WO2015097550A1

WO2015097550A1 - Sulfur trioxide decomposition catalyst, sulfur dioxide production process using sulfur trioxide decomposition catalyst, hydrogen production process using sulfur dioxide production process, and process for producing sulfur trioxide decomposition catalyst

Info

Publication number: WO2015097550A1
Application number: PCT/IB2014/002966
Authority: WO
Inventors: Shinichi Takeshima; Masato Machida
Original assignee: Toyota Jidosha Kabushiki Kaisha; National University Corporation Kumamoto University
Priority date: 2013-12-24
Filing date: 2014-12-23
Publication date: 2015-07-02
Also published as: JP2015120116A

Abstract

A sulfur trioxide decomposition catalyst includes: a composite oxide containing an alkali metal and vanadium; and a catalyst support on which the composite oxide is supported. In addition, a process for producing a sulfur trioxide decomposition catalyst includes: impregnating the catalyst support with one solution of a solution containing an alkali metal compound and a solution containing a vanadium compound, and drying and preliminarily calcining the catalyst support; and impregnating the preliminarily calcined catalyst support with the other solution of the solution containing the alkali metal compound and the solution containing the vanadium compound, and drying and calcining the catalyst support.

Description

SULFUR TRIOXIDE DECOMPOSITION CATALYST, SULFUR DIOXIDE PRODUCTION PROCESS USING SULFUR TRIOXIDE DECOMPOSITION CATALYST, HYDROGEN PRODUCTION PROCESS USING SULFUR DIOXIDE PRODUCTION PROCESS, AND PROCESS FOR PRODUCING SULFUR TRIOXIDE

DECOMPOSITION CATALYST

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to a sulfur trioxide decomposition catalyst, a sulfur dioxide production process using the sulfur trioxide decomposition catalyst, a hydrogen production process using the sulfur dioxide production process, and a process for producing a sulfur trioxide decomposition catalyst. 2. Description of Related Art

[0002] Hydrogen is attracting attention as a clean fuel because carbon dioxide is not produced during combustion. However, industrial hydrogen production depends on fossil fuel, and carbon dioxide is emitted in its production process. Accordingly, even if hydrogen is used as fuel, problems such as depletion of fossil fuel resources and global warming caused by carbon dioxide may not be solved.

[0003] In this connection, use of solar thermal energy or nuclear thermal energy has been proposed as a method for producing hydrogen without using fossil fuel (for example, refer to Japanese Patent Application Publication No. 2007-218604 (JP 2007-218604 A), A. Giaconia, et al., "Hydrogen/methanol production by sulfur-iodine thermochemical cycle powered by combined solar/fossil energy", and International Journal of Hydrogen Energy 32 (2007), 469-481).

[0004] As a method for producing hydrogen from water using thermal energy, there has been proposed a method called an S-I (sulfur-iodine) cycle process represented by the following formulae (1) to (3): (1) H₂S0₄ (liquid)→H₂0 (gas)+S0₂ (gas)+l/20₂ (gas)

(reaction temperature=about 950°C, ΔΗ= 188.8 kJ/mol-H₂)

(2) I₂ (liquid)+S0₂ (gas)+2H₂0 (liquid)→2HI (liquid)+H₂S0₄ (liquid)

(reaction temperature=about 130°C, ΔΗ=-31.8 kJ/mol-H₂)

(3) 2HI (liquid)→H₂ (gas)+I₂ (gas)

(reaction temperature=about 400°C, ΔΗ=146.3 kJ/mol-H₂)

[0005] The total reaction of the S-I (sulfur-iodine) cycle process represented by the formulae (1) to (3) is as follows:

H₂0→H₂+l/20₂

(ΔΗ=286.5 kJ/mol-H₂ (higher heating value basis))

(ΔΗ=241.5 kJ/mol-H₂ (lower heating value basis))

[0006] Here, the reaction of the formula (1) can be divided into two elementary reactions of the following formulae (1-1) and (1-2):

(1-1) H₂S0₄ (liquid)→H₂0 (gas)+S0₃ (gas)

(reaction temperature=about 300°C, ΔΗ=90.9 kJ/mol-H₂)

(1-2) S0₃ (gas)→S0₂ (gas)+l/20₂ (gas)

(reaction temperature=about 950°C, ΔΗ=97.9 kJ/mol-H₂)

[0007] That is, when hydrogen is produced by the S-I cycle process, the highest temperature is required in a sulfur trioxide (SO₃) decomposition reaction of the formula (1-2). However, in the related art, it is difficult to obtain such a high temperature required in the sulfur trioxide decomposition reaction.

[0008] Therefore, in order to decrease the temperature required in the sulfur trioxide decomposition reaction, use of a platinum catalyst has been proposed. The platinum catalyst may have high catalytic activity at the start of use; however, when the platinum catalyst is used at a temperature of, in particular, 650°C or higher, the platinum is oxidized by oxygen produced in the sulfur trioxide decomposition reaction as shown in the formula (1-2), and thereby sintering occurs relatively easily. When such sintering occurs, platinum particles are coarsened, and thus the initial activity of the platinum catalyst becomes lost. In addition, since the platinum catalyst is relatively expensive, its use on an industrial scale is difficult from the viewpoint of the material cost.

[0009] Japanese Patent Application Publication No. 2012-148268 (JP

2012- 148268 A) describes a sulfur trioxide decomposition catalyst in which a composite oxide of vanadium and at least one metal selected from the group consisting of transition metal and rare earth elements is supported on a support. Specifically, JP 2012-148268 A describes a catalyst in which, for example, a copper-vanadium composite oxide or a cerium-vanadium composite oxide is supported on a porous silica support. Likewise, Japanese Patent Application Publication No. 2013-111542 (JP 2013-111542 A) describes a sulfur trioxide decomposition catalyst in which a composite oxide of copper and vanadium is supported on a porous silica support. In addition, JP 2012-148268 A and JP

2013- 111542 A describe that, according to the sulfur trioxide decomposition catalysts thereof, the temperature required in the sulfur trioxide decomposition reaction can be decreased.

[0010] M. Machida et al., "Macroporous Supported Cu-V Oxide as a Promising Substitute of the Pt Catalyst for Sulfuric Acid Decomposition in Solar Thermochemical Hydrogen Production", Chemistry of Materials 2012, 24, 557-561 describes a catalyst in which copper pyrovanadate (Cu₂V₂0₇) is supported on a porous silica support as a substitute of the platinum catalyst. M. Machida et al., "Macroporous Supported Cu-V Oxide as a Promising Substitute of the Pt Catalyst for Sulfuric Acid Decomposition in Solar Thermochemical Hydrogen Production", Chemistry of Materials 2012, 24, 557-561 describes that: a uniform thin film of Cu₂V₂0₇ is formed on a pore and an outer surface of the porous silica support by thermal aging at 800°C; and, according to such a catalyst, high catalytic activity can be achieved in the sulfur trioxide decomposition reaction.

[0011] As the method for producing hydrogen using thermal energy, in addition to the S-I cycle process, a Westinghouse cycle process, an Ispra-Mark 13 cycle process, a Los Alamos science laboratory cycle process, and the like are generally known. However, in these processes, it is necessary to decompose sulfur trioxide into sulfur dioxide and oxygen as in the S-I cycle process.

