Inverse Catalysts Research Articles

Insights into the Surface Electronic Structure and Catalytic Activity of InOx/Au(111) Inverse Catalysts for CO2 Hydrogenation to Methanol

Insights into the Surface Electronic Structure and Catalytic Activity of InO_<i>x</i>/Au(111) Inverse Catalysts for CO₂ Hydrogenation to Methanol

Modulating charge transfer and constructing synergistic bimetallic active sites for CO-SCR reactions in inverse spinel

The selective catalytic reduction of NO by CO (CO-SCR) are frequently catalyzed by spinel due to its exceptional stability and redox capabilities. In this research, a ternary metal-based inverse spinel catalyst Cu0.67Mn0.33Fe2O4 was synthesized utilizing the glycerol self-templating technique. Some Fe3+ (Fe3+Td) sites are transferred to vacant octahedral sites through the occupancy of tetrahedral sites by Mn species. Mn3+ (Mn3+Td) sites synergistically interact with Cu2+ and Fe3+ ions on the octahedral sites to promote charge transfer, producing numerous oxygen-defective and highly mobile oxygen species and constructing Cu+-Ov-Fe2+ active sites on the surface. Various characterizations demonstrated that Cu+-Ov-Fe2+ has greater catalytic activity, resistance to alkali metals, and water toxicity than the Cu2+-Ov-Fe2+ active site of CuFe2O4. In situ DRIFTS further revealed the reaction mechanism of the ternary metal-based inverse spinel catalyst, and this study provides novel perspectives on the development of Cu-based catalysts for CO-SCR.

Atomic Level Understanding of the Structural Stability and Catalytic Activity of Nanoporous Gold/Titania Cluster Inverse Catalysts at Ambient and High Temperatures.

Nanoporous gold (NPG) exhibits exceptional catalytic performance at low temperatures, but its activity declines at elevated temperatures due to structural coarsening. Loading metal oxide nanoparticles onto NPG can enhance its catalytic activity at high temperatures. In this work, we used NPG-supported titania nanoparticles as a model system (denoted as Ti2O4/NPG) to study their catalytic activity at ambient and high temperatures with CO oxidation as a probe reaction by density functional theory (DFT) calculation and ab initio molecular dynamics (AIMD) simulations. The possible factors that may affect the CO oxidation reaction pathways and energy profiles on the Ti2O4/NPG, such as oxygen vacancies; silver impurities; Mars-van Krevelen (MvK), Eley-Rideal (ER), or trimolecular Eley-Rideal (TER) mechanisms; and catalytic active sites, were comprehensively investigated. The results showed that reaction energy barriers on Ti2O4/NPG were not significantly decreased compared to the pristine NPG, indicating that their catalytic activities at ambient temperature were comparable. At the evaluated temperature (400 °C), the Ti2O4/NPG exhibited superior thermal stability and maintained its active sites, while the NPG reduced active sites due to surface coarsening. The strong oxide-metal interaction (SOMI) effect between the NPG and Ti2O4 nanoparticles is found to be a main factor for the high structural stability and catalytic activity at high temperatures.

Inverse ceria-nickel catalyst for enhanced C-O bond hydrogenolysis of biomass and polyether.

Regulating interfacial electronic structure of oxide-metal composite catalyst for the selective transformation of biomass or plastic waste into high-value chemicals through specific C-O bond scission is still challenging due to the presence of multiple reducible bonds and low catalytic activity. Herein, we find that the inverse catalyst of 4CeOx/Ni can efficiently transform various lignocellulose derivatives and polyether into the corresponding value-added hydroxyl-containing chemicals with activity enhancement (up to 36.5-fold increase in rate) compared to the conventional metal/oxide supported catalyst. In situ experiments and theoretical calculations reveal the electron-rich interfacial Ce and Ni species are responsible for the selective adsorption of C-O bond and efficient generation of Hδ- species, respectively, which synergistic facilitate cleavage of C-O bond and subsequent hydrogenation. This work advances the fundamental understanding of interfacial electronic interaction over inverse catalyst and provides a promising catalyst design strategy for efficient transformation of C-O bond.

