Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

Design of a novel bifunctional catalyst IrFe/Al2O3 for preferential CO oxidation

In this study, a novel bifunctional catalyst IrFe/Al₂O₃, which is very active and selective for preferential oxidation of CO under H₂-rich atmosphere, has been developed. When the molar ratio of Fe/Ir was 5/1, the IrFe/Al₂O₃ catalyst performed best, with CO conversion of 68% and oxygen selectivity towards CO₂ formation of 86.8% attained at 100 °C. It has also been found that the impregnation sequence of Ir and Fe species on the Al₂O₃ support had a remarkable effect on the catalytic performance; the activity decreased following the order of IrFe/Al₂O₃ > co-IrFe/Al₂O₃ > FeIr/Al₂O₃. The three catalysts were characterized by XRD, H₂-TPR, FT-IR and microcalorimetry. The results demonstrated that when Ir was supported on the pre-formed Fe/Al₂O₃, the resulting structure (IrFe/Al₂O₃) allowed more metallic Ir sites exposed on the surface and accessible for CO adsorption, while did not interfere with the O₂ activation on the FeO_x species. Thus, a bifunctional catalytic mechanism has been proposed where CO adsorbed on Ir sites and O₂ adsorbed on FeO_x sites; the reaction may take place at the interface of Ir and FeO_x or via a spill-over process.     

Enthalpy recovery of gases issued from H2 production processes: Activity and stability of oxide and noble metal catalysts in oxidation reaction under highly severe conditions

Oxidation activity and stability under reaction was investigated for a series of mixed oxide catalysts, doped or not by a precious metal (Pd, Pt). The reaction feedstock, containing CO, H₂, CH₄, CO₂ and H₂O, simulated gases issued from H₂ production processes for fuel cells. Contrarily to conventional noble metal catalysts, mixed oxide samples present generally good stability under reaction at high temperature. The activities measured for the perovskite and hexaaluminate catalysts, are however largely lower than that of the reference Pd/Al₂O₃ catalyst. High activities were obtained after impregnation of 1.1 wt.% Pd or 0.8 wt.% Pt on the hexaaluminates samples. Even if Pd/Al₂O₃ was found to present a high activity, this sample suffered from drastic deactivation at 700 °C. Better stability were obtained on perovskite. Furthermore, doping hexaaluminate by Pt led to samples with good activities and high stability. Even if better activities were obtained by doping the hexaaluminate samples by Pd, the Pd/BaAl₁₂O₁₉ strongly deactivated, as it was previously observed for the reference catalyst. Interestingly, this Pd deactivation was not observed when Pd was impregnated on the Mn substituted hexaaluminate, leading to a stable and active catalyst. This suggests that it is possible to stabilize the palladium in its oxidized form at high temperature (700 °C) on the surface of some supports.     

Quantified NOx adsorption on Pt/K/gamma-Al2O3 and the effects of CO2 and H2O

A multi-component NO_x-trap catalyst consisting of Pt and K supported on γ-Al₂O₃ was studied at 250 °C to determine the roles of the individual catalyst components, to identify the adsorbing species during the lean capture cycle, and to assess the effects of H₂O and CO₂ on NO_x storage. The Al₂O₃ support was shown to have NO_x trapping capability with and without Pt present (at 250 °C Pt/Al₂O₃ adsorbs 2.3 μmols NO_x/m²). NO_x is primarily trapped on Al₂O₃ in the form of nitrates with monodentate, chelating and bridged forms apparent in Diffuse Reflectance mid-Infrared Fourier Transform Spectroscopy (DRIFTS) analysis. The addition of K to the catalyst increases the adsorption capacity to 6.2 μmols NO_x/m², and the primary storage form on K is a free nitrate ion. Quantitative DRIFTS analysis shows that 12% of the nitrates on a Pt/K/Al₂O₃ catalyst are coordinated on the Al₂O₃ support at saturation.

When 5% CO₂ was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by 45% after 1 h on stream due to the competition of adsorbed free nitrates with carboxylates for adsorption sites. When 5% H₂O was included in a feed stream with 300 ppm NO and 12% O₂, the amount of K-based nitrate storage decreased by only 16% after 1 h, but the Al₂O₃-based nitrates decreased by 92%. Interestingly, with both 5% CO₂ and 5% H₂O in the feed, the total storage only decreased by 11%, as the hydroxyl groups generated on Al₂O₃ destabilized the K–CO₂ bond; specifically, H₂O mitigates the NO_x storage capacity losses associated with carboxylate competition.     

