A study of the kinetics of ethanol conversion in the presence of Zr-containing zeolites BEA doped with palladium particles has revealed the order of formation of the main reaction products. It has ...been shown that the primary processes are ethanol dehydrogenation to acetaldehyde on Pd sites and ethanol dehydration to diethyl ether on the acid sites of the catalyst. After that, acetaldehyde undergoes the aldol–croton condensation reaction to form crotonal, which is hydrogenated to butanol on the metal sites. Butanol, in turn, is dehydrated into butenes, which undergo hydrogenation to butane. The presence of hydrogen in the gas phase leads to the displacement of ethanol from the metal surface and prevents the formation of surface carbonates and acetates. It has been found that hydrogen significantly accelerates ethanol dehydration owing to a decrease in the activation energy, which can be attributed to hydrogen spillover to the zeolite. The addition of water inhibits all acid-catalyzed reactions owing to competitive adsorption on acid sites and thereby decreases the butanol yield and the ethanol conversion.
Butanal conversion has been studied over Zr‐BEA zeolites with different amount of partially hydrolyzed open Zr‐sites and fully condensed closed sites. The kinetic study pointed that the main reaction ...pathway of butanal conversion involves its self‐condensation into 2‐ethyl‐2‐hexenal (2‐EN), the selectivity to 2‐EN being within 80–85 %. Besides, small amounts of 1‐butanol and butanoic acid are formed via Cannizzaro disproportionation of butanal. Secondary reactions include butanoic acid ketonization to 4‐heptanone and esterification to butyl butanoate and tandem hydrogenation/dehydration of 2‐EN into octadienes. A number of techniques, including FTIR spectroscopy of adsorbed CO, pyridine, 2,6‐di‐tert‐butylpyridine (DTBPy), TPD of isopropylamine and selective poisoning with DTBPy were applied for characterization of Zr‐sites and elucidation of their activity. Open Zr‐sites were found to be more active with respect to closed Zr‐sites in butanal condensation: the TOF values estimated at 433 K being 1.4 s−1 for open Zr‐sites and 0.6 s−1 for closed sites. On the contrary, selectivity into 2‐EN was similar for open and closed sites. The results point that Zr‐BEA catalysts with enhanced content of open sites are perspective catalysts for the selective synthesis of 2‐EN.
Intrinsic site reactivity: Zr‐doped BEA zeolite is active and selective catalyst in butanal condensation to 2‐ethyl‐hexenal. Analysis with in situ FTIR revels that condensation is followed by more high temperature Cannizzaro disproportionation. Kinetic study with selective poisoning allows to establish partially hydrolyzed Zr(OSi)3(OH) open sites as more active than fully condensed closed sites.
The mechanism of Zr-BEA hydrothermal synthesis in fluoride media has been investigated through the detailed characterization of samples obtained at different synthesis times by XRD, XRF, TGA, ...multinuclear solid-state NMR, FTIR, SEM, TEM with EDS, XAS, and nitrogen sorption. The synthetic procedure involved hydrothermal crystallization of the gel with the following composition: 1SiO2:0.54TEAOH:0.54HF:0.005ZrO2:5.6H2O. The formation of open and closed Lewis acid sites was monitored by FTIR spectroscopy of adsorbed CO, while coordination of Zr was studied by XAS. The results show that the formation of Zr-BEA proceeds by two steps. In the first step, pure silica BEA is crystallized via a solid–solid hydrogel rearrangement mechanism. Zirconium species are occluded in Si-BEA crystals in the form of Zr-rich silicate particles. These particles do not provide for any appreciable Lewis acidity. In the second step, Zr incorporation into T positions of the zeolite structure takes place, leading to the formation of closed Zr sites, which are partially converted into open sites at longer synthesis times. It is demonstrated that the content of open and closed sites can be tuned by variation of the synthesis time.