Intensified O2 activation and lattice O supply at inverse oxide-metal interface for catalytic oxidation reactions: A case study in alcohol oxidation

Inverse oxide/metal catalysts are recently employed and of great importance in catalytic oxidation reactions, due to the high activity and sintering-resistance. However, the investigations for identification of active-site structure, adsorption-activation of O2, and insight into interface-enhanced mechanism are relatively scare. Herein, the inverse “CoOx-Au” ensemble (∼10 nm CoOx nanoparticles onto ∼30 nm Au particles) for gas-phase alcohol oxidation was exampled to investigate above issues, under the premise of delivering 92 % benzyl alcohol conversion with 98 % benzaldehyde selectivity at 240 °C and atmospheric pressure. It is revealed that O2 is activated at CoOx-Au interface. The “CoO-Au ↔ Co3O4-Au” redox cycle at CoOx-Au interface proceeds over the “CoO↔Co3O4” cycle on CoOx surface, due to the weakened interfacial Co-O bond. And the enhanced redox cycle intensifies O2 activation and lattice O supply. The O2 activation at inverse oxide-metal interface provides promising clue for rational design of inverse catalysts for catalytic oxidation reactions.

Electronic Oxide-Metal Strong Interactions (EOMSI) Localized at CeOx-Ag Interface.

Electronic oxide-metal strong interactions (EOMSI) refer to the electronic oxide-metal interactions (EOMI) between oxide adlayers and underlying metal substrate that is strong enough to stabilize supported oxide adlayers in a low-oxidation state, which individually is not stable under an ambient condition, from high temperature oxidation in air to a certain extent. Herein we report the deposition and electronic structure of CeOx adlayers on capping ligand-free cubic Ag nanocrystals, i.e., CeOx/Ag inverse catalysts. The EOMI occur via the charge transfer from Ag substrate to CeOx adlayers in the CeOx/Ag inverse catalyst, and the EOMSI are observed in the CeOx/Ag inverse catalyst with the average thickness of CeOx adlayers about 0.9 nm to exclusively form Ce2O3 adlayers stable against oxidation at 400 °C. As the thickness of CeOx adlayers increases, ceria adlayers with oxygen vacancies (CeO2-x) emerge and grow in the CeOx/Ag inverse catalysts, and the Ce3+/Ce4+ ratio decreases. Catalytic performance of CeOx/Ag inverse catalysts in the CO oxidation reaction is closely linked with the thickness and electronic structure of CeOx adlayers. These results demonstrate that the EOMSI and EOMI in the oxide/metal inverse catalysts are localized at the oxide-metal interface and sensitively vary with the thickness of oxide adlayers, offering a strategy of thickness engineering to tune electronic structures of oxide adlayers in oxide/metal inverse catalysts.

Metal Supported Metal Oxide Catalysts for Hydrogen Peroxide Production Via Water Splitting

The two-electron water oxidation reaction (WOR) has drawn recent attention as an efficient, simple method of producing on-site hydrogen peroxide through electrochemical water splitting. However, finding electrocatalysts with high activity and stability in the reaction’s harsh oxidizing environment is challenging. This is made especially difficult due to the competing four-electron and one-electron WOR, which necessitate a catalyst material with high selectivity towards the hydrogen peroxide production pathway. Here, we utilize metal/metal oxide coupling interactions to tune metal oxide catalysts to be active for 2 e- WOR via the creation of a Mott-Schottky junction. Metal/metal oxide interactions have been well studied for supported metal catalysts, but their behavior has been less investigated for catalysts where the metal oxide provides the active sites, called inverse catalysts. In this case, the difference in work functions between the metal support and metal oxide semiconductor catalyst drives electron transfer between the two, facilitating electronic interactions which can modulate the binding energy of reaction intermediates at the interface and thus tune the catalytic ability of the metal oxide. In this work, we investigate a variety of metal oxide catalyst and metal support materials and find that a thin layer of indium tin oxide (ITO) supported by a layer of platinum (Pt) has excellent catalytic ability towards 2 e- WOR. The activity, selectivity, and stability of the ITO/Pt catalyst shows significant improvement over the unsupported ITO catalyst and yields hydrogen peroxide production rates greater than those reported in literature using other standard catalysts for this reaction. This method of catalyst engineering also provides a future pathway to create new catalysts for this electrochemical reaction. Figure 1

