A novel example of molecular hydrogen generation from formic Acid at visible-light-responsive photocatalyst

From photocatalysis to photon-phonon co-driven catalysis for methanol reforming to hydrogen and valuable by-products

Hydrogen energy will play a dominant role in energy transition from fossil fuel to low carbon processes, while economical, efficient, and safe hydrogen storage and transportation technology has become one of the main bottlenecks that currently hinder the application of the hydrogen energy scale. Methanol has widely been regarded as a primary liquid H2 storage medium due to its high hydrogen content, easy storage and transportation and relatively low toxicity. Hydrogen release from methanol using photocatalysis has thus been the focus of intense research and recent years have witnessed its fast progress and drawbacks. This review offers a comprehensive overview of methanol-based hydrogen production via photocatalysis, spotlighting recent developments in photocatalysts referring to thermal catalysts, including efficient semiconductors and cocatalysts, followed by the discussion of mechanistic investigation via advanced techniques and their disadvantages. Beyond this, particular focus has been placed on the discussion of co-driven processes involving coupling of photons (photocatalysis) with phonons (thermal catalysis) - the concept of photon-phonon co-driven catalysis - for methanol reforming and cutting-edge reactor design strategies, in order to enhance the overall process efficiency and applicability. Concluding with forward-looking insights, this review aims to provide valuable guidance for future research on hydrogen release through methanol reforming.

Mechanistic Study of Glucose Photoreforming over TiO2-Based Catalysts for H2 Production

Glucose is a key intermediate in cellulose photoreforming for H₂ production. This work presents a mechanistic investigation of glucose photoreforming over TiO₂ and Pt/m-TiO₂ catalysts. Analysis of the intermediates formed in the process confirmed the α-scission mechanism of glucose oxidation forming arabinose (C_n-1 sugar) and formic acid in the initial oxidation step. The selectivity to sugar products and formic acid differed over Pt/TiO₂ and TiO₂, with Pt/TiO₂ showing the lower selectivity to formic acid due to enhanced adsorption/conversion of formic acid over Pt/TiO₂. In situ ATR-IR spectroscopy of glucose photoreforming showed the presence of molecular formic acid and formate on the surface of both catalysts at low glucose conversions, suggesting that formic acid oxidation could dominate surface reactions in glucose photoreforming. Further in situ ATR-IR of formic acid photoreforming showed Pt-TiO₂ interfacial sites to be key for formic acid oxidation as TiO₂ was unable to convert adsorbed formic acid/formate. Isotopic studies of the photoreforming of formic acid in D₂O (with different concentrations) showed that the source of the protons (to form H₂ at Pt sites) was determined by the relative surface coverage of adsorbed water and formic acid.

Synergistic Generation of Radicals by Formic Acid/H2O2/g-C3N4 Nanosheets for Ultra-efficient Oxidative Photodegradation of Rhodamine B

Water pollution is a global challenge endangering people's health. In this work, an ultra-efficient photodegradation system of Rhodamine B (RhB) has been established using a graphitic carbon nitride nanosheet (CNNS) as the semiconductor photocatalyst, from which energy is harvested on both the conduction band and valence band by formic acid and hydrogen peroxide, respectively. The optimized FA/H₂O₂/CNNS system increases the apparent photodegradation rate of RhB by 25 folds, from 0.0198 to 0.4975 min^-1. Through a comprehensive investigation with reactive oxygen species scavengers, electron paramagnetic resonance, high-performance liquid chromatography-mass spectrometry, etc., an oxidative mechanism for RhB photodegradation has been proposed, which combines enhanced charge carrier migration and synergistic generation of multiple radicals. Comparable performance improvements have also been observed for similar systems with different semiconductors, suggesting that such a catalytic system could afford a general approach to enhance semiconductor-catalyzed photodegradation.

Promoting the hydrogenation of acetone C–C coupling into pinacol with dehydrogenation of formic acid over a NaOH-treated g-C3N4 photocatalyst

NaOH-treated g-C₃N₄ photocatalytic dehydrogenation of formic acid to promote the hydrogenation of acetone C–C coupling into pinacol was carried out in which photo-electrons and photo-holes were effectively used.

