Synergistic effects for enhanced catalysis in a dual single-atom catalyst

Junhong Fu, Jinhu Dong, Rui Si,* Keju Sun, Junying Zhang, Mingrun Li, Nana Yu, Bingsen Zhang,

Research Article

Synergistic Eﬀects for Enhanced Catalysis in a Dual Single-Atom

Catalyst

Mark G. Humphrey, Qiang Fu,* and Jiahui Huang *

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ABSTRACT: Synergistic eﬀects have been discussed extensively

in bimetallic heterogeneous catalysis, but it remains unclear how

the eﬀects function at the atomic scale. Here, we report a dual

single-atom catalyst (DSAC) Ir Mo /TiO displaying much greater

catalytic chemoselectivity (>96%, at 100% conversion) than

comparable single-atom catalysts (SACs) Ir /TiO (38%, at 87%

conversion) and Mo /TiO (no activity) for the hydrogenation of

-nitrostyrene (4-NS) to 4-vinylaniline (4-VA). Activation of the

TiO -supported bimetallic carbonyl cluster Ir Mo (CO) (η -

C H ) in an Ar atmosphere aﬀords the DSAC Ir Mo /TiO .

5 2

Characterization of the dual single-atom structure conﬁrms that it

consists of well-dispersed Ir single atoms (Ir ) and Mo single atoms

(

Mo ) on TiO . Density functional theory studies reveal that Ir

sites eﬀect H activation while Mo sites are responsible for 4-NS adsorption, with synergistic cooperation between the two sets of

single atoms contributing to the better catalytic performance for the hydrogenation of 4-NS. This work provides a deep

understanding of synergistic eﬀects in dual single-atom catalysis.

KEYWORDS: bimetallic catalysis, synergistic eﬀects, single-atom catalyst, dual single-atom catalyst, hydrogenation of nitrostyrene

5−19

INTRODUCTION

bonds

M(M′) linkages.

single-atom sites (speciﬁcally Ni and Ru ) anchored on CeO

or diatomic ensemble structures with M−O−

■

20−22

The synergistic eﬀect of two sets of

Heterogeneous bimetallic catalysis generally involves the

cooperation of two metals in a chemical transformation and

often displays superior catalytic performance compared to

in the dry reforming of CH₄was reported recently;

computational studies reveal that Ni activates CH and Ru

dissociates CO in the catalytic reaction. However, the

synthesis of such dual single-atom catalysts (DSACs) and the

understanding of the synergistic eﬀects of the two sets of

single-atom sites at the atomic level still remain signiﬁcant

challenges.

Organometallic clusters have long been used as precursors to

supported well-deﬁned sub-nanometer clusters.

ligands (including carbonyl and organic species) around the

organometallic clusters are easily removed in H , and the

metallic character of the metal clusters is then retained on the

monometallic catalysis due to synergistic eﬀects.

particular, supported bimetallic nanoparticles and alloys

composed of a noble metal and a non-noble metal have

been extensively studied, in which the two types of metals

function diﬀerently. The noble metals are often the active sites

for many reactions, and the non-noble metals inﬂuence the

noble metal through electronic and geometric interactions,

from involvement in the reaction by bonding to the reactants

or intermediates, or by creating interfacial sites, which in turn

24−26

The

−6

can enhance the catalytic activity and selectivity.

Downsizing metal particles to single-metal atoms is a

straightforward way to increase metal utilization eﬃciency,

and the resulting single-atom catalysts (SACs) have displayed

good catalytic performances in many reactions including CO

27,28

support.

For example, a Pt−Re bimetallic catalyst was

prepared through treatment of Re Pt(CO) supported on

Al O in H and displayed high resistance to deactivation

oxidation, the water−gas shift reaction, methane steam

Received: December 21, 2020

Revised: January 11, 2021

Published: January 29, 2021

reforming, selective nitroarene hydrogenation, and so forth.

Instead of one type of single atom (as in SACs), catalysts with

a diatomic structure can deliver superior catalytic performance

to that of SACs due to cooperation between the two

2−14

atoms.

To date, the reported diatomic catalysts mostly

feature supported diatomic-pair structures with metal−metal

2021 American Chemical Society

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Research Article

Figure 1. (a) ATR-IR spectra of Ir Mo (CO) (η -C H ) , TiO , and Ir Mo (CO) (η -C H ) /TiO . (b) Temperature-programmed desorption

results for Ir Mo (CO) (η -C H ) /TiO in an Ar ﬂow.

during the catalytic dehydrogenation of methylcyclohexane,

adsorbed on TiO₂was not completely removed after

evacuation, as shown by the presence of absorption bands

corresponding to n-pentane (Figures 1a and S2). Further

due to the role of Re in stabilizing the dispersion of Pt.

