Pd-Rh Catalysts Research Articles

PdRh-Sensitized Iron Oxide Ultrathin Film Sensors and Mechanistic Investigation by Operando TEM and DFT Calculation.

Metal oxide semiconductor (MOS) thin films are of critical importance to both fundamental research and practical applications of gas sensors. Herein, a high-performance H2 sensor based on palladium (Pd) and rhodium (Rh) co-functionalized Fe2 O3 films with an ultrathin thickness of 8.9nm deposited by using atomic layer deposition is reported. The sensor delivers an exceptional response of 105.9 toward 10ppm H2 at 230 °C, as well as high selectivity, immunity to humidity, and low detection limit (43 ppb), which are superior to the reported MOS sensors. Importantly, the Fe2 O3 film sensor under dynamic H2 detection is for the first time observed by operando transmission electron microscopy, which provides deterministic evidence for structure evolution of MOS during sensing reactions. To further reveal the sensing mechanism, density functional theory calculations are performed to elucidate the sensitization effect of PdRh catalysts. Mechanistic studies suggest that Pd promotes the adsorption and dissociation of H2 to generate PdHx , while Rh promotes the dissociation of oxygen adsorbed on the surface, thereby jointly promoting the redox reactions on the films. A wireless H2 detection system is also successfully demonstrated using the thin film sensors, certifying a great potential of the strategy to practical sensors.

Improved methanol electro-oxidation reaction on PdRh-PVP/C electrodes

Here we report on the organometallic synthesis of polyvinylpyrrolidone (PVP)-stabilized palladium-rhodium nanostructures that display high electrochemical properties when used as carbon-supported electrodes (PdxRh1-x-PVP/C) for methanol oxidation reaction (MOR). These nanostructures were synthesized by hydrogenation of the tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) and tris(allyl) rhodium (Rh(η3-C3H5)3) complexes, in tetrahydrofuran (THF) under mild reaction conditions (room temperature and 3 bar H2) and in the presence of PVP as stabilizer. The influence of methanol concentration (0.5, 1.0 and 2.0 M) as well as different scan rates (5–100 mV s−1) was evaluated to determine changes in the stability and catalytic activity. The organometallic approach and the use of PVP for the preparation of PdRh electrode materials promoted the formation of highly dispersed nanostructures, that led to a remarkably enhanced methanol electro-oxidation in alkaline medium. This high catalytic behavior can reasonably arise from a synergistic effect between Pd and Rh metals as, under the applied conditions, Rh is expected to enhance the Pd ability to oxidize methanol to CO2 (oxophilic character) as well as the catalyst stability. From all the evaluated electrode materials, Pd8Rh2-PVP/C electrode showed the highest mass activity at high methanol concentration (2.0 M) and low scan rate (10 mV s−1). This catalyst showed a performance up to 26 times higher than that of Pd-PVP/C and interestingly an electroactivity superior to that of previously reported PdRh catalysts.

Synthesis of an advanced Pd-Rh bimetallic catalyst for gasoline engine emission control

• An advanced Pd-Rh bimetallic catalyst was obtained by adjusting the Rh distribution. • The aggregation of Pd upon aging is suppressed owing to the diffusion barrier effect. • More PdO and Rh 2 O 3 species are kept due to the synergistic effect between Pd and Rh. • After aging, the modified catalyst shows superior catalytic conversion efficiency. In recent years, the dramatic increase of the prices of Pd and Rh brings about increasingly heavier cost pressure for the exhaust gas after-treatment industry, thus it is of great significance to develop advanced Pd-Rh bimetallic catalysts with low metal loading. In this work, on the basis of conventional catalyst composed of Al 2 O 3 -supported Pd and CZ (ceria-zirconia-based oxide)-supported Rh, an advanced Pd-Rh catalyst was obtained by adjusting the synthesis procedure. That was, a small portion of Rh was co-impregnated with Pd on Al 2 O 3 , while the other portion was still supported on CZ. In this way, the co-impregnated Rh may act as an auxiliary for Pd/Al 2 O 3 component. As revealed by the various characterization results, the agglomeration of Pd species is well suppressed, and larger amount of PdO species is maintained for the modified catalyst after hydrothermal aging treatment. On the other hand, the state of Rh species also appears to be somewhat modified. Consequently, after hydrothermal aging treatment, the modified catalyst behaves much better for the catalytic conversions of CO, HC and NO.

