First authors: Fang Yarong, Li Huijuan
Corresponding authors: Associate Professor Luo Zhu, Professor Guo Yanbing
Thesis DOI: 10.1021/acs-est.1c07573
Graphical Abstract
Introduction to Achievements
Recently, Professor Yanbing Guo's research group from Central China Normal University published a research paper titled "Oxygen Vacancy Governed Opposite Catalytic Performance for C3H6 and C3H8 Combustion: The Effect of Pt Electronic Structure and Chemisorbed Oxygen Specifications" in Environmental Science&Technology (DOI: 10.1021/acs. est. 1c07573), which explores the different mechanisms of oxygen vacancies on the surface of Pt/TiO2-x catalysts for the catalytic combustion of propylene (C3H6) and propane (C3H8). This article synthesized Pt/TiO2-x catalyst rich in oxygen vacancies through oxygen vacancy regulation strategy, studied the changes in Pt electronic structure caused by oxygen vacancies and their effects on surface chemisorbed oxygen species, and deeply revealed the promoting effect of oxygen vacancies on the catalytic oxidation of short chain unsaturated hydrocarbon C3H6 and the inhibitory effect of short chain saturated hydrocarbon C3H8. This study provides theoretical guidance for the synthesis of highly efficient and stable Pt based catalysts and their environmental catalytic applications.
Introduction
Straight chain hydrocarbons represented by propylene (C3H6) and propane (C3H8) are one of the main components of atmospheric pollutants VOCs, and their environmental hazards should not be underestimated. Precious metal Pt is a commonly used VOCs catalytic oxidation catalyst in industry, and constructing surface oxygen vacancies is a common strategy to improve catalytic oxidation performance. However, the mechanism by which oxygen vacancies affect short chain unsaturated hydrocarbons C3H6 and short chain saturated hydrocarbons C3H8 with different molecular structures during catalytic oxidation reactions is still unclear. Therefore, this study investigated the catalytic oxidation performance of Pt/TiO2-x catalysts rich in surface oxygen vacancies for C3H6 and C3H8; By combining theoretical calculations and spectroscopic characterization, the influence of surface oxygen vacancies on the particle size, surface valence state, and other properties of Pt nanoparticles was studied; Explored the influence of surface oxygen vacancies on the adsorption sites, adsorption energy, and activation energy of O2; Revealed the different mechanisms of action of the above properties on the catalytic oxidation of C3H6 and C3H8.
Graphic and textual guidance
Sample structure
Figure 2. High resolution transmission electron microscopy (HRTEM) images of a: Pt/TiO2-x;d: Pt/TiO2 catalysts; The high-angle annular dark-field scanning (HAADF) image of b: Pt/TiO2-x;e: Pt/TiO2; The insertion shows the corresponding size distribution of platinum nanoparticles; c: Low-temperature (liquid nitrogen, -196 °C) electron paramagnet icresonance (EPR) spectra collected in vacuum; f: The simulated Pt18(111)/TiO2-x and Pt12(111)/TiO2 models and corresponding calculated formation energy.
The HRTEM results showed that compared to the Pt/TiO2 catalyst, the Pt nanoparticles loaded on the surface of the Pt/TiO2-x catalyst were larger in size (3.31 nm vs. 1.86 nm). The EPR and DFT results indicate that the surface oxygen vacancies of TiO2 support can serve as nucleation centers for Pt nanoparticle growth and promote the growth of Pt nanoparticles.
Surface adsorbed oxygen species
Figure 3. a: O 1sXPS spectra of TiO2, TiO2-x, Pt/TiO2 and Pt/TiO2-x catalysts; b: O k-edge XANES spectra of Pt/TiO2 and Pt/TiO2-x catalysts; c: Room-Temperature electron paramagnetic resonance (EPR) spectra collected in air atmosphere; d: Three typical O2 adsorption configurations on Pt18(111)/TiO2-x surface and Pt12(111)/TiO2 surface, and corresponding oxygen molecule adsorption energies.
Combining the O 1s XPS spectrum and O K-edge XANES spectrum, we found that there are abundant adsorbed oxygen species at the interface between Pt nanoparticles and TiO2-x support in Pt/TiO2-x catalyst rich in oxygen vacancies. DFT results showed that the adsorption configuration of the adsorbed oxygen species is Pt-O-O-Ti, exhibiting the structural properties of peroxide species (O22-) and having good stability.
The electronic structure of Pt
Figure 4. a: Pt 4f X-ray photoelectron spectroscopy (XPS) of Pt/TiO2 and Pt/TiO2-x catalysts after Ar sputter; b: the 3D isosurface of local charge density difference (down) of Pt18(111)/TiO2-x and Pt12(111)/TiO2 models; electron accumulation and depletion are represented by yellow and cyan respectively. c: CO adsorption DRIFTS spectra over Pt/TiO2-x and Pt/TiO2 catalysts, respectively. d: the pristine configuration (up) and 3D isosurface of local charge density difference (down) of O2 adsorbed Pt18(111)/TiO2-x and O2 adsorbed Pt12(111)/TiO2 models; electron accumulation and depletion are represented by yellow and cyan.
The Pt 4f XPS spectra and DFT results indicate that there is charge transfer between the Pt nanoparticles loaded on the surface of the oxygen rich vacancy Pt/TiO2-x catalyst and the oxygen vacancies, and the electrons confined by the oxygen vacancies are transferred to the Pt sites, promoting the generation of reduced Pt0 species.
