Understanding mixing of Ni and Pt in the Ni/Pt(111) bimetallic catalyst via molecular simulation and experiments

Molecular dynamics (MD) simulations employing embedded atom method potentials and ultrahigh vacuum (UHV) experiments were carried out to study the mixing process between the Ni and Pt atoms in the Ni/Pt(111) bimetallic system. The barrier for a Ni atom to diffuse from the top surface to the subsurface layer is rather high (around 1.7 eV) as calculated using the nudged elastic band (NEB) method. Analysis of the relaxation dynamics of the Ni atoms showed that they undergo diffusive motion through a mechanism of correlated hops. At 600 K, all Ni atoms remain trapped on the top surface due to large diffusion barriers. At 900 K, the majority of Ni atoms diffuse to the second layer and at 1200 K diffusion to the bulk is observed. We also find that smaller Ni coverages and the presence of Pt steps facilitate the Ni-Pt mixing. By simulated annealing simulations, we found that in the mixed state, the Ni fraction oscillates between layers, with the second layer being Ni-richer at equilibrium. The simulation results at multiple time scales are consistent with the experimental data.     

Hydrogen chemisorption on Pt/Ni (111) systems

Extensive self-consistent real-space recursion-method calculations were performed for the Pt overlayer or the Pt₇ cluster on the Ni(111) surface and for hydrogen chemisorption on these systems. Correlations between the surface-atom local density of electronic-state properties before chemisorption, surface reactivity, and the initial-state contribution to the metal core-level shifts, respectively, is documented and discussed. The experimentally observed catalytic properties of Pt Ni alloy surfaces are also briefly considered. © 1996 John Wiley & Sons, Inc.     

NO Chemisorption on Pt(111), Rh/Pt(111), and Pd/Pt(111)

The chemisorption of NO on clean Pt(111), Rh/Pt(111) alloy, and Pd/Pt(111) alloy surfaces has been studied by first principles density functional theory (DFT) computations. It was found that the surface compositions of the surface alloys have very different effects on the adsorption of NO on Rh/Pt(111) versus that on Pd/Pt(111). This is due to the different bond strength between the two metals in each alloy system. A complex d-band center weighting model developed by authors in a previous study for SO2 adsorption is demonstrated to be necessary for quantifying NO adsorption on Pd/Pt(111). A strong linear relationship between the weighted positions of the d states of the surfaces and the molecular NO adsorption energies shows the closer the weighted d-band center is shifted to the Fermi energy level, the stronger the adsorption of NO will be. The consequences of this study for the optimized design of three-way automotive catalysts, (TWC) are also discussed.     

氧原子在具有Pt皮肤的Pt3Ni(111)表面的吸附和扩散

用基于密度泛函理论的第一性原理方法研究了氧原子在具有Pt皮肤的Pt3Ni(111)[记为Pt-skin-Pt3Ni(111)]表面的吸附和扩散特性. 重点研究了氧原子在Pt-skin-Pt3Ni(111)表面的扩散问题, 这对理解Pt-skin-Pt3Ni(111)催化剂的高催化活性有重要意义. 结果表明: 氧原子容易吸附在fcc位; 催化剂Pt3Ni中的Ni原子对催化剂的电子结构有很大影响, 从而改变了其对氧原子的吸附. 用推拉弹性带(NEB)方法搜索氧原子的扩散势垒, 并解释了Pt-skin-Pt3Ni(111)催化剂的高催化活性.     

Experimental and theoretical study of reactivity trends for methanol on Co/Pt(111) and Ni/Pt(111) bimetallic surfaces

Methanol was used as a probe molecule to examine the reforming activity of oxygenates on NiPt(111) and CoPt(111) bimetallic surfaces, utilizing density functional theory (DFT) modeling, temperature-programmed desorption, and high-resolution electron energy loss spectroscopy (HREELS). DFT results revealed a correlation between the methanol and methoxy binding energies and the surface d-band center of various NiPt(111) and CoPt(111) bimetallic surfaces. Consistent with DFT predictions, increased production of H2 and CO from methanol was observed on a Ni surface monolayer on Pt(111), designated as Ni-Pt-Pt(111), as compared to the subsurface monolayer Pt-Ni-Pt(111) surface. HREELS was used to verify the presence and subsequent decomposition of methoxy intermediates on NiPt(111) and CoPt(111) bimetallic surfaces. On Ni-Pt-Pt(111) the methoxy species decomposed to a formaldehyde intermediate below 300 K; this species reacted at approximately 300 K to form CO and H2. On Co-Pt-Pt(111), methoxy was stable up to approximately 350 K and decomposed to form CO and H2. Overall, trends in methanol reactivity on NiPt(111) bimetallic surfaces were similar to those previously determined for ethanol and ethylene glycol.     

