Efficient production of formic acid by simultaneous photoreduction of bicarbonate and oxidation of glycerol on gold-TiO 2 composite under solar light

Photocatalytic Recovery of Gold from a Non-Cyanide Gold Plating Solution as Au Nanoparticle-Decorated Semiconductors

In this work, a photocatalytic process was carried out to recover gold (Au) from the simulated non-cyanide plating bath solution. Effects of semiconductor types (TiO₂, WO₃, Nb₂O₃, CeO₂, and Bi₂O₃), initial pH of the solution (3-10), and type of complexing agents (Na₂S₂O₃ and Na₂SO₃) and their concentrations (1-4 mM each) on Au recovery were explored. Among all employed semiconductors, TiO₂ exhibited the highest photocatalytic activity to recover Au from the simulated spent plating bath solution both in the absence and presence of complexing agents, in which Au was completely recovered within 15 min at a pH of 6.5. The presence of complexing agents remarkably affected the size of deposited Au on the TiO₂ surface, the localized surface plasmon effect (LSPR) behavior, and the valence band (VB) edge position of the obtained Au/TiO₂, without a significant change in the textural properties or the band gap energy. The photocatalytic activity of the obtained Au/TiO₂ tested via two photocatalytic processes depended on the common reduction mechanism rather than the textural or optical properties. As a result, the Au/TiO₂ NPs obtained from the proposed recovery process are recommended for use as a photocatalyst for the reactions occurring at the conduction band rather than at the valence band. Notably, they exhibited good stability after the fifth photocatalytic cycle for Au recovery from the actual cyanide plating bath solution.

Harvesting Hot Holes in Plasmon-Coupled Ultrathin Photoanodes for High-Performance Photoelectrochemical Water Splitting

The harvesting of hot carriers produced by plasmon decay to generate electricity or drive a chemical reaction enables the reduction of the thermalization losses associated with supra-band gap photons in semiconductor photoelectrochemical (PEC) cells. Through the broadband harvesting of light, hot-carrier PEC devices also produce a sensitizing effect in heterojunctions with wide-band gap metal oxide semiconductors possessing good photostability and catalytic activity but poor absorption of visible wavelength photons. There are several reports of hot electrons in Au injected over the Schottky barrier into crystalline TiO₂ and subsequently utilized to drive a chemical reaction but very few reports of hot hole harvesting. In this work, we demonstrate the efficient harvesting of hot holes in Au nanoparticles (Au NPs) covered with a thin layer of amorphous TiO₂ (a-TiO₂). Under AM1.5G 1 sun illumination, photoanodes consisting of a single layer of ∼50 nm diameter Au NPs coated with a 10 nm shell of a-TiO₂ (Au@a-TiO₂) generated 2.5 mA cm^-2 of photocurrent in 1 M KOH under 0.6 V external bias, rising to 3.7 mA cm^-2 in the presence of a hole scavenger (methanol). The quantum yield for hot-carrier-mediated photocurrent generation was estimated to be close to unity for high-energy photons (λ < 420 nm). Au@a-TiO₂ photoelectrodes produced a small positive photocurrent of 0.1 mA cm^-2 even at a bias of -0.6 V indicating extraction of hot holes even at a strong negative bias. These results together with density functional theory modeling and scanning Kelvin probe force microscope data indicate fast injection of hot holes from Au NPs into a-TiO₂ and light harvesting performed near-exclusively by Au NPs. For comparison, Au NPs coated with a 10 nm shell of Al₂O₃ (Au@Al₂O₃) generated 0.02 mA cm^-2 of photocurrent in 1 M KOH under 0.6 V external bias. These results underscore the critical role played by a-TiO₂ in the extraction of holes in Au@a-TiO₂ photoanodes, which is not replicated by an ordinary dielectric shell. It is also demonstrated here that an ultrathin photoanode (<100 nm in maximum thickness) can efficiently drive sunlight-driven water splitting.

Photons to Formate-A Review on Photocatalytic Reduction of CO2 to Formic Acid

Rising levels of atmospheric carbon dioxide due to the burning and depletion of fossil fuels is continuously raising environmental concerns about global warming and the future of our energy supply. Renewable energy, especially better utilization of solar energy, is a promising method for CO₂ conversion and chemical storage. Research in the solar fuels area is focused on designing novel catalysts and developing new conversion pathways. In this review, we focus on the photocatalytic reduction of CO₂ primarily in its neutral pH species of carbonate to formate. The first two-electron photoproduct of carbon dioxide, a case for formate (or formic acid) is made in this review based on its value as; an important chemical feedstock, a hydrogen storage material, an intermediate to methanol, a high-octane fuel and broad application in fuel cells. This review focuses specifically on the following photocatalysts: semiconductors, phthalocyanines as photosensitizers and membrane devices and metal-organic frameworks.

