Ultrasmall Pd Clusters in FER Zeolite Alleviate CO Poisoning for Effective Low-Temperature Carbon Monoxide Oxidation

Ultrasmall Pd4 clusters form in the micropores of FER zeolite during low-temperature treatment (100 °C) in the presence of humid CO gas. They effectively catalyze CO oxidation below 100 °C, whereas Pd nanoparticles are not active as they are poisoned by CO. Using catalytic measurements, infrared (IR) spectroscopy, X-ray absorption spectroscopy (EXAFS), microscopy, and density functional theory calculations, we provide the molecular-level insight into this previously unreported phenomenon. Pd nanoparticles get covered with CO at low temperatures, which effectively blocks O2 activation until CO desorption occurs. Small Pd clusters in zeolites, in contrast, demonstrate fluxional behavior in the presence of CO, which significantly increases the affinity for binding O2. Our study provides a pathway to achieve low-temperature CO oxidation activity on the basis of a well-defined Pd/zeolite system.

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water-Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO₂-anatase SAC (Pd₁/TiO₂) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO₂ by H₂ alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H₂ is accompanied by the partial sintering of single Pd atoms (Pd₁) into disordered, flat, ∼1 nm diameter clusters (Pd_n). The highly active Pd sites in the new coordination environment under H₂ are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pd_n and facilitates the reduction of TiO₂. In contrast, Pd₁ sinters into crystalline, ∼5 nm particles (Pd_NP) during CO treatment, deactivating Pd₁/TiO₂. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H₂ dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H₂. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.

Single Ru(II) Ions on Ceria as a Highly Active Catalyst for Abatement of NO

Atom trapping leads to catalysts with atomically dispersed Ru₁O₅ sites on (100) facets of ceria, as identified by spectroscopy and DFT calculations. This is a new class of ceria-based materials with Ru properties drastically different from the known M/ceria materials. They show excellent activity in catalytic NO oxidation, a critical step that requires use of large loadings of expensive noble metals in diesel aftertreatment systems. Ru₁/CeO₂ is stable during continuous cycling, ramping, and cooling as well as the presence of moisture. Furthermore, Ru₁/CeO₂ shows very high NO_x storage properties due to formation of stable Ru-NO complexes as well as a high spill-over rate of NO_x onto CeO₂. Only ∼0.05 wt % of Ru is required for excellent NO_x storage. Ru₁O₅ sites exhibit much higher stability during calcination in air/steam up to 750 °C in contrast to RuO₂ nanoparticles. We clarify the location of Ru(II) ions on the ceria surface and experimentally identify the mechanism of NO storage and oxidation using DFT calculations and in situ DRIFTS/mass spectroscopy. Moreover, we show excellent reactivity of Ru₁/CeO₂ for NO reduction by CO at low temperatures: only 0.1-0.5 wt % of Ru is sufficient to achieve high activity. Modulation-excitation in situ infrared and XPS measurements reveal the individual elementary steps of NO reduction by CO on an atomically dispersed Ru ceria catalyst, highlighting unique properties of Ru₁/CeO₂ and its propensity to form oxygen vacancies/Ce⁺³ sites that are critical for NO reduction, even at low Ru loadings. Our study highlights the applicability of novel ceria-based single-atom catalysts to NO and CO abatement.

Effect of reaction conditions on the hydrogenolysis of polypropylene and polyethylene into gas and liquid alkanes

Hydrogenolysis of polypropylene and polyethylene provides a pathway to smaller hydrocarbons. We describe the impact of the polyolefin structure, reaction conditions, and presence of chlorine on the product distribution and branching degree.

Optimizing Active Sites for High CO Selectivity during CO2 Hydrogenation over Supported Nickel Catalysts

Controlling the selectivity of CO₂ hydrogenation catalysts is a fundamental challenge. In this study, the selectivity of supported Ni catalysts prepared by the traditional impregnation method was found to change after a first CO₂ hydrogenation reaction cycle from 100 to 800 °C. The usually high CH₄ formation was suppressed leading to full selectivity toward CO. This behavior was also observed after the catalyst was treated under methane or propane atmospheres at elevated temperatures. In situ spectroscopic studies revealed that the accumulation of carbon species on the catalyst surface at high temperatures leads to a nickel carbide-like phase. The catalyst regains its high selectivity to CH₄ production after carbon depletion from the surface of the Ni particles by oxidation. However, the selectivity readily shifts back toward CO formation after exposing the catalysts to a new temperature-programmed CO₂ hydrogenation cycle. The fraction of weakly adsorbed CO species increases on the carbide-like surface when compared to a clean nickel surface, explaining the higher selectivity to CO. This easy protocol of changing the surface of a common Ni catalyst to gain selectivity represents an important step for the commercial use of CO₂ hydrogenation to CO processes toward high-added-value products.

In Situ Dispersion of Palladium on TiO2 During Reverse Water-Gas Shift Reaction: Formation of Atomically Dispersed Palladium

The application of single-atom catalysts (SACs) to high-temperature hydrogenation requires materials that thermodynamically favor metal atom isolation over cluster formation. We demonstrate that Pd can be predominantly dispersed as isolated atoms onto TiO₂ during the reverse water-gas shift (rWGS) reaction at 400 °C. Achieving atomic dispersion requires an artificial increase of the absolute TiO₂ surface area by an order of magnitude and can be accomplished by physically mixing a precatalyst (Pd/TiO₂ ) with neat TiO₂ prior to the rWGS reaction. The in situ dispersion of Pd was reflected through a continuous increase of rWGS activity over 92 h and supported by kinetic analysis, infrared and X-ray absorption spectroscopies and scanning transmission electron microscopy. The thermodynamic stability of Pd under high-temperature rWGS conditions is associated with Pd-Ti coordination, which manifests upon O-vacancy formation, and the artificial increase in TiO₂ surface area.

