Understanding The CO Oxidation Reaction Mechansim Over Supported Gold Nanoparticle And Platanium Single-atom Catalysts

With the increasing issues of social resource depletion and environmental pollution, catalysis plays an important role in chemical production and environmental improvement. Supported metal catalysts are widely used in heterogeneous reactions due to their excellent catalytic activities. Understanding the catalyst structure-activity relationship at the atomic level is essentially important. However, the complexed sturcutres of the real catalyst system makes it extremely difficut to understand the raction mechanism. Simplifying catalyst structure is an effective way to address this issue. In this work, we applied two stratageies to simplify the supported metal catalysts system, on which the reaction mechanism in CO oxidation was investigated: first, we, for the first time, precisely tuned the Au-TiO2 interfacial length and low-coordination Au atoms to different degrees while preserving the Au particle size using atomic layer deposition (ALD). Such supported Au catalyst with TiO2 overcoat provides a new "inverse" real catalysts system for clearly understanding the individual roles of Au low-coordination sites, Au-TiO2 interface and Au size effect for Au/TiO2 catalysts in CO oxidation. Second, atomically dispersed noble metals on metal oxide supports have gained wide attention due to the unique catalytic properties and maximized atom efficiency. Compared with supported metal nanoparticle catalysts, single-atom catalysts (SACs) have well-defined structures, allowing to investigate the complex reaction process and understand the reaction mechanism at the atomic level. The mainresulsts are as follows:(1) We chose two inert supports as the starting materials to synthesize Au/Al2O3 and Au/SiO2 catalysts using the deposition-precipitation (DP) method, then different cycles of TiO2 ALD overcoat were applied onto the Au catalysts and new interfaces between Au and the TiO2 overcoat were created while maintaining the Au particle size; therein, the new Au-TiO2 interfacial length was precisely tuned to different degrees, and the exposed surface Au atoms were gradually decreased with increasing ALD cycles. High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements of CO chemisorption all demonstrated that the TiO2 overcoat preferentially decorates the low-coordinated sites of Au nanoparticles and follows an island growth mode. In CO oxidation, we observed a remarkable improvement of the catalytic activity of Au/Al2O3 and Au/SiO2 catalysts by the TiO2 overcoat. More interestingly, the activity as a function of the number of ToO2 ALD cycles showed a volcano-like behavior, providing direct evidence that the total Au-TiO2 interfacial length are the active sites in CO oxidation, instead of Au low-coordination sites.(2) The catalytic activity of Au/TiO2catalyst in CO oxidation was greatly influenced by the Au particle size. However, the origin of the Au particle size effect is still widely debated. Decreasing particle size inevitably results in an increase of the number of low-coordination sites and the total length of perimeter sites at the metal-support interfaces, or even the change of Au oxide state. It is hard to separate these three factors for understanding their individual roles. In order to solve the problem, here we further applied TiO2 ALD overcoating onto three Au/TiO2 catalysts with different particle sizes (2.9±0.6,5.0±0.8 and 10.2±1.6 nm), so that the number of exposed low-coordination sites and the length of perimeter sites are precisely tuned by varying ALD cycles, meanwhile their own particle sizes were maintained. Comparing the catalytic activity changes of these three Au/TiO2 catalysts by TiO2 overcoat, we clearly illustrated that the Au particle size effect in CO oxidation is not related with the number of the low-coordination Au sites, or the changes in oxidation states. Size-related change in the length of perimeter sites at the Au-TiO2 interface might largely contribute to the Au particle size effect.(3) Poor thermal stability of supported gold catalysts at elevated temperatures is one key issue for practical the applications in industrial processes. Here TiO2 and Al2O3 ALD overcoat with precise thickness control was deposited onto an Au/T1O2 catalyst and the thermal stability of the resulting overcoated Au catalysts was investigated. We surprisingly found that sub-nanometer-thick Al2O3 overcoat can sufficiently inhibit the aggregation of Au particles up to 600℃ in oxygen. On the other hand, the enhancement of Au nanoparticle stability by TiO2 overcoat is very limited. The catalytic activities of the Al2O3 overcoated Au/TiO2 catalysts in CO oxidation increased as increasing calcination temperature, which suggests that the embedded Au nanoparticles becomes more accessible for catalytic function after high temperature treatment. Finally, our work might open new opportunities to apply the stabilized Au catalysts in other reactions under severe conditions.(4) Atomically dispersed SACs have gained wide attention for the unique catalytic properties and maximized atom efficiency. Here we reported that single Pt atoms can be anchored on CeO2 nano powder using ALD technique (Pt1/CeO2). Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and DRIFTS of CO chemisorption both confirmed the presence of isolated Pt single atoms on the CeO2 support. In CO oxidation, we found that the presence of water could significantly improve the activity of CO oxidation, and lower the corresponding apparent activation barrier from 0.64 to 0.41 eV. Our DFT calculations and isotope-labeling experiments both suggested that water changes the reaction pathway, contributes explicitly to about half amount of CO2 production and oxygen exchange between molecule O2 and adsorbed water. The finding of direct participation of water in final CO2 production on Pt1/CeO2 was different from the water effect on Au particles catalysts, on which water mainly assists oxygen molecule adsorption, activation and the decomposition of carbonate, without directly participating in the final CO2 formation. Since water molecules are widely present in reactant gases, the new insights revealed are expected to be stimulating for other oxidation reactions.