Structural evolution of TiN catalysts during mechanocatalytic ammonia synthesis.

Mechanocatalytic ammonia synthesis is a novel approach toward ammonia synthesis under mild conditions. However, many open questions remain about the mechanism of mechanocatalytic ammonia synthesis, as well as the structure of the active catalysts during milling. Herein, the structural evolution of an in situ synthesized titanium nitride catalyst is explored during extended milling. The yield of ammonia bound to the catalyst surface was found to strongly correlate with an increase in catalyst surface area during milling, although a lower surface concentration of ammonia at earlier milling times suggests a delay in ammonia formation, corresponding to the conversion of the titanium metal pre-catalyst into the nitride. Small pores develop in the catalyst during milling due to interstitial spaces between agglomerated titanium nitride nanoparticles, as shown by SEM and TEM. In the first 6 h, the titanium is both converted to a nitride and fractured to smaller particles, before an equilibrium state is reached. After 18 h of milling, the catalyst nanoparticles appear to crystallize into a denser material, resulting in a loss of surface area and pore volume.

Insights into the Oxidative Degradation Mechanism of Solid Amine Sorbents for CO2 Capture from Air: Roles of Atmospheric Water.

Direct air capture (DAC) processes for extraction of CO2 from ambient air are unique among chemical processes in that they operate outdoors with minimal feed pretreatments. Here, the impact of humidity on the oxidative degradation of a prototypical solid supported amine sorbent, poly(ethylenimine) (PEI) supported on Al2 O3 , is explored in detail. By combining CO2 adsorption measurements, oxidative degradation rates, elemental analyses, solid-state NMR and in situ IR spectroscopic analysis in conjunction with 18 O labeling of water, a comprehensive picture of sorbent oxidation is achieved under accelerated conditions. We demonstrated that the presence of water vapor can play an important role in accelerating the degradation reactions. From the study we inferred the identity and kinetics of formation of the major oxidative products, and the role(s) of humidity. Our data are consistent with a radical mediated autooxidative degradation mechanism.

Characterization of Surface Species during Benzene Hydroxylation over a NiO-Ceria-Zirconia Catalyst.

NiO/ceria-zirconia (CZ) is a promising catalyst for the selective oxidation of benzene, as the Lewis-acidic NiO clusters can activate C-H bonds and the redox-active CZ support can activate O2 and supply active oxygen species for the reaction. In this study, we used transmission in situ infrared (IR) spectroscopy to examine surface species formed from benzene, water, oxygen, phenol, and catechol on a NiO/CZ catalyst. The formation of surface species from benzene and phenol was compared at different temperatures in the range of 50-200 °C in the presence and absence of water vapor. We also examined the role of the NiO clusters and the CZ support during benzene activation by comparing the surface species formed on NiO-CZ with those formed on a Ni-free CZ support and on a NiO/SiO2 catalyst. The spectrum of surface species from dosing benzene at 180 °C provides evidence for C-H bond activation. Specifically, the observation of C-O stretching vibrations indicates the formation of phenolate species. Introduction of water enhances these IR signals and introduces several additional peaks, indicating that a variety of different surface species are formed. These results show that NiO/CZ could catalyze direct conversion of benzene to phenol.

The "Missing" Bicarbonate in CO2 Chemisorption Reactions on Solid Amine Sorbents.

We have identified a hydrated bicarbonate formed by chemisorption of 13CO2 on both dimethylaminopropylsilane (DMAPS) and aminopropylsilane (APS) pendant molecules grafted on SBA-15 mesoporous silica. The most commonly used sequence in solid-state NMR, 13C CPMAS, failed to detect bicarbonate in these solid amine sorbent samples; here, we have employed a Bloch decay ("pulse-acquire") sequence (with 1H decoupling) to detect such species. The water that is present contributes to the dynamic motion of the bicarbonate product, thwarting CPMAS but enabling direct 13C detection by shortening the spin-lattice relaxation time. Since solid-state NMR plays a major role in characterizing chemisorption reactions, these new insights that allow for the routine detection of previously elusive bicarbonate species (which are also challenging to observe in IR spectroscopy) represent an important advance. We note that employing this straightforward NMR technique can reveal the presence of bicarbonate that has often otherwise been overlooked, as demonstrated in APS, that has been thought to only contain adsorbed CO2 as carbamate and carbamic acid species. As in other systems (e.g., proteins), dynamic species that sample multiple environments tend to broaden as their motion is frozen out. Here, we show two distinct bicarbonate species upon freezing, and coupling to different protons is shown through preliminary 13C-1H HETCOR measurements. This work demonstrates that bicarbonates have likely been formed in the presence of water but have gone unobserved by NMR due to the nature of the experiments most routinely employed, a perspective that will transform the way the sorption community will view CO2 capture by amines.

