Temperature-programmed Surface Reaction Research Articles

Comparative study of Co3O4-ZSM-5 catalysts synthesized by different hydrothermal methods for the catalytic oxidation of dichloromethane

In this work, various Co3O4-ZSM-5 catalysts were prepared by the microwave hydrothermal method (MH-Co3O4@ZSM-5), dynamic hydrothermal method (DH-Co3O4@ZSM-5), and conventional hydrothermal method (CH-Co3O4/ZSM-5). Their catalytic oxidation of dichloromethane (DCM) was analyzed. Detailed characterizations such as X-ray diffractometer (XRD), scanning microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), H2 temperature-programmed reduction (H2-TPR), temperature-programmed desorption of O2 (O2-TPD), temperature-programmed desorption of NH3 (NH3-TPD), diffuse reflectance infrared Fourier-transform spectra with NH3 molecules (NH3-DRIFT), and temperature-programmed surface reaction (TPSR) were performed. Results showed that with the assistance of microwave, MH-Co3O4@ZSM-5 formed a uniform core-shell structure, while the other two samples did not. MH-Co3O4@ZSM-5 possessed rich surface adsorbed oxygen species, higher ratio of Co3+/Co2+, strong acidity, high reducibility, and oxygen mobility among the three Co3O4-ZSM-5 catalysts, which was beneficial for the improvement of DCM oxidation. In the oxidation of dichloromethane, MH-Co3O4@ZSM-5 presented the best activity and mineralization, which was consistent with the characterizations results. Meanwhile, according to the TPSR test, HCl or Cl2 removal from the catalyst surface was also promoted in MH-Co3O4@ZSM-5 by their abundant Brønsted acid sites and the promotion of Deacon reaction by Co3O4 or the synergistic effect of Co3O4 and ZSM-5. According to the results of in situ DRIFT studies, a possible reaction pathway of DCM oxidation was proposed over the MH-Co3O4@ZSM-5 catalysts.

Self-aldol condensation of aldehydes over Lewis acidic rare-earth cations stabilized by zeolites

The self-aldol condensation of aldehydes was investigated with rare-earth cations stabilized by [Si]Beta zeolites in parallel with bulk rare-earth metal oxides. Good catalytic performance was achieved with all Lewis acidic rare-earth cations stabilized by zeolites and yttrium appeared to be the best metal choice. According to the results of several complementary techniques, i.e., temperature-programmed surface reactions, in situ diffuse reflectance infrared Fourier transform spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, the reaction pathway and mechanism of the aldehyde self-aldol condensation over Y/Beta catalyst were studied in more detail. Density functional theory calculations revealed that aldol dehydration was the rate-limiting step. The hydroxyl group at the open yttrium site played an important role in stabilizing the transition state of the aldol dimer reducing the energy barrier for its hydration. Lewis acidic Y(OSi)(OH)2 stabilized by zeolites in open configurations were identified as the preferred active sites for the self-aldol condensation of aldehydes.

Mesoporous Ce-Zr solid solutions supported Ni-based catalysts for low-temperature CO2 methanation by tuning the reaction intermediates

We facilely fabricated the mesoporous Ce-Zr solid solutions (Ce/Zr molar ratio = 0–100%) with excellent textural properties and employed them as the supports of the Ni-based catalysts for CO2 methanation. These supported catalysts were systematically measured by various techniques, such as X-ray diffraction (XRD), N2 physisorption, transmission electron microscope (TEM), selective area electron diffraction (SAED), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), CO2 temperature-programmed desorption (CO2-TPD), etc. In this catalytic system, the influencing factors of the supports on the promotion of the low-temperature catalytic performances toward CO2 methanation were carefully investigated. Besides, temperature-programmed surface reaction (TPSR) and in-situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) of the CO2 methanation were also carried out to investigate the reaction mechanism and the possible reaction intermediates. The kinetic study were also conducted to investigate the apparent activation energies of the CO2 methanation over these Ni-based catalysts with different supports. The influencing factors on the stabilization of the metallic Ni active sites were also investigated by conducting the 40 h stability test over the 15Ni/M-Ce80Zr20 and 15Ni/SiO2 catalysts. It was found that the Ni-based catalysts supported on the mesoporous Ce-Zr solid solutions were provided with advanced low-temperature activity owing to the redox property of the support, which could tune the reaction intermediates and decrease the apparent activation energy during the CO2 methanation.

