Influence of Defects and H₂O on the Hydrogenation of CO₂ to Methanol over Pt Nanoparticles in UiO-67 Metal–Organic Framework

Abstract

In catalysts for CO 2 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 H 2 O on the CO 2 hydrogenation reaction at 170 °C, through steady state and transient isotope exchange experiments, H 2 O 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 [Zr 2 (μ-O) 2 ] 4+ and [Zr 2 (μ–OH)(μ-O)(OH)(H 2 O)] 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 CH 4 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.

Synthesis, characterization and application of defective metal–organic frameworks: current status and perspectives

“…Defective MOFs and their composites have received broad research interest in the eld of heterogeneous catalysis, including oxidation, 22,25,39,40,115,116 esterication, 27 CO 2 conversion, 36,81,117 and photo/electrocatalysis. 21,23,33,118 The CUSs and/or the actives species immobilized in the local microenvironment of structural defects can serve as active sites in different catalytic processes.…”

“…121 Due to their high specic surface areas and hierarchical pores, defective MOFs enable reactant molecules to access the active sites even aer loaded with active guest species. 117 Different kinds of guest species, such as polyoxometalates (POMs) and metal nanoparticles (NPs) have been successfully incorporated into defective MOFs. [37][38][39]117,122,123 For example, Keggin-type POMs conned in defective UiO-66 were prepared via a one-pot method with an in situ addition of thiourea (Fig.…”

“…117 Different kinds of guest species, such as polyoxometalates (POMs) and metal nanoparticles (NPs) have been successfully incorporated into defective MOFs. [37][38][39]117,122,123 For example, Keggin-type POMs conned in defective UiO-66 were prepared via a one-pot method with an in situ addition of thiourea (Fig. 11c).…”

MOF-enabled confinement and related effects for chemical catalyst presentation and utilization

“…A series of studies by Gutterød and co-workers 287 revealed that Pt@UiO-67 is catalytically competent for H 2 -based reduction of CO 2 , with the initial report describing formation of CO and the classic Sabatier reaction product, methane. Subsequent mechanistic 288 and structural studies, 289 including studies of the influence of missing-linker defects, permitted the reduction to be optimized for methanol production. The researchers found that the interfacial sites where Pt NPs and Zr nodes interact thus serve as the meeting point for formate produced at the node from zirconia-sorbed CO 2 and H atoms formed by dissociation of H 2 by Pt (Fig.…”

Support–Activity Relationship in Heterogeneous Catalysis for Biomass Valorization and Fine-Chemicals Production

“…The high surface area generates a very high density of catalytic sites, allowing greater possibilities of contact with substrates and producing catalysts with high activity and excellent steric selectivity, tunable by changing linkers shape and length in the synthesis steps [178]. The range of reactions that can be performed with MOF-based catalysts is almost as wide as their synthetic tunability: industrially relevant reactions involving commodities production (butenes syntheses for synthetic rubber production) [181]; environmental remediation reactions with greenhouse gases abatement (CO 2 reduction through thermal- [182,183] or photo-catalytic processes [184]); photocatalytic [184] and electrocatalytic [185] hemi-reactions for fuel cells processes; and reactions for fine chemicals production [186]. Apart from these cases, there are only a few (yet remarkable) examples of MOFs employed as supports for catalysts employed as biomass valorization processer finalized to the production of higher added-value molecules.…”