DOI: 10.1002/cctc.201501269

Reviews

This is the peer reviewed version of the following article: ChemCatChem 2016, 8, 21 – 33, which has been published in
final form at http://dx.doi.org/10.1002/cctc.201501269. This article may be used for non-commercial purposes in
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Advances in the Design of Nanostructured Catalysts for
Selective Hydrogenation
Gianvito VilØ,[a] Davide Albani,[a] Neyvis Almora-Barrios,[b] Nfflria López,[b] and Javier PØrezRamírez*[a]

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Selective hydrogenations lay at the heart of many industrial
processes. The archetypal catalysts for this class of reactions
are generally prepared by ‘metal poisoning’ strategies: the
active metal is protected and selectively deactivated with various compounds. This approach has been applied for decades,
with limited understanding. Low product selectivity and presence of toxic elements in the catalyst pose severe constraints
in the utilization of these materials in the future. Thus, to develop more sustainable catalysts, this field has recently gained
momentum. This Review analyzes the concepts and frontiers
that have been developed in the last decade: from nanostruc-

turing less conventional metals in order to improve their ability
to activate H2, to the use of oxides as active phases, from alloying, to the ensemble control in hybrid materials, and site isolation approaches in single-site heterogeneous catalysts. Particular attention is given to the hydrogenation of alkynes and nitroarenes, two reactions at the core of the chemical industry,
importantly applied in the manufacture of polymers, pharmaceuticals, nutraceuticals, and agrochemicals. The strategies
here identified can be transposed to other relevant hydrogenations and can guide in the design of more advanced materials.

1. Introduction
Hydrogenations are among the pillars of the chemical industry.[1–4] It has been estimated that approximately 25 % of all
chemical processes include at least a catalytic hydrogenation
step.[5, 6] This, by some extent, contributes to ca. 8 % of the
world’s GDP. Being the most applied reaction for the manufacture of chemicals, it is unsurprising that hydrogenations
remain one of the widest areas of research in catalysis.
The pioneering works of Sabatier,[7] Haber,[8] Horiuti,[9]
Polanyi,[9] and Lindlar[10, 11] settled the grounds for many hydrogenation reactions; the concepts derived from their observations were successfully implemented in industrial practice.
However, after the research of Lindlar, this field has been languishing for decades, with only minor contributions, and increasing compartmentalization between reactions conducted
for bulk and fine chemical applications (Figure 1). The fine
chemical sector, in particular, has relied for decades on the use
of harmful heterogeneous catalysts and inefficient (batch) processes. This has also been ascribed to the fact that organic
chemists with little skills in catalysis have dominated this type
of practice.[12]
To address new safety and environmental challenges,[13] the
last decade has witnessed a revolution in the field. New catalytic materials have been developed and many more are awaiting further exploration. This has driven a mature ‘sleeping
beauty’ into one of the most vivid, challenging, and fast-growing research fields in catalysis, able to provide, as in the early
days, a wide, rich set of new, conceptual research directions.
By revisiting these concepts, this contribution highlights
common denominators between the strategies that have been
applied in catalyst design and the frontiers ahead. Considering
the variety of hydrogenation reactions (Figure 2) and the fact

Figure 1. The illustration depicts the strict differences in operation and
market between hydrogenation reactions applied in the manufacture of
bulk (left) and fine (right) chemicals.

that more than 15 000 publications and patents have appeared
in the last 10 years, it would be impossible to give a comprehensive description of the properties and applications of all
the materials reported. Accordingly, this review targets the hydrogenation of alkynes and nitroarenes as two case studies.
These reactions are industrially relevant, as they are involved in
the synthesis of polymers (such as polyethylene, polypropylene, polyurethanes, and rubber additives), pharmaceuticals, vitamins, nutraceuticals, fragrances, and agrochemicals, and
have been particularly studied in this decade, as more than
4000 contributions can be found in the literature.

2. Poisoning as Design Strategy for Selective
Hydrogenation

[a] Dr. G. VilØ, D. Albani, Prof. J. PØrez-Ramírez
Institute for Chemical and Bioengineering
Department of Chemistry and Applied Biosciences
ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
E-mail: jpr@chem.ethz.ch

From a mechanistic point of view, the hydrogenation of alkynes proceeds through the dissociative chemisorption of H2
and sequential addition of the dissociated H atoms to the acetylenic substrate, yielding the alkene (partial or semi-hydrogenation) and the alkane (over-hydrogenation).[9, 14–17] Similarly,
the hydrogenation of nitroarenes involves hydrogen splitting,
and reduction of the nitro group, nitroso intermediate, and hydroxylamine (Figure 3).[8, 12, 18] In both cases, oligomers, isomers,

[b] Dr. N. Almora-Barrios, Prof. N. López
Institute of Chemical Research of Catalonia (ICIQ) and Barcelona Institute
of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)

