Silica-supported tantalum clusters: catalyst for alkane conversion

Nemana, Sailendra; Gates, Bruce C.

doi:10.1039/b608794a

Cited by 8 publications

(10 citation statements)

References 10 publications

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“…Consistent with these literature reports [17,18,19], our product distribution data can be explained approximately by the disproportionation of n-butane accompanied by the reaction of methane with other alkanes. To account approximately for our observed products other than propane and pentane, we consider the reaction of methane to be primarily with propane and n-butane.…”

Section: Catalyst Performancesupporting

confidence: 91%

“…On the basis of EXAFS data, the authors concluded that the supported tantalum catalyst was a mononuclear complex anchored to the support, in contrast to our clusters. We also reported evidence of catalysis of propane disproportionation and ethane disproportionation by our SiO 2 -supported tantalum clusters [18], We infer that disproportionation of n-butane occurred similarly.…”

Section: Catalyst Performancementioning

confidence: 72%

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Silica-supported tantalum clusters: catalysts for conversion of methane with n-butane to give ethane, propane, and pentanes

Nemana

Gates

2007

Catal Lett

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Si0 2 -supported tantalum clusters were prepared by adsorption of the precursor Ta(CH 2 Ph) 5 (Ph is phenyl) on the support followed by treatments in H 2 at 523, 623, and 723 K. The resultant clusters, had approximate average diameters of 0.3, 0.8, and 2 nm, as determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. The samples were tested as catalysts for conversion of methane with n-butane in a once-through flow reactor operated at atmospheric pressure and 523 K, and EXAFS spectroscopy was used to characterize the used catalysts. The results show that (a) the catalysts are active for the conversion of methane with n-butane to give ethane, propane, and pentanes; (b) catalytic activity decreased to nearly zero over a time on stream of 22 h; (c) the catalyst incorporating the smallest clusters exhibited the highest initial activity and that incorporating the largest clusters exhibited the lowest activity; (d) each used catalyst contained clusters of approximately the same nuclearity as the respective fresh catalyst, but with Ta-Ta bond lengths approximately 0.17 Å longer than those found in the fresh catalysts. The data are consistent with catalysis by the supported clusters, and the product distributions are consistent with disproportionation of n-butane accompanied by the reaction of methane with propane to give other alkanes.

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Section: Catalyst Performancesupporting

confidence: 91%

Section: Catalyst Performancementioning

confidence: 72%

Silica-supported tantalum clusters: catalysts for conversion of methane with n-butane to give ethane, propane, and pentanes

Nemana

Gates

2007

Catal Lett

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“…Active cen ters of these catalysts are usually formed by the reac tion of a metallic salt and an organometalic compound with surface OH groups and low coordinated oxygen atoms on the surface of the support [1,3,4]. Repre sentative examples of such systems are TM species ultra dispersed as cations on high area silica surfaces [5], also in the form of hydrides [6][7][8][9][10][11], small clusters [12,13], and nanoparticles [14]. Very often these spe cies are anchored to the surface to form oxo and oxo hydroxo complexes of early transition metals [4,[15][16][17][18][19][20][21].…”

Section: Introductionmentioning

confidence: 99%

Trinuclear tantalum clusters grafted to hydroxylated silica surfaces: A density-functional embedded-cluster study

