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In this study, the growing scientific field of alternative biofuels was examined, with respect to hydrochars produced from renewable biomasses. Hydrochars are the solid products of hydrothermal carbonization (HTC) and their properties depend on the initial biomass and the temperature and duration of treatment. The basic (Scopus) and advanced (Citespace) analysis of literature showed that this is a dynamic research area, with several sub-fields of intense activity. The focus of researchers on sewage sludge and food waste as hydrochar precursors was highlighted and reviewed. It was established that hydrochars have improved behavior as fuels compared to these feedstocks. Food waste can be particularly useful in co-hydrothermal carbonization with ash-rich materials. In the case of sewage sludge, simultaneous P recovery from the HTC wastewater may add more value to the process. For both feedstocks, results from large-scale HTC are practically non-existent. Following the review, related data from the years 2014–2020 were retrieved and fitted into four different artificial neural networks (ANNs). Based on the elemental content, HTC temperature and time (as inputs), the higher heating values (HHVs) and yields (as outputs) could be successfully predicted, regardless of original biomass used for hydrochar production. ANN3 (based on C, O, H content, and HTC temperature) showed the optimum HHV predicting performance (R2 0.917, root mean square error 1.124), however, hydrochars’ HHVs could also be satisfactorily predicted by the C content alone (ANN1, R2 0.897, root mean square error 1.289).

type:Journal Article, Photocatalytic activity of bismuth vanadate (BiVO4) for acetaldehyde decomposition under visible light irradiation was improved by inclusion of a nanocomposition of silica as an adsorbent material and loading of platinum (Pt) or trivalent iron ion (Fe3+) as reduction cocatalysts. Addition of silica enhanced photocatalytic activity due to improvement of adsorption ability, but total decomposition of acetaldehyde was not observed within 24 h of visible light irradiation. For further improvement of photocatalytic activity, BiVO4 with an optimized amount of silica composition were modified with Pt or Fe3+. Photodeposition of Pt greatly increased photocatalytic activity, and acetaldehyde was totally decomposed within 24 h of visible light irradiation.

type:Journal Article, C4+ and S4+-codoped titanium dioxide (TiO2) having a rutile phase was prepared. By doping C4+ and S4+ ions into a TiO2 lattice, the absorption edge of rutile TiO2 powder was largely shifted from 400 to 700 nm. 2-Methylpyridine and methyleneblue were photocatalytically oxidized at high efficiency on C4+ and S4+-doped TiO2 under visible light at a wavelength longer than 5 nm.

type:Journal Article, S- or N-doping of TiO2 powder having an anatase or rutile phase extended the photocatalytic activity for water oxidation and reduction under UV light and visible light irradiation. For the reduction of water, anatase-doped TiO2 showed higher level of activity than that of doped TiO2 having a rutile phase using ethanol as an electron donor. Furthermore, the activity level of S-doped TiO2 for hydrogen evolution was higher than that of N-doped TiO2 photocatalysts under visible light. Photocatalytic oxidation of water on doped TiO2 having a rutile phase proceeded with fairly high efficiency when Fe3⁺ ions were used as electron acceptors compared to that on doped TiO2 having an anatase phase. In addition, water splitting under visible light irradiation was achieved by construction of a Z-scheme photocatalysis system employing the doped TiO2 having anatase and rutile phases for H2 and O2 evolution and the I⁻/IO⁻3 redox couple as an electron relay.

type:Journal Article, Fluorocarbon groups were introduced onto surfaces of SiO2-covered TiO2 particles (SiO2–TiO2). Oxidation of pentafuluorobenzaldehyde on the surface modified TiO2 powders proceeded much efficiently than that on SiO2-covered TiO2 particles without surface modification. In addition, no enhancement of activity level of surface-modified SiO2–TiO2 for oxidation of benzaldehyde was observed. The enhancement of the surface-modified SiO2–TiO2 is due to the interaction of F atoms between the substrate and fluorocarbon groups introduced on SiO2–TiO2.

