Boosting CO2 hydrogenation via size-dependent metal–support interactions in cobalt/ceria-based catalysts

Metal-support interactions (MSI) significantly influence the effectiveness of heterogeneous catalysts. Specific sites at the metal support interface can exhibit high reactivity, prompting increasing interest in optimizing not only the properties of metal particles but also the metal-support interface. Nevertheless, a precise modulation of MSI strength for optimal metal dispersion and size remains a significant challenge. Here, we demonstrate that tuning the particle size of the support by varying calcination temperature can effectively modulate the interaction between Co and CeO2-Y2O3(CY), thereby greatly enhancing the dry reforming of methane (DRM). A combination of X-ray diffraction, H2-TPR, X-ray photoelectron spectroscopy, and transmission electron microscopy reveals that cobalt nanoparticles, stabilized on CY supports with an intermediate particle size of ∼75 nm, exhibit superior strength and enhanced DRM activity due to improved MSI after reduction at 800 °C. For catalysts with optimal MSI, we demonstrate that facile formation of oxygen vacancies is pivotal for the high DRM activity.

Bimetallic Ni-Cu catalysts feature high activity in CO2 hydrogenation. However, the primary surface intermediates during reaction are still elusive, making the understanding of the reaction mechanism inadequate. Herein, taking advantage of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we focused on the mechanistic exploration of CO2 hydrogenation on the Ni/Cu(100) model catalyst under millibar pressures. We show that CO2 dissociates into CO and atomic oxygen on the Ni/Cu(100) surface and gives rise to the formation of chemisorbed O and nickel oxide (NiO). The CO3* species is formed through the reaction of CO2 with surface oxygen during CO2 activation. With the presence of H2, the conversion of adsorbed CO3* into the formate intermediate, HCOO*, is unambiguously demonstrated by the C 1s and O 1s core-level spectra as well as ultraviolet photoelectron spectroscopy. Based on these observations, we conclude that the CO2 hydrogenation route via CO2 dissociation, the formation of CO3*, the conversion of CO3* to formate, and the ensuing hydrogenation of formate to methanol on the Ni-Cu catalyst are feasible.

Catalytic hydrolysis of ammonia borane (AB) provides an effective way to generate pure H2 at ambient temperature for fuel cells. Pt-based catalysts usually exhibit great initial activity toward this reaction but deactivate quickly. Here, we report that the metal-support interactions in Pt/Co3O4 nanocages can simultaneously accelerate the H2 generation and enhance the catalyst's stability. The Pt/Co3O4 catalyst is made for the first time by embedding Pt clusters (∼1.2 nm) in a high-surface-area Co3O4 nanocage to maximize the metal-support interface. The turnover frequency of the Pt/Co3O4 catalyst is about nine times higher than that of commercial Pt/C and outperforms almost all other Pt-based catalysts. X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, in situ spectroscopy, and density functional theory calculations suggest that the Co3O4 nanocages with rich oxygen vacancies facilitate the adsorption and dissociation of H2O to give electropositive H (Hδ+), while the in situ embedded Pt clusters can accelerate the formation of electronegative H (Hδ-) from AB. Subsequently, the Hδ+ and Hδ- spill over to the abundant interfacial sites and bond into H2. In addition to this dual-function synergy effect, the strong metal-support electronic interactions between Co3O4 and Pt benefit the desorption of poisonous B-containing byproducts from Pt sites. This effect together with cluster anchoring leads to a fivefold enhancement in durability compared to commercial Pt/C. The metal-support interactions revealed in this study provide more options for catalyst design toward facile H2 production from chemical hydrogen storage materials.

The effect of SMSI (strong metal-support interaction) behavior on CO adsorption and hydrogenation on Pd catalysts: II. Kinetic behavior in the methanation reaction

Pd model catalysts: Effect of aging duration on lean redispersion

Near ambient-pressure X-ray photoelectron spectroscopy study of CO2 activation and hydrogenation on indium/copper surface

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.

