The activity of plasmonic Au−TiO 2 catalysts for solar hydrogen production from H 2 O/MeOH mixtures was found to depend strongly on the support phase (anatase, rutile, brookite, or composites thereof) as well as on specific structural properties caused by the method of Au deposition (sol-immobilization, photodeposition, or deposition−precipitation). Structural and electronic rationale have been identified for this behavior. Using a combination of spectroscopic in situ techniques (EPR, XANES, and UV−vis spectroscopy), the formation of plasmonic Au particles from precursor species was monitored, and the chargecarrier separation and stabilization under photocatalytic conditions was explored in relation to H 2 evolution rates. By in situ EPR spectroscopy, it was directly shown that abundant surface vacancies and surface OH groups enhance the stabilization of separated electrons and holes, whereas the enrichment of Ti 3+ in the support lattice hampers an efficient electron transport. Under the given experimental conditions, these properties were most efficiently generated by depositing gold particles on anatase/rutile composites using the deposition−precipitation technique.
Supported V2O5/Ce1–x
Ti
x
O2 (3, 5, and 7 wt
% V; x = 0, 0.1, 0.3, 0.5, 1) and bare supports have
been tested in the selective catalytic reduction (SCR) of NO by NH3 at different gas hourly space velocities (GHSVs) and were
comprehensively characterized using XRD, pseudo in situ XPS, and UV–vis
DRS as well as EPR and DRIFTS in in situ and operando mode. The best
V/Ce1–x
Ti
x
O2 (x = 0.3, 0.5) catalysts showed
almost 100% NO conversion and N2 selectivity already at
190 °C with a GHSV value of 70000 h–1, which
belongs to the best performances observed so far in low-temperature
NH3-SCR of NO. The corresponding bare supports still converted
around 80% to N2 under the same conditions. On bare supports,
SCR proceeds via a Langmuir–Hinshelwood mechanism comprising
the reaction of adsorbed surface nitrates with adsorbed NH3. On V/Ce1–x
Ti
x
O2, nitrate formation is not possible, and an Eley–Rideal
mechanism is working in which gaseous NO reacts with adsorbed NH3 and NH4
+. Lewis and Brønsted acid
sites, though adsorption of NH3, do not scale with the
catalytic activity, which is governed rather by the redox ability
of the materials. This is boosted in the supports by replacing Ce
with the more redox active Ti and in catalysts by tight connection
of vanadyl species via O bridges to the support surface forming −Ce–O–V(O)–O–Ti–
units in which the equilibrium valence state of V under reaction conditions
is close to +5.
Photocatalytic hydrogen evolution
rates and structural properties
as well as charge separation, electron transfer, and stabilization
have been analyzed in advanced sol–gel-derived carbon nitrides
(SG-CN) pyrolyzed at different temperatures (350–600 °C)
and in bulk polymeric carbon nitride reference samples (CN) by XRD,
XPS, FTIR, UV–vis, Raman, and photoluminescence as well as
by in situ EPR spectroscopy. SG-CN samples show about 20 times higher
H2 production rates than bulk CN. This is due to their
porous structure, partial disorder, and high surface area which favor
short travel distances and fast trapping of separated electrons on
the surface where they are available for reaction with protons. In
contrast, most of the excited electrons in bulk polymeric CN return
quickly to the valence band upon undesired emission of light, which
is responsible for their low catalytic activity.
