Ni Oxide Species Research Articles

CO2 Activation on Ni(111) and Ni(110) Surfaces in the Presence of Hydrogen

The structure sensitivity of CO2 activation in the presence of H2 has been identified by ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Ni(111) and Ni(110) surfaces under identical reaction conditions. Based on the APXPS results and computer simulations, we propose that, around room temperature, the hydrogen-assisted activation of CO2 is the major reaction path on Ni(111), while the redox pathway of CO2 prevails on Ni(110). With increasing temperature, the two activation pathways are activated in parallel. While the Ni(111) surface is fully reduced to the metallic state at elevated temperatures, two stable Ni oxide species can be observed on Ni(110). Turnover frequency measurements indicate that the low-coordinated sites on Ni(110) promote the activity and selectivity of CO2 hydrogenation to methane. Our findings provide insights into the role of low-coordinated Ni sites in nanoparticle catalysts for CO2 methanation.

Influence of Highly Stable Ni2+ Species in Ni Phyllosilicate Catalysts on Selective Hydrogenation of Furfural to Furfuryl Alcohol.

Enhancing the catalytic performance of non-noble Ni catalysts in the selective hydrogenation of furfural to furfuryl alcohol (FA) in terms of furfural conversion, selectivity, and good recyclability is challenging. Here, spherical nickel phyllosilicate catalysts (Ni_PS) with fibrous-like structures are prepared via a modified sol-gel method with Ni loadings of 2-30 wt %. Upon exposure to air, all the reduced Ni_PS catalysts exhibit more than 80% Ni0/Niphyllosilicate species on the surface, whereas a large portion of Ni oxide species (>55%) is presented on the impregnated catalyst. The Ni2+ species in nickel phyllosilicate catalysts are active and highly stable during reduction, reaction, and regeneration, yielding stable catalytic performance for multiple recycle tests in furfural hydrogenation to FA. Furfural conversion over the Ni_PS catalysts increased monotonically with increasing Ni loading without an FA selectivity drop. The presence of both metallic Ni0 and Niphyllosilicate also produces a synergistic promotional effect for FA formation.

Co-processing of hydrodeoxygenation and hydrodesulfurization of phenol and dibenzothiophene with NiMo/Al2O3–ZrO2 and NiMo/TiO2–ZrO2 catalysts

Abstract The influence of Al2O3–ZrO2 and TiO2–ZrO2 supports on NiMo-supported catalysts at a different sulfur concentration in a model hydrodeoxygenation (HDO)-hydrodesulfurization (HDS) co-processing reaction has been studied in this work. A competition effect between phenol and dibenzothiophene (DBT) for active sites was evidenced. The competence for the active sites between phenol and DBT was measured by comparison of the initial reaction rate and selectivity at two sulfur concentrations (200 and 500 ppm S). NiMo/TiO2–ZrO2 was almost four-fold more active in phenol HDO co-processed with DBT than NiMo/Al2O3–ZrO2 catalyst. Consequently, more labile active sites are present on NiMo/TiO2–ZrO2 than in NiMo/Al2O3–ZrO2 confirmed by the decrease in co-processing competition for the active sites between phenol and DBT. DBT molecules react at hydrogenolysis sites (edge and rim) preferentially so that phenol reacts at hydrogenation sites (edge and edge). However, the hydrogenated capacity would be lost when the sulfur content was increased. In general, both catalysts showed similar functionalities but different degrees of competition according to the highly active NiMoS phase availability. TiO2–ZrO2 as the support provided weaker metal-support interaction than Al2O3–ZrO2, generating a larger fraction of easily reducible octahedrally coordinated Mo- and Ni-oxide species, causing that NiMo/TiO2–ZrO2 generated precursors of MoS2 crystallites with a longer length and stacking but with a higher degree of Ni-promotion than NiMo/Al2O3–ZrO2 catalyst.

CO and CO₂ Methanation Over Ni/γ-Al₂O₃ Prepared by Deposition-Precipitation Method.