Recent Advances in Glycerol Catalytic Valorization: A Review

Once a biorefinery is ready to operate, the main processed materials need to be completely evaluated in terms of many different factors, including disposal regulations, technological limitations of installation, the market, and other societal considerations. In biorefinery, glycerol is the main by-product, representing around 10% of biodiesel production. In the last few decades, the large-scale production of biodiesel and glycerol has promoted research on a wide range of strategies in an attempt to valorize this by-product, with its transformation into added value chemicals being the strategy that exhibits the most promising route. Among them, C3 compounds obtained from routes such as hydrogenation, oxidation, esterification, etc. represent an alternative to petroleum-based routes for chemicals such as acrolein, propanediols, or carboxylic acids of interest for the polymer industry. Another widely studied and developed strategy includes processes such as reforming or pyrolysis for energy, clean fuels, and materials such as activated carbon. This review covers recent advances in catalysts used in the most promising strategies considering both chemicals and energy or fuel obtention. Due to the large variety in biorefinery industries, several potential emergent valorization routes are briefly summarized.

Sustainable Recovery of CO2 by Using Visible-Light-Responsive Crystal Cuprous Oxide/Reduced Graphene Oxide

A simple solution-chemistry method has been investigated to prepare crystal cuprous oxide (Cu2O) incorporated with reduced graphene oxide (designated as Cu2O-rGO-x, where x represents the contents of rGO = 1%, 5% and 10%) in this work. These Cu2O-rGO-x composites combine the prospective advantages of rhombic dodecahedra Cu2O together with rGO nanosheets which have been studied as visible-light-sensitive catalysts for the photocatalytic production of methanol from CO2. Among the Cu2O-rGO-x photocatalysts, the methanol yield photocatalyzed by Cu2O-rGO-5% can be observed to be 355.26 μmol g−1cat, which is ca. 36 times higher than that of pristine Cu2O nanocrystal in the 20th hour under visible light irradiation. The improved activity may be attributed to the enhanced absorption ability of visible light, the superior separation of electron–hole pairs, well-dispersed Cu2O nanocrystals and the increased photostability of Cu2O, which are evidenced by employing UV-vis diffuse reflection spectroscopy, photoluminescence, scanning electron microscopy/transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. This work demonstrates an easy and cost-effective route to prepare non-noble photocatalysts for efficient CO2 recovery in artificial photosynthesis.

Impact of Ultrasmall Platinum Nanoparticle Coating on Different Morphologies of Gold Nanostructures for Multiple One-Pot Photocatalytic Environment Protection Reactions

Here, we report the preparation of various morphologies of core-satellite hybrid plasmonic nanomaterials of gold and platinum, viz., the coating of platinum nanoparticles on gold nanostars (Pt@AuNSs), nanospheres (Pt@AuNPs), and nanorods (Pt@AuNRs) for visible-NIR-sunlight illuminated highly efficient photoconversion of CO₂ to HCOOH and visible light mediated degradation of various organic dyes (methylene blue, methyl orange, and methyl red) in a single step at room temperature. In comparison, Pt@AuNSs shows higher conversion efficiency for CO₂ photoconversion to HCOOH with 1.52% of quantum yield and 2.58% of chemical yield in visible-light illumination. There was a 200-fold increase in the conversion efficiency after PtNP coating to AuNSs. On the other hand, Pt@AuNPs (quantum yield = 1.44%) and Pt@AuNRs (quantum yield = 0.64%) also show the significant conversion rate in visible and NIR light, respectively. All the hybrid nanoparticles were found to be stable, whereas PtNPs increase the stability of AuNSs incredibly, during the photoconversion reaction and reused for five CO₂ reduction reaction cycles without losing photocatalytic activity. Moreover, Pt@AuNSs also showed higher photocatalytic activity for organic pollutant degradation and degraded methylene blue, methyl red, and methyl orange in 45, 75, and 80 min with 0.104 min^-1, 0.055 min^-1, and 0.044 min^-1 reaction rate, respectively. The results highlighted the crucial role of PtNPs as a highly efficient cocatalyst as well as the impact of increases in surface area with the change in morphology of gold nanostructures on CO₂ photoconversion and organic dye degradation.