Catalytic partial oxidation of n-octane and iso-octane: Experimental and modeling results

The Catalytic Partial Oxidation (CPO) of two octane isomers, 2,2,4-trimethyl pentane (iso-octane) and n-octane, chosen as representative of gasoline is investigated by means of adiabatic tests and mathematical modeling. CPO experiments were carried out in a lab scale auto-thermal reformer with honeycomb monolith catalysts (2% Rh/α-Al₂O₃), equipped with probes for spatially resolved measurements of temperature and concentration. Tests were performed with about 50% N₂ dilution to prevent risks of deactivation due to catalyst over temperature. The CPO of the two isomers follows similar reaction pathways, which mainly consist of the exothermic combustion reaction and the endothermic steam reforming. This results in a close similarity of the concentration profiles of the main species and of the temperature profiles obtained with the two isomers. On the other hand, gas phase reactions proceed to a different extent and bring about a different distribution of thermal cracking products, iso-octane being more reactive and selective to iso-butylene and propene, while n-octane being selective to ethylene. Coke formation was observed upon adiabatic tests which was responsible for partial deactivation of the reforming zone of the catalyst. Post mortem TPO tests show that n-octane exhibits a higher tendency to coke deposition than iso-octane in the adopted CPO conditions. Thermodynamic and modeling calculations show that the risk of coking can be reduced by using exhaust gas recycling instead of N₂ to dilute the reactants.     

The kinetic effect of H2O pressure on CO hydrogenation over different Rh cluster sizes

The effect of H₂O pressure (P_H2O) on CH₄ formation turnover rate (TOF_CH4) was evaluated as a function of Rh cluster size. In-situ DRIFTS and kinetic measurements were used to study the CO hydrogenation at methanation conditions on two Rh/Al₂O₃ catalysts with different cluster sizes (Rh-1nm and Rh-3nm). It was observed a significant effect of P_H2O on TOF_CH4 over Rh-1nm catalysts, while the rate on Rh-3nm resulted insensible to the H₂O pressure. The mean Rh cluster sizes were estimated by H₂ chemisorption, TEM and XPS analyses. It is confirmed the structural sensitivity of the CO methanation reaction on supported Rh catalysts as the TOF_CH4 resulted significantly higher in the larger clusters. Nonetheless, lower apparent activation energy was measured in smaller Rh clusters, which is successfully explained by the energies involved into the parameter of the proposed L-H kinetic model. CO adsorption isotherms were obtained from in-situ DRIFTS experiments at 280–340 °C. The enthalpy and entropy values for CO adsorption indicate that the CO binds more strongly on the low-coordinated surface of Rh atoms. In-situ DRIFTS results demonstrate that the CO coverage is unaffected by P_H2 and P_H2O, regardless of Rh dispersion, thus excluding H* and H₂O* as most abundant surface intermediates of the Kinetics. The infrared and kinetic measurements on both catalysts are consistent with a mechanism of CO bond dissociation assisted by H*. Data are well represented by a single-site-L-H model that successfully captures the effect of P_H2, P_CO and P_H2O on TOF_CH4. This model contains a parameter (k’) that represents the ratio between the rates of C* removal by O* and H* at Rh surface and explains the stronger kinetic effect of P_H2O on TOF_CH4 observed over smaller Rh clusters.     

Catalytic partial oxidation of methane over nanosized Rh supported on Fecralloy foams

Structured catalysts for the partial oxidation of methane were prepared by supporting Rh nanoparticles onto Fecralloy foams at relatively low precious metal loadings. The investigation was focused mainly on an innovative and straightforward preparation procedure consisting in the direct cathodic electrodeposition of Rh onto foam samples. For the sake of comparison, other Rh-based catalysts were prepared with a more traditional approach, by using the same foams and an AlPO₄ washcoat layer. The catalysts were characterized by SEM-EDS, XRD and cyclic voltammetry, to assess the Rh surface area, and tested in the CPO of methane to syngas under self-sustained high temperature conditions at short-contact-time. During prolonged CPO tests the performance of electrochemically prepared catalysts underwent a progressive decline, as compared to stable operation of AlPO₄ washcoated catalysts, which was mainly ascribed to sintering of Rh nanoparticles, negatively affecting the activity for methane steam reforming.     

