Effect of Coadsorbed Water on Deep Oxidation Mechanisms: Temperature-Programmed Reactions of Benzene and Hydroxyl on the Pt(111) Surface

Abstract

The deep oxidation of benzene by preadsorbed hydroxyl on the Pt(111) surface has been characterized using temperatureprogrammed reaction spectroscopy. A mechanism for benzene oxidation in the presence of coadsorbed water has been developed based on these experiments. Reaction-limited carbon dioxide and water are formed over the temperature range 330-530 K, indicating oxydehydrogenation and skeletal oxidation are occurring over this temperature range. Oxidation of benzene has been compared on the oxygen-covered Pt(111) surface with and without coadsorbed water. With preadsorbed hydroxyl, the yield of water at low-temperatures is increased, and the yield of carbon dioxide is increased. The temperature ranges for carbon dioxide and water formation above 300 K are increased by 30 K when water is coadsorbed with atomic oxygen to form hydroxyl. These results clearly suggest that hydroxyl enhances oxydehydrogenation below 300 K, which results in the formation of different dehydrogenated intermediates with increased activation energy for oxidation during the reaction. Taken together these kinetic and mechanistic results provide a clear description of the reactivity of hydroxyl during benzene deep oxidation on the Pt(111) surface.

Surface Chemistry during Atomic Layer Deposition of Pt Studied with Vibrational Sum-Frequency Generation

“…They observed additional combustion pathways opening up at >530 K in the presence of OH(ads). 51 , 52 These oxidation reactions are likely to occur during ALD and would explain the formation of CO 2 and H 2 O during the initial phase of the precursor step.…”

“… 25 , 61 Furthermore, the oxidation reactions of CH 3 and cyclic hydrocarbons are poisoned by the presence of CO(ads) and OH surface groups; however, considering the scarcity of O(ads) at the end of the precursor half-cycle, oxidation reactions are most likely of less importance. 48 , 52 In general, the formation of a carbonaceous layer—mainly consisting of unsaturated hydrocarbons and also referred to as coke—is known to form when a Pt surface is exposed to higher hydrocarbons, which reduces the reactivity of the surface. 62 − 64 For ALD, a reduced reactivity of the surface due to the built-up of a carbonaceous layer is the most likely mechanism responsible for the self-limiting nature of the precursor half-cycle.…”

“…In ALD, this manifests itself as the production of CO 2 and H 2 O as gas-phase reaction products with ample reports on the detection of the gas-phase species but with few reports directly probing the underlying surface chemistry leading to the production of CO 2 and H 2 O. ,,, The surface chemistry of hydrocarbons on Pt surfaces (in the presence of O) has been studied extensively in the field of surface science. Combustion reactions, or more generally oxidation reactions, are known to occur on various O-rich Pt surfaces at ALD relevant temperatures for both Me groups and unsaturated cyclic hydrocarbons such as MeCp. − For example, Marsh et al studied the oxidation of benzene on a Pt(111) surface and demonstrated that above >300 K benzene is oxidized, forming CO 2 and H 2 O. They observed additional combustion pathways opening up at >530 K in the presence of OH­(ads). , These oxidation reactions are likely to occur during ALD and would explain the formation of CO 2 and H 2 O during the initial phase of the precursor step.…”

H2O/D2O Exchange on SnO2 Materials in the Presence of CO: Operando Spectroscopic and Electric Resistance Measurements

“…Two mechanisms seem to be reasonable to explain the consumption of bridging hydroxyls. The first assumes direct interaction of the target gas with surface OHs, which is accepted for various oxidation mechanisms on noble metals and oxide supported catalysts. ,− This interaction apparently occurs through formation of hydrocarboxyl radical, leading to formation of formic acid or CO 2 − as shown in Figure a. In the latter case, the hydrogen atom has been proposed to recombine with the neighboring OH group and desorb as water molecule. − Another mechanism, shown in Figure b, can be speculated assuming a competition between water and CO molecules for the same surface oxygen species. , …”

NO−C2H4 Reactions on the Surface of Stepped Pt(332)

“…The adsorption and reaction of individual hydrocarbon species such as ethylene, benzene, acetylene, etc. has been more extensively studied. − …”