JP2021049491A - Methane purification catalyst having sulfur poisoning resistance - Google Patents

Abstract

To provide a methane purification catalyst having sulfur poisoning resistance that can suppress sulfur poisoning without impairing methane purification activity and is suitable for exhaust purification.SOLUTION: The present invention relates to a methane purification catalyst having sulfur poisoning resistance, characterized by that: in PdO, an atom (3f) coordinated with three oxygen atoms and an atom (4f) coordinated with four oxygen atoms are present as outermost-surface atoms of a PdO(101) face; and the atoms (3f) coordinated with three oxygen atoms are at least partially replaced with atoms M excluding Pd.SELECTED DRAWING: Figure 11

Description

The present invention relates to a methane purification catalyst having sulfur toxicity resistance, and more specifically, a methane purification catalyst having sulfur toxicity resistance suitable for exhaust gas purification applications capable of suppressing sulfur poisoning without impairing methane purification activity. It is about.

In a heat engine that uses natural gas as fuel, methane remains in the exhaust gas when it is not completely burned during operation or when the operation is stopped. Since methane is a greenhouse gas (25 times that of carbon dioxide), it needs to be purified before it is discharged into the atmosphere. Here, purification of methane means oxidation of methane to remove hydrocarbons (hereinafter, the same applies).

For purification of methane in exhaust gas, the next oxidation reaction that converts carbon dioxide and water, which are harmless and have a lower global warming potential than methane, is desirable.

CH ₄ + 2O ₂ → CO ₂ + 2H ₂ O
Currently, palladium oxide is mainly used as a methane purification catalyst for methane purification (Patent Documents 1 to 6 and the like). However, desulfurized natural gas also contains a small amount of sulfur, or artificially added odorants also contain sulfur, causing sulfur poisoning that significantly impairs the catalytic life of palladium oxide as a methane purification catalyst. There was a problem.

Therefore, in order to reduce the sulfur poisoning resistance of palladium oxide in methane purification, the present inventors have investigated the mechanism of sulfur poisoning and methane activity at the electron / atomic level, and reported the results in Non-Patent Document 1. did.

In Non-Patent Document 1, the PdO (101) surface, which can stably appear in palladium oxide, was focused on as a model surface of a sulfur poisoning catalyst in methane purification. FIG. 1 shows a lattice structure of a PdO (101) plane, the left side view is a side view, and the right side view is a top view. A large light gray sphere represents a Pd atom and a small dark gray sphere represents an oxygen atom. The outermost surface is characterized by the presence of a Pd atom coordinated by three oxygen atoms (hereinafter referred to as Pd (3f)) and a Pd atom coordinated by four oxygen atoms (hereinafter referred to as Pd (4f)). Is. The side view and the top view of FIG. 1 show each oxygen coordination surface as a square. The former is perpendicular to the surface and the latter is slightly inclined from the surface parallel surface.

The local density of states of each atom is shown in FIG. The dark solid line is the local density of states of the PdO (101) surface atom Pd (3f), and the light solid line is the local density of states of the PdO (101) surface atom Pd (4f). The density of states of the Pd (211) surface atoms of the metal is also shown for comparison. Unoccupied peak appearing on the higher energy side than the Fermi level (E _F) can be mentioned as a characteristic electronic state. As a result of the analysis, it was found that in Pd (3f), _{they belong to the d zz} orbital extending in the vertical direction of the surface, and in Pd (4f), they belong to the _{antibonding orbital due to the d yz orbital between the adjacent oxygen atoms.} It was.

FIG. 3A shows the results of determining the most stable adsorption state of methane. The upper side is a top view and the lower side is a side view. Further, FIG. 3 (b) shows electron orbitals involved in electronic transitions (donations) required for stable adsorption of methane molecules on the surface, and FIG. 3 (c) contributes to methane activity. It shows an electron orbital involved in electronic transition (back donation). Here, methane activity means that hydrogen can be easily taken from methane (hereinafter, the same applies). A methane molecule is one. It was found that it was adsorbed on one Pd (3f) and the CH binding distance on the surface side extended to 1.118 Å and was activated. The CH binding distance on the opposite side to the surface was 1.095 Å in the gas phase. There was almost no change from 1.097 Å of methane. When the electronic state at the time of adsorption was analyzed _{, electrons were donated from the σ-bonded molecular orbital of the methane molecule to the unoccupied d zz} of Pd (3f), and the molecular-surface covalent bond was formed. _{It is found that electrons are donated from the occupied d yz} orbital of Pd (3f) to the CH antibonding σ * orbital on the surface side of methane, and methane activity that weakens the CH bond occurs. It was.

