Low-temperature CO Oxidation Research Articles

Asymmetric coordinative modulation boosting the activity and thermal stability of Pt1/CeO2 for CO oxidation under harsh condition

Supported metal catalysts play a central role in chemical industry, but it remains a formidable challenge to realize high activity while maintaining atomically dispersed status to maximize atom efficiency under harsh conditions. Herein, a novel strategy of constructing asymmetric coordination Pt active sites via coating N-doped carbon onto ceria supported Pt single atoms (Pt1/CeO2@CN) is proposed, through which superior low-temperature CO oxidation activity and satisfactory thermal stability are achieved simultaneously. Experimental and density functional theory results confirm that the unique asymmetric N2-Pt1-O2 coordinative configuration on Pt1/CeO2@CN tailors the electronic properties of Pt 5d states, increasing the turnover frequency by 15 times for CO oxidation at 120 °C compared with that on Pt1/CeO2 and sustaining the atomically dispersed status of Pt at as high as 400 °C under H2-containing atmosphere. This strategy suggests a promising avenue to fabricating highly active and stable supported metal catalysts for practical applications under harsh conditions.

Exploring Pt-Cu synergistic effects on porous SiO2 support with different Pt loadings for low-temperature oxidation of CO and C3H6

To address the high cost of noble metals, this study aims to optimize Pt loading and the synergistic effect of Cu on porous SiO2 support for low-temperature oxidation of CO and C3H6. A simulated test bench system was used to evaluate the performance of PtxCu1/SiO2 (where x= 0.3, 0.5, 0.7, and 1 wt%), Cu1/SiO2 and Ptx/SiO2 (where x= 0.5 and 1 wt%) catalysts with varying Pt loadings. Comprehensive characterization, including BET, XRD, SEM, TEM, and XPS was conducted to assess particle size, structure, and surface morphology. The effects of Pt incorporation on reduction and oxidation properties were further examined using H2-TPR and CO-TPD techniques. Results demonstrated a significant enhancement in catalytic activity with the addition of Cu confirming a synergistic interaction between Pt and Cu. This synergy was particularly pronounced in lower Pt loadings, where PtxCu1/SiO2 composites outperformed Ptx/SiO2 in catalytic activity. Pt addition reduced the particle size, which resulted in the creation of PtCu alloy, with 1 and 0.7 % wt Pt loading exhibiting comparable effects in CO conversion and 0.7 % Pt loading exhibiting better activity at higher temperatures in C3H6 oxidation indicating that catalytic activity was maintained even with a reduced Pt loading. Additionally, minimal difference in activity for C3H6 oxidation was observed among PtxCu1/SiO2 catalysts with 0.7, 0.5, and 0.3 wt% Pt loadings as indicated by T50 and T80 values. Reducing Pt loading from 0.7 wt% had minimal impact on catalytic efficiency for C3H6 oxidation with catalysts maintaining high activity. 0.7 wt% Pt loadings provide sufficient active sites for effective CO and HC oxidation. The enhanced catalytic activity and stability of bimetallic catalysts over monometallic Cu/SiO2 and Pt/SiO2 result from the simultaneous presence of active sites created by the synergetic effect of PtCu alloy formation with optimal ratio and well-optimized surface. These findings demonstrate the potential to optimize Pt usage without compromising catalytic performance, offering a cost-effective approach to catalyst design.

Carbonate-mediated Mars-van Krevelen mechanism on CuxO/CeO2 catalysts for boosting CO oxidation

