Mixed Oxide Support Research Articles

A Comparative Study of Ni-Based Catalysts Prepared by Various Sol-Gel Routes.

The use of heterogeneous catalysts to increase the development of green chemistry is a rapidly growing area of research to save industry money. In this paper, mesoporous SiO2-Al2O3 mixed oxide supports with various Si/Al ratios were prepared using two different sol-gel routes: hydrolytic sol-gel (HSG) and non-hydrolytic sol-gel (NHSG). The HSG route was investigated in both acidic and basic media, while the NHSG was explored in the presence of ethanol and diisopropyl ether as oxygen donors. The resulting SiO2-Al2O3 mixed oxide supports were characterized using EDX, N2 physisorption, powder XRD, 29Si, 27Al MAS-NMR and NH3-TPD. The mesoporous SiO2-Al2O3 supports prepared by NHSG seemed to be more regularly distributed and also more acidic. Consequently, a simple one-step NHSG (ether and alcohol routes) was selected to prepare mesoporous and acidic SiO2-Al2O3-NiO mixed oxide catalysts, which were then evaluated in ethylene oligomerization. The samples prepared by the NHSG ether route showed better activity than those prepared by the NHSG alcohol route in the oligomerization of ethylene at 150 °C.

Investigation on the hydrogen production by methanol aqueous phase reforming over Pt/CexMg1-xO2 catalyst: Synergistic effect of support basicity and oxygen vacancies

Green hydrogen production from biomass oxygenated derivatives (methanol, ethanol, ethylene glycol, etc.) by low temperature aqueous phase reforming (APR) has the advantages of high hydrogen selectivity and low energy consumption. A series of CexMg1-xO2 mixed oxide supported catalysts were synthesized by a simple citric acid combustion method. The results show that these catalysts exhibit excellent water-gas shift reaction activity and CO is rarely detected in the products, which is due to their abundant oxygen vacancies of the mixed oxide supports of the Pt/CexMg1-xO2 catalysts. At the same time, the introduction of MgO provides more strong basic sites on the surface of mixed oxide support, and the methanol conversion and hydrogen yield show a volcano peak with the increasing number of basic sites and the hydrogen production reaches the highest (127.16 mmol) over Pt/Ce0·5Mg0·5O2 catalyst. The mixed oxide supported Pt/Ce0·5Mg0·5O2 catalyst enjoys the benefits of both oxygen vacancies in accelerating H2O adsorption/dissociation and strong basic sites in contributing to the formation of formate group (HCOO*), which improve the oxidation of CO* and APR activity. This synergistic effect is conducive to improve hydrogen yield of methanol APR and reduce CO selectivity to a minimum level.

Study on the effect of promotors in CO2 utilization for syngas production via dry reforming of methane over Co-MOX/TiO2-Al2O3 (MOX = La, Ce, Mg, and K) catalysts

In this study, Co-based catalysts supported over Ti-Al oxide and promoted with La, Ce, Mg, and K metals were assessed for CO2 reforming of methane reaction to produce syngas. Titania-alumina mixed oxide supports were prepared using the template-assisted-solvothermal method, and then Co and promotors were co-impregnated over the as-prepared support. Different characterizations of catalysts showed that variation in promotor metal impacts these catalysts' physical and chemical properties. The Ti-Al oxide support possessed the perfect hexagonal morphology. Potassium-promoted catalysts possessed the highest number of basic sites, whereas the La-promoted catalyst possessed the highest number of acidic sites. La promotion improved the Co dispersion, while Mg promotion enhanced the metal support integration. La-promoted catalysts are deactivated because of active metal oxidation and the generation of hard carbon. The carbon was deposited in all catalysts; however, the activity of the Mg-promoted catalyst was unaffected. The intermediate surface basicity and strong metal support interaction improved the Mg-promoted catalyst's stability. The La and Mg-promoted catalysts possessed lower apparent activation energies.

