Pd Membrane Surface Research Articles

Catalytic palladium membrane reactors for one-step benzene hydroxylation to phenol

Pd membranes can be applied to many hydrogenation or dehydrogenation reactions for chemical production. Such membrane reactors can realize the simultaneous separation and reaction in one unit. However, the Pd membrane is easily poisoned by hydrocarbons or other impurities at high temperatures and itself does not provide sufficient catalytic efficiency. To overcome these problems, here we report a novel bifunctional titanium silicalite (TS) zeolite modified Pd capillary membrane microreactor for one-step hydroxylation of benzene to phenol. Using the state of art technology to uniformly anchor the monolayer of TS nanoparticles on the Pd membrane is still a challenge. In this work, the TS nanoparticles were attached the Pd membrane surface via a novel and efficient covalently bonding method using alkoxysilane linkers. The effects of various Pd membranes with different TS zeolite loading on the reaction efficiency and long-term stability were investigated. The benzene conversion and phenol yield could be dramatically increased by applying the TS nanoparticles as the catalyst compared to the blank Pd membranes. A maximum benzene conversion of 23.4% and phenol yield of 22.3% were achieved at 473 K and 100 kPa, much higher than that typically reported in Science (Niwa et al., 2002) with the benzene conversion and phenol yield of 13.3% and 11.3%, respectively. The TS zeolite modified Pd membranes also displayed good stability for low temperature separation and reaction during 110-h long-term-test at 473 K and 100 kPa. The deposited uniform TS catalyst in a monolayer not only provides good catalytic properties to improve reaction efficiency but also works as the diffusion barrier to protect the Pd membrane by avoiding the direct contact of the hydrocarbon molecules and Pd surface. The possible reaction pathway for TS zeolite modified Pd membrane microreactor for the direct benzene conversion to phenol was also elucidated.

Influence of Hydrogen Sulfide and Redox Reactions on the Surface Properties and Hydrogen Permeability of Pd Membranes

Although hydrogen sulfide (H2S) was always a negative factor leading to the reduction of hydrogen permeability of palladium (Pd) membranes, its proper application could result in a positive effect. In this study, pure Pd membranes were firstly reacted with H2S at 23–450 °C, and then treated by redox reactions. Afterwards, the hydrogen permeability was tested under different reaction conditions using a hydrogen penetrant testing device. Moreover, both products and morphology changes occurred on the Pd membrane surface were analyzed using XRD, XPS and SEM. The results showed that H2S was dissociated to produce sulfides at 23 °C. With a rise of temperature, a regular change took place in the reaction products, morphology of the Pd membrane surface and hydrogen permeability. Adsorbed impurities such as sulfides and free carbon on the Pd membrane surface were removed by the redox treatment. The hydrogen permeability was improved by about 80% for the Pd membrane material subjected to the treatment method stated the above against the untreated one.

Confined and in-situ zeolite synthesis: A novel strategy for defect reparation over dense Pd membranes for hydrogen separation

Abstract The manufacturing of ultrathin Pd based hydrogen separation membranes with high permeation performance has great potential in industrial applications. However, there usually exists a trade-off between reduced thickness and high hydrogen permselectivity which are regarded as a pair of contradictory requirements. In this study, a novel technique of confined and in-situ zeolite synthesis for defects reparation is proposed to improve the hydrogen permselectivity of preformed Pd membranes without further increasing their thickness. Two technology routes are developed for different defect dimensions. Zeolites can only grow inside large defects in one route, which is suitable for repairing broken membranes. In the other route, a thin zeolite layer is formed on Pd membrane surface, covering small defects for further enhancing selectivity. The purity of permeate hydrogen is enhanced from 99.884 to 99.994% after zeolite reparation. High purity hydrogen (99.986%) can be separated from 75% H2/N2 mixture with a hydrogen recovery of 91% by the repaired membrane at 500 °C and 0.4 MPa.

Surface activation inspires high performance of ultra-thin Pd membrane for hydrogen separation

Pd-based membranes play a critical role in the field of hydrogen purification and small-scale hydrogen generation. The microstructure and composition of ultra-thin Pd membranes immediately determines their performance and stability, which are however complex and elusive. We applied for the first time a novel technique of environmental scanning electron microscopy (ESEM) together with EDS analysis for in-situ investigation of microstructure and composition of Pd composite membranes under gas-thermal treatment. It enables step by step analysis of the evolution process and offers more meticulous details compared to ex-situ technologies through analysis at exactly the same location of membrane samples. Here we observed the elimination of Pd crystallites as well as the homogenization/diffusion along the whole membrane surface following high T treatment under inert gas or H2 atmosphere. Gradual evolution of microhillocks on Pd membrane surface was resulted due to α−β phase transition or exposure to either O2 or H2O, which can be effectively “smoothened” out during H2 treatment. The exposure to steam exhibited a mild oxidation effect without pinhole formations, which appears a more appropriate surface cleaning process compared to air/oxygen treatment. A significant interlayer diffusion was revealed between Pd layer and ZrO2/alumina substrate in H2/O2/H2O atmosphere.

