J. Douglas Way

Preparation and characterization of Pd-Cu composite membranes for hydrogen separation

Pd–Cu composite membranes were made by successive electroless deposition of Pd and then Cu onto various tubular porous ceramic supports. Ceramic filters used as supports included symmetric -alumina (nominal 200 nm in pore size), asymmetric zirconia on-alumina (nominal 50 nm pore size), and asymmetric -alumina on -alumina (nominal 5 nm pore size). The resulting metal/ceramic composite membranes were heat-treated between 350 and 700 ◦ C for times ranging from 6 to 25 days to induce intermetallic diffusion and obtain homogeneous metal films. Pure gas permeability tests were conducted using hydrogen and nitrogen. For an 11 m thick, 10 wt.% Cu film on a nominal 50 nm pore size asymmetric ultrafilter with zirconia top layer, the flux at 450 ◦ C and 345 kPa H2 feed pressure was 0.8 mol/m 2 s. The ideal hydrogen/nitrogen separation factor was 1150 at the same conditions. The thickness of the metallic film was progressively decreased from 28 md own to 1–2m and the alloy concentration was increased to 30 wt.% Cu. Structural factors related to the ceramic support and the metallic film chemical composition are shown to be responsible for the differences in membrane performance. Among the former are the support pore size, which controls the required metal film thickness to insure a leak-free membrane and the internal structure of the support (symmetric or asymmetric) which changes the mass transfer resistance. The support with the 200 nm pores required more Pd to plug the pores than the asymmetric membranes with smaller pore sizes, as was expected. However, leak-free films could not be deposited on the support with the smallest pore size (5 nm -alumina), presumably due to surface defects and/or a lack of adhesion between the metal film and the membrane surface.

The influence of alloy composition on the H2 flux of composite PdCu membranes

Abstract PdCu composite membranes were made by successive electroless deposition of Pd and then Cu onto various tubular porous ceramic supports. Pure gas (H2, N2) permeability tests were conducted at high temperature with simultaneous annealing to induce intermetallic diffusion. Both the alloy composition and the support structure are shown to be responsible for the differences in membrane performance. The highest hydrogen permeability was observed at an alloy composition of 60 wt.% Pd at a constant temperature of 350°C. A typical H2 flux for this membrane was 0.81 mol/m2·s at 350°C and 255 kPa H2 partial pressure.

Single component and mixed gas transport in a silica hollow fiber membrane

Abstract The permeances of gases with kinetic diameters ranging from 2.6 to 3.9 A were measured through silica hollow fiber membranes over a temperature range of 298 to 473 K at a feed gas pressure of 20 atm. Permeances at 298 K ranged from 10 to 2.3· 10 5 Barrer/cm for CH 4 and He, respectively, and were inversely proportional to the kinetic diameter of the penetrant. From measurements of CO 2 adsorption at low relative pressures, the silica hollow fibers are microporous with a mean pore size estimated to be between 5.9 and 8.5 A. X-ray scattering measurements show that the orientation of the pores is completely random. Mass transfer through the silica hollow fiber membranes is an activated process. Activation energies for diffusion through the membranes were calculated from the slopes of Arrhenius plots of the permeation data. The energies of activation ranged from 4.61 to 14.0 kcal/mol and correlate well with the kinetic diameter of the penetrants. The experimental activation energies fall between literature values for zeolites 3A and 4A. Large separation factors were obtained for O 2 N 2 and CO 2 CH 4 mixtures. The O 2 N 2 mixed gas separation factors decreased from 11.3 at 298 K to 4.8 at 423 K and were up to 20% larger than the values calculated from pure gases at temperatures below 373 K. Similar differences in the separation factors were observed for CO 2 CH 4 mixtures after the membrane had been heated to at least 398 K and then cooled in an inert gas flow. The differences between the mixture and ideal separation factors is attributed to a competitive adsorption effect in which the more strongly interacting gases saturate the surface and block the transport of the weakly interacting gases. Based on Fourier transform infrared (FTIR) spectroscopy results, this unusual behavior is attributed to the removal of physically adsorbed water from the membrane surface.

