Zirconia Support Research Articles

Joining of solid oxide fuel/electrolysis cells at low temperature: A novel method to obtain high strength seals already at 300 °C

A novel method using a combination of surface three-dimensional structuring, silver nanoparticles and a silver-foam interlayer is developed for joining solid oxide fuel/electrolysis cell components at low temperature. Joints with a high mechanical strength of 24 MPa are obtained with this method already at 300 °C. The interfaces of a sealed assembly comprising ferritic stainless steel interconnect and nickel oxide-yttria stabilized zirconia (NiO-YSZ) support are analyzed by scanning electron microscopy and transmission electron microscopy to study the interfacial sintering mechanism. It is shown that the high strength of the joint is due to a combination of an optimized three-dimensional nickel/gold nanosheet array deposited on the substrates which facilitates mechanical interlocking, and a silver-foam interlayer which enhances the resistance to crack propagation. The long-term stability of the joint is evaluated by aging in a reducing atmosphere at 800 °C for 250 h. No defects indicating a possible failure are observed in the joint after this aging. An oxide layer forms along the silver/steel interface and parts of the steel are transformed to austenite due to nickel diffusion from the nanosheet array, but this does not deteriorate the joint stability.

Insight towards the role of ceria-based supports for reverse water gas shift reaction over RuFe nanoparticles

Utilization of CO2 through the reverse water gas shift (RWGS) reaction is a promising solution in managing greenhouse gas emissions. Here, we used the RWGS reaction to evaluate the catalytic conversion of CO2 over Ru-Fe nanoparticles supported on samarium-doped ceria (SDC) support. Catalysts of different RuxFe100-x compositions (x = 100, 80, 45, 20, 0 at.%) have been studied under steady-state conditions in a packed bed reactor in the temperature range 300–800 °C to determine their catalytic activity and selectivity towards CO. The metal-support interaction (MSI) effect for SDC was evaluated to determine its promotional behavior for the RWGS reaction. The catalyst was characterized using TEM, TGA, and ICP-ES techniques. Among all investigated catalysts, Ru45Fe55/SDC (2 wt.%) displayed the overall best activity and CO selectivity. A stability test of 100 h at 650 °C confirmed an excellent stability of the Ru45Fe55/SDC catalyst. Overall, the use of Ru45Fe55/SDC (2 wt.%) is a promising catalyst in the utilization of CO2, reaching a maximum CO yield of ∼47.5% at 800 °C and 100% CO selectivity above 500 °C. Furthermore, Ru45Fe55 (2 wt.%) nanoparticles were deposited on un-doped CeO2 and doped ceria: Gd-CeO2, Y2O3-CeO2, as well as yttria-stabilized zirconia (YSZ) and carbon supports. Contrary to carbon support, all catalysts containing oxygen conducting-ceramic supports displayed 100% selectivity to CO at temperatures above 600 °C, which can be attributed to the synergistic relationship between Ru-Fe nanoparticles being promoted through the MSI and thermally induced migration of promoting ionic species (Oδ−) from oxygen conducting ceramics to the nanoparticles.

Catalytic methane combustion over Pd/ZrO2 catalysts: Effects of crystalline structure and textural properties

Zirconia supports were synthesized via a precipitation method, and the effects of crystalline structure and textural properties of ZrO2 on the methane combustion reaction were investigated using the Pd/ZrO2 catalysts. Upon increasing the digestion temperature, the crystalline structure of ZrO2 was transformed from the mixed phase (monoclinic and tetragonal) to the pure tetragonal phase, and these tetragonal ZrO2 supported Pd catalysts exhibited good hydrothermal stabilities. In addition, the BET surface areas of the tetragonal ZrO2 supports could also be tuned by varying the digestion time and calcination temperature, and the catalysts supported on ZrO2 having lower BET surface areas exhibited enhanced catalytic activities. Therefore, the Pd/ZrO2(85) catalyst, which had a tetragonal structure in addition to the smallest BET surface area, gave the optimal catalytic performance. This is likely due to larger PdO particles being formed on this ZrO2(85) support, thereby resulting in a highly reducible PdO species. Finally, variations in reducibility between the monoclinic and the tetragonal supported catalysts were clearly calculated by the density functional theory.

