H2 Flux Research Articles

Quantification of the atomic hydrogen flux as a function of filament temperature and H2 flow rate

An isothermal sensor is developed to quantify the atomic hydrogen flux on a surface, which can be located at any distance from the molecular hydrogen cracking unit. This flux is determined from the measured heat effect due to recombination of atomic hydrogen at the sensor surface. The temperature of the sensor was kept constant at 350°C to keep the heat losses constant during the measurement. Other heat flows due to radiative, conductive, and convective phenomena were quantified with targeted measurements. The design of the sensor allows ample area for the atomic hydrogen recombination reaction; thus enabling the flux values to be determined with high accuracy (errors were between ±8.3×1015 and ±3.3×1016atcm-2s-1). The atomic hydrogen flux, generated with a commercial atomic hydrogen source was measured as a function of the filament temperature in the range of 1400-1950°C and H2 gas flow in the range of 7.44×10-6 to 7.44×10-5mol/s (10–100 sccm). These measurements showed that the atomic hydrogen flux increases with both filament temperature and H2 flux.

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 H2 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 H2 flux through the membranes with SV in the MRs.

Hydrogen permeability of the hollow fibers used in H2-based membrane biofilm reactors

Abstract We developed and used steady-state permeation tests and a mathematical model to determine the H2 permeabilities of three hollow fibers (composite, polyester, and polypropylene) commonly used in the H2-based membrane biofilm reactor (MBfR). The H2 permeabilities spanned a wide range: 1.6 × 10−6, 1.8 × 10−7, and 6.6 × 10−8 m3 H2 @ standard temperature and pressure m membrane thickness/m2 hollow fiber surface area d bar for the composite, polypropylene, and polyester hollow fibers, respectively; this represents a ratio of the maximum H2 flux for composite:polypropylene:polyester hollow fibers = 32:3.3:1. The H2 permeabilities were then used to correlate hollow-fiber type to contaminant-removal flux in previous MBfR experiments. Only the experiments with composite hollow fibers were not routinely limited by H2 delivery, although fouling by mineral precipitates lowered the H2 flux in some cases. Since existing contaminant-removal data for the polyester hollow fibers were not adequate, we did special experiments to investigate the effect of H2 pressure on contaminant removal in the MBfR with the polyester hollow fibers. We confirmed that this hollow fiber was operated at its maximum H2-delivery capacity based on its H2 permeability.

Hydrogen transport through a selection of thin Pd-alloy membranes: Membrane stability, H2S inhibition, and flux recovery in hydrogen and simulated WGS mixtures

Resistance to H2S is important for the use of Pd membranes in fossil-fuel power stations with integrated carbon capture. In the current article we present the H2S inhibition and subsequent H2 flux recovery of thin (∼2μm) pure Pd, Pd77Ag23, Pd85Au15 and Pd70Cu30 membranes. Upon H2S exposure in H2/N2 feed mixtures, all the investigated thin Pd-alloy membranes exhibited a sharp decrease of the H2 flux depending of the H2S concentration. At 2ppm, the H2 flux reduces to a value equal to around 20–40% compared to the H2 flux obtained in the absence of H2S; at 20 and 100ppm H2S in the feed the H2 flux only equals 10–20% of its original value. The pure Pd membrane shows an instant failure during the exposure to 100ppm H2S. After removal of 2ppm H2S from the feed stream the H2 flux through the Pd, Pd85Au15 and Pd70Cu30 membranes recovers within 2h. After the exposure to 20ppm H2S only the H2 flux through the pure Pd, Pd85Au15 and Pd70Cu30 membranes recovers to the same extent. As opposite to these three alloys, the H2 flux through the Pd77Ag23 alloy is recovering much more slowly. When exposed to a simulated sour WGS mixture, the Pd85Au15 membrane experiences a loss of H2 flux down to 10–15% of the value obtained applying 62% H2 in N2 as feed. The extent of H2S poisoning is thus of the same magnitude in WGS mixtures and H2/N2 mixtures. No sign of unselective transport is observed for this membrane after 100h of operation in a sour WGS mixture. Post-process characterisation show signs of surface roughening combined with grain growth, whilst multiple small pits are found primarily in the grain boundaries of the membrane feed surface after the long-term operation. This study demonstrates that thin Pd–Au and Pd–Cu membranes survive sour atmospheres, but at a cost of a significant loss of H2 flux.

