Coke Resistance Research Articles

Salt-assisted surface charge driven synthesis of large pores alumina as carbon tolerance support for propane dehydrogenation

Porous alumina has been widely used as catalytic support for industrial processes. Under carbon emission constraints, developing a low-carbon porous aluminum oxide synthesis method is a long-standing challenge for low-carbon technology. Herein, we report a method involving the only use of elements of the aluminum-containing reactants (e.g. sodium aluminate and aluminum chloride), sodium chloride was introduced as the coagulation electrolyte to adjust the precipitation process. Noticeably, the adjustment of the dosages of NaCl would allow us to tailor the textural properties and surface acidity with a volcanic-type change of the assembled alumina coiled plates. As a result, porous alumina with a specific surface area of 412 m2/g, large pore volume of 1.96 cm3/g, and concentrated pore size distribution at 30 nm was obtained. The function of salt on boehmite colloidal nanoparticles was proven by colloid model calculation, dynamic light scattering, and scanning/transmission electron microscopy. Afterward, the synthesized alumina was loaded with PtSn to prepare catalysts for the propane dehydrogenation reaction. The obtained catalysts were active but showed different deactivation behavior that was related to the coke resistance capability of the support. We figure out the correlation between pore structure and the activity of the PtSn catalysts associated with the maximum conversion of 53 % and minimum deactivation constant occurring at the pore diameter around 30 nm of the porous alumina. This work offers new insight into the synthesis of porous alumina.

Enhancing the Catalytic Activity of Ce(Mn, Fe)O2 on a Perovskite-Based Electrode via Spraying for High-Temperature CO2 Electrolysis

Solid oxide electrolysis cells (SOECs) have the potential to be highly efficient devices for high-temperature CO2electrolysis. However, commonly used Ni-based cathodes suffer from redox instability under CO/CO2 conditions. La(Sr)Cr(Mn)O3 (LSCM) is a promising alternative due to its coking resistance, but it has poor catalytic activity on CO2reduction. To address this, Ce(Mn, Fe)O2 (CMF) was added as a catalyst to accelerate CO2 adsorption. In this study, the non-touchable coating process by spraying was employed to fabricate the well-decorated embedded CMF on LSCM surface, without complicated pretreatment and infiltration. The performances of the highly decorated electrode surface with CMF nano catalyst were measured at 1123 K, exhibiting a maximum power density of 0.503 W cm-2 in fuel cell mode (H2) and electrolysis performance of 0.835 A cm-2 at 1.5 V in CO2 reduction. This approach shows potential for enhancing the catalytic activity of LSCM cathodes and advancing the development of SOECs for sustainable energy production.

Characterization and Testing of Exsolution-Based Solid Oxide Cells for Reversible Operations in CO2 Electrolysis

Reversible solid oxide cells (rSOCs) are promising electrochemical devices, able to work both in energy storage mode as solid oxide electrolyzes (SOECs), and in power generation mode as solid oxide fuel cells (SOFCs). rSOCs dedicated to high-temperature co-electrolysis of CO2/H2O mixtures are of interest since they could foster the utilization and consequent reduction of CO2 gas emissions while converting it and H2O into valuable fuels (i.e. syngas and H2). However, operation under CO2-rich feeds requires alternatives to the state-of-the-art Ni-Yttria-stabilized-zirconia (Ni-YSZ) cermet fuel electrode in view of its low coking and redox resistance. Perovskites with mixed ionic and electronic conductivity are major alternatives, especially when the functionality of their active surface is enhanced via metal nanoparticle exsolution. This work focuses on the investigation of the electrocatalytic properties of the SrTi0.3Fe0.7O3 (STF), and exsolution Sr0.95(Ti0.3Fe0.63Ni0.07)O3 (STF-Ni) perovskite-based fuel electrodes together with Ni-YSZ while operating with CO2-rich mixtures both in the short and long term.

