Abstract
A series of Pd/SiO2 catalysts were prepared with different Pd precursors by a dry ball-milling method and used in the catalytic oxidation of lean methane at low temperature. The effect of Pd precursors on the catalytic performance was investigated and the state of the most active Pd species was probed. The results indicate that dry ball-milling is a simple but rather effective method to prepare the Pd/SiO2 catalysts for lean methane oxidation, and palladium acetylacetonate is an ideal precursor to obtain a highly active Pd/SiO2-Acac catalyst with well- and stably dispersed Pd species, owing to the tight contact between acetylacetonate and Si–OH on the SiO2 support. Besides the size and dispersion of Pd particles, the oxidation state of Pd species also plays a crucial role in determining the catalytic activity of Pd/SiO2 in lean methane oxidation at low temperature. A non-monotonic dependence of the catalytic activity on the Pd oxidation state is observed. The activity of various Pd species follows the order of PdOx >> Pd > PdO; the PdOx/SiO2-Acac catalysts (in particular for PdO0.82/SiO2-Acac when x = 0.82) exhibit much higher activity in lean methane oxidation at low temperature than Pd/SiO2-Acac and PdO/SiO2-Acac. The catalytic activity of PdOx/SiO2 may degrade during the methane oxidation due to the gradual transformation of PdOx to PdO in the oxygen-rich ambiance; however, such degradation is reversible and the activity of a degraded Pd/SiO2 catalyst can be recovered through a redox treatment to regain the PdOx species. This work helps to foster a better understanding of the relationship between the structure and performance of supported Pd catalysts by clarifying the state of active Pd species, which should be beneficial to the design of an active catalyst in lean methane oxidation at low temperature.
Highlights
As the second most important gas after carbon dioxide that contributes to the greenhouse effect, anthropogenic methane emissions, mostly in a low concentration of about0.1–1.0 vol.%, have caused serious environmental problems [1,2]
Xiong and co-workers investigated the effect of the Pd oxidation state on the oxidation of methane, propane and propylene and found that metallic Pd was more active than PdO [15]; when the reduced catalyst was tested under lean fuel conditions, the catalytic activity declined due to the formation of PdO
A series of Pd/SiO2 catalysts were prepared with different Pd precursors by a dry ball-milling method and used in the oxidation of lean methane at low temperature
Summary
As the second most important gas after carbon dioxide that contributes to the greenhouse effect, anthropogenic methane emissions, mostly in a low concentration of about. By using Operando XANES conducted at Beamline ID24 of the European Synchrotron Radiation Facility in Grenoble, France, Nilsson and co-workers found that switching off oxygen led to a transformation of PdO to metal Pd except at a low temperature, where such a phase transformation was very slow [13] They observed that the highest catalytic activity for methane oxidation was obtained when oxygen was switched off, implying that the co-existence of metallic Pd and PdO was probably essential to achieve high reactivity at a low temperature. The effect of Pd precursors on the catalytic performance as well as the state of the most active Pd species was investigated, through a cooperative regulation and probe of the oxidation state of Pd species as well as an elaborate correlation of the catalytic performance with the Pd oxidation state
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