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provement in selectivity.[21] Hence, modifiers are required
during both gas and liquid-phase operation. These modifiers
tailor the size and energy landscape of the ensemble of atoms
where the reaction occurs, suppressing the formation of hydrides and carbides and securing a high product yield
(Figure 4).[23, 24] This approach is often referred to as ‘selective
poisoning’ and is the conventional method applied in the
design of heterogeneous catalysts for selective hydrogenations. For example, for gas-phase alkyne hydrogenation, the industrial catalyst is based on palladium (0.01–0.05 wt %) promoted with Ag, Au, or Cu.[2, 16, 17] To further enhance the selectivity, CO can be fed with the reactants as a process modifier.[14]
For liquid-phase alkyne hydrogenation, the catalyst in practice
is based on Pd (5 wt %) modified with Pb or Bi (4 wt %).[10, 11] In
some cases, quinoline is added as a selectivity enhancer but
then the total Pb loading is lower (Figure 5).[24] For nitroarene

and condensation products can be expected. Thus, a selective
catalyst preferentially adsorbs and hydrogenates the reactive
moiety, avoiding the consecutive hydrogenation of the product and of additional functional groups.[19]
Palladium and platinum are the preferred metal of choice
for the hydrogenation of alkyne and nitroarene, respectively.[16, 20, 21] Stopping the reaction at the product stage might be
challenging, especially over these metals that are highly active
in H2 splitting.[17] For this reason, a kinetic selectivity factor is
identified, defined as the ratio between the rate of selective
hydrogenation and the rate of over-hydrogenation.[15, 19] In addition, a thermodynamic selectivity factor can be defined as
the favorable adsorption of the reactant and the lack of adsorption of the product.[19, 20]
Bare palladium and platinum have intrinsic low thermodynamic and kinetic selectivities (Figure 4). In fact, at moderateto-high hydrogen partial pressures, these metals incorporate H
species in the lattice, forming hydrides.[19–23] This will catalyze
undesired reactions such as isomerization and hydrogenation
of other functional groups. Similarly, in H-poor regimes, the adsorbed reactant may not find available H species in close proximity. Thus, coking and condensation reactions between adsorbed species could be favored.[23] The presence of carbon
fragments may induce the formation of Pd or Pt carbide
phases, leading to a major drop in activity with a slight im-

Gianvito VilØ studied Chemical Engineering at the Polytechnic University
of Milan, Italy. After short stays at TU
Delft, The Netherlands, and ETH
Zurich, Switzerland, he conducted his
Ph.D. studies at ETH Zurich. His research interests embrace the design of
catalytic materials for selectivity control in organic reactions, as well as the
application of flow chemistry, structured reactors, and novel reaction
media for process intensification.

Javier PØrez-Ramírez studied Chemical
Engineering at the University of Alicante, Spain, and earned his Ph.D. at
TU Delft, The Netherlands, in 2002.
Since 2010, he has been a full professor of Catalysis Engineering at the Institute for Chemical and Bioengineering of ETH Zurich. He is engaged in
the discovery and understanding of
heterogeneous catalysts and reactor
engineering concepts devoted to sustainable technologies. Current topics
of interest include hierarchically organized materials, alkane
functionalization, selective hydrogenations, and valorization of
renewables.

Davide Albani studied Industrial
Chemistry at the University of Milan,
Italy. Since 2014, he is a Ph.D. student
at ETH Zurich, under the supervision of
Prof. Javier PØrez-Ramírez. He researches the synthesis and catalytic properties of hybrid nanocatalysts in hydrogenations.

Nfflria López studied Chemistry at the
University of Barcelona, Spain, and obtained her Ph.D. in 1999 from the
same university. Her post-doctoral
studies were carried out in the group
of Prof. Jens K. Nørskov at the Technical University of Denmark. In 2005, she
joined the Institute of Chemical Research of Catalonia (ICIQ) as a group
leader. Her field of expertise is the use
of computational simulations to model
the structure and reactivity of heterogeneous catalysts.

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Neyvis Almora-Barrios studied Chemistry at the University of Havana, Cuba,
and earned her Ph.D. in 2010, working
in the group of Prof. Nora de Leeuw at
the University College of London, UK.
Since 2011, she is a Research Fellow at
the Institute of Chemical Research of
Catalonia (ICIQ) and, in 2014, she was
the recipient of the Ayuda formación
Posdoctoral fellowship from the Spanish Ministerio de Economia y Competitividad. She applies modelling techniques to study structure-properties relationships in heterogeneous
catalysis.