(Digital Presentation) Au-Mo-Fe-Ni/CeO2(Gd2O3) As Potential Fuel Electrodes for Internal CO2 Reforming of CH4 in Single SOFCs

In this study the catalytic and electro-catalytic performance, as well as the coking resistance of un-modified and modified Ni/CeO2(Gd2O3) with 3 wt.% Au−0.4 wt.% Mo and 3 wt.% Au−0.5 wt.% Fe electrocatalysts were studied as half and full electrolyte supported cells under internal CO2 reforming of CH4 in single SOFCs, at 750-900 oC. The aim was to elucidate their activity towards the consumption of CH4, CO2, the production of H2, H2O, CO and the production of carbon, as a function of temperature and the applied current density under a biogas fuel mixture of CH4/CO2=1. Additionally, the cells comprising the electrocatalysts as fuel electrodes, 8 mol% Y2O3 stabilized ZrO2 (8YSZ) as electrolyte and La0.6Sr0.4Co0.8Fe0.2O3- δ (LSCoF) as oxygen electrode were characterized using I-V measurements and Electrochemical Impedance Spectra (EIS) analysis in order to investigate the evolution of the ohmic and polarization resistance values as a reflection of current.

Enhancing Coke Resistance of Mg1–xNixAl2O4 Catalysts for Dry Reforming of Methane via a Doping‐Segregation Strategy

AbstractDry reforming of methane (DRM) is an important reaction to utilize two greenhouse gases (CO2 and CH4) simultaneously and produce syngas (CO and H2) for the manufacture of downstream high‐value chemicals. One of the major obstacles to preventing the commercialization of Ni‐based catalysts in DRM is the serious coking problem. This work develops a doping‐segregation method to manipulate the active metal structure and metal‐support interaction of Mg1–xNixAl2O4 catalysts to enhance their coke resistance for DRM. The Mg1–xNixAl2O4 catalysts prepared via the sol‐gel method exhibit outstanding coke resistance: the coke deposition rate of supported catalyst Ni/MgAl2O4 is 326 times higher than that of the Mg0.7Ni0.3Al2O4 catalyst at 600 °C. The Ni structure and metal‐support interaction of Mg1–xNixAl2O4 catalysts have been systematically studied using various characterization. Combined with the temperature‐programmed surface reaction (TPSR) experiments, the enhanced coke resistance of Mg1–xNixAl2O4 catalysts is attributed to smaller Ni particles, higher Ni dispersion, and stronger metal‐support interaction, which significantly inhibit the direct dissociation of CH4 and promote the oxidation of formed coke. The findings of this work provide insights into the correlation between the enhanced coke resistance and the Ni structure of Mg1–xNixAl2O4 catalysts synthesized via the doping‐segregation strategy using spinel as the precursor, leading to a stable catalyst design for many dry reforming reactions.

Enhanced stability of a direct-methane and carbon dioxide protonic ceramic fuel cell with a PrCrO3 based reforming layer

Dry reforming of methane (CH4 + CO2 = 2CO + 2H2) is a very interesting approach both to reduce the overall carbon footprint of the increasing worldwide fossil-based methane consumption as well as to cut emission greenhouse gas of CO2. Utilizing the produced syngas as fuel directly in protonic ceramic fuel cell can further kill two birds with one stone: obtain power output and high purity CO. However, the drawback of the coking deposition limits the process of the above strategy. Here, we synthesis a Ni-based catalyst with high conversion rates (∼88% for CO2 and ∼89% for CH4) and excellent stability (>160 h at 700 °C) proceeded by Ce doping, and further employ it as reforming layer on solid oxide fuel cell. The results demonstrate that the Ce substitution plays an important role for homogenous Ni nanoparticles exsolution, benefiting for the coking resistance of the catalyst then the stability of the cell using CH4 and CO2 as fuel directly.

Bimetallic NiFe Nanoparticles Supported on CeO2 as Catalysts for Methane Steam Reforming.

