Achieving complete electrooxidation of ethanol by single atomic Rh decoration of Pt nanocubes.

SignificanceDirect ethanol fuel cells are attracting growing attention as portable power sources due to their advantages such as higher mass-energy density than hydrogen and less toxicity than methanol. However, it is challenging to achieve the complete electrooxidation to generate 12 electrons per ethanol, resulting in a low fuel utilization efficiency. This manuscript reports the complete ethanol electrooxidation by engineering efficient catalysts via single-atom modification. The combined electrochemical measurements, in situ characterization, and density functional theory calculations unravel synergistic effects of single Rh atoms and Pt nanocubes and identify reaction pathways leading to the selective C-C bond cleavage to oxidize ethanol to CO2. This study provides a unique single-atom approach to tune the activity and selectivity toward complicated electrocatalytic reactions.

Density functional theory study of bulk properties of transition metal nitrides.

Density functional theory (DFT) calculations are performed to compute the lattice constants, formation energies and vacancy formation energies of transition metal nitrides (TMNs) for transition metals (TM) ranging from 3d-5d series. The results obtained using six different DFT exchange and correlation potentials (LDA, AM05, BLYP, PBE, rPBE, and PBEsol) show that the experimental lattice constants are best predicted by rPBE, while the values obtained using AM05, PBE, rPBE and PBEsol lie between the LDA and BLYP calculated values. A linear relationship is observed between the lattice constants and formation energies with the mean radii of TM and the difference in the electronegativity of TM and N in TMNs, respectively. Our calculated vacancy formation energies, in general, show that N-vacancies are more favorable than TM-vacancies in most TMNs. We observe that N-vacancy formation energies are linearly correlated with the calculated bulk formation energies indicating that TMNs with large negative formation energies are less susceptible to the formation of N-vacancies. Thus, our results from this extensive DFT study not only provide a systematic comparison of various DFT functionals in calculating the properties of TMNs but also serve as reference data for the computation-driven experimental design of materials.

Tuning Reaction Pathways of Electrochemical Conversion of CO2 by Growing Pd Shells on Ag Nanocubes.

The electrochemical carbon dioxide reduction reaction (CO2RR) has been studied on Ag, Pd, Ag@Pd1-2L nanocubes using a combination of in situ characterization and density functional theory calculations. By manipulating the deposition and diffusion rates of Pd atoms on Ag nanocubes, Ag@Pd core-shell nanocubes with a shell thickness of 1-2 atomic layers have been successfully synthesized for CO2RR. Pd nanocubes produce CO with high selectivity due to the transformation of Pd to Pd hydride (PdH) during CO2RR. In contrast, PdH formation becomes more difficult in Ag@Pd1-2L core-shell nanocubes, which inhibits CO production from the *HOCO intermediate and thus tunes the reaction pathway toward HCOOH. Ag nanocubes exhibit high selectivity toward H2, and there is no phase transition during CO2RR. The results demonstrate that the CO2RR reaction pathways can be manipulated through engineering the surface structure of Pd-based catalysts by allowing or preventing the formation of PdH.

Metal-Coordinated Phthalocyanines as Platform Molecules for Understanding Isolated Metal Sites in the Electrochemical Reduction of CO2.

Single-atom catalysts (SACs) of non-precious transition metals (TMs) often show unique electrochemical performance, including the electrochemical carbon dioxide reduction reaction (CO2RR). However, the inhomogeneity in their structures makes it difficult to directly compare SACs of different TM for their CO2RR activity, selectivity, and reaction mechanisms. In this study, the comparison of isolated TMs (Fe, Co, Ni, Cu, and Zn) is systematically investigated using a series of crystalline molecular catalysts, namely TM-coordinated phthalocyanines (TM-Pcs), to directly compare the intrinsic role of the TMs with identical local coordination environments on the CO2RR performance. The combined experimental measurements, in situ characterization, and density functional theory calculations of TM-Pc catalysts reveal a TM-dependent CO2RR activity and selectivity, with the free energy difference of ΔG(*HOCO) - ΔG(*CO) being identified as a descriptor for predicting the CO2RR performance.