A comparative study of the water-gas shift activity of Pt catalysts supported on single (MOx) and composite (MOx/Al2O3, MOx/TiO2) metal oxide carriers

The water-gas shift (WGS) activity of platinum catalysts dispersed on a variety of single metal oxides as well as on composite MO_x/Al₂O₃ and MO_x/TiO₂ supports (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, La, Ce, Nd, Sm, Eu, Gd, Ho, Er, Tm) has been investigated in the temperature range of 150–500 °C, using a feed composition consisting of 3% CO an 10% H₂O. For Pt catalysts supported on single metal oxides, it has been found that both the apparent activation energy of the reaction and the intrinsic rate depend strongly on the nature of the support. In particular, specific activity of Pt at 250 °C is 1–2 orders of magnitude higher when supported on “reducible” compared to “irreducible” metal oxides. For composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts, it is shown that the presence of MO_x results in a shift of the CO conversion curve toward lower reaction temperatures, compared to that obtained for Pt/Al₂O₃ or Pt/TiO₂, respectively. The specific reaction rate is in most cases higher for composite catalysts and varies in a manner which depends on the nature, loading, and primary crystallite size of dispersed MO_x. Results are explained by considering that reducibility of small oxide particles increases with decreasing crystallite size, thereby resulting in enhanced WGS activity. Therefore, evidence is provided that the metal oxide support is directly involved in the WGS reaction mechanism and determines to a significant extent the catalytic performance of supported noble metal catalysts. Results of catalytic performance tests obtained under realistic feed composition, consisting of 3% CO, 10% H₂O, 20% H₂ and 6% CO₂, showed that certain composite Pt/MO_x/Al₂O₃ and Pt/MO_x/TiO₂ catalysts are promising candidates for the development of active WGS catalysts suitable for fuel cell applications.     

Re-investigating the CO oxidation mechanism over unsupported MnO, Mn2O3 and MnO2 catalysts

Kinetic study of CO oxidation in combination with experiments of temperature-programmed oxidation (TPO) and reduction (TPR) have been performed on various unsupported crystalline manganese oxides (MnO_x); while the reactivity shows an order of MnO ≤ MnO₂ < Mn₂O₃ in a mixture of unit ratio of O₂/CO at/below 523 K. We propose that under the current conditions the interaction of adsorbed CO and O is mainly responsible for CO₂ formation on Mn₂O₃ and MnO₂ catalysts, following either the Langmuir–Hinshelwood mechanism or Eley–Rideal mechanism. Meanwhile, direct evidence from transient CO oxidation suggests that the Mars-van-Krevelen mechanism may occur for all catalysts simultaneously, especially, it is predominant for the MnO catalyst. The evidence of structural modifications during reaction was confirmed by Raman spectra obtained from used MnO.     

Combined CO2 reforming and partial oxidation of n-heptane on noble metal zirconia catalysts

The combined CO₂ reforming and partial oxidation (POX) of n-heptane was studied on various noble metal zirconia catalysts between 700 and 900 °C. The activity order of the metals was Rh > Pd > Ir > Pt. Selectivity to syngas increased with the activity of the catalysts but the H₂ to CO molar ratio decreased. The activity and selectivity of the 0.25 wt% Rh/ZrO₂ catalyst were close to the performance of a commercial 15 wt% NiO/Al₂O₃ catalyst. The conversions and product compositions were compared to the calculated thermodynamic equilibria.     

CO oxidation and oxygen-assisted CO adsorption/desorption on Ag/MnOx catalysts

A series of Ag-doped manganese oxide catalyst were synthesized by the reflux method in an acid medium. The surface structure of the catalysts was characterized by N₂ adsorption, XRD and TEM experiments. The catalysts showed excellent catalytic activity for CO oxidation. The adsorption and oxidation of CO on a 1.0% Ag/MnO_x catalyst between 393 and 493 K were studied by means of single pulse experiments in a TAP reactor. The adsorption of CO was reversible at these temperatures and CO₂ was formed in an oxidation reaction of CO and lattice oxygen. Curve fitting to the experimental TAP response curves of the reactant and product was used to determine the kinetic parameters for the elementary steps. The activation energies were 83 kJ/mol for CO desorption, 31 kJ/mol for CO₂ desorption, and 116 kJ/mol for the surface CO oxidation by lattice oxygen. In addition, the effect of coadsorbed O₂ on CO adsorption was studied by the TAP technique. Below 353 K, there was a sharp increase, by about one order of magnitude, in the rate constant of CO adsorption promoted by the presence of coadsorbed O₂.     

