硫化氢对碳基吸附汞的影响机理研究

应用密度泛函理论B3PW91方法,计算了H2S在碳基固体表面的吸附结构和吸附能,并研究了单质汞在含硫碳基表面的吸附和HgS在碳基表面的吸附.研究结果表明,H2S倾向于解离吸附在碳基表面,属于较强的化学吸附.解离吸附形成的AC—S和AC—SH不直接与单质汞反应,但碳基表面S的存在会增加邻近吸附位的吸附活性,从而增强了碳基对汞的吸附能力,使其更倾向于化学吸附.HgS在碳基表面的吸附形式有解离吸附和非解离吸附,其中平行位解离吸附是主要的吸附形式.     

V6O15团簇钾、钠、钙中毒机理研究

为分析碱金属及碱土金属对催化剂主要活性组分V_6O_(15)分子团簇的影响,应用Gaussian软件结合密度泛函理论的B3LYP方法对V_6O_(15)分子团簇模型进行结构优化,并计算加入金属原子K、Na、Ca后钒基的分子结构参数、吸附能、LUMO轨道能量和加氢反应放热量,得出金属原子对钒基催化剂的作用机理和影响规律。结果表明:催化剂在吸附K、Na、Ca金属原子后催化活性下降;吸附碱金属后,V_6O_(15)团簇氧化性强弱顺序为:未中毒Ca中毒Na中毒K中毒;V_6O_(15)团簇中毒程度的影响顺序为:K中毒Na中毒Ca中毒未中毒。     

燃油锅炉机械未完全燃烧损失q_4的确定

在以往通常的过量空气系数α~"_n.n=1.15条件下运行时,焦炭——烟炱粒子在火焰中心业已燃尽。被烟气携带出来的残余未燃尽碳,实际上可以略而不计。通过锅炉各部烟气通道的烟色是透明的。随着过量空气系数的降低,燃烧过程拖长,焦炭——烟炱粒子被带入温度相对为较冷的区域,该处的温度水平已不足以保证碳粒的完全燃尽。因此,部分未燃尽碳被排烟带走,从而产生了机械未完全燃烧损失。在苏联,首次对燃油炉的机械未完全燃烧损失进行测定是由BTN(全苏热工研究所)作出的。按参考资料,在所研究的过量空气系数范围内(1.02及以上)。q_4<0.01%。按照较近的数据,机械未完全燃烧损失当α>1.03时等于零;当α<1.03时建议采用q_4=0.15。资料[2]中给出了更为详细的q_3、q_4与α关系     

飞灰未燃尽碳对吸附烟气汞影响的试验研究

采用HYDRA AA全自动测汞仪对3个燃煤电厂的飞灰未燃尽碳进行测试,并利用垂直炉试验系统对电厂飞灰吸附烟气汞进行了试验研究.结果表明：不同燃煤电厂飞灰中的未燃尽碳含量不同是由于各电厂不同煤种、不同燃烧工况以及机组的不同参数造成的;同一电厂的飞灰在灼烧后与原灰相比,对烟气汞的吸附效率相差不大;除了飞灰中的未燃尽碳对汞有吸附外,尾矿对汞也有一定的吸附作用;未燃尽碳含量高的飞灰对汞的吸附效率也较高.     

H_2S对Hg在CeO_2表面吸附的影响机理

建立了金属氧化物吸附剂CeO_2(111)表面的结构模型,首先计算了H_2S、HS、S在CeO_2(111)表面吸附的结合能及键长,结果表明H_2S会在CeO_2(111)表面依次分解为HS与单质S;随后计算了Hg在S-CeO_2(111)表面吸附的结合能及键长,结果表明Hg会与CeO_2(111)表面的单质S结合成键形成稳定的HgS,结合能为-52.829 e V,属于强烈的化学吸附,从微观上发现了H_2S的存在对单质汞在CeO_2表面的吸附有促进作用。     

烟气成分NO和HCl影响活性炭纤维吸附汞的量子化学计算

构建四碳环单层石墨结构作为活性炭纤维(ACF)模型,应用密度泛函理论B3LYP方法,在lanl2dz基组水平上研究了烟气成分NO和HCl对ACF表面吸附汞的微观机理.结果表明:吸附NO和HCl后,ACF表面形成稳定的新构型ACF—NO和ACF—Cl;NO和HCl在ACF表面的吸附属于较强化学吸附,吸附能分别为119.48kJ/mol和348.68kJ/mol;NO和HCl被ACF表面的活性区域直接吸附,形成稳定性较强的化学键,并均匀分布在ACF表面,从而使ACF表面活性区域增加,促进了ACF表面对单质汞的吸附.     

