Supported Ir Catalysts Research Articles

Comparative Investigation of Benzene Steam Reforming over Spinel Supported Rh and Ir Catalysts

In a combined experimental and first-principles density functional theory (DFT) study, benzene steam reforming (BSR) over MgAl2O4-supported Rh and Ir catalysts was investigated. Experimentally, it has been found that both highly dispersed Rh and Ir clusters (1–2 nm) on the spinel (e.g., MgAl2O4) support are stable during the BSR in the temperature range of 700–850 °C. Compared to the Ir/MgAl2O4 catalyst, the Rh/MgAl2O4 catalyst is more active with higher benzene turnover frequency and conversion. At steam conditions with the molar steam-to-carbon ratio >12, the benzene conversion is only a weak function of the H2O concentration in the feed. This suggests that the initial benzene decomposition step, rather than the benzene adsorption, is most likely the rate-determining step in BSR over supported Rh and Ir catalysts. To understand the differences between the two catalysts, we followed with a comparative DFT study of initial benzene decomposition pathways over two representative model systems for each supported metal (Rh and Ir) catalysts. A periodic terrace (111) surface and an amorphous 50-atom metal cluster with a diameter of 1.0 nm were used to represent the two supported model catalysts under low and high dispersion conditions. Our DFT results show that the decreasing catalyst particle size enhances the benzene decomposition on supported Rh catalysts by lowering both C–C and C–H bond scission. The activation barriers of the C–C and the C–H bond scission decrease from 1.60 and 1.61 eV on the Rh(111) surface to 1.34 and 1.26 eV on the Rh50 cluster. For supported Ir catalysts, the decreasing particle size only affects the C–C scission. The activation barrier of the C–C scission of benzene decreases from 1.60 eV on the Ir(111) surface to 1.35 eV on the Ir50 cluster while the barriers of the C–H scission are practically the same. The experimentally measured higher BSR activity on the supported highly dispersed Rh catalyst can be rationalized by the thermodynamic limitation for the very first C–C bond scission of benzene on the small Ir50 catalyst. The C–C bond scission of benzene on the small Ir50 catalyst is highly endothermic although the barrier is competitive with those of both the C–C and the C–H bond-breaking on the small Rh50 catalyst. The calculations also imply that, for the supported Rh catalysts, the C–C and C–H bond scissions are competitive, independent of the Rh cluster sizes. After the initial dissociation step via either the C–C or the C–H bond scission, the C–H bond breaking seems to be more favorable rather than the C–C bond breaking on the larger Rh terrace surface.

Structure sensitivity of hydrogenolytic cleavage of endocyclic and exocyclic C–C bonds in methylcyclohexane over supported iridium particles

Structure sensitivities, H2 pressure effects, and temperature dependencies for rates and selectivities of endo- and exocyclic C–C bond cleavage in methylcyclohexane were studied over supported Ir catalysts. The rate of endocyclic C–C bond cleavage first decreased and then increased with declining Ir dispersion from 0.65 to 0.035. The ring opening (RO) product distribution remained unchanged with varying H2 pressure on small Ir particles, while further shifting to methylhexanes with increasing H2 pressure on large particles. In contrast, the rate and selectivity of exocyclic C–C bond cleavage decreased monotonically with increasing H2 pressure and decreasing Ir particle size. The distinct dependencies of endocyclic and exocyclic C–C bond cleavage pathways on Ir dispersion and H2 pressure suggest that they are mediated by surface species with different ensemble size requirements. DFT calculations were performed on an Ir50 cluster and an Ir(111) surface, with or without pre-adsorbed hydrogen atoms, to provide insight into the observed effects of particle size and H2 pressure on RO pathways. On small Ir particles, the calculated dehydrogenation enthalpies for all endocyclic bonds were similar and affected to similar extents by H2 pressure; on large particles, the selectivity to n-heptane (via substituted C–C bond cleavage) was even lower than on small particles as a result of the least favorable adsorption and dehydrogenation energetics for hindered bonds.