Catalytic reduction of NOx with H2/CO/CH4 over PdMOR catalysts

Conversion of NO_x with reducing agents H₂, CO and CH₄, with and without O₂, H₂O, and CO₂ were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO_x to N₂ conversion with H₂ and CO (>90% conversion and N₂ selectivity) range under lean conditions. The formation of N₂O is absent in the presence of both H₂ and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H₂ and CO at 450–500 K. The positive effect of cerium is significant in the case of H₂ and CH₄ reducing agent but is less obvious with H₂/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH₄, 500 ppm NO, 5% O₂, 10% H₂O (0–1% H₂), N₂ balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO_x reduction with H₂, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K.     

NOx storage and reduction characteristics of Pd/MnOx–CeO2 at low temperature

The reaction between hydrogen and NO was studied over 1 wt.% Pd supported on NO_x-sorbing material, MnO_x–CeO₂, at low temperatures. The result of pulse mode reactions suggest that NO_x adsorbed as nitrate and/or nitrite on MnO_x–CeO₂ was reduced by hydrogen, which was spilt-over from Pd catalyst. The NO_x storage and reduction (NSR) cycles were carried out over Pd/MnO_x–CeO₂ in a conventional flow reactor at 150 °C. In a storage step, NO was removed by the oxidative adsorption from a stream of 0.04–0.08% NO, 5–10% O₂, and He balance. This was followed by a reducing step, where a stream of 1% H₂/He was supplied to ensure the conversion of nitrate/nitrite to N₂ and thus restore the adsorbability. It was revealed that the NSR cycle is much more suitable for the H₂–deNO_x process in excess O₂, compared to a conventional steady state reaction mode.     

Pt/MnOx–CeO2 catalysts for the complete oxidation of formaldehyde at ambient temperature

MnO_x–CeO₂ mixed oxides with a Mn/(Mn + Ce) molar ratios of 0–1 were prepared by a modified coprecipitation method and investigated for the complete oxidation of formaldehyde. The MnO_x–CeO₂ with Mn/(Mn + Ce) molar ratio of 0.5 exhibited the highest catalytic activity among the MnO_x–CeO₂ mixed oxides. Structure analysis by X-ray powder diffraction and temperature-programmed reduction of hydrogen revealed that the formation of MnO_x–CeO₂ solid solution greatly improved the low-temperature reducibility, resulting in a higher catalytic activity for the oxidation of formaldehyde. Promoting effect of Pt on the MnO_x–CeO₂ mixed oxide indicated that both the Pt precursors and the reduction temperature greatly affected the catalytic performance. Pt/MnO_x–CeO₂ catalyst prepared from chlorine-free precursor showed extremely high activity and stability after pretreatment with hydrogen at 473 K. 100% conversion of formaldehyde was achieved at ambient temperature and no deactivation was observed for 120 h time-on-stream. The promoting effect of Pt was ascribed to enhance the effective activation of oxygen molecule on the MnO_x–CeO₂ support.     

Influence of CO2 and H2O on NOx storage and reduction on a Pt–Ba/γ-Al2O3 catalyst

In this paper, the effect of CO₂ and H₂O on NO_x storage and reduction over a Pt–Ba/γ-Al₂O₃ (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO₂ and H₂O, NO_x is stored on BaCO₃ sites only. Moreover, H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N₂ as well as complete regeneration of stored NO. In the presence of CO₂, NO is oxidized into NO₂ and more NO_x is stored as in the presence of H₂O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO_x trapping in the presence of CO₂·NH₃ formation is seen in the rich phase and the selectivity towards N₂ is 83%. Ba(NO₃)₂ is always completely regenerated during the subsequent rich phase. In the absence of CO₂ and H₂O, both surface and bulk barium sites are active in NO_x storage. As lean/rich cycling proceeds, the selectivity towards N₂ in the rich phase decreases from 82% to 47% and the N balance for successive lean/rich cycles shows incomplete regeneration of the catalyst. This incomplete regeneration along with a 40% decrease in the Pt dispersion and BET surface area, explains the observed decrease in NO_x storage.     

CO2 reforming of methane to syngas over highly active and stable Pt/MgO catalysts

Pt/MgO catalysts were prepared by wet impregnation. At 800 °C and atmospheric pressure, Pt/MgO catalysts exhibited a high stability at high gas hourly space velocity of 36,000 ml/g h with a CH₄/CO₂ ratio of 1.0. During 72 h time on stream, the conversion of CH₄ and CO₂ remained almost constant, at about 88% and 90%, respectively. There was no loss of Pt. After reaction, the XRD peaks of MgO became broader, indicating amorphization of MgO, which was supported by TEM results. XPS indicated that the reforming reaction had little influence on Pt. CO₂-TPSR results showed that some carbon deposition occurred under stoichiometric feed of CH₄ and CO₂, but it did not result in the deactivation of the catalyst. The deposited carbon came mainly from the decomposition of methane.     