Pt- and Pd-modified transition metal nitride catalysts for the hydrogen evolution reaction.

Hydrogen production via electrochemical splitting of water using renewable electricity represents a promising strategy. Currently, platinum group metals (PGMs) are the best performing hydrogen evolution reaction (HER) catalysts. Thus, the design of non-PGM catalysts or low-loading PGM catalysts is essential for the commercial development of hydrogen generation technologies via electrochemical splitting of water. Here, we employed density functional theory (DFT) calculations to explore Pt and Pd modified transition metal nitrides (TMNs) as low-cost HER catalysts. Our calculations show that Pt/Pd binds strongly with TMs on TMN(111) surfaces, leading to the formation of stable Pt and Pd-monolayer (ML)-TMN(111) structures. Furthermore, our calculated hydrogen binding energy (HBE) demonstrates that Pt/MnN, Pt/TiN, Pt/FeN, Pt/VN, Pt/HfN, Pd/FeN, Pd/TaN, Pd/NbN, Pd/TiN, Pd/HfN, Pd/MnN, Pd/ScN, Pd/VN, and Pd/ZrN are promising candidates for the HER with a low value of limiting potential (UL) similar to that calculated on Pt(111).

Density functional theory studies of transition metal carbides and nitrides as electrocatalysts.

Transition metal carbides and nitrides are interesting non-precious materials that have been shown to replace or reduce the loading of precious metals for catalyzing several important electrochemical reactions. The purpose of this review is to summarize density functional theory (DFT) studies, describe reaction pathways, identify activity and selectivity descriptors, and present a future outlook in designing carbide and nitride catalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (N2RR), CO2 reduction reaction (CO2RR) and alcohol oxidation reactions. This topic is of high interest to scientific communities working in the field of electrocatalysis and this review should provide theoretical guidance for the rational design of improved carbide and nitride electrocatalysts.

Active sites for tandem reactions of CO2 reduction and ethane dehydrogenation.

Ethylene (C2H4) is one of the most important raw materials for chemical industry. The tandem reactions of CO2-assisted dehydrogenation of ethane (C2H6) to ethylene creates an opportunity to effectively use the underutilized ethane from shale gas while mitigating anthropogenic CO2 emissions. Here we identify the most likely active sites over CeO2-supported NiFe catalysts by using combined in situ characterization with density-functional theory (DFT) calculations. The experimental and theoretical results reveal that the Ni-FeO x interfacial sites can selectively break the C-H bonds and preserve the C-C bond of C2H6 to produce ethylene, while the Ni-CeO x interfacial sites efficiently cleave all of the C-H and C-C bonds to produce synthesis gas. Controlled synthesis of the two distinct active sites enables rational enhancement of the ethylene selectivity for the CO2-assisted dehydrogenation of ethane.

Boosting Activity and Selectivity of CO2 Electroreduction by Pre-Hydridizing Pd Nanocubes.

The electrochemical CO2 reduction reaction (CO2 RR) to syngas represents a promising solution to mitigate CO2 emissions and manufacture value-added chemicals. Palladium (Pd) has been identified as a potential candidate for syngas production via CO2 RR due to its transformation to Pd hydride under CO2 RR conditions, however, the pre-hydridized effect on the catalytic properties of Pd-based electrocatalysts has not been investigated. Herein, pre-hydridized Pd nanocubes (PdH0.40 ) supported on carbon black (PdH0.40 NCs/C) are directly prepared from a chemical reduction method. Compared with Pd nanocubes (Pd NCs/C), PdH0.40 NCs/C presented an enhanced CO2 RR performance due to its less cathodic phase transformation revealed by the in situ X-ray absorption spectroscopy. Density functional theory calculations revealed different binding energies of key reaction intermediates on PdH0.40 NCs/C and Pd NCs/C. Study of the size effect further suggests that NCs of smaller sizes show higher activity due to their more abundant active sites (edge and corner sites) for CO2 RR. The pre-hydridization and reduced NC size together lead to significantly improved activity and selectivity of CO2 RR.

