A.M.J. van der Eerden

Promotion Effects in the Oxidation of CO over Zeolite-Supported Pt Nanoparticles

Well-defined Pt-nanoparticles with an average diameter of 1 nm supported on a series of zeolite Y samples containing different monovalent (H+, Na+, K+, Rb+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations have been used as model systems to investigate the effect of promotor elements in the oxidation of CO in excess oxygen. Time-resolved infrared spectroscopy measurements allowed us to study the temperature-programmed desorption of CO from supported Pt nanoparticles to monitor the electronic changes in the local environment of adsorbed CO. It was found that the red shift of the linear Pt-coordinated CO vibration compared to that of gas-phase CO increases with an increasing cation radius-to-charge ratio. In addition, a systematic shift from linear (L) to bridge (B) bonded CO was observed for decreasing Lewis acidity, as expressed by the Kamlet-Taft parameter alpha. A decreasing alpha results in an increasing electron charge on the framework oxygen atoms and therefore an increasing electron charge on the supported Pt nanoparticles. This observation was confirmed with X-ray absorption spectroscopy, and the intensity of the experimental Pt atomic XAFS correlates with the Lewis acidity of the cation introduced. Furthermore, it was found that the CO coverage increases with increasing electron density on the Pt nanoparticles. This increasing electron density was found to result in an increased CO oxidation activity; i.e., the T(50%) for CO oxidation decreases with decreasing alpha. In other words, basic promotors facilitate the chemisorption of CO on the Pt particles. The most promoted CO oxidation catalyst is a Pt/K-Y sample, which has a T(50%) of 390 K and a L:B intensity ratio of 2.7. The obtained results provide guidelines to design improved CO oxidation catalysts.

Apparatus for In Situ X-Ray Absorption Fine Structure Studies on Catalytic Systems in the Energy Range 1000 eV < E < 3500 eV

A new apparatus for in situ x-ray absoprtion fine structure measurements in the medium energy range of 1000–3500 eV has been developed. Measurements can be performed in a gaseous environment (max. pressure 1 bar) at temperatures ranging from 80 to 750 K. Pre-treatments can be performed at 5 bar and 750 K in the same cell, after which XAFS measurements can be done without exposing the sample to ambient air. In a modular set-up several detector systems can be used: fluorescence detection using a gas proportional counter, a photodiode or a microstrip detector. All detectors are highly integrated into the cell, gaining solid angle for detection. Electron yield detection can be used simultaneously using conversion electron yield or total electron yield. The performance of the new apparatus is demonstrated by a study of the K edge of Al in Zeolite Beta. The Al content is as low as 2 wt%. It will be shown that octahedral framework Al is formed while adding gaseous water at room temperature after ammonia removal ...

Flexible aluminium coordination of zeolites as function of temperature and water content, an in-situ method to determine aluminium coordinations

The aluminium coordinations in zeolites H-Beta and H-Y have been quantitatively investigated as a function of temperature in the presence and absence of water. In-situ Al K edge X-ray Absorption Spectroscopy shows that a framework tetrahedrally coordinated aluminium is stable in inert to at least 725 K. However, in the presence of water, already at room temperature, part of the framework tetrahedral aluminium is converted to an octahedral coordination. This octahedral aluminium is not stable in inert at 375 K, where it quantitatively reverts to the tetrahedral framework coordination.

The system BaCO3 + SrCO3; crystal phase transitions: dta measurements and thermodynamic phase diagram analysis

Abstract Onset temperatures for the endothermic crystal phase transitions of nine Bal-xSrx,CO3 mixed crystals were measured with DTA in 1 atm CO2. Comparison of signal shapes with calculated phase diagrams indicates that the onset temperatures are the temperatures at which diffusionless transition takes place. On that assumption, transitional excess Gibbs energies of x(1−x)[−5160 ± 200] J-mol−1 for the orthorhombic-rhombohedral transition and x(1−x)[0 ± 200] J mol−1 for the rhombohedral-cubic transition were calculated.

Promotion effects in the oxidation of CO over zeolite-supported Pt nanoparticles

cations have been used as model systems to investigate the effect of promotor elements in the oxidation of CO in excess oxygen. Time-resolved infrared spectroscopy measurements allowed us to study the temperatureprogrammed desorption of CO from supported Pt nanoparticles to monitor the electronic changes in the local environment of adsorbed CO. It was found that the red shift of the linear Pt-coordinated CtO vibration compared to that of gas-phase CO increases with an increasing cation radius-to-charge ratio. In addition, a systematic shift from linear (L) to bridge (B) bonded CtO was observed for decreasing Lewis acidity, as expressed by the Kamlet-Taft parameter R. A decreasing R results in an increasing electron charge on the framework oxygen atoms and therefore an increasing electron charge on the supported Pt nanoparticles. This observation was confirmed with X-ray absorption spectroscopy, and the intensity of the experimental Pt atomic XAFS correlates with the Lewis acidity of the cation introduced. Furthermore, it was found that the CO coverage increases with increasing electron density on the Pt nanoparticles. This increasing electron density was found to result in an increased CO oxidation activity; i.e., the T50% for CO oxidation decreases with decreasing R. In other words, basic promotors facilitate the chemisorption of CO on the Pt particles. The most promoted CO oxidation catalyst is a Pt/K-Y sample, which has a T50% of 390 K and a L:B intensity ratio of 2.7. The obtained results provide guidelines to design improved CO oxidation catalysts.

Structure of Ti in TiCl3 Doped NaAlH4

To elucidate the structure of Ti in Ti‐doped sodium alanate used for hydrogen storage, XAFS studies were performed. In freshly prepared samples the majority of the Ti is present in interstitial positions in the NaAlH4. By increasing the desorption temperature, thus increasing the extent of hydrogen desorption, Ti migrates into the Al, finally forming a less active TiAl3 alloy.