An understanding of the adsorption of CO2, the first step in its photoreduction, is necessary for a full understanding of the photoreduction process. As such, the reactive adsorption of CO2 on ...oxidized, reduced, and platinized TiO2 nanotubes (Ti-NTs) was studied using infrared spectroscopy. The Ti-NTs were characterized with TEM and XRD, and XPS was used to determine the oxidation state as a function of oxidation, reduction, and platinization. The XPS data demonstrate that upon oxidation, surface O atoms become more electronegative, producing sites that can be characterized as strong Lewis bases, and the corresponding Ti becomes more electropositive producing sites that can be characterized as strong Lewis acids. Reduction of the Ti-NTs produces Ti3+ species, a very weak Lewis acid, along with a splitting of the Ti4+ peak, representing two sites, which correlate with O sites with a corresponding change in oxidation state. Ti3+ is not observed on reduction of the platinized Ti-NTs, presumably because Pt acts as an electron sink. Exposure of the treated Ti-NTs to CO2 leads to the formation of differing amounts of bidentate and monodentate carbonates, as well as bicarbonates, where the preference for formation of a given species is rationalized in terms of surface Lewis acidity and or Lewis basicity and the availability of hydrogen. Our data suggest that one source of hydrogen is water that remains adsorbed to the Ti-NTs even after heating to 350 °C and that reduced platinized NTs can activate H2. Carboxylates, which involve CO2 – moieties and are similar to what would be expected for adsorbed CO2 –, a postulated intermediate in CO2 photoreduction, are also observed but only on the reduced Ti-NTs, which is the only surface on which Ti3+/O vacancy formation is observed.
We investigate the performance and stability of a platinum (Pt) supported on indium tin oxide (ITO) electrocatalyst. ITO was synthesized using the co-precipitation method and uniform particles with a ...B.E.T. surface area larger than 40m2/g were obtained. Pt was dispersed onto the ITO by colloidal deposition followed by reduction with formaldehyde. Previous rotating disk electrode (RDE) work has shown that Pt/ITO possesses high activity and stability for the oxygen reduction reaction (ORR). However, this catalyst exhibited very poor performance and stability in an operating polymer electrolyte fuel cell (PEFC) membrane electrode assembly (MEA). For H2/O2 PEFC operation at 80°C and 75% relative humidity (RH), the current density obtained at 0.55V was only 90mA/cm2, and the maximum current density was only 150–160mA/cm2, when Pt/ITO was used at the cathode, whereas a limiting current density of 3900mA/cm2 was readily obtained using a benchmark Pt/C catalyst under identical conditions. The low performance with Pt/ITO was primarily due to the high overall cell resistance of the MEA (>400mOhmcm2). X-ray photoelectron spectroscopy (XPS) was employed to investigate the degradation of the Pt/ITO catalyst in the PEFC electrode during operation. The deconvolution of the indium 3d XPS peak revealed the presence of two peaks: the first was assigned to indium oxide in ITO (at 445.6eV), while the second was assigned to indium hydroxide (at 446.6eV). We observed an increase in surface hydroxide concentration (compared to pristine Pt/ITO) after the Pt/ITO catalyst was used either at the cathode or anode of an operating PEFC. We postulate that the surface hydroxides form a passivating layer that increases the electrode resistance and undermines PEFC performance.
•Contrary to RDE results, low performance was observed with a Pt/ITO electrocatalyst in an operating PEFC.•X-ray photoelectron spectroscopy (XPS) revealed Pt/ITO catalyst degradation during PEFC operation.•Indium 3d XPS peak deconvolution revealed the degradation mechanism, namely the formation of surface indium hydroxides.•We postulate that the surface hydroxides form a passivating layer that increases the electrode resistance and undermines PEFC performance.
