In-situ/Operando Spectroscopic Studies Of Heterogeneous Catalysis

In-situ/Operando studies of heterogeneous catalytic reactions is the most challenging topic in the fundamental studies of heterogeneous catalysis but is the core issue in understanding the reaction mechanism of catalytic reactions and catalysis of solid surfaces. Recent advances in the spectroscopic techniques are making the In-situ/Operando studies of heterogeneous catalytic reactions into the reality. In this thesis, In-situ/Operando spectroscopic techniques were developed on the basis of synchrotron radiation-based spectroscopies and Diffuse-reflectance infrared spectroscopy and used to study the mechanisms and structure-activity relations of catalytic oxidation of light alkanes and CO adsorption/oxidation. The main results include:1. An in-situ catalytic reaction reactor was set up in connection with the synchrotron VUV photoionization mass spectroscopy, which was used for the online detection of the gas-phase products of the oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane catalyzed by Li/MgO catalysts. Highly reactive gas-phase radicals and intermediates, including methyl radical, ethyl radical, methylperoxy radical, methylhydroperoxide and ethylhydroperoxide, were unambiguously identified. These results provide key experimental evidence for the reaction mechanisms:the activation of alkane on catalyst surface to generate gas-phase alkyl radicals followed by the reaction of radicals on the surface or in the gas-phase to produce alkenes, alkynes, CO, CO2 and et al..2. Li/MgO OCM catalysts with initial Li loadings of 1.3%Li (wt%) and 5.6% Li (wt%) were observed to contain the same Li loading after OCM reaction but exhibit different microstructures and catalytic performances.5.6%-Li/MgO exhibits an irregular morphology with high index facets (such as{111} facets) and rich defective sites while 1.3%-Li/MgO mainly exposes{100} facets, and 5.6%-Li/MgO exhibit a higher concentration of gas-phase methyl radical and is more selective in C2 formation than 1.3%-Li/MgO. The synchrotron radiation X-ray photoelectron spectroscopy results demonstrate that more Li is located within the near-surface region of 5.6%-Li/MgO than that of 1.3%-Li/MgO. These observations suggest that the presence of sufficient amount of Li in Li/MgO during the OCM reaction and its segregation and subsequent desorption from the catalyst surface play a critical role in forming the morphology exposing high-energetic crystal planes and a high density of defective sites beneficial for the OCM reaction.3. Li/MgO catalysts with the same initial Li loading employing MgO with different morphologies were synthesized and used for the OCM reaction. It was found that the MgO morphology affect the Li-MgO interaction and the final Li loading of the catalysts during the OCM reaction. MgO with a thinner thickness and a high specific surface area is capable of reversing more Li in Li/MgO catalysts. These results suggest that the MgO morphology can be tuned to optimize the catalytic performance of Li/MgO catalysts for the OCM reaction.4. Pulse chemisorption/reaction system and vacuum adsorption/reaction system were set up on the basis of Nicolet6700 Fourier transformed spectroscopy (DRIFTS) equipped with Harrick in-situ diffuse reflectance reaction cells to realize in-situ/operando DRIFTS studied of adsorption/reaction on solid catalysts without the gas-phase interferences. Employing this system pulse CO adsorption on Pd/SiO2 catalysts was in-situ and time-resolved studied, in which the simultaneous chemisorption of CO on top, twofold and threefold bridged sites of Pd particles with different adsorption energies were observed and followed by the subsequent desorption, surface diffusion, and transformation of weakly bound CO in to strongly bound CO. These results provide strong experimental evidence for the dynamic nature of adsorption processes on solid surfaces.