Ion-dipole interactions between adsorbed CO and support cations in Pt/K-LTL

Kappers, M. J.; Vaarkamp, Marius; Miller, J.T.; Modica, FS; Barr, M.K.; Koningsberger, DC Diek; Maas, JH van der

doi:10.1007/bf00769475

Cited by 26 publications

(31 citation statements)

References 37 publications

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“…The proposed coordination of water molecules with the K + ions in the zeolite pores has a dramatic effect on the CO spectrum. 25 As the amount of water in the zeolite increases, the broad low-frequency band (LFB) between 2000 and 1860 cm -1 in the spectrum of KL-dry ( Figure 1a) shifts to higher frequency with a more structured appearance. The high-frequency band (HFB) in the spectrum of KL-wet (Figure 1c) shows distinct maxima at 2049, 2031, and 2010 cm -1 and shoulders at 2065 and 1990 cm -1 .…”

Section: Resultsmentioning

confidence: 99%

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Deconvolution and Curve Fitting of IR Spectra for CO Adsorbed on Pt/K-LTL: Potassium Promoter Effect and Adsorption Site Distribution

1996

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Diffuse reflectance infrared (DRIFT) spectra have been obtained for carbon monoxide adsorbed with and without H 2 O on Pt/K-LTL zeolite. The complex spectra of CO linearly adsorbed on highly dispersed Pt have been analyzed by the combination of Fourier self-deconvolution and curve fitting. The linear-CO spectrum is composed of two broad multiplets: a high-frequency band (HFB) at 2100-1980 cm -1 and a low-frequency band (LFB) at 1990-1860 cm -1 . The relative intensities of the HFB and LFB depend on the amount of coadsorbed water in the zeolite pores. The LFB is associated with an ion-dipole interaction between the oxygen atoms of CO species with potassium cations from the zeolite support. The HFB is associated with linear-CO unperturbed by the alkali promoter effect. It is proposed that water preferentially adsorbs on the K + ions, shielding the ion-dipole interaction. The individual components of the complex HFB and LFB are proposed to be due to CO molecules adsorbed on platinum atoms with different coordination numbers. The average coordination number of the Pt atoms in K-LTL, determined in the applied deconvolution and curvefitting program, corresponds well with that obtained by EXAFS, indicating a metal particle size distribution with particles containing between 1 and 13 platinum atoms. By applying the newly developed method, several literature IR CO spectra of Pt/K-LTL with different relative intensities in the HFB region have been analyzed successfully by adjusting the particle size distribution only. In view of the new results showing the prominent effects of particle size distribution and potassium promotion on the IR spectra of adsorbed CO, previously reported conclusions concerning metal-support interactions in PT/K-LTL should be reconsidered when based on infrared spectroscopic data.

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Section: Resultsmentioning

confidence: 99%

“…In a previous IR study on the adsorption of CO on Pt/K-LTL, 25 it was noted that the CO feed used was contaminated with water. In this study we have used this knowledge to obtain CO spectra with different concentration of coadsorbed water in the zeolite.…”

Section: Methodsmentioning

confidence: 99%

Deconvolution and Curve Fitting of IR Spectra for CO Adsorbed on Pt/K-LTL: Potassium Promoter Effect and Adsorption Site Distribution

1996

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“…Subsequently at room temperature, the sample was purged with He for 10 min, followed by flowing 20% CO in He gas at atmospheric pressure for 10 min, after which the CO absorbance spectrum was collected at room temperature. The spectra for Pd/LTL and Pt/LTL were collected under wet conditions to prevent the ion-dipole interaction between zeolite cations and adsorbed CO (20). The spectra were corrected for the infrared adsorption of the specific support material and gas-phase CO.…”

