Catalytic reduction of NO by CO over NiO/CeO2 catalyst in stoichiometric NO/CO and NO/CO/O2 reaction

A new catalyst composed of nickel oxide and cerium oxide was studied with respect to its activity for NO reduction by CO under stoichiometric conditions in the absence as well as the presence of oxygen. Activity measurements of the NO/CO reaction were also conducted over NiO/γ-Al₂O₃, NiO/TiO₂, and NiO/CeO₂ catalysts for comparison purposes. The results showed that the conversion of NO and CO are dependent on the nature of supports, and the catalysts decreased in activity in the order of NiO/CeO₂ > NiO/γ-Al₂O₃ > NiO/TiO₂. Three kinds of CeO₂ were prepared and used as support for NiO. They are the CeO₂ prepared by (i) homogeneous precipitation (HP), (ii) precipitation (PC), and (iii) direct decomposition (DP) method. We found that the NiO/CeO₂(HP) catalyst was the most active, and complete conversion of NO and CO occurred at 210 °C at a space velocity of 120,000 h⁻¹. Based on the results of surface analysis, a reaction model for NO/CO interaction over NiO/CeO₂ has been proposed: (i) CO reduces surface oxygen to create vacant sites; (ii) on the vacant sites, NO dissociates to produce N₂; and (iii) the oxygen originated from NO dissociation is removed by CO.     

CH4/CO2 reforming over La2NiO4 and 10%NiO/CeO2–La2O3 catalysts under the condition of supersonic jet expansion via cavity ring-down spectroscopic analysis

CH₄/CO₂ reforming over La₂NiO₄ and 10%NiO/CeO₂–La₂O₃ catalysts under the condition of supersonic jet expansion was studied via direct monitoring of the reactants and products using the sensitive technique of cavity ring-down spectroscopy. Vibration–rotational absorption lines of CH₄, H₂O, CO₂ and CO molecules were recorded in the near infrared spectral region. Our results indicated that La₂NiO₄ is superior to 10%NiO/CeO₂–La₂O₃ in performance. In addition, we observed enhanced reverse-water-gas-shift reaction at augmented reaction temperature. The formation of reaction intermediates was also investigated by means of time-of-flight mass spectrometry and there was the detection of CH_x⁺, OH⁺ and H⁺ species.     

Kinetics of CO and H2 oxidation over CuO-CeO2 catalyst in H2 mixtures with CO2 and H2O

The kinetics of CO and H₂ oxidation over a CuO-CeO₂ catalyst were simultaneously investigated under reaction conditions of preferential CO oxidation (PROX) in hydrogen-rich mixtures with CO₂ and H₂O. An integral packed-bed tubular reactor was used to produce kinetic data for power-law kinetics for both CO and H₂ oxidations. The experimental results showed that the CO oxidation rate was essentially independent of H₂ and O₂ concentrations, while the H₂ oxidation rate was practically independent of CO and O₂ concentrations. In the CO oxidation, the reaction orders were 0.91, −0.37 and −0.62 with respect to the partial pressure of CO, CO₂ and H₂O, respectively. In the H₂ oxidation, the orders were 1.0, −0.48 and −0.69 with respect to the partial pressure of H₂, CO₂ and H₂O, respectively. The activation energies of the CO oxidation and the H₂ oxidation were 94.4 and 142 kJ/mol, respectively. The rate expressions of both oxidations were able to predict the performance of the PROX reactor with accuracy. The independence between the CO and the H₂ oxidation suggested different sites for CO and H₂ adsorption on the CuO-CeO₂ catalyst. Based on the results, we proposed a new reaction model for the preferential CO oxidation. The model assumes that CO adsorbs selectively on the Cu⁺ sites; H₂ dissociates and adsorbs on the Cu⁰ sites; the adsorbed species migrates to the interface between the copper components and the ceria support, and reacts there with the oxygen supplied by the ceria support; and the oxygen deficiency on the support is replenished by the oxygen in the reaction mixture.     

