Investigation of the Intrinsic Activity of ZrxCe1−xO2 Mixed Oxides in the CO + NO Reactions: Influence of Pd Incorporation

The intrinsic activity of various Zr _x Ce_1–x O₂ mixed oxides and after a Pd deposition has been investigated in the CO + NO reactions from temperature-programmed experiments performed under stoichiometric conditions. It has been found that the activity of Zr _x Ce_1–x O₂ depends on either the specific surface area or the number of Ce cations and their intrinsic activity, Zr_0.5Ce_0.5O₂ being the most active support. The addition of palladium strongly enhances the catalytic activity of the supports probably due to a synergistic effect between CeO₂ and the metal since the initial activity of palladium-based catalysts is directly related to their Ce content. Such a catalytic enhancement has been explained by a bifunctional mechanism involving active sites probably composed of Pd and ceria. A strong deactivation operates leading to the disappearance of the beneficial effect of ceria. Such a deactivation seems to be dependent on the support composition, Pd supported Zr_0.25Ce_0.75O₂ being the most resistant to deactivation.     

Electrochemical lean-deNOx catalyst using H-conducting solid polymer electrolyte

The reduction of NO to N₂/N₂O in the presence of excess O₂ has been successfully achieved at 70 °C using an electrochemical cell of the type, 0.1% NO, 0–10% O₂, Pt | NAFION | Pt, H₂O. An H⁺-conducting solid polymer electrolyte (SPE) plays a key role in evolving hydrogen on the Pt cathode, where the catalytic NO–H₂ takes place. It was revealed that the competitive H₂–O₂ reaction is suppressed because the Pt surface was covered with stable nitrate (NO₃) species, which blocks oxygen adsorption hereon. The inhibition of H₂–O₂ reaction becomes most efficient at 100 °C in agreement with the optimal operation temperature range of SPE. The reduction efficiency of NO in an excess O₂ could be improved by packing 1 wt% Pt/ZSM-5 catalyst in the cathode room. The combination between the SPE cell and Pt catalysts can broadly be applied to novel low-temperature deNO_x processes in a strongly oxidizing atmosphere.     

Role of La2O3 in Pd-Supported Catalysts for Methanol Decomposition

Systems of Pd supported on various La₂O₃-modified -Al₂O₃ and CeO₂–Al₂O₃ catalysts were tested for catalytic methanol decomposition and characterized by means of XRD, BET, TPR, H₂-chemisorption and CO–FTIR. The addition of lanthanum significantly improved the selectivity of CO and H₂ for all the catalysts but showed a different influence on the catalytic activity in two systems. Methanol conversion decreased on La₂O₃-modified Pd/-Al₂O₃ catalysts, in line with the reduction of Pd dispersion, while the addition of La₂O₃ improved the dispersion of Pd and reinforced Pd–CeO₂ interaction for La₂O₃-modified Pd/CeO₂–Al₂O₃ catalysts, which resulted in a high production rate of CO and H₂. Thus, a synergistic effect between CeO₂ and La₂O₃ was observed on -Al₂O₃-supported Pd catalyst for methanol decomposition.     

Synthesis and Catalytic Properties of the Electrochemical NO x Reduction System

Catalytic and electrical properties of an electrochemical NO_x reduction system were investigated. This system had a laminated structure composed of BaCo(Al,Ga)₁₁O₁₉-based catalyst layer on a Pt/YSZ/Pt sheet. The stacked catalyst system can directly reduce more than 65% of NO_x to N₂ under an external bias above 2.5 V at 650 °C. In this system, oxygen existing around the catalyst layer was removed by O²⁻ transportation through the YSZ layer.     

First In Situ Raman Study of Vanadium Oxide Based SO2 Oxidation Supported Molten Salt Catalysts

In situ Raman spectroscopy at temperatures up to 500°C is used for the first time to identify vanadium species on the surface of a vanadium oxide based supported molten salt catalyst during SO₂ oxidation. Vanadia/silica catalysts impregnated with Cs₂SO₄ were exposed to various SO₂/O₂/SO₃ atmospheres and in situ Raman spectra were obtained and compared to Raman spectra of unsupported model V₂O₅–Cs₂SO₄ and V₂O₅–Cs₂S₂O₇ molten salts. The data indicate that (1) the V^V complex V^VO₂(SO₄)₂ ^3– (with characteristic bands at 1034 cm^–1 due to (V=O) and 940 cm^–1 due to sulfate) and Cs₂SO₄ dominate the catalyst surface after calcination; (2) upon admission of SO₃/O₂ the excess sulfate is converted to pyrosulfate and the V^V dimer (V^VO)₂O(SO₄)₄ ^4– (with characteristic bands at 1046 cm^–1 due to (V=O), 830 cm^–1 due to bridging S–O along S–O–V and 770 cm^–1 due to V–O–V) is formed and (3) admission of SO₂ causes reduction of V^V to V^IV (with the (V=O) shifting to 1024 cm^–1) and to V^IV precipitation below 420°C.     

