Nano-photocatalysts in microfluidics, energy conversion and environmental applications

Extensive studies have been carried out on photocatalytic materials in recent years as photocatalytic reactions offer a promising solution for solar energy conversion and environmental remediation. Currently available commercial photocatalysts still lack efficiency and thus are economically not viable for replacing traditional sources of energy. This article focuses on recent developments in novel nano-photocatalyst materials to enhance photocatalytic activity. Recent reports on optofluidic systems, new synthesis of photocatalytic composite materials and motile photocatalysts are discussed in this article.Postprint (published version

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.Postprint (published version

Mechanochemically activated Au/CeO2 for enhanced CO oxidation and COPrOx reaction

COPrOx reaction is one of the particularly appealing and cost-effective solutions for the selective oxidation of CO in hydrogen-rich gas streams to meet the stringent requirement of the current electrocatalysts employed in low-temperature fuel cells. Herein, we synthesized ultrasmall Au clusters supported on ceria by one-step solvent-free mechanochemical method. These catalysts exhibited higher activity for COPrOx and CO oxidation compared to the sample prepared by conventional incipient wetness impregnation. The unique Au-Ce interaction caused by impact and friction between the ball, vessel, and powders, greatly promotes the generation of positive charged Aud+ active sites and, at the same time, the reducibility of the catalyst. Interestingly, an aging treatment of the ball-milled samples resulted in a significant superior performance of the catalytic activity. This enhancement has been attributed to a change in the oxidation state of Au between the fresh and the aged catalysts prepared by ball milling.Peer ReviewedPostprint (published version

CO oxidation and COPrOx over preformed Au nanoparticles supported over nanoshaped CeO2

Au/CeO2 (0.25% wt. Au) catalysts were prepared by anchoring preformed Au nanoparticles over ceria polycrystals, cubes and rods and tested in the oxidation of CO and COPrOx. The use of preformed Au nanoparticles assured a constant Au particle size (ca. 5 nm by HRTEM) for all samples, which allowed to a precise assessment of the effect of the morphology of nanoshaped ceria on catalytic activity. The catalytic performance of the Au/CeO2-rods was much better than that of the Au/CeO2-polycrystals and Au/CeO2-cubes both in the oxidation of CO and COPrOx reactions. The Au/CeO2-rods exhibited the highest amount of oxidized Au and Ce(III) species by XPS, whereas in the Au/CeO2-cubes gold was totally metallic and the amount of Ce(III) was minimum. An intermediate situation was encountered in the Au/CeO2-polycrystals. Considering the differences in the oxidation states ofAu and Ce and the factthat all samples were prepared with preformed metallic Au nanoparticles of the same size, the results indicate that the intrinsic nature of the different ceria surfaces exerts a prominent role in the gold-ceria interaction and in the electron density transfer from Au to Ce, which in turn has a strong effect on catalytic activity. Gold nanoparticles strongly interact with CeO2-{1 1 0} surfaces with respect to CeO2-{1 1 1} and CeO2-{1 0 0}, even when Au nanoparticles are prepared separately and simply deposited by impregnation.Postprint (author's final draft

Effect of temperature on the gas-phase photocatalytic H2 generation using microreactors under UVA and sunlight irradiation

The effect of temperature on the photocatalytic hydrogen generation from a gaseous water-ethanol mixture has been tested in a silicone microreactor containing nine microchannels of 500 µm (width) × 1 mm (depth) × 47 mm (length) coated with Au/TiO2photocatalyst under UVA irradiation. Kinetic analyses have indicated that the hydrogen production rate follows the Langmuir-Hinshelwood model. The effect of temperature from 298 to 348 K has been determined by thermodynamic parameters, such as enthalpy (¿H¿), entropy (¿S¿) and Gibbs free energy (¿G¿) of activation, using the transition state theory (TST). The apparent rate constants (kapp) are higher by increasing the temperature, and the activation energy has been determined to be 24 ± 1 kJ·mol-1. In order to evaluate if solar concentration could be used to enhance the photoproduction of hydrogen, the reaction has also been conducted under direct sunlight using a solar concentrator of about 1 m in diameter. Finally, the microreactor has been scaled out by a factor of ca. 10 to a device containing thirty-two microchannels of 500 µm (width) × 1 mm (depth) × 117.5 mm (length). The specific (i.e. per irradiated area of catalyst) hydrogen production rates of both microreactors using sunlight are very similar suggesting that this technology could lead to viable solar hydrogen production.Postprint (author's final draft

