We use density functional theory to examine the dissociation of halogen molecules on CeO2(111). We are interested in this because oxides are used to catalyze the oxihalogenation of alkanes. Our calculations show that the exothermicity of the dissociative adsorption is increased substantially if one of the dissociation fragments is a Lewis acid and the other is a Lewis base. Doping the CeO2 surface to turn it into an acid or a base can be used to influence strongly halogen dissociation. Finally, we show that the presence of a halogen on the oxide surface facilitates the breaking of the C–H bond in methane.

A catalyst consisting of vanadium oxide submonolayers supported on rutile titanium dioxide is used for a variety of reactions. One important question is the difference between the activity of monomeric clusters (having one vanadium atom) and polymeric clusters (having more than one vanadium atom). In the case of oxidative dehydrogenation of alkanes and methanol, the reaction produces water, oxygen vacancies, and hydrogen atoms bound to the surface. For this article we use density functional theory to examine how the presence of these species on the surface affects a V2O5 cluster, which we assume to be a representative of a polymeric species. We find that often the presence of other species on the surface can change the composition of the cluster or break it up into two monomeric clusters.

We studied propylene production by the reaction of propane with oxygen in the presence of gaseous I2 which works as a gas-phase catalyst. I2 is either introduced as a gas in the mixture of propane and oxygen, or it is produced when propane and oxygen come in contact with molten LiI or a mixture of molten LiI and LiOH. The single-pass propylene yields obtained in both types of experiments are ~64%, at 500 °C and propane partial pressure of 0.1 atm. The main role of I2 is to initiate chain reactions that lead to the formation of a propyl iodide intermediate that decomposes to form propylene. Another important intermediate is HI, which reacts very rapidly with oxygen to regenerate I2 and prevent oxygen from attacking the hydrocarbons.

Constant-temperature ab initio molecular dynamics is used to study reactions between molten LiI and gas phase molecules (O2, H2O, and I2) in an attempt to elucidate some aspects of the alkane oxidative dehydrogenation activity performed in the presence of molten LiI. We investigate the energy of reactions that produce LiIO, LiIO3, LiIO4, Li2O2, Li2O, LiOH, and I2. We find that the most favorable process is the formation of gaseous I2, coproduced with LiOH or Li2O (depending on the availability of water). If water is absent, then some LiIO4 will also be formed. However, this is unlikely to happen during oxidative dehydrogenation, because LiI is very hydroscopic and the oxidative dehydrogenation reaction produces water.

Vanadium pentoxide is a layered compound in which V2O5 monolayers are held together by van der Waals forces. It is therefore possible, in principle, to exfoliate the material and form two-dimensional monolayers. Density functional theory is used to calculate the structure and the energy of vacancy formation for hypothetical, two-dimensional V2O5 systems and compare them to the same properties of V2O5 slabs. We study a two-dimensional sheet (infinite in two directions) and two ribbons (infinite in one direction) whose edges are perpendicular to the [100] or [001] directions. These edges undergo a substantial reconstruction. When an oxygen vacancy is formed, the formal charge of two vanadium atoms is reduced from 5+ to 4+. The energy of oxygen vacancy formation is higher for the two-dimensional structures than for the corresponding slabs (i.e. it is more difficult to remove oxygen from the edge of a ribbon perpendicular to [001] than from the (001) surface of a slab). Therefore, the two-dimensional structures are less aggressive oxidants than vanadium pentoxide powders.

Oxygen vacancy formation energies are often used as a descriptor of the catalytic activity of metal oxides for oxidation reactions having the Mars–van Krevelen mechanism. When these energies are calculated, it is often assumed that they depend only on the concentration of the vacancies in the top oxygen layer. Previous work has shown that in the case of TiO2 and V2O5, the energy of vacancy formation depends not only on their concentration but also on the manner in which they are distributed on the surface. However, the energy change due to the change of configuration in these systems is very small. Here, we find that in the case of α-MoO3(010) the dependence on the energy of vacancy formation of the distribution of vacancies is very large: if the lattice made by the vacancies consists of parallelograms, the energy of vacancy formation is 0.4 eV smaller than when the lattice consists of rectangles (the two systems having the same vacancy concentration).

