Higher tolerance to sulfur poisoning in CO2 methanation by the presence

This study investigates the deactivation mechanism of CeO2-promoted catalyst for the CO2 methanation reaction. The catalytic performance was evaluated at high temperature (T=500 ºC,P=5 bar·g) and under the presence of unfavourable H2S impurities (1-5 ppm). The thermal stability of the CeO2-promoted catalyst was excellent, while the non-promoted sample suffered from nickel sintering. In contrast, the presence of H2S was detrimental for both catalysts. The tolerance to H2S of CeO2-promoted sample was higher; keeping one third of the initial catalytic activity under continuous addition of H2S. The identification of crystallographic planes associated with Ce2O2S phase (HRSTEM) evidenced that the addition of CeO2 to nickel catalyst minimized the formation of non-active NiS sites. This finding was further confirmed through DRIFT spectroscopy since for the Ni-CeO2/-Al2O3, methane formation derived from for mate dissociation was followed by hydrogenation of the adsorbed CO on the remaining available active sites

Tuning the Fermi Level and the Kinetics of Surface States of TiO₂ Nanorods by Means of Ammonia Treatments

Ammonia-induced reduction treatment of titanium dioxide rutile nanorods has been performed, where the treatment triggered a synergistic surface modification of titania electrodes that enhanced its overall photoelectrochemical performance, besides introducing a new absorption band in the 420–480 nm range. A physical model has been proposed to reveal the role of each fundamental interfacial property on the observed behavior. On the one hand, by tuning the Fermi level position, charge separation was optimized by adjusting the depletion region width to maximize the potential drop inside titanium dioxide and also filling the surface states, which in turn decreased electron–hole recombination. On the other hand, by increasing the density of surface holes traps (identified as surface hydroxyl groups), the average hole lifetime was extended, depicting a more efficient hole transfer to electrolyte species. The proposed model could serve as a rationale for controlled interfacial adjustment of nanostructured photoelectrodes tailoring them for the required application

Charge Transfer Characterization of ALD-Grown TiO₂ Protective Layers in Silicon Photocathodes

A critical parameter for the implementation of standard high-efficiency photovoltaic absorber materials for photoelectrochemical water splitting is its proper protection from chemical corrosion while remaining transparent and highly conductive. Atomic layer deposited (ALD) TiO₂ layers fulfill material requirements while conformally protecting the underlying photoabsorber. Nanoscale conductivity of ALD TiO₂ protective layers on silicon-based photocathodes has been analyzed, proving that the conduction path is through the columnar crystalline structure of TiO₂. Deposition temperature has been explored from 100 to 300 °C, and a temperature threshold is found to be mandatory for an efficient charge transfer, as a consequence of layer crystallization between 100 and 200 °C. Completely crystallized TiO₂ is demonstrated to be mandatory for long-term stability, as seen in the 300 h continuous operation test

Visible Photoluminescence Components of Solution-Grown ZnO Nanowires: Influence of the Surface Depletion Layer

Arrays of electrodeposited ZnO nanowires (NWs) were used to illustrate the dependence of the ZnO visible photoluminescence (PL) emission on the extension of the surface depletion layer and obtain further insight into the localization of the related states. With this goal in mind, three sets of measurements were carried out: (i) analysis of the PL spectra of ZnO:Cl NWs as a function of their carrier concentration; (ii) analysis of the PL spectra of ZnO:Cl/ZnO core–shell NWs as a function of the thickness of their intrinsic ZnO shell; (iii) in situ analysis of the PL dependence on the polarization of ZnO:Cl photoelectrodes. The obtained experimental results evidenced that the yellow and orange emissions from electrodeposited ZnO NWs are correlated with the extension of the NWs surface depletion region. This result points out the surface localization of the states at the origin of these transitions. On the other hand, the green emission that dominates the visible part of the PL spectra in annealed ZnO NWs showed no dependence on the surface band bending, thus pointing toward its origin in the bulk

Controlled Photocatalytic Oxidation of Methane to Methanol through Surface Modification of Beta Zeolites

The selective oxidation of methane to methanol is achieved by means of a photocatalytic process. For this purpose, designed Bi- and V-containing beta zeolites prepared by incipient wetness impregnation have been used under different test conditions. While the zeolite proves to be photoactive under UVC irradiation toward the total oxidation process, the formation of V₂O₅ on the surface is an effective alternative for modifying the acid–base surface properties, thus significantly decreasing the undesired CO₂ formation. At the same time the zeolite framework serves as a scaffold for increasing the surface area and distribution of the metal oxide. Additionally, the addition of low Bi amount favors the formation of a BiVO₄/V₂O₅ heterojunction, which acts as a visible light photocatalyst while at the same leading to total selectivity to methanol at the expense of ethylene formation

Role of Tungsten Doping on the Surface States in BiVO₄ Photoanodes for Water Oxidation: Tuning the Electron Trapping Process

The nanostructured BiVO₄ photoanodes were prepared by electrospinning and were further characterized by XRD, SEM, and XPS, confirming the bulk and surface modification of the electrodes attained by W addition. The role of surface states (SS) during water oxidation for the as-prepared photoanodes was investigated by using electrochemical, photoelectrochemical, and impedance spectroscopy measurements. An optimum 2% doping is observed in voltammetric measurements with the highest photocurrent density at 1.23 V_RHE under back side illumination. It has been found that a high PEC performance requires an optimum ratio of density of surface states (<i>N</i>_SS) with respect to the charge donor density (<i>N</i>_d), to give both good conductivity and enough surface reactive sites. The optimum doping (2%) shows the highest <i>N</i>_d and SS concentration, which leads to the high film conductivity and reactive sites. The reason for SS acting as reaction sites (i-SS) is suggested to be the reversible redox process of V⁵⁺/V⁴⁺ in semiconductor bulk to form water oxidation intermediates through the electron trapping process. Otherwise, the irreversible surface reductive reaction of VO₂⁺ to VO²⁺ though the electron trapping process raises the surface recombination. W doping does have an effect on the surface properties of the BiVO₄ electrode. It can tune the electron trapping process to obtain a high concentration of i-SS and less surface recombination. This work gives a further understanding for the enhancement of PEC performance caused by W doping in the field of charge transfer at the semiconductor/electrolyte interface

