A descriptor of active CuO-ZnO(Al₂O₃) methanol-synthesis and water–gas-shift catalysts is the presence of high-temperature carbonates (HT-CO₃) in the oxidic catalyst precursor. Previous reports have shown that such HT-CO₃ lead to an appropriate interaction between the oxides; as a result, a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) can be achieved. Yet, their nature is not well understood. In this study, the nature of these carbonates was investigated by experimental and theoretical methods in the oxidic precatalyst. A calcined Cu-Zn-Al hydrotalcite model compound revealed to have well-dispersed ZnO and CuO phases, together with highly stable HT-CO₃. It was hypothesized that these HT-CO₃ groups may be placed at critical locations at nano-scale as a glue, thus avoiding the growth of the oxide crystallites during calcination. This is an excellent pre-condition to achieve a high Cu surface area (or Cu-Zn or Cu/ZnO interphase areas) upon reduction, and therefore a high activity. To prove that, first-principles calculations were carried out based on the density functional theory (DFT); alumina was not considered in the model as the experimental data showed it to be amorphous but it may still have an effect. Comprehensive calculations provided evidence that such carbonate groups favourably bind the CuO and ZnO together at the interface, rather than being isolated on the individual oxide surfaces. The results strongly suggest that the HT-CO₃ groups are part of the structure, in the calcined precatalyst, where they would prevent thermal sintering through a bonding mechanism between CuO and ZnO particles, which is a novel interpretation of this important catalyst descriptor.

Titanium-based metal-organic framework, NH2-MIL-125(Ti), has been widely investigated for photocatalytic applications but has low activity in the hydrogen evolution reaction (HER). In this work, we show a one-step low-cost postmodification of NH2-MIL-125(Ti) via impregnation of Co(NO3)2. The resulting Co@NH2-MIL-125(Ti) with embedded single-site CoII species, confirmed by XPS and XAS measurements, shows enhanced activity under visible light exposure. The increased H2 production is likely triggered by the presence of active CoI transient sites detected upon collection of pump-flow-probe XANES spectra. Furthermore, both photocatalysts demonstrated a drastic increase in HER performance after consecutive reuse while maintaining their structural integrity and consistent H2 production. Via thorough characterization, we revealed two mechanisms for the formation of highly active proton reduction sites: nondestructive linker elimination resulting in coordinatively unsaturated Ti sites and restructuring of single CoII sites. Overall, this straightforward manner of confinement of CoII cocatalysts within NH2-MIL-125(Ti) offers a highly stable visible-light-responsive photocatalyst.

In order to capture and separate CO2 from the air or flue gas streams through nanoporous adsorbents, the influence of the humidity in these streams has to be taken into account as it hampers the capture process in two main ways: (1) water preferentially binds to CO2 adsorption sites and lowers the overall capacity, and (2) water causes hydrolytic degradation and pore collapse of the porous framework. Here, we have used a water-stable polyimide covalent organic framework (COF) in N2/CO2/H2O breakthrough studies and assessed its performance under varying levels of relative humidity (RH). We discovered that at limited relative humidity, the competitive binding of H2O over CO2 is replaced by cooperative adsorption. For some conditions, the CO2 capacity was significantly higher under humid versus dry conditions (e.g., a 25% capacity increase at 343 K and 10% RH). These results in combination with FT-IR studies on equilibrated COFs at controlled RH values allowed us to assign the effect of cooperative adsorption to CO2 being adsorbed on single-site adsorbed water. Additionally, once water cluster formation sets in, loss of CO2 capacity is inevitable. Finally, the polyimide COF used in this research retained performance after a total exposure time of >75 h and temperatures up to 403 K. This research provides insight in how cooperative CO2-H2O can be achieved and as such provides directions for the development of CO2 physisorbents that can function in humid streams.

In this study, Ni catalysts supported on Pr-doped CeO₂ are studied for the CO₂ methanation reaction and the effect of Pr doping on the physicochemical properties and the catalytic performance is thoroughly evaluated. It is shown, that Pr³⁺ ions can substitute Ce⁴⁺ ones in the support lattice, thereby introducing a high population of oxygen vacancies, which act as active sites for CO₂ chemisorption. Pr doping can also act to reduce the crystallite size of metallic Ni, thus promoting the active metal dispersion. Catalytic performance evaluation evidences the promoting effect of low Pr loadings (5 at% and 10 at%) towards a higher catalytic activity and lower CO₂ activation energy. On the other hand, higher Pr contents negate the positive effects on the catalytic activity by decreasing the oxygen vacancy population, thereby creating a volcano-type trend towards an optimum amount of aliovalent substitution.

