Catalysts used for heterogeneous processes are usually composed of metal nanoparticles dispersed over a high–surface-area support. In recent years, near-ambient pressure techniques have allowed ...catalyst characterization under operating conditions, overcoming the pressure gap effect. However, the use of model systems may not truly represent the changes that occur in real catalysts (the so-called material gap effect). Supports can play an important role in the catalytic process by providing new active sites and may strongly affect both the physical and chemical properties of metal nanoparticles. We used near-ambient pressure x-ray photoelectron spectroscopy to show that the surface rearrangement of bimetallic (rhodium-palladium) nanoparticles under working conditions for ethanol steam reforming with real catalysts is strongly influenced by the presence of a reducible ceria support.
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•Temperature favors the photogeneration of H2 over Au/TiO2.•Kinetic and thermodynamic parameters are calculated.•Silicone photo-microreactors and solar concentrators allow easy scale ...up.
The effect of temperature on the photocatalytic hydrogen generation from a gaseous water-ethanol mixture has been tested in a silicone microreactor containing nine microchannels of 500 μm (width) × 1 mm (depth) × 47 mm (length) coated with Au/TiO2 photocatalyst under UVA irradiation. Kinetic analyses have indicated that the hydrogen production rate follows the Langmuir-Hinshelwood model. The effect of temperature from 298 to 348 K has been determined by thermodynamic parameters, such as enthalpy (ΔH≠), entropy (ΔS≠) and Gibbs free energy (ΔG≠) of activation, using the transition state theory (TST). The apparent rate constants (kapp) are higher by increasing the temperature, and the activation energy has been determined to be 24 ± 1 kJ·mol−1. In order to evaluate if solar concentration could be used to enhance the photoproduction of hydrogen, the reaction has also been conducted under direct sunlight using a solar concentrator of about 1 m in diameter. Finally, the microreactor has been scaled out by a factor of ca. 10 to a device containing thirty-two microchannels of 500 μm (width) × 1 mm (depth) × 117.5 mm (length). The specific (i.e. per irradiated area of catalyst) hydrogen production rates of both microreactors using sunlight are very similar suggesting that this technology could lead to viable solar hydrogen production.
•Photogeneration of H2 from water–ethanol is enhanced in optical fiber honeycombs.•Au/TiO2 layer thickness of 1μm is optimum for 0.2mWcm−2 photon delivery.•The photoproduction of H2 is markedly ...influenced by temperature and contact time.•Evolution of photogenerated H2 is strongly affected by acetaldehyde adsorption.•Close surface sites are involved in hole scavenging and proton reduction.
Cordierite honeycombs loaded with different amounts of Au/TiO2 have been used in an optical fiber photoreactor illuminated with UV LEDs to produce hydrogen from ethanol and water–ethanol mixtures. The photoreactions have been carried out at 298–348K, volume hourly space velocities of 300–2100h−1, Au contents of 0.5–2wt.% with respect to TiO2, and ethanol molar contents of 1–100%. Excellent dispersion and homogeneous particle size of gold has been obtained by using pre-formed, dendrimer-encapsulated Au nanoparticles. The best photocatalytic performance has been obtained over monoliths loaded with ca. 0.5mgcm−2 of Au/TiO2 (1.0–1.5wt.% Au), which corresponds to a photocatalyst layer thickness of about 1μm. In gas phase, acetaldehyde adsorption onto the photocatalyst surface plays an important role in the dynamics of the photoprocess by affecting hydrogen evolution sites, which is improved with temperature and short contact times. Hydrogen photogeneration in liquid phase is significantly enhanced (fivefold increase) in the optical fiber honeycomb photoreactor with respect to a conventional slurry photoreactor.
The effect of temperature on the photocatalytic hydrogen generation from a gaseous water-ethanol mixture has been tested in a silicone microreactor containing nine microchannels of 500 μm ...(width) × 1 mm (depth) × 47 mm (length) coated with Au/TiO2 photocatalyst under UVA irradiation. Kinetic analyses have indicated that the hydrogen production rate follows the Langmuir-Hinshelwood model. The effect of temperature from 298 to 348 K has been determined by thermodynamic parameters, such as enthalpy (ΔH≠), entropy (ΔS≠) and Gibbs free energy (ΔG≠) of activation, using the transition state theory (TST). The apparent rate constants (kapp) are higher by increasing the temperature, and the activation energy has been determined to be 24 ± 1 kJ·mol−1. In order to evaluate if solar concentration could be used to enhance the photoproduction of hydrogen, the reaction has also been conducted under direct sunlight using a solar concentrator of about 1 m in diameter. Finally, the microreactor has been scaled out by a factor of ca. 10 to a device containing thirty-two microchannels of 500 μm (width) × 1 mm (depth) × 117.5 mm (length). The specific (i.e. per irradiated area of catalyst) hydrogen production rates of both microreactors using sunlight are very similar suggesting that this technology could lead to viable solar hydrogen production.
•Photogeneration of H2 from water–alcohol is enhanced in optical fiber honeycombs.•The amount of H2 produced is: methanol∼ethanol∼glycerol>bio-ethanol≫bio-glycerol. Impurities in bio-alcohols inhibit ...hole scavenging capacity.
Cordierite honeycombs loaded with 1wt.% Au/TiO2 have been used in an optical fiber photoreactor illuminated with UV LEDs to produce hydrogen at 298K from aqueous methanol, ethanol, glycerol, bio-ethanol, and bio-glycerol in liquid phase. The amount of alcohol has been varied between 1% and 100%. Under the same photoreaction conditions, the amount of hydrogen produced follows the trend: methanol∼ethanol∼glycerol>bio-ethanol≫bio-glycerol. Hydrogen photogeneration is significantly enhanced (4–5-fold increase) in the optical fiber honeycomb photoreactor with respect to a conventional slurry photoreactor.
