JP2011056342A - Catalyst and method for manufacturing catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst which can efficiently make a nitrogen oxide harmless while inhibiting the consumption of a precious metal, as well as a method for manufacturing a catalyst. <P>SOLUTION: This catalyst has a rhodium (Rh) atomic layer 1 laminated on the surface of a carrier 2. The carrier 2 is constituted of a material in which the distance between atoms in the Rh atomic layer 1 laminated on the surface of the carrier 2 is equal to or larger than the distance between atoms of Rh in bulk. On the surface of the Rh atomic layer 1 where the distance between atoms is equal to or larger than the distance between atoms of Rh in bulk, the probability of dissociative adsorption of nitrogen monoxide is high. Therefore, the nitrogen oxide (NOx) can be easily reduced and as such, the catalyst can make NOx harmless efficiently. When the number of the Rh atomic layers 1 to be laminated on the surface of the carrier 2 is lessened, the consumption of Rh can be cut more and besides, the capability of the catalyst to make NOx harmless can be increased more. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a catalyst for detoxifying harmful components contained in exhaust gas discharged from an internal combustion engine or the like, and a method for producing the catalyst.

  Exhaust gas discharged from internal combustion engines such as gasoline engines or combustion devices such as boilers contains harmful components such as nitrogen oxides, and various catalysts have been developed to render these harmful components harmless. ing. In particular, for gasoline engines provided in many automobiles, regulations on harmful components contained in exhaust gas continue to be strengthened, and development of a catalyst capable of efficiently detoxifying harmful components is desired.

  Major harmful components contained in exhaust gas discharged from a gasoline engine include nitrogen oxides (NOx), hydrocarbons (HC), and carbon monoxide (CO). In general, a method for detoxifying these harmful components changes HC and CO into carbon dioxide and water by an oxidation reaction, and changes NOx to nitrogen and oxygen by a reduction reaction. A catalyst that efficiently detoxifies NOx, HC, and CO by promoting the oxidation reaction of HC and CO and the reduction reaction of NOx is called a three-way catalyst, and detoxifies exhaust gas discharged from a gasoline engine. Is used for. Conventionally, catalysts using noble metals such as platinum (Pt), palladium (Pd) or rhodium (Rh) have been developed as three-way catalysts. Patent Document 1 discloses a catalyst using iridium (Ir), which is a kind of noble metal, as one of the materials.

JP 2009-61394 A

  Of the three harmful components of NOx, HC and CO, NOx is the most difficult to detoxify, and in particular, the development of a catalyst that effectively detoxifies NOx is desired. In addition, the conventional three-way catalyst uses a noble metal which is a scarce resource, so it is expensive and difficult to mass-produce. Therefore, development of a catalyst that reduces the amount of noble metal used is desired.

  The present invention has been made in view of such circumstances, and an object of the present invention is to produce a catalyst capable of efficiently detoxifying NOx while suppressing the amount of noble metal used, and production of the catalyst. It is to provide a method.

  The catalyst according to the present invention is a catalyst that reduces nitrogen oxides in a gas by bringing a gas containing nitrogen oxides into contact with each other. The dissociative adsorption energy of nitrogen oxide with respect to the surface of the atomic layer is a negative value, and the nitrogen is absorbed with respect to the surface of the atomic layer. The energy is lower than the molecular adsorption energy of the oxide.

  The catalyst according to the present invention is characterized in that the metal element is any one of rhodium, iridium, cobalt, or copper.

  In the catalyst according to the present invention, the carrier is a metal crystal other than the metal element, and an interatomic distance is greater than or equal to an interatomic distance in the bulk of the metal element.

  In the catalyst according to the present invention, the support is a crystal of a metal oxide, and the interatomic distance is greater than or equal to the interatomic distance in the bulk of the metal element.

  In the catalyst according to the present invention, the carrier is a crystal of a semiconductor material, and an interatomic distance is greater than or equal to an interatomic distance in the bulk of the metal element.

  The catalyst according to the present invention is characterized in that the carrier has a cluster structure.

  The catalyst according to the present invention is characterized in that the carrier has a film-like structure.

  The method for producing a catalyst according to the present invention is a method for producing the catalyst according to the present invention, wherein for each metal element having d electrons, the atomic layer of the metal element is laminated on the surface of the carrier. A carrier having an interatomic distance greater than or equal to the interatomic distance in the bulk is selected, and for each metal element, the atomic layer of the metal element stacked on the surface of the selected carrier is determined by first-principles calculation. Calculate the molecular adsorption energy and dissociative adsorption energy of nitrogen oxide on the surface of the metal, and select the metal element whose calculated dissociative adsorption energy is negative and whose dissociative adsorption energy is lower than the molecular adsorption energy. The catalyst is manufactured by laminating the selected atomic layer of the metal element on the surface of the carrier selected for the metal element.

  In the present invention, the catalyst is formed by laminating an atomic layer of a specific metal element having d electrons on the surface of the support, and the interatomic distance in the atomic layer is larger than the interatomic distance in the bulk. Thus, the dissociative adsorption energy of nitrogen oxides on the surface of the atomic layer is negative and is lower than the molecular adsorption energy. Since the dissociative adsorption energy of nitrogen oxides is negative and lower than the molecular adsorption energy, nitrogen oxides dissociate and adsorb on the surface of the atomic layer, and the dissociated and adsorbed nitrogen oxides are easily reduced. Can efficiently detoxify nitrogen oxides.

  In the present invention, the metal element constituting the atomic layer is Rh, Ir, cobalt, or copper, and the dissociative adsorption energy of nitrogen oxide is negative by stacking the atomic layer of these metal elements on the carrier. Thus, a catalyst having a lower energy value than the molecular adsorption energy can be obtained.

