Variations in Reactivity on Different Crystallographic Orientations of Cerium Oxide

Cerium oxide is a principal component in many heterogeneous catalytic processes. One of its key characteristics is the ability to provide or remove oxygen in chemical reactions. The different crystallographic faces of ceria present significantly different surface structures and compositions that may alter the catalytic reactivity. The structure and composition determine the number of coordination vacancies surrounding surface atoms, the availability of adsorption sites, the spacing between adsorption sites and the ability to remove O from the surface. To investigate the role of surface orientation on reactivity, CeO₂ films were grown with two different orientations. CeO₂(100) films were grown ex situ by pulsed laser deposition on Nb-doped SrTiO₃(100). CeO₂(111) films were grown in situ by thermal deposition of Ce metal onto Ru(0001) in an oxygen atmosphere. The chemical reactivity was characterized by the adsorption and decomposition of various molecules such as alcohols, aldehydes and organic acids. In general the CeO₂(100) surface was found to be more active, i.e. molecules adsorbed more readily and reacted to form new products, especially on a fully oxidized substrate. However the CeO₂(100) surface was less selective with a greater propensity to produce CO, CO₂ and water as products. The differences in chemical reactivity are discussed in light of possible structural terminations of the two surfaces. Recently nanocubes and nano-octahedra have been synthesized that display CeO₂(100) and CeO₂(111) faces, respectively. These nanoparticles enable us to correlate reactions on high surface area model catalysts at atmospheric pressure with model single crystal films in a UHV environment.     

The electro-oxidation of dimethyl ether on platinum-based catalyst

Cyclic voltammetry (CV) and stripping voltammetry were used to study the electro-oxidation of dimethyl ether (DME) on powder microelectrodes (PMEs) containing Pt black and Pt–Ru black. As evidenced by current–time curves of DME oxidation, Pt–Ru was better than Pt in catalytic oxidation of DME, which is due to the high concentration of OH_ads species on its surface. Results also showed that high temperature not only promotes the oxidation of DME, but also increases the concentration of OH_ads species formed by water decomposition. In addition, the performance of single direct DME fuel cell was investigated combined with gas chromatographic (GC) analyses of its anode outlet stream. Based on CV tests, a mechanism of DME electro-oxidation was tentatively proposed, indicating two kinds of DME adsorption modes, Pt–CO and Pt–COH existed on Pt surface.     

Comparison of Two Preparation Methods in the Redox Properties of Pd/CeO<Subscript>2</Subscript>/Ta/Si Model Catalysts: Spin Coating Versus Sputter Deposition

Pd/CeO₂/Ta/Si model catalysts were prepared by spin coating and sputter deposition method, and characterized by means of AFM, SEM and in situ XPS, especially focusing on the redox properties of Ce and Pd elements. Compared with thin CeO₂ films (about 2.2nm), the thicker ones (about 22nm) maintained Ce⁴⁺ oxidation state even after treatment with H₂ up to 500°C while the presence of Pd facilitated the reduction of ceria. The reduction of ceria brought about following that of PdO, which was explained by the spillover of hydride in Pd to CeO₂ originating from hydrogen adsorption on the Pd surface. Compared with the sputter deposition method, spin coating produced the smaller size of Pd particles, thus leading to formation of the stable PdO species against hydrogen. Based on these results, a schematic model of Pd/CeO₂/Ta/Si was suggested and it might be assumed that spin coating method provided with an environment similar to the conventional impregnation.     