Some features of alcohols electrooxidation process on Pd, Rh and PdRh catalysts

PdRh/C catalyst and electrodeposited Pd, Rh and PdRh particles on glassy carbon electrode were studied in methanol and ethanol oxidation reactions in comparison with Pt-containing catalysts. Some features of this process have been described: (1) the appearance of oxidation peak in acidic and neutral media is possible only when polarization on CV is prolonged to hydrogen adsorption region where the surface OH-group formation occurred and surface oxides are reduced; (2) the CV-responses of model systems (electrodeposited Pd, Rh, PdRh) corresponding to alcohol oxidation increase and deform even during relaxation period when polarization is switched off; (3) experiments with addition of corresponding aldehydes (formaldehyde, acetaldehyde) to alcohol solution with stabilized CV responses showed that only increase in currents but not deformation of CV occurred. The addition of acid residue did not change the response noticeably. The formation of the same electroactive species, gem-diolate, for both alcohol and aldehyde electrooxidation mechanisms was supposed.

Effect of operating conditions and TWC parameters on emissions characteristics of a China-VI stoichiometric natural gas engine

This study involved conducting an experimental and numerical investigation on the effects of the air-to-fuel ratio (AFR), engine speed, and engine load on the inlet gas component of a three-way catalyst (TWC) and on the effects of noble metal loading, noble metal ratio, and carrier pore density on the emission conversion efficiency. The results showed that AFR can significantly affect the raw emissions of NOx and THC, and better emission conversion efficiency of a TWC can be reached when AFR is controlled between 0.995 to 1. Compared with engine speed, engine load has a relatively small effect on exhaust temperature but greatly affects the flow velocity and NOx and THC emissions. Increasing the content of Pt in the catalyst can improve the THC conversion efficiency. For low Pt and Pd-Rh catalysts, the THC conversion effect is significantly deteriorated. The content of Rh affects the NOx conversion, and NOx conversion efficiency at high speeds is significantly reduced when Rh content is reduced. Higher carrier pore density can slightly improve the catalytic reaction rate and emission conversion efficiency at high engine speeds. However, high conversion efficiency can be maintained even after aging.

Reversible hydrogenation and dehydrogenation of N-ethylcarbazole over bimetallic Pd-Rh catalyst for hydrogen storage

• Develop a bimetallic Pd-Rh catalyst for both hydrogenation and dehydrogenation reaction. • The catalyst structure that Rh clusters isolated by the surrounding Pd atoms was proved by various ex- and in-situ characterizations. • The mechanism of NEC hydrogenation was proposed and verified by DFT calculations. Hydrogen is a promising candidate to replace the exhausting fossil fuels. However, it is still a big challenge for hydrogen storage. Liquid organic hydrogen carrier (LOHC), which possesses high safety and purity, easy transport and reversibility, is highly attractive for hydrogen storage. Herein we report a new reusable bimetallic Pd-Rh nanoparticles (NPs) catalyst with synergy effect, boosting both the hydrogenation of N-ethylcarbazole (NEC) and dehydrogenation of dodecahydro-N-ethylcarbazole (12H-NEC), which allows multiple cycles of reversible high-weight% hydrogen storage and uptake in short reaction time. Various ex- and in-situ characterizations evidenced that the superior activity was attributed to the bimetallic Pd-Rh NPs cluster structure on the surface of γ-Al 2 O 3 support. The mechanism of NEC hydrogenation over the Pd-Rh catalyst was proposed and verified by density functional theory (DFT) calculations. Such a strategy can be extended to other LOHCs and sheds light onto the development of LOHCs as energy vectors.

