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.

a b s t r a c tAn effective and versatile synthetic approach to produce well-dispersed supported intermetallic nanoparticles is presented that allows a comparative study of the catalytic properties of different intermetallic phases while minimizing the influence of differences in preparation history. Supported PdZn, Pd 2 Ga, and Pd catalysts were synthesized by reductive decomposition of ternary Hydrotalcite-like compounds obtained by co-precipitation from aqueous solutions. The precursors and resulting catalysts were characterized by HRTEM, XRD, XAS, and CO-IR spectroscopy. The Pd 2+ cations were found to be at least partially incorporated into the cationic slabs of the precursor. Full incorporation was confirmed for the PdZnAlHydrotalcite-like precursor. After reduction of Ga-and Zn-containing precursors, the intermetallic compounds Pd 2 Ga and PdZn were present in the form of nanoparticles with an average diameter of 6 nm or less. Tests of catalytic performance in methanol steam reforming and methanol synthesis from CO 2 have shown that the presence of Zn and Ga improves the selectivity to CO 2 and methanol, respectively. The catalysts containing intermetallic compounds were 100 and 200 times, respectively, more active for methanol synthesis than the monometallic Pd catalyst. The beneficial effect of Ga in the active phase was found to be more pronounced in methanol synthesis compared with steam reforming of methanol, which is likely related to insufficient stability of the reduced Ga species in the more oxidizing feed of the latter reaction. Although the intermetallic catalysts were in general less active than a Cu-/ZnO-based material prepared by a similar procedure, the marked changes in Pd reactivity upon formation of intermetallic compounds and to study the tunability of Pd-based catalysts for different reactions.

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.

Nearly 2 % of the world's primary energy is stored in the 65 million tonnes of hydrogen generated each year, almost all of which is for captive use in the chemical and refinery industries. [1] Currently, the main processes for producing industrial hydrogen are catalytic steam reforming of natural gas (48 %) and oil-derived naphtha (30 %), coal gasification (18 %), and the electrolysis of water (4 %). [2] Apart from its traditional uses, hydrogen is considered an ideal energy carrier in the future energy systems that need to be economically and environmentally sustainable. [3] The possibility of using hydrogen as an alternative energy carrier has intensified the exploration of hydrogen production processes from a wide range of primary sources such as natural gas, fuels, methanol, biomass, coal, solar, and nuclear power. [4][5][6][7][8] Although hydrogen production, storage, and distribution are commercially viable in the chemical and refining industries, the cost and efficiency of the infrastructures for its storage and distribution for energy use is currently unacceptable compared to existing petroleum distillate facilities. [9] Additionally, current commercial options for H 2 storage (high pressure or liquefaction) do not fully meet requirements for compactness, drive range, and cost in transport applications (2 kWh kg À1 and 4 $ kWh À1 ). To achieve effective hydrogen storage, research activities have focused on the development of in situ production processes based on the reforming of high-density liquids that contain hydrogen, such as methanol, ethanol, or fuels. Several studies [10][11][12][13][14] have analyzed [a] Dr.

Cd 1-x Zn x S solid solutions have been prepared by employing different thermal treatments (He flow from 773 to 1073 K and static vacuum at 1073 K). The photocatalysts were characterized by inductively coupled plasma atomic emission spectroscopy (ICP-AES), N 2 adsorption isotherms, X-ray diffraction (XRD), UV-vis absorption spectra, and X-ray photoelectron spectroscopy (XPS) techniques and tested in the production of hydrogen from aqueous solutions containing S 2-/SO 3 2as sacrificial agents under visible light irradiation. Structure, morphology, composition, and visible light absorption capacity of the Cd 1-x Zn x S solid solutions were found to be strongly dependent on the postsynthesis thermal treatments. The samples treated under He flow presented Cd loss with the temperature rise that implies formation of defects in the crystalline structure of the solid solutions. The increase in the extent of the structural defects of the Cd 1-x Zn x S solid solutions with the annealing temperature rise is related with a progressive decrease in their capacity to produce hydrogen. A significant improvement of the photoactivity was obtained for the solid solution derived from the treatment under static vacuum. The improvement in activity is related to the formation of Cd 1-x Zn x S solid solutions with higher crystallinity and less thermal Cd sublimation that inhibits the formation of structural defects and thus improves the effective utilization of the e -/hole energy carriers.

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There is a large worldwide demand for light olefins (C2=–C4=), which are needed for the production of high value-added chemicals and plastics. Light olefins can be produced by petroleum processing, direct/indirect conversion of synthesis gas (CO + H2) and hydrogenation of CO2. Among these methods, catalytic hydrogenation of CO2 is the most recently studied because it could contribute to alleviating CO2 emissions into the atmosphere. However, due to thermodynamic reasons, the design of catalysts for the selective production of light olefins from CO2 presents different challenges. In this regard, the recent progress in the synthesis of nanomaterials with well-controlled morphologies and active phase dispersion has opened new perspectives for the production of light olefins. In this review, recent advances in catalyst design are presented, with emphasis on catalysts operating through the modified Fischer–Tropsch pathway. The advantages and disadvantages of olefin production from CO2 via CO or methanol-mediated reaction routes were analyzed, as well as the prospects for the design of a single catalyst for direct olefin production. Conclusions were drawn on the prospect of a new catalyst design for the production of light olefins from CO2.

We studied different preparation methods to synthesize a series of bifunctional hybrid catalytic systems for the direct synthesis of DME from syngas. The objective was to optimize the contact and interaction between the methanol synthesis catalyst and the methanol dehydration catalyst (Cu/ZnO/Al 2 O 3 and H 3 PW 12 O 40 supported on TiO 2 , respectively) by using different mixing methods (simple mixing, mixing−milling, suspension, and mixing−pressing). It has been found that the close contact between methanol synthesis and acidic functions is highly dependent on the degree of mixing of the two catalysts. In this respect, the hybrid catalyst prepared by mixing−pressing, which represents the closest contact, shows the strongest alterations in both catalytic functions. These modifications are associated with a decrease in copper surface area and a decrease in strong acid sites caused by the physical blocking of active sites or by cation exchange of the Cu 2+ /Zn 2+ species from Cu−ZnO(Al) and the H + from the H 3 PW 12 O 40 units. The activity results demonstrated that the mixing−pressing method leading to a closer contact between the two catalytic functions led to a very low dimethyl ether time yield compared to the other bifunctional catalysts prepared by less severe mixing methods (19.4 vs 205−335 μmol/min•g cat ). This work clearly indicates the importance of the mixing method in the synthesis of the hybrid catalyst to optimize the distance and interaction between the metal and acid sites.