不同粒径的Ni/SiO2催化剂上CH4和CO2吸附活化的漫反射傅里叶变换红外光谱研究

 采用原位漫反射傅里叶变换红外光谱研究了CH4和CO2在不同粒径的Ni/SiO2催化剂上的吸附及活化. 结果表明,在不同粒径的催化剂上,检测到有CH4解离生成的CHx(x=1～3)物种,以及催化剂表面吸附的CHx物种与表面-OH 作用生成的CHx-O物种. CH4的裂解强烈依赖于催化剂表面Ni颗粒的大小,在粒径8 nm左右的Ni颗粒上, CH4较易解离; CO2难以直接在Ni/SiO2催化剂表面发生解离吸附,但CH4解离生成的吸附H对CO2的解离吸附具有明显的促进作用; CH4与CO2共吸附时,较小粒径的Ni可以促进CO2与表面氧物种发生反应,生成单齿表面碳酸盐物种.     

Pd掺杂的Ni催化剂表面CH4解离吸附的理论研究

使用密度泛函理论研究了Pd掺杂的Ni(111),Ni(100)和Ni(211)表面最稳定的结构,同时考察了干净的和Pd掺杂的Ni表面催化CH₄解离反应的活性.结果表明,由Pd原子取代最外层Ni原子而形成的表面Pd掺杂的Ni表面在热力学上最为稳定,亚表面Pd掺杂的Ni表面在热力学上都不稳定; 而对于表面Pd吸附的Ni表面,只有Pd/Ni(211)表面是稳定的.表面掺杂的Pd/Ni表面上CH₄解离中间体(CH₄,CH₃,CH,C,H)吸附能的计算结果表明,Pd的掺杂在不同程度上减弱了除CH₄之外各解离中间体的吸附能.另外,CH₄和CH均优先在Ni(211)和Pd/Ni(211)台阶面上解离,其次是在比较开阔的Ni(100)和Pd/Ni(100)表面上.Pd的掺杂不同程度上提高了CH₄和CH解离的能垒,对于活性最高的Ni(211)面,Pd的掺杂使得CH脱氢的能垒较CH₄脱氢的高,改变了其速率控制步骤,从而抑制了积碳的生成.     

流化床中 CH4/C3H8 自热重整制合成气

 在流化床反应器中, 考察了 Ni/SiO2 催化剂上 CH4 或 CH4-C3H8 临氧 CO2 重整 (自热重整) 制合成气反应性能. 结果表明, 在 CH4-C3H8 混合气自热重整反应中, Ni 粒径较小催化剂的活性和抗积炭性能较高, CH4 和 CO2 转化率分别达 75.5% 和 72.6%. C3H8 比 CH4 更易解离及被氧化, 部分 C3H8 解离出来的中间产物 CHx 物种可与吸附 H 结合为 CH4, 因而降低了 CH4 的表观转化率; CHx 也可与吸附的 CO2 物种反应生成 H2 与 CO, 从而促进了 CO2 的转化.     

助剂La2O3和载体对镍基催化剂上甲烷解离活化动力学参数的影响

 通过非稳态脉冲动力学方法,考察了载体MgAl2O4和α-Al2O3以及助剂La2O3对Ni催化剂催化解离CH4性能的影响. 在以前的积炭方法中, CH4的解离在很大程度上受积炭的影响. 这里采用非稳态脉冲方法可以得到在新鲜的Ni活性位上CH4解离的本征动力学信息. 在排除了传质和传热的影响之后考察了在Ni/α-Al2O3, Ni/La2O3/α-Al2O3, Ni/MgAl2O4和Ni/La2O3/MgAl2O4催化剂上的CH4解离动力学,求得催化剂表面上平均每个Ni活性位上CH4解离的活化能分别为90.9, 111.8, 79.5和85.9 kJ/mol. 可以看出,载体MgAl2O4比α-Al2O3更能降低Ni上CH4解离的活化能,而La2O3的加入会提高CH4在Ni上的解离活化能. 通过动力学方法,定量地描述了载体和助剂对Ni活性位上CH4解离能力的影响. 同时CH4解离的活化能和指前因子之间存在着补偿效应,这在一定程度上削弱了助剂和载体对Ni上CH4解离的影响.     