Effusive molecular beam study of C2H6 dissociation on Pt(111)

The dissociative sticking coefficient for C2H6 on Pt(111) has been measured as a function of both gas temperature (Tg) and surface temperature (Ts) using effusive molecular beam and angle-integrated ambient gas dosing methods. A microcanonical unimolecular rate theory (MURT) model of the reactive system is used to extract transition state properties from the data as well as to compare our data directly with supersonic molecular beam and thermal equilibrium sticking measurements. We report for the first time the threshold energy for dissociation, E0 = 26.5 +/- 3 kJ mol(-1). This value is only weakly dependent on the other two parameters of the model. A strong surface temperature dependence in the initial sticking coefficient is observed; however, the relatively weak dependence on gas temperature indicates some combination of the following (i) not all molecular excitations are contributing equally to the enhancement of sticking, (ii) that strong entropic effects in the dissociative transition state are leading to unusually high vibrational frequencies in the transition state, and (iii) energy transfer from gas-phase rovibrational modes to the surface is surprisingly efficient. In other words, it appears that vibrational mode-specific behavior and/or molecular rotations may play stronger roles in the dissociative adsorption of C2H6 than they do for CH4. The MURT with an optimized parameter set provides for a predictive understanding of the kinetics of this C-H bond activation reaction, that is, it allows us to predict the dissociative sticking coefficient of C2H6 on Pt(111) for any combination of Ts and Tg even if the two are not equal to one another.     

Pt oxide and oxygen reduction at Pt(111) studied by surface X-ray diffraction

The influence of the oxygen reduction reaction on the oxidation of Pt(111) is studied by surface X-ray diffraction. The oxygen reduction reaction does not significantly influence the place-exchange process during the initial stages of oxidation and there is no change in the onset potential and kinetics.     

Pt（111）和Pt3Ni（111）表面上CO催化氧化反应的密度泛函理论研究

采用密度泛函理论,对Pt(111)和Pt3Ni(111)表面上CO和O的单独吸附、共吸附以及CO的氧化反应进行了系统的研究. 结果表明, Pt3Ni(111)表面上CO的吸附弱于Pt(111)表面, O的吸附明显强于Pt(111)表面. 两个表面表现出相似的CO催化氧化活性. 表面Ni的存在不但稳定了O的吸附,同时也降低了过渡态O的能量.     

MgZnO/ZnO p–n junction UV photodetector fabricated on sapphire substrate by plasma-assisted molecular beam epitaxy

We have investigated the spectral response of back- and front-surface-illumination MgZnO/ZnO p–n ultraviolet photodetector fabricated by plasma-assisted molecular beam epitaxy on sapphire substrate. The current–voltage measurements show that the device has a rectifying behavior with a turn-on voltage of 4.5 V. The detector exhibits a broad spectral response which covers the visible and UV spectra regions (from 275 to 375 nm) and has a maximum peak response at the wavelength of 330 nm. At a reverse bias of 5 V, the visible rejection (R330 nm/R500 nm) was more than two orders of magnitude. The peak responsivity at 330 nm for the device under back-illumination is about four times larger than that of the device under front-illumination under the same reverse bias. The response mechanisms of the device under back- and front-illumination are discussed.     

