Activity For CO Oxidation Research Articles

Enhanced methane combustion and CO oxidation with Pd and Pt substitution LaSrCuO4 perovskites

The LaSrCuO4 as parent perovskite purely lattice was synthesized by new presented solid states technique and Pd and Pt substitution in LaSrCuO4 catalysts instead of Cu were prepared in this method at low temperature. All the perovskites were investigated by X-ray Diffraction, HRTEM, O2-TPD, H2-TPR, BET and XPS analyses. The CO oxidation and methane combustion catalytic activity of prepared perovskites were investigated which the LaSrCuO4 catalytic performance noticeably enhanced via Pd and Pt substitution instead of Cu. Both Langmuir and intra-facial mechanisms were intensified with increasing of surface area and mobility of lattice oxygen due to Pd and Pt doping in CO oxidation and methane combustion reactions. Pd 3d3/2 and 3d5/2 XPS spectra in LaSrCu0.9Pd0.1O4 confirmed Pd incorporation in perovskite structure due to presence of high oxidation state of Pd3+ or Pd4+. Pt 4 f5/2 and 4 f7/2 peaks approved Pt4+ oxidation state in LaSrCu1−xPtxO4 (x = 0.1,0.2) catalysts. The H2-TPR profiles of the Pt and Pd doped LSC perovskites revealed that the reduction peaks of the Cu were shifted to lower temperatures by increasing in Pd and Pt partial substitution due to H2-spillover effect of palladium and platinum.

Microwave-assisted CO oxidation over LaNiO3 and Ce-promoted LaNiO3

Microwave-assisted catalytic reactions have been investigated for energy-efficient chemical processes. However, catalysts with comparatively high heating properties, activities, and economics have been developed with limited success. In this study, we investigated the catalytic properties of Ce addition to LaNiO3 catalyst, which has high CO oxidation activity and high microwave heating properties. Ce-added catalysts exhibited high heating properties under microwave heating. Increasing the Ce content in the catalyst enhanced the dielectric loss factor, the ability to absorb microwaves and convert them to heat, and improved the microwave heating properties of the catalyst. The catalytic activity toward CO oxidation over Ce-added catalysts decreased in the order of La0.8Ce0.2NiO3 > La0.9Ce0.1NiO3 > LaNiO3 at all oxygen partial pressures when compared to the same microwave power. To clarify the role of Ce addition, temperature programmed reduction (TPR) by H2, CO, and XAFS measurements were performed. TPR results suggested that Ce promotes oxygen mobility and then affects both CO oxidation and microwave heating properties.

Catalytic behavior of Pt single-atoms supported on CeO2

In this study, we demonstrate that the CO oxidation activity of Pt/CeO2 single-atom catalysts (≤0.4 Pt/nm2) is significantly low despite the increase in reducibility, which is associated with the formation of oxygen vacancies that are critical for oxygen activation, with increasing Pt surface density. This result can be related to the negligible amount of CO adsorbed onto the Pt/CeO2 single-atom catalysts. As the Pt surface density increases to 0.8 Pt/nm2, the activity sharply increases; at this loading, Pt clusters are formed and the interactions with CO are enhanced. Notably, after the controlled reduction treatment using CO, the catalytic activity of 0.4 Pt/CeO2 increases to the level of 0.8 Pt/CeO2. The sudden increase in activity can be explained by the formation of a partially reduced Pt cluster and the enhanced CO interactions with the Pt atoms of the cluster. These results indicate that Pt cluster formation and its partial reduction are essential for low-temperature CO oxidation. Our study explains the cause of the significantly low CO oxidation activity of Pt/CeO2 single-atom catalysts and how controlled reduction treatment of these catalysts enhances their activity.

Self-Standing Pd-Based Nanostructures for Electrocatalytic CO Oxidation: Do Nanocatalyst Shape and Electrolyte pH Matter?

