10.1002/anie.202005842
Angewandte Chemie International Edition
COMMUNICATION
repulsion (Figure S17), indicating that boron atoms are separated
by at least three Pd-Pd bonds. The enhanced selectivity could
conceivably also result from modification with nitrogen, but boron
can be better incorporated into the palladium lattice due to its
smaller size and is also energetically more likely to be displaced
from distinct defective sites in the carrier (Figure S18). When
boron is found on the surface of a nanoparticle, it can diffuse into
the subsurface with a relatively low barrier (0.55 eV on Pd(111),
Figure S19). In contrast, the penetration into deeper layers
requires a much larger barrier (1.90 eV). The subsurface is the
most stable position for most Pd facets (Table S4).
The presence of interstitial boron modifies the electronic structure
of neighboring palladium atoms, which is reflected in the
adsorption energy of carbon monoxide (Figure 4b and 4c). Only
very small differences could be detected by Bader charge
analysis (Figure S20). In contrast, the projected density of states
shows that specific interactions between the p-states of boron and
mainly the dxy and dyz-states of a neighboring palladium atoms
shift the d-band center to more negative values by ~0.9 eV
(Figure S20). In situ diffuse reflection infrared Fourier transform
(DRIFT) spectroscopy confirms a decreased adsorption of carbon
monoxide on Pd/PBN-873 compared to Pd/Al2O3 (Figure 4b).
The amount of CO adsorbed is also reduced on Pd/BN-573 but to
a lesser extent. The computed adsorption energies of CO on the
surface of boron-modified palladium identifies the creation of
well-defined domains of metallic character (CO adsorption energy
on unmodified Pd(111) = −2.4 eV) confined by atoms where the
chemisorption of CO is less favorable by 0.7 eV (Table S5,
Figure 4c and S21). Since the adsorption energy of carbon
monoxide is closely related (Figure S22), similar spatially defined
regions are expected for hydrogen and acetylene, constituting
ensembles that have been demonstrated as a key feature for
selective hydrogenation catalysts.[6]
As no specific interactions of 1-hexyne are expected, the
mechanism of semi-hydrogenation is studied for acetylene to
avoid spurious rotations and simplify the study.[18] Hydrogen
dissociation occurs without barrier over both the B-doped and
pristine palladium surfaces, which is an important feature for a
high-performance catalyst. Distinctions emerge for the sequential
hydrogen transfer to the C2 backbone, evidencing decreased
barriers when boron is present in the lattice (Table S6,
Figures 4d and S23). Besides, the desorption energy of ethylene
is less endothermic in that case. Since the adsorption energy of
hydrogen is less exothemic (−0.22 eV) in the subsurface of
B-doped Pd(111) compared to pristine Pd(111) (−0.37 eV), the
potential formation of hydrides is reduced. All these factors
contribute to the increased selectivity evidenced in the catalytic
tests. When desorption of the alkene is easier than the further
of the sites implies the reaction is self-limited and the side
oligomerization paths cannot occur. All structures presented
herein can be retrieved from the ioChem-BD database.[20, 21] [Link
for
reviewers:
collection/100/23771/f2b83c409a3ac76cd79d8898].
In summary, this study demonstrated that carriers could
induce chemical modifications of supported metals, creating
intricate surface structures with tailored electronic and geometric
properties. The use of a porous boron nitride with low crystalline
order enabled the migration of heteroatoms from the carrier to
palladium. Nanoparticles of 4-5 nm displayed the highest
metal-specific rate while decreasing the size led to an increasingly
oxidized character and reduced activity. Impressively, the
optimized Pd/PBN-873 catalyst exhibited almost full selectivity at
high conversion in the continuous semi-hydrogenation of
1-hexyne, outperforming some of the best-reported Pd-based
catalysts for this reaction. Simulations showed that interatomic
repulsion determined the amount and distribution of dopant atoms
in the palladium nanoparticles, originating spatially-isolated
ensembles of metallic character. The findings highlight the
potential of tailored carriers for the design of frontier catalysts
through environmentally-benign and scalable routes without the
need for introducing further additives.
