J.L.G. Fierro

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.

Study of chemical activation process of a lignocellulosic material with KOH by XPS and XRD

Abstract Chemical activation of carbons is currently a very common method for obtaining activated carbons with very high surface areas. KOH is one of the most effective agents employed for this purpose. However, the reaction mechanism of this kind of activation it is not yet completely elucidated, although some models have been proposed. In this paper, an activated charcoal was obtained from a lignocellulosic material by impregnation with different amounts of KOH. The activation process was studied by X-ray photoelectron spectroscopy and X-ray diffraction. These techniques point to the formation of different potassium compounds at the carbon surface (mainly K 2 CO 3 and different oxides) and show the dependence between surface area development in the carbons and the amount of K 2 CO 3 formed during the activation process.

Alumina-supported manganese- and manganese–palladium oxide catalysts for VOCs combustion

Alumina-supported manganese- and palladium–manganese oxide catalysts were prepared and tested in the combustion of formaldehyde. Total combustion of formaldehyde/methanol was achieved at 220 °C over a 18.2% Mn/Al2O3 catalyst. This temperature decreased either to 90 or 80 upon adding 0.1% or 0.4% Pd, respectively, to the base 18.2% Mn/Al2O3 catalyst. The combined use of X-ray diffraction, temperature-programmed reduction and photoelectron spectroscopy (XPS) techniques revealed that a Mn4+/Mn3+ oxide(s) and PdOx species are present on the surface of the fresh catalysts and remain along the catalytic reaction.

Effect of the carbon pre-treatment on the properties and performance for nitrobenzene hydrogenation of Pt/C catalysts

Abstract This paper reports a study of the effect of the purification and functionalization treatments of a peach pit derived carbon on the properties and performance for nitrobenzene hydrogenation reaction of Pt/C catalysts. Results show that the elimination of inorganic impurities, mainly sulphur, enhances the nitrobenzene hydrogenation rate. Moreover, the functionalization treatments of purified carbon with ozone and hydrogen peroxide have a positive effect both on the Pt dispersion and on the hydrogenation capacity of the catalyst, while the HNO3-treatment has a lower effect. The effect of the different oxidants can be related to the nature of the functional groups developed on the carbon surface. Thus, HNO3-treated carbon displays a high density of both strong and weak acid sites, while H2O2- and O3-treated carbons show an important concentration of weak acid sites but a low concentration of strong acid sites, according to the TPD results. Moreover, the H2PtCl6 isotherms in liquid phase at 298 K show a stronger interaction of the metallic precursor with the carbons of low acidity (like those treated with H2O2 or O3) than with the most acidic carbon (treated with HNO3). Carbons functionalized with weak oxidants, which develop acidic sites with moderate strength and show strong interaction with H2PtCl6 during impregnation, would favour the Pt dispersion on the carbon surface and consequently the catalytic behaviour.

Immobilization of 12-molybdophosphoric and 12-tungstophosphoric acids on metal-substituted hexagonal mesoporous silica

Abstract Hexagonal mesoporous silicate (HMS) materials containing Ti, Zr and Al ions were prepared using the neutral template route. The effect of the cation on the immobilization of 12-molybdophosphoric and 12-tungstophosphoric acids on mesoporous materials was studied. The texture of the samples was studied by the N2 adsorption–desorption isotherms, and their structure by X-ray diffraction (XRD), UV-Vis diffuse reflectance (DRS), infrared (IR) and X-ray photoelectron spectroscopy (XPS), and 1 H and 31 P nuclear magnetic resonance (NMR). Incorporation of different metals into the framework of HMS led to change in the structural and surface properties of mesoporous silica. All metal-containing samples showed a lower surface area compared to that of pure silica. The introduction of Zr and Al cations led to the development of textural mesoporosity in addition to the framework mesoporosity, characteristic of HMS and Ti-containing mesoporous silica (TiHMS) materials. IR and 1 H and 31 P NMR spectroscopic data revealed that the species present on the surface of HMS and TiHMS mesoporous materials after impregnation with solutions of 12-molybdophosphoric and 12-tungstophosphoric acid are non-degraded heteropoly anions with a preserved Keggin unit. Partial degradation of the anion was observed for zirconium-containing silicate samples (ZrHMS)-supported acids, the extent of which was stronger for supported molybdophosphoric acid in comparison with that of tungstophosphoric acid.

