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.

Correction to "Optimized CO2 Capture of the Zeolitic Imidazolate Framework ZIF-8 Modified by Solvent-Assisted Ligand Exchange".

[This corrects the article DOI: 10.1021/acsomega.1c01130.].

Optimized CO2 Capture of the Zeolitic Imidazolate Framework ZIF-8 Modified by Solvent-Assisted Ligand Exchange.

Zeolitic imidazolate frameworks, like ZIF-8 and related structures, have shown great potential for the capture of carbon dioxide. Modifying their structure by exchanging part of the constituent organic ligands is a proven method for enhancing the capacity to absorb CO2. In this work, we performed solvent-assisted ligand exchange (SALE) on nanosized ZIF-8 (nZIF-8) with a series of functionalized imidazole derivatives (exchange percentages, after 24 h): 2-bromoimidazole (19%), 2-chloroimidazole (29%), 2-trifluoromethylbenzimidazole (4%), 2-mercaptobenzimidazole (4%), and 2-nitroimidazole (54%). The sodalite topology and porosity of nZIF-8 were maintained with all SALE modifications. Low-pressure CO2 adsorption of nZIF-8 (38.5 cm3 g-1) at STP was appreciably enhanced with all mixed-linker SALE products. Using halogenated (-Cl, -Br, and -CF3) imidazole derivatives in a 24 h SALE treatment resulted in increases between 11 and 22% in CO2 adsorption, while the thiol (-SH)- and nitro (-NO2)-functionalized SALE products led to 32 and 100% increases in CO2 uptakes, respectively. These CO2 uptakes were further optimized by varying the SALE treatment time. The SHbIm- and NO2Im-exchanged SALE products of nZIF-8 show 87 and 98 cm3 g-1 of CO2 uptakes after 60 and 120 h of SALE, respectively. These are record high CO2 adsorptions for all reported ZIF derivatives at low-pressure conditions.

Silica Nanopowder Supported Frustrated Lewis Pairs for CO2 Capture and Conversion to Formic Acid.

Treatment of hydroxylated silica nanopowders S1 and allyl-functionalized silica nanopowders S2 with 3-(diphenylborano)- or 3-bis(pentafluorophenylborano)propyltrimethoxysilane or 2-(diphenylphosphino)- or 2-(dicyclohexylphosphino)ethyltriethoxysilane generates silica nanopowder supported Lewis acids S3 and silica nanopowder supported Lewis bases S4. These surfaces were characterized by 13C, 11B, and 31P cross-polarization magic angle spinning nuclear magnetic resonance (CP MAS NMR), X-ray photoelectron spectroscopy (XPS), and attenuated total reflection Fourier transform infrared (ATR FTIR). When S3 is combined with solution-phase Lewis bases PR3 (R = C6F5, C6H5, mesityl), six associated silica nanopowder supported frustrated Lewis pairs (FLPs) are formed. In another set of six reactions, the interactions between the supported Lewis bases S4 and solution-phase Lewis acids BR3 with R = C6F5, C6H5, mesityl produced six more associated supported FLPs. The capture of CO2 by these FLPs producing FLP-CO2 Lewis pair adducts S5 and S6 were highlighted by ATR FTIR, and it was found that FLP S5e with R = C6H5 on both the supported Lewis acid and solution-phase Lewis base trapped the largest quantities of CO2 on the silica nanopowder supports. Conversion of CO2 to HCOOH was achieved by first activating H2 to generate activated FLP-H2 surfaces S7 and S9. Addition of CO2 then generated HCOOH via the silica nanopowder supported FLP-HCOOH adducts S8 and S10. Qualitative identification of HCOOH generation was achieved by ATR FTIR measurements, and surface 10b with R = C6H5 proved to be the most successful silica nanopowder surface bound FLP in HCOOH generation. In some cases, diborano formates (-BO(CH)OB-) S11 and S12 were also identified as side products during HCOOH formation. Spectroscopic characterization of purposefully synthesized S11 and S12 included 11B and 31P CP MAS NMR.

