Chemoenzymatic Photoreforming: A Sustainable Approach for Solar Fuel Generation from Plastic Feedstocks.

Plastic upcycling through catalytic transformations is an attractive concept to valorize waste, but the clean and energy-efficient production of high-value products from plastics remains challenging. Here, we introduce chemoenzymatic photoreforming as a process coupling enzymatic pretreatment and solar-driven reforming of polyester plastics under mild temperatures and pH to produce clean H2 and value-added chemicals. Chemoenzymatic photoreforming demonstrates versatility in upcycling polyester films and nanoplastics to produce H2 at high yields reaching ∼103-104 µmol gsub-1 and activities at >500 µmol gcat-1 h-1. Enzyme-treated plastics were also used as electron donors for photocatalytic CO2-to-syngas conversion with a phosphonated cobalt bis(terpyridine) catalyst immobilized on TiO2 nanoparticles (TiO2|CotpyP). Finally, techno-economic analyses reveal that the chemoenzymatic photoreforming approach has the potential to drastically reduce H2 production costs to levels comparable to market prices of H2 produced from fossil fuels while maintaining low CO2-equivalent emissions.

Bio-Electrocatalytic Conversion of Food Waste to Ethylene via Succinic Acid as the Central Intermediate.

Ethylene is an important feedstock in the chemical industry, but currently requires production from fossil resources. The electrocatalytic oxidative decarboxylation of succinic acid offers in principle an environmentally friendly route to generate ethylene. Here, a detailed investigation of the role of different carbon electrode materials and characteristics revealed that a flat electrode surface and high ordering of the carbon material are conducive for the reaction. A range of electrochemical and spectroscopic approaches such as Koutecky-Levich analysis, rotating ring-disk electrode (RRDE) studies, and Tafel analysis as well as quantum chemical calculations, electron paramagnetic resonance (EPR), and in situ infrared (IR) spectroscopy generated further insights into the mechanism of the overall process. A distinct reaction intermediate was detected, and the decarboxylation onset potential was determined to be 2.2-2.3 V versus the reversible hydrogen electrode (RHE). Following the mechanistic studies and electrode optimization, a two-step bio-electrochemical process was established for ethylene production using succinic acid sourced from food waste. The initial step of this integrated process involves microbial hydrolysis/fermentation of food waste into aqueous solutions containing succinic acid (0.3 M; 3.75 mmol per g bakery waste). The second step is the electro-oxidation of the obtained intermediate succinic acid to ethylene using a flow setup at room temperature, with a productivity of 0.4-1 µmol ethylene cmelectrode -2 h-1. This approach provides an alternative strategy to produce ethylene from food waste under ambient conditions using renewable energy.

Long-term solar water and CO2 splitting with photoelectrochemical BiOI-BiVO4 tandems.

Photoelectrochemical (PEC) devices have been developed for direct solar fuel production but the limited stability of submerged light absorbers can hamper their commercial prospects.1,2 Here, we demonstrate photocathodes with an operational H2 evolution activity over weeks, by integrating a BiOI light absorber into a robust, oxide-based architecture with a graphite paste conductive encapsulant. In this case, the activity towards proton and CO2 reduction is mainly limited by catalyst degradation. We also introduce multiple-pixel devices as an innovative design principle for PEC systems, displaying superior photocurrents, onset biases and stability over corresponding conventional single-pixel devices. Accordingly, PEC tandem devices comprising multiple-pixel BiOI photocathodes and BiVO4 photoanodes can sustain bias-free water splitting for 240 h, while devices with a Cu92In8 alloy catalyst demonstrate unassisted syngas production from CO2.

Insights from Operando and Identical Location (IL) Techniques on the Activation of Electrocatalysts for the Conversion of CO2: A Mini-Review.

