Effect of iridium dispersion on the catalytic activity of Ir/SiO2 for the selective reduction of NO with CO in the presence of O2 and SO2

IrSn Bimetallic Clusters Confined in MFI Zeolites for CO Selective Catalytic Reduction of NOx in the Presence of Excess O2

We developed a simple strategy for preparing IrSn bimetallic clusters encapsulated in pure silicon zeolites via a one-pot hydrothermal synthesis by using diethylamine as a stabilizing agent. A series of investigations verified that metal species have been confined successfully in the inner of MFI zeolites. IrSn bimetallic cluster catalysts were efficient for the CO selective catalytic reduction of NOx in the presence of excess O2. Furthermore, the 13CO temperature-programmed surface reaction results demonstrated that NO2 and N2O could form when most of the CO was transformed into CO2 and that Sn modification could passivate CO oxidation on the IrSn bimetallic clusters, leading to more reductants that could be used for NOx reduction at high temperatures. Furthermore, SO2 can also influence the NOx conversion by inhibiting the oxidation of CO. This study provides a new strategy for preparing efficient environmental catalysts with a high dispersion of metal species.

The generation of sulfate species on Ir-based catalysts for boosting NO reduction with CO under the coexistence of O2 and SO2 atmosphere

Generally, sulfur poisoning is considered to be one of the main factors contributing to the deactivation of selective catalytic reduction of NOx by CO (CO-SCR) catalysts, while the promotional effect of SO2 on NO reduction over Ir/SiO2 is observed which is an interesting scientific phenomenon. After the introduction of 20 ppm SO2, NOx conversion increased from ∼ 40 % to ∼ 90 % at 275 °C, and N2 selectivity increased from ∼ 80 % to 100 % at 200 ∼ 300 °C. Furthermore, the promoting effect could remain unchanged after 24 h of continuous reaction. However, the temperature point for achieving complete conversion of CO increased from 225 °C to 275 °C after the introduction of SO2. Experimental characterization and theoretical calculation jointly proved that the inhibition of CO oxidation by the generation of sulfate was the main reason for promoting NO reduction. Under the coexistence of O2 and SO2, SO2 was firstly oxidized to SO3 on the iridium surface and generated sulfate species on surface hydroxyl groups of SiO2. Some active sites for O2 adsorption were covered by the generated surface sulfate, and adsorbed CO was hard to react with adsorbed O2, resulting in Langmuir-Hinshelwood (L-H) reaction pathways for CO oxidation being inhibited. Therefore, unoxidized CO reacted with NO adsorbed species and generated N2O to generate N2 and CO2, improving NO reduction. This new insight has implications for understanding the promotional effect of SO2 on NO reduction with CO in the presence of O2.

Catalytic removal of gaseous pollutant NO using CO: Catalyst structure and reaction mechanism

Carbon monoxide (CO) has recently been considered an ideal reducing agent to replace NH3 in selective catalytic reduction of NOx (NH3-SCR). This shift is particularly relevant in diesel engines, coal-fired industry, the iron and steel industry, of which generate substantial amounts of CO due to incomplete combustion. Developing high-performance catalysts remain a critical challenge for commercializing this technology. The active sites on catalyst surface play a crucial role in the various microscopic reaction steps of this reaction. This work provides a comprehensive overview and insights into the reaction mechanism of active sites on transition metal- and noble metal-based catalysts, including the types of intermediates and active sites, as well as the conversion mechanism of active molecules or atoms. In addition, the effects of factors such as O2, SO2, and alkali metals, on NO reduction by CO were discussed, and the prospects for catalyst design are proposed. It is hoped to provide theoretical guidance for the rational design of efficient CO selective catalytic denitration materials based on the structure-activity relations.

Rational Design of Precious-Metal Single-Atom Catalysts for Methane Combustion

Supported precious-metal single-atom catalysts (PM SACs) have emerged as a new frontier of high-performance catalytic material with 100% atom utilization efficiency. However, the rational design of such material with guidance from fundamental understandings of the structure-activity relationship remains challenging. Here, we report the synthesis, characterizations, and mechanistic investigation of various PM SACs supported on nanoceria for CH₄ combustion. Using density functional theory, two descriptors as the d-band center of PMs and oxygen vacancy formation energy are established, which jointly govern the reactivity for CH₄ combustion. These descriptors are thus applied to predict a dual SAC consisting of proximate Pd and Rh sites, demonstrating a remarkable improvement versus Pd or Rh catalyst, respectively. Our results reveal the general strategy of integrating experimental and computational efforts for investigation of various PM SACs in methane combustion, thus paving the way for the next generation of advanced catalytic materials.

