over other reducible metal oxides as the support (rutile TiO2, CeO2 and WO3). This work showcases an effective strategy for tuning hydrogenation selectivity in electro-catalysis, by taking advantage of thermo-catalytically well-documented hydrogen spillover effect.

Phenol-Mediated Proton Shuttling Enables Electrochemical Hydrogenation of Phenol in Alkaline Electrolytes on Rhodium

Thermal chemical hydrogenation (TCH) is a key reaction for upgrading petroleum and biomass feedstocks to value- added fuels and chemicals. TCH requires high operating temperatures and pressures which results in high energy consumption and CO2 emissions. Electrochemical hydrogenation (ECH) is a mild alternative where applied potential drives the hydrogenation of hydrocarbons with protons from the aqueous electrolyte near ambient conditions. Phenol is a simple representative compound for renewable lignocellulosic feedstocks that has been the focus of recent ECH studies. The added complexity from the electrolyte and applied potential in aqueous ECH systems requires a fundamental understanding of these reaction parameters on ECH activity in-order to overcome current limitations, including faradaic efficiency losses to the competitive hydrogen evolution reaction (HER). Furthermore, the wide range of reaction conditions employed in the phenol ECH literature makes comparison of the ECH performance difficult. Phenol ECH on PGMs is commonly performed in electrolytes in a pH range of 1-5, assuming that the ECH of phenol follows a Langmuir Hinshelwood (LH) mechanism [1, 2]. One study reported a 95% faradaic efficiency (FE) for phenol ECH on rhodium at pH 10 [3], but no further investigation into the source of this promising activity at alkaline pH has been conducted.In this work we study the role of electrolyte pH on phenol ECH on platinum and rhodium. We show that there are differing pH trends for ECH activity between platinum and rhodium, with the highest ECH rates on platinum at pH 1 and the highest rates on rhodium at pH 9, near the pka of phenol. We show using cyclic voltammetry and in-situ electrochemical FTIR that this pH dependent behavior results from the interplay between hydrogen adsorption kinetics, differences in phenol/hydrogen coverage on each metal, and competition with HER. We show that the electrolyte pH dictates the balance between a hydrogen atom transfer (LH) mechanism and a phenol-mediated proton coupled electron transfer mechanism. With this insight, we show that the unique properties of rhodium and the acid-base chemistry of phenol and the electrolyte buffer can be leveraged for higher ECH rates and FE.[1] Singh, N., et al., Aqueous Phase Catalytic and Electrocatalytic Hydrogenation of Phenol and Benzaldehyde over Platinum Group Metals. Journal of Catalysis 2020, 382, 372–384. https://doi.org/10.1016/j.jcat.2019.12.034.[2] Song, Y., et al., Integrated Catalytic and Electrocatalytic Conversion of Substituted Phenols and Diaryl Ethers. Journal of Catalysis 2016, 344, 263–272. https://doi.org/10.1016/j.jcat.2016.09.030.[3] Song, Y., et al., Aqueous Phase Electrocatalysis and Thermal Catalysis for the Hydrogenation of Phenol at Mild Conditions. Applied Catalysis B: Environmental 2016, 182, 236–246. https://doi.org/10.1016/j.apcatb.2015.09.027.

Electrocatalytic hydrogenation of phenol by active sites on Pt-decorated shrimp shell biochar catalysts: Performance and internal mechanism

The development of highly-active electrocatalysts and identification of active sites contribution are necessary for electrocatalytic hydrogenation (ECH) of bio-oil. Herein, the Pt-decorated shrimp shell biochar (SSB) catalysts with high nitrogen content was prepared successfully. The biochar-based catalysts were first used for the ECH of bio-oil model compounds. Results showed that the Pt/SSB catalysts exhibited excellent electrocatalytic activity and stability, realizing 100 % conversion of phenol and 98 % total selectivity of cyclohexanone and cyclohexanol within 5 h. By correlation analysis and density functional theory (DFT) calculations, the Pt, Pt-Nx, and CO sites in Pt/SSB catalysts were found to play an important role in the stepwise hydrogenation of phenol. Furthermore, the Pt-Nx sites were identified as the main contributor to the high selectivity of cyclohexanol generation. This study lays the foundation for the controllable preparation of electrocatalysts with high activity and stability, which is significant for large-scale upgrading of bio-oil in the future.