Active sites and reactive intermediates in the hydrogenolytic cleavage of C–C bonds in cyclohexane over supported iridium

Hydrogenolysis of cyclohexane has been explored over supported Ir catalysts. The kinetic data are combined with modeling results to assess the structural requirements and the nature of catalytically relevant surface intermediates for endocyclic C–C bond cleavage. The turnover frequency (TOF) for cyclohexane hydrogenolysis showed complex dependence on Ir particle size, while the selectivity to the primary ring opening product, n-hexane, decreased monotonically with decreasing Ir dispersion. The decreasing TOF as the Ir dispersion decreased from 65% to 52% originates principally from the diminishing abundance of low-coordination Ir atoms at particle surfaces. The increase of the TOF with further Ir particle growth is attributed to an increased fraction of terrace planes, or step sites, and a less unsaturated nature of the most abundant reactive intermediate. Selectivities for multiple C–C bond cleavage, yielding C<6 alkanes, varies with the relative abundance of coordinatively unsaturated Ir atoms and terrace planes. The multiple hydrogenolysis depends additionally upon H2 pressure, because single and multiple C–C bond scissions are mediated by surface intermediates with different H-deficiencies.

Design of a Highly Active Ir/Fe(OH)x Catalyst: Versatile Application of Pt‐Group Metals for the Preferential Oxidation of Carbon Monoxide

The proton-exchange membrane fuel cell (PEMFC) has been regarded as one of the most promising candidates for the efficient use of hydrogen energy. However, small amounts of CO (0.3–1%) in the H2 stream from reforming processes must be selectively removed because CO is highly poisonous to the Pt anode of a PEMFC. The preferential oxidation of CO in a H2-rich gas (PROX) is presently the most effective approach to address this problem. Oxide-supported Au catalysts are highly active for the PROX reaction even at room temperature, but the lower stability and sensitivity to CO2 constrain their practical applications. Supported Pt catalysts, on the other hand, are less active and only a few have shown reasonable activity for conversion of CO at temperatures lower than 60 8C. Therefore, it is highly desirable to develop improved catalysts with better catalytic performance for the PROX reaction at lower temperatures. Ir has a higher melting point and surface energy than other metals with 5f orbitals, such as Pt and Au, and Ir can be well-dispersed on and strongly interact with the support. However, compared to Ptand Au-based catalysts, Ir-based catalysts have limited applications in heterogeneous catalysis and are rarely investigated for the PROX reaction, most probably because of its inferior activity. Although much effort has been made to improve the activity of Ir-based catalysts and remarkable progress has been achieved, their activities for the PROX reaction are still low at low temperatures. In fact, there is no report so far claiming that Ir-based catalysts can show high activity at temperatures below 80 8C; thus it remains a formidable challenge to utilize Ir-based catalysts for the PROX reaction at ambient temperatures. One basic task of modern catalysis is to rationally design catalysts based on the fundamental understanding of their reaction mechanisms. Especially, the contribution of support materials to the performance of the final catalysts should be taken into account. For the PROX reaction, the strong binding of CO to Ir poisons the surface so that O2 cannot competitively adsorb on the Ir surface and be activated at low temperatures, thereby prohibiting the conversion of CO to CO2. Therefore, weakening the adsorption strength of CO and/or promoting the activation of O2 at lower temperatures have become the crucial steps. Ferric oxide has proven effective for O2 activation and has been used extensively as an additive to Pt-based catalysts. Recently, we have designed a bimetallic catalyst by adding FeOx to a supported Ir catalyst, and the activity for the PROX reaction was improved. Further study of the catalytic reactions showed that the reaction rate of CO oxidation correlated well with the presence and amount of Fe, suggesting that Fe sites were indeed the active sites for O2 activation. [13] The coordinatively unsaturated Fe center was also recently identified as the site to activate O2, which helped the design of a highly active FeOx/Pt/SiO2 catalyst to totally convert CO at room temperature. All of these studies suggest that the presence of low-valent Fe (Fe) played a decisive role in improving the PROX activity, thus providing a clue for obtaining a highly effective Ir-based catalyst by incorporating materials containing, or easily forming, Fe species. Ferric hydroxide (Fe(OH)x) is a novel support material which has recently been adopted to stabilize various types of metal species for CO oxidation. It possesses a large surface area and a large amount of OH groups; these unique properties make Fe(OH)x a good candidate for generating highly dispersed metal clusters or even single-atom catalysts. Furthermore, the longer Fe O bonds in Fe(OH)x (compared to those in Fe2O3) make it easier to form Fe 2+

Electrocatalytic Performance of Graphene Supported Ir Catalyst for Ammonia Oxidation

Instead Vulcan XC-72 carbon(XC), grapheme(G) was used as the support to prepare the Ir/G catalyst. The electrochemical measurement indicted that the electrocatalytic performance of the Ir/G catalyst for the ammonia oxidation was better than that of the Ir/XC catalyst. XRD and TEM measurements indicated that the average sizes of Ir particles in Ir/G and Ir/XC catalysts were similar. The measurement of the Raman spectroscopy illustrates the graphitization extent of G is higher than that of XC. Thus, the conductivity of G is higher than that of XC. Therefore, the electrocatalytic performance of the Ir/G catalyst is better than that of the Ir/XC catalyst can be attributed to the high conductivity due to the high graphitization extent of G. The results show that there is the good linear relationship between the current density of the ammonia oxidation at the Ir/G catalyst electrode and the concentration of ammonia. The related coefficient(R) is 0. 99557. Thus, Ir/G catalyst electrode can be used as the working electrode in the current type of electrochemical ammonia sensor.

