Photoreforming of Organic Waste into Hydrogen Using a Thermally Radiative CdOx/CdS/SiC Photocatalyst

State-of-the-art and perspectives of hydrogen generation from waste plastics

Waste plastic utilization and hydrogen production present significant economic and social challenges but also offer opportunities for research and innovation. This review provides a comprehensive analysis of the latest advancements and innovations in hydrogen generation coupled with waste plastic recycling. It explores various strategies, including pyrolysis, gasification, aqueous phase reforming, photoreforming, and electrocatalysis. Pyrolysis and gasification in combination with catalytic reforming or water gas-shift are currently the most feasible and scalable technologies for hydrogen generation from waste plastics, with pyrolysis operating in an oxygen-free environment and gasification in the presence of steam, though both require high energy inputs. Aqueous phase reforming operates at moderate temperatures and pressures, making it suitable for oxygenated plastics, but it faces challenges related to feedstock limitations, catalyst costs and deactivation. Photoreforming and electrocatalytic reforming are emerging, sustainable methods that use sunlight and electricity, respectively, to convert plastics into hydrogen. Still, they suffer from low efficiency, scalability issues, and limitations to specific plastic types like oxygenated polymers. The challenges and solutions to commercializing plastic-to-hydrogen technologies, drawing on global industrial case studies have been outlined. Maximizing hydrogen productivity and selectivity, minimizing energy consumption, and ensuring stable operation and scaleup of plastic recycling are crucial parameters for achieving commercial viability.

Progress on Photo-, Electro-, and Photoelectro-Catalytic Conversion of Recalcitrant Polyethylene, Polypropylene, and Polystyrene - A Review

Recalcitrant waste plastics such a polyethylene, polypropylene, and polystyrene are difficult to recycle and are mostly disposed of in landfills and eventually leached into the environmental as micro- and nano-plastics. This review explores how photo-, electro-, and combined photoelectro-catalytic processes can assist in the degradation and upcycling of waste plastic into different chemicals and mitigate their release to the environment. In this work, we discuss how the different reaction mechanisms proceed, explore the current relevant literature, and highlight the developments needed to advance the field.

Photocatalytic upcycling of polylactic acid to alanine by sulfur vacancy-rich cadmium sulfide

Photocatalytic conversion has emerged as a promising strategy for harnessing renewable solar energy in the valorization of plastic waste. However, research on the photocatalytic transformation of plastics into valuable nitrogen-containing chemicals remains limited. In this study, we present a visible-light-driven pathway for the conversion of polylactic acid (PLA) into alanine under mild conditions. This process is catalyzed by defect-engineered CdS nanocrystals synthesized at room temperature. We observe a distinctive volcano-shaped relationship between sulfur vacancy content in CdS and the corresponding alanine production rate reaching up to 4.95 mmol/g catalyst/h at 70 oC. Ultraviolet-visible, photocurrent, electrochemical impedance, transient absorption, photoluminescence, and Fourier-transform infrared spectroscopy collectively highlight the crucial role of sulfur vacancies. The surface vacancies serve as adsorption sites for lactic acid; however, an excessive number of vacancies can hinder charge transfer efficiency. Sulfur vacancy-rich CdS exhibits high stability with maintained performance and morphology over several runs, effectively converts real-life PLA products and shows potential in the amination of other polyesters. This work not only highlights a facile approach for fabricating defect-engineered catalysts but also presents a sustainable method for upcycling plastic waste into valuable chemicals.

Ultrathin Porous Carbon Nitride Anchored with Pt Nanoclusters for Synergistic Enhancement of Hydrogen Production in Alkaline Photocatalytic Polyester Reforming

