R.M. Navarro

Hydrogen production from renewable sources: biomass and photocatalytic opportunities

The demand for hydrogen over the coming decade is expected to grow for both traditional uses (ammonia, methanol, refinery) and running fuel cells. At least in the near future, this thirst for hydrogen will be quenched primarily through the reforming of fossil fuels. However, reforming fossil fuels emits huge amounts of carbon dioxide. One approach to reduce carbon dioxide emissions, which is considered first in this review, is to apply reforming methods to alternative renewable materials. Such materials might be derived from plant crops, agricultural residues, woody biomass, etc. Clean biomass is a proven source of renewable energy that is already used for generating heat, electricity, and liquid transportation fuels. Clean biomass and biomass-derived precursors such as ethanol and sugars are appropriate precursors for producing hydrogen through different conversion strategies. Virtually no net greenhouse gas emissions result because a natural cycle is maintained, in which carbon is extracted from the atmosphere during plant growth and released during hydrogen production. The second option explored here is hydrogen production from water splitting by means of the photons in the visible spectrum. The sun provides silent and precious energy that is distributed fairly evenly all over the earth. However, its tremendous potential as a clean, safe and economical energy source cannot be exploited unless it is accumulated or converted into more useful forms of energy. Finally, this review discusses the use of semiconductors, more specifically CdS and CdS-based semiconductors, which are able to absorb photons in the visible region of the spectrum. The energy stored within a semiconductor as electronic energy (electrons and holes) can be used to split water molecules by simultaneous reactions into H2 and O2. This conversion of solar energy into a clean fuel (H2) is perhaps the greatest challenge for scientists in the 21st century.

Retracted article: Towards near zero-sulfur liquid fuels: a perspective review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

Photocatalytic Water Splitting Under Visible Light: Concept and Catalysts Development

Sustainable hydrogen production is a key target in the development of alternative energy systems of the future for providing a clean and affordable energy supply. The conversion of solar energy into hydrogen via a water-splitting process assisted by photosemiconductor 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. Undoubtedly, the conversion of solar energy into a clean fuel (H 2 ) under ambient conditions is the greatest challenge facing scientists in the twenty-first century. This chapter provides an overview of the principles, experimental designs, and research progress on solar-hydrogen production via the water-splitting reaction on photocatalyst surfaces. The concept of using solar energy to drive the conversion of water into hydrogen and oxygen is examined from the standpoint of both energy requirements and factors that determine the activity of photocatalysts. A survey is presented of the advances made in the development of catalysts for photochemical water splitting under visible light since the pioneering work by Fujishima and Honda in 1972. Photocatalysts for water splitting under ultraviolet light have made remarkable progress in recent years, but there are many technical challenges, mainly the low efficiency in light-to-hydrogen conversion, still facing photocatalysts under visible light. There are still major challenges in the development of photocatalysts with improved efficiencies for hydrogen production from water using solar energy. An overview is provided in this chapter about research strategies and approaches adopted in the search for photocatalysts for water splitting under visible light (new photocatalyst materials and the control of the synthesis of materials for customizing the crystallinity, electronic structure, and morphology of catalysts at nanometric scale).

Dibenzothiophene hydrodesulfurization on silica-alumina-supported transition metal sulfide catalysts

Unpromoted and Pt, Pd, Ni and Ru-promoted molybdenum catalysts supported on amorphous silica-alumina (ASA) were characterized in the hydrodesulfurization of dibenzothiophene, and their activities were compared with a commercial PtASA catalyst. The catalytic activity was found to increase in the following order: commercial PtASA ⪢ PtMo ≈ NiMo ≈ RuMo > PdMo ≈ Mo ≈ ASA. Characterization of the catalysts by XPS, TPR, and FTIR spectroscopy of adsorbed NO and pyridine confirmed that incorporation of 16.1 wt.-% Mo, minimized to a large extent the metal-support interaction, and decreased the acidity of the catalysts; although for the Ni-catalyst, the formation of Ni aluminate (and silicate) was not prevented. The incorporation of promoters enhanced molybdenum surface exposure and decreased the reduction temperature of MoO3; and for the Ni-promoted catalyst, increased the amount of sulfidable Ni species. Pd was not very effective as a promoter due to its poor dispersion, and the presence of a low percentage of active sites in the PdMo catalyst. For binary samples, a correlation was found to exist between the dispersion of MoS2 phases, the number of active sites titrated by NO, and the HDS activity of the sulfided samples.