担持イリジウム触媒上での1,3-ジメチルシクロペンタン水素化開環反応における担体効果

担持イリジウム触媒上での1,3-ジメチルシクロペンタン水素化開環反応における担体効果

Catalytic oxidation of aromatic alcohols and alkylarenes with molecular oxygen over Ir/TiO 2

A TiO 2 supported Ir catalyst prepared by the conventional impregnation method exhibited efficient catalytic activity toward the oxidation of organic compounds with molecular oxygen. Aromatic alcohols and alkylarenes selectivity afforded the corresponding carbonyl compounds in high yield under mild conditions. The reaction was quenched by removing the catalyst with filtration suggesting that the catalytic reaction proceeded heterogeneously. The highly dispersed Ir metal particles on TiO 2 surface was considered to be responsible for the catalysis.

Effect of Re and Mo addition upon liquid phase methanol reforming with water over SiO 2, ZrO 2 and TiO 2 supported Ir catalysts

Two different enhancement manners of Re and Mo additives have been studied in liquid phase methanol reforming with water over SiO 2, ZrO 2 and TiO 2 supported Ir catalysts. In the cases of Ir–Re/SiO 2 and Ir–Re/ZrO 2, hydrogen formation rate was accelerated remarkably by the formation of some nanocomposites between Ir and Re. On the other hand, in the case of Ir–Mo/TiO 2, the formation of thin monolayer-like structure of MoO 2 on TiO 2 surface was important for the acceleration of hydration process of partially dehydrogenated methanol with adsorbed water which was activated over thin MoO 2 overlayer.

Efficient Heterogeneous Dual Catalyst Systems for Alkane Metathesis

AbstractA fully heterogeneous and highly efficient dual catalyst system for alkane metathesis (AM) has been developed. The system is comprised of an alumina‐supported iridium pincer catalyst for alkane dehydrogenation/olefin hydrogenation and a second heterogeneous olefin metathesis catalyst. The iridium catalysts bear basic functional groups on the aromatic backbone of the pincer ligand and are strongly adsorbed on Lewis acid sites on alumina. The heterogeneous systems exhibit higher lifetimes and productivities relative to the corresponding homogeneous systems as catalyst/catalyst interactions and bimolecular decomposition reactions are inhibited. Additionally, using a “two‐pot” device, the supported Ir catalysts and metathesis catalysts can be physically separated and run at different temperatures. This system with isolated catalysts shows very high turnover numbers and is selective for the formation of high molecular weight alkanes.

Electrocatalytic Performance of Carbon Supported Ir Catalysts with Different Ir Loadings for Ammonia Oxidation

Electrocatalytic Performance of Carbon Supported Ir Catalysts with Different Ir Loadings for Ammonia Oxidation

Hydroconversion of Dimethylpentanes from Methylcyclohexane Using Two Consecutive Reactors Packed with Pt-modified Solid Acid Catalyst and Supported Ir Catalyst

Hydroconversion of methylcyclohexane was conducted over various noble metal-loaded catalysts at 493 K and atmospheric pressure. On the bases of the proposed reaction route and the rate of reaction over Ir/H-β zeolite, which was the best catalyst examined, we concluded that the low yield of the desired dimethylpentanes (18%) and high yield of undesired methylhexanes (25%) were due to two reasons: (1) formation of ethylcyclopentane with high selectivity at low conversions and (2) higher ring-opening rate of ethylcyclopentane (a precursor of methylhexanes) than that of dimethylcyclopentanes (a precursor of dimethylpentanes). In order to improve the yield of dimethylpentanes, two catalysts for ring contraction from methylcyclohexane to ethyl- or dimethyl-cyclopentanes (Pt-H4SiW12O40/SiO2 or Pt/H-β) and for ring opening of the produced cyclopentanes (Ir/Al2O3) were used either in one reactor as a physical mixture or in two separate reactors connected in series. When the physical mixture (Pt-H4SiW12O40/SiO2 and Ir/Al2O3) was used, there was only a slight increase in the dimethylpentanes yield (20%) with a large amount of undesired products, such as monobranched heptanes and cracked products. In contrast, when two consecutive reactors packed with Ir/H-β and Ir/Al2O3 were used, the yield of dimethylpentanes increased to 30%, which was nearly two times of that of methylhexanes, at a 65% conversion of methylcyclohexane, while the yield of undesired methylhexanes was about 15%.