FIG. 4 shows the results of investigating the reaction pathways and activation barriers of the reaction (1) that cuts the activated surface-side C-H bond and the reaction (2) that cuts the deactivated C-H on the opposite side. In the figure, TS1 is the activation barrier of the reaction (1) that cuts the activated CH on the Pd (3f) side among the CHs of the adsorbed methane molecule, and TS2 is the activation barrier of the reaction (2) that cuts the inactivated CH. It is an activation barrier. The activation barrier of the reaction (1) that cuts the activated C-H on the Pd (3f) side was as low as 0.66 eV, and it was found that the reaction easily occurs. This is the most important elementary reaction process of PdO's methane purification catalyst.

Next, FIG. 5 shows the results of investigating the reaction in which sulfur is oxidized _{and approaches the surface of the catalyst as sulfur dioxide SO 2 to be adsorbed and oxidized.} FIG. 5 shows the results of investigating the reaction pathways of three types of mechanisms: the Eley-Rideal mechanism, the Langmuir Hinshelwod mechanism, and the Mars-van Krevelen mechanism. The reaction pathway through the transition state TS-ER of oxygen molecule dissociation adsorption (dotted line in the figure below) is of the Eley-Rideal mechanism, the reaction pathway through the transition state TS-LH (solid line in the figure below) is of the Langmuir Hinshelwod mechanism, oxygen molecule The reaction pathway (broken line in the figure below) that deprives the surface of PdO of oxygen without dissociation is that of the Mars-van Krevelen mechanism. The upper figure of FIG. 5 shows the transition of the atomic position in the reaction path of each mechanism, and the lower figure of FIG. 5 shows the change of the free energy. In the reaction path of each mechanism, the higher free energy is 400 ° C, and the lower free energy is absolute zero.

As a result of comparison, it was found that _{SO 2 that came to the surface of the catalyst was easily oxidized to SO 4} by the Mars-van Krevelen mechanism that uses oxygen on the surface for the reaction. At that time, it was found that in order for the flying SO ₂ to be adsorbed on the surface, a total of three surface Pd atoms, two adjacent Pd (3f) and one adjacent Pd (4f), were required. It was.

Figure 6 shows the results of investigating a thermodynamically stable surface under a mixed gas in which two types were selected from O ₂ , CH ₄ , and SO _2. The vertical axis and the horizontal axis are the chemical potential Δμ of each gas type, and the partial pressure conversion when the temperature is 400 ° C. is also shown. Each region represents a region of chemical potential that is stable when the molecule of the indicated chemical formula is adsorbed. * Indicates that the clean surface on which molecules are not adsorbed is stable. ☆ (white ☆ symbol) indicates the gas partial pressure (P _{CH_4} = 10 ^-3 atm, P _{O_2} = 0.20 atm, P _{SO_2} = 5x10 ^-6 atm) environment under typical exhaust gas experimental conditions. It was found that the clean surface is thermodynamically stable and the methane activity is maintained in the coexistence of methane and oxygen (upper figure of FIG. 6) and in the coexistence of methane and sulfur dioxide (middle figure of FIG. 6). It was.

However, when sulfur dioxide and oxygen coexist, as shown in the lower figure of FIG. 6, _{the state where the surface is covered with SO 4} becomes thermodynamically stable, and the methane activity and the action of the methane purification catalyst are lost. I understood. _{It should be noted that the adsorption of SO 2} and SO ₄ is required to generate the surface state of this sulfur poisoning, which requires two adjacent Pd (3f) and one Pd (4f). I understand more. It has been desired to develop a catalyst that suppresses sulfur poisoning while maintaining methane purification activity in some way by utilizing the difference between the catalytic action of methane purification and the surface Pd atom required for sulfur poisoning action. ..

