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Applied Catalysis B, Environmental (v.56, #1-2)
Introduction to solid polymer membrane fuel cells and reforming natural gas for production of hydrogen by Robert J. Farrauto (pp. 3-7).
The basic principles of the solid polymer (PEM) fuel cell with special emphasis on catalysts for hydrogen generation are presented. The role of catalysts and the improvements needed are discussed.
Keywords: Fuel cell; PEM; Reforming natural gas; H; 2; generation; Catalysts
Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs by Hubert A. Gasteiger; Shyam S. Kocha; Bhaskar Sompalli; Frederick T. Wagner (pp. 9-35).
The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.
Keywords: Oxygen reduction catalysts; Non-platinum catalysts; Platinum–cobalt catalyst; Polymer electrolyte membrane fuel cell; Particle size effect; PEFC
The effect of temperature on the adsorption rate of hydrogen sulfide on Pt anodes in a PEMFC by R. Mohtadi; W.-K. Lee; J.W. Van Zee (pp. 37-42).
The coverage of a Pt anode in a proton exchange membrane fuel cell (PEMFC) by sulfur species resulting from exposure to 5ppm H2S/H2 is evaluated in situ by using cyclic voltammetry (CV). The coverage is shown to be a function of H2S dosage and temperature at 50, 70, and 90°C. The rate constant for the H2S adsorption reaction on Pt at 50°C was approximately 69% lower than at 90°C. The Pt–sulfur species accounts for 90% of the Pt after 800min (18μmol of H2S) at 50°C and 100% of the Pt after 600min (13.5μmol of H2S) at 90°C. Shifts in the sulfur oxidation potential in cyclic voltammetry to higher values at lower temperatures were observed indicating that sulfur is more strongly adsorbed at lower temperatures. A value of the activation energy of hydrogen sulfide adsorption on Pt is shown to be 28.2 ± 5.5kJ/mol.
Keywords: PEMFCs; H; 2; S poisoning; Pt anodes; Cyclic voltammetry
Quantifying the ‘reverse water gas shift’ reaction inside a PEM fuel cell by Tao Gu; W.-K. Lee; J.W. Van Zee (pp. 43-50).
Reformed gas containing CO2, N2, and H2 may be used in proton exchange membrane fuel cells (PEMFCs), and recent evidence has shown that CO2 can react in situ with H2 (i.e., a reverse water gas shift (RWGS) reaction) and produce adsorbed CO that can poison the electrode catalyst. Here, a study is presented to extend the previous observations by considering how pressure, gas composition, and temperature affect this reaction in a PEMFC for both Pt and Pt/Ru alloy catalysts. The coverage of CO produced on the electrodes was determined by stripping cyclic voltammetry (CV). The data show how the CO stripping potential depends on temperature, and how the analysis allows the determination of an activation energy. The data are shown to be consistent with a kinetic catalytic model and not with an equilibrium model.
Keywords: Proton exchange membrane fuel cell (PEMFC); Reformate; Reverse water gas shift (RWGS) reaction; CO stripping; Pressure and temperature effects
Adsorptive removal of dimethylsulfide and t-butylmercaptan from pipeline natural gas fuel on Ag zeolites under ambient conditions by Shigeo Satokawa; Yuji Kobayashi; Hiroshi Fujiki (pp. 51-56).
Desulfurization of pipeline natural gas fuel at ambient conditions before introduction to reforming processes is strongly needed for stationary applications of fuel cells. Adsorptive removal of dimethlysulfide (DMS) and t-butylmercaptan (TBM) from pipeline natural gas was efficiently carried out by using silver exchanged Y zeolites (AgNa-Y) in the presence of water at room temperature and normal pressure. The capacity and adsorptivity for DMS and TBM on AgNa-Y were improved by an increase in silver content of AgNa-Y. Adsorption runs for DMS and TBM were carried out separately to compare their adsorptivity on AgNa-Y. The sulfur adsorption capacity for DMS on Ag(18)Na-Y (1.9mmolg−1) was much higher than that for TBM (0.6mmolg−1). The clear color change of AgNa-Y during the period of adsorption run provides information regarding the formation of silver sulfide species on AgNa-Y. The used AgNa-Y was regenerated easily by heat treatment in air at 500°C.