Hydrogen Formation in the Reactions of Methanol on Supported Au Catalysts

The adsorption and reactions of methanol have been investigated on Au metal supported by various oxides and carbon Norit of high surface area. Infrared spectroscopic studies revealed the dissociation of methanol at 300 K, which mainly occurs on the oxide-supports yielding methoxy species. The presence of Au already appeared in the increased amounts of desorbed products in the TPD spectra. The reaction pathway of the decomposition and the activity of the catalyst sensitively depend on the nature of the support. As regards the production of hydrogen the most effective catalyst is Au/CeO₂ followed by Au/MgO, Au/TiO₂ and Au/Norit. In contrast, on Au/Al₂O₃ the main process is the dehydration reaction yielding dimethyl ether. On Au/CeO₂ the decomposition of methanol starts above ~500 K and approaches total conversion at 723–773 K. The products are H₂ (~68%) and CO (~27%) with very small amounts of methane and CO₂. The decomposition of methanol follows the first order kinetics. The activation energy of this process is 87.0 kJ/mol. The selectivity of H₂ formation at 573–773 K was ~90%, this value increased to 97% using CH₃OH:H₂O (1:1) reacting mixture indicating the involvement of water in the reaction. No deactivation of Au catalysts was experienced at 773 K in ~10 h. It is assumed that the interface between Au and partially reduced ceria is responsible for the high activity of Au/CeO₂ catalyst.     

Kinetics of dimethyl ether oxidation over Pt/ZSM-5 catalyst

A kinetic study of dimethyl ether (DME) combustion over Pt/ZSM-5 was performed below 423 K. Power law model and Langmuir-Hinshelwood model were established to predict the reaction rate. Reaction orders for DME and O₂ were 0.28 and 2.30, respectively. Activation energies for the two models were 99.35 and 109.30 kJ/mol. The reaction orders and adsorption constants suggested DME was more strongly adsorbed on Pt/ZSM-5 than O₂ at low temperatures. The reliability of the models was confirmed by the comparison between the predicted and experimental conversions of DME.     

Low temperature water–gas shift: Differences in oxidation states observed with partially reduced Pt/MnOX and Pt/CeOX catalysts yield differences in OH group reactivity

The Pt–ceria synergy may be described as the dehydrogenation of formate formed on the surface of the partially reducible oxide (PRO), ceria, by Pt across the interface, with H₂O participating in the transition state. However, due to the rising costs of rare earth oxides like ceria, replacement by a less expensive partially reducible oxide, like manganese oxide, is desirable. In this contribution, a comparison between Pt/ceria and Pt/manganese oxide catalysts possessing comparable Pt dispersions reveals that there are significant differences and certain similarities in the nature of the two Pt/PRO catalysts. With ceria, partial reduction involves reduction of the oxide surface shell, with Ce³⁺ at the surface and Ce⁴⁺ in the bulk. In the case of manganese oxide, partial reduction results in a mixture of Mn³⁺ and Mn²⁺, with Mn²⁺ located at the surface. With Pt/CeO_X, a high density of defect-associated bridging OH groups react with CO to yield a high density of the formate intermediate. With Pt/MnO_X, the fraction of reactive OH groups is low and much lower formate band intensities result upon CO adsorption; moreover, there is a greater fraction of OH groups that are essentially unreactive. Thus, much lower CO conversion rates are observed with Pt/MnO_X during low temperature water–gas shift. As with ceria, increasing the Pt loading facilitates partial reduction of MnO_X to lower temperature, indicating metal–oxide interactions should be taken into account.     

ZrO_2/TiO_2催化剂上一步法合成乙二醇二甲醚

采用共沉淀法制备了ZrO2/TiO_2催化剂,在固定床反应器上考察其催化二甲醚(DME)与环氧乙烷(EO)一步法合成乙二醇二甲醚(DMG)的性能,并采用X射线衍射(XRD)、氮气吸附(BET)、氨气化学吸附和氨气傅立叶红外(NH_3-FTIR)等对催化剂进行表征。结果表明,当ZrO_2质量分数为25%时,催化剂呈现无定形态,最可几孔径分布为7.5 nm,NH_3吸附酸量670μmol/g,表面分布B酸与L酸。在反应温度80℃,压力0.6 MPa,DME与EO物质的量之比为3,气体空速(GHSV)为1 800.0 h~(-1)的条件下,EO转化率100.0%,DMXG(乙二醇二甲醚、二乙二醇二甲醚和多乙二醇二甲醚)选择性89.5%以上。催化剂连续使用720 h,EO转化率99.7%,DMG选择性为71.7%。     

