Ammonia oxidation/decomposition catalyst

Abstract

Provided is an ammonia oxidation/decomposition catalyst which can decrease the reduction temperature of a support, which is required for the catalyst to have a property of being activated at room temperature, and also can render a property of being activated at a temperature lower than room temperature. The ammonia oxidation/decomposition catalyst of the present invention is an ammonia oxidation/decomposition catalyst, comprising: a catalyst support composed of a composite oxide of cerium oxide and zirconium oxide; and at least one metal selected from the group consisting of metals of group 6A, group 7A, group 8, and group 1B as a catalytically active metal deposited thereon, characterized in that the molar concentration of zirconium oxide in the catalyst support is from 10 to 90%.

Claims

1. A method for activating an ammonia oxidation/decomposition catalyst comprising: providing the ammonia oxidation/decomposition catalyst comprising a catalyst support composed of composite oxide of cerium oxide and zirconium oxide and a catalytically active metal deposited thereon, wherein the molar concentration of zirconium oxide in the catalyst support is from 10 to 90%; heating the catalyst at 200 to 400 C. in a hydrogen stream or in an ammonia stream in order to reduce a part of or the whole of the cerium oxide constituting the catalyst support to
CeO.sub.2-x (0<x<2); and then supplying oxygen and ammonia simultaneously to the catalyst at a temperature between room temperature and 30 C. in order to generate oxidation heat by the reaction of the catalyst support in a reduced state with oxygen such that the temperature of the catalyst layer is increased to a temperature at which ammonia and oxygen are reacted with each other.

2. The method according to claim 1, wherein the catalyst support is in a honeycomb form.

3. The method according to claim 1, wherein the catalyst support is in a pellet form.

4. The method according to claim 1, wherein the molar concentration of the zirconium oxide in the catalyst support is from 20% to 70%.

5. The method according to claim 1, wherein the catalytically active metals is at least one metal selected from the group consisting of metals from groups 6A, 7A, 8 and 1B.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph showing a change over time of the temperature of a catalyst layer when a test for a property of being activated at a low temperature was performed.

(2) FIG. 2 is a graph showing a change over time of the concentration of each gas when a catalyst of Example 17 was used.

DESCRIPTION OF EMBODIMENTS

(3) Hereinafter, several Examples of the present invention and Comparative Examples for comparison therewith will be described for specifically illustrating the present invention.

(4) a) Catalyst Support

(5) As the catalyst support, four types of commercially available CeO.sub.2ZrO.sub.2 (manufactured by DAIICHI KIGENSO KAGAKU KOGYO Co., LTD.) in which the molar concentration of ZrO.sub.2 is 10, 20, 50, and 80% were used.

(6) b) Deposition of Catalytically Active Metal

(7) A catalytically active metal was deposited on each of the above catalyst supports. As the catalytically active metal, Ru, Pt, Rh, and PtRh, which are noble metals, and Co, Ni, Fe, Cu, Mo, and Mn, which are base metals, were used. The catalyst deposition amount was set to 2% by weight in all cases.

(8) (Preparation of Pellet Catalyst)

(9) The deposition of each of the above metals on the catalyst support was performed as follows. Each of the metal salts, which are precursors of the respective metals, was dissolved in pure water, to which the above catalyst support was immersed so as to make the metals dispersed on the supports such that the deposition amount of the catalytically active metal was 2% by weight (in terms of metal).

(10) This dispersion was heated to gradually evaporate water (an evaporation to dryness method).

(11) The obtained powdery substance was calcinated in air at 300 C. for 3 hours.

(12) The powdery substance after firing was compression molded and the resulting molded product was sieved to 1 to 0.85 mm and used.

(13) (Preparation of Honeycomb Catalyst)

(14) The catalytically active metal was deposited on 600 cpi cordierite by a wash-coating method until the catalyst deposition amount reached about 250 g/L.

(15) As the catalyst support for Comparative Examples, commercially available CeO.sub.2 (manufactured by DAIICHI KIGENSO KAGAKU KOGYO Co., LTD.) was used. On this catalyst support, each of the catalytically active metals, Ru, Co, and Ni, were deposited. The catalyst deposition amount was set to 2% by weight.