Design of Ru-Co/MgO-Al2O3 catalyst system for CO2 reforming of methane: Performance investigation concerning the Mg/Al ratio

This study explores the design of bimetallic Co-Ru nanoclusters deposited on surface-tuned mesoporous Mg-Al mixed oxide support for the CO2 reforming of methane (DRM) process. The evaporation-induced self-assembly technique was used to create a series of MgO-Al2O3 mixed oxide supports with various Mg/Al molar ratios. Co and Ru nanoclusters over the MgO-Al2O3 supports were decorated using a urea precipitation-deposition approach. Results revealed that the activity and stability of the DRM catalyst were substantially reliant on the Mg/Al molar ratio and Co-Ru metal combination. Among the series of Co-loaded catalysts, the best performance was obtained over an unpromoted CoMg20Al catalyst (20 wt.% Mg), with only a 10 % activity loss after 50 h of study. A modification of CoMg20Al by 0.5 wt.% Ru significantly enhanced the catalytic activity, demonstrating a 93 % methane conversion with raw feed and stable activity over a 100-h extended activity analysis. The catalyst characterization showed that Mg concentration increased basic sites over Mg-Al support. Due to the separation of crystalline phases as MgO-MgAl2O4, CoMg40Al had the most basic sites but poor DRM reaction performance. Balanced acidic/basic properties and well-dispersed Co-Rh nanoclusters-maintained DRM activity and stability. Ru-promoted CoMg20Al catalyzed the lowest activation energies.

Influence of CaCe ratio on the hydrogen production from ammonia over CaO-CeO2 supported Co catalysts

Hydrogen (H2) fuel has the potential to replace fossil fuel for generating electricity, clean energy and generating power. Ammonia (NH3) has the potential to overcome the challenges associated with storage, transportation, and use of hydrogen fuel. The catalytic decomposition of ammonia into hydrogen is studied over cobalt catalysts supported on CaO-CeO2 mixed oxide. The influence of the Ca/Ce ratio in the support composition on catalytic activity was investigated. The CaO-CeO2 mixed oxide support materials prepared with different Ca/Ce ratios (1:1, 2:1 and 3:1). The overall basicity of catalyst varied with the increase of Ca to CaO-CeO2 mixed oxide and showed maximum at Ca/Ce = 2. The cobalt catalysts are prepared with a nominal 5 wt% Co over CaO-CeO2 mixed oxide support with various Ca/Ce ratios. Among all prepared catalysts 5CoCaCe-21 (5 wt%Co on CaO-CeO2 mixed oxide with Ca/Ce = 2) showed highest activity. Surface area, X-ray diffraction (XRD), Temperature Program Reduction (TPR), Scanning Electron Microscopy (SEM), and CO2 Temperature Program Desorption (TPD) methods were used to characterize the synthesized materials. The high performance of 5CoCaCe-21 catalyst is due to high basicity, suitable metal support interactions, high surface area and easy reducibility of Co species resulting from its support favorable properties. The effect of cobalt loadings also studied on CaCe-21 support. The 10 wt%Co on CaCe-21 (10CoCaCe-21) showed highest activity among all other cobalt loadings. The 10CoCaCe-21 catalyst showed excellent stability for H2 production from NH3 during 70 h of time on study with different GHSVs.

Effect of Mo content on oxidative dehydrogenation of propane with CO2 over ZnOx catalysts supported on Mo–Zr mixed–oxides

The production of propylene from readily available propane sources is of great importance as it serves as a crucial building block for producing numerous value-added chemicals. Oxidative dehydrogenation of propane with carbon dioxide (ODPC) has been proposed as a viable and eco-friendly approach for simultaneous propylene production and carbon dioxide reduction. In this study, ZnOx catalysts were prepared by supporting them on a range of molybdenum–zirconium (Mo–Zr) mixed–oxides with varying Mo contents, aiming to utilize them for ODPC applications. The incorporation of suitable quantities of Mo in zirconium dioxide (ZrO2) supports increases the surface concentration and enhances the dispersion of ZnOx, thereby demonstrating promotional effects on the surface properties of the supported ZnOx catalysts. Notably, the ZnOx catalyst supported on a Mo–Zr mixed–oxide, with an optimized Mo content, exhibits a propane conversion rate more than two-fold higher than that of the catalyst without Mo in ODPC. Furthermore, the introduction of Mo into the ZrO2 supports effectively inhibits the formation of coke by suppressing the cleavage of C–C bonds in propane. In comparison, the supported ZnOx catalysts with higher Zn loadings, show that a Mo–Zr mixed–oxide support effectively suppresses the formation of bulk ZnOx more than an unmodified ZrO2 support, leading to improved ODPC performance. The promoting effects of Mo emphasize the potential of utilizing mixed–oxide supports for ZnOx catalysts in ODPC, demonstrating a promising opportunity for enhancing catalytic performance.

