ALD/ALE 2022 Session AA1-MoA: ALD for Catalysis

The most efficient NO_x abatement process is the selective catalytic reduction using NH₃ (NO_x NH₃-SCR). The commercial V-based NH₃-SCR catalysts have a narrow high-temperature operational window of 350-450°C; this requires installing the catalyst bed before desulfurization and dust removal units, which initiates catalyst deactivation. To avoid this, the SCR unit is currently being placed in the so-called tail-end configuration, which results in exhaust gases temperature below the operational window of the commercial catalysts. Manganese oxide (MnO_x) catalysts show superior NH₃-SCR activity at low temperatures (LT). The various oxidation states (Mn²⁺,Mn³⁺,and Mn⁴⁺) of MnO_x, are known to play an essential role in the LT activity of MnO_x catalysts; hence, the existence of multiple oxidation states of Mn is pivotal for its SCR activity. On the other hand, the crystalline MnO_x does not contribute effectively to NH₃-SCR; thus, the dispersion of MnO_x strongly affects SCR activity. The routine catalyst synthesis methods like precipitation impregnation, and sol-gel, are less suitable for preparing MnO_x SCR catalysts since they often require high-temperature post-treatments, which increase the crystallinity of particles and decrease dispersion.

Atomic layer deposition (ALD) offers a reliable LT coating method, with subnanometer control over the process. ALD makes it possible to obtain metal oxide nano-coatings at temperatures significantly lower than conventional methods. In this work, we have employed fluidized bed ALD to deposit highly dispersed MnO_x on SiO₂ for LT NH₃-SCR catalysis. The ultra-fine MnO_x NPs were grown on SiO₂ at 150°C and 1 bar. Additionally, we deposited TiO₂ on ALD prepared MnO_x/SiO₂ to increase the acidity of the catalyst, which is crucial for NH₃ activation during the SCR process. The XPS analysis revealed three oxidation states of Mn²⁺, Mn³⁺,and Mn⁴⁺ in these samples, which are essential for NO_x-SCR. Also, powder XRD could not detect any crystalline phases of MnO_x, suggesting that ALD synthesis avoided the crystalline MnO_x; consistently, the MnO_x NPs were scarcely observable using TEM, demonstrating extreme dispersion of MnO_x over SiO₂. NH₃ temperature-programed desorption analysis demonstrated that the acidity of the MnO_x/SiO₂ sample is significantly increased after TiO₂ ALD, increasing the NH₃ activation on TiO₂-MnO_x/SiO₂. The obtained TiO₂-MnO_x/SiO₂ with such characteristics provide a promising catalyst for low-temperature selective catalytic reduction of nitrogen oxides. Accordingly, the catalytic activity evaluation showed~85% NO conversion for the TiO₂-MnO_x/SiO₂ sample, while the MnO_x/SiO₂ sample showed a NO conversion of~30%.

ALD of ternary compounds (A_mB_nC_o) typically combines two binary ALD processes, each consisting of a metal precursor source and a co-reactant. Recently, however, we have demonstrated that ternary compounds, in casu MRuO_x (M = Al, Pt), can be deposited using a single binary ALD process, thus reducing deposition complexity.¹ During such process, metal-organic precursors (MeCpPtMe₃ and TMA) are combined with RuO₄, the latter functioning both as metal source and co-reactant which combusts organic ligands. Yet, such multi-constituent co-reactant limits deposition to ternary compounds, such as MRuO_x, while often metallic MRu films or nanoparticles (NPs) are desired. Herein, we show that by increasing the complexity of the original MRuO_x process by adding extra ‘functionalities’, such as reduction or even etching steps, bimetallic films and NPs can be deposited with full thickness and compositional control. This approach therefore significantly extends the applicability of the original process toward bimetallic compounds.

We first show that ALD of bimetallic PdRu films can be achieved by inserting a RuO₄ step in the Pd(hfac)₂ / H₂-plasma (H₂*) process, leading to a three-step Pd(hfac)₂ / RuO₄ / H₂* process with a high growth per cycle of 0.19 nm/cycle.^{1, 2} Herein, RuO₄ acts both as an oxidizing agent and a Ru source, while H₂* reduces the surface. Thin films resulting from this process are Ru-rich, and we show that the Pd content can be increased by not including the RuO₄ step in every cycle, thus decreasing the incidence rate of the RuO₄ step in the ALD process (Fig. 1).

