CO2 adsorption on TiO2(110) rutile: Insight from dispersion-corrected density functional theory calculations and scanning tunneling microscopy experiments

Highly stable and durable ZnIn2S4 nanosheets wrapped oxygen deficient blue TiO2(B) catalyst for selective CO2 photoreduction into CO and CH4

Developing new and highly stable efficient photocatalysts is crucial for achieving high performance and selective photocatalytic CO2 conversion. In this paper, we designed a one-dimensional oxygen-deficient blue TiO2(B) (BT) catalyst for improved electron mobility and visible light accessibility. In addition, hexagonal ZnIn2S4 (ZIS) nanosheets with a low bandgap and great visible light accessibility are employed to produce effective heterostructures with BT. The synthesized materials are tested for photocatalytic conversion of CO2 into solar fuels (H2, CO and CH4). The optimized composite yields 71.6 and 10.3 μmol g-1h-1 of CO and CH4, three and ten times greater than ZIS, respectively. When ZIS nanosheets are combined with a one-dimensional oxygen-deficient BT catalyst, improved electron mobility and visible light accessibility are achieved, charge carriers are effectively segregated, and the transfer process is accelerated, resulting in efficient CO2 reduction. The photocatalytic CO2 conversion activity of the constructed BT/ZIS heterostructures is very stable over a 10-day (240-hour) period, and CO and CH4 production rates increase linearly with time; however, as time goes on, the rates of H2 production decrease. Further, a five-time recycling test confirmed this, revealing essentially equal activity and selectivity throughout the experiment. As a result, CO2 to CO and CH4 conversion has high selectivity and longer durability. The band structure of the BT/ZIS composite is determined using Mott-Schottky measurement, diffuse reflectance spectroscopy, and valence band X-ray photoelectron spectroscopy. This research demonstrates a novel approach to investigating effective, stable, and selective photocatalytic CO2 reduction systems for solar-to-chemical energy conversion.

Activation of CO2 by Direct Cleavage Triggered by Photoelectrons on Rutile TiO2(110)

The initial activation of the inert CO₂ is a key step in its photoreduction to valuable chemicals. This process was proposed to proceed mainly by CO₂ accepting a photoelectron to form a CO₂^•- radical or by CO₂ accepting two photoelectrons and a proton to form the HCOO^- anion on the prototypical rutile TiO₂(110) surface. Here, we reveal a new mechanism, in which CO₂ is directly cleaved to CO and the adsorbed O^2- anion under the trigger of two photoelectrons, by using density functional theory calculations with the HSE06 hybrid functional. The newly revealed mechanism is more favorable than the two previously proposed pathways. Furthermore, our results show that the deficiency of photoelectrons on the catalyst surface is a potential reason for the current low efficiency of CO₂ photoreduction.

Iso-valent doping of reducible oxides: a comparison of rutile (110) and anatase (101) TiO2surfaces

We studied the role of iso-valent heteroatoms replacing Ti⁴⁺cations in the lattice of two titania polymorphs, rutile (r-) and anatase (a-) by means of first principles calculations. The r-TiO₂(110) and the a-TiO₂(101) surfaces have been considered and Ti ions in the bulk, sub-surface, and surface sites have been replaced with Si, Ge, Sn, Pb, Zr, Hf, and Ce ions: surface or sub-surface sites are clearly preferred. Since the dopants have the same number of valence electrons as the replaced Ti atom, they can have only two effects: one is steric, related to the different size of the dopant compared to Ti⁴⁺; the other is an orbital effect, due to the energy levels associated to the dopant not present on the pristine surface. Both these effects can modify locally the geometric and electronic structure of the surface, in particular by introducing new states in the band gap. To check the effect of the dopants on the surface reactivity we studied as an example the decomposition of HCOOH which can follow four different paths with desorption of (a) H_2,(b) CO, (c) H₂O, or (d) CO₂. The results show the very different behavior of the two titania polymorphs considered, rutile and anatase: rutile is more reactive and more easily reduced than anatase. For specular reasons, the presence of the dopants has in general more pronounced effects on anatase, as they can deeply modify the surface reactivity and the HCOOH decomposition path.

