Preparation of Pt/Al2O3 and PtPd/Al2O3 diesel oxidation catalysts by supercritical deposition

Diesel oxidation catalysts (DOCs) are aftertreatment system parts responsible for the oxidation of CO, unburnt hydrocarbons and NO gases coming from heavy-duty diesel engines. In this study, DOCs were prepared, characterized, tested and compared with their commercial counterparts. In the first part of this study, four commercial monolithic DOCs with two different platinum group metal (PGM) loadings and Pt:Pd ratios of 1:0, 2:1 or 3:1 (w/w) were investigated systematically for NO, CO, and C3H6 oxidation, CO – C3H6 co-oxidation, and CO – C3H6 – NO oxidation reactions via transient activity measurements in a laboratory scale simulated diesel engine exhaust environment. As PGM loading increased, light-off curves shifted to lower temperatures for individual and co-oxidation reactions of CO and C3H6. CO and C3H6 were observed to inhibit the oxidation of themselves and each other. Addition of Pd to Pt was found to enhance CO and C3H6 oxidation performance of the catalysts while the presence and amount of Pd was found to increase the extent of self-inhibition of NO oxidation. NO inhibited CO and C3H6 oxidation reactions while NO oxidation performance was enhanced in the presence of CO and C3H6 due to the probable occurrence of reduced Pt and Pd sites during CO and C3H6 oxidations. The optimum Pt:Pd ratio for individual and co-oxidations of CO, C3H6 and NO was found to be Pt:Pd = 3:1 (w/w) in the range of experimental conditions investigated in this study. Pt/Al2O3 and bimetallic PtPd/Al2O3 catalysts were prepared via supercritical deposition (SCD) method using supercritical carbon dioxide. The effects of Pt loading of Pt/Al2O3 and Pd addition to Pt/Al2O3 on particle size, particle size distribution (PSD) and activity for NO and C3H6 oxidation and C3H6-selective catalytic reduction (C3H6-SCR) were investigated. Pt/Al2O3 catalysts were prepared with Pt loadings of 0.6, 1.2 and 2.1 wt% and a bimetallic PtPd/Al2O3 catalyst was prepared with total metal loading of 1.4 wt% and Pt:Pd molar ratio of 1.3:1. A small fraction of the particles agglomerated after calcination at 550 oC. Around 98% of the particles had an average particle size of 1 nm. The rest of the particles were larger and average size of these larger particles was 10 nm for monometallic catalysts and 6.5 nm for PtPd/Al2O3. All catalysts were found to be active for NO and C3H6 oxidation and C3H6-SCR reactions. NO oxidation performance of 1.2 wt% Pt/Al2O3 catalyst was the highest. C3H6 oxidation activity increased with increasing metal content. Light-off temperature for C3H6 oxidation shifted to higher temperature in the presence of NO, suggesting competitive oxidation of C3H6 and NO. Concentration profiles indicated that C3H6-SCR started when C3H6 conversion by oxidation reached 50%; C3H6 was consumed both by oxidation and C3H6-SCR at higher conversions. Morphologies of used and aged catalysts were investigated. HAADF-STEM imaging revealed that bimodality of the nanoparticles, sharp PSD and average nanoparticle sizes were maintained after usage in NO and C3H6 oxidation reactions. After thermal ageing at 800 oC, 1.2 wt% Pt/Al2O3 preserved the bimodality of the nanoparticles with slightly higher average nanoparticle size. 2.1 wt% Pt/Al2O3 prepared by SCD was compared with commercial monolithic Pt/Al2O3 DOC for NO oxidation performance. The DOC prepared by SCD performed better at temperatures lower than 325 oC. While maximum conversions achieved by both catalysts were more than 50%, commercial DOC achieved slightly higher values at a higher temperature. C3H6 oxidation kinetics of 2.1 wt% Pt/Al2O3 and bimetallic PtPd/Al2O3 prepared by SCD were investigated in a tubular flow reactor at differential conditions and in exhaust environment of diesel engines. Langmuir-Hinshelwood reaction mechanism with dissociative adsorption of O2 was assumed. C3H6-TPD results suggested dissociative adsorption of C3H6 as well. MATLAB was used to determine rate constants, adsorption equilibrium constants and exponents in the rate expressions by nonlinear regression. Pre-exponential factors and apparent activation energies were calculated using Arrhenius Law. For both catalysts, increase in C3H6 feed concentration decreased C3H6 oxidation reaction rate at isothermal conditions. The reaction rate increased with increasing O2 concentration for 2.1 wt% Pt/Al2O3 and it was zeroth order in terms of O2 for PtPd/Al2O3. Apparent activation energies of the oxidation reaction were calculated as 84.3 kJ mol-1 and 17.9 kJ mol-1 respectively for 2.1 wt% Pt/Al2O3 and PtPd/Al2O3. Average error between experimental and theoretical rates were calculated as 18.3% and 13.6% respectively. To our knowledge, this is the first study in which bimetallic PtPd/Al2O3 catalyst was prepared by SCD. This is also the first study on the kinetics of C3H6 oxidation over a DOC prepared by SCD and one of the very few studies where the presence of H2O in the feed mixture was taken into account during the investigation of C3H6 oxidation reaction kinetics.