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•Methodology in Cu species quantification with electron paramagnetic resonance described.•Higher hydrothermal stability of Cu2+-2Z than Cu(OH)+-Z established both experimentally and ...computationally.•Rational design principles for Cu/SSZ-13 described and discussed.•Strategies for cold-start NOx removal described in terms of low-temperature active SCR catalysts and passive NOx adsorbers.
Cu/SSZ-13 SCR catalysts have been extensively studied in the past decade or so. Hydrothermal stability of these catalysts has been identified as the most important criterion for application. In this perspective, we describe recent atomic-level understanding of their hydrothermal stability. In particular, electron paramagnetic resonance (EPR) is shown to rather accurately quantify isolated Cu(II) ions and CuO clusters in fresh and aged catalysts to demonstrate the remarkable hydrothermal stability for Cu2+ ions located in 6-membered ring windows, and the conversion of Cu(OH)+ ions in Chabazite cages to CuO clusters. The hydrothermal stability difference of the two isolated Cu(II) ions is confirmed with DFT simulations and the conversion of Cu(OH)+ to CuO is proposed to involve formation, migration and condensation of Cu(OH)2 intermediates. The structural destructive role of CuO clusters is attributed to mesopore formation from their migration, which more severely damages the catalysts than dealumination. Finally, perspectives are given on new strategies for low-temperature NOx removal, rational design and refinement of Cu/SSZ-13, and development of new Cu/zeolite SCR catalysts with even better performance than the state-of-the-art Cu/SSZ-13.
The catalytic performance of a series of Ru/Al2O3 catalysts with Ru content in the 0.1–5% range was examined in the reduction of CO2 with H2. At low Ru loadings (≤0.5%) where the active metal phase ...is highly dispersed (mostly atomically) on the alumina support, CO is formed with high selectivity. With increasing metal loading, the selectivity toward CH4 formation increases, while that for CO production decreases. In the 0.1% Ru/Al2O3 catalyst, Ru is mostly present in atomic dispersion, as scanning transmission electron microscopy (STEM) images obtained from the fresh sample prior to catalytic testing reveal. STEM images recorded from this same sample, following the temperature programmed reaction test, clearly show the agglomeration of small metal particles (and atoms) into 3D clusters. The clustering of the highly dispersed metal phase is responsible for the observed dramatic selectivity change during elevated temperature tests: dramatic decrease in CO and large increase in CH4 selectivity. Apparent activation energies, estimated from the slopes of Arrhenius plots, of 82 and 62 kJ/mol for CO and CH4 formation were determined, respectively, regardless of Ru loading. These results suggest that the formation of CO and CH4 follow different reaction pathways or proceed on active centers of a different nature. Reactions with CO2/H2 and CO/H2 mixtures (under otherwise identical reaction conditions) reveal that the onset temperature of CO2 reduction is about 150 °C lower than of CO reduction.
The hydrogenation of CO2 was investigated over a wide range of reaction conditions, using two Pd/γ-Al2O3 catalysts with different Pd loadings (5% and 0.5%) and dispersions (∼11% and ∼100%, ...respectively). Turnover rates for CO and CH4 formation were both higher over 5% Pd/Al2O3 with a larger average Pd particle size than those over 0.5% Pd/Al2O3 with a smaller average particle size. The selectivity to methane (22–40%) on 5% Pd/Al2O3 was higher by a factor of 2–3 than that on 0.5% Pd/Al2O3. The drastically different rate expressions and apparent energies of activation for CO and CH4 formation led us to conclude that reverse water gas shift and CO2 methanation do not share the same rate-limiting step on Pd and that the two pathways are probably catalyzed at different surface sites. Measured reaction orders in CO2 and H2 pressures were similar over the two catalysts, suggesting that the reaction mechanism for each pathway does not change with particle size. In accordance, the DRIFTS results reveal that the prevalent surface species and their evolution patterns are comparable on the two catalysts during transient and steady-state experiments, switching feed gases among CO2, H2, and CO2 + H2. The DRIFTS and MS results also demonstrate that no direct dissociation of CO2 takes place over the two catalysts and that CO2 has to first react with surface hydroxyls on the oxide support. The thus-formed bicarbonates react with dissociatively adsorbed hydrogen on Pd particles to produce adsorbed formate species (bifunctional catalyst: CO2 activation on the oxide support and H2 dissociation on the metal particles). Formates near the Pd particles (most likely at the metal/oxide interface) can react rapidly with adsorbed H to produce CO, which then adsorbs on the metallic Pd particles. Two types of Pd sites are identified: one has a weak interaction with CO, which easily desorbs into gas phase at reaction temperatures, whereas the other interacts more strongly with CO, which is mainly in multibound forms and remains stable in He flow at high temperatures, but is reactive toward adsorbed H atoms on Pd leading eventually to CH4 formation. 5% Pd/Al2O3 contains a larger fraction of terrace sites favorable for forming these more multibound and stable CO species than 0.5% Pd/Al2O3. Consequently, we propose that the difference in the formation rate and selectivity to CH4 on different Pd particle sizes stems from the different concentrations of the reactive intermediate for the methanation pathway on the Pd surface.