[0012] JP 2012-148268 A describes that, for example, in a sulfur trioxide decomposition catalyst in which a copper-vanadium composite oxide or a cerium-vanadium composite oxide is supported on a porous silica support, the sulfur trioxide decomposition reaction can be performed at a temperature of 700°C or 650°C. However, in the sulfur trioxide decomposition catalyst, the sulfur trioxide decomposition reaction may not be sufficiently performed at a temperature lower than 700°C or 650°C under conditions of a compact reactor and a high space velocity. Accordingly, in the sulfur trioxide decomposition catalyst described in JP 2012-148268 A, there is still room for improvement regarding its catalytic activity, particularly, its catalytic activity at a low temperature.

[0013] In the sulfur trioxide decomposition catalyst described in JP 2013-111542

A, in order to obtain high catalytic activity, it is necessary that the catalyst be calcined at a high temperature of 700°C or higher in a catalyst preparation step or be temporarily used at a high temperature of about 800°C. In addition, in the sulfur trioxide decomposition catalyst described in JP 2013-111542 A, even if such an activation treatment is performed, sufficient catalytic activity may not be obtained in the sulfur trioxide decomposition reaction, particularly, at a low temperature of about 600°C.

[0014] In the catalyst described in M. Machida et al., "Macroporous Supported Cu-V Oxide as a Promising Substitute of the Pt Catalyst for Sulfuric Acid Decomposition in Solar Thermochemical Hydrogen Production", Chemistry of Materials 2012, 24, 557-561, in order to obtain high catalytic activity, it is necessary that thermal aging be performed at a high temperature of about 800°C in a catalyst preparation step. In addition, in the catalyst described in M. Machida et al., "Macroporous Supported Cu-V Oxide as a Promising Substitute of the Pt Catalyst for Sulfuric Acid Decomposition in Solar Thermochemical Hydrogen Production", Chemistry of Materials 2012, 24, 557-561, even if thermal aging is performed at such a high temperature, sufficient catalytic activity may not be obtained in the sulfur trioxide decomposition reaction, particularly, at a low temperature of about 600°C.

SUMMARY OF THE INVENTION [0015] The invention provides a sulfur trioxide decomposition catalyst with catalytic activity, particularly, improved catalytic activity at a low temperature as compared to a catalyst of the related art. In addition, the invention has been made to provide a sulfur dioxide production process using the sulfur trioxide decomposition catalyst, a hydrogen production process using the sulfur dioxide production process, and a process for producing a sulfur trioxide decomposition catalyst.

[0016] A sulfur trioxide decomposition catalyst according to a first aspect of the invention includes: a composite oxide containing an alkali metal and vanadium; and a catalyst support on which the composite oxide is supported.

[0017] According to the first aspect of the invention, the composite oxide in which vanadium and the alkali metal are combined is supported on the catalyst support. As a result, the catalytic activity of the obtained sulfur trioxide decomposition catalyst can be improved under a condition of a high space velocity (SV) as compared to a catalyst of the related art. Examples of the catalyst of the related art include a catalyst in which a composite oxide of copper and vanadium is supported on a catalyst support. In addition, according to the first aspect of the invention, sulfur trioxide (S0₃) decomposition activity at a temperature of 600°C or lower can be improved.

[0018] In the first aspect of the invention, the alkali metal may be selected from the group consisting of sodium, potassium, rubidium, cesium, and a combination of at least two of sodium, potassium, rubidium, and cesium. In the first aspect of the invention, the alkali metal may be selected from the group consisting of potassium, cesium, and a combination of potassium and cesium. In the first aspect of the invention, an atom ratio of the alkali metal to vanadium in the composite oxide may be 1:1 to 1:8. In the first aspect of the invention, the composite oxide may contain an alkali metal vanadate represented by the formula A_1-xVO_3-o.5_X in which A represents the alkali metal and 0≤x<l. In the first aspect of the invention, the composite oxide may contain an alkali metal metavanadate represented by the formula AVO₃ in which A represents the alkali metal. In the first aspect of the invention, the alkali metal may be cesium, and the composite oxide may contain at least one of CsV0₃, Cs2V40n, and CsV₃08. In the first aspect of the invention, the composite oxide may contain a combination of at least two of CSVO3, Cs₂V₄0.11, and CsV₃08. In the first aspect of the invention, the catalyst support may be selected from the group consisting of silica, alumina, zirconia, titania, and a combination of at least two of silica, alumina, zirconia, and titania. In the first aspect of the invention, the catalyst support may contain at least one of a mesopore and a macropore. In the first aspect of the invention, the catalyst support may be silica.

[0019] A sulfur dioxide production process according to a second aspect of the invention includes: performing a decomposition reaction in which sulfur trioxide is decomposed into sulfur dioxide and oxygen using the sulfur trioxide decomposition catalyst according to the first aspect of the invention.

[0020] In the second aspect of the invention, the decomposition reaction may be performed at a temperature of 600°C or lower.

[0021] A hydrogen production process according to a third aspect of the invention includes: decomposing sulfuric acid into water, sulfur dioxide, and oxygen in a reaction represented by the following formula (XI) containing the following formulae (Xl-1) and (Xl-2) as elementary reactions. The elementary reaction of the formula (Xl-2) is performed using the sulfur dioxide production process according to the second aspect of the invention:

(XI) H₂S0₄→H₂0+S0₂+l/20₂

(Xl-1) H₂S0₄→H₂0+S0₃

(Xl-2) S0₃→S0₂+l/20₂

[0022] In the third aspect of the invention, one of an S-I cycle process, a Westinghouse cycle process, an Ispra-Mark 13 cycle process, and a Los Alamos science laboratory cycle process may be performed.

[0023] A process according to a fourth aspect of the invention is a process of producing a sulfur trioxide decomposition catalyst in which a composite oxide containing an alkali metal and vanadium is supported on a catalyst support. The process includes: impregnating the catalyst support with one solution of a solution containing an alkali metal compound and a solution containing a vanadium compound, and drying and preliminarily calcining the catalyst support; and impregnating the preliminarily calcined catalyst support with the other solution of the solution containing the alkali metal compound and the solution containing the vanadium compound, and drying and calcining the catalyst support.

[0024] A process according to a fifth aspect of the invention is a process of producing a sulfur trioxide decomposition catalyst in which a composite oxide containing an alkali metal and vanadium is supported on a catalyst support. The process includes impregnating the catalyst support with a solution containing an alkali metal compound and a vanadium compound, and drying and calcining the catalyst support.

[0025] According to the fourth and fifth aspects of the invention, a sulfur trioxide decomposition catalyst with improved sulfur trioxide (S0₃) decomposition activity at such a low temperature can be produced without the necessity of being calcined or used at a high temperature of 700° C or higher.

[0026] In the fourth and fifth aspects of the invention, the catalyst support may be calcined at a temperature of 400°C to 650°C. In the fourth and fifth aspects of the invention, the catalyst support may be calcined at a temperature of 500°C to 600°C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1 is a conceptual diagram illustrating a mechanism of a sulfur trioxide decomposition reaction using a sulfur trioxide decomposition catalyst according to an embodiment of the invention;

FIG 2 is a phase diagram of a CS2O-V2O5 system;

FIG 3 is a diagram illustrating a fixed bed flow reactor which was used for evaluating the activities of sulfur trioxide decomposition catalysts of Examples 1 to 4 and Comparative Examples 1 to 3;

FIG 4 is a diagram illustrating X-ray diffraction patterns regarding the sulfur trioxide decomposition catalysts of Example 4 and Comparative Examples 1 to 3; and FIG 5 is a diagram illustrating the results of analyzing the sulfur trioxide decomposition catalyst of Example 4 by Raman spectroscopy.

