Inverse Catalysts Research Articles

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.

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

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

Cu Facet-Dependent Elementary Surface Reaction Kinetics of CO2 Hydrogenation to Methanol Catalyzed by ZrO2/Cu Inverse Catalysts.

ZrO2-Cu-based catalysts are active in catalyzing the hydrogenation of CO2 to methanol. Herein, we report Cu facet effects on the catalytic performance of ZrO2/Cu inverse catalysts in CO2 hydrogenation to methanol using various Cu nanocrystals with well-defined Cu morphologies and facets. The ZrO2-Cu interface is the active site, in which the ZrO2-Cu{100} and ZrO2-Cu{110} interfaces exhibit similar apparent activation energies of ∼42.6 kJ/mol, smaller than that of the ZrO2-Cu{111} interface (∼64.5 kJ/mol). Temporal in situ diffuse reflectance infrared Fourier transform spectroscopy characterization results identify the bridge formate hydrogenation as the rate-determining elementary surface reaction under typical reaction temperatures, whose activation energy is similar at the ZrO2-Cu{100} (∼36.3 kJ/mol) and ZrO2-Cu{110} (∼40.5 kJ/mol) interfaces and larger at the ZrO2-Cu{111} interface (∼54.5 kJ/mol). This fundamental understanding suggests Cu facet engineering as a promising strategy to improve the catalytic performance of ZrO2/Cu inverse catalysts for CO2 hydrogenation to methanol.

CO self-sustained catalytic combustion over morphological inverse model CeO2/Cu2O catalysts exposing (1 0 0), (1 1 1) and (1 1 0) planes

CO self-sustaining catalytic combustion tends to be one of potential means to achieve efficient conversion of off-gases from steelmaking. Building model catalysts to investigate the reaction process academically is essential to design optimized catalysts and Cu2O exposing (1 0 0), (1 1 1) and (1 1 0) planes are excellent materials. Herein, morphological CeO2/Cu2O inverse model catalysts were synthesized to reveal the synergistic effect and reaction mechanism. It is demonstrated that the ignition temperature is lowered on the interface by synergistic effect, which dominates the ignition, while the combustion after ignition is controlled by Cu2O planes providing abundant lattice oxygen to react with CO. Additionally, exposure of different planes plays a significant role in the reaction since O-termination on (1 0 0) inhibits the interaction, whereas Cu-termination on (1 1 0) surface structure and coordinately unsaturated Cu+ on open (1 1 1) surface respectively contribute to their superior catalytic performance.

CeO2/Cu2O/Cu Tandem Interfaces for Efficient Water-Gas Shift Reaction Catalysis.

Metal-oxide interfaces on Cu-based catalysts play very important roles in the low-temperature water-gas shift reaction (LT-WGSR). However, developing catalysts with abundant, active, and robust Cu-metal oxide interfaces under LT-WGSR conditions remains challenging. Herein, we report the successful development of an inverse copper-ceria catalyst (Cu@CeO2), which exhibited very high efficiency for the LT-WGSR. At a reaction temperature of 250 °C, the LT-WGSR activity of the Cu@CeO2 catalyst was about three times higher than that of a pristine Cu catalyst without CeO2. Comprehensive quasi-in situ structural characterizations indicated that the Cu@CeO2 catalyst was rich in CeO2/Cu2O/Cu tandem interfaces. Reaction kinetics studies and density functional theory (DFT) calculations revealed that the Cu+/Cu0 interfaces were the active sites for the LT-WGSR, while adjacent CeO2 nanoparticles play a key role in activating H2O and stabilizing the Cu+/Cu0 interfaces. Our study highlights the role of the CeO2/Cu2O/Cu tandem interface in regulating catalyst activity and stability, thus contributing to the development of improved Cu-based catalysts for the LT-WGSR.

