PdAu Membranes Research Articles

Comparative analysis of conventional steam methane reforming and PdAu membrane reactor for the hydrogen production

A comparison study between conventional and PdAu-based membrane steam methane reforming (SMR) processes for small-scale hydrogen production was conducted. Both processes were simulated using a commercial simulation software Aspen Plus. For the PdAu-based membrane reactor, a 1D pseudo-homogenous model was developed via Aspen Custom Modeler (ACM). The optimum operating pressure and temperature for the membrane SMR process are 30 bar and 550˚C. While the conventional SMR process has higher performance operating at 23 bar and 900˚C at the reforming reactor outlet. The simulation results revealed that the membrane SMR process has higher methane conversion, hydrogen yield, and process energy efficiency than the conventional SMR process. Moreover, cost analysis was conducted for both processes to study their economic feasibility. The analysis revealed that hydrogen production costs for conventional and membrane SMR processes are 4.54 and 2.87 $/kg H2 respectively.

Steam reforming of simulated pre-reformed naphtha in a PdAu membrane reactor

Process intensification in a membrane reactor is an efficient and compact way to produce hydrogen. A methane-rich gas mixture that simulated the composition of pre-reformed naphtha (PRN; with a steam-to-carbon ratio of 2.7) was reformed at temperatures of 550 °C–625 °C and pressures up to 40 barg. The reactor contained commercial steam reforming catalyst and a 14.8 cm long, 2.6 μm thick Pd-1.8Au (wt. %) membrane on a porous alumina support. Methane conversions approaching 90% were obtained in the membrane reactor at a gas-hourly space velocity of 676 h−1, compared to ≤30% conversion at the same conditions in conventional reactor mode (CM) without withdrawing hydrogen through the membrane. The results were compared to steam methane reforming (SMR) in the membrane reactor at similar conditions. The nitrogen leak through the membrane increased slowly during the testing, because of both pinhole formation and some leakage through the end seals.

PdAu and PdAuAg composite membranes with reduced film thickness using YSZ as a stainless-steel support modifier

PdAu and PdAuAg alloy membranes were synthesized on top of YSZ-modified planar porous stainless steel supports by electroless deposition. A uniform, continuous and smooth coverage was observed after three YSZ deposition calcination cycles, with neither uncovered zones nor detachment after calcination at 673 K. Through this modification procedure, we could obtain thinner and more robust PdAu and PdAuAg alloy membranes. These membranes showed higher hydrogen permeance during the evaluations (623–723 K) and high thermal stability during the long-term stability test. The PdAu and PdAuAg membranes exhibited a good performance during the exposure to CO/H2O steam mixture. Contrary to the PdAu membrane, where no significant change on the atomic surface composition was observed, silver segregation was evidenced on the PdAuAg membrane after exposure to CO/H2O steam mixture.

Synthesis of Pt-zeolite coated palladium alloys as catalytic membranes for hydrogen production

PdAu composite membranes were applied for the preparation of PtNaA-PdAu catalytic membranes for hydrogen production from the water gas shift reaction. In the first stage, the deposition of NaA zeolite on top of the PdAu alloys was optimized. Functionalization employing a 1.0 v/v % 3-aminopropyltriethoxysilane solution prior to both seeding and hydrothermal synthesis allowed obtaining a homogeneous and complete PdAu alloy surface coverage. Even though the NaA zeolite coverage decreased the pure hydrogen permeance at 623 K and 50 kPa in comparison with a PdAu membrane, the resistance to a CO containing stream was higher. From direct incorporation of Pt during the zeolite synthesis, a homogeneous, well-intergrown PtNaA zeolite coverage on top of a PdAu alloy membrane was obtained. PtNaA catalytic tubular membranes were evaluated in the water gas shift reaction at 673 K. The membranes were active and stable under reaction conditions, without evidence of methane formation.

Performance of Pd-Based Membranes and Effects of Various Gas Mixtures on H2 Permeation

