Organocatalyticvs.Ru-based electrochemical hydrogenation of nitrobenzene in competition with the hydrogen evolution reaction

Article abstract of DOI:10.1039/d0dt01075h

The electrochemical reduction of organic contaminants allows their removal from water. In this contribution, the electrocatalytic hydrogenation of nitrobenzene is studied using both oxidized carbon fibres and ruthenium nanoparticles supported on unmodified carbon fibres as catalysts. The two systems produce azoxynitrobenzene as the main product, while aniline is only observed in minor quantities. Although PhNO₂hydrogenation is the favoured reaction, the hydrogen evolution reaction (HER) competes in both systems under catalytic conditions. H₂formation occurs in larger amounts when using the Ru nanoparticle based catalyst. While similar reaction outputs were observed for both catalytic systems, DFT calculations revealed some significant differences related to distinct interactions between the catalytic material and the organic substrates or products, which could pave the way for the design of new catalytic materials.

Full text of DOI:10.1039/d0dt01075h

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DOI: 10.1039/D0DT01075H
ARTICLE
Organocatalytic vs Ru-based Electrochemical Hydrogenation of
Nitrobenzene in Competition with Hydrogen Evolution Reaction
Alicia Moya,a Jordi Creus,b,c Nuria Romero,b José Alemán,d,e Xavier Solans-Monfort,b Karine
Received 00th January 20xx,
Accepted 00th January 20xx
Philippot,c Jordi García-Antón,b Xavier Sala,*b and Rubén Mas-Ballesté*a,e
DOI: 10.1039/x0xx00000x
The electrochemical reduction of organic contaminants allows its removal from water. In this contribution, the
electrocatalytic hydrogenation of nitrobenzene is studied using both oxidized carbon fibres and ruthenium nanoparticles
supported on unmodified carbon fibres as catalysts. The two systems produce azoxynitrobenzene as main product, while
aniline is only observed in minor quantities. Although PhNO2 hydrogenation is the favoured reaction, the hydrogen evolution
reaction (HER) competes in both systems under the catalytic conditions. The H2 formation occurs in larger amounts when
using the Ru nanoparticle based catalyst. While similar reaction outputs have been observed for both catalytic systems, DFT
calculations revealed some significant differences related to distinct interactions between the catalytic material and the
organic substrates or products, which could pave the way for the design of new catalytic materials.
Metal-free catalysts, such as carbon-based materials16,17 as well
as Ru-based catalysts18–20 seem to be ideal alternatives for this
purpose.
Introduction
Electrochemical reduction of organic substrates is an important
synthetic method but also, an efficient way to remove
persistent pollutants1–3 and to produce energy vectors based on
reversible hydrogenation/dehydrogenation processes.4–6 Such
processes, when carried out in the presence of proton sources,
require electrocatalysts able to hydrogenate organic
functionalities.7 Such electrocatalytic materials are intended to
increase the activity and selectivity of the reduction reaction.
Regarding the selectivity, the occurrence of Hydrogen Evolution
Reaction (HER) as a parasite process in competition with the
hydrogenation of organic reagents is ubiquitously observed.8
The characteristics of the electrocatalytic materials will have
enormous influence on the reaction output, tipping the balance
in favour of one or another process. Thus, a deep understanding
of the mechanism underlaying both pathways will enable to
rationally design electrocatalytic materials capable to
hydrogenate organic substrates or to mediate HER. Indeed, a
wide range of materials has been already studied due to the
need of clean and renewable alternatives to the use of fossil
fuels.9–11 Although Pt-based catalysts are known to be highly
performant for HER, its substitution by cheaper and
earth-abundant catalytic materials is highly demanded.12–15
In a recent report, we explored carbon microfibres (CFs)
prepared by the pyrolysis of polyacrylonitrile (PAN) as an
alternative electro-organocatalytic material for HER (Fig. 1a,
top).21 CFs with a graphene-like structure including pyridyl
moieties can be easily oxidized to generate carboxylic acid
groups at the surface, constituting nicotinic fragments in the its
structure. The resulting material that will be further referred as
ox-CF presents high HER electrocatalytic activity. Indeed, ox-CF
acts as catalytic centres for the HER in a way similar to those of
NADPH in biological systems22,23 and Hantzsch esters in
synthetic reduction procedures.24–26 In another work, we used
pristine CFs as support for Ru-NPs in order to achieve stable and
high surface area functionalized electrodes (Fig. 1a, bottom).27
Our results demonstrated the viability of such hybrid material
composed by Ru-NPs onto CFs as remarkable electrocatalysts
for the HER (referred as Ru-CF). A common feature of both
systems (ox-CF and Ru-CF) is the generation of hydride species
that can be protonated to form molecular hydrogen.
Additionally, they could hydrogenate a variety of substrates,
widening the applicability of electrochemical systems usually
employed for HER. Remarkably, such electrocatalytic
procedures are convenient strategies that avoid the
manipulation of H2 gas or metal hydride derivatives (LiAlH4 or
NaBH4), commonly used in chemical hydrogenation processes.
