Tuning CO2 hydrogenation selectivity via metal-oxide interfacial sites

CO2 hydrogenation to methanol is one of the main and valuable catalytic reactions applied on Cu/ZnO-based catalysts; the interface formed through Zn migration from ZnO support to the surface of Cu nanoparticle (ZnOx-Cu NP-ZnO) has been reported to account for methanol synthesis from CO2 hydrogenation. However, the accompanied reverse water gas shift (RWGS) reaction significantly decreases methanol selectivity and deactivates catalysts soon. Inhibition of RWGS is thus of great importance to afford high yield of methanol. The clear understanding of the reactivity of RWGS reaction on both the direct contact Cu-ZnO interface and ZnOx-Cu NP-ZnO interface is essential to reveal the low methanol selectivity in CO2 hydrogenation to methanol and look for efficient catalysts for RWGS reaction. Cu doped plate ZnO (ZnO:XCu) model catalysts were prepared through a hydrothermal method to simulate direct contact Cu-ZnO interface and plate ZnO supported 1 wt % Cu (1Cu/ZnO) catalyst was prepared by wet impregnation for comparison in RWGS reaction. Electron paramagnetic resonance (EPR), XRD, SEM, Raman, hydrogen temperature-programmed reduction (H2-TPR) and CO2 temperature-programmed desorption (CO2-TPD) were employed to characterize these catalysts. The characterization results confirmed that Cu incorporated into ZnO lattice and finally formed direct contact Cu-ZnO interface after H2 reduction. The catalytic performance revealed that direct contact Cu-ZnO interface displays inferior RWGS reaction reactivity at reaction temperature lower than 500 °C, compared with the ZnOx-Cu NP-ZnO interface; however, it is more stable at reaction temperature higher than 500 °C, enables ZnO:XCu model catalysts superior catalytic activity to that of 1Cu/ZnO. This finding will facilitate the designing of robust and efficient catalysts for both CO2 hydrogenation to methanol and RWGS reactions.

Intermetallic compounds (IMCs) are widely employed in heterogeneous catalysis. In this paper, the performance and mechanism of In–Ni IMCs with different structures (InNi3, InNi2, InNi, and In3Ni2) in the reverse water gas shift (RWGS) reaction are reported. In situ spectroscopic and microscopic characterizations combined with density functional theory (DFT) calculation demonstrated that the increase of the In/Ni ratio in IMCs enhanced the selectivity of CO by inhibiting the adsorption of CO* via “active site isolation.” Ni, as the active site of the CO2 hydrogenation reaction, was isolated by In in In–Ni IMCs. Meanwhile, DFT calculation revealed that CO2 hydrogenation preferentially produced CO rather than CH4 and/or CH3OH, and the reaction path for the RWGS reaction is the redox path over the In–Ni IMC. At 500 °C, the catalyst achieved a 99.8% CO selectivity at a CO2 conversion of 50.7% for more than 250 h without any deactivation, rendering it a promising candidate for industrial application.

Multiple products are typically formed during CO 2 hydrogenation over oxide-supported metal catalysts, making the regulation of product selectivity both practically and fundamentally important. In this work, a silver-promoted In 2 O 3 catalyst (Ag/In 2 O 3 ) was synthesized using a deposition-precipitation method and employed as a model system to elucidate the pressure-dependent reactivity of the catalyst in CO 2 hydrogenation. A strong electronic metal-support interaction (EMSI) between Ag and defective In 2 O 3 (In 2 O 3-x ) is identified, which promotes the formation of catalytically active Ag-In 2 O 3-x interfacial sites. The Ag/In 2 O 3 catalyst exhibits a 100% CO selectivity at 0.1 MPa, while achieving high methanol selectivity at 5 MPa within the temperature range investigated. Kinetic analysis reveals that CO formation and methanol synthesis share the same active sites. Combined in situ catalyst characterization and theoretical calculations demonstrate that elevated pressure enhances CO adsorption and stabilizes surface CO*, thereby promoting the further hydrogenation of CO* to CH 3 O* and ultimately leading to high methanol selectivity. This work provides mechanistic insights into pressure-controlled selectivity in catalytic CO 2 hydrogenation and offers a promising strategy for tuning product distribution via the change in the pressure. • Ag/In 2 O 3 is active for the RWGS reaction at atmospheric pressure. • 100% CO selectivity is achieved at atmospheric pressure. • An electronic metal-support interaction (EMSI) exists in Ag/In 2 O 3 . • EMSI induces partially positively charged Ag species. • Ag/In 2 O 3 shows pressure-dependent reactivity in CO 2 hydrogenation.

