Activity and characterization of Rh/Al2O3 and Rh-Na/Al2O3 catalysts for the SCR of NO with CO in the presence of SO2 and HCl

SO₂ and HCl are major pollutants emitted from waste incineration processes. Both pollutants are difficult to remove completely and can enter the catalytic reactor. In this work, the effects of SO₂ and HCl on the performance of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts for NO removal were investigated in simulated waste incineration conditions. The characterizations of the catalysts were analyzed by BET, SEM/EDS, XRD, and ESCA. Experimental results indicated the 1%Rh/Al₂O₃ catalyst was significantly deactivated for NO and CO conversions when SO₂ and HCl coexisted in the flue gas. The addition of between 2 and 10 wt.% Na promoted the activity of the 1%Rh/Al₂O₃ catalyst for NO removal, but decreased the CO oxidation and BET surface area. The catalytic activity for NO removal was inhibited by HCl as a result of the formation of RhCl₃. Adding Na to the Rh/Al₂O₃ catalyst decreased the inhibition of SO₂ because of the formation of Na₂SO₄, which was observed in the XRD and ESCA analyses. SEM mapping/EDS showed that more S was residual on the surface of the Rh-Na/Al₂O₃ catalyst than Cl.     

Improving the Activity of Rh/Al2O3 Catalyst for NO Reduction by Na Addition in the Presences of H2O and O2

The effect of Na addition on the performance of Rh/Al₂O₃ catalyst for NO reduction with CO in the presence of H₂O and O₂ was investigated. The reacted catalysts were analyzed by the FTIR technique to identify the products for further investigation on the possible catalytic reaction mechanisms and the reasons behind the H₂O poisoning. Experimental results show that the removal efficiency of NO by Rh/Al₂O₃ catalyst was 63% at 250 °C but that decreased as the H₂O content increased. Adding Na to modify the Rh/Al₂O₃ catalyst significantly enhanced the conversion of NO to 99% at 250–300 °C even as the H₂O content was 1.6 vol%. The FTIR analyses results reveal that the abundant H₂O in the flue gas can compete with NO to adsorb on the surfaces of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts and further enhance the formation of NO₃ that reacts with H. The effects of H₂O on Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts can be eliminated by increasing the reaction temperature to higher than 300 °C. Rh-Na/Al₂O₃ is a feasible catalyst for NO reduction at such condition with relative high H₂O and O₂ contents.     

Catalytic removal of NO in waste incineration processes over Rh/Al2O3 and Rh–Na/Al2O3: Effects of particulates,heavy metals,SO2 and HCl

Two novel catalysts Rh/Al₂O₃ and Rh–Na/Al₂O₃ were prepared for NO removal and tested their practical performances in a laboratory-scale waste incineration system. The effects of particulates, heavy metals, and acid gases on the catalysts were evaluated and investigated through several characterization techniques, such as SEM, EA, XRPD, ESCA, and FTIR. The results indicated that the NO conversions were increased with the accumulation of particulates on the surface of catalysts, which was attributed to the increase in carbon content. However, the increase in heavy metals Cd and Pb contents on the surface of catalysts decreased the activity of catalyst for NO removal but did not change the chemical state of Rh and Na. The Rh/Al₂O₃ catalysts were poisoned when the acid gases SO₂ and HCl were present in the flue gas, because Rh and Al reacted with S and Cl to form inactive products. Adding Na to Rh/Al₂O₃ catalysts produced a promoting effect for SO₂ removal due to the formation of Na₂SO₄. The influence levels of different pollutants on the performances of Rh/Al₂O₃ and Rh–Na/Al₂O₃ catalysts for NO removal followed the sequence of HCl > heavy metals > SO₂ > particles.     

The activity of Rh/Al2O3 and Rh-Na/Al2O3 catalysts for PAHs removal in the waste incineration processes: Effects of particulates, heavy metals, and acid gases

This study investigated the activity of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts for polycyclic aromatic hydrocarbons (PAHs) removal and the influence of particulates, heavy metals, and acid gases (SO₂ and HCl) on the performance of catalysts. The experiments were carried out in a laboratory-scale waste incineration system. Experimental results show that the destruction removal efficiency (DRE) of PAHs by Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts were 80% and 59%, respectively when the flue gas did not contain any pollutants. The concentrations of PAHs increased by using a Rh/Al₂O₃ catalyst when the flue gas contained Cd, Pb, and SO₂ and also increased by using a Rh-Na/Al₂O₃ catalyst when the flue gas contained particulates, Cd, and HCl. Adding Na to the Rh/Al₂O₃ catalyst can inhibit the increases of 3-4 ring PAHs when the flue gas contained Pb. The influence of acid gases on the performance of the Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts followed the sequence SO₂ > HCl > SO₂ + HCl. The activity of the catalysts for PAHs removal was significantly suppressed by increased concentrations in particulates and Cd, yet promoted by a high Pb concentration. The results of ESCA analysis indicated that the presence of Cd and Pb did not change the chemical states of Rh and Na, but the presence of SO₂ and HCl did.     

