Continuous hydrogen production activity over finely dispersed Ag2O/TiO2 catalysts from methanol:water mixtures under solar irradiation: A structure–activity correlation

Ag₂O/TiO₂ catalysts with varying amounts of Ag₂O 0.5, 1, 2, and 5 wt% loadings are prepared by impregnation method and Ag/TiO₂ catalyst is prepared by photo deposition method. These catalysts are characterized by XRD, SEM-EDAX, DRS, XPS and TEM techniques. DRS studies clearly showing the expanded photo response of TiO₂ into visible region on impregnation of Ag⁺ ions on surface layers of TiO₂ due to the increased number of energy states created by the silver ions in TiO₂ surface lattice. TEM images are showing the fine dispersion of silver particles on TiO₂ surface. XPS of Ag₂O/TiO₂ calcined and used catalysts along with Ag₂O/TiO₂ reduced and Ag/TiO₂ photo deposited catalysts are compared and the binding energy values of Ti 2p, O1s and Ag 3d are confirming that silver ions are in interaction with TiO₂ in Ag₂O/TiO₂ calcined catalysts. EDAX analysis supports the presence of silver species on the surface layers of TiO₂. Photocatalytic hydrogen production activity studies are conducted over Ag₂O/TiO₂, Ag₂O/TiO₂ reduced and Ag/TiO₂ photo deposited catalysts in pure water and methanol:water mixtures under solar irradiation. Maximum hydrogen production of 145 μmoles/h is observed on 0.5wt% Ag₂O/TiO₂ catalyst in pure water and the maximum hydrogen production of 3350 μmoles/h is observed on 1wt% Ag₂O/TiO₂ catalyst in methanol:water mixtures. Whereas Ag₂O/TiO₂ reduced and Ag/TiO₂ photo deposited catalysts are not showing any hydrogen production activity either in water or in methanol:water mixtures under solar irradiation. Based on the XPS, DRS, TEM, SEM-EDAX studies and the hydrogen production activity on these catalysts, a structure–activity correlation has been proposed wherein the interacted Ag⁺ ions on the surface layers of TiO₂ are playing an important role in maintaining the hydrogen production activity under solar irradiation.     

Field effect surface passivation of SiO2/Si interfaces by heat treatment with high-pressure H2O vapor

We investigated a simple field effect passivation of the silicon surfaces using the high-pressure H₂O vapor heating. Heat treatment with 2.1×10⁶ Pa H₂O vapor at 260°C for 3 h reduced the surface recombination velocity from 405 cm/s (before the heat treatment) to 38 cm/s for the thermally evaporated SiO_x film/Si. Additional deposition of 140 nm-SiO_x films (x<2) with a high density of fixed positive charges on the SiO₂/Si samples further decreased the surface recombination velocity to 22 cm/s. We also demonstrated the field effect passivation for n-type silicon wafer coated with thermally grown SiO₂. Additional deposition of 210 nm SiO_x films on both the front and rear surfaces increased the effective lifetime from 1.4 to 4.6 ms. Combination of thermal evaporation of SiO_x film and the heat treatment with high-pressure H₂O vapor is effective for low-temperature passivation of the silicon surface.     

Structural properties of CuO/TiO2 nanorod in relation to their catalytic activity for simultaneous hydrogen production under solar light

CuO was introduced into porous TiO₂ nanorod through impregnation method. Before the impregnation step, TiO₂ nanorod was hydrothermally synthesized from TiO₂ powder in aqueous NaOH solution and followed by thermal treatment at 450 °C. The structures and properties of impregnated samples were characterized using various techniques, including XRD, BET, XAS, TEM, and UV-DRS. Their photocatalytic performance on simultaneous hydrogen production from pure water and aqueous methanol solution was also investigated under solar light. It was found that CuO/TiO₂ nanorod possessed a high surface area, good photocatalytic property and excellent hydrogen generation activity. Incorporation of Cu ions into the lattice framework of anatase TiO₂ nanorod enhanced the efficiency in visible region at 438–730 nm. Moreover, the XAS results showed that some Cu ions formed solid solution in the TiO₂ nanorod (Cu_xT_1−xO₂). However, the excessive incorporation of Cu ions did not improve any ability of anatase TiO₂ nanorod for production of hydrogen from pure water splitting. This could be due to the excessive CuO agglomeration at outside-pores which blocked the sensitization of TiO₂ nanorod. Only 1% Cu/TiO₂ nanorod was found to be a remarkable and an efficient photocatalyst for hydrogen production under solar light from both pure water and sacrificial methanol splitting. The highest rate of hydrogen production of 139.03 μmol h⁻¹ g_catalyst⁻¹ was found in sacrificial methanol which was 3.24% higher than in pure water.     

