Chuin-Tih Yeh

Temperature-programmed reduction and temperature-resolved sorption studies of strong metal-support interaction in supported palladium catalysts

Abstract Palladium catalysts on various supports were prepared for strong metal-support interaction (SMSI) studies. The reduction and hydrogen sorption for these catalysts were investigated by a combination of temperature-programmed reduction and temperature-resolved sorption techniques. Palladium was found to be reduced at temperatures lower than 200 °C. Spillover of hydrogen from Pd to supports occurred at higher temperatures. Hydrogen treatment at high temperatures also induced SMSI and sintering. The extent of spillover, sintering, and SMSI are support-dependent. The absorption of hydrogen into bulk palladium was also suppressed by SMSI. Under the same reduction conditions, the sintering of supported palladium catalysts follows the trend Pd C > Pd TiO 2 > Pd Al 2 O 3 > Pd SiO 2 .

Interactions of methane with ThO2SiO2 surface at 1073 K

Abstract Methane decomposition over ThO 2 SiO 2 surface was investigated at 1073 K. Positive catalytic effect on the decomposition has been confirmed at low reaction conversions when ethane and ethylene are major products. Yields of ethylene and minor unsaturated hydrocarbons were sensitively inhibited by the presence of NO impurity in the methane feed.

Determination of the surface composition of copper-nickel alloy powders: a comparison between hydrogen chemisorption and the reaction of N2O decomposition

The surface composition of copper-nickel alloy powders was measured by chemisorption. Three different techniques were used for comparison, namely, strong hydrogen chemisorption, temperature-programmed desorption of hydrogen, and decomposition of N2O. The agreement between the compositions determined by these three techniques was excellent. A segregation of copper atoms to the powder surfaces was confirmed. The chemisorption strength of hydrogen atoms on nickel is weakened by alloying with copper.

Deuterium nuclear magnetic resonance characterization of particle size effect in supported rhodium catalysts

Deuterium adsorbed onto Rh particles of various sizes (1.1–9 nm) supported on Al2O3 was studied by 2H NMR spectroscopy. Near zero overpressure of D2, δD shifts downfield with increasing size of particles, approaching a limiting value ca. −120 ppm when particle size exceeds 2 nm. The dependence of δD on particle size of the odd-electron metal Rh(4d7) is opposite to the dependence observed for the even-electron metal Pt(5d8) which we have also investigated. δD also shifts downfield upon increased overpressure of D2. All these variations of chemical shifts can be satisfactorily accounted for by considering the spin density on the metal particle. The linewidth depends on the particle size but not on the overpressure of D2. Hence the major relaxation mechanism is not paramagnetic interaction which is responsible for the variations of δD.

Mechanisms of n-butane isomerization over superacidic sulfated metal oxides

Abstract Superacidic sulfated metal oxides were prepared by impregnating different metal oxides (or hydroxides) with sulfuric acids. The superacidic strength of prepared samples were compared by temperature-programmed desorption of ammonia (NH 3 -TPD) and showed a trend of HS/ZrO 2 >HS/MgAl 2 O 4 >HS/Al 2 O 3 >HS/Fe 2 O 3 . Catalytic properties of these superacidic samples towards isomerization of n -butane to isobutane were subsequently pursued. Both the activity and the isobutane selectivity increased with the superacidic strength of catalysts. Observed variation in the selectivity was interpreted with a difference in the reaction mechanism: A high selectivity was obtained from strong superacid sites that catalyze the isomerization through a revised monomolecular mechanism; while a low selectivity became dominated on weak superacid sites that favored a bimolecular mechanism. The relative importance of these two mechanisms was kinetically controlled by formation of different butyl carbenium ions (primary or secondary) while the n -butane reactant adsorbed on superacidic sites.

Characterization of supported-palladium catalysts by deuterium NMR spectroscopy

Alumina-supported Pd, Rh and Pd–Rh bimetallic alloys have been prepared by the method of incipient wetness. Sorption of deuterium by these samples was investigated by means of 2H NMR spectroscopy and uptake measurements at room temperature. Deuterium chemisorbed on palladium and rhodium atoms was characterized by 2H NMR peaks with chemical shifts of –12 and –170 ppm, respectively, from monometallic samples under a deuterium pressure of 1 Torr or less. These two characteristic peaks remained for the bimetallic samples and their relative intensities unambiguously revealed the surface composition of the bimetallic crystallites. Deuterium atoms were absorbed or weakly chemisorbed at greater pressures of deuterium. The weakly sorbed deuterium atoms exchanged with the strongly chemisorbed deuterium atoms, indicated by the coalescence of the 2H NMR characteristic peaks of the bimetallic samples. The position of the coalesced peak depended on the composition of the alloy crystallites.