Various Ni catalysts supported on γ-Al₂O₃ were prepared by a wet impregnation (WI) method and deposition-precipitation (DP) method with different precipitants and applied to CO and CO₂ methanation. The prepared catalysts were characterized by various techniques including nitrogen physisorption, X-ray diffraction (XRD), temperature-programmed reduction with H₂ (H₂-TPR), H₂ chemisorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Irrespective of kinds of precipitant, the Ni/γ-Al₂O₃ catalysts prepared with a DP method showed a remarkable enhanced catalytic performance in CO and CO₂ methanation compared with the Ni/γ-Al₂O₃ catalyst prepared with a WI method owing to the higher catalytic active surface area (CASA). In the case of Ni/γ-Al₂O₃ catalysts prepared with a DP method, the high calcination temperatures are not favorable for the high catalytic activity due to the decreased reduction degree of Ni oxide species and CASA. The reduction degree of Ni oxide species can be increased with reduction temperature. However, the higher reduction temperature above 500 °C is not desirable to achieve the high catalytic activity because of the decreased CASA. The selective CO methanation was also accomplished at lower temperatures over the Ni/γ-Al₂O₃ catalyst prepared with a DP method than over the Ni/γ-Al₂O₃ catalyst prepared with a WI method.

Investigation of Ni/C and Pt/C Catalyst Stability in an Anion Exchange Membrane (AEM) Fuel Cell

Anion exchange membrane fuel cells (AEMFC) have gained increasing research interest because its alkaline chemistry enables non-platinum group metal (non-PGM) catalysts to be used. Despite this opportunity, many researchers still focus on PGM material due to their performance. Often the approach is to minimize PGM content by using non-PGM supporting substrates, such as Pt-Ni foam and Pt-Ni/Ni-B catalysts in order to mitigate cost [1] [2]. Although the performances of both PGM and non-PGM catalysts have been thoroughly researched, their stability under an AEMFC environment is seldom questioned. This is because PGM materials, typically Pt, are noble metals and many non-PGM materials are considered stable under alkaline conditions, especially nickel. However, the Pt stability in PEMFCs has been severely challenged during fuel cell operations. Ferreira et al. reported the Pt catalyst particle aggregation with decreased PEMFC performance [3]. Helmly et al. reported Pt dissolution in a PEM electrolyte. In alkaline fuel cell operation, the amount of dissolved Pt was reported to be even higher than from PEM FC operation [4]. Hence, the Pt stability is of concern under AEMFC conditions. On the other hand, Ni stability does not appear to have been reported. Therefore, the focus of this study is to examine catalyst stability of Pt and Ni under identical AEMFC operating conditions. This study focuses on pure Pt and Ni carbon supported catalysts, which have been conventionally used to operate AEMFCs. To study and compare the two materials under the same test condition, both Ni/C and Pt/C catalysts were applied on the same AEMFC in separate locations as shown in Fig.1. A sandwiched AEM was used so that catalyst migration could be studied without interference from direct catalyst contact. The AEMFC was held at 0.9V by a potentiostat with H2 and O2 gas flowing at each specific electrode. Once per day, the cell open circuit potential (OCP) was measured without the imposed voltage. To identify any catalyst morphology change after 30 days (~500hrs) operation, scanning electron microscopy (SEM) was conducted on a cross section of each catalyst layer; transmission electron microscopy (TEM) was used to observe catalyst particles from each layer; inductively coupled plasma (ICP) analysis was used to determine if any catalyst could be detected in the middle AEM. The OCP was found to have dropped by 0.2V during the 30 day operation. Significant catalyst morphology changes were found at the cathode for both Ni/C and Pt/C catalysts. SEM analysis showed that the Ni material was found to develop a band like structure at the AEM-catalyst layer interface. The TEM analysis confirmed that the Ni nanoparticles developed a smeared shape on the carbon substrate, which was verified as a Ni oxide species using energy-dispersive X-ray (EDX) mapping. The SEM images showed that the Pt signal from the cathode catalyst layer was much weaker than the one from the anode. TEM analysis showed that the Pt remained as nanoparticles on the carbon substrate, but that the nanoparticle density appeared lower than on the anode, consistent with Pt dissolution. The ICP analysis showed both Ni and Pt in the middle AEM. The results indicate, substantial morphology changes occurred on both Ni/C and Pt/C catalysts during AEMFC operation. Additional research is needed to investigate the change of morphology mechanisms, which is key to developing enhanced stability of AEMFC catalysts. [1] Daping He, Libo Zhang, Dongsheng He, Gang Zhou, Yue Lin, Zhaoxiang Deng, Xun Hong, Yuen Wu, Chen Chen & Yadong Li, Amorphous nickel boride membrane on a platinum–nickel alloy surface for enhanced oxygen reduction reaction, Nature Communications volume 7, Article number: 12362 (2016) [2] Julia van Drunen, Brandy K. Pilapil, Yoseif Makonnen, Diane Beauchemin, Byron D. Gates, and Gregory Jerkiewicz, Electrochemically Active Nickel Foams as Support Materials for Nanoscopic Platinum Electrocatalysts, ACS Applied Materials & Interfaces 2014 6 (15), 12046-12061 [3] P. J. Ferreira, G. J. la O,, Y. Shao-Horna, D. Morgan, R. Makharia, S. Kocha, and H. A. Gasteiger, Instability of Pt ∕ C Electrocatalysts in Proton Exchange Membrane Fuel Cells A Mechanistic Investigation J. Electrochem. Soc. 2005 volume 152, issue 11, A2256-A2271 [4] Serhiy Cherevko, Aleksandar R. Zeradjanin, Gareth P. Keeley and Karl J. J. Mayrhofer, A Comparative Study on Gold and Platinum Dissolution in Acidic and Alkaline Media J. Electrochem. Soc. 2014 volume 161, issue 12, H822-H830 Figure 1