One-step hydroxylation of benzene to phenol via a Pd capillary membrane microreactor

A novel Pd capillary membrane microreactor for one-step hydroxylation of benzene to phenol was synthesized and investigated to showcase the effectiveness of ‘Niwa concept’. Reaction parameters including H2/O2 ratio and temperature were systematically studied for their effects on benzene conversion and phenol yield. A detailed examination of different membrane reactors, feed mode and long-term reaction stability was also conducted. Pd capillary membrane displayed good stability for low temperature separation and reaction due to the excellent anchorage of Pd layer into the porous α-alumina support. An optimum H2/O2 ratio was identified at 473 K with the benzene conversion of 19.6% and phenol yield of 18.1%. An increase in reaction temperature caused not only an increase in benzene conversion but also a decrease in phenol selectivity. A comparison between our work and the literature results was also made to discuss the feasibility of the membrane reactor concept. Experimental results proved that narrow flow channels and larger Pd membrane surface area-to-volume ratios provided more effective area of Pd interface and promoted the radial diffusion of reactants, enabling the reactive species more opportunities to react directly with benzene resulting in high benzene conversion.

Effect of surface oxidation on the surface condition and deuterium permeability of a palladium membrane

Oxidation and deoxidation of a Pd membrane was conducted in a quartz tube oven in a temperature range of 23–500°C. The micromorphology and chemical composition of the Pd membrane surface was characterized using scanning electronic microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Micropores and PdO began to form on the Pd membrane surface after oxidation at 240°C for 1h and their quantity increased gradually with increasing temperature. A rough Pd membrane surface was obtained when the temperature rose to 500°C. The PdO on the Pd membrane surface was completely deoxidized once more using H2 at room temperature, but the rough surface morphology caused by oxidation remained. The deuterium permeability of the Pd membrane was tested using special equipment in the China National Key Laboratory and the results indicated that the rough Pd membrane surface had higher deuterium permeability than the original membrane. The improved deuterium permeability could be attributed to the higher Pd membrane surface area, which provided deuterium atoms with more adsorption sites and dissociation sites.

Direct H 2O 2 synthesis over Pd membranes at elevated temperatures

H 2O 2 was directly synthesized from the elements over supported Pd membranes in O 2-saturated, aqueous H 2SO 4 (0.03 mol/l) between 303 and 348 K while feeding H 2 through the membrane. The H 2O 2 yield increased with temperature despite H 2O 2 selectivity declining simultaneously due to faster H 2O 2 decomposition. Both H 2O 2 yield and selectivity improved with H 2 feed pressure, however. Comparison of H 2 consumption during H 2O 2 synthesis with the intrinsic membrane H 2 permeability and the energetics of H 2 oxidation on Pd surfaces revealed that the H 2O 2 productivity gain with temperature was primarily owed to the acceleration of reaction kinetics. The H 2O 2 selectivity enhancement with H 2 feed pressure is attributed to the inhibition of direct H 2O formation as a result of hindered O 2 dissociation due to blocking of O atom adsorption sites on the membrane surface because of increasing occupation by H atoms. H 2O 2 decomposition and reduction on the Pd membrane surface accelerate much faster with temperature than H 2O 2 formation and need to be suppressed in order to maximize H 2O 2 yield and H 2 utilization at higher temperatures.

Improved photocatalytic deposition of palladium membranes

In order to enhance ultra-thin palladium (Pd) membranes integrity, an improved photocatalytic deposition method (PCD) was developed for the Pd/TiO 2 composite membranes. The PCD reactions for Pd membranes occurred in the solution previously and the mechanism of the improved PCD method is interpreted by the photocatalytic reaction at the TiO 2 semiconductor/Pd(II) liquid film interface by directly irradiating. The reaction processes conducted in the liquid film could effectively avoid the homogeneous reaction and increase the irradiation efficiency. Ultra-thin tubular Pd membranes with the thickness of about 0.4 μm on TiO 2 semiconductor membranes were fabricated by the method. X-ray diffraction (XRD) and SEM analysis of the Pd/TiO 2 composite membranes indicated that the metal existed in the form of pure Pd element and Pd membrane surface appeared homogeneous. A gas permeation experiment was conducted under temperature of 612–704 K and pressure of 0.05–0.12 MPa. Hydrogen permeance and selectivity of hydrogen/nitrogen (H 2/N 2) were 4.8 × 10 −6 mol m −2 s −1 Pa −1 and 120 at 704 K, respectively.

