Reactions of neopentane and neohexane on platinum/Y-zeolite and platinum/silica catalysts

Föger, K.; Anderson, John R.

doi:10.1016/0021-9517(78)90080-5

Cited by 131 publications

(28 citation statements)

References 10 publications

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“…The results of such studies are sometimes contradictory, but it is generally accepted that many reactions are sensitive to the catalyst topography at the molecular level. Noticeable changes in either reaction rates or product selectivity have been reported for conversion of molecules like butane [4] and neopentane [5,6]. Similar changes have been seen by Anderson [7] and by Santacessaria [8] for n-hexane reactions over ultra-thin films and over platinum particles smallf"r than 10 A supported on silica.…”

Section: ''supporting

confidence: 69%

Direct determination of blocking of surface defects by carbonaceous deposits during hydrocarbon reactions on platinum single crystals

Zaera¹,

Somorjai²

1986

Langmuir

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A CO titration technique has been developed to quantitatively determine the presence of different kinds of catalytic sites on single-crystal surfaces. A direct correlation was found between platinum catalytic activity for conversion of hydrocarbons and the existence of bare defects on the metal surface. Isomerization and hydrogenolysis of either isopentane or butane are enhanced by the presence of steps or kinks on the catalyst surface. The same effect is not observed for methylcyclopentane or n-hexane conversion, where carbonaceous deposits cover the surface defects within a few minutes of starting the reactions.

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Section: ''supporting

confidence: 69%

Direct determination of blocking of surface defects by carbonaceous deposits during hydrocarbon reactions on platinum single crystals

Zaera¹,

Somorjai²

1986

Langmuir

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“…For example, in ethane hydrogenolysis, a very negative order of −2.5 in P H 2 correlates with a high E app (225 kJ/mol), and a moderately negative order of −1.5 leads to a lower E app (175 kJ/mol) [23]. The same results are found for the conversion of neopentane (2,2-dimethylbutane) to isopentane (isomerization) or isobutane and methane (hydrogenolysis) [24][25][26][27]. In addition to the correlation between the order in P H 2 and E app , the compensation relation is regularly observed for hydrogenolysis reactions [28][29][30].…”

Section: Introductionmentioning

confidence: 60%

“…Thus the reaction is inhibited by hydrogen adsorbed on the surface. Indeed negative orders from −1.5 to −2.5 have been reported in hydrogen pressure [24][25][26][27], implying that n d +n s as defined in this work can vary from 3 to 5. As θ H decreases to very small values, such as at high T or low hydrogen gas pressure, the incremental change in θ H is more important, but then 1 − θ H approaches one.…”

Section: Effect Of Support Acid/base Properties On Neopentane Coveragmentioning

confidence: 84%

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In situ X-ray absorption spectroscopy as a unique tool for obtaining information on hydrogen binding sites and electronic structure of supported Pt catalysts: towards an understanding of the compensation relation in alkane hydrogenolysis

Koningsberger

Oudenhuijzen

Graaf

et al. 2003

Journal of Catalysis

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L 2 and L 3 X-ray absorption near edge spectra (XANES) on supported Pt particles, with and without chemisorbed hydrogen, are shown to reflect the type of hydrogen-binding site on the Pt surface. FEFF8 ab initio multiple scattering calculations are used to determine XANES spectral fingerprints for the atop vs threefold H binding sites on Pt. Comparison of the experimental XANES data with the theoretical fingerprints, and further theoretical results, show that the acid/base properties of the support have a profound influence on the hydrogen coverage, and therefore on the mode of hydrogen adsorption on the Pt surface. As the electron richness of the support oxygen atoms increases (i.e., with increasing alkalinity of the support), the H coverage increases and the hydrogen-binding site of the strongly adsorbed hydrogen changes from atop to threefold. This site change is primarily responsible for the observed changes in previously reported kinetic data, which show an increase in negative order (roughly from −1.5 to −2.5) in hydrogen partial pressure for neopentane hydrogenolysis with increasing support alkalinity. This change in negative order directly reflects the greater number of vacant Pt sites that must be available to allow adsorption of the neopentane. A compensation relation is found in the kinetic data of Pt on different supports resulting directly from this change in hydrogen coverage. This implies that the experimentally determined kinetic parameters are apparent values. These apparent values are correlated to the intrinsic kinetic parameters via the thermodynamic properties of the sorption of the reactants, described by the Temkin equation. The TOF of neopentane hydrogenolysis over several catalysts, measured in previous work, decreases with the increasing alkalinity of the support. This can now be directly explained as the result of the change in hydrogen coverage using a Frumkin isotherm, implying that the neopentane adsorption becomes weaker with increased hydrogen coverage. These conclusions, that hydrogen drives the catalysis, are further supported by density functional calculations on small Pt 4 clusters, which show that the acid/base properties of the support have a much larger direct influence on Pt-H bonding than on Pt-CH n bonding.

