Ming Li Ang

Ni and/or Ni–Cu alloys supported over SiO2 catalysts synthesized via phyllosilicate structures for steam reforming of biomass tar reaction

In this paper, we describe the synthesis of Ni/SiO2 and Ni–Cu/SiO2 catalysts derived from phyllosilicate structures (Ni/SiO2P and Ni–Cu/SiO2P, respectively) for steam reforming of biomass tar reaction. The steam reforming of biomass tar reaction was investigated with cellulose as a biomass model compound. The influence of steam-to-carbon ratio and reaction temperatures was also explored. Overall, the catalysts synthesized via phyllosilicate structures gave better catalytic performance than the catalysts prepared by the impregnation method. An optimum catalyst composition of 30Ni–5Cu/SiO2P gave superior catalytic performance in terms of stability and activity compared to all other catalysts. At 600 °C, about 78% of biomass was converted to gaseous products over 30Ni–5Cu/SiO2P, which is the highest among all the catalysts tested. Temperature-programmed reduction results indicate that the metal–support interaction of Ni/SiO2P catalyst prepared via phyllosilicate structures is stronger due to the unique layered structure compared to that prepared by conventional impregnation (10Ni/SiO2). The formation of a unique layered structure in Ni/SiO2P and Ni–Cu/SiO2P was also confirmed through TEM analysis. The surface elemental composition results obtained from XPS analysis show that the Cu/Ni surface molar ratio for Ni–Cu/SiO2P catalysts is consistent with the actual molar ratio values obtained from SEM-EDX analysis. This result suggests that the bimetallic catalysts synthesized via the phyllosilicate structure route can yield uniformly distributed alloy species.

Promotion of the Water-Gas-Shift Reaction by Nickel Hydroxyl Species in Partially Reduced Nickel-Containing Phyllosilicate Catalysts

The role of surface hydroxyl species generated by partially reduced Ni‐containing phyllosilicate structures (Ni/SiO2P) in promoting the water‐gas‐shift (WGS) reaction and methane suppression was investigated. To analyze the effect of the surface hydroxyl species, Ni/SiO2P catalysts reduced at various temperatures were employed. All the Ni/SiO2P catalysts showed enhanced catalytic performances and methane suppression compared to the conventional Ni/SiO2 catalyst. As revealed by diffuse‐reflectance infrared Fourier transform spectroscopy (DRIFTS), methane suppression could be attributed to the inhibition of the formation of nickel subcarbonyl species, and the promotion of WGS activity was attributed to the involvement of surface hydroxyl species (Ni−OH, 3626 cm−1, and Si−OH, 3740 cm−1). The Ni/SiO2P catalyst reduced at 600 °C showed exceptionally superior performance to the other catalysts in the water‐gas‐shift reaction in terms of turnover frequency (2.79 s−1) and hydrogen formation rates (492.63 μmol H2 g−1 s−1) at 375 °C.

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.

Bimetallic Ni–Cu alloy nanoparticles supported on silica for the water-gas shift reaction: activating surface hydroxyls via enhanced CO adsorption

Highly dispersed Ni–Cu nanoparticles supported on SiO2 were synthesized via an in situ self-assembly core–shell precursor route. Monometallic (Ni and Cu) and bimetallic (Ni–Cu) catalysts were synthesized, characterized by XRD, H2-TPR, XAS, XPS, CO-TPR, DRIFTS and N2 adsorption analysis and tested for the water-gas shift reaction. Formation of a highly dispersed Ni–Cu alloy was confirmed via XRD, H2-TPR, XAS and DRIFTS. Oleic acid was found to promote the dispersion of both monometallic and bimetallic particles, anchoring small metal particles to the support via enhanced metal–support interactions. The DRIFTS results suggest that CO is adsorbed on the Cu sites in the Ni–Cu alloy thereby suppressing methanation. Additionally, stronger CO adsorption on the 5Ni5Cu/SiO2 (OA) catalyst activates the surface terminal hydroxyl groups on silica for enhanced CO conversion. The promotional effect of OA on the WGS activity was evidenced through kinetic measurements: the 5Ni5Cu/SiO2 (OA) catalyst obtained a turnover frequency of 0.004 s−1 which is twice that of the 5Ni5Cu/SiO2 catalyst (0.002 s−1).

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.