Patent Application
20250073694
Catalytic selective hydrogenation of an alkyne to an alkene is an industrially important reaction. For example, the hydrogenation of acetylene to ethylene is a key process for removing acetylene from an ethylene feed for polyethylene production, since the catalyst used for the polyethylene production is poisoned by acetylene in the ethylene feed.
Catalysts based on nanoparticles containing noble metals, such as Pt and Pd, are used in the hydrogenation of alkyne. For example, the use of intermetallic Pt/Ga nanoparticles is reported in EP 2 060 323 A1.
Catalytic oxidation of CO to CO2 is important for pollution control in the automotive industries. Noble metals, such as Pt, Pd, Rh and Au are known to be especially active in the catalytic CO oxidation reaction. Since noble metals are expensive, CO oxidation catalysts usually contain highly dispersed noble metal nanoparticles supported on metal oxides. EP 0 602 865 A1 relates, for instance, to CeO2 as a support material for the catalyst. Compared to such nanoparticle catalysts, single atom catalysts (SACs) are generally considered to be advantageous, as they maximize the noble metal efficiency. Since catalysis is a surface reaction process, the use of smaller metal particles can save cost and/or yield better catalytic selectivity/activity.
However, as described in US 2021/0016256 A1, smaller metal particles, clusters or single atoms are not thermodynamically stable and usually sinter to form larger particles during a catalytic reaction, especially at elevated temperatures and under a reducing environment. Typical inert refractory support materials (e.g., SiO2, Al2O3, etc.) have been used as high-surface-area supports, but typically do not strongly anchor small metal clusters or single metal atoms. Against this background, US 2021/0016256 A1 proposes a method to synthesize nanocomposite catalysts using the deposition of small clusters and nanoparticles of reducible oxides (CeOx, FeOx, NbOx, etc) onto inert refractory support materials selected from silica, alumina, magnesia, zirconia, cordierite, mullite, perovskite or any combination thereof. One or more atoms of Pt, Au, Pd, etc are deposited onto such “nanocomposites” via strong electrostatic adsorption (SEA) from aqueous solutions. The synthesis of the nano composite catalysts requires calcination steps in air at e.g. 400° C. or 600° C.
US 2021/0016256 A1 asserts that 0.05 wt % Pt1/CeOx-SiO2 or 0.05 wt % Pd1/CeOx-SiO2 Single-Atom Catalysts were synthesized. Increasing the loading of Pt to 2 wt. % led to the formation of Pt clusters and nanoparticles.
There is accordingly a demand of SACs with high metal loading which do not only show excellent activity and selectivity but are also stable under catalytically relevant conditions.
An object of the present invention is to provide a catalytically active material which can be effectively used for the oxidation of CO. It is a further object to provide a catalytically active material suitable for the oxidation of CO that shows a high activity and/or stability in this reaction.
According to one further aspect it is an object of the present invention to provide a catalytically active material which can be effectively used for the hydrogenation of an alkyne. It is a further object to provide a catalytically active material suitable for the hydrogenation of an alkyne that shows a high activity, high selectivity and/or stability in this reaction.
According to one further aspect, it is an object of the present invention to provide a simple and effective method for preparing such catalytically active material that can be suitably used.
The present inventors have found that the above problem is solved by the following embodiments.
Where the present description refers to preferred embodiments/features including all levels of preference, combinations of these preferred embodiments/features shall also be deemed as disclosed, as long as this combination of preferred embodiments/features is technically meaningful.
Herein, the use of the term “comprising” or “including” should be understood as disclosing, as a more restricted embodiment, the term “consisting of” as well, as long as this is technically meaningful.
If preferred upper and lower limits are indicted for certain features, this should be understood as disclosing any combination of the upper and lower limits.
In the following, depending on the context, the term “catalytically active material” may also be replaced by “catalyst”.
(Right side) Pt 4f X-ray photoemission spectra (XPS) obtained on the Pt/CeO2-plasma sample of Example 1, the Pt/CeO2-step sample of Comparative Example 3, the Pt/CeO2-red sample of Comparative Example 2, and the Pt/CeO2(111) sample of Comparative Example 1 (from top to bottom) before and after CO oxidation reaction (10 mbar CO, 50 mbar O2, balanced by He to 1 bar, 523 K). The Pt 4f XPS before the reaction were measured after the samples were annealed at 523 K in ultrahigh vacuum (UHV) at a pressure of 10−9 mbar.
Aberration corrected scanning transmission electron microscopy (STEM) images of the plasma-treated Pt/CeO2 powder catalyst according to Example 2 before (b) and after (c) the CO oxidation reaction at 523° C. Some Pt single atoms are highlighted by circles.
Plasma-treatment of the CeO2(111) surface in the presence of O2 produces peroxo species and induces surface restructuring, resulting in small ceria NPs, which act as anchoring sites either directly upon Pt adsorption or through surface migration of peroxo-stabilized Pt single atoms (bottom row).
The catalytically active material of the present invention comprises a support comprising a metal oxide, and atomically dispersed noble metal on the surface of the support. The support comprises a metal oxide, and the metal oxide is selected from TiO2, CeO2, ZnO, SnO2, Ga2O3, In2O3, ZrO2, and Fe2O3. The noble metal is selected from Pt, Pd, Rh and Au. The catalytically active material is obtained by a method comprising a step of non-thermal plasma treatment in the presence of O2. This method comprises e.g. a step of subjecting the support comprising a metal oxide and noble metal on its surface to a non-thermal plasma treatment in the presence of O2. In the alternative, the support comprising a metal oxide is subjected to a non-thermal plasma treatment prior to the deposition of noble metal on its surface.
As “atomically dispersed” we understand the presence of single noble metal atoms on the surface of the support which can be detected by usual analytical techniques such as extended X-ray absorption fine structure (EXAFS), infrared spectroscopy (IR), or STEM.
Analysis with STEM is further described in the Experimental Section. For the avoidance of doubt and if not stated otherwise, the term “atom” is used in the description and claims to cover both neutral (oxidation state=0) and charged (ionic, e.g. oxidized) noble metal atoms.
The form of the support is not particularly limited. The support may be, for instance, in the form of particles (e.g. powder) or a film.
The present invention is characterized in that the catalytically active material is obtained by a method comprising a step of non-thermal plasma treatment in the presence of O2.
In one embodiment, the catalytically active material of the present invention shows a Raman band centered within the range of 815-845 cm−1. The Raman spectroscopy can be carried out according to the method as described in the examples. Without being bound to theory, the Raman band centered within the rage of 815-845 cm−1 is considered to be attributed to surface peroxo (O22−) species preferably formed on the surface of the metal oxide during the non-thermal plasma treatment.
