Hydrogen Surface Coverage Research Articles

Impact of promoter on the cobalt based Fischer–Tropsch synthesis: a comprehensive kinetic study

A comprehensive kinetic model for cobalt based Fischer–Tropsch synthesis products was developed based on the recently proposed generalized mechanism for monomer formation. The model was evaluated with recent kinetic data from the literature (Co–Re/Al2O3) as well as newly obtained experimental data (Co–Zr/Al2O3). The model results were in good agreement with experimental data. Also estimated model parameters were statistically and physically significant. The model can predict the effect of promoter on the species surface coverage. According to the results, the surface of both catalysts is mainly covered with CO, monomer and carbide. At the same operating condition, carbide surface coverage was significantly higher in the case of Zr promoted catalyst (13.8%) compared to that of Re promoted catalyst (3.3%). Moreover the utilization of Zr promoter resulted in less hydrogenated carbon species (16.3%) which was about 20% in the case of Re promoter. The results also indicated that the product selectivity was mainly governed with monomer, hydrogen and vacant site surface coverage ratios.

Interface binding and mechanical properties of MXene-epoxy nanocomposites

Thermosetting epoxy polymers exhibit excellent stiffness and strength and are commonly utilized as matrices to make fiber reinforced composites. However, epoxy thermosets are brittle and typically possess a low fracture toughness that restricts their applications. One promising mechanism for improving mechanical properties of epoxy is the integration of micro- and nano-scale fillers. MXenes, a large family of 2D transition-metal carbides, carbonitrides, and nitrides, can be used to produce multifunctional polymer nanocomposites due to their excellent electrical, thermal, and mechanical properties. We employed density functional theory and coarse-grained molecular dynamics simulations to evaluate binding energy and microscopic mechanisms of fracture under uniaxial tension for MXene-epoxy composites. The simulation results were verified by manufacturing Ti3C2Tx MXene-epoxy composites and studying their structure and fracture surfaces. MXene-epoxy binding energies are largely unaffected by MXene type (Ti2CTx or Ti3C2Tx). Binding between Ti3C2Tx and epoxy becomes stronger with less hydrogen coverage of Ti3C2Tx surface due to increase in favorable electrostatic interactions. The Young's modulus of MXene-epoxy composites is greater compared to the neat epoxy which originates from stress transfer between the matrix and the nanofiller, the modulus linearly increases with the filler loading up to 1 vol %. At higher filler contents, the increase of the modulus is reduced due to filler aggregation. Void formation was detected near edges of the particles in MXene-epoxy composites under deformation from both experimental and simulation studies of the fracture surfaces. From these observations, we expect the MXene fillers to improve epoxy toughness and enhance its mechanical performance.

Aqueous phase catalytic and electrocatalytic hydrogenation of phenol and benzaldehyde over platinum group metals

The mechanisms of aqueous-phase thermal catalytic hydrogenation (TCH) and electrocatalytic hydrogenation (ECH) of organic molecules over Pt group metals are not as well-understood as gas-phase thermal catalytic hydrogenation. In gas-phase, the reactions generally occur via a Langmuir-Hinshelwood mechanism with adsorbed hydrogen adding to adsorbed organics. Here, we show that the rates, reaction orders and activation energies for TCH and ECH of phenol and benzaldehyde on Pd, Pt, and Rh can be explained with a simple kinetic model based on similar Langmuir-Hinshelwood mechanisms. For Pt/C, the adsorption equilibrium constants for the organics needed to fit the rate data are consistent with independently-measured values, provided we assume that the rates are dominated by (1 1 1)-like sites, in agreement with reported particle size effects. The reaction rate of Pd in the ECH of benzaldehyde increases with the surface hydrogen coverage. The state of Pd during ECH of phenol and benzaldehyde are very different, with a high concentration of adsorbed H in the presence of phenol, but not in the presence of benzaldehyde, consistent with benzaldehyde’s stronger binding to the surface. In consequence, Pd is converted to β-PdHx during the hydrogenation of phenol but not benzaldehyde. This is proposed to explain the much lower activity of Pd for hydrogenation of phenol compared to benzaldehyde. The measured low coverage of H on Pd in the presence of benzaldehyde is in agreement with the high selectivity/Faradaic efficiency of protons to benzaldehyde hydrogenation to benzyl alcohol. The decrease in apparent activation energy for ECH versus TCH can also be understood within this same kinetic model. The combination of ECH and TCH kinetics and spectroscopy has, thus, allowed to deduce a microkinetic model for these hydrogenation reactions.

