JP6832010B2 - Methane activation catalyst with caulking resistance - Google Patents

Description

The present invention relates to a methane activation catalyst having a caulking resistance.

At present, the method of extracting hydrogen from hydrocarbons such as methane, which is the main component of natural gas, by steam reforming using a metal catalyst is mainly used for industrial production of hydrogen. When hydrogen is produced using this method, carbon dioxide is emitted as a by-product, but since it is intensively emitted at the fuel reforming plant, carbon dioxide can be easily recovered and there are few problems in global warming. Nickel is mainly used industrially as a steam reforming catalyst. When nickel is used as a catalyst, the carbon contained in the hydrocarbon is not oxidized well, and a "caulking phenomenon" in which the surface of the catalyst is covered with the precipitated carbon often appears. Since this caulking inactivates the catalytic action of nickel, there is a problem that the life of the catalyst is significantly shortened.

On the other hand, ruthenium has been found as a catalyst having a caulking resistance. Ruthenium is a precious metal of the platinum group element and is more expensive than nickel (Ruthenium 190 yen / g (March 2017), nickel ~ 1 yen / g). Ruthenium becomes _{RuO 4 when oxidized.} RuO ₄ is highly volatile (melting point 40 ° C) and has the problem that ruthenium evaporates easily.

J. R. Rostrup-Nielsen, J. Catalysis 85 (1984) 31-43, Page 32, Table 1 H. S. Bengaard, et al., J. Catalysis 209 (2002) 365384, Page 373, Section 5.2.1

The present invention has been made in view of the above problems of the prior art, and the activation of methane in the catalyst such as steam reforming (promotion of the chemical reaction accompanied by the cleavage of the carbon-hydrogen bond) remains. As it is, it is an object to provide a methane activation catalyst having a coking-resistant action at a low cost.

The present inventors have investigated and studied the reaction mechanism of the catalytic action of the steam reforming catalyst from the atomic and electronic levels. As a result, it was found that the metal surface flat on the atomic scale had no catalytic activity, and the active site was a stepped portion on the step surface shown in FIG. FIG. 1 shows methane molecules adsorbed on the surface of the Ni (221) step. As a result of studying the difference in properties at this active point between nickel, which has a remarkable coking action, and ruthenium, which has a weak coking action, the chemical species (CH _{x) from methane (x = 4) to isolated carbon (x = 0) are formed.} : It was found that in x = 1 to 4), the adsorption state of the carbon atom (C) in the final stage (x = 0) was significantly different. _{FIG. 2 shows the adsorption energy ΔE (eV) when CH 4-n} (n = 0 to 4) and n Hs are co-adsorbed on the step surface of nickel and ruthenium. In nickel, carbon forms a total of five coordination bonds with the four nickel atoms on the outermost surface and one nickel atom on the sub-surface, whereas in ruthenium, it bonds only with the four ruthenium atoms on the surface. I understood it. FIG. 3 shows the energy density of states of electrons when carbon atoms are adsorbed on the step surface of nickel and ruthenium. The spatial distribution of the peak electrons (2 peaks in (a) and 1 peak in (b)) surrounded by a dotted line is shown in the illustration. This electron is involved in (a) C-Ni or (b) C-Ru bond. Of the two peaks (a) surrounded by the dotted line, the lower peak on the high energy side is involved in the bond between carbon and the nickel atom on the sub-surface (shown in the spatial distribution of electrons indicated by the arrows). ). By bonding the sub-surface atom and the carbon atom, carbon exists extremely stably on the nickel surface, and a caulking action occurs.

It has long been speculated that the step step is the active point (for example, Non-Patent Documents 1 and 2), and as a means to prevent caulking, other elements (sulfur in Non-Patent Documents 1 and 2) are added to the step step. Although attempts have been made to adsorb and suppress caulking, the steam reforming catalytic action itself is deactivated at the same time as caulking. Therefore, in order to suppress only caulking while leaving the water vapor catalytic action on the surface of the nickel step, the present inventors have left the nickel on the outermost surface as it is and nickel at a position where it is bonded to carbon on the sub surface (directly below the step). (Sub-surface atom of) was replaced with another element M (FIG. 4 (a)), and the present invention was completed in consideration of suppressing the fifth bond.

