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[Display omitted] • Hydrogen spillover induced a cooperative catalysis between Pd particles and Pd single atoms supported on carbon supports. • Few-layer graphene (FLG) support allows more extended H-spillover than multi-walled carbon nanotubes (CNT) • Pd/FLG is more active for solvent-free total hydrogenation of squalene than Pd/CNT but deactivated. • A new mechanism of thermal catalyst deactivation induced by H-spillover is proposed. General interest in solvent-free catalytic processes is driven by the development of environmentally and economically acceptable procedures. Such processes require the use of highly active and stable catalysts. This study reports on the synthesis, characterization, and kinetic evaluation of two highly active Pd catalysts supported on different carbons support, i.e., multi-walled carbon nanotubes (CNT) and few-layer graphene (FLG). These catalysts, with similar Pd loading (∼2 wt%) and containing both Pd single atoms (Pd SA) and nanoparticles (Pd NP), were systematically tested for solvent-free total hydrogenation of squalene (SQE) to obtain high-purity squalane (SQA). Thanks to the unique cooperative catalysis between Pd SA and Pd NP involving H-spillover, the activities of both catalysts were high compared to catalysts described in the literature. Solvent-free total hydrogenation of SQE could be performed under mild conditions (120 °C, 20 bar H 2 , 4 h) using ultralow Pd loading (60 ppm). However, since this reduction reaction is highly exothermic (ΔH R = − 765 kJ.mol−1), heat management during the reactor operation with these highly active catalysts is crucial to avoid deactivation and thermal runaway. Under these conditions, Pd/FLG was highly active but deactivated, unlike Pd/CNT, which still has high activity, but does not deactivate. Characterizations (ICP, XPS, Raman, TPD-MS, HAADF-STEM) of the Pd/FLG catalyst before and after reaction, supported by calculations based on density functional theory, allowed us to propose a new mechanism of catalyst deactivation involving the chemical reduction of specific surface oxygen groups of the carbon support induced by H-spillover. [ABSTRACT FROM AUTHOR]

• Ni@Pd/C and Ni@PdRh/C core–shell nanoparticles were synthesized. • Catalytic activity toward ethanol oxidation depends on structure and composition. • The addition of Rh improves the catalytic activity and stability of Ni@Pd/C. • Ni@PdRh/C exhibited the highest catalytic activity for ethanol electro-oxidation. • The catalytic activity for Ni@PdRh/C was three times higher, compared to Pd/C. This paper describes the synthesis and catalytic activity of carbon-supported Pd/C, PdNi/C, and PdNiRh/C catalysts with core–shell morphologies towards the oxidation of ethanol in alkaline media. The catalysts are synthesised by a chemical reduction method using sodium borohydride as the reducing agent. Electrochemical characterisations show that the addition of Ni and Rh enhances remarkably the electrocatalytic activity of the materials due to the bifunctional and electronic effects in PdNi/C and PdNiRh/C catalysts, and the synergetic effect of the bifunctional and core–shell morphology on the Ni@Pd/C and Ni@PdRh/C catalysts. Electrochemical impedance spectra reveal lower values of charge transfer resistance for Pd 60 Ni 20 Rh 20 /C and Ni 20 @Pd 60 Rh 20 /C. Chronoamperometric tests also reveal the Ni 20 @Pd 60 Rh 20 /C catalyst displays current densities 9.3 times higher than the Pd/C catalyst. The superior catalytic performance of the trimetallic core–shell Ni 20 @Pd 60 Rh 20 /C catalyst can result from the synergy between the metals and the presence of core–shell morphology. [ABSTRACT FROM AUTHOR]

Co-Mo bimetallic nanoparticles (NPs) with various compositions were prepared on a CeO 2 support (Co-Mo/CeO 2 ) by sodium naphthalenide driven reduction. The Co-Mo/CeO 2 catalyst exhibits much higher activity for ammonia synthesis than monometallic catalysts such as Co/CeO 2 and Mo/CeO 2 , and the optimum activity is obtained at Co:Mo = 4:6. X-ray absorption fine structure (XAFS) analyses reveal that Co 3 Mo 3 N NPs are formed after ammonia synthesis reaction. Nitrogen temperature programmed desorption (N 2 -TPD) measurements indicate that the Co-Mo/CeO 2 catalyst possesses a large number of nitrogen adsorption sites with an intermediate nitrogen adsorption energy between that of Co and Mo. Furthermore, nitrogen vacancy sites, which are active sites for N 2 dissociation, are more easily formed on the Co-Mo/CeO 2 catalyst than on bulk-type Co 3 Mo 3 N. As a consequence, the turnover frequency of the Co-Mo/CeO 2 catalyst is much higher than that of bulk-type Co 3 Mo 3 N and is comparable to that of the efficient Ru/CeO 2 catalyst. [ABSTRACT FROM AUTHOR]

