Pd-only Catalysts Research Articles

The influence of κ-Ce2Zr2O8 content on three-way catalytic performance

The CeO2-ZrO2 based oxide (CZ) with κappa-phase CZ (κ-Ce2Zr2O8) has been examined as a promising alternative to the traditional CZ used in the three-way catalysts (TWCs). However, forming pure κ-Ce2Zr2O8 requires huge energy, and it remains obscure whether more κ-Ce2Zr2O8 is better. Herein we synthesized a series of CZ with increasing κ-Ce2Zr2O8 content by increasing reduction time, as well as the supported Pd-only catalysts, for investigating the influence of κ-Ce2Zr2O8 content on TWC performance. The results evidenced that κ-Ce2Zr2O8 started to form at surface and gradually covered the whole surface. The crystal interface between κ-Ce2Zr2O8 and the cubic-phase matrix, was thus first increased but then decreased as the κ-Ce2Zr2O8 content increased from 0 % to 34.3 %. Interestingly, the presence of 19.8 % κ-Ce2Zr2O8 could dramatically improve the reducibility of CZ, making reduction temperature decrease from 598 ℃ to 446 ℃, however, further improvement was not achieved by increasing κ-Ce2Zr2O8. More importantly, the Pd-CZ interaction strength was found to be positively corelated with the amount of the mentioned crystal interface. The catalytic activity followed the similar variation tendency with the Pd-CZ interaction. Among them, Pd/CZ1 with 19.8 % κ-Ce2Zr2O8 showed the optimal catalytic performance. Based on the analyses, the improved TWC performance by κ-Ce2Zr2O8 was ascribed to the strengthened Pd-CZ interaction. Therefore, instead of increasing the κ-Ce2Zr2O8 content by consuming huge energy, constructing larger content of the mentioned crystal interface is more efficient for improving TWC performance.

The preparation of Pd/CeO2–ZrO2–Al2O3 catalyst with superior structural stability: effect of zirconia incorporation method

A series of CeO2–ZrO2–Al2O3 (CZA) supports were prepared by coprecipitation method with different mixed modes of precursors, and the corresponding Pd-only catalysts were acquired with incipient impregnation technique. Interestingly, the CZA3 prepared by mixing of Ce(NO3)3·6H2O–ZrO(CO3)2 mixed solution and ZrO(CO3)2–Al(NO3)3·9H2O mixed solution showed better structural stability; meanwhile, the relevant Pd/CZA3 presented better catalytic activity and the operation window. The characterization results displayed that Zr was introduced into Ce and Al, respectively, could enhance the moderate interaction between Pd and support and increase oxygen vacancies as well as inhibit particle growth, agglomeration and phase separation. Moreover, the Pd–O–Ce bond could be weakened for Pd/CZA3 with appropriate Zr content, which was helpful for enhancing redox activity. Combining the results of characterizations and catalytic performance, it is revealed that the Pd/CZA3 possessed the greatest application potential for (three-way catalysts) TWCs.

Tin Dioxide as an Alternative to Platinum Promoter in Palladium-Catalyzed Wet Lean Methane Combustion

Tin dioxide was assessed as a substitute for part of platinum-group metals in a catalytic converter, and the activity of various Pd and PdPt catalysts was compared on conventional alumina support and SnO2 in wet lean methane combustion. Remarkably, Pd-only catalysts supported on SnO2 revealed higher activity compared with PdPt/Al2O3. The catalysts benefit from the strong metal-support interactions and high oxygen mobility in SnO2 with dual Sn4+/Sn2+ valency. SnO2 can thus be considered a potential replacement of Pt in a catalytic converter for a natural gas vehicle under lean burn conditions. This potentially decreases the price of the converter and eliminates the need for scarce and expensive Pt.

