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A first-principles study on the hydrogenation ofacetone on HxMoO3 surface

Qiyun Pan a, Liang Huang b,*, Zhong Li a, Juanjuan Han a, Nian Zhao a,Yunlong Xie a, Xiang Li a, Meifeng Liu a, Xiuzhang Wang a,Jun-Ming Liu a,c

a Institute for Advanced Materials, Hubei Key Laboratory of Pollutant Analysis &Reuse Technology,

Hubei Normal University, Huangshi 435002, Chinab The State Key Laboratory of Refractories and Metallurgy, College of Materials and Metallurgy,

Wuhan University of Science and Technology, Wuhan 430081, Chinac National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

a r t i c l e i n f o

Article history:

Received 19 December 2018

Received in revised form

4 February 2019

Accepted 6 February 2019

Available online 7 March 2019

Keywords:

First-principles study

Acetone hydrogenation

HxMoO3 surface

H content

* Corresponding author.E-mail address: huangliang1986@wust.ed

https://doi.org/10.1016/j.ijhydene.2019.02.0320360-3199/© 2019 Hydrogen Energy Publicati

a b s t r a c t

Hydrogenation of acetone on the (010) surface of hydrogen molybdenum bronzes was

investigated by density functional theory (DFT) calculations with periodic slab models. The

formation of H-bond between the carbonyl oxygen of acetone and the terminal OH group of

the surface leads to a stable adsorption of acetone. The effect of hydrogen concentration in

the bronzes on the hydrogenation of acetone was systematically investigated, indicating

the hydrogenation reaction is a one-step concerted and exothermic process regardless of

the hydrogen contents in the bronzes surface. The 8H surface with increased H-content

shows a significantly exothermic reaction process and exhibits the smallest kinetic barrier

compared with 4H or 6H surfaces. Additionally, the selectivity for hydrogenation acetone

could increase owing the absence of CeC bond activation. The findings in this study can

help with designing of high-efficient and low-cost metal oxide catalysts for hydrogenation

of unsaturated substances.

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction

Hydrogenation of unsaturated substances is a widely prac-

ticed process in catalytic reforming of petroleum feedstocks

and numerous other chemical production processes [1e7]. As

known, acetone is one of the typical unsaturated organic

compounds. With the development of current industrial

technology, the industrial yield of acetone has been over-

saturated, while the usage of acetone as solvent is

u.cn (L. Huang).

ons LLC. Published by Els

decreasing in recent years, so as the other fields. In order to

extend the usage of acetone, substantial experimental and

theoretical studies have been devoted onto acetone hydroge-

nation in the recent years [8e14]. Among the hydrogenated

products, isopropanol as the highly selective product of

acetone is a critical solvent in industry for fine chemical

synthesis, and enlarging the scale of isopropanol production

by various approaches has drawn a lot of attention. In addi-

tion, isopropanol is in high demand for direct isopropanol fuel

cells for hydrogen storage [15e18], and relevant technologies

evier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 0 4 4 3e1 0 4 5 210444

possess many advantages such as high-energy production,

much lower crossover current, ease to oxidize, production of

more hydrogen, and being less toxic than other alcohols like

methanol and ethanol.

However, a highly efficient hydrogenation of acetone into

isopropanol essentially relies on specific catalysts. High effi-

ciency and cost-competitive catalysts for hydrogenation of

acetone are becoming highly concerned. Conventional cata-

lysts are mainly composed of metal materials such as nickel,

copper, platinum, rhodium and ruthenium as the active in-

gredients [19e31]. These catalysts may be categorized into

three types, namely, supported metal catalysts, alloy cata-

lysts, and metal oxide catalysts. Although the metal catalysts,

especially precious metal materials, exhibit obviously high

efficiency in acetone hydrogenation, they are more expensive

and easy to be poisoned. Moreover, these solid heterogeneous

catalysts work un-effectively, due to complicated experi-

mental procedure at high operating temperature.

