Beating Heterogeneity of Single-Site Catalysts: MgO-Supported

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Beating Heterogeneity of Single-site Catalysts: MgO-supported Iridium Complexes Adam Scott Hoffman, Louise Debefve, Shengjie Zhang, Jorge Ernesto Perez-Aguilar, Edward T Conley, Kimberly R Justl, Ilke Arslan, David A Dixon, and Bruce C. Gates ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00143 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 12, 2018

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Beating Heterogeneity of Single-site Catalysts: MgO-supported Iridium Complexes Adam S. Hoffman,1,2† Louise M. Debefve,1† Shengjie Zhang,3 Jorge E. Perez-Aguilar,1 Edward T. Conley,1,4 Kimberly R. Justl,1,4 Ilke Arslan,5,6 David A. Dixon,3 and Bruce C. Gates1* 1

Department of Chemical Engineering, University of California—Davis, Davis, California, 95616, USA Present address: Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California, 94025, USA 3 Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama, 35487, USA 4 Department of Materials Science and Engineering, University of California—Davis, Davis, California, 95616, USA 5 Fundamental and Computational Sciences Directorate, Institute for Integrated Catalysis and Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, P O Box 999, Richland, Washington, 99352, USA 2

6 †

Present address: Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL, 60439, USA

These authors contributed equally to this work.

ABSTRACT: Catalysts consisting of isolated metal atoms on oxide supports have attracted wide attention because they offer unique catalytic properties, but their structures remain largely unknown because the metals are bonded at various, heterogeneous surface sites. Now, by using highly crystalline MgO as a support for metal sites made from a mononuclear organoiridium precursor and investigating the surface species with X-ray spectroscopy, atomic resolution electron microscopy, and electronic structure theory, we have differentiated among the MgO surface sites for iridium bonding. The results demonstrate the contrasting structures and catalytic properties of samples, even including those incorporating iridium at loadings as low as 0.01 wt% and showing that they are nearly ideal in the sense of having almost all the Ir atoms at equivalent surface sites, with each Ir atom bonded to three oxygen atoms of the MgO surface. These supported molecular catalysts are represented accurately with density functional theory. The results open the door to the precise synthesis of families of single-site catalysts.

KEYWORDS: single-site catalyst, iridium catalyst; MgO-supported catalyst; surface heterogeneity; metal–support interaction; electron microscopy; EXAFS spectroscopy, IR spectroscopy

INTRODUCTION

1 ACS Paragon Plus Environment

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Atomically dispersed oxide-supported metal catalysts are drawing wide attention because they offer catalytic properties that distinguish them from conventional supported metal catalysts as well as the advantage of maximally efficient use of expensive metals.1-9 Because they consist of isolated metal atoms on supports, these catalysts are referred to as single-site metal catalysts, and, alternatively, site-isolated metal catalysts and atomically dispersed metal catalysts (although these terms are sometimes used when the metal atoms are not fully isolated from each other). Catalysts with singly isolated metal atoms on supports are sometimes referred to as single-metalatom catalysts, but this term is limited, and we eschew it, because it ignores the groups (ligands) bonded to the metal, and these include the support. An alternative is “supported mononuclear metal complex catalysts,” but some authors avoid this term because it implies the presence of ligands which (in addition to the support) may not always be present. Notwithstanding the inconsistencies in terminology, almost all such catalysts are structurally nonuniform,7 consisting of metals on supports with intrinsically heterogeneous surfaces typified by metal oxides (although single-site metal catalysts supported on crystalline aluminosilicates (zeolites) meet a higher standard of uniformity7). As a consequence of the structural nonuniformity, the catalytic species are less than well understood and the opportunities to tune them limited. Our goals were to investigate single-site catalysts consisting of metal complexes on various surface sites of a metal oxide support and to determine how the catalytic activity depends on the structure of the surface species and their bonding to the support—thus, we sought to make progress toward dialling in nearly uniform catalytic sites. We report a family of site-isolated iridium catalysts made from the reactive precursor Ir(C2H4)2(acac)10 (acac = acetylacetonato) and high-surface-area powder MgO11 as a support. The catalysts were characterized with infrared (IR), X-ray absorption near edge structure