Japanese Unexamined Patent Publication No. 09-38498 Japanese Unexamined Patent Publication No. 09-229890 Japanese Unexamined Patent Publication No. 10-73556 Japanese Unexamined Patent Publication No. 2000-254505 JP-A-2007-105633

"Sulfation of a PdO (101) methane oxidation catalyst: mechanism revealed by first principles calculations", Ryan L. Arevalo, Susan M. Aspera, Hiroshi Nakanishi, Catal. Sci. Technol., 2019, 9, 232-240, 10 December 2018

The present invention has been made in view of the above-mentioned actual conditions of the prior art, and methane having sulfur toxicity resistance suitable for exhaust gas purification applications capable of suppressing sulfur poisoning without impairing methane purification activity. An object of the present invention is to provide a purification catalyst.

According to the present invention, the following invention is provided in order to solve the above problems.

[1] In PdO in which an atom (3f) having three coordinations of oxygen atoms and an atom (4f) having four coordinations are present on the outermost surface atom of the PdO (101) plane, the atom having three coordinations of oxygen atoms (1) A methane purification catalyst having sulfur toxicity resistance, characterized in that at least a part of 3f) is replaced with an atom M other than Pd.

[2] The resistance according to the first aspect of the invention, wherein M atoms are arranged on the outermost surface in an amount of 1/4 or more and less than 1/2 of the number of metal atoms on the outermost surface. A methane purification catalyst with sulfur toxicity.
[3] The methane purification catalyst according to the invention of the first [1] or [2] above, wherein the M atom is Au or Ag.

According to the present invention, by adopting the above configuration, it is possible to provide a methane purification catalyst having sulfur toxicity resistance suitable for exhaust gas purification applications capable of suppressing sulfur poisoning without impairing methane purification activity. Is possible.