Alternatively, a class of hydrogen bronzes materials in the

form of HxTMOy, where TM denotes transition metal, have

been attracting attention in recent years. They can be pre-

pared from transition metal oxides such as MoO3, WO3, and

V2O5, via hydrogen spillovermechanisms. Hydrogen spillover

phenomenon inMoO3 andWO3 was extensively studied since

the 1970s [32]. In fact, earlier investigations indicated that the

formation of hydrogen bronzes was favorable and H atoms

have high mobility in these transition metal oxides [33e37]

with low energy barrier pathways. The low barriers for pro-

ton diffusion on MoO3 (010) surface allow highly mobile pro-

tons at near ambient temperature [38]. On the other hand, a

series of previous works did demonstrate the highly selective

and effective catalytic kinetics of hydrogen bronzesmaterials

in the hydrogenation of p-conjugated organic molecules with

carbon-carbon double bond [39e42]. Recent theoretical cal-

culations also predicted that molybdenum oxide hydrogen

bronzes such as HxMoO3 were highly effective to the hydro-

genation of ethylene [42]. Compared with transition metal

catalysts, several reactions catalyzed by hydrogen bronzes

generally occur at lower temperature without unfavored by-

products [43,44]. All these earlier investigations suggest the

great potentials of hydrogen bronzes as promising hydroge-

nation catalysts of unsaturated substances. We believe,

therefore that hydrogen molybdenum bronzes may be one

kind of effective catalysts with promising applications in

acetone hydrogenation owing to the ability of hydrogen mo-

lybdenum bronzes to reversibly capture and release

hydrogen [45].

Nevertheless, so far, the hydrogenation of acetone by

hydrogen bronzes remains less touched and there is almost

no study onmicroscopic mechanism for hydrogenation of C]

O onto CeOH with these hydrogen bronzes. Therefore, the

study of hydrogenation of C]O using acetone as probe

molecule on the bronzes surface is one of the most important

topics. Given the issues discussed above, it is of essential in-

terest to address possible kinetic landscape of the acetone

hydrogenation process on the hydrogen bronzes surface,

using the first-principles calculations based on the density-

functional theory (DFT). As reported, the hydrogen concen-

trations in the lattice influence the physicochemical proper-

ties of MoO3 [45] and the reaction activity [39,40]. Therefore,

the hydrogen concentration on a specific surface of HxMoO3

and its influence on the acetone hydrogenation reaction will

be given special attention. The minimum energy pathways

caused by hydrogenation of acetone to isopropanol were

carried out. The objective of this study is to provide useful

insight into hydrogen bronzes materials as potential hydro-

genation catalysts in the hydrogenation of acetone and may

shed light on the hydrogenation mechanism of C]O.

Computational details

In our calculation, the periodic surfacemodel was constructed

by considering a (010) MoO3 surface consisting of four-atom-

layers, as done in our recent work [42] and shown in Fig. 1

for a schematic illustration. The calculated adsorption en-

ergies of acetone on the (2 � 2) MoO3 (010) slab, (3 � 3) MoO3

(010) slab and two-layer (2 � 2) MoO3 (010) slab were�4.9, �3.5

and �4.5 kcal/mol, respectively. Since MoO3 is principally

layer-packed along the [010] orientation bymeans of weak van

der Waals interactions, the effect of further increasing the

atom layers is virtually negligible. Meanwhile, high coverage

is more conducive to the adsorption of reactant. So we chose a

supercell containing one (2 � 2) MoO3 (010) slab, which is large

enough to accommodate surface reactive species. Between

adjacent slabs, a vacuum space of 15 �A was inserted to elim-

inate the artificial inter-cell interactions.

All the calculations were performed on the commercially

available code VASP (Vienna ab-initio simulation package) in

the framework of spin-polarized generalized gradient

approximation (GGA) [46,47]. The exchange-correlation po-

tential was described by the PBE functional [48], and the cut-

off energy of solving Kohh-Sham equation was chosen as

400 eV. The Monkhorst-Pack grid (3 � 3 � 1) was applied to

sample the Brillouin zone during the geometry relaxation,

while the Methfessel-Paxton technique was adopted to

include the electron smearing effect in order to minimize the

errors in Hellmann-Feynman forces and facilitate self-

consistent field convergence [49]. The geometry was consid-

ered as an equilibrium state when the energy difference in

two subsequent optimization steps was smaller than 10�3 eV

and the force was converged to be less than 0.03 eV/�A. The

structures of transition states were obtained by the climbing

image nudged elastic band (CI-NEB) method [50,51], where six

images between reactant and product were inserted to ach-

ieve a smooth energy curvewith the force tolerance of 0.05 eV/�A. The minimum energy path calculations were performed to

explore the detailed catalytic process of acetone hydrogena-

tion. This computational protocol has been proved to be well-

accurate in producing structural and energetic data for MoO3

and HxMoO3 systems [42].

