Kinetic analysis of reduction process of supported Rh/Al2O3 catalysts by time resolved in-situ UV–vis spectroscopy

2012 ◽  
Vol 419-420 ◽  
pp. 142-147 ◽  
Author(s):  
Qiang Lin ◽  
Ken-ichi Shimizu ◽  
Atsushi Satsuma
Keyword(s):  
Kinetic Analysis ◽  
Reduction Process ◽  
Time Resolved ◽  
Vis Spectroscopy

Growth of block copolymer stabilized metal nanoparticles probed simultaneously byin situXAS and UV–Vis spectroscopy

2016 ◽  
Vol 23 (1) ◽  
pp. 293-303 ◽  
Author(s):  
C. Nayak ◽  
D. Bhattacharyya ◽  
S. N. Jha ◽  
N. K. Sahoo
Keyword(s):  
Block Copolymer ◽  
Reduction Process ◽  
Pt Nanoparticles ◽  
Vis Spectroscopy ◽  
The Difference ◽  
Two Stages ◽  
Real Time Information ◽  
Kinetics Of ◽  
Time Information

The growth of Au and Pt nanoparticles from their respective chloride precursors using block copolymer-based reducers has been studied by simultaneousin situmeasurement of XAS and UV–Vis spectroscopy at the energy-dispersive EXAFS beamline (BL-08) at INDUS-2 SRS at RRCAT, Indore, India. While the XANES spectra of the precursor give real-time information on the reduction process, the EXAFS spectra reveal the structure of the clusters formed at the intermediate stages of growth. The growth kinetics of both types of nanoparticles are found to be almost similar and are found to follow three stages, though the first stage of nucleation takes place earlier in the case of Au than in the case of Pt nanoparticles due to the difference in the reduction potential of the respective precursors. The first two stages of the growth of Au and Pt nanoparticles as obtained byin situXAS measurements could be corroborated by simultaneousin situmeasurement of UV–Vis spectroscopy also.

In situ time-resolved HEROS study of Pt electronic structure during regenerative H2-O2 cycles

2015 ◽  
Vol 2 (1) ◽  
Author(s):  
Jacinto Sá
Keyword(s):  
Electronic Structure ◽  
Energy Resolution ◽  
Time Resolution ◽  
Electronic States ◽  
Reduction Process ◽  
High Energy ◽  
Single Step ◽  
High Energy Resolution ◽  
Time Resolved

AbstractWe report an in situ time-resolved high-energy resolution off-resonant spectroscopy (HEROS) study with unprecedented 100 ms time resolution revealing the unoccupied electronic states of platinum during regenerative oxidation and reduction cycles. The study depicted a slowed oxidation step in comparison with reduction. The oxidation cycle is composed of two characteristic stages, namely adsorption of dissociated oxygen followed by partial oxidation of Pt subsurface. Besides improved temporal resolution of the experiment, the detected reduction process of platinum showed no intermediate features and was completed in a single step within a few seconds.

scholarly journals Dioxygen Activation Kinetics over Distinct Cu Site Types in Cu-CHA Zeolites

2021 ◽  
Author(s):  
Daniel Bregante ◽  
Laura Wilcox ◽  
Changming Liu ◽  
Christopher Paolucci ◽  
Rajamani Gounder ◽  
...  
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Resonance Raman Spectroscopy ◽  
Structural Evolution ◽  
Synthesis Methods ◽  
Time Resolved ◽  
Vis Spectroscopy ◽  
Kinetics Of ◽  
Site Types

