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Phosphates as surface-functional materials were investigated. Boron phosphate (BPO4) , which is known to be a solid acid catalyst, showed prominent activity for the oligomerization of light olefins giving a liquid fuel, although other phosphates were inactive. Its activity was enhanced by grinding before use or by adopting a new preparation method characterized by supercritical drying As a catalyst support, AlPO4 was examined for selective catalytic reduction of NO in exhaust gas to N2 by hydrocarbon in the presence of excess oxygen. It was found that the activity of Pt(0.5wt%)/AlPO4 catalyst was higher than that of conventional Pt(0.5wt%)/Al2O3 catalyst, although AlPO4 alone was hardly inactive. Next, metal phosphates were examined as humiditysensing materials, and several new candidates including Ti(HPO4)2•E•H20 were found . The humidity-sensing characteristics of Ti(HPO4)2•EH2O was considerably improved by treating with a NaOH solution. In addition, we found that Cu-P-Y-O ceramics, which were produced by the solidstate reaction between Cu3(PO4)2•E3H2O and Y2O3, showed good humidity-sensitive characteristics; its electric resistance decreased by three orders in magnitude as the surrounding relative humidity increased from 0% to 96%. INTRODUCTION Phosphates are utilized for detergents, food additives, fertilizers, corrosion inhibitors, etc. as bulk materials, and for adsorbents, catalysts, chemical sensors, etc. as surface-functional materials. The interaction of some guest molecules with the surface of a host functional phosphate results in (1) adsorption on the surface, (2) activation of the adsorbed molecules, (3) chemical reaction on the surface, (4) change in electric or electronic properties of the host phosphate, etc. The present paper describes several surface-functional phosphates exemplified below. PHOSPHATES AS THE MATERIALS FOR CATALYST Boron phosphate, BPO4, as a solid acid catalyst Light olefins can be converted to a liquid fuel by their oligomerization over acid catalysts. There are a number of oligomerization catalysts [1] such as liquid acids, solid phosphoric acid, clays, and silica-alumina. In 1981, one of the authors [2] has reported the potentiality of high-temperature outgassed boron phosphate catalyst. Recently, zeolites, particularly H-Fesilicate [3] and H-Co-silicate [3] have been found to be very effective. The products of the oligomerization of light olefins depend on catalysts, reaction conditions, kind of olefins, and so on. In order to develop an effective olefin oligomerization process fitting one's purpose, it is necessary to understand comprehensively the activities and selectivities of various oligomerization catalysts. 100 Proceedings of IMPHOS-IWPPC-Jena 2002 Phosphorus Research Bulletin Vol. 15 (2004) Boron phosphate, BPO4(RP) was prepared by heating an equimolar mixture of boric acid and phosphoric acid at 40 •Ž in a rotary evaporator followed by drying in situ at that temperature, heating at 150 •Ž for 24 h, and finally calcining at 360 •Ž for 12 h [4]. Another boron phosphate, BPO4 (SC) was prepared by adding acetic anhydride to a mixture of boric acid, phosphoric acid, acetic acid and methyl acetate [5]. The grinding of BPO4 sample was performed with a FRITSCH planetary micro mill pulverisette [6]. The oligomerization of olefins was carried out using a closed recirculation reactor (volume = 600 ml). Figure 1 shows the results of propene oligomerization over BPO4 (RP) at 0 200 •Ž. The progress of the reaction is expressed as a rate of decrease in total pressure, 100•E(Po P)/Po, where P0 is the initial (total) pressure of the monomer and P is total pressure at any reaction time . At 0 100 •Ž, the activities of various catalysts for the oligomerization of propene decreased in the order, BPO4 (RP) > SiO2-Al2O3 > HY-zeolite > H-ZSM-5, SO4/ZrO2. Figure 2 summarizes the product distributions of propene oligomerization at 0 200 •Ž for 20 h over BPO4. At 0 •Ž the products were rich in relatively higher molecular weight substances such as C12-15 compounds. With a rise in temperature, the percentage of such substances decreased and that of C6 compound increased. At 100 •Ž, both C8 and C9 were formed in abundance. Figure 3 shows the products of propene oligomerization at 50 200 •Ž for 20 h over SiO2-Al2O3. With increasing temperature, the percentages of C6 and C7 compounds increased. As can be seen in Figure 4, the products of propene oligomerization at 150 •Ž over HY were similar to those over SiO2-Al2O3. However, at 100 •Ž, C10-15 compounds were appreciably produced. Figure 1: Oligomerization of propene over BPO4 (RP) at various temperatures. Initial pressure (Po) of propene = 600 Torr, total pressure at any time = P, weight of catalyst = 0.5 g, calcination 500 •Ž, 2 h; evacuation 500 •Ž, 2h Figure 2: Products of the oligomerization of propene over BPO4 (RP) at various