Reversible Metal Aggregation and Redispersion Driven by the

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Reversible Metal Aggregation and Redispersion Driven by the Catalytic Water Gas Shift Half Reactions: Interconversion of SingleSite Rhodium Complexes and Tetrarhodium Clusters in Zeolite HY Chia-Yu Fang, Shengjie Zhang, Yiqin Hu, Monica Vasiliu, Jorge PerezAguilar, Edward T. Conley, David A Dixon, Cong-Yan Chen, and Bruce C. Gates ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04798 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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ACS Catalysis

Reversible Metal Aggregation and Redispersion Driven by the Catalytic Water Gas Shift Half Reactions: Interconversion of Single-Site Rhodium Complexes and Tetrarhodium Clusters in Zeolite HY Chia-Yu Fang,†‡ Shengjie Zhang,# Yiqin Hu,# Monica Vasiliu,# Jorge E. Perez-Aguilar,† Edward T. Conley,‡ David A. Dixon,# Cong-Yan Chen, †§ and Bruce C. Gates†*

†Department

of Chemical Engineering, University of California, Davis, One Shields Avenue,

Davis, California 95616, United States

‡Department

of Materials Science and Engineering, University of California, Davis, One Shields

Avenue, Davis, California 95616, United States

#Department

§Chevron

of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487, United States

Energy Technology Company, 100 Chevron Way, Richmond, California 94892, United

States 1 ACS Paragon Plus Environment

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KEYWORDS Rhodium cluster synthesis. Reversible metal cluster formation and breakup. Water gas shift reaction. Synthesis in zeolite.

ABSTRACT

Rhodium gem-dicarbonyl complexes, Rh(CO)2, bonded within the pore structure of zeolite HY and formed by the reaction of Rh(CO)2(acac) (acac = acetonato) with OH groups on the zeolite surface were converted in >95% yield to Rh4(CO)12 by reaction with CO + water at 308 K, and the process was reversed by treatment of the supported clusters in helium at 353 K. The chemistry of these reactions was characterized by IR and X-ray absorption spectra recorded during the changes and by density functional theory. The cluster formation is driven by the water-gas shift half reaction, leading to generation of CO2 and zeolite surface protons, and the reverse reaction proceeds via the half reaction that completes the cycle of the water-gas shift reaction. Thus, the overall process is cyclic—catalytic. The yield in the synthesis of Rh4(CO)12 is the highest reported, and the high selectivity is facilitated by the confining environment for the clusters in the zeolite supercages and the low density of OH groups on the zeolite surface (the zeolite Si:Al atomic ratio

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was 30). The results provide insights into the first steps of sintering of atomically dispersed metals on supports.

INTRODUCTION Atomically dispersed noble metals on solid supports, including zeolites,1 carbons,2-3 metal oxides,4-8 metal-organic frameworks,9-10 and metal surfaces,11 have attracted wide attention because they offer new catalytic properties with maximum efficiency in the use of the expensive metals. These single-site catalysts are essentially molecular—some of them being the simplest structurally of any supported catalysts—and they are now illustrated by many examples.12-15 The simplicity of these catalysts facilitates fundamental understanding of catalyst structures and reaction mechanisms.16-17 The next step beyond these most highly dispersed catalysts would be those incorporating pairs of metal atoms or a few metal atoms in clusters on supports. Such catalysts are well known, but there are only a few having uniform, well-defined structures, because they are challenging to synthesize.18-19 Synthesis of metal clusters in solution benefits from purification steps to remove side products, but such steps are usually not feasible with supported species. Synthesis of well-


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defined supported metal clusters by simple adsorption of cluster precursors such as metal carbonyls typically falls short because the clusters undergo changes upon adsorption,20 and such syntheses fail when the cluster precursor is too large to fit into the pores of the support, such as a zeolite. We now report a reversible, high-yield and selective synthesis of a small metal cluster, Rh4(CO)12, in the cages of a zeolite, and the process is catalytic, with the cluster synthesis and break-up each being driven by one of the half cycles of the water gas shift reaction. Preparation of Rh4(CO)12 by conventional solution methods, in contrast, typically suffers from poor reproducibility of high-yield synthesis, with yields varying substantially (from 86-99%), with Rh6(CO)16 formed as a byproduct.21 We chose the support, dealuminated zeolite HY, because it has large enough pores for entry of the rhodium precursor Rh(CO)2(acac) (acac = acetylacetonato) and because it offers the opportunity to dial in the density of proton-donor sites for binding the precursor. We report characterization of the supported species and their reactions by infrared (IR) and X-ray absorption spectroscopies complemented by electronic structure calculations providing evidence of the intermediate species in the process of rhodium aggregation/redispersion. The results point to opportunities to better understand the evolution of the most highly dispersed metal cluster structures and their catalytic properties. 4 ACS Paragon Plus Environment

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RESULTS Formation of Rh(CO)2 Complexes in Zeolite HY. Highly dispersed single-site Rh(CO)2 with a nearly uniform structure was prepared by bringing a solution of the precursor Rh(CO)2(acac) in contact with dealuminated zeolite HY having an Si:Al atomic ratio of 30, as before.22 The rhodium loading in the zeolite was 0.5 wt%, chosen to give isolated and widely separated catalytic species (0.07 Rh atoms/supercage). IR spectra (Figure S1 in the Supporting Information, SI), show that the rhodium precursor reacted selectively with acidic sites (OH groups) in the zeolite by protonation and removal of acac ligands from the precursor, characterized by intensity decreases in the bands at 3630 cm-1 (characterizing acidic OH groups in the supercages) as a result of the chemisorption reaction. Two intense bands, at 2117 and 2053 cm-1, characterize the symmetric and asymmetric vibrations of CO ligands of rhodium gem-dicarbonyls terminally bonded to the site-isolated rhodium centers. The sharpness of these bands, having full width at half maximum (FWHM) values < 6 cm-1, indicates a high degree of structural uniformity of the supported Rh(CO)2 species.23 Extended X-ray absorption fine structure (EXAFS) spectra (Table 1) show that each Rh atom was, on average, bonded to two carbonyl ligands (confirming the IR data) and two oxygen atoms 5 ACS Paragon Plus Environment

