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.     

火焰合成Pt修饰CuO氧载体低温化学链燃烧特性

采用火焰喷雾热解方法(FSP)合成了Pt修饰CuO纳米氧载体材料(FSP-Pt/Cu O),首先通过XRD、SEM等表征手段分析了Pt/CuO的结构和组成;通过热重分析仪、化学吸附仪对比研究了FSP合成的CuO、Pt/CuO以及商用CuO纳米颗粒的化学链燃烧反应性能．结果表明,FSP-Pt/CuO氧载体材料与H₂、CO、CH₄的还原反应温度能够分别降低到200℃、105℃、290℃以下．进一步考察了H₂、O₂不同浓度对Pt/CuO氧载体还原、氧化特性的影响,并测试了Pt/CuO氧载体的低温化学链燃烧循环稳定性．最后通过Pt/CuO氧载体颗粒表面的氢溢流原理对低温化学链燃烧过程进行了模型机理解释．     

铜基氧载体化学链氧解耦的流化床实验研究

采用溶胶-凝胶法制备了CuO/CuAl2O4氧载体,在CO2气氛下和空气气氛下,分别研究了该氧载体的释氧和吸氧性能,研究结果表明,随着温度的升高,氧载体的释氧、吸氧速率不断升高.随后在N2气氛下研究了3种典型煤的化学链氧解耦燃烧过程,结果表明,煤中挥发分的含量直接影响燃烧过程的快慢,在氧解耦燃烧时,高挥发分的褐煤尾气中,CO2体积分数比无烟煤高15%左右,褐煤中碳的平均转化率为无烟煤的2～3倍.     

化学链中Cu、Fe基复合氧载体各组分间相互作用的研究进展

化学链燃烧是一种新型的燃烧方式,既可以避免氮氧化物的产生,又可以实现二氧化碳的内分离,近年来受到了国内外研究人员的广泛关注.氧载体的反应性、循环稳定性以及载氧能力等性能,是化学链燃烧系统的重要影响因素.添加惰性载体和制备多活性组分复合氧载体都是可以有效提高氧载体性能的主要方式.为了解决氧载体中各组分之间反应降低氧载体性能的问题,本文就近年来国内外有关Cu基和Fe基复合氧载体的研究进行了总结和分析,发现TiO2和SiO2会与Fe2 O3形成尖晶石,不适合做Fe2 O3的载体;添加活性组分虽然可以使氧载体释氧速率和循环反应性增强,但也会和Fe2 O3或者CuO反应产生复杂物相,使氧载体反应活性下降或者烧结.在此基础上针对活性组分与惰性组分以及活性组分与活性组分之间的相互作用两个方面进行了综述和展望.     

Cu-Mn复合载氧体释氧反应性及动力学研究

载氧体作为化学链转换技术的核心与关键,其性能决定了反应过程的经济性和稳定性。采用石墨成孔浸渍法制备了4种不同含量Cu修饰的Cu-Mn复合载氧体,对其进行了表征和抗磨损性能实验。在热重分析仪上研究了载氧体的释氧特性和循环反应特性,并采用Model-free method分析了载氧体的释氧动力学特性。结果表明,载氧体制备过程中活性组分CuO和Mn2O3并未与惰性载体MgO发生化学反应。Cu修饰对载氧体具有拓孔作用,可以显著提高载氧体比表面积和总释氧量,降低释氧所需温度,缩短释氧时间,降低释氧活化能。但随着Cu含量的增加,载氧体抗烧结性能和抗磨损性能逐渐降低,且随着循环次数增加,载氧体循环反应性能降低。     

核壳结构铜基氧载体的化学链制氧

针对化学链制氧中使用的氧载体,采用自组装模板燃烧法制备了一种具有核壳结构的高性能铜基氧载体.该氧载体颗粒具有一种微观的分层结构,其中活性成分铜均匀地负载在纳米结构的核(Al2O3)-壳(Ti O_2)上,由于二氧化钛层形成的壳阻止了氧化铝和氧化铜的接触,从而避免了CuAl_2O_4的生成,减少了活性成分损失.利用热重分析仪研究了反应温度对氧载体吸氧、释氧的影响,并在小型流化床上探究了流化气流量对释氧速率的影响.结果表明,温度越高,释氧过程越快,吸氧过程越慢.研究了释氧过程中,流化气流量存在一个最优值使释氧速率达到最大值,同时出口气体中氧气浓度较高.十次循环制氧实验结果表明,该氧载体具有很高的载氧率和稳定性,适用于化学链制氧.     