Ru/ZrO2 Catalysts for Transfer Hydrogenation of Levulinic Acid with Formic Acid/Formate Mixtures: Importance of Support Stability

AbstractThe transfer hydrogenation of levulinic acid to γ‐valerolactone using an equimolar mixture of formate/formic acid in water was investigated using Ru/ZrO2 catalysts. Ru/ZrO2 with < 3 wt. % loading were more active than those with higher loadings. The stability of the catalysts is of concern as the reaction occurs under acidic and hydrothermal conditions. To minimize metal leaching, the Ru/ZrO2 catalysts were prepared by a sol‐gel process to entrap the ruthenium nanoparticles within the support. Despite insignificant leaching of Ru, the used catalyst was deactivated after the first run. No agglomeration of Ru particles could be observed by TEM and XRD. However, the zirconia support had transformed from a predominantly tetragonal crystal phase to the monoclinic form. X‐ray photoelectron spectroscopy revealed a decrease in the surface Ru/Zr ratio. Doping with 0.1 wt. % SiO2 successfully stabilized the tetragonal phase of zirconia and the recycled catalyst maintained its high activity in reuse.

Precious Metal Doped Ni–Mg/Ceria–Zirconia Catalysts for Methane Conversion to Syngas by Low Temperature Bi-reforming

Low temperature (450–600 °C) bi-reforming of methane was studied over platinum (0.16 wt%) or palladium (0.13 wt%) along with nickel (1.4 wt%) and magnesium (1.0 wt%) immobilized onto a ceria–zirconia support (termed 0.16Pt and 0.13Pd). Bi-reforming studies using a feed ratio of 3:1:2 for CH4:CO2:H2O, respectively, showed that the H2:CO ratio neared the desired ratio of 2 between 500 and 600 °C for the 0.16Pt catalyst and consistent with bi-reforming at more traditional conditions. This H2:CO ratio was desirable (near 2) compared to dry or steam reforming alone, as well as to other CH4:CO2:H2O ratios. Reaction studies showed that CH4 conversion decreased with increasing GHSV (68,000–272,000 h−1) at 500 °C and increased with increasing temperature at a GHSV of 136,000 h−1 for both catalyst systems. However, the Pt-based catalyst had more consistent H2:CO ratios between 1.9 and 2.2 (T ≥ 500 °C) compared to its Pd counterpart (H2:CO ratios of 1.6–3.0), which was linked to the increased rWGS activity and stronger CO2 adsorption. This study indicates reasonable CH4 conversions and H2:CO ratio near 2 can be achieved despite operating in this low temperature regime and may enable intensified processes for the conversion of methane to value-added products.

Effect of alumina incorporation on the sulfur tolerance of the dual-catalyst aftertreatment system for reduction of nitrogen oxides under lean conditions

Presence of sulfur or sulfur additives in natural gas lead to the formation of sulfur dioxide in the exhaust stream of natural gas-fired lean-burn engines. The sulfur tolerance and hydrothermal stability of the dual-catalyst aftertreatment system comprising a reduction catalyst (palladium supported on sulfated zirconia- Pd/SZ) and an oxidation catalyst (cobalt oxide supported on ceria- CoOx/CeO2) for NOx reduction under lean-burn conditions has been tested under simulated engine exhaust conditions. Alumina-incorporated Pd/SZ (Pd/SZ(Alumina)) reduction catalyst as part of the dual catalyst bed demonstrated superior sulfur tolerance in the presence of 5 ppm SO2 in the feed. The ratio of the reduction to oxidation catalyst was also seen to affect the hydrothermal stability under wet conditions with SO2 in the reaction medium. The effect of hydrocarbon and water concentration in the reaction mixture on the sulfur tolerance of the dual-catalyst bed was studied. Increasing the reaction temperature from 450 °C to 500 °C demonstrated very stable NOx conversion and improved CH4 conversions when the dual-catalyst bed was kept on-stream for 40 h−1. X-ray photoelectron spectroscopy on samples poisoned with SO2 ex-situ revealed the deposition of surface sulfates on the Pd/SZ catalyst samples and a combination of sulfates and sulfites on the CoOx/CeO2 on treatment with SO2. The presence of alumina in the Pd/SZ sample protects the support from SO2 poisoning as the oxidation state of the zirconia support remained largely unchanged on poisoning while in the CoOx/CeO2 sample, sulfur poisoning likely affects the cerium redox cycle (Ce3+-Ce4+). 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) revealed redistribution of the octahedral and tetrahedral Al-O coordination on SO2 poisoning. Due to association with deposited sulfates on the catalyst, shielding effect on the Al nuclei resulted in an increased octahedral Al-O coordination in the poisoned sample.

