Propane conversion over a Ru/CGO catalyst and its application in intermediate temperature solid oxide fuel cells

A Ru/CGO catalyst was investigated in combination with a Cu current collector for the direct electro-oxidation and internal reforming of propane in a solid oxide fuel cell. The electrochemical power densities for the direct oxidation were larger than in the internal reforming process at 750 °C. The electrochemical performance in the presence of propane was significantly affected by the polarization resistance which was about three times larger than that obtained for the SOFC fed with hydrogen at 750 °C. However, out-of-cell steam reforming tests showed a C₃H₈ conversion to syngas approaching 90% at 800 °C. Thus, significant enhancements may be achieved by properly optimizing the anode structure. No formation of carbon deposits was observed both upon operation of the anode in the direct oxidation and internal reforming processes at 750 °C.     

Effect of methane concentration on reaction behavior in direct-methane solid oxide fuel cells with (Ce,Gd)O2−x-impregnated FeCr layer for fuel processing

A solid oxide fuel cell (SOFC) with a Ni-yttria-stabilized zirconia anode of 1 cm² area was set up with a porous disk of gadolinia-doped ceria-impregnated FeCr as a gas diffusion layer (GDL) under direct-methane feeding. In this setup of SOFC plus GDL, the tests at 800 °C and ambient pressure show that the current density, the methane conversion rate, the product formation rates, and the CO₂ selectivity increased with increasing methane concentration. The major reaction in the GDL is CO₂ reforming of methane to produce the syngas (CO plus H₂). The anodic electrochemical oxidation of CO from GDL results in an overall rate of CO₂ formation being much larger than that of CO formation. There is a synergy between the rate of reaction in the GDL and that over the anode.     

The effects of bimetallic interactions for CO2-assisted oxidative dehydrogenation and dry reforming of propane

The catalytic reduction of CO₂ by propane may occur via dry reforming to produce syngas (CO + H₂) or oxidative dehydrogenation to yield propylene. Utilizing propane and CO₂ as coreactants presents several advantages over conventional methane dry reforming or direct propane dehydrogenation, including lower operating temperatures and less coke formation. Thus, it is of great interest to identify catalytic systems that can either effectively break the C C bond to generate syngas or selectively break C H bonds to produce propylene. In this study, several precious and nonprecious bimetallic catalysts supported on reducible CeO₂ were investigated using flow reactor studies at 823 K to identify selective catalysts for CO₂-assisted reforming and dehydrogenation of propane.     

Modeling of a SOFC fuelled by methane: From direct internal reforming to gradual internal reforming

Natural gas appears to be a fuel of great interest for SOFC systems. The principal component of natural gas is methane, which can be converted into hydrogen by direct or gradual internal reforming (DIR or GIR) within the SOFC anode. However, DIR requires a large amount of steam to produce hydrogen. If the injected mixture contains very small quantities of steam, GIR is then obtained. With GIR, the risk of carbon formation is even greater. This paper proposes a model and simulation, using the CFD-Ace software package, of the behaviour of a tubular SOFC using GIR and a comparison between utilization in DIR and GIR. A thermal study is included in the model and a detailed thermodynamic analysis is carried out to predict the carbon formation boundary for SOFCs fuelled by methane. Thermodynamic equilibrium calculations taking into account Boudouard and methane cracking reactions allowed us to investigate the occurrence of carbon formation. Simulations were used to calculate the distributions of partial pressures for all the gas species (CH₄, H₂, CO, CO₂, H₂O), current densities and potentials in both electronic and ionic phases within the anode part (i.e., gas channel and cermet anode). The simulations indicate that there is no decrease in electrochemical performance if GIR is used rather than DIR. A thermal study appears to confirm that the cooling effect due to the endothermic reforming reaction is eliminated in GIR, but the thermodynamic study indicates that carbon formation can be suspected for x_H2O/x_CH4 ratios lower than one.     