DETAILED DESCRIPTION OF EMBODIMENTS

[0028] <Sulfur Trioxide Decomposition Catalyst>

A sulfur trioxide decomposition catalyst according to an embodiment of the invention has a characteristic in that a composite oxide containing an alkali metal and vanadium is supported on a catalyst support.

[0029] As described above, when a platinum catalyst is used in the sulfur trioxide decomposition reaction, platinum is oxidized by oxygen produced in the sulfur trioxide decomposition reaction, and thereby sintering occurs relatively easily. Accordingly, the platinum catalyst deteriorates at a relatively early stage, and it is difficult to stably use the platinum catalyst over a long period of time.

[0030] The present inventors have investigated a composite oxide in which vanadium and an alkali metal are combined. As a result, the present inventors have found that, by causing the composite oxide to be supported on a catalyst support, the catalytic activity of the obtained sulfur trioxide decomposition catalyst, in particular, the sulfur trioxide (S0₃) decomposition activity at a low temperature of about 600°C or lower can be improved under a condition of a high space velocity (SV) as compared to a catalyst of the related art. Examples of the catalyst of the related art include catalysts described in JP 2012-148268 A and JP 2013-111542 A in which a composite oxide of copper and vanadium is supported on a catalyst support.

[0031] As described above, when hydrogen is produced by the S-I (sulfur-iodine) cycle process, generally, a temperature of about 1000°C is required to decompose sulfur trioxide into sulfur dioxide and oxygen. Accordingly, in the related art, regarding the S-I cycle process, there has been studied a method of efficiently producing hydrogen using heat of such a high temperature which is extracted from a high-temperature gas-cooled reactor (HTGR).

[0032] In the related art, it is generally considered the use of solar heat is difficult in the S-I cycle process. For example, as a collector for obtaining solar thermal energy, a parabolic dish-type collector, a solar tower-type collector, and a parabolic trough-type collector are generally known. However, among these collectors, in the parabolic trough-type collector which is suitable for a large-scale plant due to its relatively simple structure and low cost, the collection of solar energy at a high temperature of about 700°C or higher, in particular, about 1,000°C is technically difficult and is financially unrealistic in view of balance between the collection of solar energy and the dissipation of energy due to radiation.

[0033] In the sulfur trioxide decomposition catalyst according to the embodiment, the temperature required in the sulfur trioxide decomposition reaction is decreased, and thus the sulfur trioxide decomposition reaction can be performed at a low temperature of, for example, about 600°C or lower under a high SV condition. Accordingly, use of the sulfur trioxide decomposition catalyst according to the embodiment has an industrially important value.

[0034] [Alkali Metal]

In the embodiment, as the alkali metal, for example, sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and combinations thereof are preferably used. Potassium (K), cesium (Cs), and a combination thereof are more preferably used, and cesium (Cs) is most preferably used.

[0035] Although not intended to be limited to a particular theory, it is considered that the sulfur trioxide decomposition reaction using the sulfur trioxide decomposition catalyst according to the embodiment is performed by, for example, a reaction mechanism described below.

[0036] FIG 1 is a conceptual diagram illustrating the mechanism of the sulfur trioxide decomposition reaction using the sulfur trioxide decomposition catalyst according to the embodiment. For easy understanding, a composite oxide (that is, AVO₃) in which an atom ratio of an alkali metal (A) to vanadium, i.e., an atom ratio of the alkali metalrvanadium, is 1:1 is used to describe FIG 1.

[0037] First, it is generally known that an alkali metal is an element with which a peroxide is relatively easily formed. Accordingly, in the composite oxide (AVO3) in which the alkali metal is combined with vanadium, the following is considered. At least a part of vanadium is present in AVO3 on the catalyst surface not in the form of VO3 but in the form of V0₄, that is, a peroxide on which gas-phase oxygen is adsorbed (refer to (I) of FIG 1) due to the action of the alkali metal (A). Next, since this adsorbed oxygen has higher reactivity than oxygen of the other vanadium-oxygen (V-O) bonds, gas-phase sulfur trioxide (SO3) is further adsorbed on the adsorbed oxygen as illustrated in (Π) of FIG 1, and then sulfur dioxide (S0₂) is separated therefrom (refer to (III) of FIG 1). Here, since oxygen remaining on V0₄ illustrated in (III) of FIG 1 is highly reactive, gas-phase sulfur trioxide is easily adsorbed on the oxygen as illustrated in (IV) of FIG 1, and lastly sulfur dioxide (S0₂) and an oxygen molecule (0₂) are separated therefrom (refer to (V) of FIG

1).

[0038] In the sulfur trioxide decomposition catalyst according to the embodiment, vanadium and the alkali metal are combined. As a result, it is considered that the adsorptivity and reactivity of the composite oxide to sulfur trioxide are high due to, for example, the oxygen adsorption and the oxygen addition mechanism described above. As a result, in the sulfur trioxide decomposition catalyst according to the embodiment, it is considered that, as compared to a catalyst of the related art, high sulfur trioxide (SO₃) decomposition activity can be achieved under a condition of a high space velocity (SV) in a relatively low temperature range, for example, in a temperature range of 600°C or lower.

[0039] Further, although not intended to be limited to a specific theory, it is considered that, in the sulfur trioxide decomposition catalyst according to the embodiment, the composite oxide containing the alkali metal and vanadium is present on the catalyst support as a uniform thin layer or is supported on the catalyst support as a microcrystal with high dispersity.

[0040] Referring to FIG 2, a case where cesium (Cs) is used as the alkali metal will be described below in more detail. FIG 2 is a phase diagram of a CS2O-V2O5 system. As illustrated in FIG 2, a composite oxide in which an atom ratio (Cs:V) of cesium to vanadium is 1:1, that is, cesium metavanadate (CSVO3) has a melting point of 642°C. However, as can be seen from FIG 2, for example, when this composition is slightly shifted to a vanadium (V)-rich side, the melting point thereof significantly decreases to about 400°C. Here, in an actual composition, there is a concentration distribution. Therefore, it is considered that, even if the atom ratio (Cs:V) of cesium to vanadium is 1:1 overall, a V-rich region is partially present, and thus a region where the melting point decreases is partially present.

[0041] Accordingly, it is considered that, in a calcining operation at a relatively low temperature of, for example, about 400°C to 600°C during the catalyst preparation, a composite oxide precursor containing cesium and vanadium forms a composite oxide on the catalyst support as a uniform thin layer while being partially melted or is supported on the catalyst support as a microcrystalline composite oxide with high dispersity. In this way, in the sulfur trioxide decomposition catalyst according to the embodiment, a highly active catalyst layer formed of the uniform this layer or the microcrystal is present. Therefore, it is considered that, as compared to a catalyst of the related art, high sulfur trioxide (S0₃) decomposition activity can be achieved under a condition of a high space velocity (SV) in a relatively low temperature range, for example, in a temperature range of 600°C or lower.

[0042] Particularly when a metal oxide such as silica is used for the catalyst support, a part of cesium (the same will be applied to other alkali metals) reacts with the silica. Therefore, the composition of a composite oxide precursor is partially in a V-rich state during the catalyst preparation. Accordingly, it is considered that, in a calcining operation at a relatively low temperature range of about 600°C or lower, the composite oxide precursor containing cesium and vanadium forms a composite oxide on the silica as a uniform thin layer while being partially melted or is supported on the silica as a microcrystalline composite oxide with high dispersity.