Photocatalytic reforming (PR) of polyester waste, fueled by renewable sources like solar energy, offers a sustainable method for producing clean H2 and valuable by-products under mild conditions. The design of high-performance photocatalyst plays a pivotal role in determining the efficacy of an alkaline polyester PR system, influencing H2 generation activity and selectivity. Here, ultrathin porous carbon nitride nanosheets (UP-CN) loaded with Pt nanoclusters (Pt NCs, average diameter of 1.7 nm) with uniform Pt NCs distribution are introduced. The resulting Pt NCs/UP-CN catalyst can accelerate charge and mass transfer while providing additional active sites, achieving superior H2 generation rates of 11.69 mmol gcat -1 h-1 and 2923 mmol gPt -1 h-1 under AM 1.5 light, which nine times higher than that of Pt nanoparticles-bulk graphitic carbon nitride composite (1.29 mmol gcat -1 h-1 and 258 mmol gPt -1 h-1) as counterpart. This performance also surpasses that of previously reported carbon nitride-based and TiO2-based photocatalysts. Moreover, the density functional theory calculations reveal a significant reduction in the energy barrier for the water dissociation step (H2O + * → *H + OH) at the interface between UP-CN and anchored Pt NCs, showcasing the synergistic effect between Pt NCs and UP-CN. This catalytic system also exhibits universality across various polyester plastics.

Photocatalytic Upcycling of Different Types of Plastic Wastes: A Mini Review

With the escalating demand and utilization of plastics, considerable attention has been given to controlling plastic pollution. Among these methodologies, photocatalytic upcycling of plastic has emerged as a promising method for plastic management due to its energy-saving and eco-friendly properties. In the past several years, great efforts have been devoted to the photocatalytic conversion of a variety of commercial plastic types. These encouraging endeavors foreshadow the continued progression and application in this field. In this review, recent advancements in the photocatalytic upcycling of plastics are reviewed. The fundamentals and principles of photocatalytic deconstruction of plastics are first introduced. Then, we summarize the works on the reforming of different types of plastic, including polyolefins, polyesters, and other types. Finally, some challenges and possible solutions are provided for the development of photocatalytic upcycling of plastics.

Photoreforming of Microplastics: Challenges and Opportunities for Sustainable Environmental Remediation

Plastics are widely used in daily lives, but unfortunately, their inadequate recycling practices have led to the accumulation of microplastics in the environment, posing a threat to public health. The existing methods for treating microplastics are energy-intensive and environmentally damaging. In this context, photoreforming has emerged as a sustainable solution to address the microplastic crisis by simultaneously recycling them into value-added chemicals. This review presents a comprehensive overview of the application of photoreforming for upcycling microplastic. The underlying mechanisms of photoreforming reaction are discussed, followed by the exploration of recent advancements and innovative strategies in photoreforming techniques with particular emphasis on their real-world applications and potential for large-scale implementation. Also, critical factors influencing the efficiency of microplastic photoreforming are identified, providing guidance for further research and optimization.

Photorefinery of Biomass and Plastics to Renewable Chemicals using Heterogeneous Catalysts

The photocatalytic conversion of biomass and plastic waste provides opportunities for sustainable fuel and chemical production. Heterogeneous photocatalysts, typically composed of semiconductors with distinctive redox properties in their conduction band (CB) and valence band (VB), facilitate both the oxidative and reductive valorization of organic feedstocks. This article provides a comprehensive overview of recent advancements in the photorefinery of biomass and plastics from the perspective of the redox properties of photocatalysts. We explore the roles of the VB and CB in enhancing the value-added conversion of biomass and plastics via various pathways. Our aim is to bridge the gap between photocatalytic mechanisms and renewable carbon feedstock valorization, inspiring further development in photocatalytic refinery of biomass and plastics.

From Plastic Waste to Green Hydrogen and Valuable Chemicals Using Sunlight and Water

Over 79 % of 6.3 billion tonnes of plastics produced from 1950 to 2015 have been disposed in landfills or found their way to the oceans, where they will reside for up to hundreds of years before being decomposed bringing upon significant dangers to our health and ecosystems. Plastic photoreforming offers an appealing alternative by using solar energy and water to transform plastic waste into value-added chemical commodities, while simultaneously producing green hydrogen via the hydrogen evolution reaction. This review aims to provide an overview of the underlying principles of emerging plastic photoreforming technologies, highlight the challenges associated with experimental protocols and performance assessments, discuss recent global breakthroughs on the photoreforming of plastics, and propose perspectives for future research. A critical assessment of current plastic photoreforming studies shows a lack of standardised conditions, hindering comparison amongst photocatalyst performance. Guidelines to establish a more accurate evaluation of materials and systems are proposed, with the aim to facilitate the translation of promising fundamental discovery in photocatalysts design.

