Controlling the Shapes of Nanoparticles by Dopant-Induced Enhancement of Chemisorption and Catalytic Activity: Application to Fe-Based Ammonia Synthesis

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Transition metal small clusters anchored on biphenylene for effective electrocatalytic nitrogen reduction

The synthesis of ammonia via an electrochemical nitrogen reduction reaction (NRR, N2 + 6H+ + 6e- → 2NH3), which can weaken but not directly break an inert NN bond under mild conditions via multiple progressive protonation steps, has been proposed as one of the most attractive alternatives for the production of NH3. However, the development of appropriate catalyst materials is a major challenge in the application of NRRs. Recently, single- or multi-metal atoms anchored on two-dimensional (2D) substrates have been demonstrated as ideal candidates for facilitating NRRs. In this work, by applying spin-polarized density functional theory and ab initio molecular dynamic simulations, we systematically explored the performances of nine types of transition metal multi-atoms anchored on a recently developed 2D biphenylene (BPN) sheet in nitrogen reduction. Structural stability and NRR performance catalyzed by TMn (TM = V, Fe, Ni, Mo, Ru, Rh, W, Re, Ir; n = 1-4) clusters anchored on BPN sheets were systematically explored. After a strict six-step screening strategy, it was found that W2, Ru2 and Mo4 clusters loaded on BPN demonstrate superior potential for nitrogen reduction with extremely low onset potentials of -0.26, -0.36 and -0.17 V, respectively. Electronic structure analysis revealed that the enhanced ability of these multi-atom catalysts to effectively capture and reduce the N2 molecule can be attributed to bidirectional charge transfer between the d orbitals of transition metal atoms and molecular orbitals of the adsorbed N2 through a "donation-back donation" mechanism. Our findings highlight the value of BPN sheets as a substrate for designing multi-atom nitrogen reduction reaction catalysts.

Single Ru-N4 Site-Embedded Porous Carbons for Electrocatalytic Nitrogen Reduction

Ammonia is an effective feedstock for chemicals, fertilizers, and energy storage. The electrocatalytic nitrogen reduction reaction (NRR) is an alternative, efficient, and clean technology for ammonia production, relative to the traditional Haber-Bosch method. Single-metal catalysts are widely studied in the field of NRR. However, very limited conclusions have been made on how to precisely modulate the coordination environment of the single-metal-atom sites to boost catalytic NRR performance. Herein, we report a 5,7-membered carbon ring-involved porous carbon (PC) preparation toward single-atom Ru-embedded PCs. As electrocatalysts, such materials exhibit surprisingly promising catalytic NRR properties with an NH₃ yield rate of up to 67.8 ± 4.9 μg h^-1 mg_cat^-1 and a Faradaic efficiency of 19.5 ± 0.6%, exceeding those of most of the reported single-atom NRR catalysts. Extended X-ray absorption fine structure demonstrates that the presence of topological defects increases the Ru-N bond from 1.48 to 1.56 Å, modulating the coordination environment of the single-atom Ru active sites. Density functional theory-calculated results demonstrate that the adsorption of N₂ onto single-atom Ru surrounded by topological defects extends the N≡N bond to 1.146 Å, weakening the strength of N≡N and making it susceptible to the NRR. All in all, this work provides a new design strategy by involving topological defects and corresponding large polarization around the Ru single atom to boost the catalytic NRR performance. Such a concept can also be applied to many other kinds of catalysts for energy storage and conversion.

Reaction Mechanisms, Kinetics, and Improved Catalysts for Ammonia Synthesis from Hierarchical High Throughput Catalyst Design

