Elucidating CO2 Hydrogenation over In2 O3 Nanoparticles using Operando UV/Vis and Impedance Spectroscopies

Importance of Metal-Support Interactions for CO2 Hydrogenation: An Operando Near-Ambient Pressure X-ray Photoelectron Spectroscopy Study on Gold-Loaded In2O3 and CeO2 Catalysts

Metal-support interactions, which are essential for the design of supported metal catalysts, used, e.g., for CO2 activation, are still only partially understood. In this study of gold-loaded In2O3 and CeO2 catalysts during CO2 hydrogenation using near-ambient pressure X-ray photoelectron spectroscopy, supported by near edge X-ray absorption fine structure, we demonstrate that the role of the noble metal strongly depends upon the choice of the support material. Temperature-dependent analyses of X-ray photoelectron spectra under reaction conditions reveal that gold is reduced on CeO2, enabling direct H2 activation, but oxidized on In2O3, leading to decreased activity of Au/In2O3 compared to bare In2O3. At elevated temperatures, the catalytic activity of the In2O3 catalysts strongly increases as a result of facilitated CO2 and (In2O3-based) H2 activation, while the catalytic activity of Au/CeO2 is limited by reoxidation by CO2. Our results underline the importance of operando studies for understanding metal-support interactions to enable a rational support selection in the future.

Boosted Sacrificial-Agent-Free Selective Photoreduction of CO2 to CH3OH by Rhenium Atomically Dispersed on Indium Oxide

Solar-energy-driven photoreduction of CO2 is promising in alleviating environment burden, but suffers from low efficiency and over-reliance on sacrificial agents. Herein, rhenium (Re) is atomically dispersed in In2O3 to fabricate a 2Re-In2O3 photocatalyst. In sacrificial-agent-free photoreduction of CO2 with H2O, 2Re-In2O3 shows a long-term stable efficiency which is enhanced by 3.5 times than that of pure In2O3 and is also higher than those on Au-In2O3, Ag-In2O3, Cu-In2O3, Ir-In2O3, Ru-In2O3, Rh-In2O3 and Pt-In2O3 photocatalysts. Moreover, carbon-based product of the photoreduction overturns from CO on pure In2O3 to CH3OH on 2Re-In2O3. Re promotes charge separation, H2O dissociation and CO2 activation, thus enhancing photoreduction efficiency of CO2 on 2Re-In2O3. During the photoreduction, CO is a key intermediate. CO prefers to desorption rather than hydrogenation on pure In2O3, as CO binds to pure In2O3 very weakly. Re strengthens the interaction of CO with 2Re-In2O3 by 5.0 times, thus limiting CO desorption but enhancing CO hydrogenation to CH3OH. This could be the origin for photoreduction product overturn from CO on pure In2O3 to CH3OH on 2Re-In2O3. The present work opens a new way to boost sacrificial-agent-free photoreduction of CO2.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Unraveling surface and bulk dynamics of iron(III) molybdate during oxidative dehydrogenation using operando and transient spectroscopies

Iron(III) molybdate (Fe2(MoO4)3) is a commercial catalyst for the oxidative dehydrogenation (ODH) of methanol, but it has recently been shown to be relevant for other substrates as well. Despite its commercial use, a detailed mechanistic understanding of Fe2(MoO4)3 catalysts at the surface and in the bulk has been lacking, largely hampered by the lack of suitable spectroscopic methods, directly applicable under reaction conditions. Using propane ODH as an example, we highlight the potential of operando Raman and impedance spectroscopy combined with transient IR spectroscopy, to identify surface active sites and monitor the hydrogen transfer and oxygen dynamics. By comparison with the behavior of reference compounds (MoO3, MoOx/Fe2O3) a mechanistic model is proposed. The presence of iron greatly influences the reactivity behavior via oxygen diffusion but is moderated in its oxidative capacity by surface MoOx. Our approach directly elucidates fundamental properties of Fe2(MoO4)3 of general importance to selective oxidation catalysis.

