AlPO4 Catalyst Providing High Resistance

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Surface Properties of Rh/AlPO4 Catalyst Providing High Resistance to Sulfur and Phosphorus Poisoning Haris Puspito Buwono,† Saki Minami,† Kosuke Uemura,† and Masato Machida*,†,‡ †

Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo, Kumamoto 860-8555, Japan ‡ Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University 1-30 Goryo-Ohara, Nishikyo, Kyoto 615-8245, Japan S Supporting Information *

ABSTRACT: A rhodium catalyst supported on AlPO4 exhibited a much higher resistance to sulfur and phosphorus poisoning compared with a reference catalyst (Rh/Al2O3). The acidic surface of AlPO4 was effective in preventing the adsorption of sulfur oxides (SO2), whereas Lewis acid/base sites on Al2O3 favored SO2 adsorption followed by the formation of sulfite, leading to deterioration of the activity of Rh/Al2O3 for the model NO−CO−C3H6−O2 reaction. Similarly, the AlPO4 support suppressed the extent of phosphorus poisoning caused by dimethylphosphite (DMP) (CH3O)2POH, which was used as a model phosphorus source. A greater amount of inactive phosphate overlayers were deposited from the gas feed containing DMP and O2 on Rh/ Al2O3 than Rh/AlPO4 because of the reaction between P2O5 vapors and Al2O3. Consequently, the active Rh surface was covered to a greater extent for Rh/Al2O3 than Rh/AlPO4.

1. INTRODUCTION Phosphorus and sulfur poisoning are major sources of severe deactivation in automotive catalysts.1 Poisoning occurs when oxidized fuel- and oil-derived contaminants and/or additives are adsorbed or deposited onto the catalyst surface. In the case of phosphorus poisoning, an engine oil additive, zinc dialkyldithiophosphate (ZDDP), is oxidized during the combustion process and deposited as glassy overlayers of phosphates that cover the catalyst surface.2−5 The dense and stable overlayers that form as a result cause pore blockage, loss of surface area, and occlusion of active precious metals.6,7 The as-deposited phosphate in part reacts with the porous Al2O3 support to yield AlPO4, and further reactions with the oxygen storage material CeO2 yield CePO4, which deteriorates the oxygen storage performance.8,9 The poisoning effects of sulfur have also been studied extensively.10−18 It is well-known that SO2 adsorption probes the surface basic site.19,20 Strong chemisorption of SO2 onto the surface of precious metals blocks the adsorption and catalytic reactions of NO, CO, and hydrocarbons. Under a reducing atmosphere, the chemisorbed SO2 is reduced to elemental sulfur, which is also strongly bound to the metal surface unless sufficient concentrations of oxygen are present in the gas phase. In addition, these chemisorbed sulfur species may influence the electronic state of the precious metals and in turn their catalytic activity. Reaction with SO2 in the presence of O2 resulting in sulfation of the metal oxide support and CeO2 is another possible contributor to catalyst deactivation. Development of automotive catalytic materials with high resistance to sulfur and phosphorus poisoning remains a challenge.21,22 The poisoning processes in many cases are associated with the conversion of metal oxide components in the catalysts to inactive sulfate/phosphate compounds. One possible approach, therefore, is to develop metal oxides and alternative materials such as oxoacid salts that are less reactive © 2015 American Chemical Society

to phosphorus and sulfur poisons. We recently reported that AlPO4 is a robust support material that exhibits optimum interactions with Rh species; reduction of the threshold Rh loading is possible owing to the high thermal stability and high dispersion of Rh species anchored onto the surface.23−26 The outstanding stability is demonstrated by aging at 900 °C for 500 h in a stream of water vapor, during which a conventional catalyst consisting of Rh loaded on γ-Al2O3 completely loses its catalytic activity. Another noteworthy feature is the very low threshold Rh loading (≤0.01 wt %), which corresponds to approximately 1/20th that of a conventional Rh/Al 2O 3 catalyst.23 We also anticipate that Rh/AlPO4 will have increased chemical stability. The surface of AlPO4 contains Lewis and Brønsted acid sites,26,27 which are expected to be efficient in suppressing the catalyst deactivation caused by the adsorption of acidic sulfur oxides (SO2). Considering the aforementioned phase change from metal oxides to metal phosphates during phosphorus poisoning, it is also expected from a thermodynamic viewpoint that the AlPO4 support may be less reactive to phosphorus oxide compared with Al2O3. In this study, we investigated the ability of Rh/AlPO4 to resist deactivation using the model poisons, SO2 and dimethylphosphite (DMP), (CH3O)2POH, using Rh/Al2O3 as a reference catalyst. The formation of adsorbates on the catalysts following exposure to these poisoning species was correlated to the surface properties of the AlPO4 and Al2O3. Their catalytic activities for simulated stoichiometric autoexhaust containing NO, CO, C3H6, and O2 with a stoichiometric Received: Revised: Accepted: Published: 7233

