Low-Temperature Ammonia Oxidation in a Microchannel Reactor with

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 9819−9828

pubs.acs.org/IECR

Low-Temperature Ammonia Oxidation in a Microchannel Reactor with Wall-Loaded X(X = Pt, Pd, Rh, PtPdRh)/TiO2 Nanotube Catalysts Qiang Liu, Wei Qiu, Pan Wu, Hairong Yue, Changjun Liu, and Wei Jiang* Low-Carbon Technology and Chemical Reaction Engineering Laboratory, School of Chemical Engineering, Sichuan University, Chengdu 610065, PR China

Downloaded via UNIV STRASBOURG on August 21, 2019 at 19:27:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: In this research, wall-loaded X(X = Pt, Pd, Rh, PtPdRh)/TiO2 nanotube (NT) catalysts were prepared by deposition via photoreduction with anodized TiO2 nanotubes and were assembled in a microchannel reactor to catalyze ammonia oxidation. Results showed that Pt/TiO2-NTs gave NH3 conversion and NOx selectivity as high as 100% at 280 and 380 °C, respectively. Simulations confirmed that the NOdissociation energy on Pt(111) is the highest, although the adsorption energy of NH3 and O2 on the Pt(111) surface was between those on Rh(111) and Pd(111), resulting in the easiest NO desorption and, consequently, the best catalytic performance of Pt/TiO2-NTs. reactor when the reaction temperature was only 425 °C, which is far below the industrial ammonia-oxidation reaction temperature of 800 °C. Jiang et al.16 loaded Pt onto a TiO2 plate to catalyze ammonia oxidation in a microchannel reactor, and NH3 conversion reached 100% at 350 °C. The above two examples exhibit the application potential of microchannel reactors for ammonia oxidation. In the same process,16 the photoreduction-deposition process of Pt is used to load it onto TiO2, a photocatalyst support, realizing reduction and high dispersion of Pt species. This method is an effective approach to deposit noble metals, such as Pt, Pd, Ag,17 and Au,18 most of which can be used in nitric acid as catalysts. The photoreduced Pt deposited on TiO2,19 CeO2,20 and ZrO221 has been prepared and used as an ammonia-oxidation catalyst, and complete NH3 conversion was reached at 300−600 °C with an NOx yield of 71% at 800 °C. The above work confirmed the effectiveness of photodeposited Pt on different photocatalyst supports, but the reaction temperature for obtaining acceptable NOx yield was still too high. Thus, it is necessary to introduce different possible ammonia-oxidation catalyst species onto photocatalyst supports and assemble them in a microchannel reactor, improving the catalyst performance and decreasing the reaction temperature. In this study, the usual ammonia-oxidation catalyst species, Pt, Pd, Rh, and PtPdRh alloy, are loaded onto TiO2 nanotubes to construct a wall-loaded catalyst by a photoreduction-

1. INTRODUCTION The demand for nitric acid has increased with the development of agriculture, medicine, military industries, and other fields.1 Ammonia oxidation is the core reaction for converting ammonia to NOx for nitric acid. However, its strong exothermicity produces high reaction temperatures and, consequently, a serious loss of platinum, which is the common catalyst for ammonia oxidation and can generate volatile PtO2 at higher temperatures. Thus, the expense of a platinum catalyst is the second highest cost for nitric acid production after the cost of the raw material, ammonia.1,2 Reducing the reaction temperature of ammonia oxidation with acceptable production yield and consequently reducing the catalyst consumption is attractive for nitric acid development.1,3,4 Some nonplatinum catalysts have been developed, but they have not yet been applied in industry because of their low NH3 conversion, low selectivity, and harsh reaction conditions.5 The current catalyst employed widely for nitric acid production is still Pt-woven gauze and its derivatives, such as a Pt−Pd−Rh gauze catalyst, because of the merits of a large specific surface area and low pressure drop.6 However, there are two unsolved difficulties waiting to be overcome: platinum loss due to high reaction temperatures and unsatisfactorily low surface areas compared with those of impregnated catalysts prepared with porous supports and nanocatalysts.7−9 The wall-loaded catalyst used in the microchannel reactor is a potential solution to the above problems in the ammoniaoxidation process. As is known, the microchannel reactor can improve the transfer efficiency of mass and heat, which is especially conducive to the improvement of highly exothermic heterogeneous reactions.10−15 Srinivasan et al.10 found that NH3 conversion reached 100% in a homemade microchannel © 2019 American Chemical Society

Received: Revised: Accepted: Published: 9819

February 28, 2019 May 13, 2019 May 19, 2019 May 22, 2019 DOI: 10.1021/acs.iecr.9b01135 Ind. Eng. Chem. Res. 2019, 58, 9819−9828

Article

Industrial & Engineering Chemistry Research

Figure 1. (A) Preparation of supports by calcination and catalysts by photodeposition. (B) Preparation of supports by anodization and catalysts by photodeposition. (C) Catalyst evaluation.

Figure 2. (A−D) XRD characterization of Pt/TiO2, Pt/FexOy, Pt/Al2O3, and Pt/TiO2-NTs. (E−H) SEM images of TiO2, FexOy, Al2O3, and TiO2NTs. (I−L) SEM images of Pt/TiO2, Pt/FexOy, Pt/Al2O3, and Pt/TiO2-NTs.

acetone, and deionized water (Figure 1A). The TiO2-nanotube (NT) support was prepared by anodization,16,21 according the path described in Figure 1B. The titanium plate, after being polished and washed, was placed into an electrolyte composed of 1.5 mL of hydrofluoric acid, 23.7 mL of phosphoric acid, and 300 mL of deionized water. After a 2 h anodization process with 20 V of external constant voltage at 30 °C, the treated Ti foil was washed ultrasonically with dehydrated ethanol for 2 min to remove the residue. The washed sample was calcined at 550 °C for 1 h to obtain the final TiO2-NT product. Platinum was loaded on the as-prepared supports via facile photocatalytic reduction deposition.19,22,23 Potassium chlor-

deposition process and assembled in a microchannel reactor to evaluate their performance. The catalyst with the best performance is selected, and the reason for the difference in performance is investigated with density-functional-theory simulations.