In one embodiment, the catalytically active material of the present invention shows an additional O 1s XPS signal at a binding energy that is 1.2 eV to 2.0 eV, preferably 1.5 eV to 1.7 eV higher than that of the main signal of oxygen in the metal oxide in the support. The XPS analysis can be carried out according to the method as described in the examples. The additional XPS signal may be observed as a shoulder of the main signal. Without being bound to theory, the additional XPS signal is considered to be attributed to surface peroxo (O22−) species, preferably formed on the surface of the metal oxide during the non-thermal plasma treatment. For example, in the case of the CeO2 support, the O 1s XPS signal of pristine (not plasma-treated) CeO2 appears at 529.2 eV, while that of the surface peroxo species appears at 530.8 eV.
In one embodiment, the non-thermal plasma treatment in the presence of O2 leads to the formation of metal oxide nanoclusters at the support surface. The average diameter of the metal oxide nanoclusters may be 2.0 nm or less, preferably 1.5 nm or less, as determined by scanning tunneling microscopy (STM); see further description below. The STM measurements can be carried out as described in the examples.
In one embodiment, the catalytically active material of the present invention shows two or more characteristics as described in the above embodiments.
According to the present invention, the support comprises a metal oxide at the surface of the support, and the metal oxide is selected from TiO2, CeO2, ZnO, SnO2, Ga2O3, In2O3, ZrO2 and Fe2O3. These metal oxides are frequently called reducible metal oxides. The reducible metal oxide is capable of losing an oxygen atom from its crystal structure at a relatively low energy, which results in the formation of an oxygen vacancy and the reduction in the oxidation number of the metal atom in said reducible metal oxide, e.g. from Ce4 to Ce31. The metal oxide is preferably selected from TiO2, CeO2, SnO2, Ga2O3, In2O3 and Fe2O3, more preferably from TiO2, CeO2, and In2O3, most preferably CeO2.
Since the support comprises the reducible metal oxide, the catalytically active material of the present invention can show excellent catalytic activity and stability and the CO oxidation reaction can be carried out at a relatively low reaction temperature. Without being bound to theory, it is believed that the metal oxide nanoclusters are formed on the support surface upon the plasma treatment in the presence of O2, and the atomically dispersed noble metals strongly interact with the metal oxide. It is believed that this interaction allows charge transfer and the presence of an oxygen atom bridging the noble metal and the metal oxide, leading to excellent stability and excellent catalytic activity, even at a low reaction temperature.
Another benefit of using specific reducible metal oxides (instead of e.g. SiO2) for the support is that reducible oxides, such as CeO2 per se, may also contribute to the catalytic activity in the reaction to be catalyzed, especially if the reducible metal oxide has been plasma-treated.
In one preferred embodiment, CeO2 clusters are created during the plasma treatment of CeO2 support and hence, metal atoms (e.g. Pt atoms) interact with the also active CeO2 support (when compared to e.g. SiO2). Furthermore, the data presented in the example section show that, even in the absence of a noble metal such as Pt, the CeO2 support may become activated by the plasma treatment. This finding can be assigned to the formation of small CeO2 clusters with a higher surface density and possibly also the formation of peroxide surface species, e.g. on the CeO2 clusters formed. These findings are transferable to other reducible metal oxides and noble metals as used in the present invention.
The catalytically active material according to the present invention comprises the atomically dispersed noble metals on the surface of the support, wherein the noble metal is selected from Pt, Pd, Rh and Au. Preferably, the noble metal is selected from Pt and Pd. Further preferably, the noble metal is Pt.
The combination of the metal oxide and the noble metal is not particularly limited. According to one preferred embodiment, the metal oxide is selected from TiO2 and CeO2, and the noble metal is selected from Pt and Pd, the combination of CeO2 and Pt being more preferred.
The number of the noble metal atoms normalized to the surface area of the catalytically active material (surface density, Ns) is not particularly limited, and preferably at least 3.0×1012 at/cm2, more preferably at least 6.0×1012 at/cm2, further preferably at least 1.2×1013 at/cm2. The upper limit for the Ns is 8.0×1014 at/cm2, and may be for example 4.0×1014 at/cm2, 2.0×1014 at/cm2, 1.0×1014 at/cm2, 8.0×1013 at/cm2, 6.0×1013 at/cm2, or 4.0×1013 at/cm2. Any combinations of the above upper and lower limits are embodiments of the present invention. Therefore, the Ns may be, for example, 3.0×1012 at/cm2 to 8.0×1014 at/cm2, 3.0×1012 at/cm2 to 4.0×1014 at/cm2, 3.0×1012 at/cm2 to 2.0×1014 at/cm2, 6.0×1012 at/cm2 to 8.0×1014 at/cm2, 6.0×1012 at/cm2 to 4.0×1014 at/cm2, and so forth. The Ns is preferably within the range of from 3.0×1012 at/cm2 to 8.0×1014 at/cm2, more preferably from 3.0×1012 at/cm2 to 4.0×1014 at/cm2, further preferably from 6.0×1012 at/cm2 to 2.0×1014 at/cm2. The method for determining the Ns is given in the examples section.
It is preferable that at least a part of the noble metal has a specific oxidation state X. The specific oxidation state X of the noble metal may result from strong interaction with the metal oxide in the support. When the noble metal is Pt or Pd, the specific oxidation state X is 2+. When the noble metal is Rh or Au, the specific oxidation state X is 3+. A relative intensity of the noble metal having the specific oxidation state with respect to the total intensity of the noble metal of all oxidation states as measured by X-ray photoelectron spectroscopy (XPS) is preferably 50% or more, more preferably 75% or more, further preferably 80% or more, particularly preferably 90% or more. For example, if the noble metal is Pt, the relative intensity of Pt2+ with respect to the total intensity of Pt0, Pt2+ and Pt4+ (i.e. all oxidation states observed) is to be determined from the Pt 4f XPS spectrum. The detailed determination method is given in the examples section.
It is preferable that an infrared reflection-absorption spectrum (IRAS) measured on the catalytically active material shows an absorption band within the range of 2085 cm−1 to 2120 cm−1 that does not change upon heating the material from 300 K to 500 K, when measured under the following condition:
For any catalytically active material of the present invention, a corresponding absorption band within the range of 2085 cm−1 to 2120 cm−1 exists and can be easily determined by the person skilled in the art. In the case of Pt/CeO2, the corresponding band (i.e. its absorption maximum) lies at 2110 cm−1 ±1 cm−1 as seen from
According to the first embodiment of the present invention, the support is present in the form of particles and may e.g. form a powder. The shape of these particles is not particularly limited, and may be selected from ellipsoidal, cubic and spherical shapes.
According to the first embodiment of the present invention, the average particle diameter of the support is not limited, and preferably 10 nm to 1 μm, more preferably 10 nm to 500 nm, further preferably 10 nm to 100 nm, particularly preferably 10 nm to 50 nm, most preferably 10 nm to 30 nm. The average particle diameter of the support can be determined by STEM by measuring diameters of 20 arbitrarily selected particles and calculating the average thereof, as explained in the experimental section. As “diameter” the longest visible axis of the particle is taken. If the accuracy of the measurement is to be further increased, the measurement can be conducted with 100 arbitrarily selected particles. If several particles form together agglomerates, the diameter of the primary particles is taken for the measurement.