Study of IrxRu1−xO2 oxides as anodic electrocatalysts for solid polymer electrolyte water electrolysis

Electrocatalysts of the general formula Ir_xRu_1−xO₂ were prepared using Adams’ fusion method. The crystallite characterization was examined via XRD, and the electrochemical properties were examined via cyclic voltammetry (CV) in, linear sweep voltammetry (LSV) and chronopotentiometry measurements in 0.5 M H₂SO₄. The electrocatalysts were applied to a membrane electrode assembly (MEA) and studied in situ in an electrolysis cell through electrochemical impedance spectroscopy (EIS) and stationary current density–potential relations were investigated. The Ir_xRu_1−xO₂ (x = 0.2, 0.4, 0.6) compounds were found to be more active than pure IrO₂ and more stable than pure RuO₂. The most active electrocatalyst obtained had a composition of Ir_0.2Ru_0.8O₂. With an Ir_0.2Ru_0.8O₂ anode, a 28.4% Pt/C cathode and the total noble metal loading of 1.7 mg cm⁻², the potential of water electrolysis was 1.622 V at 1 A cm⁻² and 80 °C.     

Effect of CeO2 on Supported Pd Catalyst in the SCR of NO: A DRIFT Study

The activity of Pd/Al₂O₃ and Pd/Al₂O₃–CeO₂ samples has been tested in the selective catalytic reduction of NO by propene. It is found that the activity of Pd/Al₂O₃ decreases with calcination temperature, while the activity of Pd/Al₂O₃–CeO₂ increases abnormally with increasing calcination temperature. Surface-area measurement shows both samples suffer a linear decrease in their surface area, so it is reasonable to attribute the activity enhancement to the effect of CeO₂. The adsorption behavior and state of surface-active sites have been characterized by diffuse reflectance FTIR spectroscopy using CO and NO as probes and the effect of CeO₂ has been revealed. The CeO₂ component increases and stabilizes the dispersion of surface Pd species to prevent it from aggregating at high temperature. CeO₂ may also act as a buffer during the redox cycle of Pd, lengthen the period of Pd redox procedure and render Pd a property of inertia in its redox process, thus increasing the activity of the Pd/Al₂O₃–CeO₂ sample. The essential feature of both effects is the strong interaction between Pd and CeO₂. The intensity of interaction increases linearly with calcination temperature and so does the sample activity.     

High Efficiency of Noble Metal and Metal Oxide Catalyst Systems for the Selective Reduction of NO with Propene in Lean Exhaust Gas

An investigation was conducted of noble metal and metal oxide catalysts deposited on Al₂O₃. The noble metals Pt, Pd, Rh the metal oxides CuO, SnO₂, CoO, Ag₂O, In₂O₃, catalysts were examined. Also investigated were noble metal Pt, Pd, Rh-doped In₂O₃/Al₂O₃ catalysts prepared by single sol–gel method. Both were studied for their capability to reduce NO by propene under lean conditions. In order to improve the catalytic activity and the temperature window, the intermediate addition propene between a Pt/Al₂O₃ oxidation and metal oxide combined catalyst system was also studied. Pt/Al₂O₃ and In₂O₃/Al₂O₃ combined catalyst showed high NO reduction activity in a wider temperature window, and more than 60% NO conversion was observed in the temperature range of 300–550 °C.     

Decomposition and oxidation of CH3 13CH2OH on A12O3, Pd/Al2O3, and PdO/Al2O3 catalysts

Temperature-programmed desorption (TPD) and oxidation (TPO) were used to investigate the decomposition and oxidation of ethanol on Al₂O₃, Pd/Al₂O₃, and PdO/Al₂O₃. Ethyl--¹³C alcohol (CH₃ ¹³CH₂OH) was adsorbed on the catalysts so that reaction pathways of the two carbons could be distinguished. Alumina was mainly a dehydration catalyst, but dehydrogenation was also observed and some carbon remained on the surface. In the presence of O₂, A1₂O₃ oxidized the decomposition products and the-carbon was oxidized faster. Ethanol, which was adsorbed on A1₂O₃, decomposed much faster on Pd/A1₂O₃ by diffusing to Pd and undergoing CO elimination to form CH₄,¹³CO, H₂, and surface carbon. On PdO/A1₂O₃, the decomposition was slower than on Pd/A1₂O₃ until lattice oxygen was extracted above 450 K; the decomposition products were oxidized by lattice oxygen. In the presence of gas phase O₂, Pd/Al₂O₃ was an active oxidation catalyst at low temperature, but lattice oxygen had to be extracted from PdO/A1₂O₃ before it had significant oxidation activity.     

