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CO self-sustaining combustion, induced by a CuCe0.75Zr0.25Oy catalyst, has been confirmed experimentally as an effective strategy to reduce serious environmental pollution and energy waste, which is caused by direct combustion of conventional converter gas in the steelmaking industry. In this paper, the effects of CuCe0.75Zr0.25Oy catalysts prepared by a sol-gel method via three different pore-forming agents (oxalic acid, cellulose and thermal decomposition) were investigated for their catalytic activity of self-sustained CO combustion. Additionally, characterization methods were used to obtain the structural properties of each catalyst. The results obtained show that the CuCe0.75Zr0.25Oy catalyst, as a sol-gel pore-forming agent, prepared from cellulose exhibits the highest activity among the three catalysts. Under the condition of a reaction gas (3% CO+5% O2/N2), the T10 (70°C), T50 (73°C) and T90 (78°C) of the cellulose catalyst are obviously lower than those of the other catalysts, where T10, T50 and T90 denote the reaction temperature corresponding to the CO conversion of 10%, 50% and 90%, respectively. The reason is that the cellulose pore-forming agent promotes the formation of a multistage porous structure, which strengthens the synergistic effect between the Cu and Ce catalysts and changes the redox property of the overall catalyst. On the one hand, the strong synergy between CuO and CeO2 adjusts the dispersion and chemical state of copper nanoparticles. On the other hand, the oxygen vacancies generated locate at the copper-cerium interface enhance the ability of oxygen storage and oxygen release of the catalyst. [ABSTRACT FROM AUTHOR]

This study focuses on the variation in activity-controlling factors during CO catalytic ignition over a CuO–CeO 2 catalyst. The activity for CO combustion follows the decreasing order of CuO–CeO 2 > CuO > CeO 2. Except for inactive CeO 2 , increasing temperature induces CO ignition to achieve self-sustained combustion over CuO and CuO–CeO 2. However, CuO provides enough copper sites to adsorb CO, and abundant active lattice oxygen, thus obtaining a higher hot zone temperature (208.3°C) than that of CuO–CeO 2 (197.3 °C). Catalytic ignition triggers a kinetic transition from the low-rate steady-state regime to a high-rate steady-state regime. During the induction process, Raman, X-ray photoelectron spectroscopy (XPS), CO temperature-programmed desorption (CO-TPD) and infrared (IR) spectroscopy results suggested that CO is preferentially adsorbed on oxygen vacancies (Cu+-[O v ]-Ce3+) to yield Cu+-[C O]-Ce3+ complexes. Because of the self-poisoning of CO, the adsorbed CO and traces of adsorbed oxygen react at a relative rate, which is entirely governed by the kinetics on the CO-covered surface and the heat transport until the pre-ignition regime. Nonetheless, the Cu+-[C O]-Ce3+ complex is a major contributor to CO ignition. The step-response runs and kinetic models testified that after ignition, a kinetic phase transition occurs from a CO-covered surface to an active lattice oxygen-covered surface. During CO self-sustained combustion, the rapid gas diffusivity and mass transfer is beneficial for handling the low coverage of CO. The active lattice oxygen of CuO takes part in CO oxidation. [ABSTRACT FROM AUTHOR]

Abstract The self-sustained combustion of carbon monoxide (CO) has been studied over the CuCe 0.75 Zr 0.25 O δ catalyst by sol–gel method, and compared with the CuCe 0.75 Zr 0.25 O δ (H) and Ce 0.75 Zr 0.25 O δ to investigate sensitivity of different active sites, such as dispersed CuO, Cu–Ce and Ce–Zr solid solution. The CuCe 0.75 Zr 0.25 O δ (H) is obtained via CuCe 0.75 Zr 0.25 O δ sonicated in nitric acid to remove surficial CuO species, and the Ce 0.75 Zr 0.25 O δ is prepared as the reference catalyst. Using the temperature programmed oxidation of CO together with an infrared camera (CO-TPO + FLIR), the catalytic activity for CO self-sustained combustion is determined not only by the ignition temperatures, following the decreasing order CuCe 0.75 Zr 0.25 O δ (81 °C) > CuCe 0.75 Zr 0.25 O δ (H) (131 °C) > Ce 0.75 Zr 0.25 O δ (167 °C) at the flow rate of 200 mL/min, but also by corresponding limits of lean combustion (equivalence ratio Φ) of 0.06–0.09, 0.10–0.13, 0.24–0.37, respectively, under the flow rate of 100–1000 mL/min. The M-K mechanism, in which adsorbed CO reacts with lattice oxygen, is crucial for all the catalysts via temperature programmed surficial reaction and in situ infrared analysis (TPSR + DRIFT). The sensitivity of active sites as follows: well-dispersed CuO > Cu–Ce solid solution > Ce–Zr solid solution. As rate-determining step for CO self-sustained combustion, CO is preferentially adsorbed on surficial dispersed CuO to yield carbonyls, and then the carbonyls interact with lattice oxygen to form CO 2 release. CO is secondly adsorbed on the surficial oxygen of copper and cerium sites in solid solution to yield carbonates. The carbonates formed are more stable and thus CO 2 is produced at the lower rate than carbonyls, indicating that the solid solution is less active than dispersed CuO. Exposed Ce3+ favors to form vacancies on the Cu–Ce solid solution surface, which is beneficial to adsorbing both CO and O 2 , thus presenting the higher activity than Ce–Zr solid solution. [ABSTRACT FROM AUTHOR]

