Weiyu Song

A mechanistic DFT study of low temperature SCR of NO with NH3 on MnCe1−xO2(111)

Mn-promoted CeO2 is a promising catalyst for the low temperature selective catalytic reduction of NO by NH3. We investigated the mechanism of this reaction for a model in which Mn cations are doped into the CeO2(111) surface by quantum-chemical DFT+U calculations. NH3 is preferentially adsorbed on the Lewis acid Mn sites. Dissociation of one of its N–H bonds results in the key NH2 intermediate that has been experimentally observed. NO adsorption on this NH2 intermediate results in nitrosamine (NH2NO) that can then undergo further N–H cleavage reactions to form OH groups. The resulting N2O product is desorbed into the gas phase and can be re-adsorbed through its O atom on an oxygen vacancy in the ceria surface, resulting from water desorption. Water desorption is the most difficult elementary reaction step. This redox mechanism involves doped Mn as Lewis acid sites for ammonia adsorption and O vacancies in the ceria surface to decompose N2O into the desired N2 product.

The selective catalytic reduction of NOx over a Cu/ZSM-5/SAPO-34 composite catalyst

A series of Cu/ZSM-5/SAPO-34 (Cu/ZS-PM) composite catalysts with fixed Cu amount and varying ZSM-5 mass fraction were synthesized using a pre-seed method, and their catalytic performances were tested for selective catalytic reduction (SCR) of NO with NH3. The catalysts were characterized by means of XRD, FT-IR, SEM, BET, UV-vis DRS, NH3-TPD, H2-TPR and in situ DRIFTS. The results indicate that the high deNOx performance of the Cu/ZS-PM-15% catalyst was due to the excellent redox property and the formation of NO2. The acidity of the catalysts was significantly influenced by the content of ZSM-5 seeds, whereas there was no obvious correlation between acid strength and NH3-SCR performance. The in situ DRIFTS spectra demonstrated that both Lewis and Bronsted acid sites are involved in the NH3-SCR reaction and the reaction on Bronsted acid sites is more active than that on Lewis acid sites. NO2 is a primary intermediate following NO adsorption on Cu/ZS-PM catalysts, which may promote a “fast” SCR reaction at low temperature.

Selective catalytic reduction over size-tunable rutile TiO2 nanorod microsphere-supported CeO2 catalysts

Size-tunable 3D rutile TiO2 spheres consisting of nanorods were controllably synthesized by adjusting the precursor hydrolysis rate. CeO2 nanoparticles were supported on TiO2 to prepare a series of Ce/Ti catalysts via an incipient wetness impregnation method. Catalytic activity tests showed that the hierarchical rutile TiO2 microspheres with a size of 1 μm containing nanorod-supported CeO2 showed excellent activity and high N2 selectivity over a wide temperature range. The novel morphology of the TiO2 nanostructures exhibits a strong interaction with CeOx species, enhancing their dispersion. The excellent catalytic activity should be mainly attributed to the enriched surface oxygen species, abundant surface acidity and high reducibility. The presence of enriched surface oxygen vacancies could facilitate the formation of active NO2 and bidentate nitrate species, leading to remarkable SCR performances. This was confirmed by in situ DRIFTS investigations.

Elucidation of the high CO2 reduction selectivity of isolated Rh supported on TiO2: a DFT study

Methanation and reverse water-gas shift reaction are two important reactions that could happen simultaneously during the process of CO2 reduction. Exploiting new catalysts with high selectivity towards one single process is highly desirable. It has been shown that isolated-Rh/TiO2 can selectively generate CO rather than CH4. A molecular level understanding would provide more insight into catalyst design for CO2 reduction. In the present contribution, the density functional theory method was employed to study the CO2 reduction reaction by H2 based on a Rh1/TiO2 (101) model. The co-adsorbed CO2 and H2 on the Rh atom can react with each other to form CO. The inhibition of further H2 adsorption on the CO pre-adsorbed Rh atom stops the following sequential hydrogenation of CO. This can explain the experimentally observed high selectivity of Rh1/TiO2 to CO. Different co-adsorption properties can be understood by the frontier orbital charge density symmetry matching principle. The same method has been extended to other metal systems (Ru, Pd and Pt) to identify candidate catalysts with high selectivity for CO2 reduction. Similar adsorption properties of isolated Pd with Rh may induce high selectivity towards CO. These results are expected to provide a prediction to find new selective catalysts for CO2 reduction.

Selective catalytic reduction of NO with NH3 over Mo–Fe/beta catalysts: the effect of Mo loading amounts

A series of Mox–Fe/beta catalysts with constant Fe and variable Mo content were synthesized and investigated for selective catalytic reduction (SCR) of NOx with NH3. It was found that the Mo0.2–Fe/beta catalyst exhibited excellent activity, N2 selectivity and preferable resistance to H2O and SO2. The Mox–Fe/beta catalysts were characterized by various analytical techniques. TEM and SEM images showed that the addition of Mo could enhance the dispersion of iron oxides. The results of NH3-TPD and Py-IR indicated that the introduction of Mo resulted in a change of Bronsted acidity, which was associated with high-temperature SCR activity. XPS and XANES results showed that the introduction of Mo resulted in a change of Fe2+ content, which determined the low-temperature activity. DFT calculations showed the strong effects of Mo on the crystal structure, charge distribution and oxygen vacancy formation energy of iron oxides, which further explained the role of Mo in the catalyst behaviors during the SCR process.

