Methanol Photoreforming Research Articles

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.

Zero carbon footprint hydrogen generation by photoreforming of methanol over Cu/TiO2 nanocatalyst

Zero carbon footprint hydrogen generation by photoreforming of methanol over Cu/TiO2 nanocatalyst

Photocatalytic effects on Au@TiO2 confined in BMIm.NTf2 ionic liquid for hydrogen evolution reactions

We report a photocatalytic device based on gold nanoparticles (AuNPs/TiO2) coated with an ionic liquid (IL) as a very active, stable, and versatile system for hydrogen evolution reactions (HERs) via methanol photo reforming (MPR) and photocatalytic water-splitting (WSR) reaction. The devices prepared by magnetron sputtering yield hybrid materials with direct Au–TiO2 interactions that were subsequently coated with BMIm.NTf2 (IL@Au@TiO2). The IL presence critically enhances the photocatalytic performance given its interaction with the AuNPs and TiO2 surfaces by decreasing the flat band energy levels and the Schottky barrier thickness. The results show an increase in the number of charge carriers yielding a capacitor-like effect generated by interactions of IL and Au@TiO2 particles. Apparent quantum yield (AQY) up to 28.3% under UV irradiation (365 nm) and 22.9% at 405 nm irradiation were observed in the HER employing the as-prepared materials.

High-Performance Photocatalytic H2 Production Using a Binary Cu/TiO2/SrTiO3 Heterojunction.

Cu/TiO2/SrTiO3 hybrid structures have been synthesized by the simple impregnation method from Cu/TiO2 and SrTiO3 systems. The structural and surface characterization stated that Cu/TiO2/SrTiO3 composites form an effective covering of SrTiO3 by Cu/TiO2. The heterostructured catalysts lead to an outstanding improved photoactivity for hydrogen production from methanol photoreforming that would be related with the efficient separation of charge pairs favored by the Cu/TiO2/SrTiO3 heterojunction. The best photoproduction is attained for the 30 wt % SrTiO3 heterojunction showing 81.7 mmol/g H2 after 6 h (leading to an apparent quantum yield of ca 1%), 1.7 times higher than that of bare Cu/TiO2.

Ultrabroadband plasmon driving selective photoreforming of methanol under ambient conditions

Liquid methanol has the potential to be the hydrogen energy carrier and storage medium for the future green economy. However, there are still many challenges before zero-emission, affordable molecular H2 can be extracted from methanol with high performance. Here, we present noble-metal-free Cu-WC/W plasmonic nanohybrids which exhibit unsurpassed solar H2 extraction efficiency from pure methanol of 2,176.7 µmol g-1h-1 at room temperature and normal pressure. Macro-to-micro experiments and simulations unveil that local reaction microenvironments are generated by the coperturbation of WC/W's lattice strain and infrared-plasmonic electric field. It enables spontaneous but selective zero-emission reaction pathways. Such microenvironments are found to be highly cooperative with solar-broadband-plasmon-excited charge carriers flowing from Cu to WC surfaces for efficient stable CH3OH plasmonic reforming with C3-dominated liquid products and 100% selective gaseous H2. Such high efficiency, without any COx emission, can be sustained for over a thousand-hour operation without obvious degradation.

Phototransformations of TiO2/Ag2O composites and their influence on photocatalytic water splitting accompanied by methanol photoreforming.

This work aimed to revise the mechanism of photocatalytic activity of the TiO2/Ag2O system in photocatalytic water splitting accompanied by methanol photoreforming. The transformation of Ag2O into silver nanoparticles (AgNPs) during photocatalytic water splitting/methanol photoreforming was monitored using XRD, XPS, SEM, UV-vis, and DRS techniques. The impact of AgNPs, grown on TiO2, on its optoelectronic properties was analysed through inter alia spectroelectrochemical measurements. The photoreduced material exhibited a significantly shifted position of the TiO2 conduction band edge. Surface photovoltage measurements revealed the lack of photoinduced exchange of electrons between TiO2 and Ag2O, indicating the absence of an efficient p-n junction. Furthermore, the impact of chemical and structural changes in the photocatalytic system on the production of CO and CO2 from methanol photoreforming was analysed. It was found that fully formed AgNPs exhibit improved efficiency in the production of H2, whereas the Ag2O phototransformation, resulting in the growth of AgNPs, promotes simultaneously ongoing photoreforming of methanol.

