CeO2-Based Heterogeneous Catalysts in Dry Reforming Methane and Steam Reforming Methane: A Short Review

Study of Mesostructured CeO2 Synthesis via Nanocasting Using SBA-15 as a Template: Influence of the Cerium Precursor

Mesoporous materials with high surface area, large pore volume, and adjustable pore size are promising in the fields of adsorption and heterogeneous catalysis. In this work, ordered mesoporous ceria structures were successfully prepared via nanocasting using SBA-15 as a template, with Ce(NO3)3·6H2O or CeCl3·7H2O as ceria precursors. The materials were characterized before and after template removal. The CeO2 crystallite size in the CeO2/SBA-15 composites increases with successive impregnations until it reaches the pore size of the SBA-15. Upon removal of the SBA-15 template, the synthesized materials exhibit pore diameters corresponding to the wall thickness of the SBA-15, evidencing that the inverted structure was obtained. Mesoporous ceria exhibits increasingly ordered structure up to five successive impregnations with 1.3 mmolCe/gSBA-15. Using cerium chloride as a precursor, highly ordered structures were reached after only three impregnations. The feasibility of this synthesis in fewer steps (1, 3, and 5), employing the same amount of Ce precursor (6.7 mmolCe/gSBA-15), was also studied. Results show a higher ordering degree and oxygen mobility capacity at higher impregnation steps. The mesostructured ceria samples exhibit significantly higher oxygen mobility than commercial bulk ceria, along with high thermal stability, which highlights the usefulness of these structures.

Investigating Ni nanoparticles on CeO2 for methane dissociation: a comparative study of theoretical calculations and experimental insights

CeO2 supported with Ni nanoparticles has emerged as a promising catalyst for enhancing the efficiency of dry reforming of methane (DRM) reaction. Methane dissociation (CH4 → CH3 + H) was reported as one of the rate-determining steps in the DRM reaction. We elucidated the reaction mechanism and explored methods for reducing the activation energy using density functional theory (DFT) calculations, where the activation energy of methane dissociation was determined at multiple Ni4 cluster sites on CeO2. In parallel, we experimentally evaluated methane dissociation based on the methane consumption rate in the DRM reaction using newly developed flower-like Ni-supported CeO2 catalyst (Ce(F)). The experimental activation energy was determined to be 0.69 eV (15.91 kcal mol-1), closely matching the DFT-calculated value of 0.80 eV (18.45 kcal mol-1) for the Ni4 cluster model, validating our theoretical predictions. Additionally, we discovered that positively charging the Ni4 can lower the activation energy of methane dissociation. These findings contribute to a deeper understanding of how to control the activation energy of the methane dissociation reaction in DRM.

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 g_cat^-1 h^-1 and with the reactant flow varying between 54 and 415 L g_cat^-1 h^-1 at 700 °C. Hydrogen production rates of 67 mol g_met^-1 h^-1 have been achieved. The performance of the ball-milled Fe_0.1Ni_0.9/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.

Direct oxidation of methane to methanol using CuMoO4

Upgrading methane into methanol or other high value-added chemicals is not only beneficial to mitigate the greenhouse effect, but also provides basic raw materials for industrial production. Nowadays, most research is limited to zeolite systems, and it is a considerable challenge to extend the support to metal oxides while achieving a high yield of methanol. In this paper, we take advantage of impregnation methods to synthesise a novel Cu/MoO3 catalyst, which can convert methane to methanol in the gaseous phase. At 600 °C, the Cu(2)/MoO3 catalyst can achieve a maximum STYCH3OH of 47.2 μmol (g-1 h-1) with a molar ratio CH4 : O2 : H2O = 5 : 1.4 : 10. Consequences of SEM, TEM, HRTEM and XRD substantiate that Cu is incorporated into the lattice of MoO3 to form CuMoO4. And transmission infrared spectroscopy, Raman spectroscopy together with XPS characterization techniques confirm the generation of CuMoO4, which is the main active site provider. This work provides a new support platform for Cu-based catalyst research in the methane-to-methanol system.

Carbon Formation during Methane Dry Reforming over Ni-Containing Ceria-Zirconia Catalysts

Two series of Ni/Ce(Ti/Nb)ZrO₂ catalysts were prepared using citrate route and original solvothermal continuous flow synthesis in supercritical isopropanol and studied in dry reforming of methane (DRM). TEM, XPS and FTIRS of adsorbed CO confirm influence of support composition and preparation method on the catalysts' morphology and surface features. The oxygen mobility was studied by isotope heteroexchange with C¹⁸O₂. After testing in DRM, carbon deposits after catalysts' testing in DRM were investigated by temperature-programmed oxidation with thermo-gravimetric analysis. The lowest amounts of carbon deposits were obtained for unmodified Ni-CeZr and Ni-CeNbZr compositions. Ti addition lead to an increased amount of carbon, which was removed at higher temperatures. The use of supercritical supports also resulted in the formation of a higher amount of coke. Catalysts prepared by the supercritical synthesis were tested in DRM for 25 h. The highest activity drop was observed in the first three hours. For all compositions, close values of carbon deposits were revealed.

Ni/CeO2 Catalyst Prepared via Microimpinging Stream Reactor with High Catalytic Performance for CO2 Dry Reforming Methane

Methane reforming with carbon dioxide (DRM) is one promising way to achieve carbon neutrality and convert methane to syngas for high-value chemical production. Catalyst development with better performance is the key to its potential large-scale industrial application due to its deactivation caused by carbon deposition and metal sintering. Hence, a Ni/CeO2 catalyst (Ni/CeO2-M) with higher CO2 conversion and better stability is prepared, supported on CeO2 precipitated via a novel microimpinging stream reactor. A series of ex-situ or in-situ characterizations, such as CO titration measurements, two-step transient surface reaction (two-step TSR), CO2 and CH4 temperature-programmed surface reaction (CO2-TPSR and CH4-TPSR), X-ray absorption fine structure (XAFS), and in-situ Raman spectroscopy study, were used to investigate its structure and mechanism. In contrast to Ni supported on commercial CeO2 (Ni/CeO2-C), the Ni/CeO2-M catalyst with stronger lattice oxygen mobility and higher oxygen storage capacity enhances its CO2 activation ability and carbon deposition. The Ni particle size of the Ni/CeO2-M catalyst decreased, and a higher oxidation state was obtained due to the strong metal–support interaction. Besides the reaction performance improvement of the Ni/CeO2-M catalyst, the novel microimpinging stream reactor could achieve catalyst continuous production with a high preparation efficiency. This work provides a novel method for the high-performance catalyst preparation for DRM reaction and its mechanism study gives a deep insight into high-performance catalyst development via bottom-up study.