Development of advanced aftertreatment catalysts via active site control for industrial emission control

Abstract

This research investigates catalyst-based emission control strategies tailored for realistic industrial exhaust conditions, emphasizing active site engineering. The main objective is to understand how different catalyst synthesis methods affect active site structures and catalytic reaction mechanisms, enabling rational design of superior emission control systems. Several studies were conducted under realistic industrial conditions, examining varying gas compositions and synthesis methods to optimize catalyst activity for pollutants such as CO and NH3. Four distinct catalytic systems, from noble metals to transition metal oxides, were explored to evaluate active site control strategies. The first study developed a Pt-V-W/TiO2 catalyst exhibiting unique bimetallic active sites from strong Pt-V interactions, significantly enhancing N2 selectivity in selective catalytic oxidation of NH3 (NH3-SCO). Under realistic conditions containing NH3 and CO, this catalyst simultaneously oxidized both pollutants effectively. It facilitated in-situ NOx reduction via internal SCR (i-SCR) and CO-assisted SCR (CO-SCR), achieving high conversion to N2. This multifunctional behavior demonstrates its effectiveness under complex exhaust conditions. The second approach involved creating a two-dimensional cobalt silicate catalyst (Co-DML) through hydrothermal delamination of an MWW-type zeolite precursor. Increasing the hydrothermal temperature transformed the structure from 3D to 2D, generating two distinct active sites: framework-incorporated Co (strong Lewis acidity) and dispersed Co3O4 (reducibility). These combined properties enabled effective NH3 adsorption and conversion predominantly via i-SCR mechanisms, enhancing both NH3 conversion and N2 selectivity under realistic conditions. Subsequent studies examined strong metal-support interactions (SMSI) to selectively form desirable active phases. Specifically, Co supported on Al2O3 typically forms inactive cobalt aluminate phases due to detrimental interactions involving surface hydroxyl groups. To overcome this, a controlled dry impregnation synthesis strategy was developed. Initially, extensive dehydroxylation of γ-Al2O3 under inert conditions removed surface hydroxyl groups, minimizing undesired SMSI. Subsequent cobalt precursor impregnation onto this modified support resulted in highly dispersed Co3O4 nanoparticles, as confirmed by XRD, HR-TEM, and XPS. This method substantially improved NH3 oxidation performance, even at lower cobalt loadings, indicating cost-effective catalyst potential. Next, to enhance low-temperature NOx reduction under oxygenated hydrocarbon-rich conditions typical in semiconductor fabrication emissions, an AgCo/Al2O3 catalyst was developed using isopropyl alcohol (IPA-SCR). This catalyst demonstrated significantly improved NOx conversion compared to conventional Ag/Al2O3. Characterizations including H2-TPR, EtOH-TPD, in-situ DRIFTS revealed Ag and Co co-impregnation promoted formation of Ag2O-Co3O4 Janus structures, essential for IPA activation. The generated enol-type intermediates facilitated reactive -NCO and -CN species formation, significantly enhancing NOx reduction under low-humidity conditions. Finally, a sustainable approach was explored to convert spent NCM cathode waste into active oxidation catalysts through chemical delithiation and oxidative heat treatment. Chemical delithiation extracted lithium ions extensively, inducing oxygen vacancies via charge compensation. Subsequent oxidative heat treatment promoted Ni exsolution, forming dispersed NiO active sites. Oxidation-induced volumetric expansion increased specific surface area significantly. The resulting upcycled catalyst showed superior performance in oxidizing CH4, CO, and NH3 compared to conventional catalysts. This approach provides a practical strategy for recycling lithium-ion battery waste, addressing environmental challenges and air pollutant mitigation simultaneously. In conclusion, this body of work demonstrates that strategic control of active sites across diverse catalytic systems can significantly enhance the selective removal of harmful emissions. By tailoring the composition, structure, and chemical state of active sites, the studied catalysts achieved superior N2 selectivity and oxidation activity, even under the complexed conditions of real-world emission control. The active site engineering strategies proposed herein offer a practical pathway for developing high-performance aftertreatment catalysts that can meet increasingly stringent environmental regulations and contribute meaningfully to industrial emission control.