Photooxidation Of Methanol Research Articles

Photo-oxidation of methanol in complexes with pyrido[2,3-b]pyrazine: a nonadiabatic molecular dynamics study.

Excited-state Proton-Coupled Electron Transfer (PCET) constitutes a key step in the photo-oxidation of small, electron-rich systems possessing acidic hydrogen atoms, such as water or alcohols, which can play a vital role in green hydrogen production. In this contribution, we employ ab initio quantum-chemical methods and on-the-fly nonadiabatic molecular dynamics simulations to study the mechanism and the photodynamics of PCET in 1 : 1 pyrido[2,3-b]pyrazine complexes with methanol. We find the process to be ultrafast and efficient when the intramolecular hydrogen bond is formed with one of the β-positioned nitrogen atoms. The complex exhibiting a hydrogen bond with an isolated nitrogen site, on the contrary, shows much lower reactivity. We explain this effect with the stabilization of the reactive ππ* charge-transfer electronic state in the former case.

Selective Photocatalytic Reduction of 3‐Nitrophenol to 3‐Aminophenol by Anatase and Rutile TiO2 – What Stands Behind the Photoactivity?

AbstractThe photocatalytic selective reduction of 3‐nitrophenol to 3‐aminophenol was studied in the presence of titanium dioxide in the form of various anatase and rutile compositions. The reaction progress is altered by various parameters, which can be classified as intrinsic (depending on the chemical, structural, and electronic features) and extrinsic (depending on the reaction and conditions). The goal of the studies was to understand the influence of intrinsic factors and to compare the performance of anatase and rutile materials. The (photo)electrochemical analysis revealed unequivocally the differences in interfacial electron transfer, charge recombination, reduction driving force, and methanol photooxidation efficiency for titania polymorphs. It appeared that all mentioned processes were phase‐dependent, and they contributed unequally to overall photocatalytic activity. In particular, methanol oxidation was the most efficient at the rutile phase, which overcame the critical limitation of the oxidation pathway and facilitated the reduction of 3‐nitrophenol. On the other hand, the dark electron transfer efficiency was highest at the anatase phase, despite the lower driving force in this case. The presented thorough and systematic analysis of the discussed photocatalytic system (the photocatalyst and the reaction) should allow the rational design of efficient and selective photocatalysts.

Unraveling the effect of low Cu2O loading on P25 TiO2 and its self-reduction during methanol photoreforming

Using noble metal-free catalysts is highly promising for sunlight-driven photocatalytic H2 production. Copper compounds have gained considerable interest in this regard. Cu2O/semiconductor composites exhibit superior photocatalytic activity compared to individual compounds. This study focuses on different CuXO/TiO2 composites synthesized via chemical reduction, using Cu2O weight ratios ranging from 1 to 0.05 concerning the TiO2 source. Characterization techniques were employed, including XRD, SEM, XPS, UV–Vis DRS, PL, and TRMC. Surface modification of TiO2 with Cu species (CuO, Cu2O, and Cu0) was observed during the photocatalytic reforming of methanol under simulated solar light. By loading small amounts of CuXO (0.05–0.1%) on TiO2, the highest H2 production and methanol mineralization were achieved, along with the formation of metallic copper. The coexistence of the Cu species and TiO2 facilitated the effective separation of photogenerated electrons and holes, promoting the photoreduction of protons and photooxidation of methanol, respectively.