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“…In order to determine the effect of polyvalent cations on the TOF, it is necessary to compare catalysts with an equivalent number of protons since the TOF is dependent on the number of protons. Previously, Ca and La ions were reported to increase the TOF of Pt in Y [46]. It is well known, however, that La ion exchange also leads to the introduction of protons [47,48].…”

Section: Catalysismentioning

confidence: 99%

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

Koningsberger

Graaf

Mojet

et al. 2000

Applied Catalysis A: General

107

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The turnover frequency (TOF) for conversion of neo-pentane was determined for Pt in Y zeolite with different numbers of protons and La +3 ions, different Si/Al ratios and with non-framework Al being present. Comparing Pt/NaY to Pt/H-NaY and Pt/K-USY with Pt/H-USY, respectively, shows an increase in the ln (TOF) which is proportional to the number of protons. Compared to NaY, the TOF of Pt in non-acidic NaLaY zeolite is about 25 times higher, which indicates also a strong influence on the charge of the cations on the TOF of Pt. The 20 times increase in the Pt TOF for K-USY compared to NaY is attributed to the effect of a higher Si/Al ratio and non-framework Al in the K-USY.EXAFS data collected on Pt/NaY and Pt/H-USY showed platinum particles consisting of 14-20 atoms on an average. These results were confirmed by HRTEM, which also showed that the Pt particles were dispersed inside the zeolite. The EXAFS data indicate that the metal particles are in contact with the oxygen ions of the support. The peak in the Fourier transform of the atomic XAFS (AXAFS) spectrum of the Pt/H-USY is larger in intensity than the corresponding peak of the Pt/Na-Y data. A detailed analysis of the L 2 and the L 3 X-ray absorption near edge structure revealed a shape resonance due to the Pt-H anti-bonding state (AS) induced by chemisorption of hydrogen on the surface of the platinum metal particles. The difference in energy (E res ) between the AS and the Fermi-level (E F ) is 4.7 eV larger for Pt/H-USY than for Pt/NaY. Both the AXAFS spectra and the shape resonances of the Pt-NaY and the Pt/H-USY catalysts provide direct experimental evidence of how the support properties determine the electronic structure of the platinum metal particles.Previous AXAFS and shape resonance work lead to a model in which the position in energy of the Pt valence orbitals is directly influenced by changes in the potential (i.e. electron charge) of the oxygen ions of the support and how the proton density affects this oxygen charge. This work shows that the potential of the oxygen ions is also a function of the Si/Al ratio of the support and the polarisation power of the charge compensating cations (H + , Na + , La 3+ and extra-framework Al); the metal particles experience an interaction which is determined by several properties of the support. The data further reveal how the change in the Pt electronic structure directly influences the catalytic properties of the catalyst.While the TOF is dependent on the metal-support interaction, the hydrogenolysis selectivity is determined by the Pt particle size, and increases linearly with increasing dispersion, or decreasing particle size.

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Reactions of neopentane and neohexane on platinum/Y-zeolite and platinum/silica catalysts

Cited by 131 publications

References 10 publications

Direct determination of blocking of surface defects by carbonaceous deposits during hydrocarbon reactions on platinum single crystals

Direct determination of blocking of surface defects by carbonaceous deposits during hydrocarbon reactions on platinum single crystals

In situ X-ray absorption spectroscopy as a unique tool for obtaining information on hydrogen binding sites and electronic structure of supported Pt catalysts: towards an understanding of the compensation relation in alkane hydrogenolysis

The metal–support interaction in Pt/Y zeolite: evidence for a shift in energy of metal d-valence orbitals by Pt–H shape resonance and atomic XAFS spectroscopy

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