In this mini-review we compare two prototypical metal foam electrocatalysts applied to the transformation of CO2 into value-added products (e.g. alcohols on Cu foams and formate on Bi foams). A substantial improvement in the catalyst performance is typically achieved through thermal annealing of the as-deposited foam materials, followed by the electro-reduction of the pre-formed oxidic precursors prior or during the actual CO2 electrolysis. Utilizing highly insightful and sensitive complementary operando analytical techniques (XAS, XRD, and Raman spectroscopy) we demonstrate that this catalyst pre-activation process is entirely accomplished in case of the oxidized Cu foams prior to the formation of hydrocarbons and alcohols from the CO2. The actually active catalyst is therefore the metallic Cu derived from the precursor by means of oxide electro-reduction. Conversely, in their oxidic form, the Cu-based foam catalysts are inactive towards the CO2 reduction reaction (denoted ec-CO2 RR). Oxidized Bi foams can be regarded as an excellent counter example to the above-mentioned Cu case as both metallic and the thermally derived oxidic Bi foams are highly active towards ec-CO2 RR (formate production). Indeed, operando Raman spectroscopy reveals that CO2 electrolysis occurs upon its embedment into the oxidic Bi2O3 foam precursor, which itself undergoes partial transformation into an active sub-carbonate phase. The potential-dependent transition of sub-carbonates/oxides into the corresponding metallic Bi foam dictates the characteristic changes of the ec-CO2 RR pathway. Identical location (IL) microscopic inspection of the catalyst materials, e.g. by means of scanning electron microscopy, demonstrates substantial morphological alterations on the nm length scale on the material surface as consequence of the sub-carbonate formation and the potential-driven oxide reduction into the metallic Bi foam. The foam morphology on a mesoscopic length scale (macroporosity) remains, by contrast, fully unaffected by these phase transitions.

Conversion of Polyethylene Waste into Gaseous Hydrocarbons via Integrated Tandem Chemical-Photo/Electrocatalytic Processes.

The chemical inertness of polyethylene makes chemical recycling challenging and motivates the development of new catalytic innovations to mitigate polymer waste. Current chemical recycling methods yield a complex mixture of liquid products, which is challenging to utilize in subsequent processes. Here, we present an oxidative depolymerization step utilizing diluted nitric acid to convert polyethylene into organic acids (40% organic acid yield), which can be coupled to a photo- or electrocatalytic decarboxylation reaction to produce hydrocarbons (individual hydrocarbon yields of 3 and 20%, respectively) with H2 and CO2 as gaseous byproducts. The integrated tandem process allows for the direct conversion of polyethylene into gaseous hydrocarbon products with an overall hydrocarbon yield of 1.0% for the oxidative/photocatalytic route and 7.6% for the oxidative/electrolytic route. The product selectivity is tunable with photocatalysis using TiO2 or carbon nitride, yielding alkanes (ethane and propane), whereas electrocatalysis on carbon electrodes produces alkenes (ethylene and propylene). This two-step recycling process of plastics can use sunlight or renewable electricity to convert polyethylene into valuable, easily separable, gaseous platform chemicals.

Suppression of the Hydrogen Evolution Reaction Is the Key: Selective Electrosynthesis of Formate from CO2 over Porous In55Cu45 Catalysts.

Direct electrosynthesis of formate through CO2 electroreduction (denoted CO2RR) is currently attracting great attention because formate is a highly valuable commodity chemical that is already used in a wide range of applications (e.g., formic acid fuel cells, tanning, rubber production, preservatives, and antibacterial agents). Herein, we demonstrate highly selective production of formate through CO2RR from a CO2-saturated aqueous bicarbonate solution using a porous In55Cu45 alloy as the electrocatalyst. This novel high-surface-area material was produced by means of an electrodeposition process utilizing the dynamic hydrogen bubble template approach. Faradaic efficiencies (FEs) of formate production (FEformate) never fell below 90% within a relatively broad potential window of approximately 400 mV, ranging from -0.8 to -1.2 V vs the reversible hydrogen electrode (RHE). A maximum FEformate of 96.8%, corresponding to a partial current density of jformate = -8.9 mA cm-2, was yielded at -1.0 V vs RHE. The experimental findings suggested a CO2RR mechanism involving stabilization of the HCOO* intermediate on the In55Cu45 alloy surface in combination with effective suppression of the parasitic hydrogen evolution reaction. What makes this CO2RR alloy catalyst particularly valuable is its stability against degradation and chemical poisoning. An almost constant formate efficiency of ∼94% was maintained in an extended 30 h electrolysis experiment, whereas pure In film catalysts (the reference benchmark system) showed a pronounced decrease in formate efficiency from 82% to 50% under similar experimental conditions. The identical location scanning electron microscopy approach was applied to demonstrate the structural stability of the applied In55Cu45 alloy foam catalysts at various length scales. We demonstrate that the proposed catalyst concept could be transferred to technically relevant support materials (e.g., carbon cloth gas diffusion electrode) without altering its excellent figures of merit.

Oxo-functionalised mesoionic NHC nickel complexes for selective electrocatalytic reduction of CO2 to formate.

