Nickel hydroxide nanosheets were prepared by a very simple direct manual grinding strategy and then calcined at 200, 300, 400, and 500°. The synthesized samples were tested in lean methane (1.0% CH 4 , air balanced) catalytic combustion and subjected to a series of physical and chemical characterizations. The sample calcined at 200 °C (Ni(OH) 2 -200) presented a typical nanosheet structure and the best methane catalytic activity in all the samples, which can completely catalyze methane at 400 °C. The crystal structure changed from β-Ni(OH) 2 to NiO at a calcination temperature of 300 °C. The β-Ni(OH) 2 nanosheets began to partially agglomerate into nanoparticles at 400 °C and almost transformed into nanoparticles at 500 °C. Interestingly, the original nanosheet samples Ni(OH) 2 -200 and NiO-300 still maintained their morphology and structure although they all went through an activity test at 500 °C in a 1.0% CH 4 atmosphere, which proves that the calcination of nanosheets in a CH 4 atmosphere tended to maintain their nanosheet morphology compared with calcination in the air. Furthermore, through the activity test, X-ray photoelectron spectroscopy results, TPx, and in situ DRIFTS characterization, it was proved that the hydroxyl groups on the Ni(OH) 2 -200 and NiO nanosheets were beneficial to the dissociation of methane on the catalyst surface, and the nanosheet structure was also prone to generating more active adsorbed oxygen, so the activation energy of methane was lowered. A methane catalytic mechanism on the Ni(OH) 2 nanosheets and NiO nanoparticles was proposed, which further proved the key role of hydroxyl groups in methane combustion.

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.

The direct oxidation of low-concentration methane (CH 4 ) to methanol (CH 3 OH) is often regarded as the “holy grail”. However, it still is very difficult and challenging to oxidize methane to methanol in one step. In this work, we present a new approach to directly oxidize CH 4 to generate CH 3 OH in one step by doping non-noble metal Ni sites on bismuth oxychloride (BiOCl) equipped with high oxygen vacancies. Thereinto, the conversion rate of CH 3 OH can reach 39.07 μmol/(g cat ·h) under 420 °C and flow conditions on the basis of O 2 and H 2 O. The crystal morphology structure, physicochemical properties, metal dispersion, and surface adsorption capacity of Ni–BiOCl were explored, and the positive effect on the oxygen vacancy of the catalyst was proved, thus improving the catalytic performance. Furthermore, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was also performed to study the surface adsorption and reaction process of methane to methanol in one step. Results demonstrate that the key to keep good activity lies in the oxygen vacancies of unsaturated Bi atoms, which can adsorb and active CH 4 and to produce methyl groups and adsorbing hydroxyl groups in methane oxidation process. This study broadens the application of oxygen-deficient catalysts in the catalytic conversion of CH 4 to CH 3 OH in one step, which provides a new perspective on the role of oxygen vacancies in improving the catalytic performance of methane oxidation.

Single-step conversion of methane to methanol in the gaseous phase is required for high value added application of methane and environmental protection, but it is challenging. Here, direct oxidation of methane to methanol under gaseous condition on iridium promoted Cu/CeO 2 catalyst, prepared using a sol-gel method, is investigated. The addition of iridium can effectively upgrade the redox properties and oxygen storage capacity due to intense metal interaction, stimulating a prominent catalytic performance for methane conversion to methanol on CuÀ Ir@CeO 2 catalyst. Approximately 26.2 μmol/g cat methanol yield and 68 % methanol selectivity are achieved on the trimetallic 5Cu0.5Ir@CeO 2 catalyst at 550 °C in 2 h. Consequences of the analysis of XRD, SEM, HRTEM, Raman, FI-IR and XPS demonstrate that highly-dispersed Ir and Cu species are uniformly distributed on CeO 2 surface, and partial Cu or Ir atoms replace Ce 4 + in CeO 2 lattice due to the metal interaction in the colloidal structure, which can impact the catalyst's electronic properties. H 2 temperature-programmed reduction (H 2 -TPR) and CH 4 temperature-programmed desorption (CH 4 -TPD) results disclose that the unique ternary surface exhibits excellent redox properties and strong adsorption capacity for methane, which can activate the first CÀ H bond of methane to methyl species, and then react with OH À to form methanol. The good stability in cyclic operation is an additional attribute, rendering this type of catalyst a "front-runner" in future catalyst development for direct methane-to-methanol. This composite catalyst design provides hope for developing ternary metaloxide catalysts for functionalization of methane.

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