Development of Surface Reaction Mechanisms of CO / O2 on Pt and Rh for Three Way Catalyst based on Gas Phase and Surface Species Analyses

ABSTRACT

In this study, detailed surface reaction mechanisms for CO/O₂ system on Pt/Al₂O₃ and Rh/Al₂O₃ have been developed based on the measurements of gaseous and surface species. The conventional conversion rate experiments for gaseous CO to CO₂ were conducted with the monolith honeycomb catalyst, and furthermore, the adsorbed surface species were identified with the powdered catalyst by in-situ FTIR under the same conditions as conversion rate experiment. As a result, it was found that only linear CO adsorption was detected for Pt, while twin CO adsorption was detected for Rh. Based on the results, detailed surface reaction mechanisms have been developed for Pt/Al₂O₃ and Rh/Al₂O₃. As a result of numerical simulation, it was confirmed that the variations of CO to CO₂ conversion rate with temperature were quantitatively reproduced for various O₂ concentration conditions. Furthermore, using those reaction mechanisms, CO conversion rate on bimetal, Pt/Rh Al₂O₃ catalysts were examined for several Pt/Rh ratios. Numerical simulation results for bimetal catalysts were also quantitatively agreed with experimental results for various Pt/Rh ratios as well as CO/O₂ ratios.

KEYWORDS:

• Catalytic combustion
• honeycomb monolith catalyst
• surface reaction mechanism
• platinum group metal
• bimetal catalyst

Nomenclatures

CR	=	: Conversion rate
CPSM	=	: Number of channels per square inch [1/in2]
s	=	: Repeat distance of the monolith [m]
δl	=	: Total thickness of the monolith’s wall [m]
δwall	=	: Thickness of the substrate wall [m]
δwcl	=	: Thickness of the washcoat layer [m]
dhyd	=	: Hydraulic channel diameter[m]
εg	=	: Volume fraction of the gas phase in the catalyst [m3/m3]
ρg	=	: Gas density [kg/m3]
${\omega }_{k,g}$	=	: Molar rate of production by chemical reaction of the kth gaseous species [mol/(m3·s)]
$v_g$	=	: Interstitial gas velocity [m/s]
$M_{k,g}$	=	: Molar mass of the species k in the gas phase [kg/mol]
$c_k^L$	=	: Concentration of species k in the reactive surface layer [mol/m3]
$c_{p,s}^{\hspace{0.17em}}$	=	: Specific heat of solid phase [J/(kg·K)]
$r_i(c_k^L,T_s)$	=	: Molar reaction rate of the catalytic surface reactions with their stoichiometric coefficients
${\nu }_{i,k}$	=	: Stoichiometric coefficient of species k in reaction i
$h_{k,g}$	=	: Enthalpy of species k in the gas phase [J/(kg·K)]
${\lambda }_g$	=	: Thermal conductivity of the gas [W/(m·K)]
$A_s$	=	: Surface of the solid part in the computation cell [m2]
$k_h$	=	: Heat transfer coefficient [W/(m2·K)]
Ts	=	: Solid temperature [K]
Tg	=	: Gas temperature [K]
$\mathrm{\Delta }{h}_i$	=	: Heat of reaction i [kJ/mol]
$D_{k,eff}$	=	: diffusion coefficient of species k (parallel pore model in this study)
${\tau }_{wcl}$	=	: Tortuosity of the washcoat layer
${\epsilon }_{wcl}$	=	: Porosity of the washcoat layer
$D_{k,g}$	=	: Diffusion coefficient of the species k in the gas mixture [m2/s]
$D_{Kn}$	=	: Knudsen diffusion coefficient [m2/s]
$d_{pore}$	=	: Pore diameter of washcoat layer [m]
$M$	=	: Molar mass of the entire gas phase [kg/mol]
$A_{PGM}$	=	: Total surface area of noble metal APGM responsible for the catalytic reaction [m2]
${\gamma }_{cat}$	=	: Metal dispersion
${\mathrm{\Gamma }}_{PGM}$	=	: Site density of noble metal [mol/m2]
$M_{PGM}$	=	: Molar mass of noble metal [kg/mol]
$W_{PGM}$	=	: Metal loading of the catalyst [g/L]
$V$	=	: Volume of computational cell [m3]
$\dot{r}$	=	: Converter (monolith) volume based species consumption and creation rate [mol/(m3·s)]
$V_{conv}$	=	: Volume of converter (monolith) [m3]
$\bar{\dot{r}}$	=	: Rate constant of surface elementary reactions [mol/(m2·s)]