In recent years, along with increasing awareness of environmental issues, attention has been focused on fuel cells as one of the clean energy technologies. A fuel cell is an energy technology that utilizes hydrogen, and can convert hydrogen and oxygen into electrical energy through an electrochemical reaction between them, without causing any direct emission of carbon dioxide, which is one of the causes of global warming. Various feedstocks have been considered as hydrogen sources for fuel cells, such as natural gas, liquid fuels, petroleum-based hydrocarbons, and biomass-derived feedstocks.
To produce hydrogen from hydrocarbon-containing feedstocks as described above, a method in which the steam reforming reaction for reacting steam and a hydrocarbon gas is performed using a steam reforming catalyst is generally employed. The chemical reaction for generating hydrogen by means of steam reforming of a hydrocarbon, for example, methane, is represented by formula (1):CH4+H2O→3H2+CO  (1)
In this reaction, if the ratio of steam to hydrocarbon gas during the steam reforming decreases, carbon will be deposited on the catalyst, causing the catalytic activity to decrease. Conversely, if the ratio of steam is high, the consumption of thermal energy for heating the steam will be high, which is disadvantageous in terms of cost. Thus, a reforming catalyst that has less carbon deposition even at a low steam ratio is required.
Hydrocarbon feedstock typically contains a sulfur compound. Such a sulfur compound may be naturally present, or can be artificially added as an odorant from a safety standpoint; in either case, however, the sulfur compound is adsorbed on a catalytic active metal, which causes the catalytic activity to decrease.
Furthermore, when, for example, natural gas, which is widespread as town gas, is used as a hydrocarbon gas feedstock, ammonia is likely to be generated from approximately several % of nitrogen contained therein, and the generated ammonia readily causes poisoning of downstream fuel cell electrodes. Thus, a steam reforming catalyst system that is unlikely to be affected even when a sulfur compound-containing hydrocarbon gas is used is required, and a steam reforming catalyst is also strongly required which does not cause the generation of ammonia, even if nitrogen gas is present in the hydrocarbon gas.
Thus, the important goals for the required performance in a steam reforming catalyst and a system using the catalyst are as follows: 1) high conversion ratio; 2) minimal adverse effect of the sulfur component, nitrogen gas or the like in the feedstock; 3) less carbon deposition during the reaction; and 4) less degradation in performance even after a long reaction time. In addition to these kinds of performance, 5) low catalyst production costs is an essential requirement for the widespread use of hydrogen energy. Conventionally, an expensive platinum-group element has been typically used as an active metal of such a steam reforming catalyst, and the proportion of the active metal in catalyst production costs is large; therefore, reducing the loading amount of the active metal is an extremely important issue for saving costs.
Among the above issues, with regard to 4) the long-term stability, no general target value is known as the time required varies depending on the use or usage conditions. In the case of a fuel cell for commercial use, however, the fuel cell is usually continuously operated throughout the night; thus, a catalyst lifetime of at least 1 year is required, and generally, a long-term stability of 10 years (approximately 90,000 hours when converted to hours) or longer is desired.
In view of these issues, catalysts in which Ru (ruthenium), a notably inexpensive noble metal which also has a high reactivity, is supported on a carrier such as alumina, for example, have been widely studied (Patent Literature 1). Ru, however, is likely to cause the generation of ammonia due to nitrogen contained in natural gas or the like. The major issue with ammonia is that it poisons the electrode catalyst in a fuel cell, which causes the system reliability to decrease.
There is a method that uses Ni (nickel) as a metal that can inhibit the generation of ammonia, and allows a reduction in costs; however, this also has a major problem in that it causes the generation of carbon (coke) during the steam reforming reaction. To remedy this problem, a catalyst has been proposed in which a combination of Ni with a minute amount of Pt (platinum) is carried on an alumina carrier having a rare-earth element and an alkaline earth metal supported thereon (Patent Literature 2). While this method can inhibit the generation of ammonia, there is a problem in that, because Ni is used as a main component, the deposition of coke cannot be completely prevented, and the activity will decrease if the reaction is continued for 1,000 hours or longer.
On the other hand, a system that uses rhodium (Rh) instead of Ru or Ni is known, and this system is considered to be preferable due to its high activity. There is, however, a problem in using Rh as an active metal in that ammonia is generated when a nitrogen-containing feed gas is used. Thus, a catalyst in which an alloy of Rh and Pt is directly supported on an α-alumina carrier having a purity of 99.99% or more has been proposed to be preferable in terms of preventing the generation of ammonia (Patent Literature 3). The catalyst of Patent Literature 3, however, has, low activity as will be shown in Comparative Example 5 below (corresponding to the catalyst disclosed in Patent Literature 3); moreover, although the catalyst can suppress the generation of ammonia, when a sulfur compound-containing hydrocarbon gas is used, the activity of the catalyst decreases due to sulfur poisoning of the catalyst. This is believed to occur because Pt readily adsorbs sulfur. That is, in conventional Rh-based catalysts, the use of Rh alone causes the generation of ammonia; however, the addition of Pt to prevent this has the problem of a trade-off relationship in that the activity tends to decrease due to sulfur poisoning.
A catalyst has been proposed in which a Rh-based metal is supported on an α-alumina carrier modified with cerium (Ce) and an alkaline earth metal, as a catalyst that can solve the aforementioned problem in the Rh system, and can be used without a desulfurization step (Patent Literature 4). In the Examples of Patent Literature 4, a system containing Rh as a single component as the active metal is shown (Example 1, for example), using LP gas containing 5 ppm of sulfur compound. This system, however, is not preferable in terms of ammonia generation, although it is resistant to sulfur poisoning. Example 3 of Patent Literature 4 also illustrates an alloy system having a loading amount of 1.5% of Rh and a loading amount of 0.5 wt % of Pt, relative to the weight of the carrier. Such a high Rh loading amount, however, is not practical for putting hydrogen fuel into widespread use, because, even if it is effective in reducing the influence of sulfur poisoning, it increases the production costs markedly.
On the other hand, a method is known in which steam reforming is performed after desulfurization with a desulfurizer placed upstream from the reformer, thereby preventing poisoning of the reforming catalyst. In such modes of desulfurization, an ordinary temperature desulfurizer using an adsorbent such as a silver zeolite, which can decrease sulfur to several ppb or less (Patent Literature 5), and a super higher-order desulfurizer using a Cu—Zn-based material (Patent Literature 6), are known. The placement of such a desulfurizer, however, increases the costs of the system as a whole, and thus, is difficult to put into practical use, unless the increase can be compensated for by decreasing the costs in the reforming catalyst's part. That is, the use of a desulfurizer in combination requires a stricter cost reduction in the reforming catalyst, and no steam reforming catalyst is known which is unlikely to be affected by the sulfur component, at such a low cost that it can meet that expectation.
As stated above, a catalyst for steam reforming of hydrocarbons has not been heretofore obtained which gives a high conversion ratio, is minimally affected by a sulfur compound, nitrogen gas or the like contained in a hydrocarbon feedstock, has less carbon deposition during the reaction, and exhibits practically stable performance even over a long time, i.e. 10 years (approximately 90,000 hours when converted to hours) or longer. Furthermore, even in combination with a desulfurizer, a catalyst that is low enough in price to put into general widespread use cannot be provided under the actual circumstances.