79 research outputs found
Highly dispersed PdS preferably anchored on In₂S₃ of MnS/In₂S₃ composite for effective and stable hydrogen production from H₂S
Novel MnS/(InCu)S composite for robust solar hydrogen sulphide splitting via the synergy of solid solution and heterojunction
Large photocatalytic hydrogen (H) production from copious waste hydrogen sulphide (HS) can meet the increasing demand for H in a sustainable manor which is beneficial from both environmental and energy standpoints. In this work, we reported a robust MnS/(InCu)S composite photocatalyst. Both experimental results and density functional theory (DFT) calculations proved that Cu does not act as a cocatalyst but forms a solid solution ((InCu)S) in the composites, which plays dual roles in improving the photocatalytic performance of H2S splitting: (i) enhancing solar light absorption, and (ii) promoting the desorption of sulfur (S) adsorbed on the catalyst surface. Moreover, the formation of a heterojunction between γ-MnS and (InCu)S can significantly improve charge separation and migration in the composites. As a result, the MnS/(InCu)S exhibits greatly extended visible light absorption up to 599 nm and extraordinarily high photocatalytic H production under visible light from HS with a maximum rate of 29,252 μmol h g. The corresponding apparent quantum efficiencies (AQE) at 420 and 450 nm are as high as 65.2% and 62.6%, respectively. They are the highest so far for the visible light photocatalytic splitting of HS in the absence of noble-metal co-catalysts
Highly dispersed PdS preferably anchored on In₂S₃ of MnS/In₂S₃ composite for effective and stable hydrogen production from H₂S
Unravelling the Different Reaction Pathways for Low Temperature CO Oxidation on Pt/CeO2 and Pt/Al2O3 by Spatially Resolved Structure–Activity Correlations
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The dynamic nature of Cu sites in Cu-SSZ-13 and the origin of the seagull NOx conversion profile during NH3-SCR
Cu-Zeolites with chabazite structure show a peculiar dual-maxima NO conversion profile, also known as a seagull profile, during the selective catalytic reduction by ammonia. In order to understand the origin of this behavior, systematic catalytic tests and operando spectroscopy were applied to derive structure–performance relationships for Cu-SSZ-13 catalysts with low and high Cu loading. Operando X-ray absorption, X-ray emission and in situ electron paramagnetic resonance spectroscopy measurements, including novel photon-in/photon-out techniques, demonstrated the interconversion of isolated Cu sites and dimeric bis(μ-oxo) Cu species, the former occurring via formation of ammonia Cu2+/Cu+ complexes and the latter in an oxidizing gas mixture. The formation of dimeric Cu+–O2–Cu+ species by involving Cu sites in close vicinity was linked to the high activity at low temperatures of the highly loaded Cu-SSZ-13 sample. In contrast, the isolated Cu sites present at very low Cu loadings are strongly poisoned by adsorbed NH3. The activity decrease around 350 °C that gives rise to the seagull shaped NO conversion profile could be attributed to a more localized structure of mono(μ-oxo)dicopper complexes. Above this temperature, which corresponds to partial NH3 desorption from Cu sites, the isolated Cu sites migrate to form additional dimeric entities thus recovering the SCR activity
NH-SCR over V-W/TiO Investigated by Operando X-ray Absorption and Emission Spectroscopy
V–W/TiO-based catalysts, which are used for the removal of NO from the exhaust of diesel engines and stationary sources via selective catalytic reduction with NH (NH-SCR), were studied by operando X-ray absorption spectroscopy (XAS) and emerging photon-in/photon-out techniques. In order to minimize the influence of highly X-ray absorbing tungsten and the fluorescence of titanium, we used a high-energy-resolution fluorescence setup that is able to separate efficiently the V Kβ emission lines and additionally allows to record valence-to-core (vtc) X-ray emission lines. High-energy resolution fluorescence-detected XAS (HERFD-XAS) and vtc X-ray emission spectroscopy (vtc-XES) proved to be the only way to perform an operando V K edge X-ray spectroscopic study on industrially relevant V–W/TiO catalysts so far. The V–W/TiO and V/TiO samples synthesized by incipient wetness impregnation and grafting exhibited high activity toward NH-SCR. Raman spectroscopy showed that they mainly contained highly dispersed, isolated, and polymeric V-oxo species. HERFD-XAS and XES identified redox cycling of vanadium species between V and V. With respect to most of the potential NH adsorption complexes, density functional theory calculations further showed that vtc-XES is more limited than surface-sensitive techniques such as infrared spectroscopy; hence, a combination of X-ray techniques with IR or similar spectroscopies is required to unequivocally identify the mechanism of NH-SCR over vanadia-based catalysts