In situ X-ray absorption spectroscopy (XAS) is a powerful technique for the investigation of heterogeneous catalysts and electrocatalysts. The obtained XAS spectra are usually interpreted from the ...point of view of the investigated chemical processes, thereby sometimes omitting the fact that intense X-ray irradiation may induce additional transformations in metal speciation and, thus, in the corresponding XAS spectra. In this work, we report on X-ray induced photochemical reduction of vanadium in supported vanadia (VO x ) catalysts under reaction conditions, detected at a synchrotron beamline. While this process was not observed in an inert atmosphere and in the presence of water vapor, it occurred at room temperature in the presence of a reducing agent (ethanol or hydrogen) alone or mixed with oxygen. Temperature programmed experiments have shown that X-ray induced reduction of VO x species appeared very clear at 30–100 °C but was not detected at higher temperatures, where the thermocatalytic ethanol oxidative hydrogenation (ODH) takes place. Similar to other studies on X-ray induced effects, we suggest approaches, which can help to mitigate vanadium photoreduction, including defocusing of the X-ray beam and attenuation of the X-ray beam intensity by filters. To recognize beam damage under in situ / operando conditions, we suggest performing X-ray beam switching (on and off) tests at different beam intensities under in situ conditions.
In situ
X-ray absorption spectroscopy (XAS) is a powerful technique for the investigation of heterogeneous catalysts and electrocatalysts. The obtained XAS spectra are usually interpreted from the ...point of view of the investigated chemical processes, thereby sometimes omitting the fact that intense X-ray irradiation may induce additional transformations in metal speciation and, thus, in the corresponding XAS spectra. In this work, we report on X-ray induced photochemical reduction of vanadium in supported vanadia (VO
x
) catalysts under reaction conditions, detected at a synchrotron beamline. While this process was not observed in an inert atmosphere and in the presence of water vapor, it occurred at room temperature in the presence of a reducing agent (ethanol or hydrogen) alone or mixed with oxygen. Temperature programmed experiments have shown that X-ray induced reduction of VO
x
species appeared very clear at 30-100 °C but was not detected at higher temperatures, where the thermocatalytic ethanol oxidative hydrogenation (ODH) takes place. Similar to other studies on X-ray induced effects, we suggest approaches, which can help to mitigate vanadium photoreduction, including defocusing of the X-ray beam and attenuation of the X-ray beam intensity by filters. To recognize beam damage under
in situ
/
operando
conditions, we suggest performing X-ray beam switching (on and off) tests at different beam intensities under
in situ
conditions.
X-ray photoreduction of V
5+
in supported vanadia catalysts was detected by XAS during
in situ
experiments at a synchrotron beamline. Practical strategies helping to recognize and mitigate such problems are suggested.
The multilayer settlement Rakushechny Yar situated in the lower Don River (Rostov region, Russia) is one of the oldest early Neolithic sites in this region, dated to the 7th and 6th millennia BC. ...Recent investigations have shown a particular importance of this site in the study of the spread of the Near Eastern “Neolithic package” and the neolithisation of Eastern Europe. Long-term study has provided unique evidence of lives of ancient communities. New 14C dates contribute to refining the chronology of the recently excavated to dating the development of cultural traditions more precisely. The excellent preservation state of organic materials led to uncovering a rich assemblage of faunal and fish remains, household constructions, hunting and fishing tools, as well as pottery. The subsistence strategies and the life cycle of these communities were reconstructed through multiple proxies, which describe a particular system of resource management determined by specific economic, environmental and cultural conditions. Rich fish remains, shell middens, site location, specific toolkit with restricted categories, and incomplete context of tool production testify all that it was a specialized site for aquatic resource procurement. Faunal remains indicated the use of resources from other ecological niches as well. Finds of bones of domesticated animals in the same Early Neolithic layers may suggest even a more complicated organization of this ancient community and may indicate the northern limit of the Neolithic package distribution.
In situ X-ray absorption spectroscopy (XAS) is a powerful technique for the investigation of heterogeneous catalysts and electrocatalysts. The obtained XAS spectra are usually interpreted from the ...point of view of the investigated chemical processes, thereby sometimes omitting the fact that intense X-ray irradiation may induce additional transformations in metal speciation and, thus, in the corresponding XAS spectra. In this work, we report on X-ray induced photochemical reduction of vanadium in supported vanadia (VOx) catalysts under reaction conditions, detected at a synchrotron beamline. While this process was not observed in an inert atmosphere and in the presence of water vapor, it occurred at room temperature in the presence of a reducing agent (ethanol or hydrogen) alone or mixed with oxygen. Temperature programmed experiments have shown that X-ray induced reduction of VOx species appeared very clear at 30–100 °C but was not detected at higher temperatures, where the thermocatalytic ethanol oxidative hydrogenation (ODH) takes place. Similar to other studies on X-ray induced effects, we suggest approaches, which can help to mitigate vanadium photoreduction, including defocusing of the X-ray beam and attenuation of the X-ray beam intensity by filters. To recognize beam damage under in situ/operando conditions, we suggest performing X-ray beam switching (on and off) tests at different beam intensities under in situ conditions.