Fixed beds of Rh/Al2O3-based catalysts for syngas production in methane SCT-CPO reactors

The present experimental work deals with methane short contact time (SCT) CPO in a fixed bed reactor considering CH₄ conversion and H₂ and CO selectivity in a wide range of weight hourly space velocity (WHSV). Two different Rh/Al₂O₃-based catalysts both loaded with 0.5% by weight of Rh were developed: one catalyst carrying Rh on the external support surface (Egg-Shell configuration), and the other one with Rh embedded into the porous support (Egg-Yolk configuration). The goal was the design of the optimal fixed bed structure (not only considering beds made of egg-shell or egg-yolk catalysts alone, but also their various combinations), able to either attain the best performance or maintain a reaction temperature along the bed without excessive variations with WHSV. The highest CH₄ conversion (>90%) and H₂ selectivity (>98%), moreover stable despite the WHSV variation of about 3.6 times, and reactor working temperature with not too large variations (maximum of about 16%) by increasing WHSV were obtained with the fixed bed of Egg-Yolk catalyst alone. Instead, the fixed bed of Egg-Shell catalyst alone showed the worst performance: CH₄ conversion and H₂ selectivity were lower of about 15% and 10%, respectively, and decreasing with the increase of WHSV; on the contrary, the CO selectivity remained practically the same, only a slightly decrease being observed. Suitable combinations of the two catalysts in the fixed bed produced intermediate performance between those of the catalysts alone. The different performance of the two catalyst types was probably due to the different structure of the particles and to the Rh position on the carrier itself. Finally, thermal and performance durability tests up to 16 working hours showed that the Egg-Yolk catalyst employed alone in the fixed bed was able to maintain the CH₄ partial oxidation activity with practically disregardable decrease.     

Effect of sulphur during the catalytic partial oxidation of ethane over Rh and Pt honeycomb catalysts

The impact of sulphur addition (2–58 ppm) during the catalytic partial oxidation (CPO) of ethane was investigated on Rh- and Pt-based honeycomb catalysts tested under self-sustained high temperature conditions. Both steady state and transient operation of the CPO reactor were investigated particularly with regards to poisoning/regeneration cycles. A detailed analysis of products distribution in the effluent and a heat balance of the CPO reactor demonstrates that sulphur reversibly adsorbed on Rh selectively inhibits the ethane hydrogenolysis and, to a lower extent, steam reforming reaction. A further, simultaneous adverse effect of S on the kinetics of the reverse water gas shift reaction on Rh catalyst operating at temperatures < 750 °C can cause an unexpected increase in the H₂ yield above its equilibrium value for low concentrations of the poison. Pt catalyst is less active for those reactions but in turn is more S-tolerant.     

Structure-dependent activity and stability of Al2O3 supported Rh catalysts for the steam reforming of n-dodecane