Enhanced Hydrothermal Stability of γ-Al2O3 Catalyst Supports with Alkyl Phosphonate Coatings.

In this study, monolayers formed from organophosphonic acids were employed to stabilize porous γ-Al2O3, both as a single component and as a support for Pt nanoparticle catalysts, during exposure to hydrothermal conditions. To provide a baseline, structural changes of uncoated γ-Al2O3 catalysts under model aqueous phase reforming conditions (liquid water at 200 °C and autogenic pressure) were examined over the course of 20 h. These changes were characterized by X-ray diffraction, NMR spectroscopy, N2 physisorption, and IR spectroscopy. It was demonstrated that γ-alumina was rapidly converted into a hydrated boehmite (AlOOH) phase with significantly decreased surface area. Deposition of alkyl phosphonate groups on γ-alumina drastically inhibited the formation of boehmite, thereby maintaining its high specific surface area over 20 h of treatment. 27Al MAS NMR spectra demonstrated that hydrothermal stability increased with alkyl tail length despite lower P coverages. Although the inhibition of boehmite formation by the phosphonic acids was attributed primarily to the formation of Al2O3-PO x bonds, it was found that use of longer-chain octadecylphosphonic acids led to the most pronounced effect. Phosphonate coatings on Pt/γ-Al2O3 improved stability without adversely affecting the rate of a model reaction, catalytic hydrogenation of 1-hexene.

Silica-Supported Sterically Hindered Amines for CO2 Capture.

Most studies exploring the capture of CO2 on solid-supported amines have focused on unhindered amines or alkylimine polymers. It has been observed in extensive solution studies that another class of amines, namely sterically hindered amines, can exhibit enhanced CO2 capacity when compared to their unhindered counterparts. In contrast to solution studies, there has been limited research conducted on sterically hindered amines on solid supports. In this work, one hindered primary amine and two hindered secondary amines are grafted onto mesoporous silica at similar amine coverages, and their adsorption performances are investigated through fixed bed breakthrough experiments and thermogravimetric analysis. Furthermore, chemisorbed CO2 species formed on the sorbents under dry and humid conditions are elucidated using in situ Fourier-transform infrared spectroscopy. Ammonium bicarbonate formation and enhancement of CO2 adsorption capacity is observed for all supported hindered amines under humid conditions. Our experiments in this study also suggest that chemisorbed CO2 species formed on supported hindered amines are weakly bound, which may lead to reduced energy costs associated with regeneration if such materials were deployed in a practical separation process. However, overall CO2 uptake capacities of the solid supported hindered amines are modest compared to their solution counterparts. The oxidative and thermal stabilities of the supported hindered amine sorbents are also assessed to give insight into their operational lifetimes.

15N Solid State NMR Spectroscopic Study of Surface Amine Groups for Carbon Capture: 3-Aminopropylsilyl Grafted to SBA-15 Mesoporous Silica.

Materials composed of high-porosity solid supports, such as SBA-15, containing amine-bearing moieties inside the pores, such as 3-aminopropylsilane (APS), are envisioned for carbon dioxide capture; solid-state 15N NMR can be highly informative for studying chemisorption reactions. Two 15N-enriched samples with different APS loadings were studied to probe the identity of the pendant molecules and structure of the chemisorbed CO2 species. 15N cross-polarization magic-angle spinning NMR provides unique information about the amines, whether they are rigid or dynamic, by measuring contact time curves and rotating frame, T1ρ(15N), relaxation. Both carbamate and carbamic acid are formed; carbamic acid is shown to be less stable than carbamate. After desorption, a steady state for the chemisorbed reaction product is reached, leaving behind carbamate. 15N NMR monitors the evolution of the species over time. During desorption, APS is regenerated, but the ammonium propylsilane intensity does not change, leading us to conclude that carbamic acid desorbs, while carbamate (to which ammonium propylsilane is ion paired) persists. A secondary ditehtered amine present does not react with CO2, and we posit this may be due to its rigidity. These findings demonstrate the versatility of solid-state NMR to provide information about these complex CO2 reactions with solid amine sorbents.

