CO as a Promoting Spectator Species of C_xH_y Conversions Relevant for Fischer–Tropsch Chain Growth on Cobalt: Evidence from Temperature-Programmed Reaction and Reflection Absorption Infrared Spectroscopy

Abstract

Cobalt-catalyzed low temperature Fischer–Tropsch synthesis is a prime example of an industrially relevant reaction in which C x H y intermediates involved in chain growth react in the presence of a large quantity of COad. In this study, we use a Co(0001) single-crystal model catalyst to investigate how CO, adsorbed alongside C x H y adsorbates affects their reactivity. Temperature-programmed reaction spectroscopy was used to determine the hydrogen content of the C x H y intermediates formed at different temperatures, and infrared absorption spectroscopy was used to obtain more specific information on the chemical identity of the various reaction intermediates formed. Ethene, propene, and but-1-ene precursors decompose below 200 K. The 1-alkyne adsorbate is identified as a major product, and some alkylidyne species form as well when the initial alkene coverage is high. The surface hydrogen atoms produced in the low temperature decomposition step start leaving the surface >300 K. When an alkyne/Had-covered surface is heated in the presence of CO, the alkyne adsorbates are hydrogenated to the corresponding alkylidyne at temperatures <250 K. This finding shows that C x H y surface species react differently in the presence of COad, a notion of general importance for catalytic reactions where both CO and C x H y species are present. In the context of Fischer–Tropsch synthesis, the observed CO-induced reaction is of specific importance for the alkylidyne chain growth mechanism. In this reaction, scheme hydrocarbon chains grow via coupling of CHad with a (C n ) alkylidyne adsorbate to produce the (C n+1) alkyne. A subsequent hydrogenation of the alkyne product to the corresponding alkylidyne is required for further growth. The present work shows that this specific reaction is promoted by the presence of CO. This suggests that the influence of CO spectators on the stability of C x H y surface intermediates is beneficial for efficient chain growth.

Mechanistic insight into carbon-carbon bond formation on cobalt under simulated Fischer-Tropsch synthesis conditions

“…CO adsorbed alongside small C x H y adsorbates attenuates absorption bands in the C-H (~2800-3000 cm −1 ) and C-C (1000-1200 cm −1 ) stretching regions, a phenomenon that has been observed before for ethylidyne adsorbed alongside CO on, e.g. Ru(0001) and Pt(111) [32][33][34] . This leaves only the C-H bending region (1500-1300 cm −1 ) as our main source of information.…”

“…Acetylene is the most stable C 2 H x adsorbate in the absence of CO 21,[38][39][40][41] , and it is therefore the sole product of ethene decomposition around 180 K. The experiments show that CO causes conversion of acetylene to ethylidyne, and the driving force for this must be that ethylidyne becomes more stable than acetylene when CO is present on the surface 32,40 . Two experimental observations give more insight into why ethylidyne is preferred when CO is present: (i) repulsive interactions between H ad and CO ad destabilize H ad (Fig.…”

“…2b. The peak around 270 K is attributed to desorption of surface-bound hydrogen, shifted downward due to the presence of CO [30][31][32] . The peak area indicates that 0.12 ML H ad desorbs around 270 K, only 50% of the 0.24 ML H ad that was present prior to CO exposure.…”

“…sputtering (1 kV Ar + ) at 650 K followed by~10 min annealing at the same temperature 21,32 . Residual carbon was most efficiently removed by dosing~1 × 10 −7 mbar O 2 at 650 K for a few minutes.…”

Room-Temperature Propylene Dehydrogenation and Linear Atomic Chain Formation on Ni(111)

“…The adsorption of lower alkanes (methane, ethane, propane) and alkenes (ethylene, propylene) on catalytically active metals (Ni, Pt, Ir, Co etc.) has been studied for a long time at various temperatures [13][14][15][16][17][18][19][20][21] . The current consen-sus is that at low temperatures (90-100 K), hydrocarbons are adsorbed molecularly; at higher temperatures (200-250 K) the hydrocarbon partially dehydrogenates; alkelidynes are formed (the methyl group either remains intact or is formed).…”