In this first issue of 2025, we would like to wish all our members a happy, healthy, and scientifically prosperous year. Starting with this issue, we have made changes to the format of our bulletin. We hope you will like it. We also plan to make changes to the content. In each issue, we aimed to publish a study discussing a current topic in Catalysis. In this issue, we are publishing an article by Dr. Mustafa Yasin Arslan on the Haber-Bosch ammonia production process, which is one of the most important catalytic processes dating back to the beginning of 20th century and still maintains its relevance today. For our early carrier members and students in catalysis, we share lecture notes from academic experts on topics such as catalyst preparation, characterization, experimental design and reactor design. In this issue, we also share the seminar notes of our founding member and honorary president, Prof. Dr. Deniz Üner, titled “Dynamic Spectroscopic Methods in Catalyst Characterization,” which she delivered at the Applied Catalysis Seminar organized by the Chemical Engineering Department of Ondokuz Mayıs University in October 2024. We also plan to feature articles by our PhD students and recent graduates, sharing their personal experiences, challenges, achievements, disappointments, hopes, and memories from their education. We look forward to receiving your contributions for this section in the upcoming issues. As a tradition, we will continue featuring articles about catalytic R&D labs and summaries of latest research works of our members
The Catalysis Society
Editorial Board
Prof. Dr. Ayşe Nilgün AKIN
Prof. Dr. N. Alper TAPAN
Dr. Merve DOĞAN ÖZCAN
Dr. Elif CAN ÖZCAN
Dr. Mustafa Yasin ASLAN
The Haber – Bosch Process
By Dr. Mustafa Yasin ASLAN
Historical Background
Most of the people in chemical synthesis area know that the journey of production of ammonia started in the beginning of 20th century. What will be described in this article may be surprising to you, but the synthesis of ammonia dates back over 200 years. The first attempt to synthesize ammonia was performed just after the discovery of nitrogen. Otto Erdman and Richard Merchand tried to synthesize ammonia from its elements in 1788. J. W. Döbereiner pioneered the use of platinum as a catalyst for ammonia synthesis in 1823. By the end of the 1870s, almost all pure metals had been tested for their ability to catalyse the formation of ammonia [1].
Towards the end of the nineteenth century, Wilhelm Ostwald, one of the most renowned physical chemists, observed that the source of nitrate in Germany had potentially become a scarce commodity in the context of wartime conflict. This was due to the fact that nitrate salts were imported from Chile through the Atlantic Ocean, and therefore vulnerable to disruption [1]. As a consequence, Ostwald started working on ammonia synthesis reaction from its elements and claimed to produce ammonia in 1900 over iron, but later, it was understood that ammonia formed by hydrogenolysis of iron nitride, which was obtained at high temperature [1]. Although it was an unsuccessful attempt for the discovery of ammonia production, the study of Ostwald put forth the ammonia synthesis conditions explicitly. It was not a surprising event for them at that time, because, equilibrium reaction thermodynamics was evolving at the same years. van’t Hoff equation was first proposed in 1884 by Jacobus Henricus van 't Hoff in his book “Études de Dynamique chimique (Studies in Dynamic Chemistry)”.
Figure 1. Trends in human population with and without synthetic nitrogen fertilizers throughout the 20th Century [4, 5]
In the beginning of 20th century, Fritz Haber and his assistants could determine the equilibrium composition of ammonia synthesis reaction for a specified temperature and pressure with the development of reaction equilibrium thermodynamics. Therefore, they could calculate the theoretical amount of produced ammonia when they performed an ammonia synthesis experiment. Finally, Fritz Haber and his assistant, Robert Le Rossignol, synthesized ammonia from its elements at a rate of ~2 ml/min in an experimental setup that had the capability of operation at high temperatures and high pressures in 1909 [2]. Later, the BASF company purchased all the rights of the experimental demonstration of ammonia synthesis to develop an industrial high pressure ammonia synthesis process. One of the process chemists working for the BASF company, Carl Bosch, was tasked with the industrialization of ammonia synthesis. In 1910, BASF company announced the first industrial high pressure ammonia production process. The process was called as the Haber-Bosch process throughout the years. After 20000 trials within almost ten years, Alwin Mittasch found the most feasible mixture as a catalyst for ammonia synthesis reaction, which was iron-aluminium-calcium mixture [3]. Just 20 years after the discovery of the synthesis of ammonia from its elements, the Haber-Bosch Process was responsible for the production of fixed nitrogen, which is contained in more than half of the synthetic fertiliser produced worldwide in a year [1].