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(XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies, aberrationcorrected scanning transmission electron microscopy (STEM), and density functional theory (DFT). Their catalytic performance was evaluated for ethylene hydrogenation, a reaction chosen because it involves small, spectroscopically identifiable ligands on the metal.12 The data demonstrate a discrimination among various support surface sites for binding iridium, providing evidence of their structures and understanding of how to synthesize nearly uniform sites in the limit of low iridium loadings. MgO was chosen as the support because it is well suited to characterization by microscopy,13 spectroscopy,14 and theory.15,16 Moreover, MgO offers the complexity of a surface with multiple bonding sites17 and is representative of high-area industrial catalyst supports. We prepared MgO samples with high degrees of crystallinity to facilitate understanding of these sites. Iridium was chosen as the catalytic metal because, in various forms, it offers catalytic activity for numerous technologically important reactions,18 and its high atomic number allows STEM imaging of individual Ir atoms on the MgO support.19 RESULTS AND DISCUSSION Structures of supported organoiridium species formed initially. Ir(C2H4)2(acac) reacted with MgO, as expected,12 as confirmed by Ir LIII edge jumps in the X-ray absorption spectra. IR spectra of the supported species show that chemisorption of Ir(C2H4)2(acac) was accompanied by the removal of acac ligands from the iridium, indicated by bands (Figure S1A in the Supporting Information, SI) nearly matching those of Hacac20 and Mg(acac)2.21 STEM images of the initially synthesized samples (Figure 1A-D) show isolated Ir atoms on the MgO (iridium clusters subsequently formed under the influence of the electron beam, as expected22,23,24), confirmed by the lack of IR bands for di-σ-bonded ethylene (which is diagnostic of neighboring Ir centers24)


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and no EXAFS evidence of Ir–Ir contributions. A signature of π-bonded ethylene was evident in the high-energy-resolution XANES spectra25 of the samples containing sufficient iridium (≥0.1 wt% Ir) (Figure 2), showing that the supported species retained these ligands. EXAFS data (Table 1) correspondingly indicate an Ir–C coordination number of nearly 4 at a distance that corresponds to two π-bonded ethylene ligands per Ir. The results are confirmed by the IR spectrum of the sample containing 1.0 wt% Ir, with the C–H stretch in π-bonded ethylene at 3032 cm-1 (Figure S1B). Bonding of Ir complexes to MgO. The EXAFS spectra provide evidence of the Ir–support interactions in all the samples (Table 1), indicating Ir–Osupport bonding distances of 1.90–1.95 Å, comparable to those in solid IrO2 (1.98 Å),26 but less than that in crystalline Ir(C2H4)2(acac) (2.04 Å).10 The average Ir–Osupport coordination number of the supported species was 3.0 ± 0.1 when the Ir loading was 0.01 wt% but only 2.4 ± 0.5 when it was 1.0 wt% (Table 1), indicating that the nature of the bonding sites changed with loading.

Figure 1. STEM Images characterizing the MgO-supported Ir(C2H4)2 with various Ir loadings and the species formed by treatment in flowing H2 at 1 bar and 573 K. The initial Ir(C2H4)2/MgO samples contained the following loadings of Ir (wt%): A,


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1.0; B, 0.1; C, 0.05; D, 0.01. Samples after treatment in H2 contained Ir in the following loadings (wt%): E, 1.0; F, 0.1; G, 0.05; H, 0.01.

To probe the Ir centers, we exposed them to CO flowing at 298 K and 1 bar, which rapidly replaced all the ethylene ligands to give anchored iridium gem-dicarbonyls, identified by (a) high-energy resolution XANES (Figure 2) and (b) two IR bands (Figures 3A and B) of the symmetric and antisymmetric C–O vibrations.10 The C–O vibrations were detected for samples containing ≥0.1 wt% Ir, and the frequencies were shifted from 2064 and 1984 cm-1 to 2059 and 1983 cm-1, respectively, as the loading was reduced from 1.0 to 0.1 wt% (Figures 3B and D). The dependence of the νCO values (Table 1) on the Ir loading confirms the variation in the bonding of the Ir center to the support. Because the values characterizing the sample containing 1.0 wt% Ir are slightly blue-shifted relative to those with lower loadings, we infer that the MgO bonding sites in the former samples had higher average electron-donor tendencies. Because of the simplicity of the structure of the supported species in the samples with the lowest loadings, we focused attention on them, testing the hypothesis that they consist of nearly ideal surface sites. For STEM characterization, ≥30 particles of each sample were imaged for samples containing ≤ 0.1 wt% Ir (Table S1 in the SI). Cubic crystals of MgO decorated with Ir were observed, tilted in various orientations. To minimize electron beam effects, the cubes were imaged without tilting of the holder, causing most to be off axis. The numbers of particles imaged and on-axis orientations are indicated in Table S1 in the SI. Only one image showed an entire (110) surface, and one showed (110) as a defect area adjacent to a particle in a different orientation (Figure S2B in the SI). The locations of Ir atoms on various MgO faces are reported in Table 2. When the Ir loading was high, 1.0 wt%, the images show the Ir was distributed almost uniformly over the MgO surface.