An overview: Comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO and NO + H2 reactions

This paper reports a comparative kinetic investigation of the overall reduction of NO in the presence of CO or H₂ over supported Pt-, Rh- and Pd-based catalysts. Different activity sequences have been established for the NO+H₂ reaction Pt/Al₂O₃>Pd/Al₂O₃>Rh/Al₂O₃ and for the NO+CO reaction Rh/Al₂O₃>Pd/Al₂O₃> Pt/Al₂O₃. It was found that both reactions differ from the rate determining step usually ascribed to the dissociation of chemisorbed NO molecules. The rate enhancement observed for the NO+H₂ reaction has been mainly related to the involvement of a dissociation step of chemisorbed NO molecules assisted by adjacent chemisorbed H atoms. The calculation of the kinetic and thermodynamic constants from steady-state rate measurements and subsequent comparisons show that Pd and Rh are predominantly covered by chemisorbed NO molecules in our operating conditions which could explain either changes in activity or in selectivity with the lack of ammonia formation on Rh/Al₂O₃ during the NO+H₂ reaction. Interestingly, Pd and Rh exhibit similar selectivity behaviour towards the production of nitrous oxide (N₂O) irrespective of the nature of the reducing agent (CO or H₂). A weak partial pressure dependency of the selectivity is observed which can be related to the predominant formation of N₂ via a reaction between chemisorbed NO molecules and N atoms, while over Pt-based catalysts the associative desorption of two adjacent N atoms would occur simultaneously. Such tendencies are still observed under lean conditions in the presence of an excess of oxygen. However, a detrimental effect is observed on the selectivity with an enhancement of the competitive H₂+O₂ reaction, and on the activity behaviour with a strong oxygen inhibiting effect on the rate of NO conversion, particularly on Rh.     

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.

     

The decomposition of NO on CNTs and 1 wt% Rh/CNTs

Carbon nanotubes (CNTs) and CNTs-supported rhodium were tested as catalysts for NO decomposition. For the fresh catalysts, 100% NO conversion was achieved at 600°C over CNTs; when 1 wt% Rh was loaded on CNTs, 100% NO conversion was achieved at 450°C. If the catalysts were pre-reduced in H₂ at or above 300°C, 100% NO conversions were observed at 300°C. XPS investigation indicated that there was still metallic rhodium (BE=307.2 eV) on Rh/CNTs after heating in air at 500°C for 2 h and after the NO decomposition reaction. As for a 1 wt% Rh/Al₂O₃ sample, the rhodium (BE = 308.2 eV) was completely in the form of Rh₂O₃ after similar treatments. These results suggest that compared to γ-Al₂O₃, the CNTs material is more capable of keeping the rhodium in its metallic state. The results obtained in H₂-TPR studies support this conclusion. In addition, TEM investigation revealed that the rhodium particles distributed rather evenly over CNTs with a particle diameter of around 8 nm. We propose that CNTs can be used as a material for the facilitation of NO decomposition. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

The catalytic removal of CO and NO over Co-Pt(Pd, Rh)/γ-Al2O3 catalysts and their structural characterizations