[0043] It is considered that the high S0₃ decomposition activity at a low temperature of the sulfur trioxide decomposition catalyst according to the embodiment is obtained, as described above, by the improvement of the adsorptivity and reactivity of the composite oxide to sulfur trioxide due to the addition of the alkali metal or by the formation of a catalyst layer having high activity based on a decrease in the melting point of an alkali metal vanadate due to the combination of the alkali metal and vanadium. Alternatively, it is considered that the high SO3 decomposition activity at a low temperature of the sulfur trioxide decomposition catalyst according to the embodiment is obtained by the improvement of the adsorptivity and reactivity and by the formation of a catalyst layer having high activity.

[0044] [Composite Oxide]

In the embodiment, an atom ratio of the alkali metal to vanadium in the composite oxide is generally 1:0.8 to 1:9.

[0045] When the atom ratio of the alkali metal to vanadium is lower than 1:0.8, the amount of the alkali metal is large. Therefore, the alkali metal may be precipitated on the catalyst surface and may strongly bind to oxygen. In this case, adsorbed oxygen species having relatively high reactivity which are described with reference to FIG 1 may not be formed. In addition, when the amount of the alkali metal is large as described above, the melting point of the alkali metal vanadate may not be sufficiently decreased. Therefore, a catalyst layer having high activity formed of a uniform thin layer or a microcrystal of the composite oxide may not be obtained.

[0046] On the other hand, when the atom ratio of the alkali metal to vanadium is higher than 1:9, the amount of the alkali metal is small. Therefore, the effect of improving the adsorptivity and reactivity of the composite oxide to sulfur trioxide due to the addition of the alkali metal may not be sufficiently obtained. In addition, when the amount of the alkali metal is small as described above, the melting point of the alkali metal vanadate is relatively high. Therefore, a catalyst layer having high activity formed of a uniform thin layer or a microcrystal of the composite oxide may not be formed.

[0047] Accordingly, in the embodiment, the atom ratio of the alkali metal to vanadium in the composite oxide is generally 1:0.8 or higher and preferably 1:1 or higher and is generally 1:9 or lower and preferably 1:8 or lower, 1:6 or lower, 1:4 or lower, 1:3 or lower, or 1:2 or lower. The atom ratio of the alkali metal to vanadium in the composite oxide is more preferably 1:1 to 1:8 and most preferably 1:1. By adjusting the atom ratio of the alkali metal to vanadium to be within the above-described range, the above-described effect obtained by the combination of the alkali metal and vanadium can be sufficiently exhibited. As a result, a sulfur trioxide decomposition catalyst in which the SO3 decomposition activity, in particular, the SO3 decomposition activity at a low temperature is improved can be obtained.

[0048] In the sulfur trioxide decomposition catalyst according to the embodiment, the composite oxide may contain various alkali metal vanadates. Generally, the composite oxide may contain an alkali metal vanadate represented by the formula A1.xVO3-0.5x in which A represents the alkali metal and 0≤x<l. Preferably, the composite oxide may contain an alkali metal metavanadate represented by the formula AVO3 in which A represents the alkali metal. In addition, when the alkali metal is cesium, the composite Oxide may contain, for example, CSVO3, Cs2V40n, CsV₃0₈, or combinations thereof and, preferably, may contain CsV0₃. These various alkali metal vanadates can be easily produced by appropriately selecting a ratio of an alkali metal compound and a vanadium compound which are to be introduced during the preparation of the composite oxide according to the embodiment.

[0049] [Catalyst Support]

In the sulfur trioxide decomposition catalyst according to the embodiment, the composite oxide is supported on the catalyst support described below in an arbitrary appropriate amount. Although not particularly limited, for example, generally, vanadium may be contained in the composite oxide in an amount of 0.01 wt or more, 0.1 wt or more, 0.5 wt% or more, 1 wt or more, or 2 wt% or more with respect to the catalyst support and may be supported on the catalyst support. In addition, vanadium may be contained in the composite oxide in an amount of 20 wt% or less, 15 wt% or less, 10 wt% or less, 8 wt or less, 7 wt% or less, or 5 wt% or less with respect to the catalyst support and may be supported on the catalyst support.

[0050] According to the embodiment, the catalyst support on which the composite oxide is supported is not particularly limited, but an appropriate metal oxide which is generally known as the catalyst support in the related art may be used. Examples of the catalyst support include silica (Si0₂), alumina (A1₂0₃), zirconia (Zr0₂), titania (Ti0₂), and combinations thereof. In the sulfur trioxide decomposition reaction, since the reaction is performed in the coexistence of sulfur trioxide and water, it is preferable that, generally, a sulfuric acid resistant material be used as the catalyst support. Accordingly, from the viewpoint of sulfuric acid resistance, it is preferable that silica (Si0₂), zirconia (Zr0₂), or a combination thereof be used as the catalyst support.

[0051] In the embodiment, it is preferable that this catalyst support be a porous material having a pore structure.

[0052] However, when a porous material having a so-called micropore such as zeolite is used as the catalyst support, and when the composite oxide of the alkali metal and vanadium is supported in the micropore, the diffusion of sulfur trioxide to the micropore is restricted. As a result, the sulfur trioxide decomposition reaction may be inhibited. This tendency is particularly significant when a metal element having a large ionic radius such as cesium is used as the alkali metal. Accordingly, in the sulfur trioxide decomposition catalyst according to the embodiment, it is particularly preferable that the catalyst support contain at least one of a mesopore and a macropore.

[0053] The term "mesopore" used in this specification generally refers to a pore having a diameter of 2 nm to 50 nm. In addition, the term "macropore" used in this specification generally refers to a pore having a diameter of more than 50 nm.

[0054] From the viewpoint of sulfuric acid resistance, the pore size of the catalyst support, and the like, as the catalyst support according to the embodiment, silica is preferably used, and a porous silica such as a mesoporous silica is more preferably used. The mesoporous silica is not particularly limited, and examples thereof include a cubic mesoporous silica such as KIT-6. Other examples of the mesoporous silica include SBA-15, SBA-16, MCM-41, MCM-48, and FMS-16. These mesoporous silicas can be produced using an arbitrary method which is well-known to those skilled in the art, and generally can be produced using a sol-gel method in which a surfactant is used as a template.

[0055] <Process for Producing Sulfur Trioxide Decomposition Catalyst> As an embodiment of the invention, a process for producing a sulfur trioxide decomposition catalyst is further provided. Using this process, the sulfur trioxide decomposition catalyst according to the embodiment in which the composite oxide having the above-described various characteristics is supported on the catalyst support can be produced.

[0056] Specifically, the sulfur trioxide decomposition catalyst according to the embodiment can be produced using a process including: impregnating the catalyst support with a solution containing an alkali metal compound, and drying and preliminarily calcining the catalyst support; and impregnating the catalyst support with a solution containing a vanadium compound, and drying and calcining the catalyst support. Conversely, the sulfur trioxide decomposition catalyst according to the embodiment can also be produced using a process including: impregnating the catalyst support with a solution containing a vanadium compound, and drying and preliminarily calcining the catalyst support; and impregnating the catalyst support with a solution containing an alkali metal compound, and drying and calcining the catalyst support.

[0057] Alternatively, for example, when a specific alkali metal compound and a specific vanadium compound are selected, and when a solvent in which both the compounds are soluble is used, the sulfur trioxide decomposition catalyst according to the embodiment can also be produced using a process including impregnating the catalyst support with a solution containing both the alkali metal compound and the vanadium compound, and drying and calcining the catalyst support.

