Lluís Soler

Ambient Pressure Photoemission Spectroscopy Reveals the Mechanism of Carbon Soot Oxidation in Ceria-Based Catalysts

Removing soot is one of the most important challenges in minimizing the impact of combustion engines on the environment. Catalysts based on CeO2 have proved suitable to oxidize soot owing to their capacity to store and release oxygen easily while maintaining structural integrity, although their mode of operation in a complex environment involving two solid phases (catalyst and soot) and a gas phase (oxygen) is not yet fully understood. Herein, we provide a study of the surface/subsurface of ceria–soot and ceria–zirconia–soot mixtures under working conditions by means of near‐ambient‐pressure photoelectron spectroscopy. Soot abatement involves two cooperative routes: one occurring at the ceria–soot interface with formation of oxygen vacancies and CeIII and the other at the surface of soot, mediated by active superoxide species, which result from the reaction between gas‐phase O2 and oxygen vacancies. The two routes occur simultaneously and mutually reinforce each other.

Ceria-zirconia particles wrapped in a 2D carbon envelope: improved low-temperature oxygen transfer and oxidation activity

Engineering the interface between different components of heterogeneous catalysts at nanometer level can radically alter their performances. This is particularly true for ceria-based catalysts where the interactions are critical for obtaining materials with enhanced properties. Here we show that mechanical contact achieved by high-energy milling of CeO2–ZrO2 powders and carbon soot results in the formation of a core of oxide particles wrapped in a thin carbon envelope. This 2D nanoscale carbon arrangement greatly increases the number and quality of contact points between the oxide and carbon. Consequently, the temperatures of activation and transfer of the oxygen in ceria are shifted to exceptionally low temperatures and the soot combustion rate is boosted. The study confirms the importance of the redox behavior of ceria-zirconia particles in the mechanism of soot oxidation and shows that the organization of contact points at the nanoscale can significantly modify the reactivity resulting in unexpected properties and functionalities.

Gold(I)-complex–titania hybrid photocatalyst for hydrogen production

The integration of TiO2 with a AuI complex containing a thiocoumarin moiety resulted in a very efficient photocatalyst for the generation of H2. The molecular structure of the complex was preserved under the photoreaction owing to the strong AuI−S bond. The AuI complex played a determinant role in the photogeneration of H2 by accepting the photoinduced electrons originated in TiO2 upon light exposure. This is the first example of a AuI complex semiconductor hybrid photocatalyst. The rate of H2 generation under dynamic conditions from water/ethanol is approximately one order of magnitude superior on a metal basis to that obtained over conventional TiO2 decorated with Au metal nanoparticles.

Preparation and photocatalytic activity of Au/TiO2 lyogels for hydrogen production

High surface area titania lyogels have been prepared, thermally transformed into TiO2 polymorphs at 400–850 °C, and decorated by ball milling with preformed Au nanoparticles of ca. 2 nm to ensure the same contact points at the metal–support interphase. The best performance in the photogeneration of hydrogen from gaseous water–ethanol under dynamic conditions in a fixed bed reactor has been obtained with the lyogel calcined at 550 °C, with a hydrogen photoproduction rate of 19.8 mmol H2 g−1 h−1 under an irradiance of 80 mW cm−2 and GHSV = 26u2006000 h−1. This photocatalyst contains 84% anatase and 16% rutile polymorphs, very similar to the standard P25. A series of photocatalysts prepared by lyophilization of a mixture of the titania gel and the preformed Au nanoparticles and calcination under the same conditions has resulted in samples with Au nanoparticles up to 18 nm strongly interacting with TiO2. The presence of Au nanoparticles in the composite lyogel has strongly retarded the transformation of anatase into rutile. The results have shown that the TiO2 polymorph has a greater influence than the Au nanoparticle size on the photoproduction of hydrogen.

