Ammonia Adsorption and Decomposition on Co(0001) in Relation to Fischer–Tropsch Synthesis

Abstract

In order to fundamentally understand cobalt catalyst deactivation in Fischer–Tropsch synthesis (FTS) due to parts per million levels of NH3 in the synthesis gas, the adsorption and decomposition of NH3 on Co(0001) are investigated experimentally under ultrahigh vacuum (UHV) conditions and theoretically using density functional theory (DFT) calculations. NH3 desorbs intact from the surface, between 100 and 270 K. In agreement with this, DFT calculations show that the activation barrier for NH3 decomposition, 105 kJ/mol, is higher than the adsorption energy of NH3, 59 kJ/mol. Neither COad nor Had block the adsorption of NH3. Instead, CO and NH3 form a stable coadsorbed layer. Preadsorbed ammonia negatively affects dissociative H2 adsorption. Electron-induced dissociation produces NH x species on the surface at low temperature. The order of stability is NH­(+2 Had) > N­(+3 Had) > NH2(+ Had) > NH3. N and NH lower the quantity of CO that can be accommodated on the surface but do not affect the adsorption energy significantly. For FTS, we conclude that (i) NH3 adsorption on cobalt is not inhibited by the other FTS reactants and thus parts per million levels of NH3 can already be detrimental, (ii) due to their high stability, NH x species are most likely responsible for catalyst deactivation.

Atomic Layer Deposition of Cobalt Using H2-, N2-, and NH3-Based Plasmas: On the Role of the Co-reactant

“…For instance, Kizilkaya et al investigated the stability of NH x groups on Co after NH 3 exposure and found that NH 3 adsorption on Co is followed by decomposition, resulting in NH x groups remaining on the surface. 67 In addition, Wang et al studied the nitridation of transition-metal surfaces and calculated that Co is not prone to nitridation, although a N-covered surface is stable after formation. 68 On the basis of these reports, it can be expected that NH x groups are present on the Co film after NH 3 or H 2 /N 2 plasma exposure.…”

Decomposition of N2O at low temperature over Co3O4 prepared by different methods

“…Instead, a small amount of Na is responsible for desorption of surface oxygen during the N 2 O decomposition process. Basically due to the presence of alkali, weakens the CoO bond which is responsible for accelerated O 2 desorption from catalyst surface leading to high N 2 O decomposition . In addition, during experiments, it was observed that at higher pH value greater than 10, no precipitates were observed indicating that the formation of some by‐product Co(OH) 2 (insoluble in dilute alkali but soluble in water and strong alkali) rather than formation of CoCO 3 (not soluble in water).…”

Toward a More Rational Design of the Direct Synthesis of Aniline: A Density Functional Theory Study

“…A recent experimental study of the decomposition of ammonia on the closed-packed surface Co(0001), which is anticipated to carry reactivity similar to that of the Ni(111) surface, has found that imide is the most stable species among the NH x species and can be selectively produced using the electron-induced dissociation technique at low temperature. 65 Gibbs free energies of the decomposition reactions of ammonia on the Ni(111) surface at 523 K predicted that these reactions are exergonic (except the dehydrogenation of imide) (see Table 10), with the imide–Ni(111) being the most favorable decomposition reaction of ammonia at this temperature. The differences between Δ H and ΔG are small as shown from the results in Table 10, indicating small contributions from the entropies.…”