Claudio Ampelli

Synthesis of solar fuels by a novel photoelectrocatalytic approach

The characteristics of nanostructured (a) TiO2 thin films (based on an ordered array of titania nanotubes) and their performances as photoanode in H2 production by water splitting or photoreforming of ethanol (in liquid or gas phase) and (b) carbon-nanotube based electrodes for the gas-phase reduction of CO2 to liquid fuels (mainly isopropanol) are discussed together with their application for the design of a novel photoelectrocatalytic approach for the synthesis of solar fuels.

Electrocatalytic conversion of CO2 to liquid fuels using nanocarbon-based electrodes

Abstract Recent advances on the use of nanocarbon-based electrodes for the electrocatalytic conversion of gaseous streams of CO2 to liquid fuels are discussed in this perspective paper. A novel gas-phase electrocatalytic cell, different from the typical electrochemical systems working in liquid phase, was developed. There are several advantages to work in gas phase, e.g. no need to recover the products from a liquid phase and no problems of CO2 solubility, etc. Operating under these conditions and using electrodes based on metal nanoparticles supported over carbon nanotube (CNT) type materials, long C-chain products (in particular isopropanol under optimized conditions, but also hydrocarbons up to C8–C9) were obtained from the reduction of CO2. Pt-CNT are more stable and give in some cases a higher productivity, but Fe-CNT, particular using N-doped carbon nanotubes, give excellent properties and are preferable to noble-metal-based electrocatalysts for the lower cost. The control of the localization of metal particles at the inner or outer surface of CNT is an importact factor for the product distribution. The nature of the nanocarbon substrate also plays a relevant role in enhancing the productivity and tuning the selectivity towards long C-chain products. The electrodes for the electrocatalytic conversion of CO2 are part of a photoelectrocatalytic (PEC) solar cell concept, aimed to develop knowledge for the new generation artificial leaf-type solar cells which can use sunlight and water to convert CO2 to fuels and chemicals. The CO2 reduction to liquid fuels by solar energy is a good attempt to introduce renewables into the existing energy and chemical infrastructures, having a higher energy density and easier transport/storage than other competing solutions (i.e. H2).

Electrocatalytic Synthesis of Ammonia at Room Temperature and Atmospheric Pressure from Water and Nitrogen on a Carbon-Nanotube-Based Electrocatalyst.

Ammonia is synthesized directly from water and N2 at room temperature and atmospheric pressure in a flow electrochemical cell operating in gas phase (half-cell for the NH3 synthesis). Iron supported on carbon nanotubes (CNTs) was used as the electrocatalyst in this half-cell. A rate of ammonia formation of 2.2×10-3  gNH3  m-2  h-1 was obtained at room temperature and atmospheric pressure in a flow of N2 , with stable behavior for at least 60 h of reaction, under an applied potential of -2.0 V. This value is higher than the rate of ammonia formation obtained using noble metals (Ru/C) under comparable reaction conditions. Furthermore, hydrogen gas with a total Faraday efficiency as high as 95.1 % was obtained. Data also indicate that the active sites in NH3 electrocatalytic synthesis may be associated to specific carbon sites formed at the interface between iron particles and CNT and able to activate N2 , making it more reactive towards hydrogenation.

CO2 utilization: an enabling element to move to a resource- and energy-efficient chemical and fuel production.

CO2 conversion will be at the core of the future of low-carbon chemical and energy industry. This review gives a glimpse into the possibilities in this field by discussing (i) CO2 circular economy and its impact on the chemical and energy value chain, (ii) the role of CO2 in a future scenario of chemical industry, (iii) new routes for CO2 utilization, including emerging biotechnology routes, (iv) the technology roadmap for CO2 chemical utilization, (v) the introduction of renewable energy in the chemical production chain through CO2 utilization, and (vi) CO2 as a suitable C-source to move to a low-carbon chemical industry, discussing in particular syngas and light olefin production from CO2. There are thus many stimulating possibilities offered by using CO2 and this review shows this new perspective on CO2 at the industrial, societal and scientific levels.

H2 production by selective photo-dehydrogenation of ethanol in gas and liquid phase on CuOx/TiO2 nanocomposites

CuOx/TiO2 nanocomposites prepared by copper photodeposition (1.0 and 2.5 wt% copper loading) on TiO2 (synthesized by three different routes) are studied in the ethanol photo-dehydrogenation in gas- and liquid-phase operations, and characterized in terms of surface area, phase composition by XRD, morphology and copper-oxide nanoparticle size distribution, and copper species by UV-visible diffuse reflectance spectroscopy. Cu2+ ions partially enter into the titania structure leading to the creation of oxygen vacancies responsible for the shift in the band gap, but also the creation of traps for photogenerated holes and electrons. While the band gap shifts to lower energies with the copper content, a maximum photocatalytic activity is shown for the intermediate copper loading. Gas-phase operations allow a higher H2 productivity with respect to liquid-phase operations, and especially a higher selectivity (about 92–93%) to acetaldehyde. It is remarked that the route of photo-dehydrogenation of ethanol to H2 and acetaldehyde has an economic value about 3.0–3.5 times higher than the alternative route of photoreforming to produce H2. Gas-phase operations would be preferable for the photo-dehydrogenation of ethanol.