Hysteresis Phenomena on Pt- and Pd-Diesel Oxidation Catalysts: Experimental Observations

In Diesel Oxidation Catalysts (DOC) platinum (Pt), palladium (Pd), or a combination of both (alloyed or multilayered) are the typical active components. During lightoff/lightout experiments, hysteresis phenomena of CO, NO, and HC conversion have repeatedly been reported and attributed to thermal effects, to surface coverage effects and/or to noble metal oxidation. A detailed understanding of the causes of these hysteresis phenomena will be crucial, especially for modeling purposes. The present contribution provides detailed experimental evidence on single component conversion as well as on the mutual influence of CO, NO, and HC on the respective conversion for close to commercial Pt- and Pd-only catalysts under isothermal conditions. It will be shown, that hysteresis effects on Pd-only catalysts mostly vanish after one lightoff. Contrarily, on Pt-only catalysts, a cyclic, stationary conversion hysteresis during lightoff/lightout experiments is observed for CO and NO. C3H6-only however does not show this phenomenon whereas small hysteresis in conversion can be observed in combination with CO and/or NO.

Hysteresis Phenomena on Platinum and Palladium-based Diesel Oxidation Catalysts (DOCs)

The diesel oxidation catalyst (DOC) plays a key role in diesel exhaust treatment systems. Typical noble metals used as active components are platinum (Pt) and palladium (Pd). During lightoff/lightout experiments, the catalyst reactivity during heating is in some cases different from the reactivity during cooling in the same reaction mixture. These so called hysteresis phenomena have repeatedly been reported for CO, NO, and HC conversion and are mostly attributed to noble metal oxidation and/or surface coverage effects. Hauff et al. have developed a kinetic model that is able to account for the hysteresis effects observed in NO conversion on Pt-only catalysts due to noble metal oxidation. The model was only validated for a limited range of NO concentrations and temperatures. In this follow-up, further experiments on Pt-only as well as on Pd-only catalysts will be presented for additional feed compositions. General transferability of the Pt-only model to Pd-only catalysts for NO-only is demonstrated. By addition of CO and propene to the feed, additional hysteresis phenomena are observed and will be discussed. On Pd–only, results indicate that under lean exhaust conditions, the noble metal can only be slightly reduced by CO. Interestingly, with a mix of CO and NO no reactivation is observed whereas the combination of CO, NO, and propene again shows reducing tendency. Based on these informations, a possible modeling approach will be proposed.

A new insight into the effects of barium addition on Pd-only catalysts: Pd-support interface and CO + NO reaction pathway

The effects of barium addition on both fresh and aged Pd/Ce0.5Zr0.5O2 catalysts were investigated. The activities of the catalysts were evaluated by CO+NO reaction and oxygen storage capacity (OSC). The features of palladium species and the properties of the supports were explored by X-ray diffraction (XRD), nitrogen physisorption, high-resolution TEM, CO chemisorption, X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR). The adsorption of CO and NO species during the reaction was especially characterized using in situ DRIFTs. The results reveal that barium addition decreases the activity of palladium on Ce0.5Zr0.5O2, especially after ageing. During the ageing, barium reacts with Ce0.5Zr0.5O2 forming barium zirconium oxides (xBaO·yZrO2), which results in a barrier layer at the interface between palladium and ceria-zirconia. Such a layer consumes the beneficial Pd–ceria interface and blocks the migration of activated oxygen supplied from Ce0.5Zr0.5O2 support to palladium, which results in the severe sintering of palladium during ageing and decreases the catalytic activity of the catalysts. In contrast, the activity of Pd/Al2O3 catalysts is improved by barium addition. The barium addition significantly suppresses the sintering of alumina, improves the dispersion of palladium and facilitates the adsorption of active surface nitrates during the reaction.

Active, selective and robust Pd and/or Cr catalysts supported on Ti-, Zr- or [Ti,Zr]-pillared montmorillonites for destruction of chlorinated volatile organic compounds