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However, when the loading was lowered to 0.1, 0.05, and then 0.01 wt%, a trend emerged with respect to the bonding of Ir on the MgO surface (Figure 4) (details of Ir on defect sites provided in the SI). On the predominant (100) face, the fraction of Ir atoms at the edges and defects increased with decreasing Ir loading. At the lowest loading, 60% of the Ir species were located at edges and defect sites, and 72% were on or within 1 nm of those sites (Figure 4D–F). On the (111) face at the low Ir loadings, Ir was found only on the terraces, and, as the loading increased, Ir increasingly populated the edges and corners. These observations support the postulate that Ir is located preferentially at sites where it bonds to 3 support O atoms (being present as tripodal species). There is no evidence of Ir atoms located on corners in these samples. Large areas were devoid of Ir with some particles containing only one to several Ir atoms. Table 1. Structural Models based on EXAFS Spectra Characterizing MgO-supported Samples Initially Present as Ir(C2H4)2 Complexes, Before and After Treatment in flowing H2 at 573 K at 1 bar, and Initial Catalytic Activities for Ethylene Hydrogenation (measured as TOF) of the Samples as Initially Formed.

Sample

Ir loading (wt%)

A

1.0

B

1.0

C

0.1

XAS Experimental Conditions

Flowing He, 50 mL/min T = 298 K; P = 1.01 bar Flowing He, 50 mL/min T = 298 K; P = 1.01 bar After treatment of Sample A in H2 at 573 K Flowing He, 50 mL/min T = 298 K; P = 1.01 bar

D

0.1

Flowing He, 50 mL/min T = 298 K; P = 1.01 bar After treatment of Sample C in H2 at 573 K

E

0.05

Flowing He, 50 mL/min T = 298 K; P = 1.01 bar

0.05

Flowing He, 50 mL/min T = 298 K; P = 1.01 bar After treatment of Sample E in H2 at 573 K

F

Shell

N

R (Å)

103 x σ2 (Å2)

∆E0 (eV)

102 x TOF (s-1) [at Steady State]

Ir–O Ir–C Ir–Mg Ir–OL

2.4 4.0 1.1 2.8

1.94 2.07 3.10 3.62

2.4 1.7 4.6 1.1

-12 0.7 -13 2.7

3.2 ± 0.46 [0.95]

2064, 1984

Ir–Ir

>>1

-

-

-

−

−

Ir–O Ir–C Ir–Mg Ir–MgL Ir–O Ir–C

2.9 4.1 4.6 1.5 1.3 2.6

1.90 2.02 3.02 3.15 1.85 2.03

3.5 1.2 7.3 0.7 7.5 3.1

-7.9 5.7 -7.7 -0.1 4.1 -6.8

0.96 ± 0.57b [0.12]

2059, 1983

Ir–Mg Ir–Ir Ir–O Ir–C Ir–Mg Ir–MgL Ir–O Ir–C Ir–Mg Ir–MgL

2.3 2.4 3.0 4.0 1.0 4.2 2.8 3.7 2.1 3.1

3.01 2.61 1.95 2.03 2.94 3.14 2.05 1.97 2.88 3.05

7.5 9.5 7.1 3.5 5.4 13.5 4.3 8.7 1.4 4.0

-7.1 -7.0 -12 3.8 8.0 -15 3.1 -13 -10 -5.1

−

−

0.25 [0.23]

−c

−

−

νCO sym., asym. (cm-1)


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G

0.01

H

0.01

Flowing He, 50 mL/min T = 298 K; P = 1.01 bar Flowing He, 50 mL/min T = 298 K; P = 1.01 bar After treatment of Sample G in H2 at 573 K