In this work, we have compared the electrocatalytic HER and
the hydrogenation of nitrobenzene (PhNO2) as model substrate,
using modified carbon fibres (ox-CF and Ru-CF) as
electrocatalytic materials (see Fig. 1b). The reduction of PhNO2
is of great interest since nitro aromatic compounds (such as
explosives, dyes, agrochemicals or pesticides) are common
water contaminants.28,29 Although many alternatives have been
a. Department of Inorganic Chemistry (module 07).Universidad Autónoma de Madrid,
28049, Madrid, Spain. E-mail: ruben.mas@uam.es
b. Departament of Chemistry. Universitat Autònoma de Barcelona, 08193,
Cerdanyola del Vallès, Barcelona, Spain. E-mail: Xavier.sala@uab.cat
c. CNRS, LCC (Laboratoire de Chimie de Coordination), UPR8241, Université de
Toulouse, UPS, INPT, F-31077 Toulouse cedex 4, France.
d. Department of Organic Chemistry (module 01). Universidad Autónoma de Madrid,
28049, Madrid, Spain.
e. Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad
Autónoma de Madrid, 28049, Madrid, Spain.
† Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/x0xx00000x
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explored,30–35 electrochemical procedures are still the most
convenient ways to reduce PhNO2 and related compounds.
However, in the presence of protic media, this process would
typically be in competition with HER. Again, platinum electrode
is often used for electrochemical reduction of nitrobenzene,36–
Experimental
Materials
View Article Online
DOI: 10.1039/D0DT01075H
All reagents, solvents and materials were purchased from
commercial sources and used without further purification.
Specifically, fabric of carbon fibre (Twill 2x2 3K weight 200 g/m2
width 1200 mm, Model HA2301) was purchased from
ClipCarbono.
38
limiting their applicability for large scale processes.39 Thus,
there is a need to find efficient, inexpensive, easy to make and
recyclable electrocatalytic materials free of noble metals40,41 for
the reduction of nitroaryl compounds. This reaction will be
deeply presented in this manuscript, considering HER as a
competitive process. We will determine the intrinsic properties
of ox-CF and Ru-CF as electrocatalysts for both reactions
(compare Fig. 1b top and bottom) by means of combination of
experimental and theoretical results. This work will provide
fundamental understandings on the distinctive catalytic
behaviour of metal-based and metal-free electrocatalysts. The
obtained fundamental insights can pave the way for the future
design of active and selective electrocatalytic materials for HER
or hydrogenation of organic substrates.
Electrode Preparation
The commercial carbon fibre is prepared in a 6 cm long bunch
of fibres. Then, it is joined to a copper wire and tight with a
Teflon tape. The hand-made electrodes are modified using two
different approaches, namely: 1) oxidative treatment of the
carbon fibre and 2) ex-situ deposition of Ru-NPs. For the
oxidized CF brushes, they are treated 30 minutes with sulfuric
acid (98%) at room temperature with stirring and later they are
immersed into a mixture 1:1 of H2SO4/H2O2. The mixture should
be freshly prepared to obtain its stronger oxidative potential
and thus, be able to oxidize the carbon fibre. Finally, the
electrodes are washed and sonicated in distilled water to
neutralize its surface, later washed in ethanol and dried with a
heat gun. Characterization of this material was previously
reported and confirms that the oxidation process changes the
chemical features of the CF surface (generation of carboxylic
groups, transforming pyridinic fragments into nicotinic ones)
whereas keep its bulk properties and morphology.21 For the Ru-
based CF, 2 cm of CF electrodes were soaked overnight in a THF
(10 mL) crude dispersion of RuPP NPs27,42 inside a Fisher-Porter
bottle under an Ar atmosphere. Then, the supernatant was
removed through cannula and the resulting CF materials were
rinsed with pentane (3x10 mL) and dried under vacuum.
RuPP@pCF. TEM: Ø = 1.8 ± 0.3 nm. ICP(Ru%): 0.47%.
Electrochemistry
Electrochemical experiments were performed under an argon
atmosphere at room temperature in CH3CN. Sodium
perchlorate (NaClO4) was used as supporting electrolytes.
Measurements were carried out using an Ivium CompaqStat
potentiostat interfaced with a computer. A standard three-
electrode electrochemical cell was used with Ag/AgCl reference
electrode. The working electrode was carbon fibre brush:
pristine, oxidized and modified with inorganic nanoparticles.
The counter electrode was a pristine carbon fibre brush.
Gas Chromatography
The evolution of the nitrobenzene during the electrocatalytic
reaction was monitored by gas chromatography, by injecting
samples of the intern atmosphere to an Agilent 7820A GC
system equipped with a mass spectroscopy detector (5977B
MSD). For gas chromatography quantification of nitrobenzene
and its derivatives, the solvent of the working electrode solution
is reduced at low pressure and then it is solved with 1 mL of
CHCl3 which is able to solve the nitrobenzene and its derivatives
but not the inorganic salt of the electrolyte. An aliquot of 0.2 mL
of the solution is mixed with 0.2 mL of methylnaphthalene as
Fig. 1 Schematic representation of a) Carbon Fibre-based electrocatalysts for HER
(previous work)21,27 and b) HER vs hydrogenation of PhNO2 using same electrocatalysts
(this work).