Photocatalytic activity of TiO2 (anatase) is appreciably enhanced by substitutional doping of Mo in anatase lattice, in conjunction with the incorporation of nanostructured MoO3 within the parent ...anatase lattice. The photocatalyst material was characterized in detail using X-ray diffraction, Raman spectroscopy, diffuse reflectance (DR-UV–Vis spectroscopy), X-ray photoelectron spectroscopy, and electron microscopy. Photocatalysis experiments were conducted using a model rhodamine-B (Rh–B) dye reaction using both UV and visible irradiation sources. The observed trends in the case of visible irradiative source can be summarized as follows: Mo-1 < Mo-2 < Mo-5 ≫ Mo-10. Attempts were made to isolate the structural factors that control photochemical behavior of these Mo–TiO2 photocatalysts and to correlate photocatalytic activity with different structural aspects like oxidation state, band gap, surface species, etc. Mechanistic insights were acquired from ex situ 1H NMR studies showing different intermediates and different probable routes for the Rh–B dye degradation with UV and visible radiations. The stable intermediates were formed by a direct oxidative fragmentation route, without any evidence of the initial deethylation route. The intermediates found were benzoic acid, different amines, diols, and certain acids (mostly formic and acetic acid). The adsorption of the Rh–B dye on the catalytic surface via the N-charge centers of the Rh–B was also observed.
Management of plastic, rubber and cellulosic waste from various industries is a challenging task. An engineering scale plasma pyrolysis based incinerator has been commissioned for incineration of ...combustible waste, including plastic, rubber and cellulose. Operational trials of wastes with simulated composition show a weight reduction factor of more than 18 and volume reduction factor of more than 30. The volume reduction factor is tenfold higher than the compaction process currently practised for rubber and plastic wastes. Representative residual ash samples derived from these runs are subjected to their elemental analysis using EDXRF technique and results are comparable with the published literature. Relative variation of individual elements is attributed to the type of waste and feed composition. Analysis is aided with the calculation of index of geoaccumulation, enrichment factor (EF), contamination factor (CF) and pollution load index (PLI). From this study, it is evident that S, Cr, Zn, As, Se, Hg and Pb are of concern for environment in residual ash from plasma incineration of combustible waste. The efficacy of the incineration process is evaluated; C, H and O reduction achieved is more than 98% and overall enrichment ratio (ER) for the inorganic elements is more than 4.5. This study highlights the importance of elemental composition for the performance analysis of the plasma based incineration as well as hazards evaluation of constituents in residual ash for its further management.
Platinum supported on mixed-metal oxides (MMOs) are a class of active and durable cathode catalysts for proton exchange membrane fuel cell (PEMFC) due to a combination of the high oxidative stability ...of the supports and strong-metal-support interactions (SMSI) that enable them to exceed the activity of Pt/C. Herein, we solve a significant remaining challenge with Pt/MMO systems, namely the relatively low surface area and porosity. This is achieved by dispersing nearly-uniform Pt clusters using atomic layer deposition (ALD) on highly conductive (6.2 S/cm) and stable antimony doped tin dioxide (ATO) support. ALD-Pt/ATO exhibited significantly higher electrochemically active surface area (ECSA) (74 m2/g) and oxygen reduction reaction (ORR) catalytic activity (102 mA/mgPt at 0.9 V vs. RHE) compared to Pt/ATO synthesized using ethylene glycol (ECSA=31 m2/gPt, mass activity=52 mA/mgPt at 0.9 V vs. RHE) and formic acid reduction methods (ECSA=28 m2/gPt, mass activity=46 mA/mgPt at 0.9 V vs. RHE). Further characterization showed that wet chemical methods resulted in poorer Pt particle dispersion, poor control over Pt particle size distribution and chemical degradation of the support (during Pt deposition). Given the near-ideal Pt particle size distribution of the ALD-Pt/ATO, particle size growth and loss of ECSA was found to be minimal over the course of rigorous potential cycling. Thus, after 10,000 potential cycles between 1V and 1.5V vs. RHE, ALD-Pt/ATO and other Pt/ATOs were found to retain 100% of their initial ECSA compared to 57.6% retention for Pt/C. Upon testing in a H2/air PEMFC, following 1,000 potential cycles, the change in ALD-Pt/ATO performance was negligible while Pt/C exhibited a 68.2% loss of initial peak power density. Thus, ALD-Pt/ATO is an active and highly durable ORR electrocatalyst in PEMFCs under start-up-shut down conditions.
