Redox Chemistry of Tantalum Clusters on Silica Characterized by X-ray Absorption Spectroscopy

Nemana, Sailendra; Gates, Bruce C.

doi:10.1021/jp0629386

Cited by 15 publications

(27 citation statements)

References 29 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The darkened sample color due to mixed-valent Ta was also reported for a dysprosium tantalum oxynitride prepared by ammonia nitridation at relatively higher temperatures [8]. It was not possible to determine the Ta valence by means of XANES spectroscopy without ambiguity, because the positions of L III -edge are close to each other for Ta 4+ and Ta 5+ [22].…”

Section: +mentioning

confidence: 81%

Structural phase transition in the perovskite-type tantalum oxynitrides, Ca1−Eu Ta(O,N)3

Motohashi

Hamade

Masubuchi

et al. 2009

Materials Research Bulletin

View full text Add to dashboard Cite

Pure Ca 1-x Eu x Ta(O,N) 3 were successfully synthesized in the whole range of Ca/Eu compositions by means of ammonia nitridation via a citrate precursor route. As-nitrided products with x < 0.4 were apparently orthorhombic, while those with x ≥ 0.4 crystallized in a cubic structure. The anionic composition was found to be essentially O 2 N and independent of Eu content (x). The as-nitrided EuTaO 2 N possessed a cubic perovskite-type structure, while high-temperature post-annealing led to a tetragonal EuTaO 2 N phase with better crystallinity. The re-nitridation transformed the post-annealed product into the original cubic lattice, which might be an average of the tetragonal micro domains. *Corresponding

show abstract

Section: +mentioning

confidence: 81%

Structural phase transition in the perovskite-type tantalum oxynitrides, Ca1−Eu Ta(O,N)3

Motohashi

Hamade

Masubuchi

et al. 2009

Materials Research Bulletin

View full text Add to dashboard Cite

show abstract

“…Changes in metal-metal bond lengths such as that reported here are indicative of changes in the oxidation state of early transition metal clusters, with longer metal-metal bonds indicating more highly oxidized clusters [20,21,22]. Thus, we suggest that the elongation of the Ta-Ta bond from 2.93 to 3.10 Å resulting from contacting of the freshly treated samples with the reactants represents an oxidation of the clusters-which was accompanied by a near total loss of activity for methane conversion after 22 h. Because the samples demonstrated the highest activity when the tantalum clusters were in the most highly reduced state (represented by a Ta-Ta bond length of 2.93 Å ), we suggest that maintaining the clusters in a reduced state might mitigate the loss in catalytic activity.…”

Section: Changes In Catalyst During Operationmentioning

confidence: 54%

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

Nemana

Gates

2007

Catal Lett

Self Cite

View full text Add to dashboard Cite

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.

show abstract

“…Accord ing to EXAFS measurements, Ta-O bond lengths in monomer and dimer tantalum species are 187 pm and 195 pm, respectively [28,30]. The Ta-O bond lengths of the trimer species are related to the Ta-O coordina tion number (CN) and vary with the synthetic condi tions [29]. Ta-O bond lengths of 193 and 184 pm were found for clusters treated in a flow of He and Н 2 , respectively, and were assigned to the formation of Ta 3 species where each Ta atom has 3 and 2 short bonds to oxygen atoms.…”

Section: Introductionmentioning

confidence: 99%

“…Pioneering works on small clusters of tantalum on silica surfaces appeared in 2006 [28,29]. On the basis of IR, EXAFS and XANES data, the existence of sur face complexes of trinuclear [28,29] and later on binuclear tantalum clusters [30] was proposed and their structural parameters were determined.…”

Section: Introductionmentioning

confidence: 99%

“…On the basis of IR, EXAFS and XANES data, the existence of sur face complexes of trinuclear [28,29] and later on binuclear tantalum clusters [30] was proposed and their structural parameters were determined. Accord ing to EXAFS measurements, Ta-O bond lengths in monomer and dimer tantalum species are 187 pm and 195 pm, respectively [28,30].…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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

et al. 2015

View full text Add to dashboard Cite

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.

show abstract

Redox Chemistry of Tantalum Clusters on Silica Characterized by X-ray Absorption Spectroscopy

Cited by 15 publications

References 29 publications

Structural phase transition in the perovskite-type tantalum oxynitrides, Ca1−Eu Ta(O,N)3

Structural phase transition in the perovskite-type tantalum oxynitrides, Ca1−Eu Ta(O,N)3

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

Contact Info

Product

Resources

About