MgO-Supported Iridium Metal Pair-Site Catalysts Are More Active and Resistant to CO Poisoning than Analogous Single-Site Catalysts for Ethylene Hydrogenation and Hydrogen-Deuterium Exchange

Article abstract of DOI:10.1021/acscatal.9b03463

Atomically dispersed supported catalysts are drawing wide attention because they offer properties different from those of conventional catalysts, with maximally efficient use of the metals. However, the performance of single-site catalysts is often limited by the lack of neighboring metal centers to cooperate in catalysis. Thus, there is motivation to extend this class to catalysts incorporating isolated metal pairs. We report pairs of iridium atoms on MgO initially stabilized by support oxygen and cyclooctadiene ligands and activated by the removal of the latter. These catalysts are stable in a range of environments, including CO, H₂, and C₂H₄ + H₂ at 298-353 K and are more active than analogous single-site catalysts in ethylene hydrogenation and hydrogen-deuterium exchange because the neighboring metal centers facilitate hydrogen activation. Moreover, the pair-site catalysts retain activity even in the presence of CO, which poisons the single-site analogues. Supported metal pair-site catalysts open pathways toward understanding and applications of supported molecular catalysts.

Full text of DOI:10.1021/acscatal.9b03463

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Article
MgO-Supported Iridium Metal Pair-Site Catalysts are More
Active and Resistant to CO Poisoning than Analogous Single-
Site Catalysts for Ethylene Hydrogenation and H-D Exchange
Erjia Guan, Louise Debefve, Monica Vasiliu, Shengjie Zhang, David A Dixon, and Bruce C. Gates
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 09 Sep 2019
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ACS Catalysis
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MgO-Supported Iridium Metal Pair-Site Catalysts are More Active
and Resistant to CO Poisoning than Analogous Single-Site Catalysts
for Ethylene Hydrogenation and H-D Exchange
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Erjia Guan,a Louise Debefve,b Monica Vasiliu,c Shengjie Zhang,c David A. Dixon,c and Bruce C.
Gatesb,*
aDepartment of Materials Science and Engineering, University of California, Davis, California 95616, United States
bDepartment of Chemical Engineering, University of California, Davis, California 95616, United States
cDepartment of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487, United States
ABSTRACT: Atomically dispersed supported catalysts are drawing wide attention because they offer properties different
from those of conventional catalysts, with maximally efficient use of the metals. However, the performance of single-site
catalysts is often limited by the lack of neighboring metal centers to cooperate in catalysis. Thus, there is motivation to extend
this class to catalysts incorporating isolated metal pairs. We report pairs of iridium atoms on MgO initially stabilized by
support oxygen and cyclooctadiene ligands and activated by removal of the latter. These catalysts are stable in a range of
environments, including CO, H2, and C2H4 + H2 at 298–353 K and are more active than analogous single-site catalyst in ethylene
hydrogenation and H-D exchange because the neighboring metal centers facilitate hydrogen activation. Moreover, the pair-
site catalysts retain activity even in the presence of CO, which poisons the single-site analogs. Supported metal pair-site
catalysts open pathways toward understanding and applications of supported molecular catalysts.
KEYWORDS: Iridium catalyst; pair-site catalyst; ethylene hydrogenation; H-D exchange; hydrogen activation; CO poisoning
INTRODUCTION
Data characterizing ethylene hydrogenation and H-D
exchange in the reaction of H2 with D2 show how the
neighboring metal centers increase activity for H2
activation, minimize catalyst poisoning by CO, and provide
stable long-term operation.
Atomically dispersed supported metal catalysts have the
advantage of full access of the metals to reactants but the
limitation of a lack of neighboring metal centers to further
activate reactants. A second metal center placed to create a
pair-site opens an almost unexplored dimension in
supported catalysts while retaining the advantage of full
access to the metal centers. The prospects of new catalytic
properties associated with the neighboring sites are
indicated by the performance of supported clusters
consisting of several metal atoms1 and soluble dinuclear
metal catalysts in biology (e.g., for methane oxidation2) and
in chemistry.3,4 Dinuclear species incorporating metal–
metal bonds offer magnetic,5 photochemical,6 electron
transfer,7 and catalytic properties8 that differ markedly
from those of comparable mononuclear species. Metal pair-
sites on supports may incorporate metal–metal bonds, but
not necessarily, as they can also be stabilized by bonding of
the metal atoms to neighboring support sites. The few
reports of pair-site catalysts on supports9 give evidence of
new catalytic properties but fall short of explaining what
distinguishes their reactivities from those of single-site
analogues.