Effect of Operating Conditions and TWC Parameters on Emissions Characteristics of a Stoichiometric Natural Gas Engine

This study involved conducting an experimental and numerical investigation on the effects of the air-to-fuel ratio (AFR), engine speed, and engine load on the inlet gas component of a three-way catalyst (TWC) and on the effects of noble metal loading, noble metal ratio, and carrier pore density on the emission conversion efficiency. The results showed that AFR can significantly affect the raw emissions of NOx and total hydrocarbon (THC), and better emission conversion efficiency of a TWC can be reached when AFR is controlled between 0.995 to 1. Compared with engine speed, engine load has a relatively small effect on exhaust temperature but greatly affects the flow velocity and NOx and THC emissions. Increasing the content of Pt in the catalyst can improve the THC conversion efficiency. For low Pt and Pd-Rh catalysts, the THC conversion effect is significantly deteriorated. The content of Rh affects the NOx conversion, and NOx conversion efficiency at high speeds is significantly reduced when Rh content is reduced. Higher carrier pore density can slightly improve the catalytic reaction rate and emission conversion efficiency at high engine speeds. However, high conversion efficiency can be maintained even after aging.

Synthesis of palladium-rhodium bimetallic nanoparticles for formic acid dehydrogenation

Herein, we report for the first time the synthesis of preformed bimetallic Pd-Rh nanoparticles with different Pd:Rh ratios (nominal molar ratio: 80–20, 60–40, 40–60, 20–80) and the corresponding Pd and Rh monometallic ones by sol immobilization using polyvinyl alcohol (PVA) as protecting agent and NaBH4 as reducing agent, using carbon nanofibers with high graphitization degree (HHT) as the desired support. The synthesized catalysts were characterized by means of Transmission Electron Microscopy (TEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES). TEM shows that the average particle size of the Pd-Rh nanoparticles is the range of 3–4 nm, with the presence of few large agglomerated nanoparticles. For bimetallic catalysts, EDX-STEM analysis of individual nanoparticles demonstrated the presence of random-alloyed nanoparticles even in all cases Rh content is lower than the nominal one (calculated Pd:Rh molar ratio: 90–10, 69–31, 49–51, 40–60). The catalytic performance of the Pd-Rh catalysts was evaluated in the liquid phase dehydrogenation of formic acid to H2. It was found that Pd-Rh molar ratio strongly influences the catalytic performance. Pd-rich catalysts were more active than Rh-rich ones, with the highest activity observed for Pd90:Rh10 (1792 h−1), whereas Pd69:Rh31 (921 h−1) resulted the most stable during recycling tests. Finally, Pd90:Rh10 was chosen as a representative sample for the liquid-phase hydrogenation of muconic acid using formic acid as hydrogen donor, showing good yield to adipic acid.

Interaction of Pd and Rh with ZrCeYLaO2 support during thermal aging and its effect on the CO oxidation activity

A commercial ceria–zirconia composition modified with yttrium and lanthanum oxides was studied as a support for palladium- and rhodium-containing three-way catalysts. The most attention was paid to an interaction of the supported active metals with the support under high-temperature conditions. It was found that both the metals affect noticeably the porous structure of the support and the loading of metal plays a role in this process. Moreover, oxygen storage capacity of the oxide composition was also influenced by the supported metals. Hydrothermal aging of the metal-loaded samples has decreased the oxygen storage capacity values in more than 2 times. The thermal stability of the monometallic samples was compared with bimetallic Pd–Rh catalysts prepared using a “single-source precursor” approach. The experiments were performed under prompt thermal aging conditions. It was found that monometallic Pd-only samples and bimetallic Pd–Rh samples exhibit excellent stability, while monometallic Rh-only catalysts undergo deactivation being heated up to 800 °C due to diffusion of rhodium into the bulk of the support. All the samples were additionally characterized by a diffuse reflectance UV–vis spectroscopy and a testing reaction of ethane hydrogenolysis. According to the results obtained, the character of the metal-support interaction was found to be strongly affected by the catalyst’s composition. Application of the bimetallic Pd–Rh particles of alloyed type was shown to result in the preferable location of the active components on the support’s surface, thus facilitating high activity and stability of the catalyst.

