J. Shu

Methane Steam Reforming in Asymmetric Pd- and Pd-Ag/Porous SS Membrane Reactors

Abstract This work is devoted to applying electrolessly deposited Pd- and Pd-Ag/porous stainless steel composite membranes in methane steam reforming. The methane conversion is significantly enhanced by the partial removal of hydrogen from the reaction location as a result of diffusion through the Pd-based membranes. For example, at a total pressure of 136 kPa, a temperature of 500°C, a molar steam-to-methane ratio of 3, and in the presence of a commercial Ni/Al2O3 catalyst together with continuous pumping on the permeation side, a methane conversion twice as high as that in a non-membrane reactor was reached by using a Pd/SS membrane. These effects were examined under a variety of experimental conditions. A computer model of the membrane reactor was also developed to predict the effects of membrane separation on methane conversion.

Structurally stable composite PdAg alloy membranes: Introduction of a diffusion barrier

Abstract This work investigates the improvement of the structural stability of PdAg alloy membranes by introduction of a diffusion barrier. Asymmetric PdAg films were deposited on porous stainless steel (SS) substrate by electroless plating. The formation of these alloys was achieved by annealing the as-deposited membranes at temperatures higher than Tamman temperatures of the alloy components in a hydrogen atmosphere. The composite PdAg membranes were characterized using XRD and Auger electron depth profiling. The atomic interdiffusion of silver and palladium resulted in PdAg alloys in an fcc structure. To improve the structural stability of PdAg alloy/SS membranes, an ultrathin intermediate layer of titanium nitride being 0.1 μm thick was introduced as a diffusion barrier between PdAg and the SS substrate. The Auger electron depth profiling analysis indicated that the improved membranes were thermally stable at temperatures as high as 973 K, and practical for the catalytic membrane reactor use. An estimation of diffusion coefficients revealed that the presence of hydrogen in the annealing atmosphere favoured the PdAg interdiffusion and thus the formation of PdAg alloys.

Asymmetric PdAg/stainless steel catalytic membranes for methane steam reforming

Abstract Pd-Ag/porous stainless steel asymmetric membranes were prepared by successive palladium and silver platings in electroless hydrazine baths, followed by a thermal treatment in hydrogen of the as-deposited membranes above the Tamman temperatures for the alloy formation. The prepared membranes were permselective toward hydrogen separation. A membrane reactor made of stainless steel was designed to perform methane steam reforming. At mild reaction conditions, methane conversion is significantly enhanced by partial removal of hydrogen from the reaction location as a result of diffusion through the Pd-based membrane. These effects were examined under a variety of experimental conditions.

Surface segregation of PdAg membranes upon hydrogen permeation

Abstract PdAg membranes are permeable to hydrogen. Hydrogen treatment results in a small chemical shift (±0.1–0.2 eV) of Pd 3d core level but no change in the Ag3d level. A new valence band in the binding energy region of 7–9 eV corresponding to the interaction between H 1s and Pd4d appears on a hydrogen permeated membrane surface. Quantitative XPS analysis reveals that Pd segregates at the membrane surface toward the high hydrogen pressure side while Ag segregation occurs at the surface on the low hydrogen pressure side after hydrogen permeation. Both surface segregations are explained based on an MTCIP-1A (modern thermodynamic calculation of interface properties — first approximation) approach. It is concluded that hydrogen chemisorption induces palladium segregation on the PdAg membrane surface.

Autocatalytic Effects in Electroless Deposition of Palladium

This paper deals with the catalytic aspect of electroless palladium deposition on Pd-Sn activated stainless steel substrates. Experimental results of anodic hydrazine polarization revealed that the shape of the polarization curves can be taken as an indicator of the catalytic activity of substrates. The exchange potential reflects the nature of the palladium species, while the reference current density at a given potential depends on the amount of the active species. X-ray photoelectron spectroscopy and polarization measurements showed that the presence of tetravalent Pd(IV) oxide limits the autocatalytic activity of activated substrates. The Pd(IV) species is very likely stabilized by interaction with the coexisting SnO 2 on the Pd-Sn activated substrate

MORPHOLOGICAL STUDY OF HYDROGEN PERMEABLE PD MEMBRANES

Abstract This work deals with the relationship between morphology of electrolessly deposited Pd membranes and hydrogen permeation. Both concentrated and dilute Pd containing electroless baths were examined. Scanning electron microscopy and X-ray diffraction line broadening analyses revealed the formation of different morphologies due to the deposition kinetics. Crystallization aspects in electroless deposition were discussed. Loosely packed palladium particles being about 420 nm in diameter were formed by repeated depositions from the dilute bath. Dense palladium membranes with primary particle sizes of about 11 nm were deposited from the concentrated bath. Only the latter composite membrane was permselective to hydrogen.

Morphological changes of Pd-Ag membranes upon hydrogen permeation

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