不同燃料SOFC的理论电池电动势及其性能

以NH₃以及3% H₂O增湿的H₂、CH₄、C₃H₈和煤炭地下气化(underground coal gasification,UCG)气为燃料,用最小Gibbs自由能法计算平衡气体组分和理论电池电动势,并测试在NiO-GDC‖GDC‖Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3-δ(B_0.9CFN)阳极支撑固体氧化物燃料电池(SOFC)中的电池开路电压、电池性能和长期稳定性。结果表明,以上述气体作燃料的SOFC热力学计算理论电动势均高于1.05 V,而由于GDC电解质在还原气氛下存在电子电导,导致碳氢燃料在NiO-GDC‖GDC‖B_0.9CFN阳极支撑电池中的开路电压略小。中低温下,碳氢燃料相对缓慢的动力学过程和GDC电解质快速的氧离子传输速率,使得以UCG气、CH₄和C₃H₈为燃料的电池实际积炭比理论预测少。以UCG气为燃料的SOFC在500、550、600和650℃的最高功率密度分别高达0.151、0.299、0.537和0.729 W·cm^-2,在0.6 V恒压放电120 h后性能没有明显衰减,且阳极表面无积炭产生,表明直接UCG气SOFC具有广阔的应用前景。     

IT-SOFC阳极表面催化反应机理与传递过程的数值模拟与分析

固体氧化物燃料电池(SOFC)具有效率高、污染低、对燃料适应性好、功率大等特点。其性能与工作状态受发生在多孔阳极的化学反应与多种传递过程耦合的影响。基于流体力学方程组和多步基元化学反应模型,建立了描述上述耦合特性的三维数学模型,并自编程序求解分析。结果显示:重整反应主要发生在靠近通道进口的多孔阳极,表面成分Ni_s的覆盖率占70%～80%,其他主要表面成分为CO_s占20%～25%,H_s占6%,O_s占1.5%; Ni_s随工作温度升高而增加;加强吸附基元反应会提高燃料利用率和工作温度;渗透率增加会提高反应气体在多孔介质内的传递效果,但催化反应会因接触不充分而减弱。通过考虑基元反应机理研究表明,在微观层面,催化剂Ni利用率不高,催化反应受温度、化学反应速率常数、孔隙率等参数影响较大。     

13C MAS NMR mechanistic study of the initial stages of propane activation over H-ZSM-5 zeolite

¹³C MAS NMR study of the early stages of propane 2-¹³C activation was performed over H-ZSM-5 catalysts with various content of protonic and aprotonic sites. The reaction mechanism was tested by addition of various probe-molecules (C₃H₆, C₆H₆, H₂, H₂O and CO). The results on tracing the fate of ¹³C label during this experiments conclude to a monofunctional mechanism involving propane protonation on the strong Brønsted sites of H-ZSM-5 and the formation of carbonium ion type transition states, which further evolve in four different ways leading to ¹³C scrambling in propane, cracking, dehydrogenation and disproportionation.     

Catalytic steam reforming of ethane and propane over CeO2-doped Ni/Al2O3 at SOFC temperature: Improvement of resistance toward carbon formation by the redox property of doping CeO2