[0058] In a catalyst of the related art using a vanadium-containing composite oxide, for example, in a catalyst in which a composite oxide of copper and vanadium is supported on a catalyst support, the sulfur trioxide decomposition reaction may not be sufficiently performed at a temperature lower than 700°C or 650°C. In addition, in such a catalyst, in order to obtain high catalytic activity, it is necessary that the catalyst be calcined at a high temperature of 700°C or higher in a catalyst preparation step or be temporarily used at a high temperature of about 800°C.

[0059] The present inventors have found that, by not combining a transition metal such as copper but combining the alkali metal with vanadium, it is possible to produce a sulfur trioxide decomposition catalyst with improved sulfur trioxide (SO₃) decomposition activity at a low temperature as compared to a catalyst of the related art without the necessity of being calcined or used at a high temperature of 700°C or higher. More specifically, the process according to the embodiment includes supporting the composite oxide containing the alkali metal and vanadium on the catalyst support with an impregnation supporting method using the alkali metal compound and the vanadium compound. Examples of the catalyst of the related art include a catalyst in which a composite oxide of copper and vanadium is supported on a catalyst support. Further, the present inventors have found that, in the sulfur trioxide decomposition catalyst which is produced using the process according to the embodiment, as compared to a catalyst of the related art, high sulfur trioxide (SO₃) decomposition activity can be achieved at such a low temperature under a condition of a high space velocity (SV).

[0060] [Alkali Metal Compound and Vanadium Compound]

The alkali metal compound is not particularly limited, and examples thereof include a nitrate, an acetate, a sulfate, and a hydroxide of the alkali metal. On the other hand, the vanadium compound is not particularly limited, and examples thereof include a vanadate such as ammonium metavanadate.

[0061] In the process according to the embodiment, the alkali metal compound and the vanadium compound are dissolved in a solvent such as water separately or together such that an atom ratio of the alkali metal to vanadium is generally 1:0.8 to 1:9 and preferably 1:1 to 1:8, thereby preparing a solution containing the alkali metal compound and/or the vanadium compound. In order to improve solubility, optionally, the vanadium compound may be dissolved in a solvent such as water along with a small amount of an acid such as oxalic acid. In addition, the vanadium compound can be dissolved in the solvent such that the amount of vanadium is within a range of 0.01 wt% to 20 wt% with respect to the catalyst support.

[0062] [Catalyst Support]

In the process according to the embodiment, as the catalyst support, an appropriate metal oxide which is generally known as the catalyst support in the related art may be used as in the case of the above-described sulfur trioxide decomposition catalyst according to the embodiment. Examples of the catalyst support include silica (Si0₂), alumina (A1₂0₃), zirconia (Zr0₂), titania (Ti0₂), and combinations thereof. Among these, silica is preferably used, and a porous silica such as a mesoporous silica is more preferably used.

[0063] [Drying and Preliminary Calcining or Calcining]

In the process according to the embodiment, in order to produce the sulfur trioxide decomposition catalyst according to the embodiment with a sequential impregnation method, the catalyst support is impregnated with a solution containing the alkali metal compound, and then the catalyst support is dried and preliminarily calcined. Next, the catalyst support is impregnated with a solution containing the vanadium compound, and then the catalyst support is dried and calcined. Alternatively, the catalyst support is impregnated with a solution containing the vanadium compound, and then the catalyst support is dried and preliminarily calcined. Next, the catalyst support is impregnated with a solution containing the alkali metal compound, and then the catalyst support is dried and calcined. On the other hand, in order to produce the sulfur trioxide decomposition catalyst according to the embodiment with a co-impregnation method, the catalyst support is impregnated with a solution containing the alkali metal compound and the vanadium compound, and the catalyst support is dried and calcined.

[0064] Such drying and preliminary calcining or calcining may be performed at a sufficient temperature for a sufficient time for decomposing and removing impurities and volatile components. Alternatively, such drying and preliminary calcining or calcining may be performed at a sufficient temperature for a sufficient time for forming the composite oxide containing the alkali metal and vanadium. Although not particularly limited, the drying may be performed under reduced pressure or normal pressure at a temperature of about 80°C to 250°C for about 1 hour to 24 hours. On the other hand, the preliminary calcining or the calcining may be performed in an inert gas atmosphere such as nitrogen or argon or an oxidizing atmosphere such as air at a temperature of generally about 400°C to less than 700°C, preferably about 400°C to 650°C or about 500°C to 600°C, and more preferably 600°C for about 1 hour to 10 hours.

I [0065] In the process according to the embodiment, by combining the alkali metal and vanadium, the melting points of composite oxide precursors thereof can be decreased. As a result, in the process according to the embodiment, the composite oxide containing the alkali metal and vanadium is supported on the catalyst support with high dispersity at a lower temperature than in a catalyst of the related art. Accordingly, a sulfur trioxide decomposition catalyst in which the sulfur trioxide (S0₃) decomposition activity at a low temperature is improved can be easily produced.

[0066] <Sulfur Dioxide Production Process>

A sulfur dioxide production process according to an embodiment of the invention includes performing a decomposition reaction in which sulfur trioxide is decomposed into sulfur dioxide and oxygen using the sulfur trioxide decomposition catalyst according to the embodiment. According to the sulfur dioxide production process according to the embodiment, by using the sulfur trioxide decomposition catalyst according to the embodiment, the sulfur trioxide decomposition reaction can be performed at a lower temperature than a process of the related art, for example, 650°C or lower, particularly, 600°C or lower.

[0067] For example, at a temperature of 650°C or higher, the alkali metal, in particular, sodium may react with silica which is the catalyst support to form glass. In this case, a sufficient sulfur trioxide decomposition activity may not be achieved due to the deterioration of the catalyst. Accordingly, from the viewpoint of stably using the sulfur trioxide decomposition catalyst according to the embodiment for a long period of time, this process is advantageous in that the sulfur trioxide decomposition reaction can be performed in a relatively low temperature range of lower than 650°C, in particular, 600°C or lower.

[0068] <Hydrogen Production Process>

A hydrogen production process according to an embodiment of the invention includes decomposing water into hydrogen and oxygen, for example, decomposing water into hydrogen and oxygen using an S-I cycle process, a Westinghouse cycle process, an Ispra-Mark 13 cycle process, or a Los Alamos science laboratory cycle process. Here, the hydrogen production process according to the embodiment includes decomposing sulfuric acid into water, sulfur dioxide, and oxygen in a reaction represented by the following formula (XI). In addition, in the hydrogen production process according to the embodiment, an elementary reaction of the following formula (Xl-2) among elementary reactions of the following formulae (Xl-1) and (Xl-2) is performed using the sulfur dioxide production process according to the embodiment, the elementary reactions of the following formulae (Xl-1) and (Xl-2) being the elementary reactions of the reaction represented by the following formula (XI).

(XI) H₂S04→H₂0+S02+l/202

(Xl-1) H₂S0₄→H₂0+S0₃

(Xl-2) S0₃→S0₂+l/20₂

[0069] (S-I Cycle Process)

For example, in an S-I (sulfur-iodine) cycle process represented by the following formulae (XI) to (X3), the hydrogen production process according to the embodiment includes performing the elementary reaction of the formula (Xl-2) among the elementary reactions of the formulae (Xl-1) and (Xl-2) using the sulfur dioxide production process according to the embodiment; the elementary reactions of the formulae (Xl-1) and (Xl-2) being the elementary reactions of the reaction represented by the formula (XI).