Ir–OL Ir–O Ir–C Ir–Mg Ir–O

2.1 3.0 3.9 2.6 3.0

3.58 1.93 2.02 3.03 2.01

2.3 4.3 2.1 5.5 2.3

5.11 -9.2 12 -4.9 -19

Ir–C

4.1

2.10

3.8

1.6

Ir–Mg

3.0

3.13

10.2

-15

−d

−c

−

−

a Abbreviations: N, coordination number; R, absorber–backscatterer distance; σ2, disorder term (Debye-Waller factor); ∆E0, inner potential correction. bThe relatively large errors in these data are attributed to the challenge of carrying out catalyst syntheses with high accuracy of the Ir loading when the loading is so low; this is the major source of the error in these catalytic reaction experiments, which were performed multiple times to determine the stated error. cNot detectable because of sensitivity limitations of method. The EXAFS parameter values indicate mixtures of Ir species in the samples with all but the lowest loadings. dTOF was not measured for this sample because of experimental limitations—too much catalyst would have been required.

Figure 2. Dynamic high-energy-resolution XANES spectra characterizing the conversion of MgO-supported Ir(C2H4)2 with loadings of (top) 0.1 and (bottom) 1.0 wt% Ir to new species at room temperature in flowing CO. Arrows indicate resonance feature at 11225 eV corresponding to the presence of π-bonded ethylene. Circles indicate isosbestic points demonstrating the stoichiometric conversion of one species to another.

Consistent with these data, the high-energy-resolution XANES data characterizing the Ir(C2H4)2to-Ir(CO)2 transformation on these sites are characterized by isosbestic points (Figure 2, top) indicating that this conversion is stoichiometric. The lack of such points in the spectra of the


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sample containing 1.0 wt% Ir (Figure 2, bottom) confirms the presence of a mixture with more than one kind of site. Structural models of the supported surface species (Figure 5) were inferred from the EXAFS data (Table 1). The Ir–O coordination number of 3.0 ± 0.1 characterizing the sample containing ≤ 0.1 wt% Ir shows that each Ir atom, on average, was bonded to 3 oxygen atoms of MgO. Samples with a loading of 1.0 wt% are characterized by an Ir–O coordination number of 2.4 ± 0.5, matching expected values20 and confirming a mixture of Ir species bonded to 2 or 3 surface O atoms. Significantly, the value of 3.0 ± 0.1 for the former samples corresponds to a limiting case, with threefold sites donating more electrons to the Ir atom than twofold sites, consistent with the IR spectra.


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Figure 3. IR spectra in the C–H stretching and C–O stretching regions characterizing the room-temperature conversion of MgOsupported Ir(C2H4)2 with Ir loadings of (A, B) 1.0 and (C, D) 0.1 wt% to new species in the presence of flowing CO. Arrows indicate directions of change in spectra with time; π-bonded ethylene was undetectable in the IR spectra of the sample containing 0.1 wt% Ir.

DFT calculations characterizing the supported species. To provide further insights into the properties of the surface species, we used electronic structure calculations at the DFT level to probe the bonding of Ir to various models of the MgO surfaces. Terraces observed on MgO crystals treated at 1273 K include (100), (110), and (111) facets. Multi-atom steps were observed by microscopy on single-crystal MgO,27 but STEM is not sensitive enough to identify them in powder MgO. Edge sites on MgO crystallites are challenging to discern experimentally; common defects such as vacancies, kinks, or etched surfaces, notably the (110) and (111) facets,28 often occur at or near the edges of MgO crystals. Table 2. Numbers of Ir Atoms on each Type of Site observed by STEM for Samples with various Ir Loadings. MgO surface location

Number of Ir atoms observed for sample containing 0.1 wt% Ir



(100) face terrace (100) face edge

3 1 + 4 within 1 nm 0 0 13 6 0 2 7 2 0 4

4 3

7 9 + 6 within 1 nm 0 3 1 0 0 0 0 0 0 1

(100) face corner (100) face defect (111) face terrace (111) face edge (111) face corner (111) face defect (110) face terrace (110) face edge (110) face corner (110) face defect

0 1 4 0 0 0 -

We used various cluster models,29 based on the observed faces and known defects, Figure 5, to probe the binding of Ir in various charge states to various surface sites. The complete computational results are available in the SI, with the model descriptions. The results show that


