Positive Reaction Order Research Articles

Adjacent MnOx clusters enhance the hydroformylation activity of rhodium single-atom catalysts

Hydroformylation reactions promoted by Rh single-atom catalysts typically exhibit a negative reaction order for CO and a positive reaction order for H2 and alkenes. To enhance the catalytic activity, efforts have been concentrated on reducing the CO adsorption strength and/or enhancing the hydrogenation capacity at Rh sites. This report introduces an optimized Rh single-atom catalyst, utilizing positively charged Mn species to weaken the CO adsorption strength. Our strategy involves placing MnOx clusters in the proximity to the Rh single atom. This arrangement allows the reduction of Rh3+ to produce electronically deficient Rhδ+ species (1 < δ < 2), rather than the usual formation of the lower valence state Rh+ species. The weakened CO adsorption strength on the more positively charged Rhδ+ results in reduced CO coverage on the Rh active site, and enhanced adsorption of propylene. Our mechanistic study reveals that H2 and CO adsorption on Rh catalyzes the reduction of adjacent Mn(IV) species, inducing the formation of a Rh-MnOx paired active site. The neighboring MnOx clusters hinder the extensive reduction of Rh3+ to Rh+. This study presents MnOx as an inorganic ligand in Rh-MnOx pair site modifies the electronic properties of Rh sites to modulate the hydroformylation activity of Rh single-atom catalysts.

Memory-dictated dynamics of single-atom Pt on CeO2 for CO oxidation

Single atoms of platinum group metals on CeO2 represent a potential approach to lower precious metal requirements for automobile exhaust treatment catalysts. Here we show the dynamic evolution of two types of single-atom Pt (Pt1) on CeO2, i.e., adsorbed Pt1 in Pt/CeO2 and square planar Pt1 in PtATCeO2, fabricated at 500 °C and by atom-trapping method at 800 °C, respectively. Adsorbed Pt1 in Pt/CeO2 is mobile with the in situ formation of few-atom Pt clusters during CO oxidation, contributing to high reactivity with near-zero reaction order in CO. In contrast, square planar Pt1 in PtATCeO2 is strongly anchored to the support during CO oxidation leading to relatively low reactivity with a positive reaction order in CO. Reduction of both Pt/CeO2 and PtATCeO2 in CO transforms Pt1 to Pt nanoparticles. However, both catalysts retain the memory of their initial Pt1 state after reoxidative treatments, which illustrates the importance of the initial single-atom structure in practical applications.

Ethene Hydroformylation Catalyzed by Rhodium Dispersed with Zinc or Cobalt in Silanol Nests of Dealuminated Zeolite Beta.

Catalysts for hydroformylation of ethene were prepared by grafting Rh into nests of ≡SiOZn-OH or ≡SiOCo-OH species prepared in dealuminated BEA zeolite. X-ray absorption spectra and infrared spectra of adsorbed CO were used to characterize the dispersion of Rh. The Rh dispersion was found to increase markedly with increasing M/Rh (M = Zn or Co) ratio; further increases in Rh dispersion occurred upon use for ethene hydroformylation catalysis. The turnover frequency for ethene hydroformylation measured for a fixed set of reaction conditions increased with the fraction of atomically dispersed Rh. The ethene hydroformylation activity is 15.5-fold higher for M = Co than for M = Zn, whereas the propanal selectivity is slightly greater for the latter catalyst. The activity of the Co-containing catalyst exceeds that of all previously reported Rh-containing bimetallic catalysts. The rates of ethene hydroformylation and ethene hydrogenation exhibit positive reaction orders in ethene and hydrogen but negative orders in carbon monoxide. In situ IR spectroscopy and the kinetics of the catalytic reactions suggest that ethene hydroformylation is mainly catalyzed by atomically dispersed Rh that is influenced by Rh-M interactions, whereas ethene hydrogenation is mainly catalyzed by Rh nanoclusters. In situ IR spectroscopy also indicates that the ethene hydroformylation is rate limited by formation of propionyl groups and by their hydrogenation, a conclusion supported by the measured H/D kinetic isotope effect. This study presents a novel method for creating highly active Rh-containing bimetallic sites for ethene hydroformylation and provides new insights into the mechanism and kinetics of this process.