Electrochemical Hydrogenation of Aromatic Hydrocarbons

Electrochemical hydrogenation (ECH) of bio-mass derived compounds is an attractive alternative to traditional thermochemical hydrogenation (TCH) methods that are used in the U.S. chemical and petroleum industries to produce value-added fuels and chemicals. TCH uses high pressures and temperatures, along with an external source of hydrogen gas typically produced via methane reformation, in order to achieve hydrogenation; these requirements make it an energy intensive process. ECH has the advantages of operating near ambient conditions and sourcing the participating hydrogen from the aqueous electrolyte solution, resulting in reduced energy costs and CO2 emissions compared to TCH. Furthermore, the applied potential provides an additional parameter for controlling selectivity, making ECH more suitable to handle the wide chemical variability of biomass-derived feedstocks.Aromatic hydrocarbons are of interest as model compounds for lignocellulosic bio-oil [1]. The ECH of aromatic hydrocarbons proceeds with high faradaic efficiency (FE) at low overpotentials, however, the ECH turnover rate remains low at these potentials. There is no simple fix to this, as any increase in overpotential in an attempt to increase the degree hydrogenation will reduce the reaction FE, due to competition with the hydrogen evolution reaction (HER). In this work we address the need for a more fundamental understanding of ECH systems to promote high turnover rates at low overpotentials. In phenol TCH, it is well known that the hydrogenation is limited by the formation/addition of surface adsorbed hydrogen. The current ECH literature draws on these TCH studies and assumes that phenol ECH is also limited by surface adsorbed hydrogen [2]. Thus, most ECH work is done is acidic media, where the kinetics of surface adsorbed hydrogen formation are fastest on platinum [3]. HER, however, is also faster on platinum in acidic media [4], making it more difficult to achieve high faradaic efficiency.We have investigated the ECH of phenol in aqueous electrolyte at varying pH (1, 3, 11, and 13) on low-index single crystal platinum electrodes using rotating disk electrode voltammetry. We have also included the ECH of acetophenone and benzaldehyde to compare the effect of different functionalities on the phenyl ring. The conversion and faradaic efficiency have been determined for these molecules at pH 1 and pH 11 at selected potentials. In this poster we will present new insights on the ECH reaction mechanism of aromatic hydrocarbons along with methods to promote higher ECH rates at low overpotentials.[1] J. Yan et al., "Characterizing Variability in Lignocellulosic Biomass: A Review," ACS Sustainable Chemistry & Engineering, vol. 8, no. 22, pp. 8059-8085, 2020, doi: 10.1021/acssuschemeng.9b06263.[2] N. Singh et al., "Aqueous phase catalytic and electrocatalytic hydrogenation of phenol and benzaldehyde over platinum group metals," Journal of Catalysis, vol. 382, pp. 372-384, 2020, doi: 10.1016/j.jcat.2019.12.034.[3] S. Intikhab, J. D. Snyder, and M. H. Tang, "Adsorbed Hydroxide Does Not Participate in the Volmer Step of Alkaline Hydrogen Electrocatalysis," ACS Catalysis, vol. 7, no. 12, pp. 8314-8319, 2017, doi: 10.1021/acscatal.7b02787.[4] I. Ledezma-Yanez, W. D. Z. Wallace, P. Sebastián-Pascual, V. Climent, J. M. Feliu, and M. T. M. Koper, "Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes," Nature Energy, vol. 2, no. 4, 2017, doi: 10.1038/nenergy.2017.31.