Retraction Note to: Effect of the Support on the Mechanism of Partial Oxidation of Methane on Platinum Catalysts

The partial oxidation of methane was studied on Pt/Al2O3, Pt/ZrO2, Pt/CeO2 and Pt/Y2O3 catalysts. For Pt/Al2O3, Pt/ZrO2 and Pt/CeO2, temperature programmed surface reaction (TPSR) studies showed partial oxidation of methane comprehends two steps: combustion of methane followed by CO2, and steam reforming of unreacted methane, while for Pt/Y2O3 a direct mechanism was observed. Oxygen Storage Capacity (OSC) evaluated the reducibility and oxygen transfer capacity of the catalysts. Pt/CeO2 catalyst showed the highest stability on partial oxidation. The results were explained by the higher reducibility and oxygen storage/release capacity which allowed a continuous removal of carbonaceous deposits from the active sites, favoring the stability of the catalyst, For Pt/Al2O3 and Pt/ZrO2 catalysts the increase of carbon deposits around or near the metal particle inhibits the CO2 dissociation on CO2 reforming of methane. Pt/Y2O3 was active and stable for partial oxidation of methane, and its behavior was explained by a change in the reaction mechanism.

Constructing highly dispersed Ni based catalysts supported on fibrous silica nanosphere for low-temperature CO2 methanation

In this work, we reported that the fibrous silica nanosphere, also called as KCC-1, was facilely synthesized by the improved microemulsion hydrothermal fabrication strategy. The obtained KCC-1 was investigated as the support of the Ni based catalysts toward CO2 methanation. The obtained catalysts with unique dendrimeric mesopore structure and outstanding structural properties could provide easily accessible as well as highly dispersed Ni nanoparticles to the gaseous reactants. Besides, the Ni based catalysts with commercial SiO2 and MCM-41 as supports were also prepared as the reference catalysts to demonstrate the advantages of the dendrimeric mesoporous channels. Various techniques, such as X-ray powder diffraction (XRD), N2 physisorption, transmission electron microscopy (TEM), H2 temperature programmed reduction (H2-TPR), CO2 temperature programmed desorption (CO2-TPD), X-ray photoelectron spectroscopy (XPS), etc., were used to characterize the catalytic supports and corresponding catalysts. It was found that the metallic Ni nanoparticles could be highly dispersed over the catalysts supported on fibrous KCC-1 with dendrimeric mesoporous channels. Furthermore, the catalytic performances toward CO2 methanation had been further evaluated over these catalysts at different temperatures. It was found that the KCC-1 supported catalyst performed much higher low-temperature catalytic activities than the reference catalysts supported on commercial SiO2 and MCM-41, suggesting that the fibrous and dendrimeric mesoporous channels displayed distinctive advantages by accommodating enough metallic Ni active sites. The kinetic study also revealed that the topology of the mesoporous channel could obviously influence the apparent activation energy in the CO2 methanation reaction. Besides, the temperature programmed surface reaction (TPSR) and in-situ Diffused Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) of the CO2 methanation were also carried out to reveal the possible reaction pathway and mechanism. We also found that the fibrous silica nanosphere (KCC-1) supported catalyst demonstrated much better sintering-proof property of the metallic Ni nanoparticles than the reference catalysts during the 40 h stability tests at 400 °C. Therefore, the present fibrous silica nanosphere (KCC-1) could be considered as the promising support for Ni based catalysts toward CO2 methanation.

