Understanding FTS selectivity:the crucial role of surface hydrogen

\u3cp\u3eMonomeric 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 C\u3csub\u3e1\u3c/sub\u3eH\u3csub\u3exad\u3c/sub\u3e 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\u3csub\u3ead\u3c/sub\u3e affects hydrogen in multiple ways. It limits the adsorption capacity for H\u3csub\u3ead\u3c/sub\u3e, lowers its adsorption energy and inhibits dissociative H\u3csub\u3e2\u3c/sub\u3e 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 C\u3csub\u3e1\u3c/sub\u3eH\u3csub\u3ex\u3c/sub\u3e stability differences found in the present work with literature reports of the reactivity of C\u3csub\u3e1\u3c/sub\u3eH\u3csub\u3ex\u3c/sub\u3e 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.\u3c/p\u3

CO as a promoting spectator species of C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e conversions relevant for Fischer-Tropsch chain growth on cobalt:evidence from temperature-programmed reaction and reflection absorption infrared spectroscopy

\u3cp\u3eCobalt-catalyzed low temperature Fischer-Tropsch synthesis is a prime example of an industrially relevant reaction in which C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e intermediates involved in chain growth react in the presence of a large quantity of CO\u3csub\u3ead\u3c/sub\u3e. In this study, we use a Co(0001) single-crystal model catalyst to investigate how CO, adsorbed alongside C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e adsorbates affects their reactivity. Temperature-programmed reaction spectroscopy was used to determine the hydrogen content of the C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e 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/H\u3csub\u3ead\u3c/sub\u3e-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\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e surface species react differently in the presence of CO\u3csub\u3ead\u3c/sub\u3e, a notion of general importance for catalytic reactions where both CO and C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e 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 CH\u3csub\u3ead\u3c/sub\u3e with a (C\u3csub\u3en\u3c/sub\u3e) alkylidyne adsorbate to produce the (C\u3csub\u3en+1\u3c/sub\u3e) 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\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e surface intermediates is beneficial for efficient chain growth.\u3c/p\u3

Modeling the surface chemistry of biomass model compounds on oxygen-covered Rh(100)

\u3cp\u3eRhodium-based catalysts are potential candidates to process biomass and serve as a representation of the class of noble metal catalysts for biomass-related processes. Biomass can be processed in aqueous media (hydrolysis and aqueous phase reforming), and in this case the surface chemistry involves hydroxyl (OH) species. In our study this was modelled by the presence of pre-adsorbed oxygen. Ethylene glycol, with a hydroxyl group on every carbon atom, serves as a model compound to understand the conversion of biomass derived molecules into desirable chemicals on catalytically active metal surfaces. Ethanol (containing one OH group) serves as a reference molecule for ethylene glycol (containing two OH groups) to understand the interaction of C-OH functionalities with a Rh(100) surface. The surface chemistry of ethylene glycol and ethanol in the presence of pre-adsorbed oxygen on a Rh(100) surface has been studied via temperature programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) using various coverages of O(ad) and ethylene glycol and ethanol. Pre-adsorbed oxygen alters the decomposition chemistry of both compounds, thereby affecting the product distribution. Under an oxygen-lean condition, the selectivity to produce methane from ethanol is enhanced significantly (4.5-fold with respect to that obtained on the oxygen-free surface). For ethylene glycol, oxygen-lean conditions promote the formation of formaldehyde, with 10-15% selectivity. In addition, with O\u3csub\u3ead\u3c/sub\u3e present the fraction of molecules that decompose on the surface increases 2-fold for ethanol and 1.5-fold for ethylene glycol, due to fast O-H bond activation by pre-adsorbed oxygen. Under oxygen-rich conditions, the decomposition products are mainly oxidized to carbon dioxide and water for both molecules. In this condition, the promotion effect provided by adsorbed oxygen for the dissociative adsorption of ethanol and ethylene glycol is reduced due to the site blocking effect of oxygen.\u3c/p\u3