The influence of the support composition and structure (MХZr1-XO2-δ) of bimetallic catalysts on the activity in methanol steam reforming

Metal oxide-stabilized zirconia supports (MxZr1-xO2-δ) with different dopants (M = Y, La, Ce) were prepared by coprecipitation method. Bimetallic CuNi and RuRh catalysts supported on MxZr1-xO2-δ were prepared by the sequential wetness impregnation method, for use in hydrogen production by methanol steam reforming. The effect of the nature and quantity of the dopant cation (M = Y, Ce) on the catalytic performance of zirconia supported metal catalysts was investigated. The activity of NiCu/YxZr1-xO2-(x/2) (x = 0.1–0.3) samples increases with an increase in yttrium concentration due to the formation of oxygen vacancies. The dependence of the catalytic activity on the ceria concentration was not monotonous. The sample containing 10% of cerium oxide showed the highest activity. The performance of a NiCu/La0·1Zr0,9O1.95 sample was compared with the performance of a Y and Ce containing samples with the same quantity of dopant cation (10%). The La doped catalyst was more active than the yttria-containing composites, but its selectivity was lower. The catalyst based on RuRh alloy differed with significantly higher activity and lower selectivity compared with NiCu samples. The selectivity of the process was not less than 99.5% for all catalysts even at the high temperatures. At the same time, the improved activity of the catalyst also results in an increase in carbon monoxide formation while the hydrogen selectivity decreases. The optimal characteristics, such as rather high hydrogen yield, good selectivity and stability were shown by the catalyst with Ce0·1Zr0·9O2-δ support.

The beneficial effects of straight open large pores in the support on steam electrolysis performance of electrode-supported solid oxide electrolysis cell

This study is aimed at improving the electrochemical performance of electrode-supported solid oxide electrolysis cells (SOECs) by optimizing the pore structure of the supports. Two planar NiO–8 mol% yttria-stabilized zirconia supports are prepared, one by the phase-inversion tape casting, and the other by conventional tape casting method using graphite as the pore former. The former contains finger-like straight open large pores, while the latter contains randomly distributed and tortuous pores. The steam electrolysis of the cells with different microstructure cathode supports is measured. The cell supported on the cathode with straight pores shows a high current density of 1.42 A cm−2 and a H2 production rate of 9.89 mL (STP) cm−2 min−1 at 1.3 V and 50 vol % humidity and 750 °C, while the cell supported on the cathode with tortuous pores shows a current density of only 0.91 A cm−2 and a H2 production rate of 6.34 mL cm−2min−1. It is concluded that the introduction of large straight open pores into the cathode support allows fast gas phase transport and thus minimizes the concentration polarization. Furthermore, the straight pores could provide better access to the reaction site (the electrode functional layer), thereby reducing the activation polarization as well.

Zirconia-supported tungstophosphoric heteropolyacid as heterogeneous acid catalyst for biodiesel production

A series of materials based on the immobilization of the 12-tungstophosphoric heteropolyacid over zirconia supports have been prepared and applied as heterogeneous acid catalysts in the esterification of palmitic acid with methanol as a model reaction for the preliminary stage of the biodiesel production. The title materials have been obtained through the sol-gel method combined with a subsequent hydrothermal treatment at mild conditions, which affords catalysts with larger porosity and higher thermal and chemical stability under the esterification reaction conditions than other preparative approaches. Generating the zirconia support by hydrolysis of an alkoxyde precursor in the presence of the heteropolyacid leads to materials with homogeneously well-dispersed clusters, as well as to an increasing contribution of the tetragonal ZrO2 crystalline phase, a decreasing size of the nanoparticles and larger microporous volumes as the loading of the Keggin-type species increases. The 12-tungstophosphoric acid retains its catalytic activity in the esterification of palmitic acid with methanol at 60°C upon immobilization over zirconia and conversions even higher than those observed under homogeneous conditions are obtained due to the active contribution of the support. The sample with a 30% mass percentage of heteropolyacid has been identified as the most efficient catalyst because it affords conversions above the 90% and shows the lower loss of activity over successive reaction runs among all of our materials. This loss of activity has been analyzed on the basis of the leaching of the catalyst and the fouling of the materials.