Spectroscopic Characterization of Adsorbed 13CO2 on 3-Aminopropylsilyl-Modified SBA15 Mesoporous Silica.

Multiple chemisorption products are found from the interaction of CO2 with the solid-amine sorbent, 3-aminopropyl silane (APS), bound to mesoporous silica (SBA15) using solid-state NMR and FTIR spectroscopy. We employed a combination of both 15N{13C} rotational-echo double-resonance (REDOR) NMR and 13C{15N} REDOR to determine the chemical identity of these products. 15N{13C} REDOR measurements are consistent with a single 13C-15N pair and distance of 1.45 Å. In contrast, both 13C{15N} REDOR and 13C CPMAS are consistent with multiple 13C products. 13C CPMAS shows two neighboring resonances, whose chemical shifts are consistent with carbamate (at 165 ppm) and carbamic acid. The 13C{15N} REDOR experiments resonant at 165 ppm show an incomplete buildup of the REDOR data to ∼90% of the expected maximum. We conclude this 10% missing intensity corresponds to a 13C NMR species that resonates at the identical chemical shift but that is not in dipolar contact with 15N. These data are consistent with the presence of bicarbonate, HCO3-, since it is commonly observed at ∼165 ppm and lacks 15N for dipolar coupling.

Conversion of Methane into Methanol and Ethanol over Nickel Oxide on Ceria-Zirconia Catalysts in a Single Reactor.

The conversion of methane into alcohols under moderate reaction conditions is a promising technology for converting stranded methane reserves into liquids that can be transported in pipelines and upgraded to value-added chemicals. We demonstrate that a catalyst consisting of small nickel oxide clusters supported on ceria-zirconia (NiO/CZ) can convert methane to methanol and ethanol in a single, steady-state process at 723 K using O2 as an abundantly available oxidant. The presence of steam is required to obtain alcohols rather than CO2 as the product of catalytic combustion. The unusual activity of this catalyst is attributed to the synergy between the small Lewis acidic NiO clusters and the redox-active CZ support, which also stabilizes the small NiO clusters.

Important roles of enthalpic and entropic contributions to CO2 capture from simulated flue gas and ambient air using mesoporous silica grafted amines.

The measurement of isosteric heats of adsorption of silica supported amine materials in the low pressure range (0-0.1 bar) is critical for understanding the interactions between CO2 and amine sites at low coverage and hence to the development of efficient amine adsorbents for CO2 capture from flue gas and ambient air. Heats of adsorption for an array of silica-supported amine materials are experimentally measured at low coverage using a Calvet calorimeter equipped with a customized dosing manifold. In a series of 3-aminopropyl-functionalized silica materials, higher amine densities resulted in higher isosteric heats of adsorption, clearly showing that the density/proximity of amine sites can influence the amine efficiency of adsorbents. In a series of materials with fixed amine loading but different amine types, strongly basic primary and secondary amine materials are shown to have essentially identical heats of adsorption near 90 kJ/mol. However, the adsorption uptakes vary substantially as a function of CO2 partial pressure for different primary and secondary amines, demonstrating that entropic contributions to adsorption may play a key role in adsorption at secondary amine sites, making adsorption at these sites less efficient at the low coverages that are important to the direct capture of CO2 from ambient air. Thus, while primary amines are confirmed to be the most effective amine types for CO2 capture from ambient air, this is not due to enhanced enthalpic contributions associated with primary amines over secondary amines, but may be due to unfavorable entropic factors associated with organization of the second alkyl chain on the secondary amine during CO2 adsorption. Given this hypothesis, favorable entropic factors may be the main reason primary amine based adsorbents are more effective under air capture conditions.