Ni/Al₂O₃ with the doping of CeO₂ was found to have useful activity to reform ethane and propane with steam under Solid Oxide Fuel Cells (SOFCs) conditions, 700-900 °C. CeO₂-doped Ni/Al₂O₃ with 14% ceria doping content showed the best reforming activity among those with the ceria content between 0 and 20%. The amount of carbon formation decreased with increasing Ce content. However, Ni was easily oxidized when more than 16% of ceria was doped. Compared to conventional Ni/Al₂O₃, 14%CeO₂-doped Ni/Al₂O₃ provides significantly higher reforming reactivity and resistance toward carbon deposition. These enhancements are mainly due to the influence of the redox properties of doped ceria. Regarding the temperature programmed reduction experiments (TPR-1), the redox properties and the oxygen storage capacity (OSC) for the catalysts increased with increasing Ce doping content. In addition, it was also proven in the present work that the redox of these catalysts are reversible, according to the temperature programmed oxidation (TPO) and the second time temperature programmed reduction (TPR-2) results.During the reforming process, in addition to the reactions on Ni surface, the gas-solid reactions between the gaseous components presented in the system (C₂H₆, C₃H₈, C₂H₄, CH₄, CO₂, CO, H₂O, and H₂) and the lattice oxygen (O_x) on ceria surface also take place. The reactions of adsorbed surface hydrocarbons with the lattice oxygen (O_x) on ceria surface (C_nH_m+O_x→nCO+m/2(H₂)+O_x−n) can prevent the formation of carbon species on Ni surface from hydrocarbons decomposition reaction (C_nH_m⇔nC+m/2H₂). Moreover, the formation of carbon via Boudard reaction (2CO⇔CO₂+C) is also reduced by the gas-solid reaction of carbon monoxide (produced from steam reforming) with the lattice oxygen (CO+O_x⇔CO₂+O_x−1).     

Electrochemical behaviour of propane-fed solid oxide fuel cells based on low Ni content anode catalysts

Two different low Ni content (10 wt.%) anode catalysts were investigated for intermediate temperature (800 °C) operation in solid oxide fuel cells fed with dry propane. Both catalysts were prepared by the impregnation of a Ni-precursor on different oxide supports, i.e. gadolinia doped ceria (CGO) and La_0.6Sr_0.4Fe_0.8Co_0.2O₃ perovskite, and thermal treated at 1100 °C for 2 h. The Ni-modified perovskite catalyst was mixed with a CGO powder and deposited on a CGO electrolyte to form a composite catalytic layer with a proper triple-phase boundary. Anode reduction was carried out in-situ in H₂ at 800 °C for 2 h during cell conditioning. Electrochemical performance was recorded at different times during 100 h operation in dry propane. The Ni-modified perovskite showed significantly better performance than the Ni/CGO anode. A power density of about 300 mW cm⁻² was obtained for the electrolyte supported SOFC in dry propane at 800 °C. Structural investigation of the composite anode layer after SOFC operation indicated a modification of the perovskite structure and the occurrence of a La₂NiO₄ phase. The occurrence of metallic Ni in the Ni/CGO system caused catalyst deactivation due to the formation of carbon deposits.     

Biogas Catalytic Reforming Studies on Nickel‐Based Solid Oxide Fuel Cell Anodes

Heterogeneous catalysis studies were conducted on two crushed solid oxide fuel cell (SOFC) anodes in fixed‐bed reactors. The baseline anode was Ni/ScYSZ (Ni/scandia and yttria stabilized zirconia), the other was Ni/ScYSZ modified with Pd/doped ceria (Ni/ScYSZ/Pd‐CGO). Three main types of experiments were performed to study catalytic activity and effect of sulfur poisoning: (i) CH₄ and CO₂ dissociation; (ii) biogas (60% CH₄ and 40% CO₂) temperature‐programmed reactions (TPRxn); and (iii) steady‐state biogas reforming reactions followed by postmortem catalyst characterization by temperature‐programmed oxidation and time‐of‐flight secondary ion mass spectrometry. Results showed that Ni/ScYSZ/Pd‐CGO was more active for catalytic dissociation of CH₄ at 750 °C and subsequent reactivity of deposited carbonaceous species. Sulfur deactivated most catalytic reactions except CO₂ dissociation at 750 °C. The presence of Pd‐CGO helped to mitigate sulfur deactivation effect; e.g. lowering the onset temperature (up to 190 °C) for CH₄ conversion during temperature‐programmed reactions. Both Ni/ScYSZ and Ni/ScYSZ/Pd‐CGO anode catalysts were more active for dry reforming of biogas than they were for steam reforming. Deactivation of reforming activity by sulfur was much more severe under steam reforming conditions than dry reforming; a result of greater sulfur retention on the catalyst surface during steam reforming.     