2. EXPERIMENT 2.1. Preparation of Wall-Loaded Catalysts. Different metal-plate supports (titanium (99.9%), iron (99.9%), and aluminum (99.95%) plates) with dimensions of 35 × 55 mm were directly calcinated in a muffle for 1 h at 550 °C after being polished with sandpaper and washed with ethanol, 9820

DOI: 10.1021/acs.iecr.9b01135 Ind. Eng. Chem. Res. 2019, 58, 9819−9828

Article


Figure 3. (A) Blank evaluation of the supports. (B) NH3 conversion and NOx selectivity of different wall-loaded catalysts.

3. RESULTS AND DISCUSSION 3.1. Influence of Support Type on Ammonia Oxidation. Four supports (TiO2, FexOy, Al2O3, and TiO2NTs) were used to prepare Pt wall-loaded catalysts. The existence of Pt species could be detected on all these plate supports except Al2O3, as shown in Figure 2A−D. In Figure 2A, strong metallic Ti peaks (PDF#44-1294) at 35.093 and 38.421° and weak rutile TiO2 peaks (PDF#21-1276) at 27.446 and 36.085° can be determined before and after Pt loading. In addition, the weak peaks of Pt (PDF#04-0802) at 39.763 and 46.243° are detected. The metallic Fe (PDF#06-0696) and two iron oxides, Fe2O3 (PDF#33-0664) and Fe3O4 (PDF#19-0629), which are mainly at 33.152 and 40.854° and at 35.422 and 62.515°, respectively, are shown in Figure 2B, and the Pt (PDF#04-0802) peaks are also at 39.763 and 46.243°. As shown in Figure 2C, only metallic Al (PDF#04-0787) was detected as a result of the low quantity of the generated Al2O3 layer. In Figure 2D, the XRD result is the same as that of thermal-treated Ti foil, although the support is prepared by anodization. The presence of Pt species on the four support surfaces can be determined by the SEM results from Figure 2E−L before and after photoreduction deposition. The first four images, Figure 2E−H, show the morphology of the original support surface before loading, which was smooth except for the wellarranged TiO2 support nanotubes with uniform diameters of ca. 50 nm. After photodepositing Pt, structures of nanoparticles of Pt/TiO2 in Figure 2I, nanoflowers of Pt/FexOy in Figure 2J, and nanosheets of Pt/Al2O3 in Figure 2K were observed to have grown on the surface, confirming the loading of Pt. However, for TiO2-NT support, two forms of deposited Pt can be detected in Figure 2L: small nanoparticles with diameters of 10−20 nm inside the tubes and loose aggregates with a diameters of 200−300 nm at the surface. The four wall-loaded catalysts were assembled in a microchannel reactor to carry out the performance evaluation, and four blank supports were used for comparison. The results shown in Figure 3A confirm no catalytic activity of the blank supports at 200−400 °C; they can be regarded as inert. Before evaluating the four catalysts, a simple experiment to eliminate external and internal diffusion was conducted, and it showed that both were eliminated completely in the experimental conditions (Figure S8). However, after loading Pt, the four catalysts could all effectively catalyze the ammoniaoxidation process. As shown in Figure 3B, when the

oplatinate, the precursor, was dissolved in 10 mL of deionized water, with 0.2 mL of formic acid serving as a scavenger. Then, the plate support was immersed in solution and irradiated for 2 h with a 500 W ultraviolet lamp. Afterward, the sample was dried at 100 °C and calcined for 1 h at 500 °C to obtain the finished catalyst product. 2.2. Performance Evaluation of Wall-Loaded Catalysts. The catalytic performance of the wall-loaded catalyst was carried out in a homemade microchannel reactor, in which each channel was 45 mm long, 0.3 mm wide, and 1 mm high, and there were seven channels per reacting unit, as shown in Figures 1C and S1. In the homemade microchannel reactor, the main bodies were the back plate, catalyst plate, channel plate, and cover plate (inside and out), and the plates were compacted by pressure. In addition, the performances of the microexchangers, fluid temperature, and pressure drop were investigated by CFD simulations, which are showed in Figures S2−S7. After being assembled in the reactor, the catalyst was activated under NH3 atmosphere for 2 h. The ammoniaoxidation process was conducted under an arbitrary temperature with an airspeed of 10 900 h−1 and a molar oxygen to ammonia ratio of 4:1, which were the reported optimal operation conditions.16 The off-gas, dried with CaO, entered a gas chromatograph (Fuli 9790II) with a GDX-102 column to separate ammonia and a 5 Å molecular sieve column to determine nitrogen. The NH3-conversion rate and NOx selectivity were calculated according to the following formulas: X(NH3) (%) =

NH3in − NHout 3 NH3in

× 100%

ÄÅ ÉÑ ÅÅ ÑÑ 2N2 ÑÑ × 100% S(NOx ) (%) = ÅÅÅÅ1 − ÑÑ in out ÅÅÅ ÑÑ NH − NH 3 3 Ñ Ç Ö

(1)

(2)

2.3. Catalyst Characterization. An X-ray diffractometer (X’per Pro MPD, Panaco) was used to determine the crystals of the catalysts. Scanning electron microscopy (JSM-5900LV, JEOL) was used to determine the surface structure. X-ray photoelectron spectroscopy (VG ESCALAB MKII, Thermo) was used to characterize the surface elements and their valence states. X-ray fluorescence spectrometry (axios-pw4400, Panaco) was used to determine the type and contents of catalyst elements. 9821


Article


Figure 4. (A) XRD of TiO2-NTs, Pt/TiO2-NTs, Pd/TiO2-NTs, and Rh/TiO2-NTs and (B) XRD of PtPdRh/TiO2-NTs. (C−F) SEM images of (C) Pt/TiO2-NTs, (D) Pd/TiO2-NTs, (E) Rh/TiO2-NTs, and (F) PtPdRh/TiO2-NTs.