Results and discussions

The structures and formation thermodynamics of HxMoO3

with various hydrogen contents can be found in details in

our previous publication [34]. In general, there are three

types of O atoms on the MoO3(010) surface, denoted as Ot

(terminal site), Oa (asymmetric site) and Os (symmetric

Fig. 1 e (a) Side view of MoO3 (010) surface, (b) top view of MoO3 (010) surface.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 0 4 4 3e1 0 4 5 2 10445

site) (Fig. 1.). Upon the hydrogenation of MoO3, the Ot sites

are most preferentially occupied by H atoms, followed by

the occupations at the Oa and Os sites. Since H atoms are

highly mobile in MoO3 lattice, we can rationally assume

that the Ot sites are always occupied by H atoms during the

hydrogenation process. As shown in hydrogenation of

Fig. 2 e (a) Calculated energy diagram of the hydrogenation of a

view of the acetone on the 4H molybdenum bronzes surface. Op

the 4H surface (c) initial state (AB), (d) transition state (TS) and (

HxMoO3 before adsorption, C þ D is the state of isopropanol an

ethylene, the increased hydrogen concentration could lead

to an enhanced kinetics [42]. A series of HxMoO3 systems

with varying H contents (labeled as 4H, 6H and 8H for

illustration (Figs. 2e4), were considered in our calculations

to evaluate the reactivity of HxMoO3 with various hydrogen

concentrations.

cetone on the 4H molybdenum bronzes surface. (b) The top

timized configurations of acetone absorption geometries on

e) final state (CD). (A þ B presents the state of acetone and

d HxMoO3 after hydrogenation).

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 0 4 4 3e1 0 4 5 210446

Hydrogenation of acetone on 4H molybdenum bronzessurface

The two-step hydrogenationmechanismwas proven to be the

dominant process for hydrogenation of acetone on transition

metal catalysts. However, this reactionmechanismwas found

inoperative on the HxMoO3 (010) surface. Indeed, the addition

of one hydrogen to the carbonyl carbonwas optimized to form

acetone, while the addition of one hydrogen to the carbonyl

oxygenwas optimized to produce isopropanol. In otherwords,

the intermediate free radical could not be stabilized by the

surface. The taken reaction route is for the two terminal

hydrogen atoms to migrate simultaneously from the HxMoO3

surface to form a new CeOH bonds. The calculated hydroge-

nation profile is shown in Fig. 2, and several key structural

parameters of the reactant, transition state, and product on

the 4H surface are displayed in Table 1. Initially, the acetone

molecule is attracted by the eOH groups on the surface with a

moderate adsorption strength of �4.9 kcal/mol, which is

much larger than the adsorption energy of ethylene (�2.1 kcal/

mol) [42]. The enhanced anchoring strength of acetone should

originate from the interaction between the electron enriched

O atom of C]O and the protonic H of eOH groups on molyb-

denum bronzes surface [42]. This allows a relatively strong H-

bonding given the calculated O3eH2 distance of 1.756 �A. On

the other hand, a relatively long distance betweenH1 andC1 is

observed due to the steric repulsion between themethyl group

of acetone and the eOH on the molybdenum bronzes surface.

Comparing with the gas phase acetone, the CeC bonds of

acetone are essentially unchanged upon the adsorption, sug-

gesting the fact that CeC bonds of acetone are not activated

via the H-bonding interaction. As a result, only the C]O bond

is accessible for hydrogenation, implying the superior selec-

tivity of HxMoO3 for the hydrogenation.

Subsequently, the isopropanol (hydrogenation product) is

structurally relaxed on the HxMoO3 surface to model the re-

action product (CD), as shown in Fig. 2(a). The optimized

adsorption structure of isopropanol on the surface is virtually

identical to its gas-phase configuration. The formation of

isopropanol is almost thermally neutral with a negligible

Table 1 e Optimized Structural Parameters of reactantcomplex(R), transition state(TS), and product complex(P)on the 4H, 6H, and 8H molybdenum bronzes surface a.

bond length (�A)

C1eO3 C1eH1 O3eH2 O1eH1 O2eH2

4H R 1.234 2.964 1.756 0.981 1.003

TS 1.309 2.074 1.036 1.002 1.555

P 1.442 1.106 0.979 2.334 2.080

6H site A R 1.234 2.787 1.745 0.977 0.999

TS 1.322 2.219 1.013 0.997 1.696

P 1.443 1.108 0.976 2.579 2.307

6H site B R 1.233 2.821 1.966 0.983 0.990

TS 1.352 1.906 0.988 1.017 2.028

P 1.444 1.104 0.976 2.226 2.307

8H R 1.235 2.815 1.765 0.978 1.001

TS 1.337 2.286 0.992 0.991 1.845

P 1.446 1.103 0.976 2.330 2.352

a C1, C2, H1, H2, O1, and O2 are denoted in Fig. 2(b).