Photodegradation of Organic Pollutants in Seawater and Hydrogen Production via Methanol Photoreforming with Hydrated Niobium Pentoxide Catalysts

In this work, the photocatalytic activity of Hydrated Niobium Pentoxide (synthesized by a simple and inexpensive method) was explored in two unknown reactions reported for this catalyst: the photodegradation of phenol in seawater and the photoreforming of methanol. The Hydrated Niobium Pentoxide (Nb1) was synthesized from the reaction of niobium ammoniacal oxalate NH4[NbO(C2O4)2·H2O]•XH2O with a strong base (NaOH). Further treatment of this catalyst with H2O2 led to a light-sensitive Hydrated Niobium Pentoxide (Nb2). The photocatalysts were characterized by XRD, DRS, SEM Microscopy, FTIR-ATR, EDX, and specific surface area (SBET). The characterization results demonstrate that the treatment of Hydrated Niobium Pentoxide sensitized the material, increased the surface area of the material, diminished the average particle size, and modified its surface charge, and formed peroxo groups on the catalytic surface. Although both photocatalysts (Nb1 and Nb2) were active for both proposed reactions, the sensitization of the photocatalyst was beneficial in distinct situations. In the photocatalytic degradation of phenol in seawater, the sensitization of the photocatalyst did not enhance the photocatalytic activity. In both photoreactions studied, the addition of the Pt° promoter readily increased the photocatalytic performance of both photocatalysts; in this case, the sensitized photocatalyst recorded the best results. The presence of OH• radicals was confirmed, and the great contribution of the Pt° promoter was in the increase in OH• radical generation; this increase was more effective in the sensitized photocatalyst. Our work demonstrated a simple and inexpensive way to synthesize niobium photocatalysts that can effectively be used in the photodegradation of phenol in seawater and in the photoreforming of methanol to produce hydrogen.

Hydrogen production by the photoreforming of methanol and the photocatalytic water–gas shift reaction

We have examined the reforming of methanol and CO on Pd/P25 TiO2 catalysts for hydrogen production, and compared it with rates for similarly supported Au and Cu catalysts. Both reactions proceed, but the photocatalytic water–gas shift (WGS) reaction is much slower than for methanol reforming. CO2 is evolved as expected, but the yields can be much lower than for the expected stoichiometry (CH3OH + H2O → CO2 + 3H2). We show that this is due to dissolution of the carbon dioxide into the aqueous phase. We have also carried out both reactions in the gas phase. Both proceed at a higher rate in the gas phase, and for methanol reforming, there is some CO evolution. In H2 + CO2 reactions, there is little sign of the reverse WGS reaction, but some photo-methanation does occur. Of the three catalysts Pd is the best for the methanol reforming reaction, while Au is best for the water–gas shift. Nonetheless, Cu works reasonably well for methanol reforming and makes a much cheaper, earth-abundant catalyst.

Ionic liquid-assisted synthesis of F-doped titanium dioxide nanomaterials with high surface area for multi-functional catalytic and photocatalytic applications

Ionic liquid (IL) assisted syntheses of ultrareactive F-doped mesoporous aggregates of titanium dioxide nanoparticles have been studied using different synthetic methods in order to obtain non-toxic, highly versatile systems able to combine the high photocatalytic activity of the F-doping with a high surface area, to be applied in different catalytic and photocatalytic processes of environmental interest. The synthesized materials were characterized by various techniques, showing that they consist of mesoporous aggregates of F-doped anatase nanoparticles with surface areas of up to ca. 200 m2/g, which, together with the incorporation of ultrareactive (001) anatase facets promoted by doping, made the synthesized titanium dioxide nanomaterials very active in photocatalytic processes. The studies systems showed in the photocatalytic degradation of organic contaminants degradations of up to 96 % of concentrated solutions of methylene blue and common emerging pollutants such as ciprofloxacin (90 % degradation) and naproxen (79 % degradation) under UV light (300 W) in 10 min. In addition, when doped with palladium nanoparticles they became effective heterogeneous catalysts in Suzuki–Miyaura C–C coupling reactions giving good to excellent halide conversions (between 60–99 % for the formation of biphenyl derivatives) and high TOF values (up to 120 h−1) at short reaction times and good degree of recyclability. In addition, the Pd- and Pt-doped systems were also tested in the green process of hydrogen production by methanol photoreforming, showing a good accumulated hydrogen amount of ca. 75 mmol after 400 min with moderate photonic efficiencies of up to ca. 4.1 %.

Hydrogen production through photoreforming of methanol by Cu(s)/TiO2 nanocatalyst: Optimization and simulation

In this research, photocatalytic generation of hydrogen from methanol has been investigated in a solution containing TiO2 nanoparticles either with or without Cu ions using kinetic Monte Carlo simulation and response surface methodology. The obtained mechanism and the kinetic parameters of this process were confirmed by the good qualitative correspondence between the simulated and the experimental data. The RSM based on the central composite design (CCD) has been used to study the main square and interaction of independent variables initial concentration of methanol, irradiation, time of reaction, Cu concentration, and TiO2 concentration. According to the results, the time of reaction is the most influential variable with a positive effect on hydrogen generation efficiency. Increasing Cu ions to the solution until a certain amount has a positive impact on hydrogen generation efficiency, and then, it has a negative impact. Using KMC and RSM together, the optimum conditions were provided in which the efficiency of hydrogen production increases about fourteen times compared with the empirical one.