temperature increased from 200 to 280 °C, NH3 conversion for all four catalysts reached 100%. However, their NOx selectivities were completely different. The NOx selectivity of Pt/Al2O3 was the worst, with values decreasing from 78 to 67%, and Pt/FexOy was the second worst, with values from 80 to 70%. For Pt/TiO2 and the Pt/TiO2-NTs, their initial NOx selectivities were both approximately 86% at 200 °C, but the declining tendency of the Pt/TiO2-NTs was significantly smaller than that of Pt/TiO2. At 260 °C, the NOx selectivity of Pt/TiO2 decreased to 68%, but that of Pt/TiO2-NTs was maintained at a higher value of 75%. Because the components and Pt loading of the two catalysts were the same, it can be determined that it is the nanotube structure of the TiO2-NT support that helps improve the catalyst thermal stability. Thus, the TiO2-NT support was selected for further research. 3.2. Influence of Active Metal on Ammonia Oxidation. Because the TiO2-NTs were selected as the support for preparing wall-loaded ammonia-oxidation catalysts, different active species, including Pt, Pd, Rh, and their codeposited ternary mixtures based on the commercial three-way catalyst PtPdRh (Pt/Pd/Rh = 1:4:3.5),24 were loaded onto TiO2-NTs. The Pt/TiO 2 -NTs, Pd/TiO 2 -NTs, Rh/TiO 2 -NTs, and PtPdRh/TiO2-NTs obtained by photoreduction deposition were characterized and evaluated.

The XRD results shown in Figure 4A confirmed that the active species of the monocomponent catalysts could all be detected, except the characteristic peaks of TiO2. The Pt peaks of the Pt/TiO2-NTs at 39.763, 46.243, and 67.454° (PDF#040802); the Pd peaks of Pd/TiO2-NTs at 40.118, 46.658, and 68.119° (PDF#46-1043); and the Rh peaks of Rh/TiO2-NTs at 41.068, 47.780, 69.877, and 84.393° (PDF#05-0685) could all be determined, although their intensities were relatively weak. For the ternary-component catalysts, the XRD results (Figure 4B) showed that only the strongest peak of Pt at 46.243° could be detected. The SEM graphs of the four wall-loaded catalysts confirmed the existence of photoreduced noble metals on anodized TiO2nanotube supports. Deposited Pt nanoparticles were distributed inside the nanotubes and on the nanotube-array surfaces, as shown in Figure 4C. The content of Pt was 0.7732 wt % as determined by XRF. In Figure 4D, the loaded Pd was present on the support surface as an agglomeration structure, with a size relatively larger than those of the Pt particles (50− 200 nm), and its content was 0.7690 wt % as determined by XRF. Rh/TiO2 in Figure 4E showed loading of ca. 20−50 nm particles distributed on the nanotube opening. Its content was 0.7795 wt % as determined by XRF. For the ternary catalysts, the merged structure was determined to be above the nanotube array. The weight contents of Pt, Pd, and Rh of 9822


Article


Figure 5. XPS analysis of four catalysts: (A) Pt of Pt/TiO2-NTs, (B) Pd of Pd/TiO2-NTs, (C) Rh of Rh/TiO2-NTs, (D) Pt of PtPdRh/TiO2-NTs, (E) Pd of PtPdRh/TiO2-NTs, and (F) Rh of PtPdRh/TiO2-NTs.

different compared with those of the single-metal catalysts, although they were in the alloy state. The comparison of the ammonia-oxidation performances of the wall-loaded catalysts is shown in Figure 6, in which the bars

the PtPdRh/TiO2-NTs were 0.7083, 0.0384, and 0.0283 wt %, respectively. The maximum loading amount of Pt explains why it was the only metal detected by XRD. The enrichment of the Pt content of the three noble metals should be ascribed to the highest redox potential of (PtCl4)2−/Pt, 0.73 eV,25 thus making it the easiest one for photoreduction. XPS analysis of the four wall-loaded catalysts was performed to determine the forms of the metal active species. As shown in Figure 5A, the peaks at 70.7826 and 71.80 eV27 for the Pt/ TiO2-NTs were assigned to Pt 4f7/2 of Pt(0) and Pt(II), respectively. Metallic Pt and PtO were the photoreduced products, and their content ratio was 1.35:1. For Pd/TiO2NTs, the peaks at 334.3428 and 335.83 eV29 in Figure 5B were ascribed to Pd 3d5/2 of Pd(0) and PdO with a ratio of 1:3, respectively. From Figure 5C, the Rh 3d5/2 peaks of Rh/TiO2NTs at 306.5030 and 308.20 eV31 were ascribed to Rh(0) and Rh2O3, respectively. Their calculated ratio was 1.35:1. However, for the ternary-component catalysts, the characteristic peaks of Pt, Pd, and Rh shown in Figure 5D−F shifted slightly positive, which is consistent with the work function of the active metals.32,33 The work function of TiO2, Pt, Pd, and Rh are 4.8,33 5.65, 5.12, and 4.98 eV,34 respectively. Although the most probable electron-donating component is TiO2, and the most probable acceptor of electrons is Pt, on the basis of this work-function sequence, Pd and Rh can still accept electrons from TiO2 because of their low work functions. This combined action caused the shift in the peak and implied the generation of the PtPdRh alloy in the photoreduction process. Under the same analysis conditions, the 4f7/2 peaks of elemental Pt are at 71.0835 and 74.28 eV,36,37 corresponding to Pt(0) and Pt(II), respectively, with a ratio 5:2. For elemental Pd, the 3d5/2 peaks are at 335.2838 and 337.10 eV,39,40 corresponding to Pd(0) and PdO, respectively, with a ratio of 2:1. Rh corresponds to Rh(0) and Rh2O3 at 307.2841 and 309.38 eV,42 respectively, with a ratio of 2.6:1. The valence states of the three metals on the PtPdRh/TiO2-NTs were not