Keywords: Adsorption; Desulfurization; Dimethylsulfide; Natural gas; Silver; t; -Butylmercaptan; Zeolite
Activity and stability of low-content gold–cerium oxide catalysts for the water–gas shift reaction by Qi Fu; Weiling Deng; Howard Saltsburg; Maria Flytzani-Stephanopoulos (pp. 57-68).
We report here on the high activity and stability of low-content gold–cerium oxide catalysts for the water–gas shift reaction (WGS). These catalysts are reversible in cyclic reduction–oxidation treatment up to 400°C, are non-pyrophoric, and are thus potential candidates for application to hydrogen generation for fuel cell power production. Low-content (0.2–0.9at.%) gold–ceria samples were prepared by single-pot synthesis by the urea gelation/coprecipitation method; and by sodium cyanide leaching of high-content (2–8at.%) gold–ceria materials prepared by various techniques. The low-content gold–ceria catalysts are free of metallic gold nanoparticles. Gold is present in oxidized form, as verified by a variety of analytical techniques. However, these materials display the same WGS activity as the high-content gold ones, and remain free of gold nanoparticles after use in a reaction gas stream composed of 11% CO–26% H2O–26% H2–7% CO2–balance He up to 300°C. We show that the determining factor for the retention of active gold in ceria is the surface properties of the latter. Measurements of lattice constant expansion indicate gold ion substitution in the ceria lattice. The turnover frequency of WGS under the assumption of fully dispersed gold is the same for a variety of low-content gold–ceria preparations. The stability of gold–ceria in various gas compositions and temperatures was good. The most serious stability issue is formation of cerium hydroxycarbonate in shutdown operation.
Keywords: Gold catalysts; Nanocrystalline cerium oxide; Water–gas shift reaction; Fuel cells; Cyanide leaching; Hydrogen generation; Stability of gold–ceria catalysts
Deactivation of Pt/CeO2 water-gas shift catalysts due to shutdown/startup modes for fuel cell applications by Xinsheng Liu; Wolfgang Ruettinger; Xiaoming Xu; Robert Farrauto (pp. 69-75).
Fuel cells and its fuel processor will be frequently shutdown and re-started depending on the power demand. Deactivation of Pt/CeO2 water-gas shift catalysts under conditions simulating shutdown was observed and studied using a combination of techniques such as Fourier-transform infrared (FT-IR) spectroscopy, high-resolution transmission electron microscopy (HRTEM), CO chemisorption and catalytic activity test. Results show that formation of carbonates occurs on the catalyst surface as a consequence of shutdown in reformate, leading to deactivation. The carbonates originating from both CO and CO2 cover not only the surface of CeO2 but also the Pt metal surface and exert an effect on its electronic properties. It is found that a correlation exists between the content of carbonates and the degree of deactivation. Regeneration by air treatment removes the carbonates and restores activity. Pt metal sintering is not found to be a factor contributing to the catalyst deactivation under the shutdown and re-startup conditions.
Keywords: Pt/CeO; 2; WGS; Deactivation; Shutdown; Surface carbonate; Regeneration; Fuel cells
CO tolerance of Pt and Rh catalysts: effect of CO in the gas-phase oxidation of H2 over Pt and Rh supported catalysts by G. Avgouropoulos; T. Ioannides (pp. 77-86).