Spectroscopic insights into cobalt-catalyzed Fischer-Tropsch synthesis:a review of the carbon monoxide interaction with single crystalline surfaces of cobalt

\u3cp\u3eThe present article summarizes experimental findings of the interaction of CO with single crystal surfaces of cobalt. We first provide a quantitative study of non-dissociative CO adsorption on Co(0001) and establish a quantitative correlation between θ\u3csub\u3eCO\u3c/sub\u3e and adsorption site occupation. In light of these findings we revisit the structure of previously reported ordered CO/Co(0001) adsorbate layers. Measurements of the CO coverage at equilibrium conditions are used to derive a phase diagram for CO on Co(0001). For low temperature Fischer-Tropsch synthesis conditions the CO coverage is predicted to be ≈0.5 ML, a value that hardly changes with p\u3csub\u3eCO\u3c/sub\u3e. The CO desorption temperature found in temperature programmed desorption is practically structure-independent, despite structure-dependent heats of adsorption reported in the literature. This mismatch is attributed to a structure-dependent pre-exponential factor for desorption. IR spectra reported throughout this study provide a reference point for IR studies on cobalt catalysts. Results for CO adsorbed on flat and defect-rich Co surfaces as well as particular, CO adsorbed on top sites, and in addition affect the distribution of CO\u3csub\u3ead\u3c/sub\u3e over the various possible adsorption sites.\u3c/p\u3

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

\u3cp\u3eAmmonia adsorption and decomposition on defect-rich hcp-Co(0001) surfaces were investigated under ultra-high vacuum conditions in order to provide a fundamental explanation for industrially observed ammonia poisoning of cobalt based Fischer-Tropsch synthesis (FTS) catalysts. Temperature-programmed desorption, infrared spectroscopy and work function measurements indicate that undercoordinated sites bind ammonia stronger than sites on flat Co(0001), and they also induce its dehydrogenation. Density functional theory calculations were employed to explore the reactivity of defective Co surfaces using the fcc-Co(211) as a model. The results indicate that the decomposition products (NH\u3csub\u3ex\u3c/sub\u3e) adsorb strongly on or around the step site on fcc-Co(211). We find that NH (+2H\u3csub\u3ead\u3c/sub\u3e), adsorbed in the threefold site on the upper terrace, is equally stable as NH\u3csub\u3e2\u3c/sub\u3e (+H\u3csub\u3ead\u3c/sub\u3e), adsorbed in the bridge position at the step edge, both being significantly more stable than the equivalent species adsorbed on the flat Co(0001). The calculated activation barriers for NH\u3csub\u3e3,ad\u3c/sub\u3e dehydrogenation steps are in reasonable agreement with the barriers obtained by fitting experimental data. Based on these fundamental insights, poisoning of cobalt nanoparticles during FTS by NH\u3csub\u3e3\u3c/sub\u3e contaminants can be linked mainly to the blocking of undercoordinated sites by strongly adsorbed NH\u3csub\u3e2\u3c/sub\u3e species.\u3c/p\u3

Hydrogen adsorption on co surfaces: a density functional theory and temperature programmed desorption study

Density functional theory (DFT) calculations and temperature programmed desorption (TPD) experiments were performed to study the adsorption of hydrogen on the Co(111) and Co(100) surfaces. On the Co(111) surface, hydrogen adsorption is coverage dependent and the calculated adsorption energies are very similar to those on the Co(0001) surface. The experimental adsorption saturation coverage on the Co(111)/(0001) surface is ¿max ˜ 0.5 ML, although DFT predicts ¿max ˜ 1.0 ML. DFT calculations indicate that preadsorbed hydrogen will kinetically impede the adsorption process as the coverage approaches ¿ = 0.5 ML, giving rise to this difference. Adsorption on Co(100) is coverage independent up to ¿ = 1.00 ML, contrasting observations on the Ni(100) surface. Hydrogen atoms have low barriers of diffusion on both the Co(111) and Co(100) surfaces. A microkinetic analysis of desorption, simulating the expected TPD experiments, indicated that on the Co(111) surface two TPD peaks are expected, while on the Co(100) only one peak is expected. Low coverage adsorption energies of between 0.97 and 1.1 eV are obtained from the TPD experiment on a smooth single crystal of Co(0001), in line with the DFT results. Defects play a important role in the adsorption process. Further calculations on the Co(211) and Co(221) surfaces have been performed to model the effects of step and defect sites, indicating that steps and defects will expose a broad range of adsorption sites with varying (mostly less favorable) adsorption energies. The effect of defects has been studied by TPD by sputtering of the Co crystal surface. Defects accelerate the adsorption of hydrogen by providing alternative, almost barrierless pathways, making it possible to increase the coverage on the Co(111)/(0001) surface to above ¿ = 0.50 ML. The presence of defects at a high concentration will give rise to adsorption sites with much lower desorption activation energies, resulting in broad low temperature TPD features