Vanadium oxide is a layered compound that forms V2O5·nH2O xerogel when intercalated by water. The xerogel consists of V2O5 bilayers with water between them. The structure of each V2O5 layer in the bilayer is close to the structure of a single layer in bulk V2O5. However, the distance between the two layers in the bilayer is much smaller than the distance between single layers in the bulk. The xerogel is a Brønsted acid that has been used as an acid catalyst and whose protons are mobile and can be exchanged with other cations. Here, we use density functional theory to examine five possible models for the structure of the xerogel. In the model that has the properties established by experiments, the vanadyl groups in the two layers point toward the outside of the bilayer, while in the bulk V2O5 they point toward the space between layers. This change in the vanadyl positions allows the two layers to get unusually close to each other. This structure is unstable in the absence of water. Water stabilizes it by reacting with bilayers to form two H3O+ ions and one oxygen atom that bridges two vanadium atoms. It is this reaction that confers acidity to the gel.

Ru0.05Ce0.95Ox is an active catalyst for methanation of CO2 with H2. Under reaction conditions one expects that oxygen vacancies are present on the oxide catalyst surface and that their steady-state concentration depends upon the relative ratio of the oxidant (CO2) to the reductant (H2). We show that the activity of the catalyst is sensitive to the degree of surface reduction: a surface that is too reduced or too oxidized loses activity. Exposing the oxidized surface to CO2 and then to H2 produces no methane, while on a reduced surface methane is produced by exposure to CO2 followed by H2. If the reaction is carried out at the steady state, purged, and then exposed to only hydrogen, methane is produced. Methane is formed through the reaction of hydrogen with surface species, whose infrared spectrum is associated with a variety of surface carbonates, and not through CO or a formate intermediate.

Molten lithium chloride, supported on various oxides, promotes the catalytic activity for oxidative dehydrogenation (ODH) of ethane. As a first step toward understanding these systems, we use ab initio molecular dynamics to examine the solvation of ethane, ethylene, oxygen, and water in molten lithium chloride supported on magnesium oxide. Among these molecules, only water dissolves readily in molten LiCl. Possible reactions between O2 and LiCl() have been studied in an effort to identify an intermediate for the ODH mechanism. We found that the formation of LiClO, LiO2, Li2O, LiOH, and Cl2 requires a substantial increase in the free energy; therefore, we propose that they are not reaction intermediates for ODH. The only process that cannot be ruled out is molecular adsorption of O2 at the LiCl/MgO interface.

Density functional theory is used to determine differences in hydrogen abstraction and ammonia binding energies between two zeolites (BEA and MFI-type) and two α-quartz surfaces doped with Al, B, Sc, or Ga. One of the questions we wanted to answer is whether the fact that zeolite cages are made of a silica monolayer plays any role in their catalytic activity. We find no important difference. Doped α-quartz has acid hydroxyls such as those in zeolites; however, their density is very low, and doped quartz is not a shape selective catalyst. Therefore, the doped silica examined here is an inferior acid catalyst when compared to BEA or MFI.

Coadsorbed water is often unavoidable in electrochemistry and low-temperature catalysis. In addition, water influences the adsorption of biomolecules on surfaces. We use ab initio DFT molecular dynamics and ground-state calculations to study the adsorption of HCl and catechol on the rutile TiO2(110) surface and at a water–rutile interface. We find that a coadsorbed water film reduces the adsorption energy of both catechol and HCl significantly because water molecules must be displaced from the surface before catechol or HCl can adsorb. The adsorption energy of catechol (or HCl) at the water–rutile interface can be estimated as the adsorption energy in vacuum minus the energy to remove two water molecules (respectively, one water molecule) from the rutile surface in vacuum and place them in liquid water. This estimate predicts the effect of a surface water film on adsorption without the need of molecular dynamics.

Density functional theory (DFT) calculations with a PBE+U functional and a larger supercell than used previously find that that the (100) and (001) surfaces reconstruct. These reconstructions involve fairly extensive rearrangements of the surface atoms and lead to changes in the density of states and the energies of oxygen-vacancy formation.

Two types of Ru–ceria catalysts were investigated, one prepared by combustion to create an atomically doped metal oxide, and the other, prepared by impregnation, as supported Ru oxide. They have different physical properties (as measured by X-ray photoelectron spectroscopy, X-ray diffraction, and Infrared spectra of adsorbed CO) but identical catalytic activity for dry reforming of methane. We show that the catalyst for dry reforming is partially reduced using XPS and IR spectroscopy. Furthermore, transient oxidation reaction spectroscopy with oxygen pulses confirms partial reduction of the catalyst is necessary for dry reforming activity.