Insights into the interfacial structure of Cu/ZrO2 catalysts for methanol synthesis from CO2 hydrogenation: Effects of Cu-supported nano-ZrO2 inverse interface

The performance of Cu-based catalysts in the low temperature CO2 conversion to methanol highly correlates with interfacial structure. In this paper, the interfacial structure of Cu/ZrO2 catalysts is tuned by changing the molar ratio and precipitation sequence of Cu and Zr precursors in the oxalate precipitation method. Structural characterizations demonstrate that the interface structure gradually transforms from the conventional ZrO2-supported nano-Cu interface to the Cu-supported nano-ZrO2 inverse interface with the ascending Cu/Zr ratio. The space–time yield (STY) of methanol over 90 molCu% Cu/ZrO2 inverse catalyst is 518 gCH3OH/kgcat/h, which far exceeds that of the optimized 40 molCu% Cu/ZrO2 catalyst (351 gCH3OH/kgcat/h). The inverse catalyst also exhibits the best methanol selectivity and lowest apparent activation energy. The correlation of the exposed Cu surface area, Cu particle size, and CO2 adsorption capacity with STYmethanol reveal that there is an optimal coverage of fine ZrO2 particles and the Cu supported nano-ZrO2 inverse configuration is a more suitable structure for methanol synthesis from CO2. High-pressure in situ DRIFTS experiment shows that the adsorbed formate and methoxy intermediates over 90 molCu% Cu/ZrO2 inverse catalyst are more reactive for further hydrogenation. In addition, the inverse configuration is more favorable for the methanol desorption from surface. The enhanced hydrogenation ability and relatively weak oxygenates adsorption of Cu-supported nano-ZrO2 interface is probably the main reasons for the improved catalytic performances of inverse catalysts.

Stable Strain State of Single‐Twinned AgPdF Nanoalloys under Formate Oxidation Reaction

Surface reconstruction as common phenomenon during catalysis complicates the prediction and modeling on catalytic activity of the nanoalloy, hence developing a stable structure to be resistant to surface restructuring would provide an ideal prototype for substantial and reliable mechanism analysis. Herein, the single‐twinned structure in inverse AgPdF catalyst is constructed to enhance the catalytic activity and stability for the formate oxidation reaction (FOR). The single‐twinned AgPdF nanoalloy (t‐AgPdF) catalyst exhibits an enhanced peak current density of 4.6 A mgPd−1, a reduced onset potential of 0.44 V, a higher activity retention of 55.7% after 600 cycles, and a longer activity retention time of 55.9 h. Additionally, the t‐AgPdF catalyst presents a higher hydrogen generation rate of 1.11 mL mgPd−1 than that of single‐crystalline AgPd nanoalloy (AgPd) catalyst, and density functional theory calculations reveal that t‐AgPdF(111) surface exhibits a reduced activation energy of 0.59 eV for formate decomposition reaction. Impressively, the t‐AgPdF maintains compressive and tensile strain state along the Σ3 twin boundaries before and after the FOR, in contrast to AgPd. This is the first time to reveal that the nanotwinned structures contribute inverse t‐AgPdF catalysts the catalytic active sites with stable strain state since starting reaction for the FOR.

Mechanistic Understanding of Ring-Opening of Tetrahydrofurfuryl Alcohol over WOx-Modified Pt Model Surfaces and Powder Catalysts

The upgrading of heterocyclic biomass-derived oxygenates such as tetrahydrofurfuryl alcohol (THFA) via ring-opening is a promising pathway to produce value-added diol molecules using renewable carbon sources. This study combines model surface experiments, first-principles calculations, and powder catalyst characterization and activity evaluation to unravel the nature of the Pt and WOx active sites and the reaction mechanism of the THFA ring-opening reaction on a WOx/Pt inverse oxide catalyst. Temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS) measurements on model surfaces demonstrated that THFA ring opened on Pt(111) but underwent further decomposition due to its strong bonding with the surface. However, WOx deposited on Pt(111) altered the interaction strength between the ring-opened intermediate and the surface to a proper extent to facilitate the facile desorption of the desired 1,5-pentanediol (1,5-PeD) product. Density functional theory (DFT) calculations showed that WOx/Pt(111) could promote ring opening of THFA via an oxocarbenium ion-like transition state, which was stabilized by hydrogen bonding with the hydroxyl groups of WOx. The hydrogenation of the ring-opened 5-hydroxyvaleraldehyde intermediate to 1,5-PeD was then feasible via Brønsted acid sites present on WOx. Steady-state activity studies on the corresponding powder catalysts showed that the 1,5-PeD selectivity increased from 20% on Pt/SiO2 to 65% on WOx/Pt/SiO2 with 1 wt % WOx loading, consistent with model surface experiments and DFT calculations. This study demonstrates the feasibility of using model surface experiments and first-principles calculations to guide practical catalyst design, and provides a design strategy that can be applied to the selective ring-opening of relevant heterocyclic biomass-derived oxygenates.

