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Communications

Heterogeneous Catalysis Hot Paper

International Edition: DOI: 10.1002/anie.201902136
German Edition:
DOI: 10.1002/ange.201902136

Atom-by-Atom Resolution of Structure–Function Relations over
Low-Nuclearity Metal Catalysts
Evgeniya Vorobyeva, Edvin Fako, Zupeng Chen, Sean M. Collins, Duncan Johnstone,
Paul A. Midgley, Roland Hauert, Olga V. Safonova, Gianvito VilØ, Nfflria López,
Sharon Mitchell,* and Javier PØrez-Ramírez*
Abstract: Controlling the structure sensitivity of catalyzed
reactions over metals is central to developing atom-efficient
chemical processes. Approaching the minimum ensemble size,
the properties enter a non-scalable regime in which each atom
counts. Almost all trends in this ultra-small frontier derive from
surface science approaches using model systems, because of
both synthetic and analytical challenges. Exploiting the unique
coordination chemistry of carbon nitride, we discriminate
through experiments and simulations the interplay between the
geometry, electronic structure, and reactivity of palladium
atoms, dimers, and trimers. Catalytic tests evidence applicationdependent requirements of the active ensemble. In the semihydrogenation of alkynes, the nuclearity primarily impacts
activity, whereas the selectivity and stability are affected in
Suzuki coupling. This powerful approach will provide practical insights into the design of heterogeneous catalysts
comprising well-defined numbers of atoms.

The dispersion of metals as tiny particles or clusters over

high-surface-area hosts is common practice in heterogeneous
catalysis.[1, 2] It provides a means to tune the geometric and
electronic characteristics compared to the bulk material while
increasing the portion of atoms exposed for the adsorption
and transformation of a substrate. The structure sensitivity of
metal catalysts in different applications has primarily been
[*] E. Vorobyeva, Dr. Z. Chen, Dr. G. VilØ, Dr. S. Mitchell,
Prof. J. PØrez-Ramírez
Institute for Chemical and Bioengineering, Department of Chemistry
and Applied Biosciences, ETH Zürich
Vladimir-Prelog-Weg 1, 8093 Zürich (Switzerland)
E-mail: msharon@chem.ethz.ch
jpr@chem.ethz.ch
E. Fako, Prof. N. López
Institute of Chemical Research of Catalonia and Barcelona Institute
of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)
Dr. S. M. Collins, Dr. D. Johnstone, Prof. P. A. Midgley
Department of Materials Science and Metallurgy, University of
Cambridge
Cambridge CB3 0FS (UK)
Dr. R. Hauert
Empa, Swiss Federal Laboratories for Materials Science and
Technology, Überlandstrasse 129, 8600 Dübendorf (Switzerland)
Dr. O. V. Safonova
Paul Scherrer Institute
5232 Villigen (Switzerland)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201902136.

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Angew. Chem. Int. Ed. 2019, 58, 1 – 7

investigated by controlling the size and/or shape of relatively
large supported nanoparticles (> 1 nm).[3, 4] In this range, sizedependent behavior can mainly be assigned to changes in the
relative amounts or different surfaces and consequent distinctions in the electron densities around edge and defect
sites. Moving to subnanometer dimensions alters these
properties because of both increased electron confinement
and interaction with the host material.[5] This non-scalable
regime originates a non-linear variation of reactivity patterns
that widens the applicability of metal catalysts. At the
ultimate limit, the use of single-atom heterogeneous catalysts
is rapidly expanding due to advances in their synthesis and
characterization facilitated by their more readily distinguishable microscopic features.[6, 7] While isolated atoms display
attractive characteristics with respect to nanoparticles in
some reactions,[8, 9] they are not always the preferred structural unit.[4, 10] In this regard, understanding the atom-by-atom
behavior of low-nuclearity clusters is fundamental, but
addressing this still presents numerous challenges.
Pioneering surface science approaches used model systems to derive trends with cluster size in reactions involving
simple substrates, such as CO oxidation,[10, 11] propane oxidative dehydrogenation,[12] O2 reduction,[13] and propene epoxidation.[14] These works suggest that the addition or removal
of a single atom can control the catalytic response.[15]
However, the preservation of the nuclearity post-deposition
and during application lacks direct evidences and is the major
concern. Alternative synthetic strategies mainly involve the
wet deposition of metal complexes of the desired number of
atoms.[16–20] In this way, iridium dimers supported on Fe2O3[17]
and WO3[18] were shown to exhibit enhanced water photooxidation performance compared to Ir atoms or nanoparticles, by reducing the energy barrier for the third protoncoupled electron transfer step in the catalytic cycle. Similarly,
Fe, Pd, and Ir dimers were immobilized on carbon nitride.[19]
Only the iron-based catalysts were active in the targeted
epoxidation of trans-stilbene and dimers yielded superior
performance to atoms. In one of few studies on metal trimers,
Ru3 stabilized on nitrogen-doped carbon was found to
efficiently catalyze the selective oxidation of alcohols.[20]
Despite this progress, the mechanistic origin of the enhanced
activity remains poorly understood. Furthermore, the lack of
systematic assessment of the dynamics of the catalyst and its
long-term stability under the reaction conditions is of major
concern.
By merging advanced experimental and theoretical
approaches including aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelec-