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single-Atom Catalysts.

The electrochemical CO2 reduction reaction (CO2 RR) to yield synthesis gas (syngas, CO and H2 ) has been considered as a promising method to realize the net reduction in CO2 emission. However, it is challenging to balance the CO2 RR activity and the CO/H2 ratio. To address this issue, nitrogen-doped carbon supported single-atom catalysts are designed as electrocatalysts to produce syngas from CO2 RR. While Co and Ni single-atom catalysts are selective in producing H2 and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm-2 ) with CO/H2 ratios (0.23-2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single-atom configurations for the H2 and CO evolution. The results present a useful case on how non-precious transition metal species can maintain high CO2 RR activity with tunable CO/H2 ratios.

SO2-Induced Selectivity Change in CO2 Electroreduction.

Electrochemical conversion of carbon dioxide (CO2) to value-added chemicals has attracted much attention in recent years as a potential alternative to fossil resources. Although significant works have studied the influence of impurities in the electrolyte (e.g., metal ions), few studies have been performed to understand the influence of gaseous impurities in CO2 electroreduction. Herein, we study the effects of sulfur dioxide (SO2) on Ag-, Sn-, and Cu-catalyzed CO2 electrolysis in a flow-cell electrolyzer in near-neutral electrolyte, representing a broad range of CO2 reduction catalysts. We show that the presence of SO2 impurity reduces the efficiency of converting CO2 due to the preferential reduction of SO2. In the cases of Ag and Sn, the effect of SO2 impurity was reversible and the catalytic activities of both catalysts were recovered. On the contrary, a shift in selectivity toward formate accompanied by a suppression of multicarbon (C2+) products was observed on Cu catalyst, demonstrating that Cu is highly sensitive to SO2 impurity. Our results suggest that CO2 obtained from direct air capture technologies or biorefineries could be more suitable for Cu-catalyzed CO2 electrolysis as these CO2 sources would be relatively cleaner (SO2-free) than fossil-derived sources such as power plants and can be directly coupled with distributed renewable energy sources such as wind and solar.

Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride.

Despite recent intense interest in the development of catalysts for the electrochemical nitrogen reduction reaction (ENRR), mechanistic understanding and catalyst design principles remain lacking. In this work, we develop a strategy to determine the density of initial and steady-state active sites on ENRR catalysts that follow the Mars-van Krevelen mechanism via quantitative isotope-exchange experiments. This method allows the comparison of intrinsic activities of active sites and facilitates the identification and improvement of active-site structures for ENRR. Combined with detailed density functional theory calculations, we show that the rate-limiting step in the ENRR is likely the initial N≡N bond activation via the addition of a proton and an electron to the adsorbed N2 on the N vacancies to form N2 H. The methodology developed and mechanistic insights gained in this work could guide the rational catalyst design in the ENRR.

Enhancing Activity and Reducing Cost for Electrochemical Reduction of CO2 by Supporting Palladium on Metal Carbides.

Electrochemical CO2 reduction reaction (CO2 RR) with renewable electricity is a potentially sustainable method to reduce CO2 emissions. Palladium supported on cost-effective transition-metal carbides (TMCs) are studied to reduce the Pd usage and tune the activity and selectivity of the CO2 RR to produce synthesis gas, using a combined approach of studying thin films and practical powder catalysts, in situ characterization, and density functional theory (DFT) calculations. Notably, Pd/TaC exhibits higher CO2 RR activity, stability and CO Faradaic efficiency than those of commercial Pd/C while significantly reducing the Pd loading. In situ measurements confirm the transformation of Pd into hydride (PdH) under the CO2 RR environment. DFT calculations reveal that the TMC substrates modify the binding energies of key intermediates on supported PdH. This work suggests the prospect of using TMCs as low-cost and stable substrates to support and modify Pd for enhanced CO2 RR activity.