  Further, in the present invention, the carrier is formed of a metal crystal other than a metal element, a metal oxide crystal, or a semiconductor material crystal having an interatomic distance larger than the interatomic distance in the bulk of the metal element constituting the atomic layer. Configure. When the interatomic distance of the carrier is larger than the interatomic distance in the bulk metal element, the interatomic distance in the atomic layer becomes larger than the interatomic distance in the bulk.

  In the present invention, a catalyst is formed by laminating an atomic layer of a metal element on the surface of a carrier having a cluster structure. By configuring in a cluster shape, the surface area of the catalyst is increased, and the efficiency of reducing NOx is improved.

  In the present invention, the catalyst is formed by laminating an atomic layer of a metal element on the surface of a support formed in a film shape. Thereby, a thin-film catalyst is formed.

  In the present invention, if the interatomic distance in the atomic layer of a specific metal element such as Rh stacked on the surface of the support is greater than or equal to the interatomic distance in the bulk state, nitric oxide is Rh. Since the probability of dissociative adsorption on the surface is high and NOx is easily reduced, the interatomic distance in the atomic layer of a specific metal element such as Rh is larger than the interatomic distance in the bulk state. By stacking the atomic layer on the surface of the support, a catalyst capable of detoxifying NOx efficiently while suppressing the amount of a specific metal element such as Rh can be realized. In particular, when the atomic layer laminated on the surface of the support is made into one layer, the amount of use of a specific metal element such as Rh can be minimized and the ability to detoxify NOx can be maximized. The present invention has excellent effects such as being possible.

It is a graph which shows the result of having calculated the molecular adsorption energy of NO with respect to the surface of a Rh metal crystal. It is a graph which shows the result of having calculated the dissociative adsorption energy of NO with respect to the surface of a Rh metal crystal. It is a schematic diagram which shows the position of the N atom or O atom which adsorb | sucks to the surface of a Rh metal crystal. It is a graph which shows the calculation result of the molecular adsorption energy calculated about each of Rh, Pt, Pd, Cu, and Ir, dissociation adsorption energy, and an activation barrier. It is an imaginary figure which shows the process in which NO is reduce | restored on the Rh surface. It is a graph which shows the calculation result of the molecular adsorption energy and dissociative adsorption energy of NO calculated about one Rh atomic layer laminated | stacked on four Cu atomic layers. It is a graph which shows the calculation result of the molecular adsorption energy and dissociation adsorption energy of NO calculated about one Rh atomic layer laminated | stacked on four Al atomic layers. It is a graph which shows the calculation result of the molecular adsorption energy and dissociation adsorption energy of NO calculated about one Rh atomic layer laminated | stacked on four Ag atomic layers. It is typical sectional drawing which shows the structure of the catalyst of this invention. It is a graph which shows the calculation result of the molecular adsorption energy and dissociation adsorption energy of NO which were calculated about one Co atomic layer laminated | stacked on four Al atomic layers. It is a graph which shows the calculation result of the molecular adsorption energy and dissociation adsorption energy of NO calculated about one Co atomic layer laminated | stacked on four Ag atomic layers. It is a graph which shows the calculation result of the molecular adsorption energy and dissociation adsorption energy of NO calculated about one Cu atomic layer laminated | stacked on four Al atomic layers. It is a graph which shows the calculation result of the molecular adsorption energy and dissociation adsorption energy of NO calculated about one Cu atomic layer laminated | stacked on four Ag atomic layers. It is a schematic diagram which shows the usage form of the catalyst of this invention. It is a characteristic view which shows the utilization conditions of a catalyst.

Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof.
(First embodiment)
Conventionally, it is known that among Rh, Pt, and Pd that are widely used in catalysts, Rh is most likely to reduce NOx. However, the reason was not clear. The inventor of the present application clarifies the reason why Rh is most likely to reduce NOx by first-principles calculation using the density functional method, and further, while maintaining the ability of Rh to reduce NOx, the amount of Rh used is reduced. The structure of the catalyst that can be reduced to the minimum source was elucidated.

  First, molecular adsorption energy and dissociative adsorption energy of nitric oxide (NO) molecules on the surface of the Rh metal crystal were calculated by first-principles calculation using a density functional method. Here, the molecular adsorption energy is energy in a state in which NO molecules are adsorbed on the Rh surface in a molecular state, and the dissociative adsorption energy is N atoms and O atoms after the NO molecules are dissociated into N atoms and O atoms. Are energy in the state adsorbed on the surface of Rh. The crystal structure of the Rh metal crystal is a face-centered cubic structure. In the calculation, NO molecules were adsorbed on the (111) surface of the bulk Rh metal crystal.

  FIG. 1 is a chart showing the results of calculating the molecular adsorption energy of NO on the surface of the Rh metal crystal. ON / Rh (111) in the first row shown in FIG. 1 shows the result of calculating the molecular adsorption energy by bringing the NO molecule closer to the (111) surface of the Rh metal crystal from the N atom side, and the NO in the second row. / Rh (111) indicates the result of calculating the molecular adsorption energy by bringing NO molecules closer to the (111) surface of the Rh metal crystal from the O atom side. FIG. 1 also shows the results of calculating the molecular adsorption energy when NO molecules are adsorbed on each of the Top site, the fcc-hollow site, the hcp-hollow site, and the Bridge site in the surface of Rh. The Top site is a position on the surface Rh atom, the Bridge site is a position between two Rh atoms, and the fcc-hollow site is a group of three or more Rh atoms corresponding to the gap of the face-centered cubic lattice. It is the position of the gap, and the hcp-hollow site is the position of the gap between three or more Rh atoms corresponding to the gap of the hexagonal close-packed lattice. The unit of calculation results is all electron volts (eV).