无烟燃烧技术CuO基氧载体的性能分析

循环热载体无烟燃烧技术不会向大气中排放有害气体,是一种清洁燃烧技术。研究了以CuO基氧载体的无烟燃烧反应体系,对燃烧过程反应的热力学参数进行了计算,并在一套热重反应装置上研究了氧载体在甲烷、空气气氛中的循环反应性能。实验表明控制一定的反应条件,CuO基氧载体可用于无烟煤燃烧技术,当氧化铜暴露在甲烷气氛中时,氧化铜失去部分晶格氧,当切换成空气时失去部分晶格氧的氧载体又可以恢复其晶格氧。当CuO基氧载体中添加SiO2黏合剂时,氧载体表现出良好的循环性能和抗破碎能力,完成循环反应的时间也有较大的减少。     

高硫煤-CuO化学链燃烧特性及硫演化热力学研究

以贵州六枝高硫煤为研究对象,通过热重实验对该典型高硫煤和CuO氧载体的化学链燃烧特性及其与空气直接接触时的燃烧特性加以定性分析和定量评估。为探索煤中硫的演化及其与CuO氧载体作用对CuO氧化活性的影响,进一步采用HSC Chemistry软件,分析了氧载体过量系数Φ对煤中硫分布的影响。研究表明:相比于空气下煤直接燃烧,CuO与六枝煤化学链燃烧的着火温度高,可燃指数和综合燃烧特性指数小,显示CuO氧载体更低的反应活性;在热力学平衡状态下,当0Φ≤1.25及Φ1.25时,系统中硫组分分别以固相Cu_2S和气相SO_2形式存在。基于上述研究结果,提出了一种煤化学链燃烧系统中同时进行脱硫和CO_2捕集的新方案。     

化学链燃烧中基于密度泛函理论的铁基载氧体研究进展

化学链燃烧技术中,铁基载氧体由于成本低、环境良好、热稳定性高和机械性能优良等优点,被认为是最有前景的载氧体之一。但其反应活性相对较低,提高其反应活性成为了研究的重点。在阅读50余篇相关文章的基础上,对密度泛函理论用于铁基载氧体微观反应机理研究进行了3方面的综述:(1)铁基载氧体表面的电子结构特性及其与燃料分子(CO、H_2、CH_4和煤等)的氧化还原反应和形成积碳的机理;(2)Al_2O_3、MgO、TiO_2和ZrO_2等惰性载体以及Co和Pb等掺杂组分对Fe_2O_3反应性能的协同作用机理;(3)化学链燃烧过程中,S和Hg等杂质对铁基载氧体反应性能的影响。据此指出:密度泛函理论在煤和生物质等固体燃料化学链燃烧的研究中应用较少,以煤为主的固体燃料化学链燃烧中固-固反应机理以及灰分与Fe_2O_3的相互作用机制尚不清楚。此外,多组分铁基载氧体的分子结构设计及性能调控等方面有待进行深入研究。     

煤基化学链燃烧技术的NiO/NiAl_2O_4氧载体研究

直接以煤为燃料的化学链燃烧技术首先需要解决的关键问题是高性能氧载体.通过溶胶-凝胶法制备了几种不同NiO含量、不同烧结温度和不同煅烧时间的NiO/NiAl2O4氧载体,并对其物化性质进行了表征.实验结果表明,超过850℃时NiO/NiAl2O4与神府煤焦的还原反应快速进行,而60%(质量分数)NiO含量、1 300 ℃烧结6 h的NiO/NiAl2O4氧载体具有更好的还原反应性;在与煤焦/空气的单循环还原/氧化反应中,NiO/NiAl2O4表现出良好的循环反应性.实验结果证明基于NiO/NiAl2O4氧载体、燃用固体燃料煤焦的化学链燃烧技术是可行的.     