Cu-exchanged zeolites activate dioxygen to form active sites for partial methane oxidation (PMO), nitrogen oxide decomposition, and carbon monoxide oxidation. Apparent rates of O<sub>2</sub> activation depend both on the intrinsic kinetics of distinct Cu site types and the distributions of such sites within a given zeolite, which depend on the density and arrangement of the framework Al atoms. Here, we use hydrothermal synthesis methods to control the arrangement of framework Al sites in chabazite (CHA) zeolites and, in turn, the distinct Cu site types formed. Time-resolved in situ resonance Raman spectroscopy reveals the kinetics of O<sub>2</sub> adsorption and activation within these well-defined Cu-CHA materials and the concomitant structural evolution of copper-oxygen (Cu<sub>x</sub>O<sub>y</sub>) complexes, which are interpreted alongside Cu(I) oxidation kinetics extracted from in situ X-ray absorption spectroscopy (XAS). Raman spectra of several plausible CuxOy species simulated using density functional theory suggest that experimental spectra (λ<sub>ex</sub> = 532 nm) capture the formation of mono(μ-oxo)dicopper species (ZCuOCuZ). Transient experiments show that the timescales required to form Cu<sub>x</sub>O<sub>y</sub> structures that no longer change in Ra-man spectra correspond to the durations of oxidative treatments that maximize CH<sub>3</sub>OH yields in stoichiometric PMO cycles (approximately 2 h). Yet, these periods extend well beyond the timescales for the complete conversion of the initial Cu(I) intermediates to their Cu(II) states (<0.3 h, reflected in XANES spectra), which demonstrates that Cu<sub>x</sub>O<sub>y</sub> complexes continue to evolve structurally following rapid oxidation. The dependence of ZCuOCuZ formation rates on O<sub>2</sub> pressure, H<sub>2</sub>O pressure, and temperature are consistent with a mechanism in which ZCuOH reduce to form ZCu<sup>+</sup> sites that bind molecular oxygen and form ZCu-O<sub>2</sub> intermediates. Subsequent reaction with proximate ZCu<sup>+</sup> form bridging peroxo dicopper complexes that cleave O-O bonds to form ZCuOCuZ in steps facilitated by water. These data and interpretations provide evidence for the chemical processes that link rapid and kinetically irrelevant Cu oxidation steps (frequently probed by XAS and UV-Vis spectroscopy) to the relatively slow genesis of reactive Cu complexes that form CH<sub>3</sub>OH during PMO. In doing so, we reveal previously unrec-ognized complexities in the processes by which Cu ions in zeolites activate O<sub>2</sub> to form active Cu<sub>x</sub>O<sub>y</sub> complexes, which under-score the insight afforded by judicious combinations of experimental and theoretical techniques.

scholarly journals Dioxygen Activation Kinetics over Distinct Cu Site Types in Cu-CHA Zeolites

2021 ◽  
Author(s):  
Daniel Bregante ◽  
Laura Wilcox ◽  
Changming Liu ◽  
Christopher Paolucci ◽  
Rajamani Gounder ◽  
...  
Keyword(s):  
Active Sites ◽  
Density Functional ◽  
Resonance Raman Spectroscopy ◽  
Structural Evolution ◽  
Synthesis Methods ◽  
Time Resolved ◽  
Vis Spectroscopy ◽  
Kinetics Of ◽  
Site Types

Cu-exchanged zeolites activate dioxygen to form active sites for partial methane oxidation (PMO), nitrogen oxide decomposition, and carbon monoxide oxidation. Apparent rates of O<sub>2</sub> activation depend both on the intrinsic kinetics of distinct Cu site types and the distributions of such sites within a given zeolite, which depend on the density and arrangement of the framework Al atoms. Here, we use hydrothermal synthesis methods to control the arrangement of framework Al sites in chabazite (CHA) zeolites and, in turn, the distinct Cu site types formed. Time-resolved in situ resonance Raman spectroscopy reveals the kinetics of O<sub>2</sub> adsorption and activation within these well-defined Cu-CHA materials and the concomitant structural evolution of copper-oxygen (Cu<sub>x</sub>O<sub>y</sub>) complexes, which are interpreted alongside Cu(I) oxidation kinetics extracted from in situ X-ray absorption spectroscopy (XAS). Raman spectra of several plausible CuxOy species simulated using density functional theory suggest that experimental spectra (λ<sub>ex</sub> = 532 nm) capture the formation of mono(μ-oxo)dicopper species (ZCuOCuZ). Transient experiments show that the timescales required to form Cu<sub>x</sub>O<sub>y</sub> structures that no longer change in Ra-man spectra correspond to the durations of oxidative treatments that maximize CH<sub>3</sub>OH yields in stoichiometric PMO cycles (approximately 2 h). Yet, these periods extend well beyond the timescales for the complete conversion of the initial Cu(I) intermediates to their Cu(II) states (<0.3 h, reflected in XANES spectra), which demonstrates that Cu<sub>x</sub>O<sub>y</sub> complexes continue to evolve structurally following rapid oxidation. The dependence of ZCuOCuZ formation rates on O<sub>2</sub> pressure, H<sub>2</sub>O pressure, and temperature are consistent with a mechanism in which ZCuOH reduce to form ZCu<sup>+</sup> sites that bind molecular oxygen and form ZCu-O<sub>2</sub> intermediates. Subsequent reaction with proximate ZCu<sup>+</sup> form bridging peroxo dicopper complexes that cleave O-O bonds to form ZCuOCuZ in steps facilitated by water. These data and interpretations provide evidence for the chemical processes that link rapid and kinetically irrelevant Cu oxidation steps (frequently probed by XAS and UV-Vis spectroscopy) to the relatively slow genesis of reactive Cu complexes that form CH<sub>3</sub>OH during PMO. In doing so, we reveal previously unrec-ognized complexities in the processes by which Cu ions in zeolites activate O<sub>2</sub> to form active Cu<sub>x</sub>O<sub>y</sub> complexes, which under-score the insight afforded by judicious combinations of experimental and theoretical techniques.

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