Effect of La Addition on the Performance of Three-Way Catalysts Containing Palladium and Rhodium

Three-way catalysts containing palladium and/or rhodium were prepared using γ­Al2O3 doped with lanthanum oxide as a support. All the samples were obtained by an incipient wetness impregnation of the support with an aqueous solution of nitrates. In order to investigate the metal–support interaction, the support was additionally calcined at 800 °C before the impregnation procedure. Characterization of the support thermally treated within a range of 600–1000 °C by low-temperature nitrogen absorption, X-ray diffraction analysis, and electron paramagnetic resonance spectroscopy has revealed that the treatment conditions strongly affect the textural properties, the phase composition and the concentration of electron-donor sites on the surface of the support. Deposition of metals by the impregnation of initial support with a joint solution of Pd and Rh nitrates has led to formation of small Pd–Rh alloyed nanoparticles with strong metal–metal interaction, which was confirmed by a testing reaction of ethane hydrogenolysis. No alloy formation was observed in the case of mechanical mixing of the separately prepared Pd-only and Rh-only catalysts as well as in the case of preliminary calcined support impregnated with a joint solution of Pd and Rh nitrates. Bimetallic Pd–Rh catalyst of alloyed type was shown to be the most promising in terms of catalytic performance and thermal stability.

New Trends in Automotive Exhaust Gas Purification Materials: Improvement of the Support against Stability of the Active Components

Lanthanum and zirconium oxides are well known to be applied as an additive to improve the properties of the commercial alumina. Such modified aluminas are widely used in different catalytic fields including three-way catalysis. In the present research we have paid attention to the possible effects of the doping on the catalytic performance and stability of bimetallic Pd-Rh catalysts. The samples were prepared via an incipient wetness impregnation of the commercial supports, tested in CO oxidation under prompt thermal aging regime and characterized by physicochemical methods.

Characterisation and modelling of the reactions in a three-way PdRh catalyst in the exhaust gas from an ethanol-fuelled spark-ignition engine

This work investigated and modelled the performance and characteristics of automotive catalytic converter formulations when subjected to a synthetic exhaust gas mixture representative of that emitted by an ethanol-fuelled spark-ignition engine. A synthetic gas reactor and exhaust gas emission analyser were used to assess the catalytic activity, the products distribution and chemical mechanisms exhibited by a commercial catalytic converter formulation when exposed to ethanol containing gas mixtures. A commercially available after-treatment modelling platform named Axisuite was used to simulate the catalyst performance. This software was used to assign the pre-exponential frequency factor and activation energy variables within the rate equations. A set of global kinetic coefficients for the relevant reactions was established and is reported.

CO2 conversion to syngas through the steam-biogas reforming process

The steam-biogas reforming (SBR) process to convert biogas to a high hydrogen syngas was studied experimentally and using Aspen Plus simulations. An integrated renewable power generation system where the SBR process was coupled with a Solid Oxide Fuel Cell (SOFC) was studied using the Aspen Plus model. The experimental work was conducted over a metal-foam-coated [Pd(7)-Rh(1)]/[CeZrO2(25)-Al2O3(75)] catalyst in a Heat Exchanger Platform (HEP) reactor. SBR simulations were conducted for biogas feeds with CH4/CO2 ratios of 40/60, 50/50 and 60/40 at S/C ratios of 1.00–2.00 over a temperature range of 873–1123 K. The experimental data show that positive CO2 conversion was attainable only at temperatures higher than 1073 K, although the equilibrium based simulation predicts positive CO2 conversion through most of the operating temperature range. Energy efficiency of the overall system was approximately 40% at temperatures of 948 K and above. Coke formation over the Pd-Rh catalyst was estimated to be 1.05–2.88% of the carbon input to the system. Fresh and used catalysts were characterized by BET adsorption, porosimetry, CO chemisorption and Scanning Electron Microscopy. The results show that the proposed system can provide a viable approach to utilizing distributed renewable methane resources for localized power generation.