(XI) H₂S0₄→H₂0+S0₂+l/20₂

(Xl-1) H₂S0₄→H₂0+S0₃

(Xl-2) S0₃→S0₂+l/20₂

(X2) I₂+S0₂+2H₂0→2HI+H₂S04

(X3) 2HI→H₂+I₂

Total reaction: H₂0→H₂+l/20₂

[0070] (Westinghouse Cycle Process)

For example, in a Westinghouse cycle process represented by the following formulae (XI), (X4), and (X5), the hydrogen production process according to the embodiment includes performing the elementary reaction of the formula (Xl-2) among the elementary reactions of the formulae (Xl-1) and (Xl-2) using the sulfur dioxide production process according to the embodiment; the elementary reactions of the formulae (Xl-1) and (Xl-2) being the elementary reactions of the reaction represented by the formula (XI).

(XI) H₂S04→H₂0+S02+l/202

(Xl-1) H₂S0₄→H₂0+S0₃

(Xl-2) S0₃→S0₂+l/20₂

(X4) S0₂+2H₂0→H₂S0₃

(X5) H₂S0₃+H₂0→H2+H₂S04 (electrolysis)

Total reaction: H₂0→H₂+l/20₂

[0071] (Ispra-Mark 13 Cycle Process)

For example, in an Ispra-Mark 13 cycle process represented by the following formulae (XI), (X6), and (X7), the hydrogen production process according to the embodiment includes performing the elementary reaction of the formula (Xl-2) among the elementary reactions of the formulae (Xl-1) and (Xl-2) using the sulfur dioxide production process according to the embodiment; the elementary reactions of the formulae (Xl-1) and (Xl-2) being the elementary reactions of the reaction represented by the formula (XI).

(XI) H₂S0₄→H₂0+S0₂+l/20₂

(Xl-1) H₂S0₄→H₂0+S0₃

(Xl-2) S0₃→S0₂+l/20₂

(X6) 2HBr→Br₂+H₂

(X7) Br₂+S0₂+2H₂0→2HBr+H₂S0₄

Total reaction: H₂0-*H₂+l/20₂

[0072] (Los Alamos Science Laboratory Cycle Process)

For example, in a Los Alamos science laboratory cycle process represented by the following formulae (XI) and (X8) to (X10), the hydrogen production process according to the embodiment includes performing the elementary reaction of the formula (Xl-2) among the elementary reactions of the formulae (Xl-1) and (Xl-2) using the sulfur dioxide production process according to the embodiment; the elementary reactions of the formulae (Xl-1) and (Xl-2) being the elementary reactions of the reaction represented by the formula (XI).

(XI) H₂S0₄→H₂0+S0₂+l/20₂

(Xl-1) H₂S0₄→H₂0+S0₃

(Xl-2) S0₃→S0₂+l/20₂

(X8) Br2+S0₂+2H₂0→2HBr+H₂S04

(X9) 2CrBr₃→2CrBr₂+Br₂

(X10) 2HBr+2CrBr₂→2CrBr₃+H₂

Total reaction: H₂0→H₂+l/20₂

[0073] Hereinafter, the embodiments of the invention will be described in more detail using examples. However, the embodiments of the invention are not limited to these examples.

[0074] In the following examples, various sulfur trioxide decomposition catalysts in which a composite oxide containing an alkali metal and vanadium was supported on a catalyst support were prepared, and sulfur trioxide (S0₃) decomposition activities and characteristics thereof were investigated.

[0075] [Example 1]

In Example 1, a sulfur trioxide decomposition catalyst in which a composite oxide of sodium (Na) and vanadium (V) was supported on a catalyst support (Si0₂: ΚΓΤ-6) was prepared.

[0076] [Preparation of Catalyst Support (Si0₂: ΚΓΓ-6)]

The catalyst support was a cubic mesoporous silica ( ΓΤ-6) and was prepared as follows. 7.9 g of 35 mass% hydrochloric acid (HC1) and 4.0 g of nonionic surfactant (Pluronic (trade name) P-123) were added to 144 mL of distilled water, and the obtained aqueous solution was stirred at a temperature of 35°C to dissolve the components therein. 4.0 g of 1-butanol was added to the obtained mixture, the mixture was stirred at a temperature of 35°C until it was transparent. As a result, the nonionic surfactant was self-aligned. 8.6 g of tetraethoxysilane (TEOS) as a silica source was added to the obtained mixture and was strongly stirred at a temperature of 35°C for 24 hours. Next, tetraethoxysilane (TEOS) was hydrolyzed by using the self-aligned nonionic surfactant as a template. Then, the obtained substance was left to stand at a temperature of 100°C for 24 hours and then was dried at a temperature of 110°C for 24 hours without being washed. Then, the obtained substance was stirred in a mixture of 8 ml of 35 mass% hydrochloric acid and 120 mL of ethanol for 1.5 hours, followed by washing. Then, the obtained substance was dried at a temperature of 110°C for 24 hours, was heated to 550°C at a temperature increase rate of 3°C/min, and was calcined at this temperature for 5 hours. As a result, a cubic mesoporous silica (KIT-6) was obtained.

[007η [Preparation of NaV0₃/Si0₂]

First, 1.0 g of the prepared catalyst support (Si0₂: KIT-6) was impregnated with a solution in which a predetermined amount of sodium nitrate (NaN0₃) was diluted with 3 mL of ultrapure water. Next, the catalyst support was dried overnight at 110°C and then preliminarily calcined in the air in a decomposition furnace at 500°C for 2 hours. Next, a predetermined amount of ammonium metavanadate (NH₄VO₃) was dissolved in an aqueous 1M oxalic acid solution. Next, the above-described catalyst support was impregnated with this aqueous solution. Finally, the catalyst support was dried overnight at 110°C and then calcined in the air in a decomposition furnace at 600°C for 5 hours. As a result, a sulfur trioxide decomposition catalyst formed of NaV0₃/Si0₂ (Na:V:Si=l:l:12) was obtained.

[0078] [Example 2]

[Preparation of KV0₃/Si0₂]

A sulfur trioxide decomposition catalyst formed of KV0₃/Si0₂ (K:V:Si=l:l:12) was obtained with the same method as Example 1, except that potassium nitrate (KN0₃) was used instead of sodium nitrate (NaN0₃).

[0079] [Example 3]

[Preparation of RbV0₃/Si0₂]

A sulfur trioxide decomposition catalyst formed of RbV0₃/Si0₂ (Rb:V:Si=l:l:12) was obtained with the same method as Example 1, except that rubidium nitrate (RbN0₃) was used instead of sodium nitrate (NaN0₃). [0080] [Example 4]

[Preparation of CsV0₃/Si0₂]

A sulfur trioxide decomposition catalyst formed of CsV0₃/Si02 (Cs:V:Si=l:l:12) was obtained with the same method as Example 1, except that cesium nitrate (CsN0₃) was used instead of sodium nitrate (NaN0₃).

[0081] [Comparative Example 1]

[Preparation of CuV₂00/Si0₂]

In Comparative Example 1, a sulfur trioxide decomposition catalyst of the related art in which a composite oxide of copper (Cu) and vanadium (V) was supported on a catalyst (Si0₂: KJT-6) was prepared.

