Frank Rosowski

Ruthenium catalysts for ammonia synthesis at high pressures: Preparation, characterization, and power-law kinetics

Abstract Supported Ru catalysts for NH3 synthesis were prepared from Ru3(CO)12 and high-purity MgO and Al2O3. In addition to aqueous impregnation with alkali nitrates, two non-aqueous methods based on alkali carbonates were used to achieve alkali promotion resulting in long-term and high-temperature stable catalysts. For the reliable determination of the Ru particle size, the combined application of H2 chemisorption, TEM and XRD was found to be necessary. The power-law rate expressions were derived at atmospheric pressure and at 20 bar which were shown to be efficient tools to investigate the degree of interaction of the alkali promoter with the Ru metal particles. The following sequence with respect to the turnover frequency (TOF) of NH3 formation was found: Cs 2 CO 3 Ru MgO > CsNO 3 Ru MgO > Ru MgO > Ru KAl 2 O 3 > Ru Al 2 O 3 . The Cs-promoted Ru MgO catalysts turned out to be more active than a multiply-promoted Fe catalyst at atmospheric pressure with an initial TOF of about 10−2 s−1 for the non-aqueously prepared Cs 2 CO 3 Ru MgO catalyst at 588 K. The strong inhibition by H2 was found to require a lower molar H2:N2 ratio in the feed gas than 3:1 in order to achieve a high effluent NH3 mole fraction. The optimum ratio for Cs 2 CO 3 Ru MgO at 50 bar was determined to be about 3:2, resulting in an effluent NH3 mole fraction which was just a few percent lower than that of a multiply-promoted Fe catalyst operated at 107 bar and at roughly the same temperature and space velocity. Thus, alkali-promoted Ru catalysts are an alternative to the conventionally used Fe catalysts for NH3 synthesis also at high pressure.

The microkinetics of ammonia synthesis catalyzed by cesium-promoted supported ruthenium

Microkinetic models have been established successfully in case of ammonia synthesis over iron (Dumesic and Trevino, 1989) and methanol synthesis over copper (Rasmussen et al., 1994) based on a detailed knowledge about the kinetics of the involved elementary steps which allow to extrapolate the results from single crystal studies to predict the performance of an industrial catalyst operating at high pressure. In the present study, it has been possible to set up a consistent microkinetic model for ammonia synthesis based on a LHHW mechanism which describes the results of steady-state and transient experiments performed with a Cs-promoted ruthenium catalyst supported on MgO in agreement with kinetic data obtained under ultrahigh vacuum conditions.

The temperature-programmed desorption of N2 from a Ru/MgO catalyst used for ammonia synthesis

The temperature-programmed desorption (TPD) of N2 from a Ru/MgO catalyst used for ammonia synthesis was studied in a microreactor flow system operating at atmospheric pressure. Saturation with chemisorbed atomic nitrogen (N-*) was achieved by exposure to N2 at 573 K for 14 h and subsequent cooling in N2 to room temperature. With a heating rate of 5 K/min in He, a narrow and fairly symmetric N2 TPD peak at about 640 K results. From experiments with varying heating rates a preexponential factor Ades = 1.5×1010 molecules/(site s) and an activation energy Edes = 158 kJ/mol was derived assuming secondorder desorption. This rate constant of desorption is in good agreement with results obtained with a Ru(0001) single crystal surface in ultra-high vacuum (UHV). The rate of dissociative chemisorption was determined by varying the N2 exposure conditions. Determination of the coverage of N-* was based on the integration of the subsequently recorded N2 TPD traces yielding Aads = 2×10−6 (Pa s)−1 and Eads = 27 kJ/mol. The corresponding sticking coefficient of about 10−14 at 300 K is in agreement with the inertness of Ru(0001) in UHV towards dissociative chemisorption of N2. However, if the whole catalytic surface were in this state, then the resulting rate of N2 dissociation would be several orders of magnitude lower than the observed rate of NH3 formation. Hence only a small fraction of the total Rumetal surface area of Ru/MgO seems to be highly active dominating the rate of ammonia formation.

The dissociative adsorption of N2 on a multiply promoted iron catalyst used for ammonia synthesis: a temperature-programmed desorption study

The temperature-programmed desorption (TPD) of N2 from a multiply promoted iron catalyst used for ammonia synthesis has been studied in a microreactor system at atmospheric pressure. From TPD experiments with various heating rates a preexponential factorA = 2 × 109 molecules/site s and an activation energyE = 146 kJ/mol was derived assuming second-order desorption. The observed dependence of the TPD peak shapes on the heating rates indicated the influence of readsorption of N2 in agreement with the results obtained for various initial coverages. Simulating the N2 TPD curves using the model by Stoltze and Nørskov revealed that the calculated TPD curves were not influenced by the molecular precursor to desorption. However, the calculated rate of readsorption was found to be overestimated at high coverage compared with the experimental results. A coverage-dependent net activation energy for dissociative chemisorption (E*) was introduced as the simplest assumption rendering the dissociative chemisorption of N2 activated at high coverage. The best fit of the experimental data yieldedE* = (−15+30θ) kJ/mol using only a single type of atomic nitrogen species. These findings are in satisfactory agreement with the parameters underlying the Stoltze-Nørskov model for the kinetics of ammonia synthesis as well as with the data reported for Fe(111) single crystal surfaces.