Effect of the combination of inverse opal photon superficial areaic crystal and heterojunction on active free radicals and its effect on the mechanism of photocatalytic removal of organic pollutants

For the inverse opal photocatalyst, hydraulic pressure and other problems caused by the ordered structure of the inverse opal photocatalyst make it difficult for the pollutants to enter the structure quickly and effectively, so the effective control of its structure has become an urgent problem to be solved. Here, multi-heterojunction inverse opal structure catalyst with multi-level branched pore structure is prepared by the sol-gel method. The special three-dimensional dendritic porous structure solves the problem that a pollutant solution is difficult to effectively enter a skeleton with the traditional ordered inverse opal photocatalyst. This structure can also lead to the nanocrystallization of TZS powders, thus promoting the formation of the intermediate product ZrTiO4 and optimizing the heterogeneous interface in the TiO2–ZrO2 heterojunction, thereby successfully constructing the multi-heterostructure and effectively improving the redox ability. Furthermore, by adjusting the loading amount of TZS, the matching of the heterojunction and the inverse opal structure can be effectively controlled, the matching of the photonic band gap and the forbidden bandwidth can be effectively adjusted, and the degradation efficiency is greatly improved. This study provides a new idea for effective collaboration between multi-heterogeneous interfaces and 3D branch fractal structures.

Efficient conversion of lignin to alkylphenols over highly stable inverse spinel MnFe2O4 catalysts

Efficient conversion of lignin to alkylphenols over highly stable inverse spinel MnFe2O4 catalysts

Syngas-to-C2 oxygenates over the inverse Mo6C4/Cu catalyst: Identifying the role of synergistic effect

Syngas transformation into C2 oxygenates has attracted a long-term interest, however, it still faces the challenge of low selectivity and yield toward C2 oxygenates. In this study, the inverse Mo6C4/Cu catalyst is reasonably modeled and predicted to promote C2 oxygenates formation in syngas transformation using DFT calculations with the PBE functional. The results confirmed that the inverse Mo6C4/Cu catalyst not only greatly elevates catalytic performance toward CHx(x = 2, 3) monomer formation, but also facilitate C2 oxygenate production compared to the previously reported Cu4/β-Mo2C, β-Mo2C and Cu catalysts, which is attributed to the synergistic effect of Mo6C4 cluster with Cu catalyst, specifically, Mo6C4 cluster easily activates CO to produce CH2 monomer, Cu offers the undissociated CO migrated toward Mo6C4 cluster, then, CO insertion into CH2 to CH2CO over Mo6C4 cluster easily occurs. Our results offer a theoretical basis for the synergistic effect between Mo6C4 cluster with Cu catalyst in tuning catalytic performance of syngas transformation and gives a new way to rationally design the inverse catalyst with excellent performance for C2 oxygenate production from syngas.

Electronic Oxide-Support Strong Interactions in the Graphdiyne-Supported Cuprous Oxide Nanocluster Catalyst.

The interfacial interaction in supported catalysts is of great significance for heterogeneous catalysis because it can induce charge transfer, regulate electronic structure of active sites, influence reactant adsorption behavior, and eventually affect the catalytic performance. It has been theoretically and experimentally elucidated well in metal/oxide catalysts and oxide/metal inverse catalysts, but is rarely reported in carbon-supported catalysts due to the inertness of traditional carbon materials. Using an example of a graphdiyne-supported cuprous oxide nanocluster catalyst (Cu2O NCs/GDY), we herein demonstrate the strong electronic interaction between them and put forward a new type of electronic oxide-graphdiyne strong interaction, analogous to the concept of electronic oxide/metal strong interactions in oxide/metal inverse catalysts. Such electronic oxide-graphdiyne strong interaction can not only stabilize Cu2O NCs in a low-oxidation state without aggregation and oxidation under ambient conditions but also change their electronic structure, resulting in the optimized adsorption energy for reactants/intermediates and thus leading to improved catalytic activity in the Cu(I)-catalyzed azide-alkyne cycloaddition reaction. Our study will contribute to the comprehensive understanding of interfacial interactions in supported catalysts.

Activating lattice oxygen of single-layer ZnO for the catalytic oxidation reaction.