The dynamic nature of Cu sites in Cu-SSZ-13 and the origin of the seagull NOx conversion profile during NH₃-SCR
Cu-Zeolites with chabazite structure show a peculiar dual-maxima NO conversion profile, also known as a seagull profile, during the selective catalytic reduction by ammonia. In order to understand the origin of this behavior, systematic catalytic tests and operando spectroscopy were applied to derive structure–performance relationships for Cu-SSZ-13 catalysts with low and high Cu loading. Operando X-ray absorption, X-ray emission and in situ electron paramagnetic resonance spectroscopy measurements, including novel photon-in/photon-out techniques, demonstrated the interconversion of isolated Cu sites and dimeric bis(μ-oxo) Cu species, the former occurring via formation of ammonia Cu2+/Cu+ complexes and the latter in an oxidizing gas mixture. The formation of dimeric Cu+–O2–Cu+ species by involving Cu sites in close vicinity was linked to the high activity at low temperatures of the highly loaded Cu-SSZ-13 sample. In contrast, the isolated Cu sites present at very low Cu loadings are strongly poisoned by adsorbed NH3. The activity decrease around 350 °C that gives rise to the seagull shaped NO conversion profile could be attributed to a more localized structure of mono(μ-oxo)dicopper complexes. Above this temperature, which corresponds to partial NH3 desorption from Cu sites, the isolated Cu sites migrate to form additional dimeric entities thus recovering the SCR activity
HERFD-XANES and XES as complementary operando tools for monitoring the structure of Cu-based zeolite catalysts during NOx-removal by ammonia SCR
In this article, we demonstrate the potential of hard X-ray techniques to characterize catalysts under working conditions. Operando high energy resolution fluorescence detected (HERFD) XANES and valence to core (vtc) X-ray emission spectroscopy (XES) have been used in a spatially-resolved manner to study Cu-zeolite catalysts during the standard-SCR reaction and related model conditions. The results show a gradient in Cu oxidation state and coordination along the catalyst bed as the reactants are consumed. Vtc-XES gives complementary information on the direct adsorption of ammonia at the Cu sites. The structural information on the catalyst shows the suitability of X-ray techniques to understand catalytic reactions and to facilitate catalyst optimization
In situ probing of Pt/TiO activity in low-temperature ammonia oxidation
The improvement of the low-temperature activity of the supported platinum catalysts in selective ammonia oxidation to nitrogen is still a challenging task. The recent developments in in situ/operando characterization techniques allows to bring new insight into the properties of the systems in correlation with their catalytic activity. In this work, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and operando X-ray absorption spectroscopy (XAS) techniques were applied to study Pt/TiO catalysts in ammonia oxidation (NH + O reaction). Several synthesis methods were used to obtain samples with different size of Pt particles, oxidation state of Pt, and morphology of the support. Metal platinum particles on titania prepared by pulsed laser ablation in liquids exhibited the highest activity at lower temperatures with the temperature of 50% conversion of NH being 150 °C. The low-temperature activity of the catalysts synthesized by impregnation can be improved by the reductive pretreatment. NAP-XPS and operando XANES data do not show formation of PtO surface layers or PtO/PtO oxides during NH + O reaction. Despite the differences in the oxidation state of platinum in the as-prepared catalysts, their treatment in the reaction mixture results in the formation of metallic platinum particles, which can serve as centers for stabilization of the adsorbed oxygen species. Stabilization of the bulk platinum oxide structures in the Pt/TiO catalysts seems to be less favorable due to the metal–support interaction
Increased NO2 concentration in the diesel engine exhaust for improved Ag/Al2O3 catalyst NH3-SCR activity
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