Microkinetic analysis of temperature-programmed experiments in a microreactor flow system

Temperature-programmed (TP) experiments provide useful information about the kinetic properties of adsorbates. The TP experiments can be carried out either in ultra-high vacuum (UHV) or at ambient pressure using flow set-ups. Thus, they can serve as a tool to bridge the material and pressure gap between surface science and heterogeneous catalysis by applying microkinetic analysis. The temperature-programmed desorption (TPD) of H 2 from Cu is used as an example to discuss the derivation of kinetic parameters from measurements in UHV. N 2 TPD experiments from a multiply promoted Fe catalyst are analyzed to demonstrate that the assessed influence of readsorption effects depends on the reactor model chosen. The transient continuous-flow stirred tank reactor (CSTR) and the transient plug-flow reactor (PFR) are used for the modeling. Finally, the temperature-programmed surface reaction (TPSR) of adsorbed atomic nitrogen with H 2 yielding NH 3 over Fe- and Ru-based catalysts is re-examined to illustrate the influence of the reactor model.

Ruthenium as catalyst for ammonia synthesis

Five Ru-based catalysts were prepared to study the effect of the support and the role of the alkali promoter in NH3 synthesis: Ru/Al2O3, Ru/MgO, Cs-Ru/Al2O3 and Cs(K)−Ru/MgO. The catalysts were characterized by N2 physisorption, H2 chemisorption and XPS. The absence of chlorine- and sulphur containing compounds turned out to be important for the preparation of highly active catalysts. Power law expressions were derived from conversion measurements at atmospheric, pressure and at 20 bar. For all catalysts, the reaction order for H2 was found to be negative suggesting that a PN2/PH2 ratio in the feed gas higher than 1/3 would be favourable for industrial NH3 synthesis at high pressure. The microkinetic analysis of the temperature-programmed desorption and adsorption of N2 and of the kinetics of isotopic exchange demonstrated the enhancing influence of the Cs promoter on the rate of N2 dissociation and recombination. XPS measurements after through reduction revealed a shift of the Ru 3d5/2 peak to lower binding energy by about 1 e V in the presence of Cs suggesting an electronic promoter effect.

The Intimate Relationship between Bulk Electronic Conductivity and Selectivity in the Catalytic Oxidation of n-Butane†