We report supported metal pair-site synthesis from a di-
nuclear organoiridium precursor; characterization by
spectroscopy and atomic-resolution microscopy; and
testing of catalyst performance with iridium pair-sites
incorporating various ligands. We chose MgO as the support
because it offers basic surface sites that facilitate precise
synthesis of well-defined supported species10 and because
the combination of the heavy catalytic Ir atom and the light
Mg and O support atoms provides strong contrast in
aberration-corrected scanning transmission electron
microscopy (STEM) for visualization of individual Ir atoms
in the pair-sites. In contrast to supports with redox surface
properties, such as CeO2,1, 11 or amorphous supports with
non-uniform, fluxional surface sites, such as SiO2,12 MgO
provides relatively simple chemistry for the investigation of
pair-site catalysts.
The catalysts were synthesized by the reaction of Ir2(μ-
OMe)2(COD)2 (OMe = methoxy; COD = cyclooctadiene) with
partially dehydroxylated MgO, with retention of the iridium
nuclearity upon chemisorption and subsequent ligand
modification. We determined the reactivity of the pair-sites
Our goals were to precisely synthesize supported pair-
site catalysts and to compare them with single-site catalysts
for test reactions that provide insight into their functions.
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in the gas atmospheres CO, H2 and D2, and in the presence of
C2H4 + H2 reacting catalytically to give ethane.
the activity increasing as CO ligands are gradually replaced
by reactants.
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The structures of the MgO-supported iridium-containing
pair-sites were determined with infrared (IR) and X-ray
absorption spectroscopies, high-angle annular dark-field
RESULTS
Synthesis and structure of MgO-supported iridium
pair-sites. The synthesis chemistry is confirmed by IR
spectra (Figure S1 in the Supporting Information, SI)
demonstrating that MgO surface OH groups react with Ir2(μ-
OMe)2(COD)2 to give surface species having C–H vibrations
of COD ligands13 remaining on the iridium; these bands
were blue-shifted by about 17 cm-1 due to the support acting
as a ligand, in agreement with density functional theory
(DFT) calculations of the νCH frequencies (Details in Table
S1 in the SI). A C–H band characterizing the bridging
methoxy ligands in Ir2(μ-OMe)2(COD)2 disappeared upon
chemisorption, as these ligands became bonded directly to
the MgO, as shown by IR spectra (Figure S2 in SI).
STEM
(HAADF-STEM),
and
electronic
structure
calculations. IR, X-ray absorption near edge structure
(XANES), and extended X-ray absorption fine structure
(EXAFS) spectroscopies were used to track structure
changes of the supported species during reactions in the gas
atmospheres and during catalysis. The results show that the
Ir pair-sites are stable on MgO and that the ligands on
iridium can be manipulated to tune the catalytic properties.
The pair-sites present initially were activated by exposure
to H2 at 353 K for 1 h, giving catalysts that are 46 times more
active than the untreated catalysts. Unlike iridium single-
site catalysts, the iridium pair-site catalysts are active for
ethylene hydrogenation even after poisoning by CO, with
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Table 1. Summary of EXAFS structure parametersa representing MgO-supported species formed from Ir2(μ-OMe)2(COD)2 and
the species after treatments in CO or H2 at 353 K.
Sample/Treatment conditions
Shell
N
R (Å)
103 x σ2
ΔE0
(eV)
(Å2)
Ir2/MgO formed by chemisorption of
Ir–Ir
0.95
1.9
3.8
1.2
3.1
2.99
1.99
2.13
2.76
3.11
3.2
0.6
1.7
1.3
3.3
3.3
-9.0
8.6
Ir2(μ-OMe)2(COD)2 on MgO
Ir–Os
Ir–CCOD1
Ir–CCOD2
Ir–CCOD2
3.1
-9.9
Ir2/MgO after exposure to flowing CO
(flow rates: CO, 5; He, 45 mL/min) at
353 K for 1 h
Ir–Ir
Ir–Os
0.96
0.90
1.8
2.80
2.25
2.04
2.96
2.11
2.4
1.9
5.5
2.4
1.8
7.9
1.1
-11
-3.6
1.0
Ir–Cter
Ir–OCO
Ir–Cbr
4.3
1.9
Ir2/MgO after exposure to flowing H2
(flow rate: 50 mL(NTP)/min) at 353 K
for 1 h
Ir–Ir
0.9
1.0
2.0
1.9
4.3
2.82
2.00
2.13
2.53
2.98
0.3
0.5
1.3
0.4
-2.6
-9.9
-5.5
1.5
Ir–Os
Ir–CCOE1
Ir–CCOE2
Ir–CCOE3
3.5.
2.0
σ2
a
Notation: N, coordination number; R, distance between absorber and backscatterer atoms; , disorder term; ΔE0, inner
20%
0.02 Å σ2 20%
20%
potential correction. Estimated error bounds: N, ±
; R, ± , ±
;
; ΔE0, ± . Os: O in MgO; OCO: O in CO ligands;
CCOD: C in cyclooctadiene ligands; Cbr: C in bridging CO; Cter: C in terminal CO; CCOE: C in cyclooctene ligands.
