Tuning CO2 Hydrogenation Selectivity through Reaction-Driven Restructuring on Cu-Ni Bimetal Catalysts

A Mechanism-Based Strategy for Controlling CH4 and CO Selectivities in CO2-H2 Reactions on Dispersed Ru, Co, and Ni Nanoparticles

The formation of CO as an intermediate in the conversion of CO2-H2 to CH4 and its strong binding on dispersed Ru, Co, and Ni nanoparticles inhibit rates of CO and CH4 formation but to different extents. CO2 conversion rates decrease and CH4 selectivities increase as CO concentration gradients evolve axially along the catalyst bed and radially within diffusion-limited porous aggregates. These trends and their interpretation in terms of the identity and kinetic relevance of surface-catalyzed elementary steps enable mechanism-based strategies for selectivity control through the purposeful introduction of CO pressures into inlet CO2-H2 streams. This strategy exploits the stronger CO inhibition of its formation (from CO2) than its conversion (to CH4), which causes the selective inhibition of CO2 conversion relative to CH4 formation. The presence of CO at levels accurately prescribed by the mechanism-derived rate equations, similar in functional form on Ru, Co, and Ni nanoparticles, and by diffusion-convection-reaction models that account for CO gradients at the bed and aggregate scales led to the exclusive formation of CH4 and to the elimination of CO gradients at both scales, as evident from measured rates and selectivities for CO2-H2 reactions on Ru, Co, and Ni nanoparticles over a broad and practical range of temperature (483-573 K), reactant pressures (4-1100 kPa CO2; 8-820 kPa H2), and nanoparticle diameter (2-30 nm). This mechanism-based strategy enables the exclusive formation of CH4 from CO2-H2 reactants, irrespective of reaction conditions or nanoparticle composition (Ru, Co, Ni) and size, without requiring complex catalyst architectures or intricate synthesis protocols.

Nearly 100% CO Selectivity for CO2 Reduction via Synergistic Engineering of Heteronuclear CuCo Dual Atoms

Monatomic catalysts demonstrate exceptional activity in CO2 hydrogenation for mitigating the greenhouse effect and achieving carbon neutrality goals. However, single-atom catalysts are limited by having only one type of active site, resulting in unsatisfactory activity and selectivity. In this work, a heteronuclear dual-atom catalyst (CuCoDA) is successfully synthesized using a dual-anchoring method and applied to CO2 hydrogenation. The synergistic effect between Cu and Co atoms results in a remarkable CO selectivity of 99.1%, with a CO2 conversion rate of 28.1%. The experimental results and theoretical calculations demonstrate that the incorporation of Co into the Cu monatomic catalyst enhances the adsorption of CO2 and H2 on the CuCoDA surface throughout the reaction, thereby significantly promoting CO2 conversion. Simultaneously, the cooperative effect minimizes the adsorption of CO* on the CuCoDA surface and inhibits the formation of *CHO (a key intermediate for methane generation), which suppresses the further hydrogenation of CO2. This results in an extremely high selectivity of CO. This study provides a general strategy for constructing dual-heteronuclear catalysts incorporating multiple metal species and highlights the critical importance of synergistic interactions between adjacent single atoms in the development of advanced catalysts.

'Tearing Effect' of Alloy-Support Interaction for Alloy Redispersion in NiRu/TiO2 Hydrogenation Catalysts

Supported alloy catalysts have been extensively applied to many significant industrial chemical processes due to the abundant active sites with distinguishable geometry and electron states. However, a detailed in situ investigation of the interaction between support and alloy nanoparticles is still lacking. Here, a subversive 'tearing effect' on the interface of TiO2-supported NiRu alloy nanoparticles is in situ discovered by environmental transmission electron microscopy (ETEM) with a dramatic redispersion process of alloy nanoparticles from ~25 nm to 2-3 nm under the repeated hydrogen reduction. Dual-driven by the distinct alloy-support interaction involving the restructuring of alloy nanoparticles and growth of TiOx overlayer, larger NiRu alloy nanoparticles spontaneously disintegrate into atoms migrating on support. Atoms are finally captured by the defects generated on TiO2 during the repeat reduction, which also confines the further growth of the newly alloy nanoparticles. Owing to this specific alloy-support interaction, smaller alloy nanoparticles on TiO2 support are much more stable than the bigger ones, which holds promise for industrial applications as durable catalysts. This novel metal-support interaction with the 'tearing effect' revealed on supported alloy catalysts provides new knowledge on the structure-performance relationships in all the alloy catalysts for hydrogenation reactions.

