CO₂ Activation over Catalytic Surfaces

Abstract

Management of carbon on Earth has become one of the central themes in science, society,a nd politics owing to continuous relocation of carbon from the underground to the atmosphere in the form of carbon dioxide (CO 2 ). This is ac onsequenceo ft he modern life of mankind largely relying on burning or utilising carbon-based fossil fuels, which also causes their depletion. Recently,g lobalw arminga nd consequent climate change have been ascribed to the increasingc oncentration of atmosphericg reen-houseg ases,m ostr epresented by CO 2 ,a nd the world is joining forces to reduce the amount of CO 2 emissiont ot he atmosphere and convertt he "waste" CO 2 into valuable chemicals like polymers and fuels.CO 2 is at hermodynamically stable molecule with the standard formation enthalpy of À393.5 kJ mol À1 . [1] However,C O 2 can be transformed with notable reactivity depending on the chemicale nvironment. Among them catalysis offerss pecific sites to activateC O 2 for its chemical transformation. While CO 2 to polymers is generally enabled by efficient homogeneous catalysts (i.e. reactants and catalyst are in the same liquid phase), large-scale production of useful chemicals like fuels necessitates continuous operation using heterogeneous catalyst to activate CO 2 over its surface. There are several activation methods overc atalyst surfacer eported to date and each methodg enerally leads characteristicr eactivityo fC O 2 and products due to the unique form of activated CO 2 during transformation. Thisa rticle aims at concisely describing the reactivity of CO 2 in general, summarising the state-of-the-art activation methods and also highlighting similarities in different modes of CO 2 activation and correlations to product selectivity to evaluatec oherent views on CO 2 transformation over catalytic surfaces.The general properties of the CO 2 molecule, associatedw ith its reactivity,are summarised in the following four points: 1) Bending of CO 2For the uncharged state, bending of the molecule from its linear equilibrium geometry induces changes in the shape and energy level of the molecular orbitals. Notably,t he more bent the geometry,t he lower the energy level of the in-plane (i.e. to the plane of bending) contribution of 2p u orbital( the lowest unoccupied molecular orbital, LUMO) as shown in Figure 1. Changing the OCO bond angle from 1808 to 1578,t he proportion of the LUMO on the carbon is increased from 61 %t o 78 %, while the distance between carbon ando xygen (< 0.01 )a nd the energy (DE < 0.5 eV) remaina lmostc onstant. [2] Importantly,t his loweringo ft he in-plane 2p u orbital (LUMO) energy upon bending makest he carbon atom electrophilic. 2) Repartition of the ChargesWhen isolated, ap ositive chargec an be found on the carbon atom (the Mulliken's population is + 0.368 e) and negative chargeso nt he two oxygen atoms (with ap opulation of À0.184 e). [3] Ap olarized mediuml ike water can increase the charge on the carbon to + 0.407 e( obtained by DFT using a polarizable continuumm odel with al inear geometry). [3]...

On the Real Nature of Rh Single‐Atom Catalysts Dispersed on the ZrO2 Surface

“…All sites have been considered on the regular (101) surface or on a stepped (156) surface, to take into account the heterogeneity of the oxide Table 3. Number of adsorbed CO molecules, No(CO), CO adsorption site, adsorption energy, E ads (eV), CÀ O bond length, R CO (Å), CÀ Rh distance, R CRh (Å), Bader charge of Rh bound to CO, Q(Rh) (j e j), harmonic CO stretching frequency scaled by 2143/2125 factor, ω e (cm À 1 ), and frequency shift Δω e (cm À 1 ) with respect to gas-phase for CO adsorption on Rh 6 Based on the CO adsorption energies and CO frequencies, and a comparison with the FT-IR spectra, we can also exclude the (Rh) ads , and (RhO 2 ) ads species. In one case, (Rh) ads , the CO stretching frequency is too low, due to the zero oxidation state of Rh and the large back donation which results in large redshifts of the CO frequency; in the other case, (RhO 2 ) ads , the frequencies are too high, and the shifts are inconsistent with the IR spectra.…”

“…Heterogeneous catalysts often consists of highly dispersed precious metal clusters deposited on more or less inert oxide supports and are used in various catalytic reactions, from oxidation, [1,2,3] to water-gas shift, [4,5] and CO 2 activation, [6] just to mention a few. The chemical activity of these systems is due to the metal centers that are able to activate, and often break, chemical bonds at low energetic cost.…”

Heterostructured Catalysts for Electrocatalytic and Photocatalytic Carbon Dioxide Reduction

“…Notably, the actual redox potentials observed experimentally to drive the electrochemical reactions are often greater in comparison to the required thermodynamically determined redox potential, concluding the need for larger than necessary amounts of energy to favor reaction progress. This difference in voltage, known as the overpotential, is often attributed to the highly stable nature of CO 2 molecules and also has a direct dependence on factors, such as the type of catalyst, electrode, and electrolyte, and other reaction conditions, temperature, and CO 2 concentration used [35] …”

Recent Developments in the Modelling of Heterogeneous Catalysts for CO2 Conversion to Chemicals

“…[12,13] The use of specific catalysts is key to overcome high activation energy barriers, to extend the electrode life and stability or to capture solar radiations that generate excitons (e À + hole) for CO 2 reduction. [14,15] For the CO 2 conversion processes heterogeneous catalysts are preferred and widely used in the industries. They exhibit high catalytic activity, robustness, high efficiency in the recovery and recycling with the possibility of simple product separation, which can be economically advantageous in industrial CO 2 conversion.…”