Hydrogen in the gas plume of an open-vent volcano, Mount Etna, Italy

[1] We report here on the first hydrogen determinations in the volcanic gas plume of Mount Etna, in Italy, which we obtained during periodic field surveys on the volcano's summit area with an upgraded MultiGAS. Using a specific (EZT3HYT) electrochemical sensor, we resolved H2 concentrations in the plume of 1–3 ppm above ambient (background) atmosphere and derived H2-SO2 and H2-H2O plume molar ratios of 0.002–0.044 (mean 0.013) and 0.0001–0.0042 (mean 0.0018), respectively. Taking the above H2-SO2 ratios in combination with a time-averaged SO2 flux of 1600 Gg yr−1, we evaluate that Etna contributes a time-averaged H2 flux of ∼0.65 Gg yr−1, suggesting that the volcanogenic contribution to the global atmospheric H2 budget (70,000–100,000 Gg yr−1) is marginal. We also use our observed H2-H2O ratios to propose that Etna's passive plume composition is (at least partially) representative of a quenched (temperatures between 750°C and 950°C) equilibrium in the gas-magma system, at redox conditions close to the nickel-nickel oxide (NNO) mineral buffer. The positive dependence between H2-SO2, H2-H2O, and CO2-SO2 ratios suggests that H2 is likely supplied (at least in part) by deeply rising CO2-rich gas bubbles, fluxing through a CO2-depleted shallow conduit magma.

The unusual protoplanetary disk around the T Tauri star ET Chamaeleontis

We present new continuum and line observations, along with modelling, of the faint (6-8) Myr old T Tauri star ET Cha belonging to the eta Chamaeleontis cluster. We have acquired HERSCHEL/PACS photometric fluxes at 70 mic and 160 mic, as well as a detection of the [OI] 63 mic fine-structure line in emission, and derived upper limits for some other far-IR OI, CII, CO and o-H2O lines. The HERSCHEL data is complemented by new ANDICAM B-K photometry, new HST/COS and HST/STIS UV-observations, a non-detection of CO J=3-2 with APEX, re-analysis of a UCLES high-resolution optical spectrum showing forbidden emission lines like [OI] 6300A, [SII] 6731A and 6716A, and [NII] 6583A, and a compilation of existing broad-band photometric data. We used the thermo-chemical disk code ProDiMo and the Monte-Carlo radiative transfer code MCFOST to model the protoplanetary disk around ET Cha. Based on these models we can determine the disk dust mass Mdust = (2.E-8 - 5.E-8) Msun, whereas the total disk gas mass is found to be only little constrained, Mgas = (5.E-5 - 3.E-3) Msun. In the models, the disk extends from 0.022 AU (just outside of the co-rotation radius) to only about 10 AU. Larger disks are found to be inconsistent with the CO J=3-2 non-detection. The low velocity component of the [OI] 6300A emission line is consistent with being emitted from the inner disk. The model can also reproduce the line flux of H2 v=1-0 S(1) at 2.122 mic. An additional high-velocity component of the [OI] 6300A emission line, however, points to the existence of an additional jet/outflow of low velocity (40 - 65) km/s with mass loss rate ~1.E-9 Msun/yr. In relation to our low estimations of the disk mass, such a mass loss rate suggests a disk lifetime of only ~(0.05 - 3) Myr, substantially shorter than the cluster age. The evolutionary state of this unusual protoplanetary disk is discussed.