The Influence of the Reaction Conditions on the Photocatalytic Gas‐Phase Conversion of Methanol with Water Vapor over Pt/SrTiO3 in a Continuously Operated Flow Reactor

AbstractContinuous methanol photooxidation in the gas phase is a promising method to produce valuable chemicals like formaldehyde or methyl formate in addition to hydrogen under mild conditions. The influence of the reaction conditions on the selectivity of methanol oxidation to formaldehyde is studied using a heated flat‐plate flow photoreactor illuminated by an LED array (λmax = 368 nm) and Pt‐modified SrTiO3. A combination of online analytical methods allowed to quantify all gaseous products during extended time‐on‐stream (> 48 h TOS). The selectivity to formaldehyde is found to be primarily determined by the residence time and the process temperature. At a low methanol to water ratio, methanol conversion and evolution of CO2 are favored, whereas the light intensity primarily influenced the apparent quantum yield from 5.1 to 1.8% at 9.36 to 52.93 mW cm−2, respectively, and the methanol conversion thus determining the economic efficiency of the process. Operation temperatures higher than 110 °C resulted in a strong deactivation of the catalyst while simultaneously the formation of CO at the expense of formaldehyde selectivity is favored. This study demonstrates the importance of understanding the influence of relevant reaction conditions and the potential of selective photocatalytic gas‐phase oxidation of small molecules.

Highly Selective Photosynthesis of Formic Acid by Unifying the Products of CO2 Reduction and Methanol Oxidation

Photocatalytic conversion of CO2 to added‐value fuels or chemicals is a promising technology for the effective utilization of the disturbing greenhouse gas, but its economic benefit is limited either by low efficiency due to the difficulty of water oxidation or by the employment of expensive sacrificial reagents. To overcome this dilemma, herein, an effective strategy is reported for the resourceful utilization of CO2 by creating an eco‐friendly bifunctional photocatalyst to achieve CO2 photoreduction concurrent with methanol photooxidation to the uniform product of formic acid for the first time. The bifunctional photocatalyst is elaborately constructed by in situ encapsulating lead‐free double perovskite (Cs2AgBiBr6, abbreviated as CABB) nanocrystals into the pores of Fe‐based metal−organic framework (MIL‐101(Fe)). The tight contact between CABB and MIL‐101(Fe) in the resultant CABB/MIL‐101(Fe) composite facilitates the efficient spatial separation of photogenerated carriers, which leads to the division of reduction and oxidation functional units. CABB/MIL‐101(Fe) composites can unify the redox products into formic acid, reaching an impressive formation yield of 1375 μmol g−1 h−1 with a high selectivity of 97.8%. This work provides a viable strategy for the efficient photosynthesis of formic acid, promoting the cost‐effective utilization of CO2.

Fabrication of nitrogen doped TiO2/Fe2O3 nanostructures for photocatalytic oxidation of methanol based wastewater

An important industrial process that often occurs on the surface of a heterogeneous catalyst using thermochemical or photochemical could help in the oxidation of methanol-based wastewater to formaldehyde. Titania-based photocatalysts have drawn a lot of interest from scientists because they are a reliable and affordable catalyst material for photocatalytic oxidation processes in the presence of light energy. In this study, a straight-forward hydrothermal method for producing n-TiO2@α-Fe2O3 composite photocatalysts and hematite (α-Fe2O3) nanocubes has been done. By adjusting the ratio of n-TiO2 in the prepared composite photocatalysts, the enhancing influence of the nitrogen-doped titania on the photocatalytic characteristics of the prepared materials was investigated. The prepared materials were thoroughly characterized using common physiochemical methods, such as transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), X-ray photoelectrons spectroscopy (XPS), physisorption (BET), and others, in order to learn more about the structure The results obtained showed that nitrogen-doped titania outperforms non-doped titania for methanol photooxidation. The addition of nitrogen-doped titania to their surfaces resulted in an even greater improvement in the photooxidation rates of the methanol coupled with hematite. The photooxidation of methanol in the aqueous solution to simulate its concentration in the wastewater has been occurred. After 3 h, the four weight percent of n-TiO2@α-Fe2O3 photocatalyst showed the highest rate of HCHO production.