Solvothermal, Chloroalkoxide-based Synthesis of Monoclinic WO₃ Quantum Dots and Gas-Sensing Enhancement by Surface Oxygen Vacancies

We report for the first time the synthesis of monoclinic WO₃ quantum dots. A solvothermal processing at 250 °C in oleic acid of W chloroalkoxide solutions was employed. It was shown that the bulk monoclinic crystallographic phase is the stable one even for the nanosized regime (mean size 4 nm). The nanocrystals were characterized by X-ray diffraction, High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV–vis, Fourier transform infrared and Raman spectroscopy. It was concluded that they were constituted by a core of monoclinic WO₃, surface covered by unstable W­(V) species, slowly oxidized upon standing in room conditions. The WO₃ nanocrystals could be easily processed to prepare gas-sensing devices, without any phase transition up to at least 500 °C. The devices displayed remarkable response to both oxidizing (nitrogen dioxide) and reducing (ethanol) gases in concentrations ranging from 1 to 5 ppm and from 100 to 500 ppm, at low operating temperatures of 100 and 200 °C, respectively. The analysis of the electrical data showed that the nanocrystals were characterized by reduced surfaces, which enhanced both nitrogen dioxide adsorption and oxygen ionosorption, the latter resulting in enhanced ethanol decomposition kinetics

Insights into the Performance of Co_<i>x</i>Ni_1–<i>x</i>TiO₃ Solid Solutions as Photocatalysts for Sun-Driven Water Oxidation

Co_<i>x</i>Ni_1–<i>x</i>TiO₃ systems evaluated as photo- and electrocatalytic materials for oxygen evolution reaction (OER) from water have been studied. These materials have shown promising properties for this half-reaction both under (unbiased) visible-light photocatalytic approach in the presence of an electron scavenger and as electrocatalysts in dark conditions in basic media. In both situations, Co_0.8Ni_0.2TiO₃ exhibits the best performance and is proved to display high faradaic efficiency. A synergetic effect between Co and Ni is established, improving the physicochemical properties such as surface area and pore size distribution, besides affecting the donor density and the charge carrier separation. At higher Ni content, the materials exhibit behavior more similar to that of NiTiO₃, which is a less suitable material for OER than CoTiO₃

Surface Modification of TiO₂ Nanocrystals by WO_<i>x</i> Coating or Wrapping: Solvothermal Synthesis and Enhanced Surface Chemistry

TiO₂ anatase nanocrystals were prepared by solvothermal processing of Ti chloroalkoxide in oleic acid, in the presence of W chloroalkoxide, with W/Ti nominal atomic concentration (<i>R</i>_w) ranging from 0.16 to 0.64. The as-prepared materials were heat-treated up to 500 °C for thermal stabilization and sensing device processing. For <i>R</i>_0.16, the as-prepared materials were constituted by an anatase core surface-modified by WO_<i>x</i> monolayers. This structure persisted up to 500 °C, without any WO₃ phase segregation. For <i>R</i>_w up to <i>R</i>_0.64, the anatase core was initially wrapped by an amorphous WO_<i>x</i> gel. Upon heat treatment, the WO_<i>x</i> phase underwent structural reorganization, remaining amorphous up to 400 °C and forming tiny WO₃ nanocrystals dispersed into the TiO₂ host after heating at 500 °C, when part of tungsten also migrated into the TiO₂ structure, resulting in structural and electrical modification of the anatase host. The ethanol sensing properties of the various materials were tested and compared with pure TiO₂ and WO₃ analogously prepared. They showed that even the simple surface modification of the TiO₂ host resulted in a 3 orders of magnitude response improvement with respect to pure TiO₂

Colloidal Counterpart of the TiO₂‑Supported V₂O₅ System: A Case Study of Oxide-on-Oxide Deposition by Wet Chemical Techniques. Synthesis, Vanadium Speciation, and Gas-Sensing Enhancement

TiO₂ anatase nanocrystals were surface modified by deposition of V­(V) species. The starting amorphous TiO₂ nanoparticles were prepared by hydrolytic processing of TiCl₄-derived solutions. A V-containing solution, prepared from methanolysis of VCl₄, was added to the TiO₂ suspension before a solvothermal crystallization step in oleic acid. The resulting materials were characterized by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared, Raman, and magic angle spinning solid-state ⁵¹V nuclear magnetic resonance spectroscopy (MAS NMR). It was shown that in the as-prepared nanocrystals V was deposited onto the surface, forming Ti–O–V bonds. After heat treatment at 400 °C, TEM/electron energy loss spectroscopy and MAS NMR showed that V was partially inserted in the anatase lattice, while the surface was covered with a denser V–O–V network. After heating at 500 °C, V₂O₅ phase separation occurred, further evidenced by thermal analyses. The 400 °C nanocrystals had a mean size of about 5 nm, proving the successful synthesis of the colloidal counterpart of the well-known TiO₂–V₂O₅ catalytic system. Hence, and also due to the complete elimination of organic residuals, this sample was used for processing chemoresistive devices. Ethanol was used as a test gas, and the results showed the beneficial effect of the V surface modification of anatase, with a response improvement up to almost 2 orders of magnitude with respect to pure TiO₂. Moreover, simple comparison of the temperature dependence of the response clearly evidenced the catalytic effect of V addition