Ni-Fe nanocatalysts supported on CeO2 have been prepared for the catalysis of methane steam reforming (MSR) aiming for coke-resistant noble metal-free catalysts. The catalysts have been synthesized by traditional incipient wetness impregnation as well as dry ball milling, a green and more sustainable preparation method. The impact of the synthesis method on the catalytic performance and the catalysts' nanostructure has been investigated. The influence of Fe addition has been addressed as well. The reducibility and the electronic and crystalline structure of Ni and Ni-Fe mono- and bimetallic catalysts have been characterized by temperature programmed reduction (H2-TPR), in situ synchrotron X-ray diffraction (SXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Their catalytic activity was tested between 700 and 950 °C at 108 L gcat-1 h-1 and with the reactant flow varying between 54 and 415 L gcat-1 h-1 at 700 °C. Hydrogen production rates of 67 mol gmet-1 h-1 have been achieved. The performance of the ball-milled Fe0.1Ni0.9/CeO2 catalyst was similar to that of Ni/CeO2 at high temperatures, but Raman spectroscopy revealed a higher amount of highly defective carbon on the surface of Ni-Fe nanocatalysts. The reorganization of the surface under MSR of the ball-milled NiFe/CeO2 has been monitored by in situ near-ambient pressure XPS experiments, where a strong reorganization of the Ni-Fe nanoparticles with segregation of Fe toward the surface has been observed. Despite the catalytic activity being lower in the low-temperature regime, Fe addition for the milled nanocatalyst increased the coke resistance and could be an efficient alternative to industrial Ni/Al2O3 catalysts.

A Novel Synergetic Effect and Photoactivation Lead to Very Effective UV–Visible–Infrared Light‐Driven Photothermocatalytic CO2 Reduction with CH4 on Ni/Ni‐Doped Al2O3

A nanocomposite of Ni nanoparticles (NPs) loaded on Ni‐doped Al2O3 (Ni/Ni‐Al2O3) is prepared. Very effective photothermocatalytic dry reforming of methane (DRM) is acquired by Ni/Ni‐Al2O3 under focused UV–vis–IR illumination with a relatively low light intensity of 80.7 kW m−2. Under focused UV–vis–IR illumination, Ni/Ni‐Al2O3 has high production rates of H2 (, 72.62 mmol g−1 min−1) and CO (rCO, 88.29 mmol g−1 min−1) as well as a large light‐to‐fuel efficiency (η) of 28.0%. Compared with a reference catalyst of Ni NPs loaded on Al2O3 (Ni/Al2O3), the photothermocatalytic activity and η of Ni/Ni‐Al2O3 are enhanced and its photothermocatalytic durability is significantly improved due to its coking rate being dramatically decreased by 42.5 times. This is attributed to a synergistic effect between Ni NPs and Ni‐Al2O3: the active oxygen of Ni‐Al2O3 participates in the oxidation of carbon species as one of decisive steps for DRM, thus not only improving the catalytic activity, but also dramatically promoting coking resistance. It is found that light not only plays a heating role, but also substantially reduces the activation energy of DRM, thus improving the catalytic activity and coking resistance due to the great acceleration of carbon oxidation upon focused illumination (photoactivation).

Molten salt induced synthesis of Ni-Sn-Al ternary oxide as highly efficient catalyst for low S/C ratio methane steam reforming

Until now almost 50% of the world's hydrogen production at industrial level is realized by methane steam reforming (SMR). Traditionally a high steam to carbon ratio (S/C = 2.5–3.0) is needed to reduce coke formation on catalyst surface while adversely derives downsides like high energy input and significant gas dilution. Such dilemma in SMR process could be relieved by using low S/C ratio (<1.0) but instead raises the bar for catalyst anti-coking ability. In this work, we employed a facile method of molten salt with ball milling pretreatment to prepare Ni-Sn-Al ternary oxide. The synthesized sample showed high reaction activity with slight carbon deposition, benefiting from the well-dispersed Ni nanoparticles and the addition of Sn promoter. The molten salt was found to provide liquid environment and thus induce space confinement for Ni dispersion via strong NiO bond. The positive role of SnOx species is evidenced by comparing the catalytic stability of NiAl binary oxide and Ni-Sn-Al ternary oxide, which the latter is with strong coke resistance under the 30-h stability test without obvious deactivation. Time-resolved in-situ DRIFTS reveals different reaction routes over the two catalysts. Our approach offers a new strategy of dissolving the severe coke formation issue during low S/C ratio SMR.