Effect of dislocation cell walls on hydrogen adsorption, hydrogen trapping and hydrogen embrittlement resistance

This study investigated the effects of dislocation patterns introduced by cold rolling deformation on the hydrogen absorption properties, hydrogen trap behaviors and hydrogen embrittlement (HE) resistance for Armco iron under electrochemical charging. The surface hydrogen coverage first increases and then decreases with increasing deformation, and reaches a maximum at 50% deformation. Dislocation cell walls appear when the deformation is greater than 50%. Furthermore, the HE resistance is improved when the deformation reaches 80%. Because the dislocation cell walls can be used as effective hydrogen traps when the dislocation density reaches a sufficiently high value.

Kinetics of Furfural Hydrogenation over Bimetallic Overlayer Catalysts and the Effect of Oxygen Vacancy Concentration on Product Selectivity

AbstractFurfural hydrogenation over silica‐supported Co@Pd and Cu@Pd bimetallic overlayer catalysts was studied in this work. Both overlayer catalysts showed enhanced reactivity compared to pure Pd in furfural hydrogenation. Langmuir‐Hinshelwood kinetic study revealed that this enhanced reactivity could be attributed to the decreased hydrogen surface coverage on catalyst surface of overlayer catalysts compared to pure Pd during reaction, releasing more available active sites for surface reaction or furfural adsorption to take place. The effect of oxygen vacancy concentration on furfural hydrogenation product selectivity was also studied. It is proposed that oxygen vacancies were crucial for both furfural hydrodeoxygenation and aromatic ring hydrogenation, depending on the concentration of oxygen vacancies. This work provides new perspective for tuning the furfural hydrogenation product selectivity by altering the oxygen vacancy concentration.

Thermodynamic and kinetic roles of H2 in structure evolution of urchin-like Co: A density functional theory study

Small gas molecules acting as good capping agents play important roles in controlling the morphologies and surface structures of metal nanocrystals. In the present work, the thermodynamic and kinetic roles of H2 molecules in the morphology of Co nanocrystals were systematically studied by density functional theory (DFT). The Gibbs surface free energies of Co(100), Co(110), and Co(111) at different hydrogen surface coverages were determined by ab initio thermodynamics. The phase diagram of stable H coverage on each plane was obtained and morphology evolution of the Co nanocrystals with various surface hydrogen coverages was predicted by the Wulff construction. Addition of H2 changes the facet stability, generating diverse morphological Co nuclei. The kinetic role of H2 in adatom Co surface diffusion at different H coverages was investigated by DFT. The results suggest that surface H hinders Co surface diffusions, except for Co(100) at 0.56 monolayer coverage. The projected density of states gives deeper insight into the electronic structures of Co adatoms with addition of the surface H atoms, which affect its surface diffusion ability.

Hydride Formation Diminishes CO2 Reduction Rate on Palladium.

The catalytic hydrogenation of CO2 includes the dissociation of hydrogen and further reaction with CO2 and intermediates. We investigate how the amount of hydrogen in the bulk of the catalyst affects the hydrogenation reaction taking place at the surface. For this, we developed an experimental setup described herein, based on a magnetic suspension balance and an infrared spectrometer, and measured pressure‐composition isotherms of the Pd−H system under conditions relevant for CO2 reduction. The addition of CO2 has no influence on the measured hydrogen absorption isotherms. The pressure dependence of the CO formation rate changes suddenly upon formation of the β‐PdH phase. This effect is attributed to a smaller surface coverage of hydrogen due to repulsive electronic interactions affecting both bulk and surface hydrogen.

