Hongsheng Chen

Synergism between Cu and Zn sites in Cu/Zn catalysts for methanol synthesis

Abstract Three Cu-based catalysts prepared using RF plasma-sputtering (Cu/ZnO and Cu/Al2O3) and coprecipitation (Cu/Zn/Al oxide) methods were employed for the investigation of the synergism between Cu and Zn sites. The static secondary ion mass spectrometry (SIMS) experiments indicate that, on both Cu/ZnO/Al2O3 and Cu/ZnO catalysts, CO is adsorbed at Cu sites (SIMS peaks appear at 91 and 93 amu), and H is bound to ZnO sites (67, 69, 70, 71 and 81, 83, 84, 85 amu) when the catalyst surfaces are exposed to H2, CO and CO2. Methoxy, the intermediate species of methanol synthesis, is found to be bound at Zn sites (95, 97, 98, 99 amu). CO and CO2 are found to induce Zn migration from sub-layers to the topmost layer while H2 is heterolyzed easier due to the presence of Cu. The coexistence of ZnO with Cu could enhance the capability of Cu to adsorb CO species and itself to adsorb H2 species. Al2O3 shows no synergetic effect with Cu in this experiment. No CO and H2 are detected on the Cu/Al2O3 catalyst exposed to syngas. The synergetic effect between Cu and Zn in the course of methanol synthesis was discussed.

Comparative studies of manganese-doped copper-based catalysts: the promoter effect of Mn on methanol synthesis

Abstract A Mn-promoted Cu/ZnO/Al2O3 catalyst with 5–10% improvement in methanol yield as compared with the undoped Cu/ZnO/Al2O3 catalyst under identical conditions has been synthesized. In order to understand the promoter effect of Mn on the methanol production, Cu/Al2O3 and Cu/SiO2 catalysts with and without various amounts of Mn were also prepared and comparatively investigated by microreactor, N2O titration, TPR (Temperature Programmed Reduction) and XPS (X-ray Photoelectron Spectroscopy). Experimental results show that these Mn-promoted catalysts have higher catalytic activity, larger surface Cu concentration, and elevated Cu reduction temperature than their Mn-undoped counterparts. XPS study gives evidence that in the reduced or reacted catalysts, the interaction between Mn and Cu occurs, resulting in the reduction of Mn4+ to Mn2+ or Mn3+, as well as the oxidation of Cu0 and Cu+ to higher oxidation states. This interaction may be responsible for the harder Cu reduction process, better Cu dispersion, higher active center concentration, and thus enhanced methanol yield.

N2O decomposition over ZrO2 : an in-situ DRIFT, TPR, TPD and XPS study

Abstract The adsorption and decomposition of N2O over ZrO2 are studied using in-situ DRIFT, TPD, TPR and XPS. N2O is found to be reversibly and molecularly adsorbed on the surface. The dissociation of N2O into adsorbed dinitrogen and oxygen can take place on Ar-pre-purged ZrO2. The adsorbed dinitrogen, which produces an IR band at 2360 cm−1 as detected by in-situ DRIFT, can be easily removed from the surface by Ar-purge or heating to high temperatures. The adsorbed oxygen, which is identified by the IR band at 810 cm−1, is strongly bonded to surface Zr ions and remain at the surface at high temperatures. TPR shows that the decomposition of N2O into N2 over ZrO2 becomes significant only at temperatures higher than 750 K, especially after the evolution of dioxygen (the desorption of adsorbed oxygen). The mechanism of decomposition is discussed.

FTIR, XPS and TPR studies of N2O decomposition over Cu‐ZSM‐5

Decomposition of N 2 O on Cu-ZSM-5 was studied by temperature-programmed reaction (TPR), in situ Fourier transform infrared spectroscopy (FTIR) and XPS. It is found that the decomposed intermediates of N 2 O-dinitrogen and oxygen ion-show absorption bands at 2161 cm -1 and 910 cm -1 in the IR spectrum, which can be assigned respectively to as N-N stretching vibration and a T-O stretching vibration perturbed by Cu 2+ . Both bands increase in intensity with temperature in the range 25-250°C. Unlike the band at 2161 cm -1 , whose intensity decreases sharply above 250°C, the band at 910 cm -1 still persists at higher temperatures. The IR results agree with the TPR profiles of N 2 and O 2 : N 2 starts to desorb at 250°C whereas O 2 remains until 310°C, where N 2 O decomposes. An oscillation in the O 2 signal is observed between 400 and 550°C, along with a more evident but opposite oscillation in the N 2 O signal. The oscillation of N 2 O decomposition and the release rates of O 2 are found to correlate with the oxidation-reduction of copper sites by XPS. Based on this evidence, Cu + is proposed to be the active centre for dissociative adsorption of N 2 O. The removal of adsorbed oxygen ions through recombination as O 2 or through interaction with protons trapped in the zeolite cavities may preserve Cu in the +1 oxidation state, which enables continuous decomposition of N 2 O.

Non-oxidative conversion of methane to aromatics over modified Mo/HZSM5 catalysts

Abstract The effects of HZSM5 Si/Al ratio, cobalt additive and HCl-acidified pretreatment on methane conversion and selectivity to aromatics have been investigated for non-oxidative conversion of methane to aromatics. The HZSM5 (E) with low Si/Al ratio (Si/Al=20) has better methane conversion and benzene selectivity than HZSM5 (D) (Si/Al=55). The synergistic effect is exhibited in 3% Mo-1% Co-HCl/HZSM5 (E) catalyst, on which a high methane conversion of 10.7% can be achieved, in comparison with 5.3% methane conversion on 3% Mo/HZSM5 (E). The catalysts have been characterized by XPS and the reaction mechanism is discussed.