energy of �0.3 kcal/mol lower than the reactant. Comparing

with the ethylene case [42], where the hydrogenation of

ethylene on the 4H molybdenum bronzes surface is energet-

ically more favorable with the calculated reaction energy of

�15.3 kcal/mol. Such a low reaction energy of acetone should

be attributed to the strong repulsion between the methyl

group and the surface. Based on the obtained reactant and

product, the transition state (TS) could be identified using the

NEB algorithm by linking AB and CD. Similar to ethylene [42],

the hydrogenation of acetone would also undergo a one-step

mechanism where the two surface H atoms (H1 and H2, as

labeled in Fig. 2(b)) attack the C]O bond simultaneously. In

the TS, the C]Obond is elongated by 0.075�A compared to that

of acetone (1.234 �A), indicating the weakness of the original

double-bonding. In themeantime, the distance of both C1eH1

and O3eH2 bonds is reduced to 2.074 �A and 1.036 �A, respec-

tively, implying the formation of hydrogenation product. The

short distance of O3eH2 (1.036 �A) in the transition state is

originated from the strong H-bonding interaction, which fa-

cilitates the H migration process. As a consequence, a mod-

erate activation barrier of 17.8 kcal/mol is required to

accomplish the hydrogenation reaction. Since the isopropanol

is still adsorbed on the surface through a relatively week H-

bonding interaction between the eOH of isopropanol and the

terminal O atom on the surface with a distance of 2.080 �A, a

small desorption energy of 2.1 kcal/mol is necessary to release

the product into gas phase. Our results suggest that HxMoO3

with the 4H surface is capable of hydrogenating acetone

molecules with superior activity. It is notable the desorption

of isopropanol is facile, reducing the likelihood of subsequent

reactions, which is helpful to improve the selectivity.

Hydrogenation of acetone on 6H and 8H molybdenumbronzes surface

Similar calculations were performed on the 6H and 8H sur-

faces to address the effect of H content on the hydrogenation

reaction. For the 6H surface, two possible starting points were

considered in our calculations due to the asymmetric location

of the H atom in the second layer, as shown in Fig. 3(b and c).

Comparing with the 4H surface case, the internal H-bonding

interaction between the H atom in the second layer and the O

atom in the third layer leads to a downward movement of the

second layer Oa atoms, resulting in two reaction sites. Upon

adsorption of acetone on the surface, the acetone is somewhat

locked by the four eOH groups on the surface. The reaction

space at site A is more open (:OeMoeO ¼ 94.1�), while the

adsorption region at site B is somewhat embedded with a

reduced :OeMoeO ¼ 85.4�. Therefore, the site A should be

more accessible for acetone molecule during the hydrogena-

tion. Indeed, the optimized adsorption structures indicate that

both the O3eH2 and C1eH1 distances at site A are much

shorter than the value at site B (Table 1). As a consequence,

the acetone adsorption at site A is energetically more favor-

able than that at site B by 1.9 kcal/mol in the energy difference.

As described in the 4H case, the acetone hydrogenation on

the 6H surface also follows the one step reaction mechanism.

Table 1 summarizes the structural parameters of the opti-

mized geometries along the reaction. At the transition state of

site A, a small surface relaxation is observed with negligible

Fig. 3 e (a) Calculated energy diagram of the hydrogenation of acetone on the 6H molybdenum bronzes surface. The two

possible reaction pathways are denoted as black for site A and red for site B. Optimized configurations of possible acetone

absorption geometries on the 6H molybdenum bronzes surface with (b) 6H site A (1) initial state(AB), (2) transition state(TS)

and (3) finial state(CD). (c) 6H site B. (4) initial state(AB), (5) transition state(TS) and (6) finial state(CD). (For interpretation of

the references to colour in this figure legend, the reader is referred to the Web version of this article.)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 0 4 4 3e1 0 4 5 2 10447