Pulsed DC bias for the study of negative-ion production on surfaces of insulating materials in low pressure hydrogen plasmas

In this work, negative-ion production on the surface of a sample negatively DC biased in a hydrogen plasma is studied. The negative ions created under positive ion bombardment are accelerated toward the plasma, self-extracted, and detected according to their energy and mass, by a mass spectrometer placed in front of the sample. The use of a pulsed bias allows applying a quasi-DC bias on insulating material during a short period of time and offers the possibility to extend the measurement method to nonconductive samples. The pulsed-bias tests were performed first with highly oriented pyrolytic graphite (HOPG), a conductive material, to demonstrate the feasibility of the method. By changing the pulsed-bias frequency, it was possible to obtain HOPG material with different hydrogen surface coverages and hence different surface states leading to an increase of negative-ion production by up to 30%–50% as compared to the continuous bias case. To establish a protocol for insulating materials, charge accumulation on the surface during the bias pulse and influence of the bias duration and frequency were explored using microcrystalline diamond (MCD) thin layers. By using a pulse short enough (10 μs) at 1 kHz frequency, it has been possible to measure negative-ions on the MCD sample at a quasi-constant surface bias of 130 V, with only 1 V variation during the measurement. Negative-ion surface production on MCD has been studied in pulsed mode with surface temperature from room temperature to 800 °C. It is shown that pulsing the bias and increasing the temperature allows limiting defect creation on MCD which is favorable for negative-ion production. Consequently, at 400 °C, the yield on MCD in pulsed mode is one order of magnitude higher than the yield on HOPG in continuous mode at room temperature.

(Invited) Low-Power Nanoscale Sensors for Low-Concentration Hydrogen Detection in Expired Air

Light-weight molecular sensors have attracted growing interests in recent years, since they have a potential to be used for easy, fast health condition check. Some molecules in breath are consider as markers of some diseases. For example, acetone is an indicator of diabetes mellitus. Nonanal is consider to be one of the biomarkers of lung cancer. If we can realize small-size and low-power sensors that detect a specific molecule selectively, the sensors will be integrated with mobile terminals such as smart phone and the concentration of several molecules in breath can be checked regularly for a long time of period. The long-time monitoring of changes in biomarker concentrations could enable extremely early-stage diagnosis of diseases. Therefore, the development of sensors to detect molecules in expired air is strongly demanded. In this work, we will introduce two type of nanoscale hydrogen sensors. One is nanoscale Pt film sensors that accurately detect the concentration of hydrogen even at high relative humidity. The other is suspended graphene sensor functionalized with Pd nanoparticles that can detect both hydrogen and water selectively depending on the bias voltage applied to the sensor. Hydrogen is an important biomarker for the human digestive system and its concentration ranges from sub-ppm to several-hundred ppm. However, accurate detection of ppm-level hydrogen in breath is difficult due to competing detection of water which is included in breath with high concentration. Here, we fabricated Pt thin films that respond to hydrogen in air at concentrations as low as 500 ppb [1]. In both dry and humid air, these films have almost identical response to hydrogen, i.e., their resistance decreases linearly with increasing hydrogen concentration regardless of relative humidity. Even at high relative humidity, these Pt thin films can detect ppm-level hydrogen. Based on the chemical kinetics, namely the adsorption and desorption of hydrogen and oxygen, the sensor response is quantitatively described by relating the hydrogen surface coverage to the magnitude of electron scattering at the Pt surface. The proposed model successfully reproduces the effects of hydrogen concentration and time on the sensitivity, particularly at hydrogen concentrations below 20 ppm. Another hydrogen sensor consisting of suspended graphene functionalized Pd nanoparticles will be also demonstrated. Various sensors using Pd as a sensing layer or as a catalyst of carbon materials have been proposed because Pd has selectivity to H2. However, most of them are affected by relative humidity variations. Therefore, high temperature operations for humidity robustness have been studied utilizing not only external heaters but also Joule heating (self-heating). However, most studies on self-heating focused on sensor response time and recovery time. No attention has been paid to self-heating-induced changes in the physical properties and device parameters except for temperature. Here, the effects of self-heating on Pd-functionalized suspended graphene sensors were investigated. Self-heating was realized by nanoscale point contacts between the graphene and Au electrodes, which suppressed heat dissipation to the electrodes. By applying an appropriate voltage bias, the graphene temperature was increased up to 180 °C. At temperatures over 100 °C, the sensor response realized by self-heating was lower than that by external heater heating. The response reduction was due to suppressed charged-carrier scatterings with H-induced potentials under high electric fields in the self-heated graphene. In addition, it should be noted that this sensor can detect hydrogen at high bias condition thanks to self-heating-induced high temperature and that it can detect water at low bias conditions because of easy adsorption of water on sensing materials. This multi-functionality of the sensors will be also promising for mobile applications where space availability is extremely limited. Reference [1] T. Tanaka et al., Sensors and Actuators B, 258, p913, 2018.

