Atom-by-Atom Resolution of Structure–Function Relations over Low-Nuclearity Metal Catalysts

Article abstract of DOI:10.1002/anie.201902136

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 application-dependent requirements of the active ensemble. In the semi-hydrogenation 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.

Full text of DOI:10.1002/anie.201902136

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Atom-by-Atom Resolution of Structure-Function Relations over
Low-Nuclearity Metal Catalysts
Authors: Evgeniya Vorobyeva, Edvin Fako, Zupeng Chen, Sean M
Collins, Duncan Johnstone, Paul A Midgley, Roland Hauert,
Olga Safonova, Gianvito Vilé, Núria López, Sharon Mitchell,
and Javier Pérez-Ramírez
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902136
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10.1002/anie.201902136
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Atom-by-Atom Resolution of Structure-Function Relations over
Low-Nuclearity Metal Catalysts
Evgeniya Vorobyeva[a], Edvin Fako[b], Zupeng Chen[a], Sean M. Collins[c], Duncan Johnstone[c], Paul A.
Midgley[c], Roland Hauert[d], Olga V. Safonova[e], Gianvito Vilé[a], Núria López[b], Sharon Mitchell*[a], and
Javier Pérez-Ramírez*[a]
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 where 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 application-dependent requirements of the active
ensemble. In the semi-hydrogenation 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.
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 like 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
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 investigated
by controlling the size and/or shape of relatively large supported
nanoparticles (>1 nm).[3,4] In this range, size-dependent 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
[17]
[18]
way, iridium dimers supported on Fe2O3
and WO3 were
shown to exhibit enhanced water photo-oxidation performance
compared to Ir atoms or nanoparticles, by reducing the energy
barrier for the third proton-coupled 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.
Uniting advanced experimental and theoretical approaches
including aberration-corrected scanning transmission electron
microscopy (AC-STEM), X-ray photoelectron 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 selective hydrogenation 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 1a). Although
attractive for comparative purposes, the controlled aggregation
of Pd atoms via thermal treatment is unsuccessful (Figure S1).
Alternatively, the deposition of dimeric and trimeric palladium
complexes appears a more promising strategy for the controlled
introduction of low-nuclearity 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)
[a]
[b]
E. Vorobyeva, Dr. Z. Chen, Dr. G. Vilé, Dr. S. Mitchell, Prof.
J. Pérez-Ramírez
Institute for Chemical and Bioengineering, Department of Chemistry
and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1, 8093
Zürich (Switzerland)
E-mails: 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, CB3 0FS Cambridge (United Kingdom)
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, including detailed description of the
experimental methods and additional characterization data, for this
article is given via a link at the end of the document.
[c]
[d]
[e]
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Figure 1. a) Approaches to prepare low-nuclearity Pd catalysts based on carbon nitride. Red arrows indicate unsuccessful routes. b-d) AC-STEM 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.
as well as complete decomposition of the precursor ligands
(Figure S3).
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 (Movies 1-3).[23] Interestingly, the
Pd-Pd distance distributions estimated by projection of the 3D
structures simulated by BOMD on the xy plane closely match
those derived by AC-STEM (Figure 1b-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 = -3.47, -0.92, and
-1.37 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).
AC-STEM imaging confirms the high metal dispersion in the
Pdx/ECN catalysts. Although features resembling dimers and
trimers can be identified (Figure 1b-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 Text) 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 Pd-Pd 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
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
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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 2b) 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 2c,
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 Å primarily originates from the first
coordination neighbor (N or O), with an associated coordination
distance of 2.019 Å. A similar Pd-N interaction appears in the
radial distribution functions (RDF, Figure 2d) 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±0.3 (EXAFS) and
