Changzhen Wang

The Properties of Individual Carbon Residuals and Their Influence on The Deactivation of Ni–CaO–ZrO2 Catalysts in CH4 Dry Reforming

Ni–CaO–ZrO2 catalysts with different properties were prepared and tested for CO2 reforming of methane. The catalysts were characterized by means of transmission electron microscopy, thermogravimetric analysis, Raman spectroscopy, X‐ray photoelectron spectroscopy, and X‐ray diffraction to reveal their distinct properties and carbon deposition behaviors in the reforming reaction. It was found that the catalyst prepared by a coprecipitation method and ageing by heating to reflux exhibited a nanocrystalline structure with strong metal–support interaction, which was responsible for both high activity and stability, but it also exhibited the highest carbon formation rate among the tested catalysts. This result suggests that catalyst deactivation might not necessarily correlate with the amount of formed carbon, and the individual properties of carbon residuals could play a more decisive role. Carbon residuals on different catalysts were identified as amorphous carbon, encapsulating carbon, whisker carbon, and graphite, which had different influence on the deactivation. On the surface of the most active and stable catalyst, the carbon species mainly consisted of amorphous and whisker carbon, suggesting that the formation of such carbon species does not necessarily lead to catalyst deactivation. In contrast, the deactivation was found to be closely related to the formation of encapsulating carbon and graphite, which could coat the catalyst surface. The accumulation of different carbon residuals was proven to follow a formation–diffusion/elimination scenario, which was significantly influenced by the Ni particle size and Ni–ZrO2 interactions.

The bi-functional mechanism of CH4 dry reforming over a Ni–CaO–ZrO2 catalyst: further evidence via the identification of the active sites and kinetic studies

A mesoporous Ni–CaO–ZrO2 catalyst which showed an excellent performance in the dry reforming of CH4 was thoroughly characterized by using a series of methods including N2 physical adsorption, temperature-programmed reduction (TPR), H2/CO chemisorptions, and so forth. Particularly, samples after different treatments such as calcination, reduction and different periods of reaction were subjected to X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis, by which changes in the phase structure and surface chemistry were followed. The results suggested that metallic Ni was gradually oxidized during the reaction, and a non-stoichiometric Ni–carbon compound was slowly formed. This latter species has a role as an important intermediate (or even active phase). Kinetic studies were then carried out based on these findings, according to which a Langmuir–Hinshelwood model was developed. Both the experimental results and the kinetic analysis provided novel evidence for the bi-functional mechanism of dry reforming over ZrO2-based catalysts.

Highly stable mesoporous NiO–Y2O3–Al2O3 catalysts for CO2 reforming of methane: effect of Ni embedding and Y2O3 promotion

A series of mesoporous NiO–Y2O3–Al2O3 composite oxides with different yttrium contents were synthesized by either a one-pot evaporation-induced self-assembly (EISA) method or impregnation for carbon dioxide reforming of methane (CRM). Their catalytic performance was evaluated and all the samples were characterized by means of N2 physisorption, XRD, XPS, H2-TPR and TEM. It was found that addition of appropriate amounts of Y2O3 (sample NYA2) has little influence on the EISA process, and thus, an ordered mesoporous structure with enhanced textural and Ni dispersive properties can be obtained. The NYA2 catalyst showed excellent performance in CRM, and no deactivation was observed for 100 h. Based on the comparative characterization of the reduced and exhausted catalysts, the good performance of NYA2 was related to its low carbon formation rate thanks to the very small and thermally stable metallic Ni particles (ca. 6.0 nm) which were well embedded in the catalyst framework and the redox properties of the Y2O3 promoter.

CO2 splitting via two step thermochemical reactions over doped ceria/zirconia solid solutions

Thermochemical CO2 splitting was carried out over Ni-, Fe-, Mg- and Mn-doped ceria/zirconia solid solutions, where the sample was thermally reduced at 1400 °C under inert atmosphere followed by the re-oxidization of CO2 to generate CO at 1100 °C in the subsequent step. Compared with the undoped sample, all the doped ceria/zirconia had a high reduction yield in the first thermal reduction step. Due to the low thermal stability, Ni-, Fe- and Mn-doped samples showed lower CO production in the CO2 splitting step than the stoichiometric amounts. In contrast, the Mg-doped sample produced more CO with the volumes of 5.64 and 5.17 mL g−1 during the two thermochemical cycles. Moreover, a 10% Mg-doped sample prepared via hydrothermal treatment with P123 showed a more stable reactivity during cycling due to the relatively stable microstructure under the successive high temperature thermal treatment.

