The performance of Ni/SiO2 catalyst in the process of combination of CO2 reforming and partial oxidation of methane to produce syngas was studied. The Ni/SiO2 catalysts were prepared by using incipient wetness impregnation method with nickel nitrate as a precursor and characterized by FT-IR, TG-DTA, UV-Raman, XRD, TEM, and H2-TPR. The metal nickel particles with the average size of 37.5 nm were highly dispersed over the catalyst, while the interaction between nickel particles and SiO2 support is relatively weak. The weak NiO-SiO2 interaction disappeared after repeating oxidation-reduction-oxidation in the fluidized bed reactor at 700°C, which resulted in the sintering of metal nickel particles. As a result, a rapid deactivation of the Ni/SiO2 catalysts was observed in 2.5 h reaction on stream.
The Ni-based catalyst has recently attracted considerable attention due to the plentiful resources of nickel, as well as its low cost and good catalytic performance comparable to those of noble metals for many catalytic reactions, such as hydrogenation of olefins and aromatics [
Support plays an important role in determining the performance of Ni-based catalyst. Generally, a support with high surface areas is very necessary since it is effective in increasing Ni dispersion and improving thermal stability, hence not only providing more catalytically active sites, but also decreasing the deactivation over time of the catalysts due to sintering and migration effects [
The method of catalyst preparation is another key parameter which needs to be optimized because it will result in different structural and textural properties of Ni-based catalyst. Therefore, numerous methods, including precipitation, homogeneous deposition-precipitation, and sol-gel techniques, have been developed to enhance the performance of Ni-based catalyst [
In addition, the choice of the precursor salt is also crucial since it determines whether the Ni-based catalyst will be prepared successfully or not. As an efficient precursor, two terms must be met: firstly, high solubility is desirable because the precursor concentration in the impregnation solution must be high [
In this paper, Ni/SiO2 catalyst was prepared by incipient wetness impregnation (IWI) with nickel nitrate as precursor and tested in the process of combination of CO2 reforming and partial oxidation of methane (CRPOM) to produce syngas. TG-DTA, HR-TEM, IR, UV-Raman
The Ni/SiO2 catalysts were prepared with IWI using nickel nitrate as precursor according to our previous works [
The catalytic reaction was performed in a fluidized-bed reactor that was comprised of a quartz tube (I.D. = 20 mm,
FTIR spectra were measured using a Nicolet 560 spectrometer equipped with a MCT detector. The samples were tabletted to thin discs with KBr.
Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on a PERKIN ELMER-TAC7/DX with a heating rate of 10°C/min under oxygen (99.99%, 20 mL/min). The samples were pretreated with oxygen flow at 383 K for 1 h.
UV-Raman spectra were carried out with a Jobin Yvon LabRam-HR800 instrument, using 325.0 nm Ar+ laser radiation. The excitation laser was focused down into a round spot approximately 2
X-ray powder diffraction (XRD) patterns of samples were obtained with an automated power X-ray diffractometer (Rigku-D/max-2550/PC, Japan) equipped with a computer for data acquisition and analysis, using Cu K
Transmission electron microscopy (TEM) images were recorded on a Philips-FEI transmission electron microscope (Tecnai G2 F30 S-Twin, Netherlands), operating at 300 kV. Samples were mounted on a copper grid-supported carbon film by placing a few droplets of ultrasonically dispersed suspension of samples in ethanol on the grid, followed by drying at ambient conditions.
H2-temperature-programmed reduction (H2-TPR) experiments were performed in a fixed-bed reactor (I.D. = 4 mm). 50 mg samples were used and reduced under a stream of 5% H2/N2 (20 mL/min) from 50°C to 800°C with a ramp of 7°C/min. Hydrogen consumption of the TPR was detected by a TCD and its signal was transmitted to a personal computer.
The experiments for reduction-oxidation cycle (redox) performance were performed as follows. The catalysts were pretreated with H2 flow at 700°C for 1 h and then were cooled down to room temperature and reoxidized in O2 at different temperature for 1 h. The reoxidized samples were then performed by H2-TPR experiments as above.