Solar-Driven Biomass Reforming for Hydrogen Generation: Principles, Advances, and Challenges

Hydrogen (H2) has emerged as a clean and versatile energy carrier to power a carbon-neutral economy for the post-fossil era. Hydrogen generation from low-cost and renewable biomass by virtually inexhaustible solar energy presents an innovative strategy to process organic solid waste, combat the energy crisis, and achieve carbon neutrality. Herein, the progress and breakthroughs in solar-powered H2 production from biomass are reviewed. The basic principles of solar-driven H2 generation from biomass are first introduced for a better understanding of the reaction mechanism. Next, the merits and shortcomings of various semiconductors and cocatalysts are summarized, and the strategies for addressing the related issues are also elaborated. Then, various bio-based feedstocks for solar-driven H2 production are reviewed with an emphasis on the effect of photocatalysts and catalytic systems on performance. Of note, the concurrent generation of value-added chemicals from biomass reforming is emphasized as well. Meanwhile, the emerging photo-thermal coupling strategy that shows a grand prospect for maximally utilizing the entire solar energy spectrum is also discussed. Further, the direct utilization of hydrogen from biomass as a green reductant for producing value-added chemicals via organic reactions is also highlighted. Finally, the challenges and perspectives of photoreforming biomass toward hydrogen are envisioned.

Investigating the Corrosion Resistance of Different SiC Crystal Types: From Energy Sectors to Advanced Applications

Silicon carbide, as a third-generation semiconductor material, plays a pivotal role in various advanced technological applications. Its exceptional stability under extreme conditions has garnered a significant amount of attention. These superior characteristics make silicon carbide an ideal candidate material for high-frequency, high-power electronic devices and applications in harsh environments. In particular, corrosion resistance in natural or artificially acidic and alkaline environments limits the practical application of many other materials. In fields such as chemical engineering, energy conversion, and environmental engineering, materials often face severe chemical erosion, necessitating materials with excellent chemical stability as foundational materials, carriers, or reaction media. Silicon carbide exhibits outstanding performance under these conditions, demonstrating significant resistance to corrosive substances such as hydrochloric acid, sulfuric acid, nitric acid, and alkaline substances such as potassium hydroxide and sodium hydroxide. Despite the well-known chemical stability of silicon carbide, the stability conditions of its different types (such as 3C-, 4H-, and 6H-SiC polycrystals) in acidic and alkaline environments, as well as the specific corrosion mechanisms and differences, warrant further investigation. This Review not only delves deeply into the detailed studies related to this topic but also highlights the current applications of different silicon carbide polycrystals in chemical reaction systems, energy conversion equipment, and recycling processes. Through a comprehensive analysis, this Review aims to bridge research gaps, offering a comparative analysis of the advantages and disadvantages between different polymorphs. It provides material scientists, engineers, and developers with a thorough understanding of silicon carbide's behavior in various chemical environments. This work will propel the research and development of silicon carbide materials under extreme conditions, especially in areas where chemical stability is crucial for device performance and durability. It lays a solid foundation for ultra-high-power, high-integration, high-reliability module architectures, supercomputing chips, and highly safe long-life batteries.

Plastic Waste Conversion by Leveraging Renewable Photo/Electro-Catalytic Technologies

Plastics have revolutionized our lives; however, the exponential growth of their usage has led to a global crisis. More sustainable strategies are needed to address this dilemma and transform the plastics economy from a linearity to a circular model. Herein, we systematically summarize the recent progress in renewable energy-driven plastic conversion strategies, including photocatalysis, electrocatalysis, and their integration. By introducing the significant works, the design principles, mechanisms, and system regulations, we decipher and compare the various aspects of plastic conversion. These approaches show high reactivity and selectivity under environmentally benign conditions and provide alternative reaction pathways for plastic conversion. Plastic upcycling as a chemical feedstock can yield value-added chemicals and fuels, contributing to the establishment of a sustainable and circular economy. Additionally, several innovations in reaction engineering and system designs are presented. Finally, the challenges and perspectives of sustainable energy-driven plastic conversion technologies are comprehensively discussed.

