Speaker: Cristina I. Q. Silva, Chemistry Ph.D. student supervised by Professor Javier Ruiz-Martínez

Title: Effect of Fe and Ce in a manganese titania catalyst for the low temperature selective catalytic reduction of NO_x with NH₃ 

Abstract

Nitrogen oxides (NO_x) are harmful pollutants present in combustion gases that can be emitted from stationary and mobile sources. Mn-based oxides are promising for the selective catalytic reduction (SCR) of NO_x with NH₃ at temperatures below 200 °C, where the current commercial deNO_x catalysts fall short.[1, 2] However, MnO_x often suffer from low N₂ selectivity and low tolerance for H₂O and SO₂, which can be improved by combining Mn with other transition metals.[1]

With this in mind, we have developed a series of catalysts based on Mn, Fe, Ce and Ti with different compositions by a co-precipitation method designed to obtain a homogeneous dispersion to understand the effect of Fe and Ce in a MnTiO_x catalyst. The catalytic activity and N₂ selectivity were tested for NH₃-SCR of NO_x and the properties of these catalysts were investigated by ICP, N₂-physisorption, XRD, H₂-TPR, NH₃-TPD, STEM, EDX, TEM, EELS, XPS and XAS.

We were able to determine that the addition of Fe to MnTiO_x led to a catalyst composed of a rutile TiO₂ structure with well-dispersed amorphous Mn and Fe (Fig.1 (A)). The addition of Fe had a detrimental effect on the specific surface area compared to the initial MnTiO_x, which had a negative effect on the catalytic activity at low temperatures (below 250 °C) (Fig.1 (C)), particularly in the presence of H₂O. On the other hand, there was an overall increase in the N₂ selectivity (lower N₂O production) across the tested temperature range (Fig.1 (D)). Based on our previous results [2], we included Ce in the catalyst formulation to improve specific surface areas and consequently catalytic activities. Adding Ce to MnTiO_x and MnFeTiO_x led to a totally or partially amorphous structure (Fig.1 (B)), higher overall surface areas, higher acidity, and a change in the distribution of the oxidation states of Mn. Regarding performance, it led to a sharper decrease in catalytic activity at low temperatures and an increase in N₂ selectivity (Fig.1 (C, D)). Through HR-EELS we could observe that MnTiO_x and MnFeTiO_x had a higher concentration of Mn on the surface (Fig.1 (E)), and that the addition of Ce led to a better distribution of Mn across the Ti support. MnCeTiO_x showed a homogeneous distribution of Mn and Ti across the whole material, whereas MnFeCeTiO_x showed a combination of the aforementioned samples (Fig.1 (F)). The addition of Fe led to an increase in the specific activity of Mn, whereas Ce led to a decrease, demonstrating that the Mn-Ce bond is detrimental to the activity.

We found an optimum formulation by adding small quantities of Ce and Fe to the MnTiO_x catalyst, more specifically in the Mn_0.31Ce_0.04Fe_0.04Ti_0.62 sample, where the catalytic performance and N₂ selectivity were maximized at 150 °C (79% and 94% in the absence of H₂O and 47% and 98% in the presence of H₂O, respectively) (Fig.1 (C, D)), while also having a broader temperature range of application. We suggest that the addition of Fe and Ce suppresses the overoxidation of reactants and thus N₂O formation by reducing the MnO_x ensembles on the catalyst’s surface, as shown by HR-EELS.

Figure 1. Representative TEM images of the metal oxides A - Mn_0.29Ti_0.71 ; B - Mn_0.31Ce_0.04Fe_0.04Ti_0.62 ; Catalytic activity (C) and N₂ selectivity (D) in the absence of H₂O ; Representative HR-EELS images of the metal oxides E - Mn_0.29Ti_0.71 ; F - Mn_0.31Ce_0.04Fe_0.04Ti_0.62 (■ - Mn_0.29Ti_0.71 ; ● - Mn_0.34Fe_0.04Ti_0.62;▲- Mn_0.37Ce_0.03Ti_0.62; ▼- Mn_0.31Ce_0.04Fe_0.04Ti_0.62).