According to the first embodiment of the present invention, the support preferably consists of the metal oxide selected from TiO2, CeO2, ZnO, SnO2, Ga2O3, In2O3, ZrO2, and Fe2O3.
According to one preferred aspect of the first embodiment of the present invention, the catalytically active material preferably consists of the metal oxide support and the noble metal which is at least in part atomically dispersed. According to one further preferred aspect of the first embodiment of the present invention, the catalytically active material preferably consists of the metal oxide support and the noble metal of which 50% or more, 75% or more, 80% or more, or 90% or more is atomically dispersed.
XPS can be used to measure the percentage of the atomically dispersed noble metals. In particular, the relative intensity of the noble metal having the specific oxidation state X with respect to the total intensity of the noble metal of all oxidation states as described above can be taken as feature indicating the percentage of the atomically dispersed noble metals.
For example, when the noble metal is Pt, the percentage of the atomically dispersed Pt can be determined from the relative intensity of Pt2+ with respect to the total intensity of Pt of all oxidation states. Pt0 and Pt4+ may be attributed to clusters of metallic Pt and PtO2, respectively.
STEM can also be used to measure the percentage of the atomically dispersed noble metals. In particular, 100 STEM pictures (magnification to e.g. 10 nm×10 nm) clearly showing noble metal species (atomically dispersed noble metals and clusters containing two or more noble metals) are arbitrarily selected, and the percentage of the atomically dispersed noble metals is determined for each STEM picture and then averaged. In STEM pictures, the noble metal species may appear brighter than the support.
The percentage of the atomically dispersed noble metals is preferably determined by XPS.
According to the first embodiment of the present invention, the surface density Ns of the noble metal is preferably 3.0×1012 at/cm2 to 4.0×1014 at/cm2, more preferably from 3.0×1012 at/cm2 to 2.0×1014 at/cm2, further preferably 3.0×1012 at/cm2 to 1.0×1014 at/cm2, even further preferably 6.0×1012 at/cm2 to 1.0×1014 at/cm2.
According to the second embodiment of the present invention, the support is in the form of a film. The film support may be provided on a base material. In one embodiment the base material provides a planar area on which the film support can be grown, e.g. by vapor deposition of the metal component of the reducible oxide on an oxidized surface of the base material. The base material is not particularly limited, as long as it does not adversely affect the catalytically active material. For example, a base material made of a metal, an alloy or a ceramic may be used. Examples of the metal used as the base material include, but are not limited to, Ru, Cu, Al, and the like. The metal used as the base material may be a single crystal, and examples thereof include a Ru(0001) single crystal and a Cu(111) single crystal. Examples of the alloy used as the base material include, but are not limited to, steel, brass, and the like. Examples of the ceramic used as the base material include, but are not limited to, alumina, silica, silicon carbide, and the like. The skilled person is able to select the base material suitable for the reaction conditions to which the catalytically active material is exposed.
According to the second embodiment of the present invention, the thickness of the film support is not particularly limited, and preferably 5 nm to 50 nm, more preferably 10 nm to 20 nm. The thickness of the metal oxide film can be determined by, for example, XPS by measuring the attenuation of an XPS signal of the base material, on which the metal oxide film is provided. For instance, if the metal oxide film is provided on a Ru base material, the attenuation of Ru 3d signal in the XPS spectra can be measured to determine the thickness of the metal oxide film support. Likewise, if a Cu base material is used, the attenuation of Cu 2p signal in the XPS spectra can be measured. Likewise, if a steel base material is used, the attenuation of Fe 2p signal in the XPS spectra can be measured. The skilled person is able to appropriately select the signal resulting from the element in the base material and measure the attenuation thereof to determine the thickness of the metal oxide film. The details of the measurement method are given in the examples section.
According to the second embodiment of the present invention, the surface density Ns of the noble metal is preferably 3.0×1012 at/cm2 to 4.0×1014 at/cm2, more preferably from 6.0×1012 at/cm2 to 4.0×1014 at/cm2, further preferably 1.2×1013 at/cm2 to 2.0×1014 at/cm2.
According to the second embodiment of the present invention, the support may consist of a substrate and clusters supported on the substrate, the substrate and the clusters comprising the same metal oxide. Preferably, the substrate and the clusters consist of the same metal oxide. The average diameter of the clusters may be 2.0 nm or less, preferably 1.5 nm or less as determined by STM by plotting a topography profile along a straight line corresponding to the length of 10 nm; measuring the distance between two adjacent minima; multiplying the measured value by a correction factor of 0.5 and recording the obtained value as a diameter; repeating the measurement on 20 arbitrarily selected surface areas showing a coverage by metal oxide clusters; and calculating the average value. The detailed method is given in the examples section.
The number of the metal oxide clusters on the metal oxide substrate and normalized to the surface area of the metal oxide substrate (surface cluster density) is not particularly limited.
The surface cluster density is preferably at least 0.05 clusters/nm2, more preferably at least 0.08 clusters/nm2, further preferably at least 0.10 clusters/nm2, and most preferably at least 0.12 clusters/nm2. The surface cluster density can be determined by STM by counting the number of the metal oxide clusters present within an arbitrarily selected area. The detailed method is given in the examples section.
According to one preferred aspect of the second embodiment of the present invention, the catalytically active material preferably comprises the noble metal of which e.g. 50% or more, 75% or more, 80% or more, or 90% or more is atomically dispersed. The percentage of the atomically dispersed noble metals can be determined as described above for the first embodiment.
The catalytically active material of the present invention is particularly suitable for the oxidation of CO and selective hydrogenation of alkynes (e.g. acetylene hydrogenation). The catalytically active material of the present invention may also be suitable for other catalytic reactions, including for instance water-gas-shift reaction, steam reforming and oxidation of natural gas.
The catalytically active material of the present invention shows an excellent stability and activity which is also reflected by suitably low onset reaction temperatures in the target reactions such as CO oxidation and selective alkyne hydrogenation. At least in part, these properties seem to be related to the atomic dispersion of noble metals on the surface of metal oxide clusters. The available experimental evidence further indicates that the uniformity of this atomic dispersion and/or the noble metal loading (in at/cm2) can be enhanced by the inventive methods for producing the catalytically active material as follows.
A method for producing the catalytically active material according to the first embodiment of the present invention may comprise the following steps:
At the end of step (1), the noble metal may be present on the surface of the support in elementary form (oxidation state=0) or as noble metal compound, e.g. as noble metal salt.