La0.7(Ca, Sr)0.3Ni0.6Mn0.5O3 perovskites for photothermal catalytic reforming of toluene: The role of oxygen species in the photothermal synergistic effect

Photothermal steam reforming (PTR) has emerged as an attractive approach for catalytic tar removal, due to the excitation of the photothermal synergistic effect. However, the role of oxygen species in PTR is still unclear. In this paper, the effect of alkaline earth metals (Ca and Sr) A-site-substitution on perovskite LaNi0.6Mn0.5O3 was investigated to reveal the role of oxygen species in PTR. The substitution of alkaline earth metals into perovskite lattice affected the dispersion of nickel and significantly adjusted the active oxygen species content on the catalyst surface. Oxygen species are crucial to the carbon deposition behavior of catalysts in PTR. La0.7Ca0.3Ni0.6Mn0.5O3 was rich in reactive oxygen species, which effectively inhibited the deposition and graphitization of coke, resulting in its good coke resistance and catalytic stability in PTR. Ultraviolet–visible light (UV–vis) supported the formation of oxygen vacancies and the dissociation of H2O to hydroxyl radicals through the photochemical reaction, which ultimately increased the surface adsorbed oxygen contents. Meanwhile, UV–vis promoted the migration and reaction of lattice oxygen. The photoactivation of oxygen species by UV–vis effectively enriched active oxygen species and improved the coke resistance, which resulted in enhanced catalytic stability of PTR at low-temperature.

Microwave-assisted hydrothermal synthesis of Ru/CeO2 catalyst for efficient and stable low-temperature dry reforming of methane

As the widely applied catalysts in dry reforming of methane (DRM), Ni-based catalysts typically suffer from severe coke formation on the catalyst leading to the deactivation of the catalyst. Meanwhile, with great trends of reducing the operating temperature for DRM reactions, the Ni-based catalysts can’t exhibit comparable reactivity in low-temperature DRM reactions. To reduce coke formation and achieve better reactivity at low temperatures, a novel Ru/CeO2 catalyst was synthesized through a microwave-assisted hydrothermal method and exploited in a low-temperature DRM reaction in a fixed bed reactor. It exhibited comparable activity of above 58 % CH4 and 71 % CO2 conversion with above 0.85 H2/CO ratio for over 120 h at 600 °C. Moreover, it remained stable without obvious coke deposition on the catalyst after the long-term experiment. The characterizations by XRD, XPS, HAADF-STEM, FTIR and some temperature-programmed experiments indicated that microwave irradiation and the introduction of glucose played important roles in achieving great thermal stability and better DRM reactivity of Ru/CeO2. Ru, as the active metal compared with Ni, exhibited moderate CH4 dissociation ability leading to excellent coke resistance. The novel Ru/CeO2 catalyst with high DRM reactivity, simple synthesis method and great coke resistance in this work offers a new opportunity for the commercialization of low-temperature DRM reaction.

A stable chromite anode for SOFC with Ce/Ni exsolution for simultaneous electricity generation and CH4 reforming

Solid oxide fuel cells (SOFCs) operating at a high temperature can be used to generate electricity and produce syngas from hydrocarbon fuels. In this work, the in-situ decoration of CeO2 and Ni nanoparticles on the surface of chromite anode has been explored; i.e., the exsolution of CeO2 from La0.75Ce0.05Ca0.2Cr0.9Ni0.1O3 (Ce-LCN) can be achieved in the oxygenation in air to boost the exsolution of Ni0 under the fuel condition. The SOFC with a Ce-LCN anode showed higher performance and better stability under propane and H2 fuel than the one La0.8Ca0.2Cr0.9Ni0.1O3 (LCN) anode because of the formation of CeO2 and Ni0 nanoparticles. Setting in the distribution of H2 in the existing natural gas pipeline, hydrogen-injection in CH4 (20% H2 in volume in CH4) was found for the first time to efficiently improve the coking resistance of the anode because the H2 was able to boost the transfer of oxygen from the anode to the direct internal reforming. The conversion of CH4 was found to be around 85% and the selectivity for CO was close to unity. This study provides a viable solution to the design of complex perovskite SOFC to produce electricity and syngas simultaneously from CH4.