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

Facile C-C bond formation is essential to the formation of long hydrocarbon chains in Fischer-Tropsch synthesis. Various chain growth mechanisms have been proposed previously, but spectroscopic identification of surface intermediates involved in C-C bond formation is scarce. We here show that the high CO coverage typical of Fischer-Tropsch synthesis affects the reaction pathways of C2Hx adsorbates on a Co(0001) model catalyst and promote C-C bond formation. In-situ high resolution x-ray photoelectron spectroscopy shows that a high CO coverage promotes transformation of C2Hx adsorbates into the ethylidyne form, which subsequently dimerizes to 2-butyne. The observed reaction sequence provides a mechanistic explanation for CO-induced ethylene dimerization on supported cobalt catalysts. For Fischer-Tropsch synthesis we propose that C-C bond formation on the close-packed terraces of a cobalt nanoparticle occurs via methylidyne (CH) insertion into long chain alkylidyne intermediates, the latter being stabilized by the high surface coverage under reaction conditions.

Synthesis, Spectroscopy and Electrochemistry in Relation to DFT Computed Energies of Ferrocene- and Ruthenocene-Containing -Diketonato Iridium(III) Heteroleptic Complexes. Structure of [(2-Pyridylphenyl)2Ir(RcCOCHCOCH3].

A series of new ferrocene- and ruthenocene-containing iridium(III) heteroleptic complexes of the type [(ppy)2Ir(RCOCHCOR')], with ppy = 2-pyridylphenyl, R = Fc = FeII(η5-C5H4)(η5-C5H5) and R' = CH3 (1) or Fc (2), as well as R = Rc = RuII(η5-C5H4)(η5-C5H5) and R' = CH3 (3), Rc (4) or Fc (5) was synthesized via the reaction of appropriate metallocene-containing ß-diketonato ligands with [(ppy)2(-Cl)Ir]2. The single crystal structure of 3 (monoclinic, P21/n, Z = 4) is described. Complexes 1-5 absorb light strongly in the region 280-480 nm the metallocenyl -diketonato substituents quench phosphorescence in 1-5. Cyclic and square wave voltammetric studies in CH2Cl2/[N(nBu)4][B(C6F5)4] allowed observation of a reversible IrIII/IV redox couple as well as well-resolved ferrocenyl (Fc) and ruthenocenyl (Rc) one-electron transfer steps in 1-5. The sequence of redox events is in the order Fc oxidation, then IrIII oxidation and finally ruthenocene oxidation, all in one-electron transfer steps. Generation of IrIV quenched phosphorescence in 6, [(ppy)2Ir(H3CCOCHCOCH3)]. This study made it possible to predict the IrIII/IV formal reduction potential from Gordy scale group electronegativities, χR and/or ΣχR' of -diketonato pendent side groups as well as from DFT-calculated energies of the highest occupied molecular orbital of the species involved in the IrIII/IV oxidation at a 98 % accuracy level.

Efficient Solar-Driven Hydrogen Transfer by Bismuth-Based Photocatalyst with Engineered Basic Sites.

Photocatalytic organic conversions involving a hydrogen transfer (HT) step have attracted much attention, but the efficiency and selectivity under visible light irradiation still needs to be significantly enhanced. Here we have developed a noble metal-free, basic-site engineered bismuth oxybromide [Bi24O31Br10(OH)δ] that can accelerate the photocatalytic HT step in both reduction and oxidation reactions, i.e., nitrobenzene to azo/azoxybenzene, quinones to quinols, thiones to thiols, and alcohols to ketones under visible light irradiation and ambient conditions. Remarkably, quantum efficiencies of 42% and 32% for the nitrobenzene reduction can be reached under 410 and 450 nm irradiation, respectively. The Bi24O31Br10(OH)δ photocatalyst also exhibits excellent performance in up-scaling and stability under visible light and even solar irradiation, revealing economic potential for industrial applications.

Can Electrochemical Measurements Be Used To Predict X-ray Photoelectron Spectroscopic Data? The Case of Ferrocenyl-ß-Diketonato Complexes of Manganese(III).

