期刊 – 2022世界杯晋级名额 - 英格兰vs伊朗预测

ALD技术助力：解密Pt-Ni催化剂DRM积碳之谜

管理员 — Mon, 20 May 2024 01:54:14 +0000

化石能源的过度消耗导致了温室气体排放的增加，进而加剧了全球变暖的程度。为了缓解这一问题，研究人员一直在探索各种替代能源和绿色技术。其中，甲烷干重整（DRM）是一种非常有前景的技术，通过将天然气甲烷重整为合成气，不仅能够减少温室气体排放，还具有天然气回收和再利用的广阔前景。然而，传统的Ni基催化剂在DRM过程中会因CH4逐渐脱氢形成碳而失活，严重限制了其工业应用。同时在异相催化领域，催化剂的颗粒大小和微观结构对催化性能起着至关重要的作用。通过精准调控催化剂纳米颗粒的大小或排布，不仅可以提高催化剂的活性和稳定性，还可以为研究微观结构与性能之间的关系提供更有利的条件。尽管在原子尺度上设计和制备催化剂仍然存在困难和挑战，但原子层沉积（ALD）技术的发展为应对这一难题提供了潜在的解决方案。

5月10日，英格兰vs伊朗预测微纳中心单斌教授团队在化工催化领域知名期刊《Chemical Engineering Journal》上发表了最新研究成果，论文标题为“Deciphering the stability mechanism of Pt-Ni/Al₂O₃ catalysts in syngas production via DRM（揭示铂-镍/氧化铝催化剂在甲烷干重整制备合成气过程中的稳定性机理）”，DOI: https://doi.org/10.1016/j.cej.2024.151966。这项研究利用ALD技术制备了Pt-Ni/Al2O3双金属催化剂，成功在650 ℃下进行了48小时的DRM反应，保持了高转化率，且未出现活性下降，但产生了与Ni/Al2O3相似的大量积碳现象。对于这一特殊的实验结果，研究人员进一步研究了积碳的性质随反应时间的变化情况，揭示了催化剂在发生积碳的同时仍能保持高稳定性的机理。这一研究成果深化了我们对Pt-Ni催化剂在DRM反应中积碳行为的理解，并为开发更稳定的Ni基催化剂系统提供了新思路。

图1 论文首页

研究人员通过调节Ni/Al2O3基底表面Ni的化学状态以及ALD的沉积温度，制备了两种不同构型的Pt-Ni双金属催化剂：Pt-Ni合金和Pt表面修饰Ni纳米颗粒。通过X射线衍射（XRD）、X射线光电子能谱（XPS）以及CO作为探针分子的漫反射傅里叶变换红外光谱（CO-DRIFTS）等手段对催化剂进行了结构表征，并揭示了不同工艺制备的Pt-Ni催化剂在构型上的差异。结果显示，Pt-Ni催化剂形成了PtNi合金，并且发生了Pt-Ni之间的电子转移。而采用预还原方法处理后在Ni/Al2O3表面进行Pt ALD沉积的PR Pt-Ni/Al2O3催化剂则未发生电子转移或形成合金相。

图2 催化剂的结构表征 (a) Pt-Ni/Al₂O₃, Ni/Al₂O₃, Pt/Al₂O₃催化剂的XRD图谱, (b) Pt/Al₂O₃, (c) PR 1% Pt-Ni/Al₂O₃, (d) 1% Pt-Ni/Al₂O₃催化剂的CO-DRIFTS光谱（自下而上，光谱分别对应CO吸附饱和后，经氩气吹扫2, 5, 8, 10, 14, 17, 20, 23, 27和30分钟后的情况。右上角的示意图展示了相应催化剂的结构，其中绿色球体代表Ni原子，黄色球体代表Pt原子）

研究人员对催化剂在650 ℃下的催化稳定性进行了详细测试。结果显示，两种构型的Pt-Ni催化剂均表现出了优异的稳定性。在最初的10小时内，催化剂经历了一个活化过程，随后在整个48小时的老化过程中保持了高活性。其中，Pt表面修饰Ni催化剂的转化率略高于PtNi合金样品的转化率，分别为74%和72%，而未加入Pt改性的Ni/Al2O3催化剂则迅速失活。这表明Pt的加入可以同时提升催化剂的初始活性及稳定性。随后，用热重分析（TGA）对催化剂的积碳含量进行了测试。结果如图3 (a)所显示，尽管Pt-Ni/Al2O3及PR 1% Pt-Ni/Al2O3样品在48小时内保持了高活性且没有出现衰减，但它们却生成了与Ni/Al2O3相近的积碳含量。这表明抑制积碳的生成并非本文所制备的Pt-Ni双金属催化剂提升DRM催化稳定性的原因。

