CO Bond Scission Research Articles

Valorization of lignin through reductive catalytic fractionation of fermented corn stover residues

The fermented corn stover residues are abundant renewable lignin-rich bioresources that show great potential to produce aromatic phenols. However, selective catalytic hydrogenolysis of this residual material still remains challenge to obtain high yields. Herein, a novel strategy to produce monophenolic compounds from the fermented stover over a commercial Pd/C catalyst was proposed. Taking the reaction temperature as the key variable, the highest monomer yield was 28.5 wt% at 220 °C in compaction with that of the pristine corn stover (22.8 wt%). The enhanced monophenol yield was due to the higher contents of lignin and less recalcitrance in the fermented stover. Moreover, the van Krevelen diagram revealed a slight selective CO bond scission of lignin macromolecular during fermentation as well as the dehydration and deoxygenation in hydrogenolysis reaction. Overall, this work opens a new avenue for the valorization of lignin through reductive catalytic fractionation of agricultural wastes.

The Vital Role of Step-Edge Sites for Both CO Activation and Chain Growth on Cobalt Fischer–Tropsch Catalysts Revealed through First-Principles-Based Microkinetic Modeling Including Lateral Interactions

Microkinetic modeling is employed to predict catalytic turnover rates, product distributions, preferred mechanistic pathways, and rate- and selectivity-controlling elementary reaction steps for the Fischer-Tropsch (FT) reaction. We considered all relevant elementary reaction steps on Co(112̅1) step-edge and Co(0001) terrace sites as well as such important aspects as coverage-related lateral interactions, different chain-growth mechanisms, and the migration of adsorbed species between the two surfaces in the dual-site model. CHx–CHy coupling pathways relevant to the carbide mechanism have favorable barriers in comparison to the overall barriers for the CO insertion mechanism. A comparison of reaction barriers indicates why cobalt is such a good FT catalyst: CO bond scission and chain growth compete, while termination to olefins has a slightly higher barrier. The predicted kinetic parameters correspond well with experimental kinetic data. The Co(112̅1) model surface is highly active and selective for the FT reaction. Adding terrace Co(0001) sites in a dual-site model approach leads to a substantially higher CH4 selectivity at the expense of the C2+-hydrocarbons selectivity. The chain-growth probability decreases with increasing temperature and H2/CO ratio, caused by faster hydrogenation of the hydrocarbon chains. The elementary reaction steps for O removal and CO dissociation significantly control the overall CO consumption rate. Chain growth occurs almost exclusively at step-edge sites, while additional CH4 stems from CH and CH3 migration from step-edge to terrace sites. Replacing CO by CO2 as the reactant shifts the product distribution nearly completely to CH4, which is related to the much higher H/CO coverage ratio during CO2 hydrogenation in comparison to CO hydrogenation. These findings highlight the importance of a proper balance of CO and H surface species during the FT reaction and pinpoint step-edge sites as the locus of the FT reaction with low-reactive terrace sites near step-edge sites being the origin of unwanted CH4.

A comprehensive DFT study of CO2 catalytic conversion by H2 over Pt-doped Ni catalysts

PtNi bimetallic catalysts show superior performance for CO2 catalytic conversion by hydrogen, but the underlying mechanism and the key elementary steps in controlling the activity and selectivity of CO2 hydrogenation remain unclear. In present work, the complete reaction network for CO2 hydrogenation has been investigated systematically over Pt/Ni (111) surface based on periodic density functional theory, and active sites and reaction mechanism have been determined. It is found that HCOOH is mainly produced by undergoing the HCOO pathways while synthesis of CH3OH and CH4 via RWGS+CO hydrogenation is the dominant reaction pathway, and their selectivity are determined by the competitive reaction between hydrogenation and CO bond scission of H2COH species. The dissociation of COOH is regarded as the rate-determining step as it has the highest barrier (2.07 eV) in RWGS+CO hydrogenation. Moreover, it is observed that the doping of Pt on Ni surface can promote the transformation of CO2 into chemisorbed CO2δ− and reduce the barrier in H2 dissociation, which further facilitate the activation and hydrogenation of CO2. More importantly, the doped Pt atom could promote HxCO hydrogenation to HxCOH, meanwhile, suppress HxCOH dissociation into CHx. Especially, the activation barrier and reaction energy for C formation is markedly enhanced, and the ability for C hydrogenation is promoted over Pt/Ni (111) surface, which could lower the possibility of coke formation. These results provide helpful information in understanding the process of CO2 hydrogenation at atomic scale, and could benefit for the synthesis of Ni-based bimetallic catalysts.

