Combining Renewable Electricity and Renewable Carbon: Understanding Reaction Mechanisms of Biomass-Derived Furanic Compounds for Design of Catalytic Nanomaterials.

ConspectusDespite the growing deployment of renewable energy conversion technologies, a number of large industrial sectors remain challenging to decarbonize. Aviation, heavy transport, and the production of steel, cement, and chemicals are heavily dependent on carbon-containing fuels and feedstocks. A hopeful avenue toward carbon neutrality is the implementation of renewable carbon for the synthesis of critical fuels, chemicals, and materials. Biomass provides an opportune source of renewable carbon, naturally capturing atmospheric CO2 and forming multicarbon linkages and useful chemical functional groups. The constituent molecules nonetheless require various chemical transformations, often best facilitated by catalytic nanomaterials, in order to access usable final products.Catalyzed transformations of renewable biomass compounds may intersect with renewable energy production by offering a means to utilize excess intermittent electricity and store it within chemical bonds. Electrochemical catalytic processes can often offer advantages in energy efficiency, product selectivity, and modular scalability compared to thermal-driven reactions. Electrocatalytic reactions with renewable carbon feedstocks can further enable related processes such as water splitting, where value-adding organic oxidation reactions may replace the evolution of oxygen. Organic electroreduction reactions may also allow desirable hydrogenations of bonds without intermediate formation of H2 and need for additional reactors.This Account highlights recent work aimed at gaining a fundamental understanding of transformations involving biomass-derived molecules in electrocatalytic nanomaterials. Particular emphasis is placed on the oxidation of biomass derived furanic compounds such as furfural and 5-hydroxymethylfurfural (HMF), which can yield value-added chemicals, including furoic acid (FA), maleic acid (MA), and 2,5-furandicarboxylic acid (FDCA) for renewable materials and other commodities. We highlight advanced implementations of online electrochemical mass spectrometry (OLEMS) and vibrational spectroscopies such as attenuated total reflectance surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS), combined with microkinetic models (MKMs) and quantum chemical calculations, to shed light on the elementary mechanistic pathways involved in electrochemical biomass conversion and how these paths are influenced by catalytic nanomaterials. Perspectives are given on the potential opportunities for materials development toward more efficient and selective carbon-mitigating reaction pathways.

Electrochemical Stability of Thiolate Self-Assembled Monolayers on Au, Pt, and Cu.

Self-assembled monolayers (SAMs) of thiolates have increasingly been used for modification of metal surfaces in electrochemical applications including selective catalysis (e.g., CO2 reduction, nitrogen reduction) and chemical sensing. Here, the stable electrochemical potential window of thiolate SAMs on Au, Pt, and Cu electrodes is systematically studied for a variety of thiols in aqueous electrolyte systems. For fixed tail-group functionality, the reductive stability of thiolate SAMs is found to follow the trend Au < Pt < Cu; this can be understood by considering the combined influences of the binding strength of sulfur and competitive adsorption of hydrogen. The oxidative stability of thiolate SAMs is found to follow the order: Cu < Pt < Au, consistent with each surface's propensity toward surface oxide formation. The stable reductive and oxidative potential limits are both found to vary linearly with pH, except for reduction above pH ∼10, which is independent of pH for most thiol compositions. The electrochemical stability across different functionalized thiols is then revealed to depend on many different factors including SAM defects (accessible surface metal atom sites decrease stability), intermolecular interactions (hydrophilic groups reduce the stability), and SAM thickness (stability increases with alkanethiol carbon chain length) as well as factors such as SAM-induced surface reconstruction and the ability to directly oxidize or reduce the non-sulfur part of the SAM molecule.

Electrochemical oxidation of pharmaceuticals in synthetic fresh human urine: Using selective radical quenchers to reveal the dominant degradation pathways and the scavenging effects of individual urine constituents.

Electrochemical oxidation of fresh human urine is a promising method to prevent pharmaceuticals from being discharged into the environment. Here, we evaluate the importance of electro-generated oxidants and direct anodic oxidation for degradation of four pharmaceutical (cyclophosphamide (CP), carbamazepine (CBZ), sulfamethoxazole (SMX) and ibuprofen (IBP)) accounting for the scavenging effect of urine constituents using boron-doped diamond (BDD) and IrO2 electrodes. Allyl alcohol and tert-butanol were used as selective quenchers for adsorbed and dissolved radicals, respectively. In electrolyte containing only chloride and pharmaceuticals, we found that CBZ and SMX are primarily oxidized by electro-generated Cl2 in the fluid boundary layer , and CP and IBP are primarily oxidized by physisorbed •OH or chemisorbed chlorine (IrO3-Cl). Regarding the effects of other fresh urine constituents, urea, creatinine, and uric acid quench the dissolved reactive chlorine species (Cl•/Cl2•‒, HOCl, Cl2, etc.). However, SO42‒ shows no effect on pharmaceutical degradation while H2PO4‒ and citrate ions quench IrO3-Cl resulting in a mixed kinetic and mass-transfer limiting oxidation of pharmaceuticals on IrO2. Citrate ions only quench the dissolved oxidants (surface adsorbed radicals are the dominant oxidants) leading to the pharmaceutical degradation limited by the mass transfer of pharmaceutical to BDD surface. This work provides an understanding of the significance of various pathways for pharmaceutical degradation, scavenging effect of urine constituents, and strategies for rapid pharmaceutical degradation in human urine.

Selective oxidation of pharmaceuticals and suppression of perchlorate formation during electrolysis of fresh human urine.

Urine comprises only a small (~1%) volumetric fraction of municipal wastewater, but represents a dominant source of pharmaceuticals, many of which may pass through conventional wastewater treatment and pose risks to aquatic ecosystems. Point-source treatment of source-separated urine presents a unique opportunity to degrade pharmaceuticals before dilution with wastewater, and electrochemical advanced oxidation processes are one increasingly investigated option. However, they often lead to the formation of oxidation byproducts including chlorate, perchlorate at very high concentrations. Here, we show that the high urea content of fresh human urine suppresses the formation of oxychlorides by inhibiting formation of HOCl/OCl‒ during electrolysis, while still enabling pharmaceutical degradation due to the slow rate of urea oxidation by •OH. This results in improved performance compared to equivalent treatment of hydrolyzed aged urine. This electrochemical oxidation scheme is shown to degrade the model contaminants cyclophosphamide and sulfamethoxazole with surface-area-to-volume-normalized pseudo-first-order rate constants greater than 0.08 cm/min in authentic fresh human urine. It results in ~100 × decrease in pharmaceutical concentrations in 2 h while generating ~1000 × lower oxychloride byproduct concentrations in synthetic fresh urine than synthetic hydrolyzed aged urine matrixes. Importantly, this proof-of-principle shows that simple and safe electrochemical methods can be used for point-source-remediation of pharmaceuticals in fresh human urine (before storage and hydrolysis), without formation of significant oxychloride byproducts.