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Ir can bond to the (100) surface in various ways. For example, it can bridge 2 surface oxygen atoms. For L2Ir–O2–{MgO}, the Ir–O bond distances are 2.09 and 2.13 Å for L = CO and C2H4, respectively. In (CO)2Ir–O2–{MgO}, the Ir–O bond distance in the structure with Ir bonded on top of a single oxygen atom is 1.92 Å for Ir(I) and 2.11 Å for Ir(0) (see SI). For (C2H4)2Ir, the Ir(I) converted to the di-oxo bridge structure on optimization, and the Ir(0)–O bond distance is 2.23 Å. For the (100) corner site, the (CO)2Ir–O3–{MgO} binds to 3 oxygen atoms with approximately equal Ir–O bond distances of 2.00 Å for Ir(III). The Ir–O bond distances increase with decreasing Ir charge, with one Ir–O distance becoming significantly longer for Ir(II); (CO)2Ir(I) binds only to 2 oxygen atoms. For (C2H4)2Ir, similar results are predicted. If the L2Ir complex binds to a surface defect site on the (100) surface, there are only two short Ir–O distances for Ir(III), near 2.10 Å. The Ir–O distance increases to ~2.2 Å for Ir(II), whereas for L2Ir(I) there is only one, short, Ir–O distance, near 2.3 Å. The binding of L2Ir for either ligand to the (110) surface occurs at 2 oxygen atoms with Ir–O distances of 2.1–2.2 Å. On the (111) surface, (CO)2Ir binds to three sites with distances of ~1.98 Å. In contrast, (C2H4)2Ir binds to sites through 2 Ir–O bonds with a somewhat longer bond to the third O site; the two shorter Ir–O bonds have distances of about 2.05 Å. In summary, the calculations show that the typical number of surface sites to which Ir bonds involves 2 Ir–O bonds, and bonding to sites via 3 Ir–O bonds likely occurs at corner sites of the (100) surface or to sites on the (111) surface. At the lowest iridium loadings, the results of these calculations are in good agreement with the locations of Ir atoms shown in the STEM images and with the best-fit EXAFS models. The calculated C–O and C–H stretches were scaled on the basis of a comparison with the isolated molecule to experiment by 0.9637 for C–O and 0.9446 for C–H.30 The symmetric


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combination of the C–O stretch is higher than the asymmetric combination. The experimental C– O stretching frequencies are matched best by the 3-fold sites on the (111) surface and by Ir(0) on the (100) 2O bridge site. Other sites with computed stretches that could possibly be consistent with the experimental vibrational results are the (100) corner sites for Ir(II) or Ir(I) and Ir(I) on (111) sites. Taken together, the results provide strong support confirming the importance of the trioxygen corner (100) and (111) sites.

Figure 4. STEM images of Ir atoms in Ir(C2H4)2/MgO with varying Ir loadings (in wt%): (A,D), 1.0; (B,E), 0.05; (C,F), 0.01, shown on various MgO faces: (A-C) unidentified and (D-F) (100).

Catalytic activities of various supported iridium species. The catalytic activities of the samples were probed with ethylene hydrogenation in an isothermal once-through plug-flow reactor operated with differential conversions. All the initially synthesized catalysts were active (Table 11 ACS Paragon Plus Environment

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1), with no side reactions. There were no induction periods, and each catalyst lost activity with time on stream, corresponding to changes in the Ir ligand sphere in the reactant stream (Figure 6). The initial TOF decreased with decreasing Ir loading (Table 1); and, after attainment of apparent steady state, each catalyst remained active (Table 1, Figure 6). Comparison of activities and stabilities of catalysts with various Ir loadings. The variations in TOF and the deactivation profiles that vary from sample to sample support the hypothesis that the bonding sites for Ir influence the catalytic activity. Nonetheless, we do not exclude the possibility that minority species—Ir bonded to 2 support oxygen atoms—were present in the samples with the relatively low Ir loadings (which predominantly incorporate Ir bonded to 3 support oxygen atoms), but that they were not evident because of the detection limits of our techniques. These suggested minority species could have been responsible for some of the catalytic activity of the samples with low Ir loadings. However, we emphasize the plausibility of the hypothesis that the Ir complexes that bond to the support via 3 Ir–O bonds are themselves active, but less active than those anchored by 2 Ir–O bonds, because they offer fewer sites for bonding of the reactants. As support for the hypothesis that the catalysts with the lowest iridium loadings were substantially distinct from those with higher loadings and not just samples with significant impurity species, we point to the differences in performance of the various catalysts in the flow reactor over hundreds of minutes of operation (Figure 6). The catalyst with the lowest Ir loading, 0.05 wt%, is evidently more stable than the ones with higher loadings in terms of fractional activity loss as a function of time on stream. (The catalysis experiment with this sample was carried out near the detection limits of the apparatus, with the scatter in the data attributed to operation of a mass flow controller in the feed stream operating near its lower flowrate limit and the conversions being more than negligibly affected by the presence of impurity


