photo Wang, Dezheng
Professor of Chemical Engineering
Ph.D. 1982, Yale University
E-mail: wangdezheng@tsinghua.edu.cn
Phone: 86 10 6279-4467
Fax: 86 10 6277-2051
 

Chinese version

 


Chemical Kinetics and Heterogeneous Catalysis

The goal of our research is to characterize the chemical kinetics of heterogeneously catalyzed reactions at the level that can provide a basis for the use of the reaction mechanism in chemical reaction engineering. Conventional kinetics characterization developed with the usual laboratory reactors use the power-law or Langmuir-Hinshelwood form that entails non-uniqueness of the underlying reaction mechanism. With these kinds of kinetics, it is not safe to extrapolate experimental kinetics data to different reaction conditions. We are interested in developing experimental and computational techniques to characterize reaction kinetics at the elementary step level ("elementary step level" as loosely defined in heterogeneous catalysis). The techniques we have developed are used for the characterization of active sites and diffusivities, and we further include the use of results in the literature that were obtained using surface science and molecular spectroscopic techniques to develop our own version of the mathematical description of the microkinetics, active site, and diffusivity characteristics of industrial chemical processes to provide mechanistic insights in the chemical reaction engineering.
   From another application point of view, the understanding of the catalytic chemistry is very useful for catalyst development.
   Challenges in the study of complex reaction systems include
reaction species identification, rate parameter measurements and kinetic parameter estimation, and the development of tools for their high precision ("high precision" as loosely defined in heterogeneous catalytic kinetic) modeling.

Simulation of surface reactions 
Solid catalysts use gas mixtures for their reactions, so calculating the surface concentrations of mixed components is important. The textbooks teach using the multicomponent Langmuir isotherm (MLI) for competitive adsorption for this. But MLI calculations can differ from those using equations that include the relationships of solution thermodynamics, e.g., ideal adsorbed solution theory (IAST), which use Raoult’s Law for relating gas and adsorbed compositions. Despite this, the MLI is used in heterogeneous catalytic kinetics because it happens to be consistent with the IAST when the monolayer capacity is the same for all species. For competitive adsorption on many adsorbents, the monolayer capacity or site density would be the same for all species, and the MLI can be validly used.
   Our recent work showed that even when the site density for all species is the same, the MLI surface concentrations are not the same as the IAST surface concentrations when the adsorbent is a zeolite. This is due to that there is no free gas movement in the zeolite such as exist over an open surface. The confinement of molecules inside the zeolite results in additional physisorbed species that do not exist on an open surface. The space above an open surface allows physisorbed species to desorb and thus its weakly physisorbed concentration will be negligibly low. In contrast, in a zeolite the adsorption of many more molecules than there are acid sites is well known. An aspect of this that our work seeks to generalize is that the adsorption of molecules proceeds in two distinct phases, and that due to this, the MLI gives wrong surface concentrations.

Selected Publications:

"The significance of a second adsorption phase with weakly adsorbed species for the calculation of the surface concentrations of a mixture: methanol-DME and methanol-ethene adsorption in SAPO-34," F Wang, Y Kobayashi, YX Li, Y Wang, DZ Wang, Research on Chemical Intermediates, 41(4), 2015; DOI 10.1007/s11164-015-1981-x.

"A microscopic model of the Tian-Calvet microcalorimeter, cell design for a faster response, and measurement by a continuous procedure," Y Kobayashi, F Wang, QX Li, DZ Wang, Review of Scientific Instruments, 85,
034101, 2014.

"Differences in the methanol-to-olefins reaction catalyzed by SAPO-34 with dimethyl ether as reactant," YX Li, MY Zhang, DZ Wang, F Wei, Y Wang, Journal of Catalysis, 311, 281, 2014.

"Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed methane flames," QX Li, TF Wang, YF Liu, DZ Wang, Chemical Engineering Journal, 207-208, 235, 2012.
        
Publications

Updated: 29/03/15

 

Tsinghua University

Department of Chemical Enginering

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