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·g-1 cat·h-1 (7.4 mmol·g-1 TiO2 ·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.Postprint (published version

Enhanced photoproduction of hydrogen on Pd/TiO2 prepared by mechanochemistry

Supported metal clusters are considered as promising cocatalysts in heterogeneous photocatalysis due to their singular geometric structures and unique reactivity. Nevertheless, to explore efficient synthetic routes that result in stable supported clusters with tailored active sites is an urgent yet challenging task. Here, a photocatalyst with highly dispersed Pd clusters onto TiO2 is synthesized through only one-step ball milling procedure. The obtained Pd clusters form a particular metal-support interface, which has the ability to rearrange the small clusters evolving into Pd nanoparticles during the photocatalytic H2 production process, and maintain a stable photocatalytic performance up to 100 h of continuous operation. Moreover, the unique interaction between Pd clusters and titania support was only observed in the ball-milled sample, and it disappeared after a calcination treatment. The mechanochemical strategy paves the way to stabilize supported metal clusters onto semiconductors without any organic compounds involved.Postprint (published version

Surface roughness-induced speed increase for active Janus micromotors

We demonstrate a simple physical fabrication method to control surface roughness of Janus micromotors and fabricate self-propelled active Janus microparticles with rough catalytic platinum surfaces that show a four-fold increase in their propulsion speed compared to conventional Janus particles coated with a smooth Pt layer.Postprint (published version

Unraveling the chemical state of cobalt in Co-based catalysts during Ethanol steam reforming: an in situ study by near ambient pressure XPS and XANES

Peer ReviewedPostprint (author's final draft

Facet-engineered TiO2 drives photocatalytic activity and stability of supported noble metal clusters during H2 evolution

Nanoparticles; PhotocatalysisNanopartículas; FotocatálisisNanopartícules; FotocatàlisiMetal clusters supported on TiO2 are widely used in many photocatalytic applications, including pollution control and production of solar fuels. Besides high photoactivity, stability during the photoreaction is another essential quality of high-performance photocatalysts, however systematic studies on this attribute are absent for metal clusters supported on TiO2. Here we have studied, both experimentally and with first-principles simulation methods, the stability of Pt, Pd and Au clusters prepared by ball milling on nanoshaped anatase nanoparticles preferentially exposing {001} (plates) and {101} (bipyramids) facets during the photogeneration of hydrogen. It is found that Pt/TiO2 exhibits superior stability than Pd/TiO2 and Au/TiO2, and that {001} facet-based photocatalysts always are more stable than their {101} analogous regardless of the considered metal species. The loss of stability associated with cluster sintering, which is facilitated by the transfer of photoexcited carriers from the metal species to the neighbouring Ti and O atoms, most significantly and detrimentally affects the H2-evolution photoactivity.This work was supported by projects MICINN/FEDER PID2021-124572OB-C31 and GC 2021 SGR 01061. Y.C. (CSC No. 201806920042) acknowledges the China Scholarship Council for Ph.D. scholarship support. L.S. and C.C. are grateful to MICINN Ramon y Cajal program for individual fellowship grant agreements RYC2019-026704-I and RYC2018-024947-I, respectively. J.L. is a Serra Húnter Fellow and is grateful to ICREA Academia program. N.G.B. and V.P. acknowledge financial support from RTI2018-099965-B-I00, AEI/FEDER,UE and 2017-SGR-1431. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. The authors thank E. Molins, I. Matas and M. Benito from ICMAB to kindly analyze BET areas