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Figure 2. Overview of the typical active phases applied in hydrogenation reactions. The figure, which is adapted from refs. [1 and 2], highlights that the
catalysts used for more than six decades in the chemical industry often contain an excessive amount of metals and promoters.

hydrogenation, Pt nanoparticles (5 wt %) are modified with Pb,
V, or other S, P, N, and Cl-containing species (1 wt %).[12, 25, 26]
The improved activity and/or selectivity of these modified
materials has been explained in terms of: 1) electronic effects,
owing to the perturbation of the metal environment by the
promoter, leading to changes in the mechanism of H2 activation, semi and over-hydrogenation;[19, 20] 2) geometric effects,
owing to the reduction of the ensemble size, and confinement
leading to a decreased number of adjacent adsorption sites,
blocking diffusion paths of the organic fragments and limiting
oligomerization and condensation reactions.[19, 20] The presence
of modifiers to protect the active phase leads, on the other
hand, to a low metal utilization. For instance, for the Lindlar
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catalyst applied in liquid-phase alkyne hydrogenation, it has
been estimated that only 0.02 wt. % of the total loading of precious metal (5 wt %) is active during catalysis.[24] This suboptimal atom efficiency and the presence of (often toxic) additives
to poison the metal encourage the search for more sustainable
and efficient hydrogenation catalysts.[27–29]

3. Advances in Catalyst Design
3.1. Promoting Poorly Active and Poorly Selective Metals
Less conventional metals, such as Au,[19, 30] Ag,[31, 32] Cu,[33] Ni,[34]
and Fe[35] can be used to catalyze selective hydrogenations.
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Figure 5. To improve product selectivity, highly active metals are generally
protected with other elements and organic or inorganic compounds. This
catalyst design strategy, which has been applied for decades in academia
and industry, is often referred to as poisoning. The figure also highlights
that the fraction of surface metal available for catalysis (ensemble) is
reduced upon increased concentration of modifiers.

Figure 3. Simplified reaction network for the hydrogenation of (a) 2-hexyne
and (b) nitrobenzene. The desired products are highlighted in blue.

Figure 4. Bare nanoparticles of highly active metals, such as palladium and
platinum, cannot catalyze hydrogenation reactions in a selective manner, because of the tendency to readily form unselective phases, such as hydrides
(left) and carbides (right).

Figure 6. (a) Selection of research contributions, highlighting the possibility
to tune the activity of less-conventional hydrogenation metals by introduction of surface defects. These defects can be created by reduction of the
average particle size of the metal, using physical and chemical methods. Influence of the (b) Au and Pd, and (c) Ag particle size on the hydrogenation
of acetylene and propyne. The figures are adapted from refs. [103] (a), [36]
(b), and [31] (c).

Some of these metals (Au, Ag) do not readily activate hydrogen. Their intrinsic low activity can be enhanced by nanostructuring the metal phase (Figure 6 a), increasing the concentration of surface defects (e.g., steps, edges, and kinks) where the
reaction occurs. In this context, Nikolaev and co-workers[36]
have prepared alumina-supported Au nanocatalysts with variable particle size (2–8 nm). Applied in acetylene hydrogenation, the materials showed a higher turnover frequency (TOF)
and ethylene selectivity over smaller particles (Figure 6 b). This
result is opposite to what is expected over Pd-based catalysts,[37–42] and could be attributed to the increased surface
concentration of active corners and edges.[30] Similarly, PØrezRamírez and co-workers demonstrated that Ag nanoparticles
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tive in the hydrogenation of propyne.[31] The maximal activity
at approximately 4.5 nm Ag particles (Figure 6 c) could be attributed to the presence of B5 sites, which combine (11 0) and
(1 0 0) facets and are active centers in the reaction. Corma
et al.[41] also observed a significant increase in selectivity in the
hydrogenation of 3-nitrostyrene and chloronitrobenzene (up
to 99.6 % at 96 % conversion) with small crystallites of TiO2-supported Au (< 5 nm). This result was also attributed to the presence of low-coordinated sites where the adsorption of the re25

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actants is favored. Cu,[33] Ni,[34] and Fe,[35] on the other hand,
are quite active in hydrogen splitting, but poorly selective. The
catalytic performance can be improved by alloying strategies.
Among the excellent examples in the literature, Studt et al.[43]
performed a theoretical screening on a broad space of alloys
and, aided by scaling relationships, discovered new selective
acetylene hydrogenation catalysts (Ni-Zn, Ni-Ga; Figure 7 a–e).
Bridier and PØrez-Ramírez made use of the unselective character of Ni and Cu surfaces, engineering a Cu-Ni-Fe catalysts that
showed full selectivity in propyne hydrogenation (Figure 7 c).[44]
In situ X-ray absorption spectroscopy experiments demonstrated that Ni enhanced the surface H coverage and the reducibility of Fe and Cu, producing a well-distributed Cu surface and
hindering the catalyst re-oxidation that would favor Fe segregation.[45] Armbrüster et al.[46] prepared a stable Al-Fe catalyst
for acetylene hydrogenation that showed high alkene selectivity (82 %) (Figure 7 d). Using a bifunctional Au-Fe2O3 catalyst,
Corma achieved up to 99 % selectivity for the hydrogenation
of nitrostyrene.[47]

