Xianglan Xu

Effects of La, Ce, and Y Oxides on SnO2 Catalysts for CO and CH4 Oxidation

SnO2‐based catalysts modified by La, Ce, and Y with a Sn/Ln (Ln=La, Ce, Y) atomic ratio of 2:1 were prepared by using a co‐precipitation method and used for CO and CH4 oxidation. The catalysts were characterized by N2 adsorption–desorption, XRD, energy dispersive X‐ray spectroscopy (EDS)‐SEM, H2 temperature‐programmed reduction (TPR), X‐ray photoelectron spectroscopy (XPS), and thermogravimetric analysis differential scanning calorimetry (TGA‐DSC) techniques. All three rare earth metal oxides were found to improve the thermal stability of SnO2, which resulted in catalysts with much higher surface areas and smaller crystallite and particle sizes. However, only the addition of Ce resulted in a catalyst with improved activity for both CO and CH4 oxidation. In contrast, La and Y modification resulted in samples with decreased activity for both reactions. For the Ce‐modified sample, Ce cations were found to dope into the lattice of rutile SnO2 to form a solid‐solution structure. As a lattice impurity, ceria, the well‐known oxygen storage component (OSC), led to the formation of more defects in the matrix of SnO2 and impeded the crystallization process, which resulted in a catalyst with a higher surface area and more active oxygen species. In contrast, XRD proved that the addition of La and Y mainly led to the formation of more stable and inert pyrochlore compounds, Sn2La2O7 and Sn2Y2O7, which disrupted a major part of the active sites based on SnO2. Consequently, the oxidation activity was impaired, although these two samples also have higher surface areas than pure SnO2. The Ce‐modified sample showed not only high activity but also good reaction durability and thermal stability. Furthermore, Sn‐Ce binary oxide is a better support than SnO2, CeO2, and traditional Al2O3 supports for Pd, which gives it the potential to be applied in some real after‐treatment applications.

Tin Modification on Ni/Al2O3: Designing Potent Coke‐Resistant Catalysts for the Dry Reforming of Methane

Sn‐modified Ni/Al2O3 catalysts for CH4 dry reforming were prepared by co‐impregnation and two‐step impregnation methods and characterized by thermogravimetric analysis with differential scanning calorimetry, SEM, TEM, high‐angle annular dark‐field scanning transmission electron microscopy mapping, XRD, X‐ray photoelectron spectroscopy, H2 temperature‐programmed reduction, and CO2 temperature‐programmed desorption. After reduction, surface Ni‐Sn alloys were formed on the Ni particles, which changed the inherent activity of Ni sites and suppressed coking effectively with a mild loss of the activity. The catalysts with different amounts of surface Ni‐Sn alloys also provided strong evidence to prove that the coking rate and activity change tendency correlate well with the amount of the surface alloys. These results are of help to develop catalysts with potent resistance to coking for industrial use.

Nickel‐Supported on La2Sn2O7 and La2Zr2O7 Pyrochlores for Methane Steam Reforming: Insight into the Difference between Tin and Zirconium in the B Site of the Compound

La2Sn2O7 and La2Zr2O7, two pyrochlore compounds with different B‐site cations, were prepared and used as supports for Ni in methane steam reforming. Compared with Ni/γ‐Al2O3, both Ni/La2Zr2O7 and Ni/La2Sn2O7 show very stable reaction performance. Whereas Ni/La2Zr2O7 also displays reasonably high activity for the reaction, the activity of Ni/La2Sn2O7 is extremely low. It was found that severe coking occurred with Ni/γ‐Al2O3, but no coke formation was observed on the two pyrochlore catalysts. On the reduced and spent Ni/La2Sn2O7 catalyst, the Ni3Sn2 and Ni3Sn alloys were detected; these alloys suppressed coke formation but also decreased the activity of the catalyst. In comparison, a large amount of La2O2CO3 was formed on the used Ni/La2Zr2O7 catalyst; these species reacted with the carbon deposits formed on the Ni particles and continuously restored the Ni sites. Thus, coking was effectively suppressed and the initial high activity of the catalyst was maintained. Thus, Ni/La2Zr2O7 is a superior catalyst having the potential for industrial use.

Ni–Co/Al2O3 Bimetallic Catalysts for CH4 Steam Reforming: Elucidating the Role of Co for Improving Coke Resistance

A series of supported Ni–Co/γ‐Al2O3 bimetallic catalysts with a fixed 12 % Ni loading but different Co contents were prepared by using the coimpregnation method and investigated for methane steam reforming. The addition of Co can significantly improve the coke resistance and the reaction stability of Ni/Al2O3 at a mild loss of the reforming activity. XPS and TEM results prove the existence of strong interaction between Ni and Co species. XRD and high‐angle annular dark‐field scanning transmission electron microscopy mapping results of the reduced catalysts provide direct evidence for surface Ni–Co alloy formation upon Co addition onto Ni/Al2O3, which can block part of the active low coordinated Ni sites and lower the metal dispersion, thus effectively suppressing coking and improving the reaction stability in comparison with the unmodified Ni/Al2O3 catalyst.