Why is the nitrogen fixation important?
Ammonia is one of the main sources of nitrogen that has been used in nature. The significant effect of the invention of ammonia production process from its elements at the beginning of the 20th Century on the rapid increase of the world population is given in Figure 1 [4]. The production of nitrogen based fertilizers and the increase in the world population have supported each other through the years.
Ammonia is the one of the most produced commodity chemicals and at the same time, it is the one of the most energy-intensive processes among petroleum-based industry [6]. The Haber-Bosch process consumes annually 2% of world energy production and approximately 4 % of the total natural gas output [7]. The above data clearly indicate that a more sustainable way of producing ammonia needs to be developed, although the Haber-Bosch process has been the best-known practice to date.
The ammonia production process operates under high temperature and high pressure operating conditions due to its mildly exothermic thermodynamics. High temperatures is needed to obtain significant reaction rates, and high pressure is needed to shift the reaction in the forward direction. R&D efforts have focused on inventing a more sustainable and energy efficient ammonia production process. Moreover, the U.S. National Academy of Engineering have set the management of the nitrogen cycle problem as one of the 14 Grand Challenges of Engineering that need to be overcome in the 21st Century [8].
Sustainability Issues
Today's economic problems, the threat of global warming and the scarcity of energy sources all over the world have prompted discussions about the sustainability of the ammonia production process. The decrease in energy requirement for production of ammonia over a number of years is given in Figure 2 [9]. It can be easily seen that discovery of the Haber-Bosch process is a milestone for ammonia production. Although there has been a great improvement within 100 years in terms of energy efficiency and even low energy need with respect to the biological systems (500 kJ/mole NH3) [10], today’s technology is insufficient for sustainable ammonia production.
High-energy intensive characteristics, CO2 emissions, extra energy requirement for hydrogen production and recycling of unreacted synthesis gas in ammonia synthesis loop were the main drawbacks of the Haber-Bosch process [7]. Large investment costs and the lack of a sustainable catalyst make the process more inefficient. There are also many efforts attempting to invent a new ammonia synthesis route that can replace Haber-Bosch technology and that can be operated at room temperature and atmospheric pressure such as electrochemical routes, photocatalytic routes, (solar) chemical looping, and biochemical routes [11]. Although these new routes are promising, they still cannot be efficient as a conventional method for production.
Under these circumstances, new R&D studies have been initiated to make the ammonia production process more sustainable. At the heart of these studies is the discovery of a catalyst that will allow the ammonia production process to operate under milder conditions. Recent research studies have shown that an agent is required to reduce molecular nitrogen, which has the stable triple bond, to atomic nitrogen for ammonia synthesis. Norskov et al. stated that a sustainable and active catalyst should synthesize ammonia at a rate of 1 
at 373 K and 1 bar [12].
Figure 2. Energy consumption for ammonia production through the years and comparison with biological ammonia production [9]
Although the Haber-Bosch process is the most efficient way to produce ammonia, there are still some problems. For example, the rates of ammonia synthesis using commercial catalysts are only sufficient at high temperatures and pressures. In this respect, the discovery of a new catalyst should make it possible to activate H2 and N2 at sufficient rates under milder conditions to produce ammonia.
This knowledge led the researchers to investigate the surface mechanism of the ammonia synthesis reaction over iron- and ruthenium-based catalysts in order to overcome the rate-limiting step(s) of the overall reaction. At the same time, the reaction mechanism via nitrogenase enzyme was targeted to learn from nature.