Tailoring the shape of Pd nanocrystals is one of the main ways to enhance catalytic activity; however, the effect of shapes and electrolyte pH on carbon monoxide oxidation (COOxid) is not highlighted enough. This article presents the controlled fabrication of Pd nanocrystals in different morphologies, including Pd nanosponge via the ice-cooling reduction of the Pd precursor using NaBH4 solution and Pd nanocube via ascorbic acid reduction at 25 °C. Both Pd nanosponge and Pd nanocube are self-standing and have a high surface area, uniform distribution, and clean surface. The electrocatalytic CO oxidation activity and durability of the Pd nanocube were significantly superior to those of Pd nanosponge and commercial Pd/C in only acidic (H2SO4) medium and the best among the three media, due to the multiple adsorption active sites, uniform distribution, and high surface area of the nanocube structure. However, Pd nanosponge had enhanced COOxid activity and stability in both alkaline (KOH) and neutral (NaHCO3) electrolytes than Pd nanocube and Pd/C, attributable to its low Pd-Pd interatomic distance and cleaner surface. The self-standing Pd nanosponge and Pd nanocube were more active than Pd/C in all electrolytes. Mainly, the COOxid current density of Pd nanocube in H2SO4 (5.92 mA/cm2) was nearly 3.6 times that in KOH (1.63 mA/cm2) and 10.3 times that in NaHCO3 (0.578 mA/cm2), owing to the greater charge mobility and better electrolyte-electrode interaction, as evidenced by electrochemical impedance spectroscopy (EIS) analysis. Notably, this study confirmed that acidic electrolytes and Pd nanocube are highly preferred for promoting COOxid and may open new avenues for precluding CO poisoning in alcohol-based fuel cells.

Exploring the key components of Au catalyst during CO oxidation using TG-MS and operando DRIFTS-MS

Nano-Au has a good catalytic activity for CO oxidation, but its catalytic mechanism is still not clear. Here, CO oxidation on Au2O3 and its thermal decomposition products were studied using thermogravimetric-mass spectrometry (TG-MS) and operando diffuse reflectance infrared Fourier transform spectroscopy-mass spectrometry (DRIFTS-MS) to explore the key active components of Au catalyst for CO oxidation. The TG-MS results showed that Au2O3 decomposed to AuO at 210 °C and Au at 375 °C and was oxidized to form Au oxide above 375 °C. When Au2O3 decomposed into AuxOy (x>y) above 210 °C, the CO oxidation activity obviously increased. Au atoms in the Au-Au system serve as electron donor and CO adsorption sites. Au atom is oxidized to AuO by the O atom from O2 decomposition following the Epling-Xu decomposition mechanism. A schematic diagram of the key components of Au2O3 catalysts during CO oxidation was proposed based on TG-MS and operando DRIFTS-MS analysis results.

Reverse oxygen spillover triggered by CO adsorption on Sn-doped Pt/TiO2 for low-temperature CO oxidation

The spillover of oxygen species is fundamentally important in redox reactions, but the spillover mechanism has been less understood compared to that of hydrogen spillover. Herein Sn is doped into TiO2 to activate low-temperature (<100 °C) reverse oxygen spillover in Pt/TiO2 catalyst, leading to CO oxidation activity much higher than that of most oxide-supported Pt catalysts. A combination of near-ambient-pressure X-ray photoelectron spectroscopy, in situ Raman/Infrared spectroscopies, and ab initio molecular dynamics simulations reveal that the reverse oxygen spillover is triggered by CO adsorption at Pt2+ sites, followed by bond cleavage of Ti-O-Sn moieties nearby and the appearance of Pt4+ species. The O in the catalytically indispensable Pt-O species is energetically more favourable to be originated from Ti-O-Sn. This work clearly depicts the interfacial chemistry of reverse oxygen spillover that is triggered by CO adsorption, and the understanding is helpful for the design of platinum/titania catalysts suitable for reactions of various reactants.

Mechanochemically activated Au/CeO2 for enhanced CO oxidation and COPrOx reaction

COPrOx reaction is one of the particularly appealing and cost-effective solutions for the selective oxidation of CO in hydrogen-rich gas streams to meet the stringent requirement of the current electrocatalysts employed in low-temperature fuel cells. Herein, we synthesized ultrasmall Au clusters supported on ceria by one-step solvent-free mechanochemical method. These catalysts exhibited higher activity for COPrOx and CO oxidation compared to the sample prepared by conventional incipient wetness impregnation. The unique Au-Ce interaction caused by impact and friction between the ball, vessel, and powders, greatly promotes the generation of positive charged Auδ+ active sites and, at the same time, the reducibility of the catalyst. Interestingly, an aging treatment of the ball-milled samples resulted in a significant superior performance of the catalytic activity. This enhancement has been attributed to a change in the oxidation state of Au between the fresh and the aged catalysts prepared by ball milling.