Abstract: We have identified a new NO x storage/reduction catalyst with the composition of Pt–KNO3 supported on a K-titanate nanobelt (KTN) prepared using hydrothermal method. Isothermal NO x storage experiments at 350°C revealed the highest storage capacity (2.27mmol/g) ever achieved over 33wt.% KNO3 impregnated catalyst, and lean–rich cycles could be operated for more than 15 times with maintaining its high storage capacity. The characterization of the spent catalysts revealed that addition of more than 33wt.% KNO3 caused the gradual destruction of the nanobelt structure during the reaction and the decrease in NO x storage capacity. High temperature operation such as at 500°C also caused the destruction of nanobelt structure. It was demonstrated that the main storage specie of NO x was potassium nitrate. Immigration of K+ from and back to K-rich layers on KTN might be the mechanism for the storage and reduction of NO x on those materials. [Copyright &y& Elsevier]

The nature of stored NOx and mechanistic aspects of the reduction of NOx stored over a model Pt—K/Al2O3 catalyst sample are investigated in this paper, and a comparison with a model Pt—Ba/Al2O3 catalyst is also made.It is found that at 350 °C on both the catalysts the storage proceeds with the initial formation of nitrites, followed by the oxidation of nitrites to nitrates. A parallel pathway involving the direct formation of nitrates species is also apparent; at saturation, only nitrates are present on the catalyst surface over both Pt—K/Al2O3 and Pt—Ba/Al2O3. However, whereas bidentate nitrates are present in remarkable amounts on Pt—K/Al2O3, along with ionic nitrates, only very small amounts of bidentate nitrates were observed on Pt—Ba/Al2O3.Under nearly isothermal conditions, the reduction of the stored NOx with H2 occurs via an in series two-steps Pt-catalysed molecular process involving the formation of ammonia as an intermediate, like for the Pt—Ba/Al2O3 catalyst sample. However, higher N2 selectivity is observed in the case of the Pt—K/Al2O3 catalyst due to the similar reactivity of the H2 + nitrate and NH3 + nitrate reactions. Accordingly ammonia, once formed, readily reacts with surface nitrates to give N2, and this drives the selectivity of the reduction process to N2. Notably, a strong inhibition of H2 on the reactivity of NH3 towards nitrates is also pointed out, due to a competition of H2 and NH3 for the activation at the Pt sites. Finally, the effect of water and CO2 on the reduction process is also addressed. Water shows a slight promotion effect on the reduction of the nitrates by H2, and no significant effect on the reduction by ammonia, whereas CO2 has a strong inhibition effect due to poisoning of Pt by CO formed upon CO2 hydrogenation. [Copyright &y& Elsevier]

Abstract: The catalytic properties of several amine-functionalized MCM-41 materials in the nitroaldol (Henry) reaction are reported. The work investigated the effects of organoamine type (primary, secondary, tertiary), amine density, and the presence of silanol groups on the catalytic activity and product selectivity. High selectivity to the nitroalcohol product was achieved by introducing secondary and tertiary amine groups on the MCM-41 surface, while the nitroalkene is the dominant product for primary amines. Steric effects appear to be significant in determining the overall reactivity as the least sterically hindered amines are the most active. The capping of silanols with trimethylsilyl groups resulted in a reduction in catalytic activity for nearly all samples, indicating the importance of surface silanols and/or the surface polarity in the reaction. [Copyright &y& Elsevier]

