Exsolution of Catalytically Active Iridium Nanoparticles from Strontium Titanate

The search for new functional materials that combine high stability and efficiency with reasonable cost and ease of synthesis is critical for their use in renewable energy applications. Specifically in catalysis, nanoparticles, with their high surface-to-volume ratio, can overcome the cost implications associated with otherwise having to use large amounts of noble metals. However, commercialized materials, that is, catalytic nanoparticles deposited on oxide supports, often suffer from loss of activity because of coarsening and carbon deposition during operation. Exsolution has proven to be an interesting strategy to overcome such issues. Here, the controlled emergence, or exsolution, of faceted iridium nanoparticles from a doped SrTiO₃ perovskite is reported and their growth preliminary probed by in situ electron microscopy. Upon reduction of SrIr_0.005Ti_0.995O₃, the generated nanoparticles show embedding into the oxide support, therefore preventing agglomeration and subsequent catalyst degradation. The advantages of this approach are the extremely low noble metal amount employed (∼0.5% weight) and the catalytic activity reported during CO oxidation tests, where the performance of the exsolved SrIr_0.005Ti_0.995O₃ is compared to the activity of a commercial catalyst with 1% loading (1% Ir/Al₂O₃). The high activity obtained with such low doping shows the possibility of scaling up this new catalyst, reducing the high cost associated with iridium-based 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.

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.

In situ Infrared Spectroscopic Studies on the Mechanism of the Selective Catalytic Reduction of NO by C3H8 over Ga2O3/Al2O3: High Efficiency of the Reducing Agent

The partial oxidation of propane and the mechanism of selective catalytic reduction (SCR) of NO by propane over Ga2O3/Al2O3 in excess of O2 have been investigated using in situ Fourier transform infrared spectroscopy. An optimized Ga2O3/Al2O3 catalyst shows high activity and efficiency of the reducing agent propane (100% conversion of NO at 623 K, GHSV: 10,000 h(-1)). One molecule of propane converts more than 4 NO molecules to N2. The reaction starts with the partial oxidation of C3H8 by O2 and carboxylates (acetate, formate) are formed on the catalyst surface above 573 K. This oxidation represents the rate-determining step of the SCR reaction. These surface carboxylates represent a dominating intermediate and (easily) react with (adsorbed) NO forming nitrogen-containing organic species. The latter are proposed to react with NO to form N2. Total oxidation of propane was enhanced at temperatures above 773 K leading to decreased reductant efficiency. Surface nitrite and nitrate species can also be observed, but they were found to be spectators only. This could be concluded from the electron balance (conversion of propane relative to NO) and from the relative rates of the single reaction steps. On the basis of these investigations and stoichiometric calculations, a conclusive reaction mechanism is proposed.