CATALYST FOR AMMONIA OXIDATION

Abstract

The present invention relates to a bimetallic catalyst for ammonia oxidation, a method for producing a bimetallic catalyst for ammonia oxidation and a method for tuning the catalytic activity of a transition metal. By depositing an overlayer of less catalytic active metal onto a more catalytic active metal, the total catalytic activity is enhanced.

Claims

1. A bimetallic catalyst for ammonia oxidation, said bimetallic catalyst comprising: a first metal layer, wherein said first metal layer is a Cu layer; a second metal layer, wherein said second metal layer is a Ru layer; and a substrate; wherein said first metal layer is located onto said second metal layer and wherein said second metal layer is located onto said substrate; and wherein said first metal layer is a layer of a metal that is less catalytically active towards ammonia oxidation than a metal of said second metal layer; and wherein said metal of said first metal layer is segregated onto said second metal layer; and wherein said metal of said first metal layer does not form an alloy with said metal of said second metal layer, thereby providing a catalyst having a catalytic activity towards ammonia oxidation that is higher than a catalyst having either a single layer of Cu or a single layer of Ru supported on the same substrate.

2-13. (canceled)

14. The bimetallic catalyst according to claim 1, wherein said first metal layer is a layer of a metal that is less catalytically active towards ammonia oxidation than a metal of said second metal layer.

15. The bimetallic catalyst according to claim 1, wherein the second metal layer is a layer of a metal that is more catalytically active towards ammonia oxidation than a metal of said first metal layer.

16. The bimetallic catalyst according to claim 1, wherein said substrate is a metal oxide.

17. The bimetallic catalyst according to claim 1, wherein said first metal layer has a thickness between 0.1 and 3 monolayers.

18. The bimetallic catalyst according to claim 1, wherein the weight ratio between said second metal layer and said first metal layer is in the area between 1:0.05 and 1:0.5.

19. A method for producing a bimetallic catalyst according to claim 1, said method comprising: depositing said second metal layer onto said substrate; and subsequently depositing said first metal layer onto said second metal layer.

20. The method according to claim 19, wherein depositing said first metal layer onto said second metal layer comprises depositing a first metal layer having a thickness between 0.1 and 3 monolayers.

21. The method according to claim 19, wherein the weight ratio between said second metal layer and said first metal layer is in the area between 1:0.05 and 1:0.5.

22. A method for tuning the catalytic activity of a transition metal layer comprising depositing a desired thickness of said metal layer as an overlayer onto a transition metal layer, thereby changing the position of the d-band centre of said metal overlayer.

23. The method according to claim 22, wherein said desired thickness is between 0.1 and 3 monolayers.

24. The method according to claim 22, wherein the weight ratio between said transition metal layer and said metal overlayer is in the area between 1:0.05 and 1:0.5.

25. A method for tuning the catalytic activity of a bimetallic catalyst according to claim 1, comprising depositing said first metal layer onto said second metal layer, thereby changing the position of the d-band centre of said metal of said first metal layer.

26. The method according to claim 25, wherein said depositing said first metal layer onto said second metal layer comprises depositing said first metal layer having a thickness between 0.1 and 3 monolayers.

27. The method according to claim 25, wherein the weight ratio between said second metal layer and said first metal layer is in the area between 1:0.05 and 1:0.5.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0057] The catalyst and the methods according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0058] FIG. 1 shows the activity of overlayer structure of Cu/Ru for varying thickness of copper on a 50 layer of ruthenium according to some embodiments of the invention.

[0059] FIG. 2 shows the activity of a co-evaporated thin film produced by co-evaporation of two metals.

[0060] FIG. 3 shows to two metal weight % ratio vs. the normalized catalytic conversion, according to some embodiments of the invention.

[0061] FIG. 4 is a flow-chart of a method according some embodiments to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

[0062] A number of Cu/Ru catalytic systems have been produced and tested.

[0063] In some examples, the material, i.e. Cu/Ru, has been synthesized in surface science experimental setup with a controlled atmosphere. The setup conducting the experiments in Ultra High Vacuum (UHV) has a base pressure 110.sup.10 Torr. It combines an E-beam evaporator and surface scientific methods such as X-Ray Photoelectron Spectroscopy (XPS) and Ion Scattering Spectroscopy (ISS) for characterization with a High Pressure Cell (HPC) allowing catalytic testing up to 1 bar of reactant gas.

[0064] A number of Cu/Ru systems are evaporated on the TiO.sub.2 substrate for varying overlayer thickness of copper on a 50 Ru thin film. After deposition the catalytic Cu/Ru spots are characterized by XPS and transferred to the HPC where the spots are exposed to reactant gas and the temperature is ramped up and down in steps. In order to test the catalytic activity, of the TiO.sub.2 samples are moved into a HPC where the composition and flow of the reactant gases can be controlled by flow controllers. The outlet of the HPC is connected to a roughing line through a pressure controller which regulates the pressure to 1 bar. For the high pressure ammonia oxidation reaction N6 gases of 5.000 ppm NH.sub.3 diluted in argon and 5.000 ppm O.sub.2 diluted in argon are used. The gas composition the HPC is 1:1. The ammonia oxidation in the HPC is tracked with a Balzers quadrupole mass spectrometer. The pressure is reduced a factor 10.sup.6 Torr with a glass capillary inserted in a nozzle device to avoid spillover effects from neighbouring metal spots. During the experiment, a number of masses connected to the ammonia oxidation are measured to make it possible to distinguish ammonia from water and to investigate the formation of NOx.

