Novel metal oxide nanocomposites for oxygen storage, sulfur dioxide adsorption and hydrogen sulfide absorption

Abstract

Increasingly stringent regulations on automotive emissions have resulted in the need for improved pollution control technology. To reduce mobile emissions, researchers have investigated alternatives such as lean-bum engines and fuel cells. This work is focused on the synthesis, characterization and testing of novel metal oxide nanocomposites to facilitate the utilization of these technologies. In lean-bum engines, the use of adsorbents to remove NOx faces two major challenges: (1) excess hydrocarbon and CO emissions during fuel-rich pulses for adsorbent regeneration, and (2) reduced NOx adsorption efficiencies due to competitive adsorption of SO2 in the gas stream. To provide for the low-temperature oxidation of hydrocarbons and CO under a reducing atmosphere, CeO2, a well-known oxygen storage material, was modified through secondary metal oxide doping to improve thermal stability and oxygen accessibility. 20 at% substitution of Pr, Sc and Zr in CeO2 successfully promoted microstructural stability, with Ceo.8Zro0.202- retaining grain size of 30 nm even after calcination at 10000C. At high doping levels, Zr improved grain size stability further, but ZrO2 phase segregation was noted in CelxZrxO2.8 with x > 0.2. TPR experiments under 2.5% H2 in He showed that Ceo8Pr0.202- provided superior low-temperature reduction and overall reducibility amongst Ce0.8M0.2026- materials. Moreover, CelxPrxO2-8 showed increased reducibility with increasing x, achieving a maximum weight loss of 4.8% at x = 1.0. CO oxidation studies over Ceo.8M0.202-8 identified Sc and Zr doping with the lowest CO light-off temperatures (247⁰C and 264⁰C, respectively).

(cont.) For CelPrxO2- and CelxZrxO2-, low levels of doping resulted in the highest CO oxidation activity; light-off was successfully achieved at 264⁰C and 252⁰C for x = 0.4 and 0. 1, respectively. Metal oxide-based materials were developed to selectively adsorb SO2 during fuel-lean conditions and desorb SO2 during fuel-rich conditions, thereby preventing the SO2 poisoning of the NOX adsorbent. Of various simple and mixed metal oxides, the Cr203-CuO system was found to provide SO2 adsorption under oxidizing conditions at 400⁰C, and SO2 evolution under reducing conditions below 350⁰C. The CuCr20O4 phase present at the optimal Cr20O3-CuO composition gave rise to improved low-temperature CO activity, which facilitated SO2 desorption. With increased CuO content, both adsorption capacity and regenerability were increased. Through the introduction of dopants, phase-pure CuCr2yCoyO4 was obtained to allow for SO2 desorption below 300Ê»C, which corresponded well with increased CO2 evolution. By introducing excess CuO onto CuCr1.9Co0.1O4 via various synthesis routes, improved SO2 sorption characteristics were attained. In pulse adsorption/desorption studies, the CuO/ CuCr.9Co0.104 materials and CuO/CuCr204 also demonstrated excellent capacity and superior regenerability relative to the conventional CuO/A1203 adsorbent. For on-board H2 production for fuel cells, the removal of H2S is paramount to avoiding poisoning of the H2 separation membrane and the fuel cell. Conventional coarse-grained ZnO is not viable for H2S ...