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of the zeolite, with no evidence of any Rh–Rh contribution, confirming the expectation of the mononuclear rhodium complexes in the zeolite. Lack of Reaction of Rh(CO)2 in Zeolite HY with H2O. As a first step toward investigating the chemistry of rhodium cluster synthesis in the zeolite, we used a once-through plug-flow reactor to characterize the reaction of the supported Rh(CO)2 complexes with water vapor. The sample (200 mg) was exposed at 308 K to a stream of helium (100 mL/min) that had been saturated with water vapor at 298 K; the flow continued for 10 min, and the effluent gas was characterized by mass spectrometry. To better resolve any signals, we diverted the stream through a bypass to isolate the sample in the reactor for 30 min, and then restarted the flow through the reactor. No gas-phase products such as CO2 and H2 were detected (Figure S2 in the SI). Reaction of Rh(CO)2 in Zeolite HY with CO + H2O.

Next, we characterized the reaction of

this zeolite-supported Rh(CO)2 sample with CO + H2O. A stream of CO saturated with water vapor at 298 K flowed into the reactor at 308 K at a constant rate of 50 mL(NTP) min-1. CO2 was detected in the effluent gas stream by mass spectrometry (m/z = 44) (Figure S3 in the SI). Comparison experiments with the zeolite lacking rhodium did not give evidence of CO2 formation. Even with the rhodium, the rate of CO2 formation was extremely low, and, consequently, to observe more 6 ACS Paragon Plus Environment

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than minimal conversions, we diverted the CO + H2O stream to a bypass, isolating the cell to make it temporarily a batch reactor. Then, after periods of 30 min, we directed the CO + H2O stream back to the cell and recorded the mass spectra signals as the accumulated CO2 was purged and flowed to the mass spectrometer. The data show that the rate of CO2 formation declined as reaction proceeded; after 48 h of exposure of the sample to CO + H2O, the CO2 signal was no longer detectable. H2 (m/z = 2), a product of the water gas shift reaction that might have been expected as a product, was not detected during the process. Structural Characterization of Supported Rhodium Species formed by Reaction of Rh(CO)2 with Water. To understand how the rhodium species changed in the reaction with water vapor, we used IR, X-ray absorption near edge structure (XANES), and EXAFS spectroscopies to characterize the supported species after various periods of exposure to water. After 10 min of contact with the stream containing water at 308 K and 1 bar, the IR bands (Figure S4(a) in the SI) characterizing Rh(CO)2 disappeared, accompanied by the appearance of four new bands in the νCO region (confirmed in experiments with 13CO (Figure S5 in the SI)), at 2106, 2088, 2037, and 2019 cm-1. The bands at 2106 and 2037 cm-1 are assigned22,24 to CO in supported Rh(CO)2(H2O) and those at 2088 and 2019 cm-1 to CO in supported Rh(CO)2(H2O)2. XANES data recorded as the 7 ACS Paragon Plus Environment

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sample was treated in the same way include isosbestic points (Figure S6 in the SI), showing that, in the transformation of Rh(CO)2 to the rhodium aquo complexes, the two products formed in a constant ratio. When the mixture of rhodium aquo complexes was exposed to a stream of dry helium, the IR bands characterizing Rh(CO)2 were fully regenerated, showing that the reaction to form the aquo complexes was fully reversible (Figure S4(b) in the SI) and implying that the reaction involved ligand association/dissociation. The supported Rh(CO)2(H2O)x (x = 1,2) was further characterized by EXAFS spectroscopy; the data (Table 1) demonstrate that the Rh–C coordination number (CN) remained unchanged (2.0 ± 0.2), and no Rh–Rh contribution was found, confirming that the rhodium complexes remain mononuclear after reacting with water. Two Rh–O contributions were observed, one with a CN of 1.0 at a distance of 1.97 ± 0.02 Å and one with a CN of 1.2 ± 0.1 at a distance of 2.11 ± 0.02 Å; however, the Rh–Os CN (Os is support oxygen) decreased from 2.0 ± 0.2 to 1.0 ± 0.1, indicating that Rh(CO)2(H2O)x was bonded to only one support oxygen atom. Reaction with CO + H2O converts Rh(CO)2 to Rh4(CO)12. After completion of the reaction of Rh(CO)2 with water, a new set of experiments was done to investigate the reaction of the 8 ACS Paragon Plus Environment

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supported species in a stream of CO + H2O. After the start of flow through the sample cell at 308 K, the IR and XANES spectra (Figure 1) slowly changed, being characterized by the appearance of isosbestic points, showing that a stoichiometrically simple conversion was occurring. After 48 h, the spectra had stopped changing, and the sample cells were purged with helium at 298 K. The final IR spectrum includes an intense band at 2075 cm-1 and two bands with medium intensities centered at 2044 and 1885 cm-1. Their frequencies match those reported for Rh4(CO)12 in solution (Figure S7 in the SI).25 The IR spectrum of the supported species also includes a weak band of unconverted Rh(CO)2, and the carbonyl band areas show that the yield of Rh4(CO)12 was >95% (Figure 2). This is the highest yield reported for synthesis of a metal cluster on a support (Table 2).


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Table 1 EXAFS parameters characterizing rhodium-containing HY zeolite after various treatments.