Steam reforming of liquid hydrocarbon fuels is an appealing way for the production of hydrogen. In this work, the Rh/Al₂O₃ catalysts with nanorod (NR), nanofiber (NF) and sponge-shaped (SP) alumina supports were successfully designed for the steam reforming of n-dodecane as a surrogate compound for diesel/jet fuels. The catalysts before and after reaction were well characterized by using ICP, XRD, N₂ adsorption, TEM, HAADF-STEM, H₂-TPR, CO chemisorption, NH₃-TPD, CO₂-TPD, XPS, Al²⁷ NMR and TG. The results confirmed that the dispersion and surface structure of Rh species is quite dependent on the enclosed various morphologies. Rh/Al₂O₃-NR possesses highly dispersed, uniform and accessible Rh particles with the highest percentage of surface electron deficient Rh⁰ active species, which due to the unique properties of Al₂O₃ nanorod including high crystallinity, relatively large alumina particle size, thermal stability, and large pore volume and size. As a consequent, Rh/Al₂O₃-NR catalyst exhibited superior catalytic activity towards steam reforming reactions and hydrogen production rate over other two catalysts. Especially, Rh/Al₂O₃-NR catalyst showed the highest hydrogen production rate of 87,600 mmol g_fuel^?1 g_Rh^?1min^?1 among any Rh-based catalysts and other noble metal-based catalysts to date. After long-term reaction, a significant deactivation occurred on Rh/Al₂O₃–NF and Rh/Al₂O₃-SP catalysts, due to aggregation and sintering of Rh metal particles, coke deposition and poor hydrothermal stability of nanofibrous structure. In contrast, the Rh/Al₂O₃-NR catalyst shows excellent reforming stability with negligible coke formation. No significantly sintering and aggregation of the Rh particles is observed after long-term reaction. Such great catalyst stability can be explained by the role of hydrothermal stable nanorod alumina support, which not only provides a unique environment for the stabilization of uniform and small-size Rh particles but also affords strong surface basic sites.     

Autothermal reforming of iso-octane and gasoline over Rh-based catalysts: Influence of CeO2/γ-Al2O3-based mixed oxides on hydrogen production

Autothermal reforming (ATR) of iso-octane in the presence of Rh-based catalysts (0.5 wt% of Rh) supported onto γ-Al₂O₃, CeO₂, and ZrO₂ were initially carried out at 700 °C with a S/C ratio of 2.0, an O/C ratio of 0.84, and a gas hourly space velocity (GHSV) of 20,000 h⁻¹. The activity of Rh/γ-Al₂O₃ was found to be higher than Rh/CeO₂ and Rh/ZrO₂, with H₂ and (H₂ + CO) yields of 1.98 and 2.48 mol/mol C, respectively, after 10 h. This Rh/γ-Al₂O₃ material, however, was potentially susceptible to carbon coking and produced 3.5 wt% of carbon deposits following the reforming reaction, as evidenced by C, H, N, and S elemental analysis. In contrast, Rh/CeO₂ catalyst exhibited lower activity but higher stability than Rh/γ-Al₂O₃, with nearly no carbon being formed within 10 h. To combine the superior activity originated from Rh/γ-Al₂O₃ with high stability from Rh/CeO₂, Rh/CeO₂/γ-Al₂O₃ catalysts with different CeO₂ contents were synthesized and examined for the ATR reactions of iso-octane. Compared to Rh/γ-Al₂O₃, the newly prepared Rh/CeO₂/γ-Al₂O₃ catalysts (0.5 wt% of Rh and 20 wt% of CeO₂) showed even enhanced activity during 10 h, and H₂ and (H₂ + CO) yields were calculated to be 2.08 and 2.62 mol/mol C, respectively. In addition, as observed with Rh/CeO₂, the catalyst was further found to be stable with less than 0.3 wt% of carbon deposition after 10 h. The Rh/γ-Al₂O₃ and Rh/CeO₂/γ-Al₂O₃ catalysts were eventually tested for ATR reactions using commercial gasoline that contained sulfur, aromatics, and other impurities. The Rh/γ-Al₂O₃ catalyst was significantly deactivated, showing decreased activity after 4 h, while the Rh/CeO₂/γ-Al₂O₃ catalyst proved to be excellent in terms of stability against coke formation as well as activity towards the desired reforming reaction, maintaining its ability for H₂ production for 100 h.     