Bimetal-organic-framework (Bi-MOF) NH2-MIL-125(Ti/Cu)-derived nanocomposites are systematically investigated to elucidate the role of individual species TiO2, CuxO and the porous carbon matrix in photocatalytic activity. Among the studied samples, the TiO2/CuxO/C nanocomposite derived from heat processing NH2-MIL-125(Ti/Cu) under Ar/H2O vapor demonstrates the highest photocatalytic H2 evolution performance due to the formation of a phasejunction between the well-crystallized anatase/rutile TiO2 polymorph, the optimized and codoped nitrogen/carbon in the composites, the formation of p-n heterojunctions between the TiO2 and CuxO nanoparticles, as well as their uniform distribution in a hydrophilic porous carbon matrix decorated with N and carboxylic functional groups. These parameters enable the in situ-formed multi-heterostructures in these nanocomposites to not only possess relatively narrower energy band gaps and improved spatial charge separation due to the formed type-II staggered p-n heterojunctions but also offer multiple pathways for charge diffusion, resulting in lower charge-transfer resistance, suppressed bulk charge recombination, and consequently, much improved visible-light absorption. Therefore, the Bi-MOF NH2-MIL-125(Ti/Cu)-derived TiO2/CuxO/C nanocomposite provides easily accessible active sites with an excellent photocatalytic H2 evolution activity of 3147 μmol gcat-1 h-1, 99 times higher than that of bare TiO2. This work provides a simple one-step approach to producing tunable novel nanocomposites for efficient photocatalytic H2 evolution without using expensive noble metals as cocatalysts.

Numerous hyper-cross-linked polymers (HCPs) have been developed as CO2 adsorbents and photocatalysts. Yet, little is known of the CO2 and H2O adsorption mechanisms on amorphous porous polymers. Gaining a better understanding of these mechanisms and determining the adsorption sites are key to the rational design of improved adsorbents and photocatalysts. Herein, we present a unique approach that combines density functional theory (DFT), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and multivariate spectral analysis to investigate CO2 and H2O adsorption sites on a triazine-biphenyl HCP. We found that CO2 and H2O adsorb on the same HCP sites albeit with different adsorption strengths. The primary amines of the triazines were identified as favoring strong CO2 binding interactions. Given the potential use of HCPs for CO2 photoreduction, we also investigated CO2 and H2O adsorption under transient light irradiation. Under irradiation, we observed partial CO2 and H2O desorption and a redistribution of interactions between the H2O and CO2 molecules that remain adsorbed at HCP adsorption sites.

Direct ink writing technique is used to 3D print Ti-metal–organic framework (MOF) NH₂-MIL-125 mixed with boehmite dispersal. Pt is also deposited onto 3D-printed monolith using atomic layer deposition (ALD) to offer additional catalytic sites. The Ti-MOF-derived powder sample and the pyrolyzed 3D-printed monolith samples are evaluated for photocatalytic H₂ evolution under UV–vis light. As a proof of concept, herein, it is demonstrated that 3D-printed MOF-derived monolith photocatalysts show five times higher H₂ evolution performance compared with TiO₂/C powder sample due to better interaction between 3D-printed photocatalysts and the incident light. The high surface area, the formation of hierarchical macro- to nanopores, and the optimizable shape/size of the 3D-printed catalyst maximize the exposure of catalytic active sites to incident photons and increase their photocatalytic H₂ evolution performance. In addition, the N-functionalized porous carbon from organic linker, and the uniformly distributed Pt/PtO_x species deposited by ALD, provide cocatalytic active sites for photocatalytic reaction and further enhance photocatalytic activity 30% of 3D-printed monoliths. This work on the 3D-printed MOF-derived free-standing monoliths for photocatalytic application provides a readily available approach to further fabricate a variety of 3D-printed MOF-based and derived materials for different energy and environment applications.

This work reports initial results on the effect of low concentrations (ppm level) of a stabilizing agent (2,6-di-tert-butyl-4-methylphenol, BHT) present in an off-the-shelf solvent on the catalyst performance for the hydrogenolysis of γ-butyrolactone over Cu-ZnO-based catalysts. Tetrahydrofuran (THF) was employed as an alternative solvent in the hydrogenolysis of γ-butyrolactone. It was found that the Cu-ZnO catalyst performance using a reference solvent (1,4-dioxane) was good, meaning that the equilibrium conversion was achieved in 240 min, while a zero conversion was found when employing tetrahydrofuran. The deactivation was studied in more detail, arriving at the preliminary conclusion that one phenomenon seems to play a role: the poisoning effect of a solvent additive present at the ppm level (BHT) that appears to inhibit the reaction completely over a Cu-ZnO catalyst. The BHT effect was also visible over a commercial Cu-ZnO-MgO-Al2O3 catalyst but less severe than that over the Cu-ZnO catalyst. Hence, the commercial catalyst is more tolerant to the solvent additive, probably due to the higher surface area. The study illustrates the importance of solvent choice and purification for applications such as three-phase-catalyzed reactions to achieve optimal performance.