Surface Pourbaix diagrams and oxygen reduction activity of Pt, Ag and Ni(111) surfaces studied by DFT

Based on density functional theory calculations we investigate the electrochemically most stable surface structures as a function of pH and electrostatic potential for Pt(111), Ag(111) and Ni(111), and we construct surface Pourbaix diagrams. We study the oxygen reduction reaction (ORR) on the different surface structures and calculate the free energy of the intermediates. We estimate their catalytic activity for ORR by determining the highest potential at which all ORR reaction steps reduce the free energy. We obtain self-consistency in the sense that the surface is stable under the potential at which that particular surface can perform ORR. Using the self consistent surfaces, the activity of the very reactive Ni surface changes dramatically, whereas the activity of the more noble catalysts Pt and Ag remains unchanged. The reason for this difference is the oxidation of the reactive surface. Oxygen absorbed on the surface shifts the reactivity towards the weak binding region, which in turn increases the activity. The oxidation state of the surface and the ORR potential are constant versus the reversible hydrogen electrode (RHE). The dissolution potential in acidic solution, on the other hand, is constant vs. the standard hydrogen electrode (SHE). For Ag, this means that where the potential for dissolution and ORR are about the same at pH = 0, Ag becomes more stable relative to RHE as pH is increased. Hence the pH dependent stability offers an explanation for the possible use of Ag in alkaline fuel cell cathodes.     

Charge state and activity of Pt/C catalysts in oxygen reduction reaction

Electrochemical experiments with a rotating disk electrode are used to measure specific catalytic activity of Pt/C structures in the oxygen reduction reaction at the density of Pt nanoparticles on the glassy carbon support surface below one monolayer. The specific activity maximum is found at the coverage of about 0.4 monolayer. An explanation of the observed dependence is suggested that is based on consideration of the relationship between the surface density and charge state of the system of metallic catalyst particles. A numeric model is developed that describes charge transfer in the catalyst structure due to the difference in the work functions between the metal nanoparticles and support with account for the discrete nature of the nanoparticle charging and their mutual polarization. Calculations show that the carbon support coverage by Pt particles of about 0.4 monolayer corresponds to the largest amount of charged particles with the maximum energy of electrons, which provides the maximum catalyst activity and explains the dependence observed in the experiment.     

甘氨酸修饰的Pt(111)电极上的氧还原

利用单晶旋转圆盘电极技术（Hanging Meniscus Rotating Disk Electrode,　HMRD）在硫酸和高氯酸溶液中分别研究了甘氨酸修饰的Pt(111)电极表面氧分子的电催化还原反应. 实验发现：在硫酸溶液中,经甘氨酸修饰的Pt(111)电极表面的氧还原活性明显提高,其中氧还原的半波电位与Pt(111)电极的相比正移约0.1 V,而在高氯酸溶液中,甘氨酸修饰的Pt(111)电极的活性几乎没有发生变化. 该实验结果表明：甘氨酸修饰的Pt(111)电极一方面抑制了SO₄^2-在电极表面的吸附,另一方面又能在电极表面提供相邻的空位供氧分子吸附. 通过与文献中报道的CN^-修饰的Pt(111)电极上的氧还原结果的对比,可以推测甘氨酸修饰的Pt(111)电极表面氧还原活性提高是由于甘氨酸在Pt(111)表面可能先被氧化成CN^-后吸附在电极表面,进而促进了氧分子的电催化还原反应.     

Activation in prochiral reaction assemblies on Pt(111)

Trifluoroacetophenone (TFAP) forms C O...H-C bonded dimers and trimers at room temperature on Pt(111). It is proposed that these systems mimic the prochiral carbonyl-chiral modifier interaction in the enantioselective hydrogenation of TFAP on cinchona-modified Pt catalysts. That is, the activation of TFAP in homomolecular assemblies at racemic sites is expected to be roughly the same as in the diastereomeric complex formed at chiral sites. This interpretation suggests a reason why alpha-phenyl ketones do not display a strong measured rate enhancement effect in the Orito reaction.     