Shapley and co-workers activated Ir Mo (CO) (η -C H ) on

5 2

Al O in H₂and the resulting catalyst maintained the

investigations of Ir Mo (CO) (η -C H ) /TiO powder

intermetallic bonds of the original cluster. In this work, we

activate Ir Mo (CO) (η -C H ) on TiO in Ar, aﬀording the

heated at 150, 300, and 450 °C in an Ar atmosphere revealed

a gradual loss of the carbonyl ligands with increasing

temperature, the ligands being completely removed at 450

°C (Figure S3 ). The sample treated at 450 °C under an Ar

atmosphere is denoted as Ir Mo /TiO . Ir /TiO and Mo /

5 2

DSAC Ir Mo /TiO that features discrete Ir single atoms (Ir )

and Mo single atoms (Mo ) anchored on TiO , distinct from

the bimetallic structures obtained via H treatment in the

7−31

previous reports.

The dual single-atom structure was

TiO were synthesized by similar methods but instead using

conﬁrmed by aberration-corrected scanning transmission

electron microscopy (STEM), X-ray absorption ﬁne structure

Ir (CO) and Mo(CO) (η -C H ) as the organometallic

precursors (Figure S1 ), respectively. IR spectra of Ir (CO)

(

XAFS) spectroscopy, and diﬀuse reﬂectance infrared Fourier

before and after loading on TiO showed identical absorption

transform spectroscopy (DRIFTS). Ir Mo /TiO shows much

bands in the ν(CO) region, meaning that the structure of

greater catalytic activity and chemoselectivity in the hydro-

genation of 4-nitrostyrene (4-NS) to 4-vinylaniline (4-VA)

than the SACs Ir /TiO and Mo /TiO prepared with the

Ir (CO) is unchanged (Figure S4), while Mo(CO) (η -

C H ) decomposed after loading on TiO as suggested by the

disappearance of the ν(CO) absorption bands and a marked

reduction in the intensity of the ν(CH) and δ (CH) absorption

same method from homometallic precursors. Density func-

tional theory (DFT) calculations demonstrate that Ir sites are

bands corresponding to η -C H (Figure S5 ). IR spectra of

responsible for H activation and Mo sites are the active sites

both Ir /TiO and Mo /TiO indicated the complete removal

1 2 1 2

for the adsorption of 4-NS via the nitro group. The synergistic

eﬀects of the two sets of single atoms give rise to an enhanced

catalytic performance.

of the ligands (Figures S4 and S5 ).

We also performed temperature-programmed desorption-

mass spectrometry (TPD−MS) experiments for

Ir Mo (CO) (η -C H ) /TiO to ascertain the fate of the

5 2

RESULTS AND DISCUSSION

organic species during the heating process under Ar (Figure

^b⁾^.^C^O^aⁿ^d^C^O2 evolution phases are observed at

■

Synthesis and Characterization of the DSAC. Both the

DSACs and SACs were synthesized by adsorption and

pyrolysis of organometallic precursors (for details, see the

Experimental Section). The bimetallic carbonyl cluster

temperatures below 450 °C, which can be assigned to the

loss of carbonyl ligands and the reaction of carbonyl ligands

with oxygen atoms from the TiO

oxygen loss from TiO gives rise to the generation of oxygen

support, respectively. The

Ir Mo (CO) (η -C H ) (Figure S1, synthesized according

5 2

to the literature ) dissolved in dry n-pentane was mixed with

fresh high-temperature-pretreated TiO₂(rutile) powder,

followed by the removal of the solvent by evacuation, the

vacancies that are detected by X-ray photoelectron spectros-

copy (XPS) and electron paramagnetic resonance (EPR)

(Figure S6). The O 1s spectra can be deconvoluted into four

peaks, with the peak at 529.3 eV corresponding to the oxygen

resultant material being denoted as Ir Mo (CO) (η -C H ) /

5 2

TiO . Attenuated total reﬂectance (ATR) infrared spectrosco-

defects. The EPR signal with a g value of 2.003 is attributed

34,35

py (IR) results (Figure 1a) show that the organometallic

to the oxygen vacancy.

attributed to the decomposition and loss of η -C

Pentane adsorbed on the support is also likely to decompose

and is believed to also contribute to the evolution of CH , CO,

and CO . TPD−MS studies for TiO -supported Ir (CO) and

The CH evolution phases can be

precursor is degraded after being deposited on TiO , as

H . n-

5 5

evidenced by the changes in the ν(C−O) stretching region and

the disappearance of bands in the ν(C−H) stretching region

that are characteristic of the cyclopentadienyl ligands. The new

−

bands (ranging from 1240 to 1160 cm : the green square area

in Figure 1a) in the spectral region corresponding to C−O

single bonding are tentatively attributed to new organic

compounds resulting from the reaction of carbonyl ligands

TiO -supported Mo(CO) (η -C H ) are shown in Figure S7.