Surface-functionalized nitrogen/carbon co-doped polymorphic TiO₂ phase junction nanoparticles uniformly distributed in porous carbon matrix were synthesized by a simple one-step pyrolysis of titanium based metal–organic framework (MOF), NH₂-MIL-125(Ti) at 700 °C under water vapour atmosphere. Introducing water vapour during the pyrolysis of NH₂-MIL-125(Ti) not only functionalizes the derived porous carbon matrix with carboxyl groups but also forms additional oxygen-rich N like interstitial/intraband states lying above the valence band of TiO₂ along with the self-doped carbon, which further narrows the energy band gaps of polymorphic TiO₂ nanoparticles that enhance photocatalytic charge transfer efficiency. Without co-catalyst, sample N-C-TiO₂/C_ArW demonstrates H₂ evolution activity of 426 µmol g_cat^-1 h⁻¹, which remarkably outperforms commercial TiO₂ (P-25) and N-C-TiO₂/C_Ar with a 5-fold and 3-fold H₂ generation, respectively. This study clearly shows that water vapour atmosphere during the pyrolysis increases the hydrophilicity of the Ti-MOF derived composites by functionalizing porous carbon matrix with carboxylic groups, as well as enhancing the electrical conductivity and charge transfer efficiency due to the formation of additional localized oxygen-rich N like interstitial/intraband states. This work also demonstrates that by optimizing the anatase–rutile phase composition of the TiO₂ polymorphs, tuning the energy band gaps by N/C co-doping and functionalizing the porous carbon matrix in the N-C-TiO₂/C nanocomposites, the photocatalytic H₂ generation activity can be further enhanced.

In situformation of p-n heterojunctions between TiO₂and Cu_xO in heteroatom-doped carbon nanocomposites and their applications in photocatalytic H₂evolution were demonstrated. One-step pyrolysis of bimetal-organic-frameworks NH₂-MIL-125(Ti/Cu) in steam at 700 °C forms a p-n heterojunction between TiO₂and Cu_xO nanoparticles. Concurrently, a phasejunction between nitrogen/carbon co-doped anatase and rutile TiO₂is formed, accompanied by the formation of Cu_xO heterostructures. These multi-heterostructures are embedded in N-containing and hydrophilic carboxyl functionalized carbon matrix. The optimized TiO₂/Cu_xO/C composite multi-heterostructures offer multiple pathways for photoinduced electrons and holes migration, absorb more visible light, and provide an increased number of active sites for photocatalytic reactions. Without loading expensive noble metals, the TiO₂/Cu_xO/C nanocomposite derived at 700 °C in steam exhibited a superior photocatalytic H₂generation activity of 3298 μmol g_cat⁻¹h⁻¹under UV-Visible light, 40 times higher than that of commercial TiO₂. This work offers a simple approach to fabricate novel photocatalytic nanocomposites for efficient H₂generation.

Deactivation of a Pd/alumina catalyst has been observed during the hydrogenation of α-methylstyrene and styrene. In both feedstocks, deactivation is caused by an additive, 4-tert-butylcatechol (TBC), a polymerization inhibitor, commonly employed at the ppm concentration level in the formulation of commercial monomers. It was found that the reaction rate in the α-methylstyrene fluctuated notably among the reactant vendors, and this was ascribed to the varying concentration of TBC, although other factors, such as the concentration of water, may play a role. The study was extended into the hydrogenation of styrene using a trickle bed reactor. The negative impact of the TBC present at the ppm level was obvious. The deactivation mechanism was complex to rationalize. A two-stage behavior was observed: a first stage of a relatively fast deactivation followed by a second stage of slow deactivation. A tentative explanation considers the presence of two types of Pd-sites, which are poisoned by TBC: the more active α-Pd-H sites and the less active β-Pd-H sites. Finally, in practical terms, it is important to emphasize that such an additive must be removed from the reactant to maximize the catalyst performance. This can be achieved by adsorption using a commercial F-200 Alcoa alumina.