Resonant catalysis theory predicts that applying an intermittent stimulus, such as charge, strain, heat or light, at frequencies on a timescale higher than the catalytic turnover frequency, can enhance activity by orders of magnitude and improve selectivity. However, experimental evidence of resonant catalysis is unfortunately lacking. This is partly due to the fact that the effect of intermittent stimulation on catalysts and reaction intermediates is not well understood. To address this challenge, time-resolved "stimulando" spectroscopic methods are needed to observe catalysts under operating conditions and during stimulation. Here, we use diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) to study the effect of intermittent light stimulation on the catalytic hydrogenation of CO2 over Ni-Ga-based catalyst materials as a model reaction. Previous research has shown that light can cause CO desorption and change the reactivity of formate intermediates during reaction. Since CO and formate species are believed to be active intermediates in CO2 hydrogenation, understanding how intermittent light affects their binding energy and surface coverage can provide insights into how to effectively stimulate catalysts to possibly achieve resonant catalysis. Ni3-Ga/SrTiO3 catalysts were synthesized using incipient wetness impregnation and tested under CO2 hydrogenation reaction conditions. Stimulando DRIFTS measurements were performed using both steady-state and rapid-scan DRIFTS to study the effect of continuous and intermittent ultraviolet (UV) light stimulation. Steady-state DRIFTS revealed that the surface coverage of CO, formate, and carbonate intermediates decreased reversibly upon UV illumination, with each species exhibiting distinct timescales to reach steady state (ranging from seconds to minutes). Furthermore, rapid-scan DRIFTS with millisecond time resolution demonstrated that spectral changes generally occurred faster when switching the UV light on compared to when switching it off in steady-state experiments. However, when using intermittent light at a 1 Hz on/off frequency, the rates of change for spectral features upon light switching on and off became comparable. This showcases the need to study catalyst stimulation under intermittent stimulation to capture the dynamic response of the system at the limit cycle. Despite the observed changes in the coverage of surface species, the CO2 hydrogenation performance of the catalyst was not significantly affected under the conditions studied herein. The stimulando spectroscopy method showcased here provides valuable insights for adjusting light stimulation parameters, such as intensity, duty cycle and light wavelength, paving the way to more effective catalyst stimulation.

Improving methanol selectivity in CO2 hydrogenation by tuning the distance of Cu on catalyst

Selective hydrogenation of CO2 into CO on a highly dispersed nickel catalyst obtained by magnetron sputtering deposition: A step towards liquid fuels

Core–shell nanoparticles are being considered for various applications due to their controllable atomic structure and improved properties compared to their bulk counterparts. In the present work, we have synthesized Cu@Mn3O4 and Cu@Co3O4 (core@shell) nanocatalysts using wet-chemical synthesis methods involving organic surfactants, and probed their surfaces using CO and H2 under reaction conditions using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The surfactant ligands used in the synthesis of the nanoparticles must be removed to allow access to the active catalyst sites. These ligands can be removed by oxidation, allowing adsorption of CO and H2. This work reports the DRIFTS results of CO adsorption and hydrogenation on Cu@Mn3O4 and Cu@Co3O4 nanoparticles after removing the ligands. The CO hydrogenation results were in agreement with the DRIFTS results, which suggested that the Cu@Co3O4 nanoparticles adsorb CO both dissociatively and associatively, creating a balance between molecular CO required for CO insertion and dissociated surface carbon species required for chain growth. This resulted in higher selectivities towards C2+ alcohols on this catalyst. On the other hand, the Cu@Mn3O4 nanoparticles showed a higher CO uptake and a lower CO dissociation activity, which resulted in a lower CHx concentration on the surface, thus limiting the rate of the CO insertion step required to form higher alcohols.

Methanol synthesis by CO and CO2 hydrogenation on Cu/γ-Al2O3 surface in liquid paraffin solution

Fischer-Tropsch (FT) synthesis has been a recursive method to form valuable molecules from syngas. Metal surfaces have been extensively studied as FT catalysts; among them, iron presents several phases under reaction conditions, oxides and carbides, as active sites for the FT and reverse water gas shift reaction. We present CO reduction on an iron sulfide phase with spinel structure, Fe3S4, also considering the pathways where C-O dissociates leaving CHx species on the surface, which may feed longer aliphatic chains via the FT process. We analysed the thermodynamic and kinetic availability of each step leading to O and OH species co-adsorbed on the surface as well as the formation of H2O from the hydrogenation of the alcohol group in the molecule. This detailed analysis led to energy profiles on both active sites of the surface, and we conclude that this Fe3S4 surface is highly selective towards the formation of methanol, in full agreement with experimental results. These findings point out that the C-C bond formation on greigite takes place through a hydroxycarbene FT mechanism.