[0082] Specifically, first, the prepared catalyst support (Si0₂: ΓΓ-6) was impregnated with a solution in which a predetermined amount of copper nitrate (Cu(N0₃)₂) was dissolved in water. Next, the catalyst support was dried at 150°C and then preliminarily calcined at 350°C for 1 hour. Next, a predetermined amount of ammonium metavanadate (NH₄V0₃) was dissolved in water. Next, the above-described catalyst support was impregnated with this aqueous solution, was dried at 150°C, and was preliminarily calcined at 350°C for 1 hour. Finally, the obtained catalyst support was calcined at 600°C for 2 hours. As a result, a sulfur trioxide decomposition catalyst formed of CuV₂06/Si0₂ (Cu:V:Si=l:2:17) was obtained.

[0083] [Comparative Example 2]

[Preparation of - Added CuV₂0₆/Si0₂]

First, 0.6 g of the prepared catalyst support (Si0₂: ΚΓΓ-6) was impregnated with a solution in which predetermined amounts of potassium nitrate (KN0₃) and copper nitrate (Cu(N0₃)₂) was diluted with 3 mL of ultrapure water. Next, the catalyst support was dried overnight at 110°C and then preliminarily calcined in the air in a decomposition furnace at 500°C for 2 hours. Next, a predetermined amount of ammonium metavanadate (NH₄VO₃) was dissolved in an aqueous 1M oxalic acid solution. Next, the above-described catalyst support was impregnated with this aqueous solution. Finally, the catalyst support was dried overnight at 110°C and then calcined in the air in a decomposition furnace at 600°C for 5 hours. As a result, a sulfur trioxide decomposition catalyst formed of K- Added CuV₂06/Si0₂ (Cu:V:K:Si=l:2:l:17) was obtained.

[0084] [Example 3]

[Preparation of Cs- Added CuV₂06/Si0₂]

A sulfur trioxide decomposition catalyst formed of Cs-Added CuV₂06/Si0₂

(Cu:V:K:Si=l:2:l:17) was obtained with the same method as Comparative Example 2, except that cesium nitrate (CSNO₃) was used instead of potassium nitrate (KNO₃).

[0085] [Evaluation of Activity of Catalyst]

Regarding the sulfur trioxide decomposition catalysts of Examples 1 to 4 and Comparative Example 1 to 3, the sulfur trioxide (SO₃) decomposition activity in the sulfur trioxide decomposition reaction of the following formula (XI -2) was evaluated. Specifically, the sulfur trioxide decomposition reaction was performed as described below using a fixed bed flow reactor illustrated in FIG 3.

(Xl-2) S0₃→S0₂+l/20₂

[0086] First, a quartz reaction tube 4 (inner diameter: 10 mm) was filled with, as a catalyst bed 10, 0.5 g of the sulfur trioxide decomposition catalyst adjusted to 14 to 20 meshes. Next, nitrogen (N₂) and an aqueous sulfuric acid (H₂S0₄) solution were supplied to the lower stage of the quartz reaction tube 4 from a nitrogen supply part 1 and a sulfuric acid supply part 3, respectively. In this case, a weight hourly space velocity (WHSV) was 110 g-I^SCVg-cat/h corresponding to a condition of a relatively high space velocity (SV).

[0087] The sulfuric acid (H₂S0₄) supplied to the lower stage of the quartz reaction tube 4 was heated in the lower and middle stages of the quartz reaction tube 4 to be decomposed into sulfur trioxide (SO₃) and oxygen (0₂) and flowed to the catalyst bed 10. Here, in the quartz reaction tube 4, the lower stage was heated to about 400°C by a heater 4a, and the middle and upper stages were heated to about 600°C by heaters 4b, 4c. The outflow gas from the quartz reaction tube 4 was air-cooled and then bubbled through a 0.05 M iodine (I₂) solution, and sulfur dioxide (S0₂) was absorbed in the iodine solution. Iodometric titration of the iodine solution having been absorbed therein sulfur dioxide was performed using a 0.025 M sodium thiosulfate (Na₂S₂C>3) solution to determine the amount of sulfur dioxide absorbed.

[0088] Also, the outflow gas after bubbling through the iodine solution was cooled with a dry ice/ethanol mixture, and the remaining sulfur dioxide and sulfur trioxide were completely removed with a mist absorber and silica gel. Next, the amount of oxygen (0₂) was determined using a magnetic pressure oxygen analyzer (MPA3000, manufactured by Horiba Ltd.) and a gas chromatograph (GC8A, manufactured by Shimadzu Corporation, molecular sieve 5A, TCD detector). The achievement ratio based on the equilibrium conversion from sulfur trioxide (S0₃) to sulfur dioxide (S0₂) was calculated from the amounts of sulfur dioxide and oxygen determined as above. The results are shown in Table 1.

Table 1: Achievement Ratio of So₃ Decomposition of Each Catalyst at 600°C

WHSV=110 g-H₂S0 /g-cat/h

[0089] Referring to Table 1, when the sulfur trioxide decomposition catalysts of Examples 1 to 4 were compared to the sulfur trioxide decomposition catalyst (CuV₂C>6/Si02) of Comparative Example 1 and the sulfur trioxide decomposition catalysts of Comparative Examples 2 and 3 in which the alkali metal was added to CuV₂0e/Si0₂ of Comparative Example 1, the achievement ratio based on the equilibrium conversion was significantly high at a relatively low temperature of 600°C. Accordingly, it can be understood that the sulfur trioxide decomposition catalysts of Examples 1 to 4 had significantly high sulfur trioxide (S0₃) decomposition activity. In addition, in the sulfur trioxide decomposition catalysts of Comparative Examples 2 and 3 in which the alkali metal was added to CuV₂0e/Si0₂ of Comparative Example 1, the S0₃ decomposition activity was higher than that of the catalyst of Comparative Example 1; however, the activity was significantly decreased as compared to the sulfur trioxide decomposition catalysts of Examples 1 to 4. It was found from the results that, in the sulfur trioxide decomposition catalysts of Examples 1 to 4, the alkali metal vanadates had high S0₃ decomposition activity.

[0090] Next, regarding the sulfur trioxide decomposition catalysts of Examples 1 to 4 and Comparative Examples 1 to 4, the achievement ratio based on the equilibrium conversion of each of the sulfur trioxide decomposition catalysts was investigated when the upper stage of the quartz reaction tube 4 was heated to 750°C by the heater 4c, the sulfur trioxide decomposition reaction was performed at this temperature for about 1 hour, and the temperature was decreased to 600°C. The results are shown in Table 2.

Table 2: Achievement Ratio of S03 Decomposition of Each Catalyst at 600°C after Being Used in Reaction at 750°C

WHSV=110 g-H₂S04/g-cat/h

[0091] It can be seen from Table 2 that, in the sulfur trioxide decomposition catalyst of Comparative Example 1, when the reaction temperature was temporarily heated to 750°C and was decreased to 600°C, the achievement ratio based on the equilibrium conversion was improved from 5.5% of Table 1 to 34.0%. The reason is considered to be that the composite oxide formed of CuV₂0₆ was spread on the catalyst support in the form of a thin layer by being used at a high temperature of 750°C; as a result, the dispersibility thereof was improved. On the other hand, in the sulfur trioxide decomposition catalysts of Examples 1 to 4, the achievement ratio based on the equilibrium conversion was decreased to some extent by the reaction temperature being temporarily increased to 750°C. In particular, in the sulfur trioxide decomposition catalyst of Example 1 in which sodium was used as the alkali metal, such a decrease was significant.

