UV Vis NIR DRS Research Articles

Stable and wide spectrum response Zn3As2/Al2O3 photocatalyst for photocatalytic overall water splitting

Constructing highly efficient and durable non-noble-metal photocatalysts with a broad spectral response for promoting sluggish photocatalyticover water splitting reaction is very desirable but still challenging. To realize scalable hydrogen production, an energy conversion efficiency of 10 % and stable photocatalysts with broad light-absorption are required. To this end, a wide spectrum response Zn3As2/Al2O3 has been successfully synthesized by a layer-by-layer coating method as a photocatalyst utilized in the hydrogen evolution reaction (HER) for the first time, which gave the HER rate of 92.5 μmol·g−1·h−1. The HER activity did not decline obviously after three runs. Characterization results and photocatalytic performance confirmed that the Al2O3 layer could protect Zn3As2 from photocorrosion, which exhibited preeminent stability and outstanding photocatalytic performance. In addition, Zn3As2/Al2O3 showed light response extending to ∼1300 nm, and more efficient separation capability of photogenerated charges benefited from the built-in electric field between Al2O3 and Zn3As2, which were verified by UV–Vis–NIR DRS and electrochemical measurements. Our research results may provide a new route of design and construction of stable and efficacious photocatalysts to realize solar energy conversion to hydrogen.

Comparison of ZrO2, TiO2, and α-Fe2O3 nanotube arrays on Cr(VI) photoreduction fabricated by anodization of Zr, Ti, and Fe foils

This paper presents the fabrication of self-organized ZrO2, TiO2, and α-Fe2O3 nanotube arrays by anodization of Zr, Ti, and Fe foils, respectively in fluoride-containing EG electrolyte at 40 V for 20 min. The as-anodized nanotubes were annealed in a tube furnace at 400 °C for 3 h to induce the crystallization of the oxide film. Morphology, crystal structure, surface properties, and optical properties of the anodic ZrO2 nanotubes (ZNTs), TiO2 nanotubes (TNTs), and α-Fe2O3 nanotubes (FNTs) were characterized by Field-Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), x-ray Diffraction (XRD), Fourier-Transform Infrared (FTIR) spectroscopy, Photoluminescence (PL) spectroscopy, and UV–visible Near-Infrared Diffuse Reflectance Spectra (UV–vis NIR DRS) spectroscopy, respectively. Based on the FESEM and TEM micrographs, ZNTs possessed the longest nanotubes (i.e. 9.6 μm) compared with TNTs and FNTs under the same anodization condition. The aspect ratio of the nanotubes can be arranged in the order of ZNTs > FNTs > TNTs. The surface of the annealed ZNTs, FNTs, and TNTs was enriched with –OH groups to facilitate the Cr(VI) adsorption. According to the UV–vis NIR DRS spectra, strong visible light absorption was observed on the FNTs due to their low band gap. Whereas, the TNTs predominantly absorbs the UV light at λmax = 360 nm. Rapid Cr(VI) removal was observed on FNTs, i.e. 100% after 2 h activated by sunlight with negligible Cr(VI) removal for ZNTs and TNTs. When exposed to UVC (λ = 254 nm), only 39% versus 37% Cr(VI) removal efficiencies were obtained on TNTs and ZNTs after 3 h suggesting sluggish electron transfer due to rapid charge carriers recombination as evident in the PL spectra.

Novel photocatalyst nitrogen-doped simonkolleite Zn5(OH)8Cl2·H2O with vis-up-conversion photoluminescence and effective visible-light photocatalysis

As photocatalysts exhibit selectivity toward various pollutants, it is necessary to develop different and novel photocatalysts. In this work, a novel photocatalyst-nitrogen-doped simonkolleite Zn5(OH)8Cl2·H2O (DSM) is prepared through a new facile method: calcinating the mixture of zinc hydroxide, urea, and guanidine hydrochloride at 575 °C for 1 h in a furnace with an air atmosphere. The as-prepared sample was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra, UV–visble near-infrared diffuse reflection spectra (UV–Vis–NIR DRS), Brunauer–Emmett–Teller (BET) method, Ramen spectra and Zeta potential measurement, photocatalytic properties, as well as active species trapping experiments. XRD and XPS show the as-prepared powder is nitrogen-doped simonkolleite Zn5(OH)8Cl2H2O (DSM) with a small ZnCl2 fraction. SEM investigation indicates that the as-prepared powder possesses a flower-like layered shape. The UV–Vis–NIR exhibits that after doping, the DSM possesses strong light absorption in the ranges of 300–500 and 1400–2500 nm, a direct electronic transition with a band gap energy of 2.469 eV. PL measurement reveals a strong photoluminescence and an up-conversion from lower to higher-energy visible light in as-prepared samples. Zeta potential investigations show that during photocatalysis, the charges on as-prepared photocatalyst are positive. The photocatalytic experiments show a good dark adsorption, a high photodegradation (99.4% at 60 min), a high pseudo-first-order constant (k) of 0.0261 min−1. Meanwhile, the active species trapping experiments suggest that hole (h+) is the dominant active species during photocatalysis. It is concluded that the doping favors in enhancing vis-light-photocatalysis. This work makes a significant contribution to the literature.