  When the adsorption energy is negative, the reaction of adsorbing NO molecules is an exothermic reaction, indicating that the state in which NO molecules are adsorbed on the Rh surface is stable. The larger the absolute value, the lower the energy. The higher the probability is, the more NO molecules are adsorbed. In addition, when the adsorption energy is positive, the reaction in which NO molecules are adsorbed on the Rh surface is an endothermic reaction, indicating that the state in which NO molecules are adsorbed on the Rh surface is unstable. do not do. According to the calculation result shown in FIG. 1, the adsorption energy at the hcp-hollow site of ON / Rh (111) is -1.974 eV, the sign is negative, and the absolute value is maximum. Accordingly, it can be seen that when NO molecules are adsorbed on the surface of the Rh metal crystal, the probability that the molecule adsorption with the N atom side located at the hcp-hollow site is the highest is generated.

  FIG. 2 is a chart showing the results of calculating the dissociative adsorption energy of NO on the surface of the Rh metal crystal, and FIG. 3 is a schematic diagram showing the positions of N atoms or O atoms adsorbed on the surface of the Rh metal crystal. . When NO is dissociatively adsorbed, N atoms and O atoms are separated and adsorbed individually on the Rh surface. FIG. 3 schematically shows the (111) surface of the Rh metal crystal, the solid circle symbol indicates the Rh atom on the surface, and the broken circle symbol indicates the second layer Rh atom from the surface. FIG. 3 shows the positions of a, b, c, and d. The positions of a and d are hcp-hollow sites, and the positions of b and c are fcc-hollow sites.

  FIG. 2A shows dissociative adsorption energy when an N atom is adsorbed at a position and an O atom is adsorbed at any position of b, c, or d. FIG. Shows the dissociative adsorption energy when O is adsorbed at the position b and the O atom is adsorbed at any position a, c or d. The combination indicated as “unstable” in FIG. 2 is a combination in which the adsorption energy cannot be calculated because the state is unstable. All other calculation result units are eV. According to the calculation result shown in FIG. 2, the adsorption energy when the N atom is adsorbed at the position a and the O atom is adsorbed at the position c is −2.190 eV, the sign is minus, and the absolute value is the maximum. , Has become the lowest energy. Thus, it can be seen that, when NO molecules are dissociatively adsorbed on the surface of the Rh metal crystal, the probability that the dissociative adsorption in which the N atom is adsorbed at the position a and the O atom is adsorbed at the position c is the highest.

  Comparing the molecular adsorption energy having the highest probability of occurrence shown in FIG. 1 with the dissociative adsorption energy having the highest probability of occurrence shown in FIG. 2, the dissociative adsorption energy has a larger absolute value. Therefore, on the surface of the Rh metal crystal, NO has a higher probability of performing dissociative adsorption than molecular adsorption.

  Next, the density of states on the surface of the Rh metal crystal was calculated before and after NO was dissociated and adsorbed by first-principles calculation using a density functional method. The state density of the Rh atom at the position e shown in FIG. 3 is the state in which NO is not adsorbed, and the state in which the N atom is adsorbed at the position a and the O atom is adsorbed at the position c shown in FIG. Calculated. The Rh atom at the position e is an Rh atom adjacent to the N atom when NO is dissociated and adsorbed on the surface of the Rh metal crystal. According to the calculation of the density of states, it is clear that the dzz orbit of the Rh atom at the position e is an unoccupied orbit after NO dissociative adsorption, whereas it is an occupied orbit after NO dissociative adsorption. It became. This suggests that d electrons possessed by Rh atoms on the surface of the metal crystal contribute to dissociate and adsorb NO.

  Next, the calculation results for Rh are compared with other metals. For each of Rh, Pt, Pd, copper (Cu), and Ir, the molecular adsorption energy and dissociative adsorption energy when NO is adsorbed on the surface are calculated, and further, activation necessary for NO to dissociate and adsorb on the surface is calculated. The barrier was calculated. Pt and Pd are materials that are often used as a catalyst, like Rh, Cu is a representative transition metal, and Ir is a material used as a catalyst in Patent Document 1. FIG. 4 is a chart showing calculation results of molecular adsorption energy, dissociative adsorption energy, and activation barrier calculated for each of Rh, Pt, Pd, Cu, and Ir. As for the molecular adsorption energy and the dissociative adsorption energy, any negative value is obtained.

  Referring to FIG. 4, for Pt, Pd and Cu, the molecular adsorption energy has a larger absolute value than the dissociative adsorption energy. Therefore, on the surface of Pt, Pd and Cu, NO has a higher probability of performing molecular adsorption than dissociative adsorption. In contrast, in Rh and Ir, the absolute value of the dissociative adsorption energy is larger than the molecular adsorption energy, and it can be seen that NO has a high probability of performing dissociative adsorption on the surface of Rh and Ir. Rh has the smallest activation barrier for dissociating and adsorbing NO on the surface, so it can be seen that NO is more easily dissociated and adsorbed than other materials.

  Dissociated and adsorbed NO is more easily reduced than NO that has been molecularly adsorbed because N and O atoms are already separated. For example, when NO reacts with CO and is reduced, O atoms already separated from N atoms react with CO rather than O atoms contained in NO molecules are separated from N atoms and react with CO. Is easier. FIG. 5 is an imaginary view showing a process in which NO is reduced on the Rh surface. As shown in FIG. 5A, when the exhaust gas containing NO and CO contacts the Rh surface, NO is dissociated and adsorbed on the Rh surface. As shown in FIG. 5B, the O atoms separated and adsorbed from the N atoms react with CO in the exhaust gas, and the adsorbed N atoms diffuse on the Rh surface and react with each other. Finally, as shown in FIG. 5C, NO is reduced to nitrogen molecules, and the nitrogen molecules are detached from the Rh surface. Further, CO that has reduced NO is itself oxidized to carbon dioxide. In this way, NO and CO in the exhaust gas are detoxified together as nitrogen molecules and carbon dioxide. NOx other than NO is also finally reduced to nitrogen molecules by repeating the reduction reaction in which O atoms are separated and react with CO.