Pd and/or Cr catalysts supported on Ti-, Zr- or mixed [Ti,Zr]-pillared montomorillonite clays (PILC), for use in combustion of dichloromethane (DCM) and trichloroethylene (TCE), were synthesized, and characterized with XRD, SEM, TEM/HRTEM, XPS, ESR and Raman spectroscopies, N2 adsorption/desorption at −196°C, NH3 TPD–MS, and FTIR of pyridine adsorption. The catalysts are porous solids with high surface area (240–390m2/g). Pd exists as fine PdOx nanoparticles, predominantly of ≤10nm size, while Cr is present as monomeric CrOx species. The highest (Pd4+ and Cr6+), or next to highest (Cr5+), oxidation states dominate at the catalysts surface. The active phases are most reducible when supported on Ti-PILC and most difficult to reduce on [Ti,Zr]-PILC. Catalysts supported on [Ti,Zr]-PILC show highest total acidity, of predominantly Lewis character. The excellent performance of the catalysts in combustion of DCM and TCE is attributed to the combination of highly porous structure of PILC supports, good dispersion of the active phases, and appropriate blend of redox and acid–base functions. In both reactions, for a given type of support, the catalysts with Cr-only active phase show best performance in terms of activity, which is assigned to the presence of well dispersed, highly oxidized Cr6+/Cr5+ species. In DCM combustion, catalysts supported on Ti-PILC show superior activity, attributed to much better reducibility of active phases deposited on this carrier. In TCE oxidation, best activity is obtained for [Ti,Zr]-PILC supported catalysts, which is correlated with the highest Lewis acidity of these samples. Pd-only catalysts are most selective, showing 100% selectivity to HCl and CO2 at T90 for both studied reactions. Cr-only catalysts are least selective, and emit substantial amounts of Cl2 and CO. Mixed Pd,Cr-catalysts represent an attractive, cheaper alternative to purely Pd systems. At T90 their selectivity to CO2 is 100% in both reactions, with the selectivity to HCl ranging from 87 to 100%. PILC-supported catalyst proved very stable in the reaction environment, both in terms of the catalytic performance and the structural identity.

Realization approach of Pd-only three-way catalysts with high catalytic performance and thermal stability

Pd-only three-way catalyst (TWC), Pd supported on washcoating (the mixture of alumina and Ce-Zr solid solution)/cordierite, was prepared and its catalytic performance and the operation window (λ-value) at 450 °C were evaluated with the simulated automotive exhaust feed gas. Surfactants such as Tween-80 and Span-20 were added in the process of preparing the catalyst in order to improve the thermal stability and catalytic performance of Pd-only TWC. The fresh and aged catalysts at 1000 °C for 4 h were characterized by low-temperature N2 adsorption, XRD, XPS, and H2-TPR techniques. The results show that the presence of surfactants in the synthesis slurry could influence the physicochemical properties of the final Pd-only TWC. The FTS catalyst prepared with the mixed surfactant of Tween-80 and Span-20 exhibited excellent three-way catalytic performance. After being aged at 1000 °C for 4 h, the catalytic performances of Pd-only TWCs slightly decreased, but the FTS catalyst still demonstrated higher catalytic performance and better thermal stability compared with the Pd-only catalysts prepared with single surfactant or without any surfactant. And the FTS catalyst has a wider λ value (operation window) than other catalysts, even after being aged at 1000 °C.

Preparation of FexCe1-xOy solid solution and its application in Pd-only three-way catalysts

AbstractFeOx-CeO2 mixed oxides with increasing Fe/(Ce+Fe) atomic ratio (1–20 mol%) were prepared by sol-gel method and characterized by X-ray powder diffraction (XRD), Brunauer-Emmett-Teller (BET) and Hydrogen temperature-programmed reduction (H2-TPR) techniques. The dynamic oxygen storage capacity (DOSC) was investigated by mass spectrometry with CO/O2 transient pulses. The powder XRD data following Rietveld refinement revealed that the solubility limit of iron oxides in the CeO2 was 5 mol% based on Fe/(Ce+Fe). The lattice parameters experienced a decrease followed by an increase due to the influence of the maximum solubility limit of iron oxides in the CeO2. TPR analysis revealed that Fe introduction into ceria strongly modified the textual and structural properties, which influenced the oxygen handling properties. DOSC results revealed that Ce-based materials containing Fe oxides with multiple valences contribute to the majority of DOSC. The kinetic analysis indicated that the calculated apparent kinetic parameters obey the compensation effect. The three-way catalytic performance for Pd-only catalysts based on the Fe doping support exhibited the redundant iron species separated out of the CeO2 and interacted with the ceria and Pd species on the surface, which seriously influenced the catalytic properties, especially after hydrothermal aging treatment.