According to studies, two surface reaction mechanisms have been proposed for ammonia formation over the catalysts: associative and dissociative mechanisms. While in the associative mechanism, nitrogen molecules are hydrogenated by H2 molecules to form ammonia, in the dissociative mechanism, nitrogen molecule is dissociated over the catalyst surface and then nitrogen atom reacts with hydrogen atoms produced by the dissociation of hydrogen molecule over the catalyst surface to synthesize ammonia. In 2007, Prof. Dr. Gerhard Ertl won the Nobel Prize for discovering the reaction mechanism of ammonia synthesis over the traditional magnetite catalyst, which was proposed as a dissociative reaction mechanism. On the other hand, recent studies on the nitrogenase enzyme suggest that ammonia synthesis occurs by an associative mechanism [13]. In the light of the above discussion, many new generation catalysts have been tried for ammonia synthesis reaction, such as supported Ru catalysts with basic and/or high electron donating properties, hydrides, nitrides, etc. Although higher catalytic activities have been achieved with new-generation catalysts, they have not yet been suitable for use in industrial applications.
Future Perspective and Challenges
A new target has been set for the production of ammonia, referred to as "green ammonia production", using only renewable resources, including hydrogen production within 30 years. This target may represent a challenge for a new generation of ammonia production methodology. At the same time, this technological change can open the doors to a new concept, such as farmers producing their own fertiliser right next to their farmland. This transformation has two main challenges: hydrogen production and ammonia synthesis loop.
Today, hydrogen is mainly produced by steam methane reforming, which uses a fossil fuel as a feedstock. In this context, green hydrogen can be produced using renewable energy resources such as solar and/or wind energy at almost the same cost as the conventional method. Although promising progress has been made in recent years, capital investment and operating costs are the most important parameters for obtaining electricity to electrolyse water to produce green hydrogen.
To reduce the energy requirements of the ammonia synthesis loop, a new catalyst is needed to operate the system at milder conditions with sufficient reaction rates. The discovery of a new catalyst would be possible with an in-depth knowledge of the surface reaction mechanism over the catalyst through experimental and theoretical studies and the elucidation of ammonia formation over the nitrogenase enzyme. In addition, new approaches to reactor operating conditions can be considered to increase single-pass reactant conversion and/or reduce reactor and/or synthesis loop operating costs.
In conclusion, the journey of ammonia production by science and industry has been going on for at least two centuries and it seems to be going on for longer times due to being the main feedstock of the nitrogen based chemicals such as fertilizers and explosives. Although there is a great development in terms of industry and catalyst science throughout the years, some new scientific and technological improvements / discoveries are needed to have a more sustainable way of ammonia production.
References
[1] V. Smil, Enriching the earth: Fritz Haber, Carl Bosch, and the transformation of world food, Cambridge: The MIT Press, 2000.
[2] J. Paull, «A Century of Synthetic Fertilizer: 1909-2009,» Journal of Bio-Dynamics Tasmania, cilt 94, pp. 16-21, 2009.
[3] T. Hager, The alchemy of air: a Jewish genius, a doomed tycoon, and the scientific discovery that fed the world but fueled the rise of Hitler, New York: Harmony Books, 2008.
[4] J. Erisman, M. Sutton, J. Galloway, Z. Klimont ve W. Winiwarter, «How a century of ammonia synthesis changed the world,» Nature Geoscience, cilt 1, pp. 636-639, 2008.
[5] H. Ritchie, «How many people does synthetic fertilizer feed?,» Our World in Data, 7 11 2017. [Çevrimiçi]. Available: https://ourworldindata.org/how-many-people-does-synthetic-fertilizer-feed. [Erişildi: 26 11 2024].
[6] M. van der Hoeven, Y. Kobayashi ve R. Diercks, «Technology Roadmap: Energy and GHG Reductions in the Chemical Industry,» IEA, DECHEMA, ICCA, 2013.
[7] J. Baltrusaitis, «Sustainable Ammonia Production,» ACS Sustainable Chemistry & Engineering, cilt 5, no. 11, p. 9527, 2017.
[8] «Grand Challenges - 14 Grand Challenges for Engineering,» National Academy of Engineering, 2018. [Çevrimiçi]. Available: https://www.engineeringchallenges.org/challenges.aspx. [Erişildi: 2024 11 26].
[9] C. Adams, «Applied Catalysis: A Predictive Socioeconomic History,» Topics in Catalysis, cilt 52, no. 8, pp. 924-934, 2009.
[10] B. Burgess ve D. Lowe, «Mechanism of Molybdenum Nitrogenase,» Chemical Reviews, cilt 96, no. 7, pp. 2983-3012, 2002.
