Ceria-supported Platinum Catalyst Research Articles

Defective ceria created by oxy-hydrogen flame and its influences on Pt dispersion, Pt-ceria interaction and catalytic hydrogenation

We employed an oxy-hydrogen flame to create defective ceria and studied the consequent changes in the physiochemical properties of ceria and ceria-supported platinum (Pt) catalysts. The defective ceria is more reduced and has greater redox capabilities than the pristine sample. The defective ceria supported Pt catalysts have stronger metal-support interactions that lead to higher Pt dispersion and chemoselectivity in 3-nitrostyrene hydrogenation compared to ceria-supported platinum catalysts. The chemoselectivity increases and then decreases with decrease in metal loadings in defective ceria supported Pt catalysts, while it monotonically increases with decreasing metal loadings in ceria-supported catalysts. The diminishing chemoselectivity of defective ceria supported catalysts can be attributed to the more reduced nature of the defective ceria that leads to metal nanoclusters with less positive charge, disfavoring adsorption of the negatively charged nitro-groups in 3-nitrostyrene in the reaction. Such effect is not obvious in the defective ceria supported catalysts with higher Pt loadings (i.e., larger Pt particle sizes).

Electronic Structure of Heteronuclear Cerium-Platinum Clusters.

Beyond the now well-known strong catalyst-support interactions reported for ceria-supported platinum catalysts, intermetallic Ce-Pt compounds exhibit fascinating properties such as heavy fermion behavior and magnetic instability. Small heterometallic Ce-Pt clusters, which can provide insights into the local features that govern bulk phenomena, have been less explored. Herein, the anion photoelectron spectra of three small mixed Ce-Pt clusters, Ce2OPt-, Ce2Pt-, and Ce3Pt-, are presented and interpreted with supporting density functional theory calculations. The calculations, which are readily reconciled with the experimental spectra, suggest the presence of numerous close-lying spin states, including states in which the Ce 4f electrons are ferromagnetically coupled or antiferromagnetically coupled. The Pt center is consistently in a nominal -2 charge state in all cluster neutrals and anions, giving the Ce-Pt bond ionic character. Ce-Pt bonds are stronger than Ce-Ce bonds, and the O atom in Ce2OPt- coordinates only with the Ce centers. The energy of the singly occupied Ce-local 4f orbitals relative to the Pt-local orbitals changes with cluster composition. Discussion of the results includes potential implications for Ce-rich intermetallic materials.

Morphological effect of ceria-supported platinum catalyst on low-temperature ethylene oxidation

Three types of Pt/CeO2 catalysts with different support morphologies (rod (CeO2-R), cube (CeO2-C), and octahedron (CeO2-O) were synthesized especially for catalytically oxidizing C2H4 at low temperatures. The as-prepared catalysts exhibited strong shape-dependent catalytic activity in the order of Pt/CeO2-O > Pt/CeO2-C > Pt/CeO2-R. Such unique performance is mainly attributed to the different steps of C2H4 oxidation associated with the variation of oxygen species activated by Pt/CeO2 catalysts. Among the three catalysts, Pt/CeO2-O had the least issue with the accumulation of intermediate species and consequently performed the highest low-temperature C2H4 catalytic oxidation activity. Moreover, the negative impact of water condensation on the catalyst can be readily eliminated by refreshing the catalyst with N2 purging. Overall, this work led to a novel catalytic system for efficient low-temperature ethylene oxidation and a better understanding of the catalytic mechanism, thus benefiting the preservation of fruits and vegetables as one of the applications.