High surface area La2Sn2O7 pyrochlore as a novel, active and stable support for Pd for CO oxidation

A high surface area mesoporous La2Sn2O7 compound with a well crystallized pyrochlore structure has been successfully prepared by a simple low temperature hydrothermal (La2Sn2O7-HT) method. As a support for Pd, a catalyst with a significantly higher activity for CO oxidation has been achieved in comparison with the other two non-mesoporous pyrochlores prepared by co-precipitation (La2Sn2O7-CP) and sol–gel (La2Sn2O7-SG) methods. The CO adsorption–desorption and STEM results demonstrate that on Pd/La2Sn2O7-HT, the highest Pd dispersion can be achieved among all of the catalysts. Moreover, compared with Pd/La2Sn2O7-CP and Pd/La2Sn2O7-SG, more active oxygen species were formed on Pd/La2Sn2O7-HT. It is believed that these are the two major reasons accounting for the superior CO oxidation activity of Pd/La2Sn2O7-HT. Furthermore, this catalyst also shows a stable reaction performance in the presence of water vapour. In conclusion, the mesoporous La2Sn2O7-HT pyrochlore is an excellent support for Pd, and has the potential to be applied in some real exhaust control processes.

SnO2 nano-rods with superior CO oxidation performance

SnO2 nanoparticles with various morphologies were successfully prepared and characterized. Although SnO2 nano-rods with preferentially exposed (110) crystal planes have the lowest surface area and lack active oxygen species, it is the most active catalyst for CO oxidation, and its catalytic behavior is similar to that of a noble metal catalyst.

Methane Dry Reforming over Coke‐Resistant Mesoporous Ni‐Al2O3 Catalysts Prepared by Evaporation‐Induced Self‐Assembly Method

Mesoporous Ni‐Al2O3 catalysts were prepared in one pot following an evaporation‐induced self‐assembly method (EISA) and used for methane dry reforming. Compared with a traditional Ni/Al2O3 catalyst prepared through impregnation method (IMP), the EISA catalysts display significantly improved coke resistance and activity. It is revealed by small‐angle XRD (SXRD), N2 adsorption–desorption, and TEM that an ordered mesoporous structure was formed in the EISA catalysts, which impedes the aggregation of the Ni sites and aids in the mass transfer of the reaction. In addition, the Ni species in the reduced EISA samples more dispersed, more uniformly distributed, and have smaller crystallite size, as evidenced by XRD, H2 adsorption–desorption, and TEM results. It is speculated that these are the major reasons accounting for the significantly improved dry reforming performance of the EISA catalysts.

Facile preparation of mesoporous Cu–Sn solid solutions as active catalysts for CO oxidation

With a facile co-precipitation method, a series of high surface area mesoporous CuxSn1−xOy solid solution catalysts have been synthesized and applied to CO oxidation. Compared with individual SnO2 and CuO, the activity of these catalysts is remarkably improved. The highest activity is achieved on Cu0.5Sn0.5Oy, a catalyst with a Cu/Sn molar ratio of 0.5/0.5 and a Caramel-Treats-like morphology. It is revealed by XRD, SEM-EDX mapping and HR-TEM results that Cu2+ cations have been incorporated into the crystal lattice of rutile SnO2 to form a uniform solid solution structure. As testified by N2 adsorption–desorption and SEM results, these CuxSn1−xOy catalysts contain well-defined mesopores and possess high surface areas and improved pore volumes, which are favourable for the dispersion of the active sites, the diffusion of the reactants and the easy interaction between the reactants and the catalyst surface. Moreover, H2-TPR and XPS results demonstrate that more active and loosely bounded oxygen species have been formed on the surface of these catalysts. It is believed that these are the predominant reasons leading to the superior CO oxidation activity over the CuxSn1−xOy catalysts. Notably, these CuxSn1−xOy catalysts are also resistant to water vapour deactivation, indicating they have the potential to be used in real exhaust control processes.

Thermally stable ultra-small Pd nanoparticles encapsulated by silica: elucidating the factors determining the inherent activity of noble metal catalysts

With an improved one-step reverse micelle method, Pd@SiO2-RM with thermally stable, 1.1 nm ultra-small Pd nanoparticles were prepared in one-pot. HRTEM results reveal that the ultra-small Pd nanoparticles are embedded in the bulk of the silica nanospheres around 30 nm to form a multi-core shell structure. Therefore, the migration and agglomeration of the ultra-small Pd nanoparticle cores can be impeded effectively at elevated temperatures. Compared with Pd/SiO2-IMP prepared by impregnation, core–shell Pd@SiO2-ST and Pd@SiO2-ME catalysts prepared by Stober and regular micro-emulsion processes, Pd@SiO2-RM possesses a much higher metal surface area. As a consequence, this catalyst shows remarkable activity and superior thermal stability for CO oxidation. It is concluded that the Pd grain size and metal surface area are the determining factors for the activity, as evidenced by the strict linear relationship between the differential rates and the Pd sizes/metal surface areas.

Elucidating the promotional effects of niobia on SnO2 for CO oxidation: developing an XRD extrapolation method to measure the lattice capacity of solid solutions

A series of Sn–Nb binary catalysts have been prepared by using a co-precipitation method and used for CO oxidation. All the catalysts show much higher specific surface areas than the individual oxides, indicating their improved thermal stability. It was found that Nb5+ cations can be doped into the lattice of tetragonal rutile SnO2 to replace a portion of the Sn4+ to form a solid solution structure. Using an XRD extrapolation method, the SnO2 lattice capacity for Nb2O5 has been quantified, which proves that only 25% of the Sn4+ in the SnO2 lattice can be replaced by Nb5+ to form a stable solid solution. For the samples with Nb contents below the capacity, the formed solid solution structure can induce the formation of a large quantity of stable and active surface deficient oxygen species due to charge imbalance and lattice defects, which improve the CO oxidation activity remarkably. For the samples with Nb contents above the capacity, the excess Nb is present as free Nb2O5 in the catalysts, which is harmful to their activity. It is concluded that Nb, as an effective promoter for SnO2, must be incorporated into its lattice as Nb5+ to form a solid solution. A Sn–Nb solid solution without excess Nb2O5 is not only a promising catalyst itself, but also a good support for precious metals to prepare catalysts for CO oxidation.