Carbon dioxide reforming of methane over glow discharge plasma-reduced Ir/Al2O3 catalyst

Abstract In this work, characterizations using X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) confirm that argon glow discharge plasma can effectively reduce the Al 2 O 3 supported Ir catalyst without the use of any conventional reducing chemicals. Such plasma-reduced Ir/Al 2 O 3 catalyst shows a significantly enhanced dispersion. High conversions have been achieved on this plasma-reduced Ir/Al 2 O 3 catalyst for CO 2 reforming of methane.

Heats of Adsorption of the Linear CO Species Adsorbed on a Ir/Al2O3Catalyst Using in Situ FTIR Spectroscopy under Adsorption Equilibrium

The adsorption of CO on a reduced 1% Ir/Al2O3 catalyst has been studied by using Fourier transform infrared spectroscopy (FTIR) at two constant CO partial pressures Pa (200 and 2000 Pa) in the adsorption temperature (denoted by Ta) range of 300−740 K. At 300 K, it is observed a strong IR band at 2044 cm-1 ascribed to linear CO species (denoted by L) associated to weak shoulders at ≈2080 and ≈2000 cm-1. The FTIR spectra show how the coverage θL of the L CO species evolves with Ta for the two Pa values. At Ta > 580 K, the curves θL= f(Ta) are in very good agreement with an adsorption model based on Temkin's approximations indicating that the heat of adsorption linearly increases with the decrease in θL from E1 to E0 at θL= 1 and 0, respectively. This allows us to evaluate the ratio E0/(E0 − E1) ≈ 2.1 and then E0 − E1 ≈ 115 kJ/mol. Moreover, it is shown that, in the Ta range of 300−740 K, the curves θL= f(Ta) for the two CO partial pressures are in very good agreement with an adsorption model considering the following: (a) an immobile adsorbed species and (b) a linear decrease in the heat of adsorption with the increase in θL from E0= 225 kJ/mol at θL= 0 to E1= 115 kJ/mol at θL= 1. Finally the two isobars are used to determine the isosteric heats of adsorption at several coverages in the range of 1−0.8 from the Clausius−Clapeyron equation. A good agreement is observed with the values determined using the adsorption model and this validates the various assumptions involved in the model. The heat of adsorption values are compared to the literature data on single crystals and supported Ir catalysts. Moreover, it is shown that the heat of adsorption values allow us to quantify several quantitative data found in the literature on the stability of the L CO species on Ir-containing solids either in a vacuum or in an inert gas.

Ammonia decomposition on Ir(100): from ultrahigh vacuum to elevated pressures

Ammonia decomposition on Ir(100) has been studied over the pressure range from ultrahigh vacuum to 1.5 Torr and at temperatures ranging from 200 to 800 K. The kinetics of the ammonia decomposition reaction was monitored by total pressure change. The apparent activation energy obtained in this study (84 kJ/mol) is in excellent agreement with our previous studies using supported Ir catalysts (Ir/Al2O3 82 kJ/mol). Partial pressure dependence studies of the reaction rate yielded a positive order (0.9±0.1) with respect to ammonia and negative order (−0.7 ±0.1) with respect to hydrogen. Temperature-programmed desorption data from clean and hydrogen co-adsorbed Ir(100) surfaces indicate that ammonia undergoes facile decomposition on both these surfaces. Recombinative desorption of N2 is the rate-determining step with a desorption activation energy of ∼63 kJ/mol. Co-adsorption data also indicate that the observed negative order with respect to hydrogen pressure is due to enhancement of the reverse reaction (NHx + H → NHx+1, x=0–2) in the presence of excess H atoms on the surface.

Catalytic ammonia decomposition: COx-free hydrogen production for fuel cell applications

Catalytic decomposition of ammonia has been investigated as a method to produce hydrogen for fuel cell applications. The absence of any undesirable by-products (unlike, e.g., COx, formed during reforming of hydrocarbons and alcohols) makes this process an ideal source of hydrogen for fuel cells. In this study a variety of supported metal catalysts have been studied. Supported Ru catalysts were found to be the most active, whereas supported Ni catalysts were the least active. The supports were found to play a profound role in the ammonia decomposition process. The activation energies for the ammonia decomposition process varied from 17 to 22 kcal/mol depending upon the catalyst employed. The activation energies of the supported Ir catalysts were found to be in excellent agreement with our recent studies addressing ammonia decomposition on single crystal Ir.