increase of angle :OeMoeO. Here, the O3eH2 distance is

slightly shorter than the value obtained on the 4H surface, in

line with the facile accessibility of site A. The calculated

activation energy of 16.8 kcal/mol (Fig. 3(a)) is also lower than

the value on the 4H surface. Thermodynamically, the hydro-

genation of acetone at site A become more exothermic than

the 4H case with the calculated reaction energy of �7.3 kcal/

mol (Fig. 3(a)). Alternatively, the hydrogenation reaction could

also proceed at site B, where the four eOH groups are pushed

slightly toward each other. In principle, the acetone molecule

should be a little bit far from the surface at the transition state

due to the poor accessibility of site B. However, the optimized

structure at the transition state shows that the acetone is very

close to the surface and both the O3eH2 and C1eH1 distances

are even shorter than the values at site A. The unexpected

approaching of acetone is attributed to the considerable sur-

face relaxation during the hydrogenation, as reflected by the

significantly increased angle :OeMoeO from 85.4� of the

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 0 4 4 3e1 0 4 5 210448

initial state to 96.1� at the transition state. It is worth noting

that the internal H-bonding between the second and third

layers is weakened since the Oa-H distance is elongated by

0.228 �A during the surface relaxation. Consequently, although

the acetone is closer to the surface at site B than that at site A,

the calculated activation barrier at site B is much higher

(20.4 kcal/mol) due to the intensive surface relaxation, which

consumes additional energies to open the reaction site. Again,

a relatively lower desorption energy (0.8 kcal/mol at site A,

1.3 kcal/mol at site B) is required to release the final product to

the gas phase at either site A or site B.

The catalytic reactivity of acetone hydrogenation on the 8H

surface is also explored and the energy profile is shown in

Fig. 4(a). Similarly to the 6H surface, the H atoms at the

asymmetric O site (Oa) in the second layer on the 8H surface

move downwards to interact with the O atom in the third

layer, leading to an open interaction space surrounded by the

four terminal OH groups (Fig. 4(b)). Considering the higher H

content, the 8H surface is expected to present favorable re-

action kinetic properties. Kinetically, the 8H surface shows a

lower activation barrier (7.9 kcal/mol) than those of the 4H and

6H (site A) surfaces. Thermodynamically, the hydrogenation

process on the 8H surface is extremely exothermic with the

calculated reaction energy of �25.2 kcal/mol. Therefore, the

highly protonic nature of the terminal H is a dominant factor

Fig. 4 e (a) Calculated energy diagram of the hydrogenation of

structure of acetone on the 8H surface (a) initial state(AB), (b) tr

in the acetone hydrogenation. Besides, higher concentration

of hydrogen bronzes materials is also necessary to exhibit

faster reaction kinetics for acetone hydrogenation [34].

Discussion

In order to analyze the distribution of electrons in each orbital

during hydrogenation, the calculated electronic density of

states (DOS) profiles for the pristine 4H bronzes surface, the 4H

surface with acetone, and the surface with isopropanol were

displayed in Fig. 5. Upon acetone adsorption, there is almost

no obvious change in the electronic structure of the 4H

bronzes surface except for 1s orbital of H, reflecting the for-

mation of H-bond between acetone and the hydrogen bronzes

surface. The two p orbitals around �7.5 and 2.5 eV are attrib-

uted to the C]O bonds of acetone, accompanied with the

minor change of orbitals of oxygen (Fig. 5(2)). After the for-

mation of isopropanol (Fig. 5(3)), the electronic structure of Mo

around the Fermi level experiences significant changes as

expected, where the Mo atoms linked to the terminal OeH

group are oxidized, and the C]O orbitals of acetone disap-

pears as one CeH bond and one OeH bond are formed.

The DOS of 6H and 8H were also calculated, as shown in

Fig. 6. Generally, the conductor-like electronic structure of

bronzes surface is preserved along the reaction pathway. The

acetone on the 8H molybdenum bronzes surface. The

ansition state(TS) and (c) finial state(CD).

Fig. 5 e Calculated electronic DOS of (1) the 4H bronzes surface, (2) the 4H bronzes surface with acetone, (3) the 4H bronzes

surface with isopropanol after reaction.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 0 4 4 3e1 0 4 5 2 10449

adsorption of acetonewould not result in any obvious changes

in the electronic structures of bronze with the selected sur-

faces, reflecting their weak physical adsorption properties.

While the formation of isopropanol could lead to significant

changes around the Fermi level, due to oxidization of the

terminal OeH group.

Fig. 7 shows the relationship between the calculated ther-

mochemical energies and activation barriers of acetone hy-

drogenation on the HxMoO3 surfaces with selected hydrogen

contents. This phenomenon fits well with the BEP (Brønsted-

Evans-Polanyi) Relations, which is able to predict energy bar-

riers of the reaction under different chemical environment of

the reactive center [52]. From the calculate results we can see

that, as the content of hydrogen in these compounds in-

creases, the catalytic activity of HxMoO3 for acetone hydro-

genation increases significantly, which fits to our previous

study for ethylene hydrogenation elucidated the potential

positive effects of increasing H content on the bronzes surface

to suppress the energy barrier for hydrogenation [34]. The

calculated activation energies range from 17.8 to 7.9 kcal/mol,

depending on the content of H in the hydrogen bronzes.