A nanoscale investigation was performed on the initial stages of the formation of nanoparticles during the preparation of NiCu/TiO2 bimetallic catalysts by incipient wetness impregnation. The evolution of the structure and chemistry of individual nanoparticles was followed in the reaction cell of an environmental transmission electron microscope. During reduction in hydrogen at 300 °C, nuclei quickly formed and grew mainly via Ostwald ripening or short-range particle–particle coalescence. The presence of Cu greatly enhanced the reducibility of the Ni species and about 85% of the particles were metallic. Most of the particles were uniform in composition but approximately 15% of the particles showed Ni enrichment on the surface. The surface enrichment of Ni was attributed to differential diffusion processes and demonstrated that, for the reduction temperature used for this experiment, the structure of the bimetallic particles was controlled by kinetics rather than by thermodynamics. [Copyright &y& Elsevier]

The propene selective catalytic reduction (C3H6-SCR) of NOx is investigated on WOx–ZrO2 and Pd/WOx–ZrO2. Temperature-programmed surface reaction (TPSR) experiments provide evidence of the decomposition of organic nitrogen-containing species formed through the interaction of C3H6 and ad-NOx, for which a C/N ratio of 3 is estimated. The TPSR of preadsorbed NOx in C3H6–O2–He also suggests the presence of two RN—Ox species, C3H5—O—NO (propenyl nitrite) and C3H5—NO2 (nitropropene), decomposing in distinct temperature regions. The addition of Pd to WOx-ZrO2 promotes the decomposition of nitropropene, whereas that of propenyl nitrite is scarcely affected. It is also suggested that RNOx are not active intermediates in C3H6-SCR on Pd/WZ catalysts but are more likely spectators. The activity for C3H6-SCR is attributed to Pd0 species, formed by in situ reduction of oxidized Pd species by C3H6, via the well-established decomposition mechanism. In light of the new findings about the formation–decomposition of R—NOx, results reported previously on a Pd/Ce0.68Zr0.32O2 catalyst [C. Thomas, O. Gorce, C. Fontaine, J.-M. Krafft, F. Villain, G. Djéga-Mariadassou, Appl. Catal. B Environ. 63 (2006) 201], for which metal–support interactions prevent PdOx reduction and C3H6-SCR reaction mechanism is different from that on Pd0 sites, are widened. [Copyright &y& Elsevier]

Uniform Co–B nanoparticles were synthesized for the first time by chemical reduction of cobalt ion with borohydride in an oil-in-water microemulsion system comprising cyclohexane, polyethylene glycol, and water. The particle size was controlled by modulating the cyclohexane content. With the characterization of X-ray diffraction, selective area electronic diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy, the resulting Co–B nanoparticles were identified to be amorphous alloys ranging in size from 6 to 20 nm. During liquid-phase cinnamaldehyde hydrogenation, the as-synthesized Co–B catalyst was extremely active and more selective than the regular Co–B prepared in aqueous solution. Furthermore, this catalyst also was found to be more durable during the hydrogenation process. [Copyright &y& Elsevier]

The selective catalytic reduction by NH3 of NO (“standard SCR”) and NO/NO2 mixtures (“fast SCR”) were compared over Fe-ZSM-5 catalysts of different Fe content ranging from 0.2 to 5 wt%. The Fe site structure was monitored by UV–vis spectroscopy. From the response of the reaction rates to the variation of the site structure, it was concluded that standard and fast SCR require different Fe sites; standard SCR proceeds both on isolated and oligomeric Fe oxo sites, whereas a small concentration of isolated Fe sites is sufficient to sustain the high reaction rates of fast SCR, with no discernible contribution of different sites. The response of both reactions to hydrothermal stress (with or without SO2 present) differs; strong deactivation in standard SCR accompanied by increasing NH3 oxidation activity being opposed to stable activity in fast SCR, suggesting that those isolated sites, which are responsible for fast SCR are very resistive toward clustering under the conditions of automobile exhaust catalysts. The potential of preparation procedures to improve the activity by increasing the Fe content is very different. The best performance was achieved using solid-state ion exchange with FeCl3, with NO x conversions >90% between 473 and 773 K obtained over the best catalyst. But if a concomitant increase of the aggregated site abundance cannot be avoided, then NH3 oxidation will effectively compete with fast SCR already at relatively low temperatures. The role of free NO2 in the reaction mechanism of standard SCR was assessed by comparing the SCR reaction rate with that of NO oxidation for all catalysts used. NO oxidation was significantly slower than NO reduction in all cases, which excludes free NO2 produced by NO oxidation as an intermediate of the standard SCR reaction. [Copyright &y& Elsevier]