Synthesis and study of Pd-Rh alloy nanoparticles and alumina-supported low-content Pd-Rh catalysts for CO oxidation

Palladium and rhodium are known to be partly miscible metals. In present work, the peculiarities of coordination compound [Pd(NH3)4]3[Rh(NO2)6]2 decomposition with formation of nanosized solid solutions under different atmospheres were studied by means of thermal gravimetry. Formation of alloy nanoparticles were confirmed by powder X-ray diffraction analysis, scanning and transmission electron microscopies. A bimetallic Pd-Rh/alumina catalyst was prepared by incipient wetness impregnation using coordination compound [Pd(NH3)4]3[Rh(NO2)6]2 as a precursor. Monometallic reference samples were obtained using [Pd(NH3)4](NO3)2 and Na3[Rh(NO2)6], correspondingly. Catalytic performance and stability of the catalysts were examined in a model reaction of CO oxidation in a prompt thermal aging regime. The environment of precursor decomposition was shown to affect noticeably both the initial activity and stability of the samples in the studied reaction. Reductive atmosphere in comparison with inert and oxidative ones facilitates the formation of the smallest active component species, which demonstrate highest initial activity but worst stability.

Effects of CeZrO2–Al2O3 support composition of metal-foam-coated Pd–Rh catalysts for the steam-biogas reforming reaction

Biogas conversion to syngas by steam reforming was studied over metal-foam-coated Pd–Rh catalysts with variable CeZrO2–Al2O3 support compositions. Catalysts with a higher CeZrO2/Al2O3 ratio exhibited greater CH4 and CO2 conversions and higher H2/CH4 yields, while displaying lower H2/CO ratios and reduced coke formation. Catalytic stability tests over 200h showed CH4 and CO2 conversion rates of 93–97% and 0–5%, respectively. CeZrO2-modification of the catalyst leads to reduction in the BET area and metal dispersion. Sintering was observed in used catalysts; however there was no clear correlation between the extent of morphological deterioration and the CeZrO2/Al2O3 ratio of the catalyst support.

Effect of Alumina Phase Transformation on Stability of Low-Loaded Pd-Rh Catalysts for CO Oxidation

Bimetallic Pd-Rh catalysts with precious metal loading of 0.2 wt% was prepared by incipient wetness impregnation of the support (γ-Al2O3 or δ-Al2O3) with dual complex salt [Pd(NH3)4]3 [Rh(NO2)6]2. Monometallic Pd and Rh catalysts as well as its mechanical mixture were used as the reference samples. All samples were exposed for in situ prompt thermal aging procedure, and characterized by EPR spectroscopy, UV–Vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. The nature of the support was found to have strong effect on high temperature stability of the samples. δ-Al2O3 having non-uniform phase structure due to presence of θ-Al2O3 and α-Al2O3 traces causes the concentrating of rhodium near the interphase boundary, thus changing the mechanism of Rh3+ bulk diffusion if compare with γ-Al2O3. No noticeable anchoring effects were observed for bimetallic Pd-Rh samples neither in terms of Rh bulk diffusion nor with regard to the Pd sintering. It has been found experimentally that phase transformation of γ-Al2O3 at high temperatures does not play dramatic role for the deactivation of bimetallic Pd-Rh active species anchored to the electron-donor site of the support.

Bimetallic Pd–Rh nanoparticles onto reduced graphene oxide nanosheets as electrocatalysts for methanol oxidation

Abstract An advanced electrode for methanol oxidation in alkali medium, comprising Pd–Rh nanoparticles and reduced graphene oxide (r-GO) sheets, has been successfully synthesized by pulse microwave polyol (MP) technique. The MP technique is capable of depositing Pd–Rh nanoparticles on the surface of r-GO sheets under microwave irradiation. The atomic ratio of Pd:Rh is chosen as a crucial factor to examine the catalytic activity toward H adsorption/desorption and methanol oxidation. The Pd–Rh catalyst (with Pd:Rh ratio = 75:25) displays the highest alloying degree (~ 41.5%), favoring the formation of Pd–Rh pairs in the binary crystals. On the Pd–Rh catalyst, the Pd site acts as major contributor toward hydrogen adsorption and methanol oxidation, whereas the neighboring Rh site serves as a promoting center to strip oxygenate species (e.g., Pd − (CO) ads sites). The enhanced catalytic activity on the Pd–Rh catalyst is attributed to the fact that the binary catalyst possesses a high alloying degree, leading to high catalytic coverage, high-level of CO tolerance, and superior cyclic durability. The Tafel slope of as-made Pd–Rh catalyst could reach as low as 63 mV dec − 1 among the electrodes, based on the analysis of Tafel plots. The Pd–Rh catalyst also exhibits both lower equivalent diffusion resistance and equivalent series resistance, as compared with pure Pd one.

