Efficient reduction of nitric oxide using zirconium phosphide powders synthesized by elemental combination method

Zirconium phosphide (ZrP) powders were synthesized by elemental combination method via the direct reaction of zirconium powders with red phosphorus, and characterized by XRD, SEM, XPS, XRF, SAED and TEM measurements. The obtained ZrP powders were found to exhibit apparent activity in the ready eliminateion of nitric oxide (NO) via facile redox reactions, and the elimination dynamics was evaluated within the context of various important experimental parameters, such as reaction temperature and gas concentration. At a fixed amount of ZrP powders, an increasing amount of NO would be eliminated with increasing reaction temperature, and complete conversion of NO to N2 could be reached in the range of 700 to 800 °C. The addition of NH3 also facilitated NO elimination at a fixed reaction temperature. Furthermore, of the products of the elimination process, zirconia (ZrO2) powder is a kind of biocompatible material, red phosphorus can be used to produce safety matches, organophosphorous pesticide and phosphor bronze, and the produced N2 might be collected and used as a protective gas or be converted into liquid nitrogen for other purposes.

Nitrogen oxides (NO x ) are harmful gases that give rise to a variety of environmental problems, such as acid rain, photochemical smog, and ozone depletion, which threaten human health to a great extent [1][2][3] . In the pursuit of a better living environment and with increasingly stringent environmental regulations, NO x emissions have become a research hotspot in the field of environmental science and engineering. NO x storage and reduction (NSR) plays an important role in controlling NO x emissions from automobile sources while permitting operation under predominantly lean-burn conditions [4][5][6] . So far, the most efficient technique to control NO x emissions from coal-fired power plants and automobiles is the selective catalytic reduction (SCR) of NO (4NO + 4NH 3 + O 2 = 4N 2 + 6H 2 O) [7][8][9][10][11] and NO 2 (NO + 2NH 3 + NO 2 = 2N 2 + 3H 2 O) with NH 3 using various catalysts 12,13 . For SCR of NO by NH 3 , it has been found that NO and NH 3 are introduced into the reaction vessel at a ratio of 1:1, and a certain amount of O 2 is essential for the reaction. However, the excessive use of NH 3 can cause air pollution and the corrosive nature of NH 3 is also harmful to the experimental apparatus, which may cause secondary pollution. Therefore, it is necessary to reduce the use of NH 3 for the elimination of NO. Also, the removal of NO x is generally carried out at high temperatures (>600 °C), where many catalysts may lose their activity [14][15][16][17][18] , leading to reduced elimination efficiency of NO x . Within this context, it is extremely urgent to find more suitable active species or to develop an efficient, green approach to the elimination of NO x at high temperatures.
Transition metal phosphides (e.g., ZrP, FeP, Ni 2 P, etc.) are known for their hardness and chemical inertness even at elevated temperatures [19][20][21] . In the past few years, substantial efforts have been devoted to the study of the catalytic properties of transition metal carbides and nitrides 22,23 . Notably, transition metal phosphides possess similar physical properties and even better catalytic activity and selectivity in many reactions such as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and have become a new research focus in the field of catalytic materials and water oxidation catalysis. For instance, Ni 2 P has been used as an excellent catalyst precursor for water oxidation catalysts 24 , and FeP as an efficient catalyst for HER 25,26 . Also, there are many studies on the catalytic performance of zirconium phosphate (ZrP 2 O 7 ) 27-31 . In contrast, whereas ZrP has been synthesized by a variety of methods in the past decades, such as the reactions of zirconium metal with red phosphorus (elemental combination method) 32,33 , zirconium or zirconium tetrachloride with Ca 3 P 2 20 , zirconium with PH 3 33 , or sodium co-reduction of ZrCl 4 and PCl 3 34 , there are few reports on the chemical properties of ZrP and, up to date, there is no study of the activity of ZrP towards NO elimination.
In this study, ZrP was synthesized by elemental combination method via the direct reaction of zirconium powders with red phosphorus in a quartz tube. Since the phosphorus atoms in the ZrP are negatively charged (P 3− ) and the nitrogen atoms in the NO are positively charged (N 2+ ), ZrP might be used to reduce NO by the reaction, ZrP + 2NO → ZrO 2 + N 2 + P. The results showed that the efficiency of NO elimination by ZrP increased with increasing temperature. In addition, it was found that when a small amount of NH 3 was added to the reaction system, the following reaction could occur, 8ZrP + 22NO + 4NH 3 → 8ZrO 2 + 13N 2 + 8P + 6H 2 O, leading to enhanced efficiency of NO elimination. The experimental results showed that at the ratio of the NH 3 to NO concentration of 2:11, 0.5 g of ZrP powders was sufficient for the complete reduction of 500 ppm NO gas for up to 14 h at 750 °C. As for the products of the elimination reaction, ZrO 2 powders can be used as a biocompatible material 35,36 and catalysts/catalyst support, red phosphorus can be used to produce safety matches and organophosphorous pesticide, and the produced N 2 may be collected and used as a protective gas or be converted into liquid nitrogen for other purposes. In this study, we not only identified an active species for NO elimination, but also reduced the amount of NH 3 for NO reduction. The elimination process was of high efficiency and all reaction products could be used for other purposes.  (Fig. 2a,b), it can be seen that ZrP was an irregular bulk material. TEM characterization. The microstructures of ZrP were also investigated by TEM measurements (Fig. 2c).