The lattice structure of the PdO (101) plane is shown, the left side view is a side view, and the right side view is a top view. A large light gray sphere represents a Pd atom and a small dark gray sphere represents an oxygen atom. On the outermost surface Pd atom, there are an atom (3f) in which the oxygen atom is coordinated three and an atom (4f) in which the oxygen atom is coordinated, and the oxygen coordination plane of each is shown by a square. The local density of states of each atom is shown. The dark solid line is the local density of the PdO (101) surface atom Pd (3f), and the light solid line is the local density of states of the PdO (101) surface atom Pd (4f). For comparison, the local density of states of the Pd (211) surface atoms of the metal is also shown. (a) shows the result of obtaining the most stable adsorption state of methane on the surface of PdO (101) (upper figure is top view, lower figure is side view), and (b) is an electronic transition that contributes to surface adsorption of methane molecules (b). The electron orbitals involved in donation are shown, and (c) shows the electron orbitals involved in electronic transitions (back donation) that contribute to methane activity. The figure showing the result of examining the reaction pathway and the activation barrier of the reaction (1) that cuts the activated surface side CH and its activation barrier and the reaction (2) that cuts the deactivated CH on the opposite side. is there. It is a figure which shows the result of having investigated the reaction that sulfur is oxidized on the surface of Pd (101) _{and is adsorbed and oxidized on the surface of a catalyst as sulfur dioxide SO 2.} The reaction pathway through the transition state TS-ER of oxygen molecule dissociation adsorption (dotted line in the figure below) is of the Eley-Rideal mechanism, the reaction pathway through the transition state TS-LH (solid line in the figure below) is of the Langmuir Hinshelwod mechanism, oxygen molecule The reaction pathway (broken line in the figure below) that deprives the surface of PdO of oxygen without dissociation is that of the Mars-van Krevelen mechanism. The upper figure of FIG. 5 shows the transition of the atomic position in the reaction path of each mechanism, and the lower figure of FIG. 5 shows the change of the free energy. In the figure showing the stable adsorption state of the surface of Pd (101), the thermodynamically stable surface adsorption state when placed in a mixed gas environment in which two types are selected from _{O 2} , CH ₄ , and SO _{2 are shown.} Mapped on the chemical potential of the gas. The regions marked with _{CH 4} *, O ₂ *, SO ₂ *, and SO _{4 * indicate that the state in which CH 4} , O ₂ , SO ₂ , and SO ₄ are adsorbed is stable, respectively. A single * that does not contain a chemical symbol indicates that the clean surface is a stable region. White stars (☆) indicate the gas partial pressure environment under typical methane purification experimental conditions. It is a figure which shows the substitution model and the stability when the surface atom is replaced by the atom M of another element. The figure on the left shows the state of the outermost surface of PdO (101) that has not been replaced from top to bottom, the state of the outermost surface in which all Pd (4f) have been replaced with M, and the state in which all Pd (3f) have been replaced with M. Indicates the surface condition. The figure on the right shows energy stability. Of the 3f ● and 4f ◆ at each substitution atom M in the graph on the right, the one on the lower side is the stable substitution state. For example, in Au, it is more stable to replace Pd (3f) with Au than Pd (4f). It is a figure which compares and shows the adsorption energy _{of CH 4} and SO ₂ when all surface atoms Pd (4f) atoms are replaced with M. It is a figure which shows the substitution model and the stability when every other surface atom is replaced by the atom M of another element. The upper left figure shows the state of the outermost surface in which every other Pd (4f) is replaced with M, and the lower left figure shows the state of the outermost surface in which every other Pd (3f) is replaced with M. The figure on the right shows energy stability. Of the 3f ● and 4f ◆ at each substitution atom M in the graph on the right, the one on the lower side is the stable substitution state. For example, in Au, it is more stable to replace Pd (3f) with Au than Pd (4f). It is a figure which compares and shows the adsorption energy _{of CH 4} and SO ₂ when every other surface atom Pd (3f) is replaced with atom M. It is a figure which shows the adsorption state _{of CH 4} and SO ₂ when Pd (3f) is replaced with a gold atom every other PdO (101) surface and Pd (3f) on the same surface. The upper figure is a side view of the (101) surface, and the lower figure is a top view of the (101) surface. In the figure when oxygen O ₂ and SO ₂ are present at the same time in FIG. 6, the boundary (broken line) of each surface stable state region when every other surface Pd (3f) is replaced with Au is added. It is a thing. It can be seen that under typical methane purification experimental conditions (☆), the clean surface (*) has changed to a stable surface state. A reaction that cuts the CH on the surface side activated by _{CH 4} adsorbed on the surface of PdO (101) (solid line ●) and the surface (broken line ◆) in which every other Pd (3f) is replaced with a gold atom on the same surface. It is a figure which shows the result of having investigated the activation barrier of. In the latter case, the stable adsorption and stable site of methane was the remaining Pd (3f), which showed methane activity comparable to that of the former. That is, it has a methane purification catalytic action. This is the experimental result of the methane purification catalytic reaction. Metal nanoparticles of Pt (□), Ir (△), Ru (◇) supported on ZrO ₂ and alloy nanoparticles of Au: Pd = 1: 4 (●) and alloy nanoparticles of 1:15 (〇) Shows the temperature dependence of the methane purification rate of the oxidized material. The experimental exhaust gas components are CH ₄ = 0.1%, O ₂ = 10%, SO ₂ = 5ppm, H ₂ O = 3%, and others He.

Hereinafter, the present invention will be described in detail based on the embodiments.

The methane purification catalyst of the present invention targets a gas to be treated containing sulfur compounds such as _{methane and SO 2 and oxygen containing stoichiometry or higher.} Here, the sulfur compound may be contained up to about 10 ppm. Oxygen of stoichiometry or higher means an amount of oxygen required for complete oxidation of reducing components such as hydrocarbons and carbon monoxide in the gas to be treated. The processing temperature of the gas to be processed is about 300 to 500 ° C.

The methane purification catalyst of the present invention is a part of Pd (3f) of PdO in one or more of oxide carriers such as _{Al 2} O ₃ , ZrO ₂ , SiO ₂ , CeO ₂ , Y ₂ O ₃ , and La ₂ O _3. Is supported by _{Au 3f} -PdO in which is replaced with Au. The amount of PdO supported is preferably about 1 to 3% by weight with respect to the carrier. When the amount of PdO supported is within the above range, appropriate methane purification activity can be obtained.

In order to create a catalyst that suppresses sulfur poisoning while maintaining methane purification activity, the following two findings were obtained through the research reported by the present inventors in Non-Patent Document 1 described above.

Condition 1. Methane purification activity requires the Pd (3f) atom on the outermost surface of PdO (101).