Understanding improved thermal stability of lanthanum-modified Cu/CeO2-ZrO2 for CO oxidation under lean-burn exhaust conditions

Rigorous emission regulations and the advancements in combustion engines have forced researchers to develop new strategies for removing CO at low temperatures. The Cu/CeO2-ZrO2 (Cu/CZ) catalyst has received an extensive attention due to its low cost and high CO oxidation activity at low temperatures. Nonetheless, the poor thermal stability of Cu/CZ remains as a major challenge for practical applications. In this study, we systematically investigated the effect of La doping on the hydrothermal stability of Cu/CZ. The La-doped catalyst, Cu/CZL, retained a high CO oxidation ability even after thermal aging at 800 °C for 20 h. La doping not only helped preserve the surface-active oxygen and oxygen storage capacity of Cu/CZ during thermal aging, but also enhanced its ability to efficiently convert intermediate surface carbonate species into CO2. These promoting effects of La resulted in the excellent CO oxidation activity over the aged-Cu/CZL under the simulated exhaust condition, significantly outperforming the conventional counterpart, PtPd/γ-Al2O3.

Ultra-Small Pt–Ni Bimetallic Nanoparticles Confined in Dendritic Mesoporous Silica with Remarkable CO Catalytic Oxidation Performance

A high-surface area (572 m2 g–1) dendritic mesoporous silica (KCC-1) was synthesized successfully and used as a support to confine Pt–Ni bimetallic nanoparticles (NPs). It is revealed that the Pt–Ni NPs are highly dispersed with an ultra-small size of 1.0 nm, and the large specific surface area as well as the abundant mesoporous structure of dendritic KCC-1 can effectively promote the dispersion and the thermal stability of Pt–Ni nanoparticles, thus significantly improving the CO oxidation activity. Compared with monometallic 1% Pt/KCC-1 and 1% Ni/KCC-1 catalysts, the combination of Pt and Ni together with a suitable ratio can further enhance the activity of the catalyst due to the synergetic effect between Pt and Ni. Over the 1% Pt7Ni3/KCC-1, the optimal catalyst in this study, 100% CO conversion was achieved at 100 °C. The incorporation of Ni into Pt leads to a reduction in the amount of noble metal required while simultaneously enhancing catalytic activity. This study offers a pathway for designing CO oxidation catalysts with low precious metal loading, yet exhibiting superior catalytic activity.

Submonolayer Cu/Pt film as bifunctional catalyst for CO oxidation: An STM study

Considering the high activity in CO oxidation as well as the related reactions, the surface structure of Cu/Pt bimetallic system and its interaction with CO and O2 molecules have been revisited with low-temperature scanning tunneling microscopy (STM) in combination with density-functional theory calculations (DFT). The extensively concerned Cu/Pt subsurface alloy (Cu-SSA) is clearly demonstrated with relative deactivation to CO and drastically weakened binding to O. In contrast, the submonolayer Cu film on Pt(111) manifests a significant enhancement in activating the O2 molecules while reserving the strong binding to CO at the uncovered Pt area. Detailed control experiments corroborated by DFT calculations revealed that the Cu/Pt interface serves as the critical sites for both O2 activation and the CO oxidation reaction. Therefore, the submonolayer Cu film may register a more promising catalyst structure than the Cu-SSA configuration for the Cu/Pt bimetallic system.

Effects of heat treatments of Pt-loaded Al2O3 on catalytic activities of CO oxidation and combustion-type CO sensors

Effects of heat treatments of Pt-loaded Al2O3 on catalytic activities of CO oxidation and combustion-type CO sensors

Tuning interaction strength between CeO2 and iridium to promote CO oxidation over Ir/TiO2

The interaction between support and noble metal plays a crucial role in heterogeneous catalysis design. However, how to tune metal support interactions to optimize the activity still needs further exploration. CeO2 was introduced to promote CO oxidation over Ir/TiO2 by adjusting the interaction strength between iridium (Ir) and CeO2. The strong interaction between Ir and CeO2 blocks CO adsorption and causes low CO oxidation activity. However, introducing CeO2 on Ir/TiO2 produces localized interaction between Ir and CeO2, which can tune the surface electronic state of Ir, so a “volcano curve” relationship between CO oxidation activity and electronic state is built. Limited amount of CeO2 on Ir/TiO2 (Ir/Ce0.2Ti) leads to CO complete oxidization at 22 °C, and a new pathway for CO oxidation was explored. The study demonstrates that the utilization of tuning interaction strength between active metal and support is a potential method to increase the catalytic activity.