Supported

Shella

EXAFS parameters

Crystallo-

species/treatment

graphic

conditions

parameters characterizing solid sample

Rh(CO)2/flowing He at 298 K

Rh(CO)2(H2O)x/support ed

Rh(CO)2

mixture/flowing H2O + He at 308 K for 40 min

Rh4(CO)12/supported Rh(CO)2 mixture/flowing H2O + He at 308 K for 40 min.

CN

R (Å)

Δσ2×103 (Å2)

ΔE0 (eV)

Rh–Os

2.0 ± 0.2

2.13 ± 0.02

6.37 ± 1.27

-3.81± 0.76

Rh–Ct

2.0 ± 0.2

1.84 ± 0.02

2.70 ± 0.54

1.92 ± 0.38

CN

R (Å)

--

--

--

--

Rh–Ot

2.0 ± 0.2

2.98 ± 0.02

0.96 ± 0.19

-6.65 ± 1.3

Rh–Al

1.1 ± 0.1

3.10 ± 0.02

0.1 ± 0.02

4.11 ± 0.82

Rh–Os

1.0 ± 0.1

1.97 ± 0.02

0.18 ± 0.04

5.30 ± 1.06

Rh–Oaquo

1.2 ± 0.1

2.11 ± 0.02

0.85 ± 0.17

-7.03 ± 1.41

Rh–Ct

2.0 ± 0.2

1.88 ± 0.02

4.71 ± 0.94

-6.08 ± 1.21

Rh–Ot

2.0 ± 0.2

2.99 ± 0.02

5.30 ± 1.06

1.84 ± 0.37

Rh–Rh

3.0 ± 0.3

2.72 ± 0.02

7.28 ± 1.46

-6.80 ± 1.36

3.0

2.70

Rh–Ct

2.2 ± 0.2

1.91 ± 0.02

8.31 ± 1.66

1.81 ± 0.36

2.2

1.90

Rh–Cb

1.5 ± 0.2

2.12 ± 0.02

6.12 ± 1.22

-5.49 ± 1.10

1.5

2.10

Rh–Ot

2.2 ± 0.2

3.03 ± 0.02

6.33 ± 1.27

-1.58 ± 0.32

2.2

3.05

Rh–Ob

1.5 ± 0.2

2.96 ± 0.02

0.87 ± 0.17

9.45 ± 1.89

1.5

3.06

The resultant sample was treated in H2O + CO for 48 h at 308.


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Notation: CN, coordination number; R, distance between the absorber and the backscatterer atoms; Δσ2, mean square relative displacement; ΔE0, inner potential correction. Error bounds are estimated as shown in the Table. aOs denotes framework oxygen atoms of HY zeolite; Oaquo denotes oxygen atoms of H2O molecules; Ct and Ot denote carbon and oxygen atoms of terminal carbonyl ligands; Cb and Ob denote carbon and oxygen atoms of bridging carbonyl ligands.

We used EXAFS spectroscopy to confirm the identification of Rh4(CO)12. A comparison of the EXAFS results with crystallographic data characterizing solid Rh4(CO)12 (Table 1) confirms the conclusion; details are presented in the SI.

Figure 1. Changes in IR spectra of sample initially containing Rh(CO)2(H2O)x (x = 1,2) during reaction with CO + H2O to make Rh4(CO)12 at 308 K for the following times (h): 0.5 (black), 1.5 (red), 3.5 (orange), 6.5 (yellow), 10.5 (green), and 48 (blue). XANES spectra near the Rh K edge


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(22320 eV) showing changes during this conversion for the following times (h): 0 (black), 2 (red), 6 (green), and 48 (blue).

Figure 2. (a) IR spectra in the C–O stretching regime of Rh(CO)2 (black line) and Rh4(CO)12 synthesized in zeolite HY (blue line). The yield estimated from the integrated intensities of the band at 2117 cm-1 ≈ 95%, and that from the corresponding peak heights ≈ 94%; (b) k0-weighted |χ(R)| plot characterizing EXAFS data comparing Rh4(CO)12 in the zeolite HY and metallic rhodium. Details are presented in the SI.

The results imply that the aggregation of atomically dispersed rhodium centers involved the water gas shift half reaction of CO and H2O reducing Rh(I)(CO)2 and forming gas-phase CO2 and clusters with rhodium formally in the zero oxidation state. Thus, we postulate that the reverse 12 ACS Paragon Plus Environment

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reaction involves CO2 and H2 and the other water gas shift half reaction and recognize the need to address the forms of hydrogen involved in the reaction. We address these points below. Table 2. Summary of the reported yields of metal clusters on supports. Metal cluster

Support

Yield

Quantification method Ref.

Rh4(CO)12

Zeolite HY

>95%

IR spectroscopy

This work

[Os10C(CO)24]2-

MgO

65-70%

EXAFS spectroscopy

26

Rh6(CO)16

Zeolite NaY

>90%

EXAFS spectroscopy

27

Ru3(CO)12

SiO2

49-93%

IR spectroscopy

28

[Pt9(CO)18]2-

MgO

50%

UV-vis spectroscopy

29

Reversing the Cluster Formation Reaction: Fragmentation of Rh4(CO)12 in Helium to Give Rh(CO)2. When the supported Rh4(CO)12 in a 30-mg sample was exposed to a stream of helium (100 mL/min) at 353 K and 1 bar, the IR spectra demonstrate that supported Rh(CO)2 was regenerated, with no evidence of supported side products (Figure 3); isosbestic points in the spectra indicate that the reaction was stoichiometrically simple. During the experiment, periodically measured mass spectra of the effluent gas showed that the gas-phase products were H2 and CO