Perovskite modified catalysts with improved coke resistance for steam reforming of glycerol to renewable hydrogen fuel

AbstractCatalytic steam reforming of renewable feedstock to renewable energy or chemicals always goes with intense coking activities that produce carbonaceous products leading to low performance and eventual catalyst deactivation. Supported metal catalyst such as Ni/Al2O3 is known to catalysed gasification and decomposition of biomass feedstock largely for renewable fuel production with promising results. Catalyst deactivation from high carbon deposition, agglomeration and phase transformations resulting to rapid deactivation are some of the issues identified with the use of the catalyst. In this work, improvement on the coke resistance and catalytic properties of Ni/Al2O3 catalyst is sought via the use of a thermally stable and coke‐resistant perovskite La0.75Sr0.25Cr0.5Mn0.5O3‐δ (LSCM) as catalyst promoter/modifier and involving Zirconia‐doped Ceria (Ce‐Zr) as alternative support in steam reforming of pure and by‐product glycerol. The stabilizing influence of the LSCM on the Ni catalyst has improved stability against agents of deactivation with a consequent significant improvement of catalytic activity of Ni/Al2O3 in H2 production and robust suppression of carbon deposition. Particularly, the synergy between the LSCM promoter and alternative Ce0.75Zr0.25O2 support enhanced the basic and redox properties known for Ce0.75Zr0.25O2 support in contrast to the week acid centres in γ‐Al2O3 support which further improved nickel stability, catalyst–support interaction with a resultant high catalytic activity and robust coke suppression as a result of enhanced oxygen mobility. There is correlation between the product distribution, nature of coke deposited and reforming temperature as well as type of support and structural modification. Hence, integration of a robust perovskite material as a catalyst promoter and choice of support could be tailored in design and development of robust catalyst systems to improve the performance of supported metal catalysts, particularly the suppression of carbon deposition for hydrocarbon and biomass conversion to renewable fuel or chemicals.

Evaluation of Engineered Biochar-Based Catalysts for Syngas Production in a Biomass Pyrolysis and Catalytic Reforming Process

Biochar, originating from biomass pyrolysis, has been proven a promising catalyst for tar cracking/reforming with great coke resistance. This work aims to evaluate various engineered biochar-based catalysts on syngas production in a biomass pyrolysis and catalytic reforming process without feeding extra steam. The tested engineered biochar catalysts include physical- and chemical-activated, nitrogen-doped, and nickel-doped biochars. The results illustrated that the syngas yields were comparable when using biochar and activated biochar as catalysts. A relatively high specific surface area (SSA) and a hierarchical porous structure are beneficial for syngas and hydrogen production. A 2 h physical-activated biochar catalyst induced the syngas with the highest H2/CO ratio (1.5). The use of N-doped biochar decreased the syngas yield sharply due to the collapse of the pore structure but obtained syngas with the highest LHVgas (18.5MJ/Nm3). The use of Ni-doped biochar facilitated high syngas and hydrogen yields (78.2 wt % and 26 mmol H2/g-biomass) and improved gas energy conversion efficiency (73%). Its stability and durability test showed a slight decrease in performance after a three-time repetitive use. A future experiment with a longer time is suggested to determine when the catalyst will finally deactivate and how to reduce the catalyst deterioration.

A review on catalyst development for conventional thermal dry reforming of methane at low temperature

AbstractCarbon dioxide (CO2) utilization and conversion, as one of the main parts of carbon capture, utilization, and storage (CCUS), is not only considered an important way to mitigate global warming but also an attractive industrial route to produce valuable fuels and chemical feedstocks. Catalytic dry reforming of methane (DRM) is a promising technology for carbon dioxide utilization and conversion as it can produce syngas, carbon monoxide (CO), and hydrogen (H2) for widespread industrial production processes. In most studies of the DRM reaction, a relatively high operational temperature (i.e., &gt;700°C) has been applied since the reactivity limitation of widely used Ni‐based catalysts at low temperatures and the extremely endothermic property of the DRM reaction. However, high cost and high requirement of thermal stability for catalysts have become a severe problem impeding the further commercialization of DRM technology. Decreasing the operational temperature (i.e., &lt;700°C) is considered a promising way for further application of the DRM route to convert CO2 and produce syngas. However, traditional Ni‐based catalysts suffered from unsatisfied reactivity and severe coke formation, leading to quick deactivation at low temperatures. Developing a catalyst with excellent catalytic activity, coke resistance, and improved thermal stability is necessary for low‐temperature DRM reactions. In recent years, with significant development in materials, catalyst design, and computational simulation, some synthesized catalysts have achieved considerable improvement in catalytic performance in low‐temperature DRM. Hence, a review of recent development on low‐temperature DRM catalysts is provided here to further guide and profoundly understand catalyst design for low‐temperature DRM.