[0065] In the UHV setup the catalytic spots are evaporated on a 1010 mm.sup.2TiO.sub.2 (110) rutile single crystal suspended on tungsten-filaments. The temperature is measured with a thermocouple glued on the side of the TiO.sub.2 substrate. The evaporator uses an E-beam to do Physical Vapour Deposition (PVD) on one or more metals at the same time. A circular mask ensures well defined circular spots of 1 mm diameter size. The rate, ratio and thickness of the deposited catalytic spots are determined using a Quartz Crystal Microbalance (QCM). The films produced have a thickness of 50 .

[0066] After testing, the catalytic spots are further characterized by XPS and ISS to investigate changes during the testing phase. The activity of the overlayer structure is presented in FIG. 1. The signal measured in the Quadrupole Mass Spectrometer (QMS) is normalized with the argon signal, as the leak through the glass capillary is temperature dependent.

[0067] The activity is measured as the increase in the 28 AMU signal, excluding the CO contribution. Equally the signal for 32 AMU decreases as a sign of the oxygen being consumed in the ammonia oxidation process. The ammonia level is measured on 17, 16 and 15 AMU and it drops as the temperature increases. The model system is 100% selective towards clean combustion as NOx is not observed for any of the catalytic Cu/Ru spots on TiO.sub.2.

[0068] The Cu/Ru system exhibits higher activity towards ammonia combustion than both pure ruthenium and copper. A volcano curve is observed of the thickness of the copper overlayer on ruthenium with an optimal thickness of 0.8 monolayer (ML). Adding a copper overlayer in the order of a monolayer to the ruthenium thin film improves the catalytic activity and the Cu/Ru system compared to the Ru catalyst almost by 100% better. Pure ruthenium exhibits high activity but is improved in both the model system and for the high surface area catalyst by having a copper overlayer. The copper overlayer thickness in FIG. 1 is varied from 0-2 ML, i.e. from 0-5 with the system performing better for a copper overlayer in the range of 0.25-2 ML, i.e. 0.6-5 . The catalytic performance of Cu/Ru system is very sensitive to the thickness of the Cu overlayer. The deposition of the Cu overlayer onto Ru leads to an enhanced catalytic activity.

[0069] Thin films of co-evaporated metals are further investigated and their activity tested in FIG. 2. The co-evaporated films are not as active as the overlayered structures but still perform better than pure Cu but worse than a pure Ru thin film. Adding copper to the bulk does not improve the catalytic performance of the thin film as in the case of copper as overlayer.

[0070] The lower activity of the co-evaporated thin films can in the light of the ISS and XPS analysis be explained by not reaching the equilibrium structure during the testing phase. The clear trend in segregation leads to the overlayer structure being the most active and stable configuration of the Cu/Ru system for ammonia oxidation.

[0071] In order to test the activity in a real catalyst, the Cu/Ru system is further applied to a high surface area alumina support. The high surface area catalysts are prepared by incipient wetness impregnation. Hydrated RuCl.sub.3.xH.sub.2O is dissolved in millipore water which is poured onto appropriate amounts depending on batch size of alumina powder targeting a Ru loading of in 1 weight percentage on alumina. The catalyst is dried at room temperature at least overnight. The catalyst is then reduced at 500 C. in a 20 ml/min pure H.sub.2 flow for 2 hours. Immediately after the reduction the catalyst is impregnated with varying amounts of copper(II)nitrate corresponding to various fractions of Cu to Ru. The catalyst is dried at room temperature overnight and reduced in-situ immediately before testing.

[0072] The high surface area catalysts are tested in a plug flow reactor setup. The catalyst is suspended in a glass tube embedded in quartz wool. The reactor is placed in an oven and the temperature of the catalyst and the oven is measured by a k-type thermocouple individually. The inlet of the reactor is connected to a gas manifold where the reactants are mixing and the composition is set by flow controllers. For experiments the flow is set at 10.75 ml/min of 5000 ppm NH.sub.3 in argon, 4 ml/min of 1% O.sub.2 in argon and 10 ml/min argon to keep stoichiometric conditions. The outlet of the reactor is measured by a FTIR spectrometer and the pressure is regulated to 1 bar by a pressure controller.

[0073] FIG. 3 shows the normalized catalytic conversion achieved by adding Cu onto a Ru layer of 50 . At 0% Cu, 100% Ru, the catalytic activity corresponds to the one of pure Ru. It can be seen that by increasing the amount of Cu up to 30 weight %, i.e. 70 weight % of Ru, the catalytic conversion is enhanced. At 100% Cu, i.e. 0% Ru, the catalytic conversion of pure Cu is around 0.5%, thus lower than the one of the overlayered structure and lower that the one of the pure Ru layer.

[0074] For the high surface area catalysts, a systematic variation of catalyst activity can be observed with the variation of Cu loading of the catalyst. One could see a marked increase in the activity of compared to the activities of both Cu and Ru when Cu is added to Ru. In this case, as shown in FIG. 1 and FIG. 3, there is a maximum when the added Ru to Cu weight ratio is 1:0.3.

[0075] The high surface area catalyst is further synthesized by co-impregnation however the Cu/Ru particles exhibited low activity. For the Cu/Ru system to exhibit high activity towards ammonia oxidation, ruthenium and copper need to be impregnated successively.

[0076] FIG. 4 is a flow-chart of a method according some embodiments to the invention.

[0077] The method for producing a bimetallic catalyst comprises the steps: S1, depositing the second metal layer, e.g. Ru layer onto the substrate, e.g. alumina;

[0078] and subsequently, S2, depositing the first metal layer, e.g. Cu layer onto the second metal layer.

[0079] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms comprising or comprises do not exclude other possible elements or steps. Also, the mentioning of references such as a or an etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.