Theoretical study of nickel phosphide ensembles for selective hydrogenation

Resumen

Scientific research is the main pillar of our modern society. Numerous scientific achievements contributed to better comprehension of nature and their exploitation brought us technological progress boosting economic development of our society. The beginning of the modern era of our society is considered to be the Industrial revolution which started in United Kingdom and soon after had been expanded on the ground of the United States of America and other countries which today are the most developed ones (Germany, France...), indicating to the importance of rapid application of new technologies for accelerated economic development. The precise start of the Industrial revolution is still debatable among historians, but as we know that those processes were relied on the steam power, using fossil fuels whose combustion produces carbon dioxide, we can relate the emission of CO2 per capita with the industrial development of a country. Unfortunately, alongside with the economic progress, industrialisation dragged its side effects of which we used to be unaware of for a long time. But now, thanks to the recent growth of environmental movements, even persons which are not even remotely related with science or technology by their call, are aware of the burning issues we are facing in the present-day as global warming and pollution, but on the other hand usually being unaware of their complexity. Here I have insinuated that the emission of carbon-dioxide as a possible historical tracker of economic development, and carbon dioxide is the main pollutant responsible for the global warming. Beside carbon dioxide, our environment has been jeopardized by many other organic and inorganic waste chemicals which have to be reduced. Those chemicals are categorized by their unfriendliness quotient and yearly production, as they have different unfriendliness impacts for our environment and they are produced in different quantities. The answer being imposed, on the first glance seems to be obvious, replacement of fossil fuels and procedures which generate waste to the clean energy sources and more environment friendly procedures which generate less amount of waste and do not use hazardous compounds (lead, cadmium, many organic compounds, asbestos etc.). Significant quantity of waste has been generating in production of fine chemicals and pharmaceuticals, where environmental factor is between 5 and 100, meaning that on one kilogram of product, it is generated between 5 and 100 kilograms of waste. Main reason for this is widespread use of stoichiometric reagents. The solution for this waste problem is replacement of these classical stoichiometric technologies with cleaner catalytic alternatives. Many factors as social, economical and scientific play their role here. As pollution is a global issue it means that is a concern of all people in our planet, but as inequities in our society are higher than ever and keep growing, it means that the same problem has been seen differently from different spots of the same iceberg. Developed countries built up their economies by exploitation of these jeopardizing procedures and energy sources and now they are in a position to make a radical change for greener future, while economies of developing countries are still highly dependent on these old-fashioned technologies. To make this transition feasible for all, it is of highest importance the usage of cheap and abundant commodities while being in line with green chemistry principles. In following, I will expose basic principles of catalysis, and how it can contribute to this cause. Catalysis is one of the nature’s basic phenomena which is essential for the existence of life. Catalysts designed by nature are called enzymes, without them life on Earth would not be possible. Conceptually, it was first time brought to light in 1794 by Elizabeth Fulhame, a Scottish chemist. Although this phenomenon was in a certain way previously observed and even systematically studied by others, she was the first one making unambiguous observation in a way which corresponds to the modern-view of a catalytic process, saying that for the oxidation of carbon monoxide is required a small quantity of water which remains unaffected in the chemical reaction. Similar observation, and the first one in organic chemistry, was made by Gottlieb Kirchhoff in 1812. He used dilute acids for hydrolyzing starch to glucose, noting that acids were not affected by chemical reaction. In following period, works by Humphrey Davy, Louis-Jacques Thénard, Edmund Davy, Wolfgang Döbereiner and Eilhard Mitscherlich were reviewed by Jöns Jacob Berzelius in a report he published in 1836. The term “catalysis” appears for the first time in this report in the way how we understand it today. It should be mentioned an attempt of Ambrogio Fusinieri from 1924 to rationalize the observations of Döbereiner and Davy through concept of “concrete laminae” as his thoughts inspired Michael Faraday to make the first step in heterogeneous catalysis. At the same time, parallel but more or less separate evolution of organic chemistry from catalysis has started. From this epoch, it has passed around two centuries. Now more than 80% of world production, from fuels to food, is depended on catalytic processes. And yet, processes depending on non-catalytic synthesis are in minority need to be replaced as they generate waste on much higher kg of waste per kg of product rate. Paradoxically, while catalysis as natural phenomenon is responsible for existing of life, now catalysis as scientific discipline have a similar role, the one which should provide the survival of its own creation. As early being observed, catalyzed reactions may occur in homogenous or heterogeneous environment, which bring us to the categorisation of catalysis. Where a catalyst is in the same phase as the reactants is known as homogenous catalysis, and where a catalyst is in different phase from the reactants is known as heterogeneous catalysis. Research in this thesis is under the frame of heterogeneous catalysis. The contemporary definition of a heterogeneous catalyst relates to the active sites on the surface of a solid substance, facilitating the kinetics of