Role of ZnO and CeO\u3csub\u3ex\u3c/sub\u3e in Cu-Based model catalysts in activation of h\u3csub\u3e2\u3c/sub\u3eO and CO\u3csub\u3e2\u3c/sub\u3e dynamics studied by in situ ultraviolet−Visible and x‑ray photoelectron spectroscopy

\u3cp\u3eFlat model and powder Cu, ZnO/Cu, and CeO\u3csub\u3ex\u3c/sub\u3e/Cu catalysts were studied by focusing on the role of the oxide phase as a promoter in the water gas shift (WGS) and its reverse reaction (RWGS). Activity measurements of the powder catalysts showed that both oxides enhance Cu reactivity, with CeO\u3csub\u3ex\u3c/sub\u3e/Cu being more active than ZnO/Cu in the WGS reaction. In situ ultraviolet−visible spectroscopy, exploiting the localized surface plasmon resonances of metallic Cu nanoparticles, together with X-ray photoelectron spectroscopy was then used to elucidate the origin of the enhanced reactivity on flat model catalysts. These experiments showed that ZnO and CeO\u3csub\u3ex\u3c/sub\u3e promote H\u3csub\u3e2\u3c/sub\u3eO and CO\u3csub\u3e2\u3c/sub\u3e dissociation, leading to oxidation of the Cu nanoparticles. CeO\u3csub\u3ex\u3c/sub\u3e performs better in this respect than ZnO. This is important because the reactivity in the WGS and RWGS reactions is related to the ability to activate H\u3csub\u3e2\u3c/sub\u3eO and CO\u3csub\u3e2\u3c/sub\u3e. The Ce\u3csup\u3e3+\u3c/sup\u3e ions are identified as the most efficient sites for H\u3csub\u3e2\u3c/sub\u3eO and CO\u3csub\u3e2\u3c/sub\u3e dissociation, while Cu\u3csup\u3e0\u3c/sup\u3e keeps Ce\u3csup\u3e3+\u3c/sup\u3e stable by promoting reduction of Ce\u3csup\u3e4+\u3c/sup\u3e during the dissociation process. In this sense, the CeO\u3csub\u3ex\u3c/sub\u3e/Cu catalyst forms a bifunctional catalyst, which is more active in the (R)WGS than CeO\u3csub\u3ex\u3c/sub\u3e and Cu catalysts separately.\u3c/p\u3

The effect of C-OH functionality on the surface chemistry of biomass-derived molecules:Ethanol chemistry on Rh(100)

