Fabrication of Stable Cu-Ce Catalyst with Active Interfacial Sites for NOx Elimination by Flame Spray Pyrolysis

The complete conversion of NOx to harmless N2 without N2O formation is crucial for the control of air pollution, especially at low temperatures. Cu-based catalysts are promising materials due to their low cost and high activity in NO dissociation, even comparable to noble metals; however, they suffer from low stability. Here, we established a Cu-Ce catalyst in one step with strong metal–support interaction by the flame spray pyrolysis (FSP) method. Almost 100% NO conversion was achieved at 100 °C, and they completely transferred into N2 at a low temperature (200 °C) for the FSP-CuCe catalyst, exhibiting excellent performance in NO reduction by CO reaction. Moreover, the catalytic performance can stay stable, while 23% NO conversion was lost in the same condition for the one made by the co-precipitation (CP) method. This can be attributed to the synergistic effect of abundant active interfacial sites and more flexible surface oxygen created during the FSP process. The flame technology developed here provides an efficient way to fabricate strong metal–support interactions, exhibiting notable potential in the design of stable Cu-based catalysts.

Investigation of catalysts M/CeO2 (M = Pt, Rh, or Pd) for purification of CO2 derived from oxycombustion in the absence or presence of water

Oxyfuel combustion is a promising technology to produce a CO₂-rich flue gas ready suitable for sequestration or valorization. But its storage as well as its further valorization requires to increase the CO₂ purification as a small amount of CO and NOx are produced during combustion. Based on the technology developed for three-way converters, similar systems, i.e., M/CeO₂ where M is Pt, Pd, or Rh, were studied for NO-CO abatement in a gas stream similar to those obtained when an oxyfuel combustion is performed. The results evidenced that the role of the metal nature influences the performances obtained on NO-CO abatement, platinum supported on ceria being the most efficient catalyst. We also measured the impact of the presence of water in the reaction stream on the catalytic activity of these materials. It appears that the presence of water has a beneficial effect on the different reactions due to a water gas shift reaction that increases the reduction of the NO and favors the formation of N₂. The study pointed out that platinum supported on ceria remained the best catalyst, under these wet operating conditions close to industrial ones, for purification of oxyfuel combustion exhausts.

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.

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).

Improved NO–CO reactivity of highly dispersed Pt particles on CeO2 nanorod catalysts prepared by atomic layer deposition

Highly dispersed platinum (Pt) nanoparticles are deposited on CeO₂ nanorods via atomic layer deposition (ALD) to improve the catalytic activity towards the NO–CO reaction.

Controlling N2O formation during regeneration of NOx storage and reduction catalysts: from impact of platinum-group metal type to rational utilization

Herein, we report an effective strategy for minimization of N₂O emissions based on elucidating the impact of the type of platinum-group metals (PGMs = Pt, Pd, or Rh) on by-product formation during regeneration of PGM-BaO/Al₂O₃ catalysts. The significant differences in N₂O or NH₃ formation were thoroughly investigated from the perspective of an in situ reaction. Kinetic analysis of NO reduction by CO shows different turnover frequency and apparent activation energy values over these catalysts. The results reveal that the apparent kinetics is dependent on the type of platinum-group metal chosen. In situ DRIFTS data indicate that the unique adsorption behaviors of reactants via which they access each PGM essentially determine their individual reaction kinetics. The preferential adsorption of NO or CO molecules on the PGM surface controls the dominant intermediate (NO_ad/N_ad, CO_ad, or NCO_ad) species, which is a major factor responsible for various yields of N₂O and NH₃ during the rich period. Finally, a feasible strategy has been proposed via optimizing catalyst formulation to effectively control the N₂O emissions.

Nanophase-separated Ni3Nb as an automobile exhaust catalyst

Nanophase-separated Ni₃Nb alloy exhibited higher performance than traditional Pt catalysts toward the remediation of automobile exhaust.

Catalytic remediation of automobile exhaust has relied on precious metals (PMs) including platinum (Pt). Herein, we report that an intermetallic phase of Ni and niobium (Nb) (i.e., Ni₃Nb) exhibits a significantly higher activity than that of Pt for the remediation of the most toxic gas in exhaust (i.e., nitrogen monoxide (NO)) in the presence of carbon monoxide (CO). When subjected to the exhaust-remediation atmosphere, Ni₃Nb spontaneously evolves into a catalytically active nanophase-separated structure consisting of filamentous Ni networks (thickness < 10 nm) that are incorporated in a niobium oxide matrix (i.e., NbO_x (x < 5/2)). The exposure of the filamentous Ni promotes NO dissociation, CO oxidation and N₂ generation, and the NbO_x matrix absorbs excessive nitrogen adatoms to retain the active Ni⁰ sites at the metal/oxide interface. Furthermore, the NbO_x matrix immobilizes the filamentous Ni at elevated temperatures to produce long-term and stable catalytic performance over hundreds of hours.

