Alhowity Samar, Balogun Kabirat, Ganesan Ashwin, Lund Colton J, Omolere Olatomide, Adesope Qasim, Chukwunenye Precious, Amagbor Stella C, Anwar Fatima, Altafi M K, D'Souza Francis, Cundari Thomas R, Kelber Jeffry A
Department of Chemistry, University of North Texas, 1155 Union Circle, No. 305070, Denton, Texas 76203-5017, United States.
ACS Appl Mater Interfaces. 2024 Jan 17;16(2):2180-2192. doi: 10.1021/acsami.3c11683. Epub 2024 Jan 4.
Significant interest in the electrocatalytic reduction of molecular nitrogen to ammonia (the nitrogen reduction reaction: NRR) has focused attention on transition metal carbides as possible electrocatalysts. However, a fundamental understanding of carbide surface structure/NRR reactivity relationships is sparse. Herein, electrochemistry, DFT-based calculations, and photoemission studies demonstrate that NbC, deposited by magnetron sputter deposition, is active for NRR at pH 3.2 but only after immersion of an ambient-induced NbO surface layer in 0.3 M NaOH, which leaves Nb suboxides with niobium in intermediate formal oxidation states. Photoemission data, however, show that polarization to -1.3 V vs Ag/AgCl restores the NbO overlayer, correlating with electrochemical measurements showing inhibition of NRR activity under these conditions. In contrast, a similar treatment of a sputter-deposited TaC sample in 0.3 M NaOH fails to reduce the ambient-induced TaO surface layer, and TaC is inactive for NRR at potentials more positive than -1.0 V even though a significant cathodic current is observed. A TaC sample with surface oxide partially reduced by Ar ion sputtering in UHV prior to transfer to UHV exhibits a restored TaO surface layer after electrochemical polarization to -1.0 V vs Ag/AgCl. The electrochemical and photoemission results are in accord with DFT-based calculations indicating greater N≡N bond activation for N bound end-on to Nb(IV) and Nb(III) sites than for N bound end-on to Nb(V) sites. Thus, theory and experiment demonstrate that with respect to NbC, the formation and stabilization of intermediate (non-d) oxidation states for surface transition metal ions is critical for N≡N bond activation and NRR activity. Additionally, the Nb suboxide surface, formed by immersion in 0.3 M NaOH of ambient-exposed NbC, is shown to undergo reoxidation to catalytically inactive NbO at -1.3 V vs Ag/AgCl, possibly due to hydrolysis or other, as yet not understood, phenomena.
对分子氮电催化还原为氨（氮还原反应：NRR）的极大兴趣使人们将注意力集中在过渡金属碳化物作为可能的电催化剂上。然而，对碳化物表面结构与NRR反应活性之间关系的基本理解却很少。在此，电化学、基于密度泛函理论（DFT）的计算和光电子能谱研究表明，通过磁控溅射沉积的NbC在pH 3.2时对NRR具有活性，但只有在将环境诱导的NbO表面层浸入0.3 M NaOH中后才具有活性，这会留下具有中间形式氧化态铌的低价铌氧化物。然而，光电子能谱数据表明，相对于Ag/AgCl极化至-1.3 V会恢复NbO覆盖层，这与电化学测量结果相关，即在这些条件下NRR活性受到抑制。相比之下，在0.3 M NaOH中对溅射沉积的TaC样品进行类似处理未能还原环境诱导的TaO表面层，并且即使观察到明显的阴极电流，TaC在电位高于-1.0 V时对NRR也无活性。在超高真空（UHV）中通过Ar离子溅射部分还原表面氧化物的TaC样品在转移至UHV后，在相对于Ag/AgCl电化学极化至-1.0 V后会出现恢复的TaO表面层。电化学和光电子能谱结果与基于DFT的计算结果一致，表明与N以端对端方式结合到Nb(V)位点相比，N以端对端方式结合到Nb(IV)和Nb(III)位点时，N≡N键的活化程度更高。因此，理论和实验表明，对于NbC而言，表面过渡金属离子中间（非d）氧化态的形成和稳定对于N≡N键的活化和NRR活性至关重要。此外，通过将暴露于环境的NbC浸入0.3 M NaOH中形成的低价铌氧化物表面在相对于Ag/AgCl为-1.3 V时会重新氧化为催化无活性的NbO，这可能是由于水解或其他尚未理解的现象所致。