Highly selective hydrogenation of 3,4-dichloronitrobenzene over Pd/C catalysts without inhibitors

A series of Pd-only catalysts supported on activated carbon pretreated with HNO 3 or EDTA-2Na for liquid phase selective hydrogenation 3,4-dichloronitrobenzene (DCNB) to corresponding 3,4-dichloroaniline (DCAN) are investigated. Characterization of the catalysts is performed by N 2 adsorption/desorption, TPD, XRD and TPD techniques. The selectivity to DCAN is not only correlate with the Pd particle average size, but also correlate with distribution of Pd particles sizes. The surface groups of activated carbon can influence the average size and distribution of Pd particles. Pretreated activated carbon with HNO 3, forms various oxygen-containing groups on the surface, results in large average size but wide-distributed Pd particles. On the contrast, pretreated activated carbon with EDTA-2Na results in large average size and narrow-distributed of Pd particles simultaneously. More than 60% Pd particles sizes are distributed in the 20–30 nm range of the 2% Pd loading catalyst pretreated with 1 M EDTA-2Na, of which the selectivity to DCAN is up to 99.61%.

Application of Rare Earth Modified Zr-based Ceria-Zirconia Solid Solution in Three-Way Catalyst for Automotive Emission Control

Automotive exhaust emission is a major cause of air pollution. Three-way catalyst (TWC) which can eliminate CO, HC (hydrocarbons), and NO(x) simultaneously has been used to control exhaust emissions. Ceria-zirconia is a key component in TWC and most researchers pay attention to Ceria-Zirconia (Ce-rich) solid solution. The research presented in this paper is focused on the intrinsic structure of Ceria-Zirconia (Zr-based) solid solution and its application in TWC. A series of Ce(0.2)Zr(0.8)O(2) modified with rare earths (La, Nd, Pr, Sm, and Y) have been prepared by coprecipitation method combined with supercritical drying technique. All samples showed single tetragonal solid solution, indicating that the rare earth ion inserted into the lattice structure completely, and an approximately linearly relationship between lattice parameter a and the ionic radius of doped rare earth was observed. The catalytic performances of corresponding Pd-only catalysts were investigated in simulated exhaust gas. The presence of La, Nd, and Pr was favorable to the catalytic activity and wide air/fuel operation window. The relationship between the intrinsic structure of the Zr-based ceria-zirconia solid solution and catalytic activity was discussed in detail, which has some reference value for catalyst design and application.

Performance of Pd/CeO 2-ZrO 2-Al 2O 3 catalyst for motorcycle

The Pd-only catalysts for motorcycle were prepared by impregnating CeO 2-ZrO 2-Al 2O 3 and CeO 2-ZrO 2+Al 2O 3 with PdCl 2 aqueous solution and characterized by X-ray diffraction (XRD), oxygen storage capacity (OSC) and H 2-temperature-programmed reduction (H 2-TPR) methods. The XRD result indicated that the CeO 2-ZrO 2-Al 2O 3 compound prepared by co-precipitation formed a single solid solution and had good thermal stability, and Pd phase was not observed in all catalysts. The TPR results showed that the reduction temperature of Pd/CeO 2-ZrO 2-Al 2O 3 catalyst was lower than that of Pd/CeO 2-ZrO 2+Al 2O 3 catalyst whether they were fresh or aged catalysts. The Pd/CeO 2-ZrO 2-Al 2O 3 exhibited high three-way catalytic activity at low temperature, high thermal stability, and wide working window, suggesting a great potential for applications.

The role of the support in achieving high selectivity in the direct formation of hydrogen peroxide

Pd-only, Au-only and bimetallic AuPd catalysts supported on a range of supports (Al2O3, TiO2, MgO, and C) have been prepared by impregnation and tested for the hydrogenation and decomposition of hydrogen peroxide under conditions similar to those used in direct synthesis of hydrogen peroxide. Hydrogenation and decomposition are the main pathways for loss of selectivity and yield in the direct synthesis reaction, and the support is found to be a crucial parameter with respect to hydrogenation and decomposition activity. We show that by making the right choice of support for both the monometallic and bimetallic Au and Pd catalysts, it is possible to achieve very low hydrogen peroxide hydrogenation and decomposition activity, thus enhancing hydrogen peroxide productivity during synthesis. Carbon is found to be the optimal support for both monometallic Au and Pd catalysts as well as Au–Pd alloys, since carbon-supported catalysts gave the lowest hydrogenation and decomposition activities. Au-only catalysts were generally less active than Pd-only catalysts when utilizing the same support and metal loading. The addition of Au to Pd catalysts supported on TiO2 and carbon resulted in a decrease in both H2O2 hydrogenation and decomposition while the reverse effect was observed for the Al2O3 and MgO-supported catalysts. These effects are discussed in terms of the basicity of the support, and in particular the isoelectronic point of the support, which is a major factor in controlling the stability of hydrogen peroxide under reaction conditions.