H2 permeation and separation properties of two Pd-based composite membranes were evaluated and compared at 400 °C and at a pressure range of 150 kPa to 600 kPa. One membrane was characterized by an approximately 8 μm-thick palladium (Pd)-gold (Au) layer deposited on an asymmetric microporous Al2O3 substrate; the other membrane consisted of an approximately 11 μm-thick pure palladium layer deposited on a yttria-stabilized zirconia (YSZ) support. At 400 °C and with a trans-membrane pressure of 50 kPa, the membranes showed a H2 permeance of 8.42 × 10−4 mol/m2·s·Pa0.5 and 2.54 × 10−5 mol/m2·s·Pa0.7 for Pd-Au and Pd membranes, respectively. Pd-Au membrane showed infinite ideal selectivity to H2 with respect to He and Ar at 400 °C and a trans-membrane pressure of 50 kPa, while the ideal selectivities for the Pd membrane under the same operating conditions were much lower. Furthermore, the permeation tests for ternary and quaternary mixtures of H2, CO, CO2, CH4, and H2O were conducted on the Pd/YSZ membrane. The H2 permeating flux decreased at the conclusion of the permeation tests for all mixtures. This decline however, was not permanent, i.e., H2 permeation was restored to its initial value after treating the membrane with H2 for a maximum of 7 h. The effects of gas hourly space velocity (GHSV) and the steam-to-carbon (S/C) ratio on H2 permeation were also investigated using simulated steam methane reforming mixtures. It was found that H2 permeation is highest at the greatest GHSV, due to a decline in the concentration polarization effect. Variations in S/C ratio however, showed no significant effect on the H2 permeation. The permeation characteristics for the Pd/YSZ membrane were also investigated at temperatures ranging from 350 to 400 °C. The pre-exponential factor and apparent activation energy were found to be 5.66 × 10−4 mol/m2·s·Pa0.7 and 12.8 kJ/mol, respectively. Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) analyses were performed on both pristine and used membranes, and no strong evidence of the formation of Pd-O or any other undesirable phases was observed.

Hydrogen production via natural gas steam reforming in a Pd-Au membrane reactor. Comparison between methane and natural gas steam reforming reactions

High-purity hydrogen to be fed directly to a PEMFC was produced by carrying out natural gas steam reforming under moderate operating conditions in a Pd-Au composite membrane reactor packed with a commercial Ni-based catalyst. The Pd-Au composite membrane with a thickness of approximately 12 µm was fabricated by using both electroless and electroplating techniques to deposit Pd and Au layers, respectively, over a porous stainless-steel support. After annealing, the membrane showed a hydrogen permeance of 1.30 × 10−3 mol/s-m2-Pa0.5 at 450 °C, and near-infinite ideal selectivity of H2/Ar at pressures lower than 300 kPa and at temperatures lower than 400 °C. The natural gas reforming reaction was performed at 450 °C with a steam-to-methane ratio of 3.5 and gas hourly space velocity of 2600 h−1 at different operating pressures varying from 100 kPa to 300 kPa. As a comparison, the steam methane reforming reaction was also carried out at the same operating conditions.The natural gas reforming reaction showed better performance than the steam methane reforming reaction and reached > 80% conversion of the higher hydrocarbons and almost 65% of hydrogen recovery at 450 °C and 300 kPa. High-purity hydrogen was obtained in all the experimental tests. No coke formation was observed. Post-reaction analysis of the membrane is discussed via scanning electron microscope and energy-dispersive X-ray spectroscopy.

Development of catalytic membranes over PdAu selective films for hydrogen production through the dry reforming of methane

PdAu composite membranes were applied in the preparation of catalytic membranes for hydrogen production. The APTES surface modification of PdAu membranes synthesized by sequential electroless deposition was carried out. Grafted samples were analyzed by means of XPS, SEM-EDS and DRIFT. XPS data showed that surface saturation was reached when a 0.5 v/v % of APTES was employed. The morphological study of the samples showed that after APTES functionalization a slightly change on the PdAu microstructure occurred. It was observed that the APTES-functionalized samples were stable after heating to 823 K for 2 h. In addition, these membranes retained their hydrogen permeability without detrimental effect over their selectivity. Catalytic Ru and Rh-based membranes were synthesized by the dip-coating method. The membranes were evaluated in the dry reforming of methane at 823 K. Even though only low conversions were obtained, these were slightly higher than those obtained for a fixed-bed reactor with the same amount of catalyst.

Hydrogen production via natural gas steam reforming in a Pd-Au membrane reactor. Investigation of reaction temperature and GHSV effects and long-term stability