NiMo catalysts supported on Al, Nb, Ti or Zr-containing MCM-41 for dibenzothiophene hydrodesulfurization

xMCM-41 materials (x = Al, Nb, Ti or Zr) with nominal x loading of 0.5 mmol/g were prepared by chemical grafting of Al2O3, Nb2O5, TiO2 and ZrO2 precursors and used as supports for NiMo/xMCM-41 catalysts. Small-angle XRD characterization of supports showed typical patterns of solids with hexagonal arrangement of mesopores. After the deposition of Ni and Mo oxide species on the supports a decrease in their long-range pore arrangement was observed. Nevertheless, the prepared NiMo catalysts possessed high specific surface areas and pore volumes. Powder XRD patterns showed the formation of the MoO3 crystalline phase in the NiMo/MCM-41 reference. This phase was not detected in the catalysts supported on xMCM-41 materials. An increase in the dispersion of MoO3 species with the incorporation of alumina, niobia, titania and zirconia in the MCM-41 support was due to the stronger interaction of Mo and Ni oxide species with xMCM-41 materials (H2-TPR and UV–vis DRS). Surface modification of the MCM-41 with different metal oxides increased the dibenzothiophene conversion and pseudo first-order rate constants. This effect was more evident for the NiMo/TiMCM-41 catalyst. Selectivity of the catalysts (product distributions) was also affected by the incorporation of Al-, Zr-, Ti- and Nb-oxide species in the support.