Methanol photo-reforming with water on pure titania for hydrogen production.

The behaviour of titania for the photo-reforming of methanol with water at ambient temperature has been examined. It is shown that the reactivity is very poor, compared with metal-loaded catalysts at low methanol levels in solution, but the rate becomes much higher at high methanol levels, such that the difference from metal-loaded samples is much less. The optimum yield is with approximately a 1 : 1 methanol/water solution. The reaction also proceeds well in the gas phase. During all such catalysis, the titania becomes blue, due to light absorption increasing across the range 400-800 nm. However, this does not result in visible range activity for the photo-reforming and is due to the reduction of the material in the presence of light and the formation of anion vacancies and Ti3+ centres. These anion vacancies are only very slowly re-oxidized in air on P25 titania, taking days to recover the original whiteness of the oxide. The performance of anatase, rutile and the mixed phase is compared. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Boosting Pt/TiO2 hydrogen photoproduction through Zr doping of the anatase structure: A spectroscopic and mechanistic study

The production of hydrogen from the photoreforming of methanol was essayed using an anatase-based system, having Pt nanoparticles and a Zr-doped anatase high surface area support. The production of hydrogen was quantitatively analyzed using the quantum efficiency parameter under UV and visible illumination conditions. The study showed that a 2.5 mol. % of Zr renders an outstanding material (4.6% quantum efficiency) for the fruitful use of sunlight as a green source of energy of the process. The outstanding promotion of activity orginated by Zr doping of the anatase structure was studied using in-situ infrared, photoluminescence and electron paramagnetic resonance spectroscopies. The multitechnique spectroscopic investigation of the reaction mechanism demonstrates that this occurs through a complex reforming plus water gas shift path. The study also unveils that optoelectronic properties of the solids derived from Zr incorporation drive activity of the system by controlling light absorption and, mainly, charge carrier recombination and interaction with the reactant molecules.

Molecular-level understanding of the deactivation pathways during methanol photo-reforming on Pt-decorated TiO2

The methanol photo-reforming offers an environmentally friendly approach for hydrogen production. However, the widely studied catalyst for this reaction, Pt loaded titanium dioxide (Pt/P25), is found to suffer from the deactivation problem. Here, we employed various techniques including in situ spectroscopy characterization to probe the underlying mechanism. The formed intermediates formaldehyde and formic acid result in the decreased activity of hydrogen production over Pt/P25. Formic acid harms the Pt co-catalyst, whereas formaldehyde can inhibit both the Pt co-catalyst and P25 photocatalyst and plays a major role in the deteriorative effect on Pt/P25. In addition, rutile in P25 is identified to be the bottleneck against the poisoning of the reaction intermediate. A deactivation mechanism is developed, which helps to troubleshoot the deactivation problem; for example, a strategy based on the chemical potential difference applied to the deactivated photocatalyst enables it to recover ∼80 % of its original hydrogen production activity.

Efficient hydrogen production by selective alcohol photoreforming on plasmonic photocatalyst comprising sandwiched Au nanodisks and TiO2

A nanocomposite comprising gold nanodisks (Au NDs) and titanium dioxide (TiO2) nanophases sandwiched between zeolite nanosheets was fabricated for hydrogen production by plasmonic photocatalytic reforming of methanol. The inter-sheet space between zeolite nanosheets served as nanoconfinement for forming the edge-attached domains of Au NDs/TiO2. Benefiting from the anisotropic shape of Au NDs and their edge attachment with TiO2, the nanocomposite exhibited superior photoactivity for visible light (>400 nm)-driven photoreforming of methanol and the hydrogen production rate was ˜ 24 times higher than that of the benchmark Au@P25 photocatalyst. Moreover, experimental results suggest that zeolite nanosheets effectively gated the access to the surface of Au NDs and allowed only methanol but not higher alcohols to react to produce hydrogen. Gradual deactivation was observed during methanol photoreforming, a phenomenon shown to be attributed to the chemisorbed CO accumulated on Au in the absence of oxygen.

Why co-catalyst-loaded rutile facilitates photocatalytic hydrogen evolution.

As the conduction band edge of rutile is close to the reduction potential of hydrogen, there is a long-lasting discussion on whether molecular hydrogen can be evolved from this semiconductor. Our study on methanol photoreforming in the ultra-high vacuum reveals that photocatalysts comprising a TiO2(110) single crystal decorated with platinum clusters indeed enable the evolution of H2. This is attributed to a new type of mechanism, in which the co-catalyst acts as a recombination center for hydrogen and not as a reduction site of a photoreaction. This mechanism is an alternative pathway to the commonly used mechanism derived from photoelectrochemistry and must particularly be considered for systems, in which reducible semiconductors enable the surface diffusion of hydrogen species.