Photocatalytic Approaches for Hydrogen Production via Formic Acid Decomposition

The photocatalytic dehydrogenation of formic acid has recently emerged as an outstanding alternative to the traditional thermal catalysts widely applied in this reaction. The utilization of photocatalytic processes for the production of hydrogen is an appealing strategy that perfectly matches with the idea of a green and sustainable future energy scenario. However, it sounds easier than it is, and great efforts have been needed to design and develop highly efficient photocatalysts for the production of hydrogen from formic acid. In this work, some of the most representative strategies adopted for this application are reviewed, paying particular attention to systems based on TiO₂, CdS and C₃N₄.

Titanium oxide based photocatalytic materials development and their role of in the air pollutants degradation: Overview and forecast

Due to the anthropogenic pollution, especially the environmental crisis caused by air pollutants, the development of air pollutant degradation photocatalyst has become one of the major directions to the crisis relief. Among them, titania (titanium dioxide, TiO₂) family materials were extensively studied in the past two decades due to their strong activity in the photocatalytic reactions. However, TiO₂ had a drawback of large bandgap which limited its applications, several modification techniques were hence developed to enhance its catalytic activity and light sensitivity. In recent years, other metal oxide based materials have been developed as replacements for TiO₂ photocatalysts. In this review, background information and developments from pure TiO₂ to chemically modified TiO₂-based materials as photocatalysts were discussed in detail, which covered their basic properties and their role in the air pollutant removal. It also proposes to solve the shortcomings of TiO₂ by developing other metal oxide-based materials and predict the future development of TiO₂ materials in future environmental applications.

CO2-based hydrogen storage – hydrogen liberation from methanol/water mixtures and from anhydrous methanol

New strategies for the reforming of methanol under mild conditions on the basis of heterogeneous and molecular catalysts have raised the hopes and expectations on this fuel. This contribution will focus on the progress achieved in the production of hydrogen from aqueous and anhydrous methanol with molecular and heterogeneous catalysts. The report entails thermal approaches, as well as light-triggered dehydrogenation reactions. A comparison of the efficiency and mechanistic aspects will be made and principles of catalytic pathways operating in biological systems will be also addressed.

Photocatalytic Dehydrogenation of Formic Acid on CdS Nanorods through Ni and Co Redox Mediation under Mild Conditions

Selective release of hydrogen from formic acid (FA) is deemed feasible to solve issues associated with the production and storage of hydrogen. Here, we present a new efficient photocatalytic system consisting of CdS nanorods (NRs), Ni, and Co to liberate hydrogen from FA. The optimized noble-metal-free catalytic system employs Ni/Co as a redox mediator to relay electrons and holes from CdS NRs to the Ni and Co, respectively, which also deters the oxidation of CdS NRs. As a result, a high hydrogen production activity of 32.6 mmol h^-1  g^-1 from the decomposition of FA was noted. Furthermore, the photocatalytic system exhibits sustained H₂ production rate for 12 h with sequential turnover numbers surpassing 4×10³ , 3×10³ , and 2×10³ for Co-Ni/CdS NRs, Ni/CdS NRs, and CoCl₂ /CdS NRs, respectively.

Multilayer core–shell MoS2/CdS nanorods with very high photocatalytic activity for hydrogen production under visible-light excitation and investigation of the photocatalytic mechanism by femtosecond transient absorption spectroscopy

Understanding the structural features and the dynamics and properties of charge carriers in photocatalysts is critical to develop them for practical applications.

Photochemical water splitting mediated by a C1 shuttle

The possibility of performing photochemical water splitting in a two-stage system, separately releasing the H₂ and O₂ components, has been probed with two separate catalysts and in combination with a formaldehyde/formate shuttling redox couple. In the first stage, formaldehyde releases hydrogen vigorously in the presence of an Na₄[Fe(CN)₆]·10H₂O catalyst, selectively affording the formate anion. In the second stage, the formate anion is hydro-genated back to formaldehyde by water and in the presence of a Bi₂WO₆ photocatalyst whilst releasing oxygen. Both stages operate at room temperature and under visible light irradiation. The two separate photocatalysts are compatible since water splitting can also be obtained in one-pot experiments with simultaneous H₂/O₂ evolution.