Redox dynamics of platinum species on CeO2 during CO oxidation reaction

Ceria supported platinum catalysts play an important role in exhaust emission catalysis. Because of the interactions between catalyst and reactants under complex reaction conditions, Pt species may exhibit dynamic structural changes as a function of the reactant pressure and reaction temperature, making it difficult to determine the structure-performance relationship. In this study, the chemical states and dynamics of several typical Pt species, including PtOx, Pt–O–Ce and reduced Pt nanoparticles (NPs), in CO oxidation reaction and their correlations with activity were explored. It was found that Pt–O–Ce species with strong Pt–O bonds remained in the ionic state during the light off reaction, while PtOx species that interacted weakly with CeO2 could be easily reduced to metallic Pt (Pt0), which also exhibited better activity due to the enhanced reducibility of the Pt NPs-CeO2 interface. Although the initial activity of the Pt/CeO2 catalyst can be improved through reduction with H2, the metallic Pt NPs, especially derived from the reduction of Pt–O–Ce dominated catalysts, can get reoxidized to the less active Ptn+–O–Ce (n = 2–δ) species during the reaction. A detailed in situ characterization revealed that the oxidation of metallic Pt NPs arises from oxygen reverse spillover from the CeO2 support. This study shows that understanding the reaction-driven structural dynamics of Pt species is essential for exploring the reaction mechanism and elucidating how the catalyst is activated and deactivated in the reaction.

Morphological Effect of Ceria-Supported Platinum Catalyst on Low-Temperature Ethylene Oxidation

Morphological Effect of Ceria-Supported Platinum Catalyst on Low-Temperature Ethylene Oxidation

Ceria supported platinum catalyst for CO oxidation reaction: Importance of metallic active species ― Commemorating the 100th anniversary of the birth of Academician Guangxian Xu

Ceria supported platinum catalyst has now been widely studied due to its excellent activity for CO oxidation. However, the electron state of active metal center is still an open question. In this work, a ceria nanorod support was prepared and platinum (Pt) with 0.9 at% was deposited using an impregnation method to obtain Pt/CeO2 catalyst. With the help of “light-off” experiment and temperature-programmed reduction under CO (CO-TPR) test, the conclusion is proposed that the process of hydrogen reduction can enhance the activity of CO oxidation reaction for the generation of optimal active Pt site. An innovative near-situ X-ray absorption fine structure (XAFS) technique was used to investigate the chemical state of central Pt atom during the reaction process, clearly demonstrating that the high oxidized state of Pt does harm to the activity for CO oxidation while the relatively reductive Pt exhibits high activity, and the different oxidized state and chemical environment of Pt during every process has been identified. Furthermore, the activity of our Pt/CeO2 catalyst is superior to that of most of the previous reports about CO catalytic oxidation by Pt based catalyst. Moreover, the optimal active species (Pt–O4) have been identified after hydrogen reduction, which could be a possible key strategy to control the oxidation of Pt.

Size effect-tuned water gas shift reaction activity and pathway on ceria supported platinum catalysts

The interfacial electronic and geometric effects behind the size effect are crucial in determining the catalytic performance of supported metal catalysts, whereas they remain still unknown for most of important heterogeneous catalytic reactions such as water-gas shift reaction (WGSR). In this work, we report a profound size effect in five Pt/CeO2 catalysts among Pt single atoms, sub-nanoclusters (ca. 0.8 nm) and nanoparticles (3.8–9.3 nm) for the WGSR. The catalyst with about 3.8 nm Pt nanoparticles had the highest activity, superior to the other four Pt/CeO2 catalysts. Ptδ+-OV-Ce3+ species served as the interfacial active sites, wherein the carboxyl pathway seemed more preferred than the formate route, and favored high activity. Such high activity and pathway preference are due to both of the electronic and geometric effects. On the one hand, the electronic metal-support interaction (EMSI) modulated by the metal deposit size tunes the activation of the CO adsorbed on Pt sites. On the other hand, the abundant interfacial oxygen vacancies for the geometric effect of providing the active sites to activate and dissociate H2O.

Catalytically Active and Spectator Ce(3+) in Ceria-Supported Metal Catalysts.

Identification of active species and the rate-determining reaction steps are crucial for optimizing the performance of oxygen-storage materials, which play an important role in catalysts lowering automotive emissions, as electrode materials for fuel cells, and as antioxidants in biomedicine. We demonstrated that active Ce(3+) species in a ceria-supported platinum catalyst during CO oxidation are short-lived and therefore cannot be observed under steady-state conditions. Using time-resolved resonant X-ray emission spectroscopy, we quantitatively correlated the initial rate of Ce(3+) formation under transient conditions to the overall rate of CO oxidation under steady-state conditions and showed that ceria reduction is a kinetically relevant step in CO oxidation, whereas a fraction of Ce(3+) was present as spectators. This approach can be applied to various catalytic processes involving oxygen-storage materials and reducible oxides to distinguish between redox and nonredox catalytic mechanisms.