Photocatalytic syngas synthesis from CO2 and H2O using ultrafine CeO2-decorated layered double hydroxide nanosheets under visible-light up to 600 nm

The rational design of photocatalyst that can effectively reduce CO₂ under visible light(l>400 nm),and simultaneously precise control of the products syngas(CO/H2)ratio is highly desirable for the Fischer-Tropsch reaction.In this work,we synthesized a series of CeO₂-decorated layered double hydroxides(LDHs,Ce-x)samples for photocatalytic CO₂ reduction.It was found that the selectivity and productivity of CO and H₂ from photoreduction of CO₂ in conjunction with Ru-complex as photosensitizer performed an obvious“volcano-like”trend,with the highest point at Ce-0.15 and the CO/H₂ ratio can be widely tunable from 1/7.7 to 1/1.3.Furthermore,compared with LDH,Ce-0.15 also drove photocatalytic CO₂ to syngas under 600 nm irradiation.It implied that an optimum amount of CeO₂ modifying LDH promoted the photoreduction of CO₂ to syngas.This report gives the way to fully utilize the rare earth elements and provides a promising route to enhance the photo-response ability and charge injection efficiency of LDH-based photocatalysts in the synthesis of syngas with a tunable ratio under visible light irradiation.     

CO oxidation at low temperature over Pd supported on CeO2-TiO2 composite oxide

Low temperature CO oxidation was carried out over CeO₂-TiO₂ composite oxide and thereon supported Pd catalysts. The effects of Ce/Ti ratio and pre-treatments of calcination and reduction on the catalytic behaviour were investigated. The CO oxidation starts at about 220 °C over CeO₂-TiO₂ and the pre-reduction treatment has little influence on the catalytic activity. Pd supported on CeO₂-TiO₂ (Pd/CeO₂-TiO₂) exhibits high activity for CO oxidation and a complete conversion of CO to CO₂ can be achieved even at ambient temperature, which suggests a synergistic effect between Pd and CeO₂-TiO₂. The activity and stability of Pd/CeO₂-TiO₂ can be further improved by the pre-reduction treatment. Ce/Ti ratio influences the catalytic behaviour significantly; the catalyst Pd/CeO₂-TiO₂ with a Ce/Ti mole ratio of 0.20 (Pd/Ce20Ti) owns the highest activity and stability, which suggests an optimization of the Pd-Ce-Ti interaction in Pd/Ce20Ti. The calcined Pd/CeO₂-TiO₂ with a Ce/Ti mole ratio higher than 0.10 shows a distorted light-off profile with the temperature, which implies an alternation of the reaction mechanism with increasing temperature.     

Tri-reforming of methane: a novel concept for catalytic production of industrially useful synthesis gas with desired H2/CO ratios

A novel process concept called tri-reforming of methane has been proposed in our laboratory using CO₂ in the flue gases from fossil fuel-based power plants without CO₂ separation [C. Song, Chemical Innovation 31 (2001) 21–26]. The proposed tri-reforming process is a synergetic combination of CO₂ reforming, steam reforming, and partial oxidation of methane in a single reactor for effective production of industrially useful synthesis gas (syngas). Both experimental testing and computational analysis show that tri-reforming can not only produce synthesis gas (CO + H₂) with desired H₂/CO ratios (1.5–2.0), but also could eliminate carbon formation which is usually a serious problem in the CO₂ reforming of methane. These two advantages have been demonstrated by tri-reforming of CH₄ in a fixed-bed flow reactor at 850 °C with supported nickel catalysts. Over 95% CH₄ conversion and about 80% CO₂ conversion can be achieved in tri-reforming over Ni catalysts supported on an oxide substrate. The type and nature of catalysts have a significant impact on CO₂ conversion in the presence of H₂O and O₂ in tri-reforming in the temperature range of 700–850 °C. Among all the catalysts tested for tri-reforming, their ability to enhance the conversion of CO₂ follows the order of Ni/MgO > Ni/MgO/CeZrO > Ni/CeO₂ ≈ Ni/ZrO₂ ≈ Ni/Al₂O₃ > Ni/CeZrO. The higher CO₂ conversion over Ni/MgO and Ni/MgO/CeZrO in tri-reforming may be related to the interaction of CO₂ with MgO and more interface between Ni and MgO resulting from the formation of NiO/MgO solid solution. Results of catalytic performance tests over Ni/MgO/CeZrO catalysts at 850 °C and 1 atm with different feed compositions confirm the predicted equilibrium conversions based on the thermodynamic analysis for tri-reforming of methane. Kinetics of tri-reforming were also examined. The reaction orders with respect to partial pressures of CO₂ and H₂O are different over Ni/MgO, Ni/MgO/CeZrO, and Ni/Al₂O₃ catalysts for tri-reforming.     