Steam-biogas reforming over a metal-foam-coated (Pd–Rh)/(CeZrO2–Al2O3) catalyst compared with pellet type alumina-supported Ru and Ni catalysts

Metal-foam-coated 0.09wt% [Pd(7)–Rh(1)]/(CeZrO2–Al2O3) catalyst was compared with commercially available alumina-supported 8.0wt% Ru and 13.0wt% Ni catalysts for steam reforming of a model biogas. The experiments were conducted in a tubular reactor using a feedstock with the steam-to-methane ratio of 1.50 at 1atm pressure and 20,000h−1 GHSV. CH4 reforming reactions by steam alone and by CO2 alone were also studied over these catalysts in order to explain the catalyst activity. The Pd–Rh catalyst is superior in a number of ways, including better activity for CH4 and CO2 conversion in spite of much less ingredient loading per unit volume. The Pd–Rh catalyst also exhibited higher syngas production index [(H2 LHV+CO LHV)/(Feed CH4 LHV)], lower syngas H2/CO ratio and coke formation. The results were supported by BET measurement, CO chemisorption and electron microscopy. The experimental results demonstrate that the metal-foam-coated Pd–Rh/(CeZrO2–Al2O3) catalyst performs better for syngas production through steam-biogas reforming compared to the Ru and Ni catalysts.

Deposition of binary Pd–Rh catalysts on nanostructured carbon supports for non-enzymatic glucose oxidation

An efficient microwave-assisted (MA) method was adopted to synthesize binary Pd–Rh catalysts onto carbon powders (CPs), carbon nanotubes (CNTs), and reduced graphene oxide (GO) sheets for glucose oxidation reaction (GOR) in alkali electrolyte. The MA method is capable of preparing binary Pd–Rh alloys with well-defined atomic ratio of Pd:Rh (75:25) under a pulse microwave irradiation. It is observed that the catalyst electrode, consisted of well-dispersed Pd–Rh alloy and CNT framework, offers higher catalytic activity and superior cyclic stability, as compared to the other supports (i.e., CPs and GO sheets). This improved catalytic activity is ascribed to the fact that the Pd–Rh/CNT catalyst creates a unique three-dimensional framework, providing one-dimensional charge transfer and more active sites for the GOR. Analyzed by the Tafel plots, the Pd–Rh/CNT catalyst delivers not only the high exchange current but also the low Tafel slope, confirming the superior catalytic activity and excellent reaction kinetics toward the GOR. Accordingly, this study proves that the selection of carbon support plays a crucial role in affecting the catalytic activity and cycleability of GOR.

Exergetic study of catalytic steam reforming of bio-ethanol over Pd–Rh/CeO2 with hydrogen purification in a membrane reactor

An exergy analysis of the ethanol steam reforming process using a Pd–Rh catalyst supported on ceria in a Pd–Ag membrane reactor has been performed based on experimental data. Both chemical and physical exergy for chemical species together with thermodynamic loss calculations have been considered to evaluate the exergy efficiency of the process. The system has been studied under different operational conditions in terms of temperature (873–923 K), pressure (4–12 bar), and feed flow rate using a water–ethanol mixture of 50–50% volume (S/C = 1.6). A significant exergy efficiency dependency on the fuel flow rate and pressure has been encountered when considering the heat loss from the reactor. The best results regarding the exergetic and thermal efficiency have been obtained at high flow rates and 8–12 bar, which has been attributed to the higher hydrogen permeation through the membrane. The chemical reactions involved in the reforming process and the heat losses are the main sources of exergy destruction.