Abstract
The synthesis of the copper-poor and aluminum-rich layered double hydroxides (LDHs) of the CuAl4 type was optimized in detail in this work, by applying an intense mechanochemical treatment to activate the gibbsite starting reagent. The phase-pure forms of these LDHs were prepared for the first time; using copper nitrate and perchlorate salts during the syntheses turned out to be the key to avoiding the formation of copper hydroxide sideproducts. Based on the use of the optimized syntheses parameters, the preparation of layered triple and multiple hydroxides was also attempted using Ni(II), Co(II), Zn(II) and even Mg(II) ions. These studies let us identify the relative positions of the incorporating cations in the well-known selectivity series as Ni2+ >> Cu2+ >> Zn2+ > Co2+ >> Mg2+. The solids formed were characterized by using powder X-ray diffractometry, UV–Vis diffuse reflectance spectroscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy. The catalytic potential of the samples was investigated in carbon monoxide oxidation reactions at atmospheric pressure, supported by an in situ diffuse reflectance infrared spectroscopy probe. All solids proved to be active and the combination of the nickel and cobalt incorporation (which resulted in a NiCoAl8 layered triple hydroxide) brought outstanding benefits regarding low-temperature oxidation and increased carbon monoxide conversion values.
Highlights
Due to the chemical diversity offered by variability in the composition of the interlamellar galleries and layers, they can be finely tuned to numerous applications, from healthcare [1] to agriculture [2] and the polymer industry [3], just to mention a few
The dry-milling of the Al(OH)3 was executed at 12 Hz grinding frequency and 100 ball/sample mass ratio. (We would like to highlight here that the specific structural changes caused by a vibration/mixer mill are in part different from those that evolve in a rolling or planetary ball mill during its circular motion [25]; here, the jars pass in radial oscillations along the horizontal axis and, inside, the balls collide with the rounded ends of the jars, resulting in intense collisions of short duration.)
To investigate the structural properties of the layered double hydroxides (LDHs), Fourier-transform infrared (FTIR) spectra were registered on a JASCO FT/IR-4700 spectrophotometer (Kyoto, Japan), accumulating 256 scans at 4 cm−1 resolution with a ZnSe ATR attachment and DTGS
Summary
Layered double hydroxides (LDH) belong to a group of anionic clay-type materials possessing a CdI2 -type structure. Due to the chemical diversity offered by variability in the composition of the interlamellar galleries and layers, they can be finely tuned to numerous applications, from healthcare [1] to agriculture [2] and the polymer industry [3], just to mention a few. Perhaps the most promising research field of the pristine and heat-treated derivatives of LDHs is their application as catalysts. The majority of LDHs have an intrinsically basic character and the cationic components, with alternating oxidation states, serve as redox centers. These compounds are commonly employed in transesterification [4], Michael addition [5], or even in photochemical transformations [6]. The following formula is used for the description of the LDHs: [M2+ 1−x M3+ x (OH)2 ]x+ [An− x/n ·mH2 O)]x− , where
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