  As described above, unlike Pt and Pd, Rh tends to dissociate and adsorb NO on the surface. Since dissociated and adsorbed NO is easier to be reduced than molecularly adsorbed NO, Rh that easily dissociates and adsorbs NO on the surface is more NO than Pt and Pd, which are more susceptible to molecular adsorption than NO. Increases ability to reduce. Thus, it became clear why Rh can reduce NOx more easily than Pt and Pd.

  Next, the inventor of the present application calculated the ease of adsorption of NO to the Rh atomic layer coated with a metal crystal other than Rh by first-principles calculation using a density functional method. When the catalyst is in a form in which a carrier such as another metal crystal is coated with Rh, the amount of Rh used in the catalyst can be reduced. The ability of the catalyst can be simulated by calculating the ease of NO adsorption on Rh coated with various supports. When the Rh atomic layer is stacked on the carrier, the Rh atomic layer can be easily stacked if the carrier and Rh have the same crystal structure. In addition, in the state where the Rh atomic layer is stacked on the carrier, the number of atomic layers of the carrier is significantly larger than the number of Rh atomic layers, so the interatomic distance in the Rh atomic layer is affected by the atomic distance in the carrier. Change. In order to reflect this, in the calculation, the interatomic distance in the Rh atomic layer was set to the same value as the interatomic distance of the carrier in the bulk state. In the calculation, Cu and aluminum (Al) and silver (Ag) and metal crystals were used as the carrier. Since these metal crystals all have a cubic fine structure, the lattice constant may be used as the interatomic distance of the carrier in the bulk state. The respective lattice constants are 3.8３ for Rh, 3.61Å for Cu, 4.05Å for Al, and 4.09Å for Ag. When a Cu metal crystal is used as a carrier, the interatomic distance in the Rh atomic layer is smaller than that in the bulk state. In addition, when an Al or Ag metal crystal is used as a carrier, the interatomic distance in the Rh atomic layer is larger than that in the bulk state.

  FIG. 6 is a chart showing calculation results of molecular adsorption energy and dissociation adsorption energy of NO calculated for one Rh atomic layer laminated on four Cu atomic layers. In the calculation, a single Rh atomic layer was stacked on the (111) surface of a Cu metal crystal composed of four Cu atomic layers, and the molecular adsorption energy and dissociative adsorption energy when NO was adsorbed to the Rh atomic layer were calculated. . The interatomic distance in the Rh atomic layer was set to 3.61 mm which is the interatomic distance of Cu. In this calculation, an Rh atomic layer is laminated on the surface of a support made of a Cu metal crystal, and the interatomic distance in the Rh atomic layer is smaller than the bulk state due to the influence of the Cu interatomic distance. Imitate. Regarding the molecular adsorption energy, the molecular adsorption energy was calculated when the N atom side of the NO molecule was adsorbed on each of the fcc-hollow site and the hcp-hollow site. As for the dissociative adsorption energy, when both N atom and O atom are located at the fcc-hollow site (N-fcc_O-fcc), the N atom is located at the fcc-hollow site and the O atom is located at the hcp-hollow site. (N-fcc_O-hcp), N atom is located at the hcp-hollow site and O atom is located at the fcc-hollow site (N-hcp_O-fcc), and both N atom and O atom are hcp- The dissociative adsorption energy of each of the cases located at the hollow site (N-hcp_O-hcp) was calculated.

  Referring to FIG. 6, the minimum value of the molecular adsorption energy is -1.787 eV, the minimum value of the dissociative adsorption energy is −1.486 eV, and the molecular adsorption energy has a larger absolute value than the dissociative adsorption energy. . Therefore, unlike bulk Rh, NO dissociative adsorption hardly occurs on the surface of one Rh atomic layer laminated on the surface of the Cu metal crystal. Therefore, the Rh atomic layer laminated on the surface of the Cu metal crystal has a low ability to reduce NO, and is insufficient as a catalyst for reducing NOx.

  FIG. 7 is a chart showing calculation results of molecular adsorption energy and dissociation adsorption energy of NO calculated for one Rh atomic layer laminated on four Al atomic layers. In the calculation, a single Rh atomic layer was stacked on the (111) surface of an Al metal crystal composed of four Al atomic layers, and molecular adsorption energy and dissociative adsorption energy when NO was adsorbed to the Rh atomic layer were calculated. . The interatomic distance in the Rh atomic layer was set to 4.05 cm, which is the interatomic distance of Al in the bulk state. In this calculation, an Rh atomic layer is stacked on the surface of a carrier made of Al metal crystals, and the interatomic distance in the Rh atomic layer is larger than the bulk state due to the influence of the interatomic distance of Al. Imitate.

  Referring to FIG. 7, the minimum value of the molecular adsorption energy is −2.533 eV, the minimum value of the dissociative adsorption energy is −3.099 eV, and the dissociative adsorption energy has a larger absolute value than the molecular adsorption energy. . For this reason, like bulk Rh, there is a high probability that NO is dissociated and adsorbed on the surface of one Rh atomic layer laminated on the surface of the Al metal crystal, and NO is easily reduced. Therefore, the Rh atomic layer laminated on the surface of the Al metal crystal has a sufficiently high ability to reduce NO, and has a sufficient ability as a catalyst for reducing NOx.