The gas-phase hydrogenation of ethyne, propyne, and propadiene was investigated over partially reduced Ni–Al mixed oxides derived from takovite, a hydrotalcite-type compound. The unique attributes of the hydrotalcite route leads to more active and selective catalysts compared to conventional Ni/Al2O3 prepared by impregnation. Tuning calcination and reduction conditions of the catalyst precursor is essential to optimize the hydrogenation performance. The best catalyst, calcined and reduced at 773 K, rendered stable propene yields up to ca. 65% and consisted of a Ni(Al)O x solid solution with 55% of the total bulk nickel in reduced form and surface enrichment by aluminum. Sintering of NiO and crystallization of NiAl2O4 at high calcination temperature induce lower activity. The alkyne or diene conversion increases with the percentage of metallic Ni in the samples, while an optimal degree of nickel reduction maximizes the monoalkene selectivity. Below the optimum, oligomer formation is favored and above the optimum, alkane production increases. A similar pattern was found for the H2/HC ratio. The alkene selectivity experiences a dramatic increase in early stages of the reaction, which correlated with the build-up of C-containing species on the catalyst (sub-)surface. These selectivity-enhancing species are formed at specific reaction temperatures, highlighting the relevance of the testing procedure on assessing hydrogenation catalysts. The catalytic performance is strongly influenced by the hydrocarbon substrate. In contrast to propyne and propadiene, ethyne hydrogenation led to a C2H4 yield of only 6%. [Copyright &y& Elsevier]

In this paper, we report a novel electrochemically assisted NO x storage/reduction catalyst (Pt–KβAl2O3) that can operate over a range of reaction conditions for the effective removal of NO x . Under negative polarization, NO x was stored on the catalyst surface in form of potassium nitrates. Under positive polarization, the catalyst was regenerated, and the stored nitrates were efficiently desorbed and reduced to N2. The variation of the current under the applied polarizations allowed monitoring the progress of both the storing and the regeneration phases, and thus optimisation of the duration of both sequences in a technically feasible manner. In addition, the possibility of electrochemical regeneration of the catalyst surface allowed work under a fixed lean gas composition, which implies an important technological advance for the NSR process. The electrochemically assisted NO x storage/reduction experiments were supported by NO x -TPD and cyclic voltammetry measurements, which also revealed useful information about the chemisorption properties of the electrochemical catalyst. [Copyright &y& Elsevier]

A detailed kinetic model of storage and reduction, in the presence of H2O and CO2, with hydrogen as the reducing agent was developed and validated in this study. The mechanism was derived from flow reactor experiments conducted at 200–400 °C over a Pt/Ba/Al monolith sample. The detailed kinetic model is divided into four sub-models: (i) NO oxidation over Pt, (ii) storage, (iii) reduction over Pt, and (iv) regeneration. The sub-model for storage is based on our earlier work and is further developed in this study to include high concentrations of CO2 and H2O in the feed and also low temperature storage. In the model is allowed to be stored on two different types of storage sites: BaCO3 and a second storage site denoted S3. Based on experimental results many studies suggests multiple storage sites one these catalysts, and there are different explanations (i) alumina and barium sites (ii) bulk and surface barium, (iii) barium close and far from the noble metal, etc. The disproportionation route, where NO2 is stored over BaCO3, is included in the storage model. To account for the storage occurring at lower temperatures NO can be stored over both BaCO3 and S3 in the presence of O2. NO adsorbed over S3 can be further oxidized to NO2 by reacting with oxygen on neighboring Pt sites. The second storage site (S3) is important in order to explain storage at low temperatures and the disproportionation reaction is essential to describe the storage at high temperatures. The reduction sub-model used here was developed earlier over Pt/Si. Additional to the reduction of into N2, it describes the formation of NH3 over Pt. In the sub-model describing the regeneration of , adsorbed species react with hydrogen adsorbed on Pt sites. Ammonia oxidation over Pt and reactions between surface species of barium and NH3 according to ammonia selective catalytic reduction (SCR) chemistry are also incorporated in the regeneration sub-model. The full model can describe the complete uptake of in the beginning of the lean period, the breakthrough, and the slow storage in the end of the lean period very well as well as the following release and reduction. It can also predict the gradual decrease in the storage capacity occurring in lean/rich cycling experiments. Furthermore, the ammonia formation predicted by the model fits well with experimental data. The model was validated with short lean (60 s) and rich (15 s) cycles which were not included in the model development. The model could predict these experiments well for all three temperatures (200, 300 and 400 °C). [Copyright &y& Elsevier]