Zirconia Support for the Stabilization of Discharged Product of Lithium-Oxygen Battery

A demand for lithium-oxygen (Li-O­2) batteries with high storage capacity is increasing due to its application for the energy storage systems and electric vehicles. However, there are still critical bottlenecks for fabrication of a commercial Li-O2 battery. The cyclability of Li-O2 battery is very poor compared to lithium-ion battery. The main reasons for this problem are a variety of side reactions during the battery operation, which inactivate the lithium anode and cathode catalyst. To solve this problem, reversible charge/discharge reactions should be presented. During discharge reaction of Li-O2 battery, various superoxide and peroxide species, such as O2 -, LiO2 and Li2O2 appear. These nucleophilic molecules could react with electrolyte and carbon electrode. A dominant portions of side-reactions originate from this process. Therefore, we adopted a catalyst support with surface oxygen-defects to stabilize the nucleophilic molecules. In this study, we used zirconia as a support and observed that electrocatalyst with zirconia support exhibits better cyclability compared to that without support. Experimental and computational approaches are exploited to understand this phenomenon and we found that the reactivity of superoxide and peroxide species decrease with the oxygen-defective support. Our concept proposed in this study could be the clue for the design of cathode catalyst for reversible charge/discharge reactions.

Surfactant templated synthesis of porous VOx-ZrO2 catalysts for ethanol conversion to acetaldehyde

Synthesis of VOx-ZrO2 catalysts with hierarchical porosity is reported here for first time. Surface areas of prepared materials reached values up to 211m2/g with the intrinsic porosity on the border between micro- and mesopores. Physico-chemical properties and catalytic activity in ethanol oxidative dehydrogenation to acetaldehyde of these materials are compared with VOx/ZrO2 catalyst obtained by vanadia deposition on the surface of the already synthesized zirconia support with the hierarchical micro-meso-macroporosity with macrochannels of 200–500nm in size. The VOx/ZrO2 sample prepared by deposition of vanadium on hierarchical zirconia exhibits high catalytic activity (TOF=106.9h−1 at 200°C), and especially high selectivity towards acetaldehyde. Acetaldehyde selectivity under reaction conditions varied between 99 and 92%. Directly synthesized VOx-ZrO2 catalysts showed higher selectivity to acetaldehyde compared to VOx/ZrO2 catalyst obtained by vanadia deposition on the surface of the already synthesized zirconia, but significantly lower activity probably due to incorporation of vanadium during the synthesis inside the channel walls where vanadium is not accessible to the gas molecules.

Highly Selective Hydrogenation of Levulinic Acid to \u03b3-Valerolactone Over Ru/ZrO2 Catalysts

We studied the catalytic hydrogenation of levulinic acid over zirconia supported ruthenium catalysts. Four different Ru/ZrO2 catalysts were prepared by different pre-treatments and using different zirconium supports (ZrOx(OH)4−2x and ZrO2). Although the final compositions of the catalysts are the same, the pre-treatments strongly affect catalytic activity. Remarkably, one of the catalysts gave >99% yield of γ-valerolactone under mild conditions. This catalyst was also robust, and could be recycled at least four times without any loss in activity or selectivity. The activity is attributed to the presence of small ruthenium particles together with acidic sites on the catalyst.Graphical Pre-treatment changes the performance for ruthenium nanoparticles on zirconia supports, giving TONs over 3500 in the hydrogenation of levulinic acid to γ-valerolactone with over 99% yield.