Condition 2. Sulfur poisoning requires two adjacent Pd (3f) and one adjacent Pd (4f).

Purifying Methane In order to suppress sulfur poisoning while maintaining the methane purification activity, it is necessary to satisfy condition 1, break condition 2, and form a stable surface. Based on these findings, the present inventors have conducted diligent studies and considered the following two measures.

Measure 1: Replace the Pd (4f) atom on the outermost surface of PdO (101) with an atom of another element to make it inactive with respect to _{SO 2.}

FIG. 7 shows the substitution model and its stability. The upper left figure of FIG. 7 shows the state of the outermost surface of PdO (101) that has not been replaced. The middle figure on the left of FIG. 7 shows the state of the outermost surface in which Pd (4f) is replaced with another element M. The lower left figure of FIG. 7 shows the state of the outermost surface in which Pd (3f) is replaced with another element M. As replacement elements, M = Cr, Mn, Fe, Co, Ni, Cu, Mo, Tc, Ru, Rh, Ag, W, Re, Os, Ir, Pt, Au were investigated. The right figure of FIG. 7 shows the energy stability. The absolute value of ΔE on the vertical axis is independent of stability. In the figure, ○ is the data of 3f and ◇ is the data of 4f. Due to the magnitude relationship between the energies of 3f and 4f, the smaller ΔE is, the more stable it is. For example, in the case of gold (Au), it is more stable to replace it with Pd (3f) rather than Pd (4f). M = Cr, Mn, W, Ir, Pt can be reliably replaced with Pd (4f).

In order to compare the purifying activity and the toxic effect, _{the adsorption energies of CH 4} and SO ₂ when Pd (4f) is replaced with another atom M are shown in FIG. Candidates are that the absolute value of the adsorption energy of CH ₄ increases and the absolute value of the adsorption energy of SO ₂ decreases (the light gray area in the figure). Copper (Cu) and nickel (Ni) are in that range, but these are unsuitable because they are more stable when replaced with Pd (3f). With this method, there was no suitable element M in the study range.

Measure 2: Partially replace the Pd (3f) atom with the atom M of another element to eliminate the adjacent Pd (3f) or reduce the adjacent Pd (3f) and suppress the activity against _{SO 2.}

FIG. 9 shows a substitution model (left figure) and its energy stability. The upper left figure of FIG. 9 shows the state of the outermost surface in which a part of Pd (3f) is replaced with another atom M, and the lower left figure shows the outermost surface in which a part of Pd (4f) is replaced with another atom M. Indicates the state of. As the atom M to be replaced, M = Cr, Mn, Fe, Co, Ni, Cu, Mo, Tc, Ru, Rh, Ag, W, Re, Os, Ir, Pt, Au were investigated. The right figure of FIG. 9 shows the energy stability. As in FIG. 7, the absolute value of ΔE on the vertical axis is independent of stability. In the figure, ○ is the data of 3f and ◇ is the data of 4f. Due to the magnitude relationship between the energies of 3f and 4f, the smaller ΔE is, the more stable it is. In this case, when M = Ni, Cu, Ag, Au, it is more stable to replace Pd (3f) than Pd (4f). _{The adsorption energies of CH 4} and SO ₂ when replaced with Pd (3f) are shown in FIG. It was found that the most favorable relationship is in the case of gold (M = Au). Although inferior to gold, M = Ag can also be expected to have some effect.

FIG. 11 shows the adsorption state _{of CH 4} and SO ₂ when the original PdO (101) surface and every other Pd (3f) are replaced with gold atoms on the same surface. In FIG. 11, the large light gray sphere in the right figure is Pd, the _{large gray sphere in which SO 2} is adsorbed in the upper and lower figures on the right in the right figure is Au, and the small dark gray sphere in which Pd is bound is PdO. of O, the center of small light gray sphere right upper and lower SO ₂ in the right figure is SO ₂ S, O small dark gray balls of the right and below the SO ₂ is SO ₂ in the right figure, the left upper and lower right figure _{The small dark gray sphere in the center of CH 4} adsorbed on Pd (3f) is C, and the small light gray sphere of _{CH 4} adsorbed on Pd (3f) on the left and right of the right figure is H. .. CH ₄ is adsorbed in the same form on both surfaces, whereas in SO ₂ , the form in which O and S are adsorbed toward the top of Pd should be directed by the two O's. It can be seen that the adjacent Pd (3f) is lost, and S is adsorbed to gold and only one O is adsorbed to Pd. This is the reason why the adsorption energy of _{SO 2 has decreased.}

