Silver Activated And Supports Effects Modulate Silver-Based Catalysts Activity For Catalytic Oxidation Of Formaldehyde

Formaldehyde（HCHO）is a common gaseous pollution in the indoor environment and is widely found in various indoor items such as furniture,interior paints and disinfectants.HCHO is highly volatile and prolonged contact with it can lead to various diseases such as leukemia,so it is important to efficiently remove the indoor HCHO.Catalytic oxidation is a highly effective method for the removal of HCHO.The two types of catalysts commonly used include transition metal oxide catalysts and noble metal-based catalysts.The former one is inexpensive but less active in low temperature oxidation while noble metal-based（Pt,Pd,Au,Rh）catalysts are excellent for low temperature oxidation but expensive.As a special noble metal,Ag has received much attention because of its low price and potential for low temperature oxidation.Therefore,this work is based on the advantages of Ag and development of its ability to oxidize indoor pollutants HCHO.The design of active or otherwise more stable catalysts and the in-depth study of the reaction mechanism during HCHO oxidation are currently hot research topics in the field of HCHO removal.However,the design of Ag-based catalysts with high oxidation activity and simple synthesis for HCHO oxidation is a major challenge,and the relative humidity in air is also a major problem for the practical application of catalysts.Based on this study,the effect of silver activation on the catalytic oxidation of HCHO by Ag-based catalysts has been investigated,and the reaction mechanism of HCHO conversion under different relative humidity conditions has been investigated in order to contribute to the solution of this problem.The relevant studies are as follows:Al₂O₃ is a stable support,the choice of Al₂O₃ as the support can eliminate the interference of the support with the activity to a great extent,and it has been demonstrated that hydrogen（H₂）activation has a significant enhancement effect on enhancing the oxidation performance of Ag/Al₂O₃ on CO.Therefore,the first part of the work focuses on the effect of H₂ activation on the oxidation of HCHO by Ag/Al₂O₃ catalysts and delves into the reaction mechanism to elucidate the conversion pathway of HCHO over Ag/Al₂O₃ catalysts and to investigate its HCHO oxidation activity at different relative humidity.Therefore,we prepared 1%Ag/Al₂O₃（Ag/Al-fresh）catalyst with Ag species in a uniform state,and then activated Ag/Al-fresh with H₂ at 600°C to obtain Ag/Al-600H₂catalyst.We observed that H₂ activation had a significant influence on the Ag/Al₂O₃ for HCHO oxidation.In dry reaction atmosphere,the Ag/Al-fresh and Ag/Al-600H₂exhibited comparable HCHO conversions,but the Ag/Al-600H₂ showed much higher CO₂ selectivity than the Ag/Al-fresh.The presence of water vaper dramatically suppressed the HCHO conversion of the Ag/Al-fresh catalyst,while the Ag/Al-600H₂showed high water resistance,maintaining the high HCHO conversion and CO₂selectivity under 35 RH%.Characterization results showed that the Ag species on Ag/Al-fresh and Ag/Al-600H₂ were the cationic Ag single-atoms and the metallic Ag nanoparticles,respectively.It was revealed that the Ag⁰ nanoparticles were more conducive than Ag⁺single-atoms for formate decomposition,as well as O₂ and H₂O activation,thus accelerating the conversion of HCOO^-and CO intermediates to CO₂,and contributing to the higher activity and water resistance of Ag/Al-600H₂ catalyst.MnO_x is an inexpensive but active reducing supports with good HCHO oxidation activity.The use of MnO_x as a support for loading Ag and its modification using the effective activation process described above is therefore considered.In addition,according to literature research,carbon monoxide（CO）is also an effective reducing gas,so an attempt was made to prepare Ag/MnO_x catalysts and activate them with H₂ and CO respectively,in order to obtain highly active and stable HCHO oxidation catalysts.In the second part of the work,the effect of different activation atmospheres（H₂ and CO）on the HCHO oxidation by Ag/MnO_x catalysts was investigated and the reaction mechanism was studied,as well as its HCHO oxidation activity at different relative humidity.The catalyst activated with CO（Ag/Mn-CO）displayed the highest activity among the tested samples with 90%conversion at 100°C under a gas space velocity of 75,000 m L/(g_cat?h).Complementary characterizations demonstrate that CO reduction treatment resulted in synergically regulated content of surface oxygen on support to adsorb/activate HCHO and size of Ag particle to dissociate oxygen to oxidize the adsorbed HCHO.In contrast,other catalysts lack for either abundant surface oxygen species or metallic silver with the appropriate particle size,so that the integrate activity is limited by one specific reaction step.In the first two parts of the study,the change in Ag properties was the main reason for the difference in catalyst activity,so we considered to keep the Ag properties consistent and to study the activity of the relevant catalysts and the reaction mechanism to provide a reference for the reasonable choice of supports in the catalyst design process.In addition to the fact that the active supports affect the catalyst activity,most of the studies in which the difference in catalytic activity between different supports has been discussed have resulted in a difference in the Ag state according to the literature,but it has been tried that the loading of Ag on P25 and Al₂O₃ respectively does not result in a difference in the Ag state.Hence,we loaded Ag onto P25 and Al₂O₃ respectively and tested the ability to oxidize HCHO.The Ag on both supports possessed similar physicochemical characteristics such as particle size and valence state,but the HCHO oxidation activity of Ag/Al₂O₃ was much higher than that of Ag/P25.The characterization results revealed that Ag/Al₂O₃ possessed a better conversion capacity to the intermediate species formate and therefore had a higher activity under the same test conditions.