Electrochemical Nanoreactors of Mixed Metal-Oxide-Supported Noble- Metal-Nanoparticles for Efficient Oxidation of Dimethyl Ether and Ethanol

Noble metal-based (e.g. Pt, Pd, PtRu or PtSn) nanoparticles have been recognized as the most active catalytic systems towards oxidation of small organic molecules (ethanol, dimethyl ether) at low and moderate temperatures. Although all of them can be ideally oxidized to carbon dioxide, their actual electrooxidation mechanisms can significantly differ and may result in distinct electrocatalytic efficiencies and final reaction products. There is a need to develop functionalized electrocatalytic systems exhibiting specific reactivity and capability to induce the slow reaction steps at ambient conditions. Platinum based nanoparticles are readily poisoned by the strongly adsorbed CO-type intermediate species requiring fairly high overpotentials for their removal. For example, to enhance activity of Pt-based catalysts toward the ethanol oxidation, additional metal or metal oxide nanostructures (rhodium, iridium, tin or molybdenum and tungsten oxides) will be intentionally introduced to the electrocatalytic interface. We are also going to demonstrate that catalytic activity of platinum-based nanoparticles can be significantly enhanced through their interfacial modification with ultra-thin monolayer-type films of mixed-metal-oxo-species of titanium, cerium, zirconium or molybdenum and tungsten in their simple and polyoxometallate forms Remarkable increases of electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account possibility of specific interactions of noble metals with transition metal oxide species as well as existence of active hydroxyl groups in the vicinity of catalytic noble metal sites. It is noteworthy that certain metal and mixed-metal oxides can generate –OH groups at low potentials: they induce oxidation of passivating CO adsorbates (e.g. on Pt) or breaking C-H bonds (e.g. during oxidation of methanol or dimethyl ether). When combined with dispersed Rh or Ir, immobilized in the organized manner in the vicinity Pt sites, they tend to weaken C-C bonds during ethanol oxidation. We will also demonstrate that deposition of Pt and bimetallic PtRu nanoparticles onto nanostructured zirconia support results in the enhancement of their catalytic activities toward electrooxidation of dimethyl ether, another possible alternate fuel. Once more, it is reasonable to expect that the abilities of zirconia to facilitate proton mobility and to donate active hydroxyl groups at the electrocatalytic interface in acid media are responsible for the observed enhanced catalytic effects. Specific interactions with platinum and ruthenium components are also postulated.

Low temperature-high selectivity carbon monoxide methanation over yttria-stabilized zirconia-supported Pt nanoparticles

The high purity hydrogen, free from any traces of carbon monoxide is crucial for the efficient operation of hydrogen proton exchange membrane fuel cells (PEMFCs). In this study we report the low temperature (25–120 °C) carbon monoxide methanation in hydrogen-rich streams over 1 wt. % Pt nanoparticles supported on yttria-stabilized zirconia (YSZ) support. Pt nanoparticles with four average particle sizes 1.9, 3.0, 4.4 and 6.7 nm are investigated. The turnover frequency at 30 °C of CO methanation reaction rate doubles with decreasing Pt mean particle size from 6.7 to 1.9 nm. The high activity of Pt/YSZ catalyst is due to the backspillover of ionic species Oδ− from YSZ support to Pt active sites. Under the reaction conditions, Oδ− forms an “effective” double layer at the catalyst surface that weakens CO (electron acceptor) adsorption and strengthens H2 (electron donor) adsorption bonds and this effect becomes more pronounced as nanoparticle size decreases.

Low-temperature carbon monoxide oxidation over zirconia-supported CuO–CeO2 catalysts: Effect of zirconia support properties

A study was conducted to investigate the effect of the preparation route of ZrO2 in CuO–CeO2/ZrO2 catalysts for the oxidation of carbon monoxide at low temperature (COX). Four ZrO2 supports were synthetized via either type sol-gel methodology or precipitation. The final Cu-Ce-Zr oxide catalysts were prepared by incipient wetness co-impregnation with copper and cerium solutions (with a loading of 6wt% of CuO and 20wt% of CeO2). The catalyst crystalline phases, texture and active species reducibility were determined by XRD, N2 physisorption at −196°C and H2-TPR, respectively; meanwhile the surface composition and copper-cerium electronic states were studied by XPS. The catalytic activity was evaluated in the oxidation of CO to CO2, in the 40–215°C temperature range. Catalytic results evidenced that the samples prepared by a sol-gel methodology showed, after the impregnation, a severe decrease of specific surface area and pore volume attributable to a wide degree of pore blockage caused by the presence of metal oxide particles and a collapse of the structure partially burying the active sites. A simple co-impregnation of a zirconia support, obtained through facile and fast precipitation, provided instead a catalyst with very good redox properties and high dispersion of the active phases, which completely oxidizes CO in the range 115–215°C with T50 of 65°C. This higher observed activity was ascribed to the formation of a larger fraction of highly dispersed and easily reducible Cu species and ceria nanocrystallites, mainly present as Ce(IV), with an average size of 5nm.