On the effect of temperature and surface oxidation on the kinetics of hydrogen electrode reactions on nickel in alkaline media

The influence of the temperature on the kinetics of the hydrogen oxidation (HOR) and evolution reactions (HER) on Ni electrodes has been explored in the range from 298 to 338 K. By combining an expexrimental cyclic voltammetry study and microkinetic modeling, the rate constants and the activation energies of the individual Volmer-Tafel-Heyrovsky steps have been extracted for two surface states of Ni electrode, namely metallic Ni and partially oxidized one. The kinetic model suggests that the HOR/HER on metallic Ni follows the Heyrovsky-Volmer mechanism characterized by strong adsorption of the hydrogen intermediate and low reaction rate. Partial coverage of the electrode surface with Ni oxide species results in a decrease of the strength of adsorption of hydrogen atoms and an increase of the rate of the Volmer step thus leading to a significant enhancement of the HOR/HER kinetics. Furthermore, the kinetic model provides evidence for a contribution of the Tafel step in the reaction mechanism for a partially oxidized electrode. The apparent activation energy is shown to be slightly dependent on the surface state, decreasing from 30 ± 1 for metallic to 26 ± 1 kJ mol−1 for partially oxidized Ni.

Sources of deactivation during glycerol conversion on Ni/γ-Al2O3

Hydrogenolysis of glycerol was studied using a diluted aqueous solution of glycerol in gas phase and atmospheric pressure on Ni/γ-Al2O3 catalyst. The catalytic transformation of glycerol generates products derived from dehydration, dehydrogenation, hydrogenolysis and condensation reactions. Deep hydrogenolysis route to produce CH4 prevails in the first few hours of reaction. As the reaction time progress, dehydration-dehydrogenation products start to appear. Here, a description of the deactivation sources and its effects on the catalytic performance of Ni catalyst was proposed. The catalyst was characterized before and after the catalytic reaction by high-resolution transmission electron microscopy (HRTEM) and by employing Fourier transformed infrared spectroscopy (FTIR) of adsorbed CO. A source of deactivation was due to carbonaceous deposition. FTIR at low CO dosing pressure reveal bands assignments species essentially due to linear and bridge carbonyls, whereas high pressure CO dosing produces a complex spectra due to polycarbonyls. X-ray absorption near edge structure (XANES) analysis was employed to reveal the initial degree of reduction of the fresh catalyst. The oxidation of metallic Ni in the course of reaction may also be considered as a source of deactivation. Ni oxide species promote dehydration routes. Alumina support facilitates nickel species to be more active toward interacting with glycerol. Dehydration, which takes place on the acid sites, is the mainly route related to the generation of carbon deposition and to the observed catalyst deactivation. Another source of deactivation was due to carbiding of Ni to form Ni3C. The regeneration of used Ni catalyst was achieved by oxidation-reduction steps at 723K.

Preparation of Cerium‐Doped Mesoporous γ‐Alumina Supported Nickel Catalysts for Pre–reforming of Liquefied Petroleum Gas under Low Steam to Carbon Ratio

AbstractCerium‐doped mesoporous γ‐alumina supported nickel oxides (NiO/Ce–γ‐MA) were prepared via one‐pot hydrolysis of inorganic salts without surfactants and applied for pre‐reforming of liquefied petroleum gas (LPG) under a low steam to carbon molar ratio of 1.2. Influences of Ce contents on catalyst structure, interaction between Ni species and support, dispersion of nickel species and Ni particle sizes were investigated in detail. The NiO/Ce–γ‐MA materials possessed large surface areas and narrow pore size distributions with homogenously‐dispersed Ni oxide species in the matrices. Upon H2 reduction, uniform Ni crystallites were formed in the γ‐alumina frameworks. The addition of Ce could significantly promote the dispersion of nickel species and lower Ni crystallite sizes, improving the catalytic properties of the reduced Ni/Ce–γ‐MA catalyst. The Ni/Ce–γ‐MA catalyst containing 7 wt% Ce produced the smallest sub‐4‐nm Ni particles, resulting in the most excellent catalytic activity and stability with less than 1 wt% carbon deposition during the 100‐h reaction time for LPG pre‐reforming.