Strained few-layer MoS2 with atomic copper and selectively exposed in-plane sulfur vacancies for CO2 hydrogenation to methanol

In-plane sulfur vacancies (Sv) in molybdenum disulfide (MoS2) were newly unveiled for CO2 hydrogenation to methanol, whereas edge Sv were found to facilitate methane formation. Thus, selective exposure and activation of basal plane is crucial for methanol synthesis. Here, we report a mesoporous silica-encapsulated MoS2 catalysts with fullerene-like structure and atomic copper (Cu/MoS2@SiO2). The main approach is based on a physically constrained topologic conversion of molybdenum dioxide (MoO2) to MoS2 within silica. The spherical curvature enables the generation of strain and Sv in inert basal plane. More importantly, fullerene-like structure of few-layer MoS2 can selectively expose in-plane Sv and reduce the exposure of edge Sv. After promotion by atomic copper, the resultant Cu/MoS2@SiO2 exhibits stable specific methanol yield of 6.11 molMeOH molMo-1 h-1 with methanol selectivity of 72.5% at 260 °C, much superior to its counterparts lacking the fullerene-like structure and copper decoration. The reaction mechanism and promoting role of copper are investigated by in-situ DRIFTS and in-situ XAS. Theoretical calculations demonstrate that the compressive strain facilitates Sv formation and CO2 hydrogenation, while tensile strain accelerates the regeneration of active sites, rationalizing the critical role of strain.

Silicalite-1 Layer Secures the Bifunctional Nature of a CO2 Hydrogenation Catalyst

Close proximity usually shortens the travel distance of reaction intermediates, thus able to promote the catalytic performance of CO₂ hydrogenation by a bifunctional catalyst, such as the widely reported In₂O₃/H-ZSM-5. However, nanoscale proximity (e.g., powder mixing, PM) more likely causes the fast deactivation of the catalyst, probably due to the migration of metals (e.g., In) that not only neutralizes the acid sites of zeolites but also leads to the reconstruction of the In₂O₃ surface, thus resulting in catalyst deactivation. Additionally, zeolite coking is another potential deactivation factor when dealing with this methanol-mediated CO₂ hydrogenation process. Herein, we reported a facile approach to overcome these three challenges by coating a layer of silicalite-1 (S-1) shell outside a zeolite H-ZSM-5 crystal for the In₂O₃/H-ZSM-5-catalyzed CO₂ hydrogenation. More specifically, the S-1 layer (1) restrains the migration of indium that preserved the acidity of H-ZSM-5 and at the same time (2) prevents the over-reduction of the In₂O₃ phase and (3) improves the catalyst lifetime by suppressing the aromatic cycle in a methanol-to-hydrocarbon conversion step. As such, the activity for the synthesis of C₂ ⁺ hydrocarbons under nanoscale proximity (PM) was successfully obtained. Moreover, an enhanced performance was observed for the S-1-coated catalyst under microscale proximity (e.g., granule mixing, GM) in comparison to the S-1-coating-free counterpart. This work highlights an effective shielding strategy to secure the bifunctional nature of a CO₂ hydrogenation catalyst.

Carbon coated In2O3 hollow tubes embedded with ultra-low content ZnO quantum dots as catalysts for CO2 hydrogenation to methanol

CO₂ hydrogenation coupled with renewable energy to produce methanol is of great interest. Carbon coated In₂O₃ hollow tube catalysts embedded with ultra-low content ZnO quantum dots (QDs) were synthesized for CO₂ hydrogenation to methanol. ZnO-In₂O₃-II catalyst had the highest CO₂ and H₂ adsorption capacity, which demonstrated the highest methanol formation rate. When CO₂ conversion was 8.9%, methanol selectivity still exceeded 86% at 3.0 MPa and 320 °C, and STY of methanol reached 0.98 g_MeOHh^-1g_cat^-1 at 350 °C. The ZnO/In₂O₃ QDs heterojunctions were formed at the interface between ZnO and In₂O₃(222). The ZnO/In₂O₃ heterojunctions, as a key structure to promote the CO₂ hydrogenation to methanol, not only enhanced the interaction between ZnO and In₂O₃ as well as CO₂ adsorption capacity, but also accelerated the electron transfer from In³⁺ to Zn²⁺. ZnO QDs boosted the dissociation and activation of H₂. The carbon layer coated on In₂O₃ surface played a role of hydrogen spillover medium, and the dissociated H atoms were transferred to the CO₂ adsorption sites on the In₂O₃ surface through the carbon layer, promoting the reaction of H atoms with CO₂ more effectively. In addition, the conductivity of carbon enhanced the electron transfer from In³⁺ to Zn²⁺. The combination of the ZnO/In₂O₃ QDs heterojunctions and carbon layer greatly improved the methanol generation activity.

The Progress of Metal-Organic Framework for Boosting CO2 Conversion

With the rapid development of modern society, environmental problems, including excessive amounts of CO2 released in the atmosphere, are becoming more and more serious. It is necessary to develop new materials and technologies to reduce pollution. Among them, metal–organic frameworks (MOFs) have shown potential for application in the area of catalysis due to their ultra-high specific surface area, structural versatility, and designability as well as ease of modification and post-synthesis. Herein, we summarize recent research advances by use of MOFs for boosting CO2 conversion. Furthermore, challenges and possible research directions related to further exploration are also discussed.