Tsong-Huei Chang

NMR characterization of the supported 12-heteropoly acids

Catalysts have been prepared by the incipient wetness method and characterized by 31P MAS NMR spectroscopy. Two species, i.e. the interacting molecular and the crystalline, were found in both the supported PMo12 and supported PW12 catalysts. Experimental results indicated that the interacting species are more thermally stable and resistant to water etching than the crystalline species. Furthermore, the interacting species also exhibited an extremely good selectivity in propan-2-ol dehydration and in the esterification of acetic acid with ethanol. The electronic properties of the interacting species were found to be support dependent.

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.

Synthesis and characterization of vanado titanium silicate molecular sieves with MEL structure

A series of vanadotitanium silicates was synthesized by a hydrothermal method. The MEL structure of these synthesized zeolites is clearly evidenced by X-ray diffraction spectroscopy. An advanced study with framework i.r., u.v.-vis, n.m.r., e.p.r., SIMS, and EDS indicates that atoms of both vanadium and titanium are simultaneously and homogeneously incorporated into the framework of the zeolite. In addition, the e.p.r. experiment also shows this zeolite to have redox properties.

Reactivity of titanosilicates-supported catalysts for the selective catalytic reduction of NO with NH3

Abstract The zeolites with MEL structure were synthesized via the hydrothermal method and the zeolites-supported catalysts, such as Cu 2+ , Ga 3+ , Co 3+ , Ce 2+ and VO 2+ /zeolites, were prepared by the incipient wetness impregnation. The structures of the synthesized zeolites were characterized by techniques of XRD, FT-IR, SIMS, 29 Si and 27 Al MAS NMR. The selective catalytic reduction (SCR) of NO by ammonia was carried out with a glass reactor under a downstream flow. The synthesized TS-2 showed no significant DeNO x activity, instead of catalyzing the ammonia oxidation at a high temperature. Furthermore, the catalytic activity of TS2 zeolite can be effectively modified and tuned up through incorporating second metal ion such as Fe 3+ , Co 3+ , and Al 3+ into the framework (i.e., [Fe,Ti]Z11, [Co,Ti]Z11, and [Al,Ti]Z11). Among the synthesized bimetallosilicates, the [Fe,Ti]Z11 zeolite is the most active catalyst for the SCR DeNO x with ammonia; the NO conversion and the N 2 yield reach around 80%. In addition, impregnating the metal ions on TS2 or bimetallosilicates is also a very effective way to improve the SCR DeNO x activity. Ga 3+ /[Fe40,Ti40]Z11 and Co 3+ /[Fe40,Ti40]Z11 are the most active catalysts and show a potential for the practical applications.

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.

119Sn NMR studies on alloying silica supported tin with nickel and palladium

Abstract Catalysts of Sn SiO 2 , Sn-Ni Si0 2 , and Sn-Pd SiO 2 were prepared with the incipient wetness method. The Knight shift in the 119Sn NMR absorption peaks from these catalysts as well as a sample of powdered tin metal were monitored by the solid state NMR spectroscopy. The peaks from the tin powders and the tin on the Sn SiO 2 catalyst were anisotropic with the Knight shifts at 0.755% (ktt) and 0.813% (K11). The Knight shift of tin was changed on alloying the supported tin with nickel or palladium. These changes in the Knight shift suggested that the electronic properties of the tin atoms in the supported alloys were affected by the presence of nickel and palladium.

A Convenient TPD Method for Calculating the Integral Heat of Sorption(Short communication)

An easy and convenient TPD method based on the Langmuir adsorption model is developed to calculate the integral heat (△?0) of gaseous adsorbate sorbed on the metallic catalysts. And, the mathematical description for this method is also established. Two systems, the hydrogen absorption into palladium black and the hydrogen adsorption on Ir/Al2O3, were taken to verify this method. The sorbed heats were calculated to be 13.2, 30.6 and 31.5 kj‧mol-1 for Pd black, 10% Ir/Al2O3 and 3% Ir/Al2O3, respectively. The calculated heats are consistent with those obtained from the isosteric and calorimetric methods.

Dynamics of bimetallic Ni–Cu catalysts

A series of unsupported nickel–copper catalysts with various Ni/Cu ratios has been prepared by coprecipitation and characterized by thermomagnetometry, (TM), temperature programmed reduction (TPR) and oxidation (TPO) and scanning electron microscopy (SEM). It was found that the magnetization of the nickel–copper catalyst depends significantly on both the Ni/Cu ratio and the reduction temperature. The magnetization is found to decrease linearly with a decrease in the Ni/Cu ratio or an increase in the reduction temperature. This is attributed to ferromagnetic decoupling between two neighbouring nickel atoms when a copper atom is introduced between them. Furthermore, magnetization of the nickel–copper catalysts decreases exponentially with the annealing time, which is a result of the structural reconstruction of nickel–copper samples during annealing. Rates of this reconstruction process are calculated to be 3.72 × 10–4 s–1(Ni/Cu = 0.96) and 1.17 × 10–4 s–1(Ni/Cu = 1.95) at 673 K; in addition, the activation energy is found to be ca. 49.0 kJ mol–1 in samples with Ni/Cu = 0.96 and 1.95.