Lean NOx reduction with CO + H2 mixtures over Pt/Al2O3 and Pd/Al2O3 catalysts

The reduction of NO_x by hydrogen under lean burn conditions over Pt/Al₂O₃ is strongly poisoned by carbon monoxide. This is due to the strong adsorption and subsequent high coverage of CO, which significantly increases the temperature required to initiate the reaction. Even relatively small concentrations of CO dramatically reduce the maximum NO_x conversions achievable. In contrast, the presence of CO has a pronounced promoting influence in the case of Pd/Al₂O₃. In this case, although pure H₂ and pure CO are ineffective for NO_x reduction under lean burn conditions, H₂/CO mixtures are very effective. With a realistic (1:3) H₂:CO ratio, typical of actual exhaust gas, Pd/Al₂O₃ is significantly more active than Pt/Al₂O₃, delivering 45% NO_x conversion at 160 °C, compared to >15% for Pt/Al₂O₃ under identical conditions. The nature of the support is also critically important, with Pd/Al₂O₃ being much more active than Pd/SiO₂. Possible mechanisms for the improved performance of Pd/Al₂O₃ in the presence of H₂+CO are discussed.     

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.     

Preparation and characterization of LaFe1−xMgxO3/Al2O3/FeCrAl: Catalytic properties in methane combustion

MnO_x–CeO₂ mixed oxides prepared by sol–gel method, coprecipitation method and modified coprecipitation method were investigated for the complete oxidation of formaldehyde. Structure analysis by H₂-TPR and XPS revealed that there were more Mn⁴⁺ species and richer lattice oxygen on the surface of the catalyst prepared by the modified coprecipitation method than those of the catalysts prepared by sol–gel and coprecipitation methods, resulting in much higher catalytic activity toward complete oxidation of formaldehyde. The effect of calcination temperature on the structural features and catalytic behavior of the MnO_x–CeO₂ mixed oxides prepared by the modified coprecipitation was further examined, and the catalyst calcined at 773 K showed 100% formaldehyde conversion at a temperature as low as 373 K. For the samples calcined below 773 K, no any diffraction peak corresponding to manganese oxides could be detected by XRD measurement due to the formation of MnO_x–CeO₂ solid solution. While the diffraction peaks corresponding to MnO₂ phase in the samples calcined above 773 K were clearly observed, indicating the occurrence of phase segregation between MnO₂ and CeO₂. Accordingly, it was supposed that the strong interaction between MnO_x and CeO₂, which depends on the preparation route and the calcination temperature, played a crucial role in determining the catalytic activity toward the complete oxidation of formaldehyde.     

Potential rare-earth modified CeO2 catalysts for soot oxidation: Part III. Effect of dopant loading and calcination temperature on catalytic activity with O2 and NO + O2

CeO₂ and CeReO_x_y catalysts are prepared by the calcination at different temperatures (y = 500–1000 °C) and having a different composition (Re = La³⁺ or Pr^3+/4+_, 0–90 wt.%). The catalysts are characterised by XRD, H₂-TPR, Raman, and BET surface area. The soot oxidation is studied with O₂ and NO + O₂ in the tight and loose contact conditions, respectively. CeO₂ sinters between 800–900 °C due to a grain growth, leading to an increased crystallite size and a decreased BET surface area. La³⁺ or Pr^3+/4+ hinders the grain growth of CeO₂ and, thereby, improving the surface catalytic properties. Using O₂ as an oxidant, an improved soot oxidation is observed over CeLaO_x_y and CePrO_x_y in the whole dopant weight loading and calcination temperature range studied, compared with CeO₂. Using NO + O₂, the soot conversion decreased over CeLaO_x_y catalysts calcined below 800 °C compared with the soot oxidation over CeO₂_y. CePrO_x_y, on the other hand, showed a superior soot oxidation activity in the whole composition and calcination temperature range using NO + O₂. The improvement in the soot oxidation activity over the various catalysts with O₂ can be explained based on an improvement in the external surface area. The superior soot oxidation activity of CePrO_x_y with NO + O₂ is explained by the changes in the redox properties of the catalyst as well as surface area. CePrO_x_y, having 50 wt.% of dopant, is found to be the best catalyst due to synergism between cerium and praseodymium compared to pure components. NO into NO₂ oxidation activity, that determines soot oxidation activity, is improved over all CePrO_x catalysts.     