Atomically precise copper dopants in metal clusters boost up stability, fluorescence, and photocatalytic activity

The structurally precise alloy nanoclusters have been emerged as a burgeoning nanomaterial for their unique physical/chemical features. We here report a rod-like nanocluster [Au12Cu13(PPh3)10I7](SbF6)2 (Au12Cu13), which was generated through a transformation of a [Au9(PPh3)8]3+ intermediate in the presence of CuI, unveiled by time-dependent UV-vis spectroscopy, electrospray ionization mass spectrometry as well as single crystal X-ray diffraction. Au12Cu13 is comprised of two pentagonal bipyramids Au6Cu units and a pentagonal prism Cu11 unit, where the copper and gold species are presented in +1 and 0 chemical states. The Cu-dopants significantly improved the stability and fluorescence (quantum yield: ~34%, 34-folds of homo-Au25(PPh3)10Br7). The high stability of Au12Cu13 is attributed to the high binding energy of iodine ligands, Au-Cu synergistic effects and its 16-electon system as an 8-electron superatom dimer. Finally, the robust Au12Cu13 exhibited high catalytic activity (~92% conversion and ~84% methyl formate-selectivity) and good durability in methanol photo-oxidation.

Methanol photooxidation in a black body like reactor using bismuth-based heterojunctions

The photocatalytic activity of different bismuth-based heterojunctions by the reaction rates (dn/dt) and quantum yields (Φλ) in aqueous suspension employing a black body like reactor (BBR), is reported. Four photocatalysts and their heterojunctions were tested while keeping the β-Bi2O3 in common: Cu2O/β-Bi2O3, ZnO/β-Bi2O3, and Na0.5Bi0.5TiO3/β-Bi2O3. The high optical density of the photocatalyst and the negative geometry of the reactor allowed measuring the quantum yields with good reproducibility since the catalyst is the only light absorbing species. The number of photons absorbed by the photocatalysts per unit time (photon flux) was determined by chemical actinometry for UV and visible light. The photocatalytic activity was measured for the oxidation of methanol as a model compound. Formaldehyde was measured as the photooxidation product employing Nash method. The charge transfer process mechanism in the photocatalytic reaction is shown based on the band gap energies, band edge potentials, and band alignment in the case of heterojunctions.

Efficient Generation of Hydrogen Peroxide and Formate by an Organic Polymer Dots Photocatalyst in Alkaline Conditions.

A photocatalyst comprising binary organic polymer dots (Pdots) was prepared. The Pdots were constructed from poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole), as an electron donor, and 1‐[3‐(methoxycarbonyl)propyl]‐1‐phenyl‐[6.6]C61, as an electron acceptor. The photocatalyst produces H2O2 in alkaline conditions (1 M KOH) with a production rate of up to 188 mmol h−1 g−1. The external quantum efficiencies were 30 % (5 min) and 14 % (75 min) at 450 nm. Furthermore, photo‐oxidation of methanol by Pdots, followed by a disproportionation reaction and an oxidation reaction, produced the high‐value chemical formate. On the basis of various spectroscopic and electrochemical measurements, the photophysical processes of the system were studied in detail and a reaction mechanism was proposed.

Promoted visible-light-assisted oxidation of methanol over N-doped TiO2/WO3 nanostructures

Alcohol oxidation is an important chemical reaction with a great deal of interest in the fields of energy and industrial sectors. Herein, n-TiO2@WO3 heterojunction photocatalysts were fabricated and their catalytic activity toward the visible-light-assisted oxidation of methanol was studied. A series of n-TiO2@WO3 photocatalysts with different weight ratios of n-TiO2 as 1, 1.5, 2.3, and 4 wt % were prepared using a simple hydrothermal approach. The prepared materials were characterized using standard physiochemical techniques including TEM, SEM, XRD, EDX, XPS, physisorption (BET), FTIR, Raman, DRS, and PL spectroscopy. The promoting effect of the n-TiO2 on photocatalytic activity was studied by varying the ratio of n-TiO2 in the composite photocatalysts. The effect of illumination time on methanol photo-oxidation was investigated up to 3 h. The interplay of n-TiO2 and WO3 could extend the photosensitivity of TiO2 into the visible region of the solar spectrum and decrease the recombination of the photogenerated electron-hole pairs. The hybrid n-TiO2@WO3 photocatalyst demonstrated enhanced photocatalytic activity for methanol oxidation compared to pure counterparts. The 2.3 wt% n-TiO2/WO3 composite photocatalyst achieved the highest photocatalytic conversion of methanol to formaldehyde of 48.4 mmol L−1 upon visible-light illumination (150 mW/cm2) for 3 h. The present paper demonstrates a non-conventional, efficient, and green approach for the photooxidation of methanol. The utilization of solar energy for alcohol oxidation can have immense advantages energetically and economically.