Recent advances in non-thermal plasma (NTP) catalysis towards C1 chemistry

C1 chemistry mainly involves the catalytic transformation of C1 molecules (i.e., CO, CO2, CH4 and CH3OH), which usually encounters thermodynamic and/or kinetic limitations. To address these limitations, non-thermal plasma (NTP) activated heterogeneous catalysis offers a number of advantages, such as relatively mild reaction conditions and energy efficiency, in comparison to the conventional thermal catalysis. This review presents the state-of-the-art for the application of NTP-catalysis towards C1 chemistry, including the CO2 hydrogenation, reforming of CH4 and CH3OH, and water-gas shift (WGS) reaction. In the hybrid NTP-catalyst system, the plasma-catalyst interactions are multifaceted. Accordingly, this review also includes a brief discussion on the fundamental research into the mechanisms of NTP activated catalytic C1 chemistry, such as the advanced characterisation methods (e.g., in situ diffuse reflectance infrared Fourier transform spectroscopy, DRIFTS), temperature-programmed plasma surface reaction (TPPSR), kinetic studies. Finally, prospects for the future research on the development of tailor-made catalysts for NTP-catalysis systems (which will enable the further understanding of its mechanism) and the translation of the hybrid technique to practical applications of catalytic C1 chemistry are discussed.

Hydrogen production by the steam reforming of ethanol over K-promoted Co/Al2O3–CaO xerogel catalysts

A series of K-promoted cobalt catalysts supported on Al2O3–CaO xerogel supports with different K loading (Co–xK/Al2O3–CaO, x = 0, 5, 10, and 15 wt.%, at fixed Co loading of 15 wt.%) were prepared to apply for ethanol steam reforming (ESR). K-promoted catalysts showed higher hydrogen yield and lower methane selectivity than did unpromoted catalyst. The promotion effect of K was investigated using various analyses, such as N2 adsorption–desorption, X-ray Diffraction (XRD), H2–Temperature-Programmed Reduction (H2–TPR), X-ray Photoelectron Spectroscopy (XPS), and Temperature-Programmed Surface Reaction (TPSR). Also, the catalysts were applied to methanation reaction, which is the main undesired reaction. It was found that lower methane selectivity during ESR over K-promoted catalysts was explained by the suppression effect of K on the methanation reaction. Also, TPSR study revealed that after K doping, starting temperature of the ESR shifted to lower temperature. However, excess amount of K doping blocked small pores of the alumina–calcium support, resulting in deterioration of the textural property of the catalysts. In addition, XPS analysis showed that as the amount of K doping increased, amount of surface concentration of cobalt decreased. Therefore, 10 wt.% K was found to be optimum loading for the ESR.

Formation and depression of N2O in selective reduction of NO by NH3 over Fe2O3-promoted V2O5-WO3/TiO2 catalysts: Roles of each constituent and strongly-adsorbed NH3 species

Roles of each composition and strongly-adsorbed NH3 in the formation and depression of N2O in selective catalytic reduction of NO by NH3 (NH3-SCR) with unpromoted and Fe2O3-promoted V2O5/TiO2-based catalysts were intensively studied. V2O5 was found to be active in N2O production at high temperatures while Fe2O3 exhibited a depressive effect. N2O reduction with NH3 and in situ N2O TPSR (temperature programmed surface reaction) measurements indicate that the Fe2O3-promoted catalysts could allow a successful reduction of N2O by strongly-adsorbed surface NH3 species. In situ diffuse reflectance infrared Fourier-transformation spectroscopy (DRIFTS) measurements disclose the presence of strongly-adsorbed NH3 species which were not desorbed even at 500 °C and these species could readily react with gaseous N2O at temperatures > 300 °C. X-ray photoelectron spectroscopy (XPS) results suggest a charge transfer from V to Fe, which may play a role in decreasing N2O formation at high temperatures. Based on these results, a reaction between N2O and strongly-adsorbed NH3 on Fe2O3 and/or Fe2O3-adjacent VOx species is proposed to be a main pathway for the depression of N2O formation in NH3-SCR reaction with the promoted catalysts.