Control of selectivity, activity and durability of simple supported nickel catalysts for hydrolytic hydrogenation of cellulose

Efficient conversion of cellulose to sorbitol and mannitol by base metal catalysts is a challenge in green and sustainable chemistry, but typical supported base metal catalysts have not given good yields of hexitols or possessed durability. In this study, it has been demonstrated that a simple carbon-supported Ni catalyst affords up to 67% yield of hexitols in the conversion of cellulose, and that the catalyst is durable in the reuse experiments 7 times. In addition, the catalyst can be separated by a magnet thanks to a high content of Ni. Physicochemical analysis has indicated that the use of carbon supports has two benefits: no basicity and high water-tolerance. CeO2, ZrO2, γ-Al2O3 and TiO2 cause side-reactions due to basicity, and SiO2, γ-Al2O3 and CeO2 are less stable in hot water. Another important factor is high Ni loading as the increase of Ni content from 10 wt% to 70 wt% significantly improves the yield of hexitols and the durability of catalysts. Larger crystalline Ni particles are more resistant to sintering of Ni and surface coverage by Ni oxide species.

Effect of titania grafting on behavior of NiMo hydrodesulfurization catalysts supported on different types of silica

In the present work, a comparison study of the NiMo hydrodesulfurization catalysts supported on different types of silica supports (nanostructured supports of MCM-41 and SBA-15-types and commercial amorphous silica) and the same ones modified by TiO2 grafting was undertaken. The aim of this study was to inquire on the effect of the characteristics of the primary silica supports on the activity and selectivity of the NiMo catalysts modified with titania in deep HDS. Supports and catalysts were characterized by nitrogen physisorption, small-angle and powder XRD, TPR, UV–vis DRS, and HRTEM, and tested in the simultaneous HDS of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). It was found that titania grafting on all silica supports resulted in a slight decrease of BET surface area and total pore volume. However, the characteristic p6mm hexagonal pore arrangement of the used nanostructured silica materials was not affected. Powder X-ray diffraction pointed out a good dispersion of Mo and Ni oxide species in all prepared catalysts. TPR characterization of the NiMo catalysts revealed some increase in the metal-support interaction after titania grafting on the silica surface. Further DRS characterization indicated that the best dispersion of Mo oxide species was obtained on the TiSBA-15 support. Titania addition to the silica supports also produced an increase in the dispersion of the sulfided NiMo phase, which was more marked for SBA-15 and commercial silica supports than for the MCM-41 (HRTEM). In line with the characterization results, titania addition to all silica supports resulted in an increase in the HDS activity of NiMo catalysts. However, this increase was smaller for the MCM-41 support than for the SBA-15 and amorphous silica supports used. The most active NiMo/Ti-SBA-15 catalyst resulted to be significantly more active (∼40%) than the reference NiMo/γ-Al2O3 catalyst in HDS of 4,6-DMDBT.

Effect of Titania Grafting on Behavior of NiMo HDS Catalysts Supported on Nanostructured Silica Materials

ABSTRACTIn the present work, a comparison study of the NiMo hydrodesulfurization (HDS) catalysts supported on different nanostructured supports of MCM-41 and SBA-15-types and the same ones modified by TiO2 grafting was undertaken. The aim of this study was to inquire on the effect of the characteristics of the primary silica supports on the activity and selectivity of the NiMo catalysts modified with titania in deep HDS. Supports and catalysts were characterized by nitrogen physisorption, small-angle and powder XRD, TPR, UV-vis DRS, and HRTEM, and tested in the simultaneous HDS of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT). It was found that titania grafting on all silica supports resulted in a slight decrease of BET surface area and total pore volume. However, the characteristic p6mm hexagonal pore arrangement of the used nanostructured silica materials was not affected. Powder X-ray diffraction pointed out a good dispersion of Mo and Ni oxide species in all prepared catalysts. TPR characterization of the NiMo catalysts revealed some increase in the metal-support interaction after titania grafting on the silica surface. Further DRS characterization indicated that the best dispersion of Mo oxide species was obtained on the TiSBA-15 support. Titania addition to the silica supports also produced an increase in the dispersion of the sulfided NiMo phase, which was more marked for SBA-15 support than for the MCM-41 (HRTEM). The most active NiMo/Ti-SBA-15 catalyst resulted to be significantly more active (∼40 %) than the conventional NiMo/γ-Al2O3 catalyst in HDS of 4,6-DMDBT.

