Competition of C-C bond formation and C-H bond formation For acetylene hydrogenation on transition metals: A density functional theory study

Selective hydrogenation of acetylene is an important reaction for production of polymer grade ethylene. The green oil formation has great influence on the selectivity and activity of acetylene selective hydrogenation. This article describes a density functional theory study on the C + H hydrogenation reaction and C + C coupling reaction on the (111) surface of Ag, Cu, Pd, Pt, Rh, and Ir. The activity of acetylene selective hydrogenation is examined by the effective barrier for ethylene formation. A comparison between the reaction barrier of ethylene hydrogenation and desorption is used to identify the selectivity for ethylene formation. The barriers of three pathways for 1,3-butadiene formation suggest that acetylene and vinyl coupling reaction is the favorable pathway. The stability of catalysts is evaluated by the selectivity of 1,3-butadiene, which follows the order of Pt(111) > Ir(111) > Rh(111) > Pd(111) > Cu(111) > Ag(111). Furthermore, the relationship between acetylene adsorption energy and effective barrier of ethylene formation and 1,3-butadiene formation has been established to well understand the catalytic properties of different metals. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1059–1066, 2019     

Surface reactions on inverse model catalysts: CO adsorption and CO hydrogenation on vanadia- and ceria-modified surfaces of rhodium and palladium

Reducible transition metal oxides are well-known promoters of the hydrogenation of CO on noble metal surfaces. In this study the promotional effect of vanadia and ceria adlayers on Rh and Pd surfaces was investigated with emphasis on the effect of the oxidation state on CO adsorption and catalytic activity. Inverse supported catalysts were prepared by UHV deposition of V and Ce on the noble metal surface (Rh(111), Pd(111) or Rh foil). After oxidation and specified reduction, the reaction kinetics on polycrystalline Rh was measured at atmospheric pressure, and the molecular and dissociative chemisorption of CO on Rh(111) and Pd(111) and the methanation kinetics on Rh(111) were investigated by molecular beam techniques. On Rh(111), the probability of CO dissociation and the reaction rate are enhanced by submonolayer VO _x deposits. Local pressures between 10^-2 and 1 mbar are sufficient to drive the methanation at 573 K with measurable amounts of products, accompanied by significant restructuring of the catalyst surface. Although the reaction on Rh is generally promoted by small quantities of vanadia and ceria, the reaction rates depend strongly on the extent and temperature of hydrogen reduction. The observed increase of the reaction rate by reduction up to 673 K can be correlated to concomitant changes of the structure and composition of the VO _x deposits. If the reduction temperature is raised above 673 K, metallic V is partially dissolved in the bulk, and the resulting V/Rh subsurface alloy exhibits a particularly high activity. Contrary to vanadia, ceria islands on Rh promote the initial reaction only after a low-temperature reduction, but the activity decreases after reduction above 573 K.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

Ethanol Synthesis from Syngas on Transition Metal-Doped Rh(111) Surfaces: A Density Functional Kinetic Monte Carlo Study

Advances in methodology, software and power of supercomputers make computational approaches, specifically, density functional theory (DFT), capable of providing qualitative, and in many cases quantitative, insights into catalysis. In this article we adopted a multiscale modeling paradigm in combination of DFT calculations and kinetic Monte Carlo (KMC) methods to provide better understanding of the promoting effect of doping metals (Fe, Mo, Mn) in ethanol synthesis from syngas on Rh(111). Our calculations show that metal-doping and the position of doped metals can have significant effects on the yield and selectivity of ethanol synthesis on Rh(111). Depending on the reaction conditions, Mo and Mn may stay either on the surface or in the subsurface region, while Fe prefers to stay at the surface and participate in the reaction directly. In term of the overall yield and ethanol yield, Mo–Rh(111) with Mo at the surface layer exhibits the highest activity, followed by Mn–Rh(111) with Mn at the subsurface > Fe–Rh(111) > Mo–Rh(111) with Mo at the subsurface, Mn–Rh(111) with Mn at the surface and Rh(111) in a decreasing sequence. In term of the ethanol selectivity, Fe–Rh(111) displays the highest to ethanol, followed by Mo–Rh(111) with Mo at the surface layer, Mn–Rh(111) with Mn at the subsurface > Mo–Rh(111) with Mo at the subsurface, Mn–Rh(111) with Mn at the surface and Rh(111) in a decreasing sequence. As long as Mo stays at the surface layer, Mo is the only dopant we studied here, being able to enhance both yield and selectivity of ethanol synthesis from syngas on Rh(111). Our results suggest that the design of alloy catalyst should be very careful and controlling the position of dopants is essential to the overall catalytic performance.     

