85 Search Results

The electrochemical oxygen evolution reaction (OER) is a critical half-reaction in a variety of energy conversion and storage applications, however, OER suffers from sluggish kinetics due to the four proton-electron transfer processes. To remedy this, interfacial engineering proves an effective strategy to design high active OER catalysts. Herein, we report a facile synthesis of Co(OH)2 nanoneedle (Co-NN) arrays grown on stainless steel (SS) via a one-step hydrothermal reaction, and the resultant impressive OER performance. Particularly, the Co-NN/SS needs 267, 307, and 576 mV overpotentials to reach 10, 100, and 1000 mA cm−2 in 1 M KOH & room temperature, and the overpotentials drop tomore » 199, 257, and 283 mV to achieve the same current densities in 30 wt% KOH & 80 °C. Additionally, an alkaline water electrolysis (AWE) cell coupled with Co-NN/SS anode and PtRu/NiMo cathode only requires 1.65 and 1.73 V iR-free cell voltage to reach 1.0 and 2.0 A cm−2, respectively. The sterling OER performance could be attributed to four possible reasons, the Fe in SS surface tailoring the electronic properties of Co(OH)2, the mixed metal oxides on SS surface accelerating surface reconstruction of Co(OH)2 nanoneedles into CoOOH active species, a Ni-rich surface layer formation cooperatively boosting the OER activity, and a local electric field effect concentrating reactants on the tip surface and accelerating the mass transfer. This work provides a facile approach for synthesizing highly efficient OER catalysts for industrial applications by interfacial engineering.« less

Captured carbon dioxide (CO₂) streams contain impurities that must be removed to meet specifications for safe transport, storage, and utilization. Among these impurities, oxygen poses challenges due to its high reactivity and potential to cause corrosion, motivating stringent purity limits below 10 ppmv. Building on recent experimental demonstrations of catalytic oxygen removal using hydrogen (H₂), carbon monoxide (CO), methanol (CH₃OH), and methane (CH₄) as reducing agents, this study presents a technoeconomic (TEA) and life cycle assessment (LCA) of these four catalytic purification pathways. Process flowsheets were developed and simulated in Aspen Plus for CO₂ streams representative of both low-temperature andmore » high-temperature capture processes, with integrated heat recovery and energy optimization. Results showed that total purification costs were dominated by feedstock procurement and electricity consumption. Among the studied reducing agents, the CH₄-assisted route achieved the lowest purification cost and highest CO₂ recovery. Sensitivity analyses showed that the H₂ route became competitive at H₂ prices below $$\$$$$0.56/kg to $$\$$$$0.84/kg, depending on the CO₂ feed temperature conditions. In conclusion, environmental impacts were primarily driven by indirect CO₂ emissions from raw material production and utility consumption.« less

Reducing pollutant emissions from heavy-duty (HD) diesel engines is critical due to their significant environmental impact, particularly concerning NOx emissions. Understanding and optimizing modern diesel oxidation catalyst (DOC) and selective catalytic reduction (SCR) performance is essential for improving exhaust aftertreatment (EAT) system efficiency to meet stringent emissions regulations. The oxidation of nitric oxide (NO) to nitrogen dioxide (NO₂) in DOC plays a key role in improving SCR efficiency in reducing NO_x. This study investigates the DOC performance in oxidizing NO to NO2 and its impact on the SCR efficiency of a 2021 MY Navistar E39 HD diesel engine. The influencemore » of engine speed, load, exhaust gas temperature, and composition on DOC efficiency is experimentally investigated. The relationships between DOC inlet temperature, oxygen availability and NO₂/NO_x ratio at the DOC inlet are examined to better understand their effects on the overall DOC efficiency. The results indicate that DOC NO oxidation efficiency is highly dependent on exhaust temperature, with optimal oxidation occurring within a specific temperature range (275-350°C). Below this threshold, the chemical reactions are kinetically limited, while at higher temperatures, thermodynamic constraints reduce the efficiency of DOC in oxidizing NO to NO₂. The experimental data further reveal that the NO₂/NO_x ratio peaks at medium loads before declining at higher loads due to reduced residence time and mass transfer effects. Additionally, the SCR NO_x conversion efficiency is significantly influenced by the NO₂/NO_x ratio, achieving peak performance when the NO₂/NO_x ratio approaches 0.5. A DOC chemistry model was developed and validated against the experimental data to predict DOC oxidation behavior under various operating conditions. The findings of this study provide insights into the interdependencies between DOC and SCR performance, contributing to the optimization of SCR systems for optimized NOx reduction.« less