Noble metal ions in CeO2 and TiO2: synthesis, structure and catalytic properties

CeO₂ and TiO₂ based noble metal ionic catalysts show very high catalytic activities toward several reactions such as auto exhaust, water gas shift, H₂ + O₂ recombination compared to supported nanometal catalysts due to their electronic interactions.

Mesoporous Pt–SiO2 and Pt–SiO2–Ta2O5 Catalysts Prepared Using Pt Colloids as Templates

Sol-gel synthesis of silica and silica-tantalum oxide embedded platinum nanoparticles is carried out using Pt colloids as templates. These colloids are prepared by reduction with Na[AlEt(3)H] and stabilized with different ligands (ammonium halide derivatives, non-ionic surfactants with polyether chains, and 2-hydroxy-propionic acid). The aim of the present study is to prepare mesoporous silica embedded Pt colloids combining the "precursor concept" with the model of catalyst preparation using preformed spheres. Nanoparticles of Pt incorporated in high surface area mesoporous materials are formed after calcination. Further, it is observed that calcination of these catalysts causes partial aggregation and oxidation of the parent colloids, a process that is largely dependent on the nature of the stabilizing ligands. Several methods have been used for characterization of these materials: adsorption-desorption isotherms at 77 K, H(2) chemisorption, X-ray diffraction(XRD), (29)Si and (13)C magic angle spinning (MAS) NMR, ammonia diffuse reflectance Fourier transform infrared spectroscopy (NH(3)-DRIFT), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It is found that both metal oxide systems exhibit Brønsted acidity (weaker for silica and quite strong for silica-tantalum oxide). In addition, NH(3)-DRIFT experiments demonstrate the oxidative properties of the surface. Part of the adsorbed NH(4) (+) species is oxidized to N(2)O. Testing these catalysts in the reduction of NO and NO(2) with isopentane under lean conditions indicate that the activity of these catalysts is indeed dependent on the size of the platinum particles, with those of size 8-10 nm demonstrating the best results. The support likely contributes to this effect, particularly after Ta incorporation into silica.

Interpretation of the Experimental Data on the Reduction Reaction of NO by CO on Rhodium by Monte Carlo Simulations and by Solving the Kinetic Equations of the Reaction Mechanism

Some mechanisms of the reduction reaction of NO by CO on rhodium are analyzed and discussed, solving the kinetics equations and using Monte Carlo simulations, in terms of its ability to interpret the recent experiments of Zaera et al., who used a molecular beam method to study experimentally the kinetics of the reaction. Critical use is also made of the information on rate constants available for this system in the literature. Uniform catalytic surfaces and the statistical incipient percolation cluster (IPC) fractal are considered in the simulations.

Surface Properties of Ni−Pt/SiO2 Catalysts for N2O Decomposition and Reduction by H2

The surface properties of bimetallic Ni-Pt/SiO2 catalysts with variable Ni/Ni + Pt atomic ratio (0.75, 0.50, and 0.25) were studied using N2O decomposition and N2O reduction by hydrogen reactions as probes. Catalysts were prepared by incipient wetness impregnation of the silica support with aqueous solutions of the metal precursors to a total metal loading of 2 wt %. For both model reactions, Pt/SiO2 catalyst was substantially more active than Ni/SiO2 catalyst. Mean particle size by TEM was about the same (in the range 6-8 nm) for all catalysts and truly bimetallic particles (more than 95%) were evidenced by EDS in the Ni-Pt/SiO2 catalysts. CO adsorption on the bimetallic catalysts showed differences in the linear CO absorption band as a function of the Ni/Pt atomic ratio. Bimetallic Ni-Pt/SiO2 catalysts showed, for the N2O decomposition, a catalytic behavior that points out an ensemble-size sensitive behavior for Ni-rich compositions. For the N2O + H2 reaction, the bimetallic catalysts were very active at low temperature. The following activity order at 300 K was observed: Ni75Pt25 > Ni25Pt75 approximately Ni50Pt50 > Pt. TOF values for these catalysts increased 2-5 times compared to the most active reference catalyst (Pt/SiO2). The enhancement of the activity in the Ni75Pt25 bimetallic catalysts is explained in terms of the presence of mixed Ni-Pt ensembles.