[0092] In the sulfur trioxide decomposition catalyst according to the invention, it is considered that the composite oxide formed of a uniform thin layer is formed on the catalyst support by being calcined at about 600°C in advance during the catalyst preparation. Accordingly, it is considered that the catalytic activity is not improved even when being used at a high temperature of 750°C, and the catalyst deteriorates to some extent by the reaction of the alkali metal and the catalyst support. Nevertheless, when all the sulfur trioxide decomposition catalysts of Examples 1 to 4 were compared to the sulfur trioxide decomposition catalyst of Comparative Example 1, high achievement ratio was maintained, and thus high sulfur trioxide decomposition activity was able to be achieved. In the sulfur trioxide decomposition catalysts of Comparative Examples 2 and 3 to which the alkali metal was added to the sulfur trioxide decomposition catalyst of Comparative Example 1, the achievement ratio based on the equilibrium conversion was decreased, and thus the improvement of the sulfur trioxide decomposition activity was not observed unlike Comparative Example 1. In addition, although not shown in Tables 1 and 2, the sulfur trioxide decomposition reaction was performed for a long period of time using the sulfur trioxide decomposition catalyst of Example 4 having the highest performance, but significant deterioration of the catalyst was not observed. Accordingly, it was confirmed that the sulfur trioxide decomposition catalyst of Example 4 exhibited high durability under a strict condition, for example, in a concentrated sulfuric acid flow.

[0093] [Structure Analysis of Catalyst by X-Ray Diffraction (XRD)]

Next, the sulfur trioxide decomposition catalysts of Example 4 and Comparative Examples 1 and 3 in which the highest sulfur trioxide decomposition activity was obtained were measured by X-ray diffraction (XRD). The results are shown in FIG 4.

[0094] FIG 4 is a diagram illustrating X-ray diffraction patterns regarding the sulfur trioxide decomposition catalysts of Example 4 and Comparative Examples 1 to 3, FIG 4 illustrates the X-ray diffraction patterns of each of the catalysts after the calcining at 600°C and after the reaction at 750°C. Referring to FIG. 4, in the catalysts of Comparative Examples 1 and 3, small diffraction peaks derived from CuO were observed, but significant peaks other than those derived from a part of impurities were not observed in all the catalysts. This result indicates that a crystal having a size that can be observed by X-ray diffraction was not present in the respective catalysts, particularly, in the catalyst of Example 4.

[0095] [Catalyst Analysis by Raman Scattering]

The sulfur trioxide decomposition catalyst of Example 4 was measured using a Raman spectrometer. The results are shown in FIG 5. FIG 5 is a diagram illustrating the results of analyzing the sulfur trioxide decomposition catalyst of Example 4 by Raman spectroscopy. For reference, FIG 5 also illustrates the analysis results of measuring samples of CsV0₃ and CsV₃Os which were not supported on a catalyst supported.

[0096] It can be seen from FIG 5 that, in the sulfur trioxide decomposition catalyst of Example 1, the peak around 940 cm^"1 assigned to the symmetric stretching of V0₃ of CsV0₃ was shifted to a high-frequency side and plotted as compared to the peak of CsV0₃ illustrated for reference. The shift to the high-frequency side indicates the V-0 bonds in VO3 were strengthened. Accordingly, it is considered that the contribution to the improvement of the sulfur trioxide decomposition activity was small. On the other hand, it is considered that the broadening of the peak was caused by the thin spreading of the composite oxide formed of CsV0₃ on the catalyst support (silica) or by the microcrystallization of the composite oxide.

Claims

CLAIMS:

1. A sulfur trioxide decomposition catalyst comprising:

a composite oxide containing an alkali metal and vanadium; and

a catalyst support on which the composite oxide is supported.

2. The sulfur trioxide decomposition catalyst according to claim 1, wherein the alkali metal is selected from the group consisting of sodium, potassium, rubidium, cesium, and a combination of at least two of sodium, potassium, rubidium, and cesium.

3. The sulfur trioxide decomposition catalyst according to claim 2, wherein the alkali metal is selected from the group consisting of potassium, cesium, and a combination of potassium and cesium.

4. The sulfur trioxide decomposition catalyst according to any one of claims 1 to 3, wherein

an atom ratio of the alkali metal to vanadium in the composite oxide is 1:1 to 1:8.

5. The sulfur trioxide decomposition catalyst according to any one of claims 1 to 4, wherein

the composite oxide contains an alkali metal vanadate represented by the formula A_1-xV0₃.o.5x in which A represents the alkali metal and 0≤x<l.

6. The sulfur trioxide decomposition catalyst according to claim 5, wherein the composite oxide contains an alkali metal metavanadate represented by the formula AV0₃ in which A represents the alkali metal.

7. The sulfur trioxide decomposition catalyst according to any one of claims 3 to 5, wherein the alkali metal is cesium, and

the composite oxide contains at least one of CSVO3, Cs2V40n, and CsV₃08.

8. The sulfur trioxide decomposition catalyst according to claim 7, wherein the composite oxide contains a combination of at least two of CsV0₃, Cs₂V₄0n, and CsV₃0₈.

9. The sulfur trioxide decomposition catalyst according to any one of claims 1 to 8, wherein

the catalyst support is selected from the group consisting of silica, alumina, zirconia, titania, and a combination of at least two of silica, alumina, zirconia, and titania.

10. The sulfur trioxide decomposition catalyst according to claim 9, wherein the catalyst support contains at least one of a mesopore and a macropore.

11. The sulfur trioxide decomposition catalyst according to claim 9 or 10, wherein the catalyst support is silica.

12. A sulfur dioxide production process comprising

performing a decomposition reaction in which sulfur trioxide is decomposed into sulfur dioxide and oxygen using the sulfur trioxide decomposition catalyst according to any one of claims 1 to 11.

13. The sulfur dioxide production process according to claim 12, wherein

the decomposition reaction is performed at a temperature of 600°C or lower.

14. A hydrogen production process comprising:

decomposing sulfuric acid into water, sulfur dioxide, and oxygen in a reaction represented by the following formula (XI) containing the following formulae (Xl-1) and (XI -2) as elementary reactions, wherein

the elementary reaction of the formula (XI -2) is performed using the sulfur dioxide production process according to claim 12 or 13:

(XI) H₂S04→H₂0+S0₂+l/20₂

(Xl-1) H₂S04→H₂0+S03

(Xl-2) S0₃→S0₂+l/20₂.

15. The hydrogen production process according to claim 14, wherein

at least one of an S-I cycle process, a Westinghouse cycle process, an Ispra-Mark 13 cycle process, and a Los Alamos science laboratory cycle process is performed.

16. A process of producing a sulfur trioxide decomposition catalyst in which a composite oxide containing an alkali metal and vanadium is supported on a catalyst support, the process comprising:

impregnating the catalyst support with one solution of a solution containing an alkali metal compound and a solution containing a vanadium compound, and drying and preliminarily calcining the catalyst support; and

impregnating the preliminarily calcined catalyst support with the other solution of the solution containing the alkali metal compound and the solution containing the vanadium compound, and drying and calcining the catalyst support.

17. A process of producing a sulfur trioxide decomposition catalyst in which a composite oxide containing an alkali metal and vanadium is supported on a catalyst support, the process comprising

impregnating the catalyst support with a solution containing an alkali metal compound and a vanadium compound, and drying and calcining the catalyst support.

18. The process according to claim 16 or 17, wherein

the catalyst support is calcined at a temperature of 400°C to 650°C.

19. The process according to claim 18, wherein the catalyst support is calcined at a temperature of 500°C to 600°C.