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ACS Catalysis
where the line scan was taken. (C) Line scan of a pair-site,
evidencing Ir atoms as bright scattering centers.
EXAFS spectra measured at the Ir LIII edge at 298 K
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provide evidence of (a) the ligands on the iridium, (b)
bonding of the iridium to the support, and (c) the iridium
nuclearity (Table 1). The best-fit model of the data shows
that two support O atoms replaced the methoxy ligands on
Ir (as shown by an Ir–O coordination number of 1.9 ± 0.4 at
a distance of 1.99 ± 0.02 Å, with the Ir–Ir coordination
number unchanged at 0.95 ± 0.2). These data demonstrate
the presence of pair-sites, with the Ir–Ir distance
corresponding to a non-bonding distance (2.99 ± 0.02 Å).
This distance is longer than that characterizing Ir2(μ-
OMe)2(COD)2 (2.90 Å),14 showing that replacement of the
bridging methoxy ligands with bridging oxygen atoms of the
support gave an Ir–Ir distance close to the distance between
support O atoms (~2.97 Å). Details of other candidate
models to represent the EXAFS data are presented in Table
S2 and Figure S3 in the SI.
The EXAFS data characterizing the sample (Table 1)
further show one Ir–C contribution with a coordination
number of 3.8 ± 0.8 at a distance of 2.13 ± 0.02 Å and two
others with coordination numbers of 1.2 ± 0.2 and 3.1 ± 0.6,
at greater-than-bonding distances of 2.76 ± 0.02 and 3.11 ±
0.02 Å. These results show that the COD ligand remained on
π
the iridium, with 4 -bonded carbon atoms and 4 carbon
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atoms per Ir atom at non-bonding distances. The remaining
protective COD ligands helped stabilize the pair-site
structure. The XANES-determined edge position of the
iridium in the pair-sites is 11216.7 eV (Figure S6 in SI),
significantly higher than that of Ir(0) in iridium foil (11215
eV) and essentially matching values reported for Ir(I)
species.15 The data show that the iridium atoms in the pair-
sites are cationic, a conclusion that is bolstered by the
results of DFT calculations summarized below.
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STEM images of an MgO crystal in the (110) orientation
confirm the presence of the Ir pair-sites (Figure 1; Figure 1C
is an intensity profile of an iridium pair); the average Ir–Ir
distance of about 2.9 Å is in good agreement with the EXAFS
value of 2.99 ± 0.02 Å. Because the electron beam quickly
induced break-up and migration of the Ir pair sites, the
images show Ir in structures other than pair sites, as
expected (Figure S4 in SI). Ir pair-sites showed no
aggregation under the influence of the electron beam, in
contrast to the sintering of single-site Ir species under the
same conditions (Figure S5 in SI).
Structures of various candidate models of the iridium
pair-sites (Figure 2, Figure S7, details in the SI) were
optimized using DFT. Two models agreeing best with the
EXAFS data were optimized on the basis of the Ir2(μ-
OMe)2(COD)2 structure bonded to the MgO. In one model,
the CH3 groups were removed and the overall system
modeled as a dianion (in the bulk, protons on the MgO
surface would balance the charge). In the other, the charge-
balancing protons were on the O atoms bonded to the Ir. In
each, the O atoms are bonded to surface Mg atoms. The first
structure has Ir–O and Ir–Ir bond distances of 2.04 and 2.80
Å, respectively, in good agreement with the EXAFS values.
The agreement with experiment is not good if the O atoms
are protonated (Figure S7, details in SI).
Figure 2. Model of iridium pair-sites on MgO. Side view
and top view of Ir2O2(COD)2 bonded to a (MgO)25 cluster.
The overall charge is -2.
To summarize, Figure 2 is a good representation of the
structure, formed as Ir2(μ-OMe)2(COD)2 reacted with OH
groups on MgO, leading to the replacement of the bridging
methoxy ligands with support O atoms.
Reactivity of iridium pair-sites on MgO. Because the
COD ligands remaining on the iridium pair-sites block
bonding of reactants and inhibit catalysis, we replaced some
of them by reaction with CO and modified the ligands by
treatment with H2. Such treatments typically cause
supported iridium clusters either to break into monomers
or to aggregate into larger clusters;16 in contrast, the pair-
sites remained stable.
Figure 1. HAADF-STEM images of sample synthesized
from Ir2(μ-OMe)2(COD)2 on MgO(110) face. (A) Iridium
pair-sites (indicated by white circles) and single-site Ir
atoms (dashed circle) shown as bright scattering centers.
(B) Detail of an individual pair-site; the dotted line shows
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