2 | J. Name., 2012, 00, 1-3
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standard.
ARTICLE
A
calibrate of nitrobenzene, nitrosobenzene,
Results and discussion
Experimental Results
View Article Online
DOI: 10.1039/D0DT01075H
azoxynitrobenzene, diphenylazobenzene and aniline was
carried out enabling the quantification of the products.
Details on synthesis and characterization of the electrocatalytic
materials used in this comparative study are reported
elsewhere.21,27 Previous mechanistic studies revealed that both,
ox-CF and Ru-CF, generate hydrides that after protonation, lead
to HER. Herein, we studied if these systems are capable to
transfer the hydride to the organic substrate: PhNO2. Although
the product typically formed from this process is simply aniline,
several other species can be generated. Therefore, as previously
reported, a more complex mechanism should be considered for
hydrogenation of nitrobenzene (see Fig. 2).56
The electrochemical behaviour of nitrobenzene has been
analysed by cyclic voltammetry (Fig. S1). A reduction signal is
observed at E1/2= -1.1 V vs Ag/AgCl, which is associated to a
reversible electron transfer process. According to this result, -1
V vs Ag/AgCl was chosen as the limit potential for all
electrochemical experiments in order to achieve
electrocatalytic hydrogenation and avoid electrochemical
electron transfer. The choice of an adequate proton source is
determinant for the efficiency of the reaction. Also, in order to
understand such effect, some factors have to be considered: 1)
Electrocatalytic hydrogenation of nitrobenzene will compete
with HER and 2) Hydride regeneration on the catalytic centres
involves injection of two electrons into the electrocatalytic
material and further protonation. Combination of both factors
will determine the effect of the proton source in the output of
the electrocatalytic PhNO2 hydrogenation. In order to optimize
the composition of our systems, we have compared linear
sweep voltammetry results of ox-CF and Ru-CF using water (1
M), trifluoroacetic acid (TFA, 20 mM) and a mixture of both as
proton sources (Fig. S2) in acetonitrile. Maximized
electrochemical response is observed for both catalytic systems
when a mixture of water and TFA is used. Fig. 3 presents the
linear sweep voltammetries recorded in the absence (top) and
in the presence (bottom) of nitrobenzene at a scan rate of 10
Computational Details
Quantum chemistry calculations were carried out using the
density functional theory (DFT). For the organocatalytic system,
geometry optimizations were performed using the M06-2X
functional43 in combination with the 6-311G** basis set.44 For
comparison some selected PBE-D2 calculations were also
carried out for this system (see S.I.).46,48 Solvent effects were
included with the solvation model density (SMD)45 and
considering acetonitrile (ε= 37.5) as solvent. Graphitic surface
was modelled using a fragment composed by 4 fused aromatic
rings (1 pyridine + 3 benzenes) as a compromise between
performance and computational cost a carboxylic group in the
meta position versus pyridine nitrogen. All optimizations were
performed without any geometrical constraint and harmonic
vibrational frequencies were used to characterize minima and
transition states in the potential energy surface. Transition
states were connected to products by optimization of
geometries slightly modified from the transition states. All the
calculations were performed using the Gaussian09 program.46
For modelling the reactivity with Ru-NPs, we used the PBE47
functional as implemented in the VASP code.48,49 Grimme (D2)
empirical correction was added to account for dispersion
interactions.50 The core electrons were described by ultrasoft
pseudopotential51,52 and the external ones by plane wave basis
set, with a kinetic energy cutoff of 500 eV. With the aim of
minimizing replica interactions, a cubic cell of 25 Å was
employed. The k point grid includes the  point only, as a
consequence of the discrete nature of the system. The energy
convergence criteria were fixed to 10–5 and 10–4 eV for
electronic and geometry relaxations, respectively. Solvent
effects were included during the optimization with the implicit
continuum model implemented in VASPsol53 and using
acetonitrile as solvent (ε= 37.5). Transition states were located
by using climbing image nudged elastic band (CI-NEB) method
without including solvent effects.54,55 Six or eight images per
NEB were used. The highest in energy image was used for
locating the final transition state by performing a gas-phase
geometry optimization with the quasi-Newton algorithm
implemented in VASP. Convergence was considered to be
achieved when forces were smaller than 0.02 eV/Å. Solvent
effects at transition state were included with a single-point
calculation at the optimizes structure. For the case of the
hydronium ion, the transition state was located by performing
VASPsol restricted optimizations, in which the O-H distance of
hydronium was fixed in each step. The highest in energy point
was used as starting point for localizing the transition state. The
energies reported along the text are all given in kcal mol-1.