High-Throughput Screening of Atomic Defects in MXenes for CO2 Capture, Activation, and Dissociation

The capture, activation, and dissociation of carbon dioxide (CO₂) is of fundamental interest to overcome the ramifications of the greenhouse effect. In this regard, high-throughput screening of two-dimensional MXenes has been examined using well-resolved first-principles simulations through DFT-D3 dispersion correction. We systematically investigated different types of structural defects to understand their influence on the performance of M₂X-type MXenes. Defect calculations demonstrate that the formation of M₂C(V_MC) and M₂N(V_MN) vacancies require higher energy, while M₂C(V_C) and M₂N(V_N) vacancies are favorable to form during the synthesis of M₂X-type MXenes. The M₂X-type MXenes from group III to VII series show remarkable behavior for active capturing of CO₂, especially group IV (Ti₂X and Zr₂X) MXenes exhibit unprecedentedly high adsorption energies and charge transfer (>2e) from M₂X to CO₂. The potential CO₂ capture, activation, and dissociation abilities of MXenes are emanated from Dewar interactions involving hybridization between π orbitals of CO₂ and metal d-orbitals. Our high-throughput screening demonstrates chemisorption of CO₂ on pure and defective MXenes, followed by dissociation into CO and O species.

Activating the FeS (001) Surface for CO2 Adsorption and Reduction through the Formation of Sulfur Vacancies: A DFT-D3 Study

As a promising material for heterogeneous catalytic applications, layered iron (II) monosulfide (FeS) contains active edges and an inert basal (001) plane. Activating the basal (001) plane could improve the catalytic performance of the FeS material towards CO2 activation and reduction reactions. Herein, we report dispersion-corrected density functional theory (DFT-D3) calculations of the adsorption of CO2 and the elementary steps involved in its reduction through the reverse water-gas shift reaction on a defective FeS (001) surface containing sulfur vacancies. The exposed Fe sites resulting from the creation of sulfur vacancies are shown to act as highly active sites for CO2 activation and reduction. Based on the calculated adsorption energies, we show that the CO2 molecules will outcompete H2O and H2 molecules for the exposed active Fe sites if all three molecules are present on or near the surface. The CO2 molecule is found to weakly physisorb (−0.20 eV) compared to the sulfur-deficient (001) surface where it adsorbs much strongly, releasing adsorption energy of −1.78 and −1.83 eV at the defective FeS (001) surface containing a single and double sulfur vacancy, respectively. The CO2 molecule gained significant charge from the interacting surface Fe ions at the defective surface upon adsorption, which resulted in activation of the C–O bonds confirmed via vibrational frequency analyses. The reaction and activation energy barriers of the elementary steps involved in the CO2 hydrogenation reactions to form CO and H2O species are also unraveled.

Surface chemistry of TiO2 connecting thermal catalysis and photocatalysis

Chemical reactions catalyzed under heterogeneous conditions have recently expanded rapidly from traditional thermal catalysis to photocatalysis due to the rising concerns about sustainable development of energy and the environment. Adsorption of reactants on catalyst surfaces, subsequent surface reactions, and desorption of products from catalyst surfaces occur in both thermal catalysis and photocatalysis. TiO₂ catalysts are widely used in thermal catalytic and photocatalytic reactions. Herein we review recent progress in surface chemistry, thermal catalysis and photocatalysis of TiO₂ model catalysts from single crystals to nanocrystals with the aim of examining if the surface chemistry of TiO₂ can bridge the fundamental understanding between thermal catalysis and photocatalysis. Following a brief introduction, the structures of major facets exposed on TiO₂ catalysts, including surface reconstructions and defects, as well as the electronic structure and charge properties, are firstly summarized; then the recent progress in adsorption, thermal chemistry and photochemistry of small molecules on TiO₂ single crystals and nanocrystals is comprehensively reviewed, focusing on manifesting the structure-(photo)activity relations and the commonalities/differences between thermal catalysis and photocatalysis; and finally concluding remarks and perspectives are given.