CO2 valorisation to methane on highly stable iron impregnated ceria-zirconia based 3D-printed catalyst

The catalytic activity of 3D-printed metal monoliths loaded with iron impregnated ceria-zirconia mixed-oxide support on CO2 conversion to methane was investigated between 300 and 500 °C under 1 bar and 20 bar pressure. The catalyst was characterised using TPR, XRD, SEM-EDX and in-situ DRIFTS. At 400 °C and atmospheric pressure, the catalyst wash-coated monoliths increased the methane yield by 3.5 times and doubled the CO2 conversion compared to the same catalyst dispersed as a powder. Methane selectivity of 95.2% was obtained at 400 °C and 20 bars pressure. This is the highest methane selectivity recorded in the literature for CO2 methanation using an iron catalyst. The catalyst loaded monoliths were stable over a continuous operation of 100 h at 500 °C and 20 bar. Such increased methane selectivity and yield combined with a long duration stability as well as an economic and easier synthesis process vouches for the great potential of catalyst loaded 3D monoliths for industrial application.

Developing magnetic, durable, agglomeration resistant and reactive copper-based oxygen carrier particles by promoting a kaolin-reinforced, manganese‑iron mixed oxide support

Chemical looping with oxygen uncoupling (CLOU) process, involves the use of two interconnected fluidized bed reactors, where oxygen carrier particles are able to release O2 at the fuel reactor operation conditions. Cu-based oxygen carriers containing a kaolin-reinforced MnFe mixed oxide as magnetic support, were prepared by granulation to improve their oxygen carrier properties by means of a simplified preparation method. A study was performed into the effect of kaolin concentration and calcination temperature on magnetic permeability, crushing strength, attrition, agglomeration resistance, and reactivity. Two oxygen carriers containing 28.5 wt% CuO-5 wt% kaolin and 30 wt% CuO-7.5 wt% kaolin, respectively, and both calcined at 1100 °C, were selected because they maintained their magnetic properties and reactivity after kaolin addition and showed improvement in their mechanical resistance. They were tested in a batch fluidized bed reactor (BFBR) to evaluate their fluidization behaviour. Low attrition rate without any agglomeration phenomena was observed for both of oxygen carriers up to temperature of 950 °C. The material prepared with 7.5 wt% kaolin demonstrated improved mechanical resistance, very low attrition rate and higher CuO utilization for the CLOU process after use in the BFBR, while maintaining reactivity during redox cycles and magnetic properties without agglomeration problems.

High Catalytic Efficiency of a Nanosized Copper-Based Catalyst for Automotives: A Physicochemical Characterization.