Nitrosobenzene
Aniline
N-phenylhydroxylamine
O
N
OH
HN
NH2
(1)
2
( )
3
( )
Azoxynitrobenzene
Diphenyldiazene
Diphenylhydrazine
N
N
N
NH
HN
O
N
4
( )
5
( )
6
( )
Fig. 2 Possible species that can be generated from hydrogenation of nitrobenzene with
species detected in this work highlighted inside the frames.
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cannot be uniquely related to the intrinsic properties of the
View Article Online
DOI: 10.1039/D0DT01075H
active sites but also to their relative concentration on the
electrode surface.
Furthermore, we investigated the reaction output after a
longer reaction time (5 vs 15 h, Fig. 4a and 4b). After 15 h, the
consumption of nitrobenzene is almost complete for both
electrocatalytic materials. The pristine carbon fibre electrode
was also tested for 15 h. In these conditions, no significant
amounts of products were found, corroborating that the
catalytic behaviour observed with the two other materials
results from either the oxidation of carbon fibres or the
presence of Ru-NPs (Fig. S3).
As shown on Fig. 4 and S4, regardless of the reaction time (5
or 15 h), the major product of the nitrobenzene reduction after
bulk electrolysis at -1 V is azoxynitrobenzene dimeric species
(4). In addition, residual amounts of aniline (3) and
diphenyldiazene (5) are also observed. Interestingly, amounts
of aniline (3) at 15 h are equal or even lower than those
detected at 5 h. In contrast, the amounts of azoxynitrobeneze
(4) and diazobenzene (5) are significantly superior at 15 h than
at 5 h. These observations indicate that, a) the generation of
aniline (3) corresponds to short reaction times, b) the formation
of dimeric species (4 and 5) needs accumulation of monomeric
species produced over the electrochemical process, thus
requiring longer reaction times. Remarkably, although the
nature of the two catalytic systems studied is very distinct,
similar product compositions are observed. However, faradaic
yields after 15 h electrolysis are approximately 75% for ox-CF
and 65% for Ru-CF, thus indicating that HER is the minor process
but it is more significant for Ru-CF, in agreement with
voltammetry data (see Fig. 3).
Fig. 3 Linear sweep voltammetries measured using pristine (solid line), oxidized (dashed
line) and Ru-based (dotted line) carbon fibre electrodes in the absence (top) and in the
presence (bottom) of 10 mM of nitrobenzene. In all cases, solvent is acetonitrile
containing H2O (1 M), TFA (20 mM) and NaClO4 (0.2 M).
mV/s. Measurements were performed in acetonitrile using
NaClO4 (0.2 M) and TFA (20 mM) and water (1 M) mixture.
The current intensity values at -1 V vs Ag/AgCl are presented
in Table 1. Electrochemical responses of the two materials (ox-
CF and Ru-CF) are much higher than that of the pristine fibre
(CF), which demonstrates their catalytic properties, either for
the proton reduction or for the nitrobenzene hydrogenation
(see -2.48 vs -6.76 and -5.77 mA; -4.85 vs -17.89 and -9.31 mA).
Current intensity is greatly enhanced upon addition of PhNO2
when using ox-CF (see -6.76 vs -17.89 mA). However, such an
effect is less pronounced for the Ru-CF (see -5.77 vs -9.31 mA).
PhNO2 hydrogenation is clearly facilitated with the purely
organic electrocatalyst while larger competition between the
nitrobenzene hydrogenation and the HER seems to take place
when using the inorganic catalyst.
With the aim of determining the amount and nature of the
final product, a 5 h electrolysis was performed at a constant
potential of -1 V vs Ag/AgCl for the two electrocatalysts in the
presence of water and TFA mixture (Fig. S3). The reaction
outputs have been analysed by gas chromatography (GC). As
expected from the current observed in the LSV, higher
conversion was found for oxidized carbon fibres (Fig. 4a).It is
important to note that the concentration of active sites is
difficult to determine accurately. Thus, the activity observed
Table 1 Current intensities at -1 V vs Ag/AgCl in 1 M H2O + 20 mM TGA
corresponding to the linear sweep voltammetries of Fig. 3.
[PhNO2]
(mM)
0
I of CF
(mA)
-2.48
-4.85
I of ox-CF
(mA)
-6.76
I of Ru-CF
(mA)
-5.77
Fig. 4 Reaction products of the electrocatalytic reduction of nitrobenzene at -1 V vs
Ag/AgCl after a) 5 h and b) 15 h of reaction using ox-CF and Ru-CF as electrodes in a
mixed solution of 1 M of water and 20 mM of TFA in NaClO4 (0.2 M, acetonitrile).