Comparison of metallic oxide,natural ore and synthetic oxygen carrier in chemical looping combustion process

As one of clean coal combustion ways, chemical looping combustion (CLC) showed high CO₂ capture efficiency with lower energy penalty. But these processes were limited by the low reaction rate between oxygen carriers (OCs) with coal char. This study evaluated the performances of Cu-based OCs with coal in in-situ gasification chemical-looping combustion (iG-CLC) and chemical-looping with oxygen uncoupling (CLOU) process. CuO modified by iron ore and chrysotile were employed as OCs which the addition of chrysolite improved the char gasification and iron ore enhanced the stability of CuO at high temperature. Results showed that CuO supported by ores (chrysolite and iron ore) had better H₂ and CO conversion under H₂O atmosphere than CuO and iron ore. Chrysolite decorated CuO can convert almost all H₂ to H₂O at 850 °C. Synthetic OCs showed better stability and high temperature tolerance during 10 redox cycles.     

Mechanism for adsorption,dissociation and diffusion of hydrogen in hydrogen permeation barrier of α-Al2O3: A density functional theory study

Toward understanding physical interaction of hydrogen isotopes with α-Al₂O₃ barrier, adsorption, dissociation and diffusion of hydrogen in α-Al₂O₃(0001) slab have been investigated by density functional theory (DFT) and rate theory. H₂ molecule, with parallel configuration, preferentially absorbs on a top Al atom site of first atomic layer on α-Al₂O₃(0001) surface, while H atom strongly bonds at a top O atom site of the second atomic layer, H atoms recombine into molecules on top Al atom sites of the third atomic layer. The barrier for H₂ exothermic dissociation on surface is 0.79 eV. The potential energy pathways of H diffusion in α-Al₂O₃ are studied, predicting that H atom diffusion preferentially occurs via surface path rather than bulk path involving elementary reorientation and hopping steps. The surface-to-subsurface diffusion is significantly endothermic except for the surface and subsurface-to-bulk path. Mechanism, in well agreement with experimental result, of α-Al₂O₃ resisting hydrogen permeation has proposed.     

The effect of Ti atom on hydrogenation of Al(111) surface: First-principles studies

First-principles calculations were performed to investigate hydrogen dissociation and subsequent diffusion over both clean and Ti-doped Al(111) surfaces. The calculations show that it is energetically favorable to dope the surface or subsurface layer of Al(111) with Ti atom. Through calculations on the detailed process associated with hydrogen dissociation and diffusion, we found that Ti doping will decrease the hydrogen dissociation barrier by about 0.6 eV. Additionally, the mobility of hydrogen atoms on surface will be easier if Ti atom is placed in subsurface layer instead of top surface layer. The present results further contribute towards understanding the improved kinetics observed in recycling of hydrogen in Ti-doped NaAlH₄.     

Identification of active sites in CO2 activation on MgO supported Ni cluster

In this work, initial activation mechanism of CO₂ over MgO supported Ni catalysts has been systematically studied through the periodic DFT calculations. In addition, the role of metal cluster, interface and support for CO₂ activation is investigated and the active site is identified. CO₂ is most favored to be activated on the interface instead of neither Ni cluster nor MgO support. The effective energy for this process is around 0.67 eV, and the dissociation of CO₂ (0.62 eV) is the rate-determining step, since it requires much higher energy than that of the CO₂ adsorption process (0.05 eV). Thus, the interface between metal cluster and support plays a key role for C=O bond activation. Moreover, CO1 is preferred to be adsorbed on the Ni cluster, while the O1 is likely to bind on Mg atom of support. To illustrate the adsorption behavior of CO₂ at different sites, the Mulliken atomic charge and electron density difference have been calculated. It was found that the total amount of electron gain for CO₂ binding at different sites follows the order of Interface (−0.03 e) < MgO support (−0.05 e) < Ni cluster (−0.07 e), and effective energy barrier rises linearly with the increase of electron gain of CO₂ binding at different sites. In addition, electron gain of oxygen atom O1 and oxygen atom O2 of CO₂ is the same for Ni cluster and MgO support, however, the electron gain of O1 and O2 is different for Interface. The difference of electron gain for two oxygen atoms shows the electron unbalance of CO₂ molecule, which is in favor of C=O activation. This study could shed some light on understanding the active sites of CO₂ thermal-catalytic activation over MgO supported Ni catalysts, and is helpful to elucidate the reaction on an atomic level.     