The use of a solar photoelectrochemical reactor for sustainable production of energy

The conversion of solar energy into H2 via water splitting process is one of the most attractive ways to obtain clean and renewable energy. Unfortunately, the fast back reaction of recombination and high band gap needed to activate the photo-catalytic materials, strongly limit the performances in conventional slurry photo-reactors. In this context we present a new photoelectrochemical approach with a double-chamber reactor configuration for H2 production by water photo-electrolysis. The core of the photo-system is a membrane electrode assembly consisting of different layers which hold distinct two areas of the reactor where the generation of O2 and H2 occurs separately. Particular attention is given to the development, on a nano-scale level, of the materials to be used as photoanode and electrocathode: nanostructured TiO2 arrays and carbon nanotubes are used respectively in the form of thin films separated by a proton conductive membrane. Results showed 3.2 mmol h−1 g−1 of H2 evolution that is about one order of magnitude higher with respect to the activity obtained with conventional slurry photoreactors. Moreover, we present the opportunity to recycle CO2 back to liquid fuels by using the same photoelectrochemical approach.

Carbon-based catalysts：Opening new scenario to develop next-generation nano-engineered catalytic materials

This essay analyses some of the recent development in nanocarbons (carbon materials having a defined and controlled nano-scale dimension and functional properties which strongly depend on their nano-scale features and architecture), with reference to their use as advanced catalytic materials. It is remarked how their features open new possibilities for catalysis and that they represent a new class of catalytic materials. Although carbon is used from long time in catalysis as support and electrocatalytic applications, nanocarbons offer unconventional ways for their utilization and to address some of the new challenges deriving from moving to a more sustainable future. This essay comments how nanocarbons are a key element to develop next-generation catalytic materials, but remarking that this goal requires overcoming some of the actual limits in current research. Some aspects are discussed to give a glimpse on new directions and needs for R&D to progress in this direction.

Mechanism of C–C bond formation in the electrocatalytic reduction of CO2 to acetic acid. A challenging reaction to use renewable energy with chemistry

Copper nanoparticles on carbon nanotubes are used in the reduction of CO2 to acetic acid (with simultaneous water electrolysis) in a flow electrocatalytic reactor operating at room temperature and atmospheric pressure. A turnover frequency of about 7000 h−1 and a carbon-based Faradaic selectivity to acetic acid of about 56% were observed, indicating potential interest in this approach for using renewable energy. The only other products of reaction detected were formic acid and methanol (the latter in some cases), besides H2. The reaction mechanism, particularly the critical step of C–C bond formation, was studied by comparing the reactivity in tests with CO2 or CO, where formic acid or formaldehyde where initially added. The results indicate the need for having dissolved CO2 to form acetic acid, likely via the reaction of CO2˙− with surface adsorbed –CH3 like species. The pathway towards formic acid is instead different from the route of the formation of acetic acid.

Nanoscale Engineering in the Development of Photoelectrocatalytic Cells for Producing Solar Fuels

Engineering at the nanoscale level is a key aspect for the design of novel devices for sustainable energy to address the changeover from fossil fuels to renewable energy sources. This perspective paper, after introducing this topic, analyses the design and development of photoelectrocatalytic cells for producing solar fuels. To overcome limitations in the design of photoelectrocatalytic cells, a different one is proposed which eliminates the need of having a liquid electrolyte, where the electrodes are immersed. This cell design requires specific characteristics in the related electrodes/materials, which in combination with the different operation conditions, determine the need to investigate new fundamental aspects in the area. Some of the aspects analyzed regard (i) the role of nanostructure for visible light absorption of the semiconductor used, (ii) the need to use catalytic concepts (photoelectro-catalysis rather than photoelectro-chemistry), (iii) the mobility of charge carriers and relation with electrode characteristics, and (iv) space charge and Helmholtz layer.

The integration of an ultraviolet-visible spectrometer and a reaction calorimeter

A small ultraviolet-visible absorption spectrometer which uses fibre optic coupled immersion probes has been incorporated into a laboratory scale reaction calorimeter. The combined instrument has been tried out using the hydrolysis of acetic anhydride as a test reaction. With the calorimeter operating in the isoperibolic mode good agreement is found for the pseudo-first order reaction rate constant as determined from spectroscopic and calorimetric measurements. Experiments have been made in order to follow the reaction indirectly using optical pH measurements with acid-base indicators. The possibility of determining the temperature dependence of the rate constant in a single experiment has also been investigated.