  FIG. 8 is a chart showing calculation results of molecular adsorption energy and dissociation adsorption energy of NO calculated for one Rh atomic layer laminated on four Ag atomic layers. In the calculation, one Rh atomic layer was laminated on the (111) surface of an Ag metal crystal composed of four Ag atomic layers, and the molecular adsorption energy and dissociative adsorption energy when NO was adsorbed to the Rh atomic layer were calculated. . The interatomic distance in the Rh atomic layer was set to 4.09 cm which is the interatomic distance of Ag in the bulk state. In this calculation, the Rh atomic layer is laminated on the surface of the support made of Ag metal crystal, and the interatomic distance in the Rh atomic layer is larger than the bulk state due to the influence of the interatomic distance of Ag. Imitate.

  Referring to FIG. 8, similarly to the single Rh atomic layer stacked on the surface of the Al metal crystal, the single Rh atomic layer stacked on the surface of the Ag metal crystal has a dissociative adsorption energy higher than the molecular adsorption energy. The absolute value is larger. For this reason, similarly to bulk Rh, there is a high probability that NO is dissociated and adsorbed on the surface of one Rh atomic layer stacked on the surface of the Ag metal crystal, and NO is easily reduced. Therefore, the Rh atomic layer stacked on the surface of the Ag metal crystal has a sufficiently high ability to reduce NO, and has a sufficient ability as a catalyst for reducing NOx.

  From the above calculation results, when the Rh atomic layer is laminated on the support surface, the ability to reduce NO is low in the state where the interatomic distance in the Rh atomic layer is smaller than the bulk state. It has been clarified that the ability to reduce NO increases when the interatomic distance of is larger than the bulk state. Therefore, the inventor of the present application has a structure in which an Rh atomic layer in which the interatomic distance is larger than the interatomic distance in the bulk is laminated on the surface of the support as a catalyst for reducing NOx. A catalyst is proposed. If the interatomic distance in the Rh atomic layer stacked on the support surface is larger than the interatomic distance in the bulk state, the probability that NO will dissociate and adsorb on the Rh surface increases, and the dissociated and adsorbed NO easily Since it is reduced, the catalyst can efficiently detoxify NOx. The greater the number of Rh atomic layers stacked on the carrier, the closer the interatomic distance in the Rh atomic layer approaches the bulk state. Therefore, the smaller the number of Rh atomic layers stacked on the support, the greater the interatomic distance in the Rh atomic layer. The ability to reduce NOx is high. Therefore, the catalyst of the present invention can reduce the amount of Rh, which is a noble metal, and at the same time reduce the number of Rh atomic layers stacked on the support as much as possible in order to improve the ability to detoxify NOx. desirable. In particular, when the Rh atomic layer laminated on the support surface is made into one layer, the amount of Rh used can be minimized and the ability to detoxify NOx can be maximized.

  The support material used for the catalyst is a metal crystal other than Rh, the crystal structure of which is a face-centered cubic structure, and the interatomic distance is larger than the interatomic distance of Rh in the bulk state. It is desirable. For example, as described above, Ag or Al metal crystals are desirable. Since the Rh metal crystal has a face-centered cubic structure, if the carrier material is a face-centered cubic metal crystal, the affinity between Rh and the carrier is high, and the atomic layer of Rh is easily formed on the surface of the support. Laminated. When the interatomic distance of the carrier is larger than the interatomic distance of Rh in the bulk state, the interatomic distance in the Rh atomic layer is larger than in the bulk state, and the ability of the catalyst to detoxify NOx is improved. Even when the interatomic distance of the carrier is the same as the interatomic distance of Rh in the bulk state, the interatomic distance in the Rh atomic layer is the same as when the bulk Rh is used as a catalyst. Can be made harmless, and the amount of Rh used can be reduced. As described above, the catalyst of the present invention can reduce the amount of Rh, which is a noble metal, as compared with the conventional one, and thus can be manufactured at a lower cost than the conventional one and can be mass-produced as compared with the conventional one. is there.

  FIG. 9 is a schematic cross-sectional view showing the structure of the catalyst of the present invention. FIG. 9A shows a cluster catalyst. The catalyst is formed by laminating two or more Rh atomic layers 1 on the surface of a carrier 2 having a cluster structure. If the carrier 2 is a metal, the cluster structure can be obtained by a method such as a method of crushing bulk metal into a powder, a method of agglomerating evaporated metal atoms in a gas phase, or a method of reducing metal ions in a solution. The carrier 2 that forms The Rh atom layer 1 can be formed by a method of aggregating Rh atoms in the gas phase on the surface of the carrier 2 having a cluster structure or reducing Rh ions in a solution. By collecting the cluster in a state where the Rh atomic layer 1 is formed on the surface of the support 2, the catalyst of the present invention as shown in FIG. 9A can be produced. In addition, the manufacturing method of the catalyst of this invention may be other. The size of the cluster catalyst is desirably a diameter of several tens of nm or less. When the diameter of the cluster-like catalyst is set to a size of several tens of nm or less, the surface area of the catalyst is increased, and the efficiency of reducing NOx is improved. The cluster-like catalyst is applied to a structural material such as ceramic formed in a honeycomb shape to create a purifier for purifying exhaust gas.

  FIG. 9B shows a catalyst having a thin film structure. A film-like carrier 2 is coated on the surface of the structural material 3 such as ceramic, and the Rh atom layer 1 is laminated on the surface of the carrier 2. The film-like carrier 2 can be formed on the surface of the structural material 3 by a method such as reduction of metal ions in a solution, physical vapor deposition or chemical vapor deposition. The Rh atomic layer 1 can also be formed on the surface of the carrier 2 by the same method, whereby a thin film catalyst can be produced. The method for producing the catalyst may be other methods. By coating the surface of the structural material 3 formed in a honeycomb shape with the carrier 2 and laminating two or more Rh atom layers 1 on the surface of the carrier 2, the catalyst of the present invention as shown in FIG. 9B is produced. A purifier is created. The thin-film catalyst may be manufactured by forming the structural material 3 with a material that can be used as the carrier 2 and directly laminating the Rh atomic layer 1 on the surface of the structural material 3.