Kinetics and mechanism of NO reduction with CO on Ir surfaces

The adsorption and thermal reactivity of NO and CO and the kinetics of the NO reduction with CO on Ir surfaces were studied using X-ray photoelectron spectroscopy, polarization modulation infrared reflection–absorption spectroscopy, and temperature programmed desorption. The NO adsorption and dissociation activity was strongly dependent on the Ir surface structure. The NO dissociation activity of the Ir planes decreased in the order (100) > (211) ? (111). In contrast, the type of the CO adsorption site was independent of the Ir surface structure. The activity of Ir(111) for N₂ and CO₂ production from the NO + CO reaction was low compared with the activities of Ir(100) and Ir(211). The kinetic data for an Ir/SiO₂ powder catalyst were similar to data obtained for Ir(211). The order of the turnover frequencies for N₂ and CO₂ formation for the Ir planes was in good agreement with the order for NO dissociation activity, and this agreement indicates that the catalytic activity for NO reduction was dependent on NO dissociation. A kinetic study of the elementary steps indicated that the rate-limiting step for NO reduction with CO was the NO dissociation step.     

Reaction Between Ethylene and Acetate Species on Clean and Oxygen-Covered Pd(100): Implications for the Vinyl Acetate Monomer Formation Pathway

Abstract  

The reaction between gas-phase ethylene and adsorbed acetate species on Pd(100)-p(2 × 2)-O and Pd(100)-c(2 × 2)-O surfaces is studied using infrared spectroscopy. It is found that acetate species are removed more rapidly by gas-phase ethylene on oxygen-covered Pd(100) than on Pd(111). However, in contrast to reaction on Pd(111), where vinyl acetate monomer (VAM) formation is detected by infrared spectroscopy, only CO is found on oxygen-covered Pd(100) surfaces. In the case of Pd(111), it has been shown that VAM is stabilized on the crowded, ethylidyne-covered surface. Since ethylidyne species do not form on Pd(100), any VAM that is formed can thermally decompose. The reaction shows an isotope effect when C₂D₄ is substituted for C₂H₄, indicating the hydrogen is involved in the rate-limiting step. Based on the surface chemistry found for VAM on a Au/Pd(111) alloy, where 30 to 40% ML of gold inhibits VAM decomposition, it is suggested that the VAM formation rate will increase on (100) alloy surfaces, while it will decrease at higher gold coverages since acetate formation is inhibited.     

Adsorbed Atoms and Molecules Destined for a Reaction

The idea of an activation complex is popular for explaining reaction rates, but the characteristics of reactions and catalysis may not be explained in this way. A predestined state for each reaction composed of surface atoms and adsorbed species is responsible for these features. Two single Sn atoms trapped in adjacent half-unit cells of an Si(111) 7 × 7 surface is an example of a predestined state. An isolated Sn atom in a half-unit cell does not migrate to other half-unit cells at room temperature, but when two single Sn atoms are in adjacent half-unit cells they undergo rapid combination to form an Sn₂ dimer. In addition, these two single Sn atoms replace the center Si adatoms and an Si₄ cluster is formed. The spatial distribution of molecules desorbing from surfaces may reflect the predestined states for the desorption processes. The spatial distribution in the temperature-programmed desorption (TPD) of NO on Pd(110) and Pd(211) surfaces and that in the temperature-programmed reaction (TPR) of NO + H₂ were studied. N₂ desorbing from Pd(110) by the recombination of N atoms obeys cos⁶ – cos⁷ but the N₂ produced by a catalytic reaction of NO with H₂ obeys cos. In contrast, the N₂ desorbing with NO at 490 K in the TPD of Pd(110) shows a sharp off-normal distribution expressed by cos⁴⁶( – 38). The adsorption of NO on Pd(211) predominantly occurs on the (111) terrace but the spatial distribution suggests that the predestined states for the reaction and desorption are formed on both the (111) terrace and (100) step surfaces.     