烟气中NO分子对碳基吸附单质铅的影响机理

采用量子化学密度泛函理论中B3PW91和B2PLYP方法,研究了NO分子对碳基吸附单质铅的影响.选定五环锯齿形苯环簇为碳基表面模型,在不同活性位点进行结构优化计算,得到2种NO分子在碳基表面的吸附构型、4种单质铅在含NO分子的碳基表面的吸附构型.计算并分析了键长、键级、电荷以及吸附能等重要参数.结果表明:NO分子比单质铅更容易吸附在碳基表面,会与单质铅竞争碳基表面的活性位;NO分子的加入会显著增强碳基表面对单质铅的吸附作用,吸附性能的提升主要缘于NO分子提高了其邻位活性位点的活性,而不是NO分子直接吸附单质铅;键级与吸附能呈现正相关趋势,键级越大,对应构型的吸附能也越大;Pb原子所带电荷量与吸附能呈明显的线性关系,Pb原子所带电荷量越多,对应吸附构型的吸附能越大.     

CuO与负载间相互作用的密度泛函理论

通过密度泛函理论(DFT)模拟了不同负载对Cu基氧载体反应性能和抗烧结性能的影响.首先通过DFT模拟计算得出了CuO纳米团簇在4种不同负载(TiO_2、ZrO_2、CuAl_2O_4和MgAl_2O_4)上的吸附能分别为-2.96eV、-5.14eV、-4.25eV和-5.42eV,其中TiO_2的吸附能最低,不利于氧载体颗粒的抗烧结性,但CuO在ZrO_2、CuAl_2O_4和Mg Al2O4上的高吸附能有助于抑制氧载体的烧结.通过计算不同负载下团簇释氧过程的能量势垒来比较负载对氧载体释氧性能的影响.结果表明,氧气分子从表面的脱附过程是整个释氧过程的速控步骤.不同负载(TiO_2、ZrO_2、CuAl_2O_4和MgAl_2O_4)下CuO纳米团簇总的释氧能量势垒分别为3.45eV、3.33eV、3.28eV和3.41eV,其中负载于CuAl_2O_4的CuO释氧能量势垒最低,反应活性最高.     

钴改性水滑石基LNT催化剂制备及其NO_x吸附-还原和碳烟催化氧化性能

采用共沉淀法和浸渍法制备了贵金属负载的Co改性水滑石基LNT(leanNO_x trap)催化剂,通过多种实验手段表征其理化特性,并研究其NO_x吸附-还原性能及碳烟催化氧化性能.结果表明,Co的引入并不会破坏前驱体水滑石晶体结构,500℃焙烧后原本的水滑石结构被破坏,并出现明显的CoAl_2O_4尖晶石相.Co含量的上升会导致催化剂表面的Co—O吸附位增加,同时催化剂表面吸附的亚硝酸盐物种会逐渐向硝酸盐物种转化.此外,随着Co含量的增加,NO_x脱附峰温度向低温区域移动,脱附峰面积先增加后降低.Pt/BaO/Co_(2.7)Mg_(0.3)AlO催化剂在NO_x吸附-还原反应中对NO_x的净化效率达到89.6%,同时针对碳烟的催化氧化反应又表现出良好的催化性能.     

载硫活性炭脱汞性能及其反应机理研究

以市售商业载硫活性炭为研究对象,在固定床实验台上进行汞吸附实验,分析吸附温度、入口汞浓度对其脱汞性能的影响,通过表征方法对失活前后吸附剂的物理化学性质进行对比,采用程序升温热脱附实验得到样品中吸附的汞形态,并探究其脱汞机理。结果表明：实验样品的适宜脱汞温度为70 ℃,脱汞效率随着汞浓度的升高而降低;活性炭样品失活后比表面积下降,表面含氧官能团减少,非氧化态硫含量降低,其汞吸附形态以HgS、HgO为主;载硫活性炭样品对汞的吸附脱除主要依靠含氧官能团和含硫官能团的化学吸附作用。     

Removal of mercury from flue gases over iron modified activated carbon made by in situ ion exchange method

Ion exchange method was applied to synthesize iron modified activated carbon in the present study. The carbon in volatile compounds of cation-exchange resin could be fixed by ferric ion, resulting in the higher carbon yields of iron contained adsorbents. The optimal temperatures for carbonization and activation of ferric ion-exchanged resin were determined to be 800 °C. The iron modified activated carbon with (Fe/AC-800) and without (Fe/C-800) activation at 800 °C showed different mercury removal mechanisms, and they could be employed to remove mercury from flue gases at reaction temperatures of 180 and 150 °C, respectively. HgO, HgS and HgSO₄ were the mainly mercury compounds generated over spent Fe/C-800, whereas only HgO and HgS were observed over spent Fe/AC-800. The formation of HgO over spent Fe/C-800 and Fe/AC-800 were mainly due to the oxidation of mercury by chemisorbed oxygen and lattice oxygen, respectively. The HgSO₄ was derived from FeS with the aid of oxygen, and the HgS was formed through the reaction between mercury and FeS and/or elemental sulfur.     