A series of Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts were prepared by successive wetness impregnation. The catalytic activities for CO oxidation, NO decomposition and NO selective catalytic reduction (SCR) by C₂H₄ over the samples calcined at 500°C and reduced at 450°C were determined. The activities of the samples calcined at 750°C and reduced at 450°C for NO selective catalytic reduction (SCR) by C₂H₄ were also determined. All the samples were characterized by XRD, XPS, XANES, EXAFS, TPR, TPO and TPD techniques. The results of activity measurements show that the presence of noble metals greatly enhances the activity of Co/γ-Al₂O₃ for CO or C₂H₄ oxidation. For NO decomposition, the H₂-reduced Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts exhibit very high activities during the initial period of catalytic reaction, but with the increase of reaction time, the activities decrease obviously because of the oxidation of surface cobalt phase. For NO selective reduction by C₂H₄, the reduced samples are oxidized more quickly by the excess oxygen in reaction gas. The oxidized samples possess very low activities for NO selective reduction. The results of XRD, XPS and EXAFS indicate that all the cobalt in Co-Pt(Pd, Rh)/γ-Al₂O₃ has been reduced to zero valence during reduction by H₂ at 450°C, but in Co/γ-Al₂O₃ only a part of the cobalt has been reduced to zero valence, the rest exists as CoAl₂O₄-like spinel which is difficult to reduce. For the samples calcined at 750°C, the cobalt exists as CoAl₂O₄ which cannot be reduced by H₂ at 450°C and possesses better activities for NO selective reduction. The results of XANES spectra show that the cobalt in Co/γ- Al₂O₃ has lower coordination symmetry than that in Co-Pt(Pd, Rh)/γ-Al₂O₃. This difference mainly results from the distorting tetrahedrally- coordinated Co²⁺ ions which have lower coordination symmetry than Co⁰ in the catalysts. The coordination number for the Co-Co shell from EXAFS has shown that the cobalt phase is highly dispersed on Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts. The TPR results indicate that the addition of noble metals to Co/γ- Al₂O₃ makes the TPR peaks shift to lower temperatures, which implies the spillover of hydrogen species from noble metals to cobalt oxides. The oxygen spillover from noble metals to cobalt is also inferred from the shift of TPO peaks to lower temperatures and the increased amount of desorbed oxygen from TPD. For CO oxidation, the Co⁰ is the main active phase. For NO decomposition and selective reduction, Co⁰ is also catalytically active, but it can be oxidized into Co₃O₄ by oxygen at high reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic decomposition of N2O over supported rhodium catalysts: high activities of Rh/USY and Rh/Al2O3 and the effect of Rh precursors

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over Al₂O₃-supported and zeolite-supported Rh catalysts. The activities of Rh/Al₂O₃ and Rh/USY (ultrastable Y zeolite) catalysts prepared from Rh(NO₃)₃ were higher than those of Rh/ZSM-5 and Rh/ZnO reported in the literature, while the activity of a Rh/Al₂O₃ catalyst prepared from RhCl₃ was suppressed severely in spite of the high H/Rh and CO/Rh values. The catalytic activity of N₂O decomposition was sensitive not only to the Rh dispersion but also to the preparation variables such as the Rh precursors and the supports used. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nitrogen Tolerant Hydrodesulfurization Catalysts Based on Rh and Ru Promoted Mo/Al2O3

Rh and Ru promoted Mo/Al₂O₃ catalysts were tested in HDS of thiophene in the presence of different amounts of pyridine and compared with CoMo/Al₂O₃. The Rh and Ru promoted catalysts were more nitrogen tolerant and in the presence of pyridine showed higher HDS activities than CoMo/Al₂O₃. This was explained by higher C–N bond hydrogenolysis activity and high nitrogen tolerance of the free Rh and Ru sulfides in the promoted catalysts.     

Hydroconversion of n-paraffins in light naphtha using Pt/Al2O3 catalysts promoted with noble metals and/or chlorine

The effect of combining 0.35 wt.% Pt with 0.35 wt.% of either Ir, Rh, Re or U on γ-Al₂O₃ support was investigated for the hydroconversion of n-pentane and n-hexane in a pulsed micro-reactor system at a temperature range of 300–500°C, except for Rh/Al₂O₃ (150–500°C). The dispersion of the metals in the catalysts under study was determined by H₂ chemisorption. The effect of chlorine addition between 1.0 and 6.0 wt.% was investigated and a content of 3.0 wt.% Cl being of optimum promotion. Highest activities for hydroisomerization, hydrocracking and hydrogenolysis were exhibited by Pt, Ir and Rh/Al₂O₃ catalysts, respectively, whether the catalysts were Cl-free or containing 3% Cl. However, Re and U catalysts were inactive. Maximum hydroisomerization selectivities using chlorinated bimetallic catalysts could be arranged in the following order: PtU/Al₂O₃>PtRe/Al₂O₃>PtIr/Al₂O₃>PtRh/Al₂O₃. However, PtRh/Al₂O₃, before and after chlorination, was the most active catalyst for hydrogenolysis. The apparent reaction rate constants as well as the apparent activation energies (E_a) for the hydroconversion of n-pentane and n-hexane were calculated and the compensation effect relationship between E_a and logarithm of the pre-exponential factor was estimated. n-hexane reaction on PtRh/Al₂O₃ catalysts deviates from this relationship for mechanistic variation.     

