Abstract
Supported metal single atom catalysts (SACs) present an emerging class of low-temperature catalysts with high reactivity and selectivity, which, however, face challenges on both durability ...and practicality. Herein, we report a single-atom Pt catalyst that is strongly anchored on a robust nanowire forest of mesoporous rutile titania grown on the channeled walls of full-size cordierite honeycombs. This Pt SAC exhibits remarkable activity for oxidation of CO and hydrocarbons with 90% conversion at temperatures as low as ~160
o
C under simulated diesel exhaust conditions while using 5 times less Pt-group metals than a commercial oxidation catalyst. Such an excellent low-temperature performance is sustained over hydrothermal aging and sulfation as a result of highly dispersed and isolated active single Pt ions bonded at the Ti vacancy sites with 5 or 6 oxygen ions on titania nanowire surfaces.
Pd/BEA is chosen as a model passive NOx adsorber (PNA) to elucidate the effect of the feed gas composition on the NOx adsorption/desorption behavior. The Brønsted acid and the partially hydrolyzed ...framework Al (P‐HAl(OH)) sites in HBEA adsorb NO and NO2 under dry conditions. Moreover, the performance of HBEA is not affected by CO, while CO inhibits nitrate formation and promotes NO adsorption via the Pd(NO)(CO) complexes formation over Pd/BEA. H2O inhibits NO adsorption over the Brønsted acid and P‐HAl(OH) sites, and ionic Pd is the only active site for NOx adsorption under wet conditions. Furthermore, NO adsorption over hydrated Pd (Pd2+(OH)(NO)(H2O)3) is weaker than NO adsorption over bare ionic Pd (Z2Pd2+(NO), ZPd2+(OH)(NO)). Dehydration of Pd2+(OH)(NO)(H2O)3 forms more stable ZPd2+(OH)(NO) during desorption. The NO adsorption capacity of Pd/BEA improves in the presence of CO under both dry and wet conditions by forming a stable carbonyl–nitrosyl complex.
Display omitted
•Three-way catalyst light-off temperatures vary significantly with fuel composition.•Most oxygenated fuels light off at lower temperatures than non-oxygenated species.•Aromatic ...species tend to light off at higher temperatures.•Alkane and alkene light-off temperatures depend on molecular structure.•Some organics inhibit CO light-off, while others have no effect on CO light-off.
The Department of Energy “Co-Optimization of Fuels and Engines” initiative aims to simultaneously develop novel high-performance fuels with advanced engine designs to reduce petroleum consumption. To achieve commercialization, advanced engines running on alternative fuels still must meet emissions regulations. Warm three-way catalysts (TWC) are very effective at meeting the stringent emissions regulations on pollutants such as nitrogen oxides (NOx), non-methane organic gases (NMOG) and carbon monoxide (CO) from gasoline-fueled spark-ignition (SI) engines operating under stoichiometric conditions; thus, most SI engine emissions occur during cold-start, when the TWC has not yet achieved light-off. In the current study, the light-off behavior of novel high-performance fuel candidates has been investigated on a hydrothermally-aged commercial TWC using a synthetic engine-exhaust flow reactor system according to industry guidelines. Over 30 potential fuel components were examined in this study, including alkanes, alkenes, alcohols, ketones, esters, aromatic ethers, and non-oxygenated aromatic hydrocarbons. Short-chain acyclic oxygenates, including alcohols, ketones, and esters, tended to light off at relatively low temperatures, while alkenes, aromatics, and cyclic oxygenates tended to light off at relatively high temperatures. The light-off behavior of alkanes and alkenes depended strongly on their size and structure. In terms of the influence on CO light-off on the TWC, the fuels fell into two distinct categories: (i) non-inhibiting species including C2-C3 alcohols, alkanes, acyclic ketones, and esters; and (ii) inhibiting species including alkenes, aromatic hydrocarbons, cyclic oxygenates, and C4 alcohols.