Silver/titania nanocomposite-modified photoelectrodes for photoelectrocatalytic methanol oxidation

Silver deposited titania (Ag/TiO₂) nanocomposite thin films were fabricated by the simple sonochemical deposition of Ag on preformed aerosol-assisted chemical vapor deposited TiO₂ thin films. The photelectrocatalytic performance of a newly fabricated Ag/TiO₂-modified photoelectrode was studied for methanol oxidation under simulated solar AM 1.5G irradiation (100 mW/cm²). The Ag/TiO₂-modified photoelectrode showed a photocurrent density of 1 mA/cm², which is four times that of an unmodified TiO₂ photoelectrode. The modification of Ag on the TiO₂ surface significantly enhanced the photoelectrocatalytic performance by improving the interfacial charge transfer processes, which minimized the charge recombination. Density functional theory (DFT) calculation studies revealed that methanol could be easily adsorbed onto the Ag surfaces of Ag/TiO₂ via a partial electron transfer from Ag to methanol. The newly fabricated Ag/TiO₂-modified photoelectrode could be a promising candidate for photoelectrochemical applications.     

Ultrasonic spray pyrolysis synthesis of Ag/TiO2 nanocomposite photocatalysts for simultaneous H2 production and CO2 reduction

Simultaneous photocatalytic hydrogen production and CO₂ reduction (to form CO and CH₄) from water using methanol as a hole scavenger were investigated using silver-modified TiO₂ (Ag/TiO₂) nanocomposite catalysts. A simple ultrasonic spray pyrolysis (SP) method was used to prepare mesoporous Ag/TiO₂ composite particles using TiO₂ (P25) and AgNO₃ as the precursors. The material properties and photocatalytic activities were compared with those prepared by a conventional wet-impregnation (WI) method. It was found that the samples prepared by the SP method had a larger specific surface area and a better dispersion of Ag nanoparticles on TiO₂ than those prepared by the WI method, and as a result, the SP samples showed much higher photocatalytic activities toward H₂ production and CO₂ reduction. The optimal Ag concentration on TiO₂ was found to be 2 wt%. The H₂ production rate of the 2% Ag/TiO₂–SP sample exhibited a six-fold enhancement compared with the 2% Ag/TiO₂–WI sample and a sixty-fold enhancement compared with bare TiO₂. The molar ratio of H₂ and CO in the final products can be tuned in the range from 2 to 10 by varying the reaction gas composition, suggesting a viable way of producing syngas (a mixture of H₂ and CO) from CO₂ and water using the prepared Ag/TiO₂ catalysts with energy input from the sun.     

Solvothermal synthesis ZnS–In2S3–Ag2S solid solution coupled with TiO2−xSx nanotubes film for photocatalytic hydrogen production

ZnS–In₂S₃–Ag₂S solid solution coupled with TiO_2-xS_x nanotubes film catalyst has been successfully prepared by a two-step process of anodization and solvothermal methods for the first time. The as-prepared photo-catalysts are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV–Visible diffuse reflectance spectra (UV–Vis DRS), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. The results show that the ZnS–In₂S₃–Ag₂S solid solution are deposited on the surface of TiO₂NTs nanotubes under the solvothermal conditions, by which S atoms are incorporated into the lattice of TiO₂ through substituting the sites of oxygen atoms. Such ZnS–In₂S₃–Ag₂S@TiO_2-xS_x nanotubes composite presents the enhanced absorption in visible region and the efficient transfer of photoelectron between the solid solution and TiO_2-xS_x nanotubes, which determines the excellent photocatalytic activity for the photocatalytic hydrogen evolution from aqueous solutions containing the sacrificial reagents of Na₂S and Na₂SO₃ under 500 W Xe lamp irradiation.     