Reaction of carbon monoxide with water on supported iridium catalysts

The adsorption of CO, the interaction and the reaction of CO and water have been investigated over supported Ir catalysts. The adsorption of CO was studied by infrared spectroscopy. The dominant spectral features for Ir/Al 2O 3 at 298 K are the bands at 2090, 2060, and 2018 cm −1 but on Ir/TiO 2, Ir/SiO 2, and Ir/MgO only a band at 2060–2075 cm −1 was detected. At higher temperature above 473 K CO induces the agglomeration of Ir, but the H 2O hindered this process. In the interaction and in the reaction of CO and water, formate formation was observed on the surface of the catalysts. The IR spectra of the adsorbed CO in the presence of water differed from that observed during the adsorption of CO; the band due to linearly bonded CO was shifted to lower wavenumbers. The reaction of CO + H 2O was studied in a fixed-bed continuous-flow reactor. The production of H 2, CO 2 (with ratio about 1) and small amount of hydrocarbons proceeded readily above 523 K. Ir/TiO 2 was found to be the most effective catalyst, its activity was 20 times higher than that of the last effective Ir/SiO 2. Possible mechanisms of the CO + H 2O reaction are discussed.

Decomposition of CH 4 over supported Ir catalysts

The decomposition of methane and its conversion into higher hydrocarbons have been investigated on supported Ir catalysts. The effects of temperature, flow rate and support materials have been examined. The interaction of CH 4 with iridium has been observed at as low a temperature as 473 K. As a result, hydrogen, a small amount of ethane and surface carbonaceous species were produced. With increase of the temperature, the extent of the decomposition significantly increased. At 773 K, the initial conversion varied between 2.0–5.0%, which decreased to low values in a short reaction time. Taking into account the dispersion of Ir, the most effective sample in the decomposition of CH 4 was Ir/MgO. By means of Fourier transform infrared spectroscopy adsorbed CH 3 was identified as a reaction intermediate of methane decomposition. Temperature programmed reactions revealed that the reactivity of surface carbon produced in the decomposition of CH 4 depends on the nature of the support. Hydrogenation of the most reactive carbonaceous species led to the production of aliphatic hydrocarbons up to six carbon atoms.

An infrared study on the formation of isocyanate in the NO + CO reaction on supported Ir catalyst

The formation of isocyanate surface species in the NO + CO reaction was investigated on supported iridium catalyst as a function of temperature and composition of the reacting gas mixture. On reduced Ir Al 2O 3 samples absorption bands due to isocyanate were observed at 2260-2240 cm −1. The isocyanate band was first detected at 200 °C, and its maximum intensity was registered at 280–300 °C. It is stable below 200 °C but decomposes rapidly above 300 °C in vacuo producing CO 2, CO, and N 2. Isocyanate species was also formed on oxidized surfaces but with considerably lower intensities. The reaction between preadsorbed NO and gaseous CO also resulted in the formation of isocyanate, but it was not identified when NO was admitted onto preadsorbed CO. During the surface interaction of NO and CO a large downscale shift of the NO band occurred which was explained by the perturbing effect of adsorbed CO. It is assumed that the primary step in the formation of isocyanate is the dissociation of NO on the reduced centers. It is postulated that isocyanate formed on the iridium migrates to the acceptor sites of the support. This explanation is backed by the observations that the different supports markedly influence the location, formation, and stability of the isocyanate band.

Catalytic oxidation: V. Mechanisms of olefin oxidation over supported iridium

The reactions of ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene, iso-butene and the two 2-pentenes were compared over supported Ir catalysts. The rates of total oxidation decreased in the order given, while the corresponding apparent activation energies increased. With ethylene, rate maxima appeared on varying the pressure of either reactant; with propylene and 1-butene, maxima were found with oxygen only; with the other olefins, the rates increased with olefin pressure and were inhibited by oxygen over the entire range studied. About 40% of all the olefin oxidized became partial oxidation products. The most important of these were acetic acid from ethylene, propylene, the 2-butenes and the 2-pentenes, and propionic acid and acetone from 1-butene and isobutene, respectively. Smaller amounts of unsaturated aldehydes, diones and other ketones and acids were also detected. The acetic acid formed from propylene-1- 14C was not radioactive. A large kinetic isotope effect was found for the total oxidation of propylene labeled with deuterium in the methyl group. Possible mechanisms are discussed.