[11] D. Ye ve S. Tsang, «Prospects and challenges of green ammonia synthesis,» Nature Synthesis, cilt 2, pp. 612-623, 2023.
[12] A. Vojvodic, A. Medford, F. Studt, F. Abild-Pedersen, T. Khan, T. Bligaard ve J. Nørskov, «Exploring the limits: A low-pressure, low-temperature Haber–Bosch process,» Chemical Physics Letters, cilt 598, pp. 108-112, 2014.
[13] T. Rod ve J. Nørskov, «Modeling the Nitrogenase FeMo Cofactor,» Journal of the American Chemical Society, cilt 122, pp. 12751-12763, 2000.
Dynamic Spectroscopic Methods in Heterogeneous Catalyst Characterization
By Prof. Dr. Deniz ÜNER
The monitoring of heterogeneous catalytic reactions through spectroscopy can provide groundbreaking insights, particularly when it allows us to observe changes occurring both in the catalyst and in the reactants. The critical question we must ask to achieve accurate information is: What is the bottleneck in this reaction? Once the bottleneck is understood, finding ways to overcome it becomes more manageable. Spectroscopic methods offer opportunities to identify such bottlenecks. This paper provides a brief summary of the lecture I delivered during the Applied Heterogeneous Catalysis School at Ondokuz Mayıs University in Samsun.
Figure 1. The energy interactions and mechanisms differ along the new pathway induced by a catalytic reaction.
Figure 2. A diagram summarizing the electromagnetic spectrum and interaction dimensions (source: https://tua.gov.tr/tr/blog/havacilik-ve-teknoloji/dalgalar-ve-elektromanyetik-tayf-spektrum).
A catalytic reaction must progress through three critical steps: adsorption of reactants, surface reaction, and desorption of products. When the energy diagram of these steps is plotted against the reaction coordinate, it becomes apparent how a catalyst reduces the activation energy of the transition state compared to the uncatalyzed reaction (Figure 1).
To enhance our understanding of catalysts, we must address key questions, such as: What intermediate species are present on the surface? What are their surface coverages? Where are these intermediates located? What is the transformation sequence of these molecules? How can I observe these phenomena? The most direct answer to the last question is through spectroscopic methods. Spectroscopic techniques enable us to study material and energy interactions by analyzing changes in electromagnetic radiation at specific frequencies as it interacts with matter.
Within the scope of this paper, I provide a review based on our studies involving UV-Vis spectroscopy and NMR spectroscopy to investigate heterogeneous catalysts and the reactions occurring on their surfaces.
UV-Vis Spectroscopy
UV-Vis spectroscopy, or ultraviolet-visible spectroscopy, is a technique used to measure the frequencies of light absorbed by a material in the ultraviolet and visible wavelength ranges. The fundamental optical principles underlying this method are depicted in Figure 3.
Figure 3. Optical pathways utilized in UV-VIS spectrometers. UV and visible light are emitted from two separate sources and directed onto a diffraction grating, where frequency selection occurs, using a mirror. The light is then split into two beams: one directed to the sample cell and the other to the reference cell. Both beams are subsequently analyzed by separate detectors to determine the wavelengths absorbed by the sample. Source: https://www.fe.infn.it/u/spizzo/met_fis/uv-visibile/uvspec.htm.
In catalytic studies, UV-Vis spectroscopy can be employed for several purposes, including determining the bandgap energy of semiconductors, identifying the oxidation states of materials, directly observing reactions triggered by UV or visible wavelengths, and performing qualitative and quantitative analyses of UV-absorbing molecules. Details on the precautions required when measuring the bandgap energy of semiconductors using UV-Vis spectroscopy can be found in the cited source [1].
Using UV-Vis spectroscopy, we determined how the bandgap energy of a material changes when prepared in a double perovskite structure [2]. PbTiO3 is a well-known perovskite, and when cobalt was added to this structure, it was able to absorb visible light. Temperature-programmed desorption studies demonstrated that this was due to weakened oxygen bonds within the structure. Solid-state 207Pb NMR and DFT analysis of the obtained signals confirmed the double perovskite structure of the material.
