First-principles study of some factors controlling the rate of ammonia decomposition on Ni and Pd surfaces

Using the plane-wave pseudopotential method within the density-functional theory with the generalized gradient approximation for exchange and correlation potential, we have calculated adsorption energies (E(ad)), diffusion barrier, and the first dissociation barrier (E(1)) for NH(3) on Ni and Pd surfaces. While the top site is found to be preferred for NH(3) adsorption on both Ni(111) and Pd(111), its calculated diffusion barrier is substantially higher for Pd(111) than for Ni(111). We also find that during the first dissociation step (NH(3)-->NH(2)+H), NH(2) moves from the top site to the nearest hollow site on Ni(111) and Pd(111) and on the stepped surfaces, Ni(211) and Pd(211), it moves from the initial top site at the step edge to the bridge site in the same atomic chain. Meanwhile H is found to occupy the hollow sites on all four surfaces. On Ni(111), E(1) is found to be 0.23 eV higher than E(ad), while at the step of Ni(211), E(1) and E(ad) are almost equal, suggesting that the probability for the molecule to dissociate is much on the step of Ni(211). In the case of Pd(211), however, we find that the dissociation barrier is much higher than E(ad). These trends are in qualitative agreement with the experimental finding that ammonia decomposition rate is much lower on Pd than on Ni.     

DFT investigation of CO adsorption on Pt(211) and Pt(311) surfaces from low to high coverage

Adsorption of CO on Pt(211) and Pt(311) surfaces has been investigated by the density functional theory (DFT) method (periodic DMol3) with full geometry optimization. Adsorption energies, structures, and C-O stretching vibrational frequencies are studied by considering multiple possible adsorption sites and comparing them with the experimental data. The calculated C-O stretching frequencies agree well with the experimental ones, and precise determination of adsorption sites can be carried out. For Pt(211), CO adsorbs at the atop site on the step edge at low coverage, but CO adsorbs at the atop and bridge sites simultaneously on both the step edge and the terrace with further increasing CO coverage. The present results interpret the reflection adsorption infrared (RAIR) spectra of Brown and co-workers very well from low to high coverage. For Pt(311), CO adsorbs also at the atop site on the step edge at low coverage. The lifting of reconstruction by CO adsorption occurs also for Pt(311), whereas the energy gain for lifting the reconstruction of the Pt(311) surface is smaller than that for Pt(110). The largest difference between the stepped Pt(211)/Pt(311) and Pt(110) surfaces is the occupation on the edge sites at higher coverage. For the stepped surfaces, the bridge site begins to be occupied at higher coverage, whereas the atop site is always occupied for the Pt(110) surface.     

氢原子在Ni(111)次表面和Ni(211)、(533)台阶面上的吸附与振动

应用原子与表面簇合物相互作用的五参数Morse势(5-MP)方法对氢原子在Ni(111)表面和次表面以及Ni(211), (533)台阶面进行了系统研究, 得到了氢原子在上述各面的吸附位、吸附几何、结合能和本征振动频率. 计算结果表明, 在Ni(111)面上, 氢原子优先吸附在三重位, 随着覆盖度的增加会吸附在次表面八面体位和四面体位. Ni(211), (533)的最优先吸附位都是四重位, 当氢原子的覆盖度增大时占据(111)平台的三重吸附位. 靠近台阶面的吸附位受台阶和平台高度的影响很大. 此外, 我们计算了氢原子在各表面的不同吸附位的扩散势垒, 获得氢原子在各表面的最低能量扩散通道.     

Adsorption of carbon monoxide on Pd (210) and (510) surfaces

Adsorption of carbon monoxide on Pd (210) and (510) stepped surfaces has been investigated by the extended London‐Eyring‐Polyani‐Sato method constructed using a five‐parameter Morse potential. Pd (210) and (510) stepped surfaces consist of terrace with (100) structure and step with (110) character. These results show that there exist common characteristics of CO adsorption on these two surfaces. At low coverage, CO adsorbs in twofold bridge site of the (100) terrace. The critical characteristics inherit that of CO molecule adsorbed in twofold bridge site of (100) original surface. When the coverage is increased, the top site of (110) step is occupied. The critical characteristics resemble that of CO molecule adsorbed in top site of (110) original surface. A number of new sites are exposed on the boundary regions, for example, the fivefold hollow site (H) of these two surfaces. There are stable adsorption sites at high coverage. Because of the different length of the (100) terrace, the (210) and (510) stepped surfaces have some different characteristics. First, CO is tilted adsorption on bridge site of terrace of (210), but perpendicular on terrace of (510) surface. Second, the bridge site (B₁) where one Pd atom at the top of the step and the other at the bottom of the step is a stable adsorption site on (210), but the same type of site on Pd (510) surface is not. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical study of NO adsorption/dissociation on Pd (100) and (111) surfaces

