Electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) surfaces

Abstract

The electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) has been studied using voltammetry, chronoamperometry, and in situ infrared spectroscopy. The oxidative adsorption of ammonia results in the formation of NH(x) (x = 0-2) adsorbates. On Pt(111), ammonia oxidation occurs in the double-layer region and results in the formation of NH and, possibly, N adsorbates. The experimental current transients show a hyperbolic decay (t(-1)), which indicates strong lateral (repulsive) interactions between the (reacting) species. On Pt(100), the NH(2) adsorbed species is the stable intermediate of ammonia oxidation. Stabilization of the NH and NH(2) fragments on Pt(111) and Pt(100), respectively, is in an interesting agreement with recent theoretical predictions. The Pt(111) surface shows extremely low activity in ammonia oxidation to dinitrogen, thus indicating that neither NH nor N (strongly) adsorbed species are active in dinitrogen production. Neither nitrous oxide nor nitric oxide is the product of ammonia oxidation on Pt(111) at potentials up to 0.9 V, as deduced from the in situ infrared spectroscopy measurements. The Pt(100) surface is highly active in dinitrogen production. This process is characterized by a Tafel slope of 30 mV decade(-1), which is explained by a rate-determining dimerization of NH(2) fragments followed by a fast decay of the resulting surface-bound hydrazine to dinitrogen. Therefore, the high activity of the Pt(100) surface for ammonia oxidation to dinitrogen is likely to be related to its ability to stabilize the NH(2) adsorbate.

Shape-controlled synthesis of Pt-Ir nanocubes with preferential (100) orientation and their unusual enhanced electrocatalytic activities

“…Vidal-Iglesias et al [77] also found the formation of N 2 H 4 during the ammonia electro-oxidation process by SERS. However, Pt (111) surface gives a deeper dehydrogenation of NH 3,ads to strongly adsorbed NH and N fragments, which fails to form N 2 at an appreciable rate [74]. Density functional theory (DFT) calculations also support that the adsorption of the NH 2 fragment is more favorable on Pt (100) than on Pt (111) [78].…”

“…The partially dehydrogenated NH 2,ads species are active intermediates to give the final product of N 2 while the fully dehydrogenated products (i.e., N ads ) poison the Pt surface. It has been widely reported that the electrocatalytic activity of the Pt (100) sites for ammonia electro-oxidation is significantly high [21,22,24,38,44,74]. This surface-structure sensitivity has been well demonstrated on both model Pt single crystal surfaces and Pt NPs with (100) preferential orientation prepared by different methods [21,22,24,38,75].…”

“…This surface-structure sensitivity has been well demonstrated on both model Pt single crystal surfaces and Pt NPs with (100) preferential orientation prepared by different methods [21,22,24,38,75]. Through the combination of electrochemical characterizations and in situ infrared spectroscopy measurements, Rosca and Koper [74] pointed out that the difference in activity between Pt (111) and Pt (100) surfaces for ammonia electro-oxidation arises from their different ability to stabilize the adsorbed NH 2 intermediates. Pt (100) surface tends to stabilize the adsorbed NH 2 intermediate, favoring the formation of hydrazine (N 2 H 4 ) which is subsequently dehydrogenated quickly to N 2 [76].…”

Influence of the metal loading on the electrocatalytic activity of carbon-supported (100) Pt nanoparticles

“…7). This peak has been previously ascribed to the oxidative (reductive) adsorption (desorption) of ammonia on Pt (100) [69]. All these findings indicate that the resulting reactivity towards ammonia oxidation will depend of different factors including (i) active surface area, (ii) metal loading and (ii) thickness of the catalytic layer.…”

Ammonia Dehydrogenation over Platinum-Group Metal Surfaces. Structure, Stability, and Reactivity of Adsorbed NHx Species

“…The difference between Pt(100) and Pt(111) and the different stability of NH 2 compared with NH and N are in very good agreement with the recent results by Rosca and Koper. 35 These authors have related the relatively high electrocatalytic activity of Pt(100) for ammonia oxidation to dinitrogen to its ability to stabilize the NH 2 adsorbate. Figure 4 presents the geometry of initial, transition, and final states corresponding to the most stable situation on the 2 × 2 unit cell for all of the elemental dehydrogenation reactions.…”