Temperature-programmed reduction and oxidation techniques help identify the reduction temperature at which centers within the catalyst exhibit maximum activity under the flow of hydrogen or another reducing molecule, such as carbon monoxide. By utilizing the maximum reduction temperature, it is possible to calculate the activation energy of the process [3].
As a result of these efforts, we were able to characterize the structure resulting from the addition of cobalt to PbTiO3, establish that it forms a double perovskite, and identify the associated physical and chemical changes.
NMR Spectroscopy
NMR spectroscopy is a versatile technique with applications ranging from solid-state physics to biomedical sciences. Its widespread medical application is referred to as Magnetic Resonance Imaging (MRI). By observing how atomic nuclei interact with magnetic fields and the shielding or deshielding effects within the nuclei, NMR spectroscopy allows for the elucidation of chemical structures. Additionally, NMR spectroscopy provides critical information about system dynamics, enabling the determination of molecular mobility and related time constants. In catalytic reactions, NMR spectroscopy has the potential to identify surface intermediates and their interactions with gas-phase molecules. The ability to utilize NMR spectroscopy systems for both solid and liquid or gas-state experiments opens new avenues for research, despite the differences in signal acquisition and analysis methods across states. Early spectrometers that used electromagnets or natural magnets allowed both solid and liquid experiments in the same system. However, the need for high resolution in liquid spectroscopy and high power for solid-state spectroscopy has necessitated extensive interface development. Recently developed low-field benchtop NMR spectrometers present significant opportunities for overcoming these challenges.
Resonance, a key parameter in measurement, refers to the natural frequency of an object at which it absorbs energy from an external signal. This energy exchange forms the basis of NMR spectroscopy. Nuclei with nonzero spin quantum numbers interact with magnetic fields and exhibit energy exchanges at a frequency known as the Larmor frequency.
This phenomenon is governed by the quantum mechanical Zeeman effect, which causes energy levels of nuclear spins to split in a magnetic field. The transitions between these energy levels occur at frequencies proportional to the magnetic field strength. The resulting net magnetic moment generates an electric current through its precessional motion, producing the NMR signal measured as the potential difference over time in a coil.
In solid-state samples, nuclei are fixed in position relative to the magnetic field, resulting in signals that also reflect orientational differences. This complexity provides substantial information but also presents challenges. Nuclei with spin quantum numbers greater than ½ are termed quadrupolar nuclei. These interact not only with magnetic fields but also with electric field gradients, enabling crystallographic structure determination. NMR spectroscopy allows for the determination of chemical and electronic structures, geometries, and molecular dynamics. Chemical shift values provide information on chemical and electronic environments. Dipolar interactions in solid and near-solid structures reveal geometric arrangements, while molecular mobility and time constants are determined using Zeeman interactions and pulse sequences.
In the study mentioned above, we utilized DFT analysis of the ½-spin 207Pb signal to determine the structural transition of PbTiO3 perovskite to a double perovskite upon cobalt addition. By analyzing the changes in dipolar interactions in the solid-state NMR signal, we confirmed this structural transformation.
NMR spectroscopy offers valuable insights into molecular mobility in catalytic reactions. For instance, experiments with Ru/SiO2 revealed heterogeneous broadening of hydrogen adsorption signals at low pressures, which hole-burning experiments confirmed was due to the heterogeneity of the catalyst particles. At slightly elevated pressures, this signal sharpened, indicating high exchange rates between gas-phase hydrogen and surface-adsorbed hydrogen. At very high pressures, a new signal was observed, attributed to hydrogen exchanging among the gas phase, the support surface, and the catalyst surface.
For catalysts with alkali additives, the hydrogen exchange remained heterogeneous even at high temperatures and pressures. This finding demonstrated that alkali additives restrict hydrogen exchange between particles [5]. Using kinetic constants derived from this study, we explained the effects of alkali additives in Fischer-Tropsch synthesis, including increased chain growth probabilities and olefin selectivity, reduced overall reaction rates, and enhanced CO2 formation from surface oxygen [6].