The global trend in restrictions on pollutant emissions requires the use of catalytic converters in the automotive industry. Noble metals belonging to the platinum group metals (PGMs, platinum, palladium, and rhodium) are currently used for autocatalysts. However, recent efforts focus on the development of new catalytic converters that combine high activity and reduced cost, attracting the interest of the automotive industry. Among them, the partial substitution of PGMs by abundant non-PGMs (transition metals such as copper) seems to be a promising alternative. The PROMETHEUS catalyst (PROM100) is a polymetallic nanosized copper-based catalyst for automotives prepared by a wet impregnation method, using as a carrier an inorganic mixed oxide (CeO2-ZrO2) exhibiting elevated oxygen storage capacity. On the other hand, catalyst deactivation or ageing is defined as the process in which the structure and state of the catalyst change, leading to the loss of the catalyst's active sites with a subsequent decrease in the catalyst's performance, significantly affecting the emissions of the catalyst. The main scope of this research is to investigate in detail the effect of ageing on this low-cost, effective catalyst. To that end, a detailed characterization has been performed with a train of methods, such as SEM, Raman, XRD, XRF, BET and XPS, to both ceria-zirconia mixed inorganic oxide support (CZ-fresh and -aged) and to the copper-based catalyst (PROM100-fresh and -aged), revealing the impact of ageing on catalytic efficiency. It was found that ageing affects the Ce-Zr mixed oxide structure by initiating the formation of distinct ZrO2 and CeO2 structures monitored by Raman and XRD. In addition, it crucially affects the morphology of the sample by reducing the surface area by a factor of nearly two orders of magnitude and increasing particle size as indicated by BET and SEM due to sintering. Finally, the Pd concentration was found to be considerably reduced from the material's surface as suggested by XPS data. The above-mentioned alterations observed after ageing increased the light-off temperatures by more than 175 °C, compared to the fresh sample, without affecting the overall efficiency of the catalyst for CO and CH4 oxidation reactions. Metal particle and CeZr carrier sintering, washcoat loss as well as partial metal encapsulation by Cu and/or CeZrO4 are identified as the main causes for the deactivation after hydrothermal ageing.

Understanding the role of SrO-ZrO2 mixed oxide support for Co3O4 based catalysts towards H2 production from steam reforming of glycerol

Cobalt dispersed on SrO-ZrO2 mixed oxide catalysts were prepared by varying the ratios of Sr and Zr. These catalysts were studied for hydrogen production from glycerol steam reforming. The physico-chemical characteristics of the catalysts were investigated by different techniques. Among all the catalysts, 15 %Co dispersed on SrO-ZrO2 with a 1:1 ratio of Sr and Zr catalyst exhibited the best activity with complete transformation of glycerol into gaseous products (H2, CH4, CO and CO2) with 73 % hydrogen yield. The catalyst activity varied with the change in Sr-Zr ratio where the redox properties of ZrO2 improved the water gas shift reaction in glycerol steam reforming. The characterization results reveal that the high activity of the catalyst is mainly due to the presence of a greater number of octahedral sites, small Co particles, Sr-Zr-Co-O solid solution, strong metal support interactions, and basicity. The catalyst is highly stable as it showed consistent performance during time on stream analysis conceded for a period of 50 h.

Complete oxidation of 1,2-dichloroethane over highly efficient Cu/Al-Ti composite metal oxide catalyst

Chlorinated volatile organic compounds (CVOCs) have received attention as hazardous air contaminants for decades, and a complete oxidation process of CVOCs using an efficient catalyst should be most effective to handle such hazardous materials. A series of CuOx-supported on Al-Ti mixed oxide catalysts were prepared by sol-gel method for the mixed oxide support and followed by wet impregnation method for the CuOx nanoparticles formation on the support. These Cu/Al-Ti composite catalysts were investigated for catalytic complete oxidation of 1,2-dichloroethane (DCE), which is one representative CVOCs. Through combining each transition metal components that having different catalytic properties with appropriate composition, excellent catalytic performances in terms of activity and selectivity for the catalytic oxidation of DCE were obtained on the resultant Cu/Al-Ti composite catalysts. The design strategy of composite catalyst studied in this work should suggest the way for efficient and sustainable removal of CVOCs using nonprecious catalytic materials.

Application of Mixed-Metal-Oxides As Active Supports for Dispersed Metal Centers: Enhancement of Electrocatalytic Reduction of Carbon Dioxide