Rhenium-promoted selective CO2 methanation on Ni-based catalyst

Re-doped Ni(111) (Re@Ni(111)) surface was used as a model to investigate the effect of Re on the CO bond scission and on the selectivity of CO2 methanation on a Ni-based catalyst. Three pathways, including CO2 dissociation into CO* followed by CO* hydrogenation, CO2 reduction through the HCOO* and COOH* intermediates, were analyzed based on the results from the density functional theory calculations. The results indicate that the presence of Re significantly lowers the activation barrier of CO bond cleavage due to the strong affinity of Re to O but has no significant effect on the hydrogenation steps. Microkinetic analysis showed that the presence of Re greatly increases the selectivity toward CH4. Analysis of surface coverage of the adsorbed species showed that CO* and H* were the most abundant species on the Ni(111) surface whereas appreciable amount of O adatoms were present on Re@Ni(111) in addition to CO* and H*, with the O adatoms on the Re sites. On both surfaces, increasing H2 partial pressure resulted in an increase in H* coverage but decreased CO* coverage. The strong affinity of Re toward O makes Re@Ni(111) more effective for CO bond scission and thereby enhances methane selectivity.

Adsorption, polymerization and decomposition of acetaldehyde on clean and carbon-covered Rh(111) surfaces

The adsorption and dissociation of acetaldehyde were investigated on clean and carbon-covered Rh(111) single crystal surfaces by electron energy loss spectroscopy (EELS), temperature programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS) and work function (∆φ) measurements. Acetaldehyde is a starting material for the catalytic production of many important chemicals and investigation of its reactions motivated by environmental purposes too. The adsorption of acetaldehyde on clean Rh(111) surface produced various types of adsorption forms. η1-(O)-CH3CHOa and η2-(O,C)-CH3CHOa are developing and characterized by HREELS. η1-CH3CHOa partly desorbed at Tp= 150K, another part of these species are incorporated in trimer and linear 2D polimer species. The desorption of trimers (at amu 132) were observed in TPD with a peak maximum at Tp= 225K. Above this temperature acetaldehyde either desorbed or bonded as a stable surface intermediate (η2-CH3CHOa) on the rhodium surface. The molecules decomposed to adsorbed products, and only hydrogen and carbon monoxide were analyzed in TPD. Surface carbon decreased the uptake of adsorbed acetaldehyde, inhibited the formation of polymers, nevertheless, it induced the CO bond scission and CO formation with 40–50K lower temperature after higher acetaldehyde exposure.

Effect of the support on the hydrodeoxygenation of m-cresol over molybdenum oxide based catalysts

The hydrodeoxygenation (HDO) of m-cresol was investigated over supported molybdenum oxide catalysts at 340°C under 4MPa as total pressure. All catalysts were fully characterized using several techniques such as atomic absorption, N2 physisorption, XRD, H2-TPR, NH3-TPD, Raman spectroscopy, TEM analysis and oxygen chemisorption. It was noted that the reducibility of molybdenum species depends on the support used and follows the same order than the one determined from the HDO activity, i.e. MoOx/Al2O3>MoOx/SBA–15>MoOx/SiO2. In addition, the use of an ordered mesoporous silica support (SBA-5) or an acidic support (Al2O3) favored significantly the dispersion of MoOx particles compared to SiO2.Under these experimental conditions, m-cresol transformation underwent through two parallel deoxygenation routes which involved either the direct CO bond scission leading to toluene (DDO route), or the total hydrogenation of the aromatic ring yielding mainly to a mixture of methylcyclohexene isomers (HYD route). Regardless of the support used, the DDO route was always predominant. A reaction mechanism was proposed to explain the formation of toluene, the main product observed from HDO of m-cresol. To explain the formation of this aromatic, a selective adsorption through the oxygen atom of the phenolic reactant on oxygen vacancies, acting as HDO active sites, was proposed.

