The effects of various catalyst synthesis parameters on the photocatalytic degradation kinetics of aqueous methyl orange dye are presented. The four factors investigated were: i) InVO4 concentration, ii) nickel concentration, iii) InVO4 calcination temperature, and iv) ballmilling time. Three levels were used for each factor. Due to the large number of possible experiments in a full factorial experiment, an orthogonal-array experimental design was used. UV-vis spectrophotometry was used to measure the dye concentration. The results show that nickel concentration was a significant parameter, with 90% confidence. The relative ranking of importance of the parameters was nickel concentration > InVO4 concentration > InVO4 calcination temperature > milling time. The results of the orthogonal array testing were used to make samples of theoretically slowest and fastest catalysts. Curiously, the predicted-slowest catalyst was the fastest overall, though both samples were faster than the previous set. The only difference between the slowest and fastest catalysts was the milling time, with the longer-milled catalyst being more reactive. From this result, we hypothesize that there is an interaction effect between nickel concentration and milling time. The slowest and fastest catalysts were characterized using energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM), x-ray powder diffractometry (XRD), BET surface area analysis, and diffuse-reflectance spectroscopy (DRS). The characterization results show that the fastest catalyst had a lower band gap than the slowest one, as well as a slightly greater pore volume and average pore diameter. The results indicate that fast kinetics are achieved with low amounts of nickel and a long ball milling time. Under the levels tested, InVO4 concentration and the calcination temperature of the InVO4 precursor were not significant.

In den letzten Jahrzehnten ist die Rückgewinnung wichtiger Elemente aus Abfallströmen wie Abwässern, Schlämmen und Abraum. Übermäßiger Bergbau, industrielle Prozesse und Überdüngung in der Landwirtschaft setzen Schadstoffe wie Phosphat, Ammonium und Übergangsmetalle in die Umwelt frei und bringen Ökosysteme aus dem Gleichgewicht. In dieser Dissertation wird die Kristallisation von Übergangsmetallphosphatverbindungen (TMPs) aus wässrigen Lösungen untersucht, darunter M-Struvit, M-Dittmarit und M-Phosphat-Octahydrat (NH4MPO4∙6H2O, NH4MPO4∙H2O, M3(PO4)2∙8H2O mit M = Ni, Co, NixCo1-x). Diese kristallinen Phasen ermöglichen die gemeinsame Ausfällung von PO43-, NH4+ und Übergangsmetallen und bieten einen vielversprechenden Weg zur Rückgewinnung von Phosphat und Übergangsmetallen aus industriellen und landwirtschaftlichen Abwässern. TMPs besitzen vielseitige Eigenschaften wie thermische und mechanische Stabilität, einfache Veränderlichkeit und Multifunktionalität, wodurch sie sich für fortschrittliche Energieumwandlungs- und -speicheranwendungen eignen. Deshalb stellt die Synthese von TMPs eine kombinierte Rückgewinnungs- und Upcycling-Methode für fortschrittliche Funktionsmaterialien dar. Detaillierte Untersuchungen des Bildungsprozesses aus wässriger Lösung wurden mit zeitaufgelösten ex- und in-situ-Elektronenbildern, spektroskopischen, spektrometrischen und beugungsbasierten Methoden durchgeführt. Die in dieser Dissertation enthaltenen Ergebnisse geben neue Einblicke in den nicht klassischen Kristallisationsmechanismus von TMPs, der eine kontrollierte Einstellung der Kristallitgröße und -morphologie ermöglicht. Darüber hinaus führt die thermische Behandlung von TMPs zu thermisch stabilen, mesoporösen und/oder protonenleitenden Materialien für elektrochemische Anwendungen. Die Ergebnisse tragen zum grundlegenden Verständnis von Keimbildung und Kristallisationsphänomenen bei und helfen bei der Entwicklung moderner Funktionsmaterialien für elektrochemische Anwendungen. / A critical issue in the 21st century is the recovery of essential elements from waste streams like wastewaters, sludges, and tailings. Excessive mining, industrial processes, and overfertilization in agriculture release pollutants such as phosphate, ammonium, and transition metals into the environment, unbalancing ecosystems. This dissertation investigates the crystallization of transition metal phosphate (TMPs) compounds from aqueous solutions, including M-struvite, M-dittmarite, and M-phosphate octahydrate (NH4MPO4∙6H2O, NH4MPO4∙H2O, M3(PO4)2∙8H2O with M = Ni, Co, NixCo1-x). These crystalline phases allow for the co-precipitation of PO43-, NH4+, and transition metals, providing a promising route for phosphate and transition metal recovery from industrial and agricultural wastewaters. TMPs possess favorable properties like thermal and mechanical stability, tunability, and multifunctionality, making them suitable for advanced energy conversion and storage applications. Accordingly, the synthesis of TMPs represents a combined recovery and upcycling method towards advanced functional materials. Detailed investigations of the formation process from aqueous solution were carried out using time-resolved ex- and in-situ, electron imaging, spectroscopic, spectrometric, and diffraction-based techniques. The results contained in this dissertation reveal new insights into the non-classical crystallization mechanism of TMPs, allowing for controlled adjustment of crystallite size and morphology. Moreover, thermal treatment of TMPs compounds yields thermally stable, mesoporous, and/or proton-conductive materials for electrochemical applications. The findings, on the one hand, can contribute to the fundamental understanding of nucleation and crystallization phenomena in aqueous solutions in general and specifically for metal phosphates. On the other hand, my findings aid applied materials chemistry in the development of advanced functional materials for electrochemical uses.