Photoelectrocatalytic analysis and electrocatalytic determination of hydroquinone by using a Cu2O-reduced graphene oxide nanocomposite modified rotating ring-disk electrode

Reduced graphene oxide (rGO)-based Cu2O nanocomposites were prepared by a facile one-pot reaction process. The surface morphology, structure and chemical composition of Cu2O-rGO nanocomposites were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The Cu2O-rGO modified Pt rotating ring-disk electrode (RRDE) was successfully fabricated for the photoelectrocatalytic analysis of hydroquinone (HQ). The photoelectrochemical behaviors of HQ were investigated by the hydrodynamic differential pulse voltammetry technique, using the Cu2O-rGO modified Pt RRDE as the working electrode. The effects of pH values, rotation rates, illumination time and applied bias potential have been discussed. The possible electroactive intermediate product, namely hydroxyhydroquinone, was obtained through the photoelectrocatalytic degradation of HQ on the Cu2O-rGO modified Pt disk electrode, which was compulsively transported and could only be detected at the bare Pt ring electrode at around +0.02 V with an oxidation signal. We found that the peak current at +0.02 V had a good linear relationship with the HQ concentration in the range from 5.0 × 10(-6) to 1.0 × 10(-3) M, with a low limit of detection and excellent reproducibility. The present work has demonstrated that Cu2O-rGO nanocomposites have enhanced photoelectrocatalytic ability for the degradation of organic pollutants and this modified RRDE technique can be potentially applied for the in situ determination of organic pollutants.

Photocatalytic Formic Acid Conversion on CdS Nanocrystals with Controllable Selectivity for H2 or CO

Formic acid is considered a promising energy carrier and hydrogen storage material for a carbon-neutral economy. We present an inexpensive system for the selective room-temperature photocatalytic conversion of formic acid into either hydrogen or carbon monoxide. Under visible-light irradiation (λ>420 nm, 1 sun), suspensions of ligand-capped cadmium sulfide nanocrystals in formic acid/sodium formate release up to 116±14 mmol H2 g(cat)(-1) h(-1) with >99% selectivity when combined with a cobalt co-catalyst; the quantum yield at λ=460 nm was 21.2±2.7%. In the absence of capping ligands, suspensions of the same photocatalyst in aqueous sodium formate generate up to 102±13 mmol CO g(cat)(-1) h(-1) with >95% selectivity and 19.7±2.7% quantum yield. H2 and CO production was sustained for more than one week with turnover numbers greater than 6×10(5) and 3×10(6), respectively.

Significant enhancement in photocatalytic hydrogen evolution from water using a MoS2nanosheet-coated ZnO heterostructure photocatalyst

Synthesis and characterization of novel noble-metal-free and stable MoS₂–ZnO composite photocatalysts for solar-to-H₂conversion.

Efficient visible light photocatalyst fabricated by depositing plasmonic Ag nanoparticles on conductive polymer-protected Si nanowire arrays for photoelectrochemical hydrogen generation

Photoelectrochemical (PEC) water splitting to produce H2 is a renewable method for addressing the worldwide energy consumption increasing and fossil fuels storage shrinking. In order to achieve sustainable PEC H2 production, the semiconductor electrodes should have good photo-absorption ability, proper band positions, and chemical stability in aqueous condition. Different from the large-band-gap semiconductors such as TiO2, which can work efficiently under UV light, Si is an narrow-band-gap semiconductor that can efficiently absorb visible light; however, Si is indirect semiconductor and susceptible to photocorrosion in aqueous solution. In this paper, we demonstrate a new strategy of first protecting and then activating to develop a stable visible light photoanode for photoelectrochemical hydrogen production. This AgNPs/PEDOT/SiNW arrays show an encouraging solar-to-chemical energy conversion efficiency of 2.86 % and a pronounced incident photo-to-current conversion efficiency (IPCE) across the whole visible region. Our strategy proposed here contributes to further improvement of corrosion protection and solar energy harvesting for narrow-band-gap semiconductors that employed in visible light photoelectrochemical and photoelectric conversion applications.

