Usman Oemar

Progress in Synthesis of Highly Active and Stable Nickel‐Based Catalysts for Carbon Dioxide Reforming of Methane

In recent decades, rising anthropogenic greenhouse gas emissions (mainly CO2 and CH4 ) have increased alarm due to escalating effects of global warming. The dry carbon dioxide reforming of methane (DRM) reaction is a sustainable way to utilize these notorious greenhouse gases. This paper presents a review of recent progress in the development of nickel-based catalysts for the DRM reaction. The enviable low cost and wide availability of nickel compared with noble metals is the main reason for persistent research efforts in optimizing the synthesis of nickel-based catalysts. Important catalyst features for the rational design of a coke-resistant nickel-based nanocatalyst for the DRM reaction are also discussed. In addition, several innovative developments based on salient features for the stabilization of nickel nanocatalysts through various means (which include functionalization with precursors, synthesis by plasma treatment, stabilization/confinement on mesoporous/microporous/carbon supports, and the formation of metal oxides) are highlighted. The final part of this review covers major issues and proposed improvement strategies pertaining to the rational design of nickel-based catalysts with high activity and stability for the DRM reaction.

Simultaneous Tuning Porosity and Basicity of Nickel@Nickel–Magnesium Phyllosilicate Core–Shell Catalysts for CO2 Reforming of CH4

Ni@Ni-Mgphy (Ni-Mgphy = Ni-Mg phyllosilicate) core-shell catalysts were designed by hydrothermally treating Ni@SiO2 nanoparticles with magnesium nitrate salt. The porosity and basicity of the catalysts were easily tuned by forming Ni-Mgphy shell using Ni originating from Ni@SiO2 during the hydrothermal treatment process and Mg(NO3)2 as the Ni and Mg sources, respectively. Among Ni@Ni-Mgphy core-shell catalysts synthesized under different hydrothermal durations, the catalyst treated for 10 h achieved the best catalytic performance for CO2 reforming of CH4 reaction with stable CO2 and CH4 conversions of around 81% and 78%, respectively, within 95 h reaction duration at 700 °C. The high Ni accessibility, strong basicity, and high structural stability for Ni@Ni-Mgphy core-shell catalyst with 10 h treatment time accounted for its superb catalytic performance. This method to simultaneously tune the porosity and basicity of Ni@SiO2 core-shell nanoparticles demonstrates a general way to modify the properties of other silica based core-shell nanoparticles through treating them with different metal salts.

CO2 reforming of methane over highly active La-promoted Ni supported on SBA-15 catalysts: mechanism and kinetic modelling

Various Ni catalysts were synthesized by combining a high surface area SBA-15 support, a novel preparation method using an oleic acid precursor to obtain highly dispersed and small Ni particles, and the basic property of La. The activity test shows that the La-promoted Ni/SBA-15 catalyst has much higher catalytic activity and stability than unpromoted Ni/SBA-15 catalysts in the CO2 (dry) reforming of methane (DRM) reaction due to the crucial role of La to simultaneously adsorb CO2 and remove deposited carbon from the Ni metal surface. The optimization of La content shows that only a small amount of La around 1 wt% is necessary to achieve the best catalytic performance. The reaction mechanism was then proposed and kinetic modeling (for pilot scale) was performed to find the rate-determining step (RDS) in the DRM reaction for this 1% La-promoted Ni/SBA-15 catalyst. The obtained fitting result shows that CH4 decomposition is the RDS for this catalyst with an apparent activation energy of around 40.8 kJ mol−1 which is similar to the activation energies reported for common Ni-based supported catalysts.

