Zhongmiao Gong,Zhigang Chen,Hao Li,Guang Yang,Min Yuan,Zhiyun Li,Xueying Wan,Yi Cui
Surface-supported isolated atoms in single-atom catalysts (SACs) grant maximum utilization of metals in heterogeneous catalysis. Herein, we report a feasible pyrolysis strategy to synthesize Pd single atoms by thermally melting Pd nanoparticles on an oxygen-vacancy-rich tungsten-oxide matrix at reduction atmosphere. Near ambient pressure X-ray photoelectron spectroscopy was used to monitor the formation of zero-valence Pd single atoms and the increased metallic feature of WO 3− x substrate. Accordingly, the as-obtained zero-valence Pd single-atom catalyst exhibits a markedly boosted HER activity with a low overpotential ( η 10 = 70 mV) at the current density of 10 mA/cm 2 and a small Tafel slope ( b = 68 mV/dec), nearly 150 mV and a 3.0-fold enhancement than those of Pd nanoparticles ( η 10 = 220 mV, b = 133 mV/dec) under the same conditions. In addition, quasi in situ XPS results suggest the hydrogen spillover effect is more likely to occur on Pd single atoms during the electrochemical process. Our work may pave an interesting route for the rational design of highly-efficient single-atom catalyst, and the elucidation of corresponding enhanced reaction mechanisms by the utilization of advanced characterization techniques.
Junguang Meng,Zengli Zhao,Xiaobo Wang,Anqing Zheng,Dongyan Zhang,Zhen Huang,Kun Zhao,Guoqiang Wei,Haibin Li
Ni/Fe bimetallic catalysts were synthesized on an olivine support using the wetness impregnation (WI) and thermal fusion (TF) methods, and catalytic cracking and steam reforming on different tar model compounds (phenol and naphthalene) were investigated in a fixed bed reactor. The effects of the reaction temperature and space-time on the catalytic cracking activity of phenol and naphthalene were tested, and the influences of steam on the carbon molar ratio (S/C) and different synthesized methods on the steam reforming of phenol and naphthalene were also studied. In addition, the carbon deposited on the catalyst was analysed using temperature program oxidation (TPO), Raman spectroscopy and transmission electron microscopy (TEM) methods. The elemental composition of the catalyst was analysed by X-ray fluorescence (XRF), and the physiochemical properties of the catalysts were characterized via X-ray diffraction (XRD), the BET surface area, scanning electron microscopy (SEM), TEM, Raman spectroscopy, temperature program reduction (TPR), and X-ray photoelectron spectroscopy (XPS). The results showed that the structure of TF-Ni/Fe/olivine changed considerably compared with that of calcined olivine (mainly the Mg 2 SiO 4 phase) and that a portion of Fe was fused into the olivine structure and reorganized into a new (Mg, Fe) Fe 2 O 4 phase. After reduction, the Ni-Fe alloy was detected on both catalysts, and the particle size of the Ni-Fe alloy on TF-Ni/Fe/olivine was smaller than that of WI-Ni/Fe/olivine. The TF-Ni/Fe/olivine cracked phenol into small-molecule gas (CO and H 2 ) and a small quantity of olefin. By contrast, phenol was polymerized to naphthalene in the absence of a catalyst. The phenol and naphthalene steam reforming conversion on WI-Ni/Fe/olivine were both higher than that of TF-Ni/Fe/olivine. A 100-h stability test of phenol steam reforming on TF-Ni/Fe/olivine and WI-Ni/Fe/olivine was conducted, and TF-Ni/Fe/olivine showed higher stability in the early stages of the experiment due to stronger interactions between the active sites and olivine support. The deposited carbon from naphthalene steam reforming was more difficult to eliminate because it contained more C β and C γ , which had higher degrees of graphitization.
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