Oxygen Vacancy Induced Strong Metal-Support Interactions on Ni/Ce0.8Zr0.2O2 Nanorod Catalysts for Promoting Steam Reforming of Toluene: Experimental and Computational Studies.

To develop an efficient Ni-based steam reforming catalyst for tar removal from the products of biomass gasification, Ni/Ce0.8Zr0.2O2 nanorods were designed. The Ni/Ce0.8Zr0.2O2 nanorod was used as a catalyst in steam reforming of toluene, which was regarded as a model compound of biomass gasification tar. At gas hourly space velocity (GHSV) of 24,000 h-1 and Ni loading of 5 wt %, the 5Ni/Ce0.8Zr0.2O2 nanorod catalyst achieved 100% of toluene conversion at 600 °C. After 10 h of operation, toluene conversion still reached 87.6%, and the carbon deposition rate was only 1.9 mg/gcat h-1. The experimental results demonstrated that the 5Ni/Ce0.8Zr0.2O2 nanorod catalyst showed much higher catalytic activity and coking resistance than other Ni-based catalysts reported in the literature. Through different characterization technologies and density functional theory calculations, it was confirmed that the excellent catalytic performance was attributed to the strong metal-support interaction (SMSI) between Ni and the {100} facet of Ce0.8Zr0.2O2. The special surface structure of {100} allowed Ni atoms to anchor to the surface oxygen vacancies and maintained its reduced state by electron transport between surface atoms. The anchored Ni facilitated oxygen vacancies formation and H2O dissociation on the support, while the support modulated the electronic structure of Ni, which promoted its ability to toluene activation.

Effect of different SAPO-34 film thickness on coke resistance performance of SAPO-34/ZSM-5/quartz film in MTA reaction

3 Brief informative subheadings: SAPO-34 film thickness, Coke deposition analysis, Rich hierarchical pores. BackgroundComposite zeolite film had a shorter lifetime due to the easy deactivation of SAPO-34. We explored the effect of SAPO-34 film thickness on anti-coking performance of composite zeolite film. MethodsThree SAPO-34/ZSM-5/quartz films with different SAPO-34 film thicknesses were prepared for MTA reaction. TG, N2 isothermal adsorption-desorption, XPS, FTIR, and GC-MS were used to explore the content, location, and composition of coke formed on the composite zeolite film. The deactivation behaviors of three catalysts were comparatively analyzed. Significant findingsThe results showed the change in SAPO-34 film thickness could affect the reactivity. The zeolite with the SAPO-34 film thickness of 8 μm had excellent coke resistance performance because of the reasonable acid distribution and rich hierarchical pores. This could improve their coke-holding capacity and hinder the formation of graphite-like carbon. The analysis of coke species indicated that the intermediate products of the composite film were mostly tetrasubstituted methylbenzene. And the composite zeolite film with medium-thickness SAPO-34 film had the simplest composition of soluble coke, such as bicyclic or tricyclic aromatic hydrocarbons. This provided a new idea for improving the anti-coking performance of composite material.

Unraveling the Unique Promotion Effects of a Triple Interface in Ni Catalysts for Methane Dry Reforming