Three CoS2-, NiS2- and (Co,Ni)S2-based thin films were prepared by magnetron sputtering and studied as catalysts for the oxygen reduction reaction (ORR). Electrochemical assessments indicate that all three films have significant ORR catalytic activities, with that of (Co,Ni)S2-type showing the best performance with regard to both open circuit potential (OCP) and current density. The ternary film has an OCP value of 0.89 V vs. the reversible hydrogen electrode, and shows a closer approach to values for Pt than have been obtained to date for other transition metal chalcogenides. The thin films assessed by electrochemistry have been characterized by a range of techniques including high-resolution X-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, X-ray photoelectron spectroscopy and scanning Auger microscopy. The CoS2-based film has a wrinkled surface, which appears relatively unchanged after an electrochemical durability test, while the NiS2-like film preferentially loses Ni and the (Co,Ni)S2-like film (based on nanoparticles with approximate composition Co0.6Ni0.4S2) undergoes a phase separation (spinodal decomposition). All these films show an excess of S in the as-prepared form and after electrochemistry; evidence is presented for the possible presence of some polysulfides. [Copyright &y& Elsevier]

Selective catalytic reduction of NO with acetylene (C2H2-SCR) over mordenite-based catalysts (HMOR, 0.5% Mo/HMOR and NaMOR) was investigated by in situ Fourier transform infrared spectroscopy. A possible mechanism was proposed to explain catalytic performance of the mordenite-based catalysts in the C2H2-SCR: Nitrosonium ions (NO+) and bidentate nitrate are reactive nitric species towards acetylene at 250 °C. Isocyanate species thus formed are then hydrolyzed to acid amide species that are crucial intermediate of the C2H2-SCR. Bridging nitrate species become reactive towards the reductant when reaction temperature increased to 300 °C. Molybdenum loading on HMOR zeolite considerably increased the population of bridging nitrate species and therefore enhanced the title reaction above 300 °C. [Copyright &y& Elsevier]

The reduction by H2 of nitrates stored on a model Pt–Ba/Al2O3 catalyst under isothermal conditions was investigated. An in-series two-step process involving at first the formation of ammonia on reaction of nitrates with H2 (step 1), followed by the reaction of ammonia with residual stored nitrates to give N2 (step 2) is proposed. Different temperature thresholds for the two steps are identified, with step 1 being much faster than step 2. The reaction of ammonia with nitrates to give nitrogen (step 2) is very selective, is rate-determining, and represents the major route for nitrogen formation. Due to the fast reduction of adsorbed nitrates by hydrogen (step 1) and to the “plug-flow” integral behavior of the reactor, an H2 front develops that travels along the reactor. Ammonia is readily produced at the front; if the temperature is high enough, then the regeneration process continues downstream of the hydrogen front due to the reaction of NH3 with the stored nitrates, leading to N2 production. This fully explains the change in product distribution with time observed at the reactor outlet during regeneration of LNT catalysts, as well as the effect of temperature on the selectivity of the process. The effects of water and CO2 on the reduction process also are addressed. Water favors the first step of the reduction process but has no significant impact on the subsequent reaction of ammonia with stored nitrates, whereas CO2 has a strong inhibiting effect both on the reduction of nitrates to ammonia by H2 (step 1) and on the subsequent reaction of ammonia with the residual stored nitrates (step 2), due to poisoning of Pt by CO. [Copyright &y& Elsevier]