Hysteresis and reaction characterization of methane catalytic partial oxidation on rhodium catalyst

Hysteresis effects and reaction characteristics of methane catalytic partial oxidation (CPO) in a fixed-bed reactor are numerically simulated. The reactions are modeled based on the experimental measurements of methane CPO with a rhodium (Rh) catalyst. Three C/O ratios of 0.6, 1.0 and 1.4 are considered in the study. When the Reynolds number is 200, the predictions indicate that the methane CPO is always triggered at around the inlet temperature of 550 K, regardless of what the C/O ratio is. It is of interest that if the inlet temperature is decreased after the methane CPO develops at higher inlet temperatures, the reversed path of methane conversion is different from the original path at lower inlet temperatures. The hysteresis effect of the methane CPO is thus observed. The hysteresis behavior implies that a higher yield of syngas or hydrogen can be achieved by controlling the reaction process. Decreasing the C/O ratio intensifies the methane CPO so that the hysteresis effect is more pronounced, and vice versa. An increase in Reynolds number delays the excitation temperature of methane CPO and lessens the hysteresis effect of methane conversion due to the growth of fluid inertial force. However, the hysteresis effect of the maximum temperature in the catalyst bed increases as a result of more methane consumption.     

Double oxalates of Rh(III) with Ni(II) and Co(II) – Effective precursors of nanoalloys for hydrocarbons steam reforming

Heteronuclear coordination compounds of d-metals are suitable single-source precursors for bimetallic nanoalloys, which often show extraordinary catalytic properties due to synergetic effect. In particular, Ni- and Rh-based catalysts are highly effective in low temperature steam reforming processes. Double oxalates of Rh with Ni and Co of the formula {[Rh(H₂O)₂(C₂O₄)μ-(C₂O₄)]₂M(H₂O)₂}·6H₂O (M = Ni, Co) were synthesized and structurally characterized. According to thermogravimetric analysis, the complexes decompose completely in He and H₂ atmospheres to form corresponding nanoalloys at ∼300 °C. The calcination in O₂ atmosphere leads to formation of spinel type mixed oxide. The supported Co–Rh/Al₂O₃ and Ni–Rh/Al₂O₃ catalysts were prepared by impregnation of double oxalate complexes in porous support with subsequent calcination and tested in propane low temperature steam reforming in CH₄ excess. The Co-containing catalyst showed comparable activity regarding to pure Rh/Al₂O₃ sample, while bimetallic Ni–Rh/Al₂O₃ catalyst revealed to be appreciably more active, than monometallic catalysts with higher active component loadings. Rh–Ni catalyst allowed for complete propane conversion at T ≈ 350 °C, whereas for Rh catalyst the temperature was T ≈ 410 °C, and Rh–Co did not reach complete C₃H₈ conversion at all.     

Catalytic partial oxidation of CH4–H2 mixtures over Ni foams modified with Rh and Pt

The catalytic partial oxidation (CPO) of methane–hydrogen mixtures in air, intended for the first stage of hybrid radiant catalytic burners, was investigated under self-sustained short contact time conditions on commercial Ni foam catalysts eventually modified with Rh and Pt. The modified catalysts were prepared by a simple novel method based on the spontaneous deposition of noble metals via metal exchange reactions onto those Ni foam substrates. SEM-EDS, electrochemical methods and H₂-TPR analysis were integrated to characterize morphology, surface area of metal deposits and reducibility of foam catalysts before and after exposure to severe conditions in the CPO reactor. In particular Rh forms finely dispersed deposits that retain their high specific surface area at temperatures up ca. 1100 °C. Modification with noble metals enhances stability and reducibility of the Ni foam whereas the overall CPO performance is not significantly improved. Safe operation of the CPO reactor with up to 70% vol. H₂ in the fuel mixture has been achieved by properly increasing the feed equivalence ratio to avoid catalyst overheating, while guaranteeing high methane conversions and a persistent net hydrogen production.     

Effect of phosphorous addition to Rh-supported catalysts for the dry reforming of methane