\u3cp\u3eThe adsorption and decomposition of ethanol on Rh(100) was studied as a model reaction to understand the role of C-OH functionalities in the surface chemistry of biomass-derived molecules. A combination of experimental surface science and computational techniques was used: (i) temperature programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS), work function measurements (Kelvin Probe-KP), and density functional theory (DFT). Ethanol produces ethoxy (CH\u3csub\u3e3\u3c/sub\u3eCH\u3csub\u3e2\u3c/sub\u3eO) species via O-H bond breaking upon adsorption at 100 K. Ethoxy decomposition proceeds differently depending on the surface coverage. At low coverage, the decomposition of ethoxy species occurs via β-C-H cleavage, which leads to an oxometallacycle (OMC) intermediate. Decomposition of the OMC scissions (at 180-320 K) ultimately produces CO, H\u3csub\u3e2\u3c/sub\u3e and surface carbon. At high coverage, along with the pathway observed in the low coverage case, a second pathway occurs around 140-200 K, which produces an acetaldehyde intermediate via α-C-H cleavage. Further decomposition of acetaldehyde produces CH\u3csub\u3e4\u3c/sub\u3e, CO, H\u3csub\u3e2\u3c/sub\u3e and surface carbon. However, even at high coverage this is a minor pathway, and methane selectivity is 10% at saturation coverage. The results suggests that biomass-derived oxygenates, which contain an alkyl group, react on the Rh(100) surface to produce synthesis gas (CO and H\u3csub\u3e2\u3c/sub\u3e), surface carbon and small hydrocarbons due to the high dehydrogenation and C-C bond scission activity of Rh(100).\u3c/p\u3

Adsorption and decomposition of ethene and propene on Co(0001):the surface chemistry of fischer-tropsch chain growth intermediates

\u3cp\u3e(Graph Presented) Experiments that provide insight into the elementary reaction steps of C\u3csub\u3ex\u3c/sub\u3eH\u3csub\u3ey\u3c/sub\u3e adsorbates are of crucial importance to better understand the chemistry of chain growth in Fischer-Tropsch synthesis (FTS). In the present study we use a combination of experimental and theoretical tools to explore the reactivity of C\u3csub\u3e2\u3c/sub\u3eH\u3csub\u3ex\u3c/sub\u3e and C\u3csub\u3e3\u3c/sub\u3eH\u3csub\u3ex\u3c/sub\u3e adsorbates derived from ethene and propene on the close-packed surface of cobalt. Adsorption studies show that both alkenes adsorb with a high sticking coefficient. Surface hydrogen does not affect the sticking coefficient but reduces the adsorption capacity of both ethene and propene by 50% and suppresses decomposition. On the other hand, even subsaturation quantities of CO\u3csub\u3ead\u3c/sub\u3e strongly suppress alkene adsorption. Partial alkene dehydrogenation occurs at low surface temperature and predominantly yields acetylene and propyne. Ethylidyne and propylidyne can be formed as well, but only when the adsorbate coverage is high. Translated to FTS, the stable, hydrogen-lean adsorbates such as alkynes and alkylidynes will have long residence times on the surface and are therefore feasible intermediates for chain growth. The comparatively lower desorption barrier for propene relative to ethene can to a large extent be attributed to the higher stability of the molecule in the gas phase, where hyperconjugation of the double bond with σ bonds in the adjacent methyl group provides additional stability to propene. The higher desorption barrier for ethene can potentially contribute to the anomalously low C\u3csub\u3e2\u3c/sub\u3eH\u3csub\u3ex\u3c/sub\u3e production rate that is typically observed in cobalt-catalyzed FTS.\u3c/p\u3

Ammonia oxidation on Ir(111): why Ir is more selective to N2 than Pt

NH3 does not dissociate on a clean Ir(1 1 1) surface, but dissociation can be induced by radiation, which yields all possible NHxad species. NHad is the most stable and it is the only intermediate found at room temperature. NHad decomposes between 350 and 500 K, yielding NH3 (g) and Nad, which desorbs as N2 between 550 and 700 K. Adsorbed oxygen atoms induce NH3 dissociation between 300 and 400 K, forming NHad and H2O. NHad decomposes further between 350 and 450 K, forming Nad and H2O. Measurements of the surface composition during ammonia oxidation showed a mismatch between the change of the surface coverage (from Nad to Oad-dominated) and that in the gas phase (from N2 to NO). This is explained by a higher barrier for NO (g) formation as compared to N2 formation on Ir(1 1 1). On Pt(1 1 1) the difference in barrier height for N2 vs. NO formation is smaller, which explains why Pt is more selective to NO than Ir