Effect of pore geometries on the catalytic properties of NiO–Al2O3 catalysts in CO2 reforming of methane

Mesoporous NiO–Al2O3 catalysts were prepared by an evaporation-induced self-assembly (EISA) method, during which the amount of HNO3 added in the precursor solution was varied. Characterization results indicated that the phase structure, component interaction and surface chemistry are fairly similar for all the samples, while the dispersion and textural properties, which are determined by the structure of the micelles and reaction rate of hydrolysis during the EISA process, changed significantly, thus leading to considerably different catalytic performance in CO2 reforming of methane (CRM). The well-known trend that carbon formation rate decreases with the decrease of Ni particle size was observed in the current NA-Hx samples, however, it is very interesting that the disordered slit-like pores endowed the NA-H32 sample with a better capability to inhibit carbon formation as it showed substantially fewer carbon deposits as compared with NA-H16 (ordered cylindrical pore), despite the fact that the Ni particles in these samples are of similar size. In summary, the excellent performance of the NA-H32 catalyst in comparison to other non-promoted NiO–Al2O3 catalysts holds promise for using this cost-effective system in practical CRM applications.

Influence of preparation conditions on the performance of Ni-CaO-ZrO2 catalysts in the tri-reforming of methane

Abstract Ni-CaO-ZrO2 catalysts were prepared by co-precipitation under different conditions and used in the tri-reforming of methane; the influence of preparation conditions on methane conversion and catalytic stability was evaluated. The results indicated that the optimal parameters used to prepare Ni-CaO-ZrO2 catalysts include a calcination temperature of 973 K, a pH value of 10–12 for co-precipitation, and a reflux duration of 24 h. The catalyst prepared under the optimal conditions has a proper surface area and Ni–ZrO2 interaction; it exhibits promising catalytic activity and stability in methane tri-reforming and the methane conversion exceeds 70% under a temperature of 973 K and atmospheric pressure.

Template-free one-pot synthesis of mesoporous Ni-CaO-ZrO2 catalyst and its application in CH4-CO2 reforming

Abstract A novel mesoporous Ni-CaO-ZrO 2 catalyst with high surface area and pore volume was prepared by one-pot template-free method and applied in CH 4 -CO 2 reforming. The mesoporous Ni-CaO-ZrO 2 catalyst was characterized by means of nitrogen sorption, SEM, TEM, XRD, H 2 -TPR and TG-DSC. The results indicated that a strong metal support interaction (SMSI) present in the Ni-CaO-ZrO 2 catalyst induces an intimate contact between Ni and ZrO 2 nano-particles, which is favorable for the surface reaction of adsorbed reactant species for CH 4 -CO 2 reforming. Owing to the mesoporous framework and SMSI, the Ni-CaO-ZrO 2 catalyst shows promising activity and stability in CH 4 -CO 2 reforming. Most of the carbonaceous deposits on the catalyst surface are in whisker form, which does not cover the active sites and then has little influence on the catalyst stability.

Geometric design of a Ni@silica nano-capsule catalyst with superb methane dry reforming stability: enhanced confinement effect over the nickel site anchoring inside a capsule shell with an appropriate inner cavity

Catalyst deactivation via severe carbon deposition and metal sintering is a significant obstacle to the industrialization of CO2 reforming of methane (CRM). Previous reports are mainly focused on metal oxide supported catalysts, which were proved to be less effective in eliminating the formation of carbon deposition completely. Here, we report a facile strategy for the preparation of a highly dispersed Ni@SiO2 nano-capsule catalyst, which features a monodisperse capsule and anchored Ni NPs in the inner porous shell. By virtue of this, carbon formation could be sterically hindered due to the sealed space for carbon formation and growth, and therefore excellent catalytic performance was achieved. Through comprehensive characterization and comparison of traditional supported catalysts as well as capsule catalysts with different architectures, the importance of a confined capsule structure in preventing carbon formation was convincingly demonstrated. Our work disclosed a morphological approach that is much more effective in preventing sintering of Ni NPs and suppressing carbon formation simultaneously, therefore shedding new light on the design and preparation of highly stable Ni-based catalysts for reforming reactions.