Binary oxides catalyze a large number of interesting reactions, but often the performance is not good enough for commercialization. Attempts have been made to improve the catalytic properties of oxides by replacing some of the cations in the surface or the subsurface layer of the oxide with cations of a different kind. Here we use density functional theory to examine oxygen adsorption on La2O3(001) doped with higher valence dopants such as Ti, Zr, Nb, Ta, Ge, Sn, As, and Sb. La2O3 was chosen because it is representative of irreducible oxides. The choice of dopants allows us to study how the O2 adsorption energy and the nature of the adsorbed O2 depend on the valence of the dopants and on their place in the periodic system.

We use density functional theory to examine methane dissociative adsorption on lanthana to produce H and CH3 bonded to the surface. Previous work suggested that the binding energy of a Lewis base and a Lewis acid to an oxide exceeds by far the value expected from adding the binding energies of the same compounds chemisorbed alone. This rule suggests that the fragments (H and CH3) formed by the dissociative adsorption of methane on an oxide will adsorb so that they could benefit from this strong acid–base interaction. To do this, one fragment must bind to oxygen and the other to a cation. Such a configuration should have lower energy than the one obtained by binding both fragments to oxygen, even though each fragment prefers to bind to oxygen when it binds alone. We suggest that this behavior is encountered in most systems.

We report the results of density-functional theory calculations for the dissociative adsorption of methane (DAM) on CaO(001) doped with Li, Na, K, and Cu. The presence of these dopants lowers the energy of oxygen-vacancy formation, increases the energy of the DAM reaction, and lowers the activation energy for DAM. We performed the same calculations for a stepped CaO(001) surface doped with Na and found that Na prefers being located at a step and the activation energy for DAM is lower at this step than on the doped, flat surface. We propose that such trends are valid for all oxides doped with lower-valence dopants.

We use density functional theory to examine the dissociative adsorption of ethane on the surface of Nb-doped NiO. We find that the Nb dopant that substitutes a Ni atom in the surface layer of NiO adsorbs O2 from gas phase and binds it so strongly that the two oxygen atoms are no longer good oxidants. It is more reasonable to consider that the dopant is the NbO2 group. We show that this group acts in many ways like a lower-valence dopant and activates the surface oxygen atoms near it.

We measure the effective activation energy of methane oxidation catalyzed by La2O3 doped with Cu, Zn, Mg, Fe, Nb, Ti, Zr, or Ta. We find that the measured activation energy is a linear function of the calculated energy of oxygen-vacancy formation.

We use density functional theory, with the GGA-PBE functional, to investigate the ability of vanadium oxide clusters, supported on Ag or Au, to break the C–H bond in methane. We perform a thermodynamic analysis to show that the VO4 cluster is the most likely oxidant and then proceed to calculate the energy of the dissociative adsorption of methane and its activation energy. We explain some peculiar features of the reaction path and propose that they are general for alkane activation on oxides.

CH3Br, like CH3OH in the Methanol-To-Gasoline process, can be readily directly converted to petrochemicals and liquid fuels. CH3Br can be obtained in high yields by the direct bromination of methane using relatively low reaction temperatures and pressure, but with the formation of dibromomethane (DBM) as a primary side product. Here, we report that DBM can be highly selectively converted to higher hydrocarbons and methyl bromide via a catalytic hydrodebromination process. Silica-supported palladium carbide shows a high selectivity for the conversion of DBM to higher hydrocarbons, mainly light olefins. Silica-supported ruthenium has a high selectivity for the conversion of DBM to methyl bromide, which can then be converted to fuels or light olefins. These reactions offer pathways to increase the overall useful product yield of the methane bromination reaction, thus taking an important step toward the potential industrial application of bromine mediated Gas-To-Liquid technology.Keywords: Fischer−Tropsch; gas-to-liquid; hydrodebromination; methane; oligomerization;

We use density functional theory to study the properties of low index flat faces (i.e., not having steps) of lanthanum oxychloride LaOCl. We calculate the surface energies and the energies to make oxygen and chlorine vacancies on the surface by producing 1/2O2 or 1/2Cl2 in the gas phase. We find that the electrons left behind when the vacancies are formed are localized at the vacancy site, making these sites very reactive with electrophiles. It is also possible to make Cl vacancies by a spillover process (a Cl atom leaves its lattice site to move onto the surface) but these vacancies are not chemically active. We show that p-doping, with dopants having lower valence than La, will facilitate oxygen or chlorine vacancy formation.