Collapsed titania nanotubes (cTiNT) were synthesized by the calcination of titania nanotubes (TiNT) at 650 °C, which leads to a collapse of their tubular morphology, a substantial reduction in ...surface area, and a partial transformation of anatase to the rutile phase. There are no significant changes in the position of the XPS responses for Ti and O on oxidation or reduction of the cTiNTs, but the responses are more symmetric than those observed for TiNTs, indicating fewer surface defects and no change in the oxidation state of titanium on oxidative and/or reductive pretreatment. The interaction of H₂O and CO₂ with the cTiNT surface was studied. The region corresponding to OH stretching absorptions extends below 3000 cm(-1), and thus is broader than is typically observed for absorptions of the OH stretches of water. The exchange of protons for deuterons on exposure to D₂O leads to a depletion of this extended absorption and the appearance of new absorptions, which are compatible with deuterium exchange. We discuss the source of this extended low frequency OH stretching region and conclude that it is likely due to the hydrogen-bonded OH stretches. Interaction of the reduced cTiNTs with CO₂ leads to a similar but smaller set of adsorbed carbonates and bicarbonates as reported for reduced TiNTs before collapse. Implications of these observations and the presence of proton sources leading to hydrogen bonding are discussed relative to potential chemical and photochemical activity of the TiNTs. These results point to the critical influence of defect structure on CO₂ photoconversion.
Subtleties in the synthesis of materials can have a profound effect on the catalytic and photocatalytic properties of materials. Black TiO2 nanotubes, demonstrating remarkable solar absorption, were ...synthesized using a stainless steel reactor. Using UV–vis diffuse reflectance spectroscopy, XPS, EDS, ICP, and TEM, the change in electronic absorption of the TiO2 nanotubes is explained by the discrete introduction of Cr concentrated particles from the stainless steel reactor. The black TiO2 nanotubes displayed significant solar-driven photocatalytic activity with the photo-oxidation of acetaldehyde under visible light (λ > 450 nm).
Low photocatalytic efficiency, rapid charge recombination and limited light absorption in wide band gap materials like ZnO severely hinder their practical applications. In order to overcome the above ...mentioned drawbacks and improve the photocatalytic efficiency, we have developed a series of Pt loaded ZnO-reduced graphene oxide (RGO) nanoheterostructures. The photocatalytic activity of these nanoheterostructures in different compositions, under sunlight irradiation, towards the reduction of 4-nitrophenol has been investigated. The prepared nanoheterostructures showed superior photocatalytic activity and composition having 5 wt% of RGO and 2 wt% of Pt in ZnO (ZPG5) was found to exhibit the highest activity. Control experiments were performed on ZnO-RGO (ZG) nanocomposites without Pt to determine the role of Pt loading. The rate constant of 4-nitrophenol reduction reaction obtained using ZPG5 nanoheterostructure was 0.4203 min−1, which is about 30 folds higher than the rate constant catalyzed using bare ZnO NR (0.0142 min−1) and 6 folds higher than ZG5. Strong enhancement in the photocatalytic activity could be evidenced for the Pt loaded ZnO-RGO nanoheterostructures under sunlight irradiation, which could be attributed to improved charge separation, enhanced charge transfer across the Schottky barrier between ZnO and Pt and the high ability of RGO to adsorb 4-nitrophenol. The developed nanoheterostructures can be used as highly efficient sunlight activated heterogeneous photocatalysts for several real life applications.