Recent advancement on photocatalytic plastic upcycling

More than 8 billion tons of plastics have been generated since 1950. About 80% of these plastics have been dumped in landfills or went into natural environments, resulting in ever-worsening contamination. Among various strategies for waste plastics processing (e.g., incineration, mechanical recycling, thermochemical conversion and electrocatalytic/photocatalytic techniques), photocatalysis stands out as a cost-effective, environmentally benign and clean technique to upcycle plastic waste at ambient temperature and pressure using solar light. The mild reaction conditions for photocatalysis enable the highly selective conversion of plastic waste into targeted value-added chemicals/fuels. Here, we for the first time summarize the recent development of photocatalytic plastic upcycling based on the chemical composition of photocatalysts (e.g., metal oxides, metal sulfides, non-metals and composites). The pros and cons of various photocatalysts have been critically discussed and summarized. At last, the future challenges and opportunities in this area are presented in this review.

Solar reforming as an emerging technology for circular chemical industries

The adverse environmental impacts of greenhouse gas emissions and persistent waste accumulation are driving the demand for sustainable approaches to clean-energy production and waste recycling. By coupling the thermodynamically favourable oxidation of waste-derived organic carbon streams with fuel-forming reduction reactions suitable for producing clean hydrogen or converting CO2 to fuels, solar reforming simultaneously valorizes waste and generates useful chemical products. With appropriate light harvesting, catalyst design, device configurations and waste pre-treatment strategies, a range of sustainable fuels and value-added chemicals can already be selectively produced from diverse waste feedstocks, including biomass and plastics, demonstrating the potential of solar-powered upcycling plants. This Review highlights solar reforming as an emerging technology that is currently transitioning from fundamental research towards practical application. We investigate the chemistry and compatibility of waste pre-treatment, introduce process classifications, explore the mechanisms of different solar reforming technologies, and suggest appropriate concepts, metrics and pathways for various deployment scenarios in a net-zero-carbon future.

Constructing the Sulfur-Doped CdO@In2O3 Nanofibers Ternary Heterojunction for Efficient Photocatalytic Hydrogen Production

An S-doped CdO@In₂O₃ nanofiber was successfully designed by in-situ electrospinning along and subsequent calcination treatment. Under artificial sunlight illumination, the S/CdO@In₂O₃-25 displayed a superior photocatalytic hydrogen evolution rate of 4564.58 μmol·g^-1·h^-1, with approximately 22.0 and 1261.0-fold of those shown by the S/CdO and S/In₂O₃ samples, respectively. The experimental and theoretical analyses illustrate that the unique one-dimensional (1D) nanofiber morphology and rich oxygen vacancies optimized the electronic structure of the nanofibers and adsorption/desorption behaviors of reaction intermediates, contributing to the realization of the remarkable solar-to-H₂ conversion efficiencies. Moreover, the staggered band structure and intimate contact heterointerfaces facilitate the formation of a type-II double charge-transfer pathway, promoting the spatial separation of photoexcited charge carriers. These results could inform the design of other advanced catalyst materials for photocatalytic reactions.

A systematic review on plastic waste conversion for a circular economy: recent trends and emerging technologies

Our biosphere has been adversely affected by plastic waste pollution, especially non-biodegradables in landfills, which induces hazardous chemical leaching and toxic gas emissions on burning into the atmosphere.