Titania-supported
vanadia (VO
x
/TiO
2
) catalysts
exhibit outstanding catalytic in a number of selective
oxidation and reduction processes. In spite of numerous investigations,
the nature of redox ...transformations of vanadium and titanium involved
in various catalytic processes remains difficult to detect and correlate
to the rate of products formation. In this work, we studied the redox
dynamics of active sites in a bilayered 5% V
2
O
5
/15% TiO
2
/SiO
2
catalyst (consisting of submonolayer
VO
x
species anchored onto a TiO
x
monolayer, which in turn is supported on SiO
2
) during the oxidative dehydrogenation of ethanol. The VO
x
species in 5% V
2
O
5
/15%
TiO
2
/SiO
2
show high selectivity to acetaldehyde
and an ca. 40 times higher acetaldehyde formation rate in comparison
to VO
x
species supported on SiO
2
with a similar density.
Operando
time-resolved
V and Ti K-edge X-ray absorption near-edge spectroscopy, coupled with
a transient experimental strategy, quantitatively showed that the
formation of acetaldehyde over 5% V
2
O
5
/15% TiO
2
/SiO
2
is kinetically coupled to the formation of
a V
4+
intermediate, while the formation of V
3+
is delayed and 10–70 times slower. The low-coordinated nature
of various redox states of VO
x
species
(V
5+
, V
4+
, and V
3+
) in the 5% V
2
O
5
/15% TiO
2
/SiO
2
catalyst
is confirmed using the extensive database of V K-edge XANES spectra
of standards and specially synthesized molecular crystals. Much weaker
redox activity of the Ti
4+
/Ti
3+
couple was also
detected; however, it was found to not be kinetically coupled to the
rate-determining step of ethanol oxidation. Thus, the promoter effect
of TiO
x
is rather complex. TiO
x
species might be involved in a fast electron
transport between VO
x
species and might
affect the electronic structure of VO
x
, thereby promoting their reducibility. This study demonstrates the
high potential of element-specific
operando
X-ray
absorption spectroscopy for uncovering complex catalytic mechanisms
involving the redox kinetics of various metal oxides.
Supported vanadia (VO
) is a versatile catalyst for various redox processes where ceria-supported VO
have shown to be particularly active in the oxidative dehydrogenation (ODH) of alcohols. In this ...work, we clarify the origin of the volcano-shaped ethanol ODH activity trend for VO
/CeO
catalysts using operando quick V K- and Ce L
- edge XAS experiments performed under transient conditions. We quantitatively demonstrate that both vanadium and cerium are synergistically involved in alcohol ODH. The concentration of reversible Ce
/Ce
species was identified as the main descriptor of the alcohol ODH activity. The activity drop in the volcano plot, observed at above ca. 3 V nm
surface loading (ca. 30 % of VO
monolayer coverage), is related to the formation of spectator V
and Ce
species, which were identified here for the first time. These results might prove to be helpful for the rational optimization of VO
/CeO
catalysts and the refinement of the theoretical models.
Supported vanadia (VOx) is a versatile catalyst for various redox processes where ceria‐supported VOx have shown to be particularly active in the oxidative dehydrogenation (ODH) of alcohols. In this ...work, we clarify the origin of the volcano‐shaped ethanol ODH activity trend for VOx/CeOx catalysts using operando quick V K‐ and Ce L3‐ edge XAS experiments performed under transient conditions. We quantitatively demonstrate that both vanadium and cerium are synergistically involved in alcohol ODH. The concentration of reversible Ce4+/Ce3+ species was identified as the main descriptor of the alcohol ODH activity. The activity drop in the volcano plot, observed at above ca. 3 V nm−2 surface loading (ca. 30 % of VOx monolayer coverage), is related to the formation of spectator V4+ and Ce3+ species, which were identified here for the first time. These results might prove to be helpful for the rational optimization of VOx/CeO2 catalysts and the refinement of the theoretical models.
Using operando time‐resolved X‐ray absorption spectroscopy, we investigated the origin of the volcano‐shaped activity trend in the oxidative dehydrogenation of ethanol over VOx/CeO2 catalysts as a function of VOx surface coverage. Vanadium and cerium synergistically change their oxidation states during the catalytic cycle. The catalytic activity correlates with the concentration of reversible Ce4+/3+ species.