Effect of inorganic ions on H2O2 production over illuminated Au/WO3 with visible light

Production of H2O2 via semiconductor photocatalysis has received an increasing attention. Herein the effects of cations in perchlorate form and anions in sodium form are examined. Experiment was conducted with Au/WO3 under visible light in presence of methanol. The production of H2O2 was greatly promoted by A (Ni2+, PO43−, CO32−, and F−), slightly increased by B (SO42−, NO3−, Cl−, and ClO4−), greatly inhibited by C (Cu2+, and Fe3+), and slightly decreased by Co2+. Moreover, the effect of Ni2+ was weakening by F−, and vice versa. Analysis showed that Ni2+ was oxidized into Ni3+, reactive to methanol, not to water. On Au/WO3 electrode, anions A inhibited O2 reduction, and promoted methanol photo-oxidation, while all anions had little influence on the pathway of O2 reduction. A plausible mechanism is proposed, involving the hole oxidation of anions (not F−), followed by methanol oxidation. This work highlights the importance of inorganic ions to the selective reduction of O2.

Formaldehyde Analysis in Non-Aqueous Methanol Solutions by Infrared Spectroscopy and Electrospray Ionization.

We present the analysis of formaldehyde (HCHO) in anhydrous methanol (CH3OH) as a case study to quantify HCHO in non-aqueous samples. At higher concentrations (C > 0.07 M), we detect a product of HCHO, methoxy methanol (MM, CH3OCH2OH), by Fourier transform infrared spectroscopy, FTIR. Formaldehyde reacts with CH3OH, CD3OH, and CD3OD as shown by FTIR with a characteristic spectral feature around 1,195 cm−1 for CH3OH used for the qualitative detection of MM, a formaldehyde derivative in neat methanol. Ab initio calculations support this assignment. The extinction coefficient for 1,195 cm−1 is in the order of 1.4 × 102 M−1cm−1, which makes the detection limit by FTIR in the order of 0.07 M. For lower concentrations, we performed the quantitative analysis of non-aqueous samples by derivatization with dinitrophenylhydrazine (DNPH). The derivatization uses an aqueous H2SO4 solution to yield the formaldehyde derivatized hydrazone. Ba(OH)2 removes sulfate ions from the derivatized samples and a final extraction with isobutyl acetate to yield a 1:1 methanol: isobutyl acetate solvent for injection for electrospray ionization (ESI). The ESI analysis gave a linear calibration curve for concentrations from 10 to 200 µM with a time-of-flight analyzer (TOF). The detection and quantification limits are 7.8 and 26 μM, respectively, for a linear correlation with R 2 > 0.99. We propose that the formaldehyde in CH3OH is in equilibrium with the MM species, without evidence of HCHO in solution. In the presence of water, the peaks for MM become less resolved, as expected from the well-known equilibria of HCHO that favors the formation of methylene glycol and polymeric species. Our results show that HCHO, in methanol does not exist in the aldehyde form as the main chemical species. Still, HCHO is in equilibrium between the production of MM and the formation of hydrated species in the presence of water. We demonstrate the ESI-MS analysis of HCHO from a non-aqueous TiO2 suspension in methanol. Detection of HCHO after illumination of the colloid indicates that methanol photooxidation yields formaldehyde in equilibrium with the solvent.

Unraveling the Origin of Photocatalytic Deactivation in CeO2/Nb2O5 Heterostructure Systems during Methanol Oxidation: Insight into the Role of Cerium Species.