The dry reforming (DR) of methane has been studied over Rh-based catalysts modified by phosphorous addition in order to investigate the possibility to enhance their resistance to sulfur poisoning. In particular, three catalysts have been prepared: i) by dispersing phosphorous with rhodium on La-stabilized γ-Al₂O₃; ii) by stabilizing γ-Al₂O₃ with phosphorous and then dispersing rhodium; iii) by dispersing rhodium on an amorphous AlPO₄ support. Rh supported on La-stabilized γ-Al₂O₃ has been used as a reference catalyst.Fresh and used catalysts and their corresponding supports have been characterized by ICP-MS, XRD, BET, H₂-TPR, CO chemisorption, CO₂-TPD, TG analysis and Raman spectroscopy. Phosphorous addition to the supports increases their surface acidity and inhibits CO₂ activation, thus depressing both activity and resistance to coke formation of the corresponding supported Rh catalysts during methane DR. On the contrary, catalysts supported on basic La-promoted alumina provide a stable syngas production approaching equilibrium at 750–800 °C. Small amounts of phosphorous co-impregnated with rhodium increase the noble metal dispersion, but do not significantly impact on the catalyst activity.Transient and steady state S-poisoning experiments during methane DR suggest that sulfur directly attacks and bonds to Rh active sites, causing a rapid drop of syngas production even at low S-contents. A secondary poisoning effect is induced by sulfur that causes the rapid formation of some amorphous coke, which is almost absent under S-free operation on the reference Rh catalyst.     

Pt-promoted α-Al2O3-supported Ni catalysts: Effect of preparation conditions on oxi-reduction and catalytic properties for hydrogen production by steam reforming of methane

The effect of Pt addition on the oxi-reduction properties of α-Al₂O₃-supported Ni catalysts, with different degrees of interaction between NiO and the α-Al₂O₃ support, was studied using atmospheres of H₂, H₂/H₂O, and CH₄/H₂O. The effect of Pt promotion on the reduction of NiO with H₂ was significant for NiO species that interacted more strongly with the alumina surface, but was much lower when a NiAl₂O₄-like bulk phase was formed. For samples activated with H₂, although metal dispersion decreased with increasing Pt content, the activity was maintained constant by the presence of Pt sites. For samples activated with a CH₄/H₂O mixture, the activity increased with increasing Pt content, due to the higher reducibility of Ni in the Pt-promoted catalysts. The Pt promotion effect was stable; there was no important decrease in the influence of Pt on NiO reduction, even after high temperature re-oxidation of the catalysts.     

H2 evolution by water SPLITTING on Rh/La2O3

The physicochemical and electrochemical properties of rhodium catalysts supported on La₂O₃ denoted XRhLa (X = 1 and 5% wt. Rh) prepared by impregnation using RhCl₆H₂O as precursor salt were studied. The solids were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal analysis (TG/TDA) and hydrogen chemisorption (HC) to evaluate the dispersion of the metal phase. The temperature-programmed reaction with hydrogen (H₂-TPR), carbon monoxide (CO-TPR) or methane (CH₄-TPR) were carried out to elucidate there effects on catalytic reaction. The adsorption and decomposition of H₂O has been investigated on the surface catalysts. The number of reduced centers of lanthanum in Rh/La₂O₃ catalysts was measured by in situ oxidation of these centers at oxydation temperature of water (T_OXtov) by water pulses according to the following reaction (Reduced centers + H₂O→Oxidized center + H₂). The amount of hydrogen Q(H), evolved in the reaction allows us to calculate the number of reduced centers of the support since the Rh metal is not oxidized. The results showed that although the conversion rate of water to H₂ is low, the 5% wt. Rh catalyst is a promising candidate in the water adsorption and dissociation compared to the 1% wt.     

Thermochemical hydrogen production using Rh/CeO2/γ-Al2O3 catalyst by steam reforming of ethanol and water splitting in a packed bed reactor

This study is focused on investigating the dual performance of Rh/CeO₂/γ-Al₂O₃ catalyst for steam reforming of ethanol (SRE) and thermochemical water splitting (TCWS) using a packed bed reactor. The catalyst is designed to be thermally stable containing an active phase of Rh and the redox component of CeO₂ for oxygen exchange, supported on γ-Al₂O₃. The catalyst has been characterised by SEM, XRD, BET, TPR, TPD, XPS and TGA before testing in the reactor. The optimal temperature for SRE reaction over this catalyst is between 700 °C and 800 °C to produce high concentrations of hydrogen (~60%), and low CO and CH₄. The selectivity towards CO and CH₄ is higher at low temperatures and drops with rise in reaction temperature. Further, Rh/CeO₂/γ-Al₂O₃ is found to be active for TCWS at relatively low temperatures (≤1200 °C). At temperatures as low as 800 °C, this catalyst is especially found suitable for multiple redox cycles, producing a total of 48.9 mmol/g_cat in four redox cycles. The catalyst can be employed for large number of redox cycles when the reactor is operated at lower temperatures. Finally, the reaction pathways have been proposed for both SRE and TCWS on Rh/CeO₂/γ-Al₂O₃ catalyst.     