Both adsorption and dissociation of the diatomic molecular NO on Pd (100) and (111) surfaces are studied using the extended London‐Eyring‐Polyani‐Sato (LEPS) method constructed by means of 5‐MP (the 5‐parameter Morse potential). All critical characteristics of the system that we obtain, such as adsorption geometry, binding energy, eigenvalues for vibration, are in good agreement with the experimental results. On Pd (100) surface, NO prefers to adsorb in fourfold hollow site (H) uprightly at low coverage. With increase in the coverage NO gradually tilts in fourfold hollow and bridge sites. For NO? Pd (111) system, two adsorption states are found at low coverage, of which one adsorption state is the B(tilt) state that the centroid of NO projects at bridge site, another (H? B? H state) that NO almost parallels to the (111) surface with the vibration frequency of 610 cm^?1, but the frequency is near to that of the atoms, which is easy to be ignored in experiments. At high coverage, two transitional states (BH and HT) are found. NO is difficult to dissociate on Pd (100) and (111) surfaces. Especially for NO? Pd (111) system, the three‐well‐potential dissociation mode is initially put forward to show the remarkable dissociation process with two dissociation transitional states of NO on Pd (111). Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

氧原子在Mo低指数表面及(211)高指数面上的吸附和振动

应用原子和表面簇合物相互作用的5参数Morse势方法(5-MP)对O-Mo低指数表面体系及(211)高指数面体系进行了系统的研究,并获得了全部临界点特性,如吸附位、吸附几何、结合能、正则振动频率等,计算结果表明：在Mo(100)面,O原子吸附在四重洞位,随着覆盖度增加,(100)面发生缺行重构,膺式三重位为稳定吸附位;在Mo(110),(111)及(211)面,O原子均趋向于吸附在膺式三重位．     

Direct sticking and differential adsorption heats as probes of structural transitions: O2 on the stepped Ni[211] surface

Coverage-dependent heats of adsorption and sticking probabilities for oxygen on Ni[211] have been measured at 300 K using single-crystal adsorption calorimetry. The data are consistent with a switch from dissociative chemisorption at low coverage to oxide formation above 2 ML adatom coverage. The initial heat of adsorption is 620 kJ mol(-)(1), considerably higher than for any low Miller index nickel surface; this is attributed to the presence of low coordination step atoms that are preferably occupied up to 1 ML. As the coverage increases, the heat is found to drop very rapidly, indicating the presence of strong lateral adatom repulsions, which ultimately drive a transition from the chemisorption regime to oxide film formation at higher coverage. The shape of the coverage-dependent sticking probability is consistent with a direct adsorption mechanism at low coverage. At higher coverage, the transition between the chemisorption and oxidation regimes is relatively complex compared with low Miller index nickel surfaces. This is discussed in terms of the influence of the step sites on the [211] surface.     

氧原子在钨低指数表面及(211)高指数表面上的吸附和振动

 应用原子和表面簇合物相互作用的5参数Morse势方法研究了氧原子在W低指数表面及(211)高指数表面上的吸附和振动. 计算结果表明,低覆盖度下,在W(100)面氧原子吸附在四重洞位,随着覆盖度增加,(100)面发生缺行重构,膺式三重位为稳定吸附位; 在W(110),(111)及(211)面,氧原子均趋向于吸附在膺式三重位. 该计算结果和实验符合得很好.     

Adsorption and Vibration of O Atoms on Fe Low-index and Fe （211） High-index Surfaces

IntroductionThe interaction of oxygen with iron in lowcoverageregimes is considered to be an important step in the for-mation of oxides in corrosion science and in Fisher-Tro-psch process for the synthesis of ammonia over the het-erogeneous catalysts[1]. …     

CO adsorption and reaction on clean and oxygen-covered Au(211) surfaces

We have used primarily temperature-programmed desorption (TPD) and infrared reflection-absorption spectroscopy (IRAS) to investigate CO adsorption on a Au(211) stepped single-crystal surface. The Au(211) surface can be described as a step-terrace structure consisting of three-atom-wide terraces of (111) orientation and a monatomic step with a (100) orientation, or 3(111) x (100) in microfacet notation. CO was only weakly adsorbed but was more strongly bound at step sites (12 kcal mol(-1)) than at terrace sites (6.5-9 kcal mol(-1)). The sticking coefficient of CO on the Au(211) surface was also higher ( approximately 5x) during occupation of step sites compared to populating terrace sites at higher coverages. The nu(CO) stretching band energy in IRAS spectra indicated that CO was adsorbed at atop sites at all coverages and conditions. A small red shift of nu(CO) from 2126 to 2112 cm(-1) occurred with increasing CO coverage on the surface. We conclude that the presence of these particular step sites at the Au(211) surface imparts stronger CO bonding and a higher reactivity than on the flat Au(111) surface, but these changes are not remarkable compared to chemistry on other more reactive crystal planes or other stepped Au surfaces. Thus, it is unlikely that the presence or absence of this particular crystal plane alone at the surface of supported Au nanoparticles has much to do with the remarkable properties of highly active Au catalysts.     