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ethane in the ethylene feed.) Our EXAFS data (Table 1) indicate that the catalytic species in the sample containing 0.1 wt% Ir were almost the same as those in the sample containing 0.05 wt% Ir, with the Ir–O coordination numbers being 2.9 and 3.0, respectively

(which are

indistinguishable from each other within error). The data showing (a) that the initial activity of the former was more than twice that of the latter and (b) that the former deactivated to give an activity close to that of the latter suggest that the former might have incorporated some of the more active di-coordinated iridium, whereas the sample containing the less active tri-coordinated sample might have contained a negligible amount of the these species. Sinter resistance of supported iridium species. Corresponding to the data showing that the Ir complexes with a higher degree of coordination to the support were more stable than the others as catalysts (in terms of fractional activity loss with respect to time on stream, Figure 6), the same pattern of stability was observed when Ir(C2H4)2/MgO samples were exposed to flowing H2 at the high temperature of 573 K for up to 3 h to replicate the harsh conditions that might be comparable to those of industrial catalytic processes—conditions under which typical supported single-site transition metal catalysts undergo reductive metal aggregation to form clusters— 23,31,32

the tripodal Ir complexes did not aggregate in the presence of H2 at the high temperature,

as shown by the EXAFS spectra (Table 1) and STEM images (Figure 1G and H) characterizing the samples after the treatment. These results support the hypothesis stated in the preceding paragraph that the samples with the lowest Ir loadings were structurally and catalytically distinct from those with higher loadings. The samples with the relatively high iridium loadings, containing 1.0 and 0.1 wt% Ir, were characterized by the formation of Ir clusters during the reduction; images show a distribution of species ranging from single-atom Ir to clusters with diameters of approximately 1 nm (Figure 1 E


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and F, and Figure S3 in the SI). (The results confirm earlier observations of a size limitation of growing Ir clusters on supports.17) In the treated samples, the atomically dispersed Ir species were observed near edges and corners of the support, just as for the samples containing only 0.05 or 0.01 wt% Ir, confirming the inference that the tripodal Ir species are more stable in reductive atmospheres than the bipodal species. The EXAFS data are consistent with these observations (details in SI). Thus, the apparently low catalytic activities at room temperature of the samples with low Ir loadings may be compensated by their higher stabilities, a potential advantage in practice, and we suggest that this pattern might extend to other supported metal catalysts.


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Figure 5. Proposed bonding sites of Ir(C2H4)2 complexes on MgO that give an Ir–O coordination number of 2 or 3 corresponding to the bipodal (I) tripodal (II) species. The atoms are labeled as oxygen (red), magnesium (green), iridium (blue),; carbon (gray), and hydrogen (white).


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Figure 6. Catalytic activities of the initially prepared MgO-supported Ir(C2H4)2 samples with Ir loadings of 1.0 (black), 0.1 (red), and 0.05 (blue) wt% Ir for ethylene hydrogenation. Temperature, 298 K; pressure, 1.01 bar, C2H4:H2 ratio (molar), 1:4; total flow rate: 50 mL min-1; the masses of catalysts were 0.1, 1.0, and 2.0 g for loadings of 1.0, 0.1, and 0.05 wt% Ir, respectively. Error bars were estimated basis of variation in ethylene signal determined in the gas chromatographic analysis of the product and are relatively large for the sample containing 0.05 wt% Ir; see text for details.