May 8, 2015 July 5, 2015 July 7, 2015 July 7, 2015 DOI: 10.1021/acs.iecr.5b01720 Ind. Eng. Chem. Res. 2015, 54, 7233−7240

Article

Industrial & Engineering Chemistry Research

programmed reaction with SO2 was performed by heating the catalyst (0.05 g) at a constant rate of 10 °C min−1 in the same gas feed. The concentration of SO2 at the reactor outlet was monitored using a nondispersive infrared analyzer (Horiba VA3000). Phosphorus poisoning was performed in a flow reactor using DMP (Sigma-Aldrich, 98%) as a model phosphorus source, which was supplied by bubbling a stream of 10 vol % O2/N2 (50 cm3 min−1) at 50 °C. The DMP concentration in the resulting gas mixture was estimated to be approximately 0.9 vol % according to the reported temperature dependence of the vapor pressure.28 The resulting gas mixture was introduced to the catalyst bed (0.1 g) in the reactor at 800 °C to cause the thermal decomposition of DMP. After a certain period of time (≤1 h), the gas feed was changed to 10 vol % O2/N2 and the catalyst was further heated at 800 °C for 2 h to ensure complete conversion of the phosphorus compound to phosphate species and removal of weakly bonding species. The amount of deposited phosphate was determined using an X-ray fluorescence analyzer (Horiba MESA500W). The phosphorus uptake by Rh/AlPO4 was determined from the difference of phosphorus contents between fresh and poisoned samples. The FT-IR spectra of the catalysts after poisoning were recorded on a Jasco FTIR610 spectrometer. Although the concentrations of these sulfur and phosphorus sources in the gas feed were much higher compared with a real exhaust, these cause poisoning in short experimental periods. Catalytic tests for as-prepared and as-poisoned catalysts were performed in a flow reactor at atmospheric pressure without further treatments. The catalyst (50 mg, 10−20 mesh) was fixed in a quartz tube (4 mm i.d.) using quartz wool at both ends of the catalyst bed. The temperature dependence of the catalytic activity was evaluated by heating the catalyst bed from room temperature to 600 °C at a constant rate of 10 °C min−1 while supplying a simulated exhaust gas mixture containing NO (500 ppm), CO (5100 ppm), C3H6 (390 ppm), O2 (4000 ppm), and He (balance) at 100 cm3 min−1 (W/F = 5.0 × 10−4 g min cm−3). Each gas composition was expressed using volumetric concentrations, and the total mixture corresponded to a stoichiometric air-to-fuel ratio. The effluent gas (NO, CO, and C3H6) was analyzed using a Pfeiffer GSD30101 mass spectrometer and a Horiba VA3000 NDIR gas analyzer.