Surface chemistry and photochemistry of small molecules on rutile TiO2(001) and TiO2(011)-(2 × 1) surfaces: The crucial roles of defects

Surface chemistry and photochemistry of small molecules on the rutile TiO₂(001) and TiO₂(011)-(2 × 1) surfaces were studied by low energy electron diffraction, thermal desorption spectroscopy, and x-ray photoelectron spectroscopy. It was found that the TiO₂(001) surface mainly exhibits the defects of Ti interstitials in the near-surface region, while the TiO₂(011)-(2 × 1) surface mainly exhibits the defects of double-oxygen vacancies. The defect structures of TiO₂ surfaces strongly affect their adsorption and thermal/photodesorption behaviors. On the TiO₂(001) surface, CH₃OH and H₂O dissociatively adsorb at the surface Ti sites near Ti interstitials; O₂ molecularly adsorbs at the surface Ti sites adjacent to Ti interstitials, forming photoactive O₂ species that undergoes a hole-mediated photodesorption process; CO adsorbs at the nearest surface Ti sites close to the Ti interstitials, but CO₂ does not, and the resulting CO species is photoactive; and both CO and CO₂ species adsorbed at the normal Ti⁴⁺ sites are photoinactive. On the TiO₂(011)-(2 × 1) surface, O₂ adsorbs only at the double-oxygen vacancy sites, and the resulting O₂ species dissociates to form two oxygen atoms to refill in the oxygen vacancies upon heating; CO₂ adsorbs at the double-oxygen vacancy sites, but CO does not, and the resulting CO₂ species is photoactive; and both CO and CO₂ species adsorbed at the surface Ti⁴⁺ sites are photoinactive. These results broaden the fundamental understandings of the chemistry and photochemistry of TiO₂ surfaces, and the established structure-reactivity relation of small molecules on TiO₂ surfaces is useful in probing complex structures of TiO₂ powder catalysts.

CO2 capture, activation and dissociation on the Ti2C surface and Ti2C MXene: the role of surface structure

Barrier-less CO₂ activation on Ti₂C(100) and MXene with preferential adsorption on the (100) surface and a lower dissociation barrier on MXene.

Molecular adsorption and dissociation of CO2 on TiO2 anatase (001) activated by oxygen vacancies

A study on the influence of oxygen vacancies on the anatase (001) surface on the CO₂ adsorption process is presented. For its realization, density functional theory (DFT) was used under the Perdew-Burke-Ernzerhof (PBE) generalized gradient and the spin-polarized approximations. Hubbard-U corrections and van der Waals interactions were also included. Three different types of oxygen vacancies were investigated at different sites on the anatase (001) surface; the formation energies in each case were 67.05, 113.84, and 93.16 kcal/mol, respectively. We identified a type of oxygen vacancy that could favor both the CO₂ adsorption and dissociation. The differences on CO₂ adsorption properties are due to electronic and structural causes, such as midgap states (Ti³⁺ polarons species) and changes in the structural properties on the TiO₂ surface, generated upon the introduction of an oxygen vacancy. It is concluded that oxygen vacancies can play an important role in both CO₂ adsorption and dissociation.

CO2 adsorption on the copper surfaces: van der Waals density functional and TPD studies

We investigated the adsorption of CO₂ on the flat, stepped, and kinked copper surfaces from density functional theory calculations as well as the temperature programmed desorption and X-ray photoelectron spectroscopy. Several exchange-correlation functionals have been considered to characterize CO₂ adsorption on the copper surfaces. We used the van der Waals density functionals (vdW-DFs), i.e., the original vdW-DF (vdW-DF1), optB86b-vdW, and rev-vdW-DF2, as well as the Perdew-Burke-Ernzerhof (PBE) with dispersion correction (PBE-D2). We have found that vdW-DF1 and rev-vdW-DF2 functionals slightly underestimate the adsorption energy, while PBE-D2 and optB86b-vdW functionals give better agreement with the experimental estimation for CO₂ on Cu(111). The calculated CO₂ adsorption energies on the flat, stepped, and kinked Cu surfaces are 20-27 kJ/mol, which are compatible with the general notion of physisorbed species on solid surfaces. Our results provide a useful insight into appropriate vdW functionals for further investigation of related CO₂ activation on Cu surfaces such as methanol synthesis and higher alcohol production.

On the structure of superbasic (MgO)n sites solvated in a faujasite zeolite

Theory and experiment reveal the structure of magnesium oxide nanoclusters in a superbasic faujasite zeolite.

