Morphology-engineered highly active and stable Pd/TiO2 catalysts for CO2 hydrogenation into formate

Unraveling the Cause of Strong Metal-Support Interaction Formation: Disparities in Metal Nanoparticle Anchoring Mechanisms

Strong metal-support interaction (SMSI) can be normally induced by the surface free energy differences between metal nanoparticles and supports. To gain deeper insights into the effect of SMSI on heterogeneous catalysis, we use prototype Pt, Pd/TiO2(anatase) systems to demonstrate different reverse water gas shift (RWGS) reaction activity changes, especially with increasing the metal nanoparticle (NP) loading. Our experiments show that the conventional surface-free-energy change law regarding the incremental NP size is no longer applicable to these systems due to the overlook of the change of support properties owing to the disparities of the metal anchoring mechanisms. Both experimental measurements and density functional theory (DFT) calculations show that Pt atoms strongly favor anchoring on the oxygen vacancies (Ov) over the OH-sites on the anatase TiO2 support. In contrast, Pd atoms lack such Ov-site preference compared to Pt atoms, thereby leaving higher content of Ov on the support than the Pt counterpart. Moreover, high density of residual Ov on the support can cause the Pd NPs to be in higher degree of contact with the support, either in NP-encapsulation state (experiment) or NP-spreading state (simulation). The enhanced CO2 conversion of Pt/TiO2A is attributed to the synergistic effect of Ov and hydrogen spillover from Pt sites.

Engineering Lattice Dislocations of TiO2 Support of PdZn-ZnO Dual-Site Catalysts to Boost CO2 Hydrogenation to Methanol

CO2 hydrogenation to methanol using green hydrogen derived from renewable resources provides a promising method for sustainable carbon cycle but suffers from high selectivity towards byproduct CO. Here, we develop an efficient PdZn-ZnO/TiO2 catalyst by engineering lattice dislocation structures of TiO2 support. We discover that this modification orders irregularly arranged atoms in TiO2 to stabilize crystal lattice, and consequently weakens electronic interactions with supported active phases. It facilitates the transformation of metallic Pd into PdZn alloy, effectively suppressing CO production through inhibiting the reverse water-gas shift reaction mediated by the carboxylate pathway on Pd0 sites. Moreover, it enables the efficient transfer of hydrogen species via hydrogen spillover from PdZn alloy to ZnO for compensating the poor hydrogen dissociation ability of ZnO, thereby creating both more oxygen vacancies essential for CO2 activation and a hydroxyl-rich environment conducive to hydrogenation of intermediates. These collective modifications on PdZn-ZnO dual sites synergistically induce the propensity of the formate pathway for methanol synthesis. Consequently, compared to the unmodified catalyst, our as-designed catalyst increases methanol selectivity from 64.2 to 80.0 %, reduces CO selectivity from 35.0 to 19.8 %, and achieves an impressive methanol space-time yield of 9028.0 mgMeOH gPd+Zn -1 h-1 at a similar CO2 conversion (~8.0 %).

The superiority of Pd2+ in CO2 hydrogenation to formic acid

The hydrogenation of CO2 to formic acid is an essential subject since formic acid is a promising hydrogen storage material and a valuable commodity chemical. In this study, we report for the first time the hydrogenation of CO2 to formic acid catalyzed by a Pd2+ catalyst, Pd-V/AC-air. The catalyst exhibited extraordinary catalytic activity toward the hydrogenation of CO2 to formic acid. The TON and TOF are up to 4790 and 2825 h-1, respectively, representing the top level among reported heterogeneous Pd catalysts. By combining a study of first-principles density functional theory with experimental results, the superiority of Pd2+ over Pd0 was confirmed. Furthermore, the presence of V modified the electronic state of Pd2+, thus promoting the reaction. This study reports the effect of metal valence and electronic state on the catalytic performance for the first time and provides a new prospect for the design of an efficient heterogeneous catalyst for the hydrogenation of CO2 to formic acid.

Feeble Single-Atom Pd Catalysts for H2 Production from Formic Acid

Single-atom catalysts are thought to be the pinnacle of catalysis. However, for many reactions, their suitability has yet to be unequivocally proven. Here, we demonstrate why single Pd atoms (PdSA) are not catalytically ideal for generating H2 from formic acid as a H2 carrier. We loaded PdSA on three silica substrates, mesoporous silicas functionalized with thiol, amine, and dithiocarbamate functional groups. The Pd catalytic activity on amino-functionalized silica (SiO2-NH2/PdSA) was far higher than that of the thiol-based catalysts (SiO2-S-PdSA and SiO2-NHCS2-PdSA), while the single-atom stability of SiO2-NH2/PdSA against aggregation after the first catalytic cycle was the weakest. In this case, Pd aggregation boosted the reaction yield. Our experiments and calculations demonstrate that PdSA in SiO2-NH2/PdSA loosely binds with amine groups. This leads to a limited charge transfer from Pd to the amine groups and causes high aggregability and catalytic activity. According to the density functional calculations, the loose binding between Pd and N causes most of Pd's 4d electrons in amino-functionalized SiO2 to remain close to the Fermi level and labile for catalysis. However, PdSA chemically binds to the thiol group, resulting in strong hybridization between Pd and S, pulling Pd's 4d states deeper into the conduction band and away from the Fermi level. Consequently, fewer 4d electrons were available for catalysis.