The catalytic behavior of Pt and Rh catalysts during the gas-phase oxidation of H2–CO mixtures was studied with commercial Pt/C, Pt–Ru/C and Pt–Sn/C electrocatalysts, as well as laboratory prepared catalysts of Pt and Rh supported on various carriers (γ-Al2O3, TiO2, TiO2(WO3), SiO2, WO3, MoO3 and carbon). The goal of this study was to assess the CO-tolerance of the catalysts, i.e. the lowering of the H2 oxidation rate in the presence of CO, generally observed both in gas-phase and electrochemical environments. A significant drop of hydrogen oxidation rate, up to 5 orders of magnitude, was observed in the presence of CO. CO inhibition was strongly dependent on the nature of the support. The standard Pt/C electrocatalyst was the least CO-tolerant catalyst among those tested, while Pt–Sn/C, as well as Pt and Rh supported on MoO3 and TiO2-based carriers were the most active and CO-tolerant catalysts. The experimental findings indicate that CO inhibition is due to CO adsorption in linear and/or bridged form depending on the type of metal and support employed. The CO tolerance of the catalysts depends on the relative adsorption strength of CO and hydrogen on the metal surface. Simulation of CO inhibition showed that Δ HCO (or the relative values of Δ HCO andΔHH2) may vary up to 20–40kJ/mol between the most and least CO-tolerant catalysts.
Keywords: Hydrogen oxidation; CO tolerance; Pt catalysts; Rh catalysts; Support effect
Influence of the preparation method on the performance of CuO–CeO2 catalysts for the selective oxidation of CO by G. Avgouropoulos; T. Ioannides; H. Matralis (pp. 87-93).
In this work we report on the influence of the preparation method on the physicochemical and catalytic properties of CuO–CeO2 catalysts for the selective CO oxidation in simulated reformate gas. Four CuO–CeO2 samples were prepared using the co-precipitation, the citrate-hydrothermal, the urea-nitrates combustion, and the impregnation methods, and characterized by N2 adsorption–desorption, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction by H2 (TPR-H2).The combustion-prepared sample exhibited the best catalytic performance, closely followed by that prepared with the citrate-hydrothermal method. The co-precipitated sample was less active followed by the impregnated one. The activity of all samples decreases in the presence of CO2 and, to a higher degree, in the simultaneous presence of both CO2 and H2O in the feed. The resistance to this deactivation was higher in the case of the sample prepared by the urea-nitrates combustion method, which remained the most active and selective, closely followed by the specimen prepared by the citrate-hydrothermal method. The superior catalytic performance of the samples prepared by the urea-nitrates combustion and the citrate-hydrothermal methods is attributed to the existence of well dispersed, strongly interacting with the ceria surface, copper oxide species.
Keywords: Selective CO oxidation; Hydrogen; Copper oxide; Cerium oxide; Co-precipitation; Combustion; Hydrothermal; XPS; XRD; TPR
Monolithic structures as alternatives to particulate catalysts for the reforming of hydrocarbons for hydrogen generation by Thomas Giroux; Shinn Hwang; Ye Liu; Wolfgang Ruettinger; Lawrence Shore (pp. 95-110).
Currently, many companies are exploring the technical and market potential of distributed hydrogen for use in such varied applications as stationary fuel cell power generation units, on-site generators of hydrogen for industrial uses, and hydrogen fueling stations for fuel cell powered automobiles.The use of particulate catalyst beds in the industrial production of hydrogen is well known. Engineers conceiving new process designs today for fuel processing can take advantage of the successful experience of environmental catalysis in automotive and stationary applications using monolithic catalysts and other substrates such as heat exchangers. In this paper, the unit operations of hydrogen generation are reviewed with the perspective of describing the opportunities of implementing such new catalyst technologies in small-scale fuel processing.
Keywords: Monolith; Reforming; Hydrogen; Fuel cell reformer
Desulfurization of transportation fuels by Ï€-complexation sorbents: Cu(I)-, Ni(II)-, and Zn(II)-zeolites by Arturo J. Hernández-Maldonado; Frances H. Yang; Gongshin Qi; Ralph T. Yang (pp. 111-126).