Oxidation of residual methane from VNG vehicles over Co3O4-based catalysts: Comparison among bulk, Al2O3-supported and Ce-doped catalysts

Cobalt oxide based catalysts with three different active phase configurations, namely bulk, alumina supported and cerium-doped Co3O4 samples were examined for the complete oxidation of methane under conditions similar to those found in the exhaust of VNG engines. The structural and redox properties of the resulting catalysts were determined by N2 adsorption-desorption, WDXRF, ICP-AES, X-Ray diffraction, temperature-programmed reactions, UV–vis-NIR DRS, XPS and Raman spectroscopy. Alumina-supported catalysts (10–40%wt. Co) were found to be less active, since the strong interactions between the alumina and the Co3O4 active phase were highly detrimental for the redox properties of these catalysts. On the other hand, doping with cerium (10%wt.) led to an increased inherent activity of the Co3O4 phase by distorting the spinel lattice, which resulted in improved structural and redox properties and enhanced mobility of the oxygen species within the spinel lattice. These catalysts were also stable over a prolonged period of time under both dry and humid conditions (150 h).

Photoelectrochemical hydrogen production at peak efficiency over 10% via PbSe QDs/TiO2 nanotube array photoanodes

Modulating the deposition of quantum dots onto TiO2 nanotube arrays provides an effective approach for expansion of visible light response, enhancement of separation and interfacial transfer of photoinduced charge carries, and improvement of photoconversion efficiency. In this paper, titanium dioxide nanotube arrays (TNAs) were modified with PbSe quantum dots (QDs) through the successive ionic layer adsorption and reaction (SILAR) under nitrogen to prepare PbSe/TNAs photoanodes. XRD, SEM, HR-TEM, UV–Vis–NIR DRS, PL, XPS, EIS, Tafel plots, Mott–Schottky analyses and J–V characteristics were used to characterize the samples. The deposition of PbSe QDs was tuned by varying the Se/Pb atomic ratios during the SILAR process. Energy band configuration, interfacial contact characteristics and photoelectrochemical properties were dissected and optimized. At the Se/Pb ratio of 5, the sample (5)PbSe/TNAs demonstrated excellent photoelectrochemical performance with a short circuit photocurrent density of 15.45 mA cm−2 and a corresponding photoconversion efficiency of 10.6%.

Effect of Ni/Al molar ratio on the performance of substoichiometric NiAl2O4 spinel-based catalysts for partial oxidation of methane

The viability of a series of nickel aluminate-based spinel catalysts with varying Ni deficiency (corresponding to a Ni/Al molar ratio in the 0.13–0.50 range) was explored for the partial oxidation of methane under different operation conditions in terms of temperature, volume hourly space velocity, O/C molar ratio and time on stream. Thus spinel-type catalysts with a Ni loading between 11 and 31wt.% were prepared by coprecipitation. A wide number of techniques including WDXRF, XRD, N2 physisorption, Raman spectroscopy, XPS, UV–vis-NIR DRS, H2-TPR, TEM and TGA-MS were used to characterise the calcined, reduced and post-run samples.With respect to the reference stoichiometric sample (Ni/Al=0.50) alumina excess in the precursor oxide provoked notable changes in the surface area, structural properties connected with the relative cation distribution between tetrahedral and octahedral coordination and reducibility of the resultant spinel phase. It was found that the catalytic performance of these non-stoichiometric samples could be optimised for a Ni/Al molar ratio of 0.25, which corresponded to a metal loading of 19wt.%Ni. The oxidation activity was associated with the remarkable intrinsic activity of nickel particles derived from Ni2+ cations with a preferential occupancy of octahedral sites in the lattice of the oxide. The promising catalytic behaviour of this sample was further proven by the notable activity and stability shown under severe reaction conditions with a reduced loss of yield of hydrogen with time on stream.

