Study on the crystal plane effect of CuO/TiO2 catalysts in NH3-SCR reaction
Graphical abstract
Cu cations locations on (101) and (100) exposed facets of TiO2 supports are different, which resulted in the different NH3-SCR performances.
Introduction
Selective catalytic reduction (SCR) technology has been used worldwide for the control of NOx emissions in exhaust gases from stationary sources and automobiles. Among the reducing agents can be utilized in SCR, ammonia has been adopted in general, which is called NH3-SCR [[1], [2], [3], [4]]. Recently, numerous NH3-SCR catalysts such as Cu [[5], [6], [7], [8], [9]], Mn [2,[10], [11], [12], [13], [14]], V [[15], [16], [17]], Fe [[18], [19], [20]], Ce [2,11,21,22] based materials, has been reported to exhibit satisfactory catalytic activities. V2O5/TiO2 is the most commonly used catalyst because of its high NO conversion, high thermal stability, and excellent anti-SO2 poisoning ability [[15], [16], [17]]. Some of V based catalysts have been commercialized. Despite these achievements, the narrow operation temperature window, as well as the volatility and toxicity of VOx, inhibits its enormous application [[23], [24], [25]]. MnOx and other manganese-containing catalysts present promising NOx conversions in the range 100–200 °C. However, improvement N2 selectivity and poisoning resistance to H2O and sulfur oxides at low temperatures is important for low temperature application [[26], [27], [28]].
Alternatively, copper-containing catalysts, mostly in the form of Cu exchanged zeolites such as Cu/SAPO-34 [[29], [30], [31]], also show superior low temperature activity. Due to the high cost of copper exchanged zeolites, copper-containing mixed oxide catalysts have also been developed. According to Weng’s report [32], CuOx impregnated on the co-precipitated WO3-ZrO2 support shows high SCR activity in the range 200–320 °C. The acidity of the WO3-ZrO2 support allows to improve ammonia adsorption, essential for high SCR activity. Sullivan et al. [33,34] reported that copper oxide based catalysts prepared from two different precursors (Cu(NO3)2 and CuSO4) with various oxide supports (SiO2, TiO2 and Al2O3) are also active in the NH3-SCR reaction. Other copper based catalysts, such as CuMnAl [35] and CuNb [36], also show remarkable NH3-SCR activities between 200 and 300 °C.
In recent years, the facet-dependent catalysis [37,38] has exerted a tremendous scientific and technological fascination. Song [39] and co-workers studied the influence of exposed (001) facet of TiO2 support on NH3-SCR performances of V2O5/TiO2 catalysts. They compared two TiO2 supports including commercial P25 and TiO2 nano-sheets exposed (001) facet of TiO2. They found that V2O5/TiO2 nano-sheets had strong interaction with active V species, which resulted in enhanced redox ability, and thus, improved NH3-SCR activity of V2O5/TiO2 nano-sheets. Deng [40] and co-workers studied the NH3-SCR performances of Mn loaded on TiO2 exposed (001) and (101) facets. MnOx could be highly dispersed on (001) facet of TiO2, which brought more acid sites on catalyst surfaces, and thus, enhanced the NH3-SCR performances of catalysts. Shen et al. [41] found that V species on different exposed facets of TiO2 support had different coordination structure, which resulted that V2O5 loaded on (001) facet of TiO2 exhibited better NH3-SCR activity than V2O5 loaded on (101) facet of TiO2. Owing to the simple and clear structure of materials with dominant exposed facets, the study of facet-dependent catalysis would be helpful to understand the influences of loading metal oxides, supports, and interaction between them on NH3-SCR performance.
Our group has focused on the study of Cu dispersion on various supports for several years [[42], [43], [44], [45], [46]], such as Al2O3, TiO2, and CeO2. It was found that the catalytic activity of Cu based catalysts were highly depended on the dispersion state of Cu species. A method to estimate the loading metal dispersion capacity on surfaces of materials was provided [46,47]. In addition, we developed a simple method to synthesize TiO2 with certain morphology [48]. In the present work, we studied the Cu ion dispersion on (100) and (101) facets of TiO2 by loading copper oxides on TiO2 nano-octahedrons and nano-rods. In addition, their effects on NH3-SCR performances were also investigated.
Section snippets
Experimental details
TiO2 supports with certain morphologies were synthesized according to Liu’s method [48]. All the agents were purchased form Aladdin, and were used without further purifying.
Structure of TiO2 supports and CuO/TiO2 catalysts
TEM analysis was employed to investigate the morphology of TiO2-NOs and TiO2-NRs samples. As it was displayed in Figs. 1a and S1, TiO2-NOs exhibits octahedron shape with an average width of 22 nm and an average length of 30 nm. According to the close-up image of TiO2-NOs (Fig. 1c), TiO2 lattice fringe of d = 0.35 nm is observed, corresponding to the anatase TiO2 (101) crystal plane. This group of lattice fringe is parallel to the exposed facet of TiO2-NOs, which demonstrates that TiO2-NOs
Conclusions
In this work, CuO loading TiO2 nano-octahedron (06CuO/TiO2-NOs) and CuO loading TiO2 nano-rods (06CuO/TiO2-NRs) were synthesized. By modifying the morphology of TiO2 support, the effects of TiO2 exposed facet on NH3-SCR performances were investigated. In results, TiO2-NOs and TiO2-NRs expose (101) and (100) facets of TiO2, respectively. In case of these two TiO2 expose different facets, the location of Cu cations on their surfaces are different. Cu cations locate in tetrahedral vacant sites on
Acknowledgements
The financial supports of National Natural Science Foundation of China (No. 21573105), Natural Science Foundation of Jiangsu Province (BK20161392) are gratefully acknowledged.
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