Figure 6. NH3 conversion and NOx selectivity of Pt/TiO2-NTs, Pd/ TiO2-NTs, Rh/TiO2-NTs, and PtPdRh/TiO2-NTs at different reaction temperatures.

represent NH3 conversion, and NOx selectivity is plotted. It should be noted that calculated equilibrium conversion of NH3 and selectivity of NO are both 100% at 100−500 °C. It can be observed that the initial NH3 conversion at 200 °C for the four catalysts, Pt/TiO2-NTs, Pd/TiO2-NTs, Rh/TiO2-NTs, and PtPdRh/TiO2-NTs, were about 2, 2, 0.4, and 5%, respectively. With increases in the reaction temperature, 100% NH3 conversion was reached at 280 °C for Pt/TiO2-NTs, at 340 °C for Pd/TiO2-NTs, and at 360 °C for PtPdRh/TiO2-NTs. However, for Rh/TiO2-NTs, 100% NH3 conversion was not achieved, even at 400 °C. NH3 conversion only reached 57%, 9823


Article


On the basis of the calculations (confidence values over 95%) listed in Table 1, it can be concluded that NH3 and O2

which indicated that pure Rh is not an acceptable active species for ammonia oxidation. On the basis of these results, the catalytic-activity sequence of the four wall-loaded catalysts was the following: Pt/TiO2-NTs > Pd/TiO2-NTs > PtPdRh/ TiO2-NTs > Rh/TiO2-NTs. The NOx-selectivity curves of the four wall-loaded catalysts shown in Figure 6 exhibited similar profiles, first decreasing and then increasing tendencies. At the starting temperature of 200 °C, the NOx selectivities of Pt/TiO2-NTs, Pd/TiO2-NTs, Rh/TiO2-NTs, and PtPdRh/TiO2-NTs, were 96, 97, 95, and 96%, respectively. After further increasing the temperature, the Pt/TiO2-NTs and PtPdRh/TiO2-NTs reached their lowest selectivity, 73 and 72%, respectively, first at approximately 280 °C. Afterward, Pd/TiO2-NTs and Rh/TiO2-NTs reached minimum NOx selectivities, 69 and 66.20%, respectively, at 300 °C. After these lowest points of NOx selectivity, the curves for all catalysts emerged upward with temperature. Eventually, the NOx selectivities of Pt/TiO2-NTs, PtPdRh/TiO2-NTs, Pd/TiO2-NTs, and Rh/TiO2-NTs reached values of 100, 82, 79, and 95% at 400 °C. In summary, the sequence of the four catalysts, on the basis of NOx selectivity, can be concluded as follows: Pt/TiO2-NTs > PtPdRh/TiO2-NTs > Pd/TiO2-NTs > Rh/TiO2-NTs. Therefore, it can be concluded that the Pt/TiO2-NTs exhibited the best performance as a wall-loaded catalyst in the microchannel reactor, although the ternary catalyst has been widely used in the nitric acid industry. In addition, on the basis of the design purpose, the wall-loaded catalyst was employed to decrease the reaction temperature and improve the catalyst performance. Here, Pt/TiO2-NTs effectively lowered the reaction temperature to 400 °C with complete NH3 conversion and 100% NOx selectivity. This result shows promise for future application because of its low-temperature advantage, as platinum loss due to high reaction temperatures has been eliminated completely. 3.3. Density-Functional-Theory (DFT) Simulation of Ammonia Oxidation. Although the experimental results confirmed the superior performance of Pt/TiO2-NTs, it is necessary to investigate the mechanism of its high performance for ammonia oxidation. Here, a DFT simulation was conducted. Only the (111) crystal faces for all active species are discussed, because the relaxed surface energy of the (111) facet of the Pt, Pd, Rh, and TiO2 support was the lowest. On the basis of the relevant literature, the adsorption of NH3 and O2 at the top43 and bridge44 on the (111) crystal surface was the most stable. The corresponding adsorption models of NH3 and O2 are shown as Figure 7.

Table 1. Energy- and Bond-Parameter Changes before and after Adsorption on Different Active Sites Pt

Pd

Rh

TiO2

Eads(NH3) (kJ/mol)

−42.331

−20.693

−48.623

−288.672

Eads(O2) (kJ/mol)

−88.160

−86.318

−110.721

−182.527

N−H (Å) ∠HNH (°) OO (Å) N−Hads (Å) ∠HNHads (°) OOads (Å) X(X = Pt, Pd, Rh)−N (Å) X(X = Pt, Pd, Rh)−O (Å) Edes(NO) (kJ/mol) Edes(N2) (kJ/mol)

1.028 107.070 1.239 1.034 111.188 1.349 2.145 2.104 −178.166 −4.773

1.028 107.070 1.239 1.029 110.169 1.321 2.148 2.106 −236.570 −33.928

1.028 107.070 1.239 1.028 109.628 1.359 2.135 2.046 −249.970 −44.117

       