Experiments suggest that adsorbing halogen atoms on an oxide surface can improve its catalytic properties. We use density functional theory to examine the dissociative adsorption of Br2 and of HBr on La2O3(001) and on La2O3(001) doped with Mg or Zr. We find that the presence of Br on the surface makes it much easier to make oxygen vacancies. In addition, there is a very strong interaction between the fragments made by the dissociative adsorption of Br2 or HBr: the presence of one fragment increases substantially the binding energy of the other one. We propose that this is a general behavior whenever one fragment is a Lewis acid and the other is a Lewis base, while the oxide is neither.

We examine a large number of DFT calculations regarding the chemistry of oxide surfaces and show that their qualitative conclusions can be predicted by using a few rules derived from the Lewis acid–base properties of the species involved. (1) The presence of a Lewis acid on an oxide surface increases substantially the binding energy of a Lewis base. (2) If an oxide has certain properties because it is a Lewis base, these properties can be suppressed by adsorbing a Lewis acid on the surface. (3) The presence of a Lewis base on an oxide surface diminishes the binding energy of another base, as compared to the binding energy on the same surface with no base on it. These rules also hold if the words “acid” and “base” are exchanged. We show that these rules apply to a large number of systems which seem to have no relationship to each other and which are important for catalysis by oxides.

We use density functional theory to examine the dissociation of halogen molecules on CeO2(111). We are interested in this because oxides are used to catalyze the oxihalogenation of alkanes. Our calculations show that the exothermicity of the dissociative adsorption is increased substantially if one of the dissociation fragments is a Lewis acid and the other is a Lewis base. Doping the CeO2 surface to turn it into an acid or a base can be used to influence strongly halogen dissociation. Finally, we show that the presence of a halogen on the oxide surface facilitates the breaking of the C–H bond in methane.

When an oxygen atom is removed from the surface of rutile TiO2(110), to make an oxygen vacancy, two unpaired electrons are left in the oxide. We perform density functional calculations with on-site repulsion (DFT+U) to find where these electrons are located. If U ≤ 2.5 eV, they are delocalized. If 3.0 ≤ U ≤ 6.0 eV, they are both localized on different Ti atoms, reducing them (formally) from Ti4+ to Ti3+. The energy of vacancy formation depends on the location of this pair of reduced Ti atoms. Three kinds of states have low energies that are very close to each other. In these states, the electrons are located on the five-coordinated Ti atoms at the surface and on Ti atoms below the surface. Previous calculations proposed that the unpaired electrons reduced two Ti atoms located near the vacancy. We find that this state has higher energy than all other states examined here.

Many recent articles have suggested that density functional theory (DFT) with the generalized-gradient approximation does not provide the correct electronic structure for the oxides of titanium. The current opinion is that a Hubbard U correction improves the DFT results. There is no generally accepted method for deciding what the value of the U parameter should be, and we propose that, if one intends to study catalysis, U should be chosen to fit the reaction energy for the oxidation of Ti2O3 to TiO2. We show that the value of U derived in this manner provides additional improvements in the description of the electronic structure.

Doping of oxides is used to modify their catalytic properties. The question we address here is whether a dopant affects only the oxygen atom next to it or also the ones further away. We find that low-valence dopants (Na and Li dopants in MgO and La and Y dopants in CeO2) have both a long-range and a short-range effect. In contrast, Pt, Ru, Zr, Ta, Mo, and W dopants in CeO2(111) affect only the oxygen atoms next to them. Furthermore, these dopants lower the energy of making an oxygen vacancy next to them by roughly the same amount. This is puzzling because these dopants have vastly different dopant–oxygen bond strength in their own oxide. By analyzing the electronic structure of the doped oxide and the changes caused by vacancy formation, we suggest that they have similar effects on vacancy formation energy because when they dope ceria, they become tetravalent, just like the Ce atom they replace.