Tuning an oxide/metal interface is of critical importance for the performance enhancement of many heterogeneous catalytic reactions. However, catalytic oxidation occurring at the interface between non-reducible oxide and metal has been challenging, since non-reducible oxides hardly lose their lattice oxygen (OL) or dissociate O2 from the gas phase. In this work, a ZnO monolayer film on Au(111) is used as an inverse catalyst to investigate CO oxidation occurring at the ZnO/Au(111) interface via high pressure scanning tunneling microscopy. Surface science experiments indicate that oxygen intercalation under the ZnO monolayer film, termed ZnO/O/Au(111), can be achieved via a surface reaction with 1 × 10-6 mbar O3. Subsequent exposure of the formed ZnO/O/Au(111) surface to mbar CO at room temperature leads to the recovery of the pristine ZnO/Au(111) surface. Theoretical calculations reveal that OL adjacent to intercalated oxygen (Oint) is activated due to the OL-Zn-Oint bonding and surface corrugation, which can be directly involved in CO oxidation. Subsequently, Oint migrates to the formed oxygen vacancy from the subsurface resuming the pristine ZnO structure. These results thus reveal that oxygen intercalation underneath single-layer ZnO will strongly boost the oxidation reaction via activating adjacent lattice oxygen atoms.

FEFOS: a method to derive oxide formation energies from oxidation states

Herein we report an interpretable, computationally efficient method to forecast formation energies from oxidation states of binary oxides from unary oxide entries in Materials Project. This new method is envisioned to guide inverse catalyst design.

Electro- and photoactivation of silver-iron oxide particles as magnetically recyclable catalysts for cross-coupling reactions.

Colloidal Ag particles decorated with Fe3O4 islands can be electrochemically or photochemically activated as inverse catalysts for C(sp2)-H heteroarylation. The silver-iron oxide (SIO) particles are reduced into redox-active forms by cathodic charging at mild potentials or by short-term light exposure, and can be reused multiple times by magnetic cycling without further activation. A negative shift in the reduction peak is attributed to an overpotential produced by surface Fe3O4 which separates residual Ag ions or clusters from bulk silver. The catalytic efficiency of SIO is maintained even with acid degradation, which can be countered simply by adding water to the reaction medium.

Stabilizing Oxide Nanolayer via Interface Confinement and Surface Hydroxylation.

Surface hydroxylation over oxide catalysts often occurs in many catalytic processes involving H2 and H2O, which is considered to play an important role in elementary steps of the reactions. Here, monolayer CoO and CoOHx nanoislands on Pt(111) are used as inverse model catalysts to study the effect of surface hydroxylation on the stability of Co oxide overlayers in O2. Surface science experiments indicate that hydroxyl groups formed on CoO nanoislands produced by deuterium-spillover can enhance oxidation resistance of the Co oxide nanostructures. Theoretical calculation shows that the interfacial adhesion between CoO and Pt is linearly strengthened with the increasing hydroxylation degree of CoO surface. Thus, the interface confinement effect between CoO and Pt can be enhanced by the surface hydroxylation due to the more reduced Co ions and stronger Co-Pt bonding at the CoOHx/Pt interface.

In Situ Spectroscopic Characterization and Theoretical Calculations Identify Partially Reduced ZnO1-x /Cu Interfaces for Methanol Synthesis from CO2.

The active site of the industrial Cu/ZnO/Al2 O3 catalyst used in CO2 hydrogenation to methanol has been debated for decades. Grand challenges remain in the characterization of structure, composition, and chemical state, both microscopically and spectroscopically, and complete theoretical calculations are limited when it comes to describing the intrinsic activity of the catalyst over the diverse range of structures that emerge under realistic conditions. Here a series of inverse model catalysts of ZnO on copper hydroxide were prepared where the size of ZnO was precisely tuned from atomically dispersed species to nanoparticles using atomic layer deposition. ZnO decoration boosted methanol formation to a rate of 877 gMeOH kgcat -1 h-1 with ≈80 % selectivity at 493 K. High pressure in situ X-ray absorption spectroscopy demonstrated that the atomically dispersed ZnO species are prone to aggregate at oxygen-deficient ZnO ensembles instead of forming CuZn metal alloys. By modeling various potential active structures, density functional theory calculations and microkinetic simulations revealed that ZnO/Cu interfaces with oxygen vacancies, rather than stoichiometric interfaces, Cu and CuZn alloys were essential to catalytic activation.