A composite palladium-gold membrane reactor (MR) was packed with a Ni-based catalyst to perform natural gas steam reforming for the purpose of producing high-purity hydrogen. In a related previous study, high hydrocarbon conversion (> 80%) and hydrogen recovery (~ 65%) were reached at 450 °C and 300 kPa, with high-purity hydrogen obtained in all the experimental tests. The membrane has shown a hydrogen permeance of 1.30 × 10−3 mol/s m2 Pa0.5 at 450 °C, and near infinite H2/Ar ideal selectivity at each operating temperature and a pressure of 50 kPa. In this study, the same membrane was used to investigate the influence of the reaction temperature and the gas hourly space velocity (GHSV) in order to optimize the membrane reactor performance. Specifically, the temperature and the GHSV were varied from 350 °C to 450 °C and from 1300 h−1 to 7800 h−1, respectively, while the reaction pressure and the steam-to-methane ratio were kept constant at 300 kPa and 3.5/1, respectively.In addition, a long-term natural gas steam reforming reaction was conducted for more than 260 h at different temperatures to evaluate the stability of the membrane reactor. The best MR performances were obtained at the lowest GHSV and highest temperature with greater than 80% hydrocarbon conversions and 80% hydrogen recovery. High-purity hydrogen was obtained in all of the experimental tests and no coke formation was observed. Finally, it is demonstrated that the Pd-Au MR, which was operated for 800 h under different reaction conditions, displayed stable performance, in terms of conversion, hydrogen recovery and product selectivity, while the membrane showed stable performance for more than 1000 h with minimal degradation.

High-temperature stability of Pd alloy membranes containing Cu and Au

High-temperature stability of Pd-based membranes benefits their application in steam reformers and sulfur-contaminated H2 streams because both membrane reforming efficiency and sulfur tolerance of Pd alloys increase much with temperature. Hence, we investigated PdCu, PdAu, and PdCuAu membranes supported on porous ceramic tubes between 500°C and 650°C. Remarkably, PdCu membranes were much more stable than Au-containing ones. The H2 permeation rates of some PdAu and PdCuAu membranes declined at 550°C with substantially increasing N2 fluxes. This was triggered by severe morphological deformation of the Au alloy films into stoichiometrically inhomogeneous, cavernous structures. The H2 fluxes of the PdCu membranes started to decline at 650°C with leak flows increasing slightly. Moreover, the PdCu layer morphology remained dense and compositionally homogeneous even after testing for up to 4800h between 500 and 650°C. The strikingly different high-temperature stability can be understood by considering the divergent surface segregation tendencies of Cu and Au and their differing impact on hydrogen solubility in Pd alloys. As a result, Au may desorb much more easily from membranes than Cu leading to structural instability above 500°C during operation in H2. The instability of PdAu membranes at high temperatures may be mitigated by addition of sufficient Cu to obtain ternary membranes with good H2 permeability and better thermal stability.

CFD analysis for the geometry effect of disc-type membrane module on separation performance

A disc-type Pd-Au membrane module was considered, and a computational fluid dynamics (CFD) model was developed to describe the actual flow dynamics and the distribution of H2 flux over the membrane. When the membrane size was increased to develop a module with a large separation capacity, the feed flow rate per unit membrane area decreased, indicating loss of utilization of the membrane area. To increase the utilization, the sizes of the feed inlet tube and retentate tube were varied (cases 1 and 2, respectively). The CFD simulation showed that the feed flow rates per unit membrane area increased by ca. 8% and 10%, respectively, whereas a change in the geometry from circular to a rectangle with rounded edges (case 3) resulted in an increase of approximately 19%. A change in the ratio of the edges (case 4) had a slight influence on the separation performance. The distribution of H2 flux where the geometries in cases 1–3 were combined clearly revealed that most of the membrane area was used to permeate H2; as a result, the number of membranes decreased by approximately 88% upon increasing their size, while the total membrane area remained the same. This indicated improved utilization of the membrane. The proposed approach is expected to be useful for acquiring valuable information on the design of a membrane module with a large separation capacity.

Integrated Experimental–Technoeconomic Analysis of the Lifetime of Pd–Au Membranes

Palladium and palladium/alloy membranes have shown to be an effective technology option for the production of pure hydrogen. In particular, Pd/Au membranes have displayed superior chemical stability and separation performance. However, the lack of industrial operating experience gives rise to technological uncertainties such as the lifetime of the membranes which could potentially prevent the deployment of this new technology option. In light of the above recognition, the primary aim of the present research work is to combine long-term H2 membrane permeance and He leak characterization tests (cumulative testing over 2.2 years) conducted to estimate the lifetime of composite asymmetric Pd/Au membranes at different thicknesses with a comprehensive economic performance assessment framework in the presence of inherent trade-offs between permeance, thickness, and membrane lifetime. The experimental results are depicted in terms of the produced H2 purity and flux at different testing times and various membrane ...