Structures of
From the HRTEM image (Fig. 2d), several lattice fringes of 3.2 Å and 2.5 Å can be observed clearly, in good agreement with the crystal planes (100) and (103) of ZrP. In addition, the corresponding SAED patterns (inset to Fig. 2d) can also be assigned to single-crystalline ZrP 42-44 . XPS characterization. In view of the same diffractions of XRD for ZrP samples prepared at different temperatures (Fig. 1a), only the typical ZrP samples synthesized at 800 °C was selected for XPS, XRF characterization. XPS analyses of the ZrP samples are shown in Fig. 3. The survey spectrum (Fig. 3a) shows that the sample surface consists of zirconium and phosphorus, and based on the integrated peak areas, the mole ratio of Zr/P is estimated to be 1.3:1 (Fig. 3b,c), which is close to that of ZrP. The oxygen may come from surface adsorption. Carbon is also found, which may be from carbon dioxide adsorption. XRF characterization. XRF analysis was then carried out to further analyze the composition of the ZrP samples, and the results are summarized in Table 1. It can be seen that for the as-prepared sample, the mole ratio of Zr/P is estimated to be 1.4:1, which is close to 1:1 (when oxygen adsorption on the sample surface was excluded). Yet, after reacting with NO at 700 °C and 750 °C, the relative content of phosphorus in the reaction products diminished with the calculated mole ratio of Zr/P increased from 2.1:1 to 3.6:1. This is consistent with the results shown in Supporting Figure S1. Figure 4 showed the reaction of ZrP with NO at different temperatures. Specifically, Fig. 4a shows that when the reaction temperature between ZrP and NO was controlled at 650 °C, it was worth noticing that ZrP samples (prepared at 800 °C, 900 °C, 1000 °C, respectively) would eliminate almost the same amount of NO from three almost identical curves, which indicated that the temperature of synthesis for ZrP had little influence on the elimination of NO at 650 °C. As shown in Fig. 4b-d, when reaction temperatures between ZrP and NO were increased to 700 °C, 750 °C and 800 °C, the elimination amount of NO over ZrP prepared at different temperatures still had no obviously difference. So ZrP samples synthesized at 800 °C was typically selected to the next study of elimination for NO. Figure 5a shows the effect of reaction temperature for the elimination of NO in the temperature range from 650 °C to 800 °C. In the course of experiment, a mixture of gases containing 500 ppm NO balanced with N 2 was introduced into the reactor at the 174th min. It can be seen that at the controlled temperature of 650 °C, after the introduction of NO, the concentration of NO at the outlet increased rapidly and equal to the concentration of NO in the inlet, which means that there was no reaction between NO and ZrP. Yet, when the temperature was raised to 800 °C, no signal of NO was detected at the outlet for 1080 min (18 h), indicating that NO was completely reduced by ZrP in the reactor within this period. At lower temperatures (700 and 750 °C), the period of time where no NO was detected was shorter, suggesting that temperature was an important factor for the elimination of NO. Such a disparity of the reaction activity can also be manifested in XRD measurements of the solids after reaction. From Supporting Figure S1, we can see that when the reaction temperature was controlled at 650 °C, the diffraction patterns of the solids after reaction were consistent with those of the ZrP, as there was no reaction with NO at this temperature. When the reaction temperature was raised to 700 °C, the solids were found to consist of a mixture of ZrP and ZrO 2 , indicating that part of the ZrP was oxidized into ZrO 2 due to reduction of NO to N 2 . At the even higher temperature of 800 °C, no diffraction pattern of ZrP was detected and only ZrO 2 diffraction patterns were observed, most likely due to complete consumption of ZrP in the reduction of NO, (ZrP + 2NO = ZrO 2 + N 2 + P).