Copper-cerium-based catalysts have gained considerable attention for their excellent performance in low-temperature CO oxidation applications. In this context, the carbonate-mediated Mars-van Krevelen (M−vK) pathway stands out as a promising route for enhancing catalytic performance. Herein, we prepared three CuxO/CeO2 nanocatalysts with varying copper content using Cu2O nanocrystals (Cu2O NCs) and Ce-BTC metal–organic frameworks (MOFs) as dual precursors. The optimal catalyst exhibits excellent performance (T100 value of 110 °C) comparable to the precious metal catalyst. Through integrating in-depth experimental results and density functional theory (DFT) calculations, we reveal the intricate interplay of interfacial synergistic catalysis and carbonate-mediated M−vK mechanisms, confirming that carbonate species generated under mild conditions significantly promote CO oxidation on CuxO/CeO2 nanocatalysts. Based on the comparison of the transition state (TS) energy barrier between the traditional and the carbonate-mediated M−vK mechanism reaction paths, we found that carbonate species is easier to form and establish the rate-determining step (viz., carbonate → CO2, the energy barrier of 1.51 eV). This study sheds light on the importance of carbonate species in modulating the catalytic behaviour of copper-cerium catalysts and provides valuable insights for designing efficient catalysts for CO oxidation applications.

Homogeneous Catalysts of RedOx Processes Based on Heteropolyacid Solutions. VI: Developing a Process for Low-Temperature Oxidation of CO with Oxygen

Research on the development of a homogeneous process for low-temperature oxidation of carbon monoxide in the presence of a “platinum group metal + vanadium-containing heteropolyacid (HPA)” catalytic system has been presented. The optimal reaction conditions, ensuring the maximum rate of CO oxidation to CO2, have been determined; for this reaction the kinetic features have been established, and its mechanism has been proposed. It has been shown that homogeneous systems based on PdII complex exhibit high activity and productivity, but have low stability and operate only at pH below 1,5. The stability of the catalyst can be increased by simultaneous introduction of σ- and π-donor ligands into the system; however, it is more effective to use the PtIV complex in the presence of catalytic amounts of palladium salt at a ratio of 100/1. The transition from HPA solutions with a low content of vanadium atoms (H7PMo8V4O40) to solutions of modified compositions (H10P3Mo18V7O84) ensures an increase in the activity and productivity of the system, with the kinetics of CO oxidation being maintained. The combined homogeneous catalyst PtIV + PdII + H10P3Mo18V7O84 remains stable during multi-cycle use without reducing activity, operates without an induction period and can be used at pH range of 1,7–2,0, which simplifies the process equipment.

Operando XANES Reveals the Chemical State of Iron-Oxide Monolayers During Low-Temperature CO Oxidation.

We have used grazing incidence X-ray absorption near edge spectroscopy (XANES) to investigate the behavior of monolayer FeOxfilms on Pt(111) under near ambient pressure CO oxidation conditions with a total gas pressure of 1 bar. Spectra indicate reversible changes during oxidation and reduction by O2 and CO at 150ºC, attributed to a transformation between FeO bilayer and FeO2 trilayer phases. The trilayer phase is also reduced upon heating in CO+O2, consistent with a Mars-van-Krevelen type mechanism for CO oxidation. At higher temperatures, the monolayer film dewets the surface, resulting in a loss of the observed reducibility. A similar iron oxide film prepared on Au(111) shows little sign of reduction or oxidation under the same conditions. The results highlight the unique properties of monolayer FeO and the importance of the Pt support in this reaction. The study furthermore demonstrates the power of grazing-incidence XAFS for in situ studies of these model catalysts under realistic conditions.

Deciphering the structural origin for the remarkable CO oxidation activities of the CuO-based catalysts with diverse morphological CeO2 supports

In this work, octahedral, cubic, particle-like, spherical and rod-shaped nano-CeO2 supports were synthesized by hydrothermal method. CuO/CeO2 catalysts were then prepared via an ultrasonic-assisted impregnation method, employing these differently shaped CeO2 supports. The resulting catalysts were tested for their performance in CO oxidation. The findings revealed that the CuO active sites significantly influenced the oxygen storage and release capacity of CeO2, thereby improving its catalytic activity for low-temperature CO oxidation. The key factors affecting the CO oxidation performance of CuO/CeO2 catalysts with different CeO2 morphologies were investigated through various characterizations. It was found that the specific surface area was not the primary factor in determining catalytic activity. Instead, the exposed crystal planes played a crucial role. By adjusting the crystal plane exposure of CeO2, the supported 1CuO/CeO2 catalysts exhibited different crystal surface configurations. Notably, the 1CuO/CeO2-S catalyst, with exposed (100) and (110) crystal planes, and the 1CuO/CeO2-P catalyst, with exposed (111), (100), and (110) planes, exhibited higher surface defect concentrations. This increased defect concentration facilitated the formation of Cu+ species, enhancing the redox properties and further improving the overall catalytic performance of CO oxidation.