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(Figure 3), further implicating water gas shift chemistry. The cluster formation/breakup cycle was reversible, being repeated multiple times. H2 Accelerates the Synthesis of Rh4(CO)12 from Rh(CO)2. To gain further insight into the cluster synthesis/breakup chemistry, we investigated the influence of H2 on the cluster formation in CO + H2O by adding H2 to the reactant gas stream. The data show that H2 markedly increased the rate of cluster formation: the reaction was virtually complete after 11 h when H2 was present at a partial pressure of 500 mbar along with CO + H2O (at 308 K and a total pressure of 1 bar), whereas 48 h was required when the H2 was absent and the conditions were otherwise the same. However, although H2 accelerated the synthesis of Rh4(CO)12, it lowered the selectivity, as evidenced by data recorded when the time of exposure to H2 exceeded 1 h. Then, in addition to the principal product Rh4(CO)12, Rh6(CO)16 was also observed, identified by the IR spectra (Figure S8 in the SI).


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Figure 3. (a) IR spectra characterizing evolution of Rh4(CO)12 containing zeolite HY in helium at 353 K for the following times (min): 0 (black), 10 (red), 20 (orange), 30 (green), and 90 (blue). (b) Mass spectra of H2 (m/z = 2) and CO (m/z = 28) in the effluent gas when the sample was treated in helium during a temperature ramp.

The results, taken together, suggest a role of the water gas shift reaction in the cluster formation/breakup chemistry, but they are not sufficient to elucidate the chemistry; in particular, the data still leave open the question of the role of hydrogen in the chemistry. We address these points below. Effect of Zeolite Si:Al Ratio on Rate of Rhodium Cluster Formation. To further investigate the intrazeolite chemistry, we carried out experiments similar to those described above with an HY 15 ACS Paragon Plus Environment

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zeolite sample having a low Si:Al ratio, now 2.6 (atomic) rather than 30. The spectra show that same chemistry again occurred, but with important differences: first, the reactions were markedly faster, with the Rh4(CO)12 synthesis being almost complete after 6 h. Figure 4 is a comparison of the rate of rhodium aggregation in zeolite HY with the Si/Al ratios of 2.6 and 30 under the water gas-shift reaction conditions. The reverse reaction in the former zeolite, however, took place at about the same rate as that observed with the zeolite having the Si:Al ratio of 30. There was another complication with the zeolite having a Si:Al ratio of only 2.6: the formation of Rh4(CO)12 did not take place in the high yield observed with the other zeolite; instead Rh6(CO)16 was a major side product (Figures S9–11 in the SI). These results demonstrate that the high-silica zeolite was nearly optimal for the high-yield synthesis of Rh4(CO)12. Further, the results suggest that Rh4(CO)12 was an intermediate in the formation of Rh6(CO)16, as it is in the solution synthesis.30 The results raise the question of why the synthesis in the high-silica zeolite HY is so highly selective. We suggest that the zeolite supercage provides a nearly optimal environment for the chemistry; it is large enough (with a diameter of about 11 Å31) to accommodate Rh4(CO)12 (diameter ≈ 9 Å32) (and also Rh6(CO)16 (diameter ≈ 10 Å)33) and also small enough to stabilize these clusters by entrapment, because the zeolite apertures have a diameter of only 7.4 Å.31 Further, 16 ACS Paragon Plus Environment

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we infer that the surface of the zeolite with a Si:Al ratio of 30 is better suited to the selective synthesis of the smaller clusters than the zeolite having the higher Al content, because the latter has a higher density of acidic OH groups that facilitate the transformation of Rh4(CO)12 to Rh6(CO)16. We suggest that the higher density of OH groups leads to a higher density of nearby Rh(CO)2 groups to facilitate the kinetics of the reaction of mononuclear rhodium species with Rh4(CO)12 to give the higher-nuclearity clusters.

Figure 4. Time-dependent integrated area of the IR peak characterizing the bridging C–O stretching frequency of synthetic Rh4(CO)12 in the zeolite HY with two Si:Al atomic ratios, 30 (▲) and 2.6 (♦).


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Tradeoffs Accounting for Rate of Rhodium Aggregation/Cluster Breakup involving the Water Gas Shift Reaction. We further examined the chemistry of Rh(CO)2 in zeolite HY under water gas shift reaction conditions. We used IR spectroscopy to track the structural evolution of Rh(CO)2 for 3 h. The rate of Rh4(CO)12 formation was highest at 308 K and declined when the temperature was either increased or decreased (Figure S12 in the SI). Rh4(CO)12 formation did not proceed measurably when the temperatures were lower than 298 K or higher than 353 K. To gain further understanding of the chemistry, we used IR spectroscopy to examine the interaction of supported Rh(CO)2 with H2O at various temperatures. Figure S13 in the SI shows the C–O stretching frequencies of the Rh(CO)2 species exposed at 1 bar to a stream of helium saturated with water vapor at 298 K. The interaction of water (as aquo ligands) with rhodium centers is evidently strongest at room temperature, indicated by the dominance of the bands centered at 2088 and 2019 cm-1, assigned to Rh(CO)2(H2O)2 species. However, when temperature was increased, the intensity of the bands at 2088 and 2019 cm-1 started to decrease as the bands at 2106 and 2037 cm-1, characterizing Rh(CO)2(H2O), became dominant at 323 K. Rh(CO)2(H2O) was completely converted back into Rh(CO)2 at 353 K. This result shows that the reactivity of Rh(CO)2 and water depends strongly on temperature (Figure S13 in the SI). Taken 18 ACS Paragon Plus Environment