Catalytic dry-reforming on Ni–zeolite supported catalyst

In the present work, the catalytic behaviour in the dry reforming of methane on Ni-based Silicalite-1 type catalyst was studied. The Silicalite-1 support has been synthesized in order to check the role of the silanol groups on the overall catalyst performance: methane and carbon dioxide conversion, hydrogen/carbon monoxide ratio and coke deposition. The population of defect groups on the Silicalite-1 surface was modified by aging of the gel, thermal treatment, ionic exchange and silylation procedure. Among these treatments, the silylation of the support surface leads to the formation of smaller and more reducible Ni-oxide species that not only improve the CH 4 and CO 2 conversion but also reduce the deactivation of the catalyst due to coke deposition and the obtained H 2/CO value is 1.04.

Zeolite-supported Ni catalyst for methane reforming with carbon dioxide

In the present work, we compare the catalytic behaviour, in the dry reforming of methane, of Ni-based Silicalite-1 type catalyst obtained by different post-synthesis treatments. The Silicalite-1 type material, used as Ni-support, has been treated in order to observe the role of the silanol groups on the Ni impregnation, and then on the overall catalyst performance. Among the applied treatments (ionic-exchange, calcinations and silylation), the silylation is the one that allows the formation of smaller and more reducible Ni-oxide species that not only improve the methane conversion but also reduce the deactivation of the catalyst, due to the coke deposition.

Exploration of electrodeposited platinum alloy catalysts for methanol electro-oxidation in 0.5�M H2SO4: Pt-Ni system

In this work, we examine the electrocatalytic activity of electrodeposited Platinum (Pt)-Nickel (Ni) alloy layers on an inert substrate electrode for methanol oxidation reaction. Analyses using energy-dispersive fluorescent X-ray analysis and powder X-ray diffractometry confirm alloying of Pt with Ni in a range of compositions. Steady-state polarisation measurements in 0.5 M methanol+0.5 M H2SO4 solutions clearly show that the onset of electro-oxidation shifts to less anodic potential values (approximately 160 mV), while also exhibiting current enhancements up to ~15 times the currents obtained for the pure Pt electrodeposit. A linear relationship between the cyclic voltammetric peak (oxidation) current and [MeOH] is observed at a scan rate of 50 mVs−1, thus indicating reduced influence of adsorbed CO (COads) surface poison. A critical composition, Pt (92%)/Ni (8%) [denoted Pt-Ni(3) alloy] is found to exhibit maximum electrocatalytic activity, beyond which the activity drops, whereas pure Ni does not catalyse the reaction. While the promotion of electro-oxidation is understood to be largely due to the alloy catalyst, surface redox species of Ni oxide formed during the electro-oxidation process may also contribute to the oxygenation of COads, thereby enhancing the oxidation current. Plausible mechanisms of methanol oxidation on Pt/ transition metal alloy electrocatalysts are discussed in terms of electron transfer (in the alloy) and the role of Ni oxide species.

An amperometric aqueous ethanol sensor based on the electrocatalytic oxidation at a cobalt-nickel oxide electrode

An electrodeposited mixed oxide film of CoNi has been applied as a sensor for the amperometric determination of aqueous ethanol samples. The electrode has a linear response over a 1.5–7.9 mM ethanol concentration range with a detection limit of 32 μM ethanol. Deposition conditions for maximum catalytic efficiency, electrode lifetime and optimum analytical analysis potential are discussed. SEM, EDAX and electrochemical kinetic studies indicate that the active catalytic species in the electrode film is a 2:1 CoNi oxide species and suggest a partial mechanism of the ethanol oxidation reaction. The feasibility of this modified electrode for the determination of atmospheric ethanol is also examined. All other organic alcohols tested were interferants. Conditions affecting the stability of the sensor are discussed.