Manganese Oxide Supported Noble Metal Nanocatalysts For The Removal Of Volatile Organic Compounds

Volatile organic compounds?VOCs?cause serious harmful effect on atmospheric environment and human health.Catalytic oxidation is belived to be one of the efficient methods for the elimination of VOCs.The key issue of such a technology is the development of catalyst with high activity,high stability,and low price.On the basis of our previous works,we herein fabricated supported noble metal nanocatalysts with ultralow loadings via the in-situ molten salt method,and investigated their catalytic performance for the complete oxidation of toluene and acetone.The main conclusions are as follows:?1?A series of Pd/Mn₂O₃-Pd?OAc?₂,Pd/Mn₂O₃-PdCl₂,and Pd/Mn₂O₃-Pd?NO₃?₂nanocatalysts were prepared via the in-situ molten salt method in the NaNO₃ and NaF mixtures,with Pd?OAc?₂,PdCl₂,and Pd?NO₃?₂ as palladium precursor,respectively.The as-prepared manganese oxide support was cubic Mn₂O₃.It exhibited one dimensional nanowire-like morphology,with length of several hundreds of nanometer,and diameter of 15²0 nm.All of the palladium nanoparticles were highly distributed on the surface of the Mn₂O₃ support.Although the palladium precursor did not influence the morphology of the Mn₂O₃ support,it gave a large effect on the average particle size distribution of palladium nanoparticles.The palladium nanoparticles in the 0.09 wt%Pd/Mn₂O₃-PdCl₂ nanocatalyst possessed relatively smaller average particle size?³.5 nm?.Under the reaction conditions of acetone or toluene concentration=1000 ppm,acetone/oxygen or toluene/oxygen molar ratio=1/400,and space velocity?SV?=40000 mL/?g h?,the loading of palladium nanoparticles influence the catalytic activities of the Pd/Mn₂O₃-Pd?OAc?₂,Pd/Mn₂O₃-PdCl₂,and Pd/Mn₂O₃-Pd?NO₃?₂ nanocatalysts for the catalytic removal of VOCs with simialr tendency.The catalytic activity firstly increased with a rise in the loading of palladium nanoparticles.When the loading of palladium nanoparticles was ca.0.10wt%,the corresponding catalyt exhibited relatively higher catalytic activity.Over the0.09 wt%Pd/Mn₂O₃-PdCl₂ nanocatalyst,T_10%,T_50%,and T_90%?the reaction temperature required for 10%,50%,and 90%conversion of VOCs?for acetone oxidation was 87,145,and 165 ^oC,and T_10%,T_50%,and T_90%for toluene oxidation was80,110,and 140^oC,respectively.A further increase in the loading of palladium nanoparticles resulted in a decrease in the catalytic activity.We duduced that it was associated with the aggregation of noble metal nanoparticles,when the loading of noble metal nanoparticles was more than a certain value in the adopted molten salt mixture.In addition to the average particle size distribution of palladium nanoparticles,the reducibility of Pd/Mn₂O₃ caused a positive effect on the catalytic activity.Compared with the Mn₂O₃ support,the low-temperature reducibilities of the Pd/Mn₂O₃ nanocatalysts were enhanced due to the interaction of palladium nanoparticles and the Mn₂O₃ support,and then their catalytic activities were improved for the oxidation of VOCs.?2?A series of 0-2.10 wt%Ag/Mn₂O₃ nanocatalysts were prepared via the in-situ molten salt method in the NaNO₃ and NaF mixtures with AgNO₃ and MnSO₄ as metal precursor.Under the reaction conditions of toluene concentration=1000 ppm,toluene/oxygen molar ratio=1/400,and SV=40000 mL/?g h?,the loading amount of silver nanoparticles influence the catalytic activities of the Ag/Mn₂O₃ nanocatalysts for the catalytic removal of VOCs.The catalytic activity firstly increased with a rise in the loading of silver nanoparticles.When the loading of silver nanoparticles was ca.0.06 wt%,the corresponding catalyt exhibited relatively higher catalytic activity.Over the 0.06 wt%Ag/Mn₂O₃ nanocatalyst,T_50%and T_90%for toluene oxidation was 170and 205 ^oC,respectively.A further increase in the loading of silver nanoparticles resulted in a decrease in the catalytic activity,due to the aggregation of silver nanoparticles.Obviously,in terms of the effect of noble metal loading on the catalytic activity,the as-prepared Pd/Mn₂O₃ and Ag/Mn₂O₃ nanocatalysts exhibited similar behavious.Hence,we believed that the in-situ molten salt method is a facile method for the preparation of supported noble metal nanocatalyst with ultralow loading.The interface of silver nanoparticles and the Mn₂O₃ support was the main reactive sites for the complete oxidation of toluene.Although the 0.06 wt%Ag/Mn₂O₃ nanocatalyst exhibited very high activity for the catalytic removal of toluene,its stability was very poor,and its activity would quickly decreased.We found that the fast deactivation of the 0.06 wt%Ag/Mn₂O₃nanocatalyst was mainly due to that the O₂ molecule in the reaction gas could not be quickly changed into active lattice oxygen over the Ag/Mn₂O₃ nanocatalyst at a low temperature,and then the consumed active lattice oxygen could not be quickly and effectively replenished,rather than the the aggregation of silver nanoparticles,carbon deposition,and the change in the crystal structure of the support or the surface composition.The doping of cerium into the 0.06 wt%Ag/Mn₂O₃ nanocatalyst could significantly enhance its catalytic stability for the removal of toluene.The O₂molecule in the reaction gas could be quickly adsorbed and activiated on the surface oxygen vacancy of CeO₂.Because the silver and cerium species were highly distributed on the Mn₂O₃ nanowires support,the active oxygen species,which located on the surface oxygen vacancy of CeO₂,could freely migrate to the reactive sites,i.e.the interface of silver nanoparticles and the Mn₂O₃ support,and then the consumed active lattice oxygen could be timely replenished.We believed that the present 0.63wt%CeO₂-0.06 wt%Ag/Mn₂O₃ nanocatalyst with high activity and stability could be an alternative for the commercial supported noble metal catalysts used in the removal of VOCs.