Abstract: Mn and Li promoted Rh catalysts supported on SiO₂ with a thin TiO₂ layer were synthesized by stepwise incipient wetness impregnation approach. The thin TiO₂ layer on the surface of SiO₂ was proved to stabilize those small Rh nanoparticles and hinder their agglomeration. The reducibility of Rh on these catalysts depends on Rh particle size as well as the position of manganese oxide, and large Rh nanoparticles with MnO on Rh nanoparticles can be only reduced at an elevated temperature. Catalyst with large Rh particles exhibits a higher CO conversion and higher products selectivity towards long chain hydrocarbons and C2-oxygenates at the expense of decreasing methane formation than a similar catalyst with smaller Rh particles. This was attributed to the synergistic effect of Mn and Li promotion and molar ratio between Rh⁰ and Rh^δ+ sites on the surface of Rh nanoparticles. Moreover, Rh nanoparticles on MnO are proved to be more efficient in promoting hydrogenation of acetaldehyde to ethanol than its counterpart with MnO on Rh nanoparticles. Finally, in order to target high C2-oxygenates selectivity, low reaction temperature together with a low H₂/CO ratio in the feed is recommended. Graphic Abstract: [Figure not available: see fulltext.].

The mechanism of methane activation on Mo/HZSM-5 is poorly described, despite the great interest in methane dehyd roa romatization (MDA) to replace oil refineries for producing aromatics. It is difficult to assess the exact nature of the active site due to fast coking. By pre-carburizing Mo/HZSM-5 with carbon monoxide, the active site for MDA can be isolated and studied without the formation of coke. This strategy helped us examine how methane is activated on the catalytic site by carrying out MDA using isotopically labelled methane (^{1 3} C H ₄ ) . We show that carbon originating from the preformed carbide is incorporated into the main products of the reaction, ethylene and benzene, demonstrating the dynamic nature of these active sites.

We tailored the size distribution of Pt nanoparticles (NPs) on graphene nanoplatelets at a given metal loading by using low-temperature atomic layer deposition carried out in a fluidized bed reactor operated at atmospheric pressure. The Pt NPs deposited at low temperature (100 °C) after 10 cycles were more active and stable towards the propene oxidation reaction than their high-temperature counterparts. Crucially, the gap in the catalytic performance was retained even after prolonged periods of time (>24 hours) at reaction temperatures as high as 450 °C. After exposure to such harsh conditions the Pt NPs deposited at 100 °C still retained a size distribution that is narrower than the one of the as-synthesized NPs obtained at 250 °C. The difference in performance correlated with the difference in the number of facet sites as estimated after the catalytic test. Our approach provides not only a viable route for the scalable synthesis of stable supported Pt NPs with tailored size distributions but also a tool for studying the structure-function relationship.

Metal-organic frameworks (MOFs) containing d⁰ metals such as NH₂-MIL-125(Ti), NH₂-UiO-66(Zr) and NH₂-UiO-66(Hf) are among the most studied MOFs for photocatalytic applications. Despite structural similarities, we demonstrate that the electronic properties of these MOFs are markedly different. As revealed by quantum chemistry, EPR measurements and transient absorption spectroscopy, the highest occupied and lowest unoccupied orbitals of NH₂-MIL-125(Ti) promote a long lived ligandto-metal charge transfer upon photoexcitation, making this material suitable for photocatalytic applications. In contrast, in case of UiO materials, the d-orbitals of Zr and Hf, are too low in binding energy and thus cannot overlap with the π orbital of the ligand, making both frontier orbitals localized at the organic linker. This electronic reconfiguration results in short exciton lifetimes and diminishes photocatalytic performance. These results highlight the importance of orbital contributions at the band edges and delineate future directions in the development of photo-active hybrid solids.

Ceria-based catalysts with different topological and textural properties have been prepared to study the role of the soot-catalyst contact on the soot oxidation reaction. The physico-chemical features of the catalysts have been investigated by means of complementary techniques, such as powder XRD, N₂ physisorption at -196 °C, optical microscopy at variable temperature, FESEM, TEM, and thermogravimetric analysis.As a whole, the best catalytic activity has been obtained with the CeO₂-nanocubes (denoted to as "Ce-NC") because of their higher intrinsic reactivity. On the other hand, high-surface area materials prepared by the cerium nitrate decomposition (denoted to as "Ce-ND") or hydrothermal route (CeO₂-stars, referred to as "Ce-SAS") resulted less effective toward the soot combustion, confirming the surface-sensitivity for this reaction.Moreover, it has been proven a higher dependence of the oxidation activity on the catalyst-to-soot ratio (wt./wt.) for the nanostructured catalyst (Ce-NC) exhibiting the lowest BET specific surface area (S_BET = 4 m² g^-1). On the other hand, the accessible (real) surface area, at variance of the BET surface area, seems to play a relevant role for this solid-solid reaction mediated by gas-phase oxygen.