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

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 = acetylacetonato) with OH groups on the zeolite surface were converted in >95% yield to Rh 4 (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 CO 2 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 Rh 4 (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 was 30). The results provide insights into the first steps of sintering of atomically dispersed metals on supports.

Synthesis of Rh6(CO)16 in Supercages of Zeolite HY: Reaction Network and Kinetics of Formation from Mononuclear Rhodium Precursors via Rh4(CO)12 Facilitated by the Water Gas Shift Half-Reaction

“…The IR spectrum had stopped changing by the end of the 8-h treatment. Consistent with reported results, 10 the IR spectra show that the major cluster product was Rh4(CO)12 (96%), with the thermodynamically more stable Rh6(CO)16 (4%) appearing as a minor product. (Figure 1) were gradually replaced by bands at 2117 and 2053 cm -1 characterizing Rh(I)(CO)2 and intense bands at 2080 and 1818 cm -1 and a weak band at 2044 cm -1 characterizing Rh6(CO)16.…”

“…The data presented in Figure S4 in the SI show that Rh6(CO)16 in zeolite HY, when exposed to dry helium at 180 C, was fully converted to anchored Rh(I)(CO)2, with the formation of effluent gas-phase H2 and CO detected by mass spectrometry. This process is evidently analogous to the chemistry observed previously 10 This observation is contrasted with ours, which show that Rh6(CO)16 clusters were completely fragmented in zeolite HY under similar conditions. We attribute the difference to the surface chemistry associated with the acidic OH groups in zeolite HY.…”

“…Tetra-and hexa-rhodium clusters can be synthesized from Rh(I)(CO)2 in the supercages of zeolite HY under conditions of the water gas shift reaction, 10 and we have reproduced the reported results as zeolite-HY-supported Rh(I)(CO)2 was exposed to flowing CO saturated with water vapor at 35 ºC for 8 h, leading to the appearance of IR peaks at 2075, 2044, and 1885 cm -1 that characterize the terminal and bridging C-O vibrations of Rh4(CO)12, along with a weak band centered at 1818 cm -1 , which characterizes the bridging C-O vibration of Rh6(CO)16 ( Figure S3 in the SI). The IR spectrum had stopped changing by the end of the 8-h treatment.…”

“…A low-silica zeolite HY (Si:Al atomic ratio = 2.6), which offers a high density of surface bonding sites (OH groups at Al sites), was chosen to facilitate the formation of Rh6(CO)16. 10 The precursor was made by chemisorption of Rh(I)(CO)2(acac) on the zeolite, as reported, 14…”

Atomically Dispersed Iridium on MgO(111) Nanosheets Catalyze Benzene-Ethylene Coupling towards Styrene

“…and amorphous structures (hydrated layers, amorphous supports etc. ), [15][16][17] (2) the complexity of surface chemistry using conventional preparation methods, where dissolution/precipitation events take place upon deposition of the active centers, (3) dynamic behavior of single atoms [18][19][20][21] and (4) the lack of available characterization techniques to probe the atoms (mostly C/N/O) directly bonded to the single atoms. [22][23][24] Overall, although atomically dispersed catalysts are usually referred to as analogs of well-defined homogenous catalysts, 25,26 the interaction between ligands (supports) and the metal centers are far less understood by comparison with what can be achieved in coordination and organometallic chemistry.…”