A comparative study of CO oxidation on Cu-doped C3N monolayer with N and C vacancies

In this study, we employ density functional theory based on first-principles calculations to investigate the CO oxidation performance of a Cu-doped C3N monolayer with N and C vacancies, which is a promising low-temperature and high-activity catalyst. We find that Cu dopants are more stably anchored at both the N and C vacancies of the C3N monolayer. To comprehensively understand the effect of N and C vacancies on CO oxidation, we investigate both the Eley–Rideal and Langmuir–Hinshelwood (LH) mechanisms. Our results indicate that the LH mechanism exhibits much lower barriers for the rate-limiting step of CO oxidation, suggesting its excellent performance in this process. Moreover, the charge state of Cu atoms and the asymmetric atomic interface at the C vacancy lead to a lower rate-limiting barrier for CO oxidation compared to that at the N vacancy (0.5 vs 0.71 eV). This study represents a promising strategy to enhance the activity and selectivity of CO oxidation by controlling the asymmetric atomic interface of single-atom catalysts.

Breaking the activity-stability trade-off of Au catalysts by depth-controlled TiO2 nanotraps

The sintering of nanoparticles has always been a great challenge for supported Au catalysts. Herein, we fabricate a depth-controlled TiO2 nanotrap via area-selective atomic layer deposition (ALD) method to balance the activity and anti-sintering properties of Au nanoparticles. Au nanoparticles anchored by a 2.31 nm deep TiO2 nanotrap prepared by 25 ALD cycles of TiO2 exhibit the best overall activity for CO oxidation, where the depth of TiO2 nanotrap is close to the geometric radius of Au nanoparticle. It exhibits enhanced sintering resistance while retaining an activity similar to that of pure Au catalyst. The average size of sintered Au (8.4 ± 2.7 nm) is three times smaller than that of pure Au catalyst (21.1 ± 7.1 nm) after calcination in air at 700 °C. The performance of TiO2 nanotrapped Au catalyst breaks the conventional trade-off between the activity and anti-sintering capability of Au by exposing active sites, and hindering Au migration-coalescence simultaneously, which serves as a promising strategy to design highly stable Au catalysts.

Activation of partial metal sites in high-entropy oxides for enhancing thermal and electrochemical catalysis

High-entropy oxides (HEOs) have been tentatively and prospectively applied for chemical catalysis and energy storage. However, further enhancing their performance is difficult owing to the difficulty in precisely regulating the physical-chemical properties. In this work, a general in-situ modulation strategy of solid-phase combustion involving thiourea addition and alkali liquor treatment is developed to activate metal sites and lattice oxygen species of CuCoNiZnAl HEOs. Consequently, compared with pristine HEOs, the activated HEOs not only display higher CO2 hydrogenation and CO oxidation activities but also significantly enhanced electrocatalytic performance (discharge/charge capacities of 12049/9901 mAh/g) with excellent cycle stability (2500 h) for Li-O2 batteries. The superior performance of the activated HEOs is attributed to its facile electron transferability. This simple and effective strategy could be easily applied on a large scale, guiding the development of highly active heterogeneous HEO catalysts for various functional applications.

Studies on High-Temperature Evolution of Low-Loaded Pd Three-Way Catalysts Prepared by Laser Electrodispersion.

Pd/Al2O3 catalyst of the "crust" type with Pd loading of 0.03 wt.% was prepared by the deposition of 2 nm Pd particles on the outer surface of the alumina support using laser electrodispersion (LED). This technique differs from a standard laser ablation into a liquid in that the formation of monodisperse nanoparticles occurs in the laser torch plasma in a vacuum. As is found, the LED-prepared catalyst surpasses Pd-containing three-way catalysts, obtained by conventional chemical synthesis, in activity and stability in CO oxidation under prompt thermal aging conditions. Thus, the LED-prepared Pd/Al2O3 catalyst showed the best thermal stability up to 1000 °C. The present research is focused on the study of the high-temperature evolution of the Pd/Al2O3 catalyst in two reaction mixtures by a set of physicochemical methods (transmission electron microscopy, X-ray photoelectron spectroscopy, and diffuse reflectance UV-vis spectroscopy). In order to follow the dispersion of the Pd nanoparticles during the thermal aging procedure, the testing reaction of ethane hydrogenolysis was also applied. The possible reasons for the high stability of LED-prepared catalysts are suggested.

