Ce Ox Catalysts Research Articles

Mn–CeOx catalyst for the simultaneous removal of NOx, C6H6, and C7H8 at low temperatures: Experimental and DFT study

Coal-fired power plants are known for emitting substantial amounts of nitrogen oxides (NOx) and volatile organic compounds (VOCs) during operation. In this study, the utilization of Mn–CeOx catalysts for the simultaneous removal of these pollutants was explored. The catalysts were synthesized through the co-precipitation technique, and their material structure and active sites were analyzed through XRD, TEM, NH3-TPD, XPS, and in-situ DRIFTs characterization methodologies. The DFT calculations indicated that VOCs and NOx were efficiently adsorbed and activated on the catalyst surface and then converted into less harmful substances (CO2, N2, and H2O). Additionally, the potential reaction pathways occurring on the catalyst surface were investigated. Furthermore, the effectiveness of the Mn–CeOx catalyst in simultaneously reducing NOx and VOCs under low-temperature conditions was assessed. The experimental results validated the exceptional performance of the Mn–CeOx catalyst in eliminating both NOx and VOCs. Furthermore, the underlying mechanisms governing the catalytic process were comprehensively investigated.

Doping Pd2+ into NinCeOx nanofibers promotes low-temperature CO2 methanation

CO2 methanation at low temperatures is still a challenge. Herein, NinCeOx (n = 1–3) and Ni2.5Pd0.1CeOx nanofibers by electrospinning is reported. The main advantage of this method is to obtain the highly dispersed precursor of palladium, nickel, and cerium, overcoming the difficulty of uniform mixing in conventional preparation methods. The Ni2.5Pd0.1CeOx nanofiber catalyst exhibited outstanding catalytic performance at low temperatures (CO2 conversion rate = 90.4 %, CH4 selectivity = 99.6 % at 230 °C) along with exceptional stability over 300 h. EPR, Raman, and O 1s XPS confirmed that Pd2+ doping increased oxygen vacancy concentration. In-situ infrared spectroscopy indicated that CO2 methanation on Ni2.5CeOx and Ni2.5Pd0.1CeOx catalysts followed the formate pathways. Pd2+ doping increased the number of surface oxygen vacancies and hydroxyl groups, thus increasing the amount of bicarbonates and formates. DFT calculations suggested that Pd2+ doping increased CO2 adsorption energy, and confirmed surface hydroxyl groups and bicarbonate being beneficial for CO2 methanation, consequently enhancing the activity of Ni2.5Pd0.1CeOx catalyst especially at low temperatures.

Improving low-temperature NH3-SCR denitration activity and resistance to H2O and SO2 over Nb-modified NbaSn0.3CeOx catalysts by enhancing acidity to compensate for its weak oxidation ability

In this paper, a series of Nb-modified NbaSn0.3CeOx (a = 0.2, 0.4, 0.5) catalysts for denitrification were prepared using a facile co-precipitation method. In order to explore the various individual properties of the samples, the catalysts were characterized by XRD, Raman, N2 adsorption and desorption, TEM, NH3-TPD, H2-TPR, XPS and in-situ DRIFTS tests to understand the influences of the Nb introduction on the NH3-SCR reaction. Among these catalysts, the Nb0.4Sn0.3CeOx catalyst exhibit excellent NH3-SCR performance in the temperature range of 155–450 °C with more than 90 % NO conversion, 100 % N2 selectivity, and possess good resistance to H2O and SO2. Compared with the Sn0.3CeOx catalyst, we believe that the improvement of the low-temperature NH3-SCR performance of the Nb0.4Sn0.3CeOx catalysts is mainly due to the change in the amount of acid, special for the increase in B acid content, which compensates for the effect of the weaken oxidizing ability on the low-temperature activity. Beyond that, the good H2O and SO2 resistance at low temperature after the introduction of Nb into Nb0.4Sn0.3CeOx catalysts was analyzed by using in-situ DRIFTS characterization in detail, and the results indicate that the incorporation of Nb can inhibit the oxidation of SO2 to form sulfate on the catalysts, and prevent the competing adsorption of H2O and NOx, but the adsorption of NH3 is not affected. This work proves that the acidity regulation can improve low-temperature SCR activity by the synergistic effect between oxidation capacity and acidity.