(16) c) Test for Property of being Activated at Room Temperature

(17) (Reduction Treatment of Catalyst)

(18) After each of the obtained pellet catalysts (1 g) and the honeycomb catalysts (4 mL) was loaded in a flow-through type reaction tube, several reduction treatments were performed at different temperatures to each treatment in a hydrogen stream. Each of temperatures of the reduction treatment was a range from 150 C. to 800 C. and each had 50 C. intervals. The reduction times for each treatment were set to 2 hours.

(19) (Validation of Property of being Activated at Room Temperature)

(20) The above catalyst loaded in the reaction tube and undergoing the reduction treatment at each temperature was kept at 25 C. in a nitrogen atmosphere, and thereafter, oxygen (air) and ammonia were simultaneously supplied to the catalyst layer. The ammonia supplying amount was kept constant at 2.5 NL/min, and the air supplying amount was set such that the volume ratio of air to ammonia was 1.0. The temperature of the catalyst layer and the gas composition at the outlet were measured by a thermocouple and a mass spectrometer, respectively.

(21) From the above results, a catalyst that satisfied the following three requirements: the temperature of the catalyst layer was increased; the production of hydrogen was observed; and hydrogen was stably produced for 30 minutes or more was determined as a catalyst that exhibits a property of being activated at room temperature, and with respect to such a catalyst, the temperature required for the reduction treatment was determined as the reduction temperature.

(22) The form and the composition of each of the catalysts of Examples and Comparative Examples and the reduction temperature thereof are shown in the following Table 1.

(23) TABLE-US-00001 TABLE 1 Reduction Form Composition temperature Example 1 Pellet Ru/CeO.sub.2ZrO.sub.2 400 C. (10 mol %) 2 Pellet Ru/CeO.sub.2ZrO.sub.2 250 C. (20 mol %) 3 Pellet Ru/CeO.sub.2ZrO.sub.2 200 C. (50 mol %) 4 Pellet Ru/CeO.sub.2ZrO.sub.2 200 C. (80 mol %) 5 Pellet Pt/CeO.sub.2ZrO.sub.2 200 C. (50 mol %) 6 Pellet Rh/CeO.sub.2ZrO.sub.2 200 C. (50 mol %) 7 Pellet PtRh/CeO.sub.2ZrO.sub.2 200 C. (50 mol %) 8 Pellet Co/CeO.sub.2ZrO.sub.2 300 C. (50 mol %) 9 Pellet Fe/CeO.sub.2ZrO.sub.2 300 C. (50 mol %) 10 Pellet Ni/CeO.sub.2ZrO.sub.2 300 C. (50 mol %) 11 Pellet Cu/CeO.sub.2ZrO.sub.2 350 C. (50 mol %) 12 Pellet Mo/CeO.sub.2ZrO.sub.2 400 C. (50 mol %) 13 Pellet Mn/CeO.sub.2ZrO.sub.2 400 C. (50 mol %) 14 Honeycomb Ru/CeO.sub.2ZrO.sub.2 200 C. (50 mol %) Compar- ative Example 1 Pellet Ru (2 wt %)/CeO.sub.2 600 C. 2 Pellet Co (2 wt %)/CeO.sub.2 600 C. 3 Pellet Ni (2 wt %)/CeO.sub.2 700 C.

(24) As apparent from the above Table 1, in the case where the catalyst support was CeO.sub.2 (Comparative Examples 1 to 3), in order to exhibit a property of being activated at room temperature, it was necessary to set the reduction temperature to 600 C. or higher, while, in the case where Zr was added at 10 mol % to CeO.sub.2 (Example 1), the reduction temperature for exhibiting a property of being activated at room temperature was 400 C., and therefore, the reduction temperature could be decreased by 200 C. as compared with Comparative Example 1.

(25) It was confirmed that in the case where ammonia oxidation/decomposition proceeded and the temperature of the catalyst layer was increased to 600 C. or higher, when the reaction was stopped, merely by blowing ammonia into the catalyst layer, the temperature of the catalyst layer was increased by the progress of the oxidation reaction (which is an exothermic reaction) with the supported catalyst, and a property of being activated at room temperature was exhibited again. Accordingly, reactivation could be achieved under a milder condition.

(26) From the above results, the molar concentration of Zr added was desirably from 20 to 70 mol %, and in the case where Ru, which is a noble metal, was used as the catalytically active metal, a property of being activated at room temperature could be exhibited when the reduction temperature was 200 C., and in the case where Ni or Co, which is a base metal, was used as the catalytically active metal, a property of being activated at room temperature could be exhibited when the reduction temperature was 300 C.