Quantitative depth profiling of Ce(3+) in Pt/CeO2 by in situ high-energy XPS in a hydrogen atmosphere.

The redox property of ceria is a key factor in the catalytic activity of ceria-based catalysts. The oxidation state of well-defined ceria nanocubes in gas environments was analysed in situ by a novel combination of near-ambient pressure X-ray Photoelectron Spectroscopy (XPS) and high-energy XPS at a synchrotron X-ray source. In situ high-energy XPS is a promising new tool to determine the electronic structure of matter under defined conditions. The aim was to quantitatively determine the degree of cerium reduction in a nano-structured ceria-supported platinum catalyst as a function of the gas environment. To obtain a non-destructive depth profile at near-ambient pressure, in situ high-energy XPS analysis was performed by varying the kinetic energy of photoelectrons from 1 to 5 keV, and, thus, the probing depth. In ceria nanocubes doped with platinum, oxygen vacancies formed only in the uppermost layers of ceria in an atmosphere of 1 mbar hydrogen and 403 K. For pristine ceria nanocubes, no change in the cerium oxidation state in various hydrogen or oxygen atmospheres was observed as a function of probing depth. In the absence of platinum, hydrogen does not dissociate and, thus, does not lead to reduction of ceria.

Structure of Platinum Catalysts under CO, Hydrogen, and Oxygen; Anomalous Behavior of Pt on Ceria after Cyanide Leaching

By means of in situ time-resolved high-energy-resolution fluorescence X-ray spectroscopy and infrared spectroscopy, the structure of platinum on alumina and ceria catalysts in a reaction mixture of 75% H2, 1% CO, and 1% O2 in He was determined as a function of temperature. After reduction, all the observed platinum in Pt/Al2O3 and Pt/Ce(La)Ox was present as Pt(0) nanoparticles covered with CO. The amount of CO depended on the temperature and CO concentration. Cyanide leaching of ceria-supported platinum catalyst yielded a high concentration of cationic platinum. Reduction occurred in the absence of oxygen; extremely small Pt clusters (≤0.5–0.7 nm) formed. Unlike nanoparticles on alumina and ceria, which chemisorb CO in the presence of CO and oxygen, these clusters are bonded to oxygen. This contrasting behavior paralleled the different selectivity for preferential oxidation of CO (24% selectivity for nonleached sample (331 K) versus 54% selectivity for leached sample (340 K)). Even without the knowledge of the exact structure of the platinum–oxygen species, this in situ study identifies the roles of reduced nanoparticles and cationic metal in generating activity and selectivity.

Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts

Fil: Vecchietti, Maria Julia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Santa Fe. Instituto de Desarrollo Tecnologico Para la Industria Quimica (i); Argentina

Sulfur promoted low-temperature oxidation of methane over ceria supported platinum catalysts

The influence of sulfur dioxide on methane oxidation over ceria supported platinum catalysts under lean conditions has been studied by transient flow reactor and in situ Fourier transform infrared spectroscopy experiments. The results show that sulfur dioxide can promote the oxidation of methane, here between 300 and 450 °C, although, as a function of time on stream, the promoting effect diminish and instead the methane oxidation becomes inhibited. We suggest that sulfate formation on ceria creates oxygen vacancies in the ceria that govern oxygen spillover leading to a lowering of detrimental oxygen coverage of platinum and/or decomposition of platinum oxide facilitating the dissociative methane adsorption. Eventually the ceria becomes saturated with sulfates and the oxygen dynamics in the platinum–ceria system settles. As a consequence, the system regresses towards highly oxidized platinum and/or otherwise active sites on the ceria or platinum–ceria boundary become blocked, which may explain the long-term inhibiting effect by SO 2 exposure.