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tron spectroscopy (XPS), X-ray absorption spectroscopy
(XAS), density functional theory (DFT), and Born–Oppenheimer molecular dynamics (BOMD), this work elucidates
the structure of three catalysts with distinct nuclearity (Pdx,
x = 1, 2, 3) on an exfoliated carbon nitride (ECN) host. This
enables rationalization of their catalytic behavior in the semihydrogenation of alkynes and Suzuki coupling, chemical
transformations of increasing complexity. To achieve this, we
exploit the multidentate coordination of the heteromacrocycles of carbon nitride (C3N4) to accommodate metal species
of different nuclearity (Figure 1 a). Although attractive for
comparative purposes, the controlled aggregation of Pd atoms
via thermal treatment is unsuccessful (Figure S1 in the
Supporting Information). Alternatively, the deposition of
dimeric and trimeric palladium complexes appears a more
promising strategy for the controlled introduction of lownuclearity clusters on ECN without alteration of the carrier
structure (Figure S2). Metal contents close to the targeted
value of 0.5 wt % are achieved in all cases (Table S1) as well as
complete decomposition of the precursor ligands (Figure S3).
AC-STEM imaging confirms the high metal dispersion in
the Pdx/ECN catalysts. Although features resembling dimers

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and trimers can be identified (Figure 1 b–d, Figure S4),
several factors hamper the clear visualization of these metal
ensembles, not least their multiple and varied orientations on
the three-dimensional irregular structure of ECN (Supporting
Information) and the dynamic connection between these
orientations (see below).[21] To gain more quantitative
insights, we adopted a statistical approach comparing the
measured and theoretical (random dispersion) nearest neighbor (NN) distance distributions between Pd atoms.[22] As
expected in the presence of isolated atoms these curves
directly overlap in Pd1/ECN (mean NN distance = 0.44 nm).
In contrast, a progressive shortening of the NN distances with
respect to the random distribution in both Pd2/ECN (0.35 nm)
and Pd3/ECN (0.26 nm) agrees with a closer proximity of Pd
centers, approaching typical values for a PdÀPd bond. To
avoid any effect of overlapping ensembles on the measured
distribution, samples with lower Pd contents (0.1 wt %) were
targeted to reduce the areal density of metal species,
evidencing similar trends (Figure S5). The multidentate sites
of the carbon nitride scaffold can accommodate Pd moieties
with various coordination structures that are dynamically
sampled (Supporting Movies 1–3).[23] Interestingly, the PdÀPd

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Figure 1. a) Approaches to prepare low-nuclearity Pd catalysts based on carbon nitride. Broken red arrows indicate unsuccessful routes. b)–d) ACSTEM images of the resulting catalysts with the measured (gray bars), predicted by BOMD (purple shaded curves), and theoretical random (red
curves) NN distance distributions. Selected Pd atoms (blue circles), dimers (pink circles), and trimers (green circles) are identified.

