Investigation of photocatalytic and photothermocatalytic ammonia production from molecular nitrogen and water under elevated temperature and pressure conditions

氨是最重要的化工产品之一。然而，它的生产也与全球二氧化碳排放量的2%有关。分子氮的光催化还原和同时水氧化成分子氧是实现可持续的NH3生产的有趣方法。然而，目前对各种反应参数对反应机理和反应速率的影响的基础知识还非常缺乏--本提案旨在填补这一空白。作为第一步，我们将研究温度如何影响反应。当然，光催化反应所需的能量是由高能光子提供的，我们最近看到，升高的反应温度对光催化反应的动力学有积极的影响，并有助于显着提高转化效率。在这方面，我们自己的初步研究已经暗示了将N2还原为NH3的巨大潜力。从那里开始，我们将进一步系统地研究这种温度效应。下一个未回答的问题是催化剂处氮的可用性是否是对NH3的选择性和总体观察到的反应速率的限制因素。为了研究这一点，我们将研究直接增加溶解在含水系统中的N2的量的所采用的氮分压的影响。特别是在这些升高的温度和压力的条件下，观察到的氨生成速率可能不完全来自N2的直接光催化还原，而是可能被光催化和/或热“Haber-Bosch”反应叠加，即，由元素形成氨，由水还原产生的副产物H2作为燃料。因此，我们将深入研究这些反应发生的条件以及它们对观察到的总反应速率的贡献。这些知识在知识驱动的催化剂设计中至关重要，因为它决定了（光）催化剂是否需要抑制H2的释放。作为光催化剂材料，我们将系统地探索卤氧化铋，因为它们是目前最有前途的材料类别。然而，目前还没有关于这类材料的合成路线、材料和性能特性的系统研究。通过改变材料中卤化物的组成以及颗粒的形态和尺寸，我们将首先通过初步筛选找到最适合作为光催化剂的候选物。我们还将研究这些材料在本文计划的整个强化反应条件范围内是否足够稳定。然后，通过分析催化剂对不同实验条件的反应速率响应，我们将能够基于详细的动力学模型计算基本的光电和催化性能。

Ammonia is one of the most important chemical commodities. However, its production is also associated with up to two percent of the worldwide CO2 emissions. The photocatalytic reduction of molecular nitrogen and simultaneous oxidation of water to dioxygen is an interesting approach to achieve a sustainable NH3 production. However, there is currently a profound lack of fundamental knowledge of the influence of the various reaction parameters on both the reaction mechanism and rate - a gap the present proposal aims to fill.As a first step we will study how temperature influences the reaction. While certainly, the energy required for the photocatalytic reaction is supplied by the high-energy photons, we have recently seen that elevated reaction temperatures positively influence the kinetics of photocatalytic reactions and can help to significantly increase the efficiency of the conversion. In this regard, our own preliminary study already hints at tremendous potential also for the reduction of N2 to NH3. Starting from there, we will further systematically investigate this temperature effect. The next unanswered question is whether the availability of nitrogen at the catalyst is a limiting factor for both the selectivity towards NH3 and the overall observed reaction rate. To study this, we will investigate the effect of the employed nitrogen partial pressure which directly increases the amount of N2 dissolved in the aqueous system.Particularly under these conditions of elevated temperature and pressure, the observed ammonia generation rate may not be exclusively from direct photocatalytic reduction of N2 but may be superimposed by photocatalytic and/or thermal “Haber-Bosch” reactions, i.e., the formation of ammonia from the elements, fueled by H2 generated as a byproduct from water reduction. We will therefore study in depth under which conditions these reactions occur and what contributions they have to the overall observed reaction rate. This knowledge will be critical in a knowledge-driven catalyst design as it determines for example if the (photo)catalysts need to suppress H2 evolution or not.As photocatalyst materials, we will systematically explore bismuth oxyhalides as they are currently the most promising material class for this reaction. Nonetheless, there is no systematic study on this material class as of yet regarding synthesis routes, materials and performance properties. By varying the compositions of the halide in the material as well as the particle morphology and size we will first find the most suitable candidate as photocatalyst through an initial screening. We will also study if these materials are sufficiently stable within the whole range of intensified reaction conditions planned herein. Then, by analyzing the reaction rate response of the catalysts towards varying experimental conditions, we will be able to calculate fundamental optoelectronic and catalytic properties based on a detailed kinetic model.