Ionic conductivity and disorder in calcium and barium nitrogen hydrogen phases.

Nitrogen-hydrogen based alkali and alkaline earth metal compounds have recently received a substantial amount of attention as co-catalysts for heterogeneous mild condition ammonia synthesis (MCAS). The incorporation of these materials has been shown to result in positive reaction orders with respect to H2, solving the issue of hydrogen poisoning, e.g., the occupation of the majority of transition metal (TM) active sites by H-adatoms due to the significantly faster kinetics of H2 dissociation as compared to N2. The mechanism that underlies this is thought to be the incorporation (sinking) of H-adatoms from the surface of TMs to the bulk of the N-H phases. Thus, the slower kinetics of N2 dissociation no longer inhibit ammonia synthesis, and improvements in the kinetics dissociation for TM can be realised without consideration for which specific gases are affected (e.g., the circumventing of scaling relations). The ability to transport H-adatoms from the surface of TM is therefore of fundamental importance to the properties of the N-H co-catalyst implying that the conductivity of these species towards H and N ions, and NHx species, is of utmost importance. As such, we investigate two N-H systems that can be prepared by reacting the respective hydrides with nitrogen resulting in nitride-hydride and imide forms for Ca and Ba, respectively. These have both been previously shown to promote ammonia synthesis and here we investigate their conductive properties, and discuss these systems in the context of activity and stability of the total system with specific focus on the rise of secondary anion species, and the presence of barium in the system.

Investigation into support effects for Pt and Pd on LaMnO3

The catalytic properties of Pt and Pd supported on thin-film LaMnO3 were investigated. The LaMnO3 film consisted of an ALD-grown 0.5-nm-thick film supported on a high-surface-area MgAl2O4. Pt and Pd were added to the LaMnO3 films by Atomic Layer Deposition (ALD) and then aged by repeated oxidation and reduction cycles at 1073 K. X-Ray Diffraction (XRD), together with Scanning Transmission Electron Microscopy (STEM)/Energy Dispersive Spectroscopy (EDS) showed that Pt underwent extensive sintering during these redox treatments and formed a Pt-Mn alloy following reduction at 1073 K. Consistent with this, the activity of the supported Pt catalysts for CO oxidation decreased progressively with the number of redox cycles. In contrast to the results for Pt, STEM/EDS results indicated that Pd remained well dispersed on LaMnO3, even after five redox cycles. The Pd catalyst showed high activity for CO oxidation at lower temperatures due to a low activation energy and positive reaction order in CO. XRD results with Rietveld analysis for the Pd catalyst indicated the presence of very large crystallites for PdO on the oxidized catalyst and metallic Pd on the reduced catalyst, inconsistent with the high dispersions measured by STEM and the high CO-oxidation rates. Possible explanations for these observations are discussed.

First-Principles Microkinetic Model for Low-Temperature NH3-Assisted Selective Catalytic Reduction of NO over Cu-CHA

A first-principles microkinetic model is developed to investigate the low-temperature ammonia-assisted selective catalytic reduction (NH3-SCR) of NO over Cu-chabazite (Cu-CHA). The reaction proceeds over NH3-solvated Cu sites by the formation of H2NNO and HONO, which decompose to N2 and H2O over Brønsted acid sites. Nonselective N2O formation is considered by H2NNO decomposition over the Cu sites. The adsorption of NH3 at oxidized Cu sites is found to inhibit the reaction at low temperatures by hindering NO adsorption. For the reactions, we find positive reaction orders with respect to NO and O2, whereas the reaction order with respect to NH3 is negative. The reaction orders and the obtained apparent activation energy are in good agreement with experimental data. A degree of rate control analysis shows that NH3-SCR over a pair of Cu(NH3)2+ is mainly controlled by NO adsorption below 200 °C, whereas the formation of HONO and H2NNO becomes controlling at higher temperatures. The successful formulation of a first-principles microkinetic model for NH3-SCR rationalizes previous phenomenological models and links the kinetic behavior with materials properties, which results in unprecedented insights into the function of Cu-CHA catalysts for NH3-SCR.