ratio were also determined to evaluate the extent of catalyst poisoning.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. AlPO4 was obtained from a mixed aqueous solution of Al(NO3)3 (Wako Pure Chemicals Ind. Ltd., 99.9%) and H3PO4 (Wako Pure Chemicals Ind. Ltd., 85 wt %). An aqueous ammonia solution (25%) was added dropwise until the pH of the supernatant was 4.5. The white gel thus obtained was recovered by filtration and washed with deionized water several times. After drying overnight at 100 °C, the solid thus obtained was calcined in air at 1000 °C for 5 h to yield AlPO4 with the tridymite-type structure. Rh-loaded on AlPO4 (0.4 wt % as Rh metal) was prepared via impregnation using an aqueous solution of Rh(NO3)3, followed by drying overnight at 100 °C and then air calcination (600 °C, 3 h). As a reference catalyst, Rh supported on Al2O3 was prepared in the same manner. The Al2O3 sample for this purpose contained 3 wt % La2O3 as an additive, which was prepared by impregnation of an aqueous solution of La(NO3)3 to γ-Al2O3 (JRC-ALO8, supplied by the Catalysis Society of Japan) and subsequent calcination at 600 °C for 5 h. The addition of La is known to stabilize γ-Al2O3 against the phase transformation to α-Al2O3. The heating and cooling rate for these calcination processes was 5 °C min−1. Powder X-ray diffraction (XRD) analysis was performed using monochromated Cu Kα radiation (30 kV, 20 mA, Rigaku Multiflex). The content of Rh was determined via X-ray fluorescence analysis (Rigaku EDXL300) using monochromated Pd L and polarized white X-ray radiation (50 kV, 1 mA). The powder sample finely ground by an agate mortar was packed into a cell with a polypropylene film window (10 mm diameter). The surface composition of the catalysts was measured by X-ray photoelectron spectroscopy (XPS) using a Thermo K-alpha spectrometer under Al Kα radiation (12 kV). The C 1s signal at 285.0 eV from adventitious carbon was used as reference to correct the effect of surface charge. The Brunauer−Emmett−Teller (BET) surface areas (SBET) were calculated using N2 adsorption isotherms measured at −196 °C (Belsorp-mini, Bel Japan). The acid/base properties of AlPO4 and γ-Al2O3 were evaluated by measuring the adsorption of NH3, CO2, and pyridine. After dehydration at 500 °C in flowing He and subsequent adsorption in flowing 5 vol % NH3/ He at 100 °C, temperature-programmed desorption curves (NH3-TPD) were obtained in flowing He at a heating rate of 10 °C min−1. Similarly, the CO2-TPD curves were obtained after adsorption in flowing CO2 at 50 °C. In situ diffuse reflectance Fourier-transform infrared (FT-IR) spectra of pyridine chemisorption were recorded on a Nicolet 6700 spectrometer at 50 °C and subsequent evacuation at room temperature. 2.2. Poisoning and Catalytic Test. Prior to the poisoning experiments and subsequent to the catalytic tests, each Rhloaded catalyst was thermally aged in a stream of 10 vol % H2O/air at 900 °C for 25 h. The SBET values of catalysts decreased from 93 to 60 m2 g−1 (Rh/AlPO4) and from 121 to 71 m2 g−1 (Rh/Al2O3) after the thermal aging. Sulfur poisoning was performed in a flow reactor under isothermic and temperature-programmed conditions. Under isothermic conditions, the breakthrough curves for SO2 adsorption were determined at 200 and 500 °C in a flowing gas mixture of 100 ppm of SO2/He (100 cm3 min−1). The temperature-

3. RESULTS AND DISCUSSION 3.1. Surface Properties of the Supports. Figure 1 shows the XRD patterns of as-prepared Rh catalysts supported on

Figure 1. XRD patterns of as-prepared (a) Rh/AlPO4 and (b) Rh/ Al2O3. 7234

DOI: 10.1021/acs.iecr.5b01720 Ind. Eng. Chem. Res. 2015, 54, 7233−7240

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aging at 900 °C was studied using a temperature-programmed technique, as shown in Figure 2. At ambient temperature, a

AlPO4 and Al2O3. The former showed weak diffraction peaks due to a low crystalline tridymite-type AlPO4 phase, and the latter showed broad peaks due to γ-Al2O3. Although the diffraction peaks from Rh species were too weak to be observed because of the small loading (0.4 wt %), they exited as Rh oxide (RhOx) after calcination as reported previously.25,26 To study the resistance to sulfur and phosphorus poisoning, the affinity of the catalyst surface for these poisoning compounds must be considered. Therefore, the surface properties of as-prepared AlPO4 and Al2O3 were investigated. TPD of preadsorbed NH3 and CO2 from the catalysts was used to estimate surface acidity and basicity (see Supporting Information for the TPD data). Table 1 compares the density of acid/base sites as the amounts Table 1. Surface Properties of the Support Materials sample

SBETa/ m2 g−1

AlPO4 Al2O3

101 178

acid typeb

acid site densityc/ μmol m−2

basic sites densityd/ μmol m−2

Lewis + Brønsted Lewis

12.0 1.0

0.01 0.38

Figure 2. Temperature-programmed reaction of Rh/AlPO4 and Rh/ Al2O3 with 100 ppm of SO2/He supplied at 100 cm3 min−1. Sample: 0.05 g; heating rate: 10 °C min−1.