References

[1] Han, L.;  Cai, S.;  Gao, M.;  Hasegawa, J.-y.;  Wang, P.;  Zhang, J.;  Shi, L.; Zhang, D. Chemical Reviews 2019, 119, 10916-10976.

[2] Gevers, L. E.;  Enakonda, L. R.;  Shahid, A.;  Ould-Chikh, S.;  Silva, C. I. Q.;  Paalanen, P. P.;  Aguilar-Tapia, A.;  Hazemann, J.-L.;  Hedhili, M. N.;  Wen, F.; Ruiz-Martínez, J. Nature Communications 2022, 13, 2960.

Bio

Cristina got her bachelor's (2016) and master’s degrees (2018) in Chemistry from the Faculty of Sciences of the University of Porto, in Portugal, where her research focused on material science, nanotechnology, and characterization through a variety of techniques. In 2021, she became a Ph.D. student in the group of Prof. Javier Ruiz-Martínez at King Abdullah University of Science and Technology (KAUST), where she is currently working on the development of optimized heterogeneous catalysts for the NH₃-SCR, to reduce NO_x emissions from mobile sources.

Speaker: Ekaterina Toshcheva, Chemistry Ph.D. student supervised by Professor Luigi Cavallo

Title: Towards sustainability by integrating DFT and ML for heterogeneous catalysis

Abstract

CO₂ reduction and ammonia synthesis are two important chemical processes with the potential to make significant contributions to the mitigation of climate change. Both CO2 reduction and ammonia synthesis are challenging processes, but there is a growing interest in developing more efficient and sustainable methods for these processes. This is because CO2 and nitrogen are abundant in the atmosphere, and their conversion into useful products could help to reduce our reliance on fossil fuels.

In this study, I investigated the reaction pathways for the aforementioned catalytic reactions. I modeled the surfaces of indium oxide, perovskite, iron oxide, and iron carbide to calculate the adsorption energy of important reaction intermediates and thereby understand different reaction pathways. Machine learning (ML) programs were integrated to predict the binding energy for structures allowed by single atom catalysts (SACs). This could help improve the speed and optimize computational power of the calculations to design new catalysts.

Bio

In 2020, Ekaterina completed her BSc followed by MSc in Physics from St. Petersburg State University in Russia. Her Ph.D. research focuses on applying Machine learning (ML) to analyze the density functional theory (DFT) calculations for heterogeneous catalysis.

Speaker: Katya Aguillar Perez, Chemistry Ph.D. student supervised by Professor Niveen Khashab

Title: Biomimetic Mineralization for Smart Biostimulant Delivery and Crop Micronutrients Fortification 

Abstract 

Sustainable and precise fortification practices are necessary to ensure food security for the increasing human population. Precision agriculture aims to minimize the use of fertilizers and pesticides by developing smart materials for real-life agricultural practices. Here, we show that biomimetic mineralization can be efficiently employed to encapsulate and controllably release plant biostimulants (MiZax-3) to improve the quality and yield of capsicum (Capsicum annum) crops in field experiments. ZIF-8 encapsulation of MiZax-3 (MiZIFs) could significantly enhance its stability up to around 679 times (p-value = 0.0072) at field conditions. Our results demonstrate that the coordinating Zn ions and the MiZax-3 play a vital role in improving Zn content in the produced fruits by 2-fold, which is the first report of this nature on Zn content in fruits. We envision this platform as a starting point to investigate other biocompatible coordination-based platforms for micronutrient delivery in precision agriculture. 

Bio 

Katya Aguilar is currently a Ph.D. candidate in Chemistry at SHMs lab under the supervision of Prof. Niveen Khashab. She is interested in the deployment and utilization of smart hybrid materials for the stabilization and target release of plant growth regulators (PGRs) and their application in precision agriculture.