benzoic acid to benzaldehyde.[50, 51] For two decades these
works were completely ignored until, very recently the field
has been revitalized. In a series of papers, PØrez-Ramírez and
co-workers demonstrated that ceria is an ultra-selective catalyst for the gas-phase hydrogenation of acetylene and propyne[52] and for the liquid-phase hydrogenation of more complex alkynes[53] (selectivity to alkene of 90–100 % at a rate of
10À3 molproduct molCeO2 À1 hÀ1). The absence of oxygen vacancies
was found to be beneficial for attaining high performances. In
addition, a large hydrogen excess in the feed mixture (H2/
alkyne > 25) and temperatures above 500 K were required. By
tuning the crystal morphology of the ceria nanocatalysts, the
authors also determined structure–performance relationships
of ceria in acetylene hydrogenations.[54] In fact, differently from
oxidation catalysis (Figure 8), the (111) surface, prevalent in
conventional CeO2 particles, appears to be more beneficial for
the reaction in comparison with to the (1 0 0) surface. The
groundbreaking nature of these investigations has opened up
a bright, new field in catalysis, in order to understand the
mechanism of hydrogenation of a metal oxide and the influence of doping strategies.
Density functional theory (DFT) calculations indicated that
the oxygen atoms were active centers in the adsorption of all
intermediates and reaction (Figure 8 a). Ceria, in fact, has the
ability to activate hydrogen (Eact > 1 eV) and stabilize acetylenic
radicals, accommodating one electron into Ce 4f level.[55] The
intrinsic anisotropy that drives site isolation of the active O
and Ce centers inhibits oligomerization pathways and is key
for the outstanding selective behavior of this metal oxide.
López and collaborators[56, 57] proposed that the mechanism of

3.2. Oxides and Carbon Materials
Oxides are traditionally considered not suitable hydrogenation
catalyst owing to the presence of oxygen species that can be
released in contact with H2.[48] The first reported hydrogenation
reaction over a metal oxide dates back to 1948, when Komarewsky and Coley discovered that vanadia-alumina catalysts
could reduce alkenes, dienes, and acetylene, with 98 % yields
at 600 K.[49] Later, Ponec et al. reported the activity of selected
metal oxides (ZrO2, ZnO, and CeO2) for the hydrogenation of

Figure 7. Selection of research contributions, highlighting the possibility to enhance the selectivity of typical hydrogenation metals by alloying. (a) Hydrogenation of 3-hexyne over Pd (red) and Pd-Bi (blue) at 373 K; the concentration of 3-hexyne and cis-3-hexene are indicated with closed and open symbols, respectively. (b) Conversion of acetylene in the presence of H2 over Ni (blue), Pd (red), and Ni-Zn (green) model (111) surfaces. (c) Alkyne conversion and alkene
selectivity in the hydrogenation of propyne as a function of the hydrogen-to-hydrocarbon ratio, over Ni (left), Cu (middle), and Fe-Ni-Cu (right) alloys at
523 K. (d) Unit cell of Al13Fe4 and acetylene hydrogenation performance of the same catalyst at 473 K and H2 :C2H2 = 5. (e) Selectivity as a function of the conversion in the hydrogenation of nitrobenzene over different Pd catalysts, at 500 K and 10 bar. The figures are adapted from refs. [79] (a), [43] (b), [44] (c), [46]
(d), [94] and [95] (e).

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gomerization at high temperature) remain to be addressed
and will require advanced characterization experiments.[64]
Following the results of ceria in alkyne hydrogenation, Yu
and co-workers have recently demonstrated that ceria nanorods can efficiently catalyze the hydrogenation of nitroaromatics to amines (selectivity of 100 %) at 300 K and 1 bar.[65] The intrinsic inability of ceria to activate hydrogen was circumvented
by using N2H2 as reducing agent. Song et al. have discovered
that tungsten oxide, another earth-abundant metal oxide with
redox properties, catalyzes the selective hydrogenation of olefins and aryl-nitro compounds.[66] Also in this case, the higher
hydrogenation activity was found over the catalyst possessing
the minimal concentration of oxygen vacancies. Metiu and coworkers have reported theoretical and experimental evidences
that La2O3 is also able to split hydrogen with a mechanism
that nicely parallel that by López.[67] Thus, La2O3 might become
another promising oxidic catalyst for new types of hydrogenations. Primo et al.[68] finally showed that pure graphene can replace metals for hydrogenation of acetylene in the presence of
a large excess of ethylene, with high selectivity and activity (selectivity to alkene of 98–100 % at a TOF of 102 hÀ1). DFT calculations are still required to rationalize the selective hydrogenation behavior of this carbon-based material.
The use of less conventional metals and metal oxides for the
hydrogenation of more functionalized substrates is limited,
owing to the required high temperatures and pressures for
catalysis.[69–71] At this condition, the high-added-value product
generally decomposes. For example, oxenin, the acetylenic intermediate involved in the synthesis of vitamin A, thermally
decomposes at approximately 350 K.[69] Thus, oxides are not
suitable heterogeneous catalysts and alternative approaches
should be investigated.

Figure 8. (a) Snapshot of selected steps involved in the mechanism of acetylene hydrogenation over CeO2. The steps highlight that the oxygen atoms
(in red) are active sites. (b) Nanocube and nanoctahedra CeO2 morphologies
with the preferentially-exposed facet and energy of oxygen vacancy formation. Reaction rate of the same materials in acetylene and CO oxidation.
(c) Turnover frequency and selectivity to ethene in acetylene hydrogenation
over CeO2 and Ce0.95Ga0.05Ox. Adapted from refs. [52], [54], [55], and [57].