Ethanol steam reforming over Ni–Fe-based hydrotalcites: Effect of iron content and reaction temperature

Ethanol steam reforming has been evaluated over nickel–iron based hydrotalcite-like compounds with Ni/Fe molar ratios of 1 and 0.5. Calcined materials have been characterized by XRD, TEM, BET and TPR. The introduction of iron leads to the formation of a mixture of Ni(Fe)O_x and spinel phase upon calcination, which results in variations of structural and catalytic properties. With a Ni/Fe ratio of 1, a remarkable improvement in catalytic activity as well as selectivity to hydrogen is observed with respect to the catalyst with Ni/Fe ratio of 0.5. This is due to the enhanced nickel dispersion, the high surface area, and small Ni⁰ crystallite size over the Ni(Fe)O_x + NiFe₂O₄ mixture. However, a further increase in iron content leads to the formation of a low surface area spinel phase (NiFe₂O₄), which results in lower activity and faster deactivation in the reaction through Ni⁰ sintering. The effect of reaction temperature has been evaluated over the most active catalyst (Ni/Fe = 1).     

Comparative XPS study of Rh/Al2O3 and Rh/TiO2 as catalysts for NaBH4 hydrolysis

The catalysts Rh/Al₂O₃ and Rh/TiO₂ for hydrogen production from NaBH₄ were prepared by deposition technique from RhCl₃ reduced by NaBH₄ and were studied by XPS and TEM. It was found that the RhCl₃/Al₂O₃ system is more stable comparing to RhCl₃/TiO₂ which starts to decompose by weak heat treatment. It was shown that NaBH₄ reduced RhCl₃/TiO₂ (Al₂O₃) to supported metal Rh nanoparticles in both cases. In the case of Rh/TiO₂ SMSI effect it was found after RT reduction. The SMSI (Strong Metal-Support Interaction) effect gave an explanation for the difference of activity between Rh/TiO₂ and Rh/Al₂O₃ catalysts in hydrolysis reaction of NaBH₄.     

Surface modification of solution combustion synthesized Ni/Al2O3 catalyst for aqueous-phase reforming of ethanol

The effect of surface modification of an alumina powder supported nano-scale nickel catalyst used in aqueous-phase reforming of ethanol has been explored in this paper. The Al₂O₃ powder was prepared by a solution combustion synthesis (SCS) route and the surface of the powder was modified by a non-thermal RF plasma treatment using nitrogen gas. Catalysts were coated by an impregnation method. The performances of the unmodified and modified Ni/Al₂O₃ catalysts have been compared from a catalytic activity, selectivity, and microstructural point of view. The catalytic activity results showed that while nature, relative ratio and selectivity of the products both in gas and liquid effluents did not change, catalytic activity (in terms of EtOH conversion and H₂ yield per g) of the sample increased after plasma modification. Microstructural (XRD, surface area) analysis showed that phase content and surface area of unmodified and modified catalysts are similar, while TEM and H₂-chemisorption showed higher metal surface area, higher metal dispersion and lower active metal particle size for the modified sample compared to the unmodified sample. The temperature programmed reduction (TPR) analysis demonstrated stronger support-metal interaction and smaller NiO particles for the modified catalyst at lower heat treatment temperature. The temperature programmed desorption (TPD) of ammonia analysis showed stronger acidity for the modified support, which can explain better dispersion of the metal particles on the modified catalyst compared to the unmodified sample.     