We use density functional theory to examine methane dissociative adsorption on La2O3 substitutionally doped with Cu, Mg, or Zn. We find that these dopants activate the surface oxygen atoms and make methane dissociation exothermic. Cu-doped lanthana is very active, but it loses oxygen too easily and the surface is likely to be reduced at the temperatures at which methane dissociates. The reduced surface has lower activity. The effect of Mg doping on methane activation is less than that of Zn. We focus, therefore, on Zn-doped lanthana and calculate the activation barrier for methane dissociation. We suggest that, for dissociative adsorption, the Brønsted–Evans–Polanyi rule needs to be modified to take into account not only the binding energy of the products but also the distance between the fragments formed by dissociation. We also show that an oxygen vacancy on the Zn-doped lanthana surface adsorbs oxygen from the gas and converts it into a species that should be described as O2–. This reacts with methane and dissociates it. Our main conclusion is that Zn-doped lanthana is a much better catalyst than lanthana, for methane activation, if one can prepare it so that the Zn dopants are in the surface layer. Unfortunately, the calculations show that the Zn atom prefers to be located in the bulk (in the absence of gases) and its influence on the chemistry of the surface, when this happens, is substantially diminished.

La2O3 is one of the more efficient oxide catalysts for oxidative methane coupling. In this article, we examine the extent to which methane activation can be improved by replacing a La cation in the surface layer with other cations. The purpose of these substitutional dopants is to make the oxygen atoms in their neighborhood more reactive, which makes the doped oxide a better oxidant. We examined doping the surface layer of La2O3(001) and (011) with Cu, Zn, Mg, Fe, and Al. We have chosen dopants whose oxide formation enthalpy is less than that of La2O3. Some (Cu, Fe) are capable of having two different valence states, whereas some (Zn, Mg, Al) have only one. All of them lower substantially the energy of vacancy formation on the two faces. We use a “moderation principle” to suggest that Cu-doped La2O3 is not a good catalyst for methane activation despite lowering the energy of oxygen-vacancy formation the most. We propose that it is likely that the experimental value for the oxygen-vacancy formation energy might be affected substantially by the presence of adventitious dopants, which will then affect catalytic activity as well. We suggest that dopants affect the energy of vacancy formation in two ways: a local modification of the bond strength of the oxygen atoms to the oxide and a global effect due to a change in the Fermi level, which, in turn, can affect the charge of the oxygen vacancy and its energy of formation.

Isolated vanadia clusters supported on titania catalyze the oxidation of methanol to formaldehyde. We used density functional theory to determine the mechanism of this reaction and found a new pathway for the dissociative adsorption of methanol and the dehydrogenation of the methyl group. In this mechanism, methanol adsorbs dissociatively, by inserting into the double bond of the vanadyl group; the methoxy radical binds to the vanadium atom, whereas the hydrogen binds to the oxygen atom of the vanadyl. The dehydrogenation of the methyl group, which is the rate-limiting step, takes place by moving a H atom from CH3 onto an oxygen atom in the −V−O−Ti− group. The V−O bond is broken, and a HO−Ti group is formed.

We use density functional theory to examine some of the important aspects of methanol oxidation to formaldehyde catalyzed by isolated MO3 (M = V, Mo, and Cr) clusters supported on rutile, TiO2(110). Thermodynamic analysis led us to conclude that in the presence of oxygen, the M (M = V, Mo, and Cr) atom takes three oxygen atoms from the gas phase and this MO3 species is the oxidant in the catalyst. We calculate the structure of these clusters, their Bader charge, the structure of the methoxide formed by methanol adsorption, and the activation energy for the dehydrogenation of the methyl group in the methoxide. We find that VO3 is a substantially better catalyst than MoO3 or CrO3.

We study the effect of gold doping on oxygen vacancy formation and CO adsorption on the (1 1 0) and (1 0 0) surfaces of ceria by using density functional theory, corrected for on-site Coulomb interactions (DFT + U). The Au dopant substitutes a Ce atom in the surface layer, leading to strong structural distortions. The formation of one oxygen vacancy near a dopant atom is energetically “downhill” while the formation of a second vacancy around the same dopant requires energy. When the surface is in equilibrium with gaseous oxygen at 1 atm and room temperature there is a 0.4 probability that no oxygen atom left the neighborhood of a dopant. This means that the sites where the dopant has not lost oxygen are very active in oxidation reactions. Above 400 K almost all dopants have an oxygen vacancy next to them and an oxidation reaction in such a system takes place by creating a second vacancy. The energy required to form a second vacancy is smaller on (1 1 0) than on (1 0 0). On the (1 1 0) surface, it is much easier to form a second vacancy on the doped surface than the first vacancy on the undoped surface. The energy required to form a second oxygen vacancy on (1 0 0) is comparable to that of forming the first vacancy on the undoped surface. Thus doping makes the (1 1 0) surface a better oxidant but it has a small effect on the oxidative power of the (1 0 0) surface. On the (1 1 0) surface CO adsorption results in formation of a carbonate-like structure, similar to the undoped surface, while on the (1 0 0) surface direct formation of CO2 is observed, in contrast to the undoped surface. The Au dopant weakens the bond of the surrounding oxygen atoms to the oxide making it a better oxidant, facilitating CO oxidation.