That is, according to the present invention, in order to solve the above problems, first, in a nickel catalyst having a step surface in which the step step portion is a catalytic activity point, the steam catalytic action remains as it is on the outermost surface of the nickel atomic layer. A methane activation catalyst having a coking-resistant action, which is characterized by suppressing the coking phenomenon by replacing the nickel atom at least at the position of bonding with the carbon atom on the sub-surface with the atom of another element M. Is provided.

Secondly, in the first invention, there is provided a methane activation catalyst having a caulking-resistant action, which comprises embedding an atomic layer of another element M in a sub-surface of a step.

Thirdly, in the first invention, there is provided a methane activation catalyst having a caulking-resistant action, which is characterized by a structure in which an atom at a sub-surface position of a step is another element M.

Fourth, there is provided a methane activation catalyst having a caulking-resistant effect, characterized in that the nickel atomic layer on the sub-surface of the step is replaced with an atomic layer of another element M.

Fifth, in the fourth invention, there is provided a methane activation catalyst having a caulking-resistant action, which is characterized by surface segregation of nickel on the surface of another element M having a step step portion.

Sixth, in any one of the first to fifth inventions, the element M is selected from the group consisting of Ir, Pt, Au, Fe, Co, Tc, Ru, Rh, Pd, Ag and W1. A catalyst for activating methane, which is a seed and has a caulking resistance, is provided.

According to the present invention, it is possible to suppress the caulking phenomenon while retaining the methane activating action, and to provide an inexpensive methane activation catalyst having a caulking resistant action.

It is a figure which shows the methane molecule adsorbed on the surface of a Ni (221) step. It is a figure which shows the adsorption energy (n = 0-4) when _{CH 4-n} and n H are co-adsorbed on the step surface of Ni (left figure) and Ru (right figure). It is a figure which shows the energy density of states of an electron when a carbon atom is adsorbed on the step surface of Ni ((a) in a figure) and Ru ((b) in a figure). It is a figure which shows three kinds of structures which replace the sub-surface atom of the Ni step surface with the atom of element M. It is a figure which shows the adsorption energy of the carbon atom in the case where the sub-surface Ni atom immediately below a step is replaced with the atom of element M (FIG. 4 (a)). M = Ni represents the case of no substitution. It is a figure which shows the energy density of states of an electron when a carbon atom is adsorbed on the surface (M = Au in the structure shown in FIG. 4A) in which the Ni atom of the sub-surface immediately below the step is replaced with the atom of Au. _{Adsorption energy (n = 0 ~) when CH 4-n} and n H are co-adsorbed when the Ni step surface (left figure) and the sub-surface atom immediately below the step are replaced with Au atoms (right figure). It is a figure which shows 4). It is a figure which shows the adsorption energy of a carbon atom when the surface second layer, that is, the sub-surface Ni atomic layer is replaced with the atomic layer of element M (the structure shown in FIG. 4B). M = Ni represents the case where no replacement is performed.

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

In the methane activation catalyst of the present invention, nickel on the outermost surface remains as it is, and nickel (sub-surface atom immediately below the step) at least at the position of bonding with carbon on the sub-surface is replaced with another element M (FIG. 4 (a)). )), It is characterized in that the fifth bond is suppressed.

By doing so, the result shown in FIG. 5 was obtained. FIG. 5 is a diagram showing the adsorption energy of carbon atoms when the sub-surface Ni atom immediately below the step is replaced with an atom of another element M (in the structure shown in FIG. 4A). -8.324 Below the eV straight line (Stronger) is when the bond with C is stronger than before replacement (M = Ni), and above (Weaker) is when the bond with C is weaker than before replacement. .. It is M = Ir, Pt, Au that the bond is completely quadruped. As an example of this case, FIG. 6 shows the electron energy density of states when the Ni atom on the sub-surface immediately below the step is replaced with an Au atom (in the structure shown in FIG. 4A). The binding force with carbon is weaker than when it is not substituted (M = Ni). When M = Fe, Co, Tc, Ru, Rh, Pd, Ag, W, it is also effective in suppressing caulking. It has also been investigated that the activation catalytic action of methane is not deactivated as in the pure Ni step surface. _{In FIG. 7, CH 4-n} (n = 0 to 4) and n Hs are co-adsorbed when the Ni step surface (left figure) and the sub-surface atom immediately below the step are replaced with Au (right figure). The adsorption energy of the case is shown.