Development of a model surface flow membrane by modification of porous γ-alumina with octadecyltrichlorosilane

Novel organic/inorganic gas-separation membranes were fabricated by modification of mesoporous γ-alumina ultrafilters with octadecyltrichlorosilane (ODS). Based on ellipsometry measurements and XPS analysis, our hypothesis is that the membranes were composed of a very thin, approximately 11 nm, layer of ODS oligimers grafted to the surface of the mesoporous substrate. However, the microstructure of the silane layer is not well understood. Pure gas permeance of the alumina membrane decreased by 2 to 3 orders of magnitude after modification with ODS. Permeance was history dependent, however the flow could be recovered by rinsing in toluene and drying at 333 K, or by exposure to flowing N2 with a pressure drop across the membrane of approximately 34.5 kPa. Following silane modification the membrane exhibited reverse selectivity, or selectivity for heavier gases such as CO2 and n-C4H10, over lighter gases such as H2, N2 CH4 and C2H6. Reverse selectivities were measured as high as 48 for n-C4H10/N2 and 24 for n-C4H10/CH4. The pure gas permeance of various gases fit an exponential relationship with critical temperature that was consistent with transport based on preferential sorption and solution diffusion often observed in rubbery polymers. A model for surface diffusion enhanced permeation provided a parametric fit to the pure gas permeance of the ODS membrane. A maximum in permeance as a function of pressure and temperature, and a change in sign for the apparent activation energy of diffusion, distinguished the sorption and surface flow of n-C4H10 from the transport of non-condensable gases and CO2.

Synthesis of β-Mo2C Thin Films

Thin films of stoichiometric β-Mo(2)C were fabricated using a two-step synthesis process. Dense molybdenum oxide films were first deposited by plasma-enhanced chemical vapor deposition using mixtures of MoF(6), H(2), and O(2). The dependence of operating parameters with respect to deposition rate and quality is reviewed. Oxide films 100-500 nm in thickness were then converted into molybdenum carbide using temperature-programmed reaction using mixtures of H(2) and CH(4). X-ray diffraction confirmed that molybdenum oxide is completely transformed into the β-Mo(2)C phase when heated to 700 °C in mixtures of 20% CH(4) in H(2). The films remained well-adhered to the underlying silicon substrate after carburization. X-ray photoelectron spectroscopy detected no impurities in the films, and Mo was found to exist in a single oxidation state. Microscopy revealed that the as-deposited oxide films were featureless, whereas the carbide films display a complex nanostructure.

Dense Carbide/Metal Composite Membranes for Hydrogen Separations Without Platinum Group Metals

This creates the need for cost-effective separation processes to purify hydrogen from these components as well as other contaminants. Hydrogen-selective membranes are of particular interest for this purpose, as they are energy–effi cient and allow for co-production of hydrogen while maintaining a CO 2 -rich retentate at high pressure for capture or use. Additionally, if a hydrogen-selective membrane is incorporated into a reforming or shift reactor, the removal of products can drive reactions (1–3) toward complete conversion. A number of materials are available that selectively transport hydrogen. [ 2–4 ] Among the options, polymers are eliminated by temperature constraints, while microporous ceramic systems are limited with regard to selectivity. An appealing option is dense metal fi lms from groups III–V of the periodic table, which transport hydrogen via a solution-diffusion mechanism. In this mechanism, hydrogen is dissociatively adsorbed onto the membrane feed surface and dissolved into the bulk. The dissociated hydrogen is then transported by site-hopping diffusion and recombined on the low-pressure membrane permeate surface. [ 5 ] As only hydrogen can be transported in this fashion, pinhole-free fi lms of these metals will therefore have theoretically infi nite selectivity. At present, palladium and its alloys dominate research and industrial practice in this class of materials due to their ability to both dissociate hydrogen and display high permeability

Palladium-Copper and Palladium-Gold Alloy Composite Membranes for Hydrogen Separations

Electroless plating was used to fabricate PdCu and PdAu alloy composite membranes using tubular Al2O3 and stainless steel microfilters to produce high temperature H2 separation membranes. The composite membranes were annealed and tested at temperatures ranging from 350 to 400°C, at high feed pressures (≤250 psig) using pure gases and gas mixtures containing H2, carbon monoxide (CO), carbon dioxide (CO2), H2O and H2S, to determine the effects these parameters had on the H2 permeation rate, selectivity and recovery.