The study provides deep insight into the origin of photocatalytic deactivation of Nb2O5 after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO2/Nb2O5 heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV–visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb2O5 and CeO2) and as-prepared CeO2/Nb2O5 heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by operando-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO2/Nb2O5 heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb2O5 to CeO2. However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.

Anatase titania activated by Cu(II) or Zn(II) nanoparticles for the photooxidation of methanol assisted by Rhodamine-B

This paper deals with the photocatalytic reactivity of anatase titania particles, modified by surface deposition of Cu2+ and Zn2+ oxide/hydroxide nanoparticles which are detected by EDS and FX techniques. The objective of the preparation steps was chosen to avoid the formation of larges Cu and Zn oxide particles, which could be identified by XRD and other techniques. Their activity for methanol photocatalytic oxidation in water was studied and Rhodamine-B was added as photo-sensibilizer. To obtain a-TiO2 support, a thermal activation of commercial Millennium-PC500 nanosized powder was achieved. 3Cu/a-TiO2 and 3Zn/a-TiO2 catalysts (3 wt% of Cu or Zn) were prepared under simple, easy and reproducible experimental conditions, namely by impregnation with nitrate salts. Catalysts and titania support were characterized by: X-rays diffraction, fluorescence spectroscopy, FEG-SEM equipped with EDS accessory, N2-sorption and UV–Vis Diffuse Reflectance spectroscopy. Only crystalline anatase TiO2was identified by XRD and large Cu and Zn oxide/hydroxide nanoparticles were easily evidenced by EDS and other techniques. Electronic band-gap was measured by UV–Vis spectroscopy using Kubelka-Munkand Tauc's methods, results varied in the order: 3Cu/a-TiO2<a-TiO2 <3Zn/a-TiO2; also, the presence of reduced Ti3+cations have been demonstrated by this technique. Two photo-activated reactions take place in parallel, on the one hand the photooxidation of methanol with titania holes, on the other hand photo-generated electrons are formed in the titania conduction band. They can react rapidly with diluted oxygen to generate superoxide radicals or with RhB molecules to generate RhB° radicals. RhB molecules are mainly mono-deethylated. After 150 min of reaction, RhB was 98% degraded with 3Cu/a-TiO2 best catalyst.

CuO Nanoparticle-Decorated TiO2-Nanotube Heterojunctions for Direct Synthesis of Methyl Formate via Photo-Oxidation of Methanol.

Methyl formate is widely employed as one of the important organic intermediates in C1 chemistry and as an environmentally benign chemical in the emerging area of “green chemistry”. We here study the catalytic effectiveness of CuO/TiO2-nanotube nanocomposites for the direct synthesis of methyl formate via methanol photo-oxidation in this paper. The CuO/TiO2-nanotube nanocomposites with 7% CuO weight loading give a promising photocatalytic performance (over 93.3% methanol conversion and 89.4% methyl formate selectivity) at 25 °C under 365 nm UV irradiation. The turnover frequency and the apparent quantum efficiency reach up to 22.9 molmethanol gcat–1 h–1 and 57.6% over 7% CuO/TiO2-nanotube, respectively, which is mainly because of the effective minimization of the recombination of photogenerated electrons and holes by the surface ultrasmall-sized CuO particles. Furthermore, these CuO/TiO2-nanotube nanocomposites exhibit excellent durability and robust nature during the catalytic processes of the methanol photo-oxidation to methyl formate. The experimental results demonstrate the promise of the robust CuO/TiO2 nanocomposites for efficient and green production of methyl formate, which may offer guidelines for the design of efficient catalysts for selective alcohol conversion to other valuable chemicals.

Light-Intensity-Responsive Changes of Products in Photocatalytic Reduction of Nitrous Acid on a Cu-Doped Covalent Triazine Framework-TiO2 Hybrid.