One-Step Preparation of FeCo Nanoparticles in a SBA-16 Matrix as Catalysts for Carbon Nanotubes Growth

Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered 3D cubic Im3m mesoporous silica (SBA-16) matrix were prepared by a novel, single-step templated-assisted sol-gel technique. Two different approaches were used in the synthesis of nanocomposites; a pure SBA-16 sample was also prepared for comparison. Low-angle X-ray diffraction, transmission electron microscopy and N2 physisorption at 77 K show that after metal loading, calcination at 500 degrees C and reduction in H2 flux at 800 degrees C the nanocomposites retain the cubic mesoporous structure with pore size not very different from the pure matrix. X-ray absorption fine structure (EXAFS) analysis at Fe and Co K-edges demonstrates that the FeCo nanoparticles have the typical bcc structure. The final nanocomposites were tested as catalysts for the production of carbon nanotubes by catalytic chemical vapour deposition and high-resolution TEM shows that good quality multi-walled carbon nanotubes are obtained.

A new estimation of the recent tropospheric molecular hydrogen budget using atmospheric observations and variational inversion

Abstract. This paper presents an analysis of the recent tropospheric molecular hydrogen (H2) budget with a particular focus on soil uptake and European surface emissions. A variational inversion scheme is combined with observations from the RAMCES and EUROHYDROS atmospheric networks, which include continuous measurements performed between mid-2006 and mid-2009. Net H2 surface flux, then deposition velocity and surface emissions and finally, deposition velocity, biomass burning, anthropogenic and N2 fixation-related emissions were simultaneously inverted in several scenarios. These scenarios have focused on the sensibility of the soil uptake value to different spatio-temporal distributions. The range of variations of these diverse inversion sets generate an estimate of the uncertainty for each term of the H2 budget. The net H2 flux per region (High Northern Hemisphere, Tropics and High Southern Hemisphere) varies between −8 and +8 Tg yr−1. The best inversion in terms of fit to the observations combines updated prior surface emissions and a soil deposition velocity map that is based on bottom-up and top-down estimations. Our estimate of global H2 soil uptake is −59±9 Tg yr−1. Forty per cent of this uptake is located in the High Northern Hemisphere and 55% is located in the Tropics. In terms of surface emissions, seasonality is mainly driven by biomass burning emissions. The inferred European anthropogenic emissions are consistent with independent H2 emissions estimated using a H2/CO mass ratio of 0.034 and CO emissions within the range of their respective uncertainties. Additional constraints, such as isotopic measurements would be needed to infer a more robust partition of H2 sources and sinks.

Impact of precipitation on the treatment of real ion-exchange brine using the H2-based membrane biofilm reactor

The H(2)-based membrane biofilm reactor (MBfR) was used to remove nitrate and perchlorate from real ion-exchange brine at two different salinities (30- and 50-g/L NaCl). Base production from nitrate reduction to N(2) gas caused the pH to increase, and this exacerbated precipitation of calcium and magnesium carbonates onto the MBfR fibers. The precipitates lowered the H(2) flux to the biofilm and caused a deterioration of denitrification performance that could be reversed by mild citric-acid washing. The addition of acid seems to be the only mechanism to avoid serious precipitation, membrane fouling, and non-optimal pH for denitrification.

Iron–cobalt nanocrystalline alloy supported on a cubic mesostructured silica matrix: FeCo/SBA-16 porous nanocomposites

A series of novel nanocomposites constituted of FeCo nanoparticles dispersed in an ordered cubic Im3m mesoporous silica matrix (SBA-16) have been successfully synthesized using the wet impregnation method. SBA-16, prepared using the non-ionic Pluronic 127 triblock copolymer as a structure-directing agent, is an excellent support for catalytic nanoparticles because of its peculiar three-dimensional cage-like structure, high surface area, thick walls, and high thermal stability. Low-angle X-ray diffraction, N2 physisorption, and transmission electron microscopy analyses show that after metal loading, calcination at 500 °C, and reduction in H2 flux at 800 °C, the nanocomposites retain the well-ordered structure of the matrix with cubic symmetry of pores. FeCo alloy nanoparticles with spherical shape and narrow size distribution (4–8 nm) are homogeneoulsy distributed throughout the matrix and they seem in a large extent to be allocated inside the pores.