The method for providing the precursor for the catalytically active material is not particularly limited, and the step (1) is preferably selected from the following steps (1a) and (1b):
The method for depositing the noble metal onto the support according to the step (1a) is not particularly limited, and examples thereof include physical vapor deposition, chemical vapor deposition and deposition from an aqueous solution of a salt of the noble metal. It is preferred that the noble metal is deposited from the aqueous solution of a salt of the noble metal onto the support, and known methods such as strong electrostatic adsorption (SEA) and impregnation may be appropriately used. The deposition is preferably conducted such that the noble metal deposition is controllable in terms of the surface density of metal atoms on the support. In step (1a), the support preferably consists of the metal oxide.
The salt of the noble metal is not particularly limited. Examples of the salt of the noble metal include, but are not limited to, tetraammineplatinum(II) nitrate, tetraammineplatinum(II) chloride, platinum(II) acetylacetonate; tetraamminepaladium(II) nitrate, tetraamminepaladium(II) chloride; rhodium(III) acetylacetonate; hydrogen tetrachloroaurate(III), gold(III)-chloride, to name few.
As used herein, the expressions “calcining” or “calcination” refers to a thermal treatment of a solid at an elevated temperature in the presence of O2 under dry conditions. As used herein, the expression “dry” indicates that no additional water is introduced to the atmosphere, in which the calcination is carried out. For example, the calcination in the atmospheric air is to be understood as being carried out under the dry conditions.
The calcination of the precursor for the catalytically active material is preferably carried out at a temperature of 573 K to 973 K, more preferably 623 K to 873 K, further preferably 673 K to 823 K. The calcination is preferably carried out at a concentration of O2 of 5 vol. % or more, more preferably 10 vol. % or more. For example, air can be used as the source of O2.
The non-thermal plasma treatment of the calcined precursor for the catalytically active material is carried out in the presence of O2. Prior to ignition of the plasma, the pressure is preferably reduced to 40 mbar or less, more preferably 20 mbar or less. The pressure during the non-thermal plasma treatment is preferably 40 mbar or less, more preferably 20 mbar or less. The non-thermal plasma treatment is preferably carried out at a concentration of O2 in an inert gas of 5 vol. % or more, more preferably 80% or more, further preferably 100% (pure oxygen). The inert gas may be selected from N2, He, Ne, Ar, and the like.
The non-thermal plasma treatment may be carried out under a static atmosphere or under a gas flow, the latter being preferred. When the non-thermal plasma treatment is carried out under a gas flow, it is preferred that the calcined precursor for the catalytically active material is blown up (levitated) by the gas flow during the non-thermal plasma treatment.
The source of the non-thermal plasma is not limited, and examples include inductively coupled plasma, capacitively coupled plasma, DC glow discharge, microwave plasma, radio-frequency plasma, cold plasma jet, and dielectric barrier discharge plasma. The source of the non-thermal plasma is preferably microwave plasma or radio-frequency plasma.
When the source of the non-thermal plasma is a microwave plasma, an anode voltage of the plasma source is preferably 0.2 kV to 2 kV, and an emission current is preferably 0.1 μA to 1 μA.
When the source of the non-thermal plasma is the radio-frequency plasma, the power of the radio-frequency plasma generator is preferably 20 W to 300 W.
A duration of the non-thermal plasma treatment can be appropriately adjusted as desired, and is usually 1 min to 300 min, preferably 5 min to 180 min, more preferably 10 min to 120 min, further preferably 30 min to 90 min.
A method for producing the catalytically active material according to the second embodiment of the present invention may be selected from the following methods A and B.
Method A comprises the following steps:
Method B comprises the following steps:
The method for providing the metal oxide film support is not particularly limited, and any method available in the art may be used. The method for providing the metal oxide film may include molecular beam epitaxy, laser ablation, chemical vapor deposition, sol-gel method, and the like. In one embodiment, the metal oxide film is provided on a base material as explained before in connection with the second embodiment of the invention.
In step (2) of method A and in step (3) of method B, the non-thermal plasma treatment is carried out in the presence of O2. The non-thermal plasma treatment is preferably carried out at a pressure of 1×10−6 mbar to 1×10−2 mbar, more preferably 1×10−6 mbar to 1×10−4 mbar, further preferably 1×10−6 mbar to 1×10−5 mbar. The partial pressure of O2 during the non-thermal plasma treatment is preferably 1×10−6 mbar to 1×10−2 mbar, more preferably 1×10−6 mbar to 1×10−4 mbar, further preferably 1×10−6 mbar to 1×10−5 mbar. In one embodiment, the non-thermal plasma treatment is preferably carried out in 100% O2 at the above-described pressure.
The non-thermal plasma treatment may be carried out under a static atmosphere or under a gas flow.
A source of the non-thermal plasma is not particularly limited, and examples include inductively coupled plasma, capacitively coupled plasma, DC glow discharge, microwave plasma, radio-frequency plasma, cold plasma jet, and dielectric barrier discharge plasma.
The source of the non-thermal plasma is preferably microwave plasma or radiofrequency plasma.
When the source of the non-thermal plasma is the microwave plasma, an anode voltage of the plasma source is preferably 0.2 kV to 2.0 kV, more preferably 0.5 kV to 1.5 kV, and an emission current is preferably 0.1 μA to 1.0 μA.
When the source of the non-thermal plasma is the radio-frequency plasma, the power of the radio-frequency plasma generator is preferably 20 W to 300 W.
The duration of the non-thermal plasma treatment can be appropriately adjusted as desired, and is usually 1 min to 300 min, preferably 5 min to 180 min, more preferably 10 min to 120 min, further preferably 30 min to 90 min.
According to step (3) of the method A, the method for depositing the noble metal onto the plasma-treated metal oxide film support in gas phase is not particularly limited, and examples thereof include physical vapor deposition and chemical vapor deposition, physical vapor deposition being preferred. According to step (2) of the method B, the method for depositing the noble metal onto the plasma-treated metal oxide film from a solution containing the noble metal is not particularly limited, and examples thereof include deposition from an aqueous solution of a noble metal salt as a precursor. When the noble metal is deposited from an aqueous solution, the methods and the materials described in the method for producing the catalytically active material according to the first embodiment of the present invention can be applied.
According to the method as described herein, it can be assumed that all the deposited noble metal is present at the surface of the metal oxide support and the metal oxide clusters.
Hence, a high catalytic activity of the catalytically active material can be achieved with a small amount of the noble metal.
The catalytically active material of the present invention can be suitably used in catalytic oxidation of CO to CO2 (also referred to as “catalytic oxidation”).
In the catalytic oxidation of CO to CO2, a reaction gas mixture comprising CO and O2 is reacted in the presence of the catalytically active material to produce CO2. The pressure of the reaction gas mixture is not particularly limited and may be 0.8 bar to 2.0 bar, preferably 0.8 to 1.5 bar, more preferably 0.9 to 1.2 bar.
The reaction temperature for the catalytic oxidation is not particularly limited and may be 323 K to 573 K, preferably 353 K to 523 K, more preferably 353 K to 473 K.
