Electrocatalytic oxidation of ammonia is an appealing, lowtemperature process for the sustainable production of nitrites and nitrates that avoids the formation of pernicious N 2 O and can be fully powered by renewable electricity. Currently, however, the number of known efficient catalysts for such a reaction is limited. The present work demonstrates that copperbased electrodes exhibit high electrocatalytic activity and selectivity for the NH 3 oxidation to NO 2 À and NO 3 À in alkaline solutions. Systematic investigation of the effects of pH and potential on the kinetics of the reaction using voltammetric analysis andin situ Raman spectroscopy suggest that ammonia electrooxidation on copper occurrs via two primary catalytic mechanisms. In the first pathway, NH 3 is converted to NO 2 À via a homogeneous electrocatalytic process mediated by redox transformations of aqueous [Cu(OH) 4 ] À /2À species, which dissolve from the electrode. The second pathway is the heterogeneous catalytic oxidation of NH 3 on the electrode surface favoring the formation of NO 3 À . By virtue of its nature, the homogeneous-mediated pathway enables higher selectivity and was less affected by electrode poisoning with the strongly adsorbed "N" intermediates that have plagued the electrocatalytic ammonia oxidation field. Thus, the selectivity of the Cu-catalyzed NH 3 oxidation towards either nitrite or nitrate can be achieved through balancing the kinetics of the two mechanisms by adjusting the pH of the electrolyte medium and potential.
Highly appreciated concert halls have their own acoustic signature. These signatures may not often be consciously appraised by general audiences, but they have a significant impact on the appreciation of the hall. Previous research indicates that two of the most important defining elements of a hall’s acoustic signature are (i) the reflection sequence and relative reflection levels at the listener position and (ii) the perceptibility of the reflections based on perception thresholds. Early research from Sir Harold Marshall identified the importance of unmasked early reflections to enhance a concert hall’s acoustic signature. The authors see an opportunity to extend the existing research by further examining the sequence of unmasked reflections. By analysing the cross-sections of three concert halls, this manuscript quantifies potential links between a hall’s architectural form, the resultant skeletal reflections, and the properties of its acoustic signature. While doing so, the manuscript identifies potential masking reflections through visual and analytical assessment of a hall’s skeletal reflections. It is hypothesized that the “rhythm” of the reflection sequence could hold key insights into the hall’s “personality” and acoustic signature. If so, this could present new design tools and considerations for new concert halls and the diagnosis of underperformance in existing halls.
The electrochemical oxidation of ammonia has been studied in detail due to its inherent utility as a fuel cell reactant 1 for the remediation of wastewater systems 2 for sensors 3 and as a hydrogen carrier.4 However, the bulk of the research conducted has been focused on the use of noble metal-based catalysts such as Pt, Pd, Rh and Ir due to their inherent activity towards the ammonia oxidation reaction (AOR).5,6 Electrochemically, the ammonia oxidation reaction on platinum is a complex multistep reaction which tends to be sluggish and suffers from poor faradaic efficiency, with a variety of products being formed. Another notable feature of noble based systems is the tendency to become poisoned, by strongly adsorbed ‘N’ species.5 These have been shown to build up over time resulting in electrode de-activation. In nature, transition metals are found at the core of AOR enzymes such as ammonia monooxygenase which suggests that it is possible to perform this reaction with non-noble catalysts.7 With the issues surrounding noble-metal catalysts and following nature as an example, we have begun investigating the oxidation of ammonia with transition-metal based catalysts. To this end our investigations have revealed that it is possible to conduct electrochemical ammonia oxidation to NOx species with faradaic efficiencies of up to 100%, at appreciable rates with no evidence of catalytic poisoning (see fig. 1). This work represents a step forward in our understanding of the electrochemical AOR reaction and may serve as a foundation for future work in this field. 1. M. H. M. T. Assumpção et al., Int. J. Hydrogen Energy, 39, 5148–5152 (2014). 2. L. Marinčić and F. B. Leitz, J. Appl. Electrochem., 8, 333–345 (1978). 3. A. Galdikas et al., Sensors Actuators B Chem., 67, 76–83 (2000). 4. F. Vitse, M. Cooper, and G. G. Botte, J. Power Sources, 142, 18–26 (2005). 5. Z.-F. F. Li, Y. Wang, and G. G. Botte, Electrochim. Acta, 228, 351–360 (2017). 6. S. Johnston, B. H. R. Suryanto, and D. R. MacFarlane, Electrochim. Acta, 297, 778–783 (2019). 7. J. A. Zahn, D. M. Arciero, A. B. Hooper, and A. A. DiSpirito, FEBS Lett., 397, 35–38 (1996). Fig. 1. (A) Chronoamperometry conducted in 1 M KOH + 0.1 M NH3 over a time of 48 hours and (B) associated yields of NOx. Figure 1
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