Enhancing Activity and Stability of Pd-on-TiO2 Single-Atom Catalyst for Low-Temperature CO Oxidation through in Situ Local Environment Tailoring.

The development of efficient Pd single-atom catalysts for CO oxidation, crucial for environmental protection and fundamental studies, has been hindered by their limited reactivity and thermal stability. Here, we report a thermally stable TiO2-supported Pd single-atom catalyst that exhibits enhanced intrinsic CO oxidation activity by tunning the local coordination of Pd atoms via H2 treatment. Our comprehensive characterization reveals that H2-treated Pd single atoms have reduced nearest Pd-O coordination and form short-distanced Pd-Ti coordination, effectively stabilizing Pd as isolated atoms even at high temperatures. During CO oxidation, partial replacement of the Pd-Ti coordination by O or CO occurs. This unique Pd local environment facilitates CO adsorption and promotes the activity of the surrounding oxygen species, leading to superior catalytic performance. Remarkably, the turnover frequency of the H2-treated Pd single-atom catalyst at 120 °C surpasses that of the O2-treated Pd single-atom catalyst and the most effective Pd/Pt single-atom catalysts by an order of magnitude. These findings open up new possibilities for the design of high-performance single-atom catalysts for crucial industrial and environmental applications.

In-situ formed stable Pt nanoclusters on ceria-zirconia solid solutions induced by hydrothermal aging for efficient low-temperature CO oxidation

Pt nanoclusters (PtC) on the surface of ceria-zirconia solid solutions exhibit superior low-temperature CO oxidation activity compared to Pt single atoms (Pt1). However, the redispersion behavior of the PtC to Pt1 lowers the efficiency of CO Oxidation. In this work, the stable surface hydroxyl groups were constructed by high-temperature hydrothermal aging treatment. Specifically, the Pt1 catalyst (Pt/CZOe-SA) prepared by atom trapping was subjected to high-temperature hydrothermal treatment to obtain the stable PtC catalyst (Pt/CZOe-HT) with in-situ formed PtC. The experimental results showed that PtC with 2–3 nm size could achieve excellent low-temperature activity and thermal stability. In addition, the ab initio molecular dynamics and density functional theory calculations revealed the specific evolution process of dynamic dispersion for PtC with or without hydroxyl groups from the femtosecond scale, suggesting that the hydroxyl groups limited the dispersion of PtC in the form of “energy fence”. The hydrothermal treatment not only introduced high-temperature stable hydroxyl groups to improve the stability of the in-situ formed PtC, but also optimized the ratio of Pt1 and PtC, and then enhanced the low-temperature activity by the synergistic effect between Pt1 and PtC. The present work can provide theoretical guidance for developing high-performance and high-stability Pt-based catalysts.

Unveiling the mechanistic synergy in Mn-doped NiO catalysts with atomic-burry structure: Enhanced CO oxidation via Ni-OH and Mn bifunctionality