Methane dry reforming (MDR) is of great interest for its efficient consumption of greenhouse gases and the production of valuable syngas. The interfaces of metal and boron nitride (BN) are expected to enhance the catalytic activity and inhibit coke formation. Herein, the composite of layered metal oxides (NiMgAlOx) and BN was constructed to form the interface-confined NiMgAlOx/BN (Ni-MAO/BN) catalysts, and the unique promotion effects of a triple interface in Ni catalysts were unraveled. The triple interface among the Ni, BN, and MgAlOx oxides enhances the sintering resistance of the developed catalysts, which endows the developed catalysts with excellent adsorption/activation capacity of CH4 and CO2 as well as superb stability during a long-term MDR activity test. The abundant bicarbonate (HCO3*) species in the Ni-MAO/BN catalysts demonstrates that the triple interface significantly enhances gas activation. Meanwhile, the dynamic variation of HCO3* and CO3* species further proves the inhibition of deep CH4 cracking and the fast reaction rate over the Ni-MAO/BN catalysts. The negligible graphitic carbon observed in the operando Raman spectra and the produced large amount of H2/CO demonstrate not only the excellent coke resistance but also the strengthened activation capability of CO2 and CH4. This work elucidates the role of interfacial effects on gas activation and provides innovative insights into the design of highly efficient Ni catalysts for MDR.

Dynamic Tracking of NiFe Smart Catalysts using In Situ X-Ray Absorption Spectroscopy for the Dry Methane Reforming Reaction

The exsolution of nanoparticles from perovskite precursors has been explored as a route to synthesize catalysts with sinter or coke resistance. The characteristics of these exsolved nanoparticles are highly dynamic depending on the redox nature of the environment to which they are subjected. To develop their properties for thermo- and electrocatalytic applications, it is necessary to track the states and behavior of exsolved catalysts with in situ and ex situ characterization. In this study, we conduct in situ X-ray absorption spectroscopy (XAS) along with ex situ scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) and energy-dispersive X-ray spectroscopy analysis of the parent perovskite oxide precursor, LaFe0.8Ni0.2O3, as its structure forms bimetallic NiFe nanoparticles and evolves in oxidative, reductive, and dry methane reforming environments. We develop a theory that NiFe exsolution is a function of the reduction potential where LaFe0.8Ni0.2O3 transforms to NiFe alloy supported on LaOx-LaFeOx. The Ni starts to exsolve at 268 °C, while most Fe exsolves at 700 °C. During dry methane reforming conditions, most of the Fe is oxidized by CO2 during the reaction and re-enters the perovskite as LaFeO3, while Ni remains on the surface as nanoparticles in the metallic state. During the oxidative regeneration phase, most of the Fe re-enters the bulk perovskite phase, while Ni is partially regenerated with a small percentage oxidized to large NiO nanoparticles. This study sheds light on the exsolution and regeneration of bimetallic alloy nanoparticles and the influence of the reaction conditions on their catalyst performance.

Improving the Coke Resistance of Ni-Ceria Catalysts for Partial Oxidation of Methane to Syngas: Experimental and Computational Study.

The synthesis of syngas (H2 : CO=2) via catalytic partial oxidation of methane (CPOM) is studied over noble metal doped Ni-CeO2 bimetallic catalysts for CPOM reaction. The catalysts were synthesized via a controlled deposition approach and were characterized using XRD, BET-surface area analysis, H2 -TPR, TEM, Raman and TGA analysis. The catalysts were experimentally and computationally studied for their activity, selectivity, and long-term stability. Although the pure 5Ni/CeO2 catalyst showed high initial activity (∼90%) of CH4 conversion, it rapidly deactivates around 20% of its initial activity within 140 hours of TOS. Doping of Ni/CeO2 catalyst with noble metal was found to be coke resistant with the best-performing Ni-Pt/CeO2 catalyst showed ∼95% methane conversion with >90% selectivity at a temperature of 800 °C, having exceptional stability for about 300 hours of time-on-stream (TOS). DFT studies were performed to calculate the activation barrier for the C-H activation of methane over the Ni, Ni3 Pt, Ni3 Pd, and Ni3 Ru (111) surfaces showed nearly equal activation energy over all the studied surfaces. DFT studies showed high coke formation tendency of the pure Ni (111) having a very small C-C coupling activation barrier (14.2 kJ/mol). In contrast, the Ni3 Pt, Ni3 Pd, and Ni3 Ru (111) surfaces show appreciably higher C-C coupling activation barrier (∼70 kJ/mol) and hence are more resistant against coke formation as observed in the experiments. The combined experimental and DFT study showed Ni-Pt/CeO2 as a promising CPOM catalyst for producing syngas with high conversion, selectivity and long-term stability suited for future industrial applications.