The hydrogen permeability of Pd4S

Hydrogen permeates rapidly through pure Pd membranes, but H2S, a common minor component in hydrogen-containing streams, produces a Pd4S film on the Pd surface that severely retards hydrogen permeation. Hydrogen still permeates through the bi-layered Pd4S/Pd structure, indicating that the Pd4S surface is active for H2 dissociation; the low hydrogen permeability of the Pd4S film is responsible for the decreased rate of hydrogen transport. In this work, the hydrogen permeability of Pd4S was determined experimentally in the 623-773K temperature range. Bi-layered Pd4S/Pd foils were produced by exposing pure Pd foils to H2S. H2 fluxes through the bi-layered Pd4S/Pd foils were measured during exposure to both pure H2 and a 1000ppm H2S in H2 gas mixture. Our results show that H2S slows hydrogen permeation through Pd mainly by producing a Pd4S film on the Pd surface that is roughly an order-of-magnitude less permeable to hydrogen (kPd4S=10−7.5exp(−0.22eV/kBT)molH2/m/s/Pa1/2) than pure Pd. The presence of H2S in the gas stream results in greater inhibition of hydrogen transport than can be explained by the very low permeability of Pd4S. H2S may block H2 dissociation sites at the Pd4S surface.

Separability of hydrogen from hydrogen–carbon dioxide mixture across silica–silicalite-1 film

Colloidal silica particles, grown on a mesoporous silica layer using macroporous alumina substrate as a support, were used to separate hydrogen from carbon dioxide. The particles transformed into rectangular interlocking silicalite-1 structures with size approximately 8×4×4μm, oriented epitaxially with film thickness of ca. 22μm. The silicalite-1 particles grew in size due to the effect of structure directing agent (SDA), Oswald ripening and hydrothermal synthesis that promoted the growth of the colloidal particles into crystals. Permeation experiment using silicalite-1 showed that CO2 flux decreased and H2 flux increased with increase in temperature. The separability of H2 that was unity at the lower temperature became increased in value as the temperature was raised.

An Atomic‐Scale View of Palladium Alloys and their Ability to Dissociate Molecular Hydrogen

AbstractPalladium and its alloys play a central role in a wide variety of industrially important applications such as hydrogen reactions, separations, storage devices, and fuel‐cell components. Alloy compositions are complex and often heterogeneous at the atomic‐scale and the exact mechanisms by which many of these processes operate have yet to be discovered. Herein, scanning tunneling microscopy (STM) has been used to investigate the atomic‐scale structure of Pd–Au and Pd–Cu bimetallics created by depositing Pd on both Au(111) and Cu(111) single crystals at a variety of surface temperatures. We demonstrated that individual, isolated Pd atoms in an inert Cu matrix are active for the dissociation of hydrogen and subsequent spillover onto Cu sites. Our results indicated that H spillover was facile on Pd–Cu at 420 K but that no H was found under the same H2 flux on a Pd–Au sample with identical atomic composition and geometry. In the case of Au, significant H uptake was only observed when larger ensembles of Pd were present in the form of nanoparticles. We report experimental evidence for hydrogen’s ability to reverse the tendency of Pd to segregate into the Au surface at catalytically relevant temperatures and our STM images reveal a novel H‐induced striped structure in which Pd atoms aggregated on top of the surface in regularly spaced rows. These results demonstrate the powerful influence an inert substrate has on the catalytic activity of Pd atoms supported in or on its surface and reveal how the atomic‐scale geometry of Pd–Au alloys is greatly affected by the presence of hydrogen.