Hydrogen Production and Storage: Analysing Integration of Photoelectrolysis, Electron Harvesting Lignocellulose, and Atmospheric Carbon Dioxide-Fixing Biosynthesis

Green hydrogen from photocatalytic water-splitting and photocatalytic lignocellulosic reforming is a significant proposition for renewable energy storage in global net-zero policies and strategies. Although photocatalytic water-splitting and photocatalytic lignocellulosic reforming have been investigated, their integration is novel. Furthermore, biosynthesis can store the evolved hydrogen and fix the atmospheric carbon dioxide in a biocathode chamber. The biocathode chamber is coupled to the combined photocatalytic water-splitting and lignocellulose oxidation in an anode chamber. This integrated system of anode and biocathode mimics a (bio)electrosynthesis system. A visible solar radiation-driven novel hybrid system comprising photocatalytic water-splitting, lignocellulose oxidation, and atmospheric CO2 fixation is, thus, investigated. It must be noted that there is no technology for reducing atmospheric CO2 concentration. Thus, our novel intensified technology enables renewable and sustainable hydrogen economy and direct CO2 capture from air to confront climate change impact. The photocatalytic anode considered is CdS nanocomposites that give a low absorption onset (200 nm), high absorbance range (200–800 nm), and narrow bandgap (1.58–2.4 V). The biocathode considered is Ralstonia eutropha H16 interfaced with photocatalytic lignocellulosic oxidation and a water-splitting anode. The biocathode undergoes autotrophic metabolism fixing atmospheric CO2 and hydrogen to poly(3-hydroxybutyrate) biosynthesis. As the hydrogen evolved can be readily stored, the electron–hole pair can be separated, increasing the hydrogen evolution efficiency. Although there are many experimental studies, this study for the first time sets the maximum theoretical efficiency target from mechanistic deductions of practical insights. Compared to physical/physicochemical absorption with solvent recovery to capture CO2, the photosynthetic CO2 capture efficiency is 51%. The maximum solar-to-hydrogen generation efficiency is 33%. Lignocelluloses participate in hydrogen evolution by (1–4)-glycosidic bond decomposition, releasing accessible sugar monomers or monosaccharides forming a Cd–O–R bond with the CdS/CdOx nanocomposite surface used as a photocatalyst/semiconductor, leading to CO32− in oxidised carboxylic acid products. Lignocellulose dosing as an oxidising agent can increase the extent of water-splitting. The mechanistic analyses affirm the criticality of lignocellulose oxidation in photocatalytic hydrogen evolution. The critical conditions for success are increasing the alcohol neutralising agent’s strength, increasing the selective (ligno)cellulose dosing, broadening the hybrid nanostructure of the photocatalyst/semiconductor, enhancing the visible-light range absorbance, and increasing the solar energy utilisation efficiency.

Recent development of organic-inorganic hybrid photocatalysts for biomass conversion into hydrogen production

Over the last few years, photocatalysis using solar radiation has been explored extensively to investigate the possibilities of producing fuels. The production and systematic usage of solar fuels can reduce the use of fossil-based fuels, which are currently the primary source for the energy. It is time for us to exploit renewable sources for our energy needs to progress towards a low-carbon society. This can be achieved by utilizing green hydrogen as the future energy source. Solar light-assisted hydrogen evolution through photocatalytic water splitting is one of the most advanced approaches, but it is a non-spontaneous chemical process and restricted by a kinetically demanding oxidation evolution reaction. Sunlight is one of the essential sources for the photoreforming (PR) of biomass waste into solar fuels, or/and lucrative fine chemicals. Hydrogen production through photoreforming of biomass can be considered energy neutral as it requires only low energy to overcome the activation barrier and an alternate method for the water splitting reaction. Towards the perspective of sustainability and zero emission norms, hydrogen production from biomass-derived feedstocks is an affordable and efficient process. Widely used photocatalyst materials, such as metal oxides, sulphides and polymeric semiconductors, still possess challenges in terms of their performance and stability. Recently, a new class of materials has emerged as organic-inorganic hybrid (OIH) photocatalysts, which have the benefits of both components, with peculiar properties and outstanding energy conversion capability. This work examines the most recent progress in the photoreforming of biomass and its derivatives using OIHs as excellent catalysts for hydrogen evolution. The fundamental aspects of the PR mechanism and different methods of hydrogen production from biomass are discussed. Additionally, an interaction between both composite materials at the atomic level has been discussed in detail in the recent literature. Finally, the opportunities and future perspective for the synthesis and development of OIH catalysts are discussed briefly with regards to biomass photo-reforming.