10
-17.89
-9.31
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Theoretical Calculations
Methodological Strategies. In order to get
View Article Online
DOI: 10.1039/D0DT01075H
a
deep
understanding of the processes occurring in the
electrocatalysis, we performed a computational DFT study. Two
sets of calculations were carried out. The full catalytic process
involving the organocatalytic system (ox-CF) was represented
with a molecular model from a previous work.21 The hybrid
M062X functional43 was used and atoms were represented with
the 6-311G** basis sets44 including solvation effects of
acetonitrile (ε= 37.5) by means of solvation model density
(SMD).45 The protonation steps of the PhNO2 hydrogenation
were studied also at this level of theory without including any
of the two catalysts in the calculations. The reactivity of Ru-NPs
was studied with VASP.48,49 For saving computational time, the
exploration was limited to the thermodynamics of all steps and
the transition states that appeared to be key for the
organocatalytic system. VASP includes periodic boundary
conditions and defines the basis set with plane wave. Thus,
interaction with neighbour images was avoided by using a 25 Å
edge cubic unit cell. These calculations were performed with
the PBE GGA functional47 and an energy cut-off of 500 eV.
Dispersion forces were taken into account with Grimme D2
empirical correction50 (see Computational Details section for
further information). Remarkably, values for the Ru-NPs do not
include thermal corrections. However, since all steps are
unimolecular, ΔE and ΔG values calculated for ox-CF system
using M062X and PBE-D2 functionals show little variations,
except for the HER, where H2 is formed (see details in S.I.). In
any case, it is worth to note that since the methodologies used
for modelling the two catalytic processes differ significantly, we
have performed some selected calculation with PBE-D2 for the
ox-CF system. Results are reported in the S.I. and show the PBE-
D2 underestimates the energy barriers as it is well-known in the
literature.57 Thus, comparison should be made with caution.
Initial Considerations. As reported previously,21 the electro-
organocatalytic behaviour of ox-CF is initiated by the injection
of two electrons on the carbon material that contains randomly
distributed nicotinic fragments. It is known that the value for
the work function of highly orientated pyrolytic graphite
(HOPG) in air is c.a. 4.8 eV,58 which is expected to be close to
that of the carbon fibre material. Thus, it should allow
stabilization of injected electrons through the graphitic π-
electron density. However, considering the reduced size of the
molecular model used in our calculations to simulate the
graphenic region, the stabilization energy after injection of two
electrons (3.13 eV) is underestimated. After injection of two
electrons, the negative charge increases the basicity of the
Fig.5 Ru57H88 model used for representing the Ru-NPs.
pyridinic nitrogen atom and results in its barrierless
protonation. Thus, we considered as the starting point the
negatively charged species shown in Scheme 1 (centre) that
contains
a protonated pyridinic nitrogen atom. Further
evolution of this reduced species consists in its protonation in
the para-position with respect to the nitrogen atom. The
corresponding species can transfer the hydride that is
responsible for the observed reactivity. Taking into account the
nature of the reaction medium, we have considered two
possible protonation agents: TFA molecule and hydrated proton
modelled as [H3O(H2O)3]+ according to previous report.59 Very
similar results have been found using both species, being
slightly more favourable the protonation with TFA (∆G= -20.9
kcal mol-1; ∆G‡= 2.9 kcal mol-1) than with [H3O(H2O)3]+ (∆G= -
16.4 kcal mol-1; ∆G‡= 5.3 kcal mol-1).
In order to study the processes mediated by Ru-NPs, we
used a model that includes 57 ruthenium (44 at the surface) and
88 hydrogen atoms (Ru57H88, Fig. 5). We have considered this
structure as a good representation of the active species
generated under reductive potentials in acidic media, able to
act as an efficient hydride transfer agent. The model choice and
the number of hydrogen atoms is based on Comas-Vives et al.
results.60 Their work showed that a) this model is a good
compromise between accuracy and computational cost and b)
the number of hydrogen atoms per surface ruthenium is close
to the double of surface ruthenium atoms in pristine
nanoparticles.
Hydride Transfer to PhNO2 vs Hydride Protonation. Once the
active species able to perform hydride transfer processes are
generated, two potential reactions can take place: hydride
transfer to nitrobenzene and HER. Fig. 6 shows the comparison
of the kinetics and thermodynamics of these processes in
presence of model systems for a) ox-CF and b) Ru-CF. Fig. 7
shows some optimized structures.
Regarding the reactivity in presence of the ox-CF,
calculations show that the hydride transfer to PhNO2 is slightly
endergonic (G= +3.6 kcal mol-1) but this is clearly compensated
by the subsequent protonation of PhNO2H, which leads to PhNO
(1) and a water molecule (origin of the exergonicity; G= -108.9
kcal mol-1). The competitive H2 formation is also
thermodynamically favourable both from TFA and hydronium
type molecule [H3O(H2O)3]+, obtaining computed G values of -
12.4 and -6.2 kcal mol-1, respectively. From a kinetic point of
view, the hydride transfer to PhNO2 is favoured with respect to
HER by at least 9.2, kcal mol-1. HER reaction is more favourable
when TFA is used as proton source. These results are in
accordance with the high faradaic yields found for nitrobenzene
reduction (see above).