Mercury emission and adsorption characteristics of fly ash in PC and CFB boilers

The mercury emission was obtained by measuring the mercury contents in flue gas and solid samples in pulverized coal (PC) and circulating fluidized bed (CFB) utility boilers. The relationship was obtained between the mercury emission and adsorption characteristics of fly ash. The parameters included unburned carbon content, particle size, and pore structure of fly ash. The results showed that the majority of mercury released to the atmosphere with the flue gas in PC boiler, while the mercury was enriched in fly ash and captured by the precipitator in CFB boiler. The coal factor was proposed to characterize the impact of coal property on mercury emissions in this paper. As the coal factor increased, the mercury emission to the atmosphere decreased. It was also found that the mercury content of fly ash in the CFB boiler was ten times higher than that in the PC boiler. As the unburned carbon content increased, the mercury adsorbed increased. The capacity of adsorbing mercury by fly ash was directly related to the particle size. The particle size corresponding to the highest content of mercury, which was about 560 ng/g, appeared in the range from 77.5 to 106 µm. The content of mesoporous (4–6 nm) of the fly ash in the particle size of 77.5–106 µm was the highest, which was beneficial to adsorbing the mercury. The specific surface area played a more significant role than specific pore volume in the mercury adsorption process.     

Effects of calcium chloride and sodium chloride on the flotation removal of unburned carbon from coal fly ash

The effects of calcium chloride and sodium chloride on the coal fly ash flotation were investigated by studying the surface properties of coal fly ash. X-ray photoelectron spectroscopy and scanning electron microscopy showed the presence of many lime particles in the coal fly ash, which, together with Ca(OH)₂(s), could be adsorbed on the rough surface of unburned carbon particles. The flotation results indicated that calcium chloride reduced the performance of unburned carbon removal from the coal fly ash, whereas sodium chloride increased froth stability thereby improving the flotation performance.     

First principles study towards the influence of interstitial nitrogen on the hydrogen storage properties of the Mg2Ni (0 1 0) surface

Doping light elements into Mg-based alloy has been viewed as an effective method for improving the hydrogen storage properties without remarkably reducing hydrogen capacity. The influences of interstitial nitrogen doping on the crystal structure, thermal stability, hydrogen adsorption energy and electronic properties of Mg₂Ni (0 1 0) surface were investigated by first principles calculations. The calculation results showed that the addition of interstitial N results in an anisotropic expansion in the crystal structure and a better improvement effect on lowering thermal stability of the Mg₂Ni surface than the commonly used transition metal. Three stable sites including the NiNi bridge site, the top sites of Mg and Ni atoms, were determined to take in hydrogen in the pure surface. When the nonmetal N is doped into the pure surface, the number of the stable adsorption sites is increased and the adsorption energy of H in the NiNi bridge site is also increased from ?0.9614 eV for the pure to ?0.5441 eV for the N-doped counterpart. The increases in both the stable adsorption sites and the energy caused by the addition of N indicate that more hydrogen could be adsorbed in the weaker NiH bonds of the N-doped Mg₂Ni alloy, thereby improving the hydrogen storage behaviors of Mg-based alloy.     

Complex interaction of hydrogen with the monolayer TiS2 decorated with Li and Li2O clusters: an ab initio random structure searching approach

We have applied ab initio random structure searching to study the structure, stability and hydrogen storage properties of monolayer TiS₂ coated with Li and small Li₂O clusters. For the low Li covered system we found a complex adsorption mechanism: some hydrogen molecules were adsorbed due to polarization with Li, others due to polarization with S near the surface of TiS₂. The peculiarities of the interaction of the H₂ molecules with each other and the preferred adsorption sites allowed us to formulate a series of recommendations that can be useful when selecting the material for the most effective support. Moreover, the findings also show that the storage capacity of this system can reach up to 9.63 wt%, presenting a good potential as hydrogen storage material. As for the Li₂O clusters supported on TiS₂, we found that the polarization of the Li–O bond increases upon the adsorption of the Li₂O nanocluster. Moreover, the polarized Li–S bonds appear in addition to the already existing Li–O bonds. All this is possible due to the extraction of 1.46 electrons from the S atom of the substrate by O atom of the cluster, and should contribute to an increase in both the adsorption energy and the maximum capacity. The adsorption energies of H₂ for the systems studied here are within 0.11–0.16 eV/H₂ which is a recommended range for reversible hydrogen physisorption under standard test conditions. This study may stimulate experimental efforts to check the claims of high-capacity, stable and reversible hydrogen adsorption reported here.     