Section: Infrared Spectroscopymentioning

confidence: 99%

A New Model Describing the Metal–Support Interaction in Noble Metal Catalysts

Mojet

Miller²,

Ramaker

et al. 1999

Journal of Catalysis

216

191

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The catalytic activity and spectroscopic properties of supported noble metal catalysts are strongly influenced by the acidity/ alkalinity of the support but are relatively independent of the metal (Pd or Pt) or the type of support (zeolite LTL or SiO 2 ). As the alkalinity of the support increases, the TOF of the metal particles for neopentane hydrogenolysis decreases. At the same time, there is a decrease in the XPS binding energy and a shift from linear to bridge bonded CO in the IR spectra. Analysis of the shape resonance in XANES spectra indicates that in the presence of chemisorbed hydrogen the difference in energy between the Pt-H antibonding orbital and the Fermi level decreases as the alkalinity of the support increases. Based on the results from the IR, XPS, and shape resonance data a new model is proposed in which the interaction between the metal and support leads to a shift in the energy of the metal valence orbitals. The EXAFS structural analysis indicates that the small metal particles are in contact only with the oxide ions of the support. Finally, a new spectroscopic characterisation, Atomic XAFS, is presented which provides new insights into the origin of the electronic changes in the metal. As the alkalinity of the support increases, there is decrease in the metal ionisation potential. The primary interaction is a Coulomb attraction between metal particle and support oxygen ions, which affects the metal interatomic potential. This model for the metal-support interaction explicitly excludes the need for electron transfer, and it can account for all observed changes in the catalytic, electronic, and structural properties of the supported metal particles induced by support acidity ranging from acidic to neutral to alkaline.

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“…Previously, the effect of water adsorption on the IR spectrum of FWK-LTL was reported. 35 Interaction of the terminal 0 of CO with cations in the zeolite channels shifts the frequency of the CO stretch vibration to lower wavenumbers by as much as 60 cm-1. 35 Upon adsorption of water the CO stretch vibration shifted to higher wavenumbers.…”

Section: Pt-pt Distance (277 A) a Contraction Of The Metal-metalmentioning

confidence: 99%

“…35 Interaction of the terminal 0 of CO with cations in the zeolite channels shifts the frequency of the CO stretch vibration to lower wavenumbers by as much as 60 cm-1. 35 Upon adsorption of water the CO stretch vibration shifted to higher wavenumbers. In contrast, Figure 11 shows that with the acidic zeolite used here a high-frequency band is observed in the absence of water Vaarkamp et al and a shift to lower frequency upon adsorption of water.…”

Section: Pt-pt Distance (277 A) a Contraction Of The Metal-metalmentioning

confidence: 99%

Hydrogen as a Modifier of the Structure and Electronic Properties of Platinum in Acidic Zeolite LTL: A Combined Infrared and X-ray Absorption Spectroscopy Study

Vaarkamp¹,

Mojet²,

Kappers³

et al. 1995

J. Phys. Chem.

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The structure and electronic properties of platinum in WH-LTL after reduction at 300 "C and heating in helium to 500 or 690 "C were determined using X-ray absorption and infrared spectroscopy. After reduction at 300 "C, the platinum particles were metallic, consisted of 4 or 5 atoms, and were located at 2.64 A from the oxygen atoms in the zeolite framework. The particles remained metallic but increased in size to e 1 3 atoms during hydrogen desorption by heating in a helium flow up to 690 "C. Simultaneously, the distance between metal particle and oxygen atoms of the zeolite framework was shortened to 2.05 A. After reduction at 300 "C and in the presence of chemisorbed hydrogen, the platinum atoms in the PT/H-LTL catalyst had more holes in the d-band than bulk platinum. Hydrogen desorption decreased the number of holes of the platinum atoms in the WH-LTL catalyst to levels lower than bulk metal values. The linear CO band shifted from 2071 to 2084 cm-' upon hydrogen desorption, due to the increased particle size andor the change in the structure of the metal-support interface. The apparent contradiction between the shift to higher wavenumbers of the linear CO band and the decreased number of holes in the d-band was attributed to the interaction of CO with filled d-orbitals and the effect of chemisorbed hydrogen on the distribution of the local density of states.

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Ion-dipole interactions between adsorbed CO and support cations in Pt/K-LTL

Cited by 26 publications

References 37 publications

Deconvolution and Curve Fitting of IR Spectra for CO Adsorbed on Pt/K-LTL: Potassium Promoter Effect and Adsorption Site Distribution

Deconvolution and Curve Fitting of IR Spectra for CO Adsorbed on Pt/K-LTL: Potassium Promoter Effect and Adsorption Site Distribution

A New Model Describing the Metal–Support Interaction in Noble Metal Catalysts

Hydrogen as a Modifier of the Structure and Electronic Properties of Platinum in Acidic Zeolite LTL: A Combined Infrared and X-ray Absorption Spectroscopy Study

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