Dry (CO2) reforming of methane over Pt catalysts studied by DFT and kinetic modeling

Dry reforming of methane (DRM) is a well-studied reaction that is of both scientific and industrial importance. In order to design catalysts that minimize the deactivation and improve the selectivity and activity for a high H2/CO yield, it is necessary to understand the elementary reaction steps involved in activation and conversion of CO2 and CH4. In our present work, a microkinetic model based on density functional theory (DFT) calculations is applied to explore the reaction mechanism for methane dry reforming on Pt catalysts. The adsorption energies of the reactants, intermediates and products, and the activation barriers for the elementary reactions involved in the DRM process are calculated over the Pt(111) surface. In the process of CH4 direct dissociation, the kinetic results show that CH dissociative adsorption on Pt(111) surface is the rate-determining step. CH appears to be the most abundant species on the Pt(111) surface, suggesting that carbon deposition is not easy to form in CH4 dehydrogenation on Pt(111) surface. In the process of CO2 activation, three possible reaction pathways are considered to contribute to the CO2 decomposition: (I) CO2*+* → CO*+O*; (II) CO2*+H* → COOH*+* → CO*+OH*; (III) CO2*+H* → mono-HCOO*+* → bi-HCOO*+* [CO2*+H* → bi-HCOO*+*] → CHO*+O*. Path I requires process to overcome the activation barrier of 1.809eV and the forward reaction is calculated to be strongly endothermic by 1.430eV. In addition, the kinetic results also indicate this process is not easy to proceed on Pt(111) surface. While the CO2 activation by H adsorbed over the catalyst surface to form COOH intermediate (Path II) is much easier to be carried out with the lower activation barrier of 0.746eV. The CO bond scission is the rate-determining step along this pathway and the process needs to overcome the activation barrier of 1.522eV. Path III reveals the CO2 activation through H adsorbed over the catalyst surface to form HCOO intermediate firstly. This reaction requires a quite high activation barrier and is a strongly endothermic process leading to a very low forward rate constant. In conclusion, Path II is the dominant reaction pathway in CO2 activation. Additionally, there are two pathways of CH oxidation by O: (A) CH*+O*→CHO*+*→CO*+H*; (B) CH*+O*→COH*+*→CO*+H*. Both the activation barriers and kinetic results demonstrate that Path A is the prior reaction pathway. Furthermore, in the two pathways of CH oxidation by OH: (C) CH*+OH*→CHOH*+*→CHO*+H*; (D) CH*+OH*→CHOH*+*→COH*+H*. Path C is easier to proceed. In conclusion, the main reaction pathway in CH oxidation according to the mechanism: CH*+OH*→CHOH*+*→CHO*+H*→CO*+2H*. These results could provide some useful information for the operation of DRM over Pt catalysts, and are helpful to understand the mechanisms of DRM from the atomic scale.

Elementary steps in Fischer–Tropsch synthesis: CO bond scission, CO oxidation and surface carbiding on Co(0001)

Dissociation of CO on a Co(0001) surface is explored in the context of Fischer–Tropsch synthesis on cobalt catalysts. Experiments show that CO dissociation can occur on defect sites around 330K, with an estimated barrier between 90 and 104kJmol−1. Despite the ease of CO dissociation on defect sites, extensive carbon deposition onto the cobalt surface up to 0.33ML requires a combination of high surface temperature and a relatively high CO pressure. Experimental data on the CO oxidation reaction indicate a high reaction barrier for the CO+O reaction, and it is argued that, due to the rather strong Co–O bond, (i) oxygen removal is the rate-limiting step during surface carbidization and (ii) in the context of Fischer–Tropsch synthesis, removal of surface oxygen rather than CO bond scission might be limiting the overall reaction rate.

The acceleration of methanol synthesis and C2 oxygenates formation on copper grain boundary from syngas

Density functional theory is employed to investigate the Fischer–Tropsch mechanism on copper ∑5(310) tilt grain boundary together with Cu(111) surface. For the methanol formation, Cu∑5(310) can effectively reduce each energy barrier through the preferred CH3O intermediate route as compared with Cu(111). Furthermore, Cu∑5(310) provides a synergetic effect to modulate the energy landscape in the methanol synthesis, where the pathway associated with CH2OH (CH2O↔CH2OH↔CH3OH) presents the clear kinetic advantage than the CH3O route. Simultaneously, the kinetically assisted intermediate CH2OH enable the CH2 production both thermodynamically and kinetically on Cu∑5(310). The formation of C2 oxygenates from CH2 provides a critical precursor to higher alcohol synthesis. The higher activity of grain boundary is attributed to the presence of low-coordinated sites with flexible local configuration. Consequently, the adsorption strength of small species is enhanced to provide a thermodynamic driving force for CO bond scission. The adaptive character of ∑5(310) grain boundary facilitates the stabilization of the transition state, therefore lowering the activation barrier. Present work unravels the microscopic origin of the higher catalytic capacity of copper ∑5(310) grain boundary which can be helpful to guide the development of novel Cu-based catalysts for higher alcohol formation from syngas.

