Oxidation of Ethylene on Oxygen Reconstructed Silver Surfaces

Abstract

We report on theoretical and experimental studies of the reactivity of ethylene with oxygen in two well-known oxygen induced surface reconstructions on silver, the p(2x1) reconstruction on the Ag(110) and the p(4x4) reconstruction on the Ag(111) surfaces. Density functional theory calculations demonstrate that ethylene can react with oxygen on both surfaces to form an oxametallacycle that can decompose into either ethylene oxide or a CO 2 precursor, acetaldehyde.The activation energy associated with acetaldehyde formation is predicted to be 0.4 eV lower than that associated with epoxide formation on both surfaces, though we find lower barriers for all elementary steps on the p(4x4) reconstruction due to its unique structural dynamics. Our calculations predict these dynamics make the p(4x4) reconstruction active in acetaldehyde formation at room temperature. Experiments performed by exposing the p(4x4) reconstruction to ethylene at room temperature support this finding with CO 2 the only carbonaceous product formed during temperature programed desorption. Our results unambiguously demonstrate that, alone, these oxygen reconstructions are not selective in ethylene epoxidation on silver.

Are multiple oxygen species selective in ethylene epoxidation on silver?

“…The generally accepted idea that reconstructed atomic oxygen, assigned to nucleophilic oxygen, participates in the complete oxidation of ethylene to CO 2 and that unreconstructed adsorbed atomic oxygen, often assigned to electrophilic oxygen, can participate in ethylene oxide (EO) production, motivated a series of theoretical investigations on the reaction mechanism for ethylene epoxidation. 17 , 31 – 33 On unpromoted silver, experimental evidence 10 , 14 , 27 shows that the reaction of ethylene with O-reconstructions favors total combustion giving CO 2 as a product, which is supported by theory. 10 , 32 For the reconstructions, DFT calculations show that the reaction of ethylene and atomic O has a low barrier towards acetaldehyde (AcH) formation, 10 which rapidly burns.…”

“…5 – 7 The former, an electron rich oxygen, has been proposed to activate C–H bond breaking 8 , 9 and has been shown to participate in the complete oxidation of ethylene. 10 The latter, an electron deficient oxygen, has been proposed to open the C C double bond of ethylene, forming the COC ring through O insertion, 9 , 11 , 12 and has been shown to participate in epoxidation. 13 Yet, while the atomic structure of nucleophilic oxygen is known, 10 , 14 , 15 the atomic structure of the active species for ethylene epoxidation is debated.…”

“… 10 The latter, an electron deficient oxygen, has been proposed to open the C C double bond of ethylene, forming the COC ring through O insertion, 9 , 11 , 12 and has been shown to participate in epoxidation. 13 Yet, while the atomic structure of nucleophilic oxygen is known, 10 , 14 , 15 the atomic structure of the active species for ethylene epoxidation is debated. 4 , 6 , 15 – 17 This electrophilic species is thought to be weakly bound oxygen and, following early assignments, 18 , 19 has been extensively interpreted as being unreconstructed adsorbed or dissolved atomic O.…”

Deciphering complex features in STM images of O adatoms on Ag(110)

“…The importance of silver in influencing these reactions has inspired researchers to put significant efforts in gaining insights into the chemical activity of surface structures of O/Ag systems. For example, the active O species responsible for ethylene oxide formation on Ag(111) [9,10] were recently identified, but the mitigating processes are still far from being fully understood [11,12]. One obstacle in the advancement of knowledge about the reactivity of the Ag catalyst in an oxidative environment is the complexity of the O-Ag system: the intricacy of the structures often involves the adsorption configurations of O species, inducing substrate reconstruction [13], oxygen migration to the Ag subsurface region [10,14], and formation of surface oxide phases [15].…”

Chemical Behaviour of CaAg2 under Ethylene Epoxidation Conditions

“…Still debated in detail, there are two fundamentally different approaches for the discussion of the ethylene epoxidation reaction: (i) the consideration of different types of oxygen species and their roles in the ethylene epoxidation;, , and (ii) the “oxametallacycle” model,, which largely relies on suitable oxygen species from model (i). Nowadays, intense research is focused on the nature of electrophilic oxygen species, theoretical predictions and experimental proofs of reaction intermediates (e.g., oxametallacycle OMC), optimization of catalyst particle size, the development of suitable supports and the search for new synthesis routes for catalysts …”