Infrared radiation active,novel nanocomposite photocatalyst for water splitting

In this report, we investigated an infrared photon active catalytic system for the conversion of solar energy into chemical energy in which we introduced an effective “Dark Photocatalyst” for photolysis of water. The Ag₂O/TiO₂ composite catalyst containing 28% of silver and 72% of titanium was synthesised by a simple chemical deposition method. The catalyst was found to be active in the infrared region of the solar spectrum or in the dark and hydrogen production was demonstrated for the first using low-energy 800–1200 nm IR radiation. Sub-band gap phonon assisted multi-photon excitation is proposed for the observed infrared photocatalytic activity of Ag₂O/TiO₂ composite.     

NOx formation and reduction mechanisms in staged O2/CO2 combustion

The purpose of this study was to investigate the NO_x formation and reduction mechanisms in staged O₂/CO₂ combustion and in air combustion. A flat CH₄ flame doped with NH₃ for fuel-N was formed over the honeycomb, and NO_x formation characteristics were investigated. In addition, chemiluminescence of OH^* distribution was measured, and CHEMKIN-PRO was used to investigate the detailed NO_x reduction mechanism. In general, the NO_x conversion ratio decreases with decreasing primary O₂/CH₄ ratio, whereas NH₃ and HCN, which are easily converted to NO_x in the presence of O₂, increases rapidly. Therefore, a suitable primary O₂/CH₄ ratio exists in the staged combustion. Our experiments showed the primary O₂/CH₄ ratio, which gave the minimum fixed nitrogen compounds in O₂/CO₂ combustion, was lower than in air combustion. The NO_x conversion ratio in O₂/CO₂ combustion was lower than in air combustion by 40% in suitable staged combustion. This could be explained by high CO₂ concentrations in the O₂/CO₂ combustion. It was shown that abundant OH radicals were formed in O₂/CO₂ combustion through the CO₂ + H → CO + OH, experimentally and numerically. OH radicals produced H and O radicals through H₂ + OH → H + H₂O and O₂ + H → OH + O, because a mass of hydrogen source exists in the CH₄ flame. O and OH radicals formed in the fuel-rich region enhanced the oxidation of NH₃ and HCN. NO_x formed by the oxidation of NH₃ and HCN was converted to N₂ because the oxidation occurred in the fuel-rich region where the NO_x reduction effect was high. In fact, the oxidation of NH₃ and HCN in the fuel-rich region was preferable to remaining NH₃ and HCN before secondary O₂ injection in the staged combustion. A significant reduction in NO_x emission could be achieved by staged combustion in O₂/CO₂ combustion.     

Study on the NOx release rule along the boiler during pulverized coal combustion

Numerical simulation and experimental study on NO _x release along the boiler during pulverized coal combustion have been conducted. With the increase of temperature the NO _x emission increased and the peak value of NO _x release moved forward. But when the temperature increased to a certain degree, NO _x emission began to reduce. NO _x emission increased with the increase of nitrogen content of coal. The peak value of NO _x release moved backwards with the increase of coal rank. NO _x emission increased obviously with the increase of stoichiometric ratio. There existed a critical average diameter of the pulverized coal (d _c ). If d ⩽ d _c , NO _x emission reduced with the decrease of pulverized coal size. If d > d _c , NO _x emission reduced with the increase of the pulverized coal size. The results showed that the simulation results are in agreement with the experimental results for concentration distribution of NO _x along the axis of the furnace. Translated from Proceeding of the CSEE, 2006, 26(1): 35–39 [译自: 中国电机工程学报]     

Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO

TiO₂ photocatalyst with deposited CuO (CuO-TiO₂) was synthesized by the impregnation method using P25 (Degussa) as support, and exhibited high photocatalytic hydrogen generation activity from methanol/water solution. A substantial hydrogen evolution rate of 10.2 ml min⁻¹ (18,500 μmol h⁻¹ g⁻¹_catalyst) was observed over this efficient CuO-TiO₂ with optimal Cu content of 9.1 mol% from an aqueous solution containing 10 vol% methanol; this improved hydrogen generation rate is significantly higher than the reported Cu-containing TiO₂, including some Pt and Pd loaded TiO₂. Optimal Cu content of 9.1 mol% provided maximum active sites and allowed good light penetration in TiO₂. Over this efficient CuO-TiO₂, the hydrogen generation rate was accelerated by increasing the methanol concentration according to Freundlich adsorption isotherm. However, the photocatalytic hydrogen generation rate was suppressed under long time irradiation mainly due to accumulation of by-products, reduction of CuO and copper leaching, which requires further investigation.     