F3CCOO-
G = 2.9
-
(H2O)3
G = 5.3
H
H
HOOC
HOOC
HOOC
H +
H
{(H2O)3
}
F3CCOO
N
N
N
H
H
H
G = -20.9
G = -16.4
Scheme 1 Generation of the species able to transfer hydrides in the ox
catalytic system (energy values are given in kcal mol-1).
-CF organo-
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favourable for nitrobenzene and TFA than for hydronium ion
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[H3O(H2O)3]+ (see S.I. for further detaDilsO)I.: 1R0.e1m03a9r/Dka0DblTy0,10t7w5Ho
adsorption isoenergetic modes are found for nitrobenzene: a
parallel adsorption mainly controlled by dispersion forces and a
perpendicular one mainly controlled by the Ru···O interactions
(see S.I. for further details). However, only the perpendicular
orientation can account for the observed process. As found for
the carbon fibres, the hydride transfer to the adsorbed
nitrobenzene is slightly unfavourable (E= 4.3 kcal mol-1), but
the high exothermicity of the protonation step makes the
formation of PhNO + H2O largely favourable. The energy barrier
for the hydride transfer to the adsorbed PhNO2 species is
computed to be 10.8 kcal mol-1, thus suggesting it is kinetically
easy.
The hydride protonation in presence of Ru-NPs is easier
when involving TFA than [(H3O)(H2O)3]+. The energy barrier is
computed to be 17.1 kcal mol-1 from adsorbed TFA. This value
is higher than that computed for the PhNO2 hydride transfer.
The energy difference between the two processes is slightly
smaller than for the ox-CF, indicating that HER (which is in both
cases kinetically less favoured) could be a better competing
reaction when using Ru-CF rather than when using ox-CF.
Similar conclusions can be inferred from PBE-D2 calculations on
the organic system. These results agree with the experimentally
measured faradaic yields for both systems. A potential
explanation of the large difference between the HER energy
barriers when involving TFA and [(H3O)(H2O)3]3+ could derive
from the nature and the strength of the substrate-Ru-NP
interaction. Indeed, the Ru···O interaction increases the acidity
of TFA favouring the hydride protonation.
Fig. 6 Comparison of energetics for the hydride transfer to PhNO2 (black line) and HER
(grey lines) using a) ox-CF and b) Ru-CF electrocatalytic models. Hydride protonation
can be carried out by TFA (solid grey line) or by [(H3O)(H2O)3]+ (dashed grey line).
Energies are given in kcal mol-1.
Nitrobenzene hydrogenation and H2 formation on Ru-NPs
start with the adsorption of the reacting substrates to the
nanoparticle surface (Table S2). This adsorption is more
From PhNO (1) to PhNH2 (3). Evolution of PhNO species
formed from the first hydrogenation process (hydride transfer
+
protonation) to aniline consists of two consecutive
hydrogenation steps (Scheme 2). Every hydrogenation step has
been considered as an initial hydride transfer with a non-
negligible energy barrier, followed by the protonation process,
which is generally very favourable, mainly barrier-less and does
Fig. 7 Optimized structures found for PhNO2 hydrogenation and HER using TFA and
hydronium ion as source of protons: a) transition states shown in Figure 6a for ox-CF
model system and b) the starting materials, transition states and final products using
Ru-CF as catalytic model system as shown in Figure 6b. Cl atom (green) has been added
to balance charge in periodic calculations. Distances are given in Å.
Scheme 2 Catalytic evolution of PhNO (1) consisting in two consecutive hydrogenation
and protonation steps: a) forming hydroxyaniline (2) and b) aniline (3). Energy values
are given in kcal mol-1.
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Scheme 4 Dimerization processes towards formation of azoxynitrobenzene (4) and
azobenzene (5) (energy values are given in kcal mol-1).
Scheme
3 Catalytic evolution of Ph-NO to aniline through two successive formation from the reaction of nitrosobenzene (1) and
hydrogenation/protonation steps with Ru-NPs. Energies in kcal mol-1
hydroxyaniline (2) intermediates. Effectively, condensation of
such compounds is highly favoured, as it should be expected for
the formation of a water molecule, which serves as an efficient
driving force of the process (Scheme 4). Thus, while
hydroxyaniline (2) is slowly being generated, it efficiently reacts
with the excess of nitrosobenzene (1) to generate the main final
product of this electrocatalytic system, azoxynitrobenzene (4).
Furthermore, formation of the diazo compound (5) that has
been detected in residual amounts can be explained by the
dimerization of two hydroxyaniline (2) molecules (Scheme 4).
Considering that during the reaction, hydroxyaniline (2) coexists
with an excess on nitrosobenzene (1), this alternative process is
less probable in agreement with the residual amounts of such
species experimentally detected. Finally, condensation of
aniline and nitrosobenzene (1) could also account for the
formation of the diazo compound (5), which could be
responsible for the disappearance over time of aniline (3).
