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.     

The effect of catalyst formulation and Rh dispersion on the performance of a CPO fuel processor investigated by operando sampling technique and predictive modelling analysis

The catalytic partial oxidation (CPO) of methane and iso-octane on Rh-coated monoliths is studied in an adiabatic reactor where axial temperature and concentration profiles are collected by a spatially resolved sampling technique. In CH₄-CPO, the Rh/MgAl₂O₄ outperforms the Rh/α-Al₂O₃ formulation, due to a significant improvement of Rh dispersion. In iso-octane CPO, the beneficial effect of the improved Rh surface is less important due to the intrinsic lower sensitivity of the system. However, a non-negligible impact of Rh dispersion on the extent of hydrocarbon side-products is observed. This factor, together with the lower acidity of the spinel support, contributes to limit the C build-up. Reactor model and kinetic schemes allow to rationalize the measurements and explore the more general effect of Rh specific surface on the key performance indicators of the CPO reformer, that is syngas productivity and hot-spot temperature. Gas-solid diffusion rate makes such indicators strictly fuel-specific.     

Methanation of CO2 over nanostructured nickel-4f block element bimetallic oxides

Nanostructure nickel-4f block element bimetallic oxides were prepared by electrospinning technique using a precursor sol-gel solution containing metal nitrates and a treatment of controlled oxidation under dry air. The heating rate of such treatment allows the control of the product morphology aiming the production of either nanoparticles or nanofibers. Catalytic hydrogenation studies of carbon dioxide were for the first time undertaken using this type of bimetallic oxides as catalysts and remarkably almost all of them present a catalytic activity superior to that of a commercial rhodium catalyst supported on alumina (5 wt % Rh/Al₂O₃) for the production of methane. Last but not the least, the nickel-4f block element bimetallic oxides present also a deactivation resistance for at least 50 h in the gaseous stream, which is an advantage for nickel-based catalysts.     

Combined dry reforming and partial oxidation of methane to synthesis gas on noble metal catalysts

In this paper CO₂ reforming of methane combined with partial oxidation of methane to syngas over noble metal catalysts (Rh, Ru, Pt, Pd, Ir) supported on alumina-stabilized magnesia has been studied. The catalysts were characterized by using BET, XRD, SEM, TEM, TPR, TPH and H₂S chemisorption techniques. The H₂S chemisorption analysis showed an active metal crystallite size in the range of 1.8-4.24 nm for the prepared catalysts. The obtained results revealed that the Rh and Ru catalysts showed the highest activity in combined reforming and both the dry reforming and partial oxidation of methane. The obtained results also showed a high catalytic stability without any decrease in methane conversion up to 50 h of reaction. In addition, the H₂/CO ratio was around 2 and 0.7 over different catalysts for catalytic partial oxidation and dry reforming, respectively.     

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.     

Preparation of methanation catalysts for high temperature SOEC by β-cyclodextrin-assisted impregnation of nano-CeO2 with transition metal oxides

The aim of this work was to prepare and examine the catalytic activity of nanometric CeO₂ decorated with transition metal oxides – Ni, Co, Cu, Fe and Mn – towards a high-temperature methanation process under SOEC CO₂/H₂O simulated co-electrolysis conditions. Samples were prepared using the wet impregnation method via the conventional process and with the addition of native cyclodextrin. The influence of β-cyclodextrin (βCD) onto the size, dispersion and integration of the obtained metal nanoparticles was investigated. The differences between the catalysts’ reducibility revealed that samples prepared from βCD-containing solutions, in most cases, resulted in the creation of smaller Me_xO_y NPs on the surface of the substrate material compared to those prepared using traditional nitrate solutions. The samples containing Ni and Co were the only ones that observably catalysed methane synthesis. The high dispersion and integration of NPs prepared via the proposed synthesis route resulted in increased catalytic activity and enhanced stability, which was most pronounced for the Co-impregnated sample. The methane production peak for Ni-βCD/CeO₂ at 375 °C was characterised by nearly 99% CO conversion and 80% selectivity towards CH₄ production. Co-βCD/CeO₂ reached 84% CO conversion and almost 60% methane selectivity at 450 °C. The usage of CeO₂ coupled with βCD for the preparation of catalysts for high-temperature methane synthesis for use in SOECs gave promising results for further application.     

Water-gas shift of reformate streams over mono-metallic PGM catalysts

Mono-metallic Pt and Rh catalysts supported on both CeO₂ and TiO₂ were prepared and tested for water-gas shift activity in a Flowrence, high throughput reactor system. The feed composition mimicked a typical fuel processor, steam methane reformer outlet stream. The Pt/CeO₂ catalyst showed the best metal activity of ～3.8 E-07 moles CO converted·gPt^-1 s^-1, at a Pt loading of 0.5 wt%, activity decreasing with increasing metal loading. Furthermore, the Pt/CeO₂ catalyst produced almost no methane while the Rh based catalysts led to substantial methanation.     