4.3±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 Pd-N
coordination number to 2.5±0.3/3.8±0.8 and 2.8±0.3/2.3±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 Pd-Pd 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 Pd-Pd bond
length variation (Figure 1b-d), the dominant Pd-scaffold
interactions strongly affect the metal-metal peaks. Additionally,
the fact that scattering signals generated by different Pd-Pd
couples destructively interfere because they 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 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 2e) in which the electron density of the
scaffold shields Pd preventing CO adsorption.
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), dashed gray 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 (Movies 1-3) illustrating
distinct possible surface and subsurface configurations of metal ensembles.
All catalysts convert 2-methyl-3-butyn-2-ol into 2-methyl-3-
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 5
catalytic runs (Figure S11). However, major differences are
evident in terms of activity versus nuclearity, with Pd3/ECN
exhibiting
a
reaction rate (0.73 × 103 mol-ene molPd-1 h-1) 3.8
times higher than that over Pd2/ECN and Pd1/ECN (Figure 3a,
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
Pd4+ (338.5 eV) and Pd2+ (336.9 eV) based on formal charges
(Figure 2a). Nevertheless, the Pd2+:Pd4+ ratio varies from
0.56 (Pd1/ECN)
evidencing
to
0.61 (Pd2/ECN)
to
0.93 (Pd3/ECN),
a
reduced degree of oxidation with increasing
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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 5 sequential runs and b) the
calculated reaction paths over the surface configurations of the Pd ensembles in Pdx/ECN. Pd, C, N, O, and H atoms are shown in green, gray, blue, red, and
white, respectively. TS, transition state. c) Rates of alkene formation in the hydrogenation of distinct functionalized alkynes (illustrated). The color code in a)
applies to b) and c).
barriers for this step as the mechanism follows a heterolytic
route involving the transfer of a H atom to the scaffold
(Figure 3b). 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
the-art homogenous and heterogeneous catalysts in this
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
hydrogenation to the alkene and ready desorption of this product. metal losses (Table S1) and the formation of nanoparticles in
Slightly enhanced performance of the dimers could be expected
due to the existence of a bridge site, leading to more exothermic
(-0.81 and -1.16 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 (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 3c).
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
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.
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 state-of-
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
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Table 1. Performance in the continuous reaction of bromobenzene with
phenylboronic acid pinacol ester (depicted).
Sample [a]
Pd1/ECN
Pd2/ECN
Pd3/ECN
Pd[a] / wt%
0.57
X[b] / %
54
S[c] / %
89
r[d] / mol mol-1 h-1
538
516
208
0.46
79
49
0.46
19
79
[a] ICP-OES. [b] Conversion and [c] selectivity determined by
high-performance liquid chromatography. [d] Rate of biphenyl formation per
mole of Pd.
and BOMD. Catalytic tests in alkyne semi-hydrogenation and Suzuki
coupling were conducted in batch continuous mode, respectively.
Acknowledgments
We are grateful to the SNSF (200021-169679, E.V., Z.C., S.M.,
J.P.-R.), the Severo Ochoa Excellence program (SEV-2013-
0319, 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).
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.
Keywords: heterogeneous catalysis • structure-activity
relationships • metal clusters • hydrogenation • C-C coupling
the low-nuclearity species provided essential insights to interpret
the experimental observations. Catalytic tests in alkyne
semi-hydrogenation and Suzuki coupling revealed distinct
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.
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Entry for the Table of Contents
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An atom apart: By exploiting the
versatile coordination chemistry
of carbon nitride to accommodate
low-nuclearity metal species, this
work interrogates the properties
and catalytic performance of
palladium atoms, dimers, and
trimers, highlighting the opposite
requirements for optimal atom
economy in C-C coupling and
alkyne semi-hydrogenation.
E. Vorobyeva, E. Fako, Z. Chen,
S. M. Collins, D. Johnstone, P. A.
Midgley, R. Hauert, O. V.
Safonova, G. Vilé, N. López,
S. Mitchell*,
Ramírez*
and
J.
Pérez-
Page No. – Page No.
Atom-by-Atom Resolution of
Structure-Function
Relations
over
Low-Nuclearity Metal
Catalysts
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