FIG. 12 shows the results of investigating the stable surface adsorption state when O ₂ and SO _{2 are present at the same time.} The white star (☆) symbol indicates the gas partial pressure environment under typical methane purification experimental conditions: T = 400 ° C: Po_ ₂ = 0.20 atm, Pso_ ₂ = ^{5x10 -6} atm. By substituting Au every other Pd (3f), it becomes difficult to oxidize _{, the adsorption stable region of O 2} , SO ₂ , and SO ₄ is reduced, and the clean surface is stable at a typical experimental partial pressure. It turned out. That is, by substituting a part of Pd (3f) with Au, _{it is suppressed that SO 2} is covered with oxidized SO ₄ , and Pd (3f) having methane purification activity remains. The purification reaction of methane is not suppressed. In this case, the number of Pd (3f) is halved, but the _{adjacent Pd (3f), which is originally adsorbed by CH 4} and the reaction is proceeding, has a purification activity due to the interaction between the adsorbed methane. I'm losing. That is, the purification activity is not lower than that of the original PdO surface.

The result of evaluating the purification activity in the remaining Pd (3f) is shown in FIG. The activation barrier from the molecular adsorption state (CH ₄ *) is 0.68 eV, which is 0.02 eV higher, but the difference is extremely low, and the reaction continues at room temperature, rather the CH ₄ molecule. The reaction activity is increased by the increase in the absolute value of the adsorption energy of the state adsorption.

In this system, gold has the property of surface segregation, so if an appropriate amount of Au is mixed with Pd to produce metal nanoparticles by the liquid phase reduction method or the like, Au will precipitate on the surface of the nanoparticles. When it is oxidized to create PdO, Au is more stable to replace with Pd (3f) than with Pd (4f), so Au is automatically placed at the position of Pd (3f). The most suitable number of Au atoms is 1/4 the amount of surface atoms, but if Pd (3f) that is not adjacent to each other remains even more than that, sulfur poisoning does not deactivate the methane purification activity. Therefore, the number of Au atoms is preferably 1/4 or more and less than 1/2 of the surface atomic number.

For example, in the case of metal nanoparticles having a diameter of 3.7 nm, 6.3% to 12.6% of Au atoms may be mixed with respect to the total number of atoms. It corresponds to Au: Pd = 1:15 to 1: 7 in atomic number ratio and 1: 8 to 1: 4 in weight ratio.

Further, the methane purification catalyst can be used after being calcined under the same temperature and time conditions as a general PdO-supported catalyst.

In the liquid phase reduction method, 1 Au and Pt in an atomic ratio: Model exhaust gas (CH ₄ = 0.1% was used to create the mixing nano alloys 4 and 1:15 catalyst supported on ZrO _2, O ₂ The methane purification activity was verified experimentally at = 10%, SO ₂ = 5ppm, H ₂ O = 3%, and others He). FIG. 14 shows the temperature dependence of the obtained methane purification rate. For comparison, the results of a catalyst in which Pt, Ir, and Ru were similarly _{supported on ZrO 2 are shown.} From Pt and Ru, it was shown that the methane purification rate was as high as Ir from low temperature, and that the methane purification catalytic action was _{also in the presence of SO 2} and O _2.

Claims (3)

In PdO in which an atom (3f) having three coordinations of oxygen atoms and an atom (4f) having four coordinations are present on the outermost surface atom of the PdO (101) plane, the atom (3f) having three coordinations of oxygen atoms A sulfur-resistant methane purification catalyst characterized in that at least a part is substituted with an atom M other than a Pd atom. The sulfur toxicity resistance according to claim 1, wherein M atoms are arranged on the outermost surface in an amount of 1/4 or more and less than 1/2 of the number of metal atoms on the outermost surface. Methane purification catalyst with. The sulfur-resistant methane purification catalyst according to claim 1 or 2, wherein the M atom is Au or Ag.

Images