2 3 7 8

The IR and TPD−MS data conﬁrm the complete removal of

the ligands of the precursors following 450 °C treatment in an

Ar ﬂow. The metal loadings of Ir (0.14 wt %) and Mo (0.06 wt

%) in Ir Mo /TiO were determined by inductively coupled

with cyclopentadienyl fragments. The solvent n-pentane

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mapping of Ir Mo /TiO by energy-dispersive X-ray spectroscopy.

Research Article

Figure 2. STEM images of (a) Mo /TiO and (b) Ir Mo /TiO . The white circles highlight single Ir or Mo atoms, and the white squares contain

clusters. (c) LS−EDS of a selected cluster from Ir Mo /TiO (the inset shows the area at which the line scanning was performed). (d) Elemental

Figure 3. (a) Normalized XANES and (b) Fourier transforms of the EXAFS spectra at the Ir L -edge of Ir Mo /TiO , Ir /TiO , IrO , and Ir foil

(EXAFS intensity × 1/2).

plasma atomic emission spectroscopy (ICP−AES) analysis

Table S1): the Ir/Mo molar ratio is 1:1, consistent with the

shown in the STEM images of Mo /TiO (Figures 2a and S8),

1 2

the Mo is slightly brighter than the lattice Ti atoms, while

atomic ratio of the deposited precursor. The metal loadings in

Ir /TiO and Mo /TiO are close to those of the individual

much brighter and well-dispersed single atoms are clearly

observed in the STEM images of Ir Mo /TiO ; only a small

metals in Ir Mo /TiO (Table S1 ).

number of clusters with dimensions of ca. 1 nm were detected

(Figures 2b and S9 ). We note that the AC−HAADF−STEM

image does not provide suﬃcient contrast to distinguish Mo

from Ir atoms but it does conﬁrm the atomic dispersion of the

Ir and Mo atoms. Linear-scanned energy-dispersive X-ray

spectroscopy (LS-EDS) of a selected cluster shows the

coexistence of Ir and Mo elements (Figure 2c). The signals

To conﬁrm the local distribution of the Ir and Mo atoms in

these structures, aberration-corrected high-angle annular dark-

ﬁeld (AC−HAADF) STEM investigations were performed.

Because the atomic numbers of Ti and Mo are relatively close,

it is challenging to distinguish Mo single atoms from the TiO₂

surface, particularly when the Mo loading is very low. As

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Research Article

Figure 4. (a) XPS spectra of Mo 3d for Ir Mo /TiO and Ir /TiO ; (b) DRIFTS spectra of the CO adsorption on Ir Mo /TiO , Ir /TiO , and

Mo /TiO .

Table 1. Catalytic Data for Hydrogenation of 4-NS

selectivity (%)

entry

catalyst

temp. ( C)

time (min)

conv. (%)

0.8

Ir Mo /TiO

120

100

83.2

87.1

99.1

90.8

74.4

18.5

96.3

96.4

37.8

3.7

2.7

32.8

Ir Mo /TiO

Ir /TiO

29.3

Mo /TiO

Ir Mo /TiO

240

600

97.3

99.2

50.8

79.8

2.7

0.8

15.8

33.5

20.2

Ir Mo /TiO

IrMo/TiO -2

120

IrMo/TiO -3

Reaction conditions: H pressure (2 MPa), toluene (10 mL), entries 1−4 and 7−8: 0.13 mmol 4-NS, Ir catalyst 0.04 mol %; entries 5−6: 0.19

mmol 4-NS, Ir catalyst 0.04 mol %. Determined by GC−MS. The synthesis procedures are described in the Experimental Section.

are relatively weak due to the small size of the cluster and the

low metal loadings. Elemental mapping by energy-dispersive

spectroscopy manifested the homogeneous distribution of Mo₁

and Ir over the TiO surface (Figure 2d). STEM images of

coordination in the second shell was observed, suggesting that

the clusters in Ir Mo /TiO and Ir /TiO are composed of

loosely isolated Ir atoms, similar to the structure and function

of the pseudo-single-atom catalyst 0.08%Pt/FeO -R250

Ir /TiO (Figure S10) reveal that the majority species are Ir

reported by Zhang et al.

atoms, with a small percentage of clusters as in Ir Mo /TiO .

We note that Shapley and co-workers reported that

intermetallic bonds were retained after treatment of

The XAFS technique was used to investigate the electronic

and coordination environment of Ir. The normalized X-ray

absorption near-edge structure (XANES) proﬁles at the Ir-L₃

edge of Ir Mo /TiO and Ir /TiO are shown in Figure 3a.