Next, by replacing part of the RuO₄ units in the three-step process by O₂*, the morphology of the PdRu is transformed from thin films to bimetallic nanoparticles (BMNPs, Fig. 2). This change of morphology is attributed to the etching of the deposited Ru as volatile RuO₄ during the O₂* steps. In situ grazing-incidence small-angle X-ray scattering and X-ray fluorescence revealed that the composition and size of the BMNPs can be adjusted independently by changing the proportion of RuO₄ versus O₂* pulses in the sequence, and the total number of ALD cycles, respectively. Finally, grazing-incidence wide-angle X-ray scattering and electron energy loss spectroscopy revealed that the RuPd BMNPs are crystalline, and Ru and Pd intimately mixed, suggesting the formation of solid-solution RuPd nanoalloys.

[1]Minjauw, M. M. et al. (2022).Dalt. Trans. Advance Article (DOI: 10.1039/D1DT03543F)

[2]Feng, J. Y. et al. (2020).Phys. Chem. Chem. Phys., 22, 9124–9136.

The demand for fresh and clean water sources increases globally, and there is a need to develop novel routes to eliminate micropollutants and other harmful species from water. Photocatalysis is a promising alternative green technology that has shown great performance in the degradation of persistent pollutants. Titanium dioxide is the most used catalyst owing to his attractive physico-chemical properties, but this semiconductor presents limitations in the photocatalysis process due to the high band gap and the fast recombination of the photogenerated carriers. Herein, a novel photocatalyst has been developed, based on titanium dioxide nanofibers (TiO₂ NFs) synthesized by electrospinning. The TiO₂ NFs were coated by atomic layer deposition (ALD) to grow boron nitride (BN) and palladium (Pd) on their surface. The UV-Vis spectroscopy measurements confirmed the increase of the band gap and the extension of the spectral response to the visible range. The obtained TiO₂/BN/Pd nanofibers were then tested for photocatalysis, and showed a drastic increase of acetaminophen (ACT) degradation (>90%), compared to only 20% degradation obtained with pure TiO₂ after 4h of visible light irradiation. The high photocatalytic activity was attributed to the good dispersion of Pd NPs on TiO₂-BN nanofibers, leading to a higher transfer of photoexcited charges carriers and a decrease of photogenerated electron-holes recombination. To confirm their reusability, recycling tests on the hybrid photocatalyst TiO₂/BN/Pd have been performed, showing a good stability over 5 cycles under UV and Visible light. Moreover, toxicity testsas well as quenching tests were carried out to check the toxicity in the formation of byproductsand to determine active species responsible for the degradation. The results presented in this work demonstrate the potential of TiO₂/BN/Pd nanomaterials, and open new prospects for the preparation of tunable photocatalysts.

Keywords: TiO₂-BN-Pd nanocomposites; nanofibers, electrospinning; atomic layer deposition; photocatalysis; acetaminophen