The composition of the reaction gas mixture is not particularly limited, as long as it comprises CO and O2. The content of CO in the reaction gas mixture may be, for instance, 5 vol. % or less, preferably 3 vol. % or less, more preferably 2 vol. % or less. The content of O2 in the reaction gas mixture may be 2 vol. % or more and 25 vol. % or less, preferably 5 vol. % or more and 20 vol. % or less. The molar ratio CO/O2 of the reaction gas mixture is preferably 2.0 or less, more preferably 1.0 or less, further preferably 0.5 or less, most preferably 0.2 or less.
The catalytically active material of the present invention can be suitably used in catalytic hydrogenation of alkyne into alkene (also referred to as “catalytic hydrogenation”). The alkyne used in the catalytic hydrogenation is preferably a C2-C5 alkyne, more preferably a C2-C3 alkyne.
In the catalytic hydrogenation of alkyne into alkene, a reaction gas mixture comprising an alkyne and H2 is reacted in the presence of the catalytically active material in gas phase to produce an alkene. For instance, when acetylene (ethyne) is used as the alkyne, ethylene (ethene) is produced as the product (alkene).
The pressure of the reaction gas mixture is not particularly limited, and may be 0.8 bar to 2.0 bar, preferably 0.8 to 1.5 bar, more preferably 0.9 to 1.2 bar.
The reaction temperature for the catalytic hydrogenation is not particularly limited and may be 323 K to 573 K, preferably 323 K to 523 K, more preferably 353 K to 473 K.
A composition of the reaction gas mixture is not particularly limited, as long as it comprises the alkyne and H2. The content of the alkyne in the reaction gas mixture may be, for example, 10 vol. % or less, preferably 5 vol. % or less, more preferably 3 vol. % or less. The content of H2 in the rection gas mixture may be, for example, 3 vol. % or more, 5 vol. % or more, and 10 vol. % or more. The molar ratio H2/alkyne is preferably 1.0 or more, more preferably 2.0 or more.
Herein below, the present invention will be described in more detail with reference to the examples. However, the present invention is not limited to the following Examples.
Scanning tunneling microscopy (STM) analysis—average diameter and number of metal oxide clusters; and low-energy electron diffraction analysis (LEED)—surface ordering in film supports
The STM measurements were performed at a sample voltage of 3.8 V and a constant tunneling current of 30 pA.
To determine the average diameter of metal oxide clusters, such as CeO2 clusters, a topography profile was plotted along a straight line corresponding to the length of 10 nm. In the topography profile, a distance between two adjacent minima was measured, and the measured value was multiplied by a correction factor of 0.5 to correct for tip/cluster convolution effect. The obtained corrected value was recorded as a diameter. The diameters were measured on 20 arbitrarily selected metal oxide clusters, and the average value was calculated.
To determine the surface density of metal oxide clusters, i.e. the number of metal oxide clusters normalized to the surface area of the metal oxide substrate, in the recorded STM images, the number of metal oxide clusters within an arbitrarily selected 100 nm×100 nm area was counted. This measurement was repeated in four different sample spots, i.e. the measurements were carried out in five different sample spots in total. The mean value was taken as “density of metal oxide clusters”.
The low-energy electron diffraction (LEED) analysis was carried on the same instrument, and the LEED patterns were recorded at 81 eV.
Scanning transmission electron microscopy (STEM) analysis—average diameter of metal oxide particles in powder
STEM images were recorded on the 200 kV JEOL JEM ARM200F probe/image-corrected TEM (JEOL Ltd.).
The average diameter of the metal oxide particles, e.g. CeO2 particles, of the metal oxide powder support, was determined by measuring diameters of arbitrarily selected 20 particles and calculating the average thereof. In the event that the STEM image showed a non-spherical shape for the selected particle, the longest axis was taken as diameter.
X-ray photoelectron spectroscopy (XPS)—elemental composition in the catalytically active material (film) and chemical (oxidation) state of the elements at surface
The relative intensity of the noble metal having the specific oxidation state (X) with respect to the total intensity of the noble metal of all oxidation sates was measured by XPS. In a case where the noble metal is Pt, the specific oxidation state is 2+. Also, in case the catalytically active material is a film, the number of the noble metals per surface area of the catalytically active material Ns (at/cm2) was measured by XPS. XPS spectra were measured with a Phoibos 150 analyzer (SPECS GmbH) using an Al Kα X-ray source (hv=1486.6 eV). Spectral analysis (background subtraction and deconvolution) was performed with the CasaXPS software (available from Casa Software Ltd; version 2.3.18), and the measured spectrum was subjected to the background subtraction using a Shirley background. In the case of Pt/CeO2 film samples Pt 4f, Ce 3d and O 1s core level spectra were recorded at pass energies of 25 eV, 50 eV and 50 eV, respectively. The skilled person is able to select core levels of target elements and attribute binding energies to oxidation states by using the common general knowledge. For instance, tabulated list of binding energies such as NIST X-ray Photoelectron Spectroscopy Database belong to the skilled person's common general knowledge.
The Pt spectrum was subjected to a spectral deconvolution analysis using a CasaXPS software. For each Pt spectrum, the background subtraction was carried out in the range of 68 eV to 80 eV. The spectra were deconvoluted using a Gaussian/Lorentzian line shape. The maximum value of full-width half maximum (FWHM) of each peak was set at 2.0 eV. The relative intensity of Pt2+ was calculated as the area percentage of the peak at 72.6 eV relative to the total peak area of the Pt 4f7/2 signal.
The thickness of the metal oxide film prepared on the base material, e.g. CeO2 film deposited on the Ru(0001) single crystal, was determined by XPS from the attenuation of the strongest signal stemming from the base material, e.g. Ru 3d signal, compared to that of the base material prior to the deposition of CeO2 film.
The Raman spectroscopy was carried out on inVia™ Raman Microscope (Renishaw) using a 532 nm excitation laser. For each sample, measurements were carried out on five different sample spots. At each sample spot, three scans were acquired, and average thereof was recorded. The acquisition time of each scan was 20 seconds. The spectra were normalized to the principal peak of the support (for instance, F2g peak at 465 cm−1 for CeO2). The normalized spectra recorded at the five different sample spots were averaged.
The specific surface area of the catalytically active material in particle form (powder) was determined by the BET method from the N2 adsorption isotherm. The N2 physisorption measurements were carried out on ASAP 2020 PLUS (Micromeritics Instrument Corporation).
Number (Ns) of Noble Metal Atoms Normalized to the Surface Area of the Catalytically Active Material (Particles) in at/Cm2
For the catalytically active material in particle form, the surface density of single atoms (i.e., the number (Ns) of the noble metal atoms normalized to the surface area of the catalytically active material) was calculated from the specific surface area (m2/g converted into cm2/g) and the amount (g converted into at) of the deposited noble metals. If the noble metal is deposited by an impregnation technique, the total amount (in g) of noble metal atoms present in the aqueous solution can be used for the calculation if the deposition was quantitative. Otherwise, the amount follows from the yield achieved.