The CuO/Ce 0.75 Zr 0.25 O 2 - δ and Ce 0.75 Zr 0.25 O δ catalysts were prepared by the sol-gel method, and Cu 0.07 Ce 0.75 Zr 0.25 O 2-δ was obtained by treating CuO/Ce 0.75 Zr 0.25 O 2 - δ with nitric acid to remove the well-dispersed CuO on the surface. Various characterizations were used to reveal the different active sites, such as surface-dispersed CuO and Cu-Ce-Zr-O δ solid solutions in CuO/Ce 0.75 Zr 0.25 O 2 - δ , Cu-Ce-Zr-O δ solid solutions in Cu 0.07 Ce 0.75 Zr 0.25 O 2-δ and Ce-Zr-O δ solid solutions in Ce 0.75 Zr 0.25 O δ . The Raman and O 2 -TPD results showed that the concentration of oxygen vacancies in Cu-Ce-Zr-O δ solid solutions was higher than that in Ce-Zr-O δ solid solutions. CO oxidation testing suggested that the catalytic activity decreases in the order of CuO/Ce 0.75 Zr 0.25 O 2 - δ > Cu 0.07 Ce 0.75 Zr 0.25 O 2-δ > Ce 0.75 Zr 0.25 O δ . Combined with the in situ diffuse-reflectance Fourier transform (in situ DRIFT) results, the reaction sensitivity followed the order of CO linear chemisorption onto dispersed CuO x species (Mars-van Krevelen mechanism) > carbonate species onto a Cu-Ce-Zr-O δ solid solution (Langmuir-Hinshelwood mechanism) > carbonate species onto a Ce-Zr-O δ solid solution (Langmuir-Hinshelwood mechanism). Kinetic studies suggested that the power-law rate expressions and apparent activation energies were r = 6.02 × 10 −7 ×P CO 0.68 P O2 0.03 (53 ± 3 kJ/mol) for CuO/Ce 0.75 Zr 0.25 O 2 - δ , r = 5.86 × 10 −7 ×P CO 0.8 P O2 0.07 (105 ± 5 kJ/mol) for Cu 0.07 Ce 0.75 Zr 0.25 O 2-δ and r = 5.7 × 10 −7 ×P CO 0.75 P O2 0.12 (115 ± 6 kJ/mol) for Ce 0.75 Zr 0.25 O δ . The Mars-van Krevelen mechanism should be the crucial reaction pathway over CuO/Ce 0.75 Zr 0.25 O 2 - δ in CO interfacial reactions, although the Langmuir-Hinshelwood mechanism cannot be ignored, and the Langmuir-Hinshelwood mechanism mainly occurred over the Cu 0.07 Ce 0.75 Zr 0.25 O 2-δ and Ce 0.75 Zr 0.25 O δ catalysts, where the contribution from the Mars-van Krevelen mechanism was negligible due to the absence of surface CuO x species. [ABSTRACT FROM AUTHOR]

The bulk-phase Cu-Ce-Zr catalyst BP-gel was prepared by the sol-gel method and investigated for the oxidation of toluene in a fixed-bed reactor. For comparison, other bulk-phase catalysts (thermal decomposition (BP-td), solid-state reaction (BP-sr)) and supported catalysts (incipient wetness impregnation (S-iwi), saturated impregnation (S-si)) were tested. The results showed that the catalytic activities were in the following order: BP-gel>S-si>S-iwi>BP-td>BP-sr. The BP-gel catalyst, which has a mesoporous structure, is beneficial for the mass transfer of reactant molecules. The thermal stability of the BP-gel catalyst was improved by the introduction of ZrO2. Bulk CuO in the catalysts reduced their activity because the synergetic effect between CuO and CeO2 decreased. The low-temperature reducibility, large pore sizes, rich oxygen vacancies, and active oxygen species contributed to the highest activity of the BP-gel, with a T90 as low as 219°C, and no deactivation was observed during the 72 h stability test at the set temperature. [ABSTRACT FROM AUTHOR]