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Angew. Chem. Int. Ed. 2019, 58, 1 – 7

Figure 2. a) Measured and b) simulated Pd 3d core level XPS spectra
of the Pdx/ECN catalysts. In (a), black lines show the fits of the raw
data (open symbols), gray lines deconvolute the components tentatively assigned to Pd4+ (dashed) and Pd2+ (dotted) species, and blue
lines show the background. The Pd2+:Pd4+ ratios are indicated in
parenthesis. In (b), colored lines represent each Pd atom in the
ensemble. c) Normalized k2-weighted Fourier Transform of the EXAFS
function of the Pdx/ECN catalysts and reference compounds. d) Sum
of the radial distribution functions over all the structures generated
during the BOMD runs, with the Pd atoms as the reference for Pdx/
ECN. e) Snapshots from the BOMD simulations (Supporting
Movies 1–3) illustrating distinct possible surface and subsurface
configurations of metal ensembles. Green Pd, pale blue N, and grey C.

are not in phase due to very broad bond-length distribution
render their detection by EXAFS almost impossible. CO
adsorption was studied as a method to discriminate the

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distance distributions estimated by projection of the 3D
structures simulated by BOMD on the xy plane closely match
those derived by AC-STEM (Figure 1 b–d).
DFT calculations support the favorable stabilization of
the dimers and trimers on ECN following removal of the
ligand. The formation energies per Pd atom, Eform = À3.47,
À0.92, and À1.37 eV for Pd1/ECN, Pd2/ECN, and Pd3/ECN,
respectively (Table S2), indicate a slightly weaker interaction
for the metal dimers and trimers than for single atoms
(Figure S6). The possible dissociation of dimers into isolated
atoms cannot be completely excluded, while for the trimer
this is less likely. Comparatively, both the agglomeration of
single atoms to dimers and of dimers to trimers are unlikely
(Table S3).
Previous studies have evidenced the cationic nature of Pd
atoms isolated on ECN.[24, 25] Similarly, analysis by XPS
distinguishes two positively charged Pd species in the cases
of Pd2/ECN and Pd3/ECN, which are tentatively assigned to
Pd4+ (338.5 eV) and Pd2+ (336.9 eV) based on formal charges
(Figure 2 a). Nevertheless, the Pd2+:Pd4+ ratio varies from 0.56
(Pd1/ECN) to 0.61 (Pd2/ECN) to 0.93 (Pd3/ECN), evidencing
a reduced degree of oxidation with increasing nuclearity,
which agrees with the predicted weaker interaction with the
host. Notably, the signal of bulk metal at 335.0 eV was not
observed in any sample. The XPS shifts simulated along the
BOMD trajectories (Figure 2 b) show that Pd atoms localized
close to the surface are less oxidized (Pd2+-like) than those
residing in the subsurface (Pd4+-like) of the C3N4 structure.
To gain further insight into the coordination environment
of the Pd species, we investigated the catalysts by XAS
(Figure 2 c, Figure S7). The fitted results (Table S4) of the
extended absorption X-ray fine structure (EXAFS) at the Pd
K-edge reveal that the strong signal at 1.7 Š primarily
originates from the first coordination neighbor (N or O),
with an associated coordination distance of 2.019 Š. A similar
PdÀN interaction appears in the radial distribution functions
(RDF, Figure 2 d) of Pd calculated by BOMD. For Pd1/ECN,
BOMD shows that the first coordination sphere is formed by
N, but full bonding to all 6 N in the cavity has low probability.
This reduces the average N (N or O for EXAFS) coordination
number of Pd to 3.4 Æ 0.3 (EXAFS) and 4.3 Æ 0.9 (BOMD,
Table S5). Distortions of the scaffold and the movement of Pd
within the cavity yield a non-equidistant first coordination
sphere even if comprised solely of nitrogen (Figure S8). For
Pd2/ECN and Pd3/ECN the less pronounced interaction with
the scaffold lowers the average PdÀN coordination number to
2.5 Æ 0.3/3.8 Æ 0.8 and 2.8 Æ 0.3/2.3 Æ 0.7 for EXAFS/BOMD,
respectively, due to the presence of Pd atoms in the first
coordination sphere (Figure S5, Figure S9). A small interaction between Pd and C atoms is also observed by BOMD
deriving from ensembles in subsurface positions. Notably, no
strong signal associated with PdÀPd bonding is visible in any
spectra. The reason for this becomes apparent when inspecting the radial distribution functions extracted from the
BOMD trajectories. Due to the low nuclearity and wide
PdÀPd bond length variation (Figure 1 b–d), the dominant
Pd-scaffold interactions strongly affect the metal-metal peaks.
Additionally, the fact that scattering signals generated by
different PdÀPd couples destructively interfere because they