Supported Inverse MnOx/Pt Catalysts Facilitate Reverse Water Gas Shift Reaction

Catalytic conversion of CO2 to CO via the reverse water gas shift (RWGS) reaction has been identified as a promising approach for CO2 utilization and mitigation of CO2 emissions. Bare Pt shows low activity for the RWGS reaction due to its low oxophilicity, with few research works having concentrated on the inverse metal oxide/Pt catalyst for the RWGS reaction. In this work, MnOx was deposited on the Pt surface over a SiO2 support to prepare the MnOx/Pt inverse catalyst via a co-impregnation method. Addition of 0.5 wt% Mn to 1 wt% Pt/SiO2 improved the intrinsic reaction rate and turnover frequency at 400 °C by two and twelve times, respectively. Characterizations indicate that MnOx partially encapsulates the surface of the Pt particles and the coverage increases with increasing Mn content, which resembles the concept of strong metal–support interaction (SMSI). Although the surface accessible Pt sites are reduced, new MnOx/Pt interfacial perimeter sites are created, which provide both hydrogenation and C-O activation functionalities synergistically due to the close proximity between Pt and MnOx at the interface, and therefore improve the activity. Moreover, the stability is also significantly improved due to the coverage of Pt by MnOx. This work demonstrates a simple method to tune the oxide/metal interfacial sites of inverse Pt-based catalyst for the RWGS reaction.

CeO2/Ni Inverse Catalyst as a Highly Active and Stable Ru-free Catalyst for Ammonia Decomposition

CeO₂/Ni Inverse Catalyst as a Highly Active and Stable Ru-free Catalyst for Ammonia Decomposition

Optimizing methanol synthesis from CO2 hydrogenation over inverse Zr-Cu catalyst

Optimizing methanol synthesis from CO2 hydrogenation over inverse Zr-Cu catalyst

A Data and DFT-Driven Framework for Predicting the Microstructure of Submonolayer Inverse Metal Oxide on Metal Catalysts.

Metal oxides on metal (inverse) catalysts can selectively drive many important reactions. However, understanding the active site under experimentally relevant conditions is lacking. Herein, we introduce a computational framework for predicting atomic models of stable inverse catalysts and demonstrate it for WOx on Pt(553) and a Pt79 nanoparticle at variable WOx coverages. An evolutionary algorithm identifies a small (5%) subset of promising atomic configurations on which DFT simulations are performed. We predict a maximum coverage of ∼50% WOx on Pt(553), consisting of small clusters (tetramers and pentamers), which preferentially reside on the terrace, with their oxygen atoms interacting with the Pt step sites. Consistently, WOx does not lie on curved and undercoordinated metal sites of Pt nanoparticles. The oxide clusters prefer a partially reduced oxidation state. Theoretical EXAFS spectra for select configurations provide insights into interpreting experimental spectra of inverse catalysts. The framework applies to other catalysts.

Controlling oxide promoter coverage and microstructure on metals of inverse catalysts: Application to liquid phase tetrahydrofurfuryl alcohol conversion to 1,5-pentanediol

Metal M1/metal oxide M2Ox (M1M2Ox) inverse catalysts, where the oxide layer rests atop metal, have gained attention for their distinct catalytic performance. They are intensively studied in biomass upgrading, e.g., the hydrogenolysis of tetrahydrofurfuryl alcohol to produce 1,5-pentanediol. Pt and MOx (M = W, Mo, Re, Nb) exhibit remarkable synergism in activity and selectivity, but the active sites remain poorly understood. Here, we examine the influence of oxide loading on PtMOx inverse catalysts and introduce a high-pressure wash treatment to leach the excess oxide from carbon and optimize their structure. The findings reveal a saturation sub-monolayer MOx coverage with 2D atomic structure on Pt that is crucial for performance; excessive loading leads to nanocrystalline of lower activity, and low loading exposes unselective metal sites. Wash treatment selectively removes MOx from carbon, enhances their dispersion on Pt, and improves, in most cases, the performance. Tuning the inverse structure advances structure-reactivity understanding.

Metal substrate engineering to modulate CO2 hydrogenation to methanol on inverse Zr3O6/CuPd catalysts.