Streams of CO₂ from various capture processes may contain several impurities of which O₂ is often overlooked as a problematic impurity. Because of its reactivity, the National Energy Technology Laboratory has recommended a stringent limit, less than 10 ppm, for the O₂ level in treated CO₂ products for safe transport, storage, and utilization. Here, in this study, a variety of catalytic oxygen reduction approaches using commercial automotive exhaust emission catalysts are evaluated for O₂ removal from CO₂ streams using different reducing agents, including H₂, CO, CH₃OH, and CH₄. When the amount of reductant added into the feed is carefully controlled,more » H₂, CO, or CH₃OH can effectively remove O₂ from 1.5% to below 10 ppm with a single-reactor design (>99.93% removal efficiency), while simultaneously meeting the impurity limits for other species. With CH₄ as a reductant, it is challenging to simultaneously meet the the O₂ and CO specifications in a single-reactor design because CH₄ also reacts with CO₂ to produce high levels of CO in the stream (at several hundred ppm). A dual-reactor design is developed to enable the use of CH₄, a readily available and low-cost reductant, for the purification of CO₂ streams to meet the specifications for O₂ and other impurities.« less

PdO/γ-Al₂O₃ catalysts are one of the most active catalytic components for the complete oxidation of methane. Under reaction conditions, especially in a wet feed, the catalysts suffer severe performance degradation. This study establishes a series of testing protocols to systematically investigate the causes of catalyst deactivation under methane oxidation reaction conditions. Four distinct catalyst deactivation modes are identified. Two of the deactivation modes are directly related to H₂O, either from the feed gas or as a part of the reaction products, with one (Mode 2) being attributed to the formation of surface hydroxyl groups and the other (Mode 3) tomore » the competitive adsorption of H₂O on the catalysts. The impact of the two deactivation modes is acute and severe but reversible. In contrast, the other two deactivation modes are gradual and persistent but irreversible. Both modes are induced by CH₄ oxidation reaction, with the impact of a wet feed (Mode 4) being substantially more severe than that of a dry feed (Mode 1). The major cause of the irreversible catalyst deactivation is attributed to surface reconstruction of PdO nanoparticles, which behaves as a passivation layer lowering the number of coordinately unsaturated Pd sites for CH₄ activation. Although the passivation layer is relatively stable against thermal or hydrothermal treatment, it is not completely inert. Formation and partial regeneration of the passivation layer is a highly dynamic process and heavily depends on the reaction temperature: a lower reaction temperature (≤ 450 ℃) can lead to quicker catalyst deactivation; but a higher reaction temperature (between 500 – 550 ℃) can result in a greater extent of catalyst deactivation.« less

Sorbent stability poses significant impacts on longterm performance of direct air capture (DAC) of CO₂ and levelized cost of capture (LCOC). We report the DAC performance degradation of amine-infused fiber sorbents based on poly(ethylenimine) (PEI), mesoporous SiO₂, and cellulose acetate (CA) over CO₂ cyclic sorption cycles in a nonoxidative environment. Infrared and nuclear magnetic resonance spectra indicate that the aminolysis reactions between CA ester moieties and PEI amine sites lead to the formation of acetamides and hence lower CO₂ affinities of the sorbents. This stability issue can be remedied by hydrolysis treatment of the CA fiber sorbents before PEI impregnationmore » or replacing CA with poly(ether sulfone). This study underscores the importance of selecting proper support or additive materials of DAC contactors that are compatible with active species of CO₂ capture.« less

Nanoparticle-supported Pt and Pd catalysts are employed industrially to convert CO and hydrocarbon residue from incomplete diesel fuel combustion into more environmentally-benign products. However, these catalysts deactivate over time due to sintering, especially for Pt nanoparticles which readily generate volatile species under high operating temperatures. Here, we turned the detrimental vapor-mediated sintering of Pt into an advantage by using a physical mixture of Pt and Pd catalysts prepared using a raspberry-colloid-templating (RCT) method. The RCT method produced Pt/Al₂O₃ and Pd/Al₂O₃ catalysts with partially embedded NPs to inhibit surface-mediated sintering pathways. As validated using an industry-defined emission control test protocol, agingmore » a physical mixture of Pt/Al₂O₃ and Pd/Al₂O₃ at high temperature produced an alloyed PtPd/Al₂O₃ catalyst that outperformed the fresh catalyst mixture and both individual catalysts for hydrocarbon conversion, while exhibiting high catalytic stability and resistance to sintering and to SO₂ poisoning. X-ray photoelectron spectroscopy revealed that in the aged catalyst mixture, half of the Pd content existed in the more active metallic state, compared to the less active oxide forms in the fresh mixture and both individual catalysts, explaining the unusual activity enhancement. Our results represent a practical approach to producing active and stable PtPd/Al₂O₃ diesel oxidation catalysts for emission control applications.« less