Electrocatalytic Hydrogenation of Phenol by Active Sites on Pt-Decorated Shrimp Shell Biochar Catalysts: Performance and Internal Mechanism

Electrocatalytic Hydrogenation of Phenol by Active Sites on Pt-Decorated Shrimp Shell Biochar Catalysts: Performance and Internal Mechanism

Mild Synthesis of Cyclohexanol and Cyclohexanone Via Electrocatalytic Reduction of Phenols Using Dispersed Metal Catalysts

Cyclohexanol and cyclohexanone are industrially important chemicals for the synthesis of Nylon polymers. On the industrial scale, they are produced by thermocatalytic processes, either phenol hydrogenation (140–170 oC, 1 atm) or cyclohexane oxidation (140–180 oC, 0.8–2 MPa), which require high temperatures and external gas supply. This work demonstrates that electrosynthesis of cyclohexanol and cyclohexanone can be done at mild conditions using diverse aqueous electrolytes and dispersed metal catalysts (Pt/C, Ru/C, and Pd/C).Electrocatalytic hydrogenation (ECH) of lignin-derived phenols (e.g., guaiacol and phenol) is performed in a stirred slurry H-cell under potentiostatic or galvanostatic control. Dilute mineral acid (H2SO4, HClO4), organic acid (CH4SO3), and inorganic salt (NaCl) worked efficiently as the electrolyte solutions. In the ECH of guaiacol at low temperatures (35–40 oC) and low Pt/C loading, high guaiacol conversions (83–96%) and Faradaic efficiencies (40–70%) were achieved with significant cyclohexanol selectivities (45–53%) after 4 h reactions. In the ECH of phenol, full conversion to cyclohexanol was achieved after 2 h with high current efficiency (90%). Remarkably, by pairing the NaCl catholyte and H2SO4 anolyte, the activity of Ru/C and Pd/C can be dramatically improved, showing the importance of electrocatalyst and electrolyte synergy in the ECH of phenolic compounds.The stirred slurry catalyst allows the cell to operate at the industrially relevant current densities (in this work, between 100–300 mA cm-2). Dispersion of the negatively charged catalyst particles in the solution also facilitates the mass transfer between the organic molecules and chemisorbed hydrogens through physical collisions during the reaction, thereby improving the ECH efficiency. Mild electrocatalytic reduction of phenolic compounds represents a promising route for selective and efficient production of renewable chemicals from biomass.

Aqueous phase catalytic and electrocatalytic hydrogenation of phenol and benzaldehyde over platinum group metals

The mechanisms of aqueous-phase thermal catalytic hydrogenation (TCH) and electrocatalytic hydrogenation (ECH) of organic molecules over Pt group metals are not as well-understood as gas-phase thermal catalytic hydrogenation. In gas-phase, the reactions generally occur via a Langmuir-Hinshelwood mechanism with adsorbed hydrogen adding to adsorbed organics. Here, we show that the rates, reaction orders and activation energies for TCH and ECH of phenol and benzaldehyde on Pd, Pt, and Rh can be explained with a simple kinetic model based on similar Langmuir-Hinshelwood mechanisms. For Pt/C, the adsorption equilibrium constants for the organics needed to fit the rate data are consistent with independently-measured values, provided we assume that the rates are dominated by (1 1 1)-like sites, in agreement with reported particle size effects. The reaction rate of Pd in the ECH of benzaldehyde increases with the surface hydrogen coverage. The state of Pd during ECH of phenol and benzaldehyde are very different, with a high concentration of adsorbed H in the presence of phenol, but not in the presence of benzaldehyde, consistent with benzaldehyde’s stronger binding to the surface. In consequence, Pd is converted to β-PdHx during the hydrogenation of phenol but not benzaldehyde. This is proposed to explain the much lower activity of Pd for hydrogenation of phenol compared to benzaldehyde. The measured low coverage of H on Pd in the presence of benzaldehyde is in agreement with the high selectivity/Faradaic efficiency of protons to benzaldehyde hydrogenation to benzyl alcohol. The decrease in apparent activation energy for ECH versus TCH can also be understood within this same kinetic model. The combination of ECH and TCH kinetics and spectroscopy has, thus, allowed to deduce a microkinetic model for these hydrogenation reactions.