NO Decomposition on an Mn-Deposited Pd(100) Surface

The influence of Mn deposited on a Pd(100) surface on the adsorption, dissociation and desorption properties of NO has been studied using infrared reflection absorption spectroscopy (IRAS), temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). On the Mn/Pd(100) surface, only the NO adsorbed on the Pd was observed at 320 K. Thermal dissociation of NO did not occur on the clean Pd(100) surface; it did occur, however, on the Mn/Pd(100) surface at 320 K. A Pd-Mn alloy was formed by deposition of Mn onto the Pd(100) surface; the formation of the Pd-Mn alloy was correlated with the activity of NO dissociation, assuming that it was the active site for this dissociation. The oxygen produced from the dissociation of NO was found to destroy the Pd-Mn alloy, forming MnOx. No desorption of oxygen from MnOx on Pd(100) was observed below 1200 K.     

A comparison of the NO–CO reaction over Rh(100), Rh(110) and Rh(111)

Reaction rates and product selectivities were measured over the Rh(100) surface as a function of temperature, and CO and NO partial pressures. These results are compared with our prior studies of the NO–CO reaction on the Rh(111) and Rh(110) surfaces. The only products detected for all three surfaces were CO₂, N₂O, and N₂. Furthermore, for the Rh(100) surface we have found a significant change in the apparent activation energy (E _a) with reaction temperature. For the Rh(100) surface it was found that the E _a can change by a factor of 2.3 in the temperature range investigated here, from 528 to 700 K, with the lower values obtained at higher temperatures. In contrast, E _a's were found to remain constant over the same temperature range for the Rh(110) and Rh(111) surfaces. The results observed for the Rh(100) surface suggest that reaction kinetics are dominated by variations in NO coverages. At low temperatures, the surface is fully saturated with NO, and dissociation is limited by the availability of vacancy sites through NO desorption. At high temperatures, the surface is still primarily covered with NO, however, the number of vacancy sites has increased substantially. In this case, we propose that the apparent activation energy is now reflecting NO dissociation kinetics rather than those for NO desorption. This revised version was published online in July 2006 with corrections to the Cover Date.     

Carbon monoxide adsorption and dissociation on Mn-decorated Rh(1 1 1) and Rh(5 5 3) surfaces: A first-principles study

Efficient CO activation on Rh particles promoted by Mn cocatalysts is important to the activity for the conversion of the syngas (CO and H₂) to hydrocarbon and oxygenates. To study the effect of the step edge and promotion of Mn cocatalysts on CO activation, we studied the CO dissociation on Mn-decorated Rh(1 1 1) and stepped Rh(5 5 3) surfaces using density functional theory calculations. We found that the presence of the step edge and Mn stabilizes the transition state and reaction products: compared to clean Rh(1 1 1), calculated barrier for CO dissociation on Mn-decorated Rh(5 5 3) is lowered by about 1.60 eV, and corresponding reaction energies with respect to CO in gas phase changes from endothermic (0.21 eV) to strong exothermic (−1.73 eV). The present work indicates that the addition of Mn cocatalysts and decrease of Rh particle sizes improves greatly the activity of CO dissociation.     

Studies of metal–support interactions with “real” and “inverted” model systems: reactions of CO and small hydrocarbons with hydrogen on noble metals in contact with oxides

Two types of model catalysts are compared: thin film catalysts consisting of polyhedral noble metal nanocrystals (Rh and Pt) supported by reducible and non‐reducible oxides, and their inverted pendants, submonolayers of titania and vanadia deposited under UHV conditions on the respective metal surfaces (Pd and Rh(111) and Rh (polycrystalline)). The structure and composition of the inverse catalysts were examined in situ by LEED and AES and the nanoparticles were characterized by HRTEM. The activity of thin film and inverse catalysts was studied in a series of reactions, such as the ring opening of methylcyclopentane and methylcyclobutane, the dissociation of CO and the CO methanation. Reaction conditions comprise atmospheric pressure but also molecular beam experiments. The reaction rates are related to the oxidation state of the supporting oxide, to the free metal surface area and to the number of sites at the interface between metal and support. This revised version was published online in June 2006 with corrections to the Cover Date.     