Improved electrochromic films of NiOx and WOxPy obtained by spray pyrolysis

Electrochromic films of NiO_x and WO_xP_y were produced by the spray pyrolysis technique. The nickel-oxide-based coatings were obtained from both an alcoholic solution of nickel nitrate and aqueous solution of the mixture nickel nitrate/cobalt nitrate. Coatings obtained from alcoholic solutions showed a noticeable contrast of optical transmittance from fully bleached to colored state. X-ray diffraction analysis showed a slight crystallization in NiO_x after electrochemical treatment: one diffraction peak for as-deposited films turned to three diffraction peaks for electrochemical treated samples. Coatings obtained from aqueous solution of mixture nickel nitrate/cobalt nitrate showed an optimized electrochromic behavior at a Ni:Co proportion of 90:10. At this condition an optical contrast of 50% is found. X-ray diffraction showed that these samples comprised a phase mixture of Co₃O₄ and NiO.WO_xP_y samples were obtained from polytungsten gel in which H₃PO₄ was added. We found that for 8.3 at% of P:W, the electrochromism was optimized. Pyrolytic coatings of WO_xP_y show superior behavior than those of WO_x obtained by spray pyrolysis, both in optical contrast and durability.     

Low thermal conductivity for Sr1−xLaxTiO3

Abstract

N type thermoelectric materials Sr_1?xLa_xTiO₃ (x?=?0·02, 0·06 and 0·10) have been obtained by hydrothermal method and sintering at 900 and 1100°C. The thermoelectric properties have been measured, and all of the samples in the measured temperature range have been found to be insulating. Very low thermal conductivity κ?=?0·75 W m^?1 K^?1 is found at 500 K for Sr_0·9La_0·1TiO₃ after sintering at 900°C. With increasing doping level x, the thermal conductivity κ decreases. With increasing sintering temperature, the grain size of samples increases, and the thermal conductivity increases. This phenomenon is ascribed to the strong suppression of the electron and phonon conductivities due to scattering by the numerous grain boundary scattering.     

Visible light-active nitrogen-doped TiO2 thin films prepared by DC magnetron sputtering used as a photocatalyst

The visible light-active nitrogen-doped TiO₂ has been prepared by dc-reactive magnetron sputtering using Ti target in an Ar+O₂/N gas mixture. The preparation of highly crystallized anatase TiO_xN_y thin films with various nitrogen concentrations allowed us to identify the optimum nitrogen flow ratio for the photocatalytic oxidation (PCO) of 2-propanol. At higher nitrogen flow rate, nitrogen is found to be difficult to substitute for oxygen having been predicted to contribute the band gap narrowing, giving rise to undesired deep level defects. In addition, Raman spectroscopy and X-ray diffraction (XRD) studies revealed that highly crystallized anatase growth of nitrogen-doped TiO_xN_y thin films are difficult at higher nitrogen flow rate. The optical band gap was found to be lower for the films deposited at 2 sccm of nitrogen flow rate. The PCO of 2-propanol was studied as a function of nitrogen flow rate using in situ FTIR spectroscopy. The PCO of 2-propanol found to proceed along two routes: one was through the chemisorbed species, 2-propoxide to form the CO₂ directly; the other was through conversion of 2-propanol to acetone, followed by formation of formate species, and finally CO₂.     