Acknowledgements
ScopeM at ETH Zurich is acknowledged for access to their
facilities. E.F. thanks MINECO La Caixa Severo Ochoa for a
predoctoral grant through Severo Ochoa Excellence Accreditation
2014–2018 (SEV 2013 0319). We thank BSC-RES for providing
computational resources, and the SuperXAS beamline at the
Swiss Light Source for granting beamtime, and Dr. Adam Clark
for help with the measurements.
Keywords: Heterogeneous catalysis • Selective hydrogenation •
Palladium • Boron nitride • Doping
[1]
[2]
[3]
[4]
T. W. van Deelen, C. Hernández Mejía, K. P. de Jong, Nat. Catal. 2019,
2, 955-970.
J. C. Matsubu, S. Zhang, L. DeRita, N. S. Marinkovic, J. G. Chen, G. W.
Graham, X. Pan, P. Christopher, Nat. Chem. 2017, 9, 120-127.
B. Qiao, B. Han, Y. Guo, Y. Huang, W. Xi, J. Xu, J. Luo, H. Qi, Y. Ren,
X. Liu, T. Zhang, Angew. Chem. Int. Ed. 2020, 10.1002/anie.202003208.
F. Huang, Y. Deng, Y. Chen, X. Cai, M. Peng, Z. Jia, P. Ren, D. Xiao, X.
Wen, N. Wang, H. Liu, D. Ma, J. Am. Chem. Soc. 2018, 140, 13142-
13146.
[5]
[6]
L. Zhang, M. Zhou, A. Wang, T. Zhang, Chem. Rev. 2020, 120, 683-733.
D. Albani, M. Shahrokhi, Z. Chen, S. Mitchell, R. Hauert, N. López, J.
Pérez-Ramírez, Nat. Commun. 2018, 9:2634.
[7]
[8]
[9]
X. Zhao, L. Zhou, W. Zhang, C. Hu, L. Dai, L. Ren, B. Wu, G. Fu, N.
Zheng, Chem 2018, 4, 1080-1091.
Y. Liu, A. J. McCue, C. Miao, J. Feng, D. Li, J. A. Anderson, J. Catal.
2018, 364, 406-414.
A. J. McCue, A. Guerrero-Ruiz, I. Rodríguez-Ramos, J. A. Anderson, J.
Catal. 2016, 340, 10-16.
hydrogenation,
over-hydrogenation. In fact, a previously reported descriptor of
selectivity[19] ΔEa (ΔEa
E(hydrogenation barrier)
the
reaction
is
selective
against
=
-
|E(ethylene desorption)| confirms the superior performance of the
examined site of B-doped palladium. E(ethylene desorption)
would need to be corrected by a constant entropic term (coming
from the transition to the gas-phase) for a correct description of
the energy competition. However, the reduction in the desorption
energy confirms the superior performance of the examined site of
B-doped palladium. This reduction can be understood in terms of
the Blyholder model, as palladium has more electron density
compromised in the bond to subsurface boron, less is available
for backdonation to alkenes, alkynes, or CO. The lack of a
continuous path for C-C coupling and the spatially resolved nature
[10] C. W. A. Chan, A. H. Mahadi, M. M.-J. Li, E. C. Corbos, C. Tang, G.
Jones, W. C. H. Kuo, J. Cookson, C. M. Brown, P. T. Bishop, S. C. E.
Tsang, Nat. Commun. 2014, 5:5787.
[11] H. Zhou, X. Yang, L. Li, X. Liu, Y. Huang, X. Pan, A. Wang, J. Li, T. Zhang,
ACS Catal. 2016, 6, 1054-1061.
[12] T. Mitsudome, T. Urayama, K. Yamazaki, Y. Maehara, J. Yamasaki, K.
Gohara, Z. Maeno, T. Mizugaki, K. Jitsukawa, K. Kaneda, ACS Catal.
2016, 6, 666-670.
[13] P. Wu, S. Yang, W. Zhu, H. Li, Y. Chao, H. Zhu, H. Li, S. Dai, Small 2017,
13, 1701857.
[14] M. Crespo-Quesada, A. Yarulin, M. Jin, Y. Xia, L. Kiwi-Minsker, J. Am.
Chem. Soc. 2011, 133, 12787-12794.
[15] N. Semagina, A. Renken, D. Laub, L. Kiwi-Minsker, J. Catal. 2007, 246,
308-314.
5
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