The non-linear behaviour of the NO-H2 reaction over Rh surfaces

In this paper we describe the non-linearity of the NO-H₂ reaction over Rh surfaces. Rate oscillations have been observed over a stepped (111) surface with (100) steps, (Rh(533) at low pressures (10^?4 Pa) below 500 K, while no oscillations could be observed under these conditions over a Rh(100) surface and a stepped (100) surface with (111) steps, Rh(711). The thermal stability of the N atoms formed during the reaction explains the observed structure sensitivity. Moreover, the results suggest that diffusion of N atoms is needed to synchronise the rate oscillations, a process that is absent on Rh(100) and Rh(711).     

Special Sites at Noble and Late Transition Metal Catalysts

An overview of recent advancements in density functional theory modeling of particularly reactive sites at noble and late transition metal surfaces is given. Such special sites include sites at the flat surfaces of thin metal films, sites at stepped surfaces, sites at the metal/oxide interface boundary for oxide-supported metal clusters, and sites at the perimeter of oxide islands grown on metal surfaces. The Newns–Anderson model of the electronic interaction underlying chemisorption is described. This provides the grounds for introducing the Hammer–N?rskov d-band model that correlates changes in the energy center of the valence d-band density of states at the surface sites with their ability to form chemisorption bonds. A reactivity change described by this model is characterized as an electronic structure effect. Br?nsted plots of energy barriers versus reaction energies are discussed from the surface reaction perspective and are used to analyze the trends in the calculated changes. Deviations in the relation between energy barriers and reaction energies in Br?nsted plots are identified as due to atomic structure effects. The reactivity change from pure Pd surfaces to Pd thin films supported on MgO can be assigned to an electronic effect. Likewise for the reactivity change from flat Au surfaces, over Au thin films to Au edges and the Au/MgO interface boundary. The reactivity enhancement at atomic step sites is of both electronic and atomic structure nature for NO dissociation at Ru, Rh and Pd surfaces. The enhancement of the CO oxidation reactivity when moving from a CO+O coadsorption structure on Pt(111) to the PtO₂ oxide island edges supported by Pt(111) is, however, identified as mainly an atomic structure effect. As such, it is linked to the occurrence of favorable pathways at the oxide island edges and is occurring despite of stronger adsorbate binding of the oxygen within the oxide edge, i.e. despite of an opposing electronic effect. As a final topic, a discussion is given of the accuracy of density functional theory in conjunction with surface reactions; adsorption, desorption, diffusion, and dissociation. Energy barriers are concluded to be more robust with respect to changes in the exchange-correlation functional than are molecular bond and adsorption energies.     

Noble metal catalysis. IV. Products from benzyl alcohol decomposition

In the presence of the noble metal chlorides of period VIII, benzyl alcohol disproportionates to benzaldehyde and toluene. The benzaldehyde can either decar-bonylate to benzene or disproportionate to benzyl benzoate. The order of catalyst activity is Pd > Rh > Ru. Benzyl alcohol also dehydrogenates to benzaldehyde or dehydrates to dibenzyl ether. The order of catalyst activity is Ru > Rh > Pd. In the absence of chloride, or in the presence of triphenylphosphine, the order across the periodic chart is not completely maintained.     

Pd(111) versus Pd–Au(111) in carbon monoxide oxidation under elevated pressures

The oxidation of CO on Pd(111) and Pd₇₀Au₃₀(111) has been studied under pressures upto 100 Torr. Gold is found to decrease the surface activity by inhibiting oxygen dissociation. For a sufficient conversion time depending on the CO coverage and the surface identity, a dramatic boost of activity occurs. This is ascribed to a switch from CO-induced inhibition of O₂ adsorption to a regime determined by CO adsorption. The other kinetic features are explained by oxidation of palladium and adsorption-induced restructuring of the surfaces.     

