The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported ...Na2WO4/SiO2 catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOx sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOx sites are responsible for selectively activating CH4 to C2Hx and over‐oxidizing CHy to CO and (ii) molten Na2WO4 phase is mainly responsible for over‐oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2 catalysts.
In the current study, with the aid of state‐of‐the‐art spectroscopic techniques, transient kinetic analysis, and implementation of robust experimental methodologies, we resolve the nature of catalytic active sites and reaction mechanism for oxidative coupling of methane over supported Na2WO4/SiO2 catalysts.
The involvement of lattice oxygen species is important toward oxidative coupling of the methane reaction (OCM) over supported Mn-Na2WO4/SiO2 catalysts, but there is no consensus regarding the types, ...role, and origin of lattice oxygen species present in supported Mn-Na2WO4/SiO2 catalysts, which hinders the understanding of the OCM reaction network. In the present study, by utilizing the temporal analysis of products technique, we show that supported Na2WO4/SiO2 catalysts possess two different types of oxygen species, dissolved O2 and atomic O, at an OCM-relevant temperature. The addition of Mn-oxide to this catalyst increases the total amount and release rate of dissolved O2 species and improves C2 selectivity of both dissolved O2 and atomic lattice O species.
The structure and promotional effect of Mn in supported Mn-Na2WO4/SiO2 catalysts for the oxidative coupling of methane (OCM) reaction has been debated for a longtime in the literature. In the current ...investigation, with the aid of multiple in-situ characterization studies, we show that the freshly calcined supported 1.2Mn-5Na2WO4/SiO2 catalyst possesses crystalline Na2WO4, Mn2O3 and SiO2 (cristobalite phase) along with surface MnOx and Na-WOx sites at low temperature and oxidizing environments. Under the OCM reaction environment (T > 800 °C), the crystalline Na2WO4 phase melts and Mn2O3 phase reduces. In contrast, the surface MnOx and Na-WOx sites exhibit excellent thermal and chemical stability. Exposure of the 1.2Mn-5Na2WO4/SiO2 catalyst to the OCM reaction environment redisperses the molten Na2WO4 phase on the SiO2 support to form new surface WOx sites. Interestingly, the stable MnOx species interacts with both molten Na2WO4 phase and surface Na-WOx sites during OCM reaction. Controlled transient kinetic experiments in TAP and detailed steady state OCM fixed-bed reaction studies reveal the role and promotional effect of Mn in the 1.2Mn-5Na2WO4/SiO2 catalyst. The W-oxides (both molten Na2WO4 and surface Na-WOx sites) are the active sites for the catalytic OCM reaction and the MnOx species only function as promoters. The promotion of MnOx strongly depends on the gas phase O2 partial pressure and the MnOx species act as mediators for oxygen exchange between the gas phase molecular O2 and catalyst lattice oxygen. The temperature dependent MnOx promotion reveals that the MnOx species selectively promote the molten Na2WO4 phase at lower reaction temperature and the surface Na-WOx sites at higher temperature.
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•Mn-oxide is present as surface MnOx sites during the OCM reaction.•MnOx sites interact with W-oxide centers (both molten Na2WO4 phase and surface Na-WOx sites) during the OCM reaction.•W-oxide centers are the active site for the OCM reaction.•MnOx species act only as promoters during the OCM reaction.•MnOx promotion is strongly influenced by gas-phase oxygen partial pressure.
The structure and promotional effect of Mn in supported Mn-Na2WO4/SiO2 catalysts for the oxidative coupling of methane (OCM) reaction has been debated for a longtime in the literature. In the current ...investigation, with the aid of multiple in-situ characterization studies, we show that the freshly calcined supported 1.2Mn-5Na2WO4/SiO2 catalyst possesses crystalline Na2WO4, Mn2O3 and SiO2 (cristobalite phase) along with surface MnOx and Na-WOx sites at low temperature and oxidizing environments. Under the OCM reaction environment (T>800°C), the crystalline Na2WO4 phase melts and Mn2O3 phase reduces. In contrast, the surface MnOx and Na-WOx sites exhibit excellent thermal and chemical stability. Exposure of the 1.2Mn-5Na2WO4/SiO2 catalyst to the OCM reaction environment redisperses the molten Na2WO4 phase on the SiO2 support to form new surface WOx sites. Interestingly, the stable MnOx species interacts with both molten Na2WO4 phase and surface Na-WOx sites during OCM reaction. Controlled transient kinetic experiments in TAP and detailed steady state OCM fixed-bed reaction studies reveal the role and promotional effect of Mn in the 1.2Mn-5Na2WO4/SiO2 catalyst. The W-oxides (both molten Na2WO4 and surface Na-WOx sites) are the active sites for the catalytic OCM reaction and the MnOx species only function as promoters. The promotion of MnOx strongly depends on the gas phase O2 partial pressure and the MnOx species act as mediators for oxygen exchange between the gas phase molecular O2 and catalyst lattice oxygen. In conclusion, the temperature dependent MnOx promotion reveals that the MnOx species selectively promote the molten Na2WO4 phase at lower reaction temperature and the surface Na-WOx sites at higher temperature.
Abstract
The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported ...Na
2
WO
4
/SiO
2
catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H
2
‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na
2
WO
4
active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WO
x
sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (
i
) surface Na‐WO
x
sites are responsible for selectively activating CH
4
to C
2
H
x
and over‐oxidizing CH
y
to CO and (
ii
) molten Na
2
WO
4
phase is mainly responsible for over‐oxidation of CH
4
to CO
2
and also assists in oxidative dehydrogenation of C
2
H
6
to C
2
H
4
. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na
2
WO
4
/SiO
2
catalysts.
In this study, we describe a novel peripheral-nerve interface which makes use of highly flexible multi-electrode arrays that are integrated into hydrogel-based scaffolds to form a hybrid ...tissue-engineered electronic construct. This tissue-engineered electronic nerve interface (TEENI) is designed to be scalable to high channel counts using multiple polyimide-based "threads" that are evenly distributed through a volume of the nerve equal to its diameter times the distance between one or more nodes of Ranvier. Such scalability could greatly increase the precision and resolution of motor-control and sensory-feedback signals exchanged between amputees and advanced upper-limb prosthetic devices.
Regenerative peripheral-nerve interfaces are a novel method for integrating with the peripheral nervous system. These devices have the potential to isolate and transduce both afferent (sensory) and ...efferent (motor) neural signals to produce fine control of advanced prosthetics. We have developed a novel regenerative device comprised of microfabricated polyimide electrode threads supported by a hydrogel scaffold containing methacrylated hyaluronic acid, collagen I, and laminin to enable intimate contact with regenerating axons. While this advanced device holds theoretical promise for establishing a stable chronic neural interface, it also requires a novel surgical approach in comparison to current existing methods of peripheral neural interface technologies. Here we describe the development of the surgical methodology required for successful chronic implantation of the TEENI device in the rat sciatic nerve.
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