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uniformity of the metal speciation, but it was not possible to
detect by pulse chemisorption measurements or IR spectroscopy (Figure S10). This relates to the fact that the Pd
ensembles coexist in subsurface configurations (Figure 2 e)
in which the electron density of the scaffold shields Pd
preventing CO adsorption.
All catalysts convert 2-methyl-3-butyn-2-ol into 2-methyl3-buten-2-ol with high stability and full chemoselectivity
(100 %) under the investigated conditions. Consistently, the
oxidation state, metal content, and atomic dispersion are
preserved after five catalytic runs (Figure S11). However,
significant differences are evident in terms of activity versus
nuclearity, with Pd3/ECN exhibiting a reaction rate (0.73 ”
103 mol-ene molPdÀ1 hÀ1) 3.8 times higher than that over Pd2/
ECN and Pd1/ECN (Figure 3 a, Figure S12). We have
explored the effect of the nuclearity when all the atoms are
exposed on the surface. The superior performance of Pd3/
ECN originates from the barrierless, homolytic H2 activation
path on hcp-like sites. This requires a minimum ensemble of
three atoms and leaves a bridge site available for adsorbing
the alkyne, resembling the Pd(111) surface.[26] In contrast, Pd1/
ECN and Pd2/ECN exhibit higher barriers for this step as the

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mechanism follows a heterolytic route involving the transfer
of a H atom to the scaffold (Figure 3 b). This opens the
coordination sphere of Pd and allows the triple-bond
adsorption and subsequent H transfer. The second H is then
recovered to the metal center, prior to the hydrogenation to
the alkene and ready desorption of this product. Slightly
enhanced performance of the dimers could be expected due
to the existence of a bridge site, leading to more exothermic
(À0.81 and À1.16 eV, Pd1/ECN and Pd2/ECN, respectively)
hydrogen activation than over single atoms. However, this is
only possible when both atoms are located in surface positions
and dimers mostly populate a configuration with one subsurface atom (Supporting Movie 2). This issue is not encountered
for Pd3/ECN because the subsurface configurations are less
favorable for the trimers (Figure S13). This nuclearity trend
was generalized to the transformation of diverse linear and
branched alkynes (Figure 3 c).
Opposite nuclearity effects are observed in the continuous
Suzuki coupling of bromobenzene with phenylboronic acid
pinacol ester (Table 1). Pd1/ECN exhibits the highest rate of
biphenyl formation, and was recently shown to surpass stateof-the-art homogenous and heterogeneous catalysts in this

Figure 3. a) Variation in the rates of 2-methyl-3-buten-2-ol formation in the hydrogenation of 2-methyl-3-butyn-2-ol (depicted) over five sequential
runs and b) the calculated reaction paths over the surface configurations (with all atoms at the surface) of the Pd ensembles in Pdx/ECN.
Green Pd, gray C, blue N, red O, and white H atoms. TS = transition state. c) Rates of alkene formation in the hydrogenation of distinct
functionalized alkynes (illustrated). The color code for data points in (a) applies to (b) and (c).

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Table 1: Performance in the continuous reaction of bromobenzene with
phenylboronic acid pinacol ester (depicted).

Sample[a]

Pd[a] [wt %]

X[b] [%]

S[c] [%]

r[d] [mol molÀ1 hÀ1]

Pd1/ECN
Pd2/ECN
Pd3/ECN

0.57
0.46
0.46

54
79
19

89
49
79

538
516
208

[a] ICP-OES. [b] Conversion and [c] selectivity determined by highperformance liquid chromatography. [d] Rate of biphenyl formation per
mole of Pd.