Hydrochemical data on groundwater quality for drinking and irrigation use around Dangila town, Northwest Ethiopia

The groundwater of volcanic aquifers, dissected by various structures and affected by several volcanic eruption events, varies in quality. A large number of rural people depend on shallow aquifers tapped by shallow hand wells and springs. On the other hand, the urban population is dependant on deep aquifers using drilled boreholes. The location of springs, shallow hand-dug wells and boreholes inside or close to farmlands, and the advancement of irrigation water use from groundwater by the government entail the assessment of groundwater quality. Therefore, the focus of the present study is to determine the quality and suitability of groundwater around Dangila Town, Northwest Ethiopia, for drinking and irrigation uses. The water quality assessment was conducted by collecting groundwater samples from 14 shallow hand-dug wells, 4 springs, and 7 deep boreholes then analysing for different physical and chemical parameters.

A total of 25 selected groundwater samples from shallow and deep aquifers were analysed in a laboratory for physical and chemical parameters. The physical parameters measured both in the field and the laboratory included pH, electrical conductivity (EC) and total dissolved solids (TDS). The chemical parameters analysed in the laboratory comprised cations of calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), Iron (Fe), manganese (Mn²⁺) and anions of bicarbonate (HCO₃^-), sulfate (SO₄^2-), carbonate (CO₃^2-), chlorine (Cl^-), nitrate (NO₃^-), fluoride (F^-), and boron (B). Based on the laboratory results, the variation in groundwater facies, and major cation and anion sources were determined. Furthermore, the groundwater quality for human consumption was assessed and sodium adsorption ratio (SAR), Na%, and the residual sodium carbonate (RSC) values, which are crucial to determine the overall groundwater quality for irrigational uses, were calculated. Detailed interpretations of the data have been presented in the paper entitled “Hydrogeological framework of the volcanic aquifers and groundwater quality in Dangila Town and the surrounding area, Northwest Ethiopia” [1]. The presented dataset demonstrates the necessity of water quality assessments that would be helpful to water sectors, government, and policymakers for sustainable groundwater management.

Catalytic activation of ethylene C–H bonds on uniform d8 Ir(i) and Ni(ii) cations in zeolites: toward molecular level understanding of ethylene polymerization on heterogeneous catalysts

The long-debated intermediates of ethylene polymerization are revealed using uniform d⁸ metal ions in zeolites.

Achieving Atomic Dispersion of Highly Loaded Transition Metals in Small-Pore Zeolite SSZ-13: High-Capacity and High-Efficiency Low-Temperature CO and Passive NOx Adsorbers

The majority of harmful atmospheric CO and NO_x emissions are from vehicle exhausts. Although there has been success addressing NO_x emissions at temperatures above 250 °C with selective catalytic reduction technology, emissions during vehicle cold start (when the temperature is below 150 °C), are a major challenge. Herein, we show we can completely eliminate both CO and NO_x emissions simultaneously under realistic exhaust flow, using a highly loaded (2 wt %) atomically dispersed palladium in the extra-framework positions of the small-pore chabazite material as a CO and passive NO_x adsorber. Until now, atomically dispersed highly loaded (>0.3 wt %) transition-metal/SSZ-13 materials have not been known. We devised a general, simple, and scalable route to prepare such materials for Pt^II and Pd^II . Through spectroscopy and materials testing we show that both CO and NO_x can be simultaneously completely abated with 100 % efficiency by the formation of mixed carbonyl-nitrosyl palladium complex in chabazite micropore.

Improved thermal stability of a copper-containing ceria-based catalyst for low temperature CO oxidation under simulated diesel exhaust conditions

CuO supported on a commercial mixed cerium–zirconium oxide shows remarkable improvement in CO oxidation after high temperature hydrothermal aging.

The interaction of reactants, intermediates and products with Cu ions in Cu-SSZ-13 NH3 SCR catalysts: an energetic and ab initio X-ray absorption modeling study

By modeling the Cu K-edge XANES of Cu-SSZ-13 from first principles, we find that the intensity and edge position does not only depend on the oxidation state of Cu, but also its coordination environment.

Characterization of Fe2+ ions in Fe,H/SSZ-13 zeolites: FTIR spectroscopy of CO and NO probe molecules

FTIR spectra of adsorbed NO and CO were used to characterize Fe²⁺ ions in different cationic positions in Fe,H/SSZ-13 zeolites.

Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials

The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu-SSZ-13 (and the isostructural Cu-SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past ∼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.

Dissecting the steps of CO2 reduction: 2. The interaction of CO and CO2 with Pd/γ-Al2O3: an in situ FTIR study

Surface species formed on Pd/γ-Al₂O₃ upon CO_x exposure will aid mechanistic studies of CO₂ reduction.

15N2 formation and fast oxygen isotope exchange during pulsed 15N18O exposure of MnOx/CeO2

Molecular nitrogen formation and extensive isotope exchange between ¹⁵N¹⁸O and surface ¹⁶O on defect-rich MnO_x/CeO₂ was observed.