The development of non-noble metal catalysts for low-temperature CO oxidation is a pivotal subject in various disciplines. In this study, an optimized Mn-doped NiO catalyst (Mn-NiO-0.5), prepared via aerosol pyrolysis with an equal molar ratio of Ni to Mn, demonstrates remarkable performance. It achieves an 82 % CO conversion at -20 °C and a 100 % conversion between 10 °C and 800 °C. The catalyst’s turnover frequency (TOF) for CO oxidation significantly surpasses that of previously reported noble-metal-based catalysts, underscoring its superior efficiency. Dynamic analyses suggest that the enhanced catalytic activity is a result of the unique atom burr structure, which presents an extensive surface area replete with abundant active sites. This structural feature is instrumental in maximizing the catalyst’s interaction with CO molecules. Experimental findings, corroborated by density functional theory (DFT) calculations, elucidate the mechanism of CO oxidation on the Mn-NiO-0.5 catalyst. The process is facilitated by the synergistic action of bimetallic sites, where the Ni–OH site acts as the primary adsorption point for CO. Subsequently, the CO is oxidized to bicarbonate through the interaction with the oxygen (O or O2) at adjacent Mn sites. This pioneering study introduces a paradigm-shifting insight: the effectiveness of non-noble metal oxide catalysts can now compete with, and in some cases, outperform their noble metal counterparts. This breakthrough is set to transform the industry, presenting a compelling alternative to conventional noble metal catalysts and propelling the evolution of state-of-the-art environmental conservation technologies.

Simultaneously activating molecular oxygen and surface lattice oxygen on Pt/TiO2 for low-temperature CO oxidation

Developing high-performance Pt-based catalysts with low Pt loading is crucial but challenging for CO oxidation at temperatures below 100 °C. Herein, we report a Pt-based catalyst with only a 0.15 wt% Pt loading, which consists of Pt–Ti intermetallic single-atom alloy (ISAA) and Pt nanoparticles (NP) co-supported on a defective TiO2 support, achieving a record high turnover frequency of 11.59 s–1 at 80 °C and complete conversion of CO at 120 °C. This is because the coexistence of Pt–Ti ISAA and Pt NP significantly alleviates the competitive adsorption of CO and O2, enhancing the activation of O2. Furthermore, Pt single atom sites are stabilized by Pt–Ti ISAA, resulting in distortion of the TiO2 lattice within Pt–Ti ISAA. This distortion activates the neighboring surface lattice oxygen, allowing for the simultaneous occurrence of the Mars-van Krevelen and Langmuir–Hinshelwood reaction paths at low temperatures.

Nature of CuxCe1-xO2 solid solution catalysts

Identifying the local environments of active sites of CuxCe1-xO2 solid solution for low-temperature CO oxidation remains significant challenges. In this study, we coupled density functional theory and microkinetic modeling to thoroughly investigate local structures of CuxCe1-xO2 solid solution catalysts, as well as its catalytic performance for low-temperature CO oxidation. Combined with the infrared spectra simulations, we propose that the substitution of Cu3 or Cu4 clusters for one Ce atom accounts for the experimentally observed vibrational peaks of the adsorbed CO, rather than the Cu single atom or dimer. Cu single-atom and Cun clusters (n = 2–4) doped into CeO2 all exhibit an excellent stability against ripening and a promising catalytic activity for low-temperature CO oxidation. This research for the first time elucidates the nature of CuxCe1-xO2 solid solution catalysts and paves the way for the rational design of Cu/CeO2 catalysts for CO oxidation.

Unveiling the Promoting Mechanism of H2 Activation on CuFeOx Catalyst for Low-Temperature CO Oxidation.

The effect of H2 activation on the performance of CuFeOx catalyst for low-temperature CO oxidation was investigated. The characterizations of XRD, XPS, H2-TPR, O2-TPD, and in situ DRIFTS were employed to establish the relationship between physicochemical property and catalytic activity. The results showed that the CuFeOx catalyst activated with H2 at 100 °C displayed higher performance, which achieved 99.6% CO conversion at 175 °C. In addition, the H2 activation promoted the generation of Fe2+ species, and more oxygen vacancy could be formation with higher concentration of Oα species, which improved the migration rate of oxygen species in the reaction process. Furthermore, the reducibility of the catalyst was enhanced significantly, which increased the low-temperature activity. Moreover, the in situ DRIFTS experiments revealed that the reaction pathway of CO oxidation followed MvK mechanism at low temperature (<175 °C), and both MvK and L-H mechanism was involved at high temperature. The Cu+-CO and carbonate species were the main reactive intermediates, and the H2 activation increased the concentration of Cu+ species and accelerated the decomposition carbonate species, thus improving the catalytic performance effectively.

