Mechanisms of the Heterogeneously Catalyzed Reaction of CO2 Hydrogenation on Transition Metal Surfaces

Renewable energy supply and energy storage in a closed materials cycle are the urgent global challenges of the 21st century. Carbon dioxide (CO2) hydrogenation over catalysts is a method to produce synthetic fuels from renewable energy in a CO2 neutral cycle. Numerous catalysts from pristine to novel materials have been investigated to achieve high CO2 conversion and selective hydrocarbon. Ahead of developing innovatively active catalyst with a high selectivity, it is crucial to understand the active sites and the related reaction mechanisms. Despite the fact that CO2 + H2 is a rather simple chemical equation, many reaction paths on a solid catalyst are possible. However, the proposed mechanisms from literatures have been controversial due to two main difficulties in 1) evidently identifying the reaction species, especially some vital transient species on the pathway of carbon hydrogenation and C-C coupling, and 2) proving the relation between surface structure of the catalysts and the reactivities. To address these difficulties, we built a new instrumental setup and developed an analysis program in order to investigate the catalyst surface and analyze the reaction mechanism in operando. We built up a diffuse reflectance infrared Fourier transform spectroscopy-mass spectroscopy-gas chromatography (DRIFTS-MS-GC) instrument for an operando study of the surface, gas, and liquid products using DRIFTS, MS, GC, and ex situ NMR. A bilevel evolutionary Gaussi-an fitting (BEGF) program was developed for dataset treatment including peak deconvolution and kinetic plot. Standard chemicals like carbonates, bicarbonates, formic acid, acetic acid, methanol, and ethanol, and isotopic spectra using 13CO2 and D2 in the reactions were em-ployed for supporting peak assignments. Based on these infrastructures, CO2 adsorption and hydrogenation reactions on pristine metals (Fe, Co, Ni, and Cu), metal hydride alloy (La-Ni4Cu), and metal-oxide (Co-CoO) were studied. We found out pure Fe, Co, Ni, Cu and La-Ni4Cu metals are not efficient for CO2 hydrogenation, and have no detectable adsorption spe-cies on the surface. High conversion (> 90%) of CO2 on Co-CoO surface emphasized the im-portance of oxide for CO2 chemisorption and for the observations of the surface species. The following investigation on oxide supported metal catalyst Ru/Al2O3 via an in situ control of the individual formation and hydrogenation of each adsorption species demonstrates that the oxide assists CO2 initial activation. CO2 methanation starts at the interface of the metal and the oxide and passes through the steps of CO2 ï® HCO3-* (and/or HCOO-* in CO2+H2 co-adsorption condition) ï® CO* ï® CH4. We further explored the relationship between met-al/(metal+oxide) ratio and their reactivities. We synthesized Co/(Co+CoO) catalysts in the DRIFTS-MS-GC instrument, and investigated the CO2 hydrogenation reaction on site. The results reveal that high concentration of the oxide in the catalyst boosts CO2 methanation, be-cause the CO2 binding is moderate and the adsorption is enhanced when the oxide concentra-tion increased. In short, we established equipment for an operando study of CO2 hydrogenation on catalyst surfaces, unraveled the surface reaction mechanisms and the active sites, developed a highly active catalyst, as well as initiated an efficient data analysis program.

Surface Reactions are Crucial for Energy Storage

Elsa Callini, Shunsuke Kato, Philippe Mauron

Reactions between gas molecules, e.g. H-2 and CO2 and solids take place at the surface. The electronic states and the local geometry of the atomic arrangement determine the energy of the adsorbate, i.e. the initial molecule and the transition state. Here we review our research to identify the surface species, their chemical state and orientation, the interaction with the neighbouring molecules and the mobility of the adsorbed species and complement the experimental results with thermodynamic modelling. The role of the Ti was found to be a bridge between the charged species preventing the individual movement of the ions including charge separation. The Ti has no catalytic effect on the hydrogen sorption reaction in borohydrides. The physisorption of molecular hydrogen is too weak at ambient temperature to reach a significant hydrogen storage density. The addition of a hydrogen dissociation catalyst to a nanoporous material with a large specific surface area may potentially enable the spillover of hydrogen atoms from the metal catalyst to the surface of the porous material and chemisorb on specific sites with a much higher binding energy compared to physisorption. The intercalation of alkali metals in C-60 fullerenes increases the interaction energy of hydrogen with the so-called metal fullerides significantly. Sterical diffusion barriers by partial oxidation of the surface of borohydrides turned out to redirect the reaction path towards pure hydrogen desorption and suppress the formation of diborane, a by-product of the hydrogen evolution reaction from borohydrides previously undetected. The combination of a newly developed gas controlling system with microreactors allows us to investigate complex reactions with small quantities of nano designed new catalytic materials. Furthermore, tip-enhanced Raman spectroscopy (TERS) will allow the investigation of the reactions locally on the surface of the catalyst and the near ambient pressure photoelectron spectroscopy enables analysis of the surfaces in ultra-high vacuum and in situ interaction with the adsorbates i.e. while the reaction takes place. This brings us in a unique position for the investigation of the heterogeneous reactive systems. The mechanism of the Ti catalysed hydrogen sorption reactions in alanates was recently established based on spectroscopic investigations combined with thermodynamic analysis of the transition states.

Schweizerische Chemische Gesellschaft2015

In Situ Control of the Adsorption Species in CO2 Hydrogenation: Determination of Intermediates and Byproducts

Ligang Wang, Kun Zhao, Andreas Züttel

CO2 hydrogenation over catalysts is a potentially exciting method to produce fuels while closing the CO2 cycle and mitigating global warming. The mechanism of this process has been controversial due to the difficulty in clearly identifying the species present and distinguishing which are reaction intermediates and which are byproducts. We in situ manipulated the independent formation and hydrogenation of each adsorption species produced in CO2 hydrogenation reaction over Ru/Al2O3 using operando diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS) and executed a novel iterative Gaussian fitting procedure. The adsorption species and their role in the CO2 hydrogenation reaction have been clearly identified. The adsorbed carbon monoxide (CO*) of four reactive structures was the key intermediate of methane (CH4) production. Bicarbonate (HCO3–), formed on the metal–support interface, appeared to be not only the primary product of CO2 chemisorption but also a reservoir of CO and consisted of the dominate reaction steps of CO2 methanation from the interface to the metal surface. Bidentate formate (Bi-HCOO–) formed on Ru under a certain condition, consecutively converting to CO to merge into the subsequent methanation process. Nonreactive byproducts of the reaction were also identified. The evolution of the surface species revealed the essential steps of the CO2 activation and hydrogenation reactions which were inevitably initiated from HCO3–* to CO* and finally from CO* to CH4.

In situ Control of the Adsorption Species in CO2 Hydrogenation: Determination of Intermediates and Byproducts

Kun Zhao

Surface Reactions are Crucial for Energy Storage

Elsa Callini, Shunsuke Kato, Philippe Mauron

Schweizerische Chemische Gesellschaft2015