Investigation of NO Adsorption and NO/O2 Co-adsorption on NOx-Storage-Components by DRIFT-Spectroscopy

NO adsorption and NO/O₂ co-adsorption on CeO₂ at different temperatures was studied by DRIFT-Spectroscopy. The results indicate that this oxide plays an important role in storing NO _x . FTIR studies show that NO adsorption is dominated by the formation of nitrite species. Furthermore, cis- and trans hyponitrite species are detected. Co-adsorption of NO/O₂ leads to the formation of nitrates. The experimental data show that the formation of nitrates is a consecutive reaction: adsorption of NO to form nitrite species (NO₂ ^–), followed by an oxidation to form nitrate species (NO₃ ^–).     

Influence of OSC Material on the Behaviour of Metallic Pd Catalysts in C2H4 and CO Oxidation as Well as in NO Reduction

The interaction of CO, C₂H₄, O₂, and NO reaction gas compounds over the metallic Pd/Al₂O₃ and Pd/OSC/Al₂O₃ monoliths was investigated in order to understand the behaviour of OSC material in the oxidation and reduction reactions. FT-IR gas analyser was used for the analysis of the product gas composition. Several activity experiments carried out with dissimilar feedstreams have revealed that the Ce _x Zr_1–x O₂ mixed oxide is an oxygen storage compound, which promotes CO and C₂H₄ oxidation as well as NO reduction in particular at low temperatures.     

Performance of Pd catalysts supported on nanocrystalline α-Al2O3 and Ni-modified α-Al2O3 in selective hydrogenation of acetylene

Nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ have been prepared by sol–gel and solvothermal methods and employed as supports for Pd catalysts. Regardless of the preparation method used, NiAl₂O₄ spinel was formed on the Ni-modified α-Al₂O₃ after calcination at 1150 °C. However, an addition of NiO peaks was also observed by X-ray diffraction for the solvothermal-made Ni-modified α-Al₂O₃ powder. Catalytic performances of the Pd catalysts supported on these nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ in selective hydrogenation of acetylene were found to be superior to those of the commercial α-Al₂O₃ supported one. Ethylene selectivities were improved in the order: Pd/Ni-modified α-Al₂O₃–sol–gel > Pd/Ni-modified α-Al₂O₃-solvothermal ≈ Pd/α-Al₂O₃–sol–gel > Pd/α-Al₂O₃-solvothermal  Pd/α-Al₂O₃-commerical. As revealed by NH₃ temperature program desorption studies, incorporation of Ni atoms in α-Al₂O₃ resulted in a significant decrease of acid sites on the alumina supports. Moreover, XPS revealed a shift of Pd 3d binding energy for Pd catalyst supported on Ni-modified α-Al₂O₃–sol–gel where only NiAl₂O₄ was formed, suggesting that the electronic properties of Pd may be modified.     

Catalytic behaviour and characterisation of CuO1−z(La2O3)z/2 based catalysts deposited on γ-Al2O3 and prepared in situ with 0.0 ⩽z⩽1.00, without and with addition of Pd

(CuO)_1–z(La₂O₃)_z/2 based catalysts with 0.0z1.0 supported on -Al₂O₃ have been prepared in situ and the phases formed have been identified by XRD, SEM and TEM/EDS studies. The catalyst with z=0.5 exhibited the best catalytic activity for oxidation of CO (T ₅₀=295 and 390C with degrees of conversions of 93 and 92% at 450C under rich and lean conditions, respectively) and C₃H₆ (291 and 414C; 93 and 83%) and reduction of NO (405C; 60 and 0%). This catalyst contained appreciable amounts of the perovskite phase LaAl_1–xCu_xO₃ and the enhanced catalytic properties are ascribed to the presence of this phase. Addition of Pd to this catalyst implied that the degree of conversion of NO increased and that the light-off temperatures for all involved gas species decreased. Ageing experiments revealed that LaAl_1–xCu_xO₃ decomposed and that Cu containing Pd particles were formed during this procedure which in turn deteriorated the catalytic properties of the catalyst.     