et al. 2015

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To identify the coordination modes of bare and hydrogenated trinuclear tantalum species on hydroxylated silica, we computationally examined models of Ta 3 H n (n = 0, 3, 5-9) species grafted to a β cris tobalite surface. Ta 3 H n clusters are bound to the surface by substitution of hydrogen atoms of vicinal (…O ) 3 SiOH and geminal (…O ) 2 Si(OH) 2 groups via three and six, respectively, Ta-O bonds of ~193 pm on average, in both types of models. The maximum Ta-O coordination number of non hydrogenated Ta 3 spe cies to a silica surface is seven for the second type model surface; the additional Ta-O bond is due to an oxy gen atom located in a bridging position to Ta-Ta bond. In the latter case, the mean Ta-O bond distance to one of =Si(O-) 2 group is increased by 15 pm. For the complexes bound via vicinal silanol groups, each addi tional unit of hydrogen loading on the metal elongated the average Ta-Ta distance by ~2 pm, covering a range of 258-277 pm. For the most stable hydrogenated trimers, Ta 3 H 9 , the desorption energies of hydrogen atoms are relatively high, above 70 kJ/mol. The average Ta-Ta distances increase by ~19 pm on going from the com plex (≡SiO-) 3 Ta 3 H 9 to complex (≡SiO-) 3 Ta 3 and by ~5 pm when the hydrogen loading is increased by one unit for (=Si(O-) 2 ) 3 Ta 3 H n complexes, reaching the maximum value 319 pm when n = 9. The desorption energies of hydrogen atoms for the most stable tantalum trimer species grafted to the surface by geminal sil anol groups, (=Si(O-) 2 ) 3 Ta 3 H 7 , are rather low, less than 40 kJ/mol.

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“…So far, metal clusters on support have been restricted almost entirely to group 8 metals but recently supported early transition metal catalysts exemplified by tantalum clusters on SiO 2 have been reported [3]. Catalyst performance is sensitive to cluster size, because the surface structure, electronic properties and cluster-support interactions depend on this size.…”

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confidence: 99%

Z-Contrast Tomography of Nanoscale Heterogeneous Catalysts

Mehraeen

Okamoto

Morgan

et al. 2007

MAM

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Most industrial catalysts are high-surface-area solids such as amorphous or crystalline oxides, onto which an active component (often clusters of a metal, metal oxide, or metal sulfide, often including high-Z contrast elements) is dispersed in the form of very small clusters or particles [1,2]. So far, metal clusters on support have been restricted almost entirely to group 8 metals but recently supported early transition metal catalysts exemplified by tantalum clusters on SiO 2 have been reported [3]. Catalyst performance is sensitive to cluster size, because the surface structure, electronic properties and cluster-support interactions depend on this size. The location of metal particles and their orientation with respect to the support material are also important in determining catalytic properties. For example, partial coverage of the metal by an amorphous oxide can influence the catalytic activity and selectivity [2].In this investigation, a silica-supported tantalum catalyst was investigated to determine the detailed 3D morphology of the nanoparticles, the degree of encapsulation of the metal cluster by the support, the location of the clusters on the support material, and the size and distribution of the clusters. For this purpose, tomography based on Scanning Transmission Electron Microscopy (STEM) was performed on a 200kV JEOL 2500SE TEM/STEM microscope. Because of the sensitivity of the image to the atomic number, the Z-contrast technique of STEM provides images of the nanoclusters on the oxide support with a high resolution (figure 1). To provide a 3D reconstructed volume, a tilt range of images from -70° to +70° with a 2° increment processed and analyzed with the Composer and Visualizer software package (JEOL Ltd.) [4].Previously reported results characterizing the sample by extended X-ray absorption fine structure (EXAFS) spectroscopy showed an average Ta-Ta coordination number of 4.8 for the clusters that had been treated only in inert atmospheres, with an average Ta-Ta distance corresponding to a chemical bond between the Ta atoms. Accordingly, a preliminary model of the tantalum clusters was suggested to be a single layer (raft) of approximately 40 Ta atoms.However, when the sample was treated in air and characterized by STEM, the tomography results showed that the shapes of the clusters tended to be almost spherical rather than raft-like. Tomography reconstruction results shown in figures 2A and B also show that the nanoclusters are not evenly distributed within the SiO 2 support. Thus, being partially encapsulated, the tantalum nanoparticles would not be expected to show the same catalytic activity as such clusters positioned on the support [5].

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Silica-supported tantalum clusters: catalyst for alkane conversion

Cited by 8 publications

References 10 publications

Silica-supported tantalum clusters: catalysts for conversion of methane with n-butane to give ethane, propane, and pentanes

Silica-supported tantalum clusters: catalysts for conversion of methane with n-butane to give ethane, propane, and pentanes

Trinuclear tantalum clusters grafted to hydroxylated silica surfaces: A density-functional embedded-cluster study

Z-Contrast Tomography of Nanoscale Heterogeneous Catalysts

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