Ying Xin

Ce-Ti amorphous oxides for selective catalytic reduction of NO with NH3: confirmation of Ce-O-Ti active sites.

The amorphous Ce-Ti mixed oxides were reported to be catalysts for selective catalytic reduction of NO(x) with NH(3), in which Ce and not Ti acts as their solvent in spite of the fact that Ce is low in content. The amorphous catalysts were characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) equipped with selective area electron diffraction (SAED). The Ce-Ti amorphous oxide shows higher activity than its crystalline counterpart at lower temperatures. Moreover, the presence of small CeO(2) crystallites as for the impregnated sample is deleterious to activity. The Ce-O-Ti short-range order species with the interaction between Ce and Ti in atomic scale was confirmed for the first time to be the active site using temperature programmed reduction with H(2) (H(2)-TPR), in situ FTIR spectra of NO adsorption, X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine-structure (XAFS). Lastly, the Ce-O-Ti structure was directly observed by field-emission TEM (FETEM).

A unified intermediate and mechanism for soot combustion on potassium-supported oxides

The soot combustion mechanism over potassium-supported oxides (MgO, CeO2 and ZrO2) was studied to clarify the active sites and discover unified reaction intermediates in this typical gas-solid-solid catalytic reaction. The catalytically active sites were identified as free K+ rather than K2CO3, which can activate gaseous oxygen. The active oxygen spills over to soot and forms a common intermediate, ketene, before it was further oxidized into the end product CO2. The existence of ketene species was confirmed by density functional theory (DFT) calculations. The oxygen spillover mechanism is proposed, which is explained as an electron transfer from soot to gaseous oxygen through the active K+ sites. The latter mechanism is confirmed for the first time since it was put forward in 1950, not only by ultraviolet photoelectron spectroscopy (UPS) results but also by semi-empirical theoretical calculations.

In situ IR studies of selective catalytic reduction of NO with NH3 on Ce-Ti amorphous oxides

A series of in situ infrared (IR) studies of the selective catalytic reduction (SCR) of NOx with NH3 on the short-range ordered structure Ce–O–Ti sites in amorphous Ce-Ti mixed oxides were performed. Under the reaction conditions, the catalyst surface was mainly covered by NH3 ad-species and no NOx ad-species were detected. The reaction order of 0.5–0.6 with respect to NO confirmed a hybrid Langmuir-Hinshelwood and Eley-Rideal mechanism. A possible route may involve the reaction of NH3 ad-species and weakly adsorbed NOx to form an active intermediate, NHyNO3 (y = 0–4); this was confirmed by GAUSSIAN calculations and the in situ IR results. The Ce–O–Ti structure, with Ce-Ti interactions on the atomic scale, enhanced the redox properties in the active temperature window of the SCR reactions.

A universal route to fabricate hierarchically ordered macro/mesoporous oxides with enhanced intrinsic activity

Hierarchically ordered macro/mesoporous oxides and mixed oxides have been fabricated by adopting a dual-templating [poly(methyl methacrylate) colloidal microspheres and Pluronic P123] strategy and a wet ammonia gas infiltration–precipitation route to solidify metal precursors in situ. This is especially suitable for mixed oxides that can be prepared by a co-precipitation method using ammonia as the precipitation agent. The as-prepared Ce–Ti mixed oxides have a desirable homogeneous distribution of the components and exhibit excellent intrinsic activity for the selective catalytic reduction of NOx with NH3 under an exceptionally high gas hourly space velocity of 500 000 mL g−1 h−1, which confirmed the contribution of the macropores to small-molecule reactions. Furthermore, hierarchically macro/mesoporous Mn, Cr, Ti, La, Y, Sn, Zn, Ni and Co oxides have been fabricated on a large-scale to illustrate the universal applicability of this method.

Coralline‐Like N‐Doped Hierarchically Porous Carbon Derived from Enteromorpha as a Host Matrix for Lithium‐Sulfur Battery

Coralline-like N-doped hierarchically porous carbon (CNHPC) was prepared through a hydrothermal carbonization process using a sea pollutant enteromorpha as the starting material. The addition of a small amount of glucose during carbonization improved the yield of carbon, and the inherent N contents, especially for pyrrolic N and pyridinic N atoms. After loading 40 wt. % sulfur, the CNHPC/S composite, as a cathode in a Li-S battery, exhibited an initial discharge capacity of 1617 mAh g-1 (96.5 % of theoretical capacity) at 0.1 C and a capacity loss of 0.05 % per charge-discharge cycle after 500 cycles at 0.5 C with a stable Coulombic efficiency of 100 % in carbonate based electrolyte. Such a great performance can be attributed to the coralline-like hierarchically porous infrastructure and inherently abundant N doping. Given the conversion of waste pollutants into valuable energy-storage materials and the easy process, this work features a promising approach to prepare C/S cathodes for Li-S batteries. The special structural and textural characteristics of CNHPC might be attractive to other practical applications such as supercapacitors and catalysis.