Activation and surface reactions of CO and H2 on ZnO powders and nanoplates under CO hydrogenation reaction conditions

Abstract Activation and surface reactions of CO and H2 on ZnO powders and nanoplates under CO hydrogenation reaction conditions were (quasi) in situ studied using temperature programmed surface reaction spectra, diffuse reflectance Fourier transform infrared spectroscopy, inelastic neutron scattering spectroscopy and electron paramagnetic resonance. CO undergoes disproportion reaction to produce gaseous CO2 and surface carbon adatoms, and adsorbs to form surface formate species. H2 adsorption forms dominant irreversibly-adsorbed surface hydroxyl groups and interstitial H species and very minor surface Zn-H species. Surface formate species and hydroxyl groups react to produce CO2 and H2, while surface carbon adatoms are hydrogenated by surface Zn-H species sequentially to produce CH(a), CH2(a), CH3(a) and eventually gaseous CH4. The ZnO nanoplates, exposing a higher fraction of Zn-ZnO(0001) and O-ZnO(000–1) polar facets, are more active than the ZnO powders to catalyze CO hydrogenation to CH4. These results provide fundamental understanding of the reaction mechanisms and structural effects of CO hydrogenation reaction catalyzed by ZnO-based catalysts.

Different mechanisms of ethane aromatization over Mo/ZSM-5 and Ga/ZSM-5 catalysts

Aromatization of light hydrocarbons can contribute to a secure supply of aromatics for the chemical industry. In this work, we investigate the influence of modification of zeolite ZSM-5 with Ga and Mo on the reaction mechanism underlying the activation and aromatization of ethane. Well-defined Mo/ZSM-5 and Ga/ZSM-5 zeolites efficiently promote ethane aromatization to benzene-toluene-xylene mixtures. Both catalysts suffer from coke formation, which leads to rapid deactivation. From catalytic tests, temperature-programmed surface reaction and pulsed reaction experiments, we infer that ethane conversion on Ga/ZSM-5 follows a conventional sequential dehydrogenation-oligomerization-aromatization mechanism, while the reaction over Mo/ZSM-5 involves reactive surface carbon (hydrocarbon pool) species.

Surface structure–activity relationships of Cu/ZnGaOX catalysts in low temperature water–gas shift (WGS) reaction for production of hydrogen fuel

A new species’ class of Cu-, Ga- and Zn-based rate catalysts was prepared by a systematic co-precipitation technique at the different related pH values (6.5–8.0) along with calcination functional conditions, influencing components’ physical properties, these were characterized, and their application performance for water–gas shift (WGS) reaction was researched. Substances were analysed by various experimental methods, namely chemisorption, temperature-programmed reduction (TPR) characterisation, diffraction, physisorption and microscopy. A homogenous size dispersion of the compounds with smaller granular particles was obtained for catalysis, implemented with high pH-resulting outputs. H2 TPR profiles revealed a tailored stronger effect of Cu–Zn on Ga for process, operated with low pH-conditioned forms. Over Cu/ZnGaOX, WGS was sensitive to Cu, which was primarily active. Catalytic chemical reactivity, activity and selectivity were also found to be critically dependent on material lattice structure, copper surface area and metal–support interaction phenomena. The temperature-programmed surface reaction with mass spectrometry (TPSR–MS) measurements showed that formulations, synthesised at the pH of 8.0, enabled reaching >99% of the equilibrium yield CO conversion at 260 °C. An increase in the converted CO, oxidation and H2 productivity with the integral steam content in gaseous feed flow was achieved. The heterogeneous phase processing at the correlated pH of 7.6 demonstrated the highest formed CO product at the temperature of 200 °C, compared with literature. This is particularly promising for reagent purity hydrogen-fed fuel cells. The kinetics for each co-precipitated solid was evaluated regarding the efficiency for the WGS in a fixed bed reactor.