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.     

Hydrogen production by coupled catalytic partial oxidation and steam methane reforming at elevated pressure and temperature

Hydrogen production by coupled catalytic partial oxidation (CPO) and steam methane reforming of methane (OSMR) at industrial conditions (high temperatures and pressures) have been studied over supported 1 wt.% NiB catalysts. Mixture of air/CH₄/H₂O was applied as the feed. The effects of O₂:CH₄ ratio, H₂O:CH₄ ratio and the gas hourly space velocity (GHSV) on oxy-steam reforming (OSRM) were also studied. Results indicate that CH₄ conversion increases significantly with increasing O₂:CH₄ or H₂O:CH₄ ratio. However, the hydrogen mole fraction goes through a maximum, depending on reaction conditions, e.g., pressure, temperature and the feed gases ratios. Carbon deposition on the catalysts has been greatly decreased after steam addition. The supported 1 wt.% NiB catalysts exhibit high stability with 85% methane conversion at 15 bar and 800 °C during 70 h time-on-stream reaction (CH₄:O₂:H₂O:N₂ = 1:0.5:1:1.887). The thermal efficiency was increased from 35.8% by CPO (without steam) to 55.6%. The presented data would be useful references for further design of enlarged scale hydrogen production system.     

Water effect in hydrogen production from methane

Ni/MgO and Ni/Al₂O₃ catalysts were prepared, by wet impregnation, to compare their performance in hydrogen production from methane CPO, wet-CPO and SR. The catalytic activity was tested at 1073 K, 1 bar and 600–1200 h⁻¹. Fresh and used catalysts were characterized by different techniques. Both supports, as expected, had a low surface area (27.1 m²/g MgO and 6.2 m²/g α-Al₂O₃), as determined by BET method. The images obtained with SEM and TEM revealed that the Ni was more dispersed in the MgO support than in the Al₂O₃ one. By XRD a strong interaction, as solid-solution, between NiO and MgO was found in the 30Ni/MgO and 40Ni/MgO catalysts. The fresh 40Ni/Al₂O₃ reduced catalyst was partially reduced. But after the activity tests the stability of the reduced Ni became bigger. Some Ni sintering was also observed in the 40Ni/Al₂O₃ after the wet-CPO and SR tests. The behaviour of the three catalysts was very good in CPO methane conversion (90–93%), but the gradual increase of the steam to carbon ratio, wet-CPO and SR, affected negatively the conversion.     

Hydrogen evolution from hydrolysis of ammonia borane catalyzed by Rh/g-C3N4 under mild conditions

Developing effective catalysts for hydrogen evolution from hydrolysis of ammonia borane (AB) is of great significance considering the useful applications of hydrogen. Herein, graphitic carbon nitride (g-C₃N₄) prepared through the simply pyrolysis of urea was employed as a support for Rh nanoparticles (NPs) stabilization. The in-situ generated Rh NPs supported on g-C₃N₄ with an average size of 3.1 nm were investigated as catalysts for hydrogen generation from the hydrolysis of AB under mild conditions. The Rh/g-C₃N₄ catalyst exhibits a high turnover frequency of 969 mol H₂· (min·mol_Rh)^?1 and a low activation energy of 24.2 kJ/mol. The results of the kinetic studies show that the catalytic hydrolysis of AB over the Rh/g-C₃N₄ catalyst is a zero-order reaction with the AB concentration and a first-order reaction with the Rh concentration. This work demonstrates that g-C₃N₄ is a useful support to design and synthesis of effective Rh-based catalyst for hydrogen-based applications.     

Well-dispersed Rh nanoparticles with high activity for the dry reforming of methane

Rh catalysts with low Rh content were prepared by incipient wetness impregnation using [NH₄]₃[RhCl₆]·3H₂O or RhCl₃·3H₂O as precursor salts, on CaO–SiO₂ supports. All solids showed a high stability after 48 h on stream for the dry reforming of methane with low carbon content, which made them suitable for obtaining ultrapure hydrogen in a membrane reactor. The methane conversion and hydrogen recovery were measured increasing the sweep gas flow rates to rise the driving force for hydrogen permeation. The catalyst with 0.36 wt.% of Rh showed a slight deactivation. However, the Rh(0.6)/CaO–SiO₂ solid, in which the Rh impregnation was performed using [NH₄]₃[RhCl₆]·3H₂O, exhibited an increase on CH₄ conversion of 77% and a hydrogen recovery equal to 84%.Nanoparticles of about 1.4–1.7 nm surface average diameter were detected for the reduced and used solids indicating that Rh is well dispersed and sintering was not produced after the catalytic tests. Rh particle sizes calculated by CO chemisorption were coincident with those measured by Transmission Electron Microscopy. Characterization by this technique and Laser Raman Spectroscopy of the solids used in membrane reactor revealed the formation of scarce carbon filaments. However, a surface re-oxidation was detected in the low loading catalyst used in the membrane reactor suggesting that it is the main cause for the decrease in the activity of the highly dispersed catalyst.     