Spill-Over Effects on Bimetallic Pt/Ru(0001) Surfaces

The concept of spill-over of adsorbed species has a long tradition in Heterogeneous Catalysis and has been explored also for adsorption on bimetallic surfaces, in particular by the Goodman group. In the present paper, we report results of a comprehensive temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy study on spill-over effects in the adsorption and desorption of CO on structurally well defined bimetallic Pt/Ru(0001) surfaces, where part of the substrate is covered by monolayer Pt islands. While upon adsorption at 90 K, the mobility of CO_ad molecules on the surface is very limited, it is activated when the adlayer is annealed to 150 K or, more directly, if CO exposure is done at 150 K or higher temperatures. This enables diffusion of CO_ad molecules to the Pt free Ru(0001) areas, even at local CO_ad coverages which preclude further adsorption from the gas phase on the Ru parts of the surface. Spill-over processes are shown to have significant impact on the TPD spectra; furthermore they provide an additional adsorption channel for adsorption on the bare Ru(0001) areas, allowing uptake of CO at local coverages where adsorption from the gas phase is precluded. This indicates that the apparent CO saturation coverage of 0.68 ML determined for direct adsorption on Ru(0001) under UHV conditions is limited by kinetics rather than thermodynamics. The data are discussed in comparison with results and interpretations in earlier studies, which indicate that these effects are not limited to the Pt/Ru(0001) surface, but may be found on a wide range of bimetallic systems.     

Basic oxide-supported Ru catalysts for liquid phase glycerol hydrogenolysis in an additive-free system

Several basic oxide-supported Ru catalysts (Ru/CeO₂, Ru/La₂O₃ and Ru/MgO) were prepared and evaluated for the hydrogenolysis of glycerol. The Ru catalysts were characterized by inductively coupled plasma–atomic emission spectroscopy, nitrogen adsorption, powder X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Ru/CeO₂ showed the best performance in the reaction, which is associated with its smaller metal particle size and the weak surface basicity feature of CeO₂. 1,2-Propanediol is obtained as the main product through a dehydrogenation–dehydration–hydrogenation mechanism. The oxidation product lactic acid can be formed by a Cannizzaro reaction from the pyruvaldehyde intermediate.     

Adsorption of weak bases from the gas phase on organic ion exchangers

The adsorption of dimethyl ether and tetrahydrofurane has been investigated at 150 °C on macroreticular ion exchangers of the styrene-divinylbenzene type containing SO₃H, PO(OH)₂ or P(OH)₂ groups. Dual site adsorption without dissociation is suggested on the basis of the analysis of adsorption isotherms and the investigation of the effect of the number of acid groups on the initial adsorption values. The adsorption coefficients found can be used to express the acid strength of the ion exchangers interacting with gaseous molecules at elevated temperature; the relative logarithmic values for the ion exchangers, respectively, with SO₃H, PO(OH)₂ and P(OH)₂ groups were 0; ?1.64; and ?2.51 (from dimethyl ether adsorption). These results are consistent with those calculated from ethyl acetate adsorption [Komers and Tomanová, Collect. Czech. Chem. Commun.37, 774 (1972)]. On the basis of the adsorption data, the relative basicities of the adsorbed molecules could also be compared.     

Fischer-Tropsch synthesis and the generation of DME in situ

Ternary physical mixtures comprised a Fischer-Tropsch catalyst, a methanol synthesis catalyst and a zeolite employed in the hydrocarbon synthesis from syngas. Two Fe-based catalysts (i.e., one promoted by K and the other by Ru), two HY zeolites with different acidities, a commercial HZSM-5 and Cu/ZnO/Al₂O₃ (methanol synthesis catalyst) were used in these systems. The main products obtained were dimethyl ether, methanol and hydrocarbons. First of all, it was observed that by adding Cu/ZnO/Al₂O₃ catalyst to a binary physical mixture comprised of a Fischer-Tropsch catalyst and HZSM-5, the CO conversion increases more than 20 times. Second, during the reaction transient period the dimethyl ether selectivity decreases as the conversion increases. Third, the hydrocarbons synthesized followed the ASF distribution in the C₁-C₁₂ range and finally, it was also verified that the Y zeolites and the Fischer-Tropsch synthesis catalyst promoted by Ru generated the most active physical mixtures. The results showed that the role of zeolites in the ternary physical mixture is only associated with the dimethyl ether synthesis. The following reaction pathway was suggested: first, methanol is synthesized from syngas using Cu/ZnO/Al₂O₃ catalyst; after that, this alcohol is dehydrated by an acid catalyst generating DME; and lastly, DME initiates Fischer-Tropsch synthesis, which is then propagated by CO.     