The release of carbon dioxide (CO2) into the atmosphere has led to effects of climate change resulting in an increase in global temperature, ocean acidification and many other environmental issues. Hydrogenation of CO2 into synthetic hydrocarbons is a promising solution in decreasing anthropogenic dependence on fossil fuels and providing an energy source that is carbon-neutral. The reverse water gas shift (RWGS) reaction is a feasible hydrogenation reaction that requires a 2-electron transfer to yield syngas (CO + H2) to be used in the Fischer-Tropsch reaction to synthesize synthetic hydrocarbons. In previous work (submitted to the Journal of CO2 Utilization), the conversion of CO2 into CO using Ru-nanostructured metal nanoparticles dispersed on ionically conducting ceramic supports like ceria (CeO2), doped-ceria (x-CeO2) and yttria-stabilized zirconia (YSZ) was studied with promising results. The activity of the Ru-based nanoparticles was improved due to the ionically conductive properties of the support, which contain oxygen (Oδ-) ionic species that promote the reaction. This promotional effect is known as the metal-support interaction (MSI) where nanoparticles are dispersed on a powder support, allowing Oδ- species to migrate from support to nanoparticle by an increase in temperature [1,2]. The MSI effect has been observed using the best Ru-based powder catalyst supported on samarium-doped ceria (SDC) - Ru45Fe55/SDC (2 wt.%), which yielded high CO amounts between 300-750°C. Current research aims at improving the overall RWGS reaction at lower temperatures through the utilization of the electrochemical promotion of catalysis (EPOC) or non-faradaic electrochemical modification of catalytic activity (NEMCA) effect [3,4]. EPOC allows to control in-situ the migration of ionic species to and from the metal surface through the application of a potential difference or current between the catalyst-working electrode and an inert counter electrode. This migration of species leads to the formation of a neutral double layer encapsulating Ru nanoparticles, promoting the reaction. The catalyst setup resembles an electrocatalytic cell where metal nanoparticles act as the working electrode deposited on a solid support in the form of a disc. The support (YSZ in this case) represents a fixed layer of electrolytes that conducts Oδ- ions to migrate to and from the active catalyst. As shown in Fig. 1, a promotional effect is observed for Ru on YSZ at 350°C under constant potential of 0.25 V, favoring the formation of CO through an enhancement ratio of ~2 and Faradaic efficiency of ~19, which is attributed to the synergistic effect between the metal and promoted ionic species Oδ­-. Additionally, density functional theory (DFT) calculations are being conducted for the hydrogenation of CO2 on Ru nanoparticles and will be discussed in correlation with the experimental findings to confirm the mechanisms occurring during the reaction. [1] P. Vernoux, M. Guth, X. Li, Ionically Conducting Ceramics as Alternative Catalyst Supports, Electrochem. Solid-State Lett. 12 (2009) E9–E11.[2] S. Ntais, R.J. Isaifan, E.A. Baranova, An X-ray photoelectron spectroscopy study of platinum nanoparticles on yttria-stabilized zirconia ionic support: Insight into metal support interaction, Mater. Chem. Phys. 148 (2014) 673–679.[3] D. Vayenas, C.G., Bebelis, S., Pliangos, C., Brosda, S., Tsiplakides, Electrochemical Activation of Catalysis, Springer US, (2001).[4] P. Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-Consuegra, J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou, E.A. Baranova, Ionically Conducting Ceramics as Active Catalyst Supports, Chem. Rev. 113 (2013) 8192–8260. Figure 1

Revealing the effect of metal-support interactions at the Ni/In2O3(111) interface on the selective CO2 hydrogenation