Mechanistic aspects of the oxidative dehydrogenation of propane over an alumina-supported VCrMnWOx mixed oxide catalyst

Mechanistic and kinetic aspects of the catalytic oxidative dehydrogenation of propane (ODP) were studied within a wide range of temperatures (673–773 K), partial pressures of oxygen (0–20 kPa), propane (0–40 kPa) and propene (0–4 kPa) under both steady-state ambient-pressure and transient, vacuum conditions in the temporal analysis of products (TAP) reactor. A Mn_0.18V_0.3Cr_0.23W_0.26O_x–Al₂O₃ catalyst was identified as a selective catalyst for ODP by high-throughput experiments. For comprehensive catalyst characterization, XRD, BET, and in situ UV–visible techniques were applied. The results from transient experiments in combination with UV–visible tests reveal that selective and non-selective propane oxidation occurs on the same active surface sites, i.e., lattice oxygen. CO_x formation takes place almost exclusively via consecutive propene oxidation, which involves both lattice and adsorbed oxygen species, with the latter being active in CO formation. However, the adsorbed species play a minor role. CO₂ formation was found to increase in the presence of propene in the reaction feed. Optimized operating conditions for selective propane oxidation were derived and discussed based on the experimental observations with respect to the influence of temperature and partial pressures of O₂, C₃H₆ and C₃H₈ on the reaction. In co-feed mode with a propane to oxygen ratio of 2, optimal catalytic performance is achieved at low partial pressures of oxygen and high temperature. Propene selectivity can be also improved by carrying out the ODP reaction in a periodic mode; that is an alternate feed of propane and air.     

Dehydrogenation of propane with selective hydrogen combustion: A mechanistic study by transient analysis of products

The role of lattice and adsorbed oxygen species in propane dehydrogenation in a perovskite hollow fiber membrane reactor containing a Pt–Sn dehydrogenation catalyst was elucidated by transient analysis of products with a sub-millisecond time resolution. Propane is mainly dehydrogenated non-oxidatively to propene and hydrogen over the catalyst, while lattice oxygen of the perovskite oxidizes preferentially hydrogen to water. For achieving high propene selectivity at high propane conversions, the formation of gas phase O₂ on the shell side of the membrane reactor should be avoided. Otherwise, oxygen species adsorbed over the Pt–Sn catalyst participate in non-selective C₃H₈/C₃H₆ transformations to C₂H₄ and CO_x.     

Comparison of the structures of methane-air and propane-air partially premixed flames

A comparison of flame structures between methane-air and propane-air laminar partially premixed flames has been made through the centerline concentration distributions of selected species measured using gas chromatography. The concentrations of fuel, major species like O₂, CO and CO₂ and those of the intermediate hydrocarbons like C₂H₆, C₂H₄, C₂H₂ and CH₄ (for the propane flame only) have been compared. Distinct double flame structures are observed for the experimental conditions under study. With approximately the same equivalence ratio and jet velocity for the primary mixture, the height of the inner flame for propane is less than that of methane. The peak concentration of C₂H₆ in the propane flame is found to be only a little higher than that in the methane flame, while the peak concentrations of C₂H₄ and C₂H₂ are much greater in the propane flame than in the methane flame. In a methane partially premixed flame, the hydrocarbon concentrations drop from their peak values very rapidly at the inner flame tip, but in the propane flames it is more gradual. In a methane partially premixed flame, CO is formed at the inner flame and burns at the outer flame to CO₂. Similar distributions of CO and CO₂ are found in the propane flame also. However, the peak CO concentration in the propane flame is found to be higher than in methane flame. A radial measurement of species distribution for a particular case in the propane partially premixed flame is also done to ascertain the species distributions across the flame.     