Barium chromium nitride-hydride for ammonia synthesis

Early 3d metals such as chromium can easily dissociate N 2 , but the subsequent hydrogenation to ammonia is difficult because they bind nitrogen too strongly. Hence, investigation of Cr-based catalysts for ammonia synthesis is very rare. Here we show that when Cr compounds with Ba, N, and H forming a nitride-hydride, effective ammonia synthesis catalysis can be achieved under mild conditions. Under 573 K and 10 bar, this catalyst has an ammonia synthesis rate (6.8 mmol NH3 g cat −1 h −1 ) that is about four times that of the Cs-Ru/MgO catalyst. With low apparent activation energy (50.1 kJ mol −1 ) and positive reaction orders of H 2 and N 2 , it can produce observable ammonia at 373 K and 1 bar. The active phase has a Ba 5 CrN 4 H-like structure containing reactive hydrogen ( H - ) and nitrogen, which are involved in the ammonia formation. This work discloses a strategy to “activate” the inactive early transition metals for effective ammonia catalysis. • A quaternary barium chromium nitride-hydride is synthesized • The hydride acts as electron and hydrogen carriers for N 2 activation • Reactive lattice hydrogen ( H - ) and nitrogen are involved in the formation of NH 3 • The nitride-hydride structure enables Cr to be active for NH 3 synthesis Ammonia is a key ingredient for manufacturing fertilizers and nitrogen-containing chemicals and has also been considered recently as a promising energy carrier. Current NH 3 production is dominated by the well-developed Haber-Bosch process operated under harsh conditions. The development of more active catalysts under mild conditions is of important scientific significance and practical value. Early 3d metals such as Cr were regarded as catalytically inactive for ammonia synthesis so that the main research efforts have been focused on Ru-, Fe-, and Co-based catalysts over the years. Here we report a strategy to “activate” Cr for effective ammonia catalysis by the construction of a nitride-hydride structure with Ba, N, and H. This unique structure renders Cr highly active for ammonia synthesis especially under lower temperatures. It is envisaged that this strategy offers new opportunities for the development of novel catalysts for mild-condition and sustainable ammonia synthesis. Early 3d metals such as Cr have very low activity for ammonia synthesis catalysis. The formation of a barium chromium nitride-hydride structure leads to great change, resulting in an activity enhancement of more than three orders of magnitude compared with the Cr nitride. The barium chromium nitride-hydride structure comprising reactive hydrogen ( H - ) and nitrogen species creates a chemical environment favorable for N 2 activation and hydrogenation under mild conditions.

Large Impact of Approximate Exchange-Correlation Functionals on Modeling the Water Gas Shift Reaction on Copper

We investigate the water gas shift reaction (WGSR) on copper using three levels of exchange-correlation (XC) functionals of increasing complexity: the Perdew–Burke–Ernzerhof (PBE) functional, the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional, and the exact exchange plus the random phase approximation (RPA) correlation functional. We show that DFT predictions for the kinetics of the WGSR strongly depend on the choice of XC functionals. It is important to have accurate CO adsorption energy. Due to PBE’s overestimation of CO’s adsorption energy, it predicts small free surface and a negative (incorrect) reaction order for CO. HSE and RPA largely avoid such overestimation and predict large free surface and positive reaction orders for CO. The key finding in this work is that the prediction for the WGSR mechanism also depends on the choice of XC functionals. PBE and HSE predict the carboxyl mechanism, while RPA predicts that both redox and carboxyl mechanisms are important. These results suggest that caution should be paid when using approximate XC functionals to model heterogeneous catalysis (such as the WGSR investigated here) in which several mechanisms compete. In addition, we also observed one problem for RPA: It underestimates the overall WGSR energy and predicts a negative reaction order for CO2. In addition, we examine formate’s role in the WGSR over copper. Both DFT and previous experiments suggest that formate does not participate much in the reaction and cannot cause a negative reaction for CO2 for the WGSR over copper.

