Effect Of The Support And Reductant On NO Reduction On Single-Pd Atom Catalysts

Emission sources of nitrogen oxides（NO_x）include mobile and stationary sources,such as motor vehicles and power plants.With the development of society,people’s demand for motor vehicles is increasing,vehicles are increasing rapidly year by year,so the exhaust pollution of motor vehicles has become an important environmental challenge.In order to meet increasingly stringent automobile emission regulations,researchers have made many efforts to develop catalysts with higher conversion rates,thermal stability,and lower conversion temperatures.It has attracted wide attention that the three-way catalyst（TWC）has high efficiency and high selectivity for the simultaneously removal of NO_x,carbon monoxide（CO）and hydrocarbons（C_xH_y）in the exhaust gas.Pd based catalysts have abundant reserves and can adsorb NO at low temperature to reduce NO_x emission during vehicle start-up.However,Pd has lower catalytic activity at low temperature and low selectivity for NO reduction products compared with Rh.Therefore,it is of great significance to find Pd based catalysts with high efficiency for NO reduction.In this work,density functional theory（DFT）calculation method is used to explore the reduction mechanism of NO on carbon-based carrier and oxide carrier supported single-atom Pd under different reducing atmospheres of reducing agents H₂,CO and NH₃.In order to find suitable and efficient carriers and reducing agents for Pd-based catalysts,the influence of reducing agents on the same kind of catalyst and the influence of different carriers on NO reduction under the same kind of reducing atmosphere were studied.The main conclusions are as follows:1.The reaction mechanism of NO reduction with H₂,CO and NH₃ on carbon-based single-atom Pd catalysts is clarifiedThe NO reduction on the single-atom Pd catalyst supported on different modified carbon-based supports SVG,DVG,BVG and NVG was studied,and the reduction mechanism of different reducing agents was obtained.The favorable formation route of N₂ is the hydrogenation of NO to HNO,HNO continues to hydrogenate through intermediates NH₂O and NH₂OH,and then NH₂OH dissociates to generate NH₂ and OH.The generated NH₂ intermediate combines with NO to form NH₂NO,and then NHNOH is formed by the isomerization of NH₂NO,finally NHNOH is dissociated to N₂ and H₂O.From NO activation to the formation of intermediate NH₂,the favorable route of NH₃ is the same as that of N₂,and then NH₂ hydrogenation leads to NH₃.NO is not easy to activate on Pd/DVG in H₂ atmosphere.The reaction mechanism of NO reduction with CO is that two molecules of NO in the form of dimers dissociate to N₂O,and then O atom combines with CO to form CO₂.The dissociation of NH₃ on these four catalysts is more difficult.2.The effect of different modification methods of carbon-based carrier on NO reductionBy studying the NO reduction reaction on Pd/SVG,Pd/DVG,Pd/BVG and Pd/NVG catalysts,it is concluded that different catalysts have different NO reduction activities.The energy barrier of NO reduction by H₂ to N₂ is 124.9 kJ·mol^-1 on Pd/NVG,which is lower than that of Pd/SVG and Pd/BVG(144.3 and 184.7 kJ·mol^-1).The energy barrier for NO reduction with CO is 81.9 kJ·mol^-1 on Pd/NVG.The energy barriers are 219.2,252.9 and 220.8 kJ·mol^-1 on Pd/SVG,Pd/DVG and Pd/BVG,respectively.Similarly,the dissociation energy barrier of NH₃ on Pd/NVG is also the lowest compared to the other three catalysts.Therefore,the modification of N improves the activity of the carbon-based carrier supported catalyst.The selectivity of NO reduction products is different on Pd single-atom catalysts supported by different modified carbon-based supports.The NO reduction with H₂ on Pd/SVG and Pd/BVG is easy to produce NH₃.On Pd/NVG,NO reduction with H₂ to N₂ and NH₃ at the same rate-limiting step.NO reduction with CO to N₂O on Pd/SVG,Pd/DVG,Pd/BVG,while on Pd/NVG,both N₂O and CO₂ are easily produced.It can be seen that different carriers have different selectivity to NO reduction products.3.Obtain the reduction mechanism of CO and NH₃ on the single-atom Pd-doped TiO₂（101）catalyst and the effect of different reducing agents on the productThe reduction mechanism of CO and NH₃ on the Pd/TiO₂（101）catalyst is different from that on the carbon-based single-atom Pd catalyst..In CO reducing atmosphere,CO and lattice oxygen to CO₂ and O vacancies on Pd/TiO₂（101）,which is the rate-determining step of the reaction,with the energy barrier of 104.4 kJ·mol^-1.The formation of O vacancies is a key step in the reduction of NO.The dissociation of cis-N₂O₂ to N₂O only needs an energy barrier of 27.2 kJ·mol^-1,and the reduction products CO₂ and N₂O all obtained.On the carbon-based supported monoatomic Pd catalyst,the dimer dissociates to form N₂O,and then O combines with CO to form CO₂.The reaction mechanism of NO reduction with NH₃ on the Pd/TiO₂（101）catalyst includes three parts,the activation and dissociation of NH₃,the formation of NH₂NO,and the generation of N₂.On Pd/TiO₂（101）,the NH₂ generated by the dissociation of NH₃ is easy to combine with NO to generate NH₂NO,and NH₂NO is easy to generate N₂ through the NONH path.The dissociation of NH₃ is the rate-determining step with an energy barrier of 151.0 kJ·mol^-1.4.The activity and selectivity of single-atom Pd catalysts supported on different supports under the same reducing atmosphere are obtained.The Pd/SVG catalyst has the highest activity of NO reduction with H₂ to NH₃,while the doping of N improves the activity of NO reduction with H₂ to N₂.Pd/NVG and Pd/TiO₂（101）catalysts have higher NO reduction activity in CO reduction atmosphere,and easy to generate N₂O and CO₂.In the NH₃ atmosphere,the Pd/TiO₂（101）catalyst has a higher NO reduction activity,and N₂ are produced at the same time.The activation and dissociation of NH₃ on the carbon-based carrier supported single Pd catalyst is more difficult to occur.