New π-complexation-based sorbents were studied for desulfurization of diesel, gasoline, and jet fuels. The sorbents were obtained by ion exchanging faujasite type zeolites with Cu+, Ni2+ or Zn2+ cations using different techniques, including liquid phase ion exchange (LPIE). Hydrolysis of the cations in aqueous solutions should be avoided in order to increase the ion exchange level when aqueous solutions are used. Cation hydrolysis limitation could be avoided when vapor phase (VPIE) and solid-state (SSIE) ion exchange techniques are used instead. The deep-desulfurization (sulfur levels of <1ppmw) tests were performed using fixed-bed adsorption techniques. The treated and untreated fuels were fully characterized using the latest generation of FPD detectors in our laboratory, which are capable of eliminating quenching effects. After proper optimization and calibration of the detector non-linear response, as done in all our previous work, it was possible to detect sulfur concentration as low as 20ppbwS. The data was verified using total sulfur analyzers. The π-complexation sorbents desulfurization performance decreases as follows: Cu(I)-Y(VPIE) > Ni(II)-Y(SSIE) > Ni(II)-X(LPIE) > Zn(II)-X(LPIE) > Zn(II)-Y(LPIE). Molecular orbital calculations indicate the sorbents performance decreases as follows: Cu+ > Ni2+ > Zn2+. This agrees well with the experimental results. The best sorbent, Cu(I)-Y(VPIE), has breakthrough adsorption capacities of 0.395 and 0.278mmolS/g of sorbent for commercial jet fuel (364.1ppmwS) and diesel (297.2ppmwS), respectively.
Keywords: Desulfurization; Transportation fuels; Dibenzothiophenes; Benzothiophenes; Pi-complexation; Zeolite
A study of the adsorption of thiophenic sulfur compounds using flow calorimetry by Flora T.T. Ng; Ataur Rahman; Tomotsugu Ohasi; Ming Jiang (pp. 127-136).
Selective adsorption of sulfur compounds from gasoline and diesel fuel has potential to produce ultra clean fuels for on-board fuel cell applications and also to meet the upcoming legislation for clean fuels. Removal of thiophenic sulfur compounds in a hexadecane solution using commercial zeolites, NaY, USY, HY and 13X, has been investigated by adsorption and flow calorimetry techniques. The S compounds chosen were thiophene (T), benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). The adsorption studies were carried out in the liquid phase at 55°C. Among the zeolites studied, NaY has the highest saturation sorption capacity for the sulfur compounds. The overall heat of adsorption of sulfur compounds in a hexadecane solution was measured at 30°C using a flow microcalorimeter. A linear correlation between the heat of adsorption and the amount of S adsorbed was found for NaY. Competitive adsorption using a mixture of anthracene, DBT and quinoline indicates that NaY selectively adsorbs quinoline while anthracene and DBT have similar affinity to NaY. Liquid flow calorimetry combined with the analysis of the effluent from the calorimeter is a promising technique to aid the development of selective sorbents for S removal and other sorption processes since it provides information on the relative heats of adsorption, the sorption capacity and the breakthrough curves.
Keywords: Sulfur adsorption; Adsorbents; Flow calorimetry; Desulfurization; Zeolites; Fuel cell
Deep desulfurization of gasoline by selective adsorption over solid adsorbents and impact of analytical methods on ppm-level sulfur quantification for fuel cell applications by Xiaoliang Ma; Subramani Velu; Jae Hyung Kim; Chunshan Song (pp. 137-147).
The objectives of this work were to comparatively study the performance of a Ni-based adsorbent and a Cu(I)Y-zeolite for the desulfurization of a commercial gasoline by fixed-bed adsorption experiments at room temperature and 200°C, and to clarify the impacts of analytical methods on the ppm-level sulfur quantification in desulfurized liquid fuels for fuel cell applications. A series of standard fuel samples containing known amounts of sulfur compounds in n-decane was prepared and was analyzed by using gas chromatograph coupled with a flame photometric detector (GC-FPD), pulsed flame photometric detector (GC-PFPD) and a total sulfur analyzer. The results show that the GC-FPD and GC-PFPD are not suitable for quantitative estimation of total sulfur concentration in complex hydrocarbon fuels at low ppm-level without considering both the nonlinear response and the quenching effect. The adsorptive desulfurization of a commercial gasoline over the Cu(I)Y-zeolite and a Ni-based adsorbent was conducted and compared using a fixed-bed adsorption system. The Cu(I)Y-zeolite prepared in the present study showed a breakthrough capacity of 0.22mgS/g of adsorbent (mg/g) at room temperature for removing sulfur in a commercial gasoline to less than 1ppmw. Under the same experimental conditions, the Ni-based adsorbent exhibited a breakthrough capacity of 0.37mg/g. The breakthrough capacity of the Ni-based adsorbent was increased by 38% at 200°C. Moreover, the breakthrough capacity of the Ni-based adsorbent corresponding to the outlet sulfur level of 10ppmw was 7.3mg/g, which was over an order of magnitude higher than that of Cu(I)Y-zeolite.