Summarising, dimerization processes require significant
concentrations of monomeric species. Therefore, at short
reaction times, when concentration of products is still low,
direct formation of aniline is plausible through the three
consecutive hydrogenation steps presented above. However,
after longer times of electrolysis, concentration of products
increases, and dimeric structures can appear through
exothermic condensations. Thus, the amounts of aniline
experimentally found have been firstly formed (Fig. 4) whereas
dimeric products have been furtherly produced. The fact that
the major product found is the azoxynitrobenzene (4) species,
indicates that its hydrogenation is unfavourable as is
demonstrated in the next section.
Evaluation of possible evolution of azoxynitrobenzene (4). The
hydride transfer to azoxynitrobenzene (4) and to the other
dimeric species (diphenyldiazene (5) and diphenylhidrazine (6))
involving the two catalytic systems are summarized in Schemes
5 and 6. With ox-CF, the hydride transfer to azoxynitrobenzene
(4) (Scheme 5a) is endergonic (G=+8.1 kcal mol-1) and the
computed Gibbs energy barrier is G‡= 22.1 kcal mol-1. These
values indicate that the process is both kinetically and
thermodynamically challenging and comparable with the first
hydride transfer step. This suggests that azoxynitrobenzene (4)
would hardly be hydrogenated, even though protonation of the
resulting product to form a water molecule and the diazo
compound is highly exergonic. Therefore, it agrees with the fact
not involve the electrocatalyst. The hydride transfer to PhNO (1)
results in deprotonated hydroxyaniline. This process is
exergonic in presence of ox-CF by -11.7 kcal mol-1. Moreover,
the energy barrier is low (G‡= 12.6 kcal mol-1), which indicates
that the process is kinetically favourable (Scheme 2a). In
addition, the protonation of deprotonated hydroxyaniline is
also very favourable, suggesting that once PhNO (1) is formed,
it easily evolves to PhNHOH (2).
Calculations for the second hydrogenation step indicate that
hydride transfer to the OH group of hydroxyaniline to generate
a water molecule and deprotonated aniline is very exergonic (-
36.1 kcal mol-1, Scheme 2b). In this case, transition state has not
been located, indicating that this reaction is barrierless or
proceeds through a very shallow barrier. Further protonation of
deprotonated aniline is very favourable. Consequently,
nitrobenzene hydrogenation (Fig. 6) seems to be the rate-
determining step. Once PhNO (1) is generated, its reduction to
aniline (3) should consecutively occur.
With the aim of analysing if these two steps are also
straightforward in presence of Ru-NPs, we computed the
corresponding reaction energies. Results are reported in
Scheme 3 and show that PhNO (1) hydrogenation is similar to
that found for the ox-CF and it is favourable by -14.9 kcal mol-1.
On the other hand, the hydrogenation of PhNHOH shows larger
differences. The resulting products are the same for the two
catalytic systems. However, the strong Ru-NPs-H2O interaction
and the Ru-PhNH makes the reaction even more exothermic
(E= -50.3 kcal mol-1). The ability of Ru-NPs to coordinate the
reacting substrates changes the thermodynamics in comparison
with the ox-CF system. In any case, the initial PhNO2
hydrogenation to PhNO (1) appears to be the more challenging
step also when using Ru-NPs (Fig. 6b).
Overall, calculations suggest that aniline (3) should be easily
formed with the two catalytic systems. However, according to
the experimental data, aniline (3) is only a minor product
detected in the reaction outcome. Justification of this
observation is found in the next section.
Dimerization process. The major product observed in the
electrolysis experiments (see Experimental Results section
above) is the dimeric compound azoxynitrobenzene (4).
Therefore, we decided to evaluate the thermodynamics of its
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Scheme
6 Azoxynitrobenzene conversion to aniline through three successive
hydrogenation/protonation steps with Ru-NPs. Energies in kcal mol-1
organocatalytic system, has also been calculated. Fig. 8 shows
the optimized structures of azoxynitrobenzene (4),
diphenyldiazene (5) and the transition state connecting these
two intermediates.
The hydrogenation pathways found for azoxynitrobenzene
(4) using Ru-CF are significantly different than those calculated
for ox-CF. The resulting products (PhNNPh + Ru-NPs-OH) using
Ru-CF are the consequence of the formation of a Ru-OH bond
at the nanoparticle surface. Remarkably, despite the large
exothermicity of the reaction (E= - 31.9 kcal mol-1), the
computed energy barrier is high (+24.9 kcal mol-1) due to the
large electronic reorganization to favour the concerted
formation/cleavage of the Ru···O and N···O bonds. Moreover,
Scheme 5 Catalytic evolution of hydrogenation of azoxynitrobenzene towards a) the
diazo compound, b) the diphenyl hydrazine product and c) aniline (energy values are
given in kcal mol-1).
that azoxynitrobenzene (4) is the major product of
nitrobenzene hydrogenation. In addition, the small portions of
diazo compound observed experimentally are likely formed by
the dimerization processes.
the
two
subsequent
hydrogenation
steps
are
The potential formation of aniline from the diazo compound
(5) in presence of ox-CF has also been considered (Scheme 5b
and 5c). The obtained results indicate that under the reaction
conditions, diphenyldiazene (5) would easily evolve to
diphenylhidrazine (6). Finally, the hydride transfer to this
hydrazine derivative (6) intermediate leading to aniline
formation is very endergonic (G= + 26.5 kcal mol-1) and
kinetically hampered (G‡= 30.5 kcal mol-1, Scheme 5c).