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•Pt loaded ZnO-RGO nanoheterostructures have been used for photocatalytic reduction of 4-nitrophenol.•Nanoheterostructure with 5 wt% of RGO and 2 wt% of Pt exhibited highest photocatalytic activity.•Synergetic effect of fast charge transfer across Schottky barrier and high pollutant adsorption ability of RGO has been studied.•Mechanism for enhanced photocatalytic activity has been proposed.
Developing highly active oxygen evolution and reduction reaction (OER/ORR) bifunctional electrocatalysts is key to multiple technologies, including regenerative fuel cells and metal-air batteries. To ...this end, we have investigated structure–activity relationships in Pb2Ru2O7–x having pyrochlore structure by tuning the structural oxygen vacancy (Ovac) and metal oxidation states. Increase in Ovac with temperature boosts the ORR activity by facilitating molecular oxygen dissociation via decrease in work function. Ovac formation is accompanied by lowering of the Ru(V)/Ru(IV) ratio due to charge-compensation which leads to decreased OER activity. Air-annealing of Pb2Ru2O7–x accelerates the formation of Ovac in comparison to Ar-annealing since atmospheric oxygen facilitates the reduction and phase-segregation of Ru from Pb2Ru2O7–x as RuO2. A maximum bifunctionality index and specific bifunctionality index of 0.69 V and 274.0 μA/cm2 BET, respectively, are observed for pristine Pb2Ru2O7–x . However, the activity is skewed toward OER for pristine Pb2Ru2O7–x , creating an asymmetric bifunctional property which is not desirable for practical applications. To reduce the asymmetric behavior, pristine and air-annealed Pb2Ru2O7–x samples at 700 °C are physically mixed which yields a higher symmetric OER/ORR activity (|Δi OER(η = 0.25 V)‑ORR(η = −0.45 V) specific|: pristine = 0.25 mA/cm2 BET, Air-700 °C = 0.20 mA/cm2 BET, physical mixture = 0.037 mA/cm2 BET). The inverse OER/ORR relationship in Pb2Ru2O7–x is attributed to the presence of an optimal ratio of 0.75 for Ru(V)/Ru(IV) and Ovac/Olattice, which provides symmetric bifunctional activity essential in electrochemical devices. An increase in Ru(V)/Ru(IV) ratio in pristine Pb2Ru2O7–x with no detectable Ru dissolution in the electrolyte observed subsequent to a 5-h OER hold-test, confirming high stability.
The development of noble metal‐free catalysts for hydrogen evolution is required for energy applications. In this regard, ternary heterojunction nanocomposites consisting of ZnO nanoparticles ...anchored on MoS2–RGO (RGO=reduced graphene oxide) nanosheets as heterogeneous catalysts show highly efficient photocatalytic H2 evolution. In the photocatalytic process, the catalyst dispersed in an electrolytic solution (S2− and SO32− ions) exhibits an enhanced rate of H2 evolution, and optimization experiments reveal that ZnO with 4.0 wt % of MoS2–RGO nanosheets gives the highest photocatalytic H2 production of 28.616 mmol h−1 gcat−1 under sunlight irradiation; approximately 56 times higher than that on bare ZnO and several times higher than those of other ternary photocatalysts. The superior catalytic activity can be attributed to the in situ generation of ZnS, which leads to improved interfacial charge transfer to the MoS2 cocatalyst and RGO, which has plenty of active sites available for photocatalytic reactions. Recycling experiments also proved the stability of the optimized photocatalyst. In addition, the ternary nanocomposite displayed multifunctional properties for hydrogen evolution activity under electrocatalytic and photoelectrocatalytic conditions owing to the high electrode–electrolyte contact area. Thus, the present work provides very useful insights for the development of inexpensive, multifunctional catalysts without noble metal loading to achieve a high rate of H2 generation.
Hello sunshine: A new ternary ZnO–MoS2–RGO nanocomposite (RGO=reduced graphene oxide) is designed and synthesized through a facile hydrothermal method for photocatalytic H2 evolution. The optimized photocatalyst shows a remarkably enhanced H2 evolution rate of 28.616 mmol h−1 gcat−1 under solar irradiation.
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