Isolation of intermediate compounds of catalytic reactions on single crystal surfaces

A key intermediate of the methanation reaction on nickel catalyst is a carbidic carbon. Accumulated carbidic intermediates on Ni(100) gives a p(2×2) p4g structure, whereas that on Ni(111) is too complex to be solved. A single domain carbide layer accidentally on Ni(111) allowed us to solve the structure explicitly. Comparison of the carbide layer on Ni(100) and that on Ni(111) showed that the carbon atoms are arranged by forming the same ordered structure. The carbide layers prepared on Ni(100), Ni(111) and Ni(110) have almost equal decomposition temperatures. Consequently, we can conclude that the same overlayer compound is formed on the three surfaces. Furthermore, the hydrogenation of the p4g carbide on Ni(100) occurs at almost equal rate to the turnover frequency of the catalytic methanation reaction. The structure insensitive methanation reaction on Ni(100), Ni(111) and Ni(110) is responsible for the same intermediate compound on these surfaces.The same strategy was applied to the reaction of NO with H₂ on Pd(100), Rh(100) and Pt-Rh(100) surfaces, and ac(2×2)-N overlayer was isolated on each of these. The hydrogenation of the isolatedc(2×2)-N produced predominantly NH species, which indicates slow NH species hydrogenation.     

Support effects in the dissociation of CO on Rh/CeO2(111)

The adsorption and reaction of CO on Rh particles supported on stoichiometric and partially reduced CeO₂(111) surfaces was studied using a combination of HREELS and TPD. A fraction of the CO adsorbed on the supported Rh particles was found to undergo dissociation to produce adsorbed C and O atoms. TPD results for isotopically labeled CO demonstrated that O atoms produced by CO dissociation rapidly exchange with the oxygen in the ceria lattice. The fraction of adsorbed CO which dissociated was found to increase significantly with the extent of reduction of the CeO₂(111) surface, suggesting that oxygen vacancies on the surface of the support play a direct role in the CO dissociation reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogenation of Nitrobenzene with Supported Transition Metal Catalysts in Supercritical Carbon Dioxide

Transition metal catalysts such as Pd, Pt, Ru, and Rh supported on carbon, silica and alumina have been examined for the hydrogenation of nitrobenzene (NB) in supercritical carbon dioxide (scCO₂) and in ethanol. The order of hydrogenation activity is Pt>Pd>Ru, Rh in scCO₂ and in ethanol. The effectiveness of the support is C>Al₂O₃, SiO₂ for either Pt or Pd in scCO₂. For all the catalysts, higher selectivity to aniline has been obtained in scCO₂ compared with ethanol. Hydrogenation of nitrobenzene catalyzed with Pd/C and Pt/C catalysts was successfully conducted in scCO₂ with a 100% yield to aniline at a lower reaction temperature of 35 °C. The product aniline (organic phase) can be easily separated from the side‐product water (aqueous phase), solvent (scCO₂), and catalyst (solid) by a simple phase separation process. The hydrogenation of NB is a structure‐sensitive reaction in ethanol as well as in scCO₂ except for a few Pt/C catalysts in which the degree of metal dispersion is small (<0.08).     

Surface intermediates in the catalytic reaction of NO+H2 on Rh studied by method of interacting bonds calculations

The semi-empirical method of interacting bonds was used to clarify the mechanism of oscillatory behavior of the catalytic system (NO+H₂)/Rh. Various rhodium planes and surface defect regions were characterized by the strength of the nitrogen bond to the surface, the stability of the adsorbed NH_n species (n=0, 1, 2, 3), and the reactivity of NH_n species towards hydrogen. Calculations admit the earlier suggested reaction mechanism, which attributes the surface wave propagation to the intermediate formation of NH_a species. The activity of the rhodium surface in oscillations is expected to increase in a row of planes: (100)<(111)<(335). The activity of Rh(335) single crystal in the reaction rate oscillations is probably governed by the presence (in contrast to ideal terraces) of gradient and broad range of the reaction intermediate properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

甲醛在Pd(111)与Pd(100)晶面上氧化的计算化学研究

使用密度泛函理论对甲醛分子在Pd(111)晶面与Pd(100)晶面的氧化反应机理进行研究,结果表明,Pd(100)晶面相比于Pd(111)晶面更有利于甲醛分子的氧化,为进一步合理设计甲醛氧化贵金属催化剂提供了理论依据。