The binding energies of the O 1s level at the surface were calculated by using density functional theory (DFT) within the final-state approximation and reference to the O 1s signal oxygen in a ceria bulk (2×2×2) supercell. The details for the DFT are given in a later section.
The reactivity measurements were performed in a high-pressure cell (SPECS HPC 20) connected to the main UHV analysis chamber. The catalytically active material was heated by a halogen lamp outside of the chamber through a quartz window. The reaction mixture consisted of 10 mbar of CO, 50 mbar of O2 and balanced by He to 1 bar. The gas composition in the reactor was analyzed by gas leaking through a quartz micro-capillary into a quadrupole mass spectrometer (QMS, MKS Instruments). Starting from room temperature, the temperature of the catalytically active material was increased stepwise with a 50 K increment, and the CO2 production was monitored by QMS. In experiments aimed at testing the stability of the atomically dispersed noble metal in the catalytically active material in the CO oxidation reaction, the sample was heated to 523 K with a rate of 1 K/s, and kept under reaction conditions for 10 min before the sample was cooled down to 300 K and evacuated, and then transferred to the analysis chamber for post-characterization.
The IRAS measurements were performed on the catalytically active material in the film form with a Bruker 66 ivs FTIR spectrometer in an UHV chamber hosting a “high-pressure” cell (reaction volume 1 I) for exposing the samples to gas mixtures at near atmospheric pressures.
The IRAS spectra of CO adsorption were measured following the steps described below:
The IRAS measurements were performed also on the catalytically active material after exposure to a reaction mixture at 1 bar consisting of 1% CO and 5% O2 balanced by Ar.
The measurement was carried out as follows:
The reactivity of the catalytically active material (powder) in the CO oxidation reaction was measured in a tubular packed-bed reactor. The gas phase composition was analyzed with a QMS (Hiden 20). The catalytically active material was sieved to below 75 μm, and 1 part by weight of the catalytically active material was physically mixed with 4 parts by mass of silica gel. The obtained mixture (5 parts by mass) was then mechanically mixed with 1 part by mass of acid-purified SiO2 and loaded into the glass tube reactor. The CO oxidation reaction rate was measured at steady state in the mixture of 1% CO and 20% O2, (He balance) at 1 bar at temperatures of 373 K, 383 K, 403 K, 423 K, 443 K and 473 K, increased stepwise, at least for 1 h at each temperature.
The reactivity of the catalytically active material in the particle form (powder) in the alkyne hydrogenation reaction was measured in a tubular packed-bed reactor, using acetylene as the alkyne. The gas phase composition was analyzed with a QMS (Hiden 20). The catalytically active material was sieved to below 75 μm and loaded into the glass tube reactor without dilution. The alkyne hydrogenation rate was measured at steady state in a mixture of 1% C2H2 and 5% H2 (Ar balance) at 1 bar at temperatures of 373, 393, 413, 433, 453 and 473 K, increased stepwise, for 12 h at each temperature. The average ethene production rate observed during the hydrogenation reaction, for 12 h at each temperature, was recorded as the hydrogenation rate of acetylene.
The experiments were performed in an ultrahigh vacuum (UHV) chamber equipped with low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM), all from SPECS GmbH. The stoichiometric well-ordered CeO2(111) films were grown on a Ru(0001) single crystal as follows. The Ru(0001) single crystal (9 mm in diameter, 1.5 mm in thickness, from MaTeck GmbH) was mounted onto a stainless-steel sample holder having a hole of 9 mm in diameter for heating the sample from the backside using an electron beam from a W filament. A type K thermocouple was spot-welded to the edge of the crystal. The surface of the Ru(0001) was oxidized in 10−6 mbar of O2 at 1000 K. Ce was vapor-deposited onto the oxidized Ru(0001) surface using an electron beam assisted evaporator (Focus EMT3) from a Mo crucible filled with Ce (99.9%, Sigma Aldrich) in 10−6 mbar of O2 at 90 K in amounts equivalent to form 4-5 monolayers (MLs) of CeO2(111). Subsequently, the temperature of the Ru(0001) was increased at a rate of 1 K/s, and kept at 673 K during the deposition of further CeO2 layers. The sample was then oxidized at 1000 K in 10−6 mbar of O2. The thickness of the prepared CeO2(111) film was about 5 nm. The prepared sample is denoted as “CeO2(111)”.
The reduced CeO2−x(111) surface (henceforth referred to as “CeO2-red”) was prepared by UHV annealing of the CeO2(111) film at 1200 K for 5 min.
The CeO2(111) surface enriched with monoatomic steps (denoted as “CeO2-step”) was prepared following the procedure described in detail in Nat. Commun. 7 (2016) 10801. In particular, it was obtained by depositing 0.3 monolayer (ML) of Ce onto the CeO2(111) film and subsequent oxidation at 673 K. This resulted in the formation of small ML-high islands. This homoepitaxy of CeO2 on CeO2 yields clearly arranged samples with high step density.
The oxygen plasma treatment of the CeO2(111) films was carried out with a microwave plasma generator with a commercial plasma source (OSPrey, from Oxford Scientific,
Pt was deposited onto the CeO2-plasma film using an electron beam-assisted evaporator (Omicron EMT3) from a Pt rod. In order to minimize metal aggregation during the deposition at room temperature, Pt was deliberately deposited at low metal flux, i.e., 0.03 ML/min, as determined by a quartz microbalance (McVac), where 1.0 ML of Pt corresponds to one Pt atom per CeO2(111) surface unit cell, i.e., 7.9×1014 Pt atoms/cm2. During the deposition, the sample was biased at the same potential as the Pt rod. The amount of Pt was controlled to be 0.20 ML by XPS through analysis of the Pt 4f signal intensity. The obtained sample is denoted as “Pt/CeO2-plasma”.
Subsequently an UHV annealing was applied to the obtained “Pt/CeO2-plasma” sample. The sample was exposed to UHV and heated to 523 K at a heating rate of 120 K/min and kept at 523 K for 5 min in UHV, and then cooled down at a cooling rate of 60 K/min. The obtained sample is denoted as “annealed-Pt/CeO2-plasma”.
The resulting annealed-Pt/CeO2-plasma film catalyst showed the following features. The average diameter of the CeO2 clusters was about 1.0 nm, as measured by STM. The number of Pt atoms at the surface of the sample (surface density, Ns) was 1.6×1014 at/cm2 as calculated from the amount of Pt deposited (0.2 ML). The relative intensity of Pt2+ with respect to the total intensity of Pt0, Pt2+ and Pt4+ in the catalytically active material was 100%, as measured by XPS. By STM measurements, the number of the CeO2 clusters normalized to the surface area of the CeO2 substrate was determined to be 0.15 clusters/nm2.
Pt was deposited onto the CeO2(111) film prepared in Reference Example 1 in the same manner as described in Example 1. The obtained sample is denoted as “Pt/CeO2(111)”. The UHV annealing was applied to the Pt/CeO2(111) in the same manner as in Example 1. The obtained sample is denoted as “annealed-Pt/CeO2(111)”.