In order to better understand intramolecular communication between molecular fragments, a series of ferrocene-functionalized ß-diketonato manganese(III) complexes, [Mn(FcCOCHCOR)3] with R = CF3, 1, CH3, 2, Ph = C6H5, 3, and Fc = FeII(η5-C5H4)(η5-C5H5), 4, the mixed ligand ß-diketonato complex [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)], 5, as well as the acac complex [Mn(CH3COCHCOCH3)3], 6, were subjected to an electrochemical and X-ray photoelectron spectroscopy (XPS) study. The ferrocenyl (FeII) and MnIII redox potentials, E°', and photoelectron lines were sufficiently resolved in each complex to demonstrate a linear correlation between E°' and group electronegativities of ligand R groups, χR, or ΣχR, as well as with binding energies of both the Fe 2p3/2 and Mn 2p3/2 photoelectron lines. These relationships are consistent with effective communication between molecular fragments of 1-5. From these relationships, prediction of Mn and Fe core electron binding energies in [Mn(R1COCHCOR2)3] complexes from known manganese and/or ferrocenyl redox potentials are, therefore, now possible. Ligand infrared carbonyl stretching frequencies were successfully related to binding energy as a measure of the energy required for inner-sphere reorganization. In particular it became possible to explain why, upon electrochemical oxidation or photoionization, the ferrocenyl FeII inner-shell of 1-5 needs more energy in complexes with ligands bearing electron-withdrawing (CF3) groups than in ligands bearing electron-donating groups such as ferrocenyl. The XPS determined entity Iratio (the ratio between the intensities of the satellite and main metal 2p3/2 photoelectron lines) is an indication not only of the amount of charge transferred, but also of the degree of inner-sphere reorganization. Just as for binding energy, the quantity Iratio was also found to be related to the energy requirements for the inner-sphere reorganization depicted by the vibrational frequency, vco.

Photosystem II Acts as a Spin-Controlled Electron Gate during Oxygen Formation and Evolution.

The oxygen evolution complex (OEC) of photosystem II (PSII) is intrinsically more active than any synthetic alternative for the oxygen evolution reaction (OER). A crucial question to solve for the progress of artificial photosynthesis is to understand the influential interactions during water oxidation in PSII. We study the principles of interatomic electron transfer steps in OER, with emphasis on exchange interactions, revealing the influence of delocalizing ferromagnetic spin potentials during the catalytic process. The OEC is found to be an exchange coupled mixed-valence electron-spin acceptor where its orbital physics determine the unique activity of PSII. The two unpaired electrons needed in the triplet O2 molecule interact with the high spin state of the catalyst via exchange interactions; the optimal ferromagnetic catalyst and the resulting radical intermediates are spin paired. As a result, the active center of the CaMn4O5 cofactor, stimulated by the driving potential provided by photons, works as a spin valve to accelerate the formation and release of O2 from diamagnetic H2O.

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 CHx species on the surface. At low coverage, CHx dehydrogenation dominates and surface carbon is produced alongside H2 and CO. At high coverage, about 60% of the CHx hydrogenates to form methane, whereas only 40% of the CHx 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 CHx, 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 CHx 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.

Activation and Deactivation of Gold/Ceria-Zirconia in the Low-Temperature Water-Gas Shift Reaction.

Gold (Au) on ceria-zirconia is one of the most active catalysts for the low-temperature water-gas shift reaction (LTS), a key stage of upgrading H2 reformate streams for fuel cells. However, this catalyst rapidly deactivates on-stream and the deactivation mechanism remains unclear. Using stop-start scanning transmission electron microscopy to follow the exact same area of the sample at different stages of the LTS reaction, as well as complementary X-ray photoelectron spectroscopy, we observed the activation and deactivation of the catalyst at various stages. During the heating of the catalyst to reaction temperature, we observed the formation of small Au nanoparticles (NPs; 1-2 nm) from subnanometer Au species. These NPs were then seen to agglomerate further over 48 h on-stream, and most rapidly in the first 5 h when the highest rate of deactivation was observed. These findings suggest that the primary deactivation process consists of the loss of active sites through the agglomeration and possible dewetting of Au NPs.

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 C1Hxad 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 COad affects hydrogen in multiple ways. It limits the adsorption capacity for Had, 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 C1Hx stability differences found in the present work with literature reports of the reactivity of C1Hx 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.

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

Correction for 'Modeling the surface chemistry of biomass model compounds on oxygen-covered Rh(100)' by B. Caglar et al., Phys. Chem. Chem. Phys., 2016, 18, 23888-23903.