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together, the data imply that the rate of rhodium cluster formation is influenced significantly by both the bonding of water to the rhodium center and by the activation energy barrier for the reaction that involves the half reaction of the water gas shift. Thus, there appears to be a tradeoff. As the rate of the latter process increases with increasing temperature, the number of aquo ligands bonded the rhodium centers decreases. Therefore, an intermediate temperature, approximately 308 K, was found to be optimal for this process. Theoretical Assessment based on DFT Calculations. To further assess the chemistry, we used density functional theory (DFT) and correlated molecular orbital theory at the CCSD(T) level to predict the structural evolution of Rhx(CO)y clusters following the approach used for the corresponding iridium carbonyl clusters up to the hexamer.34 Previous DFT calculations of the structures of oxide-supported aquo rhodium complexes35 predict the length of the Rh–Oaquo bond to be about 2.12 Å and the length of the Rh–Os bond to be about 1.96 Å, consistent with our EXAFS results. The average CN of the Rh–Oaquo was found to be 1.2 ± 0.1, in agreement with the inference that the supported species were a mixture of Rh(CO)2(H2O) and Rh(CO)2(H2O)2. The predicted energetics for the addition of H2O to Rh(CO)2 are given in reactions (1) and (2). As shown in the reactions, the H2O is not strongly bonded to the Rh(CO)2, and so the dissociation of 19 ACS Paragon Plus Environment

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water is expected as the temperature is increased. The addition of a second water molecule is not as energetically favored as the addition of the first, again consistent with experiment (energies were determined at the (MP2+ ωB97X-D)/2 level and CCSD(T)/aug-cc-pVDZ(-PP) with the latter values in parentheses in the following equations). Rh(CO)2 + H2O → Rh(CO)2(H2O)

ΔH(298) = -5.1(-6.3), ΔG(298) = 2.8(2.1)kcal/mol (1)

Rh(CO)2 + 2 H2O → Rh(CO)2(H2O)2

ΔH(298) = -9.3(-10.4), ΔG(298) = 7.1(6.0) kcal/mol (2)

The corresponding B3LYP/aug-cc-pVDZ(-PP) values are ΔH(298) = -7.0 and ΔG(298) = 0.9 kcal/mol and ΔH(298) = -8.8 and ΔG(298) = 5.9 kcal/mol. Thus the DFT results are in satisfactory agreement with the higher-level correlated molecular orbital theory CCSD(T) results. We also examined the following reactions for Rh(I). For these cases, there are two electronic states for these +I oxidation state complexes, a singlet and a triplet. For Rh(I)(CO)2 and Rh(I)(CO)2(H2O), the triplet is lower in energy than the singlet by 3.9 kcal/mol and 5.3 kcal/mol, respectively, whereas for Rh(I)(CO)2(H2O)2, the singlet is lower in energy by 13.6 kcal/mol at the CCSD(T) level (the singlet-triplet splittings and energies at the B3LYP level are comparable). As shown in reactions (3)–(6) at the same levels as for reactions (1) and (2), the reactions are more exothermic, 1Rh(I)(CO)2 + H2O → 1Rh(I)(CO)2(H2O) 20 ACS Paragon Plus Environment

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ΔH(0) = -30.0/-26.6, ΔG(298) = -23.3/-19.8 kcal/mol (3) 3Rh(I)(CO)

2

+ H2O

→ 3Rh(I)(CO)2(H2O) ΔH(0) =-27.0/-28.0, ΔG(298) = -18.2/19.1 kcal/mol (4)

1Rh(I)(CO)

2

+ 2 H2O → 1Rh(I)(CO)2(H2O)2 ΔH(0) = -66.5/-61.6, ΔG(298) = -50.2/-45.4 kcal/mol (5)

3Rh(I)(CO)

2

+ 2 H2O → 3Rh(I)(CO)2(H2O)2 ΔH(0) = -45.0/-44.1, ΔG(298) = -27.8/-26.9 kcal/mol (6)

consistent with the presence of a +1 charge which will lead to tighter binding of the H2O molecules. However, the Rh(I) species are more likely to be bonded to the acidic site of the zeolite as shown in reactions (7) and (8). The results characterizing reaction (7) and all subsequent reactions are at the B3LYP level. Bonding to the acidic site changes the energetics so that they are more like that [Rh(I)(CO)2(Al(OH)4)-]0 + H2O → [Rh(I)(CO)2(Al(OH)4)-]0(H2O) ΔH(0) = -9.1, ΔG(298) = 0.0 kcal/mol (7) [Rh(I)(CO)2(Al(OH)4)-]0 + 2 H2O → [Rh(I)(CO)2(Al(OH)4)-]0(H2O)2 ΔH(0) = -14.3, ΔG(298) = 1.4 kcal/mol (8)


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of the bare Rh(0)(CO)2 complex. In both cases, for bare Rh(0) and Rh(I) complexes bonded to the acidic site of the zeolite, the computational results show that there is likely to be a Rh(CO)2 species with one or two H2O ligands, consistent with experiment in view of the errors in the calculations in terms of the model chosen and the computational level. We previously calculated the energetics of the low energy isomers of the Irn(CO)m complexes (n = 1, 2, 3, 4, and 6) and showed that the average value of the reaction energies at the MP2 and ωB97X-D levels provide the best qualitative match to the CCSD(T) energies of the monomers.34 Consequently, we used this approach to calculate the nucleation reaction energies to form Rh4(CO)12 and Rh6(CO)16 from smaller rhodium carbonyl species. The energetics characterizing all the structures and reactions investigated in this work are shown in the SI. The calculations predict that the C3v structure of Rh4(CO)12 is more stable than the Td structure, by 21.0 kcal/mol, consistent with experiment.36-37 This result is significantly different from that characterizing Ir4(CO)12, for which the Td structure is of lower energy.34,38-43 Reactions (9)–(12) account for formation of the cluster from anchored Rh(CO)2 (Figure 5) with values at the (MP2+ ωB97X-D)/2 level and CCSD(T)/aug-cc-pVDZ(-PP) values in parentheses. Rh(CO)2 + 2 CO → Rh(CO)4