Conversion of CO2 to CO using H2, known as the reverse water gas shift (RWGS), is one of the promising ways to utilize CO2 as a renewable feedstock. This chapter reviews the performance of different heterogeneous catalysts reported so far for RWGS reaction. CO or syngas is already being used as a feedstock for the production of many chemicals and fuels. Different categories of catalysts such as supported noble metal catalysts (Au, Pd, Pt, Rh, and Ru), supported non-noble metal catalysts (Cu and Ni), bimetallic catalysts, oxide systems such as perovskites, BaFe–hexaaluminate catalysts, iron-based oxides, and cobalt-based oxides and transition metal carbides have been studied for RWGS reaction. Supported metal catalysts with dual functionalities from the support and the metal sites that can activate CO2 and dissociate H2 in a synergistic manner are extensively studied for the RWGS reaction. A range of supports such as Al2O3, TiO2, CeO2, SiO2, ZnO, MoOx, ZrO2, MgO, zeolites, and Fe2O3; transition metal carbide (TMC), SBA-15, and a metal–organic framework (MOF) were used to stabilize different metal nanoparticles and to achieve high metal dispersion and optimum particle size so as to enhance their catalytic performance with maximum CO selectivity. The structural and electronic characteristics of the metal and support, particle size, as well as the dispersion of the active metal component, metal loading, metal–support interaction, and nature of active species formed at the metal–support interfacial sites are all major factors that influence the catalytic activity and long-term stability for RWGS reaction. The supported noble metal catalysts, particularly Pt, Rh, and Ru, are resistant to coking and corrosion. They also display high activity toward H2 dissociation, CO2 conversions close to equilibrium, and avoid the formation of the main side product CH4 above 500°C. However, the high cost and limited availability of noble metals necessitate the development of cheaper and more abundant non-noble metal catalysts, particularly 3d transition elements such as Cu and Ni. However, Cu nanoparticles have poor thermal stability and have strong tendency to sinter at reaction temperatures above 300°C, whereas Ni-based catalysts give relatively low CO selectivity because of the favorable methanation reaction during the hydrogenation of CO2. Efforts have been made to improve the activity, stability, and CO selectivity over Cu- and Ni-based catalysts, such as by dispersing the active metal components on different kinds of supports, alloying Cu or Ni species with other metals (Ru, Zn, Fe, etc.), using promoters such as Cs and K, controlling the metal particle size, metal loading, the oxidation state of metal, and the metal–support interaction. Transition metal carbides (TMCs) with noble metal-like properties are also reported in the literature, which are active at low temperatures with relatively lower CO selectivity. They can also act as a promising support for dispersing active metal species for RWGS. Metal oxides are attractive for RWGS reaction because of their low cost, resistance to sintering when compared to noble metals, and metal-supported catalysts. Among the metal oxide catalysts, perovskite (ABO3) compounds have been effective and emerge as one of the potential systems for RWGS reaction because of their structural flexibility, thermal stability, high oxygen mobility and stability under nonstoichiometric conditions, and harsh reaction conditions. Two different mechanisms have been postulated for RWGS reaction in the literature, namely, dissociative (also known as regenerative or redox mechanism) and associative. In the redox (dissociative) mechanism, the reactants are oxidized or reduced separately on the catalyst surface, whereas in the associative mechanism, the reaction occurs via a surface intermediate such as bicarbonate species from adsorbed CO2, followed by its conversion to formate species in the presence of hydrogen.

The research in heterogeneous catalysis is aimed at the design of tailored ultra-high selective catalysts to promote green chemistry, and supported metals constitute an important class of these materials. The properties of a supported metal particle can be fine-tuned by (i) control of particle size, (ii) forming “bimetallic” phases, and (iii) use of the metal-support interaction. The support acts as a supramolecularuar ligand and has been claimed to promote specific electronic properties and/or geometrical features of the nano-sized supported metal particles. In the electronic “theory” of catalysis, the key point lies in the strength of interaction between the d-band of the metal and the molecular orbitals of reactants and products which is for instance reflected by the heat of adsorption. The keystone in the geometric “theory” of catalysis claims that the rate is function of the probability to encounter n free neighbor metal atoms constituting the active sites with specific topological requirements, on which the reactive adsorption can occur. Some typical situations of metal-support interaction/cooperation are then described: 1) The occurrence of electronic modifications of the metal is obvious since the EF level should have been equalized at the metal-support interface. However, the extent of electron transfer seems quite small and the screening high with respect to neighboring atoms; 2) Morphological and topological features of surface metal sites are strongly affected by the so-called Strong Metal Support Interaction (SMSI). SMSI occurs after reduction at high temperature in H2 of noble metals on some reducible supports. The keystone in SMSI is the decoration of metal particles by suboxide species coming from the support, e.g. TiOx. Some typical examples of the hydrogenation of C=C, C=O, N=O bonds, as well as the hydrogenolysis of C-C and C-halogen bonds are described in relation with the occurrence of metal-support interaction.

The manifestations of strong and reactive metal–support interaction after reduction at elevated temperatures in Pt/V2O3 and Pd/V2O3 systems were studied using a dedicated thin film model system. Pt and Pd particles were prepared by electron-beam evaporation on NaCl(001) growth templates and subsequently embedded in a crystalline V2O3 matrix, prepared by thermal evaporation of V metal in 10−4 mbar oxygen pressure. Template temperatures of 600 K were used to induce the formation of epitaxially-ordered metal–oxide systems. Engineering of the metal–support interface by distinct annealing treatments allows steering the extent and quality of metal–support interaction. Whereas for Pt/V2O3 catalysts, high-temperature reduction at 773 K in hydrogen causes the epitaxial formation of a well-ordered body-centered tetragonal Pt3V intermetallic phase, the Pd/V2O3 system is mostly unaffected by similar treatments and remains in a metal–oxide state. Nevertheless, oxidation at 773 K of both catalysts prior to the hydrogen treatments lifts the epitaxial relation between metal and oxide and in turn, subsequent reduction at high-temperatures (T ≥ 773 K) yields only polycrystalline Pt3V and Pd3V intermetallic phases without particular ordering with respect to the former growth substrate. Along with this formation of intermetallic phases goes a transformation of the support stoichiometry from V2O3 to VO. Catalyst regeneration by partial oxidative decomposition of the intermetallic state is only possible at high-temperatures (T ≥ 750 K), yielding mostly metal particles and vanadium oxides with oxygen contents higher than V:O = 1:2, in particular V3O7. Adjusting the extent of metal–support contact area by annealing treatments allows for easy steering the structure and morphology of well-defined intermetallic compounds in Pt–VOx and Pd–VOx systems. Well-defined Pt3V intermetallic compounds are formed by direct reduction, whereby lifting the ordering by pre-oxidation yields less-defined compounds with altered metal–support contact area and consequently, strong metal–support interaction.