Enhancing performance of Ni/La2O3 catalyst by Sr-modification for steam reforming of toluene as model compound of biomass tar

Steam reforming of biomass tar with toluene as the model compound was studied using Sr-doped Ni/La2O3 catalysts prepared using two methods, i.e. co-impregnation of Sr and Ni on La2O3 support (Ni–Sr/La2O3 catalyst) and sequential impregnation of Sr on Ni/La2O3 catalyst (SNL catalyst), which were then calcined at various temperatures (500, 700, and 900 °C). These two types of catalysts are found to possess better catalytic performance than the undoped Ni/La2O3 catalyst at the same calcination temperature due to the presence of Sr, which helps in water adsorption at low steam/carbon (S/C) ratio. Moreover, the catalytic performance for catalysts calcined at various temperatures decreases following this trend: 500 °C > 700 °C > 900 °C due to lower BET surface area and lower surface active metal available for reaction. In addition, it is also observed that the Sr/Ni/La2O3 catalyst has better performance than the Ni–Sr/La2O3 catalyst at the same calcination temperature. Further characterization results suggest that in the Ni–Sr/La2O3 catalyst, the Sr is present between Ni and La2O3 support. On the other hand, Sr in the Sr/Ni/La2O3 catalyst is thought to be located on the surface of Ni/La2O3 due to the preparation method. This study shows that more Sr on the catalyst surface has better catalytic activity and stability in steam reforming of toluene.

Role of lattice oxygen in oxidative steam reforming of toluene as a tar model compound over Ni/La0.8Sr0.2AlO3 catalyst

Catalytic steam reforming of tar with toluene as a model compound for production of synthesis gas (H2 and CO) was studied using Ni/LaAlO3, Ni/La0.8Sr0.2AlO3, Ni/La2O3, and Ni/α-Al2O3 catalysts prepared using a wet impregnation method. The Ni/La0.8Sr0.2AlO3 catalyst demonstrated the most superior catalytic performance in terms of both catalytic activity and coke resistance in the steam reforming of toluene. The presence of gas phase oxygen enhanced the catalytic performance of all four catalysts, with the extent of improvement being the greatest over the Ni/La0.8Sr0.2AlO3 catalyst. Catalyst characterization by X-ray diffraction (XRD), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption of oxygen (TPD-O2) revealed that the superior catalytic performance of the Ni/La0.8Sr0.2AlO3 catalyst was a result of lattice distortion caused by strontium doping, which produced a higher concentration of oxygen vacancies on the catalyst surface. This lowered the activation energy of the migration of lattice oxygen, enhancing the mobility of lattice oxygen species, and also improved the adsorption abilities of gas phase oxygen species. Mobile lattice oxygen species (Olattice) favored the direct partial oxidation of toluene, whereas gas-phase oxygen possessed stronger oxidative abilities and favored the complete oxidation of toluene. Both mobile lattice oxygen and gas phase oxygen species actively suppressed coke formation and oxidized coke deposited on the catalyst surface, conferring coking resistance.

Highly Active and Stable Bimetallic Nickel–Copper Core–Ceria Shell Catalyst for High‐Temperature Water–Gas Shift Reaction

Highly dispersed bimetallic Ni‐Cu core encapsulated by a CeO2 shell catalyst has been synthesized by a combination of positive emulsion and the self‐assembly method. Several catalyst characterization techniques were implemented to investigate the core–shell structure and its unique properties. Field‐emission TEM, X‐ray diffraction, X‐ray photoelectron spectroscopy, and N2O chemisorption analyses showed that uniform bimetallic Ni‐Cu particles with an average size of 3.4 nm and narrow size distribution encapsulated by CeO2 shell with an average size of 4.3–5.4 nm were formed. 10 wt % bimetallic Ni‐Cu catalyst encapsulated by CeO2 exhibited high catalytic activity and stability at 500 °C in the high‐temperature water–gas shift reaction. This could owe to the contributing factors of a high level of metal–support interaction, small bimetallic Ni‐Cu particle size, and high surface lattice oxygen concentration enhancing the water–gas shift reaction. Moreover, strongly adsorbed CO and the presence of type I OH on the core–shell catalyst implied that these two active species could be the most important species in the formation of active intermediate species for the water–gas shift reaction, as evidenced by CO temperature‐programmed reduction–MS and in situ diffuse‐reflectance IR Fourier transform spectroscopy.