Ir Mo (CO) (η -C H ) on Al O under a H atmosphere,

5 2

Gates et al. reported retention of the Ir cluster framework

following decarbonylation of Ir (CO) on Al O and MgO in

4,37

Both the absorption energy peaks are very close to that of IrO₂,

indicative of structural similarity. The energy of Ir Mo /TiO

H ,

and the Ir cluster can be generated from a zeolite-

38,39

supported Ir complex in H₂.

We infer that the diﬀering

is slightly lower than that of Ir /TiO , indicating a lower

activation procedures (in H or Ar) between these precedents

and our studies have resulted in diﬀerent structures. In the

oxidation state for Ir in Ir Mo /TiO . The coordination

environments of Ir in Ir Mo /TiO and Ir /TiO were

present studies, we propose that after loss of the protecting

determined by extended X-ray absorption ﬁne structure

EXAFS) spectroscopy. Fourier transform (FT) EXAFS

ligands, the carbonyl cluster Ir Mo (CO) (η -C H ) under-

5 2

(

goes decomposition, redistribution, and migration on the TiO₂

surface which is rich in O vacancies (Figure S6) or OH groups

(Figure S3) under an Ar atmosphere and then transforms to

atomically dispersed Ir and Mo species in a process driven by

thermodynamics. Noble metal nanoparticles similarly trans-

spectra at the Ir−L edge show the typical Ir−O coordination

environment in the ﬁrst shell (Figure 3b). The best-ﬁ t EXAFS

results and parameters are summarized in Figure S11 and

Table S2, respectively. The lack of measurable FT peaks

corresponding to Ir−Ir, Ir−Mo, and Ir−Ti shells is consistent

with an absence of metal−metal bonds in Ir Mo /TiO and

forming to single atoms under Ar or O atmospheres have been

,40−46

observed in previous reports.

We therefore propose that

Ir /TiO and thereby with the Ir structure that was observed

treatment of clusters or nanoparticles in a suitable gaseous

in STEM images. No evidence for Ir−O−Ir and Ir−O−Mo

atmosphere as in the present work could aﬀord single atoms.

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The oxidation states of Mo in Ir Mo /TiO and Mo /TiO

Research Article

First-Principles Calculations. DFT calculations were

carried out to elucidate the dual single-atom structure and

understand the superior catalytic behavior of Ir Mo /TiO

were investigated by XPS. The Mo 3d spectra were

deconvoluted into Mo⁵⁺and Mo components for both

Ir Mo /TiO and Mo /TiO , with the former possessing a

toward selective hydrogenation of 4-NS. As shown in Figure

S14 , model Ti(M)O catalysts were constructed by assuming

that one or two Ti cations from the rutile TiO (100) surface

were substituted by one Ir atom [Ti(Ir)O (100)] or one Mo

slightly higher percentage of Mo than the latter (Figure 4a).

Due to the low loading of Ir and the overlap of Ir 4f and Ti 3s

peaks, it was not possible to obtain eﬀective deconvoluted

peaks of Ir 4f₇_/₂and 4f₅_/₂(Figure S12).

atom [Ti(Mo)O (100)] or one Ir atom and one Mo atom

DRIFTS investigations of CO adsorption revealed two

[Ti(Ir, Mo)O (100)], and the substitution energies of the

−

bands at 2070/1999 cm for Ir Mo /TiO and at 2069/1997

resultant catalysts were then calculated (Table S4). The results

show that two heteroatoms Ir₁and Mo₁are more

thermodynamically preferable than two Mo atoms or two Ir

atoms because the substitution energy of the former (0.55 eV)

is lower than those of the latter (0.61 and 1.35 eV). This

means that the presence of two distinct single atoms on

−

cm for Ir /TiO (Figure 4b). The two vibrational bands can

be assigned to the symmetric and asymmetric stretching modes

of the two carbonyl ligands in the resultant Ir gem-dicarbonyl

(

Ir(CO) ) units. No CO bands were detected for Mo /TiO .

DRIFTS studies of atomically dispersed Ir on a variety of

supports have aﬀorded two bands assigned to similar gem-

TiO

atoms, and this provides additional support for the proposed

dual single-atom structure of Ir Mo /TiO . We emphasize here

that the role of the small cluster content in Ir Mo /TiO is

(100) is more favorable than the homometallic single

−

dicarbonyl moieties [e.g., MgO (2051/1967 cm ), γ-Al O

−

−1 47,48

(

2075/1996 cm ), and MgAl O (2070/1989 cm )],

allowing us to conﬁrm that the Ir atoms in Ir Mo /TiO and

Ir /TiO remain atomically dispersed. Additionally, no bands

ignored in this model because it does not contribute to the

enhanced catalytic performance, as we discuss below.

Table 2 lists the adsorption energies of 4-NS via the nitro

−

were observed between 1800−1900 cm , a region that

corresponds to CO adsorbed on large Ir nanoparticles,

consistent with the absence of Ir−Ir bonding in both catalysts.