A platinum catalyst deposited on a zirconia support for the design of lithium-oxygen batteries with enhanced cycling ability.

A platinum catalyst supported on zirconia is proposed as a cathode in lithium-oxygen batteries. Experimental and theoretical studies show that zirconia suppresses the side-reactions of the intermediate (O2-) and the final product (Li2O2) by the stabilization of their reactivity. Thus, it is able to enhance the reversibility during charge/discharge in lithium-oxygen batteries.

Morphology effect of zirconia support on the catalytic performance of supported Ni catalysts for dry reforming of methane

An immature pinecone shaped hierarchically structured zirconia (ZrO2-ipch) and a cobblestone-like zirconia nanoparticulate (ZrO2-cs), both with the monoclinic phase (m-phase), were synthesized by the facile hydrothermal method and used as the support for a Ni catalyst for the dry reforming of methane (DRM) with CO2. ZrO2-ipch is a much better support than ZrO2-cs and the traditional ZrO2 irregular particles made by a simple precipitation method (ZrO2-ip). The supported Ni catalyst on ZrO2-ipch (Ni/ZrO2-ipch) exhibited outstanding catalytic activity and coke-resistant stability compared to the ones on ZrO2-cs (Ni/ZrO2-cs) and ZrO2-ip (Ni/ZrO2-ip). Ni/ZrO2-ip exhibited the worst catalytic performance. The origin of the significantly enhanced catalytic performance was revealed by characterization including XRD, N2 adsorption measurement (BET), TEM, H2-TPR, CO chemisorption, CO2-TPD, XPS and TGA. The superior catalytic activity of Ni/ZrO2-ipch to Ni/ZrO2-cs or Ni/ZrO2-ip was ascribed to a higher Ni dispersion, increased reducibility, enhanced oxygen mobility, and more basic sites with a higher strength, which were due to the unique hierarchically structural morphology of the ZrO2-ipch support. Ni/ZrO2-ipch exhibited better stability for the DRM reaction than Ni/ZrO2-ip, which was ascribed to its higher resistance to Ni sintering due to a strengthened metal-support interaction and the confinement effect of the mesopores and coke deposition resistance. The higher coking resistance of Ni/ZrO2-ipch for the DRM reaction in comparison with Ni/ZrO2-ip orignated from the coke-removalability of the higher amount of lattice oxygen and more basic sites, confirmed by XPS and CO2-TPD analysis, and the stabilized Ni on the Ni/ZrO2-ipch catalyst by the confinement effect of the mesopores of the hierarchical ZrO2-ipch support. The superior catalytic performance and coking resistance of the Ni/ZrO2-ipch catalyst makes it a promising candidate for synthesis gas production from the DRM reaction.

Direct methane operation of a micro-tubular solid oxide fuel cell with a porous zirconia support

A novel micro-tubular solid oxide fuel cell (SOFC) design with an inert support was proposed for operation on direct hydrocarbon fuels with an improved stability. In this design, the inert support also serves as a diffusion barrier between the fuel stream and Ni cermet anode. The barrier effect leads to higher local steam to carbon ratios in the anode, thus inhibiting carbon deposition. To demonstrate this concept, we fabricated micro-tubular SOFCs with a porous yttria-stabilized zirconia (YSZ) support. Ni, Ni-scandia-stabilized zirconia (ScSZ), ScSZ, strontium-doped lanthanum manganite (LSM)–ScSZ, and LSM were used as the anode current collector, anode, electrolyte, cathode, and cathode current collector, respectively. Good electrochemical performance was achieved with hydrogen and methane fuels in a temperature range 600–750 °C. Continuous cell operation on direct methane fuel for >40 h at 750 °C under moderate current densities delivered stable voltage without any evident performance degradation due to carbon deposition. The absence of carbon deposition on the anode and anode current collector layers was also confirmed by scanning electron microscope images and energy-dispersive X-ray spectra. We further discuss oxidation mechanism of the direct methane fuel and removal of the carbon possibly formed in the anodic layers during stability testing.