Manufacturing Single-Atom Alloy Catalysts for Selective CO2 Hydrogenation via Refinement of Isolated-Alloy-Islands

Single-atom alloy (SAA) catalysts exhibit huge potential in heterogeneous catalysis. Manufacturing SAAs requires complex and expensive synthesis methods to precisely control the atomic scale dispersion to form diluted alloys with less active sites and easy sintering of host metal, which is still in the early stages of development. Here, we address these limitations with a straightforward strategy from a brand-new perspective involving the 'islanding effect' for manufacturing SAAs without dilution: homogeneous RuNi alloys were continuously refined to highly dispersed alloy-islands (~1 nm) with completely single-atom sites where the relative metal loading was as high as 40 %. Characterized by advanced atomic-resolution techniques, single Ru atoms were bonded with Ni as SAAs with extraordinary long-term stability and no sintering of the host metal. The SAAs exhibited 100 % CO selectivity, over 55 times reverse water-gas shift (RWGS) rate than the alloys with Ru cluster sites, and over 3-4 times higher than SAAs by the dilution strategy. This study reports a one-step manufacturing strategy for SAA's using the wetness impregnation method with durable high atomic efficiency and holds promise for large-scale industrial applications.

Formate and CO* Radicals Intermediated Atmospheric CO2 Conversion over Co-Ni Bimetallic Catalysts Assembled on Diatomite

There exists an imperative exigency to ascertain catalysts of cost-effectiveness and energy efficiency for the facilitation of industrial CO2 methanation. In this area, the dual metal synergistic enhancement of the metal-support interaction emerges as a highly promising strategy. Here, Diatomite (Dt) was used as the support, and a series of CoyNi/Dt (Co as the first component and Ni as the second component) composite catalysts were constructed using an ultrasound-assisted coimpregnation method. Different Co/Ni molar ratios had a significant impact on the phase structure, chemical properties, morphological characteristics, and NiCo crystal structure of the xCoyNi/Dt materials. When the Co/Ni molar ratio was set to 2.0, a Ni-Co alloy was obtained, which is the key to improve the catalytic activity. Compared to the other xCoyNi/Dt catalysts, the bimetallic catalyst 2Co1Ni/Dt exhibited superior CO2 catalytic performance and stability, achieving a 76% CO2 conversion and 98% CH4 selectivity at 425 °C. The in situ DRIFTS results indicated that CO2 methanation over the 2Co1Ni/Dt catalyst followed the reaction pathway with formate and CO* radicals as the intermediates.

Bimetallic CuPd nanoparticles supported on ZnO or graphene for CO2 and CO conversion to methane and methanol

Carbon dioxide (CO2) and carbon monoxide (CO) hydrogenation to methane (CH4) or methanol (MeOH) is a promising pathway to reduce CO2 emissions and to mitigate dependence on rapidly depleting fossil fuels. Along these lines, a series of catalysts comprising copper (Cu) or palladium (Pd) nanoparticles (NPs) supported on zinc oxide (ZnO) as well as bimetallic CuPd NPs supported on ZnO or graphene were synthesized via various methodologies. The prepared catalysts underwent comprehensive characterization via high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX) mapping, electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), hydrogen temperature-programmed reduction and desorption (H2-TPR and H2-TPD), and deuterium temperature-programmed desorption (D2O-TPD). In the CO2 hydrogenation process carried out at 20 bar and elevated temperatures (300 to 500 °C), Cu, Pd, and CuPd NPs (<5 wt% loading) supported on ZnO or graphene predominantly yielded CH4 as the primary product, with CO generated as a byproduct via the reverse water gas shift (RWGS) reaction. For CO hydrogenation between 400 and 500 °C, the CO conversion was at least 40% higher than the CO2 conversion, with CH4 and CO2 identified as the main products, the latter from water gas shift. Employing 90 wt% Cu on ZnO led to an enhanced CO conversion of 14%, with the MeOH yield reaching 10% and the CO2 yield reaching 4.3% at 230 °C. Overall, the results demonstrate that lower Cu/Pd loading (<5 wt%) supported on ZnO/graphene favored CH4 production, while higher Cu content (90 wt%) promoted MeOH production, for both CO2 and CO hydrogenation at high pressure.