Abstract: The deactivation of Au/CeO2 catalysts during low-temperature water–gas shift reaction and the underlying physical origin were investigated, comparing the deactivation in different reaction atmospheres and subsequent regeneration of the catalysts by reactive treatment in O2/N2 or H2O/N2 gas mixtures. Based on TEM and XRD measurements, Au and ceria particle sintering (irreversible deactivation) can be excluded as reasons for the deactivation. In addition to the formation of adsorbed monodentate carbonate species, which we recently identified as a major reason for catalyst deactivation under present reaction conditions [Denkwitz et al., J. Catal. 246 (2007) 74], XPS measurements show significant variations in Ce3+ and Au n+ content during reaction in different atmospheres and on regeneration. Reaction generally leads to increasing catalyst reduction (i.e., decreased Au n+ content and increased Ce3+ content); this is reversed during regeneration. The correlation between the relative content of these ionic species and the catalytic activity is not as clear as that between carbonate formation/decomposition and deactivation/regeneration, leaving monodentate carbonate formation as the dominant but not sole reason for catalyst deactivation. [Copyright &y& Elsevier]

Abstract: Mechanistic aspects of the reduction of stored NO x with hydrogen were analyzed over model Ba/γ-Al2O3 and Pt–Ba/γ-Al2O3 LNT trap catalysts and over a Ba/γ-Al2O3–Pt/γ-Al2O3 physical mixture. NO x species were stored and stabilized at 350 °C under controlled conditions, and their stability/reactivity was then analyzed under different atmospheres (He and hydrogen) either at constant temperature or with temperature programming under nearly isothermal conditions, that is, in the absence of significant thermal effects. The collected data indicate that over the Pt–Ba/Al2O3 catalyst, the reduction of stored NO x occurs at temperatures below those corresponding to their thermal stability. This indicates that the reduction of stored nitrates occurs in typical NSR catalysts according to a Pt-catalyzed surface reaction, which does not involve the thermal decomposition of the adsorbed NO x species as a preliminary step. The occurrence of such a pathway also requires the co-presence of the storage element and of the noble metal on the same support, because this route does not operate when they are dispersed on different support particles. [Copyright &y& Elsevier]

Abstract: In situ Fourier transform infrared spectroscopy, coupled with mass spectrometry and time-resolved X-ray diffraction, were used to study the efficiency of nitrate reduction with CO and H2 on Pt/Al2O3 and Pt/BaO/Al2O3 NO x storage reduction (NSR) catalysts. Surface nitrates were generated by NO2 adsorption, and their reduction efficiencies were examined on the catalysts together with the analysis of the gas-phase composition in the presence of the two different reductants. H2 was found to be a more effective reducing agent than CO. In particular, the reduction of surface nitrates proceeds very efficiently with H2 even at low temperatures (∼420 K). During reduction with CO, isocyanates form and adsorb on the oxide components of the catalyst; however, these surface isocyanates readily react with water to form CO2 and ammonia. The NH3 thus formed in turn reacts with stored NO x at higher temperatures (>473 K) to produce N2. In the absence of H2O, the NCO species are stable to high temperatures and are removed from the catalyst only when they react with NO x thermal decomposition products to form N2 and CO2. The results of this study point to a complex reaction mechanism involving the removal of surface oxygen atoms from Pt particles by either H2 or CO, the direct reduction of stored NO x with H2 (low-temperature NO x reduction), the formation and subsequent hydrolysis of NCO species, and the direct reaction of NCO with decomposing NO x (high-temperature NO x reduction). [Copyright &y& Elsevier]

Abstract: In this study data are presented which support the view that a special configuration of dimeric Cu(I) species in Cu-ZSM-5 catalysts may be the active site for NO decomposition. Such dimeric Cu(I) species, produced by self-reduction of Cu(II) during the thermal activation of Cu-ZSM-5 catalysts in He or under vacuum, are the sites where N2 is generated from two adsorbed NO molecules, and from which O2 desorbs from the catalyst at the steady state. It was found that these dimeric Cu(I) species adsorb N2 irreversibly at low temperature (273–325 K). DFT calculations for cluster models obtained from the crystallographic structure of orthorhombic ZSM-5 suggested that Cu(I)–N2–Cu(I) species may be IR-silent or have a very low extinction coefficient, as suggested by previous diffuse reflectance FT results. The isosteric heat of adsorption of N2 on Cu-ZSM-5, obtained with the Clausius–Clapeyron equation from adsorption isotherms in the temperature range of 273–325 K, is 10.3 kcal mol−1 at a coverage of 0.4 mol N2/mol Cu. This value is much larger than the heat of condensation of N2 at 77 K (1.33 kcal mol−1) and significantly higher than the heat of adsorption of N2 on Li-ZSM-5 (8.68 kcal mol−1 at zero coverage) obtained with the Clausius–Clapeyron equation from adsorption isotherms in the temperature range of 196–273 K. The turnover frequency (TOF) for the NO decomposition, calculated assuming that the active sites are those titrated by the irreversible adsorbed N2 at 273 K, is independent of the copper loading and of the ZSM-5 Si/Al atomic ratio. The present model is also supported by the evidence that Cu-Y, Cu-S-1, and Cu-[Al]-MCM-41 catalysts, containing Cu(I) species that do not adsorb irreversibly N2 at 273–325 K, were found to be inactive or scarcely active (Cu-Y) for the NO decomposition reaction under the same experimental conditions as employed for Cu-ZSM-5. [Copyright &y& Elsevier]