Surface Spectroscopy on UHV-Grown and Technological Ni-ZrO2 Reforming Catalysts: From UHV to Operando Conditions.

Ni nanoparticles supported on ZrO2 are a prototypical system for reforming catalysis converting methane to synthesis gas. Herein, we examine this catalyst on a fundamental level using a 2-fold approach employing industrial-grade catalysts as well as surface science based model catalysts. In both cases we examine the atomic (HRTEM/XRD/LEED) and electronic (XPS) structure, as well as the adsorption properties (FTIR/PM-IRAS), with emphasis on in situ/operando studies under atmospheric pressure conditions. For technological Ni–ZrO2 the rather large Ni nanoparticles (about 20 nm diameter) were evenly distributed over the monoclinic zirconia support. In situ FTIR spectroscopy and ex situ XRD revealed that even upon H2 exposure at 673 K no full reduction of the nickel surface was achieved. CO adsorbed reversibly on metallic and oxidic Ni sites but no CO dissociation was observed at room temperature, most likely because the Ni particle edges/steps comprised Ni oxide. CO desorption temperatures were in line with single crystal data, due to the large size of the nanoparticles. During methane dry reforming at 873 K carbon species were deposited on the Ni surface within the first 3 h but the CH4 and CO2 conversion hardly changed even during 24 h. Post reaction TEM and TPO suggest the formation of graphitic and whisker-type carbon that do not significantly block the Ni surface but rather physically block the tube reactor. Reverse water gas shift decreased the H2/CO ratio. Operando studies of methane steam reforming, simultaneously recording FTIR and MS data, detected activated CH4 (CH3 and CH2), activated water (OH), as well as different bidentate (bi)carbonate species, with the latter being involved in the water gas shift side reaction. Surface science Ni–ZrO2 model catalysts were prepared by first growing an ultrathin “trilayer” (O–Zr–O) ZrO2 support on an Pd3Zr alloy substrate, and subsequently depositing Ni, with the process being monitored by XPS and LEED. Apart from the trilayer oxide, there is a small fraction of ZrO2 clusters with more bulk-like properties. When CO was adsorbed on the (fully metallic) Ni particles at pressures up to 100 mbar, both PM-IRAS and XPS indicated CO dissociation around room temperature and blocking of the Ni surface by carbon (note that on the partially oxidized technological Ni particles, CO dissociation was absent). The Ni nanoparticles were stable up to 550 K but annealing to higher temperatures induced Ni migration through the ultrathin ZrO2 support into the Pd3Zr alloy. Both approaches have their benefits and limitations but enable us to address specific questions on a molecular level.

Role of the Three-Phase Boundary of the Platinum-Support Interface in Catalysis: A Model Catalyst Kinetic Study.

A series of microstructured, supported platinum (Pt) catalyst films (supported on single-crystal yttria-stabilized zirconia) and an appropriate Pt catalyst reference system (supported on single-crystal alumina) were fabricated using pulsed laser deposition and ion-beam etching. The thin films exhibit area-specific lengths of the three-phase boundary (length of three-phase boundary between the Pt, support, and gas phase divided by the superficial area of the sample) that vary over 4 orders of magnitude from 4.5 × 102 to 4.9 × 106 m m–2, equivalent to structural length scales of 0.2 μm to approximately 9000 μm. The catalyst films have been characterized using X-ray diffraction, atomic force microscopy, high-resolution scanning electron microscopy, and catalytic activity tests employing the carbon monoxide oxidation reaction. When Pt is supported on yttria-stabilized zirconia, the reaction rate clearly depends upon the area-specific length of the three-phase boundary, l(tpb). A similar relationship is not observed when Pt is supported on alumina. We suggest that the presence of the three-phase boundary provides an extra channel of oxygen supply to the Pt through diffusion in or on the yttria-stabilized zirconia support coupled with surface diffusion across the Pt.