In the methane activation catalyst of the present invention, as a substitution structure similar to that in FIG. 4A, an atom of element M is embedded in the step sub surface as an impurity, and an atom layer of element M is replaced with an atomic layer of element M on the sub surface. Those (FIG. 4 (b)) and those in which Ni is surface segregated on the step surface composed of the atoms of the element M (FIG. 4 (c)) have the same coking suppressing effect. Further, in the methane activation catalyst of the present invention, the same caulking inhibitory effect is obtained even when nanoparticles having a core-shell structure, the core of which is an atomic layer composed of element M and the outermost shell (one layer) of which is a Ni atomic layer. There is.

FIG. 8 shows the carbon adsorption energy when the second surface layer, that is, the sub-surface atomic layer, which is shown in FIG. 4 (b), is replaced with the atomic layer of element M. In the figure, when the adsorption energy (-8.324 eV) is lower than M = Ni (Stronger), the bond with C is stronger than before the replacement, and the upper (Weaker) is the bond with C than before the replacement. This is the case when it becomes weak. It can be seen that W has a more caulking-suppressing effect in this sub-surface layer substitution structure.

The methane activation catalyst according to the present invention can be produced by a liquid phase reduction method. That is, metal nanoparticles are produced by reducing a solution containing metal ions, and by controlling the composition of Ni and M during the growth process of the particles, FIGS. 4 (a), (b), and (c) Structure can be made.

As for the structure of FIG. 4 (a), if the amount of M is reduced in the manufacturing process of FIG. 4 (b) described later, M is stably adsorbed on the step from the terrace, so only the step is filled with M. There is a surface. If Ni is further grown on it, the structure shown in FIG. 4 (a) can be manufactured.

The structure of FIG. 4 (b) can be produced by forming nanoparticles with Ni, growing the M phase on the nanoparticles, and further growing the Ni phase.

In the structure of FIG. 4 (c), nanoparticles are first made from M alone, and then a solution containing Ni ions is sprayed or immersed in a solution containing Ni ions to reduce the particles, or the surface is covered with Ni. It can be produced by mixing M and M, mixing nanoparticles, raising the temperature, and segregating the surface.

In the case of embedding as an impurity, it can be manufactured by simply mixing Ni with a small amount of M and alloying.

In addition to the liquid phase reduction method, it can be manufactured by simple mechanical alloying, mechanical milling, or the like.

The catalyst of the present invention has been described above as a catalyst for extracting hydrogen by steam reforming, but the catalyst of the present invention can also be used as a catalyst for a chemical reaction involving carbon-hydrogen bond cleavage such as dry reforming and partial oxidation. It is useful. Therefore, in addition to hydrogen as a product, it is also useful as a catalyst for producing high value-added chemical products such as methanol.

Claims (6)

In a nickel catalyst having a step surface in which the step step portion is a catalytic active site, the nickel atom layer on the outermost surface is at a position where it is bonded to a carbon atom at least on the sub surface while retaining the methane activation catalytic action. A methane activation catalyst having a coking-resistant action, characterized in that the coking phenomenon is suppressed by replacing the nickel atom with another element M. The methane activation catalyst having a caulking-resistant action according to claim 1, wherein an atom of another element M is embedded as an impurity in the sub-surface of the step. The methane activation catalyst having a caulking-resistant action according to claim 1, wherein the atom at the sub-surface position of the step is another element M. The methane activation catalyst having a caulking-resistant action according to claim 1, wherein the nickel atomic layer on the sub-surface of the step is replaced with an atomic layer of another element M. The methane activation catalyst having a caulking-resistant action according to claim 1, wherein nickel is surface-segregated on the surface of another element M having a step step portion. The caulking resistance according to any one of claims 1 to 4, wherein one or more of the elements M are selected from the group consisting of Ir, Pt, Au, Fe, Co, Tc, Ru, Rh, Pd, Ag and W. Activating catalyst for methane.