Carbon-supported small Rh nanoparticles prepared with sodium citrate: Toward high catalytic activity for hydrogen evolution from ammonia borane hydrolysis

Hydrogen generation from the hydrolysis of ammonia borane (AB) over heterogeneous catalysts is essential for practical applications. Herein, efficient hydrogen evolution from AB hydrolysis over the carbon-supported Rh nanoparticles synthesized with sodium citrate (Rh/C-SC) was achieved at 25 °C. The turnover frequency value of Rh/C-SC was 336 mol H₂ (mol_Rh min)^?1, whereas that of Rh/C catalyst only yielded a value of 134 mol H₂ (mol_Rh min)^?1. The improvement of the catalytic performance of Rh/C-SC catalyst could be attributed to the small Rh particles with highly active surface areas, which were prepared by using sodium citrate as the stabilizing agent. This result indicates that sodium citrate can be applied as a useful stabilizing agent for synthesizing active metal nanoparticles, thus highly promoting the practical application of AB system for fuel cells.     

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.     

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.     

Carbon dioxide reforming of methane over nickel catalysts supported on TiO2(001) nanosheets

As we all know, the critical problem of nickel catalysts for carbon dioxide reforming of methane is the deactivation of catalysts due to the carbon deposition and sintering of the active components under high temperature. It was reported that anatase TiO₂ nanosheets with high-energy (001) facets had strong interaction with nickel, which was probably beneficial to resist sintering of nickel nanoparticles and to eliminate deposited carbon via oxygen migration. In this study, Ni nanoparticles were supported on TiO2 nanosheets with exposed high-energy (001) facets. The Ni/TiO₂(001) catalysts were characterized by means of X-ray diffraction, transmission electron microscopy, physisorption of N₂, X-ray photoelectron spectroscopy and H₂ temperature-programmed reduction, and the spent catalysts were characterized by Roman and thermogravimetry analysis. The catalytic performance of Ni/TiO₂(001) catalysts were measured for carbon dioxide reforming of methane reaction. It was found that the prepared Ni/TiO₂(001) catalysts showed reasonably higher catalytic activity and stability compared with the nickel catalyst supported on commercial titanium oxide (P25). The high dispersion of nickel nanoparticles of Ni/TiO₂(001) catalysts was helpful to the resistance towards carbon deposition and the strong metal-support interaction was helpful to the resistance towards nickel sintering on account of the unusual surface properties of TiO₂(001).     

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.     

H2-rich syngas production from biogas reforming: Overcoming coking and sintering using bimetallic Ni-based catalysts

Dry reforming of methane is a very appealing catalytic route biogas (mainly composed by greenhouse gases: carbon dioxide and methane) conversion into added value syngas, which could be further upgraded to produce liquid fuels and added value chemicals. However, the major culprits of this reaction are coking and active phase sintering that result in catalysts deactivation. Herein we have developed a highly stable bimetallic Ni–Rh catalyst supported on mixed CeO₂–Al₂O₃ oxide using low-noble metal loadings. The addition of small amounts of rhodium to nickel catalysts prevents coke formation and improves sintering resistance, achieving high conversions over extended reaction times hence resulting in promising catalysts for biogas upgrading.     

Preparation and detailed characterization of zirconia nanopowder supported rhodium (0) nanoparticles for hydrogen production from the methanolysis of methylamine-borane in room conditions

Uniformly dispersed Rh (0) nanoparticles supported on zirconia nanopowder were synthesized by a two-step and simple ex-situ method summarized by mixing rhodium (III) chloride hydrate with zirconia (nano-ZrO₂) aqueous solution in ambient conditions followed by reduction with NaBH₄. The ex-situ prepared nano-ZrO₂ supported Rh (0) nanoparticles (Rh/nano-ZrO₂) were characterized by various spectroscopic methods, including TEM, TEM-EDX, HR-TEM, P-XRD, XPS and ICP-OES. The catalytic activity of Rh (0) nanoparticles is 1050 h^?1 in terms of initial turnover frequency (TOF), which is the first study in the literature to produce hydrogen by catalytic methanolysis of methylamine-borane. In addition, the catalytic methanolysis of methylamine-borane by using Rh (0) nanoparticles was carried out in different catalyst/substrate concentrations and different temperatures to reveal rate equation and kinetic parameters. Consequently, Rh (0) nanoparticles are taken into account as an encouraging catalyst for the methanolysis of methylamine-borane and for providing a more fertile hydrogen storage gain in fuel cell operations.