Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes

Guo Shiou Foo; Guoxiang Hu; Zachary D. Hood; Meijun Li; De En Jiang; Zili Wu

doi:10.1021/acscatal.7b01456

Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes

Guo Shiou Foo, Guoxiang Hu, Zachary D. Hood, Meijun Li, De En Jiang, Zili Wu

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Abstract

The kinetics and mechanism of methanol dehydration, redox, and oxidative coupling were investigated at 300 °C under dilute oxygen concentration over anatase TiO₂ nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO₂. The two TiO₂ nanoshapes displayed both (001) and (101) facets, with a higher fraction of the (001) facet exposed on the nanoplates, while truncated-bipyramidal nanocrystals were dominated by the (101) facet. A kinetic study using in situ titration with ammonia shows that the active sites for methanol dehydration are acidic and nonequivalent in comparison to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions, while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO₂ surfaces. Density functional theory calculations show that the formation of dimethyl ether involves the C-H bond dissociation of an adsorbed methoxy, followed by coupling with another surface methoxy on the 5-fold-coordinated Ti cations on the (101) surface, similar to the mechanism reported on the (001) surface. Kinetic isotope effects are observed for dimethyl ether, formaldehyde, and methyl formate in the presence of deuterated methanol (CD₃OH and CD₃OD), confirming that the cleavage of the C-H bond is the rate-limiting step for the formation of these products. A comparison between estimated kinetic parameters for methanol dehydration over various TiO₂ nanocrystals suggests that (001) has a higher dehydration reactivity in comparison to (101), but the surface density of active sites could be limited by the presence of residual fluorine atoms originating from the synthesis. The (001) surface of TiO₂ is also more active than the (101) surface in redox and oxidative coupling of methanol, which is due to the reactive surface oxygen on (001) in comparison to the (101) surface. (Chemical Equation Presented).

Original language	English
Pages (from-to)	5345-5356
Number of pages	12
Journal	ACS Catalysis
Volume	7
Issue number	8
DOIs	https://doi.org/10.1021/acscatal.7b01456
State	Published - Aug 4 2017

Funding

This research was sponsored by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Parts of the work, including XRD, FTIR spectroscopy, and kinetic measurement, were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Z.D.H. gratefully acknowledges a graduate fellowship from the National Science Foundation under Grant No. DGE-1148903 and the Georgia Tech-ORNL Fellowship. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Funders	Funder number
Georgia Tech-ORNL	DE-AC02-05CH11231
National Science Foundation	DGE-1148903
U.S. Department of Energy
Office of Science
Basic Energy Sciences
Chemical Sciences, Geosciences, and Biosciences Division

Keywords

TiO
dehydration
dimethyl ether
formaldehyde
methyl formate
oxidative coupling
redox
surface structure

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10.1021/acscatal.7b01456

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title = "Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes",

abstract = "The kinetics and mechanism of methanol dehydration, redox, and oxidative coupling were investigated at 300 °C under dilute oxygen concentration over anatase TiO2 nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO2. The two TiO2 nanoshapes displayed both (001) and (101) facets, with a higher fraction of the (001) facet exposed on the nanoplates, while truncated-bipyramidal nanocrystals were dominated by the (101) facet. A kinetic study using in situ titration with ammonia shows that the active sites for methanol dehydration are acidic and nonequivalent in comparison to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions, while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO2 surfaces. Density functional theory calculations show that the formation of dimethyl ether involves the C-H bond dissociation of an adsorbed methoxy, followed by coupling with another surface methoxy on the 5-fold-coordinated Ti cations on the (101) surface, similar to the mechanism reported on the (001) surface. Kinetic isotope effects are observed for dimethyl ether, formaldehyde, and methyl formate in the presence of deuterated methanol (CD3OH and CD3OD), confirming that the cleavage of the C-H bond is the rate-limiting step for the formation of these products. A comparison between estimated kinetic parameters for methanol dehydration over various TiO2 nanocrystals suggests that (001) has a higher dehydration reactivity in comparison to (101), but the surface density of active sites could be limited by the presence of residual fluorine atoms originating from the synthesis. The (001) surface of TiO2 is also more active than the (101) surface in redox and oxidative coupling of methanol, which is due to the reactive surface oxygen on (001) in comparison to the (101) surface. (Chemical Equation Presented).",