Keywords: Adsorption; Adsorbent; Desulfurization; Gasoline; Sulfur analysis; Fuel cell
Comparison of methanol-based fuel processors for PEM fuel cell systems by James R. Lattner; Michael P. Harold (pp. 149-169).
The deployment of proton exchange membrane (PEM) fuel cells requires efficient conversion of fuels to hydrogen in distributed facilities. Methanol is often considered as a fuel source because it is stored as a liquid and can be reformed to hydrogen at relatively mild conditions. We have compared steam reforming (SR), autothermal reforming (ATR), and ATR membrane reactor based fuel processors using a commercial CuO/ZnO/Al2O3 catalyst. In our approach, we first optimize the flowsheet and identify reaction conditions that maximize overall system efficiency. Reaction kinetics and heat transfer are then incorporated into the process efficiency analysis as well as volume requirements for each fuel processor. We show that the SR and ATR fuel processors coupled with a PEM fuel cell achieve about 50% overall efficiency (LHV basis), with roughly equal fuel processor volumes of about 29 and 22 liters for 50kW net power generation, respectively. The ATR fuel processor requires distributed air injection in order to avoid overheating the copper catalyst and the formation of excessive CO. The ATR membrane reactor combines the same features with hydrogen removal in the reforming section of the reactor. The main benefit of the ATR membrane reactor is a reduction of the fuel processor volume to 13 liters, at the expense of a more complex steam system and a small reduction in overall efficiency.
Keywords: Fuel processors; Methanol; Membrane reactors; PEM fuel cells; Hydrogen; Reactor engineering; Steam reforming; Autothermal reforming
A review of catalytic issues and process conditions for renewable hydrogen and alkanes by aqueous-phase reforming of oxygenated hydrocarbons over supported metal catalysts by R.R. Davda; J.W. Shabaker; G.W. Huber; R.D. Cortright; J.A. Dumesic (pp. 171-186).
We have recently developed a single-step, low-temperature process for the catalytic production of fuels, such as hydrogen and/or alkanes, from renewable biomass-derived oxygenated hydrocarbons. This paper reviews our work in the development of this aqueous-phase reforming (APR) process to produce hydrogen or alkanes in high yields. First, the thermodynamic and kinetic considerations that form the basis of the process are discussed, after which reaction kinetics results for ethylene glycol APR over different metals and supports are presented. These studies indicate Pt-based catalysts are effective for producing hydrogen via APR. Various reaction pathways may occur, depending on the nature of the catalyst, support, feed and process conditions. The effects of these various factors on the selectivity of the process to make hydrogen versus alkanes are discussed, and it is shown how process conditions can be manipulated to control the molecular weight distribution of the product alkane stream. In addition, process improvements that lead to hydrogen containing low concentrations of CO are discussed, and a dual-reactor strategy for processing high concentrations of glucose feeds is demonstrated. Finally, various strategies are assembled in the form of a composite process that can be used to produce renewable alkanes or fuel-cell grade hydrogen with high selectivity from concentrated feedstocks of oxygenated hydrocarbons.
Keywords: Hydrogen production; Fuel cells; Reforming; Renewable energy; Supported metal catalysts; Hydrocarbon fuels; Renewable alkanes