Strategies for the conversion of CO2 to valuable products are paramount for reducing the environmental risks associated with high levels of this greenhouse gas and offer unique opportunities for transforming waste into useful products. While catalysts based on nickel as an Earth-abundant metal for the sustainable reduction of CO2 are known, the vast majority produce predominantly CO as a product. Here, efficient and selective CO2 reduction to formate as a synthetically valuable product has been accomplished with novel nickel complexes containing a tailored C,O-bidentate chelating mesoionic carbene ligand. These nickel(ii) complexes are easily accessible and show excellent catalytic activity for electrochemical H+ reduction to H2 (from HOAc in MeCN), and CO2 reduction (from CO2-saturated MeOH/MeCN solution) with high faradaic efficiency to yield formate exclusively as an industrially and synthetically valuable product from CO2. The most active catalyst precursor features the 4,6-di-tert-butyl substituted phenolate triazolylidene ligand, tolerates different proton donors including water, and reaches an unprecedented faradaic efficiency of 83% for formate production, constituting the most active and selective Ni-based system known to date for converting CO2 into formate as an important commodity chemical.

Catalyst Development for Water/CO2 Co-electrolysis.

Herein, we discuss recent research activities on the electrochemical water/CO2 co-electrolysis at the Department of Chemistry and Biochemistry of the University of Bern (Arenz and Broekmann research groups). For the electrochemical conversion of the greenhouse gas CO2 into products of higher value catalysts for two half-cell reactions need to be developed, i.e. catalysts for the reductive conversion of CO2 (CO2RR) as well as catalysts for the oxidative splitting of water (OER: Oxygen Evolution Reaction). In research, the catalysts are often investigated independently of each other as they can later easily be combined in a technical electrolysis cell. CO2RR catalysts consist of abundant materials such as copper and silver and thus mainly the product selectivity of the respective catalyst is in focus of the investigation. In contrast to that, OER catalysts (in acidic conditions) mainly consist of precious metals, e.g. Ir, and therefore the minimization of the catalytic current per gram Ir is of fundamental importance.

Selective Electrochemical Reduction of CO2 to CO on Zn-Based Foams Produced by Cu2+ and Template-Assisted Electrodeposition.

In this work, we aim to develop a Zn-based metal foam catalyst with very large specific area suitable for efficient CO production. Its manufacture is based on the dynamic hydrogen bubble template method that consists of the superposition of metal deposition and hydrogen evolution at the solid-liquid interface. We employed Cu ions in the Zn2+-rich electroplating bath as foaming agent. The concentration of Cu as foaming agent was systematically studied and an optimized Zn94Cu6 foam alloy was developed, which, to the best of our knowledge, is the most selective Zn-based CO2 electrocatalyst toward CO in aqueous bicarbonate solution (FECO = 90% at -0.95 V vs reversible hydrogen electrode). This high efficiency is ascribed to the combination of high density of low-coordinated active sites and preferential Zn(101) over Zn(002) texturing. X-ray photoelectron spectroscopy investigations demonstrate that the actual catalyst material is shaped upon reduction of an oxide/hydroxide-terminating surface under CO2 electrolysis conditions. Moreover, intentional stressing by oxidation at room conditions proved to be beneficial for further activation of the catalyst. Identical location scanning electron microscopy imaging before and after CO2 electrolysis and long-term electrolysis experiments also showed that the developed Zn94Cu6 foam catalyst is both structurally and chemically stable at reductive conditions.

Size-Dependent Activity of Palladium Nanoparticles: Efficient Conversion of CO2 into Formate at Low Overpotentials.

Remarkable size-dependent activity of palladium nanoparticles (PdNPs) towards formate production is evident at very low overpotentials (-0.1 to -0.5 V vs. RHE). Size-selective PdNPs, chemically synthesized at sizes of 3.8-10.7 nm, effected an electrochemical CO2 reduction reaction in aqueous 0.5 m NaHCO3 . The faradaic efficiency of formate production (FEformate ) on 3.8 nm PdNPs exceeded 86 % at E=-0.1 V versus RHE, whereas on 6.5 nm PdNPs an even higher FEformate of 98 % was observed. However, FEformate decreased for larger PdNPs. The superior efficiency towards formate production at low overpotentials is rationalized in terms of a changed catalytic pathway through PdH phases. The observed maximum in the formate efficiency for a mean particle size of about 6.5 nm is discussed in terms of counterbalancing the size-dependent effects of a competing CO2 reduction reaction and a parasitic hydrogen evolution reaction. Production rates of formate are also remarkably high at -0.3 V versus RHE with 539.9 and 452.3 ppm h-1 mgPd-1 for the 6.5 and 3.8 nm PdNPs, respectively.