Preparation Of Ammonia Decomposition Catalytic Materials Derived From Layered Double Hydroxides

Due to the global energy and environment crises caused by the extensive use of fossil fuel,clean and sustainable energy are constantly being explored.Hydrogen is considered as an ideal energy carrie.Technically,however,there are difficulties in storage and transportation of hydrogen due to its highly flammable,explosive,and low compressed characteristics,which limited its specific applications.Hydrogen energy must therefore be stored and transported,for example in the form of hydrogen compounds by so-called hydrogen carriers such as methanol,formic acid,and ammonia.Among these carriers,ammonia is a promising candidate because ammonia has many advantages such as high gravimetric（17.7 wt%H₂）and volumetric H₂ density,high energy density 3000 Wh/kg _NH3,liquid state under mild conditions（20°C,0.8 MPa）with low production cost.Furthermore,the ammonia decomposition process generates a solo by-product of N₂ which avoid the poisoning effect of cell electrodes by CO_x commonly existing in the hydrogen resources produced by using carbonaceous raw material like methanol and formic acidLayered double hydroxides（LDHs）,also known as hydrotalcite-like anionic clays.The general formula of LDHs is[M²⁺_1-xM³⁺x（OH）₂]^x+[A_x/n]^n-·m H₂O.It generally consists of replaceable divalent and trivalent metals in brucite-like hydroxide layers and negatively charged interlayer anions.The structural reconstruction of LDHs can ensure the immobilization and dispersion of different metal cations like Ni,Fe on hydroxide brucite-like layers via the dissolution-reprecipitation process in aqueous solution.Because of the 6-fold coordinated hydroxyl groups structure can anchor the metal cations and enhance its dispersion over the layers.When LDHs are calcined at high temperature under reducing atmosphere,the hydroxide brucite-like layers will undergo dehydroxylation,dehydration,or reduction processes,thereby transforming into metal supported layered double oxides（LDOs）which can significantly increase the dispersion and stability of the active component.These characteristics mentioned above make these materials rather suitable for a fine modulated heterogeneous precursors or catalysts.Here,a new way of constructing SMSI is reported,where the key is to employ the surface structural reconstruction of layered double hydroxides（LDHs）in the aqueous solution and the subsequent hydroxide-to-oxide support transformations.Owing to this,the Ru NPs were uniformly embedded with the LDOs,forming a ruthenium nanolayer that contained dense spherical or elliptical particles.The covering of Ru nanoparticles by layered double oxides（LDOs）,electronic interactions,and changes in CO adsorption behavior confirmed the presence of SMSI between Ru and LDOs.In all these efforts,The supported Ru catalysts with SMSI are outperforming in NH₃ decomposition,especially at relatively low temperature（300°C）.This study gives rise to new possibilities to synthesize relatively inert oxides supported metal catalysts with SMSI.Herein we report that the structure of hydrocalumite-layered double hydroxides（Ca Al-LDHs）can be tuned by gradually introducting Ni²⁺.It was revealed that the gradual intruduction of Ni²⁺in hydrocalumite led to a structure-transformation process,which introduced hydrombobomkulite and eventually evolved into takovite.Compared with the case without structure-transformation process,the structure-transformation catalysts showed excellent NH₃ decomposition activity after high-temperature reduction.Moreover,the obtained catalysts showed a relatively high conversion per Ni weight compared with other reported oxide-sopported Ni catalysts.Finally,we introduced different Fe active species into the LDHs by the endogenous construction method（hydrothermal method）.We introduced Fe active species into the layers and interlayers,respectively.The results of ammonia decomposition showed that catalysts had better catalytic activity when Fe was introduced into the layers.At the same time,there is no inhibition of activity at high Fe loading when Fe was introduced into the layers.The relationship between active species and structure-activity of Fe based catalysts in this experiment will be further discussed in the following characterization and experiments.