Theoretical investigation of the decarboxylation and decarbonylation mechanism of propanoic acid over a Ru(0 0 0 1) model surface

The hydrodeoxygenation of organic acids is often found to be a rate-controlling process during upgrading of biomass feedstocks into fuels. We developed a microkinetic model based on data obtained from density functional theory calculations for the decarboxylation and decarbonylation mechanisms of propanoic acid (CH3CH2COOH) over a Ru(0001) model surface. The model predicts that the decarbonylation mechanism is two orders of magnitude faster than the decarboxylation mechanism. The most favorable decarbonylation pathway proceeds via removal of the acid –OH group to produce propanoyl (CH3CH2CO) followed by C–CO bond scission of propanoyl to produce CH3CH2 and CO. Finally, CH3CH2 is hydrogenated to CH3CH3. Dehydrogenation reactions that have been observed to be important over Pd catalysts play no role over Ru(0001), and a sensitivity analysis indicates that removal of the acid –OH group is the rate-controlling step in the deoxygenation. Overall, our results suggest that to improve the Ru catalyst performance for the decarbonylation of organic acids, the free site coverage needs to be increased by, for example, adding a catalyst promoter that decreases the hydrogen and CO adsorption strength (without significantly affecting the C–OH bond scission rate), or by raising the reaction temperature and operating at relatively low CO and H2 partial pressures.

Microkinetic modeling of the decarboxylation and decarbonylation of propanoic acid over Pd(1 1 1) model surfaces based on parameters obtained from first principles

We have studied the reaction mechanism of deoxygenation of propanoic acid to alkanes over Pd(111) model surfaces by a combination of microkinetic modeling and density functional theory calculations. Approximate, coverage-dependent adsorption energies of CO and H have been implemented in a microkinetic model that shows that the decarbonylation mechanism is slightly preferred over the decarboxylation mechanism at various H2 partial pressures. The most significant decarbonylation pathway proceeds via dehydrogenation of the acid to yield CH3CHCOOH which illustrates the important role of α-carbon dehydrogenation steps, followed by dehydroxylation to yield CH3CHCO which further dehydrogenates to CHCHCO. Finally, facile C–CO bond scission occurs to yield CO and acetylene which gets hydrogenated to ethane. Overall, the dehydroxylation of CH3CHCOOH and to a lower degree the removal of the hydrocarbon pool from the surface and the dehydrogenation of the α-carbon of the reactant are found to be the rate-controlling steps.

Theoretical Investigation of the Reaction Mechanism of the Decarboxylation and Decarbonylation of Propanoic Acid on Pd(111) Model Surfaces

Conversion of biomass into fuels or chemicals often requires a processing step limited by hydrodeoxygenation of organic acids. Various pathways have been proposed for the deoxygenation of these acids into hydrocarbons, with the decarboxylation and decarbonylation requiring less hydrogen than the reductive deoxygenation without C–C bond cleavage. In this paper, we present the reaction mechanism for the decarboxylation and decarbonylation of propanoic acid over Pd(111) model surfaces determined by first-principles electronic structure calculations based on density functional theory. Our calculations suggest that the most significant decarbonylation pathways proceed via a dehydroxylation of the acid to produce propanoyl (CH3CH2CO) followed by either full α-carbon dehydrogenation and CH3C–CO bond scission to produce CH3C and CO, or first α-carbon dehydrogenation followed by β-carbon dehydrogenation and CH2CH–CO bond scission to produce CH2CH and CO. The decarboxylation mechanism starts with O–H bond cleavage followed by direct C–CO2 bond scission or possibly α-carbon dehydrogenation prior to C–CO2 bond cleavage. As a result, in both mechanisms the most favorable pathways likely involve some level of α- and/or β-carbon dehydrogenation steps prior to C–C scission, which distinguishes these deoxygenation pathways from the reductive deoxygenation without C–C bond cleavage that has previously been shown to not involve dehydrogenation steps.