Investigating Reaction Mechanisms for Furfural Hydrodeoxygenation on Ni and the Effect of Boron Doping on the Activity and Selectivity of the Catalyst

Fast pyrolysis is a promising route for the production of fuels and high value chemicals from non-fossil fuel resources like biomass. However, the design of active and inexpensive catalysts that can selectively convert furfural, an important component in pyrolysis derived bio-oil, to target chemicals remains a challenge. In this context, Ni-based catalysts are potential candidates for the vapor phase activation of furfural in the presence of H2. In this paper, mechanisms and energetics (kinetics and thermodynamics) of the catalytic conversion of furfural to furans, furfuryl alcohol, and C4 products in the presence of H2 on Ni(111) are established, and the experimentally observed change in the selectivity with temperature is explained, using first-principles density functional theory. Hydrogen adsorbs stronger than furfural on the Ni surface. At low operating temperatures, hydrogen adsorption is spontaneous, leading to high hydrogen surface coverages that favor furfural hydrogenation and decarbonylation ov...

Experimental and Computational Investigation of Hydrogen Evolution Reaction Mechanism on Nitrogen Functionalized Carbon Nanotubes

AbstractDesigning earth‐abundant element based efficient and durable electrocatalysts for hydrogen evolution reaction (HER) is attracting growing attention as the renewable electricity supply sector urgently needs sustainable methods for storing energy. Nitrogen functionalized carbon nanomaterials are an interesting electrocatalysts option because of their attractive electrical properties, excellent chemical stability and catalytic activity. Hence, this study reports the HER mechanism on nitrogen functionalized few‐walled carbon nanotubes (N‐FWCNT). With this earth‐abundant element based catalyst 250 mV overpotential is required to reach 10 mA cm−2 current density and so its HER activity is comparable to other non‐noble metal catalysts, and clearly among the highest previously reported for N‐FWCNTs. To gain fundament insight on their functioning, computational analysis has been carried out to verify the effect of nitrogen and to analyze the reaction mechanism. The reaction mechanism has also been analyzed experimentally with a pH series, and both the methods suggest that the HER proceeds via the Volmer‐Heyrovský mechanism. Overall hydrogen surface coverage on N‐FWCNT is also suggested to affect the HER rate. Interestingly, in the studied structure, carbons in vicinity of nitrogen atoms, but not directly bound to nitrogen, appear to promote the HER most actively. Furthermore, durability of N‐FWCNTs has been demonstrated by operating a full electrolyzer cell for five weeks.

Controlled Selectivity of CO2 Reduction on Copper by Pulsing the Electrochemical Potential.

We demonstrate a simple strategy to enhance the CO2 reduction reaction (CO2 RR) selectivity by applying a pulsed electrochemical potential to a polycrystalline copper electrode. By controlling the pulse duration, we show that the hydrogen evolution reaction (HER) is highly suppressed to a fraction of the original value (<5 % faradaic efficiency) and selectivity for the CO2 RR dramatically improves (>75 % CH4 and >50 % CO faradaic efficiency). We attribute the improved CO2 RR selectivity to a dynamically rearranging surface coverage of hydrogen and intermediate species during the pulsing. Our finding provides new insights into the interplay of transport and reaction processes as well as timescales of competing pathways to enable new opportunities to tune CO2 RR selectivity by adjusting the pulse profile. Additionally, the pulsed potential method we describe can be easily applied to other catalysts materials to improve their CO2 RR selectivity.