ΔG = -36.5 (-27.4) kcal/mol (9) 22 ACS Paragon Plus Environment

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2 Rh(CO)4 → Rh2(CO)8

ΔG = -45.2(-36.3) kcal/mol (10)

2 Rh2(CO)8 → Rh4(CO)12(C3v) + 4 CO

ΔG = -29.5 kcal/mol (11)

Rh4(CO)12(C3v) + 2 Rh2(CO)8 → Rh6(CO)16 + 2 CO

ΔG = -21.0 kcal/mol (12)

The energy values shown in Figure 5 represent ΔG at 298 K in kcal/mol for the average of the MP2 and ωB97X-D electronic values with the free energy corrections obtained at the SVWN5 level. The values in parentheses are for CCSD(T)/aD with the SVWN5 corrections. As noted above, the presence or lack of additional H2O ligands will not significantly affect the overall energetics as shown in reaction (13) and (14), as CO readily displaces H2O as a ligand. Rh(CO)2(H2O) + 2CO → Rh(CO)4 + H2O

ΔG = -39.8 (-29.5) kcal/mol (13)

Rh(CO)2(H2O)2 + 2CO → Rh(CO)4 + 2 H2O

ΔG = -43.9 (-33.3) kcal/mol (14)


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Figure 5. Calculated energetics characterizing the structures and reactions involved in the rhodium cluster formation and break-up involving the water gas shift reaction. Energies are given in kcal/mol.

Overall, the addition of 4 times reaction (9) plus 2 times reaction (10) plus reaction (11) is highly exothermic. This exothermicity must be coupled with an endothermic process, which is the one that generates the initial Rh(CO)2. The generation of Rh(CO)2 from Rh(I) at the acidic site of the zeolite was modeled as an Al(OR)4- site (reactions (15) and (16)), and on a Si(OR)3O- site with R


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= H or R = Si(OH)3 and a Rh(I)(CO)2 species, reactions (17) and (18), at the B3LYP/aD level. (The formation of Rh(CO)2(H2O)x complexes does not substantially affect the energetics or the overall mechanistic results as shown above). 2 [Rh(I)(CO)2(Al(OH)4)-]0 + H2 → 2 Rh(CO)2 + 2 [H+(Al(OH)4)-]0 ΔG = 86.9 kcal/mol (15) 2 [Rh(I)(CO)2(Al(OSi(OH)3)4)-]0 + H2 → 2 Rh(CO)2 + 2 [HAl(OSi(OH)3)4-]0 ΔG = 75.1 kcal/mol (16) 2 [Rh(I)(CO)2(SiO(OH)3)-]0 + H2→ 2 Rh(CO)2 + 2 [H+(SiO(OH)3)-]0 ΔG = 36.5 kcal/mol (17) 2 [Rh(I)(CO)2(SiO(OSi(OH)3)3)-]0 + H2 → 2 Rh(CO)2 + 2 [H+(SiO(OSi(OH)3)3)-]0 ΔG = 42.0 kcal/mol (18) The H2 can be provided either as an added reactant or generated by the water gas shift reaction (19): H2O + CO ↔ CO2 + H2

ΔH (298 K) = -9.8 kcal/mol (19)

The generation of the Rh(CO)2 as a reactant is an endothermic redox process involving a protoncoupled electron transfer at an anion site in the zeolite. The generation of the initial reactant is 25 ACS Paragon Plus Environment

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more endothermic at an Al acidic anion site than at a SiO- site. However, the reactions are significantly less endothermic than the exothermic reaction are exothermic, so that the formation of the Rh4(CO)12 cluster is thermodynamically favored. The formation of Rh(CO)4 and the formation of Rh2(CO)8 are highly exothermic, and the final dimerization step to form Rh4(CO)12 is much less exothermic. The binding energy of a CO ligand is much greater than that of a H2O ligand, and so the presence of H2O ligands does not affect the overall energetics by more than a few kcal/mol, and the H2O ligands can be displaced by CO. DISCUSSION Analogy to known Solution Chemistry and Catalytic Cycle for Rhodium Cluster Formation/Breakup and Water Gas Shift Reaction. To resolve the chemistry more fully, we need to consider the redox character of the cluster formation/breakup process and understand the role of hydrogen. On the basis of reported work with Rh(I)(CO)2(acac) in solution, which (via the water gas shift reaction) leads to the formation of H2 that reduced the Rh(I) and generated H+, we suggest that chemistry in the zeolite is related to the following solution chemistry:44 4 Rh(I)(CO)2 + 2 H2O + 6 CO → Rh4(CO)12 + 4 H+ + 2CO2

(20)

6 Rh(I)(CO)2 + 3 H2O + 7 CO → Rh6(CO)16 + 6 H+ + 3CO2

(21)


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We work from the inference that the formation of Rh4(CO)12 in the zeolite takes place via chemistry similar to that described above and like the reactions for Rh(I) salts in aqueous solution given in Eqs. (20) and (21),21,30 whereby Rh(I) is reduced by CO + H2O, generating Rh4(CO)12 along with CO2 and H+. We infer that the product H+ becomes part of the zeolite framework, balancing the charge of Al sites by forming Brønsted acid (OH) sites. The mobility of the Rh species, which sparsely populate the intrapore region of the zeolite (0.07 Rh atoms/supercage), is inferred to be high enough for these species to migrate and coalesce in the supercages. This high mobility is found in earlier work demonstrating the rapid aggregation of Rh species in this zeolite triggered by exposure to atmospheres of ethylene + H2, as ethylene hydrogenation catalysis was occurring.16 To explain the oxidative fragmentation of Rh4(CO)12 in the zeolite, we postulate that H2 was released as a result of charge transfer between Rh(0) and H+. By this postulate, we see the completion of the cycle of the water gas shift reaction, as summarized schematically in Figure 6. This whole process constitutes a catalytic cycle. Several observations of oxidative fragmentation of metal clusters have been reported, exemplified by reactions of Ir4 and Rh4-6 clusters with CO45 and ethylene16 and by platinum nanoclusters in NO + CO46 and air,47 Support surface OH groups have been invoked to explain 27 ACS Paragon Plus Environment

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such fragmentations—45 and we suggest that they could be involved in the breakup of our encaged Rh4(CO)12 clusters.

