Transformation of reduced graphene aerogel-supported atomically dispersed iridium into stable clusters approximated as Ir6 during ethylene hydrogenation catalysis
Graphical abstract
Introduction
Atomically dispersed supported metal catalysts provide opportunities for the most efficient use of expensive metals while offering unprecedented properties [1], [2], [3], [4], [5]. Related to these catalysts are those incorporating clusters consisting of only a few metal atoms [6], which offer the advantages of neighboring metal centers (e.g., for activating H2) [7], [8] while exposing essentially all the metal atoms to reactants [9], [10], [11], [12], [13], [14], [15]. Supported metal clusters typically differ from isolated dispersed metals not just in metal nuclearity, but also in metal oxidation state and coordination to the support. A variety of comparisons showing the catalytic superiority of supported clusters over atomically dispersed metals are summarized in Table S1 in the Supplementary Material (SM).
The properties of metal clusters are typically sensitive to the metal nuclearity, and so there are strong incentives to synthesize and stabilize supported clusters with specific metal nuclearities. However, controlled synthesis of uniform, well-defined metal clusters on high-surface-area supports is challenging, especially when the metal loadings are high [16] and the support is non-crystalline and lacks 3D nanoporous structures for compartmentalizing the clusters [17].
Such metal clusters have been made by adsorption of organometallic precursors with desired metal frames or by treatment of atomically dispersed metals on supports under controlled conditions [18]. Clusters made from organometallic precursors typically retain some of their original ligands that stabilize the metal framework—but these are reaction inhibitors and may need to be removed for catalysis. Removal of the ligands is usually accompanied by changes in the metal nuclearity and possibly the metal oxidation state, especially when the metal loadings are high. Further changes in metal nuclearity commonly result from aggregation of the metal into nanoparticles, especially under reducing conditions and when metal loadings are high [19], [20], [21], [22], [23], [24]. For example, Imaoka et al. [24] prepared carbon-supported platinum clusters having nuclearities between about 5 and 13 with a loading of 0.4 wt%; octanethiol ligands on the adsorbed platinum precursor were removed in H2 at 250 °C, leading to the formation of platinum clusters with almost unchanged nuclearities. But when the platinum loading was increased to 1.8 wt%, the treatment led to the formation of platinum nanoparticles approximately 2 nm in average diameter [24], illustrating the generally important point that both the initial surface density of the precursor metal complex and the reactive atmosphere influence the metal aggregation. This inference is also consistent with the work of Gai et al. [25], demonstrating that the final nuclearity of the gold clusters formed from the site-isolated Au centers during the catalytic water gas shift reaction depends strongly on the Au loading on the initial sample and the reaction conditions. In part for these reasons, almost all the reported supported metal catalysts with high dispersions have metal loadings less than about 1 wt% (Table S2 in the SM).
The support type plays a crucial role in determining the maximum metal loading that can be achieved on a support surface with control of the metal nuclearity. In this regard, carbon-based supports are highly promising because they offer high specific surface areas, high porosities, easily tailorable surface chemistries (including chemical inertness), and low costs relative to metal oxides [26], [27]. In particular, graphene, which is a two-dimensional sp2-hybridized carbon network with excellent electrical and thermal conductivity and high mechanical strength, has drawn significant interest [28]. Graphene aerogels (GAs) are an even more attractive class of carbon materials, as they combine some of the properties of graphene with those of aerogels, such as low density and high porosity [29], [30]. These properties can be tuned by simple treatments. For instance, reduced graphene aerogels (rGAs) obtained by treating GAs in N2 [31], [32], [33], [34], He [33], Ar [35], [36], [37], [38], [39], H2 [40] or under vacuum [41] at elevated temperatures were shown to have improved thermal stability, specific surface area, Young's modulus, resistance to air oxidation, and electrical conductivity [33], [34], [38], [39], [41], [42]. Besides, such simple treatments provide opportunities to control the surface density of oxygen-containing functional groups offering routes to tune the catalytic properties. In this regard, we recently demonstrated that rGA is a promising high-surface-area support that can strongly bond single noble metal atoms at high loadings through its oxygen-containing sites. rGA’s high specific surface areas, exceeding 700 m2/g, and the presence of oxygen sites at high surface densities made it possible to maintain the atomic dispersion of IrI(CO)2+ complexes at an Ir loading of 14.8 wt% [36]. However, the unreactive CO ligands are inhibitors that significantly hindered the catalytic ethylene hydrogenation. Moreover, because of these strongly bound ligands, high treatment temperatures (up to 400 °C [18]) were needed to controllably form iridium clusters. In prospect, it would be advantageous to prepare supported metals with ligands having higher reactivities than CO to facilitate precise cluster formation.
We have now investigated rGA-supported iridium complexes with reactive ethylene ligands and their transformation into iridium clusters, with the goal of preparing supported metal clusters with high loadings and uniform nuclearity. We chose a metal precursor with reactive ethylene ligands, Ir(C2H4)2(acac) (acac is acetylacetonato) to meet this goal.
X-ray absorption spectra show how clusters approximated as Ir4 and Ir6 form from an adsorbed mononuclear Ir complex obtained by the reaction of Ir(C2H4)2(acac) with the rGA surface and remain stable under conditions of ethylene hydrogenation catalysis in a stoichiometric excess of H2 at 100 °C and even in pure H2 at this temperature.
Section snippets
Synthesis of rGA
A modified Hummer’s method was used to synthesize graphite oxide powder [43]. In brief, NaNO3 (Alfa Aesar) was dissolved in highly concentrated H2SO4 (Sigma-Aldrich). Then the sample was oxidized to form graphite oxide by adding graphite powder (Alfa Aesar) and KMnO4 (Merck Millipore). Distilled water was added to the solution slowly to increase the temperature to 75 °C. After 30 min, more distilled water was introduced, followed by introduction of H2O2 to terminate the synthesis reaction. The
Characterization of rGA support
Characterization of rGA, prepared by reducing GA in Ar at 700 °C, was performed to reveal the physical and chemical properties of the support. The BET surface area, average pore diameter, and pore volume of rGA were determined to be 786.7 m2/g, 7.8 nm, and 1.02 cm3/g, respectively (Table S3).
The XP C1s spectrum of rGA (Fig. 1a) was deconvoluted, giving four peaks, with binding energies of 284.3, 285.8, 287.7, and 290.7 eV, assigned to C−C/C=C/C−H, C−O (hydroxyl or epoxy), C=O (carbonyl or
Conclusions
rGA-supported IrI(C2H4)2+ complexes were synthesized as the precursors of iridium cluster catalysts for ethylene hydrogenation. They underwent structural changes leading to the formation of supported clusters well approximated as Ir4 as the temperature was raised from room temperature to 100 °C with the sample in equimolar H2:C2H4. Doubling of the H2 concentration in the feed at the same temperature led to another change, forming clusters well approximated as Ir6. These changes were tracked
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Sefa Öztulum from Koç University Boron and Advanced Materials Application and Research Center (KUBAM) for the XRD measurements; Dr. M. Barış Yağcı from Koç University Surface Science and Technology Center (KUYTAM) for XPS measurements; Gülcan Çorapçıoğlu for the STEM imaging; and F. Eylül Saraç-Öztuna for fruitful discussions. The work conducted at Koç University was supported by the Scientific and Technological Research Council of Turkey (TUBITAK) (217M547) and Koç University TÜPRAŞ
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