Insights into Self-Poisoning during Catalytic Hydrogenation on Platinum Surfaces Using ATR-IR Spectroelectrochemistry

Attenuated total reflection infrared (ATR-IR) spectroscopy has been combined with electrochemical methods to investigate molecular decomposition and self-poisoning processes on platinum surfaces under the conditions of catalytic hydrogenation. In aqueous 0.1 M H2SO4 the α-keto ester ethyl pyruvate (EP) is found to decompose on polycrystalline platinum electrodes to yield surface-adsorbed CO, but the observed behavior is highly dependent on the electrode potential, a parameter intimately linked to the surface-adsorbed hydrogen coverage. In the potential range −0.2 to −0.4 V (vs mercury/mercurous sulfate electrode) where the hydrogen coverage is negligible, CO is readily produced at the platinum surface along with other molecular fragments but the decomposition process becomes inhibited at high EP solution concentrations. At −0.5 V only very low coverages of CO are observed due to competing hydrogen adsorption at Pt(100) step sites which most favor EP decomposition. At more negative potentials, during the onset of catalytic EP hydrogenation, CO is generated rapidly but other intermediates or products are not observed in the ATR-IR spectra. Together these observations suggest two different mechanisms of EP decomposition, the first occurring directly upon EP adsorption and the second occurring after a single hydrogen atom transfer under hydrogen rich conditions. This ability to control substrate decomposition by tuning the surface hydrogen coverage may be used as a potential route to mitigating catalyst poisoning and deactivation during hydrogenation reactions.

Nanoscale Pt thin film sensor for accurate detection of ppm level hydrogen in air at high humidity

Abstract Hydrogen is an important biomarker for the human digestive system. However, accurate detection of ppm-level hydrogen in breath is difficult due to the competing detection of high concentration water. We fabricated Pt thin films that respond to hydrogen in air at concentrations as low as 500 ppb. In both dry and humid air, these films have almost identical response to hydrogen, i.e., their resistance decreases linearly with increasing hydrogen concentration regardless of relative humidity. Even at high relative humidity, these Pt thin films can detect ppm-level hydrogen. Furthermore, it was strongly suggested that these films can be applied to low-level hydrogen in the air expired by a healthy human. Based on the chemical kinetics, namely the adsorption and desorption of hydrogen and oxygen, the sensor response is quantitatively described by relating the hydrogen surface coverage to the magnitude of electron scattering at the Pt surface. The proposed model successfully reproduces the effects of hydrogen concentration and time on the sensor response, particularly at hydrogen concentrations below 20 ppm. Based on this model, these Pt thin film sensors have the potential to detect 1 ppm hydrogen in expired air within 30 s.

(Invited) Non-Thermal Plasmas for the Production of Sustainable Functional Materials