were both adsorbed onto the surfaces of Pt, Pd, and Rh. After adsorption, N−H, ∠HNH, or OO was stretched, and the bond energy of X(X = Pt, Pd, Rh)−N or X(X = Pt, Pd, Rh)−O increased as the corresponding bond length decreased. The calculated adsorption energy of NH3 onto Pt(111) is −42.331 kJ/mol, which is between the energy onto Rh(111) (−48.623 kJ/mol) and the energy onto Pd(111) (−20.693 kJ/mol). The adsorption-energy sequence of O2 is the same as that for NH3 adsorption. These moderate adsorption energies of NH3 and O2 onto Pt(111) suggest its relatively proper adsorption ability but cannot yet explain its performance as the best catalyst. Another fact was the higher adsorption energy of NH3 onto TiO2(111), −288.672 kJ/mol, compared with other active metals, confirming the strong adsorption ability of TiO2 as an acidic support. The adsorption energy of O2 onto TiO2(111) was −182.527 kJ/mol, the strongest value of all the metals. Such strong adsorption performance of TiO2 implies a stable adsorption state and, consequently, an inert catalytic effect on ammonia oxidation, which is consistent with the results in Figure 4A. Thus, TiO2 was excluded in successive simulations. Then, the ammonia-oxidation reactions with the three metal species were simulated according to the L−H mechanism and noncompetitive adsorption.45,46 As shown in Figure 8, the different transition states of NH3 onto Pt, including different adsorbates such as NH3ads, O2ads, Oads, NH2ads, NHads, Nads, Hads, OHads, H2Oads, NOads, and N2ads, and on different adsorption sites, involving top, bridge, fcc, bridge, fcc, bridge, fcc, fcc, fcc, fcc, and bridge, 47 were simulated. The corresponding reaction heat and energy barriers of the elementary reactions were obtained, as listed in Table 2. It should be noted that the H radical has a very flexible adsorption orientation without a specific adsorption position. In Figure 8A, the adsorption and dissociation of NH3 onto Pt are illustrated. NH3 tended to adsorb on the top site, and NH2 tended to adsorb at the bridge between adjacent atoms after dissociation. However, the H radical is adsorbed at fcc. After further dissociation of NH2, the generated NH tended to adsorb at fcc, causing the adsorbed H to migrate from fcc to the top for a more stable adsorption. Similarly, the H radical migrated along the top site to the hcp site for energy balance after further dissociation of NH because of the repulsive effect between dissociated N and H. However, it is necessary to

Figure 7. Adsorption models of NH3 and O2 on (A) active metals and (B) TiO2-NTs. 9824


Article


Figure 8. (A) Direct NH3 dissociation. (B) NH3 dissociation under O. (C) NH3 dissociation under OH. (D) Formation of NH3 and NO. (E) Charge-density distributions of NH3−Pt(111), NH2−Pt(111), and NH−Pt(111).

Table 2. Reaction Heat and Activation Energy of NHx-Dissociation Elementary Reactions Pt (kJ/mol)

Pd (kJ/mol)

Rh (kJ/mol)

elementary reaction

reaction heat (ΔEr)

activation energy (Eb)

activation energya





NH3NH2 + H NH2NH + H NHN + H NH3 + ONH2 + OH NH2 + ONH + OH NH + ON + OH NH3 + OHNH2 + H2O NH2 + OHNH + H2O NH + OHN + H2O N + ONO N + NN2

112.161 31.783 98.323 27.300 −59.828 −53.714 47.152 −16.134 −34.619 −143.355 −292.727

151.874 132.896 143.727 50.535 42.088 81.486 91.201 46.558 64.754 135.702 106.227

11248 13148 13448 4249 8749 8449 7349 2249 3548  

69.509 69.739 88.353 53.408 −30.943 0.105 24.737 −16.828 −40.507 −61.467 −286.664

183.669 142.129 161.316 68.041 28.358 43.271 85.999 27.965 52.078 117.527 94.668

88.194 93.204 91.577 46.638 −6.151 24.289 32.047 −3.216 3.258 −99.251 −231.135

143.990 143.409 147.077 57.753 31.708 44.873 41.339 24.900 27.752 63.190 46.830

a

References are included.

the dissociation energy of NHx on the crystal surfaces of Pt(111), Pd(111), and Rh(111) decreased by 84.796, 115.815, and 100.047 kJ/mol on average, respectively. Under the promotion of OH, the dissociation energy of NHx on the crystal surfaces of Pt(111), Pd(111), and Rh(111) decreased by 75.328, 107.024, and 113.495 kJ/mol on average, respectively. Thus, it was confirmed that both O and OH can effectively promote NHx dissociation. The dissociation of NH3 was decided as the rate-determining step because the calculated dissociation activation energy is the largest, compared with those of NH2 and NH. To further explain the dissociation-reaction-energy-barrier sequence of (NH3)abs > (NH)abs > (NH2)abs, the Mulliken charge movement was conducted with Pt(111) as the example. Figure 8E shows the charge-density distribution. (Red represents the increasing area of electron density, and gray represents the decreasing area of electron density.) It could be determined that electrons were focused toward N and away from H atoms when NHx adsorbed onto Pt(111). (NH3)ads lost 0.07e−, which were transferred to Pt(111), but (NH2)ads and (NH)ads gained 0.23e− and 0.40e− from Pt(111),

further determine the effect of O2 on NH3 adsorption and the dissociation process, because O2 adsorption is easier than NH3 adsorption. As shown in Figure 8B, adsorbed O at the fcc site captured the dissociated H of NH3, repulsed by adsorbed N at the top, to generate OH, and the generated OH migrated from fcc to the top for a more stable adsorption. After repeating this process, N stably existed at the fcc position, and OH remained at the top position as a result. Similarly, H of the adsorbed NH3, NH2, and NH can also be seized easily by adsorbed OH to form H2O, as shown in Figure 8C. Generated H2O tended to desorb from the Pt surface because of low desorption energy. The generation mechanism of possible products, NO and N2, are shown in Figure 8D. After losing all H by the adsorbed O and OH, adsorbed N radicals tended to combine with adsorbed O radicals to form adsorbed NO at the adjacent fcc sites. However, if two adsorbed N radicals dissociated, the byproduct adsorbed N2 could be generated. The two products desorbed to form the products NO and N2. The reaction-heat (Er) and activation-energy (Eb, confidence values over 95%) values of the elementary reactions on the three catalysts are listed in Table 2. Under the promotion of O, 9825