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

The 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 (CH3CH2O) 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 viaß-C-H cleavage, which leads to an oxometallacycle (OMC) intermediate. Decomposition of the OMC scissions (at 180-320 K) ultimately produces CO, H2 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 viaα-C-H cleavage. Further decomposition of acetaldehyde produces CH4, CO, H2 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 H2), surface carbon and small hydrocarbons due to the high dehydrogenation and C-C bond scission activity of Rh(100).

Properties of Manganese(III) Ferrocenyl-ß-Diketonato Complexes Revealed by Charge Transfer and Multiplet Splitting in the Mn 2p and Fe 2p X-Ray Photoelectron Envelopes.

A series of ferrocenyl-functionalized ß-diketonato manganese(III) complexes, [Mn(FcCOCHCOR)3] with R = CF3, CH3, Ph (phenyl) and Fc (ferrocenyl) was subjected to a systematic XPS study of the Mn 2p3/2 and Fe 2p3/2 core-level photoelectron lines and their satellite structures. A charge-transfer process from the ß-diketonato ligand to the Mn(III) metal center is responsible for the prominent shake-up satellite peaks of the Mn 2p photoelectron lines and the shake-down satellite peaks of the Fe 2p photoelectron lines. Multiplet splitting simulations of the photoelectron lines of the Mn(III) center of [Mn(FcCOCHCOR)3] resemble the calculated Mn 2p3/2 envelope of Mn3+ ions well, indicating the Mn(III) centers are in the high spin state. XPS spectra of complexes with unsymmetrical ß-diketonato ligands (i.e., R not Fc) were described with two sets of multiplet splitting peaks representing fac and the more stable mer isomers respectively. Stronger electron-donating ligands stabilize fac more than mer isomers. The sum of group electronegativities, ΣχR, of the ß-diketonato pendant side groups influences the binding energies of the multiplet splitting and charge transfer peaks in both Mn and Fe 2p3/2 photoelectron lines, the ratio of satellite to main peak intensities, and the degree of covalence of the Mn-O bond.

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

Rhodium-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 Oad 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.

Early stages of catalyst aging in the iridium mediated water oxidation reaction.

When exposed to a potential exceeding 1.5 V versus RHE for several minutes the molecular iridium bishydroxide complex bearing a pentamethylcyclopentadienyl and a N-dimethylimidazolin-2-ylidene ligand spontaneously adsorbs onto the surface of glassy carbon and gold electrodes. Simultaneously with the adsorption of the material on the electrode, the evolution of dioxygen is detected and modifications of the catalyst structure are observed. XPS and XAS studies reveal that the species present at the electrode interface is best described as a partly oxidized molecular species rather than the formation of large aggregates of iridium oxide. These findings are in line with the unique kinetic profile of the parent complex in the water oxidation reaction.

Consequences of Electron-Density Manipulations on the X-ray Photoelectron Spectroscopic Properties of Ferrocenyl-ß-diketonato Complexes of Manganese(III). Structure of [Mn(FcCOCHCOCH3)3].

Reaction of [Mn3(OAc)6O·3H2O](+) (1) with ferrocenyl ß-diketones of the type FcCOCH2COR with R = CF3 (2a) and CH3 (2b), Ph = C6H5 (2c), and Fc = Fe(II)(η(5)-C5H4)(η(5)-C5H5) (2d) yielded a series of ferrocene-functionalized ß-diketonato manganese(III) complexes 3a-3d, respectively, of general formula [Mn(FcCOCHCOR)3]. The mixed-ligand ß-diketonato complex [Mn(FcCOCHCOFc)2(FcCOCHCOCH3)] (4) was obtained by reacting mixtures of diketones 2b and 2d with 1. A single-crystal X-ray structure determination of 3b (Z = 2, triclinic, space group P1̅) highlighted a weak axial elongating Jahn-Teller effect and a high degree of bond conjugation. An X-ray photoelectron spectroscopic study, by virtue of linear relationships between group electronegativities of ligand R groups, χR, or ∑χR, and binding energies of both the Fe 2p3/2 and Mn 2p3/2 photoelectron lines, confirmed communication between molecular fragments of 2a-2d as well as 3a-3d. This unprecedented observation allows prediction of binding energies from known ß-diketonato side group χR values.