Iridium (Ir) and ruthenium (Ru) are crucial ingredient in various applications, and their proper utilization is essential due to the extreme rarity of those elements. For example, in the case of electrocatalysis, only the surface participate in the reaction, and whatever lies beneath (within the nanoparticle or thin film) are un-utilized. Therefore, fabrication of Ir/Ru thin-film with high uniformity and conformality onto high surface area porous substrate is beneficial to reduce the metal content and increase its utilization. Atomic layer deposition (ALD) can be used to deposit uniform and conformal films with precise thickness control. In this study, we report the fabrication of ultra-low dimension (~5 nm) Ir-Ru thin film on to porous Ti felt substrate using a sequential approach with two precious metal ALD processes. The nano-secondary ion mass spectrometer depth-profiling (Nano-SIMS) results confirm the homogeneous, conformal and uniform growth of Ir-Ru thin film as predicted from the ideal ALD growth characteristic and XTEM on Si substrate. The electrochemical performance of Ir-Ru nanolayer electrode towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are evaluated in an acid electrolyte (0.5 M H₂SO₄). The Ru film shows the highest onset for HER (-112 mV), the Ir (5 nm) and Ir-Ru (5 nm) has an HER onset potential of ~-50 mV. In the case of the OER voltammogram, all the electrodes with precious metal films show the current due to OER. Contrary to the HER, here Ru shows the highest activity both in terms of OER onset potential and the overpotential at a specific current density; noteworthy the slightly better activity of 5 nm Ir film is attributed to the Ru nucleation layer (~2 nm) deposited on to the Ti substrate for uniform deposition of Ir, since, there is a huge nucleation delay for Ir ALD on Ti-based substrate. The stability of the film is characterized by chrono-potentiometric stability analysis for 24 hours at an OER current density of 10 mA cm^-2. As expected the Ru film is very unstable during OER and complete dissolution of 5 nm film happens at around ~4 hour of operation. However, the Ir-Ru (5 nm) display enhanced stability and is attributed to the inclusion of Ir. In conclusion, ALD can be used as an efficient technique to coat ultra-low dimension Ir-Ru thin film/nano-materials on to porous substrate with precise thickness control and high conformality, which is beneficial in terms of utilization and cost-effectiveness of electrocatalyst.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2021R1A2C1007601 and 2021M3H4A3A02099209.

A promising approach to answer the challenge of intermittency of renewable sources such as wind and solar energy, is to store green electricity into H₂ via H₂O electrolysis. Sustainable electrolysis can be achieved, for example, under alkaline conditions, with the sluggish oxygen evolution reaction (OER) representing the main limitation. Mixed oxides of the earth-abundant cobalt and nickel are considered promising OER electrocatalysts. Cobalt nickel oxide (Co_xNi_1-xO_y) can adopt both the Co₃O₄ spinel and the NiO rock-salt structure depending on the cobalt concentration. The present study aims at exploring the effect of cobalt concentration in Co_xNi_1-xO_y on the OER catalytic activity to expand on previous literature studies which are either limited to single phase structures[1, 2] or disregard the effect of bulk (film thickness >5 nm) effects[3].

Atomic layer deposition (ALD) offers the opportunity to tune the stoichiometry and therefore the structure of Co_xNi_1-xO_y. This work therefore employs plasma-enhanced ALD by combining a cobalt cyclopentadienyl (Co(Cp)₂)[4] based process for CoO_x with a nickel methylcyclopentadienyl (Ni(MeCp)₂) process[5] for NiO_x. Energy Dispersive X-ray mapping shows no variation in elemental composition, indicating that nickel intermixes with cobalt to form an alloy. X-ray spectroscopy (XPS) furthermore reveals the presence of mixed Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺ valence states, with an increase in the Ni³⁺-to-Ni²⁺ ratio for increasing cobalt concentrations. Preliminary x-ray diffraction measurements furthermore suggest a transition from rock-salt to spinel phase with increasing cobalt concentration, indicating that nickel occupies the octahedral sites in the spinel structure at high cobalt concentrations. The OER activity of Co_xNi_1-xO_y films (~30 nm) is determined by cyclic voltammetry (CV) in 1M KOH. The current density at 1.8 V vs RHE increases whilst the onset potential decreases, with a decrease in cobalt concentration in the film. An increased number of CV cycles leads to an increase in the current density, suggesting the activation of the bulk of the electrocatalyst. After CV, XPS reveals a dominant Ni³⁺ oxidation state in the film and an increase in oxygen concentration and hydroxide phase. These results indicate that the rock-salt phase is favourable for OER, implying that further research should focus on low cobalt- content Co_xNi_1-xO_y.

[1]R. N. Singh et al., Electrochim. Acta, 2000.

[2]A. Asho etl al., Int. J. Hydrogen Energy, 2019.

[3]L. Trotochaud et al., J. Am. Chem. Soc., 2012.

[4]M. E. Donders et al. J. Electrochem. Soc., 2011.