Article

Industrial & Engineering Chemistry Research respectively. Therefore, the N−H bond of absorbed NH3 is relatively difficult to break, with higher dissociation activation energy due to the reduced electron density and, consequently, the lower activity. However, the N−H bonds of (NH2)ads and (NH)ads are relatively easily damaged, with lower dissociation activation energies due to the increased electron density after being absorbed and higher N−H bond activity. The total overlapping charge-population-analysis results show that the qN−H (overlapping charges of N and H) of (NH2)ads was −0.54e−, whereas the qN−H of NH was −0.40e−, indicating the stronger covalence of (NH2)ads with a lower H association activation energy. Comparing the three catalysts, it is observed that the Oassisted dissociation energy of NH3 on Pt(111) is the smallest, which explains the highest NH3 conversion of ammonia oxidation catalyzed by Pt/TiO2-NTs. Furthermore, the calculated desorption-energy sequence of NOabs and N2abs on Pt(111), Pd(111), and Rh(111) is Rh(111) > Pd(111) > Pt(111), which reaffirms the superiority of Pt as the catalyst for ammonia oxidation. However, the reaction-energy barrier of generating NO on all three catalysts is greater than that of generating N2, which means N2ads is unstable and more easily forms free radicals while releasing 4.773 kJ/mol of energy. Therefore, the generation of N2 is easier than that of NO on the basis of thermodynamics. However, the formation of NO is easier at higher temperatures, which is consistent with the result at 300−400 °C in Figure 6. Nevertheless, because the combination of two dissociated N needs the assistance of O, it can be regarded that the generation of N2 requires the pregeneration of NO. Therefore, ammonia oxidation can be regarded as a consecutive reaction: the generation of NO comes before that of N2.16 Thus, at low temperatures, the N− N association is relatively difficult, and the O-assisted dissociated N tends to directly depart from the catalyst surface to form NO. With the increase in temperature, the vibration of the associated N becomes stronger, increasing its probability of meeting and combining with another adsorbed N to produce N2. By further increasing the temperature, the probability of the direct departure of NO as a product sharply increases because of the high temperature. This mechanism explains why the NO selectivity decreased as the reaction temperature increased from 200−300 °C but increased in the hightemperature zone at temperatures greater than 300 °C. In concluding the reaction route of ammonia oxidation for the three catalysts, it can be observed from Figure 9 that the desorption-energy barrier of Rh(111) is larger than those of the other two, and that of Pt(111) was the smallest. When N and O radicals combine to form (NO)abs, (NO)abs can also be dissociated in reverse to form N and O again. According to the energy barriers, the dissociation-energy-barrier sequence of (NO)abs was Pt(111) < Pd(111) < Rh(111). Therefore, Rh is less selective in the catalytic ammonia-oxidation reaction. In general, the dissociation of NH3 and NO desorption on Pt(111) was the easiest, but NO dissociation on Pt(111) was the most difficult. Therefore, Pt/TiO2-NTs possessed the best NH3 conversion and NO selectivity.

Figure 9. Energy barriers of NO and N2 production and desorption.

microchannel reactor, it was observed that the best catalyst was Pt/TiO2-NTs, in which the Pt content was reduced to 0.7732 wt %, and the NH3-conversion rate reached 100% at 280 °C with NOx selectivity up to 100% at 380 °C. The DFT simulation confirms that the adsorption energy of NH3 and O2 on Pt(111) falls in between those on Rh(111) and Pd(111). Oxygen radicals and OH are greatly beneficial to the dissociation of NHx. The dissociation of (NH3)abs is the ratedetermining step of ammonia oxidation. The superb catalytic performance of Pt/TiO2-NTs is ascribed to the Pt/TiO2-NTs having the lowest energy barrier of NH3 dissociation and NO desorption and to the largest energy barrier of NO dissociation being that on Pt(111). The obtained wall-loaded Pt/TiO2-NTs effectively decrease the catalyst cost and improve the catalyst performance, exhibiting promising prospects for future applications in nitric acid production.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01135. Homemade microchannel-reactor prototype, mode of a microcrossflow heat exchanger, three-dimensional temperature distribution, model of a micro reaction channel, fluid-temperature distribution, graph of the temperature distribution in the center channel, distribution of the fluid-pressure drop, conversion rate of different catalysts at different airspeeds, and full-spectrum XPS analysis of Pt−4Pd−3.5Rh−TiO2 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-28-85990133. Fax: +86-28-85460556. E-mail: [email protected]. ORCID

Hairong Yue: 0000-0002-9558-0516 Wei Jiang: 0000-0002-5560-0885 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Q.L. and W.Q. conducted the experiments and characterizations; P.W., H.Y., C.L., and W.J. gave supports on experiment guides and facilities.

4. CONCLUSION In this research, TiO2-NTs were screened as supports of wallloaded ammonia-oxidation catalysts, and photoreduced Pt, Pd, Rh, and PtPdRh were deposited on the TiO2-NTs to form the corresponding catalysts. After evaluation in a homemade