Electrocatalytic Hydrogenation of Benzaldehyde: Unexpected Enhancing Effect By Co-Adsorbed Proton Donor

Electrocatalytic hydrogenation (ECH) is a promising route for low-temperature bio-oil treatment, wherein renewable electricity can be used to store hydrogen in hydrocarbon energy carriers used as transportation fuels.1 ECH of phenol and benzaldehyde (model compounds) shows rates and selectivity to hydrogenation that depend on the metal.2 As bio-oil is a mixture of compounds with different functionalities, development of a sustainable process towards bio-oil stabilization often requires the fundamental understanding of competitive hydrogenation pathways in presence of mixtures of model compounds. Thus, the present study aims to explore the hydrogenation kinetics and reaction pathways of simultaneous conversion of phenol and aromatic carbonyls on carbon supported metal catalysts. Under ECH, all catalysts tested (Rh/C, Pd/C, Ru/C, Cu/C) were active for benzaldehyde hydrogenation to benzyl alcohol, however, only Rh/C was active for the hydrogenation of phenol which yielded cyclohexanone and cyclohexanol. The hydrogenation activity of benzaldehyde increases in the following order Ru/C < Rh/C < Pd/C < Cu/C. H2 evolution reaction (HER) is the prevalent side reaction. Therefore, an important variable to study is the selectivity (Faradaic efficiency) defined as fraction of total electron used for hydrogenation. Hydrogenation selectivity was ~90% for both Ru/C and Pd/C, however, Rh/C and Cu/C showed ~50% selectivity. When phenol was co-fed alongside benzaldehyde, no phenol hydrogenation was noted on all catalysts tested. However, an enhancement in ECH rate of benzaldehyde (~2-4 fold depending on metal catalyst) was observed in case of Pd/C, Ru/C and Cu/C (Figure 1a). ECH of benzaldehyde was very similar on Rh/C in presence and absence of phenol. The presence of phenol in the feed increased Faradaic efficiency in case of Cu/C and decreased it on Rh/C and Ru/C. We hypothesize that phenol, co-adsorbed on the metal, affects the rates of HER hindering the reaction of hydronium ions with the metal. On the other hand, phenol interacts with benzaldehyde and thus, facilitates the hydrogenation and results in an enhancement in rate. Indeed, when the benzaldehyde hydrogenation was carried out on Pd/C with different phenolic compounds with varying pKa, increasing hydrogenation rates were noted with decreasing pKa of the phenolic compounds (Figure 1b). Although, an enhancement (x2) in benzaldehyde hydrogenation was noted with 20 mM of phenol, increasing phenol concentration further (until 200 mM) barely have an impact on the hydrogenation rate. However, no such enhancement was observed when the hydrogenation was studied with molecular H2 as the reduction equivalent instead of cathodic potential. This suggests that benzaldehyde hydrogenation might proceeds through an electrochemical step with direct involvement of the phenolic compounds. The results of the detail kinetic studies, and the corresponding reaction mechanisms will be addressed in this presentation. Figure 1

Electrocatalytic Hydrogenation of Phenol over Platinum and Rhodium: Unexpected Temperature Effects Resolved

Both electrocatalytic hydrogenation (ECH) and thermal catalytic hydrogenation (TCH) of phenol by Pt and Rh show a rollover in rate with increasing temperature without changing the principal reaction pathways. The negative effect of temperature for aqueous-phase phenol TCH and ECH on Pt and Rh is deduced to be from dehydrogenated phenol adsorbates, which block active sites. ECH and TCH rates increase similarly with increasing hydrogen chemical potential whether induced by applied potential or H2 pressure, both via increasing H coverage, and indirectly by removing site blockers, a strong effect at high temperature. This enables unprecedented phenol TCH rates at 60–100 °C.

Electrocatalytic Hydrogenation of Lignin-Derived Phenol into Alkanes by Using Platinum Supported on Graphite

In this work, electrocatalytic hydrogenation (ECH) of phenol to cyclohexane and cyclohexanol was studied. Experiments were run in a H-type cell with a Pt sheet as anode. The cathode material and structure were found to have a large effect on the ECH of phenol. Among the cathodes studied, the 1.5% Pt supported on graphite gave the highest product yield and current efficiency. Temperature was another important variable, the yield of cyclohexane increased from 44.1% at 20°C to 63.7% at 60°C, but then dropped back to 50.3% at 80°C. Effects of electrolyte composition on ECH of phenol were also investigated. The yield of cyclohexane was highest when 0.2 mol/L HClO4 as electrolyte. Variable current studies in the range of 10–90 mA showed an increase in product yields with increasing current from 10–30 mA, but yields decreased when current above 70 mA. The suitable starting concentration of phenol was 50 mmol/L.