PALLADIUM/COPPER ALLOY COMPOSITE MEMBRANES FOR HIGH TEMPERATURE HYDROGEN SEPARATION FROM COAL-DERIVED GAS STREAMS

For hydrogen from coal gasification to be used economically, processing approaches that produce a high purity gas must be developed. Palladium and its alloys, nickel, platinum and the metals in Groups 3 to 5 of the Periodic Table are all permeable to hydrogen. Hydrogen permeable metal membranes made of palladium and its alloys are the most widely studied due to their high hydrogen permeability, chemical compatibility with many hydrocarbon containing gas streams, and infinite hydrogen selectivity. Our Pd composite membranes have demonstrated stable operation at 450 C for over 70 days. Coal derived synthesis gas will contain up to 15000 ppm H{sub 2}S as well as CO, CO{sub 2}, N{sub 2} and other gases. Highly selectivity membranes are necessary to reduce the H{sub 2}S concentration to acceptable levels for solid oxide and other fuel cell systems. Pure Pd-membranes are poisoned by sulfur, and suffer from mechanical problems caused by thermal cycling and hydrogen embrittlement. Recent advances have shown that Pd-Cu composite membranes are not susceptible to the mechanical, embrittlement, and poisoning problems that have prevented widespread industrial use of Pd for high temperature H{sub 2} separation. These membranes consist of a thin ({le} 5 {micro}m) film of metal deposited on the inner surface of a porous metal or ceramic tube. With support from this DOE Grant, we have fabricated thin, high flux Pd-Cu alloy composite membranes using a sequential electroless plating approach. Thin, Pd{sub 60}Cu{sub 40} films exhibit a hydrogen flux more than ten times larger than commercial polymer membranes for H{sub 2} separation, resist poisoning by H{sub 2}S and other sulfur compounds typical of coal gas, and exceed the DOE Fossil Energy target hydrogen flux of 80 ml/cm{sup 2} {center_dot} min = 0.6 mol/m{sup 2} {center_dot} s for a feed pressure of 40 psig. Similar Pd-membranes have been operated at temperatures as high as 750 C. We have developed practical electroless plating procedures for fabrication of thin Pd-Cu composite membranes at any scale.

Un-supported Palladium Alloy Membranes for the Production of Hydrogen

Thin self-supported permeable membranes of palladium alloys such as Pd60Cu40 have many applications in which hydrogen separation is required. Magnetron sputtering onto selected flexible or non-flexible substrates, for example using a watersoluble release agent or backing, has allowed lift-off and production of high quality films of controllable alloy composition down to 3μm in thickness. The targeted manufacturing process is a low-cost reel-to-reel process that will theoretically meet the targets established by DOE. The results from a recently completed project and some of the remaining problems are discussed.

Energy Saving Separations Technologies for the Petroleum Industry: An Industry-University-National Laboratory Research Partnership

This project works to develop technologies capable of replacing traditional energy-intensive distillations so that a 20% improvement in energy efficiency can be realized. Consistent with the DOE sponsored report, Technology Roadmap for the Petroleum Industry, the approach undertaken is to develop and implement entirely new technology to replace existing energy intensive practices. The project directly addresses the top priority issue of developing membranes for hydrocarbon separations. The project is organized to rapidly and effectively advance the state-of-the-art in membranes for hydrocarbon separations. The project team includes ChevronTexaco and BP, major industrial petroleum refiners, who will lead the effort by providing matching resources and real world management perspective. Academic expertise in separation sciences and polymer materials found in the Chemical Engineering and Petroleum Refining Department of the Colorado School of Mines is used to invent, develop, and test new membrane materials. Additional expertise and special facilities available at the Idaho National Engineering and Environmental Laboratory (INEEL) are also exploited in order to effectively meet the goals of the project. The proposed project is truly unique in terms of the strength of the team it brings to bear on the development and commercialization of the proposed technologies.