DEEP HYDRODESULFURIZATION OF DBT AND DIESEL FUEL ON SUPPORTED PT AND IR CATALYSTS

Abstract Deep hydrodesulfurization (HDS) of dibenzothiophene (DBT) and diesel fuel (0.08 wt.−% S) has been carried out on Pt and Ir supported on amorphous silica-alumina (ASA) and on a stabilized HY zeolite under standard industrial conditions. The effect of temperature, two different supports and feedstocks on HDS and hydrogenation (HYD) product selectivities are investigated. Normalized activity data indicate that in the HDS of DBT and of diesel fuel, the platinum catalysts are much more active than the iridium counterparts. In HDS of diesel fuel (at 623 K), both Pt HY and Pt ASA are a little more active than a commercial Co-Mo Al 2 O 3 catalyst. The normalized activity in the HDS of DBT (593 K) increases in the following order: Pt HY > Pt ASA ⪢ Ir HY > Ir ASA , and in the HDS of diesel fuel (623 K), increases according to: Pt HY ⪢ Pt ASA ⪢ Ir HY > Ir ASA . All spent catalysts were characterized by X-ray photoelectron spectroscopy (XPS). In view of their superior performance, both platinum catalysts were characterized by FTIR spectroscopy of CO probe.

Silica–alumina-supported transition metal sulphide catalysts for deep hydrodesulphurization

Abstract Deep hydrodesulphurization (HDS) of dibenzothiophene (DBT) and gas-oil has been carried out on amorphous-silica–alumina (ASA)-supported transition metal sulphides (TMS) under conditions which approach industrial practice. The activity and selectivity of the binary Ni-, Ru- and Pd-promoted Mo catalysts were compared with the monometallic ones (Ru, Ir, Pd, Ni, Mo on ASA). For both HDS of DBT and gas-oil, the observed activity trends were similar; thus, all catalysts were more active with model feed than with gas-oil, and less active than commercial CoMo/Al 2 O 3 . The binary catalysts showed larger activity than monometallic ones, with Ni–Mo catalyst being more effective than Ru–Mo or Pd–Mo. For Ni–Mo sample, the X-ray photoelectron and temperature-programmed reduction techniques confirmed that incorporation of Mo minimises metal–support interaction, although the formation of nickel hydrosilicate was not prevented. The consecutive impregnation of calcined Mo/ASA catalyst with precursor solution followed by calcination enhances molybdenum surface exposure in binary samples. As a consequence, the temperature of reduction of MoO 3 to molybdenum suboxides is decreased.

Dibenzothiophene hydrodesulfurization on HY-zeolite-supported transition metal sulfide catalysts

Noble and semi-noble metals (Pt, Pd, Ru, Ir, Ni) were incorporated on a HY zeolite support, and tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). Under steady-state conditions, the intrinsic activity of the catalysts was found to display the following order of reactivity: Ir>Pt>Pd≫Ni. While deactivation on Ir, Pt and Pd catalysts was very low, the significant differences in the activity of Ni and Ru zeolites could be attributed to catalyst deactivation. Temperature-programmed reduction studies revealed that noble metals were reduced at moderately low temperatures, whereas Ni catalyst was much more difficult to reduce as a consequence of the much stronger interaction with the zeolite substrate. For the most active Pt/HY and Ir/HY zeolites, photoelectron spectra of the used catalysts revealed that Pt is in metallic state (100% Pt°) and Ir is present as a mixture of metal (62% Ir°) and sulfide.

Studies of molybdenum sulfide catalyst ex ammonium tetrathiomolybdate: effect of pretreatment on hydrodesulfurization of dibenzothiophene

Alumina-supported molybdenum sulfide catalyst was prepared by the decomposition of ammonium tetrathiomolybdate (ATTM) and tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The HDS activity of unpromoted and Co-promoted molybdenum sulfide catalysts was found to depend strongly on the activation procedure of the catalyst. In all cases, the activity of the Co-promoted catalyst was superior to that of the ATTM counterpart. Surface characterization of the catalyst subjected to different pretreatments was carried out by X-ray photoelectron spectroscopy and FTIR spectroscopy of NO probe. The pretreatments were effective in obtaining a highly dispersed heterometallic sulfide phase and a good synergy between Co and molybdenum sulfide.

Chiral Pd organometallic complexes as catalysts in cyclopropanation reactions

Asymmetric cyclopropanation of styrene with diazoacetic esters is performed firstly using chiral amino acidato complexes of Pd(II) containing a C,N-cyclometallated group as an ancillary ligand as catalyst precursors. In general these catalytic systems provide good conversion to the corresponding cyclopropyl derivatives with a moderate trans selectivity, although the stereoselectivities obtained were low. With the use of chiral catalysts and chiral diazoacetic esters the diastereoselectivities were slightly improved.