Mechanistic Insights into Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride Nanoparticles.

Renewable production of ammonia, a building block for most fertilizers, via the electrochemical nitrogen reduction reaction (ENRR) is desirable; however, a selective electrocatalyst is lacking. Here we show that vanadium nitride (VN) nanoparticles are active, selective, and stable ENRR catalysts with an ENRR rate and a Faradaic efficiency (FE) of 3.3 × 10-10 mol s-1 cm-2 and 6.0% at -0.1 V within 1 h, respectively. ENRR with 15N2 as the feed produces both 14NH3 and 15NH3, which indicates that the reaction follows a Mars-van Krevelen mechanism. Ex situ X-ray photoelectron spectroscopy characterization of fresh and spent catalysts reveals that multiple vanadium oxide, oxynitride, and nitride species are present on the surface and identified VN0.7O0.45 as the active phase in the ENRR. Operando X-ray absorption spectroscopy and catalyst durability test results corroborate this hypothesis and indicate that the conversion of VN0.7O0.45 to the VN phase leads to catalyst deactivation. We hypothesize that only the surface N sites adjacent to a surface O are active in the ENRR. An ammonia production rate of 1.1 × 10-10 mol s-1 cm-2 can be maintained for 116 h, with a steady-state turnover number of 431.

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 in a weakly adsorbed two-dimensional film have been identified for the first time using ambient-pressure scanning tunneling microscopy (AP-STM) and X-ray photoelectron spectroscopy (AP-XPS). As of late, great effort has been devoted to the capture, activation and conversion of carbon dioxide (CO2), a ubiquitous greenhouse gas and by-product of many chemical processes. The high stability and non-polar nature of CO2 leads to weak bonding with well-defined surfaces of metals and oxides. CO2 adsorbs molecularly on the rutile TiO2(110) surface with a low adsorption energy of ∼10 kcal mol-1. In spite of this weak binding, images of AP-STM show that a substantial amount of CO2 can reside on a TiO2(110) surface at room temperature forming two-dimensionally ordered films. We have employed microscopic imaging under in situ conditions, soft X-ray spectroscopy and theory to decipher the unique ordering behavior seen for CO2 on TiO2(110).

Tuning Selectivity of CO2 Hydrogenation Reactions at the Metal/Oxide Interface.

The chemical transformation of CO2 not only mitigates the anthropogenic CO2 emission into the Earth's atmosphere but also produces carbon compounds that can be used as precursors for the production of chemicals and fuels. The activation and conversion of CO2 can be achieved on multifunctional catalytic sites available at the metal/oxide interface by taking advantage of the synergy between the metal nanoparticles and oxide support. Herein, we look at the recent progress in mechanistic studies of CO2 hydrogenation to C1 (CO, CH3OH, and CH4) compounds on metal/oxide catalysts. On this basis, we are able to provide a better understanding of the complex reaction network, grasp the capability of manipulating structure and combination of metal and oxide at the interface in tuning selectivity, and identify the key descriptors to control the activity and, in particular, the selectivity of catalysts. Finally, we also discuss challenges and future research opportunities for tuning the selective conversion of CO2 on metal/oxide catalysts.

Optimizing Binding Energies of Key Intermediates for CO2 Hydrogenation to Methanol over Oxide-Supported Copper.

Rational optimization of catalytic performance has been one of the major challenges in catalysis. Here we report a bottom-up study on the ability of TiO2 and ZrO2 to optimize the CO2 conversion to methanol on Cu, using combined density functional theory (DFT) calculations, kinetic Monte Carlo (KMC) simulations, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements, and steady-state flow reactor tests. The theoretical results from DFT and KMC agree with in situ DRIFTS measurements, showing that both TiO2 and ZrO2 help to promote methanol synthesis on Cu via carboxyl intermediates and the reverse water-gas-shift (RWGS) pathway; the formate intermediates, on the other hand, likely act as a spectator eventually. The origin of the superior promoting effect of ZrO2 is associated with the fine-tuning capability of reduced Zr(3+) at the interface, being able to bind the key reaction intermediates, e.g. *CO2, *CO, *HCO, and *H2CO, moderately to facilitate methanol formation. This study demonstrates the importance of synergy between theory and experiments to elucidate the complex reaction mechanisms of CO2 hydrogenation for the realization of a better catalyst by design.