reaction.[23] A similar rate is evidenced over Pd2/ECN, but this
catalyst is found to be more active and less selective, while
Pd3/ECN displays only limited conversion. Analysis of the
used catalysts confirms a high stability of single atoms, but
reveals metal losses (Table S1) and the formation of nanoparticles in the case of dimers and trimers (Figure S14). The
different performance originates in the first oxidative addition of bromobenzene, and relate to a nuclearity-dependent
chemoselectivity (Figure 4). Single atoms tend to activate the
Br-phenyl bond and provide the adaptive coordination
environment for the following transmetallation and reductive

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elimination steps. However, on dimers and especially trimers
the aromatic ring adsorbs to the nanocluster strongly, poisoning the active site, and weakening the interaction of the metal
cluster to the carbon nitride scaffold. This induces the
extraction of Pd from the anchoring site and consequent
aggregation.
In conclusion, by exploiting the versatile coordination
chemistry of carbon nitride, palladium atoms, dimers, and
trimers were anchored on this host enabling comparison of
their structure and reactivity. Simulations of the dynamic
structure of the low-nuclearity species provided essential
insights to interpret the experimental observations. Catalytic
tests in alkyne semi-hydrogenation and Suzuki coupling
revealed opposite application-dependent nuclearity effects.
Pd trimers were more active in the selective hydrogenation of
various functionalized alkynes, which was linked to the
reduced hydrogen activation barrier with respect to single
atoms and dimers. In contrast, Pd single atoms surpass
ensembles in Suzuki coupling exhibiting distinct chemoselectivity to the dimers and trimers, which also ensured
higher stability. While all evidence points towards the
successful stabilization of the desired ensembles, establishing
the uniformity of the metal speciation remains an important
challenge that deserves urgent attention.

Experimental Section
Full details on the catalyst synthesis, characterization, testing, and
calculations are given in the Supporting Information. Pd1/ECN was
prepared following a previously reported recipe.[23] Pd2/ECN and Pd3/
ECN were obtained by microwave-assisted deposition of the corresponding precursors (allyl palladium chloride dimer or palladium
acetate trimer) in toluene onto ECN. The resulting solids were
collected by filtration, washed, dried, and subsequently thermallytreated to ensure removal of the ligands. The structure and properties
of the Pd species of different nuclearity were studied in depth by ACSTEM, XPS, XAS, DFT (PBE + D3), and BOMD. Catalytic tests in
alkyne semi-hydrogenation and Suzuki coupling were conducted in
batch continuous mode, respectively.

Acknowledgements
We are grateful to the SNSF (200021-169679, E.V., Z.C., S.M.,
J.P.-R.), the Severo Ochoa Excellence program (SEV-20130319, E.F.), the EPSRC (Grant no. EP/R008779/1, P.A.M.),
and the Henslow Research Fellowship of Girton College,
Cambridge (S.M.C.) for funding this work. We thank ScopeM
and BSC-RES for access to facilities, and Diamond Light
Source for access and support in the use of the electron
Physical Science Imaging Centre (EM17997).

Conflict of interest
The authors declare no conflict of interest.

Angew. Chem. Int. Ed. 2019, 58, 1 – 7

Keywords: CÀC coupling · heterogeneous catalysis ·
hydrogenation · metal clusters · structure–activity relationships

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Figure 4. Reaction paths of the coupling of bromobenzene with
phenylboronic acid pinacol ester over the surface configurations of the
Pd ensembles in Pdx/ECN. Atom coloring as in Figure 3, Br in dark
red.

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Manuscript received: February 18, 2019
Accepted manuscript online: May 2, 2019
Version of record online: && &&, &&&&

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Communications
E. Vorobyeva, E. Fako, Z. Chen,
S. M. Collins, D. Johnstone, P. A. Midgley,
R. Hauert, O. V. Safonova, G. VilØ,
N. López, S. Mitchell,*
J. PØrez-Ramírez*
&&&&—&&&&
Atom-by-Atom Resolution of Structure–
Function Relations over Low-Nuclearity
Metal Catalysts

Angew. Chem. Int. Ed. 2019, 58, 1 – 7

An atom apart: Exploiting the versatile
coordination chemistry of carbon nitride
to accommodate low-nuclearity metal
species allows the properties and catalytic performance of palladium atoms,
dimers, and trimers, to be interrogated.
The results show that in CÀC coupling
and alkyne semi-hydrogenation reactions
opposite requirements give the optimal
atom economy.

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Heterogeneous Catalysis

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