the reaction which takes place in a gas or liquid phase, but do not affect its overall thermodynamics, which is in accordance with Oswald’s definition; the catalyst is not consumed, and it is regenerated at the end of the catalytic circle. On those active sites the reactants are adsorbed forming chemical bonds in a monolayer on the surface, in other words we have chemisorbed species in a monolayer, while in a multilayer it is also possible physisorption. The key for understanding heterogeneous catalysis is the nature of this adsorption. In order to facilitate a reaction and to be even possible, adsorption of all reactants has to be strong enough, and at the same time, not to be too different among all the reactants. In a similar way, products cannot be strongly adsorbed on the surface in order to prevent poisoning of the catalyst which would prevent further catalytic cycles from taking place. The Sabatier principle expresses this more concretely by proposing that an unstable intermediate between the reactant and catalyst surface has to exist, but stable enough to be generated in high quantities on the catalyst surface, while also sufficiently unstable to easily decompose and form the products. Interpreting the Sabatier principle, it can be said that the optimal stability of the intermediate leads to the maximization of the catalytic activity. Stability of the intermediate species can be studied through thermodynamic quantities as for instance is the adsorption heat of a reactant or through other descriptors. In academic research there were developed different types of descriptors for different types of reactions and different types of substrates. Those descriptors usually imply different thermodynamic quantities like heat of adsorption or Gibbs free energy of different atomic, molecular or radical species. Developing of a valid descriptor which is going to correspond the robustness quantifier of a catalyst for a certain reaction it is necessary go through a large number of different samples and carefully analyze obtained data. Process of hydrogenation is among the most important and applicable procedures for reducing different functionalities of a chemical compound. It is widely used in many areas of chemical industry, such as drug synthesis, oil refining, and biomass conversion. Hydrogenation used to be conducted using either stoichiometric reagents or reducible substances, which release hydrogen in alkaline or acidic media. Nowadays, commercially this process is usually catalyzed over heterogeneous catalyst based on transition metals as platinum, palladium or nickel most commonly, weather supported or unsupported, depending on the hydrogenising compound. Because of scarcity and high cost of precious metals as palladium and platinum, these catalysts are usually loaded on different carriers in form of nanoparticles. Depending on the nature of reactants, their concentration and required purity of products, a catalyst based on transition metal might be poisoned with other chemical elements. Special case of During decades, in academic research there were reported many promising attempts for substitution of this archetypal catalyst, reporting high activity, selectivity and stability, that in principle can be applied for semi-hydrogenation of alkynes, and yet none of them found commercial application. Generally, in order to render a catalytic process technically viable and to make application of a heterogeneous catalyst commercially possible for any kind of fine chemical there are some critical points. Some of these factors are: deficient catalytic performance of a catalyst developed in academic research (stability, activity, productivity, selectivity); lack of resources (time, money or required commodity) for making a catalyst commercially suitable; complex synthesis of the substrate for highly selective catalyst leading to lack of predictability for getting one; high demands on purity of starting materials and control of reaction conditions. A transition metal is by current IUPAC’s definition “An element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell.” Already in definition of transition metals itself, we can get a feeling that they might be good catalysts, knowing that they have an incomplete d-shell where can fit up to ten electrons which opens space for electronic recombination and bonds formation on their surface. Catalytic properties of transition metals are known almost from the inception of catalysis, and they are one of the first materials investigated in the field. In present-day, they find many applications in catalysis. The main reason that makes them good catalysts is mobility of the electrons in their valence shell, in terms of landing or withdrawing them from the reagent, so many of them have already found their applications in catalytically driven processes. A common case is that we usually face with some of the issues, either they are related to their cost and scarcity (e.g. Ir, Pt, Pd, Au) making them inadequate from economical aspect, or their hazardous nature (Cd, Cr, Hg) is environmentally unsatisfactory, or most commonly their catalytic properties simply fit poorly for desired reaction in which case we implement different strategies to modify their properties. Their thermo-stability does not put a lot of additional limitations for research in the field of heterogeneous catalysis, which is just one more reason making them attractive substrates. Designing a catalyst can be like a chase, consuming a lot of resources like energy, human power, time and money. Moreover, many different approaches and strategies have been tried out in catalytic design which steer into different directions. Since density functional theory with combination of kinetic models stepped in, theoretical methods have been introduced enabling rational understanding of activity volcano plots and selectivity profiles. In following, I am going to give a brief review of the most important contributions closely related to the research conducted in this thesis. In 2008, in the report from Nørskov and collaborators, they introduce a new descriptor for the identification of selective hydrogenation catalysts. They had scanned more than 70 metallic systems and alloys, having alloys of palladium and silver as the referent