Modeling Mechanochemical Depolymerization of PET in Ball-Mill Reactors Using DEM Simulations.

Developing efficient and sustainable chemical recycling pathways for consumer plastics is critical for mitigating the negative environmental implications associated with their end-of-life management. Mechanochemical depolymerization reactions have recently garnered great attention, as they are recognized as a promising solution for solvent-free transformation of polymers to monomers in the solid state. To this end, physics-based models that accurately describe the phenomena within ball mills are necessary to facilitate the exploration of operating conditions that would lead to optimal performance. Motivated by this, in this paper we develop a mathematical model that couples results from discrete element method (DEM) simulations and experiments to study mechanically-induced depolymerization. The DEM model was calibrated and validated via video experimental data and computer vision algorithms. A systematic study on the influence of the ball-mill operating parameters revealed a direct relationship between the operating conditions of the vibrating milling vessel and the total energy supplied to the system. Moreover, we propose a linear correlation between the high-fidelity DEM simulation results and experimental monomer yield data for poly(ethylene terephthalate) depolymerization, linking mechanical and energetic variables. Finally, we train a reduced-order model to address the high computational cost associated with DEM simulations. The predicted working variables are used as inputs to the proposed mathematical expression which allows for the fast estimation of monomer yields.

Synergy between Brønsted Acid Sites and Carbonaceous Deposits during Skeletal 1-Butene Isomerization over Ferrierite.

During skeletal 1-butene isomerization over ferrierite carbonaceous deposits block 98% of the micropores within 24 h, rendering them effectively inaccessible to reactants, while the catalytic activity improves continuously for 100 h on stream. Ex-situ pyridine adsorption shows that the concentration of conventional Brønsted acid sites in the 10-R channels decreases below the detection threshold of infrared spectroscopy within 2 h. However, the operando addition of the base triethyl amine to the feed quenches the reaction, showing that mediated acidity is necessary. The larger base 2,2,6,6-tetramethyl piperidine only deactivates catalytic activity after several hours because it cannot directly bind to active sites at the sterically restricted pore mouths. The communication of internal Brønsted acid sites to the external reactants via a concerted mechanism involving protonated monoaromatic deposits trapped in the pore mouths explains the promoting effects of coke species in zeolite-catalyzed skeletal butene isomerization. This work presents a consolidated explanation of the synergy of solid acidity, structural confinement, and carbonaceous deposits in zeolites.

Poisoning of Pt/γ-Al2O3 Aqueous Phase Reforming Catalysts by Ketone and Diketone-Derived Surface Species.

Strong adsorption of ketone and diketone byproducts and their fragmentation products during the aqueous phase reforming of biomass derived oxygenates is believed to be responsible for the deactivation of supported Pt catalysts. This study involves a combined experimental and theoretical approach to demonstrate the interactions of several model di/ketone poisons with Pt/γ-Al2O3 catalysts. Particular di/ketones were selected to reveal the effects of hydroxyl groups (acetone, hydroxyacetone), conjugation with C=C bonds (mesityl oxide), intramolecular distance between carbonyls in diketones (2,3-butanedione, 2,4-pentanedione), and length of terminal alkyl chains (3,4-hexanedione). The formation of adsorbed carbon monoxide (1900-2100 cm-1) as a decarbonylation product was probed using infrared spectroscopy and to calculate the extent of poisoning during subsequent methanol dehydrogenation based on the reduction of the ν(C≡O) band integral relative to experiments in which only methanol was dosed. Small Pt particles appeared less active in decarbonylation and were perhaps poisoned by strongly adsorbed di/ketones on undercoordinated metal sites and bulky conjugated species formed on the γ-Al2O3 support from aldol self-condensation. Larger Pt particles were more resistant to di/ketone poisoning due to higher decarbonylation activity yet still fell short of the expected yield of adsorbed CO from subsequent methanol activity. Vibrational spectra acquired using inelastic neutron scattering showed evidence for strongly binding methyl and acyl groups resulting from di/ketone decarbonylation on a Pt sponge at 250 °C. Adsorption energies and molecular configurations were obtained for di/ketones on a Pt(111) slab using density functional theory, revealing potential descriptors for predicting decarbonylation activity on highly coordinated metal sites. Calculated reaction energies suggest it is energetically favorable to reform surface methyl groups into adsorbed CO and H. However, the rate of this surface reaction is limited by a high activation barrier indicating that either improved APR catalyst designs or regeneration procedures may be necessary.