Low-temperature oxidation of carbon monoxide and methane over alumina and ceria supported platinum catalysts

The ignition and extinction processes for total oxidation of CO, CH4 and CO+CH4 mixture in oxygen excess over Pt/Al2O3 and Pt/CeO2 catalysts with the platinum phase distributed either homogeneously or heterogeneously (i.e., locally high platinum concentration) in the support have been studied by temperature programmed oxidation experiments. Following the preparation methods by Arnby et al. [42], the samples have the same Pt load and dispersion. Generally the catalytic activity follows the order: Pt/CeO2 (heterogeneous)>Pt/CeO2 (homogeneous)>Pt/Al2O3 (heterogeneous)>Pt/Al2O3 (homogeneous) as indicated by lower ignition and/or extinction temperatures. For Pt/Al2O3, the addition of NO2 to the reactant stream increases the rate of oxidation of CO in the pre-ignition regime although the light-off temperature T50 is shifted towards higher temperatures (except for low CO concentrations). In the case of the Pt/CeO2, the CO conversion generally decreases. For CH4 oxidation in the presence of NO2, the conversion increases for Pt/Al2O3 and decreases for Pt/CeO2. The addition of CO2 in the reactant stream has minor influence on CO oxidation over Pt/Al2O3 while for Pt/CeO2, T50 is shifted towards higher temperatures. For the simultaneous oxidation of CO and CH4, a reverse hysteresis for methane oxidation is observed, i.e., the extinction process occurs at higher temperature than the corresponding ignition process. The improved activity for CO oxidation over samples with heterogeneous Pt distribution is likely due to less tendency towards CO self-poisoning through the development of steeper concentration gradients in the Pt containing regions in the porous support material. The significant increase of activity for both reactions over ceria-supported Pt is here assigned to highly active sites at the platinum–ceria boundary but also, to some extent, the oxygen storage and release function and dynamics of the transport of oxygen in the Pt/CeO2 system.

CO multipulse TAP studies of 2% Pt/CeO 2 catalyst: Influence of catalyst pretreatment and temperature on the number of active sites observed

CO multipulse temporal analysis of products (TAP) experiments were used to characterize a ceria-supported platinum catalyst after various oxidative and reductive pretreatments using O 2, H 2O, CO 2, and H 2. Based on the amount of CO consumed, using the final CO-saturated catalyst composition as the common state point, the oxidatively pretreated catalyst could be described using a general scale. From a kinetic analysis of the CO multipulse responses, two kinetic regimes corresponding to two types of active sites could be identified. As the temperature was raised, the number of the most active sites did not change while the amount of the less active site increased. Comparison of the number of active sites determined from the TAP data reported herein with that determined by a previous steady-state isotope transient kinetic analysis experiment showed excellent agreement. This correlation indicates that the (very fast response) TAP experiments can provide information regarding the number and type of active sites that are relevant to a catalyst under real reaction conditions.

Analysis of Gd Level and Pt Dispersion on Ceria Support for Isobutane Steam Reforming

Ceria-supported platinum catalysts were used to produce hydrogen by isobutane steam reforming. The influence of gadolinia, as a trivalent cationic oxide (M2O3) dopant at 0–20%, is investigated here. The gadolinia content in the catalysts did not correlate well with reforming activity. The platinum dispersion ranged from 29 to 38% as measured by the propylene hydrogenation method and it did correlate well with reforming activity.

Development of a LPG fuel processor for PEFC systems: Laboratory scale evaluation of autothermal reforming and preferential oxidation subunits

Polymer electrolyte fuel cell (PEFC) offers the possibility of generating power with high efficiency and low emissions for mobile and stationary applications. A very crucial factor to use the PEFC-based engine for transportation purposes is the choice of fuel and fuel processor, proper for a realistic, on-board, employment. This paper covers the in-progress activities performed by CNR-ITAE on experimental studies of the autothermal reforming of propane on a proprietary ceria-supported platinum catalyst: this study represents the first step of a project oriented for the development of a compact and reliable fuel processor, fed by LPG, to be used in a PEFC vehicle. The main issues that should be satisfied by the hydrogen generator will be high fuel conversion, stable performance for repeated start-up and shut-down cycles, capability to process different hydrocarbons, etc. In this respect, the CO preferential oxidation step has also been studied by the performance analysis of commercial Pt / Al 2 O 3 and Ru / Al 2 O 3 catalysts (Engelhard Co.).