This talk will highlight the potential of non-thermal plasmas as a viable synthetic approach for materials that are difficult to produce using other techniques. Materials such as titanium and zirconium nitride are impossible to produce in nanoparticle form using liquid phase chemistry techniques, and are not compatible with approaches such as flame and spray pyrolysis because they are not oxide-based ceramics. At the same time, they are quickly attracting interest because they are predicted to show plasmonic response in the visible, making them great candidates for the replacement of silver and gold-based plasmonics. Our recent investigation in this material system confirms that low-temperature, non-thermal plasma processes are compatible with the synthesis of titanium and zirconium nitride nanocrystals. Particles are produced using metal chlorides and ammonia as precursors. The plasma reactor is designed in a continuous flow configuration, where precursors are continuously fed into the plasma volume and nanoparticles are continuously collected downstream of the plasma by filtering. This technique allows producing TiN and ZrN nanocrystals with average size below 10 nm, with narrow size distribution (all particles are within 5 nm and 15 nm) and with perfect stoichiometry. In partial agreement with the theoretical predictions, such particles show plasmon resonance in the visible and in the near-infrared part of the spectrum. Yet the plasmon energy is lower than theoretically predicted, a behavior that has not been observed before and that we attribute to the presence of a native oxide shell. This observation is justified by simple experiments in which the particles are annealed in air at moderate temperature (<300°C), leading to the progressive growth of an oxide shell, as confirmed by elemental analysis and by XPS. The plasmon peak energy continuously red shifts during the annealing process, suggesting that a strategy for limiting oxide growth needs to be developed to enable the use of these nanomaterials for refractory plasmonic applications. In addition to discussing the properties of these novel plasma-produce nanomaterials, this talk will summarize our most recent findings with respect of their nucleation and growth. A combination of in-situ FTIR and optical emission spectroscopy measurements suggests that the nucleation process is initiated via atomic hydrogen-induced dissociation of the chlorine-based metal precursor, in this case titanium tetrachloride. This talk will also cover our most recent studies on the interaction between nanoparticles and plasmas. Despite operating near room temperature, and despite the short residence time (tens to hundreds of milliseconds) these systems are capable of producing nanocrystals of high melting point materials. Moreover, there is mounting evidence that such systems are compatible with the production of a broad range of high melting point materials. Therefore it is necessary to achieve a detailed understanding of the phenomena occurring in such reactors to fully take advantage of their capabilities as a nanomaterial synthetic approach. Using the silane-based chemistry as a testbed, we have performed careful in-situ FTIR characterization of the surface of silicon nanoparticles immersed in a non-thermal plasma. The hydrogen surface coverage decreases as the input power density increases. We attribute this to plasma-induced heating of nanoparticles to temperatures sufficiently high to lead to hydrogen desorption. This observation confirms the prediction of several theoretical studies that there is a non-equilibrium between nanoparticle temperature in a dusty plasma and the background gas temperature.

Hydrogen-enhanced catalytic hydrothermal gasification of biomass

The impact of hydrogen co-feeding on the catalytic hydrothermal gasification of wet biomass was explored in a continuous test rig using a feed of 10 wt% glycerol in water and a fixed bed of a carbon-supported ruthenium catalyst. The reactor was operated at a nominal temperature of 400 °C and at pressures of 26–28 MPa. Variation of the hydrogen-to-glycerol ratio as well as of the total pressure showed clearly the methanation reaction to be promoted at the expense of carbon dioxide and hydrogen formation. This is explained by a higher hydrogen surface coverage and consecutively higher rates for hydrogenation of surface-bound carbon. An increase in peak temperature of ca. 75 K occurred in the catalytic fixed-bed when co-feeding hydrogen. The measured product gas composition was close to the thermodynamic equilibrium calculated at the outlet temperature of the reactor. A maximum methane concentration of 86 vol% in the raw gas was obtained at 28 MPa with a stoichiometric addition of hydrogen. Full catalytic activity was maintained during and after the hydrogen co-feeding experiments, verified by comparing the performance of a run with a 10 wt% glycerol in water feed after co-feeding hydrogen, for which the product distribution was very close to the experiments before hydrogen co-feeding.

A Study of the Mechanism of the Hydrogen Evolution Reaction on Nickel by Surface Interrogation Scanning Electrochemical Microscopy.

The hydrogen evolution reaction (HER) on Ni in alkaline media was investigated by scanning electrochemical microscopy under two operating modes. First, the substrate generation/tip collection mode was employed to extract the "true" cathodic current associated with the HER from the total current in the polarization curve. Compared to metallic Ni, the electrocatalytic activity of the HER is improved in the presence of the low-valence-state oxide of Ni. This result is in agreement with a previous claim that the dissociative adsorption of water can be enhanced at the Ni/Ni oxide interface. Second, the surface-interrogation scanning electrochemical microscopy (SI-SECM) mode was used to directly measure the coverage of the adsorbed hydrogen on Ni at given potentials. Simulation indicates that the hydrogen coverage follows a Frumkin isotherm with respect to the applied potential. On the basis of the combined analysis of the Tafel slope and surface hydrogen coverage, the rate-determining step is suggested to be the adsorption of hydrogen (Volmer step) in the investigated potential window.