Kinetic Phenomena in Mechanochemical Depolymerization of Poly(styrene).

Synthetic polyolefinic plastics comprise one of the largest shares of global plastic waste, which is being targeted for chemical recycling by depolymerization to monomers and small molecules. One promising method of chemical recycling is solid-state depolymerization under ambient conditions in a ball-mill reactor. In this paper, we elucidate kinetic phenomena in the mechanochemical depolymerization of poly(styrene). Styrene is produced in this process at a constant rate and selectivity alongside minor products, including oxygenates like benzaldehyde, via mechanisms analogous to those involved in thermal and oxidative pyrolysis. Continuous monomer removal during reactor operation is critical for avoiding repolymerization, and promoting effects are exhibited by iron surfaces and molecular oxygen. Kinetic independence between depolymerization and molecular weight reduction was observed, despite both processes originating from the same driving force of mechanochemical collisions. Phenomena across multiple length scales are shown to be responsible for differences in reactivity due to differences in grinding parameters and reactant composition.

Surface interactions of C and C(3) polyols with γ-Al2O3 and the role of coadsorbed water.

The formation of surface species from two- and three-carbon polyols on γ-Al(2)O(3) in the presence and absence of coadsorbed water is investigated. Aqueous-phase adsorption isotherms indicate that competitive adsorption between water and polyol inhibits the uptake of the polyol molecules on γ-Al(2)O(3) and that the polyol with the most hydroxyl groups, glycerol, experienced the greatest uptake. Deuterium solid echo pulse NMR measurements support the fact that glycerol strongly interacts with γ-Al(2)O(3) in the presence of physisorbed water and that ethylene glycol interacts with γ-Al(2)O(3) only after the physisorbed water has been removed. In situ high-vacuum FT-IR analysis combined with DFT simulations demonstrate that glycerol readily forms a multidentate alkoxy species through its primary hydroxyl groups with coordinatively unsaturated Al atoms of γ-Al(2)O(3) in the presence of physisorbed water. This surface species exhibits a bridging alkoxy bond from one of its primary hydroxyl groups and a strong interaction with the remaining primary hydroxyl group. FT-IR analysis of 1,3-propanediol on γ-Al(2)O(3) also demonstrates the formation of a multidentate alkoxy species in the presence of coadsorbed water. In contrast, polyols with hydroxyl groups only on the one- and two-carbon atoms, ethylene glycol, and 1,2-propanediol do not form alkoxy bonds with the γ-Al(2)O(3) surface when coadsorbed water is present. These polyols will form alkoxy bonds to γ-Al(2)O(3) when coadsorbed water is removed, and these alkoxy species are removed when water is readsorbed on the sample. The formation of strongly bound, stable multidentate alkoxy species by ethylene glycol and 1,2-propanediol on γ-Al(2)O(3) is prevented by steric limitations of vicinal alcohol groups.

Differences in solvation thermodynamics of oxygenates at Pt/Al2O3 perimeter versus Pt(111) terrace sites.

A prominent role of water in aqueous-phase heterogeneous catalysis is to modify free energies; however, intuition about how is based largely on pure metal surfaces or even homogeneous solutions. Using multiscale modeling with explicit liquid water molecules, we show that the influence of water on the free energies of adsorbates at metal/support interfaces is different than that on pure metal surfaces. We specifically compute free energies of solvation for methanol and its constituents on a Pt/Al2O3 catalyst and compare the results to analogous values calculated on a pure Pt catalyst. We find that the more hydrophilic Pt/Al2O3 interface leads to smaller (more positive) free energies of solvation due to an increased entropy penalty resulting from the additional work necessary to disrupt the interfacial water structure and accommodate the interfacial species. The results will be of interest in other fields, including adsorption and proteins.