  When an Al metal crystal is used as the carrier 2, the ability of the catalyst to reduce NOx is improved as described above. In addition, since Al and Rh have the same face-centered cubic structure in crystal structure, the affinity between the Rh atom layer 1 and the carrier 2 is increased, and the Rh atom layer 1 is easily laminated on the surface of the carrier 2. Since the technology for forming Al metal crystals in a cluster and the technology for forming Al metal crystals in a thin film have been established, the catalyst of the present invention having the structure shown in FIG. 9 can be easily produced. is there. Further, since Al is present at a lower price and abundantly than noble metals, the use of Al metal crystals as the support 2 makes it possible to produce the catalyst of the present invention at a low cost and to enable mass production.

  In addition, the material of the support | carrier 2 for comprising the catalyst of this invention is not restricted to a metal crystal, The crystal | crystallization of a metal oxide may be sufficient. Similar to the technique of laminating an atomic layer of a metal element on the surface of a metal crystal, the technique of laminating an atomic layer of a metal element on the surface of a metal oxide is an established technique, and the Rh atomic layer 1 is composed of a metal oxide. It can be easily laminated on the surface of the carrier 2 used. Among the metal oxides, NaCl-type metal oxides have a face-centered cubic structure and can be easily stacked with the Rh atomic layer 1, and therefore are desirable as the material for the carrier 2. More specifically, nickel oxide (NiO) has a lattice constant of 4.26Å, and cobalt oxide (CoO) has a lattice constant of 4.17Å, both of which are larger than bulk rhodium. When the Rh atom layer 1 is laminated with respect to the (111) plane of the NaCl type metal oxide, the Rh atoms are arranged at the positions of metal atoms or oxygen atoms having a face-centered cubic structure, and between the Rh atoms. This distance is a value corresponding to the lattice constant of the metal oxide. When a metal oxide having a lattice constant larger than that of bulk rhodium such as NiO or CoO is used as the material of the carrier 2, the interatomic distance in the Rh atomic layer 1 is larger than the interatomic distance in bulk rhodium. Thus, the ability to reduce NOx on the surface of the Rh atomic layer 1 is improved. Therefore, even when the metal oxide whose interatomic distance in the Rh atomic layer 1 is larger than the interatomic distance in bulk rhodium is used as the material of the carrier 2, the amount of Rh used is suppressed. In addition, a catalyst capable of detoxifying NOx efficiently can be realized.

The material of the carrier 2 for constituting the catalyst of the present invention may be a crystal of a semiconductor substance. Similar to metal crystals or metal oxides, the technology for stacking atomic layers of metal elements on the surface of a semiconductor material is an established technology, and the crystal of a semiconductor material has good crystallinity, so that the distance between Rh atoms is It is possible to stack the Rh atomic layer 1 on the surface of the carrier 2 using a crystal of a semiconductor material so as to be larger than the interatomic distance in bulk rhodium. Specifically, examples of the semiconductor material include group 4 semiconductors such as silicon (Si) and germanium (Ge), and group 2-6 compounds such as zinc selenide (ZnSe), cadmium sulfide (CdS), and zinc oxide (ZnO). Semiconductors, Group 3-5 compound semiconductors such as gallium arsenide (GaAs), indium phosphorus (InP) and gallium nitride (GaN), Group 4 compound semiconductors such as silicon carbide (SiC) and silicon germanium (SiGe), indium copper selenide It is possible to use a 1-3-6 compound semiconductor such as (CuInSe ₂ ). In this way, even when a semiconductor substance whose interatomic distance in the Rh atomic layer 1 is larger than the interatomic distance in bulk rhodium is used as the material of the carrier 2, the amount of Rh used is suppressed. However, a catalyst capable of detoxifying NOx efficiently can be realized.

  In addition, the material of the support 2 for constituting the catalyst of the present invention is such that the Rh atoms are formed on the surface of the support 2 so that the interatomic distance in the Rh atomic layer 1 is larger than the interatomic distance in the bulk Rh. Any material other than those listed above may be used as long as the layer 1 can be laminated. For example, it may be a metal crystal or metal oxide having a structure other than a face-centered cubic structure, may be an amorphous metal or metal oxide, and is an amorphous or polycrystalline semiconductor material. Also good. Moreover, other materials, such as a ceramic or resin, may be sufficient. Even if the material of the carrier 2 is such a substance, the Rh atomic layer 1 is formed on the surface of the carrier 2 so that the interatomic distance in the Rh atomic layer 1 is larger than the interatomic distance in bulk Rh. If the material can be laminated, a catalyst capable of detoxifying NOx efficiently can be realized.

(Other embodiments)
In the present invention, the catalyst of the present invention can be configured using Ir instead of Rh as the metal element laminated on the surface of the support 2. As shown in FIG. 4, Ir has a negative value for dissociative adsorption energy, as in Rh, and the dissociative adsorption energy has a value lower than the molecular adsorption energy. For this reason, Ir has a high ability to reduce NO similarly to Rh. Therefore, the catalyst in which the Ir atomic layer is stacked on the surface of the carrier 2 can efficiently detoxify NOx in the same manner as the catalyst in which the Rh atomic layer 1 is stacked on the surface of the carrier 2. In addition, since the amount of Ir, which is a noble metal, is small, this catalyst can be manufactured at a lower cost than in the past, and can be mass-produced more than in the past.

  In the present invention, it is also possible to constitute the catalyst of the present invention using cobalt (Co) instead of Rh as the metal element laminated on the surface of the support 2. Co is the same group 9 element as Rh and Ir, and is similarly expected to have a high ability to reduce NOx.