The self-sustained combustion of CO was studied over a series of CuCe1–xZrxOy/ZSM-5 catalysts with different Ce/Zr molar ratios to evaluate the correlation of their structural characteristics with catalytic performance. The obtained results demonstrated that the activity of the cerium-containing catalysts was much higher than that of the CuZr1/Z, with the activity of CO combustion following the order: CuZr1/Z < CuCe0.25Zr0.75/Z < CuCe0.5Zr0.5/Z < CuCe1/Z < CuCe0.75Zr0.25Z. After cerium addition, the cerium-zirconium mixed oxides formed promoted the copper species well dispersed on the catalyst surface. The high activities of cerium-containing catalysts were closely related to the synergetic effects among the copper, cerium, and zirconium species, as the surface copper species provided sites for CO adsorption and the cerium-zirconium mixed oxides promote the activation of oxygen. The activities were probably also attributed to some copper species present as substitutional defects in the ceria lattice, which were more reducible than the copper clusters, minicrystals, and bulk CuO particles. The CuCe0.75Zr0.25/Z possessed excellent redox capacity, adsorption/activation properties for O and CO species, and therefore induced a self-sustained combustion of CO at the lowest temperature than that of other CuCe1-xZrxOy/ZSM-5 catalysts. [ABSTRACT FROM PUBLISHER]

Large-scale internally circulating flows of gas and particles might form under unevenly distributed fluidized air in a fluidized bed. Compared with bed materials, the present study investigated dissimilar particles with different sizes and density moving and mixing through the developed image technique in an internally circulating fluidized bed (ICFB). The concentrations of the tracer particles were obtained by the image analysis method. The inhomogeneity degree and lateral dispersion coefficients of the tracer particles were also calculated, with the results indicating that the degree of inhomogeneity decreases with the increasing dispersed time. The lateral dispersion coefficients of tracer particles with a lower density (for a similar diameter) are larger than those with a higher density. It is clear that the different air distribution methods play an important role in the lateral dispersion of the tracer particles. [ABSTRACT FROM AUTHOR]

The reaction pathways of CO oxidation over the (1 0 0), (1 1 0) and (1 1 1) crystal planes of Cu 2 O. • Morphologies of Cu 2 O micro/nanocrystals were controlled by simply changing precursor concentration. • Increase of precursor concentration results in more exposure of active crystal plane of Cu 2 O. • Activity of CO self-sustained catalytic combustion on Cu 2 O increases with precursor concentration. • Coordination unsaturated Cu and O on the (1 1 1) and (1 1 0) planes promote CO oxidation. The self-sustained catalytic combustion is one of the most effective ways to remove high concentration CO at low temperature. In this paper, Cu 2 O micro/nanocrystals with different morphologies were successfully synthesized by changing the precursor concentration using liquid phase reduction method. The obtained Cu 2 O were characterized using SEM, XRD, XPS, H 2 -TPR and O 2 -TPD, and the relationship between the catalytic performance and morphology was analyzed based on CO-TPD-MS and activity evaluation results. It was found that high precursor concentration leads to more exposure of active crystal planes of Cu 2 O. Compared with Cu 2 O-1 exposing only (1 0 0) crystal planes, Cu 2 O-5, the precursor concentration of which is 5 times of Cu 2 O-1, exposes (1 0 0) and (1 1 0) crystal planes. Cu 2 O-9, with 9 times of precursor concentration of Cu 2 O-1, exposes (1 0 0), (1 1 0) and (1 1 1) crystal planes simultaneously. All the obtained Cu 2 O with different precursor concentrations can achieve self-sustained CO catalytic combustion, and the catalytic activity increases with increasing precursor concentration (Cu 2 O-1 < Cu 2 O-5 < Cu 2 O-9). The results prove that unsaturated coordination of Cu and O on the (1 1 1) and (1 1 0) planes can enhance the corresponding reducibility, adsorption and activation of gaseous oxygen, consequently promoting the CO oxidation to CO 2 over Cu 2 O-9. [ABSTRACT FROM AUTHOR]

It remains a major challenge to abate efficiently the harmful nitrogen oxides (NO x ) in low-temperature diesel exhausts emitted during the cold-start period of engine operation. Passive NO x adsorbers (PNA), which could temporarily capture NO x at low temperatures (below 200 °C) and release the stored NO x at higher temperatures (normally 250-450 °C) to downstream selective catalytic reduction unit for complete abatement, hold promise to mitigate cold-start NO x emissions. In this review, recent advances in material design, mechanism understanding, and system integration are summarized for PNA based on palladium-exchanged zeolites. First, we discuss the choices of parent zeolite, Pd precursor, and synthetic method for the synthesis of Pd-zeolites with atomic Pd dispersions, and review the effect of hydrothermal aging on the properties and PNA performance of Pd-zeolites. Then, we show how different experimental and theoretical methodologies can be integrated to gain mechanistic insights into the nature of Pd active sites, the NO x storage/release chemistry, as well as the interactions between Pd and typical components/poisons in engine exhausts. This review also gathers several novel designs of PNA integration into modern exhaust after-treatment systems for practical application. At the end, we discuss the major challenges, as well as important implications, for the further development and real application of Pd-zeolite-based PNA in cold-start NO x mitigation.