temperatures. Reac. time = 20 h Proceedings of IMPHOS-IWPPC-Jena 2002 101 Phosphorus Research Bulletin Vol. 15 (2004) Figure 3: Products of the oligomerization of propene over silica-alumina at various temperatures. Reac. time = 20 h Figure 4: Products of the oligomerization of propene over HY zeolite at various temperatures. Reac. time = 20 h Table 1 The specific surface area and the activity of BPO4(RP) for the oligomerization of cis-2-butene before and after grinding Amount of cis-2-butene = 70 Torr, 0.5 l; weight of catalyst = 0.20 g; reac. temp. = 0 •Ž 102 Proceedings of IMPHOS-IWPPC-Jena 2002 Phosphorus Research Bulletin Vol. 15 (2004) Generally the catalytic activity (per weight) of a solid catalyst depends on its specific surface area. As seen in Table 1, the specific surface area of BPO4(RP) was increased by grinding it and as a result its activity for the oligomerization of cis-2-butene was also increased [6]. In order to prepare highly porous BPO4, a new preparative method has been developed. The method is characterized by both the reaction of H3PO4 and H3BO3 in a mixture of acetic acid and methyl acetate and the subsequent supercritical drying [5]. Clearly, the activity of BPO4 (SC) was much higher than that of BPO4 (RP) for propene oligomerization at 50 •Ž (Figure 5). Aluminum phosphate (amorphous), AlPO4, as a catalyst support Selective catalytic reduction of NOx to N2 by a hydrocarbon (HC) in the presence of excess oxygen, HC-SCR, has been investigated over various catalysts such as zeolites, metal oxides, and noble metals as one of important reactions for purifying the automotive exhaust gas of a diesel engine or a lean-burn engine [7]. One of the most extensively studied metal oxides is alumina and it is often used as a metal-loaded alumina (M/Al2O3) [8-10]. Aluminum phosphate, however, has seldom been used as a catalyst material for HC-SCR catalysts. Aluminum phosphate was prepared by neutralizing a solution containing an equal molar ratio of aluminum nitrate and phosphoric acid with ammonia solution [11]. Catalytic reactions were carried out with a fixed bed flow reactor by passing a reactant gas mixture at a total flow rate of 130 ml•E•min-1 over a catalyst (0.40g). The gas mixture contained 1000 ppm NOx, 2.0% O2,500 ppm C2H4 or C3H6 as a reductant and balance helium. Contrary to our expectation, AlPO4 alone was inactive for both C2H4-SCR and C3H6-SCR, while Al203 was considerably active. As a result of examining many metal-loaded catalysts, it was found that the activity of Pt (0.5wt%)/AlPO4 catalyst was higher than that of conventional Pt (0.5wt%)/Al2O3 catalyst for both C2H4-SCR and C3H6-SCR as shown in figure 6. In C2H4-SCR, the maximal NO conversion value of Pt(0.5wt%)/AlPO4 is similar to that of Pt (0.5wt%)/Al2O3, but the temperature window of Pt (0.5wt%)/AlPO4 shifts to lower temperature by 50•Ž compared with that of Pt (0.5wt%)/ Al2O3. The superiority in activity of Pt (0.5wt%)/AlPO4 is more evident for C3H6-SCR. Figure 5: Oligomerization of propene over-BPO4 (SC) and BPO4 (RP). Pressure of propene = 600 Torr, weight of catalyst = 0.5 g calcination 500 •Ž, 2 h; evacuation 500 •Ž, 2 h; reac. temp. = 50 •Ž Proceedings of IMPHOS-IWPPC-Jena 2002 103 Phosphorus Research Bulletin Vol. 15 (2004) PHOSPHATES AS THE MATERIALS FOR CHEMICAL SENSORS Humidity-Sensitive Characteristics of Various Metal Phosphates A ceramic humidity sensor has advantages of both chemical and thermal stability; thus, many metal oxides such as ZnCr2O4/LiZnVO4 [12], niobia [13], Al2O3-TiO2-SnO2 [14] have been used as suitable materials. Needless to say, humidity-sensor materials should be insoluble in water, because they must be durable for adsorption/desorption cycles of water vapor. From this viewpoint, metal phosphate can be another candidate. Actually, hydroxyapatite [15], AlPO4 [16], and Zr(HPO4) 2 [17,18] have been investigated as a ceramic humidity-sensing material. However, there still seems to be room for finding of other metal phosphates as a ceramic humidity-sensing material. All metal phosphates were prepared by the methods given in the literature [19] . Sodium cation exchanged metal phosphates were also prepared [19]. Every precalcined sample was pulverized , pressed into a disc with 13 mm in diameter and 0.5-1 mm in thickness. Finally every disc was sintered at 800 •Ž for 3 h in air. An OS-type element was prepared by painting the comb-pattern electrodes shown in figure 7 on the one side of a disc with a silver paste and then by heating at 700•Ž for 10 min. A sensor element was placed in a test chamber (200 ml) containing a suitable saturated salt solution (40 ml), which gives a specified relative humidity . The four-point resistance measurements of sensor elements were performed at 25 •Ž by using a LCR meter (YHP4261A) under a applied voltage of 1 V AC (1 kHz). a)

Phosphates and Hydrogeno-Phosphates as Materials for Catalysts and Humidity Sensors