High-temperature water gas shift reaction on Ni–Cu/CeO2 catalysts: effect of ceria nanocrystal size on carboxylate formation

Thermally stable CeO2 nanospheres of various controllable sizes were successfully synthesized via a PVP-assisted hydrothermal method to study the effect of ceria crystal size in high-temperature water gas shift reaction. The intrinsic properties of ceria crystal size effect was explored using X-ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Brunauer, Emmett and Teller Surface area (BET), X-ray Photon Spectroscopy (XPS), Carbon monoxide-Temperature Programmed Reduction-Mass Spectrometry (CO-TPR-MS), and in situ Diffuse Reflectance Infra-red Fourier Transform Spectroscopy (DRIFTS) techniques. The XRD, FESEM and BET results indicate that the ceria with the largest particle size and the smallest crystal size of 12 nm shows a high specific surface area of 50m2 g−1 after calcination at 700 °C. After impregnation, high metal dispersion (15%) and a high amount of surface lattice oxygen are observed on the Ni–Cu bimetallic catalyst supported on ceria with the largest particle size. This Ni–Cu/CeO2 catalyst presents high reaction rates with low apparent activation energy as compared to other Ni–Cu/CeO2 catalysts, revealing the important effect of ceria crystal and Ni–Cu alloy sizes. A further study shows that the high amount of carboxylate species on the 5Ni5Cu/CeO2 catalyst with the biggest ceria crystal size could be the inhibitor or the real intermediate species. In addition, the reaction mechanism strongly depends on the Ni–Cu surface composition.

Sulfur resistant LaxCe1−xNi0.5Cu0.5O3 catalysts for an ultra-high temperature water gas shift reaction

A series of LaxCe1−xNi0.5Cu0.5O3 catalysts was synthesized to study the effect of Ce substitution for La. XRD results show that only a small amount of Ce (10%) is allowable to substitute La to maintain the perovskite structure. The La0.9Ce0.1Ni0.5Cu0.5O3 catalyst has the smallest metal particle size and the highest oxygen mobility among all tested catalysts as observed from the XRD, TPD-O2, and XPS results of reduced catalysts. These two factors are very important in achieving the highest catalytic activity of the La0.9Ce0.1Ni0.5Cu0.5O3 catalyst in a water gas shift reaction at 450–650 °C. In the presence of H2S, the catalytic activity at lower temperature was suppressed due to the formation of stable SO42− species on the metal. However, since the amounts of surface oxygen species and adsorbed H2S are much lower at high temperature, the formation of SO42− species is not observed, resulting in higher catalytic activity. The presence of H2S at high temperature enhances the formation of formate species, which can decompose to produce methane as the side product of the water gas shift reaction.

Catalytic Biomass Gasification to Syngas Over Highly Dispersed Lanthanum-Doped Nickel on SBA-15

Catalytic biomass gasification is an environmentally benign solution to energy problems. A series of Ni catalysts on mesoporous SBA‐15 are synthesized with various La2O3 loadings. The catalytic activity of unpromoted Ni/SBA‐15 catalyst indicates that it has a high carbon formation rate, resulting in fast deactivation. In contrast, addition of La2O3 to Ni/SBA‐15 catalyst helps to remove deposited carbon by formation of oxycarbonate, resulting in higher CO production. A 1 % La doping in the Ni/SBA‐15 catalyst is sufficient to achieve high activity and stability in biomass gasification with various raw materials and in the steam reforming of toluene as a tar model compound. The combination of small metal particle size, high metal dispersion, high surface area of SBA‐15, the crucial role of La in carbon removal, and the novel synthesis method result in a highly active, stable, and effective 1 % La‐Ni/SBA‐15 catalyst for biomass gasification.