Application of metal oxides as active matrices in electrocatalysis is particular importance. The hydrous behavior, which favors proton mobility and affects overall reactivity, reflects not only the oxide’s chemical properties but its texture and morphology as well. What is of importance to electrochemical science and technology, certain nonstoichiometric mixed-valence oxides could exhibit pseudo-metallic conductivity and possess appreciable catalytic activity.For example, mixed oxide systems stabilized through Zr-O-W bonds have been demonstrated to be very attractive acid catalysts exhibiting high catalytic activities and good stabilities in many demanding industrial reactions. For electrocatalytic applications, mixed-valent tungsten(VI,V) oxide and zirconium(IV) oxide have been sequentially deposited and integrated through voltammetric potential cycling to form sub-microstructured films on glassy carbon electrode. The mixed WO3/ZrO2 systems are characterized by fast charge (electron, proton) propagation during the system’s redox transitions. By dispersing metallic copper electrocatalytic nanoparticles (generated from Cu2O) over such active WO3/ZrO2 supports, the electrocatalytic activities of the respective systems toward the reduction of oxygen or carbon dioxide have been enhanced even at decreased loadings in acid media. The enhancement effects should be attributed to features of the mixed metal oxide support such as porosity and high population of hydroxyl groups (due to presence of ZrO2), high Broensted acidity of sites formed on mixed WO3/ZrO2, fast electron transfers coupled to unimpeded proton displacements (e.g. in HxWO3), as well as strong metal-support interactions between nanosized metals (Pt or Cu) and the metal (W, Zr) oxo species. The fact that WO3/ZrO2 nanostructures are in immediate contact with the metallic catalytic sites leads to the specific interactions (via the surface hydroxyl groups) with the reaction intermediates (e.g. CO adsorbates). Mechanistic studies involving chronoamperometric, voltammetric and gas-diffusion electrode measurements are pursued, in addition to spectroscopic and XPS measurements. Results of comparative measurements, involving single components only, will be provided as well. Acknowledgements: This work was supported by the National Science Center (Poland) under Opus Project (2018/29/B/ST5/02627.

Comparison of a commercial water-gas shift catalyst and La modified Cu-based catalysts prepared by deposition-precipitation in methanol steam reforming.

Herein, a performance analysis of La-doped copper-based catalysts (CuO/ZrO2/La-Al2O3) in methanol steam reforming (MSR) was conducted and compared with a commercial low temperature water-gas shift catalyst (HiFUEL W220) to produce H2 with low CO selectivity. The physicochemical properties of as-obtained catalysts were characterized by N2 adsorption, XRD, and ICP-OES. Effect of calcination temperature (750 °C and 1000 °C) on the properties of mixed oxide support (La-Al2O3) were discussed based on catalytic activity. The optimum conditions of H2O/CH3OH ratio (1.0-3.0), space-time ratio (WFA0) (40-120 kg s mol-1), and reaction temperature (180-310 °C) were evaluated by a parametric study using the commercial catalyst (HF220). Additionally, thermodynamic equilibrium calculations of experimentally identified components by using Aspen HYSYS process simulation software were also performed to analyze MSR process. The results were indicated that the calcination temperature significantly affected the structural properties and the activity with respect to CO selectivity. An increasing trend in CO selectivity for catalysts with supports calcined at 750 °C and a decreasing trend for catalysts with supports calcined at 1000 °C were observed. Hence, CZ30LA750 and CZ30LA1000 catalysts were selected to attain low CO selectivity and comparable activity when compared to other catalysts and the simulated thermodynamic calculation results.

Development of nanocrystalline mesoporous Pt promoted Co-based catalysts for carbon dioxide reforming of methane

In this study, a series of mesoporous Co-Pt/TiO2-Al2O3 catalysts were synthesized and evaluated for CO2 reforming of methane for synthesis gas production. The physicochemical properties of catalysts were assessed using various analytical techniques. Results showed higher activity and stability for bimetallic Co-Pt/TiO2-Al2O3 catalysts than the corresponding monometallic catalysts. The synergistic combination of Co and Pt, with the TiO2 mixed oxide support, results in the fine-tuned catalysts for the sustainable production of syngas. The TiO2 supported catalysts didn't show any deposition of hard carbon. TiO2-Al2O3 support excellently prohibits the carbon deposition and sintering of metal up to 10 wt% of Co active metal loading. Further increasing the metal loading resulted in the deactivation of catalyst by coke deposition. The study revealed that the 5 wt% Co- 0.5 wt% Pt/TiO2-Al2O3 catalyst is an excellent catalyst for the dry reforming reaction. The fine distribution of Pt alongside Co metal in the bimetallic system promotes the dry reforming reaction. The maximum syngas production rate of 72 mL g−1 h−1 with 65% to 85% of reactants conversions was obtained at 700 °C and 1 bar. The generation of Co2+ species in Co/Al2O3 catalyst was the reason for activity loss, whereas the mixed oxide support supremely prevented the formation of Co2+ species.