large portion of the SO2 in the gas feed (100 ppm of SO2/He) was removed by adsorption onto Rh/Al2O3. The effluent SO2 concentration then gradually increased with an increase in the temperature, and more SO2 than was in the gas feed was observed at temperatures ≥350 °C, indicating that the desorption of adsorbed SO2 occurred. However, the thermal desorption of SO2 from Rh/Al2O3 was not complete even at 700 °C, suggesting the high stability of the adsorbed SO2. In complete contrast, Rh/AlPO4 showed a negligible amount of SO2 adsorption over the entire temperature range (25−800 °C). Figure 3 shows the breakthrough curves for SO2 measured at a constant temperature of 200 °C when a mixture of 100 ppm

a

BET surface area. bDetermined by in situ FT-IR of adsorbed pyridine (Supporting Information). cDetermined from NH3 uptake at 100 °C divided by SBET (Supporting Information). dDetermined from CO2 uptake at 50 °C divided by SBET (Supporting Information).

of chemisorbed NH3 and CO2 normalized by the SBET values. AlPO4 exhibited an acid site density more than 10-fold greater than that of Al2O3, and the NH3-TPD profile gave rise to a broad desorption peak centered at 190 °C attributed to weak or moderate strength acid sites. In contrast, the basic sites density of AlPO4 was much lower than that of Al2O3. Therefore, the surface of AlPO4 exhibited an acidic character, while that of Al2O3 was much less acidic. The acidic nature of AlPO4 was also evaluated using in situ FT-IR of pyridine chemisorption, which was performed at 50 °C after dehydration at 600 °C in vacuo (Supporting Information). Pyridine chemisorption onto AlPO4 yielded FT-IR spectral bands assignable to pyridine molecules coordinated to Lewis acid sites (1615, 1494, and 1453 cm−1) and to pyridinium ions adsorbed on Brønsted acid sites (1545 cm−1) as reported by several researchers.29−31 On the other hand, the FT-IR spectrum of Al2O3 showed only the former bands, suggesting the presence of only Lewis acid sites. Both support materials contain coordinatively unsaturated Alδ+ on the surface, which may play the role of the Lewis acid sites. However, a large portion of the AlPO4 surface is terminated by phosphate units that bear P−OH groups,25 which are able to exhibit Brønsted acidity. As shown in Table 1, the basic sites density of Al2O3 is more than that of AlPO4, although it is less than the acid site density of Al2O3. The CO2-TPD profile of Al2O3 exhibited two desorption peaks (Supporting Information): The stronger peak at 120 °C corresponded to weak or moderate strength basic sites, and the weak peak at ≥650 °C represented stronger basic sites. As revealed by previous studies,32−34 CO2 molecules adsorbed on the γ-Al2O3 surface interact with isolated surface O2− ions, Al−O2− pair sites, and/or surface hydroxyls to form unidentate carbonate, bidentate carbonate, and/or bicarbonate species, respectively. Although the bidentate carbonate species desorb near room temperature (≤50 °C), the others (unidentate carbonate and bicarbonate species) are stable and exhibit higher desorption temperatures. 3.2. Effect of Sulfur Poisoning. The SO2 adsorption behavior of the Rh/AlPO4 and Rh/Al2O3 catalysts after thermal

Figure 3. Breakthrough curves for SO2 adsorption onto Rh/AlPO4 and Rh/Al2O3 at 200 °C. 100 ppm of SO2/He supplied at 100 cm3 min−1; sample: 0.05 g.

of SO2 in He was used as the gas feed. For Rh/Al2O3, nearly all the SO2 in the gas feed was removed by adsorption at the beginning of the experiment. The effluent SO2 concentration then gradually increased up to 100 ppm as saturation of adsorption was reached within 1 h. With the Rh/AlPO4 catalyst, however, a very small amount of SO2 was adsorbed. The cumulative SO2 uptakes were determined from breakthrough curves measured at 200 and 500 °C, as summarized in Table 2. The SO2 uptake per unit surface area for Rh/Al2O3 was approximately 16-fold higher than that on AlPO4 at 200 °C. Even at 500 °C, where desorption of SO2 was preferable, Rh/ Al2O3 exhibited more than an 8-fold higher uptake. Figure 4 shows the FT-IR spectra of the catalysts after exposure to SO2 at 200 °C referenced to those taken before SO2 supply. In the spectrum of only the Rh/Al2O3 catalyst, absorption bands 7235