type:Journal Article, Photocatalytic oxidation of methylpyridine isomers (2-methylpyridine, 3-methylpyridine, and 4-methylpyridine) was investigated in a mixed solution of acetonitrile and water or acetonitrile using various kinds of TiO2 powders as photocatalysts. The main products from methylpyridine isomers were pyridinecarboxaldehyde isomers (2-pyridinecarboxaldehyde, 3-pyridinecarboxaldehyde, and 4-pyridinecarboxaldehyde). Rutile large TiO2 particles showed the highest level of activity for oxidation of 2-methylpyridine probably because band bending was necessary for the oxidation of 2-methylpyridine. On the other hand, a fine particle having an anatase or rutile phase showed a higher level of activity than large TiO2 particles for oxidation of 3-methylpyridine. A rutile fine particle showed the highest level of activity for the reaction. It was found that pure rutile or pure anatase particles were inactive for oxidation of 4-mathylpyridine. If the particles are not extremely small, pure rutile and pure anatase powders show fairly high levels of activity, and those containing both anatase and rutile phases show the highest level of activity. The activity of pure rutile particles was also enhanced by physically mixing them with a small amount of small anatase particles, which were inactive for this reaction. These results can be explained by the synergism between rutile and anatase particles. All of these reactions effectively proceeded even under anaerobic conditions. Photocatalytic reduction of methylpyridine isomers concomitantly proceeded on TiO2 particles under the conditions used. These results suggest that the activities of TiO2 photocatalysts for oxidation of methylpyridine isomers are dominated by the oxidation potential of alkylpiridine and band bending of TiO2 particles.

type:Journal Article, Graphitic carbon nitride (g-C3N4) has attracted much attention as a metal-free semiconductor having visible light absorption and relatively high chemical stability under visible light irradiation. Graphitic carbon nitride (g-C3N4) and boron-doped g-C3N4 (B-doped g-C3N4, BCNx) were prepared by heating melamine and a mixture of dicyanodiamide and BH3NH3, respectively. X-ray diffraction, a Brunauer, Emmett and Teller (BET) apparatus, and UV–vis spectra were used to analyze the physical properties of the prepared samples. Electrodes of these samples were prepared by using the electrophoresis method. X-ray photoelectron spectroscopy analyses confirmed the incorporation of boron atoms in the g-C3N4 framework as well as the amount of boron atoms., Au, Ag or Rh as a co-catalyst was coated on the surface of g-C3N4 and B-doped g-C3N4 by using the magnetron sputtering method. The photocurrent response was observed using a solar simulator as a light source. The photocurrent response of B-doped g-C3N4 was about 5-times larger than that of pure g-C3N4. B-doped g-C3N4 coated with Rh as a co-catalyst showed the highest photocurrent response under solar light irradiation, its photocurrent being about 10-times larger than that of original g-C3N4. Under photoelectrochemical conditions, we also observed the products in gas phase and aqueous phase. C2H5OH was observed as a main product, while small amounts of CO and H2 were observed in gas phase. We also discuss the relationship between co-catalysts and photocurrent responses and the carbon source of C2H5OH as a main product. The source of carbon of C2H5OH obtained by CO2 reduction is discussed on the basis of results of a labeling experiment using 13CO2.

type:Journal Article, Quaternary ammonium base groups were introduced onto surfaces of SiO2-covered TiO2 particles (SiO2–TiO2). Oxidation of p-toluenesulfuric acid on the surface-modified TiO2 powders proceeded more efficiently than that on SiO2-covered TiO2 particles without surface modification. On the other hand, the photocatalytic activity of surface-modified SiO2–TiO2 for oxidation of 1,4-dimethylpyridinium iodide was much lower than that of pure SiO2–TiO2. In addition, no difference in photocatalytic activity for oxidation of toluene was observed between surface-modified SiO2–TiO2 and pure SiO2–TiO2. The enhancement of the photocatalytic activity of surface-modified SiO2–TiO2 is due to electrostatic interaction between the substrate and functional groups introduced on SiO2–TiO2.