A study on the characteristics of the SO2 reduction using coal gas over SnO2-ZrO2 catalysts

SO₂, which is an air pollutant causing acid rain and smog, can be converted into elemental sulfur in direct sulfur recovery process (DSRP). SO₂ reduction was performed over catalyst in DSRP. In this study, SnO₂-ZrO₂ catalysts were prepared by a co-precipitation method, and CO and coal gas, which contains H₂, CO, CO₂ and H₂O, were used as reductants. The reactivity profile of the SO₂ reduction over the catalysts was investigated at the various reaction conditions as follows: reaction temperature of 300–550 °C, space velocity of 5000–30,000 cm³/g_-cat. h, [reductant]/[SO₂] molar ratio of 1.0–4.0 and Sn/Zr molar ratio of SnO₂-ZrO₂ catalysts 0/1, 2/8, 3/5, 5/5, 2/1, 3/1, 4/1 and 1/0. SnO₂-ZrO₂ (Sn/Zr = 2/1) catalyst showed the best performance for the SO₂ reduction in DSRP on the basis of our experimental results. The optimized reaction temperature and space velocity were 325 °C and 10,000 cm³/g_-cat. h, respectively. The optimal molar ratio of [reductant]/[SO₂] varied with the reductants, that is, 2.0 for CO and 2.5 for coal gas. SO₂ conversion of 98% and sulfur yield of 78% were achieved with the coal gas.     

Role of CeO2 in Ni/CeO2–Al2O3 catalysts for carbon dioxide reforming of methane

Ni catalysts supported on γ-Al₂O₃, CeO₂ and CeO₂–Al₂O₃ systems were tested for catalytic CO₂ reforming of methane into synthesis gas. Ni/CeO₂–Al₂O₃ catalysts showed much better catalytic performance than either CeO₂- or γ-Al₂O₃-supported Ni catalysts. CeO₂ as a support for Ni catalysts produced a strong metal–support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO₂ had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/γ-Al₂O₃ catalysts for this reaction. A weight loading of 1–5 wt% CeO₂ was found to be the optimum. Ni catalysts with CeO₂ promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO₂-promoted catalysts are attributed to the oxidative properties of CeO₂.     

Tuning porosity of coal-derived activated carbons for CO2 adsorption

A simple method was developed to tune the porosity of coal-derived activated carbons, which provided a model adsorbent system to investigate the volumetric CO₂ adsorption performance. Specifically, the method involved the variation of the activation temperature in a K₂CO₃ induced chemical activation process which could yield activated carbons with defined microporous (< 2 nm, including ultra-microporous < 1 nm) and meso-micro-porous structures. CO₂ adsorption isotherms revealed that the microporous activated carbon has the highest measured CO₂ adsorption capacity (6.0 mmol∙g^–1 at 0 °C and 4.1 mmol∙g^–1 at 25 °C), whilst ultra-microporous activated carbon with a high packing density exhibited the highest normalized capacity with respect to packing volume (1.8 mmol∙cm⁻³ at 0 °C and 1.3 mmol∙cm^–3 at 25 °C), which is significant. Both experimental correlation analysis and molecular dynamics simulation demonstrated that (i) volumetric CO₂ adsorption capacity is directly proportional to the ultra-micropore volume, and (ii) an increase in micropore sizes is beneficial to improve the volumetric capacity, but may lead a low CO₂ adsorption density and thus low pore space utilization efficiency. The adsorption experiments on the activated carbons established the criterion for designing CO₂ adsorbents with high volumetric adsorption capacity.     

Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst

A series of CeO₂ promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N₂O). Addition of CeO₂ to Co₃O₄ led to an improvement in the catalytic activity for N₂O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N₂O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O₂, H₂O or NO. Methods of XRD, FE-SEM, BET, XPS, H₂-TPR and O₂-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO₂ could increase the surface area of Co₃O₄, and then improve the reduction of Co³⁺ to Co²⁺ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N₂O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO₂, are responsible for the enhancement of catalytic activity of Co₃O₄.     

Nano-structured CeO2 supported Cu-Pd bimetallic catalysts for the oxygen-assisted water–gas-shift reaction

The present work focuses on the development of novel Cu-Pd bimetallic catalysts supported on nano-sized high-surface-area CeO₂ for the oxygen-assisted water–gas-shift (OWGS) reaction. High-surface-area CeO₂ was synthesized by urea gelation (UG) and template-assisted (TA) methods. The UG method offered CeO₂ with a BET surface area of about 215 m²/g, significantly higher than that of commercially available CeO₂. Cu and Pd were supported on CeO₂ synthesized by the UG and TA methods and their catalytic performance in the OWGS reaction was investigated systematically. Catalysts with about 30 wt% Cu and 1 wt% Pd were found to exhibit a maximum CO conversion close to 100%. The effect of metal loading method and the influence of CeO₂ support on the catalytic performance were also investigated. The results indicated that Cu and Pd loaded by incipient wetness impregnation (IWI) exhibited better performance than that prepared by deposition–precipitation (DP) method. The difference in the catalytic activity was related to the lower Cu surface concentration, better Cu–Ce and Pd–Ce interactions and improved reducibility of Cu and Pd in the IWI catalyst as determined by the X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) studies. A direct relation between BET surface area of the CeO₂ support and CO conversion was also observed. The Cu-Pd bimetallic catalysts supported on high-surface-area CeO₂ synthesized by UG method exhibited at least two-fold higher CO conversion than the commercial CeO₂ or that obtained by TA method. The catalyst retains about 100% CO conversion even under extremely high H₂ concentration.     

Roles of CO2 and H2O as oxidants in the plasma reforming of aliphatic hydrocarbons

Roles of CO₂ and H₂O as oxidants are discussed based on the data on the substrate conversions and the product distributions in the nonthermal plasma reforming of aliphatic hydrocarbons such as methane, propane, and neopentane from 303 K to 433 K. Only small effects of initial concentrations of hydrocarbons and types of oxidants are observed on hydrocarbon conversions, and the initial chemical interaction between hydrocarbons and the oxidants unlikely occurs. CO₂ and H₂O have shown the comparable oxidation powers in the hydrocarbon reforming. Two molar excess of CO₂ or H₂O to methane is required to oxidize methane carbon atoms to CO and CO₂, and larger amounts of CO₂ or H₂O for propane and neopentane. The different natures of CO₂ and H₂O are reflected in the synthesis gas composition as in the ordinary catalytic reforming processes at higher temperatures: higher H₂ yields and higher H₂ to CO ratios on addition of H₂O.     

Efficient MgO-doped CaO sorbent pellets for high temperature CO2 capture

Novel MgO-doped CaO sorbent pellets were prepared by gel-casting and wet impregnation. The effect of Na⁺ and MgO on the structure and CO₂ adsorption performance of CaO sorbent pellets was elucidated. MgO-doped CaO sorbent pellets with the diameter range of 0.5-1.5 mm exhibited an excellent capacity for CO₂ adsorption and adsorption rate due to the homogeneous dispersion of MgO in the sorbent pellets and its effects on the physical structure of sorbents. The results show that MgO can effectively inhibit the sintering of CaO and retain the adsorption capacity of sorbents during multiple adsorption-desorption cycles. The presence of mesopores and macropores resulted in appreciable change of volume from CaO (16.7 cm³∙mol⁻¹) to CaCO₃ (36.9 cm³∙mol⁻¹) over repeated operation cycles. Ca2Mg1 sorbent pellets exhibited favorable CO₂ capture capacity (9.49 mmol∙g⁻¹), average adsorption rate (0.32 mmol∙g⁻¹∙min⁻¹) and conversion rate of CaO (74.83%) after 30 cycles.     