Pt was deposited onto the CeO2-red film prepared in Reference Example 1 in the same manner as described in Example 1. The obtained sample is denoted as “Pt/CeO2-red”. The UHV annealing was applied to the Pt/CeO2(111) in the same manner as in Example 1. The obtained sample is denoted as “annealed-Pt/CeO2-red”.
Pt was deposited onto the CeO2-step film prepared in Reference Example 1 in the same manner as described in Example 1. The obtained sample is denoted as “Pt/CeO2-step”. The UHV annealing was applied to the Pt/CeO2(111) in the same manner as in Example 1. The obtained sample is denoted as “annealed-Pt/CeO2-step”.
First, the CeO2(111) support was prepared in the same manner as described in Reference Example 1. Then, the surface thereof was roughened by bombarding the CeO2(111) film with 1 keV Ar+ ions at 300 K for 5 min. The surface became considerably reduced as determined by XPS due to the preferential sputtering of lighter 0 atoms. To re-oxidize the ceria surface, the film was exposed to 10−6 mbar O2 at 500 K. The resulting support is denoted as “CeO2-sputter”.
Subsequently, deposition of 0.2 ML Pt onto this “CeO2-sputter” support was carried out in the same manner as described in Example 1. The resulting sample is denoted as “Pt/CeO2-sputter”. The UHV annealing was applied to the Pt/CeO2(111) in the same manner as in Example 1. The obtained sample is denoted as “annealed-Pt/CeO2-sputter”.
The results obtained for the film samples are discussed in the following.
First, the results of Comparative Example 1 are discussed. As deposited Pt species in the Pt/CeO2(111) were characterized by a binding energy (BE) of 71.8 eV (
Analysis of the Ce 3d and O 1s XPS spectra (
As seen from
Turning to Comparative Example 3, in the Pt/CeO2-step sample, the Pt atoms were partially in the 2+ and metallic states as deposited (BEs at 72.8 and 71.9 eV, respectively, see
As shown above, in Comparative Examples 1-3, Pt deposits always formed Pt NPs upon UHV 5 annealing. A certain amount of thermally stable Pt2+ species found in the annealed-Pt/CeO2-step sample can be explained by a strong adsorption of Pt single atoms at step edges. When exposed to the CO oxidation atmosphere, metal clusters became oxidized, thus giving rise to the 4+ and 2+ states, albeit their ratio depended on the initial particle size. The formation of Pt4+ and Pt2+ species on the Pt NPs in oxidizing atmosphere is well-documented in the literature, and is commonly attributed to PtO2/PtO clusters or a thin PtOx oxide film on a large Pt NPs.
Finally,
To shed light on the origin of the exceptional stability of Pt on the annealed-CeO2-plasma surface, the film morphology was investigated by using STM. STM images (
Importantly, the O 1s XPS spectra of the CeO2-plasma support and the Pt/CeO2-plasma sample (
To identify Pt species formed on the CeO2-plasma films, IRA-spectroscopy was employed using CO as a probe molecule. The obtained CO IRAS results of the (annealed-)Pt/CeO2-plasma sample of Example 1 and (annealed-)Pt/CeO2(111) sample of Comparative Example 1 are discussed. The bare ceria surfaces were found not to adsorb CO under the conditions studied.
For the Pt/CeO2(111) sample of Comparative Example 1, adsorption of CO (at 10−6 mbar) on Pt deposited onto a well-ordered CeO2(111) surface at 200 K (
In contrast, for the Pt/CeO2-plasma sample of Example 1, CO adsorbed on the Pt species formed on the CeO2-plasma surface showed a single band at 2104 cm−1 after Pt deposition (
These two systems (Example 1 and Comparative Example 1) were also investigated with IRAS after exposure to 1 bar of the reaction mixture consisting of 1% CO and 5% O2 (balanced by Ar) in the “high-pressure” cell. After 5 min of reaction at 500 K and sample cooling to room temperature, the cell was pumped out and the IRAS spectra were recorded in UHV at 300 K without additional exposure to CO. Interestingly, the spectra revealed similar bands at 2110 cm−1 in both systems (a weak and broad band at 2070 cm−1 observed on annealed-Pt/CeO2(111) can be assigned to traces of CO residing at the edges of metallic Pt NPs, see above). However, the spectra showed a different behavior on slow heating the sample to 500 K. On annealed-Pt/CeO2(111) of Comparative Example 1 (see
Based on structural characterization of the CeO2-plasma surface by LEED and STM, one may suggest that the enhanced stability of Pt originates from surface roughening that suppresses the diffusivity of the Pt ad-atoms and hence their aggregation. To examine this scenario, a rough CeO2 surface was produced by bombarding a well-ordered CeO2(111) film with 1 keV Ar+ ions at 300 K for 5 min. The obtained “CeO2-sputter” support was employed to produce the (annealed-)Pt/CeO2-sputter samples in Comparative Example 4.
As seen from the Pt 4f XPS spectra (
Finally, activity of the annealed-Pt/CeO2-plasma sample of Example 1 in the CO oxidation reaction is compared with that of the annealed-Pt/CeO2(111) sample of Comparative Example 1. Steady state CO2 production rates were measured on the catalysts of Example 1 and Comparative Example 1 in a high-pressure cell filled with 10 mbar of CO and 50 mbar of O2 balanced by He to 1 bar at different sample temperatures increased stepwise. The results are shown in
The CeO2 powder with high surface area (50 m2/g) were purchased from US Research Nanomaterials (Stock no. US3037) and used as received.
The powder Pt/CeO2 sample was synthesized via the strong electrostatic adsorption (SEA) method. The high purity (99.995%) tetraammineplatinum(II) nitrate (TAPN) (Sigma Aldrich No. 482293) was used as the Pt precursor, and ammonium hydroxide (28%˜30% solution in water) was purchased from ACROS Organics. A pristine Pt/CeO2 powder catalyst was obtained by the following steps.
First, a pristine Pt/CeO2 powder catalyst was prepared in the same manner as in Comparative Example 5. An O2-plasma treatment was then carried out on the pristine Pt/CeO2 powder catalyst in the following manner to obtain a plasma-treated Pt/CeO2 powder sample.
The O2-plasma treatment was carried out in a setup which consisted of a glass tube with frits, a funnel shape glassware, a mechanical pump, a radiofrequency plasma generator and a high voltage power supply. A schematic illustration of the setup for the O2-plasma treatment is provided in
The plasma was generated by a 20˜60 kHz high voltage power supply (PVM500). The peak voltage was measured by an oscilloscope with a high voltage probe and the power of the plasma was the product of the square root of the mean square (RMS) voltage and RMS current, which were measured by a multimeter. The power and frequency outputs of the powder supply were adjusted until a plasma with a desirable power was formed.