Nanocrystalline Co3O4 catalysts for toluene and propane oxidation: Effect of the precipitation agent

A set of cobalt oxide catalysts was prepared via precipitation using various precipitants and evaluated in the total oxidation of toluene and propane. A commercial Co3O4 catalyst was used as a reference. Various techniques were used to characterize the catalysts before and after testing. The results showed that the physicochemical properties and the catalytic performance of these cobalt oxide catalysts were greatly influenced by the nature of the precipitation agent used upon preparation. All catalysts exhibited a Co3O4 cubic spinel structure, with different crystallite sizes (27–41 nm) and specific surface areas (15–40 m2 g−1). The optimal catalyst, obtained via carbonate-precipitation and named Co-CO3, achieved 90 % toluene and propane conversion at 256 and 226 °C, respectively. This Co-CO3 catalyst performed better than the commercial catalyst in the toluene oxidation and similarly to the commercial one in the propane oxidation. All catalysts maintained constant performance upon three consecutive catalytic cycles; however, they deactivated to a different extent upon longer-term stability tests depending on the molecule to be oxidized. In toluene oxidation, Co-CO3 exhibited good stability upon cycling but a poor long-term durability. To overcome this issue, a dual-bed catalytic system was proposed. In propane oxidation, Co-CO3 showed stable performance in both cases. The effects of propane and oxygen concentrations on the catalytic performance of Co-CO3 in propane oxidation were also investigated. A positive reaction order with respect to oxygen and the results of oxygen-free catalytic test revealed the dominant role of chemisorbed oxygen species in the oxidation of propane. The excellent performance of Co-CO3 was tentatively ascribed to the highly strained nanocrystalline structure, the high concentration in Co2+ and the superior reducibility.

Kinetics of Water Gas Shift Reaction on Au/CeZrO4: A Comparison Between Conventional Heating and Dielectric Barrier Discharge (DBD) Plasma Activation

The kinetics of the low-temperature forward water gas shift (LT-WGS) reaction have been studied over a 2 wt% Au/CeZrO4 (Au/CZO) catalyst using both thermal and dielectric barrier discharge plasma heterogeneous catalyst systems. Using the energy density (e), the apparent activation energy has been calculated under plasma and thermally activated conditions. A substantially lower apparent activation energy is observed in the plasma activated system (9.5 kJ/mol) compared with the thermal-catalysed reaction (132.9 kJ/mol). Different kinetic isotope effect (KIE) values for water were found in thermal (1.43) and plasma (1.89) activated catalytic systems which infer different mechanisms between the two activation processes and also shows the importance of water activation. Furthermore, negative and positive reaction orders with respect to CO and H2O are found for both conditions which are − 1.30, 0.28 under thermal and − 1.53, 0.35 under plasma processes, respectively. The reaction order with respect to H2O and KIE studies demonstrate that the bond cleavage in H2O molecule is a rate determining step in the plasma-assisted LT-WGS, similar to that in the thermal-assisted reaction.

Non-porous shrinking particle model of leaching at low liquid-to-solid ratio

A generalized non-porous shrinking particle model (NSPM) of leaching is proposed, which - unlike commonly used “simplified” mathematical models - takes into account the influence of the L/S ratio (by means of the so-called reduced A/B molar ratio, ϕ, which indicates how many times the leaching agent (A)-to-valuable substance (B) molar ratio actually used exceeds its stoichiometric value) and non-ideal behaviour of concentrated lixiviants. Moreover, n-th order chemical reaction with power-law kinetics is considered, which makes determination of the rate-determining step of the overall process more reliable.Correlation between the proposed model and experiment was tested using the hydrochloric acid leaching of dead-burned magnesite, which provides the opportunity to experimentally verify how the mathematical model fits the leaching data for both the positive and negative reaction order. Calculated values of the reaction order for HCl, n, different from unity (n = 0.28 and n = −0.23 obtained for low (0.20–1.03 M) and high HCl concentrations (2.06–3.83 M), respectively), together with relatively high values of the activation energy (57.5 J mol−1 and 56.6 J mol−1) indicate that the leaching rate is controlled by the intrinsic chemical reaction under the conditions considered in the present work. Simulations made using the calculated values of the parameters have shown that the proposed generalized NSPM model clearly reflects two most important experimental facts: (a) L/S ratio affects the leaching process; and (b) qualitatively different leaching behaviour has been observed for the positive and negative reaction order. Unlike the simplified NSPM model, the generalized model describes the leaching process quantitatively in the whole range of the parameter ϕ, for both n > 0 and n < 0. The simplified NSPM (which does not take into account the L/S ratio) describes the leaching process with sufficient accuracy when ϕ ≥ 10, but at ϕ < 10 the error grows exponentially with decreasing ϕ and can reach up to 11–17% rel. under the stoichiometric conditions (i.e., at ϕ = 1), in dependence on the value of n. It is assumed that the proposed generalized NSPM model provides a useful tool for planning kinetic experiments, identifying the kinetic model parameters and simulation of the leaching process even under stoichiometric conditions.