We used density functional theory to study CO oxidation catalyzed by TiO2(110), in which some Ti atoms on the surface are replaced with V, Cr, Mo, W, or Mn. We find that in the presence of O, V, Cr, Mo, and W dopants at the surface bind an oxygen atom so that the dopant has formula MO (M = V, Cr, Mo, W). Rutile doped with Mn does not take an oxygen atom from the gas phase. We find that these materials oxidize CO by a Mars−van Krevelen mechanism in which the role of the dopant is to facilitate the formation of oxygen vacancies. The energy of CO reaction with an oxygen atom from the surface layer decays linearly with the energy of vacancy formation ΔEv, whereas the energy of adsorption of O2 at a vacancy is a linear function of ΔEv. These are the only two reactions in the mechanism whose energy varies from one doped oxide to another. Because they both depend on the energy of oxygen vacancy formation, the latter quantity is a good descriptor of catalytic activity. In deciding which intermediate reactions are most likely from an energetic point of view, we impose a “spin conservation” rule: a reaction that requires “flipping a spin” is too slow for catalysis. Because of this, we only consider reactions that conserve spin. We find that all the dopants studied here lower the energy of vacancy formation; therefore, the doped oxides are better oxidants than the undoped ones.

We use density functional theory (DFT) with the generalized gradient approximation (GGA) and the revised Perdew–Burke–Ernzerhoff (rPBE) functional, to study the surface composition of the (1 1 1) and (1 0 0) dilute Pd/Au alloy. We find that the energy of Pd atoms is lower when they substitute an Au atom in the bulk than when they substitute an Au atom in the surface layer, or when they are adsorbed on the surface. Whether they are in the surface layer or in the bulk, the Pd atoms interact very weakly with each other. CO adsorbs on the Pd atom in the surface layer and the energy of this complex is lower than that of CO in gas and Pd atom in the bulk. The interaction between the PdCO complexes formed when CO adsorbs on a Pd atom imbedded in the surface layer, is also negligible. We use these energies, equilibrium thermodynamics, and a simple lattice–gas model to examine the equilibrium composition of the surface layer, as a function of temperature, CO pressure and the Pd/Au ratio. We find that the surface Pd concentration for a nanoparticle of an Au/Pd alloy differs from that in a bulk sample. The difference is due mainly to the fact that in a nanoparticle the migration of Pd atoms to the surface depletes the bulk concentration while in a large sample; the bulk provides an infinite source of Pd atoms to populate the surface sites. This system is of interest because Pd/Au alloys are selective catalysts for vinyl acetate synthesis when the Pd concentration on the surface is very low.

We review recent theoretical and experimental work on the catalytic properties of Au clusters that contain a few atoms and are supported on an oxide surface. The clusters are mass-selected and landed slowly on the oxide surface in ultra-high vacuum. STM measurements show that the clusters do not fragment and do not damage the surface when they are deposited nor do they coarsen after deposition. Their catalytic activity changes non-monotonically with the number of atoms and is sensitive to the nature of the support and to additives (hydroxyls, water, Na, Cl) present on the surface. Binary clusters (e.g. AunSr) can be more active than unary ones. Very recent work has managed to study catalysis by such clusters under realistic pressure conditions; their performance is very different from (and sometimes better than) that of large clusters.

We use density functional theory to examine the binding of O2 to Aun and Aun− clusters. O2 binds more strongly to clusters having an odd number of electrons than to those with an even number. A second O2 molecule binds more weakly than the first.

Experiments have shown that the folding rate constants of two dozen structurally unrelated, small, single-domain proteins can be expressed in terms of one quantity (the contact order) that depends exclusively on the topology of the folded state. Such dependence is unique in chemical kinetics. Here we investigate its physical origin and derive the approximate formula ln(k) = ln(N) + a + bN, were N is the number of contacts in the folded state, and a and b are constants whose physical meaning is understood. This formula fits well the experimentally determined folding rate constants of the 24 proteins, with single values for a and b.