keywords = "TiO, dehydration, dimethyl ether, formaldehyde, methyl formate, oxidative coupling, redox, surface structure",

author = "Foo, {Guo Shiou} and Guoxiang Hu and Hood, {Zachary D.} and Meijun Li and Jiang, {De En} and Zili Wu",

note = "Publisher Copyright: {\textcopyright} 2017 American Chemical Society.",

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AU - Foo, Guo Shiou

AU - Hu, Guoxiang

AU - Hood, Zachary D.

AU - Li, Meijun

AU - Jiang, De En

AU - Wu, Zili

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N2 - The kinetics and mechanism of methanol dehydration, redox, and oxidative coupling were investigated at 300 °C under dilute oxygen concentration over anatase TiO2 nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO2. The two TiO2 nanoshapes displayed both (001) and (101) facets, with a higher fraction of the (001) facet exposed on the nanoplates, while truncated-bipyramidal nanocrystals were dominated by the (101) facet. A kinetic study using in situ titration with ammonia shows that the active sites for methanol dehydration are acidic and nonequivalent in comparison to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions, while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO2 surfaces. Density functional theory calculations show that the formation of dimethyl ether involves the C-H bond dissociation of an adsorbed methoxy, followed by coupling with another surface methoxy on the 5-fold-coordinated Ti cations on the (101) surface, similar to the mechanism reported on the (001) surface. Kinetic isotope effects are observed for dimethyl ether, formaldehyde, and methyl formate in the presence of deuterated methanol (CD3OH and CD3OD), confirming that the cleavage of the C-H bond is the rate-limiting step for the formation of these products. A comparison between estimated kinetic parameters for methanol dehydration over various TiO2 nanocrystals suggests that (001) has a higher dehydration reactivity in comparison to (101), but the surface density of active sites could be limited by the presence of residual fluorine atoms originating from the synthesis. The (001) surface of TiO2 is also more active than the (101) surface in redox and oxidative coupling of methanol, which is due to the reactive surface oxygen on (001) in comparison to the (101) surface. (Chemical Equation Presented).

AB - The kinetics and mechanism of methanol dehydration, redox, and oxidative coupling were investigated at 300 °C under dilute oxygen concentration over anatase TiO2 nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO2. The two TiO2 nanoshapes displayed both (001) and (101) facets, with a higher fraction of the (001) facet exposed on the nanoplates, while truncated-bipyramidal nanocrystals were dominated by the (101) facet. A kinetic study using in situ titration with ammonia shows that the active sites for methanol dehydration are acidic and nonequivalent in comparison to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions, while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO2 surfaces. Density functional theory calculations show that the formation of dimethyl ether involves the C-H bond dissociation of an adsorbed methoxy, followed by coupling with another surface methoxy on the 5-fold-coordinated Ti cations on the (101) surface, similar to the mechanism reported on the (001) surface. Kinetic isotope effects are observed for dimethyl ether, formaldehyde, and methyl formate in the presence of deuterated methanol (CD3OH and CD3OD), confirming that the cleavage of the C-H bond is the rate-limiting step for the formation of these products. A comparison between estimated kinetic parameters for methanol dehydration over various TiO2 nanocrystals suggests that (001) has a higher dehydration reactivity in comparison to (101), but the surface density of active sites could be limited by the presence of residual fluorine atoms originating from the synthesis. The (001) surface of TiO2 is also more active than the (101) surface in redox and oxidative coupling of methanol, which is due to the reactive surface oxygen on (001) in comparison to the (101) surface. (Chemical Equation Presented).

KW - TiO

KW - dehydration

KW - dimethyl ether

KW - formaldehyde

KW - methyl formate

KW - oxidative coupling

KW - redox

KW - surface structure

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DO - 10.1021/acscatal.7b01456

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Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes

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