A simple proportional integral (PI) controller with scheduled gain has been developed and implemented in a catalytic membrane reactor (CMR) to obtain pure hydrogen from a methanol steam reforming ...process. The controller is designed to track the setpoint of the pure hydrogen flow rate in the permeate side of the CMR via the manipulation of the fuel inlet flow rate. Therefore, the controller actuator is the liquid pump that provides the mixture of methanol and water to the reactor. Within the CMR, the catalytic pellets of PdZn/ZnAl2O4/Al2O3 have been used to facilitate the methanol steam-reforming reaction under stoichiometric conditions (S/C = 1), and Pd–Ag metallic membranes have been employed to simultaneously separate the generated hydrogen. The PI controller design is based on a mathematical model constructed using transfer functions acquired from dynamic experiments conducted with the CMR. The controller has been successfully implemented, and experimental validation tests have been carried out at 450 °C and relative pressures of 6, 8, 10, and 12 bar.
In the present work, we report a solution-based strategy to produce crystallographically textured SnSe bulk nanomaterials and printed layers with optimized thermoelectric performance in the direction ...normal to the substrate. Our strategy is based on the formulation of a molecular precursor that can be continuously decomposed to produce a SnSe powder or printed into predefined patterns. The precursor formulation and decomposition conditions are optimized to produce pure phase 2D SnSe nanoplates. The printed layer and the bulk material obtained after hot press displays a clear preferential orientation of the crystallographic domains, resulting in ultralow thermal conductivity of 0.55 Wm-1K-1 in the direction normal to the substrate. Such textured nanomaterials present highly anisotropic properties, with best thermoelectric performance in plane, i.e. in the directions parallel to the substrate, which coincide with the crystallographic bc plane of SnSe. This is an unfortunate characteristic because thermoelectric devices are designed to create/harvest temperature gradients in the direction normal to the substrate. We further demonstrate that this limitation can be overcome with the introduction of small amounts of tellurium in the precursor. The presence of tellurium allows reducing the band gap, increase both charge carrier concentration and mobility, especially cross plane, with a minimal decrease of the Seebeck coefficient. These effects translate into record out of plane ZT values at 800 K.
Semipermanently polarized hydroxyapatite, named SP/HAp(w), is obtained by applying a constant dc electric field of 1–10 kV/cm at 300–850 °C to the samples previously sintered in water vapor, while ...permanently polarized hydroxyapatite, PP/HAp(a), is produced by applying a dc electric field of 3 kV/cm at 1000 °C to the samples sintered in air. SP/HAp(w) has been used for biomedical applications, while PP/HAp(a) has been proved to be a valuable catalyst for N2 and CO2 fixation. In this work, structural differences between SP/HAp(w) and PP/HAp(a) have been ascertained using Raman microscopy, wide-angle X-ray diffraction, scanning electronic microscopy, high-resolution transmission electron microscopy, and grazing incidence X-ray diffraction. Results prove the existence of crystal distortion in the form of amorphous calcium phosphate and β-tricalcium phosphate (β-TCP) phases close to the surface because of the atmosphere used in the sintering process. The existence of an amorphous layer in the surface and the phase transition through β-TCP of SP/HAp(w) are the structural factors responsible for the differences with respect to PP/HAp(a). Moreover, a superstructure has been identified in PP/HAp(a) samples, which could be another structural factor associated with enhanced conductivity, permanent polarization, and catalytic activity of this material.
Tin disulfide (SnS
) is attracting significant interest because of the abundance of its elements and its excellent optoelectronic properties in part related to its layered structure. In this work, we ...specify the preparation of ultrathin SnS
nanoplates (NPLs) through a hot-injection solution-based process. Subsequently, Pt was grown on their surface via in situ reduction of a Pt salt. The photoelectrochemical (PEC) performance of such nanoheterostructures as photoanode toward water oxidation was tested afterwards. Optimized SnS
-Pt photoanodes provided significantly higher photocurrent densities than bare SnS
and SnS
-based photoanodes of previously reported study. Mott-Schottky analysis and PEC impedance spectroscopy (PEIS) were used to analyze in more detail the effect of Pt on the PEC performance. From these analyses, we attribute the enhanced activity of SnS
-Pt photoanodes reported here to a combination of the very thin SnS
NPLs and the proper electronic contact between Pt nanoparticles (NPs) and SnS
.