Enhancing the Carbon Monoxide Oxidation Performance through Surface Defect Enrichment of Ceria-Based Supports for Platinum Catalyst.

Effective synthesis and application of single-atom catalysts on supports lacking enough defects remain a significant challenge in environmental catalysis. Herein, we present a universal defect-enrichment strategy to increase the surface defects of CeO2-based supports through H2 reduction pretreatment. The Pt catalysts supported by defective CeO2-based supports, including CeO2, CeZrOx, and CeO2/Al2O3 (CA), exhibit much higher Pt dispersion and CO oxidation activity upon reduction activation compared to their counterpart catalysts without defect enrichment. Specifically, Pt is present as embedded single atoms on the CA support with enriched surface defects (CA-HD) based on which the highly active catalyst showing embedded Pt clusters (PtC) with the bottom layer of Pt atoms substituting the Ce cations in the CeO2 surface lattice can be obtained through reduction activation. Embedded PtC can better facilitate CO adsorption and promote O2 activation at PtC-CeO2 interfaces, thereby contributing to the superior low-temperature CO oxidation activity of the Pt/CA-HD catalyst after activation.

Enhanced low-temperature CO oxidation activity through crystal facet engineering of Pd/CeO2 catalysts

Cerium dioxide (CeO2) supported palladium (Pd) catalysts have found widespread application in CO oxidation reaction owing to their exceptional catalytic performance. The morphology of the CeO2 support dictates the exposed crystal plane, exerting a profound influence on surface structure and redox properties. This is crucial as the atomic arrangement at the surface directly impacts catalytic activity. In this study, a range of CeO2 supports with diverse morphologies (e.g. nano-octahedra, nano-cubes, nano-particles, nano-spheres, and nano-rods) were successfully synthesized through hydrothermal methods and subsequently supported with Pd for CO oxidation. The spherical 1Pd/CeO2–S catalyst, revealing exposed (111) + (100) crystal planes, exhibited significantly higher CO catalytic oxidation activity compared to the catalysts with other morphological CeO2 supports. The results indicated a positive correlation between the content of PdxCe1-xO2-y species and catalytic activity. Notably, this species was most abundantly formed in the spherical 1Pd/CeO2–S catalyst with exposed (111) + (100) crystal planes. The presence of this active species facilitated the formation of surface defects, increased oxygen vacancy concentration, and enhanced metal-support interaction, consequently greatly improving the CO low-temperature catalytic oxidation activity. Consequently, the oxidation state of Pd in the Pd/CeO2 catalyst could be effectively regulated by controlling the exposure of (111) + (100) crystalline surfaces of the CeO2 support, further optimizing the catalytic performance of the Pd/CeO2 catalytic system.

Bridging Atom Engineering for Low-Temperature Oxygen Activation in a Robust Metal-Organic Framework.

Achieving active site engineering at the atomic level poses a significant challenge in the design and optimization of catalysts for energy-efficient catalytic processes, especially for a reaction with two reactants competitively absorbed on catalytic active sites. Herein, we show an example that tailoring the local environment of cobalt sites in a robust metal-organic framework through substituting the bridging atom from -Cl to -OH group leads to a highly active catalyst for oxygen activation in an oxidation reaction. Comprehensive characterizations reveal that this variation imparts drastic changes on the electronic structure of metal centers, the competitive reactant adsorption behavior, and the intermediate formation. As a result, exceptional low-temperature CO oxidation performance was achieved with T25(Temperature for 25 % conversion)=35 °C and T100 (Temperature for 100 % conversion)=150 °C, which stands out from existing MOF-based catalysts and even rivals many noble metal catalysts. This work provides a guidance for the rational design of catalysts for efficient oxygen activation for an oxidation reaction.