The Haber-Bosch (HB) process is the primary chemical synthesis technique for industrial production of ammonia (NH₃) for manufacturing nitrate-based fertilizer and as a potential hydrogen carrier. The HB process alone is responsible for over 2% of all global energy usage to produce more than 160 million tons of NH₃ annually. Iron catalysts are utilized to accelerate the reaction, but high temperatures and pressures of atmospheric nitrogen gas (N₂) and hydrogen gas (H₂) are required. A great deal of research has aimed at increased performance over the last century, but the rate of progress has been slow. This Account focuses on determining the atomic-level reaction mechanism for HB synthesis of NH₃ on the Fe catalysts used in industry and how to use this knowledge to suggest greatly improved catalysts via a novel paradigm of catalyst rational design.We determined the full reaction mechanism on the two most active surfaces for the HB process, Fe(111) and Fe(211)R. We used density functional theory (DFT) to predict the free-energy barriers for all 12 important reactions and the 34 most important 2 × 2 surface configurations. Then we incorporated the mechanism into kinetic Monte Carlo (kMC) simulations run for several hours of real time to predict turnover frequencies (TOFs). The predicted TOFs are within experimental error, indicating that the predicted barriers are within 0.04 eV of experiment.With this level of accuracy, we are poised to use DFT to improve the catalyst. Rather than forming bulk alloys with uniform concentration, we aimed at finding additives that strongly prefer near-surface sites so that minor amounts of the additive might lead to dramatic improvements. However, even for a single additive, the combinations of surface species and reactions multiplies significantly, with ∼48 reaction steps to examine and nearly 100 surface configurations per 2 × 2 site. To make it practical to examine tens of dopant candidates, we developed the hierarchical high-throughput catalysis screening (HHTCS) approach, which we applied to both the Fe(111) and Fe(211) surfaces. For HHTCS, we identified the most important 4 reaction steps out of 12 for the two surfaces to examine >50 dopant cases, where we required performance at each step no worse than for pure Fe. With HHTCS, the computational cost is about 1% of that for doing the full reaction mechanism, allowing us to do ≈50 cases in about 1/2 the time it took to do pure Fe(111). The new leads identified with HHTCS are then validated with full mechanistic studies.For Fe(111), we predict three high-performance dopants that strongly prefer the second layer: Co with a rate 8 times higher, Ni with a rate 16 times higher, and Si with a rate 43 times higher, at 400 °C and 20 atm. We also found four dopants that strongly prefer the top layer and improve performance: Pt or Rh 3 times faster and Pd or Cu 2 times faster. For Fe(211), the best dopant was found to be second-layer Co with a rate 3 times faster than that for the undoped surface.The DFT/kMC data were used to predict reshaping of the catalyst particles under reaction conditions and how to tune dopant content so as to maximize catalytic area and thus activity. Finally, we show how to validate our mechanistic modeling via a comparison between theoretical and experimental operando spectroscopic signatures.

Understanding Reaction Networks through Controlled Approach to Equilibrium Experiments Using Transient Methods

We report a combined experimental/theoretical approach to studying heterogeneous gas/solid catalytic processes using low-pressure pulse response experiments achieving a controlled approach to equilibrium that combined with quantum mechanics (QM)-based computational analysis provides information needed to reconstruct the role of the different surface reaction steps. We demonstrate this approach using model catalysts for ammonia synthesis/decomposition. Polycrystalline iron and cobalt are studied via low-pressure TAP (temporal analysis of products) pulse response, with the results interpreted through reaction free energies calculated using QM on Fe-BCC(110), Fe-BCC(111), and Co-FCC(111) facets. In TAP experiments, simultaneous pulsing of ammonia and deuterium creates a condition where the participation of reactants and products can be distinguished in both forward and reverse reaction steps. This establishes a balance between competitive reactions for D* surface species that is used to observe the influence of steps leading to nitrogen formation as the nitrogen product remains far from equilibrium. The approach to equilibrium is further controlled by introducing delay timing between NH₃ and D₂ which allows time for surface reactions to evolve before being driven in the reverse direction from the gas phase. The resulting isotopic product distributions for NH₂D, NHD₂, and HD at different temperatures and delay times and NH₃/D₂ pulsing order reveal the role of the N₂ formation barrier in controlling the surface concentration of NH_x* species, as well as providing information on the surface lifetimes of key reaction intermediates. Conclusions derived for monometallic materials are used to interpret experimental results on a more complex and active CoFe bimetallic catalyst.

Fe-Doping induced divergent growth of Ni–Fe alloy nanoparticles for enhancing the electrocatalytic oxygen reduction

Separate (111)- and (200)-faceted Ni–Fe nanoparticles were synthesized and their oxygen reduction reaction activity studied via density functional theory calculations and experiments.