Surface modification of LiNi0.5Mn1.5O4 by ZrP2O7 and ZrO2 for lithium-ion batteries

The spinel LiNi_0.5Mn_1.5O₄ has been surface modified separately with 1.0 wt.% ZrO₂ and ZrP₂O₇ for the purpose of improving its cycle performance as a cathode in a 5-V lithium-ion cell. Although the modifications did not change the crystallographic structure of the surface-modified samples, they exhibited better cyclability at elevated temperature (55 °C) compared with pristine LiNi_0.5Mn_1.5O₄. The material that was surface modified with ZrO₂ gave the best cycling performance, only 4% loss of capacity after 150 cycles at 55 °C. Electrochemical impedance spectroscopy demonstrated that the improved performance of the ZrO₂-surface-modified LiNi_0.5Mn_1.5O₄ is due to a small decrease in the charge transfer resistance, indicating limited surface reactivity during cycling. Differential scanning calorimetry showed that the ZrO₂-modified LiNi_0.5Mn_1.5O₄ exhibits lower heat generation and higher onset reaction temperature compared to the pristine material. The excellent cycling and safety performance of the ZrO₂-modified LiNi_0.5Mn_1.5O₄ electrode was found to be due to the protective effect of homogeneous ZrO₂ nano-particles that form on the LiNi_0.5Mn_1.5O₄, as shown by transmission electron microscopy.     

Role of lattice oxygen in the partial oxidation of methane over Rh/zirconia-doped ceria. Isotopic studies

Isotopic tracer and nuclear reaction analysis (NRA) are used to probe the identity of oxygen for CO formation during the catalytic partial oxidation (CPOX) of methane to synthesis gas on ¹⁸O₂ labeled Rh (1 wt.%)/(Ce_0.56Zr_0.44)O_2−x. Results reveal that methane is selectively oxidized by lattice oxygen ions from the catalyst to form carbon monoxide. ¹⁸O₂ isotopic exchange experiments, as a function of temperature in the 0–850 °C range, were performed on Rh (1 wt.%)/(Ce_0.56Zr_0.44)O_2−x, and (Ce_0.56Zr_0.44)O_2−x. It was observed that the presence of rhodium considerably accelerates the oxygen exchange with the support; the maximal exchange rates could be observed at lower temperatures, 250 °C. This may be due to oxygen spillover from the metal particles to the oxide. Comparing results from the isotopic exchange experiments on Rh/γ-alumina and Rh (1 wt.%)/(Ce_0.56Zr_0.44)O_2−x. It was revealed that oxygen conducting materials have a much higher oxygen storage capacity and isotopic exchange rate than non-oxygen conducting materials.     

Comparison between Ni-Rh/gadolinia doped ceria catalysts in reforming of propane for anode implementations in intermediate solid oxide fuel cells

Steam and autothermal reforming of propane over Ni-Rh/GDC catalysts prepared by coprecipitation and by Pechini method were investigated in the temperature range 873-1073 K. The weight ratio for Ni, Rh and Ce_0.8Gd_0.2O₂ (45:5:50) and the operating temperatures were chosen in order to gain propaedeutical information on fuel reactivity under typical intermediate solid oxide fuel cell (IT-SOFC) operating conditions.The Pechini synthesis allows to obtain catalysts with lower surface area, smaller nickel crystallites and a bimodal distribution of rhodium in comparison to the coprecipitation method. Despite the different methods of synthesis lead to catalysts with different morphological and structural properties, the activity of catalysts is quite similar.At reaction temperature higher than 973 K, under both steam reforming (SR) and autothermal reforming (ATR), the catalysts show high propane conversion and syngas (H₂ + CO) productivity.Deactivation of catalysts was observed at 873 and 973 K under SR conditions due to coke formation.In ATR, coke formation was almost completely depressed and the catalysts resulted to be very stable even at low reaction temperature (873 K). In SR coke formation occurs with higher rate on the catalyst having higher Ni dispersion, probably since propane cracking reaction is the pre-eminent phenomenon in promoting coke formation.