Hydrogen selective thin palladium-copper composite membranes on alumina supports

Thin and defect-free Pd-Cu composite membranes with high hydrogen permeances and selectivities were prepared by electroless plating of palladium and copper on porous alumina supports with pore sizes of 5 and 100 nm coated with intermediate layers. The intermediate layers on the 100 nm supports were prepared by the deposition of boehmite sols of different particle sizes, and provided a graded, uniform substrate for the formation of defect-free, ultra-thin palladium composite layers. The dependence of hydrogen flux on pressure difference was studied to understand the dominant mechanism of hydrogen transport through a Pd-Cu composite membrane plated on an alumina support with a pore size of 5 nm. The order in hydrogen pressure was 0.98, and indicated that bulk diffusion through the Pd-Cu layer was fast and the overall process was limited by external mass-transfer or a surface process. Scanning electron microscopy (SEM) images of the Pd-Cu composite membrane showed a uniform substrate created after depositing one intermediate layer on top of the alumina support and a dense Pd-Cu composite layer with no visible defects. Cross-sectional views of the membrane showed that the Pd-Cu composite layer had a top layer thickness of 160 nm (0.16 mu m), which is much thinner than previously reported

An operability level coefficient (OLC) as a useful tool for correlating the performance of membrane reactors

An operability level coefficient (OLC), defined as the ratio of product permeation and product formation rates, and related to the inverse combination of the Damkohler number and the Peclet number (1/DaPe), is suggested as a useful tool for estimating performances of membrane reactors (MRs) operating as separators in equilibrium-limited reactions. The OLCs for product hydrogen formation in previously reported MRs for methane dry-reforming (MDR), methane steam-reforming (MSR), methanol steam-reforming (MeOHSR), and ethanol steam-reforming (EtOHSR) were correlated with conversion and yield enhancements. For values of OLCs ranging from 0.03 to 0.78, a clear universal trend for increasing conversions and hydrogen yields with increasing OLC was observed for these different types of reforming reactions. The OLC curve calculated from a numerical simulation without adjustable parameters was found to closely approximate experimental data obtained from the MRs, and was shown not to depend on the assumed kinetics. This study confirms that hydrogen selectivity (from the ratio of single-gas permeances) has a substantial influence on conversion and hydrogen yield enhancements in a MR, and demonstrates that a hydrogen selectivity of 100 is sufficient to achieve high performance in a MR

New catalysts for coal processing: Metal carbides and nitrides

The subject of this research project was to investigate the catalytic properties of a new class of materials, transition metal carbides and nitrides, for treatment of coal liquid and petroleum feedstocks. The main objectives were: (1) preparation of catalysts in unsupported and supported form; (2) characterization of the materials; (3) evaluation of their catalytic properties in HDS and HDN; (4) measurement of the surface properties; and (5) observation of adsorbed species. All of the objectives were substantially carried out and the results will be described in detail below. The catalysts were transition metal carbides and nitrides spanning Groups 4--6 in the Periodic Table. They were chosen for study because initial work had shown they were promising materials for hydrotreating. The basic strategy was first to prepare the materials in unsupported form to identify the most promising catalyst, and then to synthesize a supported form of the material. Already work had been carried out on the synthesis of the Group VI compounds Mo{sub 2}C, Mo{sub 2}N, and WC, and new methods were developed for the Group V compounds VC and NbC. All the catalysts were then evaluated in a hydrotreating test at realistic conditions. It was found that the most active catalyst was Mo{sub 2}C, and further investigations of the material were carried out in supported form. A new technique was employed for the study of the bulk and surface properties of the catalysts, near edge x-ray absorption spectroscopy (NEXAFS), that fingerprinted the electronic structure of the materials. Finally, two new research direction were explored. Bimetallic alloys formed between two transition metals were prepared, resulting in catalysts having even higher activity than Mo{sub 2}C. The performance of the catalysts in hydrodechloration was also investigated

Experimental and kinetic studies of the ethanol steam reforming reaction equipped with ultrathin Pd and Pd-Cu membranes for improved conversion and hydrogen yield