Hydrogen‐rich syngas production from bi‐reforming of greenhouse gases over zirconia modified Ni/ MgO catalyst

Bi-reforming of methane (BRM) is gaining an increase interest due to the critical requirements to mitigate global warming and provide alternative energy resources. However, there has been a serious challenge to the scale-up of the process to commercial production due to the catalyst deactivation. In the present study, the influence of ZrO2 modifications on the activity and stability of MgO-supported Ni catalyst in the BRM reaction was investigated. The ZrO2-MgO mixed oxide support was prepared by co-precipitation method with variation in the ZrO2 composition and subsequently impregnated with Ni. The characterization of the freshly prepared Ni/MgO and Ni/MgO-ZrO2 catalysts using N2 physisorption analysis, X-Ray Diffraction (XRD), FESEM, XPS, H2-TPR, and CO2-TPD techniques revealed suitable physicochemical properties for the BRM reaction. The Ni/MgO-ZrO2 catalysts showed an improved performance in the BRM reaction in terms of activity and stability compared to the Ni/MgO at 800°C and CH4, H2O, CO2 ratio of 3:2:1, respectively. The best performance was obtained using the Ni/15%ZrO2-MgO for the BRM with CO2 and CH4 conversion of 81.5% and 82.5%, respectively. The characterization of the spent Ni/MgO catalyst using Raman spectroscopy, FESEM, and High Resolution Transmission Electron Microscopy (HRTEM) analysis revealed the formation of amorphous carbon that could be responsible for its fast deactivation.

Understanding the role of Al2O3-ZrO2 mixed oxides as support for vanadium catalysts for the selective ammoxidation of o-chlorotoluene to o-chlorobenzonitrile

o-Chlorotoluene (o-CT) ammoxidation to prepare o-chlorobenzonitrile (o-CBN) was investigated on Al2O3-ZrO2 supported V2O5 catalysts in the temperature range of 360 to 420 °C. A set of vanadium oxide supported on mixed oxides with varying its loadings were prepared by wet impregnation method. The catalysts surface and structural characteristics were derived from BET-surface area, X-ray diffraction, FT-infrared, temperature programmed reduction, temperature programmed desorption of ammonia and UV–Vis diffuse reflectance spectroscopy. The characterization results revealed that vanadium species were well dispersed and strongly interacted with mixed oxide support. The ammoxidation activity was depended on V2O5 content and its nature of surface species existing in the catalyst. The catalyst with 10 wt% of V2O5 on Al2O3-ZrO2 showed high ammoxidation activity with 100% selectivity to o-CBN. Moreover, the catalyst showed consistent activity during time on stream study.

Syngas production from greenhouse gases using Ni–W bimetallic catalyst via dry methane reforming: Effect of W addition

DMR is a promising technique to utilize the rising greenhouse gases and produce an alternative energy source. The main hurdle in the commercialization of DMR is the catalyst deactivation. Presently, the effect of Tungsten (W) addition on Ni-based catalysts supported on mixed oxide (Al2O3–MgO) support is tested for DMR. The Ni–W bimetallic catalysts are synthesized via co-precipitation followed by the impregnation technique. An equimolar stream of feed (CH4:CO2) is used for DMR at 800 °C. The Ni–W catalyst with 4 wt% of W showed steady performance with elevated conversions of CH4 and CO2, even after 24 h of DMR. The freshly and spent catalysts are characterized by BET, XRD, FESEM, EDX, elemental mapping, TPR-H2, TPD-CO2, and XPS to confirm the elemental composition and the type of carbon formed on the catalysts. The activity of Ni catalyst declined due to formation of amorphous carbon-nanosheets, whereas Ni–W catalyst remained active due to formation of MWCNT.