type:Journal Article, {1 1 1} exposed crystal faces of shape-controlled rutile titanium(IV) oxide (TiO2) were site-selectively modified with trivalent iron(III) (Fe3+) ions by utilizing adsorption property of iron(III)/iron(II) (Fe3+/Fe2+) ions. The rutile TiO2 with site-selective modification of Fe3+ ions showed high photocatalytic activity under visible-light irradiation as a result of separation of redox sites, i.e., oxidation and reduction proceed over Fe3+ ions on {1 1 1} faces and the bare TiO2 surface on {1 1 0} faces, respectively. Double-beam photoacoustic spectroscopic analyses suggest that the high activity of TiO2 with site-selective modification of Fe3+ ions is attributed to not efficient electron injection from Fe3+ ions but efficient reduction by injected electrons on {1 1 0} faces.

type:Journal Article, Graphitic carbon nitride (g-C3N4) has attracted much attention as a metal-free semiconductor having visible-light absorption and relatively high chemical stability. In the present study, we hybridized g-C3N4 with sulfur-doped TiO2, which is a visible light-responsive photocatalyst with high oxidation ability, in order to improve photocatalytic activity under visible-light irradiation. Hybrid photocatalysts were prepared by three methods: agate mortar, sonication, and planetary mill. Activities of the hybrid photocatalysts depended on the mixing method. The sample prepared by a planetary mill showed the highest photocatalytic activity, 4-times higher than that of sulfur-doped TiO2. We concluded that the high activity of the hybridized sample under visible-light irradiation is induced by charge transfer between the two photocatalysts mimicking the Z-scheme in photosynthesis.

type:Journal Article, Decahedral anatase titanium(IV) oxide (TiO2) with {1 0 1} and {0 0 1} exposed crystal faces was prepared by hydrothermal treatment of peroxo titanic acid (PTA) solution with polyvinyl alcohol (PVA) as a shape-control reagent. pH of the PTA solution and amounts of PVA and amorphous titania included in the PTA solution had a large influence on size and shape of the prepared particles, and particle width of the decahedral anatase TiO2 was controllable between 25 and 60 nm. Photocatalytic activity of the decahedral anatase TiO2 was examined in terms of the relationship between particle size and photocatalytic activity. Decahedral anatase TiO2 with particle width of ca. 40 nm showed excellent activity because of the optimized balance between efficient separation of redox sites and large specific surface area.

type:Journal Article, We have synthesized S (S4+)-doped and N (N3−)-doped TiO2 photocatalysts. S-doped and N-doped TiO2 loaded with Fe2O3 nanoparticles have also been prepared. These photocatalysts showed activity under a wide range of wavelengths of irradiation. Action spectra of photochemical reaction rate as a function of the incident light wavelength for oxidation of acetic acid on S-doped TiO2 was studied.The photocatalytic activities of S-doped and N-doped TiO2 photocatalysts loaded with Fe2O3 nanoparticles for oxidation of acetic acid in aqueous phase and acetaldehyde in gas phase are markedly improved compared to those of doped TiO2 without loading of Fe2O3 nanoparticles under a wide range of incident light wavelengths. The optimum amount of Fe2O3 nanoparticles loaded on S-doped TiO2 particles was different from that on N-doped TiO2 for oxidation of organic compounds. The relationship between reaction rate of photocatalytic oxidation of acetaldehyde on doped TiO2 loaded with Fe2O3 nanoparticles and amount of Fe2O3 nanoparticles is discussed.

type:Journal Article, Photocatalytic reduction of carbon dioxide (CO2) was carried out using exposed-crystal- face-controlled titanium(IV) oxide (TiO2) having a brookite phase. Methanol (CH3OH) was detected as the main product, and trace amounts of formic acid, carbon monoxide, methane, and hydrogen were also detected in some cases. The prepared nanorod-shaped brookite TiO2 with large {2 1 0} and small {2 1 2} exposed crystal faces showed larger CH3OH generation than that of commercial brookite TiO2 powder (Kojundo Chemical Laboratory Co., Ltd.). The activity of a brookite TiO2 nanorod for CO2 reduction depended on its aspect ratio because the {2 1 0} crystal faced worked as a reduction site, whereas an oxidation site was assigned to {2 1 2} crystal faces. Photodeposition of gold (Au) or silver (Ag) nanoparticles on the nanorod-shaped brookite TiO2 induced a dramatic increase in CH3OH production because the deposited metal particles work as reductive sites for multi-electron reduction of CO2. Among the co-catalyst-loaded brookite TiO2 nanorods, nanorod-shaped brookite TiO2 loaded with Ag showed higher activity. The source of carbon of CH3OH obtained by CO2 reduction is discussed on the basis of results of a labeling experiment using 13CO2.