Cu-Ce-O/γ-Al_2O_3的CO氧化性能

采用柠檬酸络合法制备一系列不同铜铈比的Cu-Ce-O/γ-Al_2O_3催化剂,用XRD、H2-TPR对其进行表征,采用连续固定床微反装置对Cu-Ce-O/γ-Al_2O_3催化剂CO催化氧化活性进行评价。结果表明,Cu-Ce-O/γ-Al_2O_3催化剂的XRD图谱中除归属于γ-Al_2O_3的晶相峰外,还出现CuO和CeO_2的晶相峰。高温水热引起活性组分CeO_2的晶粒聚集、长大和尖晶石结构CuAl2O4物质的生成;CuO-CeO_2之间的共生共存与相互作用,使得Cu-Ce-O/γ-Al_2O_3催化剂中具有非完整结构的[Cu2+1-xCu+x][O1-12x12x]增多,Cu+离子和氧空位增多,有利于其H2-TPR还原峰温度向低温区偏移,有利于提高其CO的催化氧化活性,使得Cu-Ce-O/γ-Al_2O_3催化剂的TCO50和TCO90降低。Cu与Ce物质的量比为5∶5制备的Cu-Ce-O/γ-Al_2O_3-55催化剂的TCO50和TCO90分别降至最低的162℃和199℃,表明此时的Cu-Ce-O协同效应最佳;CuO-CeO_2二相的共生共存与相互作用有利于减少高温水热环境下活性组分的聚集和晶粒长大,有利于Cu-Ce-O/γ-Al_2O_3催化剂能够保持较高的CO催化氧化活性。     

CeO2改性Ni/γ-Al2O3催化剂上顺酐加氢制备丁二酸酐

采用等体积浸渍法制备CeO₂改性Ni/γ-Al₂O₃催化剂,通过BET、XRD、H₂-TPR和SEM等对催化剂结构及物化性能进行表征,考察Ni-CeO₂/γ-Al₂O₃催化剂对顺酐催化加氢制备丁二酸酐催化性能的影响。结果表明,引入适量CeO₂可提高催化剂活性组分Ni的分散度,增加催化剂比表面积,提高催化剂热稳定性。采用负载CeO₂质量分数5%的Ni-CeO₂/γ-Al₂O₃催化剂,在反应温度120 ℃、反应压力2.0 MPa和空速0.6 h^-1条件下,顺酐转化率为99.5%,丁二酸酐选择性为99.4%。     

The oxidizing role of CO2 at mild temperature on ceria-based catalysts

For thermodynamic reasons, CO₂ has always been considered as inert at mild reaction temperatures (300 °C). In this study, we show that CO₂ may be used as a valuable compound for the catalytic combustion of methane (CCM), if ceria-based materials are used as support for the palladium active phase. Adding CO₂ in the feed significantly improves performances of ceria-zirconia supported catalysts. On the contrary, catalytic performances are inhibited on Pd/γ-Al₂O₃. Inhibition can be avoided by mixing the Pd/γ-Al₂O₃ catalyst with some CeO₂ evidencing cooperation phenomena between both catalysts. In situ DRIFTS experiments show that the inhibition of the alumina-supported catalyst is not due to formation of carbonates species. After an in situ reducing pre-treatment, pure CO₂ is able to rapidly oxidize reduced Pd/Ce_0.21Zr_0.79O₂ catalyst at 300 °C. Dissociation of CO₂ on Ce_0.21Zr_0.79O₂ would be responsible for the oxidation process. Thus, CO₂ helps in replenishing the O reservoir (OSC) of the Ce-Zr-O support which is normally consumed by reductants such as CH₄, H₂ or other HC's. XPS experiments show enrichment in oxygen species bound to Ce (Low BE O1s) on the surface of ceria-zirconia when working in the presence of CO₂. Implications of these results on the behavior of ceria-containing catalysts can be important for practical applications, e.g., in automotive exhaust catalysis.     