Therefore, although further protonation is calculated to be very
exothermic, hydrogenation of diphenylhidrazine (6) does not
seem the path for the formation of aniline (3). Indeed, the
observed aniline is expected to be formed by successive
hydrogenation of PhNO2 without dimerization, while the major
dimeric product is azoxynitrobenzene (4), which presents a high
hydride transfer Gibbs energy barrier.
thermodynamically favourable (Scheme 6), indicating that
aniline formation from azoxynitrobenzene (4) is
thermodynamically feasible. However, as in the case of ox-CF,
this is not expected to be the main route for aniline formation.
This is due to the large energy barrier of the hydride transfer to
azoxynitrobenzene (4), which should be the major dimeric
species, as found experimentally.
Summarizing, calculations on the evolution of
azoxynitrobenzene (4) suggest that this species could hardly be
The reactivity of azoxynitrobenzene (4) with the Ru-CF has
also been explored, considering the thermodynamics of the
three potential hydrogenation steps (Scheme 6a to 6c).
Furthermore, the transition state for the hydride transfer to
azoxynitrobenzene (4), which appeared to be a key step in the
Fig, 8 Optimized structures for azoxynitrobenzene intermediate adsorbed on H88Ru57
nanoparticle, the diphenyldiazene resulting from its hydrogenation and the transition
state connecting them. Distances in Å and energies in kcal mol-1
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PhNH(OH) to PhNH2) are highly favoured. In paraVllieewl AtorticalenOilnilninee
DOI: 10.1039/D0DT01075H
formation, dimeric azoxynitrobenzene compound is generated
as a consequence of the highly favourable condensation of
PhNO and PhNH(OH).
Overall, this work presents an in-depth understanding of the
catalytic behaviour of an organic- and a metal-based material
that have been previously used as electrocatalysts for HER.
Furthermore, we have shown the efficient competition
between hydrogenation of PhNO2 with HER. Thus, the
methodology reported herein can be applied to the
electrochemical remediation of an important class of pollutants
(nitro aryl compounds). In a more general perspective, these
insights may contribute to the future designs of more efficient
catalytic systems for hydrogen production, electrochemical
Scheme 7 General scheme for the nitrobenzene reduction reaction proposed for the
two electrocatalytic approaches studied.
waste-water
treatments
or
use
of
reduced regardless the nature of the electrocatalyst. Thus,
despite the different reactivity expected for ox-CF and Ru-CF,
the chemical identity of the final products should not vary,
being azoxynitrobenzene (4) the major product.
hydrogenation/dehydrogenation based energetic vectors.
Conflicts of interest
Overall, our calculations show a reaction mechanism for the
nitrobenzene hydrogenation that is consistent with the
experimental data. Despite the different reactivities observed
for the two electrocatalysts for some particular steps, the
mechanisms for the two systems share the main general issues,
which are summarized in Scheme 7. First, nitrobenzene is
hydrogenated following a first 2-electron and 2 proton transfer
steps leading to the formation of nitrosobenzene (1), which is
converted to hydroxyaniline (2) following a further 2-electron
and 2 protons reduction. While this species is being formed, two
processes can occur: a) further hydrogenation until aniline
product (3) (detected in low amounts) or b) condensation with
nitrosobenzene (1) to produce azoxynitrobenzene (4) (observed
as main product). This dimeric species can hardly be further
reduced, appearing as the final product. In addition, residual
amounts of diazo compound (5) found in the reaction outcome
seems to be the result of dimerization of residual amounts of
hydroxyaniline (2). Controversially, according to our theoretical
data, diazo species (5) should be easily reduced to hydrazine
compound, which should accumulate because its further
reduction is unfavourable. However, the known high reactivity
of hydrazine compounds can account for its further evolution
being impossible its detection under the working conditions.
There are no conflicts to declare.
Acknowledgements
This work was supported by Spanish Ministerio de Economia y
Competitividad (MINECO) (CTQ2015-64261-R, CTQ2017-89132-
P and RTI2018-095038-B-I00), and the Generalitat de Catalunya
(2017SGR1323). We acknowledge the generous allocation of
computer time at the Centro de Computación Científica at the
Universidad Autónoma de Madrid (CCC-UAM) and the Consorci
de Serveis Universitaris de Catalunya (CSUC). “Comunidad de
Madrid” and European Structural Funds (S2018/NMT-4367) are
also acknowledged. J.G.-A. acknowledges Serra Húnter Program
and KP CNRS.
Notes and references
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