Ni/La2O3/α-Al2O3中的高温脱附氢促进CH4/CO2重整反应的初活性

 通过H2程序升温脱附实验,在H2还原的Ni/La2O3/α-Al2O3催化剂上可以明显观察到高温脱附氢(高温氢). 动力学实验结果表明,随催化剂上高温氢含量的增加, CH4/CO2重整反应的初始活性升高,同时高温氢也可在重整反应过程中原位生成,并使重整反应的活性最终达到稳定. 脉冲实验结果表明,随催化剂上高温氢含量的增加, CH4解离后生成的活性中间体CHx物种的x值也增大,进而降低了CHx与CO2反应的活化能,提高了CHx与CO2反应的速率. La2O3助剂的添加提高了Ni/La2O3/α-Al2O3催化剂上逆水气变换反应的速率,并且对CO2的活化也有促进作用. La2O3助剂的加入对于CH4/CO2重整反应的重要作用是使高温氢的数量增多且稳定性提高,有利于保持CHx物种中较高的x值,促进重整反应.     

Co／Pd催化剂上CH4／CO2等温两步反应直接合成乙酸的热力学

采用量子化学密度泛函理论对CH4/CO2两步法合成乙酸反应中表面碳化物CHx (x=0～3)在Co和Pd模型表面上不同吸附活性位上的吸附能、空间构型和反应吉布斯自由能进行了系统性的比较研究. 计算结果表明, CH4/CO2两步反应在单一金属Co或Pd催化剂上在常压等温条件下不能有效进行,但在Co和Pd组成的双金属催化剂上,两步反应在常压等温下可以进行. 在Co和Pd双金属催化剂上,金属Co活化CH4生成金属碳化物CHxCo(x=0, 1)为热力学允许反应,其后CHx溢流到金属Pd上形成CHyPd (y=1～3)碳化物,最后CO2插入CHyPd生成乙酸,后两者在常压等温情况下也为热力学允许反应,并且在435 K以上可以与前者构成等温循环. 计算结果与实验结果吻合.     

A DFT theoretical study of CH_4 dissociation on gold-alloyed Ni(111)surface

A density-functional theory(DFT)method has been conducted to systematically investigate the adsorption of CHx(x=0～4)as well as the dissociation of CHx(x=1～4)on(111)facets of gold-alloyed Ni surface.The results have been compared with those obtained on pure Ni(111)surface.It shows that the adsorption energies of CHx(x=1～3)are lower,and the reaction barriers of CH4 dissociation are higher in the first and the fourth steps on gold-alloyed Ni(111)compared with those on pure Ni(111).In particular,the rate-determining step for CH4 dissociation is considered as the first step of dehydrogenation on gold-alloyed Ni(111),while it is the fourth step of dehydrogenation on pure Ni(111).Furthermore,the activation barrier in rate-determining step is higher by 0.41 eV on gold-alloyed Ni(111)than that on pure Ni(111).From above results,it can be concluded that carbon is not easy to form on gold-alloyed Ni(111)compared with that on pure Ni(111).     