In Situ Spectroscopic Characterization and Theoretical Calculations Identify Partially Reduced ZnO1−x/Cu Interfaces for Methanol Synthesis from CO2

AbstractThe active site of the industrial Cu/ZnO/Al2O3 catalyst used in CO2 hydrogenation to methanol has been debated for decades. Grand challenges remain in the characterization of structure, composition, and chemical state, both microscopically and spectroscopically, and complete theoretical calculations are limited when it comes to describing the intrinsic activity of the catalyst over the diverse range of structures that emerge under realistic conditions. Here a series of inverse model catalysts of ZnO on copper hydroxide were prepared where the size of ZnO was precisely tuned from atomically dispersed species to nanoparticles using atomic layer deposition. ZnO decoration boosted methanol formation to a rate of 877 gMeOH kgcat−1 h−1 with ≈80 % selectivity at 493 K. High pressure in situ X‐ray absorption spectroscopy demonstrated that the atomically dispersed ZnO species are prone to aggregate at oxygen‐deficient ZnO ensembles instead of forming CuZn metal alloys. By modeling various potential active structures, density functional theory calculations and microkinetic simulations revealed that ZnO/Cu interfaces with oxygen vacancies, rather than stoichiometric interfaces, Cu and CuZn alloys were essential to catalytic activation.

Modulating the dynamics of Brønsted acid sites on PtWOx inverse catalyst

Brønsted acid sites on the oxide overlayers of metal–metal oxide inverse catalysts are often hypothesized to drive selective C–O bond activation. However, the Brønsted acid site nature and dynamics under working conditions remain poorly understood due to the functionalities of the constituent materials. Here we investigate the formation and the dynamics of Brønsted acid and redox sites on PtWOx/C under working conditions. Density functional theory-based thermodynamic calculations and microkinetic modelling reveal a complex interplay between Brønsted acid and redox sites and potentially fast catalyst dynamics at comparable timescales to the Brønsted acid catalysed dehydration chemistry. Combining in situ characterization and probe chemistry, we demonstrate that the density of Brønsted acid sites on the PtWOx/C inverse catalyst could be modulated by up to two orders of magnitude by altering the reaction parameters and by the chemistry itself. We elicit an order of magnitude increase in the acid-catalysed dehydration average reaction rate by periodic hydrogen pulsing. Metal–metal oxide inverse catalysts are an intriguing class of materials, although the understanding of their structure–activity properties remains elusive. Here, Vlachos and colleagues unravel the complex dynamic interplay between Brønsted acid and redox sites at the surface of a PtWOx/C inverse catalyst, offering a strategy to tune its acid-catalysed dehydration reactivity.

Unravelling the synergy of oxygen vacancies and gold nanostars in hematite for the electrochemical and photoelectrochemical oxygen evolution reaction

The development of hematite-based electrocatalysts (EC) and photoelectrocatalysts (PEC) for oxygen evolution reaction (OER) is highly promising on account of the low-cost and favorable chemical properties. Herein, we report a unique inverse opal framework hematite-based bi-functional catalyst for both EC and PEC water oxidation in alkaline media. Under the combined action of oxygen vacancies (Vo) and gold nanostars (AuNSs) on hematite, the catalyst exhibited excellent activity and stability on both EC and PEC. The composite showed superior electrocatalytic performance for OER with a low overpotential of 281 mV at 10 mA cm−2. Density functional theory (DFT) studies reveal that the coverage of Vo controls the d-band center of surface Fe sites, and the OER activity displays a volcano relationship with the Vo coverage. The addition of gold nanoparticles on the hematite with low Vo coverage improves the adsorption strength of oxygen-containing intermediates to the optimal point and increases the OER activity. Furthermore, the as-prepared photoanode exhibits a ∼3.13 fold increase in current (1.46 mA cm−2) at 1.23 V versus RHE. It is proposed that Vo promotes bulk conductivity and surface catalysis and exhibits reduced activation energy under high light intensity. AuNSs efficiently inhibits the bulk recombination and improves carrier concentration because of the Fermi level equilibration and plasmonic resonance, and the surface catalysis compensates the deterioration of interfacial recombination of carriers induced by Vo, playing a synergistic role.