Reaction Kinetics And Performance Regulation Of Copper-based Catalysts For Oxidation Of Low-Concentration Methane

Low-concentration methane with a volume concentration of 0.5-3%methane is widely present in coal mine methane and chemical waste gas.Most of the low-concentration methane is directly discharged into the atmosphere due to the difficulty of treatment,which exacerbates the greenhouse effect.The low-concentration methane catalytic combustion technology is very important for mitigating the greenhouse effect,saving energy and reducing emissions,and alleviating the energy crisis.Cu-based transition metal catalysts have received more and more attention in catalysts for the combustion and utilization of low-concentration methane due to their good reactivity and economy,but low-concentration methane is represented by Cu-based The reaction path on the transition metal catalyst,the intrinsic reaction kinetics,the mechanism of the catalyst metal-metal interaction on its reaction performance,the influence of the impurity gas on the catalytic reaction performance,etc.are important for improving the activity and efficiency of the reaction with low concentration methane The impact needs further study.This article studies the above problems and has very important academic significance and research value.In this paper,a combination of experimental research,theoretical analysis and quantum chemistry simulation was used to study the catalytic performance,reaction kinetics and control of reaction performance of Cu-based catalysts for the oxidation of low-concentration methane.First,the density functional theory(DFT)was used to study the mechanism of C-H bond breakage in the process of dehydrogenation of methane on Cu-based catalysts and the mechanism of C-O bond formation during oxidation and its rapid control steps.The main reaction path was obtained,which was used to modify Cu The catalytic performance of the base catalyst provides theoretical guidance.Second,the experimental study of the catalytic combustion characteristics and stability of low-concentration methane on Cu-based catalysts,and the establishment of two different types of intrinsic kinetic models.Then,the catalytic combustion characteristics of Cu-based catalysts were analyzed using Mn,Fe,La,Ce,and Pd as metal additives,and the Mn-Ce additives were discussed from the perspectives of metal-metal interaction,oxygen species,and grain dispersion.The effect of the agent on the Cu-based catalytic performance.Finally,the tolerance and regeneration performance of Cu-based catalysts for impurity gases(SO₂ and water vapor)were studied,and the root cause of catalyst deactivation under the action of SO₂ and water vapor was clarified.Regulation of catalytic performance.The main research results of this article are as follows:(1)Using density functional theory(DFT)to elucidate the dehydrogenation and oxidation reaction mechanism of methane Cu-based catalytic combustion process,and establish the main reaction path.The first step of dehydrogenation of methane on a Cu-based catalyst to form CH₃-Cu-H has the highest activation energy(E_a=492.15 k J/mol)and the least endothermic energy(E_r=81.99 k J/mol)during the entire reaction process.It is the rate-determining step of the reaction process.Subsequent dehydrogenation to CH-Cu-H requires a higher activation energy,so the reaction is very difficult,mainly at CH₂(s),where CH₂(s)is first oxidized to HCHO,then to CHO,and then to CO(E_a=10.74 k J/mol,E_r=-12.04 k J/mol),the path for the final oxidation of CO to CO₂is:CO(s)+OH(s)?CO₂+H(s),the activation energy of this reaction is low(E_a=90.67 k J/mol),and the heat released is 111.85 k J/mol,the reaction is more likely to occur.The main reaction path of low concentration methane catalyticcombustiononCu-basedcatalystis:CH₄?CH₃?CH₂?HCHO?CHO?CO?CO₂,the rate-determining step is:CH₄+Cu?CH ₃Cu+H.(2)The study obtained the catalytic combustion characteristics and reaction stability of low-concentration methane on Cu-based catalysts,and established an intrinsic reaction kinetic model.The loading condition of Cu is 10 wt.%,The reaction space velocity is 10000 ml/(g·h).The methane conversion rate is the highest,and the catalytic activity of Cu/Al₂O₃ catalyst is the best.The stability of the 10Cu/Al₂O₃catalyst was measured.It was found that the catalytic activity fluctuated greatly at 2-7 h,and the catalytic activity was relatively stable at 7-12 h.The overall stability of the10Cu/Al₂O₃ catalyst was good,and no large-scale deactivation was found.High temperature is beneficial to the stability of the catalyst.This is because the adsorbate covering the surface of the Cu-based catalyst desorbs faster at high temperature,and more active sites on the catalyst surface are exposed to continue the catalytic reaction.From the study of intrinsic dynamics,we can see that the calculation accuracy of the hyperbolic model is high,and the parameters of the power series model are few and easy to calculate.(3)From the aspects of metal-metal/support interaction,oxygen species and dispersion degree,the regulation effect of Mn/Ce additives on the catalytic combustion characteristics of low concentration methane in Cu-based catalysts was revealed.The Mn additive improves the catalytic activity at low temperature(450-600°C).Compared with 10Cu/Al₂O₃ at 500°C,the CH₄ conversion rate of3Mn-10Cu/Al₂O₃ is increased by 7%.This is due to the interaction between Cu-Mn promotes the formation of Cu Mn₂O₄ and the dispersion of Cu,increasing the proportion of surface oxygen Osur.Ce additives improve the catalytic activity at high temperatures(600-700°C).Compared with 10Cu/Al₂O₃ at 700°C,the CH₄ conversion rate of5Ce-10Cu/Al₂O₃ is increased by 8%.This is because Ce reduces Cu²⁺to Cu⁺and increases the proportion of lattice oxygen O_latt,while Ce suppresses Cu Al₂O₄ produced by the Cu-support interaction.Cu-Mn-Ce three-component catalyst has the best catalytic activity.At 600°C,the CH₄ conversion rate of 3Mn-5Ce-10Cu/Al₂O₃increased by 11%.This is because the synergy between Mn-Ce increases the proportion of surface oxygen O_sur and promotes the formation of Mn Ce O_x solid solution.(4)The root cause of the inhibition of Cu-based catalytic activity by the impurity gases SO2 and water vapor was found.The SO2 absorption characteristics and the regeneration performance under the effect of water vapor were regulated by Ni additives and intermittent purging.Both catalytic activity and sulfur poisoning resistance decrease with increasing SO₂ concentration.The main reason for the deactivation of Cu-based catalysts is that SO₂ reacts with Cu O to produce inert copper sulfate Cu SO₄.Ni additives can form Ni-with Cu-based catalysts.The Cu bimetallic site inhibits the reaction of SO₂ and Cu O to regulate sulfur poisoning resistance.The methane conversion rate of the 10Ni-10Cu/Al₂O₃ catalyst is increased by 7%,and the deactivation rate is the smallest.When the SO₂ concentration fluctuates in the range of 0-0.02%,the fluctuation of the methane conversion rate of the Ni-modified Cu-based catalyst does not exceed 4%,which is more resistant to the fluctuation of SO₂ concentration than 10Cu/Al₂O₃.Based on the duration of the intermittent purge,the regeneration performance was adjusted.After 20 minutes of air purge,the methane conversion rate increased from 29%to 31%.After 40 minutes of air purge,the conversion rate increased by about 7%,and 60 minutes of air purge After the catalyst activity gradually recovered.The main reason is that the hydroxyl group is adsorbed and covered on the surface of the catalyst,effectively blocking the contact between the catalyst and methane.The high temperature causes the hydroxyl group to proceed in the direction of desorption,and the surface hydroxyl group coverage is reduced.The catalyst that has been gradually deactivated under the action of water vapor cannot be restored to its original activity after being purged with air.This is due to the fact that water vapor promotes the growth of crystal grains and the sintering of the catalyst surface,as well as the generated Cu₂(OH)CO₃ and Cu(OH)₂ will cause irreversible deactivation.