The issue of selectivity in the direct synthesis of H2O2 from H2 and O2: the role of the catalyst in relation to the kinetics of reaction

The kinetic behavior in the direct synthesis of H2O2 with Pd–Me (Me = Ag, Pt) catalysts prepared by depositing the noble metals by electroless plating deposition (EPD) or deposition–precipitation (DP) methods on α-Al2O3 asymmetric ceramic membrane with or without a further surface coating by a carbon thin layer is reported. The effect of the second metal with respect to Pd-only catalysts considerably depends on the presence of the carbon layer on the membrane support. Several factors in the preparation of these membranes as well as the reaction conditions (temperature, concentration of Br−, pH) determine the selectivity in H2O2 formation, influencing the rate of the consecutive reduction of H2O2 (which is faster with respect to H2O2 decomposition on the metal surface) and/or of direct H2 + O2 conversion to H2O. Defective Pd sites are indicated to be responsible for the two unselective reactions leading to water formation (parallel and consecutive to H2O2 formation), but the rate constants of the two reactions are differently influenced from the catalytic membrane characteristics. Increasing the noble metal loading on the membrane not only increases the productivity to H2O2, but also the selectivity, due to the formation of larger, less defective, Pd particles.

Performances of Pd-Me (Me = Ag, Pt) catalysts in the direct synthesis of H 2O 2 on catalytic membranes

The performances in the direct synthesis of hydrogen peroxide (H 2O 2) with Pd-Me (Me = Ag, Pt) catalysts prepared by depositing the noble metals by electroless plating deposition (EPD) or deposition–precipitation (DP) methods on α-Al 2O 3 asymmetric ceramic membrane with or without a further surface coating by a carbon thin layer are reported. The effect of the second metal with respect to Pd-only catalysts considerably depends on the presence of the carbon layer on the membrane support. On ceramic membranes the use of Pd-Ag bimetallic catalysts avoids the formation of β-PdH and improves the catalytic performances, while in the case of Pd-Pt bimetallic catalysts small defective nanoparticles are formed which catalyze the unselective formation of water. Conversely, on carbon-coated membranes the latter effect is not present and good H 2O 2 productivities and selectivities are shown for the Pd-Pt bimetallic systems, in particular those synthesized with the EPD method, although for both types of preparations the Pd-Pt bimetallic catalyst show improved behavior with respect to Pd-only systems.

Performance of La 2O 3- or Nb 2O 5-added Pd/SiO 2 catalysts in acetylene hydrogenation

Pd catalysts promoted by La 2O 3 and Nb 2O 5, which were selected among metal oxides showing a strong metal-support interaction (SMSI), were tested for acetylene hydrogenation and their kinetic behaviors were compared with those of Pd-only and TiO 2-added catalysts. The La 2O 3-added catalyst showed lower acetylene conversions but a higher ethylene selectivity and slower deactivation rates than the Pd-only catalyst. The improvement in ethylene selectivity and catalyst lifetime was significant, particularly when the catalysts were reduced at a temperature of 500 °C. On the other hand, Nb 2O 5-added catalysts showed higher acetylene conversions as well as both improved ethylene selectivity and catalyst lifetime when compared with the Pd-only catalyst. In this case, the improved selectivity attained by reducing the catalyst at 500 °C was significant when the reaction was conducted at low temperatures, i.e. 40 °C and 50 °C instead of 60 °C. Based on the characterization of catalysts by temperature-programmed reduction (TPR), H 2 chemisorption, and the temperature-programmed desorption (TPD) of ethylene, the La 2O 3 added to the catalyst modified the Pd surface in a manner similar to TiO 2, which interacted strongly with Pd, both geometrically and electronically, after reducing the catalyst at 500 °C. La 2O 3 interacted with Pd more strongly and, consequently improved catalyst performance to a greater extent than TiO 2. Nb 2O 5 interacted with Pd similar to the other two oxides but also showed activity for hydrogenation, which contributed to the beneficial performance of Nb 2O 5-added catalysts: a higher activity for acetylene hydrogenation than that of Pd-only catalysts; remarkably low deactivation rates compared with the other catalysts. Among the three types of catalysts studied, La 2O 3-added catalysts that were reduced at 500 °C showed the highest ethylene selectivity at a reaction temperature of 60 °C. On the other hand, the Nb 2O 5-added catalysts showed the highest activity and longest catalyst lifetime due to the additional activity of Nb oxides for hydrogenation. Consequently, the former catalysts would be advantageous for use at high temperatures and the latter at low temperatures.