图3 (a) 催化剂在老化48 h后的热重分析(TGA)及 (b) 拉曼光谱（其中I_D/I_G表示D带与G带的强度之比），(c) Ni/Al₂O₃及PR 1% Pt-Ni催化剂随着反应时间变化的积碳含量(TGA)及拉曼光谱中的D带与G带的强度之比(I_D/I_G)

图4 催化剂老化48 h后的形貌表征 (a-g) Ni/Al₂O₃催化剂在老化48 h后的TEM及EDS mapping图像，其中(f)为形貌结构示意图，(h-n) PR 1% Pt-Ni/Al₂O₃催化剂在老化48 h后的TEM及EDS mapping图像，其中(m)为形貌结构示意图

进一步研究表明，尽管产生了与Ni催化剂相近的积碳含量，但在老化48小时后，性能最优的Pt表面修饰Ni催化剂（PR 1% Pt-Ni）与Ni催化剂在DRM稳定性上存在显著差异（见图4）。这一差异归因于积碳的结构性质与位点的动态变化。具体来说，对于Pt表面修饰Ni催化剂，观察到CNTs逐渐沉积在Al2O3载体上，且石墨化程度较低（见图5）。由此产生的积碳未完全覆盖CH4的吸附、活化位点。然而，对于Ni催化剂，随着反应时间的延长，Ni纳米颗粒逐渐被大量低缺陷、高度石墨化的积碳包裹，阻碍了反应物在活性位点上的吸附，导致了催化剂的快速失活。

图5 Pt表面修饰Ni (PR Pt-Ni)与Ni/Al₂O₃催化剂的积碳机理示意图（图中展示了反应2 h与48 h的示意图，其中粉色长方体代表Al₂O₃载体，绿色小球代表Ni原子，黄色小球代表Pt原子，黑色不规则物体代表积碳）

本项工作通过精准制备Pt-Ni双金属催化剂，显著改善了Ni基催化剂在DRM反应中的性能。利用ALD技术在Al2O3载体上构筑了两种不同构型的Pt-Ni双金属催化剂：PtNi合金和Pt表面修饰的Ni催化剂。在DRM严重积碳的高温条件下（650 ℃），我们成功实现了Pt-Ni双金属催化剂的DRM催化活性和稳定性的双重提升。此外，我们首次对Pt/Ni和Ni催化剂的积碳性质随反应时间的动态变化进行了深入研究，并提出了新的抗积碳机理。我们揭示了Pt/Ni和Ni催化剂表面积碳的结构性质与活性位点在反应过程中的动态变化，为延长催化剂的使用寿命、解决DRM积碳失活问题提供了新的策略。

微纳中心硕士研究生赵瑞为第一作者，曹坤副教授、单斌教授为本文的共同通讯作者，英格兰vs伊朗预测材料科学与工程学院材料成形于模具技术国家重点实验室为论文的第一完成单位（图1）。工作得到了加州大学河滨分校Fudong Liu教授和昆明贵金属研究所赵云昆研究员的大力支持。

Investigation of self-assembled monolayer resists for hafnium dioxide atomic layer deposition

管理员 — Wed, 03 Sep 2014 04:44:46 +0000

In this paper, a series of self-assembled monolayers (SAMs) has been investigated as deactivating agents for the atomic layer deposition (ALD) of HfO2. Three important factors that determine the capacity of self-assembled monolayers to prevent ALD of HfO2, chain length, tailgroup structure, and reactive headgroup, have been investigated. We show that to achieve satisfactory deactivation toward atomic layer deposition, it is crucial to form densely packed, highly hydrophobic monolayers. This in turn requires deactivating agents with high reactivity and nonbulky tailgroups comprised of a minimum chain length. A mechanism for the blocking process is proposed in which the role of the SAMs is both to remove reactive Si−OH/Si−O−Si groups at the SiO2 surface and to prevent ALD precursors from reaching the SiO2−SAMs interface.

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管理员 — Wed, 03 Sep 2014 04:44:18 +0000

Recent experiments showed beneficial influence of vanadium doping on the electrochemical performance of lithium iron phosphate (LiFePO4) cathode materials. First-principles calculations have been performed to investigate the stability, electronic structure and lithium diffusivity of vanadium-doped LiFePO4 and to elucidate the origin of such improvement. It is found that vanadium prefers occupying Fe sites and leads to additional density of states within the intrinsic bandgap. By the nudged elastic band method, we show that the barrier of Li ions diffusion along the one dimensional channel in both LiFePO4 and FePO4 phases can be effectively reduced by vanadium doping. Structural analysis shows the lower diffusion barrier ties closely to a volumetric expansion of the diffusion channel.