Revealing the anti-sintering phenomenon on silica-supported nickel catalysts during CO2 hydrogenation

The CO2 catalytic hydrogenation represents a promising approach for gas-phase CO2 utilization in a direct manner. Due to its excellent hydrogenation ability, nickel has been widely studied and has shown good activities in CO2 hydrogenation reactions, in addition to its high availability and low price. However, Ni-based catalysts are prone to sintering under elevated temperatures, leading to unstable catalytic performance. In the present study, various characterization techniques were employed to study the structural evolution of Ni/SiO2 during CO2 hydrogenation. An anti-sintering phenomenon is observed for both 9% Ni/SiO2 and 1% Ni/SiO2 during CO2 hydrogenation at 400°C. Results revealed that Ni species were re-dispersed into smaller-sized nanoparticles and formed Ni0 active species. While interestingly, this anti-sintering phenomenon leads to distinct outcomes for two catalysts, with a gradual increase in both reactivity and CH4 selectivity for 9% Ni/SiO2 presumably due to the formation of abundant surface Ni° from redispersion, while an apparent decreasing trend of CH4 selectivity for 1% Ni/SiO2 sample, presumably due to the formation of ultra-small nanoparticles that diffuse and partially filled the mesoporous pores of the silica support over time. Finally, the redispersion phenomenon was found relevant to the H2 gas in the reaction environment and enhanced as the H2 concentration increased. This finding is believed to provide in-depth insights into the structural evolution of Ni-based catalysts and product selectivity control in CO2 hydrogenation reactions.

Harnessing the Synergy of Fe and Co with Carbon Nanofibers for Enhanced CO2 Hydrogenation Performance

Amid growing concerns about climate change and energy sustainability, the need to create potent catalysts for the sequestration and conversion of CO2 to value-added chemicals is more critical than ever. This work describes the successful synthesis and profound potential of high-performance nanofiber catalysts, integrating earth-abundant iron (Fe) and cobalt (Co) as well as their alloy counterpart, FeCo, achieved through electrospinning and judicious thermal treatments. Systematic characterization using an array of advanced techniques, including SEM, TGA-DSC, ICP-MS, XRF, EDS, FTIR-ATR, XRD, and Raman spectroscopy, confirmed the integration and homogeneous distribution of Fe/Co elements in nanofibers and provided insights into their catalytic nuance. Impressively, the bimetallic FeCo nanofiber catalyst, thermally treated at 1050 °C, set a benchmark with an unparalleled CO2 conversion rate of 46.47% at atmospheric pressure and a consistent performance over a 55 h testing period at 500 °C. Additionally, this catalyst exhibited prowess in producing high-value hydrocarbons, comprising 8.01% of total products and a significant 31.37% of C2+ species. Our work offers a comprehensive and layered understanding of nanofiber catalysts, delving into their transformations, compositions, and structures under different calcination temperatures. The central themes of metal-carbon interactions, the potential advantages of bimetallic synergies, and the importance of structural defects all converge to define the catalytic performance of these nanofibers. These revelations not only deepen our understanding but also set the stage for future endeavors in designing advanced nanofiber catalysts with bespoke properties tailored for specific applications.

Facet-Controlled Cu2O Support Enhances Catalytic Activity of Pt Nanoparticles for CO Oxidation

The metal-support interaction plays a crucial role in determining the catalytic activity of supported metal catalysts. Changing the facet of the support is a promising strategy for catalytic control via constructing a well-defined metal-support nanostructure. Herein, we developed cubic and octahedral Cu₂O supports with (100) and (111) facets terminated, respectively, and Pt nanoparticles (NPs) were introduced. The in situ characterizations revealed the facet-dependent encapsulation of the Pt NPs by a CuO layer due to the oxidation of the Cu₂O support during the CO oxidation reaction. The CuO layer on Pt at cubic Cu₂O (Pt/c-Cu₂O) significantly enhanced catalytic performance, while the thicker CuO layer on Pt at octahedral Cu₂O suppressed CO conversion. The formation of a thin CuO layer is attributed to the dominant Pt-O-Cu bond at the Pt/c-Cu₂O interface, which suppresses the adsorption of oxygen molecules. This investigation provides insight into designing high-performance catalysts via engineering the interface interaction.