H2 activation involves a heterolytic pathway owing to the polarization and redox capacity of the ceriumÀoxygen bond. The
presence of acid-base properties in ceria facilitating the splitting of hydrogen has been recently confirmed with spectroscopic techniques.[58] After heterolytic splitting of the HÀH
bond, the H atom on the cerium is transferred to the oxygen
atom, reducing a Ce center and yielding the two hydroxyl
groups. A similar activation path was later reported by Fernµndez-Torre et al.[59] and Fabris et al.[60]
Unfortunately, compared to metallic catalysts, pure oxides
lead to low reaction rates, and require high temperatures and
H2 partial pressures. By tuning the reducibility of ceria, PØrezRamírez and co-workers were able to demonstrate that trivalent cations, such as gallium and indium, enhance the reducibility of ceria and promotes the semi-hydrogenation of acetylene and propyne (Figure 8 c).[61] This enables a reduction of
the operating temperature from 523 to 423 K, while maintaining an outstanding ethylene and propylene selectivity (80–
97 %), even in the presence of excess alkene in the feed. This
ability of doped ceria to better activate hydrogen was corroborated with infrared spectroscopy, DFT calculations, and isotopic
exchange experiments.[62–64] Several open questions on to the
proposed hydrogenation paths (i.e., the interplay between the
radical[55] and concerted paths,[57] the possible path of the oliChemCatChem 2016, 8, 21 – 33

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3.3. Palladium and Platinum Alloys
The selectivity provided by palladium and platinum catalysts
can be enhanced by alloying strategies. This approach is particularly beneficial when mild reaction conditions are desired.
Besides, the secondary or ternary metals provide ensemble
control and electronic contributions on the adsorption of reactants and products. Many multimetallic materials have been
developed and excellent reviews are available on the design
and performance of active and selective hydrogenation catalysts.[72–74] For alkyne hydrogenation, the majority of works has
focused on modified Pd, since this active phase enables operation at temperatures below 300 K; the catalysts developed
were Pd-Ag,[75, 76] Pd-Au,[77] Pd-Bi,[78–80] Pd-Cu,[81, 82] Pd-Ga,[83] PdSn,[84] and Pd-Zn.[85, 86] Among the promoters, Bi and Sn behave
similarly to Pb in the Lindlar catalyst because of their efficiency
in reducing the binding energy of the double and triple bonds
and suppressing hydride formation.[74] For this reason, in both
gas and liquid-phase, catalysts based on Bi and Sn showed
a high degree of alkene selectivity (> 95 %) and low oligomer
and alkane formation, even at low modifier content (weight
modifier/Pd ratio < 1) (Figure 7 a). However, the distribution of
the secondary metal is not as homogeneous as in Pd-Pb.[74]
The use of Ag, Au, Cu, and Ga as modifiers, on the other hand,
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from the N-donor complexes are requested.[104] However,
owing to the high cost and potential toxicity of the ionic liquids,[105] catalytic applications are still limited. In general, the
combination of organic and inorganic moieties in hybrid catalysts complicates the materials characterization; thus, as highlighted in Figure 9, complementary techniques are necessary

generally alters the electronic properties of palladium, reducing the binding energies of both alkynes and alkenes. In general, the secondary metals are less prone to adsorb the reactants
and/or block diffusion paths that potentially lead to oligomerization.[74] Moreover, the dynamic response of secondary metals
to form hydride and carbide is very poor as the high pressures
can bury the secondary metal on the surface.[74] Thus, a high
loading of Ag, Au, Cu, and Ga is preferred (weight modifier/Pd
ratio > 1), in order to obtain a better isolation of the active palladium sites. In some cases, intermetallic Pd-based compounds
have been also studied with the aim of improving the activity
and/or selectivity of the individual active phases.
For nitroarene hydrogenation, the catalysts developed were
based on PtÀCr,[87, 88] PtÀMn,[88] PtÀFe,[88] PtÀCo,[88] PtÀNi,[89] PtÀ
Au,[90, 91] PtÀPd,[92] and, more recently, PtÀZn.[93] Similarly to the
industrially-relevant PtÀPb and PtÀV catalysts, the function of
the secondary metal is to increase the rate of hydroxylamine
hydrogenation through electronic contributions played on Pt
by the secondary metal. The weight modifier/Pt ratio is generally < 1. Recently, Kiwi-Minsker, Cardenas-Lizana, et al. have
also reported 100 % selectivity in the hydrogenation of chloronitroanilines over Pd-Au and Pd-Zn catalysts (Figure 7 e).[94, 95]
Sintering by metal agglomeration and Ostwald ripening as
well as metal leaching are the main causes of catalyst deactivation in hydrogenation catalysis. In this respect, researchers are
currently investigated new approaches to immobilize the metallic phase. Silica-based core–shell nanomaterials have
emerged as promising materials because of their high thermal
and leaching resistance and high tolerance to acidic conditions.[96, 97] Several synthetic methods to prepare silica shell
nanomaterials with core metals and metal alloys have been reported.[98] In some cases, magnetic nanoparticles (e.g., Co) are
chosen for the core and palladium- or platinum-based alloys
are employed on the surface.[98–101] Hydrogenation tests over
these materials have shown high activity, selectivity, and stability, although diffusion limitations might be present, particularly
when bulky reactive substrates are selected.[96]