Abstract: The influence of reduction temperature on the catalytic behavior of 10 wt% Co/TiO2 catalysts, for CO2 reforming of methane to synthesis gas under atmospheric pressure, was investigated. Co/TiO2-anatase catalysts reduced at lower temperatures () showed stable activities. On the other hand, the catalyst reduced at higher temperatures (), where the crystal phase of TiO2 is transformed from anatase to rutile during the reduction, provided almost no activity. In addition, Co/TiO2-rutile also showed very low activity, regardless of the reduction temperature. No carbon deposition () was observed for any of the Co/TiO2 catalysts. XPS and XRD analysis revealed that the main cause of low activity and deactivation of the catalyst was the oxidation of metallic cobalt. Experimental observations suggest that the large difference in initial activity corresponds to the different crystal structure of TiO2 after reduction. [Copyright &y& Elsevier]

The stability and reactivity of NOx stored over Pt/Ba/Al2O3 storage and reduction catalysts have been compared in the presence of hydrogen, carbon monoxide, and propene as reductants under stoichiometric conditions while heating between 298 and 873 K. The order of efficiency in terms of conversion of stored NOx to N2 was H2>CO> propene. Additionally the use of hydrogen led to the lowest NO2: NO ratio. Four potential mechanisms to explain the initial release of stored NOx are discussed. In the absence of reductant, less than 10% of released NOx was in the form of NO, whereas the presence of reductant led to significant NO release which was independent of the presence of Pt. Results suggest that while formation of N2 from stored NOx requires the presence of reduced noble metal sites, formation of NO can take place over the support alone, probably by direct interaction between reductant and stored nitrate. The role of Pt in providing activated spill over reductant is important for the decomposition of certain forms of NOx stored by the alumina. Both hydrogen and propene oxidation were inhibited by the presence of NOx whereas CO was able to compete for Pt sites, even in the presence of high NOx concentrations. [Copyright &y& Elsevier]

The influence of cocations (Cr and Ag) on the activity of Cu/CeO2 catalysts in the selective reduction of NOx with C3H6 has been investigated in this study. The reduction of NOx under lean conditions was greatly improved by loading Cr and Ag onto a Cu/CeO2 catalytic system. All the synthesized bimetal catalysts were characterized using XRD and TPR-H2. The better metal–support interaction and metal dispersion shown by Cu/Ag/CeO2 have elevated its performance compared to the Cu/Cr/CeO2 catalyst. By having such salient features, the competitiveness factor (SSCR-HC) assigned by Cu/Ag/CeO2 was generally better than that of the other tested catalysts. The presence of O2 was proven to be a very crucial factor for promoting the selective reduction of NOx with C3H6. [Copyright &y& Elsevier]

This work provides evidence that reduction of Pd/SiO2 catalysts in dihydrogen at temperatures as low as 450 and 500 °C may already lead to a considerable interaction between palladium and silica, identified by the formation of palladium silicides (Pd4Si and Pd3Si). This result seems significant in view of the frequent usage of this temperature level in activation and pretreatment of Pd/SiO2 catalysts. Application of a high flow of a reducing gas mixture and different reduction time periods indicates that an efficient removal of water vapor from the catalyst zone during hydrogen pretreatment facilitates reduction of silica to silicon, which, in turn, interacts with nearby palladium species. Silation of palladium in Pd/SiO2 catalysts during a prolonged reduction at 450 and 500 °C has a considerable effect on the catalytic properties in the reaction of 2,2-dimethylbutane with dihydrogen. Incorporation of silicon into palladium lowers the catalytic activity, decreases the activation energy, increases the selectivity toward isomerization (at the expense of hydrogenolysis), and also changes the type of 2,2-dimethylbutane interaction with the surface of catalyst (from αγ to αγ′ mode). [Copyright &y& Elsevier]

