Model Inverse Nanocluster Catalysts: The Role of Size, Shape and Composition on the Catalytic Activity of Small Metal Oxide Clusters on Metal Surfaces

化学催化涉及一种称为催化剂的化学物质，它通过为感兴趣的化学反应提供另一种途径来降低能源成本并创造更有选择性的产物分布。 催化剂通常用于产生环境友好的燃料，例如氢气，它可以清洁地燃烧成水，也可以用于生产增值化学品，如一氧化碳和甲醇，这些化学品可以从可持续的作物来源中生产。由于其重要性，催化剂功能是化学界研究的主要驱动力。过渡金属和过渡金属氧化物具有许多催化剂所需的化学性质，可以活化CO2、H2O和CH 4中的键，这些键是特别有前途的原料，用于更可持续地生产燃料和增值化学品。 这些金属和它们的氧化物在0.00000005英寸或纳米尺寸范围内的小原子团簇中特别活跃。纳米团簇由于其独特的几何和电子特性，如配位不足的表面原子，改性的原子间间距和大的表面体积比，表现出增强的反应性。 在这个项目中，Buratto，Bowers和Metiu博士通过将定义明确的原子级精确纳米团簇从气相沉积到金属和金属氧化物载体上来生产金属和金属氧化物的模型催化剂。然后测试这些模型系统在氢气和甲醇生产中的反应性，氢气和甲醇作为清洁燃烧燃料和化学原料都很重要。该研究小组控制纳米团簇组成原子的独特能力提供了必要的细节水平，以了解原子水平上的化学，并为新催化系统的开发提供了重要的见解。Buratto博士、Bowers博士和Metiu博士以及该项目支持的研究生积极与圣巴巴拉县和文图拉县的高中生开展外联活动，讨论他们的研究及其影响，并促进科学教育。在化学部化学催化计划的资助下，Buratto博士、Bowers博士和Metiu博士编写、表征、并测试了两类新的纳米级催化剂，它们基于小的金属氧化物和金属氧化物簇的原子-原子组装，具有一个共同的特征;它们具有非常小的、孤立的、明确的催化活性位点和增强的催化活性。 本论文的主要研究内容是以TiO_2（110）为载体，制备PdAun和PtSnm团簇，以及以Pt（111）为载体，制备FexOy团簇。这些模型系统是通过从气相沉积质量选择簇到单晶表面上来控制催化剂的尺寸和组成而制备的。然后在超高真空中通过X射线光电子能谱（XPS）和扫描隧道显微镜（STM）研究样品以确定组成和结构。 采用程序升温反应（TPR）对催化剂的水煤气变换反应、CO氧化反应和甲醇合成反应活性进行了考察。用密度泛函理论计算了团簇的结构、XPS谱和化学活性，并与实验结果进行了比较。这些结果用于在原子水平上对催化化学进行详细的基本理解，从而有助于优化重要的工业过程，提高现有催化剂的性能或发现新的催化剂。 Buratto博士、Bowers博士和Metiu博士的研究小组致力于K-12的推广和科学的推广。他们正在将他们在多相催化方面的研究纳入加州大学圣巴巴拉分校（UCSB）的五年级外展计划。 这个项目将圣巴巴拉地区小学的五年级学生带到UCSB校园进行动手科学活动。 研究小组还与教师合作，开发适合五年级课程的催化课程，然后将其纳入推广计划。 此外，该项目支持的研究生积极与圣巴巴拉和文图拉县的高中生进行外联，讨论他们的研究及其影响，并促进科学教育。

Chemical catalysis involves a chemical substance, called a catalyst, which lowers energy costs and creates more selective product distributions by providing another pathway for the chemical reaction of interest. Catalysts are often employed to generate environmentally friendly fuels, such as hydrogen which burns cleanly to water, and are also used to produce value-added chemicals, like carbon monoxide and methanol which can be made from sustainable crop sources. Because of the importance, catalyst function is a major driving force of research in the chemistry community. Transition metals and transition metal oxides possess many of the desired chemical properties for catalysts that can activate the bonds in CO2, H2O and CH4, which are particularly promising feedstocks for a more sustainable production of fuels and value-added chemicals. These metals and their oxides can be especially active as small atomic clusters in the 0.00000005 inch or nanometer size range. Nanoclusters exhibit enhanced reactivity due to their unique geometric and electronic characteristics such as under-coordinated surface atoms, modified inter-atomic spacings and large surface to volume ratio. In this project, Drs. Buratto, Bowers and Metiu produce model catalysts of both metals and metal oxides by deposition of well-defined, atomically-precise nanoclusters from the gas phase onto metal and metal oxide supports. These model systems are then tested for their reactivity in the production of hydrogen and methanol, important as both clean-burning fuels and chemical feedstocks. The research team's unique capability to control nanocluster composition atom-by-atom provides the requisite level of detail to understand the chemistry on the atomic level and provides important insight into the development of new catalytic systems. Drs. Buratto, Bowers and Metiu and graduate students supported by this project are active in outreach to high school students in the Santa Barbara and Ventura Counties to discuss their research and its impact as well as promote science education.With funding from the Chemical Catalysis Program of the Chemistry Division, Drs. Buratto, Bowers and Metiu prepare, characterize, and test two new classes of nanoscale catalysts based on the atom-by-atom assembly of small bimetallic and metal oxide clusters having one feature in common; they have very small, isolated, well-defined, catalytically-active sites and enhanced catalytic activity. The research centers on the preparation of well-defined PdAun and PtSnm clusters supported on single crystal TiO2(110) and well-defined FexOy supported a single crystal Pt(111) in the inverse catalyst geometry. These model systems are prepared by depositing mass-selected clusters from the gas phase onto single crystal surfaces to control catalyst size and composition. Samples are then studied in ultra-high vacuum by x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) to determine composition and structure. Temperature programmed reaction (TPR) is used to probe the activity to the water gas shift reaction, CO oxidation, and methanol synthesis. Density functional theory (DFT) is used to calculate the structure of the clusters, their XPS spectrum and their chemical activity, and these data are then compared to experiment. The results are used to develop a detailed fundamental understanding of the catalytic chemistry at the atomic level that will in turn help optimize important industrial processes, and improve the performance of the existing catalysts or uncover new ones. The research groups of Drs. Buratto, Bowers and Metiu are committed to K-12 outreach and the promotion of science in general. They are incorporating their research in heterogeneous catalysis into the University of California Santa Barbara's (UCSB's )5th grade outreach program. This program brings 5th grade students from elementary schools in the Santa Barbara area to the UCSB campus for hands-on science activities. The research team is also working with the teachers to develop a lesson in catalysis that is appropriate for the 5th grade curriculum and then incorporate it into the outreach program. In addition, graduate students supported by this project are active in outreach to high school students in the Santa Barbara and Ventura Counties to discuss their research and its impact, as well as to promote science education.