A kinetic study of vinyl acetate synthesis over Pd-based catalysts: kinetics of vinyl acetate synthesis over Pd–Au/SiO 2 and Pd/SiO 2 catalysts

Kinetic studies of vinyl acetate (VA) synthesis were carried out for Pd (1.0 wt%)/SiO 2 and Pd (1.0 wt%)–Au (0.5 wt%)/SiO 2 catalysts, with highly dispersed metal particles (Pd and Pd–Au) characterized by X-ray diffraction and transmission electron microscopy–energy dispersive spectroscopy (TEM-EDS). The kinetics of the related CO oxidation reaction were also explored. The kinetics of VA synthesis and CO oxidation reactions proceed via a Langmuir–Hinshelwood mechanism. For Pd-only catalysts, dissociative adsorption of O 2 is believed to be the rate-determining step. This step is suppressed by adsorbed CO/C 2H 4 and gives rise to a negative reaction order with respect to CO/C 2H 4 and a positive order with respect to O 2. However, the reaction mechanism was significantly modified by the addition of Au to Pd, as indicated by the change in the reaction order with respect to CO/C 2H 4; in particular, the order with respect to C 2H 4 becomes positive. The modification of the mechanism for the Pd–Au catalyst correlates with the reduction in the concentration of Pd–Pd ensembles upon alloying with Au. By TEM-EDS, Au surface enrichment was detected for the Pd–Au alloy, and the interaction between Au and Pd leads to a formation of more active Pd ensembles, such as PdAu 5 and PdAu 6. The surface properties of Pd versus Pd–Au catalysts were explored by CO/C 2D 4-TPD on thick films of Pd and Pd–Au. These studies indicate that the adsorption sites on Pd are significantly modified by Au; concurrently, the bonding of CO and C 2D 4 to Pd in the Pd–Au alloy also varied. As a result, the coverage of CO and C 2H 4 on the Pd–Au surface decreased markedly. The enhanced capacity of the Pd–Au surface for oxygen and/or the enhanced mobility of adsorbed oxygen under the reaction conditions are likely responsible for the unusually high reactivity of Pd–Au alloy catalysts for VA synthesis.

Correlation between activity and oxygen storage capacity of ceria on Pd-only three-way catalysts

One way of enhancing the thermal stability and NOx, THC and CO conversion of a catalyst is to improve the thermal stability and oxygen storage capacity (OSC) of the ceria. The appropriate mixing ratios of bulk and stabilized ceria are especially very important for designing Pd-only three-way catalysts. In this paper, we discuss the surface phenomena of stabilized and unstabilized (bulk) ceria, the OSC of catalysts, and the correlation between activity and OSC of Pd-only catalysts with mixing ratios of bulk ceria and stabilized ceria.

Comparative investigation of a Pd/Al2O3 catalyst system with TEM and STEM

Pd-only catalysts are attractive for automotive applications because of the low cost and wide availability of Pd compared with that of Pt and Rh. Analytical electron microscopy may be used to investigate the dispersion, which is a very important parameter in catalyst characterization, of the Pd particles on γ-Al2O3 supports. It was of interest to compare how two different techniques, TEM and STEM, might be used for the Pd-particle size determination in a model catalyst. The model system of 1wt%Pd/γ-Al2O3 used in the present study was prepared by the incipient wetness technique from a palladium nitrate solution. It was then dried at 120°C and calcined at 400°C, followed by steam aging at 600 °C for 24 hours. The TEM used was a JEOL 2000FX operated at 200 kV whereas the STEM used was a VG Microscopes HB-501UX operated in the Z-contrast mode at 100 kV.