Cage diameter ≈ 11 Å

CO + H 2O

CO2 + H+

T = 308 K

H2 + CO

T = 353 K

Rhodium

H+

Carbon

Oxygen

Figure 6. Schematic representation of mechanism of the reversible cycle of formation and fragmentation of Rh4(CO)12 via the water gas shift reaction.

Cluster formation represents a route to the initial steps of metal aggregation (metal cluster formation)—sintering—as might occur to cause catalyst activation/deactivation. But, according to this picture, ligands other than CO need to bond to the rhodium for the reactions to occur, and thus we might expect that CO could play the role of an inhibitor as too-high partial pressures of CO would hinder both the cluster formation and break-up reactions.


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We can infer that Rh4(CO)12 clusters were highly dispersed in the supercages of the zeolite on the following. (a) The IR spectrum of Rh4(CO)12 is characterized by extremely sharp and highly symmetrical C–O stretching bands that indicate highly dispersed and isolated Rh4(CO)12 species. (b) The supercages of the zeolite provide an optimal environment (with a diameter of about 11 Å) to entrap Rh4(CO)12 clusters (diameter 9 Å). (c) If clusters had formed on the outside surfaces of the zeolite, we would expect the sample to have turned dark red, which is the color of aggregated Rh4(CO)12 in the solid state; instead, our sample color is orange/beige (Figure S14 in the SI), consistent with our inference of molecularly dispersed rhodium clusters.48 Comparable Chemistry of Cluster Syntheses in Zeolites. The data indicate that the Rh(CO)2/Rh4(CO)12 conversion involves the half-reactions of the water gas shift reaction, and in this respect it is comparable to the chemistry of the synthesis of Rh6(CO)16 from Rh(I)(CO)2 on the surface of γ-Al2O3 and in zeolite NaY.49-50 The early investigations of these reactions of supported rhodium carbonyls indicate that moisture on the surfaces, as evidenced by IR spectroscopy, played a role.50-51 Our data demonstrate that a highly selective synthesis of Rh4(CO)12 was achieved in zeolite HY, with stability of these clusters at mild temperatures (< 353 K). The results are significantly 29 ACS Paragon Plus Environment

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different from reported results characterizing rhodium clusters on other supports, including γAl2O3, SiO2, and zeolite NaY, as Rh6(CO)16 was the major product, with Rh4(CO)12 being only a minor, intermediate species. Consistent with these results, Theolier et al.49 deposited Rh4(CO)12 on γ-Al2O3 and observed its conversion into Rh6(CO)16 when the sample was in a CO-poor atmosphere. In contrast, our report of the chemistry in HY zeolite indicates a highly selective synthesis of Rh4(CO)12 and its stabilization on the support.

EXPERIMENTAL AND COMPUTATIONAL METHODS Synthesis of Rh(CO)2 in Zeolite HY. Sample handling and treatments in the syntheses were carried out with standard air-exclusion techniques. The HY zeolite samples (Zeolyst International CBV760 and CBV600, having Si:Al atomic ratios of 30 and 2.6, respectively) were calcined at 773 K for 2 h in flowing O2 followed by 16 h in dynamic vacuum in a once-through plug-flow reactor. The precursor (acetylacetonato)dicarbonyl rhodium(I) (Rh(CO)2(acac), 99%, Strem) was mixed with the calcined zeolite and dried deoxygenated n-pentane, and the mixture was stirred in a slurry at 298 K under argon for 24 h. After the n-pentane had been removed by overnight


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evacuation, leaving all of the rhodium on the support, the resultant sample, containing 0.5 wt% Rh, was recovered and stored in an argon-filled glovebox. Infrared Spectroscopy. IR spectra of the supported species were recorded with a Bruker IFS 66v/S spectrometer with a resolution of 4 cm-1. Approximately 30 mg of sample was pressed into a wafer and loaded into a cell that was a flow reactor. The cell was sealed and connected to the flow system, allowing recording of transmission spectra of the sample as the reactant gases flowed around and through it at a set temperature. Each recorded spectrum is the average of 64 scans. Mass Spectrometry. Mass spectra of gases flowing from the IR cell, some produced by reaction of inflowing gases with the sample, were acquired with an online Balzers Omnistar mass spectrometer running in a multi-ion detection mode. The data were collected periodically as IR spectra were recorded. X-ray Absorption Spectroscopy. X-ray absorption spectroscopy (XAS) was carried out at beamline 4-1 at the Stanford Synchrotron Radiation Lightsource (SSRL). The storage ring energy and current were 3 GeV and 500 mA, respectively. A double-crystal Si(220) monochromator was detuned by 20% at the Rh K edge. Fluorescence XANES and EXAFS spectra were collected for the Rh-containing samples with a Lytle detector. In an argon-filled glovebox at SSRL, each 31 ACS Paragon Plus Environment