Model development and validation of hydrogen transport through supported palladium membranes

Abstract Porous matrices are often used to provide structural support to thin Pd-based metallic membranes in H2 separation applications. Optimizing such composite membranes requires detailed understanding of all possible rate-controlling processes including surface and bulk processes in the metal and diffusion of gases through the porous media. In the work described in this paper, we fabricate a composite membrane by depositing a thin (∼5–6 μm) Pd film on a porous α-Al2O3 tube and then measure the H2 permeance of this composite membrane over a range of operating conditions. The rate-controlling processes for the H2 permeation are evaluated with a computational model which combines a detailed thermo-kinetic Pd–H2 interaction model and a porous media transport model. The Pd–H2 thermo-kinetic model is validated against literature data, and the porous media transport model is independently calibrated using experimental measurements. The combined composite membrane model gives good agreement with experiments over a large range of temperatures (250–450 °C) and H2 partial pressures (100–385 kPa). This validated model is then used to analyze the importance of design parameters such as Pd thickness and support micro-structure on H2 flux through the membrane. These parametric studies will also aid in assessing trade-offs between membrane structural robustness and overall performance.

XPS Study of the Influence of CO2 on the H2 Flux through a Composite Membrane Made of Palladium and Porous Stainless Steel

Plating a Pd membrane on the outer surface of a porous stainless steel (PSS) tube facilitates rapid H2 permeation with an absolute selectivity; the permeation is based on the solution-diffusion transport mechanism. Reforming of carbon hydride is a well-known method of manufacturing H2. CH4 dry reforming is performed in a membrane reactor using a thin Pd membrane. In this reaction, chemisorbed CO2 is known to affect H2 permeation behavior. The effect on H2 permeation and membrane stability were investigated by performing a permeation test and by XPS. After H2 was permeated through the Pd membrane, CO2 was flowed over the membrane for 1 h, and H2 was once again permeated through the membrane. In the experiment, the temperatures were 623, 723, and 823 K, the differential pressure was 0.1 MPa, and the feed rate was 10 mL/min. CO2 was replaced by He and the experiment was performed under the same conditions. After the second H2 permeation, the H2 flux increased with the time and attained its original value. The time required to attain the original value was dependent on the gas and the temperature. In the case of the He flow, the time required was only 5 min and was independent of the temperature. On the other hands, in the case of the CO2 flow, the required times were 10 min and 25 min at 623 K and 723 K or 823 K, respectively. After the CO2 flow, the XPS spectra of the surface of the Pd membrane were measured. The spectra showed that PdO was formed on the Pd membrane by oxidation of the Pd surface. The PdO was formed by chemisorbed CO2, and the H2 permeability was degraded by PdO; however, PdO was easily reduced to Pd by the H2 flow. When the experiment involving CH4 dry reforming was performed in the Pd membrane reactor with conventional Pt-based catalysts and a sweep gas for 12 h at 873 K, the H2 flux, H2 selectivity, and H2 recovery were ca. 6 mL/min, 98% (calculated by excluding the sweep gas), and 80%, respectively.

Hydrogen Permeability and Membrane Durability of Novel Pd/.GAMMA.-Alumina Graded Membrane When a Sweep Gas is Used

In this study, Pd/γ-alumina graded membranes were fabricated by an electroless plating method on nanoporous γ-alumina formed by anodic oxidation of an etched aluminum support, and the H2 permeability of the membranes was determined. The Pd/γ-alumina graded membrane on the etched aluminum support was prepared by carrying out electroless plating for 30 min, and for 60 min. In the first case of electroless plating for 30 min, Pd/γ-alumina graded membrane with a thickness of 2.5 μm, H2 flux of 0.28 mol m−2 s−1, and H2/N2 permselectivity of 817 were obtained. In the second case of electroless plating for 60 min, Pd/γ-alumina graded membrane with a thickenss of 4.5 μm, H2 flux of 0.17 mol m−2 s−1, H2/N2 permselectivity of 5259 were obtained. In durability tests, it was observed that the H2 flux increased with time in the initial stage and then become approximately constant after 70 h. The graded membrane exhibited high durability. The membrane formed by 60 min of electroless plating was heated at 773 K for 12 h in a furnace (preheated Pd/γ-alumina graded membrane). The H2 flux of the preheated graded membrane remined approximately constant during the durability test. This result implies that the Pd/γ-alumina graded membrane for the optimum H2 permeation was not obtained by heating because the Pd crystals formed by electroless plating, and recrystallized to form larger grains; this happened because of heating, hybridization of γ-alumina and Pd, and influence of annealing. Finally, we successfully fabricated novel Pd/γ-alumina graded membranes that exhibited high H2/N2 permselectivity, H2 permeability, and durability.