The metal supported catalysts are emerging catalysts that are receiving a lot of attention in CO2 hydrogenation to C1 products. Numerous experiments have demonstrated that the support (usually an oxide) is crucial for the catalytic performance. The support metal oxides are used to aid in the homogeneous dispersion of metal particles, prevent agglomeration, and control morphology owing to the metal support interaction (MSI). MSI can efficiently optimize the structural and electronic properties of catalysts and tune the conversion of key reaction intermediates involved in CO2 hydrogenation, thereby enhancing the catalytic performance. There is an increasing attention is being paid to the promotion effects in the catalytic CO2 hydrogenation process. However, a systematically understanding about the effects of MSI on CO2 hydrogenation to C1 products catalytic performance has not been fully studied yet due to the diversities in catalysts and reaction conditions. Hence, the characteristics and modes of MSI in CO2 hydrogenation to C1 products are elaborated in detail in our work.

Time-resolved FT-IR spectra of carbon monoxide hydrogenation over alumina-supported ruthenium particles were recorded on the millisecond time scale at 700 K using pulsed release of CO and a continuous flow of H(2)-N(2) (ratio 0.067 or 0.15, 1 atm total pressure). Adsorbed carbon monoxide was detected along with gas phase products methane (3016 and 1306 cm(-1)), water (1900-1300 cm(-1)), and carbon dioxide (2348 cm(-1)). Aside from adsorbed CO, no other surface species were observed. The rate of formation of methane is 2.5 +/- 0.4 s(-1) and coincides with the rate of carbon dioxide growth (3.4 +/- 0.6 s(-1)), thus indicating that CH(4) and CO(2) originate from a common intermediate. The broad band of adsorbed carbon monoxide has a maximum at 2010 cm(-1) at early times (36 ms) that shifts gradually to 1960 cm(-1) over a period of 3 s as a result of the decreasing surface concentration of CO. Kinetic analysis of the adsorbed carbon monoxide reveals that surface sites absorbing at the high frequency end of the infrared band are temporally linked to gas phase product growth. Specifically, a (linear) CO site at 2026 cm(-1) decays with a rate constant of 2.9 +/- 0.1 s(-1), which coincides with the rise constant of CH(4). This demonstrates that the linear CO site at 2026 cm(-1) is the kinetically most relevant one for the rate-determining CO dissociation step under reaction conditions at 700 K.

Iron-based catalysts are considered active for the hydrogenation of CO2 toward high-order hydrocarbons. Here, we address the structural and chemical evolution of oxide-supported iron nanoparticles (NPs) during the activation stages and during the CO2 hydrogenation reaction. Fe NPs were deposited onto planar SiO2 and Al2O3 substrates by dip coating with a colloidal NP precursor and by physical vapor deposition of Fe. These model catalysts were studied in situ by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in pure O2, then in H2, and finally in the CO2 + H2 (1:3) reaction mixture in the mbar pressure range and at elevated temperatures. The NAP-XPS results revealed the preferential formation of Fe(III)- and Fe(II)-containing surface oxides under reaction conditions, independently of the initial degree of iron reduction prior to the reaction, suggesting that CO2 behaves as an oxidizing agent even in excess of hydrogen. The formation of the iron carbide phase, often reported for unsupported Fe catalysts in this reaction, was never observed in our systems, even on the samples exposed to industrially relevant pressure and temperature (e.g., 10 bar of CO2 + H2, 300 °C). Moreover, the same behavior is observed for Fe NPs deposited on nanocrystalline silica and alumina powder supports, which were monitored in situ by X-ray absorption spectroscopy (XAS). Our findings are assigned to the nanometer size of the Fe particles, which undergo strong interaction with the oxide support. The combined XPS and XAS results suggest that a core (metal-rich)–shell (oxide-rich) structure is formed within the Fe NPs during the CO2 hydrogenation reaction. The results highlight the important role played by the oxide support in the final structure and composition of nanosized catalysts.