Hicham Idriss

The effect of gold loading and particle size on photocatalytic hydrogen production from ethanol over Au/TiO2 nanoparticles

Catalytic hydrogen production from renewables is a promising method for providing energy carriers in the near future. Photocatalysts capable of promoting this reaction are often composed of noble metal nanoparticles deposited on a semiconductor. The most promising semiconductor at present is TiO₂. The successful design of these catalysts relies on a thorough understanding of the role of the noble metal particle size and the TiO₂ polymorph. Here we demonstrate that Au particles in the size range 3-30 nm on TiO₂ are very active in hydrogen production from ethanol. It was found that Au particles of similar size on anatase nanoparticles delivered a rate two orders of magnitude higher than that recorded for Au on rutile nanoparticles. Surprisingly, it was also found that Au particle size does not affect the photoreaction rate over the 3-12 nm range. The high hydrogen yield observed makes these catalysts promising materials for solar conversion.

Hydrogen production by ethanol reforming over Rh/CeO2-ZrO2 catalysts

Abstract Reforming of ethanol in excess of water (1–8 molar ratio) has been investigated on Rh/CeO 2 , Rh/ZrO 2 and Rh/CeO 2 –ZrO 2 (Ce/Zr=4, 2 and 1). Catalysts characterization was conducted by X-ray diffraction, BET surface area measurements, CO 2 adsorption, and temperature programmed reduction (TPR). At 400–500 °C all catalysts showed high activity and selectivity towards hydrogen production (between 5 and 5.7 mol of H 2 per mol of ethanol inlet) despite the considerable textural differences of the oxides (fluorite, monoclinic and tetragonal). The large variations of Rh dispersion (as monitored by TPR) between all catalysts had a small effect on H 2 production. Although it appears that the reaction is not sensitive to either the oxide or the metal structure Rh/CeO 2 (the most basic catalyst investigated) was the least reactive.

Principles and mechanisms of photocatalytic dye degradation on TiO2 based photocatalysts: a comparative overview

The total annual production of synthetic dye is more than 7 × 105 tons. Annually, through only textile waste effluents, around one thousand tons of non-biodegradable textile dyes are discharged into natural streams and water bodies. Therefore, with growing environmental concerns and environmental awareness there is a need for the removal of dyes from local and industrial water effluents with a cost effective technology. In general, these dyes have been found to be resistant to biological as well as physical treatment technologies. In this regard, heterogeneous advanced oxidation processes (AOPs), involving photo-catalyzed degradation of dyes using semiconductor nanoparticles is considered as an efficient cure for dye pollution. In the last two decades TiO2 has received considerable interest because of its high potential as a photocatalyst to degrade a wide range of organic material including dyes. This review starts with (i) a brief overview on dye pollution, dye classification and dye decolourization/degradation strategies; (ii) focuses on the mechanisms involved in comparatively well understood TiO2 photocatalysts and (iii) discusses recent advancements to enhance TiO2 photocatalytic efficiency by (a) doping with metals, non-metals, transition metals, noble metals and lanthanide ions, (b) structural modifications of TiO2 and (c) immobilization of TiO2 by using various supports to make it a flexible and cost-effective commercial dye treatment technology.

Combined infrared spectroscopy, chemical trapping, and thermoprogrammed desorption studies of methanol adsorption and decomposition on ZnAl2O4 and Cu/ZnAl2O4 catalysts

Abstract It is shown that FT-IR spectroscopy permits discrimination to be made between methoxy (methanol) and formate species adsorbed on ZnAl 2 O 4 and CuZnAl 2 O 4 catalysts. These species were found to be less stable on copper than on ZnAl 2 O 4 . The presence of reduced copper promotes methanol transformation into formates and then into C0 2 : (i) FT-IR results show that copper formate formation from methanol adsorption occurs even at room temperature and that surface oxygen ion participates in its formation; (ii) chemical trapping experiments demonstrate that increasing copper percentage destabilizes formate species, while TPD experiments correlatively indicate an accelerated transformation of formate into CO 2 . Formyl species are detected by chemical trapping only at the end of the reaction and are therefore assumed not to participate in the decomposition reaction.

A Phenomenological Study of the Metal-Oxide Interface : The Role of Catalysis in Hydrogen Production from Renewable Resources

The truth about Cats: The metal-oxide interface of a Pd-Rh/CeO{sub 2} catalyst was studied in the context of developing active, selective and durable solid catalytic materials for the production of hydrogen from renewables. The presence of a stable contact between finely dispersed transition-metal clusters (Pd and Rh) on the nanoparticles of the CeO{sub 2} support leads to a highly active and stable catalyst for the steam reforming of ethanol.

