Homolytic Products from Heterolytic Paths in H2 Dissociation on Metal Oxides: The Example of CeO2

TLDR: In this paper, it was shown by means of density functional theory that oxides with polar M-O bonds might favor heterolytic dissociation, provided that metal ions are reducible.

Abstract: Mechanisms for H2 dissociation on metal oxides have been typically inferred from the infrared spectra of reaction products on the basis of the presence or lack of M–H fingerprints. Here, we demonstrate by means of density functional theory that oxides with polar M–O bonds might favor heterolytic dissociation. Moreover, we report that the resulting heterolytic product can further evolve to the homolytic one provided that metal ions are reducible. Hence, it follows that the redox capacity of the metal determines the reaction outcome. This finding sheds light on why both M–H and O–H bands appear in the infrared spectra of nonreducible oxides such as MgO or γ-Al2O3, while only O–H bands are observed for reducible oxides like CeO2. It results in a unified mechanism for polar oxides that can be generalized to other materials exhibiting significant charge separation. Importantly, we also show that the low activity of CeO2 toward H2 can be improved by enhancing the basicity of surface O atoms upon lattice expansi...

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Homolytic Products from Heterolytic Paths in H
2
Dissociation on Metal Oxides: The example of CeO
2
Journal:
The Journal of Physical Chemistry
Manuscript ID:
jp-2014-02309r.R1
Manuscript Type:
Article
Date Submitted by the Author:
n/a
Complete List of Authors:
García-Melchor, Max; Institute of Chemical Research of Catalonia (ICIQ),
Lopez, Nuria; Institute of Chemical Research of Catalonia (ICIQ),
ACS Paragon Plus Environment
The Journal of Physical Chemistry

1
Homolytic Products from Heterolytic Paths in H
2
Dissociation on Metal Oxides: The example of CeO
2
Max García-Melchor* and Núria López*
Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans, 16, E-43007
Tarragona, Spain.
Keywords: density functional theory, metal oxides, ceria, H
2
dissociation, homolytic,
heterolytic,
Abstract
Mechanisms for H
2
dissociation on metal oxides have been typically inferred from the infrared
spectra of reaction products on the basis of the presence or lack of M–H fingerprints. Here, we
demonstrate by means of Density Functional Theory that oxides with polar M–O bonds favor
heterolytic dissociation. Moreover, we report that the resulting heterolytic product can further
evolve to the homolytic one provided that metal ions are reducible. Hence, it follows that the
redox capacity of the metal determines the reaction outcome. This finding sheds light on why
both M–H and O–H bands appear in the infrared spectra of non-reducible oxides such as MgO or
γ-Al
2
O
3
, while only O–H bands are observed for reducible oxides like CeO
2
. It results in a
unified mechanism for polar oxides that can be generalized to other materials exhibiting
significant charge separation. Importantly, we also show that the low activity of CeO
2
towards
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H
2
can be improved by enhancing the basicity of surface O atoms upon lattice expansion. This
may pave the way for the efficient use of CeO
2
in selective hydrogenation reactions and for the
further advance on processes involving dissociation of non-polar bonds like C–H.
1. Introduction
Molecular hydrogen dissociates through one of the mutually exclusive homolytic or heterolytic
pathways. On metal oxides, MO
x
,
1,2
homolytic (radicalary) dissociation produces two hydrogen
atoms that combine with two oxygen sites leading to the formation of two O–H groups, with the
concomitant reduction of two surface metal ions (Eq. 1). Alternatively, heterolytic (polar)
dissociation entails the formation of a hydride, H
-
, and a proton, H
+
, which adsorb on metal and
oxygen centers yielding M–H and O–H species, respectively (Eq. 2).
So far, the mechanistic proposal for dissociative H
2
adsorption on MO
x
has been based on the
existence in the infrared spectra of either both O–H and M–H bands for the heterolytic pathway,
or only O–H bands for the homolytic path.
3-8
Hence, heterolytic cleavage has been assumed to
occur on non-reducible oxides such as MgO or γ-Al
2
O
3
,
3-5
whereas homolytic cleavage has been
assumed for reducible oxides such as TiO
2
or CeO
2
.
6-8
Herein, we provide firm theoretical evidence that the above a priori excluding mechanisms can
be interconnected. More specifically, we report that heterolytic dissociation of H
2
can also afford
the homolytic product, and that the preference for one pathway or the other, as well as the
reaction outcome, depends on the physicochemical properties of the given MO
x
. Therefore,
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experimental evidences for the products do not ensure that the activation mechanism can be
successfully traced back.
For the present study, we chose ceria, CeO
2
, as a representative reducible MO
x
(Figure 1).
Ceria participates in a number of industrially attractive applications involving H
2
formation or
dissociation such as ethanol steam reforming,
9-10
the water-gas shift reaction,
11-12
and the
selective hydrogenation of alkynes.
13-14
Furthermore, CeO
2
has a prominent role in several
oxidation processes like three-way automotive-exhaust catalysis,
15
solid oxide fuel cells,
16-17
or
HCl oxidation.
18-19
These applications depend in turn on the oxygen storage capacity, which is
directly linked to the presence of oxygen vacancies that can be created by reduction with H
2
.
20
Figure 1. Representation of (a) the CeO
2
lattice and (b) a CeO
2
(111) surface.
H
2
adsorption on CeO
2
has been extensively investigated by means of different experimental
techniques including Temperature-Programmed Reduction (TPR) and Fourier-Transform
Infrared (FTIR) spectroscopy.
6,21-23
The TPR measurements
21-23
typically show two peaks around
750 and 1000 K, which are attributed to surface and bulk reduction, respectively. The stretching
bands appearing in the 3500-3700 cm
-1
region of the FTIR spectra
6,21
are assigned to free
hydroxyl groups with different coordination and to bound water molecules, while no Ce–H bands
have been assigned in the region where they should appear (ca.1500 cm
-1
).
24
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Figure 2. Energy profiles for the homolytic (in red) and heterolytic (in blue) reaction pathways
in the H
2
dissociation on CeO
2
(111). The transition state linking the heterolytic and homolytic
products is shown in black. Optimized geometries with relevant bond distances (Å) are also
shown.
On the basis of the FTIR data, and as with all reducible oxides,
6-8
a homolytic pathway has
been presumed for H
2
dissociation on CeO
2
.
7,13-14,25-29
This mechanistic proposal is, however,
indirect as none of the above experimental observations can provide conclusive evidences of the
operating mechanism. While the use of computational methods might greatly contribute to shed
light on this issue, all the theoretical works reported to date
7,14,25-29
have only considered a
homolytic path, neglecting heterolytic activation. Hence, the aim of the present work is to
compare and evaluate the homolytic and heterolytic paths for H
2
activation on CeO
2
and use this
knowledge to set the basis for predicting the reactivity of H
2
on any MO
x
.
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Frequently asked questions