Collectively, the AC−HAADF−STEM, XAFS, and DRIFTS

studies have conﬁrmed atomically dispersed Ir and/or Mo on

group, molecular H , and dissociated H , as well as the reaction

2 2

Table 2. Calculated Adsorption Energies (E ) of 4-NS via

ads

Ir Mo /TiO , Ir /TiO , and Mo /TiO .

the Nitro Group, Molecular H and Dissociated H , and

Catalytic Behavior of the DSAC. Selective hydrogenation

Reaction Barrier for H Dissociation From the Designated

Catalyst Site (Unit: eV)

of 4-NS was used as a probe reaction to evaluate these

catalysts. Due to the presence of two reducible groups (nitro

and CC) in a 4-NS molecule, it is challenging to

simultaneously achieve high catalytic chemoselectivity and

high conversion. In the present work, comparison reactions

ads

speciﬁc site of

designated catalyst

molecular dissociated barrier for H₂

ads

4-NS

dissociation

Ti of TiO (100)

−0.42

−0.99

−1.05

−0.04

−0.90

−0.10

0.58

−2.06

0.04

1.09

0.46

0.85

were carried out at T = 120 °C under a H pressure of 2 MPa,

Ir of Ti(Ir)O (100)

the results being listed in Table 1. When Mo /TiO was used

Mo of

Ti(Mo)O (100)

as the catalyst, no products were observed. Ir Mo /TiO

Ir of

−0.77

−1.65

0.42

displayed an excellent chemoselectivity of 96.3% for 4-VA at

00% conversion of 4-NS and within 1 h. In contrast, under

the same reaction conditions, Ir /TiO showed poor chemo-

Ti(Ir, Mo)O (100)

Mo of

Ti(Ir, Mo)O (100)

−1.13

selectivity (37.8% for 4-VA at 87.1% conversion of 4-NS); two

other products, 4-ethylnitrobenzene from selective hydro-

genation of the CC group and 4-ethylaniline from the

complete hydrogenation of both nitro and CC groups, were

also formed (29.3 and 32.8% selectivities, respectively).

Ir Mo /TiO retained its excellent chemoselectivity to 4-VA

barriers for H dissociation on the various model catalysts.

Figures S15 and S16 display the adsorption structures of 4-NS

and the dissociation process of H . The calculation results

demonstrate that the adsorption of 4-NS is via the nitro group

at the designated surfaces rather than via the vinyl group or the

arene ring because of the signiﬁcantly lower adsorption

energies via the nitro group (−0.42 ∼ −1.13 eV) compared

to those via the vinyl group or the arene (0 ∼ −0.2 eV). The

when the hydrogenation reactions were carried out at 80 and

00 °C. No intermediates such as hydroxylamine and azoxy

were detected after completion of the reactions.

A comparison with the previous reports on selective

hydrogenation of nitrostyrenes shows that the catalytic

performance of Ir Mo /TiO is very competitive (Table

doped Mo at TiO (100) has a stronger ability than Ir or Ti to

adsorb 4-NS via the nitro group, which means that the

adsorption of 4-NS via the nitro group would preferentially

9−53

S3 ).

Recycling stability tests of Ir Mo /TiO showed a

1 1 2

gradual loss of activity (Figure S13 ). ICP investigations of the

used catalyst show that the Ir and Mo metal loadings are

greatly reduced (Table S1). It is noteworthy that the excellent

selectivity to 4-VA remained even after the ninth run,

indicating that the two sets of well-dispersed single atoms

remain present. Hydrogenation of various substituted nitro-

arenes, including those with a range of functional groups

occur at the Mo atoms. The adsorption of molecular H and

dissociated H on Ir is signiﬁcantly stronger than at Mo or Ti,

and the barrier of H dissociation from Ir is much lower than at

Mo or Ti, indicating that Ir atoms would activate H rather

than Mo or Ti atoms. We therefore propose that during the

hydrogenation of 4-NS over Ir Mo /TiO , Mo adsorbs 4-NS

via the nitro group and Ir activates H .

(

−CO, −CC, −CN, −CH OH, and −X), was also

The more negative adsorption energy of dissociated H₂

(−2.06 eV) compared to that of 4-NS via the nitro group

(−0.99 eV) (Table 2) is consistent with a signiﬁcantly stronger

adsorption of H species rather than 4-NS at the Ir atoms. The

explored to study the broad-spectrum catalytic behavior of the

DSAC Ir Mo /TiO . In most cases, the selectivity for the

formation of functionalized anilines was excellent (>98% at a

nitroarene conversion of ca. 99%: Table S4).