Pt0 promoted C3H6 catalytic oxidation
Figure 5. In situ DRIFTS spectra of propene adsorption under pure C3H6 stream (down) and propene oxidation under C3H6+O2 stream (upper) on a: Pt/TiO2-x catalyst and b: Pt/TiO2;c: Left: Projected density of states (PDOS) plots of Pt d orbitals (blue) and adsorbed carbon (in C3H6 molecule) 2p orbitals (purple) on Pt18(111)/TiO2-x(upper) and Pt12(111)/TiO2 (down) surface; Right: the typical C3H6 adsorption configurations and corresponding adsorption energy (Ea); d: Dependence of reaction rate onpartial pressure of O2 of Pt/TiO2-x and Pt/TiO2 catalysts. The outlet gas components of e: Pt/TiO2-x and f: Pt/TiO2 catalysts during C3H6 temperature programmed surface reaction (C3H6-TPSR) experiments.
The results of C3H6-DRIFTS and DFT indicate that the active reduced Pt0 sites on the surface of Pt/TiO2-x catalyst can promote the chemical adsorption and activation of C3H6 molecules by effectively activating C=C bonds, promoting the generation of intermediate products such as carboxylate and carbonate, and further complete oxidation to CO2.
C3H8 catalytic oxidation inhibited by interface adsorption of Pt-O-O-Ti
Figure 6. In situ DRIFTS spectra on a: Pt/TiO2-x catalyst and b: Pt/TiO2 during propane adsorption and propane oxidation; c: Left: projected density of states (PDOS) plots of Pt d orbitals (blue) and adsorbed carbon (in C3H8 molecule) 2p orbitals (purple) on Pt18(111)/TiO2-x (upper) and Pt12(111)/TiO2 (down) surface; Right: the typical C3H8 adsorption configurations and corresponding adsorption energy (Ea); d: Dependence of reaction rate on partial pressure of O2 of Pt/TiO2-x and Pt/TiO2 catalysts. The outlet gas components of e: Pt/TiO2-x and f: Pt/TiO2 catalysts during C3H8 temperature programmed surface reaction (C3H8-TPSR) experiments.
The C3H8-DRIFTS and DFT results indicate that oxygen vacancies promote the adsorption of O2 in the form of Pt-O-O-Ti species, while the competitive adsorption between O2 and C3H8 molecules severely inhibits the effective adsorption of C3H8 molecules, thereby suppressing the catalytic combustion of propane.
Summary
Scheme 1. The catalytic mechanism of the promotion of C3H6 combustion (left) and suppression of C3H8 combustion (right) over the surface of Pt/TiO2-x catalyst.
This work synthesized Pt/TiO2-x catalysts with oxygen vacancies through oxygen vacancy regulation strategy, and further investigated the effects of oxygen vacancies on Pt electronic properties and chemisorbed oxygen species, revealing the different catalytic combustion mechanisms of C3H6 and C3H8. There is charge transfer between the oxygen vacancies on the surface of Pt/TiO2-x catalyst and the supported Pt nanoparticles, leading to the generation of a large number of reducing Pt0 species and chemically adsorbed peroxide species O22- (Pt-O-O-Ti). The reducing Pt0 species can serve as adsorption and activation sites for propylene molecules, effectively promoting the cleavage of C=C double bonds and significantly improving the catalytic oxidation efficiency of C3H6. On the contrary, the competitive adsorption between O2 and C3H8 molecules on the surface of Pt/TiO2-x catalyst inhibits the physical adsorption and activation of C3H8 molecules, thereby reducing the catalytic combustion efficiency of C3H8. This study clearly elucidates the structure-activity relationship between oxygen vacancies on the catalyst surface and the catalytic oxidation of short chain straight chain hydrocarbons, providing theoretical guidance for the design of efficient VOCs catalytic oxidation catalysts.
Author's Introduction
Dr. Yanbing Guo, a professor and doctoral supervisor, currently works at the School of Chemistry, Central China Normal University as the Vice Dean. National Youth Thousand Talents Program, Hubei Province Hundred Talents Program, member of the Youth Committee of the Chinese Chemical Society, and young editorial board member of the Chinese Chemical Letter journal. Previously engaged in efficient catalyst preparation and research and development of automotive exhaust aftertreatment systems at the University of Connecticut, Massachusetts Institute of Technology (MIT), and 3D Array Technology Co., Ltd. in the United States. At present, we are mainly engaged in research on the creation of high stability gas-phase pollutant purification catalysts, and have led more than 10 national level youth talent start-up funds, national natural science foundation general projects, etc. More than 70 SCI papers have been published in total, with over 20 corresponding and first author papers in first tier journals such as Nature Commun., J. Am. Chem. Soc., Adv. Mater., Environment. Sci. Tech. cited over 3000 times, with an H-factor of 27.
Dr. Zhu Luo, Associate Professor, Master's Supervisor, currently works at the School of Chemistry, Central China Normal University. Graduated from the University of Connecticut in 2016 and was selected for the Chutian Scholar Program in Hubei Province. At present, the main research field is the modification of nanomaterials and their application in gas-phase catalytic reactions. Published over 30 papers in academic journals such as J. Am. Chem. Soc., Adv. Energy Mater., Chem. Mater., Angew. Chem., cited over 1200 times, with an H-factor of 19. Obtained 1 US patent and 1 domestic patent.
Article link：https://pubs.acs.org/doi/10.1021/acs.est.1c07573
Original link：https://mp.weixin.qq.com/s/IrejQDA5CEn6Y6BwplRWiw
