Electrothermal Catalysis For Elimination Of Diesel Pollutants

Two major pollutants emitted from diesel vehicles,soot and nitrogen oxides（NO_x）,pose serious threats to human health and the atmospheric environment.Conventional thermal catalytic purification is one of the most effective methods to eliminate the two pollutants.For the high spontaneous soot ignition temperature（>600℃）,catalytic soot combustion can reduce the combustion temperatures to within the exhaust temperatures（200-500℃）,and thereby the emitted soot can be continuously removed.For the low NO_x reduction efficiency in the oxygen-rich exhaust,NO_x storage and reduction（NSR）technology can remove NO_x efficiently in two periods,i.e.,the NO_x is adsorbed on NSR catalysts in the fuel-lean period and desorbed in the fuel-rich period and then reduced to N₂ by the introduced reducing gas.NSR has become one of the main lean-burn NO_x elimination technologies owing to its high-efficiency fuel economy.Currently,the catalytic purification technologies for diesel vehicles face the challenge of low-temperature emissions.The catalytic soot combustion has a minimum ignition temperature of 200℃（temperature for 10%soot conversion）and NO_x elimination temperature is as low as 125℃（temperature for 90%NO_x conversion）,while higher energy consumption is required to maintain the reaction.However,urban diesel vehicles often spend a considerable amount of time idling in traffic and thus exhaust temperatures often reach as low as 100℃,which is too low for conventional catalytic purification technologies to work.To breakthrough limit of lower work temperatures,we proposed an electrothermal catalysis strategy,in which several conductive metal oxides were selected as catalysts and a low voltage was applied onto the catalysts,forming electric current throughout them,generating electrothermal effect and electronic effect to trigger and maintain catalytic reactions.Based on the strategy,an Electric-Powered Programmed Oxidation strategy（EPPO）was developed to break through the limit of initiation temperature of soot,in which more than half of the soot was combusted<75℃.Utilizing the flexible response of conductive catalysts to the electrothermal effect,electrothermal pulses of NSR catalysts were synchronously coupled with the NSR cycles in period,overcoming the difficulty to optimize the two processes of NO_x storage and reduction at uniform temperatures in time.Firstly,an electrothermal catalytic reaction system was constructed,which consisted of a homemade electrothermal catalytic reaction tube,a DC power supply,a software to control electric input,a temperature displayer,a gas circuit system,and a gas analysis instrumentation.A conductive catalyst was packed into the reaction tube,which was connected to gas circuit system allowing reaction gas to pass and was connected to the power supply forming a circuit and then allowing electric current to pass for catalytic reactions.Three typical conductive oxide catalysts including antimony tin oxide（ATO）,indium tin oxide（ITO）,and LaCoO₃ perovskite were prepared.These typical conductive oxide catalysts were studied by the crystal structure characterization,microscopic morphology observation,textural structure analysis,and redox properties analysis,confirming their structures.Moreover,the electric conductivities of the catalysts were also measured.The EPPO reactions for soot combustion over the three catalysts were conducted,behaving the activity for soot conversion.In particular,the potassium-supported antimony-tin oxides（K/ATO）was tested for the EPPO reactions,in which more than half of the soot in the mixtures was combusted below 75℃ within only a few minutes,being far superior to that with conventional thermal catalytic soot combustion（generally with the temperature for 50%soot conversion of>300℃）.The energy consumption was reduced by two orders of magnitude with EPPO strategy.The mechanism of electrothermal catalysis for soot combustion was further investigated.For this,we designed and conducted an electric-powered programmed reduction to study the interaction between the soot and the lattice oxygen of the K/ATO catalyst.Combined with the results of an isotopic oxygen exchange test,and ex-situ characterizations of X-ray diffraction,X-ray photoelectron spectroscopy and Transmission Electron Microscope and in-situ Raman,we revealed that the conductive oxide catalysts were activated by the electrically driven release of lattice oxygen.Furthermore,an electrophoretic experiment with optical microscopy to observe was designed and conducted the movement of the conductive catalyst particles and soot particles under electric fields.The opposite electrodynamic fluidization between the two kinds of particles can improve the contact conditions between them.The electrothermal catalysis strategy was applied to the NSR reaction to eliminate NO_x,in which the Pt and K co-supported ATO was a typical conductive catalyst.The comparison between the electrothermal catalysis under constant power and the thermal catalysis at the same temperatures was performed.The result shows that the suitable operating temperature was decreased by 100℃ with electrothermal catalysis with higher N₂ selectivity.Furthermore,we developed a strategy of synchronous coupling of electrothermal pulse and NSR cycles in period,in which low power pulses were applied in the fuel-lean periods while high power pulses were applied in the fuel-rich periods.By the strategy,the optimalization of two processes was achieved during NO_x storage and reduction,improving the activity of catalyst and reducing the energy consumption of the NSR reaction.The technology of electrothermal catalysis to the catalytic elimination of soot and NO_x,the two important pollutants of diesel vehicle,was developed innovatively.The mechanism of electrothermal catalysis for soot combustion was revealed.This work lays a good foundation for the practical applications of electrothermal catalysis and will lead and expand the research of other catalytic reactions.