A new 3DOM Ce-Fe-Ti material for simultaneously catalytic removal of PM and NOx from diesel engines

A new 3DOM material was designed and synthesized for the simultaneous removal of PM (soot particulates) and NOx from diesel engine exhausts. The catalytic purification taking place over the material with double efficacy is cost-efficient. The contact between solid PM and catalyst active site has been process intensified by 3DOM unique structure. 3DOM Ce0.7Fe0.2Ti0.1O2 catalyst possess a high SCR activity and an excellent selectivity to N2, giving a maximum concentration of CO2 at 385°C for PM combustion and 100% NO conversion in the temperature range of 281-425°C. The dual redox cycles (Fe3++Ce3+↔Fe2++Ce4+,Fe3++Ti3+↔Fe2++Ti4+) and the excellent reducibility and sufficient acid sites of catalysts play key roles for the highly catalytic performance.

The simultaneous purification of PM and NOx in diesel engine exhausts over a single 3DOM Ce0.9−xFe0.1ZrxO2 catalyst

A novel catalytic purification process SCRPF (selective catalytic reduction and particulate filter) over a 3DOM catalyst was designed and used for the simultaneous removal of PM (particulate matter) and NOx from diesel engine exhausts. It is a combination of DPF and SCR of NOx reduction technology advantages. The catalytic purification taking place over a SCRPF reactor is cost-efficient. The contact between the solid PM and the catalyst active sites is process intensified by the 3DOM unique structure. The 3DOM Ce0.85Fe0.1Zr0.05O2 catalyst gave a maximum concentration of CO2 at 415 °C for PM combustion and showed 100% NO conversion in the temperature range of 365–503 °C. The different Ce/Zr ratio and the introduction of Fe species present various Ce3+/Ce4+, abundant oxygen vacancies, excellent reducibility and sufficient acid sites in the catalyst, which are effective for the simultaneous removal of PM and NOx. The use of a low cost catalyst may be more desirable.

Synthesis of a chabazite-supported copper catalyst with full mesopores for selective catalytic reduction of nitrogen oxides at low temperature

Abstract A series of meso-microporous copper-supporting chabazite molecular sieve (Cu-SAPO-34) catalysts with excellent performance in low-temperature ammonia selective catalytic reduction (NH 3 -SCR) have been synthesized via a one-pot hydrothermal crystallization method. The physicochemical properties of the catalysts were characterized by scanning electron microscopy, transmission electron microscopy, N 2 adsorption-desorption measurements, X-ray diffraction, 27 Al magic angle spinning nuclear magnetic resonance, diffuse reflectance ultraviolet-visible spectroscopy, inductively coupled plasma-atomic emission spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction measurements, and electron paramagnetic resonance analysis. The formation of micro-mesopores in the Cu-SAPO-34 catalysts decreases diffusion resistance and greatly improves the accessibility of reactants to catalytic active sites. The main active sites for NH 3 -SCR reaction are the isolated Cu 2+ species displaced into the ellipsoidal cavity of the Cu-SAPO-34 catalysts.

Mesoporous Co3O4 supported Pt catalysts for low-temperature oxidation of acetylene

Three-dimensionally ordered mesoporous Co3O4 (meso-Co3O4) and its supported Pt catalysts were synthesized for the catalytic oxidation of acetylene. All the catalysts formed mesoporous structures and possessed high surface areas of 110–122 m2 g−1. Meso-Co3O4-supported Pt catalysts exhibited highly catalytic performances, and the 0.6Pt/meso-Co3O4 catalyst gave the lowest temperature of 120 °C for acetylene oxidation. It was concluded that the ordered mesoporous structure, with plenty of structural defects, good low-temperature reducibility and the high concentration of active oxygen species were responsible for the excellent catalytic performance of 0.6Pt/meso-Co3O4.

The catalytic performances and reaction mechanism of nanoparticle Cd/Ce–Ti oxide catalysts for NH3-SCR reaction

Ce0.3–TiOx nanoparticle carrier was prepared by the sol–gel method, and a series of Cd–Ce–Ti nanoparticle catalysts with variable Cd contents were prepared by the means of an improved incipient-wetness impregnation. The catalysts were characterized by means of XRD, N2 adsorption–desorption analysis, SEM, TEM, NH3-TPD and in situ DRIFTS. The catalytic activities for deNOx were evaluated by NH3-SCR reaction. All these nanoparticle catalysts contain mesopores with a pore size around 7 nm, and the average particle size is 20 nm. Among the catalysts, 2 wt% Cd/Ce0.3TiOx catalyst exhibits the best NH3-SCR performance with a wide temperature window of 250–400 °C for NO conversion above 90%. Moreover, in situ DRIFTS spectra of NOx reduction over 2 wt% Cd/Ce0.3TiOx catalyst were also investigated. The results show that this reaction mainly follows the Langmuir–Hinshelwood mechanism at room temperature, while Eley–Rideal mechanism plays more important role when the reaction temperature is higher than 150 °C. The adsorbed NH3 coordinately linked to Lewis acid site is easy to react with NOx at high temperature.