The role of oxygen vacancies in the CO2 methanation employing Ni/ZrO2 doped with Ca

The Ni/ZrO2 catalyst doped with Ca and Ni/ZrO2 were employed in the CO2 methanation, a reaction which will possibly be used for storing intermittent energy in the future. The catalysts were characterized by X-ray photoelectron spectroscopy (XPS, reduction in situ), X-ray diffraction (XRD, reduction in situ and Rietveld refinement), electron paramagnetic resonance (EPR), temperature-programmed surface reaction, cyclohexane dehydrogenation model reaction, temperature-programmed desorption of CO2 and chemical analysis. The catalytic behavior of these catalysts in the CO2 methanation was analyzed employing a conventional catalytic test. Adding Ca to Ni/ZrO2, the metallic surface area did not change whereas the CO2 consumption rate almost tripled. The XRD, XPS and EPR analyses showed that Ca+2 but also some Ni2+ are on the ZrO2 surface lattice of the Ni/CaZrO2 catalyst. These cations form pairs which are composed of oxygen vacancies and coordinatively unsaturated sites (cus). By increasing the number of these pairs, the CO2 methanation rate increases. Moreover, the number of active sites of the CO2 methanation rate limiting step (CO and/or formate species decomposition, rls) is enhanced as well, showing that the rls occurs on the vacancies-cus sites pairs.

Low Temperature Dehydration of Glycerol to Acrolein in Vapor Phase with Hydrogen as Dilution: From Catalyst Screening via TPSR to Real-Time Reaction in a Fixed-Bed

Temperature programmed surface reaction (TPSR) was developed as a method for rapid screening of catalysts. In this study, a series of acid catalysts was screened for the low-temperature dehydration of glycerol to acrolein via TPSR. Results suggested that most catalysts show activity of glycerol conversion to acrolein at a greatly different temperature range. HY, SiO2 supported H4SiW12O40 (STA/SiO2), SO42−/ZrO2, and SO42−/TiO2 were observed to be efficient for the conversion of glycerol into acrolein at 210 °C, which was significantly lower than that generally reported (250–340 °C). Moreover, high selectivity of acrolein was gained at 85% and 86% over SiW/SiO2 and SO42−/TiO2, respectively. A new style catalyst, ZnCl2/SiO2, was also found to be highly selective to acrolein and evaluated in a conventional fixed-bed reactor. Especially, stability tests showed that the catalyst life was up to 300 h with no clear deactivation on ZnCl2/SiO2 with hydrogen as dilution.

Influence of reduction temperature on Ni particle size and catalytic performance of Ni/Mg(Al)O catalyst for CO2 reforming of CH4

Hydrotalcite-derived Ni/Mg(Al)O is promising for CH4–CO2 reforming. However, the catalysts reported so far suffer from sever coking at low temperatures. In this work, we demonstrate that a significant improvement of coke-resistance of Ni/Mg(Al)O can be achieved by fine tuning the Ni particle size through adjusting the reduction condition of catalyst. Ni particles having average size within 4.0–7.1 nm are in situ generated by reducing the catalyst at a selected temperature within 923–1073 K. Controllability of Ni particle size is related to the formation of Mg(Ni,Al)O solid solution upon hydrotalcite decomposition. It is found that the catalyst reduced at 973 K exhibits high activity, stability, and coke-resistance even at reaction temperature as low as 773 K. In contrast, the catalyst reduced at 923 K has low activity and deactivates due to Ni oxidation, while those reduced at 1023 and 1073 K suffer from sintering and severe coking. STEM and O2-TPO reveal that coke deposition is directly proportional to the Ni particle size but becomes negligible at a size below 6.2 nm. It is evidenced that a critical size of about 6 nm is required to inhibit coking effectively. CO2 temperature-programmed surface reaction indicates that the deposited carbon on small Ni particles can be easily removed by the CO2 activated at the Ni–Mg(Al)O interfaces, accounting for the better resistance to coking.

Partial oxidation of methane over SiO2 supported Ni and NiCe catalysts

Abstract Nickel and nickel-ceria catalysts supported on high surface area silica, with 6 wt% Ni and 20 wt% CeO2 were prepared by microwave assisted (co) precipitation method. The catalysts were investigated by XRD, TPR and XPS analyses and they were tested in partial oxidation of methane (CPO). The catalytic reaction was carried out at atmospheric pressure in a temperature range of 400–800 °C with a feed gas mixture containing methane and oxygen in a molecular ratio CH4/O2 = 2. The Ni catalyst exhibited 60% methane conversion with 60% selectivity to CO already at 500 °C. On the contrary, the Ni–Ce catalyst was inert to CPO up to 700 °C. Moreover, the former catalyst reproduced its activity at the descending temperatures maintaining a good stability at 600 °C, over a reaction time of 80 h, whereas the latter one completely deactivated. Test of CH4 temperature programmed surface reaction (CH4-TPSR) revealed a higher methane activation temperature (> 100 °C) for the Ni–Ce catalyst as compared to the Ni one. Noticeable improvement of the ceria containing catalyst occurred when the reaction test started at a temperature higher than the methane decomposition temperature. In this case, the sample achieved the same catalytic behavior of the Ni catalyst. As confirmed by XPS analyses, the distinct electronic state of the supported nickel was responsible for the differences in catalytic behavior.