[5]D. Koushik et al., J. Mater. Chem. C, 2019.

One of our greatest challenges for the upcoming decades is the transition to a sustainable way of generating, storing, and converting energy. Green hydrogen produced by PEM Water Electrolyzers (PEMWE’s) is part of the solution, but still faces several challenges. Among the major challenges towards widespread adoption and commercialization of PEMWE’s are the cost, performance, and durability of iridium (Ir) material as used to catalyze the oxygen evolution reaction (OER) ^[1]. Sluggish OER kinetics and ensuing large overpotential above the thermodynamic equilibrium level required to drive the reaction will inevitably result in PEMWE inefficiency along with high catalyst loadings. It has previously been demonstrated that atomic layer deposition (ALD) can be used to deposit high quality and stable electrocatalyst layers with a low Ir loading ^[2], but the low deposition rate of ALD limits up-scaling for mass production. Spatial atomic layer deposition (sALD) has emerged as a viable tool for the atomically precise design and synthesis of materials with high deposition rates on both large substrates (square meters) and roll-to-roll ^[3].

Herein, we present an atmospheric-pressure spatial ALD process of Ir/IrO_x ultra-thin films on flat substrates. We investigate the temperature window of the ALD process down to 80 degrees. We study the growth characteristics as well as the thin film properties using ellipsometry, inductively coupled plasma mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (GIXRD). Furthermore, we evaluate the OER catalytic behaviour and durability using rotating disc electrode (RDE). Our study sheds light on structure, thickness, and morphology of Ir deposits and corresponding OER catalytic properties. The findings will pave the way towards identifying optimal catalytic system with the promise of ultra-low Ir loading at high OER performances.

[1] Shirvanian, P. and van Berkel, F., 2020. Novel components in Proton Exchange Membrane (PEM) Water Electrolyzers (PEMWE): Status, challenges and future needs. A mini review. Electrochemistry Communications, 114, p.106704.

[2] Laube, A., Hofer, A., Ressel, S., Chica, A., Bachmann, J. and Struckmann, T., 2021. PEM water electrolysis cells with catalyst coating by atomic layer deposition. international journal of hydrogen energy, 46(79), pp.38972-38982.

[3] Poodt, P., van Lieshout, J., Illiberi, A., Knaapen, R., Roozeboom, F. and van Asten, A., 2010. On the kinetics of spatial atomic layer deposition. Journal of Vacuum Science & Technology A, 31, pp. (01A108-1)-(01A108-7).

Hydrogen is a good, sustainable alternative energy that can replace fossil fuels that cause air pollution and global warming. To produce hydrogen, electrochemical water splitting is the promising sustainable method without producing carbon containing pollutant.¹

Platinum has been widely used for hydrogen evolution reaction(HER) for its high performance compared to other materials. However, platinum faces the problem in commercial use because of its low cost efficiency. In order to replace platinum, Transition Metal DichalCogenides(TMDCs) have been actively researched, which have a high surface-to-volume ratio and provide a sufficient number of active sites.² To further enhance the HER performance of TMDCs, many research has focused on the structure of TMDCs, defect engineering and heterojunction with graphene. However, this research will highlight the signficance of Pt and its better performance than those methods. To prove, this study will funtionalize TMDCs with the very small amount of noble metals, including Pt and Ru. Atomic Layer Deposition(ALD) will be a key technique to control the amount of noble metals and synthesize the uniform, conformal TMDCs on graphite foil.

Here we prepared catalyst using the noble metal functionalized MoS₂ synthesized on graphite foil by ALD. Various experimental methods were performed to analyze film growth characteristics including X-ray photoelectron spectroscopy (XPS) , Raman spectroscopy, Scanning Electron Microscope (SEM), X-ray diffraction and etc. The electrochemical properties of HER catalyst show high exchange current density and low Tafel slope compared with ALD MoS₂ film.

References

¹D.H Kim, J.G. Song, H.M. Yang, H.K. Lee, J.S. Park. H.J. Kim, Nanoscale, 2019, 11, 844

²Xiaxin Zou, Yu Zhang, Chem. Soc. Rev., 2015, 44, 5148