The adsorption and reaction of vinyl acetate on Au/Pd(100) alloy surfaces

The surface chemistry of vinyl acetate monomer (VAM) is studied on Au/Pd(100) alloys as a function of alloy composition using temperature-programmed desorption and reflection–adsorption infrared spectroscopy. VAM adsorbs weakly on isolated palladium sites on the alloy with a heat of adsorption of ~55kJ/mol, with the plane of the VAM adsorbed close to parallel to the surface. The majority of the VAM adsorbed on isolated sites desorbs molecularly with only a small portion decomposing. At lower gold coverages (below ~0.5ML of gold), where palladium–palladium bridge sites are present, VAM binds to the surface in a distorted geometry via a rehybridized vinyl group. A larger proportion of this VAM decomposes and this reaction is initiated by CO bond scission in the VAM to form adsorbed acetate and vinyl species. The implication of this surface chemistry for VAM synthesis on Au/Pd(100) alloys is discussed.

Bond activation in complexes of Pb(II) with 15 deprotonated amino acids: correlation with gas-phase acidity

Deprotonated amino acids and their complexes with the lead dication have been electrosprayed from solution and then subjected to collision-induced dissociation in a tandem mass spectrometer. The nature of the observed primary dissociation products and the relative onset energies for dissociation in the absence and presence of the lead dication were used as a signature and measure of bond activation by the lead dication. Bond-activation by Pb2+ appears to be the largest with deprotonated glycine and is also large with the other deprotonated amino acids containing hydrocarbon sidechains (alanine, proline and valine). Deprotonated methionine, aspartic and glutamic acid also exhibit significant bond activation with dissociation occurring at the same bond in the absence and presence of Pb2+. A correlation was uncovered between the dissociation onset energies of the eigth Pb2+ complexes that exhibit backbone dissociation and the gas-phase acidities of the corresponding amino acids. Backbone dissociation typically involves C-αC bond scission of the lead complex leading to loss of CO2+Pb or H2O+CO. The activation of the C-αC bond by Pb2+ was found to be tempered by the same factor that influences the gas-phase acidities of the amino acids, namely the presence of electron-donating groups on the sidechain. In the other seven Pb2+ complexes, which exhibit sidechain dissociation through CC, CN, CS and CO bond scission, no such correlation was found.

Aqueous‐Phase Hydrogenation of Acetic Acid over Transition Metal Catalysts

Catalytic hydrogenation of acetic acid to ethanol has been carried out in aqueous phase on several metals, with ruthenium being the most active and selective. DFT calculations suggest that the initial CO bond scission yielding acetyl is the key step and that the intrinsic reactivity of the metals accounts for the observed activity.

Radical cation ester cleavage in solution: mechanism of the mesolytic O-CO bond scission.

A wide range of enol carbonate, carbamate, and ester radical cations is characterized in solution by cyclic voltammetry and EPR spectroscopy. Preparative transformations using one-electron oxidants or anodic oxidation yield benzofurans after O-CO bond cleavage. Mechanistic investigations and direct detection of radical intermediates reveal that all enol radical cations undergo exclusively O-CO bond cleavage to provide alpha-carbonyl cations and acyl (or alternatively, alkoxyacyl and aminoacyl) radicals, respectively. The kinetics of the mesolytic fragmentation and the influence of nucleophilic additives have been determined using fast-scan cyclic voltammetry.

Methanol-induced hydrogen permeation through a palladium membrane

The dehydrogenation of methanol and the subsequent permeation of hydrogen through a 25 μm thick palladium film has been studied in a catalytic membrane reactor. At the temperature studied, 350°C, the decomposition pathway for methanol on clean palladium surfaces is believed to lead to H ad and a carbonaceous overlayer. The released hydrogen can either desorb or permeate the palladium membrane. During a continuous supply of methanol hydrogen permeation is reduced and, eventually, totally quenched by the growing carbon monoxide/carbon coverage. Adding oxygen in the methanol supply can balance the increasing carbonaceous coverage through the production of carbon dioxide. In such a case, it is concluded that no CO bond scission occurs. The methanol/oxygen ratio is crucial for the hydrogen permeation rate. Isotope-labelled methanol, CH 3OH, CH 3OD, CD 3OH and CD 3OD, shows that it is preferentially the methyl (or methoxy) hydrogen that permeates the membrane.