Rh／SiO2催化剂上甲烷部分氧化制合成气反应

 利用程序升温脱附、程序升温还原、程序升温表面反应、程序升温反应和化学捕获反应等手段，对Rh／SiO2催化剂上甲烷部分氧化制合成气反应进行了研究．结果表明，Rh／SiO2催化剂上甲烷部分氧化制合成气机理属于热解－氧化反应机理．甲烷首先在催化剂上发生解离吸附，产生具有不同H／C比的化学吸附物种CHx（x＝1～3）．其中，具有较高H／C比的CHx可能是甲烷部分氧化反应的活性物种，而具有较低H／C比的CHx可能是催化剂上积碳并导致催化剂失活的来源．活性物种CHx在活性氧物种的作用下，生成含氧中间体物种CHxO或继续脱氢．含氧中间体物种进一步分解，即生成CO和H2；CO2也可由CHx或CHxO物种进一步氧 化生成．     

乙烷在Ni(111)表面的吸附和分解

研究了乙烷在Ni(111)表面解离的可能反应机理, 使用完全线性同步和二次同步变换(complete LST/QST)方法确定解离反应的过渡态. 采用基于第一性原理的密度泛函理论与周期平板模型相结合的方法, 优化了C2H6裂解反应过程中各物种在Ni(111)表面的top, fcc, hcp和bridge位的吸附模型, 计算了能量, 并对布居电荷进行分析, 得到了各物种的有利吸附位. 结果表明, 乙烷在Ni(111)表面C—C解离的速控步骤活化能为257.9 kJ·mol-1, 而C—H解离速控步骤活化能为159.8 kJ·mol-1, 故C—H键解离过程占优势, 主要产物是C2H4和H2.     

甲烷催化部分氧化制合成气的反应机理

借助脉冲反应、质谱-程序升温表面反应（MS-TPSR）等技术研究了Ni／α-Al2O3催化剂上甲烷催化部分氧化制合成气（POM）的反应机理．结果表明,NiO上CH4不能解离产生H2只有当NiO被CH4还原为Ni0后,CH4才能解高产生H2,Ni0是CH4活化和POM反应的活性相;POM反应机理遵循直接氧化机理,CH4和O2均在Ni0上活化,活化过程形成的Ni…C和Niδ…Oδ物种是反应历程中的关键物种,Niδ …Oδ物种高选择性地与CH4解离产生的碳物种Ni…C反应生成CO．     

氢在Ni(511)台阶面的吸附与解离研究

The adsorption and dissociation of hydrogen on stepped surface (511) of nickel are studied with the embedded-atom model (EAM) method. The adsorption energy, the length of the adsorption bond and the adsorption height for a single hydrogen atom are calculated. Three kinds of stable sites are found for hydrogen adsorption. There are the double-fold bridge site B on the step edge, the three-fold hollow site H3′ on the step surface and the four-fold hollow sites H1 and H2 on the terrace surface. Compared with a hydrogen atom adsorbed on low-index (001) surface, there are two other adsorption sites near the step: the two-fold bridge site B on the step edge and the three-fold hollow site H3′ on the step surface. At the same time, the absorbability of the hydrogen atom at the site H1 is intensified. The results show that hydrogen adsorption on Ni (511) is affected by the existence of the step. The active barriers, adsorption energy and corresponding bond length for dissociation of a hydrogen molecule on the stepped surface are presented. The results show that the dissociation is easier at the bottom of the step. It is shown that the steps are the active sites for hydrogen adsorption and dissociation.     

First-principles study of C adsorption, O adsorption, and CO dissociation on flat and stepped Ni surfaces

The adsorption of atomic oxygen and carbon was studied with plane wave density functional theory on four Ni surfaces, Ni(110), Ni(111), Ni(210), and Ni(531). Various adsorption sites on these surfaces are examined in order to identify the most favorable adsorption site for each atomic species. The dependence of surface bonding on adsorbate coverage is also investigated. Adsorption energies and structural information are obtained and compared with existing experimental results for Ni(110) and Ni(111). In addition, activation barriers to CO dissociation have been determined on Ni(111) and Ni(531) by locating the transition states for these processes. Our results indicate that the binding energies of C are comparatively stronger on stepped surfaces than on flat surfaces, and the energy barriers associated with CO dissociation strongly favor reactions occurring near surface steps.     