Q1. What are the contributions mentioned in the paper "Homolytic products from heterolytic paths in h2 dissociation on metal oxides: the example of ceo2" ?

Here, the authors demonstrate by means of Density Functional Theory that oxides with polar M–O bonds favor heterolytic dissociation. Moreover, the authors report that the resulting heterolytic product can further evolve to the homolytic one provided that metal ions are reducible. Importantly, the authors also show that the low activity of CeO2 towards Page 1 of 21 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 

Q2. How was the cutoff energy for the HSE06 functional calculated?

As hybrid functionals are computationally much more demanding than DFT+U, calculations with the HSE06 functional were carried out with a reduced cutoff energy of 400 eV and with a Г−point sampling. 

Q3. What is the optimum spectral distance for the FTIR measurements?

The TPR measurements21-23 typically show two peaks around 750 and 1000 K, which are attributed to surface and bulk reduction, respectively. 

Q4. What is the common method of dissociation of metal oxides?

On metal oxides, MOx, 1,2 homolytic (radicalary) dissociation produces two hydrogen atoms that combine with two oxygen sites leading to the formation of two O–H groups, with the concomitant reduction of two surface metal ions (Eq. 1). 

Q5. What is the energy penalty for TS2?

for MOx with such polarized M–O bonds, heterolytic dissociation will predominate as only electrostatic contributions are enough to compensate the energy penalty associated with this process. 

Q6. What is the energy barrier for the transition state located for this step?

The transition state located for this step (TS1) displays a highly stretched H–H distance of 2.024 Å, and has a relatively high energy barrier of 1.21 eV.46 

Q7. What was the value for Ce atoms?

For Ce atoms, a value of Ueff = 4.5 eV was chosen on the basis of the satisfactory results reported in previous theoretical works. 

Q8. What is the common method of dissociation of hydrogen?

heterolytic (polar) dissociation entails the formation of a hydride, H-, and a proton, H+, which adsorb on metal and oxygen centers yielding M–H and O–H species, respectively (Eq. 2). 

Q9. How has the H2 adsorption on CeO2 been investigated?

H2 adsorption on CeO2 has been extensively investigated by means of different experimental techniques including Temperature-Programmed Reduction (TPR) and Fourier-Transform Infrared (FTIR) spectroscopy.6,21-23 

Q10. What is the isocontour of the H2 -bonding orbital?

The isocontour corresponds to 0.010 Å-3. (b) Schematic representation of the polarization of the H2 σ-bonding orbital caused by the surface electric field in the heterolytic path. 

Q11. What is the name of the journal?

The Journal of Physical Chemistry1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60Supporting Information. 

Q12. What is the reaction mechanism for H2 dissociation on CeO2?

H2 does not dissociate homolytically on CeO2(111), but via a heterolytic pathway (TS1’) followed by the transfer of a hydrogen atom (TS2) that finally yields the homolytic product (I2). 

Q13. What was the DFT calculation used in this work?

All the DFT calculations presented in this work were performed using the Vienna ab initio simulation package (VASP, version 5.3.2).30,31 

Q14. What is the energy of the O(2p) band?

Figure 4. Activation energy, Ea, for the H2 dissociation on strained CeO2(111) surfaces as a function of the center of the O(2p) band, εO(2p), and the corresponding Lattice Strain (top xaxis).