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The Cu/ZSM-5, Ce/ZSM-5 and Cu-Ce/ZSM-5 catalysts were prepared and characterized in this investigation and the catalytic activity of carbon monoxide (CO) combustion under these catalysts was determined by temperature-programmed oxidation. The activity for the CO combustion follows the decreasing order: Cu-Ce/ZSM-5 > Cu/ZSM-5 > Ce/ZSM-5, indicated by lower ignition, light-off, extinction temperature and broader hysteresis determined via both heating and cooling feeding process. The CO adsorbed on the copper sites to form Cu + –CO complexes, monodentate and bidentate carbonates was considered to be the crucial step for CO catalytic combustion. At the CO concentration ≥5 vol.%, the CO self-sustained combustion was achieved over the Cu-Ce/ZSM-5 catalyst. One reason is due to formation of Cu 2+ ions incorporated into cerium oxides, which are more reducible than the copper clusters, minicrystals and bulk CuO particles. Another reason is attributable to the formation of Ce 4+ /Ce 3+ redox couple, which facilitates oxygen transport on the catalyst surface. [ABSTRACT FROM AUTHOR]

[Display omitted] • The L-H mechanism dominates the low-temperature induction stage. • Coexistence of M−K and L-H mechanisms in ammonia catalytic combustion. • (M−K) /(L-H) = 0.427 (Induction stage) and 0.627 (continuous combustion stage). • The NH and NH 2 are the key species for the two stages of N 2 production (i-SCR). • The CuO-CeO 2 achieves self-sustained catalytic combustion at NH 3 = 14%. Carbon-free hydrogen-rich ammonia (NH 3) is a potential next energy generation source, where a high ignition point and high nitric oxide contents, thereby limiting further development. This study aims to investigate the mechanism evolution during the catalytic ignition process of high NH 3 concentration over copper-cerium catalyst to address the demerits of flame combustion. The phase composition, elemental valence, and active species of the copper-cerium oxide (CuO-CeO 2) catalyst are investigated using various characterization techniques. The results indicated that Cu species are mainly present in copper-cerium solid solutions and highly dispersed CuO clusters, providing sufficient Cu sites to adsorb NH 3 and abundant reactive lattice oxygen. The catalytic ignition triggered a kinetic transition from low-rate to high-rate steady-state. During the induction process, the predominance of the L-H mechanism is implied by the kinetic modeling and transient experiments (low-rate induced phase). Combined with IR spectroscopy and isotope (18O 2)-transient response study, the results indicated that adsorbed NH 3 reacted with surface adsorbed oxygen, simultaneously pulling out lattice oxygen to react with adsorbed NH 3 , confirming the coexistence of L-H and M−K mechanism over CuO-CeO 2. The L-H mechanism (18O, contribution of adsorbed oxygen) plays a relatively dominant role in the induction stage, while the involvement of the M−K mechanism (16O, contribution of lattice oxygen) is significantly increased in the sustained combustion stage. At both stages, NH and NH 2 served as critical species for N 2 generation (i-SCR), respectively, playing an important role in N 2 selectivity. Moreover, in the self-sustained combustion stage for CuO-CeO 2 (14%NH 3), the rapid gas diffusion and mass transfer facilitated the adsorption and activation of NH 3. These results are envisaged to provide theoretical support for the handling and application of high concentrations of NH 3. [ABSTRACT FROM AUTHOR]

CO self-sustaining catalytic combustion tends to be one of potential means to achieve efficient conversion of off-gases from steelmaking. Building model catalysts to investigate the reaction process academically is essential to design optimized catalysts and Cu 2 O exposing (1 0 0), (1 1 1) and (1 1 0) planes are excellent materials. Herein, morphological CeO 2 /Cu 2 O inverse model catalysts were synthesized to reveal the synergistic effect and reaction mechanism. It is demonstrated that the ignition temperature is lowered on the interface by synergistic effect, which dominates the ignition, while the combustion after ignition is controlled by Cu 2 O planes providing abundant lattice oxygen to react with CO. Additionally, exposure of different planes plays a significant role in the reaction since O-termination on (1 0 0) inhibits the interaction, whereas Cu-termination on (1 1 0) surface structure and coordinately unsaturated Cu+ on open (1 1 1) surface respectively contribute to their superior catalytic performance. [Display omitted] ● Inverse model CeO 2 /Cu 2 O catalysts with regular morphology were successfully prepared. ● Synergistic effect on Cu-Ce interface lowered ignition temperature, dominating the ignition. ● The catalytic activity and synergistic effect vary among planes with (1 1 1) performing the best. ● Exposure of crystal planes with higher activity assists in designing optimized catalysts. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The catalytic self-sustained combustion of toluene was successfully developed. • CuCeZr/TiO 2 catalyst showed the excellent performance in self-sustained combustion. • TPO in N 2 atmosphere was used to investigate the role of lattice oxygen in toluene combustion. Abstract The catalytic self-sustained combustion of toluene over Cu x Mn 1-x Ce 0.75 Zr 0.25 /TiO 2 catalysts (x = 1, 0.5, 0) was studied in a microscale combustor. The catalytic activity is determined not only by the temperature with toluene conversion of 90% (T co), but also by the corresponding lean-combustion limits. It was found that the self-sustained combustion was achieved successfully over the catalysts, with the toluene concentrations of 0.35 con.%, 0.50 con.% and 1.00 con.%. According to T co, the activity decreased in the order of CuCe 0.75 Zr 0.25 /TiO 2 (234 °C)＞Cu 0.5 Mn 0.5 Ce 0.75 Zr 0.25 /TiO 2 (238 °C) ＞MnCe 0.75 Zr 0.25 /TiO 2 (284 °C). The excellent activity of CuCe 0.75 Zr 0.25 /TiO 2 catalyst could be ascribed to the high content of active lattice oxygen, good low-temperature reducibility and homogeneous dispersion, combined with the results of H 2 -TPR, O 2 -TPD and XPS. Furthermore, the results of in-situ DRIFT and temperature-programmed oxidation (TPO) of toluene in N 2 atmosphere suggested that toluene was adsorbed on the catalyst surface by removing α-H from methyl and combining with active oxygen specie to form benzyl group, which was finally oxidized into CO 2 and H 2 O with lattice oxygen. [ABSTRACT FROM AUTHOR]