The conditions of the O2 plasma treatment were as follows:
A plasma-treated CeO2 powder support was prepared by carrying out the O2 plasma treatment in the same manner as in Example 2 to the CeO2 nanoparticles of Reference Example 5.
First, a pristine Pt/CeO2 powder catalyst was prepared in the same manner as described in Comparative Example 5, with the sole difference that the amount of TAPN dissolved in 5 mL of deionized water in step 2) was varied as show below in Table 2. An O2-plasma treatment was then carried out on the pristine Pt/CeO2 powder catalyst in the same manner as in Example 2 to prepare a plasma-treated Pt/CeO2 powder catalyst.
A pristine Pt/CeO2 powder catalyst was produced in the same manner as described for Comparative Example 5 with the sole difference that the amount of the TAPN solution added to the 25 mL NH4OH solution in step 2) was varied in Comparative Examples 4 and 5 as shown below in Table 2. In Example 4 and Comparative Example 7, 20 mg of TAPN was dissolved in 10 mL of deionized water to prepare the aqueous TAPN solution.
Due to the impregnation preparation method used, it can be assumed that all the deposited Pt atoms are located on the surface of the CeO2 support and the CeO2 clusters in the prepared catalytically active materials. The surface density Ns of Pt atoms in the catalytically active materials are shown in Table 3.
TiO2(110) single crystal was purchased from SurfaceNet. The surface of the TiO2(110) single crystal was cleaned by cycles of ion sputtering—oxygen annealing treatments, which are well known and documented in the literature (Diebold et. al., Surface Science, 1995, 331-333(PART B), 845-854), until no contamination was detected by XPS to obtain a clean TiO2(110) single crystal.
The oxygen plasma treatment of the clean TiO2(110) single crystal of Reference Example 7 was carried out in the same manner as in Reference Example 4. The obtained sample is denoted as “TiO2-plasma”.
Pt/TiO2-plasma sample was obtained in the same manner as in Example 1, except that Pt was deposited onto the TiO2-plasma film support.
Pt/TiO2(110) sample was obtained in the same manner as in Example 5, except that Pt was deposited onto the clean TiO2(110) film support. Accordingly, no plasma treatment was carried out.
The studies presented herein describe mechanistic considerations in respect to one embodiment of the invention. They are not to be understood as describing the actual situation or as limiting the scope of the present invention but may serve to illustrate the same.
The calculations were performed with the Vienna Ab initio Simulation Package (VASP, version 5.4.4), employing the generalized gradient functional by Perdew, Burke and Ernzerhof (PBE). Core electrons were treated within the projector-augmented wave (PAW) method, while valence electrons were expanded in plane-waves with a basis set cut-off of 500 eV. An additional Hubbard correction (DFT+U) was applied to the Ce(4f) band following Dudarev et al. (Phys. Rev. B, 1998, 57, 1505-1509) using an on-site, effective U-parameter of 4.5 eV.
Optimization of bulk ceria yielded a theoretical lattice parameter of 5.491 Å. The (111), (110) and (100) surfaces were modelled using (2×2) slabs. Stepped surfaces were prepared accordingly as (1×2) supercells. At least 10 Å of vacuum were added on top of the slabs. The bottom layers of the slabs (approx. half of the total number) were kept fixed at the optimized bulk positions, while the upper layers were allowed to relax. A dipole correction along the surface normal was applied throughout. Sampling of the Brillouin zone was performed with G-centred k-point grids with a reciprocal grid spacing of about 0.025 Å−1. Convergence of atomic positions was assumed when the absolute forces acting on each atom fell below 0.15 eV/A. Spin polarization was accounted for when necessary. Peroxide formation energies and adsorption energies of Pt and CO were obtained by stepwise structural relaxations, starting from the pre-optimized pristine surfaces. For the ceria NP/slab composite model, we employed an initial octahedral nanoparticle, following the well-known Wulff-construction and applied several cuts, ensuring CeO2 stoichiometry. The resulting ceria nanoisland was anchored on a CeO2(111)-(7×7) slab of nine atomic layers. The full system consisted of 178 cerium and 356 oxygen atoms (film —Ce147O294; NP —Ce31O62) and had a total expansion of 27.18×27.18×7.94 Å3. Due to the large cell dimensions, optimizations were carried out at Gamma-point.
Evaluation of CO vibrational frequencies was done by applying small, step-wise displacements of the relevant platinum, carbon and oxygen atoms (VASP-tag NFREE=2). Due to the high computational cost, a modified version of the NP/slab composite system, only keeping the surface layer of the underlying slab, was used and the Pt was kept fixed. Reference calculations have shown that deviations incurred by this approximation stay below 10 cm−1, validating the approach. Additionally, in the case of the extended low-index surfaces, higher precision was applied by setting the VASP-tag PREC to Accurate. Lastly, all frequencies were scaled by referencing with the theoretical and experimental value of molecular CO (see Table 4). Table 4 shows the calculated energies (in eV) for Pt adsorption (referenced against gaseous single-atom Pt) on extended low-index ceria surfaces, CO-adsorption on single-atom Pt and corresponding CO vibrational frequencies (in cm−1). The calculated vibrational frequencies were scaled using the scaling factor 1.008 based on the experimental (2143 cm−1) and theoretical (2125 cm−1) values obtained for CO in the gas phase.
The plasma was modelled as isolated oxygen atoms (radicals). Binding energies of the O 1s level at surface were calculated within the final-state approximation and referenced to the O 1s signal of oxygen in a ceria bulk (2×2×2) supercell.
To understand the rationale behind the formation of ceria NPs observed by STM (see
Not surprisingly, peroxo species affected the Pt adsorption on the CeO2 surfaces (Table 12 and
Next, the interaction of Pt single atoms with ceria NPs formed on the CeO2(111) surface, as shown by STM, was investigated. To this end, a model system was constructed consisting of a ceria cluster anchored on an extended CeO2(111)-(7×7) slab. The particle size was adapted to that measured experimentally (i.e., about 1.0 nm, see
Depending on their location on the ceria NP the coordination environments for adsorbed Pt atoms were chemically different from those of their symmetrically equivalent counterparts on the extended ceria surfaces. In particular, at (100)-40 sites, the difference amounted to 0.9 eV (i.e., —1.76 vs −0.85 eV, on NP vertices and the corresponding extended surface, respectively).
The NP/terrace boundary and the square-planar “nanopocket” Pt-binding sites of this system for CO adsorption were also investigated (
In an attempt to identify the nature of strongly bonded CO ad-species remained on the Pt/CeO2-plasma surface after reaction, several structures were computed derived from the Pt/nanopocket site by sequential removal of 0 atoms from the Pt-40 coordination as a result of their reaction with CO. The resulting structures, denoted Pt-30 and Pt-20 in
Based on the above observations, the overall impact of the plasma treatment on the structure of the Pt/CeO2(111) surface is rationalized as shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 21218306.5 | Dec 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/087481 | 12/22/2022 | WO |