Mechanism of Carbon Monoxide Dissociation on a Cobalt Fischer-Tropsch Catalyst.

The way in which the triple bond in CO dissociates, a key reaction step in the Fischer–Tropsch (FT) reaction, is a subject of intense debate. Direct CO dissociation on a Co catalyst was probed by 12C16O/13C18O scrambling in the absence and presence of H2. The initial scrambling rate without H2 was significantly higher than the rate of CO consumption under CO hydrogenation conditions, which indicated that the surface contained sites sufficiently reactive to dissociate CO without the assistance of H atoms. Only a small fraction of the surface was involved in CO scrambling. The minor influence of CO scrambling and CO residence time on the partial pressure of H2 showed that CO dissociation was not affected by the presence of H2. The positive H2 reaction order was correlated to the fact that the hydrogenation of adsorbed C and O atoms was slower than CO dissociation. Temperature‐programmed in situ IR spectroscopy underpinned the conclusion that CO dissociation does not require H atoms.

Kinetic Parameters for the Selective Hydrogenation of Acetylene on GaPd2 and GaPd.

Intermetallic GaPd2 is a highly selective and stable catalyst for the semi-hydrogenation of acetylene. Knowledge of the underlying reaction kinetics is essential to gain a deeper understanding of the selective hydrogenation on this catalytic material. To date, there has been no experimental kinetic data published for this reaction on a well-defined intermetallic catalyst possessing isolated active sites. Kinetic measurements are performed at 140-200 °C, revealing an apparent activation energy of 29(2) kJ mol-1 . GaPd2 is shown to be the first binary catalyst material, which shows a positive reaction order (0.89) with respect to acetylene at 200 °C. The influences on the extent of acetylene conversion, specific activity and selectivity to ethylene, ethane, and higher hydrocarbons are determined by a 24 factorial experiment following a design of experiments approach. Temperature and pressure have the strongest impact on these values. The results allow optimal operation for achieving high ethylene yields. A comparison of the reaction kinetics on GaPd2 with experimental results obtained for GaPd reveals different orders of reaction of H2 and C2 H2 on the two compounds.

Direct amination of dodecanol with NH3 over heterogeneous catalysts. Catalyst screening and kinetic modelling

The direct amination of dodecanol with ammonia to dodecylamine was investigated over carbon supported ruthenium, palladium, platinum, iridium and osmium catalysts. The influence of ammonia and hydrogen pressures was investigated. It was found that ammonia pressure did not influence the reaction, while extra hydrogen in the reaction mixture was beneficial for dodecanol conversion and dodecylamine selectivity. A dodecylamine yield of 83.8% was achieved with Ru/C catalyst at 150°C, 4bar ammonia and 2bar hydrogen after 24h reaction.Zero reaction order in ammonia typically implies that the rate limiting step is not related to amination, but rather to a preceding dehydrogenation step. Experimentally observed unusual positive order in hydrogen in dehydrogenation was related to deactivation by surface coking. Partial surface regeneration requires presence of hydrogen, thus explaining a positive reaction order towards this reactant. A detailed analysis of kinetic regularities, which due to specificity of the reaction mechanism could be done separately for activity and selectivity, allowed advancing a mathematical model adequately describing experimental observations.