Imaging the ordering of a weakly adsorbed two-dimensional condensate: ambient-pressure microscopy and spectroscopy of CO2 molecules on rutile TiO2(110)

Disorder–order transitions found for CO₂ on titania.

Revealing the influence of Cyano in Anchoring Groups of Organic Dyes on Adsorption Stability and Photovoltaic Properties for Dye-Sensitized Solar Cells

Determining an ideal adsorption configuration for a dye on the semiconductor surface is an important task in improving the overall efficiency of dye-sensitized solar cells. Here, we present a detailed investigation of different adsorption configurations of designed model dyes on TiO2 anatase (101) surface using first principles methods. Particularly, we aimed to investigate the influence of cyano group in the anchoring part of dye on its adsorption stability and the overall photovoltaic properties such as open circuit voltage, electron injection ability to the surface. Our results indicate that the inclusion of cyano group increases the stability of adsorption only when it adsorbs via CN with the surface and it decreases the photovoltaic properties when it does not involve in binding. In addition, we also considered full dyes based on the results of model dyes and investigated the different strength of acceptor abilities on stability and electron injection ability. Among the various adsorption configurations considered here, the bidentate bridging mode (A3) is more appropriate one which has higher electron injection ability, larger VOC value and more importantly it has higher dye loading on the surface.

Ambient Carbon Dioxide Capture Using Boron-Rich Porous Boron Nitride: A Theoretical Study

The development of highly efficient sorbent materials for CO₂ capture under ambient conditions is of great importance for reducing the impact of CO₂ on the environment and climate change. In this account, strong CO₂ adsorption on a boron antisite (B_N) in boron-rich porous boron nitrides (p-BN) was developed and studied. The results indicated that the material achieved larger adsorption energies of 2.09 eV (201.66 kJ/mol, PBE-D). The electronic structure calculations suggested that the introduction of B_N in p-BN induced defect electronic states in the energy gap region, which strongly impacted the adsorption properties of the material. The bonding between the B_N defect and the CO₂ molecule was clarified, and it was found that the electron donation first occurred from CO₂ to the B_N double-acceptor state then, followed by electron back-donation from B_N to CO₂ accompanied by the formation of a B_N-C bond. The thermodynamic properties indicated that the adsorption of CO₂ on the B_N defect to form anionic CO₂^δ- species was spontaneous at temperatures below 350 K. Both the large adsorption energies and the thermodynamic properties ensured that p-BN with a B_N defect could effectively capture CO₂ under ambient conditions. Finally, to evaluate the energetic stability, the defect formation energies were estimated. The formation energy of the B_N defects was found to strongly depend on the chemical environment, and the selection of different reactants (B or N sources) would achieve the goal of reducing the formation energy. These findings provided a useful guidance for the design and fabrication of a porous BN sorbent for CO₂ capture.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.

Surface Adsorption Energetics Studied with "Gold Standard" Wave-Function-Based Ab Initio Methods: Small-Molecule Binding to TiO2(110)

Coupled-cluster theory with single, double, and perturbative triple excitations (CCSD(T)) is widely considered to be the "gold standard" of ab initio quantum chemistry. Using the domain-based pair natural orbital local correlation concept (DLPNO-CCSD(T)), these calculations can be performed on systems with hundreds of atoms at an accuracy of ∼99.9% of the canonical CCSD(T) method. This allows for ab initio calculations providing reference adsorption energetics at solid surfaces with an accuracy approaching 1 kcal/mol. This is an invaluable asset, not least for the assessment of density functional theory (DFT) as the prevalent approach for large-scale production calculations in energy or catalysis applications. Here we use DLPNO-CCSD(T) with embedded cluster models to compute entire adsorbate potential energy surfaces for the binding of a set of prototypical closed-shell molecules (H₂O, NH₃, CH₄, CH₃OH, CO₂) to the rutile TiO₂(110) surface. The DLPNO-CCSD(T) calculations show excellent agreement with available experimental data, even for the "infamous" challenge of correctly predicting the CO₂ adsorption geometry. The numerical efficiency of the approach is within 1 order of magnitude of hybrid-level DFT calculations, hence blurring the borders between reference and production technique.