Improving hydrogen production via water splitting over Pt/TiO2/activated carbon nanocomposite

Mesoporous TiO₂/AC, Pt/TiO₂ and Pt/TiO₂/AC (AC = activated carbon) nanocomposites were synthesized by functionalizing the activated carbon using acid treatment and sol–gel method. Photochemical deposition method was used for Pt loading. The nano-photocatalysts were characterized using XRD, SEM, DRS, BET, FTIR, XPS, CHN and ICP methods. The hydrogen production, under UV light irradiation in an aqueous suspension containing methanol has been studied. The effect of Pt, methanol and activated carbon were investigated. The results show that the activated carbon and Pt together improve the hydrogen production via water splitting. Also methanol acts as a good hole scavenger. Mesoporous Pt/TiO₂/AC nanocomposite is the most efficient photocatalyst for hydrogen production compared to TiO₂/AC, Pt/TiO₂ and the commercial photocatalyst P25 under the same photoreaction conditions. Using Pt/TiO₂/AC, the rate of hydrogen production is 7490 μmol (h g catal.)⁻¹ that is about 75 times higher than that of the P25 photocatalyst.     

Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water

Efficient Cu incorporated TiO₂ (Cu–TiO₂) photocatalysts for hydrogen generation were fabricated by four methods: in situ sol–gel, wet impregnation, chemical reduction of Cu salt, and in situ photo-deposition. All prepared samples are characterized by good dispersion of Cu components, and excellent light absorption ability. Depending on the preparation process, hydrogen generation rates of the as-prepared Cu–TiO₂ were recorded in the range of 9–20 mmol h⁻¹ g_catalyst⁻¹, which were even more superior to some noble metal (Pt/Au) loaded TiO₂. The various fabrication methods led to different chemical states of Cu, as well as different distribution ratio of Cu between surface and bulk phases of the photocatalyst. Both factors have been proven to influence photocatalytic hydrogen generation. In addition, the Cu content in the photocatalyst played a significant role in hydrogen generation. Among the four photocatalysts, the sample that was synthesized by in situ sol–gel method exhibited the highest stability. High efficiency, low cost, good stability are some of the merits that underline the promising potential of Cu–TiO₂ in photocatalytic hydrogen generation.     

Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite

The photocatalytic hydrogen production from aqueous methanol solution was investigated with ZnO/TiO₂, SnO/TiO₂, CuO/TiO₂, Al₂O₃/TiO₂ and CuO/Al₂O₃/TiO₂ nanocomposites. A mechanical mixing method, followed by the solid-state reaction at elevated temperature, was used for the preparation of nanocomposite photocatalyst. Among these nanocomposite photocatalysts, the maximal photocatalytic hydrogen production was observed with CuO/Al₂O₃/TiO₂ nanocomposites. A variety of components of CuO/Al₂O₃/TiO₂ photocatalysts were tested for the enhancement of H₂ formation. The optimal component was 0.2 wt% CuO/0.3 wt% Al₂O₃/TiO₂. The activity exhibited approximately tenfold enhancement at the optimum loading, compared with that with pure P-25 TiO₂. Nano-sized TiO₂ photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

First-principles studies of the structures and properties of Al- and Ag-substituted Mg2Ni alloys and their hydrides

The structures and properties of hydrogen storage alloy Mg₂Ni, of aluminum and silver substituted alloys Mg_2−xM_xNi (M = Al and Ag, x = 0.16667), and of their hydrides Mg₂NiH₄, Mg_2−xM_xNiH₄ (M = Al and Ag, x = 0.125) have been calculated from first-principles. Results show that the primitive cell sizes of the intermetallic alloys and hydrides were reduced by substitution of Mg with Al or Ag. Also, the interaction of Ni–Ni was weakened by the substitution. A strong covalent interaction between H and Ni atoms forms tetrahedral NiH₄ units in Mg₂NiH₄. The NiH₄ unit near the Al/Ag atom became tripod-like NiH₃ in Mg_2−xM_xNiH₄ (M = Al, Ag), indicating that the hydrogen storage capacity was decreased by the substitution. The calculated enthalpies of hydrogenation for Mg₂Ni, Mg_2−xAl_xNi and Mg_2−xAg_xNi are −65.14, −51.56 and −53.63 kJ/mol H₂, respectively, implying that the substitution destabilizes the hydrides. Therefore, the substitution is an effective technique for improving the thermodynamic behavior of hydrogenation/dehydrogenation in magnesium-based hydrogen storage materials.     