Role of ceria-supported noble metal catalysts (Ru, Pd, Pt) in wet air oxidation of nitrogen and oxygen containing compounds

Catalytic wet air oxidation (CWAO) of aniline, phenol, carboxylic acids and ammonia was carried out in a batch reactor over noble metals (Ru, Pd, Pt) supported on ceria. Ruthenium is very active for the conversion of a wide range of organic compounds and selective into carbon dioxide. The ability of ceria to transfer oxygen is essential for good performances in CWAO. However, Ru/CeO₂ is not selective for ammonia oxidation into N₂. Addition of small amount of Pd enhances both activity and selectivity of Ru in this reaction. Finally, oxidation of nitrogenous organic compounds requires moderate temperature and oxygen pressure and needs to adjust the oxidizing capacity of the catalyst.     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

Enhanced CeO2-supported Pt catalyst for water-gas shift reaction

Sodium-promoted, CeO₂-supported, Pt catalyst were synthesized and investigated for water-gas shift (WGS) reaction. The ceria supports were synthesized by conventional precipitation method. The effect of the basic aqueous solutions used in the preparation procedure on the nature of Pt/CeO₂ catalyst was examined by physical and chemical characterization analyses. The catalytic activity for the WGS reaction was observed. In the Pt/CeO₂ catalyst prepared by NaOH, the presence of sodium (1.64 wt.%) on the support modified the base and electronic properties and consequently enhanced the catalytic activity of Pt/CeO₂. Based on temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared spectroscopy (DRIFTS) analyses, we concluded that sodium introduced through the ceria preparation procedure played an important role in the WGS reaction as a promoter to induce the fast build-up of surface intermediates of the WGS reaction and also as an electron donor to modify the adsorption strength between CO and Pt.     

Gold nanoparticles on ceria: importance of O vacancies in the activation of gold

Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO₂ and the water-gas shift (WGS, CO + H₂O → H₂ + CO₂) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO₂ was investigated on Au/CeO₂(111) and AuO _x /CeO₂ surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO₂(111) was 4–7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO₂ on the Au/CeO₂(111) surfaces. The full decomposition of SO₂ was observed only after introducing O vacancies in the ceria support. AuO _x /CeO₂ surfaces were found to be much less chemically active than Au/CeO₂(111) or Au/CeO_2−x (111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured {Au + AuO _x }–CeO₂ catalysts under the WGS reaction. At temperatures above 250 °C, a complete AuO _x → Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO₂(111) single crystal corroborate that cationic Au^δ+ species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in {Au + AuO _x }/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.     

Synthesis of AlOOH slurry catalyst and catalytic activity for methanol dehydration to dimethyl ether

AlOOH slurry catalysts were prepared by complete liquid-phase technology from aluminum iso-propoxide (AIP). Dehydration of methanol to dimethyl ether (DME) over these catalysts was investigated in slurry reactor. The catalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption, temperature-programmed desorption of ammonia (NH₃–TPD). The results showed that the slurry catalysts had high specific surface area and pore volume, and the specific surface area and the strength of weak acidic sites were influenced considerably by the molar ratio of H₂O/AIP and HNO₃/AIP. Activity tests indicated that AlOOH slurry catalysts had excellent catalytic activity and stability in slurry reactor for the dehydration of methanol to dimethyl ether, and the activity correlated well with the strength of weak acidic sites of catalysts, which can be controlled by changing the H₂O/AIP and HNO₃/AIP molar ratios. The average methanol conversion at even stage reaches nearly 80% and DME selectivity almost 100% over CAT-P1 catalyst. No deactivation was found during the reaction of 500 h. It is also expected that CAT-P1 becomes a promising methanol dehydration catalyst for the STD process based on CuZuAl methanol synthesis catalyst.     