分子束弛豫谱研究Ge(111)表面与氯分子热反应动态学

本文用分子束弛豫谱研究了Ge(111)表面与氯分子热反应动态学。采用种子束技术,发现增加氯分子的入射平动能, 将有效地增加Cl2与Ge表面的化学蚀刻反应, 对分子束弛豫谱的数据分析表明, 在表面温度Ts为700-900K的范围内, GeCl2为该反应体系的唯一产物, 它的脱附过程是蚀刻反应的速率控制步骤; 并且测得脱附活化能Ed为66.0±2.0kJ·mol^-^1。此外, 对该化学蚀刻的反应机理进行了讨论。     

Oxygen island formation on Pt(111) studied by dynamic Monte Carlo simulation

The formation of oxygen islands on the Pt(111) surface has been studied as a function of temperature by low energy electron diffraction (LEED) experiments and dynamic Monte Carlo (DMC) simulations. By raising the temperature, the (2 x 2) LEED spot intensity increases gradually and decays after a peak at around 255 K (T(p)) with full width of half maximum of 160 K. This behavior is interpreted by DMC simulations with the kinematical LEED analysis. In the DMC simulation, an oxygen atom hops to the neighboring site via the activation barrier of the saddle point. The potential energies at initial, saddle, and final points are changed at each hopping event depending on the surrounding oxygen atoms. By comparing the observed T(p) with the simulated one, the interaction energy E of oxygen atoms on Pt(111) was determined to be 25+/-3 meV at 2a(0). The DMC simulations visualize how the oxygen islands are formed and collapse on Pt(111) with increase of the temperature and well reproduce the surface configurations observed by scanning tunneling microscopy.     

Molecular dynamics simulations of surface oxide-water interactions on Pt(111) and Pt/PtCo/Pt3Co(111)

Classical molecular dynamics simulations of the interactions of water with oxidized Pt(111) and Pt/PtCo/Pt(3)Co(111) surfaces are performed by modeling water with the CF1 central force model that allows molecular dissociation and therefore the presence of other intermediates of the oxygen reduction reaction different from atomic oxygen. It is found that the water-surface oxide interactions do not affect the overall structure of the catalyst represented by an extended periodic slab. However, such interactions are affected by changes in the electrochemical potential which are simulated by higher values of the surface and atomic oxygen charges at increased oxygen coverage. Thus, electrochemical potential as well as the presence of protons and anions products of acid dissociation define the identity and the amount of oxygen reduction reaction intermediates such as OH or H(3)O. We observe agglomerations of water molecules over regions of the surface and the presence of OH and H(3)O in their vicinity. Our simulation model is able to qualitatively reproduce features of the degradation of the catalyst surface after oxidation and reduction cycles.     

Mechanistic study of the electrochemical oxygen reduction reaction on Pt(111) using density functional theory

Density functional theory (DFT) was used to study the electrolyte solution effects on the oxygen reduction reaction (ORR) on Pt(111). To model the acid electrolyte, an H(5)O(2)(+) cluster was used. The vibrational proton oscillation modes for adsorbed H(5)O(2)(+) computed at 1711 and 1010 cm(-1), in addition to OH stretching and H(2)O scissoring modes, agree with experimental vibrational spectra for proton formation on Pt surfaces in ultrahigh vacuum. Using the H(5)O(2)(+) model, protonation of adsorbed species was found to be facile and consistent with the activation barrier of proton transfer in solution. After protonation, OOH dissociates with an activation barrier of 0.22 eV, similar to the barrier for O(2) dissociation. Comparison of the two pathways suggests that O(2) protonation precedes dissociation in the oxygen reduction reaction. Additionally, an OH diffusion step following O protonation inhibits the reaction, which may lead to accumulation of oxygen on the electrode surface.     

甲醇浓度对Pt/C催化剂氧还原活性的影响

采用半池考察了Pt/C催化剂在含不同浓度甲醇的0.5mol/L硫酸中的氧还原活性(ORR).研究发现,当甲醇浓度为0.1mol/L时,Pt/C催化剂的ORR活性最高,在催化层上热压商品NafionNRE-212膜后也出现同样趋势.线性扫描伏安曲线显示,压膜前后的Pt/C催化剂的ORR活性在含0.1mol/L甲醇的0.5mol/L硫酸中几乎没有变化.电化学阻抗谱结果表明,在该溶液中,Nafion膜的电阻比在其它电解液中低,这可能是导致Pt/C催化剂ORR活性提高的主要原因.有必要关注Nafion膜的这一异常性质并通过特殊设计后用于电池堆,以提高燃料电池性能.