It is well known that the performance of some key catalytic reactions has a strong dependence on metal catalyst surfaces. In the current work, this concept is further extended to the CuPd alloy-supported zirconium oxide inverse catalyst for CO2 hydrogenation to methanol. A combined DFT and microkinetic simulation study reveal that both the metal substrate surface and the precise exposed Cu or Pd metal atoms on the substrate have a pivotal influence on the catalytic mechanism and performance of the inverse catalyst for CO2 hydrogenation to methanol. Herein, CuPd(100), (111), and (110) surfaces with either Cu and Pd terminations have been examined, which provided five metal substrates as support for the inverse catalyst. Three different mechanisms, including the formate pathway, RWGS + CO-hydro pathway, and CO2 direct activation pathway, are explored under the same conditions; they take place at the interfacial sites between the metal alloy and oxide. The calculations indicated that the inverse catalyst with the CuPd(100) substrate demonstrates better performance than those with CuPd(110) and (111) for both formate and RWGS + CO-hydro mechanisms. Conversely, the reaction pathway is more sensitive to exposed atoms on the metal substrate. The best inverse catalyst, Zr3O6/CuPd(100) with either Cu or Pd terminations, demonstrated a methanol formation TOF above 0.30 site-1 s-1 and the selectivity was above 90% at 573 K, as evaluated from microkinetic simulation. The coverage analysis indicates the most populated species is HCOO*, which is consistent with experimental reports. Both kinetic and thermodynamics control steps are identified from DRC analysis for the best performing catalysts. Overall, the current study confirms the catalytic performance of the inverse Zr3O6/CuPd catalyst and demonstrates the tunable effects of the metal alloy substrate, which can facilitate effective optimization.

Cu-supported nano-ZrZnOx as a highly active inverse catalyst for low temperature methanol synthesis from CO2 hydrogenation

Hydrogenation of CO2 into methanol at low-temperature on Cu-based catalysts is of great significance, but remains challenging to enhance activity. In this paper, we report an inverse catalyst constructed with nano-ZrZnOx supported on Cu particles with outstanding methanol synthesis performance at 220 ℃, two times higher than that of commercial Cu/ZnO/Al2O3 catalysts under the same conditions. Detailed structure characterization and performance evaluation demonstrate that the ZrZnOx mixed oxide serves as the most active oxide-metal interface site for CO2 hydrogenation. The ZrZnOx/Cu inverse catalyst increases the weak and medium CO2 adsorption sites which are further demonstrated responsible to the methanol productivity. In situ DRIFTs studies reveal that the inverse interface accelerates the reduction of asymmetric formate intermediates and prevents the generation of CO. The combination of enhanced CO2 activation capability and accelerated hydrogenation rate of intermediates over the ZrZnOx/Cu inverse catalyst probably contribute to the remarkable methanol synthesis performance from CO2.

CO2 activation and dissociation on the Fe2O3/Cu(111) inverse catalyst: A dispersion-corrected DFT study

Density functional calculations have been performed to study the adsorption and dissociation of CO2 on a model of Fe2O3/Cu(111) inverse catalyst employing the PBE-D3 functional. The adsorption of the Fe2O3 cluster on Cu(111) surface was found to be highly exothermic accompanied by a noticeable deformation of its structure. An electronic charge transfer from Cu to Fe2O3 was observed and the appearance of a partially occupied Fe 4 s electronic state around the Fermi level was evidenced. This available electronic charge plays a significant role in the subsequent CO2 adsorption. This molecule adsorbs preferentially at the oxide–metal interface with a bent geometry. The adsorption involves a charge transfer from Fe2O3/Cu(111) to CO2, yielding an activated CO2δ- species. On pristine Cu(111) and Fe2O3(0001) surfaces, the CO2 adsorption is weaker, and the almost null charge transfer indicates a non-activated CO2 molecule. The dissociation of CO2 on Fe2O3/Cu(111) was calculated to be slightly exothermic, endothermic on Cu(111) and strongly endothermic on Fe2O3(0001). The activation barrier for CO2 dissociation was found to be 0.90 eV on Fe2O3/Cu(111), significantly lower than on Cu(111) and Fe2O3(0001). Our calculations show a great potential for the use of Fe2O3/Cu(111) inverse catalysts in reactions involving CO2 activation and dissociation.

Nature of Zirconia on a Copper Inverse Catalyst Under CO2 Hydrogenation Conditions.

The growing concern over the escalating levels of anthropogenic CO2 emissions necessitates effective strategies for its conversion to valuable chemicals and fuels. In this research, we embark on a comprehensive investigation of the nature of zirconia on a copper inverse catalyst under the conditions of CO2 hydrogenation to methanol. We employ density functional theory calculations in combination with the Grand Canonical Basin Hopping method, enabling an exploration of the free energy surface including a variable amount of adsorbates within the relevant reaction conditions. Our focus centers on a model three-atom Zr cluster on a Cu(111) surface decorated with various OH, O, and formate ligands, noted Zr3Ox (OH)y (HCOO)z/Cu(111), revealing major changes in the active site induced by various reaction parameters such as the gas pressure, temperature, conversion levels, and CO2/H2 feed ratios. Through our analysis, we have unveiled insights into the dynamic behavior of the catalyst. Specifically, under reaction conditions, we observe a large number of composition and structures with similar free energy for the catalyst, with respect to changing the type, number, and binding sites of adsorbates, suggesting that the active site should be regarded as a statistical ensemble of diverse structures that interconvert.