Figure 9. Overview of the complementary characterization methods required
to assess structure and composition of the metal, ligand, and metal-ligand
interphase of a ligand-modified catalyst.

to assess the structure and composition of the metal, ligand,
and metal–ligand interphase. Table 1 collects some of the most
exciting contributions in the field. In general, high activity and
product selectivity (> 95 %) can be obtained in very mild conditions, independently of the stabilizer applied. A number of
studies, in particular, have shown that the performance of
these catalysts is highly sensitive to the nature of the organic
modifiers;[117] thus, there has been a growing interest in developing ordered structures that can provide a systematic approach for having materials with well-defined structure for
mechanistic investigations. In this context, self-assembled
monolayers (i.e., organic compounds containing a sulfur headgroup that strongly interacts with the metal) have been used
to modify the metal phase.[117] Pioneering works of Marshall,
Schwartz, and Medlin have demonstrated that the modification
of metals by self-assembled monolayers can dramatically improve the product selectivity in hydrogenation.[117–119] The enhanced selectivity was found to be independent of the tail
functionality and attributed to electronic effects induced by
the presence of the sulfur head-group, steric contributions,
created by the presence of capping layers on the surface,
active-site selection, owing to the preferential poisoning of unselective metallic sites, and molecular recognition, owing to
the exploitation of intermolecular interactions between the reactants and modifiers to orient the reactants in a desirable
way.[117–119]
Despite the great versatility of these synthetic approaches,
the industrial scale up of these nanomaterials has been long
hampered by several disadvantages, including the need for
low-boiling point organic solvents and the addition of expensive or toxic reductants. The identification of hexadecyl-2-hydroxyethyl-dimethyl ammonium dihydrogen phosphate
(HHDMA) as a ligand that combines both reducing and stabilizing functions in a single, water-soluble molecule, has enabled
the first synthesis at industrial scale of colloidally prepared Pd,

3.4. Ensemble Control in Hybrid Materials
Hybrid nanomaterials for selective hydrogenations are typically
prepared through wet chemical routes or electrochemical dissolution of a sacrificial bulk anode.[103] In the first case, an organically soluble reducing agent (e.g., hydrazine, borohydride,
or superhydrides such as LiB(C2H4)3 H) is mixed with a metal
precursor in the presence of a stabilizing agent. Upon stirring,
colloidal metal nanoparticles are formed, which can be deposited on a carrier. Alternatively, the anode of an electrolytic cell
can be dissolved in the presence of water and electric current.
The migration of the metal to the cathode creates metal nanoparticles that are stabilized by the ligand in solution. Independently of the strategy utilized, the stabilizer (e.g., surfactant,
polymer, dendrimer, etc.) controls the growth of the nanoclusters, preventing the colloidal nanoparticles from agglomeration
and subsequent catalyst deactivation. The use of ionic liquids
as modifier has been also demonstrated in recent years, with
benefits particularly when electron-transfer properties derived
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Table 1. Window of operation in terms of temperature and pressure and catalytic performance of several hybrid nanomaterials in the selective hydrogenation of alkynes and nitroarenes.
Catalyst

Modifier

Reactant

T [K]

P [bar]

X [%]

S [%]

ref.

Pd
Pd
Pt
Pt
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pt
Pt-Ru
Pt

sodium 2-ethylhexylsulfosuccinate
poly(vinylpyrrolidone)
cetyltrimethylammonium bromide
poly(vinylpyrrolidone)
poly(vinylpyrrolidone)
poly(vinylpyrrolidone)
ionic liquids
poly(ethylene oxide)-block-poly-2-vinylpyridine
poly(vinylpyrrolidone)
cetyltrimethylammonium bromide
sodium dodecyl sulfate
poly(vinylpyrrolidone)
sodium 2-ethylhexylsulfosuccinate
cetyltrimethylammonium bromide
hyper cross linked polysterene
poly(vinylpyrrolidone)
hexadecyl-2-hydroxyethyl-dimethyl dihydrogenphospahte

1-hexyne
1-hexyne
1-hexyne
2-hexyne
2-hexyne
3-hexyn-1-ol
3-hexyn-1-ol
2-butyne-1,4-diol
2-butyne-1,4-diol
4-octyne
4-octyne
2-methyl-3-butyn-2-ol
2-methyl-3-butyn-2-ol
2-methyl-3-butyn-2-ol
p-chloronitrobenzene
o-chloronitrobenzene
nitrobenzene

298–303
298–303
298
333–393
298
283–298
283–298
303–323
323
283–303
283–303
308–348
308–348
308–348
303–348
298
303–343