Promoting and inhibiting effect of SO2 on propane oxidation over Pt/Al2O3

SO₂ adsorption, SO₂ oxidation and oxidation of propane with oxygen in the absence and the respective presence of SO₂ in the feed gas were studied over unsulfated and sulfated 1% Pt/γ-Al₂O₃.Results showed that the promoting effect of SO₂ in the reaction flux on C₃H₈ oxidation over 1% Pt/γ-Al₂O₃ depends on the presulfating temperature. Catalytic activity measurements and FTIR absorption spectra showed that during propane oxidation, Pt/support interfacial adsorbed species were formed at temperatures 25–300 °C, inhibiting C₃H₈ oxidation. However, at higher temperatures these Pt/support interfacial adsorbed species were oxidized, leading to Pt/support interfacial sulfate species, which strongly promote propane oxidation.     

Additive effect of Ce, Mo and K to nickel-cobalt aluminate supported solid oxide fuel cell for direct internal reforming of methane

Direct internal reforming of methane (steam/carbon=0.031, 850 °C) is tested using button cells of Ni-YSZ/ YSZ/LSM in which the anode layer is supported either on Ni-YSZ or on Ni-CoAl₂O₄. The Ni-CoAl₂O₄ supported cell shows little degradation with operating time, as a result of higher resistance against carbon deposition, whereas the Ni-YSZ supported cell deactivates quickly and suffers fracture in 50 h. Upon incorporation of additives such as K, Ce, or Mo into the Ni-CoAl₂O₄ support, cells with 0.5 wt% CeO₂ exhibit the best stable performance as a result of reduced coke formation. Cells with 0.5 wt% Mo exhibit the lowest performance. Although no carbon deposit is detected in the cells with K₂CO₃ additives, their performance is worse than that in the CeO₂ case, and, in constant-current mode, there is a sudden voltage drop to zero after a certain period of time; this time becomes shorter with increasing K content. The injection of potassium into the anode side facilitates the generation of OH^? and CO ₃ ^2? in the anode and promotes the diffusion of these ions to the cathode. Increased polarization resistance at the cathode and increased electrolyte resistance result in such a sudden failure.     

Reforming methanol to electricity in a high temperature PEM fuel cell

In this paper we demonstrate for the first time a compact power unit, where a methanol reforming catalyst is incorporated into the anode of a PEMFC. The proposed internal reforming methanol fuel cell (IRMFC) mainly comprises: (i) a H₃PO₄-imbibed polymer electrolyte based on aromatic polyethers bearing pyridine units, able to operate at 200 °C and (ii) a 200 °C active and with zero CO emissions Cu–Mn–O methanol reforming catalyst supported on copper foam. Methanol is being reformed inside the anode compartment of the fuel cell at 200 °C producing H₂, which is readily oxidized at the anode to produce electricity. The IRMFC showed promising electrochemical behavior and no signs of performance degradation for more than 72 h.     

Ru-catalyzed anode materials for direct hydrocarbon SOFCs

A solid oxide fuel cell using a thin ceria-based electrolyte film with a Ru-catalyzed anode was directly operated on hydrocarbons, including methane, ethane, and propane, at 600 °C. The role of the Ru catalyst in the anode reaction was to promote the reforming reaction of the unreacted hydrocarbons by the produced steam and CO₂, which avoided interference from steam and CO₂ in the gas-phase diffusion of the fuels. The resulting peak power density reached 750 mW cm⁻² with dry methane, which was comparable to the peak power density of 769 mW cm⁻² with wet (2.9 vol.% H₂O) hydrogen. More important was the fact that the cell performance was maintained at a high level regardless of the change in the methane utilization from 12 to 46% but was significantly reduced by increasing the hydrogen utilization from 13 to 42%. While the anodic reaction of hydrogen was controlled by the slow gas diffusion, the anodic reaction of methane was not subject to the onset of such a gas-diffusion process.     