Supported Pd-Au Membrane Reactor for Hydrogen Production: Membrane Preparation, Characterization and Testing

A supported Pd-Au (Au 7wt%) membrane was produced by electroless plating deposition. Permeation tests were performed with pure gas (H2, H2, N2, CO2, CH4) for long time operation. After around 400 h under testing, the composite Pd-Au membrane achieved steady state condition, with an H2/N2 ideal selectivity of around 500 at 420 °C and 50 kPa as transmembrane pressure, remaining stable up to 1100 h under operation. Afterwards, the membrane was allocated in a membrane reactor module for methane steam reforming reaction tests. As a preliminary application, at 420 °C, 300 kPa of reaction pressure, space velocity of 4100 h−1, 40% methane conversion and 35% hydrogen recovery were reached using a commercial Ni/Al2O3 catalyst. Unfortunately, a severe coke deposition affected irreversibly the composite membrane, determining the loss of the hydrogen permeation characteristics of the supported Pd-Au membrane.

Effect of a Pt–ZrO2 protection layer on the performance and morphology of Pd–Au alloy membrane during H2S exposure

We prepared a disc-type Pd–Au alloy membrane, on which a Pt–ZrO2 protection layer was deposited to protect the hydrogen membrane from H2S poisoning. The effect of the protection layer on the performance and morphology of the Pd–Au alloy membrane during H2S exposure was investigated. The hydrogen flux did not decrease significantly in the presence of the protection layer, which covered the surface of the membrane. When the membrane with the protection layer was exposed to a mixture of H2S, the hydrogen flux decreased to 35% of the original value, which is half of the decrease observed with the bare Pd–Au membrane. The hydrogen flux of the poisoned membrane was almost completely recovered after changing back to pure H2 gas. Under harsh conditions, the membrane with the protection layer was found to retain the hydrogen flux and selectivity, while the selectivity of the bare membrane decreased to zero, indicating that it was disintegrated. This means that the Pt–ZrO2 layer prevents direct contact between Pd and H2S, thus protecting the Pd membrane from poisoning.

Enhancement of the Long-Term Permeance, Selectivity Stability, and Recoverability of Pd–Au Membranes in Coal Derived Syngas Atmospheres

Two new large scale (200 cm2) composite Pd and Pd–Au membranes were prepared and tested in an actual coal derived, but desulfurized, syngas in order to further quantify permeance loss due to species other than sulfur and to determine the nature of the contaminants. As in our previous work, membranes were tested at the National Carbon Capture Center (NCCC) in Wilsonville, Alabama. Before the syngas test, the Pd and Pd–Au membranes had thicknesses of 7 and 6.6 μm, H2 permeances at 450 °C of 17.7 and 29.2 N m3 m–2 h–1 bar–0.5, and H2/He selectivities higher than 2700 and 160 000, respectively. The two membranes produced H2 at an exceptionally high purity level of 99.8–99.9%. The selectivity of the Pd–Au membrane was stable for over 473 h in an actual syngas atmosphere at 450 °C and 12.6 bar demonstrating the high robustness and suitability of these membranes in industrial environments. However, as seen in our previous study, the two membranes showed a decrease in H2 permeance upon syngas introduction (rangin...

Synthesis, Annealing, and Performances of Pd–Au Asymmetric Composite Membranes for Hydrogen Purification

Composite asymmetric Pd–Au membranes were prepared on porous Inconel and Hastelloy tubular supports by electroless deposition and galvanic displacement techniques and tested both before and after Au deposition in pure H2 atmosphere. The final membranes with average Au bulk content up to 16.7 wt % were 9–15 μm thick and showed H2/He ideal selectivities in excess of 900 for the entire period of their characterization. The annealing of the as-prepared membranes was conducted through the coating and diffusion mechanism in He and H2 atmospheres at 500 °C. The annealing conditions were chosen after nonisothermal and isothermal HT-XRD studies were carried out on Pd–Au coupons prepared with the same techniques used for the membranes fabrication. After the formation of the Pd–Au layer, the membranes showed steady H2 flux and stable Au gradient on the membrane top layer. Comparisons between permeance for Pd–Au and pure Pd membranes indicated that the membranes with an average Au bulk content of 4.5 and 5.4 wt % had...

PdAu membranes supported on top of vacuum-assisted ZrO2-modified porous stainless steel substrates

To enable the formation of defect-free PdAu composites by electroless plating, the vacuum-assisted ZrO2-modified method was optimized on top of porous stainless steel disks. Both the ZrO2-modified supports and the PdAu synthesized membranes were characterized by XRD (X-ray diffraction), SEM (scanning electron microscopy), EDS (energy dispersive X-ray analysis) and XPS (X-ray photoelectron spectroscopy). SEM cross-section results showed that dense, continuous, defect-free PdAu films were deposited on top of the ZrO2-modified PSS disks, and the EDS data indicated that no significant concentration gradient was present on the thickness. At 400°C and 100kPa, the pure hydrogen permeation flux for a 10μm thick PdAu membrane was about 0.14mols−1m−2 and the H2/N2 ideal selectivity was higher than 10,000.