Three-dimensional ruthenium-doped TiO2 sea urchins for enhanced visible-light-responsive H2 production.

Three-dimensional (3D) monodispersed sea urchin-like Ru-doped rutile TiO2 hierarchical architectures composed of radially aligned, densely-packed TiO2 nanorods have been successfully synthesized via an acid-hydrothermal method at low temperature without the assistance of any structure-directing agent and post annealing treatment. The addition of a minuscule concentration of ruthenium dopants remarkably catalyzes the formation of the 3D urchin structure and drives the enhanced photocatalytic H2 production under visible light irradiation, not possible on undoped and bulk rutile TiO2. Increasing ruthenium doping dosage not only increases the surface area up to 166 m(2) g(-1) but also induces enhanced photoresponse in the regime of visible and near infrared light. The doping introduces defect impurity levels, i.e. oxygen vacancy and under-coordinated Ti(3+), significantly below the conduction band of TiO2, and ruthenium species act as electron donors/acceptors that accelerate the photogenerated hole and electron transfer and efficiently suppress the rapid charge recombination, therefore improving the visible-light-driven activity.

CO2 Hydrogenation over Oxide-Supported PtCo Catalysts: The Role of the Oxide Support in Determining the Product Selectivity.

By simply changing the oxide support, the selectivity of a metal-oxide catalysts can be tuned. For the CO2 hydrogenation over PtCo bimetallic catalysts supported on different reducible oxides (CeO2 , ZrO2 , and TiO2 ), replacing a TiO2 support by CeO2 or ZrO2 selectively strengthens the binding of C,O-bound and O-bound species at the PtCo-oxide interface, leading to a different product selectivity. These results reveal mechanistic insights into how the catalytic performance of metal-oxide catalysts can be fine-tuned.

Low Pressure CO2 Hydrogenation to Methanol over Gold Nanoparticles Activated on a CeO(x)/TiO2 Interface.

Capture and recycling of CO2 into valuable chemicals such as alcohols could help mitigate its emissions into the atmosphere. Due to its inert nature, the activation of CO2 is a critical step in improving the overall reaction kinetics during its chemical conversion. Although pure gold is an inert noble metal and cannot catalyze hydrogenation reactions, it can be activated when deposited as nanoparticles on the appropriate oxide support. In this combined experimental and theoretical study, it is shown that an electronic polarization at the metal-oxide interface of Au nanoparticles anchored and stabilized on a CeO(x)/TiO2 substrate generates active centers for CO2 adsorption and its low pressure hydrogenation, leading to a higher selectivity toward methanol. This study illustrates the importance of localized electronic properties and structure in catalysis for achieving higher alcohol selectivity from CO2 hydrogenation.

Identifying Different Types of Catalysts for CO2 Reduction by Ethane through Dry Reforming and Oxidative Dehydrogenation.

The recent shale gas boom combined with the requirement to reduce atmospheric CO2 have created an opportunity for using both raw materials (shale gas and CO2 ) in a single process. Shale gas is primarily made up of methane, but ethane comprises about 10 % and reserves are underutilized. Two routes have been investigated by combining ethane decomposition with CO2 reduction to produce products of higher value. The first reaction is ethane dry reforming which produces synthesis gas (CO+H2 ). The second route is oxidative dehydrogenation which produces ethylene using CO2 as a soft oxidant. The results of this study indicate that the Pt/CeO2 catalyst shows promise for the production of synthesis gas, while Mo2 C-based materials preserve the CC bond of ethane to produce ethylene. These findings are supported by density functional theory (DFT) calculations and X-ray absorption near-edge spectroscopy (XANES) characterization of the catalysts under in situ reaction conditions.