Facile synthesis of a novel full-spectrum-responsive Co2.67S4 nanoparticles for UV-, vis- and NIR-driven photocatalysis

Sparked by growing pollution issues, research aiming at a better harvesting of solar energy in photocatalysts for environmental remediation has been thriving. In this study, a novel mixed valence state of Co2.67S4 nanoparticles with full-spectrum-responsive photocatalytic activity had been fabricated via a facile solvothermal route. The as-synthesized samples were systematically characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV–vis-NIR diffuse reflection spectroscopy (UV–vis-NIR DRS). The photocatalytic performance of as-obtained samples had been investigated by the degradation of methylene blue (MB) in aqueous solution. The Co2.67S4 nanoparticles with the particle size of 5–20nm could degrade MB with the efficiency of 64%, 84% and 68% under the UV light, visible light and near-infrared light exposure, respectively. Furthermore, a possible photocatalytic mechanism toward the near-infrared region had been proposed to be that the Co2+/Co3+ redox couple played vital parts in the photocatalytic activity of Co2.67S4. This study provides a novel full solar spectrum-responsive photocatalyst for solar-light utility and environmental remediation.

Synthesis, characterisation and behaviour of Co/hydroxyapatite catalysts in the oxidation of 1,2-dichloroethane

Calcium-deficient hydroxyapatite (HAP) loaded with different amounts of cobalt are synthesised and characterised by several techniques including N2 physisorption, XRD, TEM, H2-TPR, UV–vis-NIR DRS, XPS spectroscopy and volumetric adsorption of CO2 and NH3 molecules techniques. The Co(x)/HAP catalysts are tested in the oxidation of 1,2-dichloroethane. Due to significant enhancement of the textural and structural properties the resulting Co/hydroxyapatite catalysts achieve higher activity compared to bulk Co3O4 catalyst confirming the advantage of dispersing cobalt on a relatively high surface area HAP. Hydroxyapatite support shows a high selectivity towards intermediate reaction products; but its activity suffers a significant decay with time due to chlorination. However, high stability and high selectivity towards deep oxidation products are favoured by Co-rich catalysts.

Exploiting the Photonic Crystal Properties of TiO2 Nanotube Arrays To Enhance Photocatalytic Hydrogen Production

Two series of self-assembled TiO2 nanotube (NT) arrays were grown by electrochemical anodization on a metallic titanium substrate with different anodization times and applied potentials in HF-containing ethylene glycol electrolyte solutions and postcalcined at 450 °C. The obtained thin films were characterized by FESEM, XRD, and UV–vis–NIR DRS analyses and tested as photoanodes in incident photon to current efficiency (IPCE) measurements and in a two-compartment photoelectrochemical cell (PEC) for separate H2 and O2 production. The photocatalytic performance of the NT arrays significantly increased with an increase in the potential applied during anodization (i.e., with increasing the NT inner diameter) and the incident angle of the light. IPCE measurements revealed that such unexpected behavior is due to a red shift of the activity threshold that allows harvesting and converting a larger portion of the solar spectrum. This phenomenon is ascribed to the parallel shift of the photonic band gap position ori...

Role of surface vanadium oxide coverage support on titania for the simultaneous removal of o-dichlorobenzene and NOx from waste incinerator flue gas

The catalytic activity of VOx species supported on TiO2 was investigated in the simultaneous destruction of NO and 1,2-dichlorobenzene (o-DCB), as typical pollutants molecules in the off-gases from municipal waste incinerator (MWI) plants. Catalysts with different vanadium loading were prepared in order to obtain different VOx species and characterized by ICP-AES, XPS, N2 adsorption at −196°C, XRD, H2-TPR, Raman and UV–vis–NIR DRS spectroscopy. The characterization results show that molecularly dispersed isolated and polymeric vanadia species form below the dispersion limit loading (“monolayer coverage”), while crystalline species form above it. We used moderate HNO3 treatment to partially leach vanadium oxide species, creating a series of catalysts with variable vanadia loading. The catalytic activity of the VOx/TiO2 catalyst shows that it is able to catalyze the destruction of both pollutants, although higher temperature is required for o-DCB oxidation than for NO reduction. Surface vanadia coverage has a clear effect on TOF and activation energy values, which underline that isolated vanadia species are more efficient for o-DCB oxidation, while the polymeric ones are more efficient for NO reduction.

Phosphotungstic Acid Nanoclusters Grafted onto High Surface Area Hydrous Zirconia as Efficient Heterogeneous Catalyst for Synthesis of Octahydroquinazolinones and β-Acetamido Ketones