(27) d) Test for Property of being Activated at Low Temperature

(28) (Reduction Treatment of Catalyst)

(29) After each of the obtained pellet catalysts (1 g) and the honeycomb catalysts (4 mL) was loaded in a flow-through type reaction tube, a reduction treatment was performed in a hydrogen stream at 600 C. for 2 hours.

(30) (Validation of Property of being Activated at Low Temperature)

(31) The above catalyst loaded in the reaction tube and undergoing the reduction treatment was cooled in a nitrogen atmosphere, and after the temperature was reached a predetermined temperature, oxygen (air) and ammonia were simultaneously supplied thereto. The ammonia supplying amount was kept constant at 1 NL/min, and the air supplying amount was set such that the volume ratio of air to ammonia was 1.0. The temperature of the catalyst layer and the hydrogen generation amount at the outlet of the catalyst layer were measured by a thermocouple and a mass spectrometer, respectively.

(32) The form and the composition of each of the catalysts of Examples and Comparative Example are shown in the following Table 2.

(33) TABLE-US-00002 TABLE 2 Initial temper- Ammonia ature of decompo- catalyst sition Form Composition layer ratio (%) Example 15 Pellet Ru/CeO.sub.2ZrO.sub.2 15 C. 41 (10 mol %) 16 Pellet Ru/CeO.sub.2ZrO.sub.2 15 C. 40 (20 mol %) 17 Pellet Ru/CeO.sub.2ZrO.sub.2 15 C. 36 (50 mol %) 18 Pellet Ru/CeO.sub.2ZrO.sub.2 15 C. 33 (70 mol %) 19 Pellet Ru/CeO.sub.2ZrO.sub.2 15 C. 35 (90 mol %) 20 Pellet Rh/CeO.sub.2ZrO.sub.2 15 C. 37 (50 mol %) 21 Pellet Pt/CeO.sub.2ZrO.sub.2 15 C. 39 (50 mol %) 22 Pellet PtRh/CeO.sub.2ZrO.sub.2 15 C. 42 (50 mol %) 23 Pellet Co/CeO.sub.2ZrO.sub.2 15 C. 33 (50 mol %) 24 Pellet Fe/CeO.sub.2ZrO.sub.2 15 C. 32 (50 mol %) 25 Pellet Ni/CeO.sub.2ZrO.sub.2 15 C. 35 (50 mol %) 26 Pellet Cu/CeO.sub.2ZrO.sub.2 15 C. 20 (50 mol %) 27 Pellet Mo/CeO.sub.2ZrO.sub.2 15 C. 28 (50 mol %) 28 Pellet Mn/CeO.sub.2ZrO.sub.2 15 C. 29 (50 mol %) 29 Pellet Ru/CeO.sub.2ZrO.sub.2 25 C. 32 (50 mol %) 30 Pellet Ru/CeO.sub.2ZrO.sub.2 30 C. 30 (50 mol %) 31 Pellet Co/CeO.sub.2ZrO.sub.2 30 C. 25 (50 mol %) 32 Honeycomb Ru/CeO.sub.2ZrO.sub.2 30 C. 33 (50 mol %) Compar- ative Example 4 Pellet Ru (2 wt %)/CeO.sub.2 15 C. 0

(34) A change with time of the temperature of the catalyst layer when performing the above test for a property of being activated at a low temperature is shown in FIG. 1 for each of the case where the catalyst (Ru/CeO.sub.2ZrO.sub.2 (50 mol %)) of Example 17 was used and the case where the catalyst (Ru (2 wt %)/CeO.sub.2) of Comparative Example 4 was used. Further, a change with time of the concentration of each gas when the catalyst of Example 17 was used is shown in FIG. 2.

(35) As apparent from FIG. 1, in the case of Comparative Example 1 in which the catalyst support was CeO.sub.2, a property of being activated was not exhibited when the initial temperature of the catalyst layer was 15 C.

(36) On the other hand, in the case of Examples 15 to 32, by adding ZrO.sub.2 to CeO.sub.2 at 10 to 90 mol %, desirably at 20 to 70 mol %, it was found that a property of being activated was exhibited even at a temperature lower than room temperature.

(37) In particular, regardless of the types of metals, i.e., noble metals such as Pt, Rh, and a PtRh alloy, and base metals such as Co, Fe, Ni, Cu, Mo, and Mn, a property of being activated at a low temperature could be exhibited.