High H2 permeable SAPO-34 hollow fiber membrane for high temperature propane dehydrogenation application

SAPO-34 hollow fiber zeolite membranes are successfully synthesized on α-Al₂O₃ hollow fiber ceramic substrates by secondary growth method, and used to separate H₂ from a binary mixture (H₂, C₃H₈) or ternary mixture (H₂, C₃H₈, and C₃H₆) under a wide temperature range (25–600°C) with the aim of using them for propane dehydrogenation (PDH) reactions at high temperature. The results show excellent performance for H₂/C₃H₈ and H₂/ C₃H₈ & C₃H₆ separation, with high H₂ permeance of 3.1 × 10⁻⁷ mol/m²/s/Pa and H₂/C₃H₈ selectivity of 41 at 600°C. Additionally, the membrane shows stable performance for 140 hr of H₂/C₃H₈ separation test at 600°C. The high performance of this membrane is mainly attributed to the thin (∼2 μm) zeolite layer and asymmetric-wall of the hollow fiber support. So far, this membrane offers the highest hydrogen permeation and selectivity for H₂/C₃H₈ separation at high temperature (600°C) compared to those reported in literature.     

Thermodynamic Equilibrium Analysis of Propane Dehydrogenation with Carbon Dioxide and Side Reactions

A thermodynamic analysis of propane dehydrogenation with carbon dioxide was performed using constrained Gibbs free energy minimization method. Different reaction networks corresponding to different catalytic systems, including non-redox and redox oxide catalysts, were simulated. The influences of CO₂/C₃H₈ molar ratio (1–10), temperature (700–1000 K), and pressure (0.5–5 bar) on equilibrium conversion and product composition were studied. In the presence of CO₂ with a molar ratio of CO₂/C₃H₈ = 1, the temperature of dehydrogenation can be 30 K lower than that of dehydrogenation in the presence of steam (H₂O/C₃H₈ = 1) and about 50 K lower than that of simple dehydrogenation without dilution to achieve 60% propane conversion. It was found that the occurrence of dry reforming of propane and coke-forming side reactions could strongly impact the equilibrium product composition of the multireaction system and, therefore, these reactions should be kinetically controlled. Comparison of the simulated reactant conversions with those reported in the literatures revealed that the experimental conversion levels of propane are far below the corresponding equilibrium values due to rapid catalyst deactivation by coke, implying that research efforts should be directed toward formulation of more active and selective catalysts.     

SOFC fuelled by methane without coking: optimization of electrochemical performance

In recent years, fuel cell technology has attracted considerable attention from several fields of scientific research as fuel cells produce electric energy with high efficiency, emit little noise, and are non-polluting. Solid oxide fuel cells (SOFCs) are particularly important for stationary applications due to their high operating temperature (1,073–1,273 K). Methane appears to be a fuel of great interest for SOFC systems because it can be directly converted into hydrogen by direct internal reforming (DIR) within the SOFC anode. Unfortunately, internal steam reforming in SOFC leads to inhomogeneous temperature distributions which can result in mechanical failure of the cermet anode. Moreover this concept requires a large amount of steam in the fed gas. To avoid these problems, gradual internal reforming (GIR) can be used. GIR is based on local coupling between steam reforming and hydrogen oxidation. The steam required for the reforming reaction is obtained by the hydrogen oxidation. However, with GIR, Boudouard and cracking reactions can involve a risk of carbon formation. To cope with carbon formation a new cell configuration of SOFC electrolyte support was studied. This configuration combined a catalyst layer (0.1%Ir–CeO₂) with a classical anode, allowing GIR without coking. In order to optimise the process a SOFC model has been developed, using the CFD-Ace+ software package, and including a thin electrolyte. The impact of a thin electrolyte on previous conclusions has been assessed. As predicted, electrochemical performances are higher and carbon formation is always avoided. However a sharp decrease in the electrochemical performances appears at high current densities due to steam clogging.