Using a MAGRITEK Spinsolve benchtop NMR spectrometer, we studied molecules adsorbed on Pd/TiO2. We demonstrated the formation of hydride signals at low temperatures and their intensity correlation with three-dimensional catalyst structures in temperature-programmed reduction experiments. Pd, capable of forming hydrides at low temperatures, was shown to simultaneously reduce surface oxygen in TiO2, confirmed by the hydrogen consumption observed in TPR experiments (300 K). Our quantitative analysis linked the extent of surface reduction to two-dimensional (2D) Pd structures, which do not form hydrides, as established using NMR [7].
We further analyzed the alpha and beta phases of Pd hydrides using NMR, DFT NMR, and adsorption calorimetry. Our findings revealed that the alpha phase corresponds to hydrogen atoms trapped between a metal oxide shell and a metal core. This study highlighted the utility of benchtop NMR spectrometers in surface studies traditionally requiring high-field systems.
The proximity of these low-field systems to adsorption units minimizes gas transport issues under low-pressure conditions, facilitating equilibrium measurements and advancing catalytic research.
References
[1] Makula, P., Pacia, M. Ve Macyk, W., How to correctly determine the band gap energy of modified semiconductor photocatalysts based on UV VIS spectraJ.Phys.Chem.Lett. 9(2018) 6814-6817
[2] Mete E., Odabasi S., Mao H., Chung T., Ellialtioglu S., Reimer J. A., Gulseren, O. And Uner D. Double Perovskite Structure Induced by Co Addition to PbTiO3: Insights from DFT and Experimental Solid-State NMR Spectroscopy J.Phys.Chem. C. 123 (2019)27132
[3] B. Dernaika and D. Uner, “A simplified approach to determine the activation energies of uncatalyzed and catalyzed combustion of soot” Applied Catalysis B: Environmental, 40(3), 219 – 229 (2003)
[4] E. Mete, S. Odabasi, H. Mao, T. Chung, S. S. Ellialtioglu, J. A Reimer, O. Gülseren & D. Üner, Double Perovskite Structure Induced by Co Addition to PbTiO sub3/sub : Insights from DFT and Experimental Solid-State NMR Spectroscopy/title, Journal of Physical Chemistry C, , 123 (44) 27132-27139 (2019).
[5] D. O. Uner, N Savargoankar, M. Pruski and T.S. King, “ The effects of alkali promoters on the dynamics of hydrogen chemisorption and syngas reaction kinetics on Ru/SiO2 catalysts”, Stud. in Surf. Sci. and Catalysis, 109, 315-324 (1997).
[6] D.O. Uner “A sensible mechanism of alkali promotion in Fischer Tropsch synthesis:Adsorbate mobilities”, Ind. Eng. Chem. Res. 37, 2239-2245 (1998).
[7] Yarar M., Bouzani A., Üner D. Pd as a reduction promoter for TiO2: Oxygen and hydrogen transport at 2D and 3D Pd interfaces with TiO2 monitored by TPR, operando 1H NMR and CO oxidation studies, Catalysis Communications , vol.174, 106580 (2023)
[8] Mete E., Yilmaz B., Üner D. PdH α-phase is associated with residual oxygen as revealed by in situ 1H NMR measurements and DFT-NMR estimations, Applied Surface Science , vol.641, 158421 (2023)
Previous issue answers Newsletter #9
1. Synchrotron, 2. QMS, 3. Chemiluminescence, 4. ReLu, 5. leaf size, 6. ANN, 7.TPD, 8. LEED
Announcement
In 2025, there will be a highly active program of events in the field of catalysis.
• In June, 10th National Catalysis Conference (NCC10), organized by the Catalysis Society and Cumhuriyet University, will take place in Sivas. We look forward to welcoming you there. (The exact dates of the conference for June will be announced soon.)
• From August 31 to September 5, 16th Europacat Conference, organized by EFCATS, of which our society is a member, will be held in Trondheim, Norway.
• From September 1 to 4, 36th National Chemistry Congress, organized by Van Yüzüncü Yıl University will feature a "Catalysis" session supported by Catalysis society."
• From September 9 to 12, the 16th National Chemical Engineering Congress, organized by Bolu Abant İzzet Baysal University, will host a special session on "Catalysis and Reaction Engineering," supported by Catalysis society.
We wish everyone a successful and fulfilling year in catalysis and look forward to seeing you at these events!