Improving hydrogen storage in modified carbon materials

The hydrogen adsorption capacity of different types of carbon nanofibers (Platelet, Fishbone and Ribbon) and amorphous carbon has been measured as a function of pressure and temperature. Results have showed as the more graphitic/ordered carbon materials adsorbed less hydrogen than the more amorphous ones. After that and, with the aim of improve the hydrogen adsorption capacity of these carbon materials, they were functionalizated (oxygen surface groups incorporation) and Ni-modificated. Results also showed an important increase of the H₂ adsorption capacity despite the porosity loss that took place after the treatments. Due to the advantages of functionalization and Ni-modification, both treatments were applied at the same time over the most promising carbon materials from the H₂ adsorption point of view, observing again an improvement of the hydrogen adsorption capacity. Finally, the H₂ adsorption capacity of chemically activated carbon materials increased considerably due the pore structure development and even more if activated materials were Ni-modificated.     

A comparative study of the electrochemical hydrogen storage properties of activated carbon and well-aligned carbon nanotubes mixed with copper

We studied the electrochemical hydrogen storage properties of activated carbon (AC) material mixed with copper. The discharge capacity of AC–Cu electrode which reached 510 mAh/g after 384 cycles, is much higher than that of the CNT–Cu electrodes. The plateau of discharge potential for AC–Cu electrode was very long and flat and reached −0.88 V vs. Hg/HgO, which was far from the potential of copper oxidation. The discharge plateau gradually appeared and continually lengthened with the increase of cycle number. Cyclic voltammetric experiments showed that the adsorption and desorption of hydrogen occurred on the surface of activated carbon and the active site increased with the increase of cycle number. The mechanism for electrochemical storage of hydrogen in AC–Cu electrode may be mainly physisorption.     

A reversible hydrogen storage material of Li-decorated two-dimensional (2D) C4N monolayer: First principles calculations

Two-dimensional (2D) materials can be regarded as potential hydrogen storage candidates because of their splendid chemical stability and high specific surface area. Recently, a new dumbbell-like carbon nitride (C₄N) monolayer with orbital hybridization of sp³ is reported. Motivated from the above exploration, the hydrogen adsorption properties of Li-decorated C₄N monolayer are comprehensively investigated via first principles calculations based on the density functional theory (DFT). It is found that the Dirac points and Dirac cones exists in the Brillouin zone (BZ) from the calculated electronic structure and indicates the C₄N can be used as an excellent topological material. Also, the calculated phonon spectra demonstrate that the C₄N monolayer owns a strong stability. Moreover, the calculated binding energy of decorated Li atom is bigger than its cohesive energy and results in Li atoms disperse over the surface of C₄N monolayer uniformly without clustering. In addition, the Li₈C₄N complex can capture up to 24H₂ molecules with an optimal hydrogen adsorption energy of −0.281 eV/H₂ and achieves the hydrogen storage density of 8.0 wt%. The ab initio molecular dynamics (AIMD) simulations suggest that the H₂ molecules can be desorbed quickly at 300 K. This study reveals that Li-decorated C₄N monolayer can be served as a promising hydrogen storage material.     

Bonding in PdH2 and Pd2H2 systems adsorbed on carbon nanotubes: Implications for hydrogen storage

This work presents a bonding study of hydrogen adsorption processes on palladium decorated carbon nanotubes by using the density functional theory (DFT). First, we considered simple decoration models involving single palladium atoms or palladium dimers, and then we analyzed the adsorption of several molecular and dissociated hydrogen coordination structures, including Kubas-type complexes. In all cases we computed the energy, bonding and electronic structure for the different nanotube-supported hydrogen–palladium systems. Our results show that Pd(H₂) and Pd₂(H₂) complexes with relaxed but not dissociated H–H bonds are the most stable adsorbed systems. The role of s, p and d orbitals on the bonding mechanism for all adsorbates and substrates was also addressed. We found intermolecular donor–acceptor C–Pd and Pd–H delocalizations after adsorption. We also studied the palladium clustering effect on the hydrogen uptake based on Kubas-type bonding.