The lean NOx performance and catalytic properties of In2O3/Al2O3 catalysts were investigated. High lean NOx activity was observed when propene was used as a reductant in the presence of 9% O2 and 7% H2O at a space velocity of 30,000 h−1. The optimum lean NOx activity of In2O3/Al2O3 catalysts was observed at a loading of 2.5 wt% indium on γ-Al2O3 which was prepared by a sol–gel technique (230 m2/g). When propane was used as a reductant, the In2O3/Al2O3 catalyst did not promote NOx reduction compared to the alumina substrate. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and temperature-programmed reduction (TPR) have been used to characterize a series of In2O3/Al2O3 catalysts to increase understanding of the surface structure of indium oxide species on the alumina support. The XRD data indicated that crystalline In2O3 was present at In2O3 loadings >5 wt% and the quantity of the crystalline phase increased as a function of indium loading. XPS results suggested that indium oxide existed as a well-dispersed phase up to 10 wt% indium. The well-dispersed or reducible indium oxide species below 400°C in TPR experiments were assigned as the sites which activate propene to oxygenated hydrocarbons such as acetaldehyde and acrolein. Alumina sites readily utilize the oxygenated hydrocarbons to reduce NOx. A bifunction mechanism was proposed to explain high NOx reduction over In2O3/Al2O3 catalysts. [Copyright &y& Elsevier]

The kinetics of reduction of chromium oxide catalysts was studied by applying nonlinear regression analysis to the temperature-programmed reduction (TPR) data of six different CrOx/Al2O3 samples (1.2–14 wt% Cr). Different kinetic models were tested. Three heating rates were required to discriminate between the different kinetic models. The best-fit kinetic models and the estimated parameters for each sample are reported. The two-dimensional nuclei growth model suitably described the kinetics of reduction of samples with loading close to or beyond the monolayer content (>5 wt%). The overall reduction mechanism seemed to be the same irrespective of the method of catalyst preparation. The chromium content was important, however. Reducibility increased regularly with chromium loading, supporting the idea of a topochemical reduction mechanism. The sample with low chromium content (∼1 wt%) was reduced by a different overall mechanism than were the other samples and its reduction behaviour was best described by homogeneous first-order kinetics (random nucleation). Complete line-shape analysis of TPR patterns offers a good means for quantitative characterisation of alumina-supported chromium oxide catalysts, since it provides information on dynamic redox behaviour of the active sites. [Copyright &y& Elsevier]

A novel composite catalyst system for the selective catalytic reduction (SCR) of NOx by NH3 is described operating at temperatures lower than 470 K in the presence of water with NO conversions of 80–100% at space velocities of 30,000–50,000 h−1. The catalyst is prepared by egg-shell precipitation of MnO2 on the external surface of zeolite NaY. Structural and thermal stability of precipitated MnO2 as well as of the MnO2/NaY composite catalyst were characterized by N2 adsorption, X-ray diffraction, laser Raman spectroscopy, temperature-programmed reduction, and electron microscopy. MnO2 precipitated on zeolite NaY (15 wt% loading) retained its amorphous state up to calcination temperatures of 775 K. The zeolite component remained structurally intact. Calcination at higher temperatures destroyed the zeolite structure and transformed MnO2 into Mn3O4. DRIFT spectroscopic investigations revealed the presence of symmetric O&z.dbnd;N—O—N&z.dbnd;O species formally corresponding to N2O3 on the composite catalyst after contact with NO. Catalytic measurements under integral flow conditions showed that the catalyst performance is associated with a close coupling of nitrite formation and its drain off from equilibria with NO/NO2 and nitrate by ammonia. Several results are in line with the “diazotation” mechanism, including NH3 protonation to NH4+, whereas prevailing Lewis acid sites should enable NH3 activation via amide species, thus leading to a parallel “amide/nitrosamide” SCR reaction route. The activity-temperature profile fulfills the requirements of a low-temperature NOx reduction catalyst for mobile diesel engines if an ammonia supply is implemented “on board,” e.g., by urea decomposition. [Copyright &y& Elsevier]