Notes

The authors declare no competing financial interest. 9826


Article


■

(20) Liu, Q.; Pu, X.; Yue, H.; Jiang, W.; Liang, B. Ammonia Oxidation Process Catalyzed by Pt@XO2 (X = Ti, Zr, Ce, and Ce/Zr) Prepared by Photoreduction Process. Ind. Eng. Chem. Res. 2018, 57, 7752−7763. (21) Wang, Y. Controllable Preparation and Gas Sensing Properties of TiO2 Nanotube Arrays, Ph.D. Thesis, Hefei University of Technology, Hefei, China, 2012. (22) Muñoz-Noval, Á .; Fukami, K.; Koyama, A.; Gallach, D.; Hermida-Merino, D.; Portale, G.; Kitada, A.; Murase, K.; Abe, T.; Hayakawa, S.; et al. Accelerated Growth from Amorphous Clusters to Metallic Nanoparticles Observed in Electrochemical Deposition of Platinum within Nanopores of Porous Silicon. Electrochem. Commun. 2016, 71, 9−12. (23) Zhang, Q.; Gao, L. Preparation and the Enhanced Photocatalytic Activity of Highly Dispersed Pt Loaded on TiO2 Nanocrystals. Acta Chim. Sinica 2005, 63, 70−75. (24) Liu, X.; Liu, X.; Liu, Z.; Shi, Y. Researches on Thermodynamics of PtRhPd Alloy in Ammonia Oxidation. Ind. Catal. 2012, 20, 62−64. (25) Dean, J. Handbooks of Lan-Shi Chemistry; The Science Publishing Company, 2003 (in Chinese). (26) Miller, D. J.; Ö berg, H.; Kaya, S.; Sanchez Casalongue, H.; Friebel, D.; Anniyev, H.; Ogasawara, H.; Bluhm, H.; Pettersson, L. G. M.; Nilsson, A. Oxidation of Pt(111) under Near- ambient Conditions. Phys. Rev. Lett. 2011, 107, 195502. (27) Porsgaard, S.; Merte, L. R.; Ono, L. K.; Behafarid, F.; Matos, J.; Helveg, S.; Salmeron, M.; Roldan Cuenya, B.; Besenbacher, F. Stability of Platinum Nanoparticles Supported on SiO2/Si(111): a High-pressure X-ray Photoelectron Spectroscopy Study. ACS Nano 2012, 6, 10743−10749. (28) Tura, J. M.; Regull, P.; Victori, L.; de Castellar, M. D. XPS and IR (ATR) Analysis of Pd Oxide Films Obtained by Electrochemical Methods. Surf. Interface Anal. 1988, 11, 447−449. (29) Shafeev, G. A.; Themlin, J. M.; Bellard, L.; Marine, W.; Cros, A. Enhanced Adherence of Area-Selective Electroless Metal Plating on Insulators. J. Vac. Sci. Technol., A 1996, 14, 319−326. (30) Dennis, A. M.; Howard, R. A.; Kadish, K. M.; Bear, J. L.; Brace, J.; Winograd, N. X-ray Photoelectron Spectra of Some Dirhodium Carboxylate Complexes. Inorg. Chim. Acta 1980, 44, L139−L141. (31) Okamoto, Y.; Ishida, N.; Imanaka, T.; Teranishi, S. Active States of Rhodium in Rhodium Exchanged Y Zeolite Catalysts for Hydrogenation of Ethylene and Acetylene and Dimerization of Ethylene Studied with X-ray Photoelectron Spectroscopy. J. Catal. 1979, 58, 82−94. (32) Navarro, R. M.; Pawelec, B.; Trejo, J. M.; Mariscal, R.; Fierro, J. L. G. Hydrogenation of Aromatics on Sulfur-Resistant PtPd Bimetallic Catalysts. J. Catal. 2000, 189, 184−194. (33) Kiss, J.; Németh, R.; Koós, A.; Raskó, J. Characterization of AuRh/TiO2 Bimetallic Nanocatalysts by CO and CH3CN Adsorption: XPS, TEM and FTIR Measurements. J. Nanosci. Nanotechnol. 2009, 9, 3828−3836. (34) Williams, M. L. Review of CRC Handbook of Chemistry and Physics, 76th edition. Occup. Environ. Med. 1996, 53, 504. (35) Légaré, P.; Lindauer, G.; Hilaire, L.; Maire, G.; Ehrhardt, J. J.; Jupille, J.; Cassuto, A.; Guillot, C.; Lecante, J. On the Interaction of O2 with Pt(111) and Pt(557) Surfaces: Core-level Shift Study Using Conventional and Synchrotron Radiation Sources. Surf. Sci. 1988, 198, 69−78. (36) Drawdy, J. E.; Hoflund, G. B.; Gardner, S. D.; Yngvadottir, E.; Schryer, D. R. Effect of Pretreatment on a Platinized Tin Oxide Catalyst Used for Low-temperature Co Oxidation. Surf. Interface Anal. 1990, 16, 369−374. (37) Dauscher, A.; Hilaire, L.; Le Normand, F.; Müller, W.; Maire, G.; Vasquez, A. Characterization by XPS and XAS of Supported Pt/ TiO2/CeO2 Catalysts. Surf. Interface Anal. 1990, 16, 341−346. (38) Kishi, K.; Motoyoshi, Y.; Ikeda, S. Deposition of Nickel from Ni(CO)4 on Palladium and Iron surfaces Studied by X-ray Photoelectron Spectroscopy. Surf. Sci. 1981, 105, 313−324.

ACKNOWLEDGMENTS We appreciate the financial support from the National Natural Science Foundation of China (Grant Nos. 21676168 and 21176157). In addition, we would like to thank the Institute of New Energy and Low-Carbon Technology, Sichuan University, for SEM-image capturing and XRD analysis.