The effects of hydrogen dilution, carbon monoxide poisoning for a Pt–Ru anode in a proton exchange membrane fuel cell

The effects of carbon monoxide (CO) poisoning and hydrogen dilution are investigated for a series of simulated reformate gases. A simple mathematical model with an internal air bleed term is proposed to understand the interaction between CO poisoning, hydrogen dilution, and the internal air bleed (IAB). According to the steady-state modeling results, the presence of nitrogen can be considered as a diluting agent for a relatively dilute H2–N2 mixture. For a CO–H2–N2 mixture, hydrogen dilution and CO poisoning “magnify” each other in their combined effect on the surface coverage of hydrogen. The beneficial effect of IAB was found to be affected by the competitive adsorption between CO and hydrogen. The steady-state surface coverage of CO is influenced by the adsorption and desorption of CO, the interaction between CO and the electro-oxidation of hydrogen, the interaction between IAB/hydrogen dilution/CO, the interaction between IAB/CO. However, the steady-state surface coverage of CO is largely determined by its own adsorption and desorption properties. Analytical solutions are also derived from the proposed mode for the transient surface coverage of hydrogen and CO. The mathematical treatment has been proved extremely effective for the understanding of the steady-state and transient behavior of the CO–H2 and CO–H2–N2 mixtures.

H2 flux inhibition and stability of Pd-Ag membranes under exposure to trace amounts of NH3

The long-term performance and flux inhibition of a 10μm-thick Pd77Ag23 membrane under the exposure to varying concentrations of NH3 ranging from 10 to 500ppm at a temperature between 300 and 450°C was investigated. At 450°C no H2 flux inhibition was found in the presence of 200–500ppm NH3. However, an inhibition of the H2 flux was observed at lower operating temperatures in the presence of 200ppm NH3. In addition, a further gradual H2 flux decline is observed giving evidence for NH3 adsorption on the membrane surface. A H2 flux reduction of ~36% was observed after 20h of exposure at 300°C compared to the H2 flux obtained at this temperature in the absence of any NH3. The subsequent increase in operating temperature back to 450°C, however, quickly recovered the H2 flux back to its original H2 flux value obtained prior to any NH3 exposure at 450°C. First-principles calculations, however, indicated that H2 flux decrease upon NH3 exposure could not be accounted for by a simple lowering of the hydrogen surface coverage due to the competitive adsorption of NH3 related species under the experimental conditions. The role of NH3 therefore seems more complex, for instance related to hydrogen dissociation kinetics and incorporation or changes in the Pd-alloy membrane due to surface segregation, but this remains uncertain at this stage.

Interplay between propylene and H2S co-adsorption on the H2 flux characteristics of Pd-alloy membranes employed in propane dehydrogenation (PDH) processes

The role of propylene and H2S exposure on Pd-Ag membranes was investigated by hydrogen flux measurements of a magnetron sputtered Pd77Ag23 film and first-principles calculations of hydrogen, sulfur and propylene adsorption on the membrane surface. Upon exposure to 10% propylene, the H2 flux was lowered to 12% of its initial value prior to exposure, which was associated with propylene adsorption and coke formation based on qualitative assessments of the flux degradation kinetics and calculated propylene and hydrogen surface coverages. In the presence of 50–75ppb H2S, the H2 flux showed a slower gradual decrease upon propylene exposure. Although sulfur adsorption also inhibits H2 flux, this slower decrease resulted in a higher absolute H2 flux in the presence H2S already after 1h of propylene exposure. Adsorbed sulfur on the membrane surface was suggested to limit coke formation due to a lower calculated surface coverage of propylene as well as a possibly lower migration of propylene and other intermediates across the membrane surface necessary for coke formation.