Gas Permeable‐Membrane for Hydrogenotrophic Denitrification

AbstractThe membrane biofilm reactor (MBfR) involves a process in which the membrane provides effective H2 utilization as an electron donor. MBfRs overcome the problems caused from the particularly low solubility of the gas delivered into a liquid by means of a gas‐permeable membrane. The study demonstrates that the MBfR is successful in the effective removal of nitrate from drinking water or nitrified water, involving a hydraulic retention time of 25 min for a maximum nitrate removal of 98%. The H2 flux reaches a level of 1.24 g H2/m2 d when the denitrification rate achieves 3.5 g N/m2 d at 0.2 atm H2 pressure. The results indicate that the biofilm on outer walls of the gas permeable membrane utilizes all of the H2 gas effectively as an electron donor since the H2 is not detectable in the bulk liquid. In the future, the MBfR could be an attractive process for water and wastewater engineers due to its applicability for treatment of secondary effluent from industrial and municipal wastewater and drinking water.

Morphological changes of InGaN epilayers during annealing assessed by spectral analysis of atomic force microscopy images

During annealing, the morphologies of thin InGaN epilayers have been observed to change from a terraced structure to a network of interlinking InGaN strips separated by troughs. This change in morphology may contribute to high efficiencies in some GaN-based light emitting diodes (LEDs) if the InGaN is exposed to elevated temperatures without a protective GaN capping layer. Here, we investigate the changes in morphology which occur when InGaN epilayers are annealed at their growth temperature under NH3, N2, and a small H2 flux. We observe that while the layers initially roughen, more extended anneals lead to the surface becoming smooth and terraced once again. Power spectral density analysis of atomic force microscopy data is used to show that the dominant mechanism for roughening is loss of material from pre-existing pits, while the dominant smoothening mechanism is surface diffusion. This mechanistic analysis may be relevant to the growth of InGaN quantum wells in LED structures.

Thermokinetic modeling and parameter estimation for hydrogen permeation through Pd0.77Ag0.23 membranes

Optimizing thin-film Pd-alloy membranes for H2 purification requires detailed understanding of all possible rate-controlling processes including surface adsorption, solution into the bulk metal, bulk diffusion, dissolution, and desorption. H2 permeation through Pd0.77Ag0.23 alloy is analyzed by extending pure Pd diffusion models in the literature with a microkinetic model for surface and bulk reactions between H2 and the alloy. Experimental data and first principles studies from literature are used to estimate and critically modify model parameters. Model results compare extremely well with literature over a broad range of membrane thicknesses (1–20 μm) and operating conditions. The importance of surface processes increase with decreased film thicknesses (below 10 μm) characteristic of recent advances in membrane technology. Previously observed enhancement of H2 flux after heat treatment in air is explained by increased surface area. Sensitivity of H2 flux to various thermokinetic parameters provides further insight as to when different processes are rate-controlling.

Control of Ge/Si intermixing during Ge island growth

The surface energy and growth kinetics during Ge deposition on Si(001) were modified by growing the films in a phosphine environment. Islands were formed under a H2 flux as well as in a PH3/H2 atmosphere, but the morphologies were different. The presence of PH3 not only affects the island shape and size but also the composition profile. The dramatical inhibition of Ge/Si intermixing during growth leads to islands richer in Ge compared to undoped islands.