Self-sustained catalytic combustion is a sustainable approach to deal with exhaust gas with high concentration CO, and revealing its reaction process is necessary and challenging. Herein, cube (Cu 2 O-C), octahedron (Cu 2 O-O) and dodecahedron (Cu 2 O-D) exposing different crystal planes were used to explore the catalytic combustion mechanism. The catalytic combustion can be self-sustained on the Cu 2 O surface and the activities decrease in the order of Cu 2 O-O > Cu 2 O-D > Cu 2 O-C, contributing to the different exposing planes with (1 1 1), (1 1 0) and (1 0 0), respectively. In-situ DRIFTS results prove that the catalytic combustion of CO to CO 2 on Cu 2 O is prone to follow the MvK mechanism. Comparing with Cu 2 O-D and Cu 2 O-C, the relatively open surface of Cu 2 O-O plane composed of unsaturated copper and oxygen atoms facilitates the CO adsorption on Cu (I) and the mobility of lattice oxygen, leading to the highest low temperature reducibility and catalytic activity. [Display omitted] • CO self-sustained catalytic combustion were achieved on cube, octahedron and dodecahedron Cu 2 O. • Octahedron Cu 2 O with (1 1 1) planes exhibits the best activity for high concentration CO. • Coordination unsaturated Cu and O on the (111) and (110) planes promote CO oxidation. • Catalytic combustion reaction of CO on Cu 2 O mainly follows the MvK mechanism. [ABSTRACT FROM AUTHOR]

Cubic Cu 2 O micro/nanocrystals were successfully synthesized by liquid-phase reduction using copper salt of CuSO 4 or CuCl 2 ·2H 2 O, and glucose or ascorbic acid as reducing agent, respectively. The activity of the catalysts was evaluated by light-off curves of CO self-sustained catalytic combustion via temperature-programmed oxidation of CO (CO-TPO), with the results showing the activity of catalysts following the order of Cu 2 O–Cl-GLU > Cu 2 O– S -GLU > Cu 2 O– S -AA > Cu 2 O–Cl-AA, (Cl denotes CuCl 2 ·2H 2 O, GLU denotes glucose, S denotes CuSO 4 and AA denotes ascorbic acid, respectively), corresponding to the ignition temperature of 109 °C, 122 °C, 137 °C and 186 °C, respectively. The crystal structure, elemental valence, morphology and redox property of the prepared catalysts were analyzed by using various characterization techniques. Combined with in situ infrared spectrum, the CO self-sustained catalytic combustion over Cu 2 O catalysts mainly follows the Mars-van-Krevelen (M-v-K) mechanism: the adsorbed and activated CO reacts with lattice oxygen to yield CO 2 and oxygen vacancy, and then the oxygen vacancy can be replenished by gaseous oxygen. Combined with catalytic performance of high-concentration CO, it is found that the catalysts prepared using glucose as reducing agent are more angular compared with ascorbic acid. The Cu 2 O–Cl-GLU synthesized with glucose and CuCl 2 ·2H 2 O exhibits the best catalytic activity among all the catalysts tested, attributing to its more obvious edge and rough crystal surface. The unique structure of Cu 2 O–Cl-GLU leads to the high exposure rate and coordination unsaturation of atoms on the cubic Cu 2 O micro/nanocrystals that can improve the ability of activating gaseous O 2 and low temperature reducibility, and consequently facilitating the catalytic activity. Cycle path diagram of CO oxidation reaction (a) and schematic diagram of CO oxidation reaction on Cu 2 O (b). [Display omitted] • Effects of reducing agent on catalytic properties is greater than that of copper salt. • High concentration CO can achieve self-sustained catalytic combustion on obtained Cu 2 O. • Glucose enhances unsaturation of Cu atoms contributing to high activity. • Chemically adsorbed oxygen on Cu 2 O surface promotes CO ignition. [ABSTRACT FROM AUTHOR]