Professor Peter Stair of Northwestern is supported by the Chemical Catalysis (CAT) program in the Division of Chemistry to investigate the surface chemistry associated with the synthesis of catalytic structures using low-temperature atomic layer deposition (ALD) techniques. The specific aims of the proposed research are: (1) to prepare heterogeneous catalysts with controlled structures of materials deposited on flat substrates using low-temperature atomic layer deposition (ALD) techniques; (2) to characterize the surface chemistry of the ALD-grown materials using a suite of surface analysis tools; and (3) to study the catalytic chemical transformations of simple molecules relevant to energy and the environment and correlate the catalytic performance with the structure and composition of the ALD-synthesized catalysts.
The project will provide fundamental information on elementary bond activation reactions and on the nature of the catalytically active sites of these nanoscale catalytic systems at the atomic level. It will also lead to a better understanding of the structure and activity of practical catalysts of relevance to energy and the chemical industry. The students involved in the project will be provided with highly technical training in surface science and catalysis. They will also gain a broader perspective on heterogeneous catalysis as a result of interactions with scientists at Argonne National Laboratory.
Heterogeneous catalysis is crucial to the conversion of natural resources into chemicals and fuels. The importance and ubiquity of catalysis in our everyday lives has led to significant efforts to improve the efficiency of these processes. The invention of new supports, new architectures, different catalyst combinations, alloying, fundamental research and many more different approaches has been devoted to creating and improving heterogeneous catalysts. Our research has contributed to better understanding of surface chemistry that takes place during the synthesis of heterogeneous catalytic materials by atomic layer deposition (ALD). The application of ALD to catalysis is a recent advancement. During ALD a material is deposited layer-by-layer with atomic control. The work under this grant focused on three studies: 1) how the substrate geometry controls the atomic structure of supported VO4 catalyst species, 2) the chemistry of titanium tetraisopropoxide (an ALD precursor) on oxidized molybdenum and 3) the influence of zeolite structure on the nature of deposited Pt species. Study 1. Supported VO4 monomers are catalysts for the oxidative transformations of organic molecules. It has been demonstrated that a bidentate VO4 monomer (Figure 1, structure 2) is more active than the other known structures. However, the tridentate species (structure 3) is most commonly observed on typical metal oxide supports such as Al2O3. With X-ray photoelectron spectroscopy (XPS) measurements and ALD we have shown that bidentate VO4 monomers can be preferentially formed by the deposition of vanadyl triisopropoxide onto SrTiO3 (001). The key to forming this bidentate structure is in choosing a support whose oxygen atoms have a geometry and spacing that only allow the formation of two vanadium-surface bonds. Study 2. Titanium tetraisopropoxide (TTIP) is a commonly used reagent in atomic layer deposition (ALD) of TiO2. As depicted on the left side of Figure 2, the reaction proceeds between TTIP and surface hydroxyl groups to form a bound Ti species and release isopropanol. The ALD reaction stops when the surface hydroxyls are covered by these species and no longer accessible for further reaction. However, upon reacting TTIP with an oxidized molybdenum surface we observed a process in which TiO2 grows continuously. The reaction never stops. This competing reaction pathway, which proceeds through dehydration of nascent TiO2, becomes dominant when the TTIP flux onto the surface is very low. This unexpected chemistry is likely to play a more significant role in transport-limited ALD processes such as at the bottom of a long thin pore in a microporous support. Project 3. Single-site and small cluster catalysts are of interest because their catalytic behavior is unusual and they represent the ultimate in efficiency for the use of expensive noble metals in catalysis. However, the synthesis of materials that are stable under reaction conditions remains a great challenge. We have begun to explore how the atomic surface structure and composition in mesoporous zeolites can influence ALD reactions in hopes of achieving the controllable deposition of single-site catalysts. Zeolites are promising reaction platforms because: 1) their reactive sites are strictly isolated in the zeolite structure and 2) the reactivity of these sites is exceptional in comparison to the surrounding zeolite matrix due to their high Brønsted acidity. These two properties make it possible to selectively deposit late transition metal clusters and single atoms by ALD. The deposited metal species are stable because they are trapped by strong binding to the reactive sites. Our initial effort has focused on the growth of Pt in mesoporous zeolite H-MFI. Two types of Pt species, single atoms and clusters, were identified by spectroscopic measurements using CO as a probe molecule. The ratio between these two types of Pt species can be widely tuned by the ALD chemistry. We also observed that only the Pt clusters were active catalysts for CO oxidation and that the single atoms were easily poisoned by CO binding. These results emphasize the importance of catalyst architecture at the atomic scale in determining the surface species and the corresponding chemistry/catalysis.