A review of the catalysts used in the reduction of NO by CO for gas purification

The reduction of NO by the CO produced by incomplete combustion in the flue gas can remove CO and NO simultaneously and economically. However, there are some problems and challenges in the industrial application which limit the application of this process. In this work, noble metal catalysts and transition metal catalysts used in the reduction of NO by CO in recent years are systematically reviewed, emphasizing the research progress on Ir-based catalysts and Cu-based catalysts with prospective applications. The effects of catalyst support, additives, pretreatment methods, and physicochemical properties of catalysts on catalytic activity are summarized. In addition, the effects of atmosphere conditions on the catalytic activity are discussed. Several kinds of reaction mechanisms are proposed for noble metal catalysts and transition metal catalysts. Ir-based catalysts have an excellent activity for NO reduction by CO in the presence of O₂. Cu-based bimetallic catalysts show better catalytic performance in the absence of O₂, in that the adsorption and dissociation of NO can occur on both oxygen vacancies and metal sites. Finally, the potential problems existing in the application of the reduction of NO by CO in industrial flue gas are analyzed and some promising solutions are put forward through this review.

NOx reduction by CO over Fe/ZSM-5: A comparative study of different preparation techniques

Fe/ZSM-5 catalysts were prepared by three kinds of ion exchange methods: aqueous ion-exchange (AI), hydrothermal ion-exchange (HI) and solid-state ion-exchange (SI). Their catalytic activities were tested for NOx reduction by CO in a separated NOx adsorption-desorption process. In this paper, performances of adsorption, reduction and dynamic adsorption-reduction were all investigated. All three catalysts exhibited good reduction activity at above 300 °C. Fe/ZSM-5(SI) exhibited excellent NOx removal efficiency in the dynamic adsorption-reduction experiments. However, in the dynamic process the adsorption efficiency of Fe/ZSM-5(AI) and the reduction efficiency of Fe/ZSM-5(HI) is not very good. The catalysts were further characterized by SEM, BET, XRD, XRF, XPS and TPD. It was found that the Fe content of the Fe/ZSM-5(SI) was the highest. Further, Fe is supported in the form of Fe2O3 particles. Bronsted acid sites were also playing a major role in the high catalytic activity. TPD and in situ DRIFT experiments show that more Fe loading in α acid sites could result in a higher NOx removal efficiency.

Kinetic and DRIFTS studies of IrRu/Al2O3 catalysts for lean NOx reduction by CO at low temperature

This study employs a series of bimetallic IrRu/Al₂O₃ catalysts with differing Ir:Ru compositions for lean NO_x reduction by CO (CO-SCR).

Lean NOx reduction by CO at low temperature over bimetallic IrRu/Al2O3 catalysts with different Ir : Ru ratios

Lean NO reduction by CO at low temperature.

Reduction of nitric oxide by acetylene on Ir surfaces with different morphologies: comparison with reduction of NO by CO

Reduction of nitric oxide (NO) by acetylene (C(2)H(2)) has been investigated by temperature-programmed desorption (TPD) on planar Ir(210) and faceted Ir(210) with tunable sizes of three-sided nanopyramids exposing (311), (311[overline]), and (110) faces. Upon adsorption, C(2)H(2) dissociates to form acetylide (CCH) and H species on the Ir surfaces at low C(2)H(2) precoverage. For adsorption of NO on C(2)H(2)-covered Ir, both planar and faceted Ir(210) exhibit high reactivity for reduction of NO with high selectivity to N(2) at low C(2)H(2) precoverage, although the reaction is completely inhibited at high C(2)H(2) precoverage. Coadsorbed C(2)H(2) significantly influences dissociation of NO. The N-, H-, and C-containing TPD products are dominated by N(2), H(2), CO, and CO(2) together with small amounts of H(2)O. For adsorption of NO on C-covered Ir(210) at fractional C precoverage, formation of CO(2) is promoted while production of CO is reduced. Reduction of NO by C(2)H(2) is structure sensitive on faceted Ir(210) versus planar Ir(210), but no evidence is found for size effects in the reduction of NO by C(2)H(2) on faceted Ir(210) for average facet sizes of 5 nm and 14 nm. The results are compared with reduction of NO by CO on the same Ir surfaces. As for NO+C(2)H(2), the Ir surfaces are very active for reduction of NO by CO with high selectivity to N(2) and the reaction is structure sensitive, but clear evidence is found for size effects in the reduction of NO by CO on the nanometer scale. Furthermore, coadsorbed CO does not affect dissociation of NO at low CO precoverage whereas coadsorbed CO considerably influences dissociation of NO at high CO precoverage. The adsorption sites of CCH+H on Ir are characterized by density functional theory.

Reduction of NO by CO on unsupported Ir: bridging the materials gap

Temperature programmed desorption (TPD) and density functional theory (DFT) are used to investigate adsorption sites and reaction of coadsorbed NO and CO on planar Ir(210) and faceted Ir(210) with tailored sizes of three-sided nanopyramids exposing (311), (31 1) and (110) faces. Both planar and faceted Ir(210) are highly active for reduction of NO by CO with high selectivity to N(2), which is accompanied by simultaneous oxidation of CO. Evidence is found for structure sensitivity in adsorption sites and reaction of coadsorbed NO and CO on faceted Ir(210) versus planar Ir(210). Strong interaction between NO and CO at high NO exposure and one-monolayer CO pre-coverage results in "explosive" evolution of N(2) and CO(2) on planar Ir(210) and size effects in reduction of NO by CO on faceted Ir(210) for average facet size ranging from 5 to 14 nm without change in facet structure.