The efficient and direct functionalization of alkanes from natural gas or future regenerative carbon-based resources is impeded by the lack of suitable catalysts and the absence of a detailed mechanistic understanding of the few efficient alkane oxidation reactions. The selective oxidation of nbutane to maleic anhydride (MA), an important basic chemical with an annual global production of 1.4 Mt, is one of such scarce commercialized examples. The industrially used vanadium-phosphorous-oxide (VPO) catalyst enabling a maximum MA yield of 65% mainly consists of (VO)2P2O7 (vanadyl pyrophosphate, VPP). Based on experimental evidence it is generally agreed that the reaction proceeds via a two-step mechanism, in which, in Step 1: “lattice” oxygen from the catalyst (or oxygen from an active surface site) is abstracted to oxidize the alkane, and in Step 2: the catalyst is subsequently reoxidized by gas-phase O2. It is however still debated, whether the reaction proceeds on noninteracting single surface sites with the bulk being only an inert support, or if the bulk supplies charge carriers and oxygen. 11–15] The single-site concept would demand spacious active sites to provide the large number of 14 electrons and seven oxygen atoms needed per converted n-butane molecule. In contrast, an unlimited bulk–surface charge and oxygen transfer contradicts the fundamental site-isolation principle of selective oxidation catalysis, which presumes that the (stoichiometric) limitation and spatial isolation of active oxygen prevents the further oxidation of the desired product to COx. Clearly, the investigation of charge-carrier dynamics in selective catalysts is of fundamental importance to disentangle the surface and bulk influence on the catalytic performance. Unfortunately, unstable (Schottky) contact resistances between catalyst particles, electrodes, and at grain boundaries hamper quantitative and sensitive electrical-conductivity investigations by DC or AC contact methods. Although such studies have provided valuable information on the electrical properties of VPO catalysts, the direct participation of bulk charge carriers in the catalytic reaction has not been demonstrated unequivocally yet. Herein, the disadvantages of contact methods could be circumvented by using a noncontact conductivity method based on the microwave cavity perturbation technique (MCPT). MCPT is a highly sensitive technique 21] allowing the non-invasive quantitative measurement of the permittivity and electrical conductivity of polycrystalline samples in a fixed-bed flow-through reactor. The excitation of free charge carriers in the investigated sample by microwaves (at 9.2 GHz) in a calibrated resonant cavity enables the determination of absolute conductivity values. By measuring the change of the resonance frequency and the quality factor of the cavity with and without the sample, its complex permittivity e = e1 + ie2 can be deduced (Figure 1). [20–22] The imaginary part e2 is composed of the dielectric loss, ionic, and electronic conductivity. A major contribution of ionic charge carriers (e.g. O ions) is negligible because their high masses are not able to follow the high-frequency microwave excitation. In addition, no response of the real permittivity e1 of VPO to variations of the gas-phase chemical potential was observed, hence a significant influence of dielectric relaxations (through dipoles) can be excluded. Consequently, the major contribution to e2 in VPO is electronic conductivity. Moreover, pre-investigations showed that the catalyst behaves as a p-type semiconductor with an increasing conductivity in oxidizing and a decreasing conductivity in reducing mixtures (Figure S2 in the Supporting Information). These results are in agreement with contact conductivity investigations. According to X-ray powder diffractometry, the VPO catalyst was phase pure and consisted of vanadyl pyrophosphate (Supporting Information, Figure S1). The catalytic performance and conductivity of VPP were probed simultaneously with the in situ MCPT/GC setup at constant temperature, but in different gas mixtures and at different gas hourly space velocities (GHSV), i.e. at different reaction gas contact times. The catalyst was preheated in air to 400 8C (GHSV: [*] Dr. M. Eichelbaum, Dr. M. H vecker, C. Heine, Dr. A. Trunschke, Prof. Dr. R. Schlçgl Department of Inorganic Chemistry Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4–6, 14195 Berlin (Germany) Fax: (49)30-84134401 E-mail: me@fhi-berlin.mpg.de

The Electronic Factor in Alkane Oxidation Catalysis

This article addresses the fundamental question of whether concepts from semiconductor physics can be applied to describe the working mode of heterogeneous oxidation catalysts and whether they can be even used to discriminate between selective and unselective reaction pathways. Near-ambient-pressure X-ray photoelectron spectroscopy was applied to the oxidation of n-butane to maleic anhydride on the highly selective catalyst vanadyl pyrophosphate and the moderately selective MoVTeNbO(x) M1 phase. The catalysts were found to act like semiconducting gas sensors with a dynamic charge transfer between the bulk and the surface, as indicated by the gas-phase-dependent response of the work function, electron affinity, and the surface potential barrier. In contrast, only a minor influence of the gas phase on the semiconducting properties and hence no dynamic surface potential barrier was monitored for the total oxidation catalyst V2O5. The surface potential barrier is hence suggested as descriptor for selective catalysts.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4 Nanorods

The electronic and structural properties of vanadium-containing phases govern the formation of isolated active sites at the surface of these catalysts for selective alkane oxidation. This concept is not restricted to vanadium oxide. The deliberate use of hydrothermal techniques can turn the typical combustion catalyst manganese oxide into a selective catalyst for oxidative propane dehydrogenation. Nanostructured, crystalline MnWO4 serves as the support that stabilizes a defect-rich MnOx surface phase. Oxygen defects can be reversibly replenished and depleted at the reaction temperature. Terminating MnOx zigzag chains on the (010) crystal planes are suspected to bear structurally site-isolated oxygen defects that account for the unexpectedly good performance of the catalyst in propane activation.

Enhancing of catalytic properties of vanadia via surface doping with phosphorus using atomic layer deposition

Atomic layer deposition is mainly used to deposit thin films on flat substrates. Here, the authors deposit a submonolayer of phosphorus on V2O5 in the form of catalyst powder. The goal is to prepare a model catalyst related to the vanadyl pyrophosphate catalyst (VO)2P2O7 industrially used for the oxidation of n-butane to maleic anhydride. The oxidation state of vanadium in vanadyl pyrophosphate is 4+. In literature, it was shown that the surface of vanadyl pyrophosphate contains V5+ and is enriched in phosphorus under reaction conditions. On account of this, V2O5 with the oxidation state of 5+ for vanadium partially covered with phosphorus can be regarded as a suitable model catalyst. The catalytic performance of the model catalyst prepared via atomic layer deposition was measured and compared to the performance of catalysts prepared via incipient wetness impregnation and the original V2O5 substrate. It could be clearly shown that the dedicated deposition of phosphorus by atomic layer deposition enhances ...