We study the methanation of CO2 catalyzed by ceria doped with Ni, Co, Pd, or Ru. Ce0.96Ru0.04O2 and Ce0.95Ru0.05O2 perform best, converting 55% of CO2 with a 99% selectivity for methane, at a temperature of 450 °C. This is comparable to the best catalysts found previously for this reaction. Ce0.95Ru0.05O2 was characterized by XRD, electron microscopy, BET, XPS, IR spectroscopy, and temperature-programmed reaction with Ar, H2, CO, and CO2 + H2. Steady-state methanation was studied at several temperatures between 100 and 500 °C. We find that the methanation reaction takes place on the reduced Ce0.95Ru0.05O2, and the role of the dopant is to make the reduction possible at lower temperature than on pure ceria. We discuss the potential for local and global effects of the dopant on catalytic chemistry.Graphical abstractThe performance of doped ceria for CO2 reaction with CH4.Download high-res image (82KB)Download full-size imageResearch highlights► CeO2 doped with Ru is a good catalyst for CO2 methanation. ► The catalyst is the reduced Ce0.95Ru0.05O2. The Ru dopant facilitates the reduction. ► The hydrogenation mechanism is not the usual CO2 to CO, to CH4. ► Ce0.95Ru0.05O2 is very easily contaminated by traces of CO2 to make carbonates.

We have studied catalytic activity of Pt-doped CeO2 for the oxidation and the dry reforming of methane. The catalyst was prepared by three methods resulting in ceria containing different ratios of ionic Pt versus Pt0. We show that these materials have different catalytic activity for methane oxidation and dry reforming and that they are more active when the fraction of ionic Pt is increased. Density functional theory was used to help understand the role of Pt dopant. It was found that the presence of Pt activates the oxygen atoms next to it in the surface layer and this decreases the activation energy for dissociative adsorption of methane (which is the rate-limiting step in the reaction).Energy level diagram (calculated) for breaking the C–H bond of methane. Steady state output from the reactor at different temperatures, for methane dry reforming. The catalyst is Pt-doped CeO2.Download high-res image (75KB)Download full-size image

Using a combination of theory and experiments, we show that ZnO substitutionally doped with Ti or Al oxidizes CO by several parallel reaction pathways that differ from the traditional Mars–van Krevelen (MVK) mechanism. In one, the dopant atoms at the surface of the doped oxide adsorb and activate O2 so that it reacts with CO. In the other, a surface oxygen atom from the lattice next to the dopant, D, moves onto the dopant and creates an O-D group and an oxygen vacancy. The O-D group is capable of oxidizing a reductant. To test these predictions made by theory, we have prepared Ti- and Al-doped ZnO and have shown that these compounds oxidize CO at temperatures at which pure-phase ZnO, Al2O3, or TiO2 do not. The proposed mechanisms were made plausible by studying CO oxidation with 18O2.ZnO doped with Ti oxidizes CO by a new mechanism: O2 adsorbs on the dopant and is activated. This is likely to work when high-valence dopants replace low-valence cations.Download high-res image (68KB)Download full-size image

Density functional theory calculations for the CeO2(111) surface doped with Au, Ag, and Cu show that the bond between the oxygen atoms and the oxide is weakened by presence of the dopant. In CO oxidation, doping of CeO2 with Au allows the oxide to react readily with CO and make carbonates. These decompose to release CO2 and form an oxygen vacancy on the surface. The vacancy adsorbs oxygen from the gas and weakens its bond, so that it reacts with CO to form a carbonate, which decomposes to release CO2 and heal the oxygen vacancy. To be a good oxidation catalyst, a doped oxide must achieve a balance between two conflicting requirements: It must make surface oxygen reactive but not so much that it will hinder the healing of the oxygen vacancies created by the oxidation reaction.

Ru0.05Ce0.95Ox is an active catalyst for methanation of CO2 with H2. Under reaction conditions one expects that oxygen vacancies are present on the oxide catalyst surface and that their steady-state concentration depends upon the relative ratio of the oxidant (CO2) to the reductant (H2). We show that the activity of the catalyst is sensitive to the degree of surface reduction: a surface that is too reduced or too oxidized loses activity. Exposing the oxidized surface to CO2 and then to H2 produces no methane, while on a reduced surface methane is produced by exposure to CO2 followed by H2. If the reaction is carried out at the steady state, purged, and then exposed to only hydrogen, methane is produced. Methane is formed through the reaction of hydrogen with surface species, whose infrared spectrum is associated with a variety of surface carbonates, and not through CO or a formate intermediate.