Phostungstic acid (PWA) nanoclusters grafted onto high surface area polycrystalline hydrous zirconia powder (PWA/ZrO2) was prepared by wet impregnation method. The zirconia particles were synthesized using a modified sol–gel route. The obtained material was characterized by X-ray diffraction (XRD), UV–Vis–diffuse reflectance spectroscopy (UV–Vis–NIR–DRS), infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and sorptometric techniques. XRD study revealed the presence of tetragonal phase of zirconia in the synthesized sample. TEM study indicates presence of small PWA clusters with size in the range of 5–15 nm well dispersed on the surface of the ZrO2 particles. The characteristic UV and IR absorption feature of the PWA was retained in the PWA/ZrO2 material. The PWA/ZrO2 material was used as an efficient catalyst for the preparation of octahydroquinazolinones and β-acetamido ketones. The octahydroquinazolinones were synthesized by the multicomponent condensation of dimedones, urea and aryl aldehydes in ethanol. Similarly, β-acetamido ketones were synthesized with high yield and purity by four component condensation of aryl aldehydes, enolizable ketones, and acetyl chlorides in acetonitrile. The protocols developed in this investigation using the PWA/ZrO2 catalyst is advantageous in terms of simple experimentation, high yield and purity of the products and recovery and reutilization of the heterogeneous catalyst.

Cu-ZSM-5 Zeolites for the Formation of Methanol from Methane and Oxygen: Probing the Active Sites and Spectator Species

A series of Cu-ZSM-5 zeolites was prepared by varying nature of the charge compensating cation, copper precursor, copper loading, and pH. The materials were tested for the oxidation of methane to methanol using oxygen. A linear relationship between the amount of methanol produced over Cu-ZSM-5 zeolites from methane and oxygen and a UV–Vis-NIR DRS charge transfer band at 22,700 cm−1 is reported irrespective of the synthesis route used. The absolute intensity of the 22,700 cm−1 band is always low, indicating a low number of active sites in the samples. In all studied Cu-ZSM-5 zeolites at least two copper species were present: (a) Cu–O clusters dispersed on the outer surface of ZSM-5 and (b) highly dispersed copper-oxo species inside the channels, a minority fraction in the sample. By relating catalytic activity to FT-IR data of adsorbed pivalonitrile, visualizing Cu–O particles on the outer surface of the zeolite, and subsequently adsorbed NO, indicative of the Cu–O species inside the zeolite channel, it was concluded that Cu–O species on the outer surface are not involved in the oxidation reaction, while copper inside the channels are responsible for the selective conversion of methane to methanol.

Understanding the effect of postsynthesis ammonium treatment on the catalytic activity of Au/Ti-SBA-15 catalysts for the oxidation of propene

Postsynthesis ammonium treatment induces a substantial increase in the catalytic activity of Au/Ti-SBA-15 catalysts for the direct vapor-phase epoxidation of propylene using hydrogen and oxygen. The PO formation rate of a calcined Au/Ti-SBA-15 catalyst prepared by this method increased from 4.3 mg PO h −1 g −1 cat to 37.2 mg PO h −1 g −1 cat at 200 °C compared with a catalyst prepared in an identical manner without this treatment. The catalysts were characterized by XRD, N 2-sorption, UV–vis–NIR DRS, 29Si MAS NMR, FT-IR spectroscopy, and TEM to gain insight into the relationship between ammonia treatment and catalyst activity. 29Si MAS NMR measurements proved that the ammonium nitrate solution caused hydrolysis of Si O Si or Ti O Si bonds, resulting in a Ti-SBA-15 surface with a greater amount of surface hydroxyl groups. FT-IR measurements indicated the presence of amine species that favor the homogeneous deposition of Au nanoparticles. This was confirmed by TEM measurements showing greater metal dispersion for NH 4NO 3-treated Au/Ti-SBA-15 materials.

Transient responses of the local electronic and geometric structures of vanado-molybdo-phoshate catalysts H 3+ nPV nMo 12− nO 40 in selective oxidation

The thermal stability of the series of title compounds was investigated with thermogravimetry (TG), differential thermal analysis (DTA), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and in situ UV–VIS–NIR difference reflectance spectroscopy (UV–VIS–NIR–DRS). The results were combined with temperature programmed reaction experiments (TPR) of dehydration and catalytic tests involving methanol oxidation. The HPA are fully dehydrated and autoreduced at the temperature of maximum conversion. The presence of gas phase oxygen and of the organic substrate have only a limited influence on the autoreduced state. The oxygen storage function was probed by pulse reduction experiments which gave evidence for the existence of two kinetically different supply processes. The active state of the HPA is a partially reduced oligomer of polyoxoanions bridged by vanadyl groups. During catalytic action a steady-state exists between the precursor monomer and the oligomerised state. This steady-state forms as consequence of the thermal load in water and oxygen atmospheres and is qualitatively not altered upon addition of the organic substrate. Above 673 K the steady-state turns irreversibly into defective oxides associated with the loss of selective oxidation activity. A consistent picture is concluded of the solid state chemistry occurring during response of the precursor HPA to thermal and chemical conditions of selective oxidation catalysis. The HPA solid appears as inherently unstable material during operation as gas phase oxidation catalyst.