José Luis G. Fierro

Structure and Reactivity of Perovskite-Type Oxides

Publisher Summary This chapter discusses the structure and reactivity of perovskite-type oxides. Perovskite-type oxides have the general formula ABO 3 (A, cation of larger size) and are structurally similar to CaTiO 3 , the mineral that gave its name to that group of compounds. These materials are first studied because of their important physical properties such as ferro-, piezo-, and pyroelectricity, magnetism and electrooptic effects. The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure. The chapter reviews only the study of oxides and their behavior in the gas solid interface and in heterogeneous catalysis. An important characteristic of perovskites, mentioned in the chapter, is their susceptibility of partial substitution in both A and B positions. This provides a wealth of isomorphic compounds that can easily be synthesized. Given the extensive range of possibilities in the tailoring of their chemical and physical properties, there is no doubt that new reactions can be studied, where these oxides can participate as catalytic agents.

Water Splitting on Semiconductor Catalysts under Visible-Light Irradiation

Sustainable hydrogen production is a key target for the development of alternative, future energy systems that will provide a clean and affordable energy supply. The Sun is a source of silent and precious energy that is distributed fairly all over the Earth daily. However, its tremendous potential as a clean, safe, and economical energy source cannot be exploited unless the energy is accumulated or converted into more useful forms. The conversion of solar energy into hydrogen via the water-splitting process, assisted by photo-semiconductor catalysts, is one of the most promising technologies for the future because large quantities of hydrogen can potentially be generated in a clean and sustainable manner. This Minireview provides an overview of the principles, approaches, and research progress on solar hydrogen production via the water-splitting reaction on photo-semiconductor catalysts. It presents a survey of the advances made over the last decades in the development of catalysts for photochemical water splitting under visible-light irradiation. The Minireview also analyzes the energy requirements and main factors that determine the activity of photocatalysts in the conversion of water into hydrogen and oxygen using sunlight. Remarkable progress has been made since the pioneering work by Fujishima and Honda in 1972, but he development of photocatalysts with improved efficiencies for hydrogen production from water using solar energy still faces major challenges. Research strategies and approaches adopted in the search for active and efficient photocatalysts, for example through new materials and synthesis methods, are presented and analyzed.

Retracted article: Towards near zero-sulfur liquid fuels: a perspective review

We, the named authors, hereby wholly retract this Catalysis Science & Technology article, due to significant similarity with previously published work. Signed: B. Pawelec, R. M. Navarro, J. M. Campos-Martin and J. L. G. Fierro, October 2012. Retraction endorsed by Jamie Humphrey, Editor, Catalysis Science & Technology.

A framework for visible-light water splitting

This review article reports the most significant advances made in H2 production via water-splitting and the challenges that need to be addressed over the coming years to verify the feasibility of H2 production by both inorganic semiconductors and living microorganisms as a competitive process in the hydrogen economy.

Delamination of Layered Covalent Organic Frameworks

Nanomaterials, understood as the study of materials having at least one dimension on the nanometer scale, has been a fruitful area of research during the past few years. [ 1 ] The main reason for this increasing interest is that, at this small scale, qualitative changes in physicochemical properties such as surface plasmon resonance in metal nanoparticles, quantum confi nement in semiconductor particles, and superparamagnetism in magnetic nanomaterials, are connected to the number of atoms or molecules forming the material. [ 2 ]

Production of hydrogen from methanol over binary Cu/ZnO catalysts - Part II. Catalytic activity and reaction pathways