•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 development of better catalysts is a passionate topic at the forefront of modern science, where operando techniques are necessary to identify the nature of the active sites. The surface of a ...solid catalyst is dynamic and dependent on the reaction environment and, therefore, the catalytic active sites may only be formed under specific reaction conditions and may not be stable either in air or under high vacuum conditions. The identification of the active sites and the understanding of their behaviour are essential information towards a rational catalyst design. One of the most powerful operando techniques for the study of active sites is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which is particularly sensitive to the surface and sub-surface of solids. Here we review the use of NAP-XPS for the study of ceria-based catalysts, widely used in a large number of industrial processes due to their excellent oxygen storage capacity and well-established redox properties.
•Mixed iron/erbium vanadates over TiO2–WO3–SiO2 support are active NH3-SCR catalysts.•Iron enhances low-temperature NH3-SCR activity while erbium improves thermal stability.•Erbium vanadate enhances ...thermal stability of SCR catalyst up to 800°C by preventing anatase–rutile transformation.•Redox and acid/base functions of the iron/erbium vanadate catalysts are important in controlling catalyst performance.
A series of SCR catalysts of mixed iron–erbium vanadates supported on TiO2–WO3–SiO2 were prepared and their reduction, textural, structural and morphological properties characterized by temperature programmed reduction, X-ray powder diffraction, B.E.T. methods and transmission electron microscopy. The influence of Fe/Er ratio in the vanadate as well as the effect of aging conditions on their activity in ammonia SCR reaction was investigated. SCR activity at medium/low temperature was found to correlate directly with the Fe loading of the catalyst, with supported FeVO4 resulting the most active material. This behavior might be correlated with the characteristics of Fe3+OV5+ bond and the acidity of the VO moiety. Supported FeVO4 suffers a strong deactivation following thermal aging due to rutile formation in TiO2-based support and consequent collapse of surface area. The effect of Er is that of blocking transformation to rutile, thus enhancing activity after thermal treatment. Mixed composition FexEr1−xVO4 with x=0.5 represents the best compromise between activity and stability and are potential candidates for deNOx process when a higher stability is requested.
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► After high temperature calcination ceria and ceria–zirconia undergo structural evolution with exposure of more reactive surfaces. ► The exposure of reactive {100} and {110} type ...surfaces in ceria–zirconia is dependent on the amount of CeO2. ► {100} and {110} family type surfaces of ceria and ceria–zirconia are more reactive toward carbon soot oxidation. ► Any processing condition which favor the formation of (100) and (110) surfaces in ceria will result in enhanced activity toward soot oxidation.
High temperature treatment under air of samples of polycrystalline ceria and ceria–zirconia is shown to induce a rearrangement of crystals, with a surface structural evolution of the oxides toward the formation of more reactive exposed planes belonging to the {100} and {110} families, associated with a decrease of the exposure of the less reactive {111} facets. This is dependent on the amount of ceria, being more evident with pure CeO2, and less important with increasing ZrO2 content. A correlation between the exposure of more reactive planes and the specific rate of carbon soot oxidation is found, in agreement with previous results obtained in monodimensional nanoshaped and polycrystalline ceria samples for CO oxidation. This suggests that Diesel soot oxidation catalysts might be designed by keeping into consideration also these effects.
The catalytic production of syngas by the partial oxidation of methane (POM) was investigated over Pd supported on ceria (0.5–2 Pd wt.%) prepared by incipient wetness impregnation and by ...mechanochemical methods. The performance of the Pd/CeO2 catalyst prepared by milling CeO2 and Pd acetate was superior to that prepared by milling CeO2 and Pd nitrate and to Pd/CeO2 prepared by impregnation from Pd acetate. The best catalytic activity of the Pd/CeO2 catalyst prepared from CeO2 and Pd acetate was obtained by milling at 50 Hz for 5 min. Two-step combustion and reforming reaction mechanism were identified. Remarkably, methane conversion increased progressively with Pd loading for the catalysts prepared by incipient wetness impregnation, whereas low metal loading showed better conversion of methane for the catalysts prepared by ball milling using Pd acetate. This was explained in terms of an impressive dispersion of Pd species with a strong interaction with the surface of ceria, as deduced from transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy characterization, which revealed a large quantity of highly oxidized species at the surface.
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