■

REFERENCES

(1) Ning, Y. The Centennial Development of Platinum Alloy Catalytic Gauzes for Ammonia Oxidation in Nitric Acid Plants II. Performances, Main Species and Reconstruction of Platinum Alloy Catalytic Gauzes. Precious Met. 2008, 29, 41−46. (2) Dai, H.; Ning, Y. Oxidation and Surface Chemical Species of Palladium. Rare Met. Mater. Eng. 2011, 40, 1256−1259. (3) Schäffer, J.; Kondratenko, V. A.; Steinfeldt, N.; Sebek, M.; Kondratenko, E. V. Highly Selective Ammonia Oxidation to Nitric Oxide over Supported Pt Nanoparticles. J. Catal. 2013, 301, 210−216. (4) Sarioğlan, Ş . Recovery of Palladium from Spent Activated Carbon-Supported Palladium Catalysts. Platinum Met. Rev. 2013, 57, 289−296. (5) Hickman, D. A.; Huff, M.; Schmidt, L. D. Alternative Catalyst Supports for HCN Synthesis and NH3 Oxidation. Ind. Eng. Chem. Res. 1993, 32, 809−817. (6) Ning, Y. Structure and Performances of Platinum based Alloys as Ammonia Oxidation Catalysts. Precious Met. 2011, 32, 7−15. (7) Liu, X.; Liu, X.; Li, Z. The Optimum Design of Ammonia Oxidation Catalyst System. Precious Met. 2014, 6−9. (8) Yang, Z.; Ning, Y. Surface State of Industrial Pd -Ni Alloy Catchment Gauzes. Precious Met. 1999, 20, 1−9. (9) Li, M.; Ning, Y.; Song, Y.; Cui, S.; Lin, Z. Research Progress of Palladium Alloy Catchment Gauzes in Nitric Acid Industry. Precious Met. 2005, 26, 71−74. (10) Srinivasan, R.; Hsing, I.-M.; Berger, P. E.; Jensen, K. F.; Firebaugh, S. L.; Schmidt, M. A.; Harold, M. P.; Lerou, J. J.; Ryley, J. F. Micromachined Reactors for Catalytic Partial Oxidation Reactions. AIChE J. 1997, 43, 3059−3069. (11) Wang, G.; Johannessen, E.; Kleijn, C. R.; de Leeuw, S. W.; Coppens, M. O. Optimizing Transport in Nanostructured Catalysts: A Computational Study. Chem. Eng. Sci. 2007, 62, 5110−5116. (12) Mills, P. L.; Quiram, D. J.; Ryley, J. F. Microreactor Technology and Process Miniaturization for Catalytic Reactions-A Perspective on Recent Developments and Emerging Technologies. Chem. Eng. Sci. 2007, 62, 6992−7010. (13) Roberge, D. M.; Ducry, L.; Bieler, N.; Cretton, P.; Zimmermann, B. Microreactor Technology: A Revolution for the Fine Chemical and Pharmaceutical Industries? Chem. Eng. Technol. 2005, 28, 318−323. (14) Yao, X.; Zhang, Y.; Du, L.; Liu, J.; Yao, J. Review of the Applications of Microreactors. Renewable Sustainable Energy Rev. 2015, 47, 519−539. (15) Rebrov, E. V.; Duinkerke, S. A.; de Croon, M. H. J. M.; Schouten, J. C. Optimization of Heat Transfer Characteristics, Flow Distribution, and Reaction Processing for A Microstructured Reactor/ Heat-Exchanger for Optimal Performance in Platinum Catalyzed Ammonia Oxidation. Chem. Eng. J. 2003, 93, 201−216. (16) Jiang, W.; Qiu, J.; Qiu, W.; Yuan, S.; Lu, H.; Liang, B. Wallloaded Pt/TiO2/Ti Catalyst and ts Application in Ammonia Oxidation Reaction in Microchannel Reactor. RSC Adv. 2016, 6, 26637−26649. (17) Chan, S. C.; Barteau, M. A. Preparation of Highly Uniform Ag/ TiO2 and Au/TiO2 Supported Nanoparticle Catalysts by Photodeposition. Langmuir 2005, 21, 5588. (18) Dinh, C. T.; Nguyen, T. D.; Kleitz, F.; Do, T.-O. A New Route to Size and Population Control of Silver Clusters on Colloidal TiO2 Nanocrystals. ACS Appl. Mater. Interfaces 2011, 3, 2228−2234. (19) Xu, X.; Pu, X.; Yuan, S.; Jiang, W.; Liang, B. Preparation and Application of Core-shell Pt /TiO2 Catalyst for Ammonia Oxidation Process. J. Mol. Catal. 2014, 28, 75−81. 9827


Article

Industrial & Engineering Chemistry Research (39) Fleisch, T. H.; Zajac, G. W.; Schreiner, J. O.; Mains, G. J. An XPS study of the UV Photoreduction of Transition and Noble Metal Oxides. Appl. Surf. Sci. 1986, 26, 488−497. (40) Solymosi, F.; Pasztor, M. Infrared Study of the Effect of Hydrogen on Carbon Monoxide- induced Structural Changes in Supported Rhodium. J. Phys. Chem. 1986, 90, 5312−5317. (41) Rothe, J.; Pollmann, J.; Franke, R.; Hormes, J.; Bönnemann, H.; Brijoux, W.; Siepen, K.; Richter, J. X-ray absorption and X-ray photoelectron Spectroscopy of a Rhodium Colloid. Anal. Bioanal. Chem. 1996, 355, 372−374. (42) Barth, G.; Linder, R.; Bryson, C. Advances in Charge Neutralization for XPS Measurements of Nonconducting Materials. Surf. Interface Anal. 1988, 11, 307−311. (43) Guo, X. J.; Liu, W.; Fang, W.; Cai, L.; Zhu, Y.; Lu, L.; Lu, X. DFT Study of Coverage- depended Adsorption of NH3 on TiO2-B (100) Surface. Phys. Chem. Chem. Phys. 2012, 14, 16618−16625. (44) Zhou, R.; Cao, P. Atom Superposition and Electron Delocalization-molecular Orbital Theoretical Study of O2 Adsorption and Dissociation. Acta Phys. Sin. 1993, 42, 470−476. (45) Kraehnert, R.; Baerns, M. Kinetics of Ammonia Oxidation over Pt Foil Studied in a Micro- structured Quartz-reactor. Chem. Eng. J. 2008, 137, 361−375. (46) Rebrov, E. V.; de Croon, M.; Schouten, J. C. Development of the Kinetic Model of Platinum Catalyzed Ammonia Oxidation in a Microreactor. Chem. Eng. J. 2002, 90, 61−76. (47) Offermans, W. K.; Jansen, A. P. J.; van Santen, R. A.; NovellLeruth, G.; Ricart, J. M.; Pérez- Ramírez, J. Ammonia Dissociation on Pt{100}, Pt{111}, and Pt{211}: A Comparative Density Functional Theory Study. J. Phys. Chem. C 2007, 111, 17551−17557. (48) Baerns, M.; Imbihl, R.; Kondratenko, V. A.; Kraehnert, R.; Offermans, W. K.; van Santen, R. A.; Scheibe, A. Bridging the Pressure and Material Gap in the Catalytic Ammonia Oxidation: Structural and Catalytic Properties of Different Platinum Catalysts. J. Catal. 2005, 232, 226−238. (49) Offermans, W. K.; Jansen, A. P. J.; van Santen, R. A. Ammonia Activation on Platinum {1 1 1}: A Density Functional Theory Study. Surf. Sci. 2006, 600, 1714−1734.

9828


Low-Temperature Ammonia Oxidation in a Microchannel Reactor with

Recommend Documents