Visualization of on-surface ethylene polymerization through ethylene insertion

Polyethylene production through catalytic ethylene polymerization is one of the most common processes in the chemical industry. The popular Cossee-Arlman mechanism hypothesizes that the ethylene be directly inserted into the metal-carbon bond during chain growth, which has been awaiting microscopic and spatiotemporal experimental confirmation. Here, we report an in situ visualization of ethylene polymerization by scanning tunneling microscopy on a carburized iron single-crystal surface. We observed that ethylene polymerization proceeds on a specific triangular iron site at the boundary between two carbide domains. Without an activator, an intermediate, attributed to surface-anchored ethylidene (CHCH₃), serves as the chain initiator (self-initiation), which subsequently grows by ethylene insertion. Our finding provides direct experimental evidence of the ethylene polymerization pathway at the molecular level.

Effect of ammonia on cobalt Fischer–Tropsch synthesis catalysts: a surface science approach

Undercoordinated (defect) sites on cobalt surfaces favor NH₃ decomposition and dehydrogenated products adsorb strongly around these defect sites.

Effect of Aldehyde and Carboxyl Functionalities on the Surface Chemistry of Biomass-Derived Molecules

The adsorption and decomposition of acetaldehyde and acetic acid were studied on Rh(100) to gain insight into the interaction of aldehyde and carboxyl groups of biomass-derived molecules with the surface. Temperature-programmed reaction spectroscopy (TPRS) was used to monitor gaseous reaction products, whereas Reflection absorption infrared spectroscopy (RAIRS) was used to determine the nature of surface intermediates and reaction paths. The role of adsorbate interactions in oxygenate decomposition chemistry was also investigated by varying the surface coverage. Acetaldehyde adsorbs in an η²(C, O) configuration for all coverages, where the carbonyl group binds to the surface via the C and O atoms. Decomposition occurs below room temperature (180-280 K) via C-H and C-C bond breaking, which releases CO, H, and CH_x species on the surface. At low coverage, CH_x dehydrogenation dominates and surface carbon is produced alongside H₂ and CO. At high coverage, about 60% of the CH_x hydrogenates to form methane, whereas only 40% of the CH_x decomposes further to surface carbon. Acetic acid adsorbs dissociatively on the Rh(100) surface via O-H bond scission, forming a mixture of mono- and bidentate acetate. The decomposition of acetate proceeds via two different pathways: (i) deoxygenation via C-O and C-C bond scissions and (ii) decarboxylation via C-C bond scission. At low coverage, the decarboxylation pathway dominates, a process that occurs at slightly above room temperature (280-360 K) and produces CO₂ and CH_x, where the latter decomposes further to surface carbon and H₂. At high coverage, both decarboxylation and deoxygenation occur, slightly, above room temperature (280-360 K). The resulting O adatoms produced in the deoxygenation path react with surface hydrogen or CO to form water and CO₂, respectively. The CH_x species dehydrogenate to surface carbon for all coverages. Our findings suggest that oxygenates with a C═O functionality and an alkyl end react on the Rh(100) surface to produce synthesis gas and small hydrocarbons whereas CO₂ and synthesis gas are produced when oxygenates with a COOH functionality and an alkyl end react with the Rh(100) surface. For both cases, carbon accumulation occurs on the surface.

Understanding FTS selectivity: the crucial role of surface hydrogen

Monomeric forms of carbon play a central role in the synthesis of long chain hydrocarbons via the Fischer-Tropsch synthesis (FTS). We explored the chemistry of C₁H_xad species on the close-packed surface of cobalt. Our findings on this simple model catalyst highlight the important role of surface hydrogen and vacant sites for product selectivity. We furthermore find that CO_ad affects hydrogen in multiple ways. It limits the adsorption capacity for H_ad, lowers its adsorption energy and inhibits dissociative H₂ adsorption. We discuss how these findings, extrapolated to pressures and temperatures used in applied FTS, can provide insights into the correlation between partial pressure of reactants and product selectivity. By combining the C₁H_x stability differences found in the present work with literature reports of the reactivity of C₁H_x species measured by steady state isotope transient kinetic analysis, we aim to shed light on the nature of the atomic carbon reservoir found in these studies.

NH3 adsorption and decomposition on Ir(110): A combined temperature programmed desorption and high resolution fast x-ray photoelectron spectroscopy study

The adsorption and decomposition of NH3 on Ir(110) has been studied in the temperature range from 80 K to 700 K. By using high-energy resolution x-ray photoelectron spectroscopy it is possible to distinguish chemically different surface species. At low temperature a NH3 multilayer, which desorbs at approximately 110 K, was observed. The second layer of NH3 molecules desorbs around 140 K, in a separate desorption peak. Chemisorbed NH3 desorbs in steps from the surface and several desorption peaks are observed between 200 and 400 K. A part of the NH3ad decomposes into NH(ad) between 225 and 300 K. NH(ad) decomposes into N(ad) between 400 K and 500 K and the hydrogen released in this process immediately desorbs. N2 desorption takes place between 500 and 700 K via N(ad) combination. The steady state decomposition reaction of NH3 starts at 500 K. The maximum reaction rate is observed between 540 K and 610 K. A model is presented to explain the occurrence of a maximum in the reaction rate. Hydrogenation of N(ad) below 400 K results in NH(ad). No NH2ad or NH3ad/NH3 were observed. The hydrogenation of NH(ad) only takes place above 400 K. On the basis of the experimental findings an energy scheme is presented to account for the observations.

The role of Oad in the decomposition of NH3 adsorbed on Ir(110): a combined TPD and high-energy resolution fast XPS study

High energy resolution fast XPS combined with TPD experiments were used to study the effect of chemisorbed oxygen on the adsorption and dissociation of NH(3) on Ir(110). Below 250 K the presence of O(ad) does not influence NH(3) decomposition. Above 250 K O(ad) enhances NH(3) dissociation, which results in three times as much N(2) formation and less molecular NH(3) desorption compared to the experiments without O(ad). The effect of O(ad) can be attributed to destabilization of NH(ad) on the surface, resulting in a further dehydrogenation towards N(ad). The presence of O(ad) on the surface lowers the temperature at which the N(ad) combination reaction takes place by as much as 200 K, due to repulsive interaction between N(ad) and O(ad). NO is formed above 450 K if both N(ad) and O(ad) are present on the surface.