The
hydrogenation of CO2 on Rh/Al2O3 catalysts
modified with Ni and K was studied by in situ and operando
DRIFTS spectroscopy comprising transient and isotopic exchange experiments
to study the influence of this modification on the catalytic performance
in CO and methane formation at 250–350 °C and to gain
mechanistic insight. Catalytic testing and spectroscopic studies revealed
that the modification with particularly K promotes the formation of
CO being the highest over Rh, K, Ni/Al2O3, whereas
methane formation is preferred over the unmodified catalyst. It was
found that CO2 does not dissociatively adsorb but is adsorbed
at the support, forming mainly hydrogen carbonate, and in the presence
of K, also carbonate species. The dissociative adsorption of H2 proceeds on Rh. The activated H2 reacts mainly
with the hydrogen carbonate species forming CO adsorbed on Rh and
formate (F1) species stably adsorbed on the support. On the K-containing
catalysts, an additional formate species (F2) was identified as more
reactive than F1 formate and can act as a reaction intermediate in
the CO formation pathway. Furthermore, adsorbed formyl species were
detected, which are assumed to be intermediates in the methanation
reaction. The modifying additives change the surroundings of the Rh
particles. This influences the strength of CO adsorption and the activation
ability of Rh for H2 dissociation. Thus, desorption of
the formed CO from the catalyst surface is favored, and the methanation
of CO is hindered. The modification with K enhances the ability for
CO2 fixation by formation of additional carbonate species
which cover adsorption sites for unreactive F1 formate species and
favors the formation of reactive F2 formate species.
Several in situ techniques are known which allow investigations of catalysts and catalytic reactions under real reaction conditions using different spectroscopic and X-ray methods. In recent years, specific set-ups have been established which combine two or more in situ methods in order to get a more detailed understanding of catalytic systems. This tutorial review will give a summary of currently available set-ups equipped with multiple techniques for in situ catalyst characterization, catalyst preparation, and reaction monitoring. Besides experimental and technical aspects of method coupling including X-ray techniques, spectroscopic methods (Raman, UV-vis, FTIR), and magnetic resonance spectroscopies (NMR, EPR), essential results will be presented to demonstrate the added value of multitechnique in situ approaches. A special section is focussed on selected examples of use which show new developments and application fields.
Carbon dioxide can be used in various ways as a cheap C1 source. However, the utilization of CO2 requires energy or energy-rich reagents, which leads to further emissions, and therefore, diminishes the CO2-saving potential. Therefore, life cycle assessment (LCA) is required for each process that uses CO2 to provide valid data for CO2 savings. Carbon dioxide can be incorporated into epoxidized fatty acid esters to provide the corresponding carbonates. A robust catalytic process was developed based on simple halide salts in combination with a phase-transfer catalyst. The CO2-saving potential was determined by comparing the carbonates as a plasticizer with an established phthalate-based plasticizer. Although CO2 savings of up to 80 % were achieved, most of the savings arose from indirect effects and not from CO2 utilization. Furthermore, other categories have been analyzed in the LCA. The use of biobased material has a variety of impacts on categories such as eutrophication and marine toxicity. Therefore, the benefits of biobased materials have to be evaluated carefully for each case. Finally, interesting properties as plasticizers were obtained with the carbonates. The volatility and water extraction could be improved relative to the epoxidized system.
Environmentally friendly and low-cost catalysts are required for large-scale non-oxidative dehydrogenation of propane to propene (PDH) to replace currently used CrO x -or Pt-based catalysts. This work introduces ZnO-containing ZrO 2 -or MZrO x -supported (M=Ce, La, Ti or Y) catalysts. The most active materials outperformed the state-of-the-art catalysts with supported CrO x , GaO x , ZnO x or VO x species as well as bulk ZrO 2 -based catalysts without ZnO. The spacetime yield of propene of 1.25 kg C3H6 •kg -1 cat •h -1 at a propane conversion of about 30% with propene selectivity of 95% was obtained over Zn(4 wt%)/TiZrO x at 550°C.For deriving key insights into the structure of active sites, reactivity, selectivity and onstream stability, the catalysts were characterized by XRD, HRTEM, EDX mapping, XPS, X-ray absorption, CO-TPR, CO 2 -TPD, NH 3 -TPD, Pyridine-FTIR, operando UV-Vis spectroscopy, Raman spectroscopy, TPO and temporal analysis of products. In contrast with previous reports used bulk ZrO 2 -based catalysts without ZnO, coordinatively unsaturated Zr cations are not the main active sites in the ZnO-containing catalysts.Supported ZnO x species were concluded to participate in the PDH reaction. The current X-ray absorption analysis proved that their structure is affected by the type of metal oxide used as dopant for ZrO 2 and on crystallinity of ZrO 2 . Isolated tricoordinated Zn 2+ species
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