Photocatalytic conversion of CH4 and CO2 to oxygenated compounds over Cu/CdS–TiO2/SiO2 catalyst

Coupled semiconductor (CS) Cu/CdS–TiO₂/SiO₂ photocatalyst was prepared using a mutli-step impregnation method. Its optical property was characterized by UV–vis spectra. BET, XRD, Raman and IR were used to study the structure of the photocatalyst. Fine CdS was found dispersed over the surface of anatase TiO₂/SiO₂ substrate. Chemisorption and IR analysis showed methane absorbed in the molecular state interacted weakly with the surface of catalyst, and the interaction of CO₂ with CS produced various forms of absorbed CO₂ species that were primarily present in the form of formate, bidentate and linear absorption species. Photocatalytic direct conversion of CH₄ and CO₂ was performed under the operation conditions: 373 K, 1:1 of CO₂/CH₄, 1 atm, space velocity of 200 h⁻¹ and UV intensity of 20.0 mW/cm². The conversion was 1.47% for CH₄ and 0.74% for CO₂ with a selectivity of acetone up to 92.3%. The reaction mechanisms were proposed based on the experimental observations.     

Ni/Al_2O_3催化剂的CO_2-TPD表征

Ni/Al_2O_3催化剂是甲烷二氧化碳重整反应制取合成气研究最多、最具应用潜力的一种催化剂。通过对催化剂进行CO_2-TPD研究,考察还原态Ni/Al_2O_3催化剂的CO_2脱附特性。结果表明,浸渍法制备的Ni/Al_2O_3催化剂CO_2脱附曲线呈现双峰,分别在(60~65)℃和(350~380)℃出现高低温两个活性位;高温CO_2吸附量为3.0 cm~3·g~(-1),低温CO_2吸附量为24.0 cm~3·g~(-1)。催化剂的CO_2吸附量与其Ni含量无关。考察选用不同载体的CO_2脱附行为,发现以Al_2O_3为载体的催化剂CO_2吸附量是MgO和SiO_2为载体催化剂的2~4倍,以TiO_2为载体的催化剂几乎不吸附CO_2。     

Interface engineering for enhancing electrocatalytic oxygen evolution reaction of CoS/CeO2 heterostructures

To realize renewable energy conversion,it is important to develop low-cost and high-efficiency electrocatalyst for oxygen evolution reaction.In this communication,a novel bijunction CoS/CeO₂ electrocatalyst grown on carbon cloth is prepared by the interface engineering.The interface engineering of CoS and CeO₂ facilitates a rapid charge transfer from CeO₂ to CoS.Such an electrocatalyst exhibits outstanding electrocatalytic activity with a low overpotential of 311 mV at 10 mA·cm?2 and low Tafel slope of 76.2 mV·dec?1,and is superior to that of CoS(372 mV)and CeO₂(530 mV)counterparts.And it has long-term durability under alkaline media.     

The effect of CeO2 structure on the activity of supported Pd catalysts used for methane steam reforming

Palladium (Pd) supported on CeO₂-promoted γ-Al₂O₃ with various CeO₂ (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO₂ structure and dispersion on the γ-Al₂O₃ support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO₂ structure. Pd is present on the oxidized CeO₂-promoted catalysts as Pd⁰, Pd⁺ and Pd²⁺, at ratios strongly dependent on CeO₂ structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO₂ and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO₂. FTIR spectra of adsorbed CO revealed that after pre-treatment under H₂ or in the presence of amorphous CeO₂, partial encapsulation of Pd particles occurs. CeO₂ structure influences the CH₄ steam reforming reaction rates. Crystalline CeO₂ and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO₂ as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al₂O₃ or on amorphous (reduced) CeO₂/Al₂O₃ catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO₂/γ-Al₂O₃ than on Pd/γ-Al₂O₃, which is attributed to a catalytic synergism between Pd and CeO₂. The low rates on the reduced Pd/CeO₂/Al₂O₃ catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.