CuO/LaCeOx catalysts with enhanced metal–support interactions for CO2 methanolization

The activity of CO2 methanolization depends on the realization of metal–oxide interactions with different strengths. The CuO/La0.25CeOx catalyst (La content: 25%) demonstrated superior catalytic activity under the reaction conditions of 260 °C, with a methanol space–time yield (STY) of 0.281 gCH3OH·gcat−1h−1 and a methanol selectivity of 83.3%. This study elucidates the correlation of the metal–support interactions in CuO/LaCeOx catalysts and the promotion of catalytic activity, which mainly shows that the introduction of La leads to enhanced conversion of Ce4+ into Ce3+ in the catalyst, facilitates oxygen vacancy generation, and activates the formate reaction pathway of CO2 hydrogenation. Thus, this study has considerable significance for the progress of efficient and new catalysts for CO2 methanolization.

Atomically Incorporating Ni into Mesoporous CeO2 Matrix via Synchronous Spray-Pyrolysis as Efficient Noble-Metal-Free Catalyst for Low-Temperature CO Oxidation

Low-temperature catalytic CO oxidation is an important chemical process in versatile applications, such as the H2 utilization for low-temperature H2 air fuel cells. Pt-group metal catalysts are efficient but highly cost-consuming. This work demonstrates an excellent and sixpenny catalyst with earth-abundant Ni and Ce, in which Ni ions are atomically incorporated into the CeO2 matrix (Ni-Ce-Ox) by synchronous spray-pyrolysis (SSP) of mixture nitrates of Ni and Ce. The Ni-Ce-Ox catalyst presents a mesoporous structure. Revealed by a model reaction of 1% CO, 1% O2, and 98% balance He at a space velocity of 13,200 mL/gcat/h, Ni-Ce-Ox catalysts display a typical volcano-shaped relationship between reactivity and Ni incorporation amount. The optimized Ni incorporation appears with a high Ni/Ce atomic ratio of 0.25, endowing the T50 (temperature corresponding to a CO conversion of 50%), which is lower-shifted by 165 °C than that of pristine CeO2 (266 °C). The density functional theory (DFT) calculations further indicate that the much-reduced oxygen vacancy formation energy at Ni-Ce single-atom sites boosted the adsorption activation of the CO molecule and therefore promoted the CO oxidation process. Besides, the2 Ni-Ce-Ox from the SSP method presents better performance than the counterparts from immersion and hydrothermal methods. This work paves a way to access efficient noble-metal-free catalysts for low-temperature CO oxidation.

CeOx as Surface Passivation and Hole Transfer Catalyst Layer Boosting Solar Water Oxidation of ZnFe2O4 Nanorods Photoanode

AbstractSevere surface photocarrier recombination and poor electronic conductivity are the major factors behind the sluggish photoelectrochemical water oxidation kinetics of the ZnFe2O4 photoanode. Here, the CeOx catalyst overlayer has been coupled with the reduced ZnFe2O4 nanorods (NRs) to reduce the surface charge recombination on the photoanode significantly. The density functional theory (DFT) studies indicate that the oxygen vacancy defect‐rich CeOx catalyst constructs a favorable band alignment with ZnFe2O4 promoting rapid photocarrier separation and serves as a conducting photocarrier transfer pathway accelerating the hole transportation toward the electrode/electrolyte interface. The ZnFe2O4/CeOx nano‐heterostructure photoanode exhibits a current density of 0.64 mA cm−2 at 1.23 V versus RHE under AM 1.5 G illumination, which corresponds to >167% increase over that of the ZnFe2O4 NRs photoanode. The CeOx coupling reduces the onset potential cathodically by 180 mV over the ZnFe2O4 NRs photoanode. The ZnFe2O4/CeOx nano‐heterostructure photoanode also exhibits excellent charge transfer efficiency (≈64% at 1.23 V vs RHE) and photostability. The results indicate the superior catalytic performance of oxygen vacancy defect‐rich CeOx in the PEC process. This work demonstrates the multifunctional role of CeOx as a surface passivation overlayer, hole transfer layer, and efficient oxygen evolution reaction catalyst.