Better understanding of true electrochemical reaction behaviors in electrochemical energy devices has long been desired. It has been assumed so far that the reactions occur across the entire catalyst ...layer (CL), which is designed and fabricated uniformly with catalysts, conductors of protons and electrons, and pathways for reactants and products. By introducing a state-of-the-art characterization system, a thin, highly tunable liquid/gas diffusion layer (LGDL), and an innovative design of electrochemical proton exchange membrane electrolyzer cells (PEMECs), the electrochemical reactions on both microspatial and microtemporal scales are revealed for the first time. Surprisingly, reactions occur only on the CL adjacent to good electrical conductors. On the basis of these findings, new CL fabrications on the novel LGDLs exhibit more than 50 times higher mass activity than conventional catalyst-coated membranes in PEMECs. This discovery presents an opportunity to enhance the multiphase interfacial effects, maximizing the use of the catalysts and significantly reducing the cost of these devices.
Catalytic oxidation of methane (CH4) over nonprecious Ni/CeO2 catalysts has received a lot of attention due to the large natural gas reserves found in North America and the prohibitive cost of ...palladium-based catalysts, commonly used for CH4 oxidation. However, the catalytic mechanism of CH4 oxidation over Ni/CeO2 still remains unclear. Moreover, the parameters affecting the reaction rates, the interaction between nickel and CeO2, and the reaction intermediates are still not well understood. Herein, kinetic model fitting, CH4 temperature-programmed reduction-mass spectroscopy (CH4 TPR-MS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations were combined to elucidate the mechanism of complete oxidation of CH4 over Ni/CeO2. CH4 TPR-MS showed that the complete oxidation of CH4 over Ni/CeO2 requires 55–120 °C lower compared to bare CeO2 or Ni/quartz sand; complete oxidation of CH4 took place when the surface oxygen species were abundant, while partial oxidation products (CO, H2) were formed when the oxygen species were depleted. In situ DRIFTS showed that CH3, CH2, CO, and CO2 were formed after CH4 activation over Ni/CeO2, while CH3O species were not observed. Combining those findings with kinetic model fitting, a redox Mars–van Krevelen (MvK) mechanism showed the best description of the experimental observations. The MvK mechanism involves the reaction of dissociated oxygen species with gas-phase CH4 while water inhibits the reaction rate by adsorbing on the oxidized sites. Moreover, CH4 activation leads to the reduction of the active sites and oxygen vacancy formation followed by reoxidation of the active sites by gas-phase O2. A CH4 oxidation reaction pathway over Ni/CeO2 is proposed by DFT calculations. In summary, the findings shown here suggest that CH4 oxidation over Ni/CeO2 follows a redox MvK mechanism and provides guidance for the rational design of non-precious-metal catalysts for CH4 oxidation reactions.
•Novel TT-LGDLs with different surface treatments are investigated for the first time.•Superior PEMEC performance with a value of 1.63V at 2.0A/cm2 and 80°C is obtained.•Ohmic losses is reduced from ...0.0925Ωcm2 to 0.0700Ωcm2.•Hydrogen production rate can be greatly increased by 28.2%.•Au thin film surface treatment on titanium material shows good stability.
A proton exchange membrane electrolyzer cell (PEMEC) is one of the most promising devices for high-efficiency and low-cost energy storage and ultrahigh purity hydrogen production. As one of the critical components in PEMECs, the titanium thin/tunable LGDL (TT-LGDL) with its advantages of small thickness, planar surface, straight-through pores, and well-controlled pore morphologies, achieved superior multifunctional performance for hydrogen and oxygen production from water splitting even at low temperature. Different thin film surface treatments on the novel TT-LGDLs for enhancing the interfacial contacts and PEMEC performance were investigated both in-situ and ex-situ for the first time. Surface modified TT-LGDLs with about 180nm thick Au thin film yielded performance improvement (voltage reduction), from 1.6849V with untreated TT-LGDLs to only 1.6328V with treated TT-LGDLs at 2.0A/cm2 and 80°C. Furthermore, the hydrogen/oxygen production rate was increased by about 28.2% at 1.60V and 80°C. The durability test demonstrated that the surface treated TT-LGDL has good stability as well. The gold electroplating surface treatment is a promising method for the PEMEC performance enhancement and titanium material protection even in harsh environment.
PDF