Ordered mesoporous NiMg bimetal oxides confined KMoS catalyst for selective CO hydrogenation into higher alcohols

The reverse water gas shift (RWGS) reaction holds significant industrial importance. While directly incorporating alkali metals can enhance the catalytic activity, its contribution to enhancing stability remains rather limited. Encapsulating metal centers within zeolites offers a solution to increase stability, however the fine structure of inner active units is hard to control, bringing negative impact on activity. In this work, a dual ligand-protected strategy to control the hybridization of Pt and Na species within silicalite-1 (S1) frameworks is reported, which possesses simultaneously-improved activity and stability compared with conventional RWGS catalysts. Specifically, the catalyst achieves an impressive turnover frequency (TOF) of 207,156.4 h-1 with 100% CO selectivity at 350°C. Moreover, it retains high activity even after undergoing accelerated aging tests at 800°C. Further mechanistic investigations uncover the formation of a novel Pt─O─Na─Si unit within the confined space of S1, where a skeletal oxygen atom simultaneously coordinates with one Na⁺ ion and one Pt atom. In this hybrid structure, the Pt centers exhibit a significantly-raised oxidation state, which was identified as the key factor for the enhancement in RWGS activity: the positive oxidation state facilitates CO desorption, thereby promoting the forward progression of the reaction.

The reverse water-gas shift (RWGS) reaction provides an important pathway for converting CO2 to CO, which can then be used to produce high-value chemicals through Fischer-Tropsch synthesis. Metal oxide materials, especially Fe2O3, offer considerable potential for use in the RWGS reaction at high temperatures, but their catalytic performance is hindered by the inadequate activation and dissociation capabilities of H2 at low temperatures. Herein, we report the introduction of Fe4 with high H2 activation capability near Fe2O3, which shows a synergistic effect and achieves excellent activity and selectivity in the RWGS reaction at low temperatures. Fe2O3 nanoparticles and Fe4 clusters supported on carbon (Fe2O3-Fe4/C) were prepared via KCl-assisted pyrolysis of NH2-MIL-88B(Fe) precursors. Fe2O3-Fe4/C delivers a remarkable CO selectivity of 99.4% and a space-time yield of 15.2 μmolCO gcat-1 s-1 at 350 °C under atmospheric pressure. In situ diffuse reflection infrared Fourier-transform spectra and density functional theory calculations demonstrate that the introduction of Fe4 clusters can activate H2 and significantly lower the reaction energy barrier of CO2 hydrogenation on Fe2O3 sites, thus boosting the RWGS performance at low temperatures.

Selective hydrogenation of CO2 into dimethyl ether over hydrophobic and gallium-modified copper catalysts

The reverse water gas shift (RWGS) reaction is an important method for converting carbon dioxide (CO2) into valuable chemicals and fuels by hydrogenation. In this paper, the catalytic activity of single-atom metal-doped (M = Pt, Ir, Pd, Rh, Cu, Ni) indium oxide (c-In2O3) catalysts in the cubic phase for the RWGS reaction was investigated using density functional theory (DFT) calculations. This was achieved by identifying metal sites, screening oxygen vacancies, followed by further calculating the energy barriers for the direct and indirect dissociation pathways of the RWGS reaction. Our results show that the single-atom dopant in the indium oxide lattice promotes the creation of oxygen vacancies on the In2O3 surface, thereby facilitating the adsorption and activation of CO2 by the oxide surface and initiating the subsequent RWGS reaction. Furthermore, we find that the oxygen vacancy (OV) formation energy on the surface of the single-atom metal doped c-In2O3(111) surface can be used as a descriptor for CO2 adsorption, and the higher the OV formation energy, the more stable the CO2 adsorption structure is. The Cu/In2O3 structure has relatively high energy barriers for both direct (1.92 eV) and indirect dissociation (2.09 eV) in the RWGS reaction, indicating its low RWGS reactivity. In contrast, the Ir/In2O3 and Rh/In2O3 structures are more conducive to the direct dissociation of CO2 into CO, which may serve as more efficient RWGS catalysts. Furthermore, microkinetic simulations show that single atom metal doping to In2O3 enhances CO2 conversion, especially under high reaction temperatures, where the formation of oxygen vacancies is the limiting factor for CO2 reactivity on the M/In2O3 (M = Cu, Ir, Rh) models. Among these three single-atom catalysts, the Ir/In2O3 model was predicted to have the best CO2 reactivity at reaction temperatures above 573 K.

Supported rhodium nanoparticles (NPs) are well-known for catalyzing methanation in CO2 hydrogenation. Now we demonstrate that the selectivity in this process can be optimized for CO production by choice of molecular sieve crystals as supports. The NPs are enveloped within the crystals with controlled nanopore environments that allow tuning of the catalytic selectivity to minimize methanation and favor the reverse water-gas shift reaction. Pure silica MFI (S-1)-fixed rhodium NPs exhibited maximized CO selectivity at high CO2 conversions, whereas aluminosilicate MFI zeolite-supported rhodium NPs displayed high methane selectivity under the equivalent conditions. Strong correlations were observed between the nanoporous environment and catalytic selectivity, indicating that S-1 minimizes hydrogen spillover and favors fast desorption of CO to limit deep hydrogenation. Materials in this class appear to offer appealing opportunities for tailoring selective supported catalysts for a variety of reactions.