Promotion effect of Cr addition on the activity and SO2 tolerance of CeOx catalysts for the NH3-SCR at middle-low temperature

CeO2 as selective catalytic reduction (SCR) catalyst has attracted wide attention due to its abundant surface acidity and excellent oxygen storage/release capacity. However, its further implication was hindered owing to insufficient catalytic activity and SO2 resistance. Novel Cr doped CeOx catalysts were synthesized by solid phase method and used for NH3-SCR of NOx at middle-low temperature. The effect of Cr addition on the SCR performance and physicochemical properties of CeO2 catalyst was investigated by activity test and characterizations. Among the tested catalysts, Cr(0.4)-CeOx catalyst exhibited excellent NOx removal efficiency, with >90% NO conversion within 180–300 °C, 100% NO conversion at 220 °C and outstanding SO2 resistance at 240 °C under 100 or 600 ppm SO2. The characterizations results revealed that Cr modified CrCeOx catalyst could effectively increase BET surface area, pore volume, surface acidity and redox capacity. In situ DRIFTS indicated that SCR reactions over Cr(0.4)-CeOx catalyst followed Eley-Rideal mechanism and active –NH2 was the major intermediate species. After sulfuration, Cr(0.4)-CeOx still exhibited excellent SCR activity due to the increase of Cr6+, Ce3+ and Oα concentration and the formation of Cr–O–Ce bond, new Brønsted acid site and more crystallized Cr2O3.

CeO -supported monodispersed MoO3 clusters for high-efficiency electrochemical nitrogen reduction under ambient condition

Abstract Developing efficient and low-cost electrocatalysts is essential for the electroreduction of N2 to NH3. Here, highly monodispersed MoO3 clusters loaded on a coral-like CeOx compound with abundant oxygen vacancies are successfully prepared by an impregnation–reduction method. The MoO3 clusters with small sizes of 2.6 ± 0.5 nm are induced and anchored by the oxygen vacancies of CeOx, resulting in excellent nitrogen reduction reaction (NRR) performance. Additionally, the synergistic effects between MoO3 and CeOx lead to a further improvement of the electrochemical performance. The as-prepared MoO3–CeOx catalyst shows an NH3 yield rate of 32.2 μg h−1 mgcat−1 and a faradaic efficiency of 7.04% at −0.75 V (vs. reversible hydrogen electrode) in 0.01 M Dulbecco's Phosphate Buffered Saline. Moreover, it displays decent electrochemical stability over 30,000 s. Besides, the electrochemical NRR mechanism for MoO3–CeOx is investigated by in-situ Fourier transform infrared spectroscopy. N–H stretching, H–N–H bending, and N–N stretching are detected during the reaction, suggesting that an associative pathway is followed. This work provides an approach to designing and synthesizing potential electrocatalysts for NRR.

In Situ DRIFTS Investigation on CeOx Catalyst Supported by Fly-Ash-Made Porous Cordierite Ceramics for Low-Temperature NH3-SCR of NOX

A series of CeOx catalysts supported by commercial porous cordierite ceramics (CPCC) and synthesized porous cordierite ceramics (SPCC) from fly ash were prepared for selective catalytic reduction of NOx with ammonia (NH3-SCR). A greater than 90% NOx conversion rate was achieved by the SPCC supported catalyst at 250–300 °C when the concentration of loading precursor was 0.6 mol/L (denoted as 0.6Ce/SPCC), which is more advantageous than the CPCC supported ones. The EDS mapping results reveal the existence of evenly distributed impurities on the surface of SPCC, which hence might be able to provide more attachment sites for CeOx particles. Further measurements with temperature programmed reduction by hydrogen (H2-TPR) demonstrate more reducible species on the surface of 0.6Ce/SPCC, thus giving rise to better NH3-SCR performance at a low-temperature range. The X-ray photoelectron spectroscopy (XPS) analyses reveal that the Ce atom ratio is higher in 0.6Ce/SPCC, indicating that a higher concentration of catalytic active sites could be found on the surface of 0.6Ce/SPCC. The in situ diffused reflectance infrared fourier transform spectroscopy (DRIFTS) results indicate that the SCR reactions over 0.6Ce/SPCC follow both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms. Hence, the SPCC might be a promising candidate to provide support for NH3-SCR catalysts, which also provide a valuable approach to recycling the fly ash.