Sol–Gel Pd Exhaust Catalysts and N2O Production

Al₂O₃, CeO₂–Al₂O₃, CeO₂–Tb₄O₇–Al₂O₃ and ZrO₂–Al₂O₃ supported Pd samples have been prepared by sol–gel methods. Extents and mechanisms of N₂O production in CO–NO and CO–NO–O₂ reactions on these have been considered. This occurs most selectively under oxidising (lean-burn) conditions or in the presence of CeO₂ and CeO₂–Tb₄O₇ promoters near the CO–NO light off temperature. Over Pd/ZrO₂–Al₂O₃ the CO–NO reaction at 573 K has CO and NO conversions that are second order with respect to p _CO and p _NO. Over this catalyst NO conversion is faster than that of CO until O_2(g) is added, causing CO conversion and N₂O production at 573 K to rise simultaneously. CeO₂ or CeO₂–Tb₄O₇ incorporation into a Pd/Al₂O₃ catalyst enhances N₂O production near the CO–NO light-off temperature in the absence of added O₂ without CO conversion being raised. There is current attention on pollution control opportunities through lean-burn conditions, Pd catalysts and oxygen storage capacity enhancement. The present work suggests that their role in N₂O production may need to be better understood and controlled. For the moment N₂O formation provides a window on mechanisms of TWC operation.     

NO2 inhibits the catalytic reaction of NO and O2 over Pt

The rate equation for the overall reaction of NO and O₂ over Pt/Al₂O₃ was determined to be r=k_f[NO] ^1.05±0.08[O₂]^1.03±0.08[NO₂]^0.92±0.07(1-), with k_f as the forward rate constant, =([NO₂]/K[NO][O₂]^1/2), and K as the equilibrium constant for the overall reaction. An apparent activation energy of 82 kJ mol^–1 ± 9 kJ mol^–1 was observed. The inhibition by the product NO₂ makes it imperative to include the influence of NO₂ concentration in any analysis of the kinetics of this reaction. The reaction mechanism that fits our observed orders consists of the equilibrated dissociation of NO₂ to produce a surface mostly covered by oxygen, thereby inhibiting the equilibrium adsorption of NO, and the non-dissociative adsorption of O₂, which is the proposed rate determining step.     

Electrical properties of V2O5–WO3/TiO2 EUROCAT catalysts evidence for redox process in selective catalytic reduction (SCR) deNOx reaction

Electrical conductivity measurements on EUROCAT V₂O₅–WO₃/TiO₂ catalyst and on its precursor without vanadia were performed at 300°C under pure oxygen to characterize the samples, under NO and under NH₃ to determine the mode of reactivity of these reactants and under two reaction mixtures ((i) 2000 ppm NO + 2000 ppm NH₃ without O₂, and (ii) 2000 ppm NO + 2000 ppm NH₃ + 500 ppm O₂) to put in evidence redox processes in SCR deNO_x reaction.It was first demonstrated that titania support contains certain amounts of dissolved W⁶⁺ and V⁵⁺ ions, whose dissolution in the lattice of titania creates an n-type doping effect. Electrical conductivity revealed that the so-called reference pure titania monolith was highly doped by heterovalent cations whose valency was higher than +4. Subsequent chemical analyses revealed that so-called pure titania reference catalyst was actually the WO₃/TiO₂ precursor of V₂O₅–WO₃/TiO₂ EUROCAT catalyst. It contained an average amount of 0.37 at.% W⁶⁺dissolved in titania, i.e. 1.07 × 10²⁰ W⁶⁺ cations dissolved/cm³ of titania. For the fresh catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved in titania were found to be equal to 1.07 × 10²⁰ and 4.47 × 10²⁰ cm⁻³, respectively. For the used catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved were found to be equal to 1.07 × 10²⁰ and 7.42 × 10²⁰ cm⁻³, respectively. Since fresh and used catalysts have similar compositions and similar catalytic behaviours, the only manifestation of ageing was a supplementary progressive dissolution of 2.9 × 10²⁰ additional V⁵⁺ cations in titania.After a prompt removal of oxygen, it appeared that NO alone has an electron acceptor character, linked to its possible ionosorption as NO⁻ and to the filling of anionic vacancies, mostly present on vanadia. Ammonia had a strong reducing behaviour with the formation of singly ionized vacancies. A subsequent introduction of NO indicated a donor character of this molecule, in opposition to its first adsorption. This was ascribed to its reaction with previously adsorbed ammonia strongly bound to acidic sites. Under NO + NH₃ reaction mixture in the absence of oxygen, the increase of electrical conductivity was ascribed to the formation of anionic vacancies, mainly on vanadia, created by dehydroxylation and dehydration of the surface. These anionic vacancies were initially subsequently filled by the oxygen atom of NO. N^o atoms, resulting from the dissociation of NO and from ammonia dehydrogenation, recombined into dinitrogen molecules. The reaction corresponded to

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. In the presence of oxygen, NO did not exhibit anymore its electron acceptor character, since the filling of anionic vacancies was performed by oxygen from the gas phase. NO reacted directly with ammonia strongly bound on acidic sites. A tentative redox mechanism was proposed for both cases.     