1–10.5
1–10.5
1
1
1–10.5
1–2.8
1–2.8
1–6
2.4
1–8
1–8
2–10
2–10
2–10
1–20
10
1–20

85–100
85–100
33–78
7–90
85–100
40–100
40–100
80
100
4–100
4–100
10–99
10–99
10-99
95
100
10–100

99
99
99
99
95
100
100
59
100
100
100
99
99
99
100
100
100

[90]
[106]
[107]
[108]
[106]
[106]
[109]
[110]
[111]
[112]
[113]
[40]
[114]
[85]
[115]
[116]
[116]

Pt, and mixed Pd-Pt nanoparticles supported on titanium silicate and activated carbon.[120–123] VilØ et al.[124, 125] have shown
the versatility of this new generation of catalysts for the semihydrogenation of functionalized alkynes and nitroaromatics,
with full selectivity (ca. 100 %) at high reaction rates
(103 molproduct molPdÀ1 hÀ1) (Figure 10). Notably, the reactions
were conducted in flow chemistry mode, in view of the obvious benefits in terms of quality, safety, and energy saving.[124]
The outstanding performance exceeds the activity and selectivity of state-of-the-art commercial catalysts for selective hydrogenations. Complementary theoretical calculations and detailed characterization studies have shown that the ligand not
only isolate and tailor the accessibility and the adsorption
modes of reactants of the active site (geometric effects) but
also tune the energy landscape felt by reactants and products
close to the active site (electronic effects) (Figure 10 a and
b).[124–126] In some cases, the ligand may also participate in the
reaction, absorbing some of the H species after hydrogen activation (co-catalytic effects),[127] and preventing the surface from
forming hydride structures. A recent work from our groups has
also demonstrated that the concentration-induced ligand ordering on the metal nanoparticles determines the catalytic response of hybrid palladium nanoparticles in hydrogenation.[139]
In fact, at low ligand content, the chains are almost flattened
on the surface and the catalyst has a 2D-type surface, with
only small pockets where non-poisoned Pd atoms can catalyze
the reaction. At high ligand density, the aliphatic chains
assume a more extended configuration, giving rise to a 3D-like
material with larger ensemble for catalysis. These counterintuitive results are also related to the interplay of geometric and
electronic effects and were never demonstrated before as the
experimental and theoretical challenges reached are important.
Still, many hurdles in the controlled synthesis of hybrid catalysts with variable ligand content and same metal concentration and average particle size, in the characterization of the
ligand organization exist, and in the rationalization of its catalytic impact remain in the cradle. Extending this technology to
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Figure 10. (a) Hydrogenation (left) and adsorption configuration (right) of 2methyl-3-butyn-2-ene over Pd-HHDMA and Pd-Pb. (b) Adsorption energy of
acetylene, ethylene, chloronitrobenzene and chloronitroaniline over different
Pd and Pt model surfaces. (c) Hydrogenation of 4-chloronitrobenzene over
Pt-HHDMA and Pt-Pb. The figures are adapted from ref. [124] and [125].

29

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Reviews
other hydrogenation metals (e.g., Ag, Au, Ir, Ni, Rh, and Ru)
and discovering alternative ligands with similar properties to
HHDMA are important outlooks in the field.

bility of the work. Zhang and co-workers recently prepared
a single-site platinum catalyst anchoring Pt species on a FeOx
support (Figure 11 b).[132, 133] Applied in the hydrogenation of
functionalized nitroarenes, the materials demonstrated high activity (TOF = ca. 104 hÀ1) and chemoselectivity to the desired
product (ca. 100 %), which was attributed to the presence of
positively charged platinum atoms in the system (Figure 11 c).
The catalyst, however, showed deactivation within the first
four cycles of reaction.
A stable single-site catalyst has been recently prepared by
embedding atomic palladium into mesoporous polymeric
graphitic carbon nitride (mpg-C3N4),[134] a material with hierarchical level of porosity and intriguing electronic properties
(Figure 11 d). The authors demonstrated that the isolated
atoms were tenaciously attached to the nitrogen atoms of the
cavities of mpg-C3N4. The absence of palladiumÀpalladium
bonds was confirmed by X-ray-absorption spectroscopy and
aberration-corrected transmission electron microscopy. In
alkyne and nitroarene hydrogenation, the material surpassed
the activity of conventional heterogeneous catalysts based on
nanoparticles, while maintaining an outstanding degree of
product selectivity (> 90 %) and leaching/agglomeration resistance for more than 20 h. The performance was rationalized for
the first time at a molecular level, employing DFT calculations,
showing that the high activity and selectivity can be attributed
to the facile hydrogen activation and reactant adsorption on
the atomically dispersed Pd sites.[134]
Following this work, Lu et al. have reported that atomically
dispersed Pd species can also be incorporated by atomic layer
deposition, after introduction of anchoring sites on pristine
graphene by high-temperature oxidation.[135] This material was