Complete Dehydrogenation of N2H4BH3 over Noble‐Metal‐Free Ni0.5Fe0.5–CeOx/MIL‐101 with High Activity and 100% H2 Selectivity

AbstractEfficient and selective dehydrogenation of hydrazine borane (HB), a novel hydrogen storage material with very high hydrogen content (HB, 15.4 wt%), is a key challenge for a fuel‐cell‐based hydrogen economy. However, even using the noble metal catalysts for HB decomposition, the activities are still far from satisfying, to say nothing of non‐noble‐metal‐containing catalysts. In response, as a proof‐of‐concept experiment, herein, noble‐metal‐free NiFe–CeOx nanoparticles are successfully immobilized on an MIL‐101 support without surfactant by a simple liquid impregnation method. Unexpectedly, the resultant Ni0.5Fe0.5–CeOx/MIL‐101 catalyst shows good performance, including 100% H2 selectivity, 100% conversion, and record catalytic activity (351.3 h−1) for hydrogen generation at mild temperature, which is even better than most of the noble metal heterogeneous catalysts and might be attributed to the good dispersion and uniform particle size of the Ni0.5Fe0.5–CeOx nanoparticles due to steric restrictions effect of the MIL‐101 support. Additionally, extending MIL‐101 to some other important kinds of metal–organic framework (MOF) structures, the resultant NiFe–CeOx/MOF catalysts all show good catalytic activity toward HB decomposition, showing the universality of the MOF supported NiFe–CeOx catalysts.

Low-Temperature Selective Catalytic Reduction of NO with NH3 over Fe–Ce–O x Catalysts

In this study, we used a simple impregnation method to prepare Fe–Ce–Ox catalysts and tested them regarding their low-temperature (200–300 °C) selective catalytic reduction (SCR) of NO using NH3. We investigated the effects of Fe/Ce molar ratio, the gas hourly space velocity (GHSV), the stability and SO2/H2O resistance of the catalysts. The results showed that the FeCe(1:6)Ox (Ce/Fe molar ratio is 1:6) catalyst, which has some ordered parallel channels, exhibited good SCR performance. The FeCe(1:6)Ox catalyst had the highest NO conversion with an activity of 94–99% at temperatures between 200 and 300 °C at a space velocity of 28,800 h−1. The NO conversion for the FeCe(1:6)Ox catalyst also reached 80–98% between 200 and 300 °C at a space velocity of 204,000 h−1. In addition, the FeCe(1:6)Ox catalyst demonstrated good stability in a 10-h SCR reaction at 200–300 °C. Even in the presence of SO2 and H2O, the FeCe(1:6)Ox catalyst exhibited good SCR performance.

Enhancement in oxidative property on amorphous rare earth doped Mn catalysts

Rare earth metal (Ce, La, or Pr) doped Mn-based catalysts were prepared to obtain amorphous Mn–Ce–Ox, Mn–La–Ox and Mn–Pr–Ox. Promotional effects of NO conversion at low temperature were observed after rare earth metal doping. The Mn–Ce–Ox catalyst had the best oxidation performance and the maximum NO oxidation conversion was 94.0% at the reaction temperature of 239°C. Among the reported Mn-based catalysts for NO oxidation, the Mn–Ce–Ox catalyst showed superior low-temperature activity. The SEM, XRD, BET and XPS analyses further confirmed that the amorphous structure of the catalyst contributed a lot to the enhancement of activity.