Adsorption and diffusion on a stepped surface: atomic hydrogen on Pt(211)

We present density functional theory calculations for atomic hydrogen interacting with a stepped surface, the Pt(211) surface. The calculations have been performed at the generalized gradient approximation level, using a slab representation of the surface. This is the state-of-the-art method for calculating the interaction of atoms or molecules with metal surfaces, nevertheless only few studies have used it to study atoms or molecules interacting with stepped surfaces, and none, to the best of our knowledge, have considered hydrogen interacting with stepped platinum surfaces. Our goal has been to initiate a systematic study of this topic. We have calculated the full three-dimensional potential energy surface (PES) for the H/Pt(211) system together with the vibrational band structure and vibrational eigenfunctions of H. A deep global minimum of the PES is found for bridge-bonded hydrogen on the step edge, in agreement with experimental results for the similar H/Pt(533) system. All the local vibrational excitations at the global minimum have been identified, and this will serve as a helpful guide to the interpretation of future experiments on this (or similar) system(s). Furthermore, from the calculated PES and vibrational band structure, we identify a number of consequences for the interpretation or modelling of diffusion experiments studying the coverage and directional dependence of atomic hydrogen diffusion on stepped platinum surfaces.     

Isothermal kinetic study of nitric oxide adsorption and decomposition on Pd(111) surfaces: molecular beam experiments

The kinetics of NO adsorption and dissociation on Pd(111) surfaces and the NO sticking coefficient (s(NO)) were probed by isothermal kinetic measurements between 300 and 525 K using a molecular beam instrument. NO dissociation and N2 productions were observed in the transient state from 425 K and above on Pd(111) surfaces with selective nitrogen production. Maximum nitrogen production was observed between 475 and 500 K. It was found that, at low temperatures, between 300 and 350 K, molecular adsorption occurs with a constant initial s(NO) of 0.5 until the Pd(111) surface is covered to about 70-80% by NO. Then s(NO) rapidly decreases with further increasing NO coverage, indicating typical precursor kinetics. The dynamic adsorption - desorption equilibrium on Pd(111) was probed in modulated beam experiments below 500 K. CO titration experiments after NO dosing indicate the diffusion of oxygen into the subsurface regions and beginning surface oxidation at > or = 475 K. Finally, we discuss the results with respect to the rate-limiting character of the different elementary steps of the reaction system.     

Studies of CO adsorption on Pt(100), Pt(410), and Pt(110) surfaces using density functional theory

Adsorption of CO on Pt(100), Pt(410), and Pt(110) surfaces has been investigated by density functional theory (DFT) method (periodic DMol(3)) with full geometry optimization and without symmetry restriction. Adsorption energies, structures, and vibrational frequencies of CO on these surfaces are studied by considering multiple possible adsorption sites and comparing them with the experimental data. The same site preference as inferred experiments is obtained for all the surfaces. For Pt(100), CO adsorbs at the bridge site at low coverage, but the atop site becomes most favorable for the c(2 x 2) structure at 1/2 monolayer. For Pt(410) (stepped surface with (100) terrace and (110) step), CO adsorbs preferentially at the atop site on the step edge at 1/4 monolayer, but CO populates also at other atop and bridge sites on the (100) terrace at 1/2 monolayer. The multiple possible adsorption sites probably correspond to the multiple states in the temperature-programmed desorption spectra for CO desorption. For Pt(110), CO adsorbs preferentially at the atop site on the edge for both the reconstructed (1 x 2) and the un-reconstructed (1 x 1) surfaces. When adjacent sites along the edge row begin to be occupied, the CO molecules tilt alternately by ca. 20 degrees from the surface normal in opposite directions for both the (1 x 2) and (1 x 1) surfaces.     

NO在氧预吸附Ir(100)表面吸附和解离的第一性原理研究

采用第一性原理密度泛函理论和周期性平板模型研究了NO在O预吸附Ir(100)表面的吸附和解离, 并考察了预吸附的O对可能产物N₂, N₂O和NO₂的选择性的影响. 优化得到反应过程中初态、 过渡态和末态的吸附构型, 并获得反应的势能面信息. 计算结果表明, NO在O预吸附表面最稳定的吸附位是桥位, 其次是顶位. 桥位和顶位的NO在表面存在两条解离通道, 即直接解离通道和由桥位和顶位扩散到平行空位, 继而发生N-O键断裂生成N原子和O原子的解离通道. 此分离机理与洁净表面上NO解离机理相同, 但后一种解离方式优于前一种, 是NO在表面上解离的主要通道. 预吸附的O原子在不同程度上抑制了NO的解离, 导致桥位和顶位NO解离互相竞争. 在O预吸附Ir(100)表面, N₂气是唯一的产物, 不会有副产物N₂O和NO₂的生成, 与实验结果一致. 预吸附的O在N/O低覆盖度下几乎不影响N₂气的生成, 但在较高覆盖度下则促进了N₂气的生成.     