Hydrogen production by tuning the photonic band gap with the electronic band gap of TiO2

Tuning the photonic band gap (PBG) to the electronic band gap (EBG) of Au/TiO2 catalysts resulted in considerable enhancement of the photocatalytic water splitting to hydrogen under direct sunlight. Au/TiO2 (PBG-357 nm) photocatalyst exhibited superior photocatalytic performance under both UV and sunlight compared to the Au/TiO2 (PBG-585 nm) photocatalyst and both are higher than Au/TiO2 without the 3 dimensionally ordered macro-porous structure materials. The very high photocatalytic activity is attributed to suppression of a fraction of electron-hole recombination route due to the co-incidence of the PBG with the EBG of TiO2 These materials that maintain their activity with very small amount of sacrificial agents (down to 0.5 vol.% of ethanol) are poised to find direct applications because of their high activity, low cost of the process, simplicity and stability.

Active sites on oxides: From single crystals to catalysts

Abstract Metal oxides are ubiquitous in heterogeneous catalysis, serving as catalysts, as catalyst supports, and as modifiers and promoters, among other roles. The surface science approach to understanding the structure and reactivity of metal oxide surfaces has blossomed during the preceding decade. We consider here important concepts drawn from catalysis by metal oxides and their connections to reaction pathways and principles of oxide surface reactivity revealed by experimental and theoretical studies of well-defined oxide surfaces. The applications of reaction mechanisms and site requirements from surface science studies of carboxylic acid chemistry, in particular, to emerging catalysts for dehydration, coupling, and reduction reactions provide important examples of contributions to heterogeneous catalysis from oxide surface science. We also consider relationships between oxide surface reactivity and the physical and electronic properties of oxides as represented by characteristics such as electronegativity, bond energies, ionicity, Madelung potential, and polarizability. This analysis highlights the need for reaction data on well-defined single-crystal surfaces in order to distinguish structural and electronic effects when comparing patterns of catalytic activity and selectivity among different metal oxides. This review of the connetions between metal oxide single-crystal surfaces and high surface area catalysts (or their components) demonstrates the potential of surface science approaches to elucidate the chemical and physical bases for catalysis by metal oxides, in pursuit of the goal of catalyst design.

Fast and efficient hydrogen generation catalyzed by cobalt talc nanolayers dispersed in silica aerogel

Cobalt talc nanolayers dispersed in silica aerogel constitute an active nanocomposited material with outstanding catalytic properties for the generation of hydrogen by ethanol steam reforming at low temperature. Delamination of talc particles into individual nanolayers of ca. 1.8 nm thick readily occurs under the reforming conditions, which results in a strong enhancement of the exposed catalytic active area. The presence of aerogel assures the immobilization of the talc nanolayers resulting from the delamination while maintaining an excellent mass transfer of products and reactants to the surface of the talc nanolayers. A fast and reversible surface activation for the steam reforming of ethanol occurs at 580–590 K under the reforming conditions. Magnetic characterization and in situ X-ray photoelectron spectroscopy (XPS) show the non-reversible formation of metal cobalt ensembles during activation, which escape HRTEM detection. This material appears as a good candidate for on-board hydrogen generation from ethanol–water mixtures for mobile and portable fuel cell applications.

The photoreaction of TiO2 and Au/TiO2 single crystal and powder surfaces with organic adsorbates. Emphasis on hydrogen production from renewables

TiO2 and TiO2-based materials are prototypes for photo-catalytic reactions as they have been shown for many decades to be active for total oxidation of hydrocarbons to clean the environment. In the last decade or so there has been a shift in the objectives for photoreactions mainly towards hydrogen production from renewables. Here we review the fundamentals behind the reactivity of model TiO2 surfaces with simple organic compounds in the dark and under photo-irradiation, then we consider the case of ethanol photo-reaction to hydrogen at its fundamental and applied levels. The review starts with an overview of the bulk and surface structures of rutile TiO2, as it is the most studied phase in surface science despite its lower activity for hydrogen production compared to the anatase phase. We then focus on the gold/TiO2 system where both phases (anatase and rutile) of TiO2 have received more attention. In the gold/TiO2 system emphasis is mainly devoted to understanding the effect of particle dimensions on the electronic conduction as well as the oxidation state of Au particles. The most recent observation using environmental X-ray Photoelectron Spectroscopy indicated the absence of charge transfer from the support to the metal. The photoreaction of ethanol is presented in more detail. The rate for hydrogen production on Au/TiO2 catalysts does not change if TiO2 particles are of macro-size or nano-size once normalised by surface area. Also, it was clearly seen that Au/TiO2 anatase is about two orders of magnitudes higher compared to a similar system where TiO2 is in the rutile phase. The mechanism for hydrogen production is presented and discussed on the metal/semiconductor systems. Of particular interest is the observation of synergistic effect between the anatase and rutile nanoparticles and one explanation involving electron transfer from one phase to the other is invoked.

Characterization of dioxymethylene species over cu-zn catalysts

Abstract Dioxymethylenic species have been characterized upon adsorption of formaldehyde on zinc aluminate and copper supported on zinc aluminate, by both FTIR spectroscopy and chemical trapping. Dioxymethylene could be an intermediate in the synthesis of methanol on these catalysts in CO + H 2 and CO 2 + H 2 reactions, as shown by chemical trapping experiments.