Overall, the reaction pathway to hydrogenate acetone on the

8H surface is the most feasible in kinetics, with a barrier of

only 7.9 kcal/mol. It fits to our previous study for ethylene

hydrogenation elucidated the potential positive effects of

increasing H content on the bronze surface to suppress the

Fig. 6 e Calculated electronic DOS. (1)the bronze surface of 6H, (2) the 6H bronze surface upon acetone adsorption, (3) the 6H

bronze surface upon hydrogenation to form isopropanol; (4)the bronze surface of 8H, (5) the 8H bronze surface upon acetone

adsorption, (6) the 8H bronze surface upon hydrogenation to form isopropanol.

Fig. 7 e The relationship between calculated

thermochemical energies (DEr) and the activation energies

(Ea) of acetone hydrogenation on the HxMoO3 with various

hydrogen concentrations.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 0 4 4 3e1 0 4 5 210450

energy barrier for hydrogenation. It is worth mentioning that,

compared to the reported activation energies of acetone hy-

drogenation on the Pt(111) surface (14.3 kcal/mol) and on the

Ni tetramer cluster (17.4 kcal/mol), hydrogenation of acetone

catalyzed by HxMoO3 appears to be kineticallymore favorable.

Compared with ethylene hydrogenation on hydrogen

bronzes HxMoO3 surface, it can be observed that the acetone

hydrogenation is quite different from the case of ethylene

hydrogenation (Table 2). Different from ethylene that requires

the C]C bond activation, the polar C]O bond in acetone

would definitely facilitate the proton transfer from the ter-

minal OH group to the carbonyl oxygen via an H-bond attack,

so that hydrogenation of acetone exhibits lower activation

energy. It is noteworthy that the hydrogenation activity and

activation energy between hydrogenation of acetone and

ethylene on hydrogen bronzes HxMoO3 surface are different,

indicating HxMoO3 may be a promising and effective catalyst

for selective hydrogenation of unsaturated substances con-

taining C]O and C]C bond simultaneously. It may be a

meaningful research for HxMoO3 as the hydrogenation cata-

lyst for methyl vinyl ketone [53].

Table 2 e The comparison about hydrogenation of acetone and ethylene on molybdenum bronzes surface.

Contrast Unsaturated bond Reaction mechanism Lowest activation energy(kcal/mol) H concentration effect

Acetone C¼O One step 7.9 Fits to BEP

Ethylene C¼C One step 9.3 þ 0.9 Fits to BEP

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 1 0 4 4 3e1 0 4 5 2 10451

Conclusion

The surface catalytic activity of hydrogen bronzes MoO3 with

different hydrogen contents towards acetone hydrogenation

was investigated by means of plane wave density functional

theory. The adsorption of acetone on the HxMoO3 surfaces is

favorable due to the formation of H-bond between acetone

and the surfaces. There is virtually no CeC bond activation

during the adsorption of acetone, allowing for excellent

selectivity for hydrogenation. Our calculations suggest that

the hydrogenation of acetone is a one-step concerted and

exothermic reaction. Increase of H concentration (i.e., 8H

surface) expectedly decreases the kinetic barrier. The hydro-

genation of acetone on 8H surface is extremely exothermic

(�25.2 kcal/mol) with the smallest activation barrier (7.9 kcal/

mol), even appearing to be kinetically more favorable than Pt

(111) surface and Ni tetramer cluster. Additionally, the dif-

ference of hydrogenation activity of acetone and ethylene on

HxMoO3 surface indicate the potential application of

hydrogen bronzes as selective hydrogenation catalysts. Our

results suggest HxMoO3 could be an effective catalyst for

acetone hydrogenation and provide a novel insight for

designing low-cost hydrogen bronzes MoO3 catalysts.

Acknowledgements

This work was financially supported by National Natural Sci-

ence Foundation of China (Grant No. 51702241), Hubei Key

Laboratory of Pollutant Analysis & Reuse Technology (Grant

No. PA20170203), the Research Project of Hubei Provincial

Department of Education (Grant No. B2018147).

Appendix A. Supplementary data

Supplementary data related to this article can be found at

https://doi.org/10.1016/j.ijhydene.2019.02.032.

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