Potential rare-earth modified CeO2 catalysts for soot oxidation part II: Characterisation and catalytic activity with NO + O2

Ceria (CeO₂) and rare-earth modified ceria (CeReO_x with Re = La³⁺, Pr^3+/4+, Sm³⁺, Y³⁺) supports and Pt impregnated supports are studied for the soot oxidation under a loose contact with the catalyst with the feed gas, containing NO + O₂. The catalysts are characterised by XRD, H₂-TPR, DRIFT and Raman spectroscopy. Among the single component oxides, CeO₂ is significantly more active compared with the other lanthanide oxides used in this study. Doping CeO₂ with Pr^3+/4+ and La³⁺ improved, however, the soot oxidation activity of the resulting solid solutions. This improvement is correlated with the surface area in the case of CeLaO_x and to the surface area and redox properties of CePrO_x catalyst. The NO conversion to NO₂ over these catalysts is responsible for the soot oxidation activity. If the activity per unit surface area is compared CePrO_x is the most active one. This indicates that though La³⁺ can stabilise the surface area of the catalyst in fact it decreases the soot oxidation activity of Ce⁴⁺. The lattice oxygen participates in NO conversion to NO₂ and the rate of this lattice oxygen transfer is much faster on CePrO_x. In general, the improvement of the soot oxidation is observed over the Pt impregnated CeO₂ and CeReO_x catalysts, and can be correlated to the presence of Pt°. The surface reduction of the supports in the presence of Pt occurred below 100 °C. The surface redox properties of the support in the Pt catalysts do not have a significant role in the NO to NO₂ conversion. In spite of the lower surface area, the Pt/CeYO_x and Pt/CeO₂ catalysts are found to be more active due to larger Pt crystal sizes. The presence of Pt also improved the CO conversion to CO₂ over these catalysts. The activation energy for the soot oxidation with NO + O₂ is found to be around 50 kJ/mol.     

Catalytic reduction of sulfur dioxide using hydrogen or carbon monoxide over Ce1−xZrxO2 catalysts for the recovery of elemental sulfur

This study focuses on the direct sulfur recovery process (DSRP), in which SO₂ can be directly converted into elemental sulfur using a variety of reducing agents over Ce_1−xZr_xO₂ catalysts. Ce_1−xZr_xO₂ catalysts (where x = 0.2, 0.5, and 0.8) were prepared by a citric complexation method. The experimental conditions used for SO₂ reduction were as follow: the space velocity (GHSV) was 30,000 ml/g_-cat h and the ratio of [CO (or H₂, H₂ + CO)]/[SO₂] was 2.0. It was found that the catalyst and reducing agent providing the best performance were the Ce_0.5Zr_0.5O₂ catalyst and CO, respectively. In this case, the SO₂ conversion was about 92% and the sulfur yield was about 90% at 550 °C. Also, a higher efficiency of SO₂ removal and elemental sulfur recovery was achieved in the reduction of SO₂ with CO as a reducing agent than that with H₂. In the reduction of SO₂ by H₂ over the Ce_0.5Zr_0.5O₂ catalyst, SO₂ conversion and sulfur yield were about 92.7% and 73%, respectively, at 800 °C. Also, the reduction of SO₂ using synthetic gas with various [CO]/[H₂] molar ratios over the Ce_0.5Zr_0.5O₂ catalyst was performed, in order to investigate the possibility of using coal-derived gas as a reducing agent in the DSRP. It was found that the reactivity of the SO₂ reduction using the synthetic gas with various [CO]/[H₂] molar ratios was increased with increasing CO content of the synthetic gas. Therefore, it was found that the Ce_1−xZr_xO₂ catalysts are applicable to the DSRP using coal-derived gas, which contains a larger percentage of CO than H₂.     

Strong rhodium–niobia interaction in Rh/Nb2O5, Nb2O5–Rh/SiO2 and RhNbO4/SiO2 catalysts: Application to selective CO oxidation and CO hydrogenation

The extent of Rh–niobia interaction in niobia-supported Rh (Rh/Nb₂O₅), niobia-promoted Rh/SiO₂ (Nb₂O₅–Rh/SiO₂) and RhNbO₄/SiO₂ catalyst after H₂ reduction has been investigated by H₂ and CO chemisorption measurements. These catalysts have been applied to selective CO oxidation in H₂ (CO+H₂+O₂) and CO hydrogenation (CO+H₂), and the results are compared with those of unpromoted Rh/SiO₂ catalysts. It has been found that niobia (NbO_x) increases the activity and selectivity for both the reactions.