Preferential oxidation of carbon monoxide in the presence of hydrogen (PROX) over ceria–zirconia and alumina-supported Pt catalysts

One crucial requirement for the operation of proton exchange membrane fuel cells (PEMFC) is to feed carbon monoxide free hydrogen to the anode. This need can be achieved by using catalysts able to selectively oxidize CO in the presence of excess hydrogen. Herein we report the preferential CO oxidation (PROX) in the presence of hydrogen over Pt/Ce x Zr x−1 O 2 ( x=0, 0.15, 0.5, 0.68, 1) catalysts. A comparison with results observed on a Pt/Al 2O 3 catalyst is also presented. We examined the effect of temperature (90–300 °C) and O 2 excess ( λ=0.8–2). Ceria-supported platinum catalysts were more active than Pt/Al 2O 3 in both CO and H 2 oxidation. The result was a sharp “light off” around 90 °C for the oxygen conversion. The maxima, which appeared in the CO conversion and in the selectivity toward CO oxidation as a function of temperature on Pt/Al 2O 3, did not show up in the case of ceria-supported samples. Chloride ion-containing Pt/CeO 2 catalysts showed lower performances in the PROX reaction, especially at low temperatures. Four types of reaction mechanisms were suggested for Pt/Ce x Zr x−1 O 2 samples: (i) competitive Langmuir–Hinshelwood CO and hydrogen oxidation on Pt particles, (ii) noncompetitive Langmuir–Hinshelwood mechanism on the metal/oxide interface, (iii) hydrogen oxidation on the support, and (iv) water–gas-shift reaction at high temperatures. The second reaction route predominated at low temperatures (90–130 °C) and was found preferential to CO oxidation rather than hydrogen–oxygen reaction. This process was selectively blocked by Cl ions. The possible application of Pt/CeO 2 and Pt/Al 2O 3 catalysts was discussed. Ceria appeared as a suitable support for the preferential CO oxidation catalysts at low temperatures.

Selective CO oxidation in the presence of hydrogen: fast parallel screening and mechanistic studies on ceria-based catalysts

An accelerated investigation of various ceria-supported platinum catalysts by means of parallel synthesis and testing reactors is reported. The aim is to discover new catalysts for purifying hydrogen for fuel cells and other applications by studying three reactions: selective CO oxidation in the absence and in the presence of hydrogen (SelO x), water–gas shift (WGS), and reverse water–gas shift (RWGS). A set of catalysts is prepared by impregnating platinum on various doped ceria supports. These catalysts display, despite their nearly identical X-ray crystal structures, diverse redox/acidic/basic properties and different selectivities in the three test reactions. The best SelO x catalysts are also the most active for WGS/RWGS equilibration. In contrast to the conventional two-stage approach of discovery screening followed by lead optimization, we demonstrate that it is possible to extract valuable mechanistic information directly from the discovery stage. Specifically, we elucidate the elementary steps coupling the SelO x and WGS processes and gain insight as to the combined effects of hydrogen presence and cation doping on the CO oxidation reaction.

Catalytic partial-oxidation of methane on a ceria-supported platinum catalyst for application in fuel cell electric vehicles

Fuel cell developers are investigating the generation of hydrogen from light hydrocarbons, such as methane or natural gas, for fuelling polymer electrolyte fuel cells (PEFCs). This study demonstrates generation of H 2 and CO by catalytic partial-oxidation of CH 4 in air at atmospheric pressure on a ceria-supported platinum catalyst prepared by a novel solution–combustion method where platinum is present in ionically-substituted form. These catalysts at different platinum loadings showed methane conversion and hydrogen selectivity of 90 and 97%, respectively. Interestingly, there is little carbon deposition in the catalytic reactor even after prolonged reaction time.

98/04320 Simulation of dynamic technical systems for gas utilisations

Fuel cell developers are investigating the generation of hydrogen from light hydrocarbons, such as methane or natural gas, for fuelling polymer electrolyte fuel cells (PEFCs). This study demonstrates generation of H2 and CO by catalytic partial-oxidation of CH4 in air at atmospheric pressure on a ceria-supported platinum catalyst prepared by a novel solution–combustion method where platinum is present in ionically-substituted form. These catalysts at different platinum loadings showed methane conversion and hydrogen selectivity of 90 and 97%, respectively. Interestingly, there is little carbon deposition in the catalytic reactor even after prolonged reaction time.