Insight into the mechanism about the initiation, growth and termination of the C–C chain in syngas conversion on the Co(0001) surface: a theoretical study

A mechanism is proposed for the initiation, growth and termination of the C–C chain from syngas on the Co(0001) surface. R represents hydrogen or an alkyl group.

Insight into both coverage and surface structure dependent CO adsorption and activation on different Ni surfaces from DFT and atomistic thermodynamics

CO adsorption and activation from low to high coverage on Ni catalyst.

Mechanistic and microkinetic analysis of CO2 hydrogenation on ceria

We evaluate the formate and carbonate routes for CO₂ hydrogenation to methanol on oxygen-deficient ceria using thermochemistry and microkinetic analyses.

Adsorption and interaction of CO2 on rutile TiO2(110) surfaces: a combined UHV-FTIRS and theoretical simulation study

P-polarized RAIR spectra of CO₂ adsorbed on reduced rutile TiO₂(110) surfaces as a function of CO₂ dosage and adsorption temperature. The incidence plane is along (a) the [001] azimuth and (b) the [11̄0] azimuth respectively.

Adsorption properties of p-methyl red monomeric-to-pentameric dye aggregates on anatase (101) titania surfaces: first-principles calculations of dye/TiO₂ photoanode interfaces for dye-sensitized solar cells

The optical and electronic properties of dye aggregates of p-methyl red on a TiO2 anatase (101) surface were modeled as a function of aggregation order (monomer to pentameric dye) via first-principles calculations. A progressive red-shifting and intensity increase toward the visible region in UV-vis absorption spectra is observed from monomeric-to-tetrameric dyes, with each molecule in a given aggregate binding to one of the four possible TiO2 (101) adsorption sites. The pentamer exhibits a blue-shifted peak wavelength in the UV-vis absorption spectra and less absorption intensity in the visible region in comparison; a corresponding manifestation of H-aggregation occurs since one of these five molecules cannot occupy an adsorption site. This finding is consistent with experiment. Calculated density of states (DOS) and partial DOS spectra reveal similar dye···TiO2 nanocomposite conduction band characteristics but different valence band features. Associated molecular orbital distributions reveal dye-to-TiO2 interfacial charge transfer in all five differing aggregate orders; meanwhile, the level of intramolecular charge transfer in the dye becomes progressively localized around its azo- and electron-donating groups, up to the tetrameric dye/TiO2 species. Dye adsorption energies and dye coverage levels are calculated and compared with experiment. Overall, the findings of this case study serve to aid the molecular design of azo dyes toward better performing DSSC devices wherein they are incorporated. In addition, they provide a helpful example reference for understanding the effects of dye aggregation on the adsorbate···TiO2 interfacial optical and electronic properties.

ReaxFF molecular dynamics simulations of CO collisions on an O-preadsorbed silica surface

A quasiclassical trajectory dynamics study was performed for carbon monoxide collisions over an oxygen preadsorbed β-cristobalite (001) surface. A reactive molecular force field (ReaxFF) was used to model the potential energy surface. The collisions were performed fixing several initial conditions: CO rovibrational states (v = 0-5 and j = 0, 20, 35), collision energies (0.05 ≤ E(col) ≤ 2.5 eV), incident angles (θ(v) = 0°, 45°) and surface temperatures (T(surf) = 300 K, 900 K). The principal elementary processes were the molecular reflection and the non-dissociative molecular adsorption. CO₂ molecules were also formed in minor extension via an Eley-Rideal reaction although some of them were finally retained on the surface. The scattered CO molecules tend to be translationally colder and internally hotter (rotationally and vibrationally) than the initial ones. The present study supports that CO + O(ad) reaction should be less important than O + O(ad) reaction over silica for similar initial conditions of reactants, in agreement with experimental data.

