Nanometer Catalytic Materials As Catalysts For Hydrogen Production Via Decomposition Of Ammonia

Hydrogen as a clean and sustainable energy source has attracted widely attention.However,the safe storage and transportation of hydrogen are still difficult,limiting its applications.Among the intensively studied attractive storage media,ammonia has received extraordinary interest due to its relatively high H2 storage capacity(17.7 wt.%),easy liquidation under considerably moderate conditions and transportation and industrial production by the Haber-Bosch process.In addition,catalytic decomposition of ammonia can produce high purity hydrogen,which showing super advantage of avoiding the Pt poisoning in proton exchange membrane fuel cells(PEMFC).Meanwhile,the unconverted NH3 can be reduced to less than 200 ppb by the suitable adsorbents.Therefore,catalytic decomposition of ammonia to high purity hydrogen is a promising method for hydrogen production.However,till now,it is still challenging to achieve a high yield of hydrogen at relatively low temperature via catalytic decomposition of ammonia.In this dissertation,many attempts including optimizing catalyst preparations,using various promoters,choosing proper supports were used to prepare the highly efficient catalysts.Various characterization methods including the aberration-corrected high-angle annular dark-field scanning transmission electron micrograph(HAADF-STEM),X-ray absorption fine structure(XAFS)spectrum,X-ray diffraction(XRD),N2 adsorption-desorption,transmission electron microscopy(TEM),X-ray photoelectron spectroscopy(XPS),temperature-programmed reduction and temperature-programmed desorption(TPR/TPD),ex-situ/in-situ Raman spectrum and in-situ diffuse reflectance infrared Fourier transform spectroscopy(in-situ DRIFTS)were used to investigate the structure and the structure-activity relation of catalysts in NH3 decomposition.This provides a deep understanding of the nature of catalytic behavior,also offers guidance to the design of highly efficient catalysts.There are four main experimental studies in this thesis as follows:1.The high surface area Co-SiO2 nanocomposite catalysts have been prepared by a simple two-step procedure with activated carbon as template for ammonia decomposition to produce CO,free hydrogen.The texture of the catalysts was characterized via various techniques containing XRD,N2 adsorption-desorption and TEM.The Co nanoparticles are uniform distribution in the SiO2 matrix and no distinct Co-rich or Si-rich area was noticed in the resulting catalysts.Furthermore,the temperature-programmed reduction by hydrogen(H2-TPR)combining the corresponding in-situ XRD under the H2-TPR conditions was performed to investigate reducing capacity of the as-prepared catalysts and the phase change during the reduction,respectively.The strong interaction between cobalt species and silica can effectively prevent the active cobalt species from agglomerating during calcination and ammonia decomposition reaction.In addition,the active species in the Co-SiO2 catalysts for ammonia decomposition are CoO and the metallic Co phase.The prepared Co-SiO2 catalysts display high catalytic performance and excellent long-duration stability for ammonia decomposition,much higher than other transition metal catalysts reported in the literature.The ammonia conversion reached about 80%with a very high GHSV of 124,000 cm3 gnat-1 h-1 at 600 ? and no any observable deactivation during a 48-h long-term stability test.2.The uniformly dispersed transition metal(Co,Ni and Fe)nanoparticles supported on the surface of La-promoted MgO were synthesized via deposition-precipitation method for hydrogen production from decomposition of ammonia.Various characterization methods including TEM,ex-situ/in-situ XRD,N2 adsorption-desorption,H2-TPR,NH3-TPD were used to investigate the structure-activity relation of catalysts in NH3 decomposition.The results show that the strong interaction between active species and MgO support can stabilize the structure of the catalysts and effectively prevent the active species from agglomerating during ammonia decomposition reaction.In addition,the La species work as a structural promoter and the introduction of the La species leads to the enhanced catalytic activity,which can be attributed to the increasing the number of active sites,facilitating the adsorption and decomposition of NH3.The prepared catalysts showed very high catalytic activity for ammonia decomposition compared with the same active composition samples that reported previously.Meanwhile,the catalysts showed excellent high-temperature stability and no any deactivation was observed,which are very promising candidates for the decomposition of ammonia to hydrogen.This method of adding promoter to improve the catalytic activity of catalysts provides a basis for the preparation of high efficient transition metal catalysts.3.The supported Ru single atoms on the surface of CeO2 have been synthesized by a modified colloidal deposition method.Then,structure uniform and thermally stable Ru cluster catalysts have been successfully prepared by reducing atomically dispersed Ru species on CeO2 under ammonia atmosphere at 550 ?.The supported Ru cluster catalysts show outstanding activity for decomposition of ammonia with an extremely high hydrogen yield of 9,924 mmolH2 gRu-1 min-1 at 450?.Compared to the previously reported results,such a value shows at least one-order enhancement on the hydrogen formation yield.Various characterization methods including the aberration-corrected HAADF-STEM,XAFS,XRD,N2 adsorption-desorption,XPS,H2-TPR,NH3-TPD,ex-situ/in-situ Raman spectrum and in-situ DRIFTS were used to investigate the structure and the structure-activity relation of catalysts during NH3 decomposition.We revealed that the very good NH3 adsorption ability of CeO2 together with excellent stability of Ru clusters were in synergy to promote the catalytic performance.The ceria support itself has very good adsorption ability to ammonia;meanwhile the Ru clusters stabilized by ceria effectively decompose ammonia to produce hydrogen.Using isolated single atoms as precursors to prepare uniform clusters provides a promising strategy for construction of high quality nanocatalysts.4.The high surface area Ru/MgO catalysts have been prepared by a modified colloidal deposition method.The structure of the catalysts was characterized via various techniques including XRD,N2 adsorption-desorption,TEM and H2-TPR.The Ru species are very small and highly dispersed on the surface of MgO support.Moreover,the strong interaction between the Ru species and MgO support can effectively inhibit the sintering of Ru species and MgO support during the ammonia decomposition reaction,maintaining the structure stability of the catalyst.The Ru/MgO catalysts prepared by colloidal deposition method show the excellent catalytic activity and long-term stability compared with those of Ru/MgO catalysts prepared by other methods reported in the literature.This method of preparing Ru-based catalysts provides a promising strategy for the preparation of high efficient noble metal catalysts.