Developing robust Pt/CeO₂-based three-way catalysts (TWCs) with enhanced oxygen buffering capability and low-temperature activity is highly desirable. In this study, a new TWC family, Pt/(1 − x)CeO₂(core)@xZrO₂(shell) (where x = 0–0.5), was prepared and evaluated at degreened (DG) and hydrothermally aged (HTA) states. Incorporation of 0.1 molar concentration of ZrO₂ resulted in a decreased temperature that 50% (T₅₀) (CO: 167 °C, THCs: 218 °C, NO: 228 °C) and 90% (T₉₀) (CO: 207 °C, THCs: 237 °C, NO: 244 °C) conversions achieved over HTA 1.8 wt% Pt/0.9CeO₂@0.1ZrO₂ compared to the HTA 1.8 wt% Pt/CeO₂ sphere (CO: T50,90 = 179, 222 °C,more » THCs: 234, 252 °C, NO_x: 240, 260 °C). An enhanced oxygen storage capacity and oxygen release rate were observed over Pt/0.9CeO₂@0.1ZrO₂ compared to the Pt/CeO₂ sphere. Increasing the ZrO₂ molar concentration to values greater than 0.2 resulted in increased T₅₀s (224, 265 274 °C) and T₉₀s (251, 289, 292 °C) for CO, THCs, and NO_x, respectively, over 1.8 wt.% Pt/0.5CeO₂@0.5ZrO₂. Overall, this work highlights the potential of forming a ZrO₂ shell on CeO₂ spheres as a support for TWC applications.« less

In the past few decades, tremendous attention has been devoted to enhancing the activity of oxygen evolution reaction (OER) catalysts for hydrogen production, while the cost and long-term stability of catalysts, which can play an even more important role in industrialization, have been much less emphasized. Herein, we engineered an OER electrode from abundant stainless steel (SS) via facile approaches, and the obtained electrode consists of a Ni-rich oxide surface layer with a Fe-rich metal substrate. An outstanding activity was observed with an overpotential of 316 mV at 100 mA cm⁻² in 1 M KOH electrolyte. Additionally, an electrode self-replenishingmore » concept is proposed in which a Ni-rich catalyst layer can be regenerated from a metallic substrate due to the difference in diffusion and dissolution rates of metal oxides/hydroxides, and this regeneration is validated by various characterizations. A recorded degradation rate of 0.012 was observed at 1000 mA cm⁻² for 1000 h. The facile engineering of OER electrodes from SS combined with the self-replenishing catalyst can potentially address the cost, activity, and long-term stability barriers.« less

A PtPd/Al₂O₃ catalyst developed for the complete oxidation of methane from the ventilation air of underground coal mines is compared against a model PdO/Al₂O₃ catalyst. Although the PtPd/Al₂O₃ catalyst is substantially more active and stable than the model catalyst, the nature of active sites between the two catalysts is deemed to be fundamentally the same based on their response to different feed gas compositions and the evolution of surface CO adsorption complexes during time-resolved CO adsorption DRIFTS experiment. For both catalysts, coordinatively unsaturated Pd sites are considered the active centers for methane activation and the subsequent oxidation reaction. H₂O competesmore » with CH₄ for the same active sites, resulting in severe inhibition. Additionally, the CH₄ oxidation reaction also causes self-inhibition. Taking both inhibition effects into consideration, a relatively simple kinetic model is developed. The model provides a good fit of the 72 sets of kinetic data collected on the PtPd/Al₂O₃ catalyst under practically relevant reaction conditions with CH₄ concentration in the range of 0.05–0.4%, H₂O concentration of 1.0–5.0%, and reaction temperatures of 450–700 °C. Kinetic parameters based on the model suggest that the CH₄ activation energy on the PtPd/Al₂O₃ catalyst is 96.7 kJ/mol, and the H₂O adsorption energy is –31.0 kJ/mol. Both values are consistent with the parameters reported in the literature. The model can be used to develop catalyst sizing guidelines and be incorporated into the control algorithm of the catalytic system.« less