A single Mn atom supported by a boron-vacancy in a BN monolayer: An encouraging catalyst for CO oxidation

Single-atom catalysts (SACs) are known for their exceptional activity, primarily attributed to the abundance of low-coordinated metal atoms, which enable efficient activation of robust chemical bonds, including CO bonds. In this work, we explore the prospective utilization of a specific SAC, isolated Mn atoms supported on Boron nitrogen (Mn/BN), for CO oxidation. By analyzing the adsorption/co-adsorption energies of CO, O2, O2+CO, and 2CO, we create a set of screening criteria and conduct a comparative analysis of the chemical processes involved in CO oxidation. The Langmuir-Hinshelwood (LH), Eley-Rideal (ER), termolecular Langmuir-Hinshelwood (TLH) and termolecular Eley-Rideal (TER) mechanisms are investigated. Results indicate the ER and LH mechanisms feature pronouncedly lower barrier of 0.33 and 0.41 eV, respectively, for the rate-limiting step, than those of the TLH (0.72 eV) and TER (3.00 eV) mechanisms, suggesting that the ER and LH mechanisms are favorable at normal temperature. This study offers valuable insights that can inform future developments in the design of low-temperature CO oxidation processes utilizing SACs.

Synthesis of thin-film CuMn2O4 for low-temperature CO oxidation

Thin films, 0.5- and 1.0-nm thick, of CuMn2O4 were synthesized on γ-Al2O3 using Atomic Layer Deposition (ALD); and their catalytic activities for CO oxidation were measured and compared to that of bulk CuMn2O4. For bulk CuMn2O4, the surface area decreased dramatically when the sample was calcined at 1073 K; but the area-specific rates for CO oxidation were the same for bulk samples calcined at 473 K and 1073 K. For thin films of CuMn2O4 on γ-Al2O3, both the surface area and spinel structure were maintained following oxidation at 1073 K or reduction at 973 K. Area-specific, CO-oxidation rates on the ALD-prepared catalysts were somewhat lower than those found on bulk CuMn2O4; however, adding an additional ALD cycle of Cu on the CuMn2O4/γ-Al2O3 increased the rates to the same level as that of the bulk CuMn2O4.

Pinpointing the catalytic role of water in low-temperature CO oxidation on gold catalysts at a molecular level

Water substantially promotes the low-temperature redox reactions catalyzed by gold. However, clarifying the catalytic role of water is challenging due to co-existing OH and H2O species on Au catalysts under working conditions. Here, via controlled preparation of OH and adsorbed H2O species on a NiO/Au(111) inverse model surface combined with isotope-labelling reactants, we provide unambiguous spectroscopic experimental evidence to identify adsorbed H2O species, instead of OH groups, to promote O2 activation for low-temperature CO oxidation at the NiO-Au interface. In combination with theoretical calculations, adsorbed H2O-stabilized O2 species was found to be facilely activated into reactive OOH species but OH stabilized-O2 species could not. These results successfully discriminate the role of adsorbed H2O species and OH group in O2 activation in gold catalysis, which significantly advances our fundamental understanding of both Au catalysis and catalytic role of H2O.

SnO2-modified CuMnOx catalysts for humid CO oxidation at low temperature

Low-temperature CO oxidation has been widely applied in environmental pollution control, but challenges remain in CO purification in high-humidity environments. In this study, the CuMnSn8 catalyst exhibited excellent low-temperature CO oxidation performance, achieving complete CO oxidation at temperatures below 55 °C in 85 % relative humidity. A series of characterizations indicated that an appropriate amount of SnO2 doping could promote the formation of more amorphous oxides, increase the oxygen species content, and facilitate the dispersion of Cu and Mn elements. The catalytic active sites for CO oxidation and the influence of SnO2 doping on the CO oxidation mechanism were investigated through in-situ DRIFTS experiments. The results indicated that Sn enhanced the synergistic effect between Cu and Mn, altered the pathway of CO oxidation reaction, generated more easily decomposable bidentate carbonate intermediates. This work provides insight into the design of CO oxidation catalysts with high activity in high humidity.

Highly active copper-intercalated weakly crystallized δ-MnO2 for low-temperature oxidation of CO in dry and humid air

Highly active copper-intercalated weakly crystallized δ-MnO2 for low-temperature oxidation of CO in dry and humid air