Nanosized HY zeolite-alumina composite support for hydrodesulfurization of FCC diesel

A series of hydrodesulfurization (HDS) NiMo catalysts supported on Al₂O₃-NY (denoted as ANY) composites with various amounts of nanosized H-type Y zeolite (denoted as NY) were prepared. The samples were characterized by XRD, BET, TPD, TPR, HRTEM, and FT-IR spectroscopy of pyridine adsorption. The characterization results showed that, compared with NiMo/Al₂O₃, the addition of NY reduced the Metal-support interaction and made the MoS₂ stacking degree higher, slabs shorter and dispersion of edge and corner Mo atoms bigger. The addition of NY also enhanced the overall acidity and the ratio of Brönsted acid to Lewis acid of these catalysts. The fluidized catalytic cracking (FCC) diesel HDS activity was increased with the addition of NY in these catalysts compared with NiMo/Al₂O₃ catalysts. The optimal NY content was found to be 20 wt% in ANY composite support. The highest HDS activity of NiMo/ANY20 was attributed to the synergy of hydrogenation activity, acid amount and textural properties.     

Pretreatment effects and NO x decomposition on alumina supported Rh/MoO3 catalysts

Rh-MoO₃/Al₂O₃, Rh/Al₂O₃ and MoO₃/Al₂O₃ catalysts have been prepared and subjected to various pretreatments including high temperature reduction, high temperature oxidation and oxidation/ reduction cycles. After each series of treatments, surfaces were analysed by XPS and FTIR of adsorbed NO. The effectiveness of these surfaces in dissociating NO was studied by TPRS in the temperature range 300–773 K. Reduction rates of Mo oxides in H₂ were determined gravimetrically while rhodium dispersion was determined from H₂ adsorption isotherms at 298 K. Results indicate that molybdenum and rhodium exist in close contact in both oxidised and reduced forms. H₂ chemisorption was suppressed following HTR of the catalyst due to coverage of rhodium by molybdenum oxide species but this could be reversed by HTO (773 K) followed by low temperature reduction. Although a small proportion of Mo could be reduced following HTR, cycles of HTR/HTO produced Rh/Mo oxide phases in which a proportion of Mo could be reduced to Mo° at 473 K. The presence of reduced Mo would appear to play an important role in the improved performance of Rh-MoO₃/Al₂O₃ catalysts.     

Characterization and Catalytic Performance of PtSn Catalysts Supported on Al2O3 and Na-doped Al2O3 in n-butane Dehydrogenation

PtSn/Al₂O₃ and PtSn/Al₂O₃–Na catalysts display important modifications of the metallic phase with respect to Pt/Al₂O₃ one. In this sense, TPR and XPS results show the presence of strong interactions between Pt and Sn, with probable alloy formation, which would be responsible for the decrease of the reaction rate and the increase of the activation energy in cyclohexane dehydrogenation. Besides the experiments of cyclopentane hydrogenolysis show that the alkali metal addition to bimetallic PtSn/Al₂O₃ catalysts completely eliminates the hydrogenolytic ensembles, which could be due to a geometric modification of the metallic phase. These important modifications in the nature of the metallic function due to the simultaneous addition of Na and Sn to Pt/Al₂O₃ are responsible for the excellent catalytic performance in the n-butane dehydrogenation, thus giving high conversions, selectivities to butenes higher than 95%, and lower deactivation capacity than those corresponding to bimetallic PtSn catalysts (with different Sn contents) supported on undoped alumina. The excellent stability of PtSn/Al₂O₃–Na catalysts would be due to a low carbon formation during the reaction, such as it was observed from pulse experiments.     

Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd

The present work has been undertaken to tailor Pt/Al₂O₃ catalysts active for NO oxidation even after severe heat treatments in air. For this purpose, the addition of Pd has been attempted, which is less active for this reaction but can effectively suppress thermal sintering of the active metal Pt. Various Pd-modified Pt/Al₂O₃ catalysts were prepared, subjected to heat treatments in air at 800 and 830 °C, and then applied for NO oxidation at 300 °C. The total NO oxidation activity was shown to be significantly enhanced by the addition of Pd, depending on the amount of Pd added. The Pd-modified catalysts are active even after the severe heat treatment at 830 °C for a long time of 60 h. The optimized Pd-modified Pt/Al₂O₃ catalyst can show a maximum activity limited by chemical equilibrium under the conditions used. The bulk structures of supported noble metal particles were examined by XRD and their surface properties by CO chemisorption and EDX-TEM. From these characterization results as well as the reaction ones, the size of individual metal particles, the chemical composition of their surfaces, and the overall TOF value were determined for discussing possible reasons for the improvement of the thermal stability and the enhanced catalytic activity of Pt/Al₂O₃ catalysts by the Pd addition. The Pd-modified Pt/Al₂O₃ catalysts should be a promising one for NO oxidation of practical interest.     