Structural and transport evolution in the LixAg2V4O11 system

We investigated the effect of inserting lithium into Ag₂V₄O₁₁ (-SVO) on the structure, electronic properties and redox committed by combining in situ XRD measurements, ESR spectroscopy and 4 probes DC conductivity coupled with thermopower measurements. The electrochemical discharge occurs in three consecutive steps above 2 V (vs. Li⁺/Li). The first one, between 0 < x < 0.7 in Li_x-SVO, has been ascribed to the V⁵⁺ reduction through a solid solution mechanism. This reduction competes with a Li⁺/Ag⁺ displacement reaction which leads to a structural collapse owing to the ionic radii mismatch between the withdrawn Ag⁺ and the inserted Li⁺. The silver reduction progresses continuously with two different slopes along two composition–potential plateaus at 2.81 V and 2.55 V. Finally, the reduction continues until we obtain an amorphous structure with V⁴⁺ and a of V³⁺. Although, the silver re-enters the structure during the subsequent recharge, the original structure is not recovered. The reduction of silver forming silver metal nano-clusters acts to increase the electronic conductivity from 3.8 × 10⁻⁵ S cm⁻¹ to 1.4 × 10⁻³ S cm⁻¹. In complement to this study, we also report on a low temperature hydro-(solvo)-thermal approach using HF_(aq) as a mineralizer, which enables the synthesis of nano-sized -SVO particles that exhibit superior electrochemical performances compared to conventional particles synthesized by solid-state reaction.     

Ag-doped TiO2 photocatalysts with effective charge transfer for highly efficient hydrogen production through water splitting

The development of efficient metal doped semiconductors for solar energy harvesting to produce hydrogen has attracted significant attention. Herein, the H₂ generation over Ag-doped TiO₂ photocatalyst, synthesized using a simple and cost-effective method based on chemical reduction, was reported. The Ag/TiO₂ exhibited an absorption peak in the visible region and the reduction of the bandgap to 2.5 eV due to surface plasmonic resonance (SPR). X-ray photoelectron spectroscopy revealed the presence of oxygen vacancies and 11% of Ag in Ti–Ag–O phase. The effect of reaction time and photocatalyst loading in the absence and presence of sacrificial reagents (alcohols and sulfur) on water splitting was studied and compared the activity of Ag/TiO₂ with that of bare TiO₂. The H₂ production rate of 23.5 mmol g⁻¹ h⁻¹ (with an apparent quantum yield of 19%), over 1.5Ag/TiO₂, was the highest ever reported so far. The observed higher activity could mainly be attributed to the existence of oxygen vacancies and the Ti–Ag–O phase. The photocatalyst was stable for three consecutive cycles in both the presence and absence of sacrificial reagents. This study offers new insights into the rational design of metal-support hybrid structures for hydrogen production through photocatalytic water splitting.     

Ternary AgInSe2 film electrode created using selenization of RF magnetron sputtered Ag–In metal precursor for photoelectrochemical applications

AgInSe₂ ternary semiconductor thin films are deposited on glass substrates and indium-doped-tin-oxide (ITO)-coated glass substrates using the selenization of magnetron sputtered Ag–In metal precursors. X-ray diffraction (XRD) and energy-dispersive analysis of X-ray (EDAX) results show that the crystal phase of samples changed from AgInSe₂ to a solid mixture of AgInSe₂ and Ag₂Se with a decrease in the [In]/[In + Ag] molar ratio in samples. The direct and indirect energy band gaps of the samples vary in the ranges of 1.27–1.45 eV and 0.91–1.17 eV, respectively, depending on the [In]/[In + Ag] molar ratio in samples. The flat-band potentials of samples are in the range of −0.47 to −0.71 V (vs. normal hydrogen electrode) in a solution containing Na₂S (0.35 M) + K₂SO₃ (0.25 M) obtained using Mott–Schottky measurements. The maximum photocurrent density of the samples on ITO-coated glass substrates is 31.7 mA/cm² at an external potential of +1.0 V (vs. Ag/AgCl) in the solution containing Na₂S (0.35 M) + K₂SO₃ (0.25 M) ions.