CONCLUSIONS The data demonstrate the heterogeneity of the surface sites on the crystalline particles of MgO powder, showing that multiple sites react with Ir(C2H4)2(acac) to give supported catalysts incorporating isolated Ir(C2H2)2 groups. When the Ir loading is low, ≤0.05 wt%, essentially all of the supported species are present at sites near MgO crystal edges, with each Ir atom bonded to 3 oxygen atoms of the support, as shown by the STEM and EXAFS data. At higher loadings, the Ir species are also bonded to terrace sites through 2 Ir–O bonds. The Ir complexes bonded to 3 support oxygen atoms are more stable than those bonded to 2 support oxygen atoms. Thus, the samples with low Ir loadings are single-site catalysts in the truest sense because they lack the heterogeneity of conventionally made single-site supported metal catalysts. Their characterization by experimental and theoretical methods determines their structures and catalytic properties in greater depth than is attainable with conventionally made catalysts. The


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ACS Catalysis

results guide the synthesis of a class of materials having nearly uniform catalytic sites. In prospect, the high degrees of uniformity may imply high catalytic selectivities because of the uniqueness of the catalytic sites and may provide a fundamental contrast to the typical supported metal catalysts (including single-site catalysts) that are heterogeneous and, for many reactions, correspondingly unselective. We posit that this inference may extend to a large class of supported catalysts. EXPERIMENTAL AND COMPUTATIONAL METHODS MgO Pretreatment. High-surface-area MgO was synthesized as follows: Commercial MgO was slurried in water overnight at 350 K forming Mg(OH)2; separated from the water; and stored in an oven at 413 K. Mg(OH)2 was heat treated under vacuum (pressure = 10-4 mbar) to form MgO in a two-step process: (1) temperature was increased to 523 K over 75 min and held constant for 16 h; (2) temperature was increased to 1273 K over 210 min and held constant for 1 h. Under vacuum, the sample was cooled to room temperature, and recovered and stored in a glovebox. The MgO powder had a morphology of interlaced cubes with edges approximately 17 nm long on average, as shown by STEM images (Figure S4 in the Supporting Information, SI) and confirmed by X-ray diffraction crystallography (Figure S5 in the SI). Catalyst Synthesis. MgO-supported single-site catalysts were synthesized by reaction of Ir(C2H4)2(acac) with MgO giving samples with Ir loadings of 0.01, 0.05, 0.1, and 1.0 wt%. Ir(C2H4)2(acac) was synthesized as reported.10 The catalyst containing 1.0 wt% Ir was synthesized by sealing 1.964 g of MgO and 36 mg Ir(C2H4)2(acac) in a Schlenk flask in the glovebox. With air-exclusion techniques, approximately 30 mL of n-pentane was added to the flask. For catalysts containing 0.1 and less Ir, approximately 2 g of MgO was sealed in a Schlenk flask and 10 mg Ir(C2H4)2(acac) was sealed in a round bottom flask. With air-exclusion


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techniques n-pentane (approximately 30 mL) was added to the Schlenk flask and 15.0 mL to the round bottom flask to dissolve the Ir complex. Between 0.5 and 6 mL of the Ir complex solution was injected into the Schlenk flask to give the desired Ir loading. Each sample was slurried for 24 h, and then evacuated for 24 h to remove solvent. The samples were recovered and stored in the glovebox. X-ray Diffraction Crystallography: Powder X-ray diffraction patterns were collected on a Siemens D-500 instrument using the Cu k-α radiation scanning 2θ from 20 to 90°. IR Spectroscopy. IR spectra were recorded on a Bruker IFS-66VS spectrometer. Each spectrum was an average of 64 scans collected over 2 min. In an argon-atmosphere glovebox, powder samples were pressed into self-supporting wafers with masses of 30–50 mg and loaded into an air-tight cell to prevent oxygen and moisture contamination. X-ray Absorption Spectroscopy. X-ray absorption spectra were recorded at the Stanford Synchrotron Radiation Lightsource (SSRL), beamline 11-2 with a Si(220) monochromator and focused beam. The sample containing 1.0 wt% Ir was measured in transmission mode using N2 ion chambers. For samples with ≤0.1 wt% Ir, fluorescence spectra were collected with a 100element Ge solid state detector array. High-energy-resolution XANES spectra were collected at SSRL beamline 6-2 using the high-energy-resolution X-ray emission spectrometer tuned to the peak of the Ir-Lα emission line. XANES scans were collected every 2 min and EXAFS scans every 20–40 min, with 3–6 scans averaged to improve the signal to noise ratio when the samples were stable. In a glovebox at SSRL, each powder sample was packed into a XAS cell that functioned as a flow reactor. The mass of the 1.0 wt% Ir sample was chosen to be approximately 200 mg on the basis of the sample having an absorbance of approximately 2.5, calculated 50 eV above the Ir LIII 18 ACS Paragon Plus Environment

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ACS Catalysis

edge. There was no need to dilute samples with

Beating Heterogeneity of Single-Site Catalysts: MgO-Supported

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