Catalytic supercritical water gasification of microalgae: Comparison of Chlorella vulgaris and Scenedesmus quadricauda

Batch-wise supercritical water gasification (SCWG) of two types of microalgae was carried out in the absence and the presence of nickel catalysts in a laboratory scale reactor at 385°C and 26MPa. The microalgae used include Chlorella vulgaris (high protein and low carbohydrate contents) and Scenedesmus quadricauda (low protein and high carbohydrate contents). Time course trend of the total carbon-based gas yield of C. vulgaris and S. quadricauda was similar, showing asymptotic convex changes that may be attributed to the positive reaction order of SCWG of microalgae. In the presence of catalyst, the yield increased to 80–90% with time in an asymptotic way, whereas it reached only 12% in the absence of catalyst. For non-catalytic SCWG, the predominant gas product was carbon dioxide, which may be formed by decarboxylation of biomass. For catalytic SCWG, the major gas products were CH4, CO2, H2, and CO, of which composition can be explained by successive reactions of (1) hydrolytic breakdown of macromolecules to smaller organics, (2) gasification of organics to CO, CO2 and H2, (3) shift reaction of CO, and (4) methanation of CO and CO2, and their equilibrium.

Probing the effect of aqueous impurities on the leaching of chalcopyrite under controlled conditions

Chalcopyrite-containing deposits frequently comprise other sulfide and various silicate and carbonate minerals. Under acidic leaching conditions, as encountered in acid mine drainage or during industrial hydrometallurgical processing, these minerals can dissolve and release various impurities that may subsequently impact the leach behaviour of chalcopyrite. To gain further insight into these effects, batch leaching of chalcopyrite under controlled conditions of 75°C, pH1 and Eh 750mV SHE in the presence of selected cationic impurities, that are common components of these associated minerals, has been examined.The addition of pre-equilibrated Fe2+ in the range of 20–50mM gave rise to a negative reaction order using the initial rate method (IRM). In contrast, the IRM analysis indicated a positive reaction order for added Fe2+ over the range 5–20mM; however, the overall leach processes generally slowed compared to leaching in the absence of added cations. This may be due to increasing iron concentrations, due to chalcopyrite dissolution, as leaching progressed. Results from the IRM analysis also indicated that Al3+ (5–50mM) and K+ (5–150mM) both resulted in positive reaction orders and accelerated chalcopyrite leaching, while Ca2+ (5–20mM), Si oxyanions (5–50mM), Na+ (5–50mM) and Mg2+ (5–50mM) all slowed the leach process.Porous sulfur layers, observed by scanning electron microscopy, do not appear responsible for these trends as their morphologies appear almost the same for leaching with and without added impurities. Time-of-flight secondary ion mass spectroscopic analyses showed no significant differences between samples collected from leaching in the absence and presence of impurities indicating that surface species within the top one to two monolayers do not contribute to the reduced leach efficiency. X-ray photoelectron spectroscopic analyses suggested that Fe(III)–O/–OH surface speciation may be associated with the reduced leach efficiency observed in the presence of some of the impurities.

Mechanistic Aspects of Submol % Copper‐Catalyzed CN Cross‐Coupling

AbstractCarbonnitrogen bond formation can be catalyzed by copper in very low concentrations (≈100 ppm), with mechanistic features that are distinct from those in the high‐concentration regime. The reaction was studied by initial rate kinetics, competitive Hammett studies, and DFT calculations. The deprotonation of the model nucleophile, pyrrole, is limited by mass transfer with the heterogeneous base. The positive reaction order in dimethylethylenediamine was explained by this reagent working not only as a ligand to Cu, but also as a facilitator for mass transfer. The selectivity‐determining step in the competitive Hammett study is oxidative addition. Alternative mechanisms for this step, such as single‐electron transfer, atom transfer, or σ‐bond metathesis, can be excluded based on the observed Hammett behavior and DFT calculations.