Pd-Pt bimetallic Nb-doped TiO2 for H2 photo-production: Gas and liquid phase processes

The performance of binary PtPd co-catalysts supported on Nb-TiO2 was tested in the gas and liquid phase photo-reforming of methanol. Co-catalyst formulations having Pt:Pd ratios from 2:1 to 1:4 were tested and the activity measured through the calculation of the quantum efficiency. Characterization of the initial and post-reaction materials showed that optimum activity is achieved in both processes by samples containing binary co-catalysts where Pd suffers an oxidation process and a PtPd alloying metallic phase is formed under reaction conditions. The analysis of catalytic output through an experimental design considering tree factors (methanol:water ratio, irradiance level, and catalyst volumetric concentration at liquid phase or surface concentration at gas phase) shows that the liquid phase process presents a rather high quantum efficiency value compared with the gas phase but the latter process displays higher atom efficiency as well as renders products with lower toxicity.

Operational Conditions Affecting Formaldehyde and Formic Acid Formation as By‐Products of Hydrogen Production via Photo‐Reforming of Methanol Using Nanoparticles of TiO2

AbstractIt is important to understand how the operational conditions affect the photocatalytic hydrogen production and how to increase the production of valuable by‐products, such as formaldehyde and formic acid from the photo‐reforming of methanol. It is difficult to optimize operational conditions in photoreactors because several factors affect the process. To prioritize them, we focused on the quantification of the effect of four factors on CH2O and CH2O2 production: (A) presence of gold as co‐catalyst, (B) intensity of UV‐light, (C) methanol concentration, and (D) nanoparticle concentration. A main and interaction effects analysis is presented with fixed effect models for four responses: total production and catalyst productivity for both by‐products. Factor A showed the highest effect, followed by factors B and C, and the interactions AB and AC showed signs of influencing the catalyst selectivity with possible mechanistic variations. Factor D showed negative effect on catalyst productivities possibly related to shielding effect between particles.

Photocatalytic decomposition of gaseous methanol over anodized iron oxide nanotube arrays in high vacuum

Real-time monitoring of photocatalytic methanol decomposition over anodized iron oxide nanotube arrays (FNAs) in high vacuum was performed. Switching phenomena in the partial pressures of H2, CO and H2O were observed simultaneously according to the ON/OFF sequence of visible and ultraviolet light illuminations over Pt-loaded FNAs prepared with photodeposition, while no change in these gas species appeared for bare FNAs and iron foils. The absence of formaldehyde (CH2O) in the intermediates during methanol photodecomposition suggests that Pt-loaded FNAs can possibly work as a catalyst for oxidation of CH2O. The photoreforming of methanol to hydrogen without any external bias in Pt-loaded FNAs presumably indicate some differences in photocatalytic reaction mechanisms between in liquid and in high vacuum. The dependence of the Pt photodepositon time on gas phase photocatalysis suggests that the selective deposition of Pt cocatalyst particles on the aperture parts of the nanotubes is necessary for efficient gas phase photoreforming.

Band alignment investigations of heterostructure NiO/TiO2 nanomaterials used as efficient heterojunction earth-abundant metal oxide photocatalysts for hydrogen production.

Earth-abundant NiO/anatase TiO2 heteronanostructures were prepared by a straightforward one-pot sol-gel synthetic route followed by a suitable thermal post-treatment. The resulting 0.1-4 wt% NiO-decorated anatase TiO2 nanoparticles were characterized by X-ray diffraction, electron microscopy, Raman and UV-visible spectroscopy and N2 sorption analysis, and showed both nanocrystallinity and mesoporosity. The careful determination of the energy band alignment diagram by a suitable combination of XPS/UPS and absorption spectroscopy data revealed significant band bending at the interface of the p-n NiO/anatase TiO2 heterojunction nanoparticles. Furthermore, these heterojunction photocatalysts exhibited an improved photocatalytic activity in H2 production by methanol photoreforming compared to pure anatase TiO2 and commercial P25. Thus, an average H2 production rate of 2693 μmol h-1 g-1 was obtained for the heterojunction of a 1 wt% NiO/anatase photocatalyst, which is one of the most efficient NiO/anatase TiO2 systems ever reported. An enhanced dissociation efficiency of the photogenerated electron-hole pairs resulting from an internal electric field developed at the interface of the NiO/anatase TiO2 p-n heterojunctions is suggested to be the reason of this enhanced photocatalytic activity.

Continuous flow gas phase photoreforming of methanol at elevated reaction temperatures sensitised by Pt/TiO2

Gas phase photoreforming of methanol using a Pt/TiO2 photocatalyst has been performed under flow conditions at elevated temperatures.