Palladium Encapsulated by an Oxygen‐Saturated TiO2 Overlayer for Low‐Temperature SO2‐Tolerant Catalysis during CO Oxidation

AbstractThe development of oxidation catalysts that are resistant to sulfur poisoning is crucial for extending the lifespan of catalysts in real‐working conditions. Herein, we describe the design and synthesis of oxide‐metal interaction (OMI) catalyst under oxidative atmospheres. By using organic coated TiO2, an oxide/metal inverse catalyst with non‐classical oxygen‐saturated TiO2 overlayers were obtained at relatively low temperature. These catalysts were found to incorporate ultra‐small Pd metal and support particles with exceptional reactivity and stability for CO oxidation (under 21 vol % O2 and 10 vol % H2O). In particular, the core (Pd)‐shell (TiO2) structured OMI catalyst exhibited excellent resistance to SO2 poisoning, yielding robust CO oxidation performance at 120 °C for 240 h (at 100 ppm SO2 and 10 vol % H2O). The stability of this new OMI catalyst was explained through density functional theory (DFT) calculations that interfacial oxygen atoms at Pd−O−Ti sites (of oxygen‐saturated overlayers) serve as non‐metal active sites for low‐temperature CO oxidation, and change the SO2 adsorption from metal(d)‐to‐SO2(π*) back‐bonding to much weaker σ(Ti−S) bonding.

Palladium Encapsulated by an Oxygen-Saturated TiO2 Overlayer for Low-Temperature SO2 -Tolerant Catalysis during CO Oxidation.

The development of oxidation catalysts that are resistant to sulfur poisoning is crucial for extending the lifespan of catalysts in real-working conditions. Herein, we describe the design and synthesis of oxide-metal interaction (OMI) catalyst under oxidative atmospheres. By using organic coated TiO2 , an oxide/metal inverse catalyst with non-classical oxygen-saturated TiO2 overlayers were obtained at relatively low temperature. These catalysts were found to incorporate ultra-small Pd metal and support particles with exceptional reactivity and stability for CO oxidation (under 21 vol % O2 and 10 vol % H2 O). In particular, the core (Pd)-shell (TiO2 ) structured OMI catalyst exhibited excellent resistance to SO2 poisoning, yielding robust CO oxidation performance at 120 °C for 240 h (at 100 ppm SO2 and 10 vol % H2 O). The stability of this new OMI catalyst was explained through density functional theory (DFT) calculations that interfacial oxygen atoms at Pd-O-Ti sites (of oxygen-saturated overlayers) serve as non-metal active sites for low-temperature CO oxidation, and change the SO2 adsorption from metal(d)-to-SO2 (π*) back-bonding to much weaker σ(Ti-S) bonding.

The role of the metal core in the WOx inverse catalyst performance

The role of the metal core in the WOx inverse catalyst performance

Dynamic Structural Evolution of CeO2 in CuO−CeO2 Catalyst Revealed by In Situ Spectroscopy

AbstractThe oxides and active metals at the interface synergistically activate reactants and thus promote the reaction, but the interface structure often changes dynamically during the reaction. In the conventional supported catalysts, the metals at the interface have been extensively studied, while the structural evolution of oxides is often overlooked due to the interference of the bulk phase signal. In this work, CeO2−CuO inverse catalysts are designed to reveal the dynamic structure evolution of CeO2 in the CeO2−CuO system during the water gas shift (WGS) reaction by in situ Raman, in situ XRD, quasi in situ XPS, and near ambient pressure XPS (NAP‐XPS). CeO2 is partially concealed in the CuO phase in the un‐pretreated catalyst and gradually exposed to the surface, forming an inverse CeOx/Cu structure during the reducing process. This structure exhibits a high catalytic activity in the WGS reaction and remains durable under the reductive conditions. When the inverse CeOx/Cu structure is exposed to the non‐redox conditions, the reconfiguration of the reduced oxide is observed which is caused by the oxygen migration of CeO2. This work explores the structure evolution of CeO2 in CeO2−CuO inverse catalyst under different conditions by in situ characterization technique and provides a reference for monitoring the dynamic changes of oxide structure.