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rhodium-containing sample (400 mg) was packed into a once-through flow reactor. Spectra were collected in the presence of reactive atmospheres for real-time and in-situ observation. For calibration, measurement of the absorption of a rhodium foil, placed between the upbeam and downbeam ion chambers, was carried out simultaneously. EXAFS Data Analysis. Analysis of the EXAFS data was carried out with Athena of the software package Demeter and with the software XDAP. Athena was used for edge calibration and deglitching. XDAP was used for background removal, normalization, and conversion of the data into an EXAFS function file. Reference backscattering phase shifts were calculated from crystallographic data determined with the software FEFF7: the Rh–Os and Rh–Oaquo (Os and Oaquo denote oxygen atoms of zeolite framework and aquo ligands, respectively) contributions were calculated on the basis of the structural parameters of Rh(CO)2(acac);52 the Rh–Rh, Rh–Ct, Rh– Cb, Rh–Ot, and Rh–Ob (Ct and Cb denote carbon atoms of terminal and bridging carbonyl ligands, respectively; Ot and Ob denote oxygen atoms of terminal and bridging carbonyl ligands respectively) contributions were calculated for Rh4(CO)12.32 Multiple backscattering paths were considered in Rh–CO moieties. The coordination numbers characterizing the Rh–C contribution (the backscatterer is a carbon atom of a terminal or bridging CO ligand) and Rh–O (the 32 ACS Paragon Plus Environment

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backscatterer is an oxygen atom of a terminal or bridging CO ligand) were set to be the same in the fits. The number of parameters used in the fitting was always less than the statistically justified number, computed with the Nyquist theorem: n = 2ΔkΔr/π + 2 (where Δk and Δr, respectively, are the wave vector and distance in the real space ranges used in the fitting). Data fitting was based on an iterative process with a difference-file technique to determine a model comparing the overall fits and fits of individual shells as well. The model was chosen as the best-fitting model when the k1- and k3-weighted EXAFS data, Fourier-transform data, and Fouriertransform data of each shell contribution were all in best agreement with the calculated fits. Quality of fits was evaluated by the value of “goodness of fit”, defined below: NPTS     mod el,i  v exp,i Goodness of fit     NPTS(v-N free ) i1   exp,i 

2

where χexp and χmodel are the experimental and calculated EXAFS functions, respectively; σexp is the error in the experimental results; ν is the number of independent data points in the fit range; Nfree is the number of free parameters, and NPTS is the number of data points in the fit range. The estimation of the error bounds is based on the reported results and a statistical analysis;53 the values are approximate. Complementary information is presented in the SI. Our approach to


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data fitting, which is standard, involves selection of plausible structural models—models that make good chemical sense with physically and chemically realistic parameters and appropriate values of goodness of fit and consistency with other data, such as IR data—and then a comparison of the candidate models on the basis of the goodness of fit. DFT Calculations. The geometries of the Rhn(CO)m clusters were optimized at the DFT54 level with the local density approximation functional SVWN5.55-56 Second-derivative calculations were performed to predict zero point energies to obtain H(0 K), H(298 K) and G(298 K). Single-point energies were calculated using the optimized structures with the CAM-B3LYP,57 B97-D,58 ωB97X-D59 functionals and at the correlated molecular orbital theory MP260-61 level. For the mono- and di-rhodium species, single-point energy calculations were also performed using the CCSD(T)62-65 method. The open-shell calculations were performed at the restricted ROMP266 level and R/UCCSD(T)67-68 levels. The reactions on the model zeolite and silicon clusters were performed at the DFT level with the B3LYP functional69-70 as there are no Rh–CO bonds being broken. All calculations were done with the augmented correlation consistent double-ζ (aug-ccpVDZ) basis set71-73 for carbon and oxygen, and a relativistic pseudopotential with the aug-ccpVDZ-pp basis set for rhodium. The pseudopotential includes 28 electrons of Rh: 34 ACS Paragon Plus Environment

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1s22s22p63s23p63d10. The active electrons in the self-consistent field calculations include the remaining 17 electrons: 4s24p64d85s1. The 4s24p6 electrons on Ir and 1s2 electrons on C and O are in the core orbitals in the MP2 and CCSD(T) calculations. The DFT and the MP2 calculations were performed with the Gaussian16 suite of programs74 and the CCSD(T) calculations were done with the MOLPRO2015 suite of programs.75

CONCLUSIONS The experimental and theoretical results reported here demonstrate a high-yield synthesis of metal clusters—Rh4(CO)12—in the supercages of zeolite HY starting from structurally welldefined Rh(CO)2 complexes bonded to the zeolite. The process is reversible, and the full cycle of cluster formation and break-up is coupled with the water gas shift reaction, with proton-donating OH groups of the zeolite support playing a role as intermediates. Thus, the overall chemistry, depicted in Figure 6, is cyclic—catalytic. That is, the water gas shift catalysis is driven by repeated cycles of metal aggregation and redispersion. Our results also demonstrate that the selectivity of the synthesis of rhodium clusters—Rh4 vs. Rh6—depends on the population of bonding sites for rhodium in the zeolite. The observations suggest that the density of surface metal atoms is critical 35 ACS Paragon Plus Environment

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to the initial steps of metal aggregation (metal cluster formation)—sintering—as might occur to cause catalyst activation/deactivation.76

ASSOCIATED CONTENT

Supporting Information. Additional IR and mass spectrometry data and details of calculations. Details of the EXAFS data analysis.

AUTHOR INFORMATION Corresponding Author

*Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) Grant FG02-04ER15513 and by BES under the DOE BES Catalysis Center Program by a subcontract from Pacific Northwest National Laboratory (KC0301050-47319). We 36 ACS Paragon Plus Environment

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acknowledge beam time and support of beamline 4-1 at the Stanford Synchrotron Radiation Lightsource (SSRL), supported by DOE, Office of Science, BES, Contract DE-AC02-76SF00515.

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