point, which are most commonly used in industrial purification process of ethylene from acetylene. The concept had been developed under the frame of Sabatier principle, calculating the adsorption energies of acetylene and ethylene, which are in this case reactant and product and the methyl group. They plotted the adsorption heat of the reactants against the adsorption heat of methyl group, the slopes of scaling relationships between the adsorption heat of reactants and methyl group were linear and corresponded to the ratio of number of bonds the adsorbed molecule form with the substrate’s surface which was in case of acetylene 4 and in case of ethylene 2. In this way methyl heat of adsorption can serve as quantifier Sabatier, not-too-weak, not-too-strong, principle. In this report they also suggested an intermetallic compound NiZn as a promising candidate for selective hydrogenation of ethylene. In study of Albani et. al. Form 2018, they showed that strategy of controlled ensemble design can be successfully implemented for selective hydrogenation. They treated palladium nanoparticles supported on graphitic carbon nitride with aqueous sodium sulfide which directed to the formation of nanoparticles of Pd3S that with controlled crystallographic orientation, (202) in particular, promoted a robust catalyst for selective hydrogenation. This study gives us an incentive to push further and explore the limits of the controlled ensemble design strategy with p-block elements. It should also mention that beside metals, alloys and metals modified with p-block elements, there are also other families of hydrogenation catalyst based on ligand modified nanoparticles and supported single atoms. Majority of reports claiming successfulness of their catalysts in process of selective hydrogenation involve the usage of palladium. Avoiding the usage of this precious metal is one of the main objectives in this thesis while not stepping back from principles of green chemistry. Therefore, beside the fundamental aspect there is also a featuring practical aspect of this research in the form of searching a more affordable catalyst and in that way making it available for widespread usage. Here I am going present the ensemble design on nickel phosphides with an assumption that is possible to obtain desired catalytic properties for selective hydrogenation which might match or even surpass the efficiency of palladium based catalysts. This assumption is based, as they are neighboring elements in the periodic table, on similarity in electronic configuration of nickel and palladium as well as sulfur with phosphorous. Since nickel phosphides are reported occurring in various ratios including different space groups, either they could be found in nature or they were synthesized in laboratory, they give us wide pool of possibilities for controlled ensemble design. The other concept I am going to rely on is the descriptor related to the robustness of catalyst for selective hydrogenation, methyl group heat adsorption, which I have previously described. Here I also want to bring these two individually developed concepts together and to inspect their compatibility, in order to reveal some critical point if this descriptor can be successfully applied on other systems beside metals and alloys and be reliably used in future research of semi-hydrogenation reaction. Controlled ensemble design on surfaces of transition metals by p-block elements in known to be effective in modulating catalytic properties for alkyne semi-hydrogenation. This approach is applied here on nickel and phosphorous as its modifier from p-block. Here the limitations of this strategy are tested on nickel and phosphorous as its modifier from p-block. Detailed investigation was conducted on two nickel phosphides (Ni2P and Ni5P4) alongside with unmodified nickel. There will be shown that phosphorous is an effective geometrical modifier since it forms spatially-isolated nickel trimers. And more importantly there will be revealed its limitations in tuning the electronic properties related to binding energies of participating organic species in alkyne semi-hydrogenation reaction. A possibility to substitute the archetypical catalyst for selective hydrogenation with more affordable materials was investigated through theoretical study using Density Functional Theory approach supported with experimental evidence. The family of materials being evaluated were nickel phosphides and strategy used for tuning their catalytic properties was controlling ensemble design. It is shown that bulk structures of nickel phosphides are stable enough to be appropriate for usage in selective hydrogenation reaction, but that that varying Ni/P ratio does not enhance dramatically their performance. The universality of methyl group adsorption heat as a descriptor for finding a successful catalyst for selective hydrogenation was also under the test. It was shown that this descriptor has its limitations when being used for materials which are not transition metals or their alloys. It is possible to use it as a qualitative descriptor as the scaling relationships are partially preserved. This means that they remain linear even when different active sites on the same catalytic surface are compared but the ratio between the slopes does not coincide with the one obtained on the transition metals and their alloys. In that case the slope ratio corresponds to the ratio of bond being formed between the surface and adsorbed species, but in our case the situation is more complex. By applying controlled ensemble design strategy we increased the distance between the same type of adsorption sites and in order to prevent the C–C coupling pathways but in that way the two C atoms from adsorbed species change their confirmation forming asymmetric intermediates on the surface. This is the reason why the scope ratio in our case does not correspond to the expected bond ratio as different kinds of bonds on opposite sides of a molecule are formed. It is shown that this descriptor is not suitable to use for quantifying the activity for selective hydrogenation when dealing with complex materials which involve two sites of different kind to adsorb a molecule and yet it can be used for rough estimation of their catalytic properties for the