Hard-template synthesis of three-dimensional mesoporous Cu–Ce based catalysts with tunable architectures and their application in the CO catalytic oxidation

In this paper, 3D Cu–Ce-Ox catalysts with different pores were synthesized by controlling the calcination temperaturre. The CeCu20-600 showed the highest catalytic activity, where CO can be fully oxidized at 55 °C and expressed good stability.

Synthesis of mixed Mn–Ce–Ox one dimensional nanostructures and their catalytic activity for CO oxidation

One dimensional Mn–Ce–Ox composite nanomaterials with various Mn contents were synthesized successfully by a hydrothermal method. The impacts of Mn contents on their morphologies and physical properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The analysis reveals that the obtained composites are one-dimensional (1D) rod-like nanostructures. The catalytic behaviors of Mn–Ce–Ox nanomaterials were investigated for CO oxidation. The Mn–Ce–Ox catalysts were found to be more active than catalysts based on ceria alone. The catalytic activities of all one dimensional Mn–Ce–Ox materials for CO conversion are quite high (142<T50%<229°C). Among the obtained nanocatalysts, the Mn–Ce–Ox composite nanomaterials with 40wt% Mn exhibit the best catalytic activity and the 50% CO conversion can be reached at 142°C. The presence of transition metal element Mn can promote the production of oxygen vacancies and improve oxygen mobility, which result in enhancing the oxygen-storage capacity of the one dimensional Mn–Ce–Ox materials and its catalytic performance for CO conversion.

Catalysis in Energy Generation and Conversion: How Insight Into Nanostructure, Composition, and Electronic Structure Leads to Better Catalysts (Perspective)

Catalysts are essential for the generation of energy carriers like hydrocarbon fuels, hydrogen, and electrical current. The performance of catalysts can be related to their nanostructure (i.e., size and shape) and composition. To rationally design catalysts by tuning these properties, they should be measured in a meaningful way using surface-sensitive spectroscopic tools under reaction conditions. In this perspective, we provide case histories of recently published research aimed at understanding these properties using a spectroscopic strategy under reaction conditions. We limit this perspective to studies whose main focus was to understand how the nanostructure and composition impact the active phase and/or efficiency of catalysts for the generation and conversion of energy carriers. We discuss studies of a Pd/Ga2O3 catalyst for the generation of hydrogen fuel from methanol and water, a PtMo catalyst for the generation of hydrogen fuel from biomass and water, Pt/Rh catalysts for the conversion of hydrogen into electrical current, a CeOx catalyst for the conversion of hydrogen into electrical current, and Fe and Co/CoPt catalysts for the generation of hydrocarbon fuel from carbon monoxide and hydrogen. Each study emphasizes how the use of spectroscopic tools under reactive conditions is beneficial for making rational decisions for improving catalysts. The studies demonstrate how different synthesis methods dictate the nanostructure and distribution of alloy components in the catalyst, certain pretreatment conditions create the active surface phase, while reactions and post-treatments can destroy it, and the nanostructure and composition change the electronic structure and alter the selectivity and activity.

Catalytic oxidation of NO over Mn–Co–Ce–Ox catalysts: effect of reaction conditions

Manganese–cobalt–cerium oxide (Mn–Co–Ce–Ox) catalysts were synthesized by the co-precipitation method and tested for activity in low-temperature catalytic oxidation of NO in the presence of excess O2. With the best Mn–Co–Ce mixed-oxide catalyst, approximately 80 % NO conversion was achieved at 150 °C and a space velocity of 35,000 h−1. The effect of reaction conditions (reaction temperature, volume fractions of NO and O2, gas hourly space velocity (GHSV), and catalyst stability) was investigated. The optimum reaction temperature was 150 °C. Increasing the O2 content above 3 % results in almost no improvement of NO oxidation. This catalyst enables highly effective removal of NO within a wide range of GHSV. Furthermore, the stability of the Me–Co–Ce–Ox catalyst was excellent; no noticeable decrease of NO conversion was observed in 40 h.

