Computerunterstütztes Mikroreaktorsystem zur Untersuchung heterogen katalysierter Gasreaktionen

Turner

Lebedev

et al. 2010

104

Two different Pd/In2O3 samples including a thin film model catalyst with well-defined Pd particles grown on NaCl(001) supports and a powder catalyst prepared by an impregnation technique are examined by Electron Microscopy, X-ray Diffraction and catalytic measurements in methanol steam reforming in order to correlate the formation of different oxide-supported bimetallic Pd-In phases with catalytic activity and selectivity. A PdIn shell around the Pd particles is observed on the thin film catalyst after embedding the Pd particles in In2O3 at 300 K, likely because alloying to PdIn and oxidation to In2O3 are competing processes. Increased PdIn bimetallic formation is observed up to 573 K reduction temperature until at 623 K the film stability limit in hydrogen is reached. Oxidative treatments at 573 K lead to decomposition of PdIn and to the formation of an In2O3 shell covering the Pd particles, which irreversibly changes the activity and selectivity pattern to clean In2O3. PdIn and Pd2In3 phases are obtained on the powder catalyst after reduction at 573 K and 673 K, respectively. Only CO2-selective methanol steam reforming is observed in the reduction temperature range between 473 K and 573 K. After reduction at 673 K encapsulation of the bimetallic particles by crystalline In2O3 suppresses CO2 formation and only activity and selectivity of clean In2O3 are measured.

Section: Methodsmentioning

confidence: 99%

Pd–In2O3 interaction due to reduction in hydrogen: Consequences for methanol steam reforming

Turner

Lebedev

et al. 2010

104

“…Catalytic measurements in methanol steam reforming as well as catalyst activation treatments of thin film and powder samples were performed in an NI Labviewautomatized re-circulating batch reactor of about 8 ml volume [29].…”

Section: Catalytic Characterizationmentioning

confidence: 99%

Origin of different deactivation of Pd/SnO2 and Pd/GeO2 catalysts in methanol dehydrogenation and reforming: A comparative study

Zhao

Turner

et al. 2010

Pd particles supported on SnO2 and GeO2 have been structurally investigated by X-ray diffraction, (High-Resolution) transmission and scanning electron microscopy after different reductive treatments to monitor the eventual formation of bimetallic phases and catalytically tested in methanol dehydrogenation/reforming. For both oxides this included a thin film sample with well-defined Pd particles and a powder catalyst prepared by incipient wetness impregnation. The hexagonal and the tetragonal polymorph were studied for powder GeO2. Pd2Ge formation was observed on all GeO2-supported catalysts, strongly depending on the specific sample used. Reduction of the thin film at 573 K resulted in full transformation into the bimetallic state. The partial solubility of hexagonal GeO2 in water and its thermal structural instability yielded Pd2Ge formation at 473 K, at the cost of a structurally inhomogeneous support and Ge metal formation at higher reduction temperatures. Pd on tetragonal GeO2 entered a state of strong metal-support interaction after reduction at 573-673 K, resulting in coalescing Pd2Ge particles on a sintered and re-crystallized support, apparently partially covering the bimetallic particles and decreasing the catalytic activity. Pd2Ge on amorphous thin film and hexagonal GeO2 converted methanol primarily via dehydrogenation to CO and H2. At 573 K, formation of Pd2Sn and also PdSn occurred on the Pd/SnO2 thin film. Pd3Sn2 (and to some extent Pd2Sn) were predominantly obtained on the respective powder catalyst. Strong deactivation with increasing reduction temperature was observed, likely not based on the classical strong metal-support interaction effect, but rather on a combination of missing active structural ensembles on Sn-enriched bimetallic phases and the formation of metallic β-Sn. Correlations to Pd and its bimetallics supported on ZnO, Ga2O3 and In2O3 were also discussed.

“…The Pd/Ga 2 O 3 powder samples were prepared following a standard incipient wetness routine followed by calcination in air at 873 K for 1h. Catalytic measurements in methanol steam reforming as well as catalyst activation treatments of all samples were performed in an NI Labview-automatized recirculating batch reactor of about 8 ml volume [7]. The system allows automated pretreatment cycles (oxidative and reductive) and reaction sequences.…”

Section: Methodsmentioning

confidence: 99%

Pd/Ga2O3 methanol steam reforming catalysts: Part II. Catalytic selectivity

Penner

Jochum

et al. 2009

Structure, morphology and composition of two Pd/Ga2O3 methanol steam reforming catalysts were correlated with the associated activity and selectivity changes in the methanol steam reforming reaction and linked to studies on pure Ga2O3 supports. For both systems, that are, a Pd/Ga2O3 thin film model catalyst and a powder-supported Pd catalyst, we identified a temperature range in which the reduction with hydrogen yields a single Pd-Ga bimetallic on a reduced oxide support, which in turn suppresses methanol dehydrogenation and results in a high CO2-selectivity in methanol steam reforming. For the thin film catalyst, this included the Pd5Ga2 bimetallic present after reduction in the temperature range 523-600 K, for the powder-supported catalyst, Pd2Ga, formed after reduction between 523 and 773 K, was found to account for the high CO2-selectivity. In contrast to studies on the corresponding Pd/ZnO catalysts, sintering and metal decoration by reduced GaOx species will additionally have to be considered for discussions about structure-activity correlations in Pd/Ga2O3 thin film model catalysts. Reduction at 673 K causes catalyst deactivation and loss of CO2-selectivity due to encapsulation of catalytically active bimetallic particles by mobile GaOx species and hampers oxidative catalyst regeneration. No such behavior has been observed for the powder-supported catalyst. This difference in catalytic activity and selectivity between the two catalysts is interpreted in terms of their different (bi-) metallic -oxide contact area. Formic acid formation has been observed on pure Ga2O3 supports, which yields additional CO by formic acid decarbonylation. For the CO2-selective methanol steam reforming reaction over a Pd-Ga bimetallic particle, formic acid is not a gas phase species, which indicates a preferential decarboxylation pathway of adsorbed formate species at low temperatures.