Active centers in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been recently identified as isolated Cu2+ and CuII(OH)+ ions. A redox reaction mechanism has also been established, ...where Cu ions cycle between CuI and CuII oxidation states during SCR reaction. While the mechanism for the reduction half-cycle (CuII → CuI) is reasonably well-understood, that for the oxidation half-cycle (CuI → CuII) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temperature standard NH3-SCR, supplemented by DFT calculations, as strong evidence that the low-temperature oxidation half-cycle occurs with the participation of two isolated CuI ions via formation of a transient CuI(NH3)2+–O2–CuI(NH3)2+ intermediate. The feasibility of this reaction mechanism is confirmed from DFT calculations, and the simulated energy barrier and rate constants are consistent with experimental findings. Significantly, the low-temperature standard SCR mechanism proposed here provides full consistency with low-temperature SCR kinetics.
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•The mechanism of CO2 methanation is independent of Ru particle size.•H-assisted dissociation of CO∗ is the rate-limiting step in CO2 methanation.•Geminal CO∗ on low-coordinated Ru ...site is unreactive toward CO2 methanation.•Smaller Ru particles contain a higher concentration of inactive surface sites.
CO2 methanation was investigated on 5% and 0.5% Ru/Al2O3 catalysts (Ru dispersions: ∼18% and ∼40%, respectively) by steady-state kinetic measurements and transient DRIFTS–MS. Methanation rates were higher over 5% Ru/Al2O3 than over 0.5% Ru/Al2O3. The measured activation energies, however, were lower on 0.5% Ru/Al2O3 than on 5% Ru/Al2O3. Transient DRIFTS–MS results demonstrated that direct CO2 dissociation was negligible over Ru. CO2 has to first react with surface hydroxyls on Al2O3 to form bicarbonates, which, in turn, react with adsorbed H on Ru to produce adsorbed formate species. Formates, most likely at the metal/oxide interface, can react rapidly with adsorbed H forming adsorbed CO, only a portion of which is reactive toward adsorbed H, ultimately leading to CH4 formation. The unreactive CO molecules are in geminal form adsorbed on low-coordinated sites. The measured kinetics are fully consistent with a Langmuir–Hinshelwood type mechanism in which the H-assisted dissociation of the reactive CO∗ is the rate-determining step (RDS). The similar empirical rate expressions (rCH4=kPCO20.1PH20.3-0.5) and DRIFTS–MS results on the two catalysts under both transient and steady-state conditions suggest that the mechanism for CO2 methanation does not change with Ru particle size under the studied experimental conditions. Kinetic modeling results further indicate that the intrinsic activation barrier for the RDS is slightly lower on 0.5% Ru/Al2O3 than on 5% Ru/Al2O3. Due to the presence of unreactive adsorbed CO on low-coordinated Ru sites under reaction conditions, the larger fraction of such surface sites on 0.5% Ru/Al2O3 than on 5% Ru/Al2O3 is regarded as the main reason for the lower rates for CO2 methanation on 0.5% Ru/Al2O3.
Because of their heterogeneous nature, supported metal catalysts always contain metal centers in a rather broad dispersion range, and the presence of even atomically dispersed metals has been ...reported on oxide supports. The role of the atomically dispersed metal centers in the overall catalytic performances of these supported metal catalysts, however, has not been addressed to date. In this study, temperature programmed reaction and scanning transmission electron microscopy experiments were applied to show the fundamentally different reactivity patterns exhibited by Pd metal in atomically dispersed and traditional 3D clusters in the demanding reaction of CO2 reduction. The requirement for two different catalyst functionalities in the reduction of CO2 with hydrogen on Pd/Al2O3 and Pd/MWCNT catalysts was also substantiated. The results obtained clearly show that the oxide support material, even when it is considered inert like Al2O3, can function as a critical, active component of complex catalyst systems.
Abstract
Catalytic CO
2
conversion to energy carriers and intermediates is of utmost importance to energy and environmental goals. However, the lack of fundamental understanding of the reaction ...mechanism renders designing a selective catalyst inefficient. Here we show the correlation between the kinetics of product formation and those of surface species conversion during CO
2
reduction over Pd/Al
2
O
3
catalysts. The
operando
transmission FTIR/SSITKA (Fourier transform infrared spectroscopy/steady-state isotopic transient kinetic analysis) experiments demonstrates that the rate-determining step for CO formation is the conversion of adsorbed formate, whereas that for CH
4
formation is the hydrogenation of adsorbed carbonyl. The balance of the hydrogenation kinetics between adsorbed formates and carbonyls governs the selectivities to CH
4
and CO. We apply this knowledge to the catalyst design and achieve high selectivities to desired products.
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•Two structural polymorphs of δ- and θ-Al2O3 form during high temperature calcination of γ-Al2O3.•Both polymorphs accommodate a high degree of structural intergrowth and ...disorder.•Evolution of γ-Al2O3 to δ- or θ-Al2O3 can proceed through gradual ordering and poorly defined structural states.•Explicitly accounting for the disorder is necessary to reliably model the high temperature transition aluminas.•High temperature transition aluminas share many similarities with γ-Al2O3 in terms of their surface properties.
High temperature treated transition aluminas display adsorptive and catalytic properties that are in many ways comparable to those of their low temperature counterpart γ-Al2O3. While being important industrial catalysts as well as catalytic supports, their very basic crystallographic and structural characteristics remain actively studied. In this review, we critically examine the crystallography and structural complexity of these materials. Specifically, we review the crystallography of δ- and θ-Al2O3 polymorphs and show how structural intergrowth and disorder are accommodated in these phases. The structural complexity at the scale of overall microstructure is also examined, and the challenges and recent progress in quantification of the structure at ensemble level are discussed. Most pertinently to catalysis, we review the surfaces properties of high temperature treated Al2O3 and discuss the implications for understanding attributes relevant to heterogeneous catalysis.
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.
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