周期性密度泛函理论研究氯气在CuCl(111)表面上的吸附与解离

运用广义梯度密度泛函理论(Generalized Gradient Approximation，GGA)的PBE(Perdew-Burke-Ernzerh)方法结合周期性平板模型，研究了氯气分子和氯原子在CuCl(111)表面上的吸附。通过对不同吸附位和不同单层覆盖度下的吸附能和几何构型参数的计算和比较发现：氯气分子在CuCl(111)表面的吸附为解离吸附；单层覆盖度为0.50时的吸附构型为稳定的吸附构型；氯气分子平行吸附在CuCl(111)表面时最稳定，吸附能最大，达364.5 kJ·mol^-1；伸缩振动频率的计算结果表明，吸附后的氯气分子的伸缩振动频率与自由氯气分子的伸缩振动频率相比，都发生了红移；布居分析结果表明整个吸附体系发生了由Cu原子向氯气分子的电荷转移。氯原子吸附的计算结果显示氯原子以穴位稳定的吸附在CuCl(111)表面。     

Effect of subsurface oxygen on the reactivity of the Ag(111) surface

Periodic, self-consistent, density functional theory calculations have been performed to demonstrate that subsurface oxygen (O(sb)) dramatically increases the reactivity of the Ag(111) surface. O(sb) greatly facilitates the dissociation of H2, O2, and NO and enhances the binding of H, C, N, O, O2, CO, NO, C2H2, and C2H4 on the Ag(111) surface. This effect originates from an O(sb)-induced upshift of the d-band center of the Ag surface and becomes more pronounced at higher O(sb) coverage. Our findings point to the important role that near-surface impurities, such as O(sb), can play in determining the thermochemistry and kinetics of elementary steps catalyzed by transition metal surfaces.     

Effect of strontium substitution on the activity of La_(2-x)Sr_xNiO_4(x=0.0-1.2)in NO decomposition

The aim of this work is to study the effect of Sr substitution on the redox properties and catalytic ac- tivity of La2-xSrxNiO4(x=0.0-1.2)for NO decomposition.Results suggest that the x=0.6 sample shows the highest activity.The characterization(TPD,TPR,etc.)of samples indicates that the x=0.6 sample possesses suitable abilities in both oxidation and reduction,which facilitates the proceeding of oxygen desorption and NO adsorption.At temperature below 700℃,the oxygen desorption is difficult, and is the rate-determining step of NO decomposition.With the increase of reaction temperature(T> 700℃),the oxygen desorption is favorable and,the active adsorption of NO on the active site(NO Vo Ni2 →NO~--Ni3 )turns out to be the rate-determining step.The existence of oxygen vacancy is the prerequisite condition for NO decomposition,but its quantity does not relate much to the activity.     

氢原子在Fe低指数面及高指数面(211)上的吸附和振动

摘要应用原子和表面簇合物相互作用的5参数Morse势方法(5-MP)对H-Fe低指数表面体系及高指数表面体系进行了系统研究, 并获得了吸附位、 吸附几何、 结合能和正则振动频率等全部临界点特性. 理论计算结果表明, 在Fe(100)面, H原子吸附在四重洞位, H-Fe的垂直振动频率为1 009 cm^-1; 在Fe(110)和Fe(211)表面, 趋向于吸附在赝式三重位, H-Fe的垂直振动频率分别为1 054和1 046 cm^-1; 而在Fe(111)表面最稳定的吸附位是近桥位, 频率为1 030 cm^-1.     

Adsorption of carbon monoxide on Pd low‐index surfaces and (110) reconstructed surface

Adsorption and diffusion of carbon monoxide on Pd low‐index surfaces and missing‐row Pd (110) reconstructed surface have been investigated by the extended London–Eyring–Polyani–Sato (LEPS) method constructed by means of a five‐parameter Morse potential. All critical characteristics, such as adsorption site, adsorption geometry, binding energy, CO vibrational frequency have been obtained and compared with the experimental and theoretical data. On these surfaces, the stable adsorption sites of CO are changed with increasing CO coverage. On the missing‐row Pd (110) reconstructed surface, there are five stable adsorption sites: H₁, H₂ (H₁ and H₂ are threefold hollow sites on (111) subsurface), B (bridge site on the second layer), SB (short‐bridge site), and T (top site). Copyright © 2010 John Wiley & Sons, Ltd.