Ir atoms of Ti(Ir)O (100) are indeed likely to be largely or

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pathway for key aspects of the catalytic hydrogenation of 4-NS,

Research Article

fully covered by H species, rendering it diﬃcult to adsorb 4-NS

via the nitro group. Consequently, both the nitro and the C

C groups of 4-NS can react with the H species on Ir via the

Eley−Rideal mechanism, which would likely result in no

chemoselectivity between the three possible hydrogenation

products. This is consistent with our experimental results,

which show that the catalytic hydrogenation of 4-NS over Ir₁/

We note that the majority species in the structure of Ir Mo /

TiO are well-dispersed Ir and Mo atoms that are the active

sites for the enhanced catalytic performance, but a small

percentage of clusters are also observed in the HAADF−

STEM images. To rule out the impact of the clusters in the 4-

NS hydrogenation reaction, we synthesized IrMo/TiO -2 via

treatment of a mixture of Ir (CO) and Mo(CO) (η -C H )

TiO displayed no signiﬁcant selectivity (4-VA 37.8%, 4-

on TiO at 450 °C under an Ar atmosphere. The majority

ethylnitrobenzene 29.3%, and 4-ethylaniline 32.8%).

species in the structure of IrMo/TiO -2 are clusters, only a

Figure 5 shows the calculated energy proﬁle and reaction

small number of single atoms being present, as is shown in the

HAADF-STEM images of IrMo/TiO -2 (Figure S18 ).

DRIFTS spectra of CO adsorption on IrMo/TiO -2 (Figure

−

S19 ) show a pair of bands at 2067/1998 cm which can be

assigned to atomically dispersed Ir atoms, similar to those in

−

Ir Mo /TiO and Ir /TiO . Two bands at 2051 and 2039 cm

are also observed, corresponding to Ir clusters, according to

previous reports. IrMo/TiO -2 shows a selectivity to 4-VA of

0.8% at a conversion of 74.4% under the same reaction

conditions as that of Ir Mo /TiO (Table 1), which is a

signiﬁcantly poorer performance than that of Ir Mo /TiO .

These results strongly suggest that the two sets of highly

dispersed single atoms in Ir Mo /TiO are the active sites for

superior catalytic performance, while the large number of

clusters in IrMo/TiO -2 do not favor the desired high

selectivity for 4-VA.

Figure 5. Energy proﬁles (unit: eV) for catalytic deoxygenation of 4-

We emphasize that because of the limitations of the

characterization techniques employed, no unambiguous

evidence is available for the existence of bonding (e.g., Ir−

NS to 4-nitrosostyrene on Ir Mo /TiO . The blue, green, gold, red,

gray, deep blue, and brown balls are Ti, Mo, Ir, O, H, N, and C atoms,

respectively. NS represents 4-NS and NSS represents 4-nitro-

sostyrene.

O−Mo) between the randomly distributed Ir and Mo , but

the presence of such interactions can be deduced from the

XANES and XPS results: XANES displays a slightly lower

exploiting the well-dispersed Ir and Mo atoms on TiO . It is

oxidation state for Ir in Ir Mo /TiO than in Ir /TiO (Figure

widely accepted that the critical step in hydrogenation of

nitrobenzene is deoxygenation of nitrobenzene to nitro-

sobenzene, which then undergoes multistep reduction to

a), and the XPS Mo 3d spectra show a higher oxidation state

for Mo in Ir Mo /TiO than in Mo /TiO (Figure 4a). We

conclude that weak interactions exist between the Ir atoms and

Mo atoms in Ir Mo /TiO , but the precise nature of their

aniline. 4-Nitrosostyrene is hence considered as the ﬁnal

state of this crucial part of the reaction pathway. Adsorption

and dissociation of H on Ir decrease the total energy of the

interaction is uncertain and needs further investigation.

In addition, we also activated Ir Mo (CO) (η -C H ) on

system to −1.65 eV. The H₂molecule then undergoes

heterolytic cleavage, resulting in the formation of Ir−H and

5 2

TiO in hydrogen at 450 °C, the resulting product being

denoted as IrMo/TiO -3. The high-resolution transmission

H−O−Ti bonds. Adsorption of the 4-NS molecule on Mo

electron microscopy (HRTEM) images of IrMo/TiO -3

reduces the total energy further to −1.86 eV. The oxygen atom

removed from 4-NS leads to the formation of an Ir−O−H

bond, which overcomes a signiﬁcant energy barrier of 0.80 eV

display the presence of nanoclusters (Figure S20), likely

resulting from the agglomeration of the original clusters. In the

selective hydrogenation of 4-NS, IrMo/TiO -3 shows a poor

(

and is, as a result, the rate-determining step of the reaction;

selectivity to 4-VA (79.8% at a conversion of 18.5%) under the

note, though, that the energy barrier here is much lower than

that of nitrobenzene to nitrosobenzene on Pd(111) at 1.23

eV). The ﬁnal step is the formation of water on Ir , which is

same reaction conditions as Ir Mo /TiO (Table 1). We infer

that activation of the catalyst in H results in the retention of

intermetallic bonds as in Shapley’s work, which is not

followed by facile desorption. The calculated energy of the

reaction is −3.15 eV, indicating that the pathway is

thermodynamically feasible. This suggested reaction pathway

emphasizes the diﬀerent roles of Mo and Ir in the catalytic

favorable for the targeted high selectivity for hydrogenation of

4-NS.

