Abstract
The effect of La content and its incorporation route on physicochemical properties of ZnO/Zn(Al,La)2O4 or La2O3–ZnO/ZnAl2O4 mixed oxides with a spinel structure obtained from ZnAlLa Layered double hydroxides (LDHs) or ex-ZnAl LDH materials was investigated. The heterostructural nanocomposites with the similar Zn/Al molar ratio and varied La content were prepared by two techniques: via co-precipitation and thermal treatment of ZnAlLa LDHs at 500 °C or via incipient wetness impregnation of ex-ZnAl LDHs with aqueous solutions of lanthanum nitrate and subsequent thermal treatment. The obtained series of materials were characterized by the following techniques: X-ray fluorescence (XRF), N2 adsorption (BET), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis with evolved gas analysis (TG/DTG/EGA), scanning transmission electron microscopy (STEM) energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM) and Fourier-transform infrared spectroscopy (FFT). The evaluation of activity toward the high-temperature water gas shift (HT-WGS) within the temperature range of 350–420 °C was carried out on the basis of rate constant measurements in the kinetic mode using a differential reactor. The co-precipitation technique allowed for a better distribution of La in bulk and on the spinel surface than in case of lanthanum incorporation via impregnation. ZnO/Zn(Al,La)2O4 or La2O3–ZnO/ZnAl2O4 mixed oxides were characterized by moderate activity in the HT-WGS reaction. The results reveal that introduction of lanthanum oxide over 2.4–2.8 wt% induces the phase separation of the ZnAl2O4 spinel, forming ZnO on the ZnAl2O4 spinel surface.
Highlights
Layered double hydroxides (LDHs) are commonly applied as catalyst supports, catalyst precursors and nanocatalysts [1,2,3]
Series I was prepared via co-precipitation and thermal treatment of ZnAlLa LDHs at 500 ◦ C
Two major stages of decomposition processes may be distinguished from DTG/EGA curves regardless of La concentration: low- (100–200 ◦ C) and medium-temperature processes (210–410 ◦ C)
Summary
Layered double hydroxides (LDHs) are commonly applied as catalyst supports, catalyst precursors and nanocatalysts [1,2,3]. An additional advantage of LDHs is the ability to incorporate a wide range of various cations with a similar ionic radius, e.g., Mg2+ , Cu2+ , Ni2+ , into the brucite-like layers [4]. During thermal treatment, they decompose forming nano-sized simple and mixed oxides with unique properties. Mixed Zn–Al oxides obtained from LDHs are used in a number of applications as sensors, high temperature ceramic material, photocatalysts [6], supports and catalysts for various chemical reactions [2]. Numerous synthesis methods are applied, i.e., coprecipitation [7,8], hydrothermal [9], microemulsion techniques [10], allowing to obtain the specific defective structures with enhanced catalytic properties [11]
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