Palladium
nanoparticles (NPs) were successfully deposited on surface-modified
metal oxides (mod-MO
x
, M = Hf, Ti, Zr,
Ce, and Al) and applied as catalyst materials for lean methane combustion.
It was found that the surface modification of support materials improved
the light-off performance of 1.0Pd/mod-HfO2 (palladium
catalyst supported on surface-modified HfO2 with a content
of 1.0 wt %), 1.0Pd/mod-ZrO2, and 1.0Pd/mod-CeO2, but lowered the purification efficiency of 1.0Pd/mod-TiO2 and 1.0Pd/mod-Al2O3 when compared with their
1.0Pd/MOx counterparts. Over the best-performing 1.0Pd/mod-HfO2 material, 90% of methane was removed at 317 °C and a
space velocity of 60 000 mL g–1 h–1, which was 120 °C lower than that required for the untreated
1.0Pd/HfO2 sample. Detailed characterization of representative
HfO2-related materials showed that the introduced silicon
modifier materials, which existed as an amorphous phase covering the
HfO2 surface, could improve the dispersion of palladium
nanoparticles due to their steric confinement and strengthen the generation
of surface-adsorbed oxygen species via electron transfer. We believe
that this surface modification strategy, which could promote the catalytic
performance of palladium nanoparticles supported on other cost-effective
host materials as well, provides a feasible method for the design
of methane combustion catalysts with excellent low-temperature performance.
The catalytic performance of MnO 2 (x)−CeO 2 (x = Mn/Ce molar ratio) and the synergistic effect have been investigated in formaldehyde removal. The results showed that formaldehyde achieved 100% conversion at 60 °C for MnO 2 (1.5)−CeO 2 , with the gas hourly space velocity = 120 000 mL h −1 g cat −1. It was lower than the formaldehyde removal temperature of MnO 2 (130 °C) and CeO 2 (170 °C). The apparent activation energy for HCHO oxidation with MnO 2 (1.5)−CeO 2 was 34.2 kJ mol −1 . The partial mutual replacement of Mn x+ and Ce x+ decreases the crystallite size due to the MnO 2 / CeO 2 interaction. The X-ray photoelectron spectroscopy and the Raman analyses show that the Mn 3+ /Mn and O ads /O latt ratios and the relative concentrations of oxygen vacancies for MnO 2 (1)−CeO 2 and MnO 2 (1.5)− CeO 2 were higher than those of the rest of the catalysts. Thus, the lattice oxygen (O latt ) from CeO 2 readily transfers to the oxygen vacancy, achieving O latt activation to surface-adsorbed oxygen (O ads ).
NiO nanomaterials prepared using a solid−liquid NH 3 • H 2 O precipitation method (NiO-NSL) were tested in the catalytic combustion of methane. The NiO-NSL presented a characteristic rodlike nanostructure with a length of about a few hundred nanometers except for a part of the nanoparticles. For comparison, the NiO nanomaterials prepared by the traditional liquid-phase NH 3 •H 2 O precipitation method (NiO-NLL) were tested in the same reaction conditions. NiO-NSL exhibited significantly higher methane combustion activity than NiO-NLL and achieved the complete combustion of methane at 390 °C, which was outstanding in non-noble metal-based catalyst. X-ray photoelectron spectroscopy (XPS) and hydrogentemperature-programmed reduction (H 2 -TPR) results indicate that the surface Ni 2+ content of NiO-NSL was higher than that of NiO-NLL, and the presence of more Ni 2+ might be responsible for the enhanced activity. DFT calculations prove that the energy barrier for C−H bond activation on Ni 2+ was lower than that on Ni 3+ , which was consistent with the higher methane catalytic combustion activity of NiO-NSL. In addition, when the precipitating agent was replaced with NaOH and (NH 4 ) 2 CO 3 , the generalization of the solid−liquid precipitation method in the preparation of the NiO catalysts was also tested. The results show that the solid−liquid precipitation method proposed in this work was still applicable when NaOH was used as a precipitant. However, with the use of (NH 4 ) 2 CO 3 as a precipitant, the methane catalytic activity of the NiO nanoparticles prepared by the solid−liquid precipitation method was reduced to a certain extent compared with the traditional liquid-phase precipitation method. This research can open up a highly efficient and environmentally friendly method for the synthesis of methane combustion catalysts.
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