Water Adsorption at the Tetrahedral Titania Surface Layer of SrTiO3(110)-(4 × 1)

The interaction of water with oxide surfaces is of great interest for both fundamental science and applications. We present a combined theoretical (density functional theory (DFT)) and experimental (scanning tunneling microscopy (STM) and photoemission spectroscopy (PES)) study of water interaction with the two-dimensional titania overlayer that terminates the SrTiO3(110)-(4 × 1) surface and consists of TiO4 tetrahedra. STM and core-level and valence band PES show that H2O neither adsorbs nor dissociates on the stoichiometric surface at room temperature, whereas it does dissociate at oxygen vacancies. This is in agreement with DFT calculations, which show that the energy barriers for water dissociation on the stoichiometric and reduced surfaces are 1.7 and 0.9 eV, respectively. We propose that water weakly adsorbs on two-dimensional, tetrahedrally coordinated overlayers.

Carbon dioxide activation and dissociation on ceria (110): a density functional theory study

Ceria (CeO(2)) is a promising catalyst for the reduction of carbon dioxide (CO(2)) to liquid fuels and commodity chemicals, in part because of its high oxygen storage capacity, yet the fundamentals of CO(2) adsorption, activation, and reduction on ceria surfaces remain largely unknown. We use density functional theory, corrected for onsite Coulombic interactions (GGA+U), to explore various adsorption sites and configurations for CO(2) on stoichiometric and reduced ceria (110), the latter with either an in-plane oxygen vacancy or a split oxygen vacancy. We find that CO(2) adsorption on both reduced ceria (110) surfaces is thermodynamically favored over the corresponding adsorption on stoichiometric ceria (110), but the most stable adsorption configuration consists of CO(2) adsorbed parallel to the reduced ceria (110) surface at a split oxygen vacancy. Structural changes in the CO(2) molecule are also observed upon adsorption. At the split vacancy, the molecule bends out of plane to form a unidentate carbonate with the remaining oxygen anion at the surface; this is in stark contrast to the bridged carbonate observed for CO(2) adsorption at the in-plane vacancy. Also, we analyze the pathways for CO(2) conversion to CO on reduced ceria (110). The subtle difference in the energies of activation for the elementary steps suggest that CO(2) dissociation is favored on the split vacancy, while the reverse process of CO oxidation may favor the formation of the in-plane vacancy. We thus show how the structure and properties of the ceria catalyst govern the mechanism of CO(2) activation and reduction.

Computational approaches to the chemical conversion of carbon dioxide

The conversion of CO₂ into fuels and chemicals is viewed as an attractive route for controlling the atmospheric concentration and recycling of this greenhouse gas, but its industrial application is limited by the low selectivity and activity of the current catalysts. Theoretical modeling, in particular density functional theory (DFT) simulations, provides a powerful and effective tool to discover chemical reaction mechanisms and design new catalysts for the chemical conversion of CO₂, overcoming the repetitious and time/labor consuming trial-and-error experimental processes. In this article we give a comprehensive survey of recent advances on mechanism determination by DFT calculations for the catalytic hydrogenation of CO₂ into CO, CH₄, CH₃OH, and HCOOH, and CO₂ methanation, as well as the photo- and electrochemical reduction of CO₂. DFT-guided design procedures of new catalytic systems are also reviewed, and challenges and perspectives in this field are outlined.

Supramolecular ordering of PTCDA molecules: the key role of dispersion forces in an unusual transition from physisorbed into chemisorbed state

Adsorption and self-assembly of large π-conjugated 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) molecules on rutile TiO(2)(110) surface have been investigated using a combination of high-resolution scanning tunneling microscopy (STM), low-energy electron diffraction, and density functional theory calculations with inclusion of Grimme treatment of the dispersion forces (DFT-D). Evolution of the STM images as a function of PTCDA coverage is caused by transition of the adsorption mode from physisorbed single adspecies and meandering stripes into spontaneously ordered chemisorbed molecular assemblies. This change in the adsorption fashion is accompanied by significant bending of the intrinsically flat, yet elastic, PTCDA molecule, which allows for strong electronic coupling of the dye adspecies with the TiO(2) substrate. Extensive DFT-D modeling has revealed that adsorption is controlled by interfacial and intermolecular dispersion forces playing a dominant role in the adsorption of single PTCDA species, their self-organization into the meandering stripes, and at the monolayer coverage acting collectively to surmount the chemisorption energy barrier associated with the molecule bending. Analysis of the resulting density of states has revealed that alignment of the energy levels and strong electronic coupling at the PTCDA/TiO(2) interface are beneficial for dye sensitization purposes.