Elimination of NO.
The tests were also carried out to elucidate the influence of NH 3 addition in NO elimination. As depicted in Fig. 5b, prior to the addition of NH 3 , 0.5 g of ZrP powders can reduce 500 ppm NO gas for 5 h at 750 °C. With the addition of NH 3 into the reactor where the molar ratio of NH 3 :NO increased from 1:3 to 2:3, the time for the complete conversion of NO into N 2 was increased from 6 h to 8 h. Yet, too much NH 3 is not conductive to the elimination of NO. For instance at the mole ratio of NH 3 :NO = 3:3, the time for the complete conversion of NO by ZrP actually decreased to only 3 h, even shorter than that without any ammonia addition at the same condition (~5 h). The optimal NH 3 :NO ratio was identified at 2:11, where 0.5 g of ZrP powders were found to reduce 500 ppm NO gas for up to 14 h at 750 °C. This may be attributed to the following reaction, 8ZrP + 22NO + 4NH 3 → 8ZrO 2 + 13N 2 + 8 P + 6H 2 O. From this reaction, we can see that at a fixed amount of ZrP, the addition of a small amount of NH 3 could facilitate the reduction of NO. In fact, this elimination process not only reduces the use of NH 3 , but also greatly improves the elimination efficiency of NO as compared to SCR of NO by NH 3 .
Verification of red phosphorus. After elimination reactions of ZrP with NO, in addition to the product of ZrO 2 , some red brown powders were found adhering to the inner wall at the end of the quartz tube (Supporting Figure S2a), which were collected, dried, and subject to a combustion test. The results showed that the powders burned violently, accompanied by the generation of white smoke (Supporting Figure S2b) and irritating smell, forming a white solid product. These observations were consistent with the combustion of red phosphorus, suggesting red phosphorus as the part of the elimination reaction of NO by ZrP.

Discussion
ZrP powders were prepared by thermal treatment of zirconium and red phosphorus in an argon atmosphere at controlled temperatures, which exhibited apparent activity in the reductive elimination of NO to N 2 . It was found that the reaction temperature and concentration of NH 3 were important factors that affected the elimination efficiency of NO by ZrP. The products of the elimination process included ZrO 2 , N 2 , H 2 O (if NH 3 was added) and red phosphorus. In summary, a new method based on ZrP was developed for the reductive elimination of NO, where the reaction products might be collected and used for other applications.

Methods
Material Preparation. All chemicals were purchased from Alfa Aesar and used as received without further purification. To prepare ZrP, zirconium powders were placed in a quartz boat at one end of a quartz tube and the required quantity of 99.99% pure red phosphorus powders were placed at the other end under an atmosphere  of purified argon. The temperature was slowly raised (2 °C/min) from room temperature to 800 °C~1000 °C and kept for 6 h 37,38 .
Elimination of NO. The elimination of NO was carried out in a fixed-bed quartz tube reactor with an internal diameter of 6 mm 39 . 0.5 g of ZrP powders were sieved with a 40-60 mesh and placed on the quartz wool held in the reactor, and the reactor was heated by a vertical electrical furnace. The total flow rate was 198 mL·min −1 (room temperature), the mass of ZrP was 500 mg, and the corresponding gas hourly space velocity (GHSV) was 6 × 10 4 cm 3 ·g −1 ·h −1 , which was evaluated by the equation (1): where q v is the total flow rate, h is the height of the reactant in the reactor and r is the radius of the reactor 40 . The feed contained 500 ppm of NO, 500 ppm of NH 3 (when used), and balance of N 2 . The concentration of NO was continuously detected by a gas chromatographic analyzer equipped with a flame photometric detector (Beijing Beifen-Ruili 3420 A). The NO conversion was calculated according to the equation (2): Data Availability. All data generated or analysed during this study are included in this article (and its Supplementary Information files).