In the design of solar-energy conversion electrochemical systems, it is important to consider that natural sunlight fluctuates. By taking nitrous acid photoreduction as an example, this study has shown that the reaction pathway, and hence the reaction products, dynamically respond to variations in light intensity. Under irradiation, the photooxidation of methanol (as sacrificial agent) on TiO2 and the reduction of HNO2 on a Cu-modified covalent triazine framework (Cu-CTF) are electrically coupled, which leads to the photoreduction of HNO2 without an external bias. The major product of the reaction changes from N2 O to NH4 + with an increase in the light intensity. The operating potential also shifts negatively (or positively) when the light intensity is increased (or decreased). These results indicate that a change in the reaction pathway is triggered by a change in the operating potential of the Cu-CTF catalyst under varying light intensity. Such a light-intensity-dependent change in the reaction pathway is particularly important in systems that use photoresponsive electrodes and where multiple products can be obtained, such as the solar-driven reduction of carbon dioxide and nitrogen oxides.

Experimental and mechanistic understanding of photo-oxidation of methanol catalyzed by CuO/TiO2-spindle nanocomposite: Oxygen vacancy engineering

We report experimental and mechanistic understanding of methanol oxidation to produce methyl formate using CuO/TiO2-spindle composite as a promising photocatalyst under mild conditions with over 97% conversion and 83% selectivity. The catalysts are obtained via precise depositing of CuO nanoclusters (size: ~ 3.5 nm) at the {101} facet of the TiO2 to optimally tune exciton recombination through oxygen vacancies generation, evidenced by photoluminescence and Raman spectroscopy measurements. The turnover frequency (TOF) and the apparent quantum efficiency (AQE) of the 7%CuO/TiO2-spindle composites reach up to 23.8 molmethanol·gcat−1·h−1 and 55.2% at 25 °C, respectively, which are substantially higher than these previously reported photocatalysts. Further, the in-situ attenuated total reflection infrared spectroscopy analysis reveals that the methanol oxidation most likely takes place through the conversion of adsorbed methoxy (CH3O*) to formaldehyde (CHO*) intermediate, a subject of major debate for a long time. The adsorbed formaldehyde (CHO*) thus produced reacts with another CH3O* species in its close proximity to form the final product of methyl formate. Results of this study provide insights into the reaction mechanism, and offer guidelines to systematically develop and apply photocatalysts for methanol conversion and related reactions via surface engineering.

Critical role of the defect state in the photo-oxidation of methanol on TiO2(110)

Photo-oxidation of CH3OH on TiO2 has been extensively studied in order to understand the fundamental principles of heterogeneous photocatalysis.

On the Mechanism of Methyl Formate Production Initiated by Photooxidation of Methanol on Rutile TiO2(110) and TiO2(011)-(2 × 1) Surfaces

The formation of HCOOCH3 initiated by photooxidation of CH3OH on TiO2 surfaces is interesting, but the mechanism remains ambiguous. Using thermal desorption spectroscopy and X-ray photoelectron spe...

Insight into methanol photooxidation over mono- (Au, Cu) and bimetallic (AuCu) catalysts supported on niobium pentoxide — An operando-IR study

The aim of this study was to gain deeper insight into the factors determining the activity and selectivity of mono- (Au, Cu) and bimetallic (AuCu) catalysts supported on Nb2O5 in methanol photooxidation. For this purpose, the prepared catalysts were characterized with the use of XRD, nitrogen sorption, ICP-OES, UV–vis, XPS, TEM and FTIR spectroscopy. Photocatalytic activity of materials was evaluated with the use of operando-IR system under UV and visible light irradiation. It was demonstrated that under visible light irradiation, the decoration of the niobia with bimetallic nanoparticles of gold and copper led to a synergic enhancement of the photocatalyst performance relative that of monometallic and non-doped niobia photocatalysts, tested under the same reaction conditions. It was also demonstrated that selectivity over the bimetallic gold-copper/Nb2O5 catalyst was strongly affected by the nature of light source: visible light promoted a total oxidation versus a partial oxidation of methanol under UV light.