type:Journal Article, A titania nanotube (TNT), which was obtained by calcinations of a titanate nanotube, was modified with two kinds of transition metal ions, iron(III) (Fe3+) and zinc(II) (Zn2+) ions. TNT with site-selective modification with metal ions showed higher photocatalytic activity than that of bare TNT, presumably due to separation of redox sites, and oxidation on the outside surface and reduction on the inside surface were the preferable separated redox sites on the tubular structure for decomposition of acetaldehyde. Modification of TNT with Fe3+ ions induced improvement of photocatalytic activity under visible-light irradiation as well as ultraviolet (UV) irradiation. On the other hand, TNT modified with Zn2+ ions showed the largest enhancement of photocatalytic activity under UV irradiation, though increase in visible-light activity was hardly observed. Double-beam photoacoustic spectroscopic analyses indicated that Zn2+ ion works efficiently as an electron acceptor, while Fe3+ ion is an effective sensitizer for visible light.

type:Journal Article, Amorphous niobium oxide (NbOx) as an electron collection layer in inverted polymer solar cells was prepared by a solution process. The power conversion efficiency of inverted polymer solar cells based on a blend of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester was improved to 2.22% by inserting an NbOx layer between the active layer and indium tin oxide electrode. An energy level diagram of component materials in the inverted polymer solar cell indicated that the NbOx layer works as both an electron collection layer and hole blocking layer in polymer solar cells., source:https://doi.org/10.1016/j.cplett.2013.08.015

type:Journal Article, Graphitic carbon nitride (g-C3N4) was prepared by heating melamine and then its specific surface area was enlarged by hydrothermal treatment in aqueous sodium hydroxide solution. The g-C3N4 samples were blended with tungsten(VI) oxide (WO3) using a planetary mill in order to improve photocatalytic activity. The composite photocatalyst with optimized amounts of these contents showed higher photocatalytic activity for decomposition of acetaldehyde under visible-light irradiation than did original samples. From the results, we concluded that the composite photocatalyst utilizes both high oxidation ability of WO3 and high reduction ability of g-C3N4 by Z-scheme charge transfer., source:https://doi.org/10.1016/j.apcatb.2013.12.048

type:Journal Article, Exposed crystal face-controlled brookite titanium(IV) oxide (TiO2) nanorods with various aspect ratios were prepared by a hydrothermal process with or without PVA or PVP as an aspect reagent. The nanorod-shaped brookite TiO2 had larger {2 1 0} and smaller {2 1 2} exposed crystal faces, which were assigned by TEM with the SAED technique. Their aspect ratios were greatly influenced by the addition of PVA or PVP as an aspect ratio control reagent to the reaction solution used in the hydrothermal treatment. The photocatalytic activity for decomposition of acetaldehyde increased with increase in the aspect ratio because the surface area ratio of {2 1 0} to {2 1 2} exposed crystal faces, which are attributed to reduction and oxidation sites, respectively, became more optimal. The {2 1 2} exposed crystal faces of surface-controlled brookite TiO2 were site-selectively modified with trivalent iron(III) (Fe3+) ions by utilizing the adsorption property of iron(III)/iron(II) (Fe3+/Fe2+) ions. The brookite TiO2 nanorod with site-selective modification of Fe3+ ions showed much higher photocatalytic activity than that of commercial brookite TiO2 loaded with Fe ions under visible-light irradiation because of the separation of redox sites. In other words, oxidation and reduction proceed over Fe3+ ion-modified {2 1 2} faces of the TiO2 surface and on {2 1 0} faces of the TiO2 surface without modification of Fe3+, respectively., source:http://dx.doi.org/10.1016/j.molcata.2014.09.036