Synthetic gas production by dry reforming of methane over Ni/Al2O3–ZrO2 catalysts: High H2/CO ratio

Alumina prepared by the sol-gel method, was impregnated with zirconia (5, 15 and 30 wt.%). Subsequently, the resulting Al2O3–ZrO2 supports were impregnated with 15% Ni to obtain the Ni/Al2O3–ZrO2 catalysts. The obtained catalysts were characterized by BET, SEM, XRD, H2-TPR and TPD- CO2. The catalytic activity was studied by means of dry reforming of methane (DRM) for syngas production. The catalysts displayed different physicochemical properties and trends of their catalytic activity as a function of the ZrO2 content in the mixed oxide supports. For instance, ZrO2 (5 wt %) in the catalyst, led to enhanced concentration of the medium strength basic sites and increased specific surface area, yielding thus the best performance in the DRM, with low carbon deposition after 36 h of reaction, compared with the other catalysts. This indicates that during the DRM reaction, this catalyst can provide more surface oxygen to prevent carbon deposits that could deactivate the catalyst.

Effect of Different Catalyst Supports on the Quality, Yield and Morphology of Carbon Nanotubes Produced from Waste Polypropylene Plastics

The role of the effect of the support on the reactivity of heterogeneous catalysts cannot be over-emphasized. Therefore, the study documented in this article investigated the effect of different metal oxide supports (MgO, CaO and TiO2) and mixed oxide supports (CaTiO3) on the performance of a bimetallic NiMo catalyst prepared via the sol–gel method during the catalytic growth of carbon nanotubes (CNTs) from waste polypropylene (PP). Waste PP was pyrolyzed at 700 °C in a single-stage chemical vapor deposition reactor and off-gas was utilized in-situ as a cheap carbon feedstock for the growth of CNTs under similar conditions for all the prepared NiMo catalysts (supported and unsupported). The structures of the prepared catalysts and deposited carbon were extensively characterized using X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), etc. The catalytic performance of NiMo supported and unsupported catalysts was evaluated in terms of the yield, purity, and morphology of synthesized CNTs. The results revealed that the stabilizing role of supports is fundamental in preventing nanoparticle agglomeration and aggregation, thereby resulting in improved yield and quality of CNTs. Supported NiMo catalysts produced better aligned graphitic and high-quality CNTs. The NiMo/CaTiO3 catalyst produced the highest carbon of 40.0%, while unsupported NiMo produced low-quality CNTs with the lowest carbon yield of 18.4%. Therefore, the type of catalyst support and overall stability of catalytic materials play significant roles in the yield and quality of CNTs produced from waste PP.

Adsorption and activation of CO2 on Pt/CeOx/TiO2(110): Role of the Pt-CeOx interface

The adsorption and dissociation of CO2 on TiO2(110), CeOx/TiO2(110) and Pt/CeOx/TiO2(110) surfaces has been examined using Ambient Pressure X-ray Photoelectron Spectroscopy (AP-XPS). The substrates under study exhibited different degrees of complexity which were tested for the binding of the adsorbate and the cleavage of C-O bonds. The surfaces were prepared by depositing CeOx (0.1 ML) onto TiO2(110) to form a mixed oxide support, onto which Pt nanoparticles (0.2 ML) were deposited. This configuration yields a complex set of interfaces between metal and oxides and we have systematically titrated the active role of each component (Pt 4f, Ce 3d and Ti 2p regions) and the arising surface intermediates (C 1 s and O 1 s regions). CO2 barely bonds to stoichiometric TiO2(110). It heals oxygen vacancies of this oxide surface (CO2,gas → COgas + Oa) and does not form stable carbonates. A stable carbonate was seen upon adsorption of CO2 on CeOx/TiO2(110) and on this type of substrate the adsorbate also removed O vacancies leading to the oxidation of Ti3+and Ce3+ sites. Pt nanoparticles dispersed on CeOx/TiO2(110) were highly effective for the binding and dissociation of CO2, with the formation of CO3, CO, C and CHx species on the Pt/CeOx/TiO2(110) system. The results of theoretical calculations based on density-functional theory (DFT) show that Pt/CeOx/TiO2(110) binds CO2 much stronger than surfaces of bulk platinum {(111), (100), (110)} or other late transition metals. On a Pt-CeOx interface, the molecule adsorbs with a bent configuration (~130° O-C-O bond angle) and with a substantial elongation (~ 0.1 Å) of the C-O bonds, facilitating its transformation into high value chemicals.