Au/TiO
2
, Au
0.75
Cu
0.25
/TiO
2
, Au
0.5
Cu
0.5
/TiO
2
and Au
0.25
Cu
0.75
/TiO
2
photocatalysts prepared from pre-formed Au and Au–Cu alloy nanoparticles of controlled composition and size were ...loaded over ceramic honeycombs (2 mg cm
−2
) and tested in an optical fiber photoreactor illuminated with UV LEDs (2.6 mW cm
−2
) to continuously produce hydrogen from water and ethanol mixtures in gas phase at
W
/
F
= 4 g min L
−1
and 298 K (where
W
is the weight of the catalyst and
F
is the flow rate). The photocatalytic honeycombs were characterized by high resolution transmission electron microscopy, high-angle annular dark-field imaging, energy dispersive X-ray, X-ray photoelectron spectroscopy, and UV–Vis spectroscopy. The yield of hydrogen generation was Au
0.75
Cu
0.25
/TiO
2
> Au
0.5
Cu
0.5
/TiO
2
~ Au/TiO
2
> Au
0.25
Cu
0.75
/TiO
2
≫ bare TiO
2
, thus demonstrating that the addition of small quantities of copper to conventional TiO
2
-supported gold photocatalysts promotes the photocatalyic activity, likely by providing effective charge transfer between Au and Cu in the alloy nanoparticles.
Onion structures: Carbosilane dendrimers containing up to four metal layers have been constructed (see picture). The metal layers are linked by di‐ or tritopic N,P ligands. Selective deconstruction ...of the dendrimers is achieved by using salts of chloride or iodide anions.
Metallodendrimers are an important class of materials with valuable properties and applications in a large number of areas. Herein, we report a highly efficient route for the synthesis of carbosilane dendrimers containing multimetal layers. The procedure involves the use of heteroditopic and tritopic P,N ligands as connectors of the metal layers. The synthetic strategy is based on the ability of the latter ligands to react selectively with the metal complexes AuCl(tht) (tht: tetrahydrothiophene), {RuCl2(p‐cymene)}2 and Pd(η3‐2‐MeC3H4)(cod)(OTf) (cod: 1,5‐cyclooctadiene; OTf: triflate). In this way, metallodendrimers containing trimetallic Ru‐Au‐Pd or tetrametallic Ru‐Au‐Au′‐Pd layers have been formed and characterized. The trimetallic dendrimer 9 can be selectively deconstructed by cleavage of the RuN or AuN bonds by reaction with chloride or iodide salts, respectively.
Onion structures: Carbosilane dendrimers containing up to four metal layers have been constructed (see picture). The metal layers are linked by di‐ or tritopic N,P ligands. Selective deconstruction of the dendrimers is achieved by using salts of chloride or iodide anions.
Abstract
The dimethyl ether steam reforming reaction to generate hydrogen was tested over 1 wt % Pd/Al
2
O
3
catalysts prepared from different precursors. The conventional catalyst prepared by ...incipient wetness impregnation from palladium nitrate underwent strong deactivation under reforming conditions at 823 K because of metal sintering and carbon deposition. The same occurred over the catalyst prepared from preformed Pd nanoparticles protected with dodecanethiol. In contrast, catalysts prepared from Pd nanoparticles protected with carbosilane dendrons showed enhanced stability and good performance for the production of hydrogen. This was because of the formation of SiO
2
at the Pd–Al
2
O
3
interface that acted as a pinning center and prevented Pd migration under the reaction conditions as well as the accumulation of carbon.
The dimethyl ether steam reforming reaction to generate hydrogen was tested over 1 wt % Pd/Al2O3 catalysts prepared from different precursors. The conventional catalyst prepared by incipient wetness ...impregnation from palladium nitrate underwent strong deactivation under reforming conditions at 823 K because of metal sintering and carbon deposition. The same occurred over the catalyst prepared from preformed Pd nanoparticles protected with dodecanethiol. In contrast, catalysts prepared from Pd nanoparticles protected with carbosilane dendrons showed enhanced stability and good performance for the production of hydrogen. This was because of the formation of SiO2 at the Pd–Al2O3 interface that acted as a pinning center and prevented Pd migration under the reaction conditions as well as the accumulation of carbon.
Reform with a tree‐form: Pd/Al2O3 catalysts prepared from preformed Pd nanoparticles protected with carbosilane dendrons show an outstanding stability for the steam reforming of dimethyl ether compared to conventional catalysts. Upon calcination, the protecting shell decomposes and leaves the Pd nanoparticles anchored to the support in contact with SiO2, which prevents metal sintering under reaction conditions and carbon accumulation.
An efficient strategy for the synthesis of carbosilane dendrimers containing double metallic layers has been developed. The method consists of the attachment of the Ru(p-cymene)Cl2 unit to the ...branches of a phosphino-terminated dendrimer, followed by abstraction with AgOTf of the chloride ligands from the ruthenium and selective incorporation, through the nitrogen pyridine atom, of the bifunctional 4-pyPPh2 molecule. Finally, metalation of the resulting species with the complexes (tht)AuCl, PdCl(η3-2-MeC3H4)2, and RhCl(cod)2 renders the targeted Ru/Au, Ru/Pd, and Ru/Rh dendrimers. An unexpected migration of the chloride ligand bonded to the peripheral metals to the internal ruthenium atoms was observed and discussed in terms of the results obtained from density functional calculations. In the case of the Ru/Au dendrimers, the different abilities for the ligand transfer process shown by the dendrimers in comparison with a mononuclear Ru/Au model compound are explained in terms of dendritic effects.
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