The catalytic performance of Ni/SiO2 was shown in Figure
CH4 conversion versus time on 3NiSN catalyst for combination of CO2 reforming and partial oxidation of methane to produce syngas (reaction temperature: 700°C, CH4/CO2/O2 = 1/0.4/0.3, and GHSV = 9000 h−1).
The FT-IR spectra of 3NiSN before calcination and Ni(NO3)2 precursor were illustrated in Figure
FT-IR spectra of 3NiSN (dried at 100°C) and nickel nitrate (Ni(NO3)2).
In order to study the formation of NiO from precursor, thermal analysis of 3NiSN before calcination was carried out (shown in Figure
(a) TG + DTG and (b) DTA thermogram of 3NiSN dried at 100°C.
Further evidence for the formation of NiO might be drawn from UV-Raman spectra exhibited in Figure
UV-Raman spectra of 3NiSN (calcined at 700°C for 4 h) and NiO (as a reference).
XRD measurements were carried out to understand the crystalline structure of 3NiSN catalysts, and the results were presented in Figure
XRD patterns of 3NiSN before and after reduction in H2 for 4 h.
Further insight on the aggregation of Ni particles over the 3NiSN could be obtained by TEM analysis. Figures
TEM images of (a) reduced 3NiSN and (b) deactivated 3NiSN, and histogram of the particle size distribution obtained from sampling of nanoparticles from TEM data (c) for reduced 3NiSN and (d) for deactivated 3NiSN.
TPR is an efficient method to characterize the reducibility of supported nickel-based catalysts. TPR profiles of 3NiSN catalysts were depicted in Figure
The reduction-oxidation cycle (redox) performance of 3NiSN catalysts with different reoxidization temperature (A: 300°C; B: 400°C; C: 500°C; D: 600°C; E: 700°C; F: fresh, just calcined).
It is generally accepted that the crystalline size of metallic nickel plays an important role in the catalytic performance for nickel-catalyzed reactions: smaller metallic Ni size helps to provide more active sites to reach the much better catalytic activity. Our previous works had also demonstrated this view [
The effect of reaction time on the XRD patterns of 3NiSN.
Crystalline size of nickel as a function of reaction time.
By comprehensively analyzing the characterization results, important information could be concluded. On one hand, graphic carbon was not detected in the spent 3NiSN catalyst by XRD and TEM, suggesting that no carbon deposition was formed during the reaction. On the other hand, except for the characteristic XRD peak of metallic nickel, no other nickel species (such as NiO) was detected, indicating that the transformation of active metallic Ni was not the reason for deactivation of 3NiSN. Importantly, the weak interaction between Ni and support disappeared as the reaction proceeding, resulting in sintering of active nickel particles. This was the reason that 3NiSN catalyst showed a rapid deactivation in the CRPOM reaction.
In this work, Ni/SiO2 catalysts were prepared with nickel nitrate precursor by IWI method and characterized by FT-IR, TG-DTA, UV-Raman, XRD, TEM, and H2-TPR. By being calcined around 380°C, water and nitrate were volatilized absolutely to form NiO, which could be reduced into metallic Ni after being treated with H2 at 700°C. The active nickel particles (around 37.5 nm) of 3NiSN catalyst were dispersed highly but weakly interacted with SiO2 support. However, this weak interaction disappeared after repeating oxidation-reduction-oxidation in the fluidized bed reactor at 700°C. Therefore, 3NiSN catalyst suffered from obvious sintering of the active nickel particle. In light of these, a rapid deactivation of 3NiSN was shown in the process of combination of CO2 reforming and partial oxidation of methane (CRPOM) to produce syngas.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank the financial supports of National Natural Foundation of China (nos. 21003066, 21367015, and 51068010) and Zhejiang Province Key Science and Technology Innovation Team (2012R10014-03).