Improvement of Air/Fuel Ratio Operating Window and Hydrothermal Stability for Pd-Only Three-Way Catalysts through a Pd–Ce2Zr2O8 Superstructure Interaction

The extremely severe and persistent haze problems in some parts of the world including China have prompted the implementation of increasingly stringent tailpipe regulations. This places increasingly higher performance requirements for three-way catalysts, and in particular a widening of the air/fuel (λ) ratio operating window to facilitate operation of the on-board diagnostic system. A new pathway is presented here by tuning the nanostructure of TWCs to improve their λ activities and hydrothermal stability. High-temperature reduction and a mild-temperature reoxidation treatment for alumina-modified ceria-zirconia brought about the formation of a cubic, fully oxidized, pyrochlore-like superstructure, Ce2Zr2O8. The combination of Pd and the Ce2Zr2O8 superstructure greatly improved the λ window for Pd-only three-way catalysts. X-ray powder diffraction (XRD), temperature-programmed reduction with H2 (H2-TPR) and high-resolution transmission electron microscopy (HRTEM) characterization confirmed the interaction between Pd and the Ce2Zr2O8 superstructure, which modifies the dynamic oxygen storage capacity in comparison to the conventional Pd-Ce(Zr)O2 interaction, due to higher low-temperature reducibility for the Ce2Zr2O8 superstructure than for Ce(Zr)O2. Furthermore, the retention of the Ce2Zr2O8 superstructure derived from the interaction with Pd results in superior λ and light-off performances after hydrothermal aging treatment at 1000 °C for 12 h in air containing 10% H2O.

Zeolitic Materials for DeNOx Selective Catalytic Reduction

Selective catalytic reduction (SCR) of NOx has been proven to be the most efficient technique for the removal of NOx (deNOx) from flue gas of stationary sources and diesel engine exhaust of mobile sources. Among various SCR catalysts, zeolites are considered as the most promising candidates for heavy‐duty trucks owing to their benign nature, very high NOx conversion and N2 selectivity, as well as excellent hydrothermal stability at high temperatures. In recent years, a tremendous number of research articles have reported on the evolution of zeolitic materials for deNOx, which makes a critical review timely to provide inspiration for future development. This paper first gives a chronological progress of zeolite catalysts specifically highlighting the CHA zeolites, the hotspot of present research and application, followed by a summary on the synthesis of CHA zeolites, the structure–activity relationships, as well as the reaction mechanisms and kinetics. Last but not least, a critical outlook is given together with expected research directions in developing zeolite‐based catalysts for deNOx.

The Potential of Cu‐SAPO‐44 in the Selective Catalytic Reduction of NOx with NH3

Nitrogen oxides (NOx) contribute much to acid rain, photochemical smog, and the depletion of tropospheric ozone. A novel, small‐pore Cu‐exchanged chabazite (Cu‐CHA) zeolite, Cu‐SAPO‐44, was first studied for the selective catalytic reduction of NOx with ammonia (NH3‐SCR), and exhibits excellent activity and N2 selectivity over the wide temperature window from 200–550 °C. The Cu content in Cu‐SAPO‐44 plays a significant role in the NH3‐SCR reactions. Two kinds of isolated Cu2+ species inside the large cages and in the six‐membered rings of the CHA structure were verified as the active sites, which are responsible for the low‐temperature and high‐temperature activity, respectively. Cu‐SAPO‐44 is shown to be a promising candidate as a SCR catalyst for deNOx with great potential in after‐treatment systems for either mobile or stationary sources.

Synthesis of CeO2-Based Quantum Dots through a Polyol-Hydrolysis Method for Fuel-Borne Catalysts

The transparent colloidal solutions of monodisperse CeO2‐based quantum dots (QDs) were prepared by heating a triethylene glycol (TEG) solution of Ce(NO3)3⋅6 H2O (and Fe(NO3)3⋅9 H2O) at 180 °C. The CeO2‐based QDs were characterized by X‐ray powder diffraction (XRD), dynamic light scattering (DLS), transmission electron microscopy (TEM), UV/vis absorption spectra and Brunauer–Emmett–Teller (BET) surface area. CeO2‐based QDs with uniform particle size below 5 nm exhibit narrow size distribution, quantum effect, good re‐dispersion ability and high surface area. For the formation of these QDs, a polyol‐hydrolysis mechanism is proposed. CeO2 QDs are applied in fuel borne catalysts (FBCs) for diesel soot combustion, which exhibit excellent activity at a rather low temperature, owing to the homogeneous and large number of contact points between the catalyst and the soot. The doping with Fe can further improve the selectivity to CO2.

An oxygen pool from YBaCo4O7-based oxides for soot combustion

Soot, often referred to as black carbon emitted from diesel engines, is not only a particulate matter pollutant but also a light-absorbing agent that may affect global climate, but can be effectively controlled using a catalytic diesel particulate filter (DPF). A new YBaCo4O7+δ-type oxygen storage material is reported as an effective catalyst for soot combustion. Isotopic isothermal reactions demonstrate the activation of gaseous oxygen and subsequent oxygen storage and reaction/desorption during an oxidation process. High activity and structural stability are achieved by the substitution of Co with Al and Ga to form YBa(Co0.85Al0.075Ga0.075)4O7+δ. The specific rates at 300 °C of YBaCo4O7+δ and YBa(Co0.85Al0.075Ga0.075)4O7+δ, normalized by surface areas, are an order of magnitude higher than those of CeO2-based oxides. This kind of oxygen-storage material acts as an oxygen pool, which ensures that the accumulated soot on a DPF can be promptly combusted.