The effect of silanol groups on the metal-support interactions in silica-supported cobalt Fischer-Tropsch catalysts. A temperature programmed surface reaction

The effect of H bonded and isolated silanol groups on the metal support interactions of SiO2-supported Co catalysts in Fischer-Tropsch (FT) synthesis are reported. SiO2 supports were pre-treated in order to improve the concentration of isolated SiOH on the surface of the SiO2. Isolated SiOH groups act as an aid to achieving relatively good metal dispersions by creating metal anchoring sites, and enhances smaller metal crystallites deposition on the surface of the SiO2. SiO2 supports were pre-treated in two different ways: (a) pre-treatment by thermal method (HT); (b) pre-treatment by ethylene glycol (EG). The incipient wetness impregnation technique was adopted for the preparation of the catalysts, each catalyst loaded with approximately 10 wt% Co on each of the three supports designated as Co/SiO2 – UT, Co/SiO2 – HT, and Co/SiO2 – EG. Co/SiO2 – UT was used as a reference for this study. A number of characterization methods were applied to study the chemical and physical properties of the supports and the catalysts namely BET, XRD, ICP-OES, FTIR, H2 chemisorption, TEM, TPR, and TPSR. The results from H2 chemisorption, TEM and XRD indicated that the Co/SiO2 – EG showed relatively smaller metal crystallites and a better cobalt crystallite dispersion while TPR, CO TPD, and TPSR studies indicated that Co/SiO2 – EG catalyst exhibited stronger metal-support interaction, unlike the other two catalysts. Catalyst evaluation experiments showed that Co/SiO2-HT catalysts showed better FT activity while the Co/SiO2 – EG catalyst favored light olefins. FT catalyst evaluation results were in agreement with the TPSR studies.

Y-Modified MCM-22 Supported PdOx Nanocrystal Catalysts for Catalytic Oxidation of Toluene

A series of rare earth elements (REEs)-modified and Mobil Composition of Matter (MCM)-22-supported Pd nanocrystal catalysts were synthesized via a high-temperature solution-phase reduction method and tested for toluene complete oxidation. These catalytic materials were systematically characterized by N2 adsorption/desorption, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive spectroscopy (EDS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), temperature-programmed surface reaction of toluene (toluene-TPSR) and X-ray photoelectron spectroscopic (XPS) techniques in order to investigate the structure–catalytic property relationship. Moreover, catalysts with an appropriate yttrium content greatly improved the catalytic activity of 0.2%Pd/MCM-22. PdOx (x = 0, 1) nanoparticles, ranging from 3.6 to 6.8 nm, which were well distributed on the surface of MCM-22. Efficient electron transfer from the Pd2+/Pd0 redox cycle facilitated the catalytic oxidation process, and the formation of Pd (or Y) –O–Si bonds improved the high dispersion of the PdOx and Y2O3 particles. Toluene–TPSR experiments suggested that the addition of Y2O3 improved the physical/chemical adsorption of 0.2%Pd/MCM-22, thus increasing the toluene adsorption capacity. Then, 0.2%Pd/7.5%Y/MCM-22 exhibited the highest catalytic performance. In addition, this catalyst maintained 95% conversion with high resistance to water and chlorine poisoning, even after toluene oxidation at 210 °C for 100 h, making it more valuable in practical applications.