Hydrogen generation in a Pd membrane fuel processor: assessment of methanol-based reaction systems

The feasibility of a Pd membrane fuel processor that integrates several methanol-based chemistries and hydrogen purification steps is assessed. The assessment involves membrane reactor simulations to determine the effects of operating and design parameters on performance metrics including hydrogen utilization, hydrogen productivity, device volume, and Pd requirements. Methanol decomposition (direct and oxidative) on Pd/SiO 2, methanol steam reforming (MSR) on Cu/ZnO/Al 2O 3, and methanol partial oxidation (MPOX) on Cu/Al 2O 3 are evaluated. The membrane reactor model includes detailed treatments of the catalytic kinetics from the literature, accounts for reaction on the Pd membrane and hydrogen permeation inhibition by site blockage, among other features. The simulations reveal that a maximum in the hydrogen productivity occurs at an intermediate value of the space velocity, implying a trade-off between reactor size, methanol conversion and hydrogen utilization. The assessment involves a determination of the Pd membrane surface to reactor volume ratio that maximizes productivity and the requisite Pd to realize that productivity. We show that MSR on Cu/ZnO and MPOX on Cu are promising reaction systems to practice the membrane concept for fuel processing, whereas direct methanol decomposition is reaction limited, making it infeasible. Several approaches for improving membrane fuel processor performance are evaluated and discussed. We show that oxygen addition can increase the hydrogen productivity in the Pd system, while water addition is beneficial for the MPOX system. The extent of enhancement in both cases depends on supply rate and kinetic factors.

Hydrogen permeation through surface modified Pd and PdAg membranes

The hydrogen permeation through surface modified Pd and Pd 70Ag 30 membranes has been studied at temperatures between 100 and 350°C. Silver has been evaporated on Pd and Pd 70Ag 30 foils with a thickness of 25 μm in order to study the role of the surface composition in comparison with the membrane bulk composition. The Pd 70Ag 30-based membranes display the largest permeation rates at temperatures below 200°C, while Pd membranes with 20 Å silver evaporated on the upstream side show the largest permeation rates above 200°C. There are, consequently, different rate limiting processes above and below 200°C: at temperatures below 200°C, the bulk diffusion through the membrane is rate limiting, while at temperatures above 200°C, the influence of the surface composition starts to become significant. It has further been concluded that a sharp silver concentration gradient from the surface to the bulk is important for the hydrogen permeation rate at temperatures above 200°C. Adding oxygen to the hydrogen supply will almost totally inhibit the hydrogen permeation rate when a pure Pd membrane surface is facing the upstream side, while for silver-containing surfaces the presence of oxygen has almost no effect. On a clean Pd surface, oxygen effectively consumes adsorbed hydrogen in a water forming reaction. With Ag on the surface, no water formation is detected. Co-supplied CO inhibits the permeation of hydrogen in a similar manner on all studied membrane surfaces, independent of surface silver content.

Hydrogen permeation through palladium after hydrogen-thermal treatment and exposure to a plasma of glow discharge in hydrogen, helium and argon

Hydrogen permeability of palladium membranes was investigated before and after their exposure of to a glow-discharge plasma. A preliminary treatment of activated (subjected to a long-time annealing in vacuum at about 1000 K) palladium by a glow-discharge plasma does not cause any changes in permeability. The results of plasma treatment of a partially activated (heated for 1 h in vacuum) palladium much depend on irradiation conditions. For treatment in the cathode regime (Pd-membrane serves as a cathode) the rates of permeability surface processes increase. As opposed to hydrogen-heat treatment, the time for reaching the permeability characteristics corresponding to the activated state is essentially reduced. Experiments on irradiating Pd-membranes with different mass ions (H +, He +, Ar +) and the determination of the degree of surface decarburization from the methane yield during annealing of irradiated samples in the hydrogen atmosphere indicated that the permeability activation effect is caused by decarburization phenomena due to irradiation. The treatment in the anode regime causes a decrease in the rate of hydrogen permeation through Pd. This is due to the occurrence of thin films deposited from the sputtered chamber wall on the Pd-membrane surface.

Isotopic product distributions of CO2 electrochemical reduction on a D flowing-out Pd surface in protonic solution and reactivities of “subsurface” hydrogen

The electrochemical reduction of carbon dioxide in 0.1 M KHCO 3 + H 2 O was investigated on a Pd membrane surface to which deuterium was supplied continuously from the back face. Isotope distributions of hydrogen and formic acid were measured. In comparison with simple hydrogen evolution under an N 2 atmosphere, the formation of HD was suppressed. This suggests the existence of hydrogen-rich and deuterium-rich regions on the electrode surface and is attributable to poisoning by the adspecies. More than 95% of the formic acid was HCOO − , i.e. it was formed by the incorporation of hydrogen from H 2 O. Considering that no formic acid was formed in the absence of hydrogen supplied from the back face, the surface hydrogen converted to formic acid is probably unstable, and is generated only on the hydrogen “flowing-out” surface. The effect of thiourea was investigated for poisoning or promotion of hydrogen-related processes and CO 2 reduction. The mechanism of the electrochemical reduction of CO 2 is discussed in relation to the role of surface and subsurface hydrogen.