The role of polaronic states in the enhancement of CO oxidation by single-atom Pt/CeO2

Single Atom Catalysts (SACs) have shown that the miniaturization of the active site implies new phenomena like dynamic charge transfer between isolated metal atoms and the oxide. To obtain direct proof of this character is challenging, as many experimental techniques provide averaged properties or have limitations with poorly conductive materials, leaving kinetic measurements from catalytic testing as the only reliable reference. Here we present an integrated Density Functional Theory-Microkinetic model including ground and metastable states to address the reactivity of Pt1/CeO2 for CO oxidation. Our results agree with experimentally available kinetic data in the literature and show that CO oxidation activity of Pt1/CeO2 is tunable via the electronic properties of the support. Particularly, samples with higher n-doping via oxygen depletion should be better in CO oxidation, as they help maintain the active state Pt0 of the catalyst. This provides an alternative strategy for tuning the performance of low-temperature oxidations in single-atom catalysts via charge transfer control.

Hydrogen-controlled structural reconstruction of palladium-bismuth oxide cluster to single atom alloy for low-temperature CO oxidation

Palladium (Pd) has been widely regarded as a high-performance catalyst for various oxidative reactions, however, the actual structure of active site remains controversial due to structural evolution under operation conditions. Herein, we prepared a series of bismuth (Bi)-doped silica-supported Pd catalysts and found a hydrogen-controlled structural reconstruction mechanism of palladium-bismuth oxide cluster to single atom alloy to efficiently catalyze low-temperature CO oxidation. The formation of PdxBiyOz clusters with unique Pd−O−Bi coordination structure could enhance the sinter-resistance ability of Pd species. This structural evolution of active site is clearly uncovered by in-situ XAFS results, in which metallic Bi−Pd shell gradually generates as the increase of reduction temperature without any metallic Bi−Bi bond. More importantly, PdBi1 single atom alloy exhibits a good CO oxidation activity with a CO2 production rate of 413 μmolCO2·gPd−1·s−1 at 100 °C and excellent catalytic stability. Density function calculation (DFT) results indicate that there are geometric and electronic effects between Bi and Pd atoms, which favor total linear-CO adsorption, activate CO and O2 molecules, and reduce the barrier for the formation of OO-CO intermediates in PdBi1 single atom alloy.

Origin of Higher CO Oxidation Activity of Pt/Rutile than That of Pt/Anatase

Herein, we show that the weak interaction of CO with Pt/TiO2 under the CO oxidation condition is the origin of higher CO oxidation activity of Pt/rutile than that of Pt/anatase. The results of CO temperature-programmed desorption (TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicate that the onset temperatures of CO desorption on freshly prepared Pt/rutile and Pt/anatase are the same. However, the CO-TPD curves of Pt/rutile after the reaction test show that the desorption temperature of CO shifts to a lower temperature, while that for Pt/anatase does not change. The in situ pulse reaction using DRIFTS reveals that CO on Pt/rutile reacted with oxygen faster than CO on Pt/anatase. IR spectra with peak deconvolution of adsorbed CO on Pt/rutile exhibit that CO adsorbed on the terrace sites of Pt clusters on rutile (2089 cm–1) reacts readily with O2. These results indicate that the higher low-temperature activity of Pt/rutile is related to its weaker interaction with CO compared with Pt/anatase under the reaction conditions. Our findings deepen the fundamental understanding of metal–support interaction and CO oxidation on Pt/TiO2 catalysts.

Tailoring the Composition of BaxBO3 (B = Fe, Mn) Mixed Oxides as CO or Soot Oxidation Catalysts in Simulated GDI Engine Exhaust Conditions.

Mixed oxides with perovskite-type structure (ABO3) are promising catalysts for atmospheric pollution control due to their interesting and tunable physicochemical properties. In this work, two series of BaxMnO3 and BaxFeO3 (x = 1 and 0.7) catalysts were synthesized using the sol-gel method adapted to aqueous medium. The samples were characterized by μ-XRF, XRD, FT-IR, XPS, H2-TPR, and O2-TPD. The catalytic activity for CO and GDI soot oxidation was determined by temperature-programmed reaction experiments (CO-TPR and soot-TPR, respectively). The results reveal that a decrease in the Ba content improved the catalytic performance of both catalysts, as B0.7M-E is more active than BM-E for CO oxidation, and B0.7F-E presents higher activity than BF for soot conversion in simulated GDI engine exhaust conditions. Manganese-based perovskites (BM-E and B0.7M-E) achieve better catalytic performance than iron-based perovskite (BF) for CO oxidation reaction due to the higher generation of actives sites.