  FIG. 10 is a chart showing calculation results of molecular adsorption energy and dissociation adsorption energy of NO calculated for one Co atomic layer stacked on four Al atomic layers. In the calculation, a single Co atomic layer was stacked on the (111) surface of an Al metal crystal composed of four Al atomic layers, and molecular adsorption energy and dissociative adsorption energy when NO was adsorbed to the Co atomic layer were calculated. . The interatomic distance in the Co atomic layer was set to the interatomic distance of Al in the bulk state. FIG. 11 is a chart showing calculation results of molecular adsorption energy and dissociation adsorption energy of NO calculated for one Co atomic layer stacked on four Ag atomic layers. In the calculation, one Co atomic layer was laminated on the (111) surface of an Ag metal crystal composed of four Ag atomic layers, and the molecular adsorption energy and dissociative adsorption energy when NO was adsorbed to the Co atomic layer were calculated. . The interatomic distance in the Co atomic layer was set to the interatomic distance of Ag in the bulk state.

  Referring to FIGS. 10 and 11, the dissociative adsorption energy is a negative value in the Co atomic layer laminated on either the Al metal crystal or the Ag metal crystal, and the dissociative adsorption energy is lower than the molecular adsorption energy. Is the value of For this reason, as in the case of Rh, there is a high probability that NO is dissociated and adsorbed on the surface of one Co atom layer stacked on the surface of an Al metal crystal or an Ag metal crystal, and NO is easily reduced. Therefore, a catalyst in which an atomic layer of Co is laminated on the surface of the carrier 2 so that the interatomic distance is larger than that in the bulk state is as efficient as a catalyst in which the Rh atomic layer 1 is laminated on the surface of the carrier 2. Can be detoxified.

  In the present invention, it is also possible to constitute the catalyst of the present invention using Cu as the metal element laminated on the surface of the carrier 2. FIG. 12 is a chart showing calculation results of molecular adsorption energy and dissociation adsorption energy of NO calculated for one Cu atomic layer laminated on four Al atomic layers. In the calculation, one Cu atomic layer was laminated on the (111) surface of an Al metal crystal composed of four Al atomic layers, and molecular adsorption energy and dissociative adsorption energy when NO was adsorbed to the Cu atomic layer were calculated. . The interatomic distance in the Cu atomic layer was set to the interatomic distance of Al in the bulk state. FIG. 13 is a chart showing calculation results of molecular adsorption energy and dissociation adsorption energy of NO calculated for one Cu atomic layer laminated on four Ag atomic layers. In the calculation, one Cu atomic layer was laminated on the (111) surface of an Ag metal crystal composed of four Ag atomic layers, and molecular adsorption energy and dissociative adsorption energy when NO adsorbed to the Cu atomic layer were calculated. . The interatomic distance in the Cu atomic layer was set to the interatomic distance of Ag in the bulk state.

  Referring to FIGS. 12 and 13, the dissociative adsorption energy is a negative value in the Cu atomic layer laminated on either the Al metal crystal or the Ag metal crystal, and the dissociative adsorption energy is lower than the molecular adsorption energy. Is the value of For this reason, as in the case of Rh, there is a high probability that NO is dissociated and adsorbed on the surface of one Cu atom layer laminated on the surface of an Al metal crystal or an Ag metal crystal, and NO is easily reduced. Therefore, the catalyst in which the atomic layer of Cu is laminated on the surface of the carrier 2 so that the interatomic distance is larger than that in the bulk state is efficiently NOx as the catalyst in which the Rh atomic layer 1 is laminated on the surface of the carrier 2. Can be detoxified.

  Ir, Co and Cu are all metallic elements having d electrons, and it is considered that dissociative adsorption of NO is promoted by the contribution of d electrons, as in the case of Rh. The material of the carrier 2 on which the atomic layer of Ir, Co or Cu is laminated is the same material as in the case of Rh. However, the material of the carrier 2 needs to be a material capable of laminating the atomic layer on the surface of the carrier 2 so that the interatomic distance in the atomic layer is larger than the interatomic distance in the bulk. The structure of the catalyst using Ir, Co or Cu as the metal element of the atomic layer is the same as that in the case where the metal element shown in FIG. 9 is Rh. In addition, in the case of a catalyst using Ir, Co, or Cu as the metal element of the atomic layer, the amount of the metal element used can be reduced by making the atomic layer of the metal element laminated on the surface of the carrier 2 into one layer, as in the case of Rh. And the ability to detoxify NOx is maximized.

  Further, the catalyst of the present invention is a metal element such that the dissociative adsorption energy of nitrogen oxides on the surface of the atomic layer laminated on the support 2 is a negative value and a value lower than the molecular adsorption energy. A catalyst using a metal element other than Rh, Ir, Co and Cu may be used. Below, the manufacturing method of the catalyst of this invention containing the catalyst using metal elements other than Rh, Ir, Co, and Cu is demonstrated. First, for each metal element having d electrons, when the atomic layer of each metal element is stacked on the surface of the carrier 2, the interatomic distance in the atomic layer is greater than the interatomic distance in the bulk. Carrier 2 is selected. As the carrier 2, for example, a carrier 2 having the same crystal structure as that of a simple metal element and having a larger lattice constant than the simple metal element is selected. Next, for each metal element, the molecular adsorption energy and dissociative adsorption energy of NO on the surface of the atomic layer of the metal element stacked on the surface of the selected carrier 2 are calculated by first-principles calculation using the density functional method. . Note that a calculation method other than the density functional method may be used as long as the calculation method can execute the first principle calculation. Next, among the metal elements whose molecular adsorption energy and dissociative adsorption energy are calculated, the calculated dissociative adsorption energy is a negative value, and the dissociative adsorption energy is lower than the calculated molecular adsorption energy. Select elements. Next, the atomic layer of the selected metal element is laminated on the surface of the selected carrier 2. By the above production method, the catalyst of the present invention in which an atomic layer of a specific metal element such as Rh, Ir, Co or Cu is laminated on the surface of the carrier 2 can be produced. The catalyst produced by the above production method can efficiently detoxify NOx as described in the case where the metal elements are Rh, Ir, Co, and Cu.