Interaction of SO2 with CeO2 and Cu/CeO2 catalysts: photoemission, XANES and TPD studies

CeO₂ and Cu/CeO₂ are effective catalysts/sorbents for the removal or destruction of SO₂. Synchrotron‐based high‐resolution photoemission, X‐ray absorption near‐edge spectroscopy (XANES), and temperature‐programmed desorption (TPD) have been employed to study the reaction of SO₂ with pure and reduced CeO₂ powders, ceria films (CeO₂, CeO_2−x, Ce₂O_3+x) and model Cu/CeO₂ catalysts. The results of XANES and photoemission provide evidence that SO₄ was formed upon the adsorption of SO₂ on pure powders or films of CeO₂ at 300 K. The sulfate decomposed in the 390–670 K temperature range with mainly SO₂ and some SO₃ evolving into gas phase. At 670 K, there was still a significant amount of SO₄ present on the CeO₂ substrates. The introduction of O vacancies in the CeO₂ powders or films favored the formation of SO₃ instead of SO₄. Ceria was able to fully dissociate SO₂ to atomic S only if Ce atoms with a low oxidation state were available in the system. When Cu atoms were added to CeO₂ new active sites for the destruction of SO₂ were created improving the catalytic activity of the system. The surface chemistry of SO₂ on the Cu‐promoted CeO₂ was much richer than on pure CeO₂. The behavior of ceria in several catalytic processes (oxidation of SO₂ by O₂, reduction of SO₂ by CO, automobile exhaust converters) is discussed in light of these results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Defect induced magnetic transition in Co doped CeO2 sputtered thin films

Cobalt-doped cerium dioxide thin films exhibit room temperature ferromagnetism due to high oxygen mobility in doped CeO₂ lattice. CeO₂ is an excellent doping matrix as there is a possibility of it losing oxygen while retaining its structure. This leads to increased oxygen mobility within the fluorite CeO₂ lattice, leading to formation of Ce³⁺ and Ce⁴⁺ species. Magnetic ceria materials are important in several applications from magnetic data storage devices to magnetically recoverable catalysts. In this paper, the room temperature ferromagnetism of rf sputtered Co doped CeO₂ thin films is reported whereas undoped CeO₂ thin films exhibit paramagnetic behavior. The ferromagnetic properties of the Co doped films were explained based on oxygen vacancies created by Co ions in Ce sites. This is further supported by X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman. Change in surface morphology due to Co doping of the samples were analyzed using atomic force microscopy (AFM).     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

Site requirements and elementary steps in dimethyl ether carbonylation catalyzed by acidic zeolites

Steady-state, transient, and isotopic-exchange studies of dimethyl ether (DME) carbonylation, combined with adsorption and desorption studies of probe molecules and infrared (IR) spectroscopy, were used to identify methyl and acetyl groups as surface intermediates within specific elementary steps involved in the synthesis of methyl acetate from DME–CO mixtures with >99% selectivity on H-zeolites. Carbonylation rates increased linearly with CO pressures but did not depend on DME pressures, suggesting that the addition of CO to CH₃ groups present at saturation coverage controls catalytic carbonylation rates. These reactions lead to acetyl groups that subsequently react with DME to form methyl acetate (423–463 K; >99% selectivity) and regenerate methyl intermediates, consistent with kinetic studies of CO reactions with CH₃ groups previously formed from DME and with kinetic and IR studies of DME reactions with acetyl groups formed by stoichiometric reactions of acetic anhydride. These studies show that CO reacts with DME-derived intermediates bound on zeolitic Al sites from the gas phase or via weakly held CO species adsorbed non-competitively with CH₃ groups. These reactions, in contrast with similar reactions of methanol, occur under anhydrous conditions and avoid the formation of water, which strongly inhibits carbonylation reactions.