Study on factors controlling catalytic activity for low-temperature water–gas-shift reaction on Cu-based catalysts

Influence of metal oxide added to Cu catalysts on the catalytic activity of the low-temperature water-gas-shift (LT-WGS) reaction was investigated. Catalytic activity was linearly correlated with both surface area of metallic copper (Cu0) and surface Cu content in the identical copper-metal oxide system. However, activity per Cu0 surface area changed depending on the metal oxide added. CO2-TPD profile demonstrated that the amount of strong basic sites on Cu–ZnOx and Cu–AlOx catalysts with high activity LT-WGS reactions was much less than those on Cu–FeOx and Cu–CeOx catalysts with low activity. Activity per Cu0 surface area increased with increasing amount of weak basic sites on Cu–AlOx and Cu–ZnOx catalysts. From these results, it is suggested that in addition to Cu0 surface area, basicity of a catalyst is one of the factors controlling the catalytic activity of Cu-based catalysts for LT-WGS reaction.

Low-temperature catalytic oxidation of NO over Mn–Co–Ce–Ox catalyst

A series of manganese–cobalt–cerium oxide (Mn–Co–Ce–Ox) catalysts were prepared by co-precipitation method and tested for low temperature (<200°C) catalytic oxidation of NO in the presence of O2. The experimental results showed that the best catalyst yielded about 80% NO conversion at 150°C with a space velocity of 35,000h−1. As the cerium content was increased from 0% to 20% (mass%), NO conversion increased significantly, while decreased at higher cerium contents (30–40%). As comparison, single oxides were also investigated, and their activities were slower than those of Mn–Co–Ce–Ox catalyst. The effect of O2 (0–11%) was evaluated, and it is almost constant when the concentration of O2 was above 3%. The activity was suppressed with the coexisting of H2O and SO2 due to the generation of nitrates and sulfates, and the activity was restored to 45% after turning them off. The excellent low-temperature catalytic activity is mainly due to the amorphous phase, pore structure and high specific areas.

Water–gas shift reaction over Pt and Pt–CeOx supported on CexZr1−xO2

The water–gas shift (WGS) reaction was examined over Pt and Pt–CeOx catalysts supported on CexZr1−xO2 (Ce0.05Zr0.95O2, Ce0.2Zr0.8O2, Ce0.4Zr0.6O2, Ce0.6Zr0.4O2, Ce0.7Zr0.3O2 and Ce0.8Zr0.2O2) under severe reaction conditions, viz. 6.7 mol% CO, 6.7 mol% CO2, and 33.2 mol% H2O in H2. The catalysts were characterized with several techniques, including X-ray diffraction (XRD), CO chemisorption, temperature-programmed reduction (TPR) with H2, temperature-programmed oxidation (TPO), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and bright-field transmission electron microscopy (TEM). Among the supported Pt catalysts tested, Pt/Ce0.4Zr0.6O2 showed the highest WGS activity in all temperature ranges. An improvement in the WGS activity was observed when CeOx was added with Pt on CexZr1−xO2 supports (x = 0.05 and 0.2) due to intimate contact between Pt and CeOx species. Based on CO chemisorptions and TPR profiles, it has been found that the interaction between Pt species and surface ceria-zirconia species is beneficial to the WGS reaction. A gradual decrease in the catalytic activity with time-on-stream was found over Pt and Pt–CeOx catalysts supported on CexZr1−xO2, which can be explained by a decrease in the Pt dispersion. The participation of surface carbonate species on deactivation appeared to be minor because no improvement in the catalytic activity was found after the regeneration step where the aged catalyst was calcined in 10 mol% O2 in He at 773 K and subsequently reduced in H2 at 673 K.

Mechanism of Catalytic Oxidation of NO over Mn–Co–Ce–Ox Catalysts with the Aid of Nonthermal Plasma at Low Temperature

NO catalytic oxidation performances have been investigated on a Mn–Co–Ce–Ox catalyst by using nonthermal plasma (NTP) reactor: the “PRFC” reactor. In the PRFC system, the plasma reactor was followe...