Nanostructured N-doped TiO₂ marigold flowers for an efficient solar hydrogen production from H₂S

Nitrogen-doped TiO₂ nanostructures in the form of marigold flowers have been synthesized for the first time using a facile solvothermal method. The structural analysis has shown that such an N-doped TiO₂ system crystallizes in the anatase structure. The optical absorption spectra have clearly shown the shift in the absorption edge towards the visible-light range, which indicates successful nitrogen doping. The nitrogen doping has been further confirmed by photoluminescence and photoemission spectroscopy. Microscopy studies have shown the thin nanosheets (petals) of N-TiO₂ with a thickness of ∼2-3 nm, assembled in the form of the marigold flower with a high surface area (224 m(2) g(-1)). The N-TiO₂ nanostructure with marigold flowers is an efficient photocatalyst for the decomposition of H₂S and production of hydrogen under solar light. The maximum hydrogen evolution obtained is higher than other known N-TiO₂ systems. It is noteworthy that photohydrogen production using the unique marigold flowers of N-TiO₂ from abundant H2S under solar light is hitherto unattempted. The proposed synthesis method can also be utilized to design other hierarchical nanostructured N-doped metal oxides.

Bi-component semiconductor oxide photoanodes for the photoelectrocatalytic oxidation of organic solutes and vapours: a short review with emphasis to TiO2-WO3 photoanodes

The use of binary semiconductor oxide anodes for the photoelectrocatalytic oxidation of organic species (both in solution and gas phase) is reviewed. In the first part of the review, the principle of electrically assisted photocatalysis is presented, the preparation methods for the most common semiconductor oxide catalysts are briefly mentioned, while the advantages of appropriately chosen semiconductor combinations for efficient UV and visible (vis) light utilization are highlighted. The second part of the review focuses on the discussion of TiO(2)-WO(3) photoanodes (among the most studied bi-component semiconductor oxide systems) and in particular on coatings prepared by electrodeposition/electrosynthesis or powder mixtures (the focus of the authors' research during recent years). Studies concerning the microscopic, spectroscopic and photoelectrochemical characterization of the catalysts are presented and examples of photoanode activity towards typical dissolved organic contaminants as well as organic vapours are given. Particular emphasis is paid to: (a) The dependence of photoactivity on catalyst morphology and composition and (b) the possibility of carrying out photoelectrochemistry in all-solid cells, thus opening up the opportunity for photoelectrocatalytic air treatment.

Hydrogen production by photoelectrochemically splitting solutions of formic acid

A TiO₂/FTO (FTO=fluorine-doped tin oxide) electrode was prepared by dip-coating FTO in a suspension of TiO₂ prepared from a sol-gel method and was used as a photoanode to split an aqueous solution of formic acid to produce hydrogen. The surface of the TiO₂/FTO film was covered with assemblies of TiO₂ nanoparticles with a diameter of approximately 20 nm. Under irradiation by using a Xe lamp, splitting of formic acid was performed at different applied current densities. Compared to splitting water or utilizing FTO and Pt foil as the anode, the splitting voltage is much lower and can be as low as -0.27 V. The results show that the splitting voltage is related to the concentration of free formate groups. The evolution rate of hydrogen measured by using gas chromatography is 130 μmol h⁻¹ at a current density of 20 mA cm⁻² and the energy-conversion efficiency can be 1.79 %. Photoelectrolysis of formic acid has the potential to be an efficient way to produce hydrogen with a high energy-conversion efficiency.

Heterointegration of Pt/Si/Ag Nanowire Photodiodes and Their Photocatalytic Properties

Photocatalyst mediated photoelectrochemical processes can make use of the photogenerated electrons and holes onsite for photocatalytic redox reactions, and enable the harness and conversion of solar energy into chemical energy, in analogy to natural photosynthesis. However, the photocatalysts available to date are limited by either poor efficiency in the visible light range or insufficient photoelectrochemical stability. We show that a Pt/Si/Ag nanowire heterostructure can be rationally synthesized to integrate a nanoscale metal-semiconductor Schottky diode encased in a protective insulating shell with two exposed metal catalysts. The synthesis of Pt/Si/Ag nanowire diodes involves a scalable process including the formation of silicon nanowire array through wet chemical etching, electrodeposition of platinum and photoreduction of silver. We further demonstrated that the Pt/Si/Ag diodes exhibited highly efficient photocatalytic activity for a wide range of applications including environmental remediation and solar fuel production in the visible range. In this article, photodegradation of indigo carmine and 4-nitrophenol were used to evaluate the photoactivity of Pt/Si/Ag diodes. The Pt/Si/Ag diodes also show high activity for photoconversion of formic acid into carbon dioxide and hydrogen.