(Usage form of the present invention)
Next, the usage pattern of the catalyst of the present invention will be described. FIG. 14 is a schematic view showing a usage pattern of the catalyst of the present invention. The purifier 4 constructed using the catalyst of the present invention is used by being mounted on a gasoline engine. The gasoline engine includes an air supply pipe 52 and an exhaust pipe 53 connected to a cylinder 51. The air supply pipe 52 is provided with a flow meter 54 and a fuel injector 55 for measuring the flow rate of the supplied air. The exhaust pipe 53 is provided with an oxygen sensor 57 for measuring the oxygen concentration in the exhaust gas and the purifier 4. The purifier 4 is composed of a structural material having an inside formed in a honeycomb shape, and the cluster-like catalyst of the present invention is applied to the structural material, or the thin-film catalyst of the present invention is formed on the structural material. The exhaust gas can pass through the inside. The purifier 4 provided in the exhaust pipe 53 is arranged so that exhaust gas passes through the exhaust pipe 53 and the exhaust gas sufficiently contacts the catalyst of the present invention. The flow meter 54, the fuel injector 55, and the oxygen sensor 57 are connected to the controller 56. The controller 56 executes processing for controlling the operation of the fuel injector 55 based on the measurement results of the flow meter 54 and the oxygen sensor 57.

  FIG. 15 is a characteristic diagram showing conditions for using the catalyst. The horizontal axis in FIG. 15 indicates the air-fuel ratio in the gasoline engine, and the vertical axis indicates the rate at which each of CO, HC, and NO is converted and rendered harmless by the purifier 4 under each air-fuel ratio condition. Under the optimum conditions shown by the oblique lines in FIG. 15 near the air-fuel ratio of 14.6, the conversion rates of CO, HC and NO are close to 100%, and are made harmless efficiently. However, when the air-fuel ratio exceeds the optimal condition, the NO conversion rate decreases rapidly, and when the air-fuel ratio falls below the optimal condition, the CO and HC conversion rates decrease rapidly. Accordingly, in order to effectively detoxify CO, HC, and NO using the purifier 4 using the catalyst of the present invention, it is necessary to use the purifier 4 under the optimum conditions shown by hatching in FIG. There is. The controller 56 performs processing for controlling the operation of the fuel injector 55 so that the air-fuel ratio is included in the optimum conditions shown in FIG. 15 based on the measurement results of the flow meter 54 and the oxygen sensor 57. Thus, by using the catalyst of the present invention, it is possible to effectively detoxify harmful components contained in the exhaust gas discharged from the gasoline engine.

  In addition, the catalyst of this invention can also be used so that the harmful component contained in the exhaust gas discharged | emitted from internal combustion engines other than gasoline engines, such as a diesel engine, may be detoxified. The catalyst of the present invention is not limited to an internal combustion engine, and can be used so as to effectively detoxify harmful components contained in exhaust gas discharged from a combustion apparatus such as a boiler. In any use form, the catalyst of the present invention can efficiently detoxify harmful components contained in exhaust gas while suppressing the amount of Rh, which is a noble metal.

  The catalyst of the present invention has a particularly high ability to reduce NOx, and can be used for efficiently detoxifying harmful components such as NOx contained in exhaust gas discharged from a gasoline engine. Similarly, the catalyst of the present invention can be used for detoxifying harmful components contained in exhaust gas discharged from an internal combustion engine other than a gasoline engine or a combustion device other than an internal combustion engine.

1 Rh atomic layer 2 Carrier 3 Structural material 4 Purifier

Claims (8)

In the catalyst for reducing the nitrogen oxide in the gas by contacting the gas containing nitrogen oxide,
An atomic layer of a specific metal element having d electrons is stacked on the surface of the carrier in a state in which the interatomic distance is greater than or equal to the interatomic distance in the bulk. A catalyst characterized in that the dissociative adsorption energy of an object is a negative value and is lower than the molecular adsorption energy of nitrogen oxide on the surface of the atomic layer.   The catalyst according to claim 1, wherein the metal element is any one of rhodium, iridium, cobalt, and copper.   The catalyst according to claim 1 or 2, wherein the carrier is a metal crystal other than the metal element, and an interatomic distance is greater than or equal to an interatomic distance in the bulk of the metal element.   3. The catalyst according to claim 1, wherein the carrier is a metal oxide crystal and has an interatomic distance greater than or equal to an interatomic distance in the bulk of the metal element.   3. The catalyst according to claim 1, wherein the carrier is a crystal of a semiconductor substance, and an interatomic distance is greater than or equal to an interatomic distance in the bulk of the metal element.   The catalyst according to any one of claims 1 to 5, wherein the carrier has a cluster structure.   The catalyst according to any one of claims 1 to 5, wherein the carrier has a film-like structure. A method for producing the catalyst of claim 1, comprising:
For each metal element having d electrons, a carrier whose atomic distance in the atomic layer is greater than or equal to the interatomic distance in the bulk when the atomic layer of the metallic element is stacked on the surface of the carrier. Selected,
First-principles calculation, for each metal element, calculate the molecular adsorption energy and dissociative adsorption energy of nitrogen oxide on the surface of the atomic layer of the metal element laminated on the surface of the selected carrier,
The metal element whose calculated dissociative adsorption energy is a negative value and whose dissociative adsorption energy is lower than the molecular adsorption energy is selected.
A method for producing a catalyst, comprising: stacking an atomic layer of a selected metal element on a surface of a support selected for the metal element.

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