CO-coverage-dependent oxygen dissociation on Pt(111) surface

管理员 — Wed, 03 Sep 2014 04:39:42 +0000

Oxygen dissociation is one of the most critical steps in the CO oxidation reaction on transition metal surfaces. It has been shown both experimentally and theoretically that oxygen dissociation on clean platinum (Pt) surface proceeds via a precursor-mediated reaction path, with negligible activation barrier. On the other hand, the oxygen dissociation pathway under diesel engine operating conditions, where the metal surface is packed with CO molecules, is understood less clearly. In this paper, we report density functional theory calculations for O2 dissociation on Pt(111) in the presence of varying CO coverage. Classical Monte Carlo simulations have been used to get an estimate of coadsorbed CO and O2 configurations. Oxygen molecular precursor states binding energies were found to shift up in energy with increasing CO coverage, with transition state energies and final product energies following the same trend. The dissociated product state becomes endothermic beyond a critical CO coverage of 0.44 monolayer, where oxygen dissociation is no longer energetically favored on Pt(111). The origin of the change in activation barrier can be attributed to the limited space available for oxygen dissociation and the lateral repulsion from neighboring CO molecules. A linear correlation exists between the oxygen dissociation barrier and the molecular precursor binding energy. These findings give useful insight into the CO oxidation mechanism under realistic diesel engine operating conditions.

NO oxidation catalysis on copper doped hexagonal phase LaCoO3: a combined experimental and theoretical study

管理员 — Wed, 03 Sep 2014 04:37:33 +0000

Cobalt-based perovskite catalysts showed excellent performance towards NO–NO2 oxidation. We systematically investigated the influence of different levels of Cu-doping on the catalytic performance of hexagonal phase LaCoO3(LaCo1-xCuxO3 (x= 0.1, 0.2, 0.3)) for NO oxidation. The catalytic activities of the oxide catalysts followed the sequence: LaCo0.9Cu0.1O3 > LaCoO3 > LaCo0.8Cu0.2O3 > LaCo0.7Cu0.3O3 where the highest NO conversion for LaCo0.9Cu0.1O3was 82% at 310C. The relevant structural characterizations were conducted by XRD, BET, FTIR and TEM. The interaction between Co and Cu promoted the conversion of NO to NO2. Upon increasing the Cu doping content, a decrease of the performance resulted from the generation of isolated CuO on the surface of the oxides, confirmed using H2-TPR and XPS. Combined with first-principle calculations, we explored the reaction mechanism of NO oxidation on the surface and found that Cu doping would facilitate the reaction by decreasing the energy of oxygen vacancy formation and the NO2desorption barrier from Co- or Cu-nitrite.

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管理员 — Wed, 03 Sep 2014 04:26:20 +0000

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管理员 — Wed, 03 Sep 2014 04:22:06 +0000

The impact of sulfur passivation on the structural and electronic properties of the HfO2 /GaAs interface is investigated by density functional theory with a hybrid functional. The gap states at the HfO2 /GaAs interface arise from three major contributions: Ga 3+ and partial oxidation, As–As dimers, and Ga dangling bonds. By introducing S atoms at the interface, the removal of the gap states within the lower half of the GaAs band gap is observed, while the gap states in the upper half are pushed upward by ~0.15 eV.

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管理员 — Wed, 03 Sep 2014 04:21:02 +0000

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Ab initio Modeling of the Interaction of Electron Beams and Single-Walled Carbon Nanotubes

管理员 — Wed, 03 Sep 2014 04:20:14 +0000

Single-walled carbon nanotubes are readily observable in a scanning electron microscope, which traditional models fail to explain. We present an ab initio model to explain how the electron beam can interact with these structures despite the very small, nanoscale, interaction volume. In particular, we show how the electron beam can generate very strong secondary electron emission from the tip of a nanotube under external electric field. The approach may also be used in modeling the interaction of charged particles with nanostructures in other applications such as electron detection.

Modulation of contact resistance between metal and graphene by controlling the graphene edge, contact area, and point defects: An ab initio study

管理员 — Wed, 03 Sep 2014 04:17:31 +0000

A systematic first-principles non-equilibrium Green’s function study is conducted on the contact resistance between a series of metals (Au, Ag, Pt, Cu, Ni, and Pd) and graphene in the side contact geometry. Different factors such as the termination of the graphene edge, contact area, and point defect in contacted graphene are investigated. Notable differences are observed in structural configurations and electronic transport characteristics of these metal-graphene contacts, depending on the metal species and aforementioned influencing factors. It is found that the enhanced chemical reactivity of the graphene due to dangling bonds from either the unsaturated graphene edge or point defects strengthens the metal-graphene bonding, leading to a considerable contact resistance reduction for weakly interacting metals Au and Ag. For stronger interacting metals Pt and Cu, a slightly reduced contact resistance is found due to such influencing factors. However, the wetting metals Ni and Pd most strongly hybridize with graphene, exhibiting negligible dependence on the above influencing factors. This study provides guidance for the optimization of metal-graphene contacts at an atomic scale.