Abstract
In catalysts for CO2 hydrogenation, the interface between metal nanoparticles (NPs) and the support material is of high importance for the activity and reaction selectivity. In Pt NP-containing UiO Zr-metal–organic frameworks (MOFs), key intermediates in methanol formation are adsorbed at open Zr-sites at the Pt–MOF interface. In this study, we investigate the dynamic role of the Zr-node and the influence of H2O on the CO2 hydrogenation reaction at 170 °C, through steady state and transient isotope exchange experiments, H2O cofeed measurements, and density functional theory (DFT) calculations. The study revealed that an increased number of Zr-node defects increase the formation rates to both methanol and methane. Transient experiments linked the increase to a higher number of surface intermediates for both products. Experiments involving either dehydrated or prehydrated Zr-nodes showed higher methanol and methane formation rates over the dehydrated Zr-node. Transient experiments suggested that the difference is related to competitive adsorption between methanol and water. DFT calculations and microkinetic modeling support this conclusion and give further insight into the equilibria involved in the competitive adsorption process. The calculations revealed weaker adsorption of methanol in defective or dehydrated nodes, in agreement with the larger gas phase concentration of methanol observed experimentally. The microkinetic model shows that [Zr2(μ-O)2]4+ and [Zr2(μ–OH)(μ-O)(OH)(H2O)]4+ are the main surface species when the concentration of water is lower than the number of defect sites. Lastly, although addition of water was found to promote methanol desorption, water does not change the methanol steady state reaction rate, while it has a substantial inhibiting effect on CH4 formation. These results indicate that water can be used to increase the reaction selectivity to methanol and encourages further detailed investigations of the catalyst system.
Highlights
The hydrogenation of CO2 is receiving attention as a key reaction for sustainable production of fuels and value-added chemicals.[1−3] The activity and selectivity of CO2 hydrogenation catalysts is strongly influenced by the nature of the metal− support interface.[4−8] For Cu-based catalysts, the presence ofZrO2 or isolated Zr moieties close to Cu facilitates methanol formation by forming low coordinated Lewis acidic Zr-sites, where formate and methoxy groups form in the presence of CO2 and H2.9−11 Formate and methoxy groups are observed at sites at the interface of Cu/Al2O3: at 4-coordinated sites, methanol forms from bidentate CO2 bridged between two sites, via formate, while at 3-coordinated sites, CO formation from monodentate CO2 is favored.[12]
XRD and N2 adsorption of UiO-67 before and after the BPYDC healing procedure, called UiO-67 and UiO-67(LD), respectively, showed that the procedure had no effect on the crystallinity of the metal−organic frameworks (MOFs), nor any significant effect on the porosity and surface area of the material (SI Figures benzoic acid amount was 60% lower in the UiO-67(LD) sample
The dynamic role of the Zr-node during CO2 hydrogenation over UiO-67-Pt and the influence of defects and water on the reaction have been investigated by steady state and isotope transient kinetic studies, as well as water cofeed experiments
Summary
The hydrogenation of CO2 is receiving attention as a key reaction for sustainable production of fuels and value-added chemicals.[1−3] The activity and selectivity of CO2 hydrogenation catalysts is strongly influenced by the nature of the metal− support interface.[4−8] For Cu-based catalysts, the presence of. The molar amount of H2O formed until it appeared in the GC (calculated from reaction stoichiometry), corresponds to about 40% of the estimated number of Zr-μ3−OH groups in the MOF This observation indicates a correlation between the transient regime with higher methanol formation rate and the Zr-nodes’ state of hydration and/or the presence of H2O. In the case of adjacent linkers, the adsorption energies change drastically, especially in the case where water is coordinated to one Zr-atom attached to an OH group (ΔG = +66.9 kJ/mol) This result strongly suggests that having a larger number of defects increases the number of active sites to produce methanol and facilitates methanol desorption. It would not influence the rate-limiting step of methanol, which has been proposed to be the hydrogenation of formate
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