3.5. Single-Atom Catalysts
To maximize the product selectivity in hydrogenations, strategies to improve the isolation of the active phases have been
often adopted.[124] The ultimate frontier in this attempt is the
single-atom catalyst, which contains isolates metals atomically
embedded in a high-surface-area carrier.[128, 129] This type of materials combines the benefits of heterogeneous and homogeneous catalysis, with none of the drawbacks existing from organometallic design. The structure and properties of the carrier
are important to entrap the catalytically active phase in an isolated form. Single-site catalysts have sporadically appeared in
hydrogenation research and only few success stories can be
mentioned. In fact, their preparation is challenging because
the thermodynamic tendency of isolated metals to leach and/
or agglomerate.[130] The pioneering work of Flytzani-Stephanopoulos and co-workers demonstrated that a single Cu(111)
crystal doped with atomically-dispersed Pd species can activate
hydrogen and display activity in the hydrogenation of acetylene and styrene under ultra-high vacuum conditions (Figure 11 a).[131] However, the palladium atoms on this copper surface are incorporated by heating a palladium source in a highvacuum chamber, using an electron beam. This method is suitable for preparing single-atom crystals but does not readily
translate to the production of industrial-scale quantities of catalysts.[131] Besides, the relatively low ethylene selectivity (30 %)
at moderate reactant conversions (10-20 %) limits the applica-

Figure 11. Uses of single-site heterogeneous catalysts in selective hydrogenations of alkynes and nitroarenes. (a) Atomic-force microscopy of Pd/Cu(111) demonstrated the possibility of hydrogen splitting over atomically-dispersed palladium atoms. (b) High-resolution transmission electron microscopy of Pt/FeOx.
The atomic platinum species anchored to the support are encircled. (c) Hydrogenation of nitrostyrene over Pt/FeOx showing the stability and high product selectivity of the catalyst. (d) Structure of [Pd]mpg-C3N4, extended X-ray absorption fine structure spectroscopy, and hydrogenation of 1-hexyne over [Pd]mpgC3N4. The results for Pd-Pb and Pd are also plotted. The Figure is adapted from refs. [131] (a), [132] (b,c), and [134] (d).

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Reviews
tested in the hydrogenation of butadiene, showing 100 %
product selectivity at 95 % conversion at mild reaction conditions (300–350 K); in addition, excellent durability during 50 h
of reaction time was achieved, demonstrating also in this case
the benefits of atom immobilization in defined cavities in
order to prepare single-site heterogeneous catalysts.
The incorporation of a metal in an appropriate porous carrier such as carbon nitrides is generally performed post-synthetically, by impregnation of minute amount of a metal salt, followed by metal reduction and/or drying. This synthetic methodology, however, could lead to an undesirable formation of
clusters during synthesis.[136] An alternative approach to better
control the introduction of the metals into the carrier network
potentially comprises the addition of a metal salt during the
synthesis of the support. Such method can provide a more homogeneous metal distribution. In this direction, unpublished
results from our groups have demonstrated that silver tricyanomethanide can be used as a reactive co-monomer during
carbon nitride synthesis to introduce both negative charges
and silver atoms to the system. The successful introduction of
the extra electron density has been proven by electron microscopy evaluation, photoluminescence measurements, X-ray
photoelectron spectroscopy investigations, and measurements
of surface Zeta-potential. These results will be soon disclosed.
Extrapolating this synthetic approach to prepare a wide range
of single-site heterogeneous catalysts for hydrogenations (e.g.,
based on Ag, Au, Ir, Ni, Rh, and Ru) and identifying alternative
carriers with similar properties to carbon nitrides remain few of
the most urgent scopes. In the long term, it can be envisioned
the development of ‘structured reactors’ containing isolated
metal atoms in the washcoat, to permit almost total control
over all relevant length scales for mass transfer and
catalysis.[137, 138]

We expect that the next decade will foresee new, targeted
applications in biomass research. Many biomass-related reactions are hydrogenations and, in most cases, high-loading Ru
catalysts with undefined nanostructure are applied. The use of
hybrid and single-site catalysts has not been demonstrated to
date, but could be major breakthroughs in the next years. Applications in the pharmaceutical and fine chemical industries
will also remain important scope of applications of hydrogenation catalysis, considering the large (and yearly increasing)
number of high added-value compounds with biological functions. The dream of a green, sustainable, and intensified hydrogenation chemistry producing no byproducts and operating
with minimal energetic impact has never been so close to
reality.
Keywords: heterogeneous catalysis · hydrogenation ·
nanostructured materials · selectivity control · single-site
catalysis
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4. Conclusions and Outlook
Hydrogenation catalysis stands at the basis of our chemical industry. This mature field, for which core research was developed more than half a century ago, is now facing new challenges owing to environmental and health regulations. The
field has reemerged with unusual strength and is benefiting
from advances in materials synthesis, improved characterization techniques, new reactor developments, and the robustness of the most advanced theoretical simulations that can
provide an integrated approach towards the molecular understanding of the catalytic phenomena. More importantly, in
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there is a bright future ahead of us and many more materials
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this interest, summarizing the approaches developed over the
past 10 years and showing how the traditional families of hydrogenation catalysts, for bulk and fine chemicals, can be understood using the same conceptual principles.
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Received: November 18, 2015
Published online on December 17, 2015

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