The activity for conversion of methanol into hydrogen was investigated over binary Cu/ZnO catalysts derived from precursors prepared by two different techniques, viz. oxalates formed in microemulsion and hydroxycarbonates formed in aqueous solution. Some distinct differences in the reaction pathways were observed. During partial oxidation of methanol under a sub-stoichiometric oxygen/methanol ratio, the microemulsion materials exhibited considerably higher combustion activity in the low-temperature region than a catalyst prepared in aqueous solution. Over the former, oxygen was quickly converted by methanol combustion, after which steam reforming was initiated, producing hydrogen at the expense of water and gradually decreasing the net heat of reaction. Hence, a reaction sequence for the partial oxidation reaction over microemulsion catalysts is proposed, consisting of consecutive methanol combustion and steam reforming, followed by decomposition when all oxygen has been consumed. Over the hydroxycarbonate catalyst, the reaction ignited at a higher temperature, directly producing hydrogen by partial oxidation of methanol. When the two types of catalysts were evaluated in the steam reforming reaction, all catalysts displayed the typical S-shaped dependence of methanol conversion on temperature. However, there was a downward shift in the temperature at which methanol reached complete conversion, favouring the hydroxycarbonate material. Hydrogen was produced selectively over all catalysts, but carbon monoxide formation was more pronounced over the microemulsion materials. The differences in catalytic behaviour are discussed in terms of catalyst morphology and the valence state of Cu in the working catalyst.

Catalytic combustion of methane over cerium-doped palladium catalysts

Various Pd-supported catalysts have been prepared using three different types of alumina as support material: (a) gamma-alumina, (b) Ba-stabilized alumina, and

Hydrogen peroxide decomposition over Ln1−xAxMnO3 (Ln = La or Nd and A = K or Sr) perovskites

The following perovskites La1−xKxMnO3 (x=0–0.15), La1−xSrxMnO3 (x=0–0.3), Nd1−xSrxMnO3 (x=0–0.4) and Nd1−xKxMnO3 (x=0–0.15) have been prepared by the freeze-drying method. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy spectra (XPS) analyses showed the presence of the single crystalline perovskite phases with an enrichment of the surface with potassium for the Ln1−xKxMnO3 samples. δ-Oxygen non-stoichiometries of the perovskites were studied by chemical analyses, temperature programmed reduction (TPR) and oxygen desorption (TPD) experiments. The results obtained suggest that the catalytic H2O2 decomposition over these perovskites is not determined by the surface K, Ln and Mn concentration nor by the Mn4+/Mn3+ surface ratio. On the other hand, a good correlation between the catalytic H2O2 decomposition and the δ-oxygen non-stoichiometries was found, especially for La perovskites. A suprafacial mechanism where the oxygen vacancies participate in the H2O2 decomposition is suggested.

Mechanistic Aspects of the Ethanol Steam Reforming Reaction for Hydrogen Production on Pt, Ni, and PtNi Catalysts Supported on γ-Al2O3†

Mechanistic aspects of ethanol steam reforming on Pt, Ni, and PtNi catalysts supported on gamma-Al(2)O(3) are investigated from the analysis of adsorbed species and gas phase products formed on catalysts during temperature-programmed desorption of ethanol and during ethanol steam reforming reaction. DRIFTS-MS analyses of ethanol decomposition and ethanol steam reforming reactions show that PtNi and Ni catalysts are more stable than the Pt monometallic counterpart. Ethanol TPD results on Ni, Pt, and NiPt catalysts point to ethanol dehydrogenation and acetaldehyde decomposition as the first reaction pathways of ethanol steam reforming over the studied catalysts. The active sites responsible for the acetaldehyde decomposition are easily deactivated in the first minutes on-stream by carbon deposits. For Ni and PtNi catalysts, a second reaction pathway, consisting in the decomposition of acetate intermediates formed over the surface of alumina support, becomes the main reaction pathway operating in steam reforming of ethanol once the acetaldehyde decomposition pathway is deactivated. Taking into account the differences observed in the mechanism of ethanol decomposition, the better stability observed for PtNi catalyst is proposed to be related with a cooperative effect between Pt and Ni activities together with the enhanced ability of Ni to gasify the methyl groups formed by decomposition of acetate species. On the contrary, monometallic catalysts are believed to dehydrogenate these methyl groups forming coke that leads to deactivation of metal particles.

Production of hydrogen from methanol over binary Cu/ZnO catalysts - Part I. Catalyst preparation and characterisation

Mixed copper-zinc oxide catalysts (Cu/ZnO) were prepared by two different techniques, i.e. from hydroxycarbonate precursors formed in aqueous solution and from oxalate precursors formed in water-in ...