Agglomerative Sintering of an Atomically Dispersed Ir-1/Zeolite Y Catalyst: Compelling Evidence Against Ostwald Ripening but for Bimolecular and Autocatalytic Agglomeration Catalyst Sintering Steps

2015-06-01
Bayram, Ercan
Lu, Jing
Aydin, Ceren
Browning, Nigel D.
Özkar, Saim
Finney, Eric
Gates, Bruce C.
Finke, Richard G.
Agglomerative sintering of an atomically dispersed, zeolite Y-supported catalyst, Ir-1/zeolite Y, formed initially from the well-characterized precatalyst [Ir(C2H4)(2)]/zeolite Y and in the presence of liquid-phase reactants, was monitored over three cycles of 3800 turnovers (TTOs) of cyclohexene hydrogenation at 72 degrees C. The catalyst evolved and sintered during each cycle, even at the relatively mild temperature of 72 degrees C in the presence of the cyclohexene plus H-2 reactants and cyclohexane solvent. Post each of the three cycles of catalysis, the resultant sintered catalyst was characterized by extended X-ray absorption fine structure spectroscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. The results show that higher-nuclearity iridium species, Ir-n, are formed during each successive cycle. The progression from the starting mononuclear precursor, Ir-1, is first to Ir-similar to 46; then, on average, Ir-similar to 40; and finally, on average, Ir-similar to 70, the latter more accurately described as a bimodal dispersion of on-average Ir similar to 40-50 and on-average Ir-similar to 1600 nanoparticles. The size distribution and other data disprove Ostwald ripening during the initial and final stages of the observed catalyst sintering. Instead, the diameter-dispersion data plus quantitative fits to the cluster or nanoparticle diameter vs time data provide compelling evidence for the underlying, pseudoelementary steps of bimolecular agglomeration, B + B -> C, and autocatalytic agglomeration, B + C -> 1.5C, where B represents the smaller, formally Ir(0) nanoparticles, and C is the larger (more highly agglomerated) nanoparticles (and where the 1.5 coefficient in the autocatalytic agglomeration of B + C necessarily follows from the definition, in the bimolecular agglomeration step, that 1C contains the Ir from 2B). These two specific, balanced chemical reactions are of considerable significance in going beyond the present state-of-the-art, but word-only, mechanismthat is, actually and instead, just a collection of phenomenafor catalyst sintering of Particle Migration and Coalescence. The steps of bimolecular plus autocatalytic agglomeration provide two specific, balanced chemical equations useful for fitting sintering kinetics data, as is done herein, thereby quantitatively testing proposed sintering mechanisms. These two pseudoelementary reactions also define the specific words and concepts for sintering of bimolecular agglomeration and autocatalytic agglomeration. The results are also significant as the first quantitative investigation of the agglomeration and sintering of an initially atomically dispersed metal on a structurally well-defined (zeolite) support and in the presence of liquid reactants (cyclohexene substrate and cyclohexane solvent) plus H-2. A list of additional specific conclusions is provided in a summary section.
ACS CATALYSIS

Suggestions

Ethylene and diethyl-ether production by dehydration reaction of ethanol over different heteropolyacid catalysts
Varisli, Dilek; Doğu, Timur; Dogu, Gulsen (2007-09-01)
Dehydration reaction of ethanol was investigated in a temperature range of 140-250 degrees C with three different heteropolyacid catalysts, namely tungstophosphoricacid (TPA), silicotungsticacid (STA) and molybdophosphoricacid (NIPA). Very high ethylene yields over 0.75 obtained at 250 degrees C with TPA was highly promising. At temperatures lower than 180 degrees C the main product was diethyl-ether. Presence of water vapor was shown to cause some decrease of catalyst activity. Results showing that product...
Syntheses of conducting polymers of 3-ester substituted thiophenes and characterization of their electrochromic properties
Çamurlu, Pınar; Toppare, Levent Kamil; Department of Chemistry (2006)
In this study three different 3-ester substituted thiophene monomers were synthesized via esterification reaction of 3-thiophene ethanol with adipoyl chloride or sebacoyl chloride or octanoyl chloride in the presence of triethylamine at 00C. Characterizations of the monomers were performed by 1H-NMR, 13C-NMR, FTIR, DSC, TGA techniques. Electrochemical behavior of the monomers both in presence or absence of BFEE were studied by cyclic voltammetry. Results showed the astonishing effect of BFEE on the polymeri...
Dry reforming of methane over CeO2 supported Ni, Co and Ni-Co catalysts
Ay, Hale; Üner, Deniz (2015-12-01)
Ceria supported Ni, Co monometallic and Ni-Co bimetallic catalysts were prepared by incipient wetness impregnation method, calcined at two different temperatures (700 degrees C and 900 degrees C) and tested for dry reforming of methane reaction at 700 degrees C. The activities of ceria-based Ni containing catalysts decreased with increasing calcination temperature accompanied by a decrease in coke deposition. While Ni/CeO2 and Ni-Co/CeO2 catalysts exhibited comparable high activities, Co/CeO2 catalysts exhi...
Thermal catalytic hydrosilylation of conjugated dienes with triethylsilane in the presence of tricarbonyl(o-xylene)metal (metal = Cr, Mo, W) complexes
Kayran, C; Rouzi, P (Walter de Gruyter GmbH, 2001-11-01)
The thermal catalytic hydrosilylation of 1,3-butadiene (1), trans-2-methyl-1, 3-pentadiene (2), 2,3-dimethyl-1,3-butadiene (3), and isoprene (4), with triethylsilane were studied in the presence of M(CO)(3) (o-xylene) (M = Cr, Mo, W) complexes in polar and nonpolar solvents such as tetrahydrofuran, hexane and toluene. Mo(CO)(3) (o-xylene) was found to be the only active catalyst for the hydrosilylation of 3 with triethylsilane, which gave 1-triethylsilyl-2,3-dimethyl-2-butene (3a), as hydrosilylated product...
Raman Spectroscopy Investigation of Nano Hydroxyapatite Doped with Yttrium and Fluoride Ions
Yilmaz, Bengi; Evis, Zafer (Informa UK Limited, 2014-01-02)
In this study, nano hydroxyapatite doped with yttrium (2.5, 5, and 7.5mol%) and fluoride (2.5mol%) ions were synthesized by precipitation method and sintered at 900 degrees C, 1100 degrees C, and 1300 degrees C. Raman spectroscopy was applied to track the structural modifications in pure and doped hydroxyapatites. The results showed that the main characteristic band of pure hydroxyapatite at 963cm(-1) was not affected significantly by ion doping but exhibited higher intensity with increasing sintering tempe...
Citation Formats
E. Bayram et al., “Agglomerative Sintering of an Atomically Dispersed Ir-1/Zeolite Y Catalyst: Compelling Evidence Against Ostwald Ripening but for Bimolecular and Autocatalytic Agglomeration Catalyst Sintering Steps,” ACS CATALYSIS, pp. 3514–3527, 2015, Accessed: 00, 2020. [Online]. Available: https://hdl.handle.net/11511/37528.