Reusable Photocatalytic Optical Fibers for Underground, Deep‐Sea, and Turbid Water Remediation

Abstract An approach for underground, deep, and turbid water remediation is presented based on optical fibers with a photocatalytic coating. Thus, photocatalytic TiO2 P25 nanoparticles immobilized in a poly(vinylidene difluoride) (PVDF) matrix are coated on polymeric optical fibers (POFs) and the photocatalytic performance of the system is assessed under artificial sunlight. To the best of our knowledge, poly(methyl methacrylate)‐POF coated with TiO2/PVDF and the reusability of any type of POF for photocatalytic applications are not previously reported. The photocatalytic efficiency of the hybrid material in the degradation of ciprofloxacin (CIP) and its reusability are evaluated here. It is shown that 50 w/w% of TiO2 P25 achieves a degradation of 95% after 72 h under artificial sunlight and a reusability of three times leads to a loss of activity inferior to 11%. The efficient removal of ciprofloxacin and the stability of the POF coated with TiO2 P25 successfully demonstrate its suitability in the degradation of pollutants with potential application in regions with low light illumination, as in underground and deep water.

Fast and simple microwave synthesis of TiO2/Au nanoparticles for gas-phase photocatalytic hydrogen generation

The fabrication of small anatase titanium dioxide (TiO2) nanoparticles (NPs) attached to larger anisotropic gold (Au) morphologies by a very fast and simple two-step microwave-assisted synthesis is presented. The TiO2/Au NPs are synthesized using polyvinylpyrrolidone (PVP) as reducing, capping and stabilizing agent through a polyol approach. To optimize the contact between the titania and the gold and facilitate electron transfer, the PVP is removed by calcination at mild temperatures. The nanocatalysts activity is then evaluated in the photocatalytic production of hydrogen from water/ethanol mixtures in gas-phase at ambient temperature. A maximum value of 5.3 mmol·gcat-1·h−1 (7.4 mmol·gTiO2-1·h−1) of hydrogen is recorded for the system with larger gold particles at an optimum calcination temperature of 450°C. Herein we demonstrate that TiO2-based photocatalysts with high Au loading and large Au particle size (≈50 nm) NPs have photocatalytic activity.

Remarkable carbon dioxide hydrogenation to ethanol on a palladium/iron oxide single-atom catalyst

The hydrogenation of CO2 into value‐added chemicals is one of the most investigated methods to reduce CO2 emissions in the atmosphere and thereby contributes to a sustainable chemical industry. Whereas the catalytic hydrogenation of CO2 into methanol and synthetic hydrocarbons is well established, the effective and selective transformation of CO2 into higher alcohols is still challenging. Here, we show that Pd single atoms anchored on the surface of Fe3O4 are very active for the hydrogenation of CO2 to ethanol at 300u2009°C, even at atmospheric pressure. By comparing various Pd/MOx catalysts, we conclude that the metal–oxide interface has a strong influence on catalytic behavior.

Outstanding Methane Oxidation Performance of Palladium-Embedded Ceria Catalysts Prepared by a One-Step Dry Ball-Milling Method

By carefully mixing Pd metal nanoparticles with CeO2 polycrystalline powder under dry conditions, an unpredicted arrangement of the Pd-O-Ce interface is obtained in which an amorphous shell containing palladium species dissolved in ceria is covering a core of CeO2 particles. The robust contact that is generated at the nanoscale, along with mechanical forces generated during mixing, promotes the redox exchange between Pd and CeO2 and creates highly reactive and stable sites constituted by PdOx embedded into CeO2 surface layers. This specific arrangement outperforms conventional Pd/CeO2 reference catalysts in methane oxidation by lowering light-off temperature by more than 50°C and boosting the reaction rate. The origin of the outstanding activity is traced to the structural properties of the interface, modified at the nanoscale by mechanochemical interaction.

Transforming a Compact Disk into a Simple and Cheap Photocatalytic Nanoreactor

A commercial compact disk has been converted into an effective photocatalytic nanoreactor by depositing a catalyst layer inside the nanochannels by means of an electrophoretic method. The resultant device has been tested for water splitting, obtaining a high yield of hydrogen at an unbeatable low cost.