Catalytic removal of nitrates from waters in a continuous flow process: The remarkable effect of liquid flow rate and gas feed composition

The selective catalytic reduction of nitrates (NO3−) in water mediums towards N2 formation by the use of H2 and in the presence of O2 (air) in the gas feed has been investigated under a continuous flow process over Pd–Cu supported on various mixed metal oxides, xwt.% MxOy/γ-Al2O3 (MxOy=CeO2, MgO, Mn2O3, Cr2O3, Y2O3, MoO2, Fe2O3 and TiO2). It is demonstrated for the first time that a remarkable improvement of both catalysts activity and N2 reaction selectivity can be achieved when increasing the liquid flow rate in the continuous flow process. In particular, it was found that NO3− reduction rates up to more than two times higher and NH4+ selectivity values up to twenty times lower can be obtained when increasing the liquid flow rate from 2 to 6mL/min. Moreover, it was proven for the first time, in a continuous flow process, that the presence of oxygen (or air) has a remarkable positive effect on the reaction's selectivity towards nitrogen. The reaction's selectivity towards NH4+ can be decreased by up to three times when 20vol.% air is added in the gas feed of the NO3−/H2 continuous flow reaction. The Pd–Cu clusters supported on TiO2-, CeO2- and MgO-coated γ-Al2O3 spheres showed the best catalytic behaviour compared with the rest of supports examined, both in the presence and in the absence of oxygen in the reducing feed gas stream. In addition, it was found that the initial concentration of nitrates in the liquid feed can significantly affect catalysts activity and reaction's selectivity. A positive apparent reaction order towards nitrates of 0.9 was calculated on Pd–Cu/TiO2/Al2O3.

Co-αAl 2O 3-Cu as shaped catalyst in NaBH 4 hydrolysis

A study about catalytic films of Co-supported-over-αAl 2O 3 fabricated by electrophoretic deposition (EPD) is reported, the as-prepared shaped catalysts being intended to catalyze NaBH 4 hydrolysis. Co-αAl 2O 3 supported over Cu substrate can be prepared by a 2-step route: (i) preparation of the supported catalyst Co-αAl 2O 3 (in powder form) by wet impregnation of CoCl 2 over αAl 2O 3, followed by a reduction, and (ii) fabrication of Co-αAl 2O 3-Cu (thin film over Cu) by EPD. Both types of catalysts, whatever their form, are highly efficient in hydrolyzing NaBH 4, conversions of 100% and HGRs of tens of mL(H 2) min −1 being achieved at 60–80 °C. The Co-αAl 2O 3-Cu catalysts are even more reactive than the Co-αAl 2O 3 catalysts because the surface of the former materials becomes much more acid than that of the latter ones in the course of the EPD process. The respective rate laws and reaction kinetics have been determined. Independently on the catalyst form, apparent activation energies of about 52 kJ mol −1 and positive reaction orders versus the initial NaBH 4 concentration (i.e. 0.3–0.7) were calculated, suggesting that the EPD does not affect the reaction mechanisms. Besides, it is showed that the hydrolysis is really catalytic as well as typical of a heterogeneous process. For example, an apparent reaction order versus the Co content of 0.9 was calculated. All of these results among others are reported and discussed in the present article.

Attainable regions of reactive distillation—Part I. Single reactant non-azeotropic systems

Reactive distillation (RD), a promising multifunctional reactor, can be used to improve the selectivity of the desired product by manipulating the concentration profiles in the reactive zone of the column. In this work, a new approach has been proposed to obtain the feasible regions of RD for the reactive systems involving single reactants, e.g. dimerization, aldol condensation, etc. Two new models namely the reactive condenser and the reactive re-boiler have been proposed. These models indicate the best location of the reactive zone in a column. Multistage versions of these models namely, reactive rectification and reactive stripping further expand the feasible region and are capable of representing the performance offered by a conventional RD unit. Several hypothetical non-azeotropic ideal systems have been extensively studied using these models and it has been shown that selectivity close to 100% is attainable over the entire range of conversion for a series as well as a combination of series and parallel reactions with positive reaction orders. Two industrially important cases of aldol condensation of acetone and dimerization of isobutylene have also been addressed using this approach. For porous catalysts, the presence of intra-particle diffusion resistance may limit the feasible region and even in the case of ideal non-azeotropic systems it may not be possible to obtain 100% selectivity. A methodology to incorporate pore diffusion effects is also illustrated.