Gas permeation field tests of composite Pd and Pd–Au membranes in actual coal derived syngas atmosphere

Three large scale (with each a permeable area of 200 cm2) highly structured composite Pd and Pd–Au membranes were prepared and tested in an actual stream of coal derived, but desulfurized, syngas at the National Carbon Capture Center (NCCC) in Wilsonville, Al. The objective of the study was to investigate the long term membrane stability and to establish the long term effects of syngas contaminants in the coal derived syngas other than sulfur compounds on H2 permeance and selectivity. The large scale membranes had thicknesses ranging from 7 to 14 μm, H2 permeances ranging from 20 to 28 Nm3 m−2 h−1 bar−0.5, and an undetectable He leak before the actual syngas test. In the syngas atmosphere, composite Pd and Pd–Au membranes showed an outstanding H2 permeance stability at 450 °C and 12.6 bar for approximately 200 h albeit a fast initial decline upon syngas introduction. The fast permeance decline observed at the introduction of the actual syngas was attributed to possible surface and/or bulk poisoning. An exceptionally high H2 purity level of 99.89% was achieved at 450 °C and 12.6 bar during the entire period of the measurement of over 200 h in syngas atmosphere representing a breakthrough result in the field never reported before with actual syngas from a coal gasification unit.

Effects of heat treatment in air on hydrogen sorption over Pd–Ag and Pd–Au membrane surfaces

Abstract The effects of heat treatment in air on H 2 sorption over the free-standing 25-μm cold-worked Pd–Ag25 wt% and Pd–Au10 wt% membrane surfaces were investigated. The equilibrium sorption results demonstrated that H 2 solubility did not change for both Pd–Ag and Pd–Au membranes before and after air oxidation at 300 °C. The sorption kinetics data revealed a much faster H 2 sorption rates for the as-received Pd–Au membrane than the as-received Pd–Ag alloy. The heat treatment in air accelerated the H 2 sorption kinetics for both membranes at small H 2 equilibrium pressure (EP) of 0.16 kPa. However, the oxidation enhancement extent is greater for Pd–Ag and the disparity becomes more significant when EP increases. The sorption rate enhancement for Pd–Ag membrane existed over a wide range of EPs even though the enhancement effect decreased with increasing EPs. For Pd–Au membrane, the enhancement induced by air oxidation decreased sharply when EP increased because the sorption rate is already very fast for the as-received Pd–Au alloy and there is less latitude for further improvement than Pd–Ag membrane. It is found that the variation of H 2 sorption rates over membrane surfaces correlates to the change of H 2 permeability. After heat treatment in air, the Pd–Ag membrane experienced a systematic increase in H 2 permeability with higher permeability increase at lower pressures while there was basically no permeation increase due to air oxidation effect for the Pd–Au membrane within the pressure range of practical H 2 permeation tests, which are consistent with the previously reported sorption rates hypothesis and results.

Hydrogen production and carbon dioxide enrichment using a catalytic membrane reactor with Ni metal catalyst and Pd-based membrane

We prepared a catalytic membrane reactor (CMR) by adopting a high-performance metal catalyst and Pd–Au membrane to investigate the possibility of hydrogen production concurrently with carbon dioxide enrichment (up to >80%) in a single-stage reactor from a simulated syngas of a coal gasification, via simultaneous WGS reaction and hydrogen separation process. The CO conversion was above 99% and the H2 recovery was above 94% at del-P = 30 bar in a CMR. The best result for the concentration of the enriched CO2 in the retentate side was 85.3% under the conditions of 350 °C, del-P = 30 bar and steam to carbon ratio of 2.0. These results show promise for a feasible simplified process able to achieve CO removal from a high-concentration CO mixture gas coming out of coal gasification via a water-gas shift reaction (WGS), to separate hydrogen and also to enrich CO2 for pre-combustion capture and storage of CO2 (CCS) in substitution for the conventional WGS and CO2 separation stages in integrated gasification and combined cycle process integrated with CCS.

First-principles study of hydrogen permeation in palladium-gold alloys

Density functional theory and lattice model calculations are combined to study the permeability of hydrogen in Pd lightly alloyed with Au. This study shows that small amounts of Au substitutions in Pd leads to, respectively, an increase and decrease of the diffusivity and solubility of hydrogen in the alloy. The competition between these two phenomena depends on temperature and can yield dilute PdAu membranes with a hydrogen permeability higher than pure Pd.