Probing the origin of the enhanced catalytic performance of sp3@sp2 nanocarbon supported Pd catalyst for CO oxidation

Tuning the fine structure of carbon support is crucial for modifying the metal-support interface (MSI) in order to harvest a high-performance catalysis. Herein, a core–shell sp3@sp2 nanocarbon (nanodiamond@graphene, ND@G) and a pure sp2 carbon derivative (onion-like carbon, OLC) were applied to support Pd nanoparticles. We found that Pd/ND@G displayed a superior catalytic activity for CO oxidation reaction with a TOF of 2.9 times higher than that of Pd/OLC at 46 °C. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and ambient pressure X-ray photoelectron spectroscopy (AP-XPS) revealed that, different with the Pd/OLC system, a unique interface microstructure was formed in Pd/ND@G, which not only provides a high exposure of active sites, but also enhances the Pd surface reactivity toward oxygen species, thus leading to a superior catalytic activity of Pd/ND@G. Moreover, the temperature-programmed surface reaction (TPSR) results showed that CO oxidation on Pd/ND@G undergoes an unusual termolecular Eley–Rideal (TER) mechanism, which has a lower energy barrier as compared to the traditional Langmuir-Hinshelwood (LH) and ER mechanism.

Catalytic combustion of methane over a highly active and stable NiO/CeO2 catalyst

In the last decades, many reports dealing with technology for the catalytic combustion of methane (CH4) have been published. Recently, attention has increasingly focused on the synthesis and catalytic activity of nickel oxides. In this paper, a NiO/CeO2 catalyst with high catalytic performance in methane combustion was synthesized via a facile impregnation method, and its catalytic activity, stability, and water-resistance during CH4 combustion were investigated. X-ray diffraction, low-temperature N2 adsorption, thermogravimetric analysis, Fourier transform infrared spectroscopy, hydrogen temperature programmed reduction, methane temperature programmed surface reaction, Raman spectroscopy, electron paramagnetic resonance, and transmission electron microscope characterization of the catalyst were conducted to determine the origin of its high catalytic activity and stability in detail. The incorporation of NiO was found to enhance the concentration of oxygen vacancies, as well as the activity and amount of surface oxygen. As a result, the mobility of bulk oxygen in CeO2 was increased. The presence of CeO2 prevented the aggregation of NiO, enhanced reduction by NiO, and provided more oxygen species for the combustion of CH4. The results of a kinetics study indicated that the reaction order was about 1.07 for CH4 and about 0.10 for O2 over the NiO/CeO2 catalyst.

Effect of calcination temperature on steam reforming activity of Ni-based pyrochlore catalysts

This work served as the second part of a study evaluating the effect of calcination temperature (700–1000 °C) on Ni-based lanthanum zirconate pyrochlore catalysts for methane steam reforming. A previous study (Haynes et al. Ceram. Int. 2017 (43) 16744) provided a thorough characterization of the material properties for the catalysts used here, and this study focuses on the evaluation of catalytic activity. The activity was assessed by two different experimental studies: the effect of reaction temperature using a temperature programmed surface reaction (TPSR), and the effect of reaction pressure. The results demonstrate a complex interaction between the Ni particles and surface LaOx species under the methane steam reforming conditions. Specifically, the material calcined at the lowest temperature (700 °C) possesses the highest activity and selectivity, which is attributed to smaller and more well-dispersed Ni particles on the surface, and, more importantly, a lesser degree La enrichment at the surface. All catalysts were deactivated by steam to NiO under all conditions tested, but at certain low reaction pressure (p = 0.23 MPa) conditions the materials calcined at 700–900 °C are able to completely recover equilibrium activity in-situ that is then robust and stable under both low and high reaction pressures (p = 1.8 MPa) suggesting the formation of a synergistic relationship between Ni and La for syngas production. However, exposure of a fresh material to high reaction pressures leads to a rapid and irreversible loss in both CH4 conversion and syngas selectivity whether in the fresh (no pretreatment), or pretreated (steam, H2 or Ar only at 800 °C) form for any catalyst. The mechanism for deactivation appears to be due to the presence of LaOx species that become mobile, possibly by the formation of La–OH, and covers the active Ni particles and inhibits sites responsible for the CH4 decomposition.