CONCLUSIONS

reaction.

■

Discussion. The combination of experimental and

theoretical results presented above has provided a clear picture

of the atomic-level synergism operative in the hydrogenation of

We have presented a methodology for the synthesis of DSACs

via the activation of supported bimetallic carbonyl clusters in

Ar. The well-dispersed dual single-atom structure of Ir Mo /

-NS over Ir Mo /TiO . Atomically dispersed Ir and Mo play

TiO has been shown to synergistically eﬀect the chemo-

diﬀerent roles in the catalytic reaction, Ir activating H and

Mo₁adsorbing 4-NS via the nitro group. The reaction

mechanism over Ir Mo /TiO in the selective hydrogenation

of 4-NS is clearly diﬀerent from the proposed Eley−Rideal

mechanism over Ir /TiO , further conﬁrmed by the wide gap

in calculated activation energies (the former: 43.4 ± 6.7 kJ/

mol vs the latter: 91.3 ± 6.8 kJ/mol) (Figure S17).

selective hydrogenation of 4-NS. The highly dispersed discrete

single atoms Ir and Mo anchored on a TiO support have

been identiﬁed by AC−HAADF−STEM, XAFS, and DRIFTS.

DSAC Ir Mo /TiO displays a superior catalytic performance

for selective hydrogenation of 4-NS to 4-VA than the single-

atom catalyst Ir /TiO . Computational results suggest that H

activation occurs on Ir and 4-NS adsorption via the nitro

957

ACS Catal. 2021, 11, 1952−1961

ACS Catalysis

https://pubs.acs.org/doi/10.1021/acscatal.0c05599.

Research Article

group preferentially occurs on Mo , with the synergistic eﬀect

gas was introduced into the reaction cell until the CO

of Ir and Mo leading to enhanced catalytic performance. Our

adsorption peak intensity no longer increased. N gas was then

work elucidates the atomic-level advantages of DSAC in

promoting reaction mechanisms for eﬃcient heterogeneous

bimetallic catalysis.

introduced to the reaction cell to remove CO from the surface

of the catalyst, and this was continued until the CO adsorption

peak intensity stabilized. At this time, the CO adsorption IR

spectra were recorded. TPD−MS was performed on an

AutoChem II chemisorption analyzer. Samples were treated

in an Ar environment under programmed temperatures: a

sample was typically pretreated at 100 °C for 1 h to remove

EXPERIMENTAL SECTION

■

Sample Preparation. Sample synthesis and handling were

performed with the exclusion of moisture and air in an argon-

ﬁlled glovebox. The TiO support (rutile, 25 nm, Aladdin) was

surface H O and solvent, and then the temperature was

treated at 600 °C for 2 h under argon before use.

ramped up to 550 °C at a rate of 5 °C/min.

X-ray absorption ﬁne structure (XAFS) spectra at the Ir L₃

Ir Mo (CO) (η -C H ) (1.7 mg) was dissolved in dry n-

5 2

pentane (50 mL), resulting in the appearance of an orange

color. Freshly prepared TiO₂(1.0 g) was added to the

abovementioned solution (the theoretical loading of Ir is about

(E = 11215 eV) edge were obtained at the BL14W1 beamline

of the Shanghai Synchrotron Radiation Facility (SSRF),

operating at 3.5 GeV with a “top-up” mode and a constant

current of 240 mA. The XAFS data were recorded in the

ﬂuorescence mode with a seven-element Ge solid-state

detector. The energy was calibrated to the absorption edge

of pure Ir powder. Athena and Artemis codes were used to

extract the data and ﬁt the proﬁles. For XANES, the

experimental absorption coeﬃcients as a function of energies

μ(E) were processed by background subtraction and normal-

ization procedures and reported as “normalized absorption”.

For EXAFS, the Fourier-transformed data in R space were

.1 wt %), followed by vigorous stirring overnight until the

orange color disappeared. The solvent was then removed by

evacuation using Schlenk techniques. The resultant solid was

heated under argon at 450 °C to aﬀord Ir Mo /TiO .