Essential effect of proton coupled electron sequent transfer on photocatalytic water complete dissociation: A DFT study

Photocatalytic water splitting for hydrogen energy is one of the most promising ways to solve the energy crises. The mechanism is unclear on the sequence of proton coupled electron transfer (PCET) in photocatalytic water dissociation catalyzed by organic material. Here, the water splitting catalyzed by zinc porphyrin with boron dipyrrin (ZnPP-BDP) is systematically investigated by density functional theory (DFT) calculations. The H₂O/ZnPP in valence state based on the electron transfer pulls the trigger on the water splitting process. For the oxygen evolution reaction (OER) step on ZnPP, the processes for the two H ions dissociate from water are exothermic with the −1.06 and −0.95 eV energies respectively, and the O/ZnPP system is formed. The second H₂O molecule on O/ZnPP system can provide the extra oxygen atom to produce the free O₂ with 1.22 eV energy barrier. For the hydrogen evolution reaction (HER) step on BDP, the H ions dissociated from OER process can be captured by BDP to form free H₂ with 2.24 eV/molecule energy released. In this attractive clean cyclic process, the ZnPP-BDP can continuously catalyze the H₂O dissociated into free H₂ and O₂ by the shifting potential barrier. Besides, the proton coupled electron sequent transfer possess an essential role in photocatalytic water dissociation. It is very instructive to design sustainable photodecomposition catalysts for both OER and HER, and to design extraordinary biomimetic photosynthesizers for clean energy.     

Spontaneous dissociation of H2 and high energy barrier of H invasion on W doped Al2O3(0001) surface

In view of the wide use of tungsten in fusion experimental devices and the importance of hydrogen isotopes permeation, here we studied the adsorption, dissociation, diffusion and invasion behavior of hydrogen on W doped α-Al₂O₃ (0001) surface. Based on the first-principle approaches, we found the W substitution for a top surface Al atom is the most energetically favorable. H₂ molecule prefers to be adsorbed on the surface W and spontaneously dissociates into two H anions. Near the W defects, H atoms favor to be adsorbed at the W and Al sites rather than O sites on the surface, and within the subsurface layer H can only bond to W stably. As a result, H migration to subsurface should occur around W with an energy barrier as large as 4.22 eV which is much larger than the 1.91 eV around the O atom on undoped α-Al₂O₃ (0001) surface. These findings suggest that W surface doping is beneficial to α-Al₂O₃ as tritium permeation barrier.     

Storing-hydrogen processes on graphene activated by atomic-vacancies

We investigated the minimum energy pathways and energy barriers of reversible reaction (V₁₁₁ + H₂?V₂₂₁) based upon calculations using density functional theory. We find a comparable activation barrier of around 1.3 eV for both the dissociative chemisorption and desorption processes. The charge transfer rate from a reacting hydrogen atom to the graphene is around 0.18 e per hydrogen atom in the final state. A subsequent reaction path to recover the initial structure of V₁₁₁ is realized by the migration of hydrogen atoms from V₂₂₁ onto the graphene surface. The comparable energy barrier of 1.3 eV for both adsorption and desorption suggests that this novel storage and release concept has the potential to act as a hydrogen storage system for certain applications.     

Theoretical study of acetate decomposition on Au(111) surface: Oxygen-assisted γ-CH activation mechanism

Inspired by the recent experimental results that the oxygen atoms adsorbed on Au(111) surface have a great influence on the mechanism and path of the decomposition of acetate(ACS Catal, 2014, 4(9): 3281?3288), the density functional theory was performed to simulate the decomposition of acetate on Au(111) surface with and without oxygen atom. The present calculation resents show that the pre-adsorbed oxygen atoms on Au(111) surface can activate the γ-CH bond of acetate and reduce the activation energy of the reaction, then finally get the product of carbon dioxide and formaldehyde. While without adsorbed oxygen atoms, acetate on Au(111) surface breaks down into carbon dioxide and methyl through C-C bond cleavage. In addition, the decomposition of acetate on Ag(111) surface with pre-adsorbed oxygen atoms has also been simulated and that of Au(111), and it was found that the oxygen atoms on Ag(111) assist γ-CH bond activation more efficiently due to its more negatively charged. The present study highlights the importance of oxygen-assisted γ-CH bond activation for oxygen-containing molecular on Au(111) and provide a new route for the synthesis of ester.