Carbonate Hydrogenated to Formate in the Aqueous Phase over Nickel/TiO2 Catalysts

Carbonate hydrogenation to formate is a promising route to convert captured carbon dioxide into valuable chemicals, thus reducing carbon emissions and creating a revenue return. Developing inexpensive catalysts with high activity, selectivity, and stability remains challenging. We report a supported non-noble metal catalyst, Ni/TiO2 , with great selectivity over 96 % and excellent stability in catalyzing the conversion of carbonate into formate in aqueous solution. Ni0 and Ni2+ species are both observed in Ni/TiO2 catalysts, and the synergistic effect of these two Ni components leads to high activity and high selectivity of carbonate hydrogenation to formate.

Constructing an oxygen vacancy- and hydroxyl-rich TiO2-supported Pd catalyst with improved Pd dispersion and catalytic stability

Surface property modification of catalyst support is a straightforward approach to optimize the performance of supported noble metal catalysts. In particular, oxygen vacancies and hydroxyl groups play significant roles in promoting noble metal dispersion on catalysts as well as catalytic stability. In this study, we developed a nanoflower-like TiO2-supported Pd catalyst that has a higher concentration of oxygen vacancies and surface hydroxyl groups compared to that of commercial anatase and P25 support. Notably, due to the distinctive structure of the nanoflower-like TiO2, our catalyst exhibited improved dispersion and stabilization of Pd species and the formation of abundant reactive oxygen species, thereby facilitating the activation of CO and O2 molecules. As a result, the catalyst showed remarkable efficiency in catalyzing the low-temperature CO oxidation reaction with a complete CO conversion at 80 °C and stability for over 100 h.

Constructing a Pd-Co Interface to Tailor a d-Band Center for Highly Efficient Hydroconversion of Furfural over Cobalt Oxide-Supported Pd Catalysts

Cobalt is an alternative catalyst for furfural hydrogenation but suffers from the strong binding of H and furan ring on the surface, resulting in low catalytic activity and chemoselectivity. Herein, by constructing a Pd-Co interface in cobalt oxide-supported Pd catalysts to tailor the d-band center of Co, the concerted effort of Pd and Co boosts the catalytic performance for the hydroconversion of furfural to cyclopentanone and cyclopentanol. The increased dispersion of Pd on acid etching Co3O4 promotes the reduction of Co3+ to Co0 by enhancing hydrogen spillover, favoring the creation of the Pd-Co interface. Both experimental and theoretical calculations demonstrate that the electron transfer from Pd to Co at the interface results in the downshift of the d-band center of Co atoms, accompanied by the destabilization of H and furan ring adsorption on the Co surface, respectively. The former improves the furfural hydrogenation with TOF on Co elevating from 0.20 to 0.62 s-1, and the latter facilitates the desorption of formed furfuryl alcohol from the Co surface for subsequently hydrogenative rearrangement of the furan ring to cyclopentanone on acid sites. The resultant Pd/Co3O4-6 catalyst delivers superior activity with a 99% furfural conversion and 85% overall selectivity toward cyclopentanone/cyclopentanol. We anticipate that such a concept of tailoring the d-band center of Co via interface engineering provides novel insight and feasible approach for the design of highly efficient catalysts for furfural hydroconversion and beyond.

Enhanced Photocatalytic Coupling of Benzylamine to N-Benzylidene Benzylamine over the Organic-Inorganic Composites F70-TiO2 Based on Fullerenes Derivatives and TiO2

The organic-inorganic composites F70-TiO₂, based on fullerene with carboxyl group derivatives and TiO₂ semiconductor, have been designed and constructed to become an optical-functional photocatalyst via the facile sol-gel method. The composite photocatalyst obtained shows excellent photocatalytic activity for the high-efficiency conversion of benzylamine (BA) to N-benzylidene benzylamine (NBBA) with air pressure at a normal temperature under visible light irradiation. By optimizing the composition, the composites with the 1:15 mass ratio of F70 and TiO₂, denoted as F70-TiO₂(1:15), demonstrated the highest reaction efficiency for benzylamine (>98% conversion) to N-benzylidene benzylamine (>93% selectivity) in this study. However, pure TiO₂ and fullerene derivatives (F70) exhibit decreased conversion (56.3% and 89.7%, respectively) and selectivity (83.8% and 86.0%, respectively). The UV-vis diffuse reflectance spectra (DRS) and Mott-Schottky experiment's results indicate that the introduction of fullerene derivatives into anatase TiO₂ would greatly broaden the visible light response range and adjust the energy band positions of the composites, enhancing the sunlight utilization and promoting the photogenerated charge (e^--h⁺) separation and transfer. Specifically, a series of results on the in situ EPR tests and the photo-electrophysical experiment indicate that the separated charges from the hybrid could effectively activate benzylamine and O₂ to accelerate the formation of active intermediates, and then couple with free BA molecules to form the desired production of N-BBA. The effective combination, on a molecular scale, between fullerene and titanium dioxide has provided a profound understanding of the photocatalysis mechanism. This work elaborates and makes clear the relationship between the structure and the performance of functional photocatalysts.