Similarly, TiO -supported Ir (CO) (Strem Chemicals) and

Mo(CO) (η -C H ) (Alfa Aesar) were each heated at 450 °C

under an argon atmosphere to give Ir /TiO and Mo /TiO ;

both have a theoretical metal loading of 0.1 wt %. In addition,

IrMo/TiO -2 was synthesized by treatment of a mixture of

Ir (CO) and Mo(CO) (η -C H ) on TiO at 450 °C under

an argon atmosphere. IrMo/TiO -3 was obtained via the

activation of Ir Mo (CO) (η -C H ) on TiO at 450 °C

analyzed by applying a ﬁrst-shell approximate model for the

Ir−O contribution. The passive electron factor, S , was

5 2

determined by ﬁtting the experimental data of the Ir powder

and ﬁxing the coordination number (CN) of Ir−Ir for further

analysis of the measured samples. The parameters describing

the electronic properties (e.g., correction to the photoelectron

under hydrogen. The theoretical metal loadings of Ir and Mo

are 0.1 wt % in IrMo/TiO -2 and IrMo/TiO -3.

Catalytic Reactions. Hydrogenation of 4-NS was carried

out in a stainless-steel autoclave (25 mL) equipped with a

pressure gauge and magnetic stirrer (Beijing Shiji Senlang

Experimental Instrument Co Ltd). Before the reaction, a

mixture of 4-NS (0.13 mmol), toluene (10 mL), and catalyst

was placed in a quartz vessel. After being sealed, the autoclave

was ﬂushed with hydrogen at least 5 times and then the

pressure increased to 2 MPa. The reaction time commenced

after reaching the set temperature. After the reaction was

complete, the autoclave was cooled to room temperature and

the remaining hydrogen gas was discharged. The product was

then condensed and analyzed by gas chromatography (7890B)

mass spectrometry (5977A) (GCMS, Agilent).

energy origin, E ) and local structure environment including

CN, bond distance (R), and Debye−Waller factor (σ ) around

the absorbing atoms were allowed to vary during the ﬁt

process. The ﬁtted range for k space was selected to be k = 3−

−

0 Å (k weighted).

Computational Details. Periodic DFT calculations were

performed using the Vienna Ab initio Simulation Package

VASP). The exchange−correlation energy and potential were

(

described by the generalized gradient approximation in the

form of PBE. Plane waves with a cutoﬀ of 400 eV were used for

projector augmented wave (PAW) potentials. The criteria for

−

the convergence were residual force less than 0.02 eV Å . The

thickness of the vacuum layer was 20 Å. The optimized lattice

Material Characterization. High-angle annular dark-ﬁeld

(

HAADF)−STEM images for Ir /TiO and Ir Mo /TiO were

obtained on a JEOL JEM ARM300F instrument with a

resolution of 0.063 nm. Samples were dispersed by ultra-

sonication in ethanol and then dropped on to Cu mesh with

carbon microgrids. XPS studies were conducted on a Thermo

Fisher ESCALAB 250Xi XPS spectrometer with a mono-

chromatic Al Kα (1486.6 eV) X-ray source. The binding

energies of all samples were calibrated by taking the carbon 1s

peak as a reference (284.6 eV). The concentrations of iridium

and molybdenum were determined by inductively coupled

plasma atomic emission spectroscopy (ICP−AES) on a

Shimadzu ICPS-8100. Prior to ICP−AES measurement, all

samples were dissolved in aqua regia. Attenuated total

reﬂectance infrared spectroscopy (ATR−IR) spectra were

collected on a Bruker VERTEX 70 FT-IR spectrometer scaled

constants of bulk rutile TiO were a = 4.660 Å and c = 2.974 Å,

in good agreement with the experimental values (a = 4.594 Å

and c = 2.958 Å). The eﬀect of spin polarization was

considered. The k points mesh was set by Monkhorst−Pack

methods as 8 × 4 × 1 for the (3 × 1)−Ti(M)O (100) surface

with a four-trilayer slab. The upper two trilayers were relaxed,

while the lower two trilayers were ﬁxed at their bulk positions.

ASSOCIATED CONTENT

■

sı Supporting Information

The Supporting Information is available free of charge at

_a_t₄₀₀₀_t_o₆₄₀_c_m₋1 with a resolution of 4 cm−1. DRIFTS

spectra to assess CO adsorption were recorded on a Bruker

IR spectra, TPD−MS, AC−STEM images, XPS, DFT

calculations, ICP, EXAFS ﬁtting results, recycling

stability studies, calculated apparent activation energies,

and hydrogenation performance of nitroarenes sub-

stituted with various groups (PDF)

VERTEX 70 FT-IR spectrometer. The catalyst was ﬁrst

−

pretreated at room temperature with N (30 mL min ) for 0.5

h, followed by a collection of the background spectrum. CO

958

ACS Catal. 2021, 11, 1952−1961

ACS Catalysis

Shanghai 201204, China; orcid.org/0000-0003-0732-

Research Article

AUTHOR INFORMATION

Corresponding Authors

Rui Si − Shanghai Synchrotron Radiation Facility, Shanghai

Institute of Applied Physics, Chinese Academy of Sciences,

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