Rh–Fe/Ca–Al2O3: A Unique Catalyst for CO-Free Hydrogen Production in Low Temperature Ethanol Steam Reforming

Low temperature ethanol steam reforming (ESR) was studied over a series of 1 wt% Rh–x % Fe catalysts with various Fe loading (x = 0–10 wt%) and on different supports (Ca–Al₂O₃, SiO₂ and ZrO₂). The results show that close interaction between Rh and Fe is required to reduce the CO selectivity to almost negligible values. In addition, Rh–Fe supported on Ca–Al₂O₃ exhibits the best performance in terms of CO selectivity and hydrogen yield as compared to other supports. Characterization by XPS and XANES indicates the presence of Fe_xO_y species upon reduction, resulting in the formation of coordinatively unsaturated ferrous (CUF) active sites along the Rh–Fe_xO_y interface. These CUF sites promote water–gas shift reaction during low temperature ESR. Temperature programmed oxidation and Raman spectroscopy of spent catalysts also indicate that the addition of iron oxide reduces coke deposition and forms more reactive coke. Hence, the catalyst lifespan is significantly extended.     

Reactivity of surface isocyanate species with NO, O2 and NO+O2 in selective reduction of NOχ over Ag/Al2O3 and Al2O3 catalysts

Reactivity of surface isocyanate (NCO(a)) species with NO, O₂ and NO+O₂ in selective reduction of NOχ over Ag/Al₂O₃ and Al₂O₃ catalysts was studied by a pulse reaction technique and an in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The NCO(a) species on Ag/Al₂O₃ reacted with O₂ or NO+O₂ mixture gas to produce N₂ effectively above 200°C, while the reaction of NCO(a) with NO hardly produced N₂ even at 350°C. In the case of Al₂O₃ alone, less N₂ was detected in the reaction of NCO(a) with NO+O₂, indicating that silver plays an important role in the N₂ formation from NCO(a). These behaviors of the reactivity of NCO(a) species with reactant gases were in good agreement with the changes in NCO(a) bands shown by in situ DRIFT measurements. Based on these findings, the role of NCO(a) species in the selective reduction of NOχ on Ag/Al₂O₃ and Al₂O₃ catalysts is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fischer–Tropsch Synthesis on SiC-Supported Cobalt Catalysts

Seven SiC supports provided by SICAT with different surface areas and pore volumes were impregnated with 12,5 wt% Co. H₂-chemisorption, N₂-adsorption, temperature programmed reduction and Fischer–Tropsch synthesis in a fixed-bed reactor at 483 K, 20 bar and H₂/CO = 2.1 were performed in order to characterize and test the samples. The performances were compared with well characterized Co/Al₂O₃ and Co-Re/Al₂O₃ reference catalysts. The selectivity towards heavier hydrocarbons (C₅₊) was found to be moderately higher for the SiC supported catalysts while the site-time yields was 20 to 66 % lower than the Co-Re/Al₂O₃ catalyst. Elemental analysis showed the presence of several impurities in the SiC material. Alkali and alkaline earth elements, such as Na, K and Ca, are all known to lower the catalytic activity and also to influence the selectivity. It is proposed that these impurities in addition to sulfur and phosphorus known to be present in SiC, are responsible for the significantly lower catalytic activity of the SiC supported catalysts.     

NO reduction by C3H6 in excess oxygen over fresh and sulfated Pt‐ and Rh‐based catalysts

The selective catalytic reduction (SCR) of NO with C₃H₆ was studied over three noble‐metal‐based catalysts: 2% Pt/γ‐Al₂O₃, 2% Rh/γ‐Al₂O₃ and 1.5% Rh/TiO₂(4% WO₃). The SO₂ effect on the catalyst activity was examined using sulfated samples of the above catalysts and SO₂‐containing feeds. Temperature‐programmed desorption and oxidation studies were carried out to examine the adsorption characteristics of NO and C₃H₆, respectively, in the absence or the presence of SO₂. The adsorption data were linked to variations in the NO reduction rates over fresh and sulfated samples. Modification of the support surface as a result of the SO₂ presence affects the NO and propene sorption characteristics, the NO oxidation and the propene consumption rates. This revised version was published online in August 2006 with corrections to the Cover Date.