Effect of support conductivity of catalytic powder on electrocatalytic hydrogenation of phenol

Metallic nanoaggregates deposited on non-conductive oxides powders as catalysts have shown good efficiency in electrocatalytic hydrogenation (ECH). In this process, the polarization of the metallic nanoaggregates is very important. This polarization can be improved when the electrode material is conductive. Thus, the goal of this work was to study the effect of the conduction of the supported material on the ECH process. Tin dioxide was chosen as oxide because it can be obtained in non-conductive or conductive form by doping with fluorine. Palladium supported catalysts powders were prepared by the sol–gel method. These electrocatalysts were characterized by XRD, SEM, TGA/DSC, FTIR and electrical conductivity. The effects of temperature and time of calcination were also investigated. Comparison of non-conductive and conductive catalysts for ECH of phenol shows that conductive F-doped SnO2 increases the rate of electrohydrogenation.

Electrocatalytic hydrogenation of phenol in aqueous solutions at a Raney nickel electrode in the presence of cationic surfactants

The electrocatalytic hydrogenation (ECH) of phenol at a Raney nickel cathode was studied in aqueous solutions. At 30 °C, without surfactants, cyclohexanol was obtained with low or medium yields. The best results were observed in alkaline solutions (pH 9). At pH 2 the efficiency of the hydrogenation reaction is significantly improved by low amounts of didodecyldimethylammonium bromide (DDAB). This surfactant effect is studied in relation to the substrate and hydrogen adsorption phenomena.

Electrocatalytic hydrogenation of phenol on dispersed Pt: reaction mechanism and support effect

The electrocatalytic hydrogenation of phenol on highly dispersed Pt on carbon has been investigated. The comparison of the hydrogenation performance of Pt/C and Pt/Pt electrodes containing the same amount of Pt or the same real electrochemical surface area shows that Pt/C electrode is electrocatalytically more active. Explanations related to this difference in activity are discussed. The study of the reaction mechanism suggests that the surface reaction between adsorbed species is the rate determining step which is the same as that proposed for platinized platinum electrode.

Electrocatalytic hydrogenation of phenol on various electrode materials

The electrocatalytic hydrogenation (ECH) of phenol was investigated at room temperature under galvanostatic control in aqueous sulfuric acid solutions on platinized platinum (Pt/Pt) electrodes and on composite Rh/Ni and Ru/Ni electrodes consisting of rhodium or ruthenium chemically deposited on nickel particles dispersed in a lanthanum polyphosphate matrix. The order of electrocatalytic activity at a current density (based on the geometric area) of 1 mA/cm2 was found to be Ru/Ni &gt; Pt/Pt &gt; Rh/Ni. The efficiency decreased with increased current density. For the Ru/Ni electrodes, the efficiency increased with the percentage of Ru (2.3 to 5%). On Ru 5%/Ni electrodes and at a current density of 5 mA/cm2, cyclohexanol was obtained with a high selectivity of 91 % and a current efficiency of 23% after 94% conversion. In neutral aqueous boric acid containing sodium chloride as supporting electrolyte, the Rh/Ni and Ru/Ni electrodes showed very low activity. The influence of the supporting electrolyte, periodic current control, and temperature was studied in the neutral medium with composite Raney nickel (RaNi) electrodes consisting of Raney nickel particles dispersed in a nickel matrix. At 60 °C and a current density of 10 mA/cm2, the selectivity of cyclohexanol formation was 100% with a 11% current efficiency after 92% conversion with sodium chloride as supporting electrolyte. Keywords: electrocatalytic hydrogenation, phenol, ruthenium on nickel cathodes, rhodium on nickel cathodes, Raney nickel cathodes.

Electrocatalytic hydrogenation of phenol on dispersed Pt: Effect of metal electrochemically active surface area and electrode material

The influence of the electrochemically active surface area on the electrocatalytic activity for phenol hydrogenation has been investigated. It is shown that the current efficiency of the reaction does not change significantly for Pt loadings between 2% and 30% whereas a significant decrease in activity is observed for 60% Pt loading. This is used to classify this reaction in terms of its sensitivity. The influence of the electrode material and the alloying component on both current efficiency and on the reaction selectivity is determined. The material and the alloying component which exhibited the best efficiency and selectivity are deduced.