Influence of solvent and cation on the properties of oxygen-containing organic anions. Part 4. Mechanism and reactivity of tetraaryloxirane cleavage with alkali metals

Six tetraaryloxiranes 1a–f(Scheme 4) were reduced (Schemes 1–3) with alkali metals (M = Li, Na, K, Cs) in eight polar aprotic solvents under an inert atmosphere. The organometallic solutions thus obtained were hydrolysed and the reaction products analysed. Similar experiments were carried out where the same solutions were quenched with D2O or Mel. In some cases the same solutions were studied by NMR and ESR spectroscopy before quenching. A stepwise reduction mechanism was established where the transfer of a first electron produces CO-bond scission in the oxirane ring, yielding a short-lived radical anion 4 or 5(Scheme 1), i.e. a tetraaryl-β-oxidoethyl radical. This intermediate can either eliminate oxygen as metal oxide (MO) to produce a tetraarylethylene 24(Scheme 2) or be further reduced to a dianion 8 or 9(Scheme 1). This anion yields, upon hydrolysis, low yields, if any, of the corresponding tetraphenylethanol 15 or 16(Z = H). The larger proportion of the dianion, after the first protonation step, yielding anion 11 or 12, undergoes CC-bond scission which leads eventually to the corresponding ketone and diarylmethane 19+ 20 or 21+23(Z = H)(Scheme 2). Other possible pathways were excluded through experiments where other possible intermediates were generated. These led to different end products. A triparametric linear correlation as a function of solvent parameters ETN and DN, as well as the cationic radius, was established for the influence of the nature of the solvent and counter-ion on the ratio between the rates of formation of products stemming from metal oxide (MO) elimination by the ring-opened radical anion 4 or 5(Schemes 1 and 2) and rates of formation of products stemming from further reduction of the same radical anion to the dianion 8 or 9, thus confirming the mechanism established.

Mechanism of CO bond scission at alkali promoter sites—IRAS study of the system CO/K/Ni(111)

IRAS (infrared reflection absorption spectroscopy) has been employed to study the isotopic mixing reaction: 13C16O(a)+12C18O(a)→13C18O(a)+12C16O(a) on Ni(111) with preadsorbed K adatoms. Under high temperature conditions (T>450 K) where the isotopically mixed CO is being desorbed, it has been shown that the isotopic exchange reaction cannot be detected by IRAS on the surface among CO molecules strongly interacting with K adatoms. This result implies that dissociation of CO occurs at K-promoted Ni sites followed by surface diffusion of C(a) and O(a) away from the K promoter site. Statistical recombination of C(a)+O(a) occurs with concomitant CO desorption on Ni(111) sites some distance from K centers. The empty K-promoted sites may be refilled by surface diffusion of CO from outside. Thus, alkali metal promotion of C–O bond scission may act via a ‘‘feeder-site’’ mechanism connecting promoter atoms to external surface sites via surface diffusion of atomic C and O species.

CO adsorption on (111) and (100) surfaces of the Pt 3Ti alloy: Evidence for parallel binding and strong activation of CO

An atom superposition and electron delocalization molecular orbital (ASED-MO) study has been made of CO adsorption on a 40-atom cluster model of the (111) surface and a 36-atom cluster model of the (100) surface of the Pt 3Ti alloy. Parallel binding to high-coordinate sites associated with Ti and low CO bond scission barriers are predicted for both surfaces. The preference for parallel adsorption is a consequence of the nature of the CO π-to-surface donation interactions. On Ti sites the π orbitals donate to the nearly empty Ti 3 d band and the antibonding counterpart orbitals are empty. Thus the π donation makes substantial contributions to the adsorption bond order that are in addition to the contributions from 5σ donation and metal backbonding to the π ∗ orbitals. Altogether these bonding interactions favor the lying down orientation. On Pt sites, on the other hand, the π donation antibonding counterpart orbitals are occupied so that the net interaction with Pt is a closed-shell repulsion. CO bonds upright in order to minimize the π interaction and, concomitantly, the closed-shell repulsion, while maintaining 5σ donation and π ∗ backbonding stabilizations. Comparisons are made with the results for a 40-atom cluster model of the unalloyed Pt(111) surface. It is shown that the extended Hückel parameterization is inappropriate for studying CO adsorption to Pt with the ASED-MO theory because it incorrectly favors adsorption bonding through the oxygen end.