Preparation and characterization of supported bimetallic Pd(IV)-Co(III) model catalyst from organometallic single source precursor for aerobic oxidation of alcohols.

The bimetallic paddlewheel catalyst precursor, [Pd(II)Co(II)(µ-OOCCH(3))(4)] H(2)O·2CH(3)COOH (1), prepared from [Pd(3)(µ-OOCCH(3))(6)] and [Co(OOCCH(3))(2)], was used as a single source precursor to prepare, after binding to a surface-hydroxylated silicon wafer and oxidation, the bimetallic oxides of Pd(IV)Co(III)/SiO(2) catalyst supported on a model planar (i.e., two-dimensional) silicon wafer. This catalyst catalyzes the aerobic oxidation of alcohols to its corresponding carbonyl compounds. The bimetallic tetracarboxylato catalyst precursor was bonded to the surface-hydroxylated silicon wafer by spin-coating and also by grafting. X-ray photoelectron spectroscopy (XPS) revealed that one of the four µ-acetato bridging ligands was substituted by Si-O fragments in a covalent bond formation process during grafting of 1 onto the wafer. In contrast, during the spin-coating process, all four acetato ligands remained intact during fixation on the silicon surface. Upon oxidation and workup, the grafted sample's Pd:Co ratio remained unchanged (1.0:1.3), whereas the spin-coated sample's Pd content decreased with respect to Co content. XPS determined binding energies were interpreted to imply that after oxidation in an oxygen/argon mixture of the grafted sample both Pd(II) and Co(II) were oxidized to produce PdO(2) (337.5 eV) and Co(III)(2)O(3) (781.1 eV) which most probably interacts with the silicon surface via Pd(IV)-O-Si and Co(III)-O-Si bonds. Solvent free aerobic oxidation of octadecanol to its corresponding carbonyl compound was achieved on this oxidized Pd(IV)Co(III)/SiO(2) model catalyst using molecular oxygen as oxidant under solvent-free conditions. The use of the single source catalyst precursor, 1, resulted in a Pd(IV)Co(III)/SiO(2) catalyst with superior catalytic activity toward the oxidation of octadecanol over a catalyst prepared from a physical mixture of the separate reactant compounds tripalladium(II) hexaacetate and cobalt(II) diacetate.

A density functional theory study on the effect of zero-point energy corrections on the methanation profile on Fe(100).

The thermodynamics and kinetics of the surface hydrogenation of adsorbed atomic carbon to methane, following the reaction sequence C+4H(-->/<--)CH+3H(-->/<--)CH(2)+2H(-->/<--)CH(3)+H(-->/<--)CH(4), are studied on Fe(100) by means of density functional theory. An assessment is made on whether the adsorption energies and overall energy profile are affected when zero-point energy (ZPE) corrections are included. The C, CH and CH(2) species are most stable at the fourfold hollow site, while CH(3) prefers the twofold bridge site. Atomic hydrogen is adsorbed at both the twofold bridge and fourfold hollow sites. Methane is physisorbed on the surface and shows neither orientation nor site preference. It is easily desorbed to the gas phase once formed. The incorporation of ZPE corrections has a very slight, if any, effect on the adsorption energies and does not alter the trends with regards to the most stable adsorption sites. The successive addition of hydrogen to atomic carbon is endothermic up to the addition of the third hydrogen atom resulting in the methyl species, but exothermic in the final hydrogenation step, which leads to methane. The overall methanation reaction is endothermic when starting from atomic carbon and hydrogen on the surface. Zero-point energy corrections are rarely provided in the literature. Since they are derived from C-H bonds with characteristic vibrations on the order of 2500-3000 cm(-1), the equivalent ZPE of 1/2 hν is on the order of 0.2-0.3 eV and its effect on adsorption energy can in principle be significant. Particularly in reactions between CH(x) and H, the ZPE correction is expected to be significant, as additional C-H bonds are formed. In this instance, the methanation reaction energy of +0.77 eV increased to +1.45 eV with the inclusion of ZPE corrections, that is, less favourable. Therefore, it is crucial to include ZPE corrections when reporting reactions involving hydrogen-containing species.