FT-IR spectroscopic investigation of the surface reaction of CH4 with NO x species adsorbed on Pd/WO3–ZrO2 catalyst

The interaction of methane at various temperatures with NO _x species formed by room temperature adsorption of NO + O₂ mixture on tungstated zirconia (18.6 wt.% WO₃) and palladium(II)-promoted tungstated zirconia (0.1 wt.% Pd) has been investigated using in situ FT-IR spectroscopy. A mechanism for the reduction of NO over the Pd-promoted tungstated zirconia is proposed, which involves a step consisting of thermal decomposition of the nitromethane to adsorbed NO and formates through the intermediacy of cis-methyl nitrite. The HCOO⁻ formed acts as a reductant of the adsorbed NO producing nitrogen.     

Catalytic NO–H2–CO–O2 reactions over Pt-supported mesoporous yttrium oxide

Catalytic light-off of a stream of NO, H₂, CO in an excess O₂ has been studied over various metal oxides loading 1 wt% Pt. Because a low-surface area Y₂O₃ (<5 m² g⁻¹) was found to exhibit the highest de-NO_x activity, a mesoporous Y₂O₃ was then synthesized from an yttrium-based surfactant mesophase templated by dodecyl sulfate , which was anion-exchanged by acetate (AcO = CH₃COO⁻). The product showed a 3-D mesoporosity with a large surface area (396 m² g⁻¹) and the Pt-supported catalyst achieved much improved light-off characteristics suitable for the low-temperature de-NO_x in the presence of CO and excess O₂.     

ESR study of alumina-supported palladium catalyst

The ESR spectra of differently pretreated 0.97 wt% Pd/Al₂O₃ catalyst showed very broad signal atg 2.10 assigned to Pd⁺ ions. The intensity of this signal is stronger after pretreatments at higher temperatures (500–600 °C). This result appears to support our earlier idea (ref. [2]) as to an important role of electron-deficient palladium as an active centre in catalyzing the reaction of neopentane hydrogenolysis.     

Studies on the redox behaviour of La1.867Th0.100CuO4 and its catalytic performance for NO decomposition

La_1.867Th_0.100CuO₄ was prepared by means of the citric acid complexing method. The reduction–oxidation (redox) properties of this composite oxide have been investigated by using the XRD, TGA, EPR, TPD, and SEM methods. The fresh (non-reduced) La_1.867Th_0.100CuO₄ catalyst is single phase with tetragonal K₂NiF₄-type structure. There were three reduction steps observed over La_1.867Th_0.100CuO₄ in the temperature ranges of 25–100, 100–300, and 300–500 °C, respectively. After reduction at 300 °C, the material still retained its original single phase but there were oxygen vacancies generated in the lattice. After reduction at 500 °C, it decomposed to a mixture of oxides. In the course of reduction, trapped electrons were generated. During the oxidation of the reduced sample, O ₂ ^– was detected. Apparently, oxygen vacancies are able to stabilise O ₂ ^– on the surface of the -1ptcatalyst. NO adsorption on both the fresh and reduced La_1.867Th_0.100CuO₄ samples generated NO radicals and O ₂ ^– species. On a La_1.867Th_0.100CuO₄ sample reduced at 300 °C, [O₂NO₂]^2– was generated in NO adsorption and decomposed to N₂ and O^2– at ca. 730 °C. After reduction, the O ₂ ^– inside the La_1.867Th_0.100CuO₄ lattice became more mobile and participated in the decomposition of [O₂NO₂]^2–. The fresh (non-reduced) La_1.867Th_0.100CuO₄ sample with cation defects in its lattice shows higher NO decomposition activity than the fresh La₂CuO₄ sample in which there are no cation defects. The 300 °C-reduced La_1.867Th_0.100CuO₄ with cation defects and oxygen vacancies is more active than the fresh one for NO decomposition. The redox action between Cu⁺ and Cu²⁺ is an essential process for NO decomposition.