Catalytic mechanism, multifunctionality and structural design of iron-based metallic glasses

Alloys with a well-defined long-range ordered crystalline structure and glasses with a highly disordered amorphous structure as two uncorrelated categories of materials have a long history with a widespread use for different purposes. With the common sense of highly ordered structure in metals/alloys, it is until the 1960s that the first metallic glass (MG, also known as amorphous alloy) has been produced by fast quenching technique to “freeze” their metallic liquid at very high critical cooling rate, realizing glass-like (amorphous/disordered atomic) structure in alloys with extraordinary properties (extreme strength at low temperature, high flexibility at high temperature, etc.) as structural and functional materials. While part of current studies focus on their fundamental and important issues including glass nature and its supercooled liquid state, many application-oriented research endeavors have achieved great successes. Motivated by the pressure of global environmental issues and potential crisis level of fresh water, recently an increasing interest of MGs in attractive catalytic applications has suggested the superior performance of MGs than their crystalline counterparts. Although the application of MGs as catalysts was firstly attempted in almost 40 years ago, their potential value in environmental and energy science has not been recognized until in recent 10 years. As such, studies of catalytic properties of MGs are still very limited and the catalytic mechanism to understand their superior performance is far from easy to achieve. A great effort is still needed for the achievement of the practical catalytic industrialization using massive produced MGs. Chapter 1 introduces the current challenges of MGs in future development and practical applications, which present as the contradictions between massive production technique and narrow practical application range, between low development as structural materials and high promise as functional materials, and between high catalytic performance and low understanding of mechanism. Accordingly, the significance and innovation of using MGs as functional catalysts will be also described as following: 1) strategies for enhancing catalytic performance of Fe-based MGs without structural change for degradation of organic pollutants; 2) the catalytic application of Fe-based MGs for purifying diversified inorganic contaminants; 3) the potential optimized structure of Fe-based MGs for highly promoting catalytic performance. Chapter 2 overviews the MGs with structural origin (e.g. short-to-medium-range atomic arrangement with analysis and characterization techniques), glass-forming ability as one of important characteristics for material design, manufacturing methods (i.e. for ribbons, powder and bulk MGs), mechanical and chemical properties, current catalytic properties and applications of MGs (including wastewater treatment, water splitting and fuel cell). The structural heterogeneity in catalysis and strategies to engineer catalytic structure of MGs are also shown in order to develop their future catalytic applications. Chapter 3 shows the research methods according to different chapters, which contain materials and chemicals, characterization methods, catalytic analysis process, kinetic study methods and other measurements. Chapter 4 shows the strategies of enhanced catalytic performance of MGs. Fe73.5Si13.5B9Cu1Nb3 MG ribbons are employed for photo-enhanced activation of persulfate (PS), indicating that 100% color removal of malachite green dye can be achieved within 30 min under optimized parameters, and the inclusion of Nb in Fe73.5Si13.5B9Cu1Nb3 MG ribbons promotes enrichment of Si to further improve the surface stability. Yet, the catalytic mechanism of MG ribbons in advanced oxidation processes (AOPs) is not sufficiently understood. As such, Fe78Si9B13 MG ribbons have been applied for the activation of three peroxides: H2O2, PS and peroxymonosulfate (PMS) to investigate catalytic mechanism. The dominant reactive radicals (•OH and/or SO4•−) in AOPs are investigated by competition kinetics using probe reaction. The order of predominant radical generation rate by Fe78Si9B13 under UV-vis irradiation is PS\u3eH2O2\u3ePMS, all with a radical generation rate at least ~2 times higher than other iron-containing materials. The radical evolution mechanism for H2O2, PS and PMS activation has also been investigated. On the other hand, the role of surface to enhance catalytic performance of MGs is suggested. Fe50Ni30P13C7 MG ribbons are found to have the superior corrosion resistance and an effective elimination of surface layer by chemical dealloying can highly promote the catalytic degradation rate of brilliant black BN dye from 20 min to only 10 min, which is attributed to reactivation of surface by chemical dealloying without generating nano-porous structured surface. The reactivation of ribbon surface effectively optimizes active reaction sites and the re-exposure of Fe, Ni and P with zero-valent state forms galvanic cells by atomic clusters leading to the acceleration of catalysis. Chapter 5 indicates the novel catalytic application of MGs against diversified contaminants. As an advanced alternative of heterogeneous crystalline iron material, low-cost Fe78Si9B13 MG ribbons with mature production by melt spinning is employed in real industrial contaminated water to investigate effective separation of arsenic (As) and reduction of nitrate (NO3−). Fe-based MG ribbons demonstrate attractive high removal rate of As in 30 min with low soluble Fe (1.5 mg/L), which is ascribed to synergistic effect of reduction/adsorption by MG ribbons, precipitation of arsenic sulfide and adsorption of generated iron sulfide. On the other hand, a remarkable sustainability up to 20 reused times of Fe-based MG ribbons for NO3− reduction suggests a promising economic value of MG ribbons in industrial applications. Surface area normalized rate coefficient indicates the superior catalytic capacity of Fe-based MG ribbons compared with other iron materials. Chapter 6 presents the strategies to engineer catalytic structure correlated to MGs. It is reported that the excellent catalytic behavior in Fe-based MGs goes through a detrimental effect with the partial crystallization but receives a compelling rejuvenation in the full crystallization. Further investigation reveals that multiple crystalline phases with electric potential differences induced by high-temperature annealing facilitate the formation of galvanic cells. The extensively reduced grain boundaries due to grain growth greatly weaken electron trapping and promote inner electron transportation. The relatively homogenous grain-boundary corrosion in the multiphase contributes to well-separated phases after reactions, leading to refreshment of surface active sites, quickly activating H2O2 and rapidly degrading organic pollutants. On the other hand, 3D printing that revolutionizes the way of material manufacturing with functional applications is employed in the manufacturing of an Fe-based MG matrix composite with three-dimensional rhombic dodecahedron microstructure. The 3D-printed porous Fe-based MG matrix composite has been employed into catalytic activation in Fenton-like process and sulfate radical-based reaction. Results demonstrate that extremely high reusability (45 times) is achieved in sulfate radical-based reaction without any apparent efficiency decay. The remarkable catalytic reusability originates from extremely low surface decay. Structural analysis indicates the α-Fe nanocrystals serve as trigger of easy electron transfer but a large amount of α-Fe lead to an inhibitive effect in the MG matrix composite. The overall catalytic ability also demonstrates the excellent catalytic performance of SLM-produced porous Fe-based MG composite in the wastewater remediation. Chapter 7 concludes the present findings in this thesis and suggests the future challenges and development using MGs as catalysts. With the investigation of enhanced catalytic performance, catalytic degradation of diversified pollutants and optimization of catalytic structure of Fe-based MGs, this thesis aims to further understand the catalytic mechanism of Fe-based MGs at the atomic size in wastewater treatment, to assess applicability of MGs in practical applications, to provide a novel clue of extending their multifunctional catalytic properties and to suggest the new developing catalyst design with ordered and/or disordered atomic arrangement in the future development