合成环氧乙烷新途径的从头算研究

在MP4/6-31G*//RHF/6-31G*理论水平上，对单态甲撑插入甲醛中碳氧双链生成环氧乙烷的反应(CH2_(~1A)＋CH_2O→C_2H_4O)进行了从头算研究。发现其反应历程由两步组成：1) 反应物沿反应坐标接近，体系能量单调下降，生成平而松散环状的分子复合物(MC)；2) 分子复合物沿反应坐标经由一过渡态(TS)重排为产物环氧乙烷，此步的势垒只有36.99 kJ·mol~(-1)。进而计算了该反应的热力学函数和动力学性质，并进行了讨论。当设法将甲醛引入产生活性中间体CH_2(~1A)的体系中，该反应有可能成为在室温下制备环氧乙烷的非催化途径。     

甲醇在Ru(0001)表面吸附的密度泛函研究

采用密度泛函理论(DFT)的B3LYP方法,以原子簇Ru15为模拟表面,对甲醇在理想的Ru(0001)面三种吸附位置(top,fcc,hcp)的吸附模型进行了几何构型优化,能量计算,Mu lliken布局分析以及振动频率计算,结果表明顶位为最有利的吸附位.这些变化与实验观察到的甲醇在过渡金属表面解离的结果相一致.同时通过对吸附过程的分析推测其可能的解离途径.     

甲胺在清洁及磷改性Mo(100)表面的解离

采用广义梯度近似(GGA)的密度泛函理论(DFT)(DFT-GGA)并结合平板模型, 研究了甲胺在清洁及磷(P)改性的Mo(100)表面(P-Mo(100))发生C—N键断裂的反应历程(CH3NH2→CH3+NH2). 优化了裂解过程中反应物、过渡态和产物的几何构型, 获得了反应路径上各物种的吸附能及反应的活化能数据. 计算结果表明, 在清洁和磷改性的Mo(100)表面, 甲胺均稳定吸附在顶位, 甲基和氨基最稳定的吸附位置均为桥位. 甲胺的C—N键在P-Mo(100)表面裂解的活化能为2.39 eV, 高于其在清洁表面的活化能(1.99 eV). 这表明Mo(100)表面被预吸附的P原子钝化了. 电子结构分析表明, 改性P原子使得金属Mo的供电子能力减弱, 导致它的d带中心下移, 从而降低了该表面的反应活性, 提高了甲胺的C—N键裂解的活化能. 活化能的分解表明, C—N键在P-Mo(100)与Mo(100)表面裂解的活化能的差异主要体现在初态到过渡态时甲胺的结构变化引起的能量变化(△EdefCH3NH2)、过渡态仅有甲基存在时的吸附能(ETSCH3)和过渡态甲基和氨基的相互作用(EintCH3…NH2). △EdefCH3NH2和ETSCH3使活化能升高幅度大于EintCH3…NH2使活化能降低幅度, 最终导致甲胺的C—N键在P-Mo(100)表面裂解的活化能要高于在Mo(100)表面裂解的活化能.     

CO2 reforming of CH4 on Ni(111): a density functional theory calculation

CO(2) reforming of CH(4) on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH(4) sequential dissociation into surface CH (CH(4) --> CH(3) --> CH(2) --> CH) and hydrogen, and CO(2) dissociation into surface CO and O (CO(2) --> CO + O). The second step is CH oxygenation into CHO (CH + O --> CHO), which is more favored than dissociation into C and hydrogen (CH --> C + H). The third step is the dissociation of CHO into surface CO and H (CHO --> CO + H). This can explain the enhanced selectivity toward the formation of CO and H(2) on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO = C((ads)) + CO(2)) is more favored than by CH(4) sequential dehydrogenation. The major problem of CO(2) reforming of CH(4) is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH(4) activation into CH(3) and H as the rate-determining step of CO(2) reforming of CH(4) on Ni(111), in agreement with the isotopic experimental results.