The ethanol steam reforming (EtOH SR) reaction was carried out over a Co-Na/ZnO catalyst both in a packed bed reactor (PBR) and in membrane reactors (MRs) equipped with ultrathin Pd or Pd-Cu membranes to evaluate the benefits of employing membranes. For all conditions, ethanol conversion and hydrogen production were significantly higher in the MRs than in the PBR. Average ethanol conversion enhancement and hydrogen production enhancement were measured to be 12% and 11% in the Pd MR and 22% and 19% in the Pd-Cu MR, respectively. These enhancements of the conversion and product yield can be attributed to the shift in equilibrium by continuous hydrogen removal by the Pd based membranes. A significant contamination of the Pd layer by CO or carbon compounds during the reaction can be the reason for the comparatively low enhancement in the Pd MR compared to the Pd-Cu MR in spite of the high H-2 permeability of the original Pd membrane. A one-dimensional modeling study of the MRs and the PBR was conducted and their predicted results were compared to those obtained from the experimental study. The model was developed using a simplified power law expression but the predicted values fit the experimental data with only minor deviations. Enhancements of ethanol conversion and hydrogen yield with space velocity (SV) could be explained by the increased H-2 flux through the membranes with SV in the MRs

Reaction of primary and secondary products in a membrane reactor: Studies of ethanol steam reforming with a silica-alumina composite membrane

The steam reforming of ethanol over Na-Co/ZnO catalysts was investigated in a conventional packed-bed reactor (PBR) and in a membrane reactor (MR) fitted with silica-alumina composite membranes. Promotion with a moderate amount of Na (0.2-1.0 wt%) produced catalysts with stable ethanol conversion and product selectivity. At 523K, the main products were H-2 (51%) and CH3CHO (42%) with minor amounts of CH4 (0.7%) while above 573 K the main products were H-2 (70%) and CO2 (15%) with a small amount of CH4 (2.4%). Higher cobalt loading, higher W:E ratio, higher reaction temperature and lower space velocity enhanced the conversion of ethanol to H-2 and CO2 while reducing the formation of undesirable acetaldehyde. Contact time studies indicated that acetaldehyde was a primary product of ethanol reforming, while CO2 and CH4 were secondary products. The composite membranes were prepared by a chemical vapor deposition (CVD) method and had moderate H-2 permeances ((5.2-6.8) x 10(-8) mol m(-2) s(-1) Pa-1 at 623 K). The MR produced positive yield enhancements of the secondary products, including hydrogen, but yield reductions for the primary product, acetaldehyde, and this is explained from the effect of the membrane separation on the reaction network. Two membranes with similar permeances but different H-2 selectivity ratios (H-2/CH4 = 60 and 350) were compared and it was found that the membrane with a higher H-2/CH4 selectivity ratio gave a higher yield enhancement. A survey of membrane reactor studies indicated that the conversion enhancement in reforming reactions could be described by a parameter denoted the operability level coefficient (OLC) which is related to 1/DaPe

Studies of the effect of pressure and hydrogen permeance on the ethanol steam reforming reaction with palladium- and silica-based membranes

The effects of hydrogen permeance and selectivity in the performance of membrane reactors for the ethanol steam reforming (EtOHSR) were studied at 1-10 atm and 623 K. The studies were performed with Pd-Cu and SiO2-Al2O3 composite membranes prepared by depositing the permselective components onto porous alumina supports with intermediate layers. The hydrogen permeances of the membranes varied from 5.2 x 10(-8) to 3.9 x 10(-6) mol m(-2) s(-1) Pa-1 and the selectivity ranged from 200 to 1000 (H-2/CO2) at 623 K, which allowed a broad set of conditions to be probed. Comparison studies with packed-bed reactor (PBR) and membrane reactor (MR) operation showed that higher ethanol conversions and hydrogen molar flows were obtained in the MRs for all pressures studied. It was determined that both hydrogen permeance and selectivity had a favorable effect on the EtOHSR reaction, with the highest ethanol conversion enhancement of 44% and hydrogen molar flow enhancement of 69% obtained in a MR fitted with a membrane with the highest hydrogen permeance. A criterion for the required permeance for industrial applications is developed, which for 1 cm diameter membrane tubes is 2.5 x 10(-7) mol m(-2) s(-1) Pa-1