Thiophene hydrodesulfurization on sulfided Ni, W and NiW/USY zeolite catalysts: effect of the preparation method

Abstract The effect of the preparation method and metal loading on the hydrodesulfurization ⊙HDS) of thiophene, and on the dispersion and location of Ni and/or W species in Ni, W and NiW sulfides supported on USY zeolite, was investigated by temperature-programmed reduction ⊙TPR), X-ray photoelectron spectroscopy, N 2 adsorption and acidity determination. The monometallic Ni catalyst prepared by ion exchange [Ni ⊙int)] exhibited higher initial HDS activity than those prepared by impregnation [Ni ⊙imp)] and by ion exchange and subsequent formation of small NiO clusters by NaOH treatment [Ni ⊙clus)]. For the binary NiW catalyst series with different metal contents, the catalysts prepared by impregnating W onto the Ni ⊙int) one showed activities comparable to those of the catalysts prepared by impregnating Ni onto the W/USY sample. These catalyst series showed much higher activities than those prepared by impregnating W onto the Ni ⊙clus). The characterization results revealed that catalysts displayed significant differences in dispersion, location and sulfidation of the Ni and W phases dependent on the preparation method. However, none of these factors alone accounted for the activity trend. In general, it was found that a combination of high acidity and metal dispersion led to catalysts with high initial HDS activity.

The nature of cobalt species in Co and PtCoZSM5 used for the SCR of NOx with CH4

Abstract A thorough characterization of CoZSM5 and PtCoZSM5 before and after catalytic use was carried out using a battery of techniques. The bimetallic solid was more selective for N2 production. The TPR profiles showed significant differences. No solid, either fresh or used, exhibited any of the characteristic cobalt oxide X-ray reflections. The XPS data provided information concerning cobalt dispersion. The Raman spectroscopy clearly indicated that Co3O4 species were present only in the monometallic zeolites while a form of highly dispersed CoxOy moieties became dominant in the PtCoZSM5. The diffuse reflectance spectroscopy showed that Co2+ species in the monometallic solids were preferentially located at the main channels while in PtCoZSM5 these cations moved to higher coordination lattice sites. Through the combination of these tools, a much better understanding of the synergetic effect of Pt incorporated to CoZSM5 has been achieved. In view of these findings, related work previously published is revisited.

Aromatics hydrogenation on silica-alumina supported palladium-nickel catalysts

Abstract Nickel- and palladium–nickel catalysts supported on silica–alumina have been studied in the simultaneous hydrogenation of naphthalene and toluene. These catalysts were characterised by means of X-ray diffraction, CO chemisorption, temperature-programmed reduction, NH 3 temperature-programmed desorption, diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy of adsorbed CO and X-ray photoelectron spectroscopy techniques. All the catalysts studied showed higher initial intrinsic activity in the hydrogenation of toluene than of naphthalene. Regarding the hydrogenation of toluene, the 1Pd–8Ni/SA sample displayed the strongest resistance to deactivation by coke precursors as compared with the Ni-free 1Pd/SA catalyst. For the 1Pd–8Ni catalyst, the characterisation data pointed not only to a high degree of reducibility of nickel but also the greatest exposure of Pd species. Both findings appear to be related to the development of nickel hydrosilicate species at the support interface. Indeed, a better resistance towards deactivation was obtained by Pd incorporation and by increasing Ni-loading.

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).

Acid-redox properties of titania-supported 12-molybdophosphates for methanol oxidation

Physicochemical and catalytic properties of titania-supported 12-molybdophosphates have been investigated. The samples were characterized by the S(BET) method, Fourier-transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopies, temperature-programmed desorption (TPD) of ammonia and temperature-programmed reduction (TPR). The effect of temperature pretreatment on the catalytic behaviour of the samples in methanol oxidation was investigated. A significant change of selectivity to the main reaction products: dimethyl ether (DME) and formaldehyde (HCHO), as a function of the acid properties of the catalysts was observed. The highest selectivity to DME at 523 K suggested that the undecomposed molybdophosphoric acid (HPMo) on titania behaves as an acid catalyst. Pretreatment of the samples at higher temperatures (up to 723 K) and replacement of the protons in HPMo by cations (Co and Ni) lead to development of the redox properties (HCHO formation) of the catalyst, due to the suppression of the Bronsted acidity. The IR and XPS results provided clear evidence for the preservation of Keggin unit up to 623 K pretreatment after test reaction.