DOI: 10.1021/acs.iecr.5b01720 Ind. Eng. Chem. Res. 2015, 54, 7233−7240

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cm−1). The former is therefore responsible for the high stability of adsorbed SO2 observed in Figure 2. Sulfate species (SO42−) was not observed, because SO2 oxidation did not occur in the absence of O2 under the present condition. Because similar SO2 adsorption behavior was observed for Al2O3 without loaded Rh, the interaction with the Lewis acid/base sites on Al2O3 plays a key role. On the surface of AlPO4, however, SO2 adsorption is not favored because of the lack of basic sites. Although AlPO4 also contains Lewis acid sites as shown in Table 1, they do not contribute to the SO2 adsorption, probably because of the weaker Lewis acidity. The different SO2 adsorption behaviors between Rh/AlPO4 and Rh/Al2O3 are in accordance with the acid/base character of these support materials as revealed by the NH3-TPD and CO2-TPD analyses (Table 1). The SO2 uptake per unit surface area for Rh/Al2O3 is much more than that found for CO2, because SO2 is able to adsorb on almost all the basic sites whatever their strength. The negligible SO2 uptake of Rh/AlPO4 is therefore indicative of the complete absence of basic sites on the surface of AlPO4. Other metal oxides such as ZrO2 and CeO2, which are conventionally used in automotive catalysts, also adsorbed considerable amounts of SO2 at ≤400 °C (Supporting Information). Under such circumstances, the adsorption of SO2 onto catalysts comprising Rh dispersed on these oxide supports will be facilitated and result in a severe poisoning effect, as described below. Figure 5 compares the catalytic light-off data for NO, CO, and C3H6 before and after sulfur poisoning at 200 °C. Although both catalysts before sulfur poisoning exhibited light-off at approximately 300 °C, Rh/AlPO4 exhibited steeper increases in the conversions compared with Rh/Al2O3, which is a characteristic of Rh catalysts supported on phosphate supports.23−26 The different light-off characteristics can be explained by the fact that Rh oxide (RhOx) is more easily reduced to the active metallic state when supported on AlPO4 compared with Al2O3. A more drastic difference was observed after treatment with SO2: the activity of Rh/Al2O3 was completely lost in the temperature range, while the steep rise in the conversion efficiencies within a narrow temperature range was preserved for the Rh/AlPO4 catalyst; although the

Table 2. Adsorption of SO2 onto the Catalysts at 200 and 500 °C SBETa/ m2 g−1

temp./ °C

Rh/ AlPO4

60

200

9.6

0.16

Rh/Al2O3

71

500 200 500

7.5 180 72

0.13 2.54 1.01

sample

SO2 uptakeb/ μmol g−1

SO2 uptakec/ μmol m−2

BET surface area after thermal aging at 900 °C for 25 h in a stream of 10 vol % H2O/air. b100 ppm of SO2, He balance, 1 h. cSO2 uptake divided by SBET. a

Figure 4. FT-IR spectra of (a) Rh/AlPO4 and (b) Rh/Al2O3 after sulfur poisoning at 200 °C for 1 h.

appeared in the region of S−O stretching vibration modes. According to literature data,35−39 these bands are assigned to SO2 strongly adsorbed onto Lewis basic O2− sites leading to the formation of sulfite (1300 and 1060−1100 cm−1) and SO2 weakly adsorbed onto Lewis acidic Al3+ sites (1180−1190

Figure 5. Catalytic light-off curves for NO, CO, and C3H6 before (solid lines) and after (dotted/dashed lines) sulfur poisoning. The catalyst was aged in a stream of 10 vol % H2O/air at 900 °C for 25 h and subsequently in a stream of 100 ppm of SO2/He at 200 °C for 1 h prior to the catalytic test. The catalyst (0.05 g) was heated at a constant rate of 10 °C min−1 in a mixed gaseous stream containing NO (500 ppm), CO (5100 ppm), C3H6 (390 ppm), O2 (4000 ppm), and He as the balance supplied at 100 cm3 min−1 (W/F = 5.0 × 10−4 g min cm−3). 7236

DOI: 10.1021/acs.iecr.5b01720 Ind. Eng. Chem. Res. 2015, 54, 7233−7240

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Industrial & Engineering Chemistry Research light-off temperature was shifted approximately 80 °C higher after sulfur poisoning. These results provide a clear indication that the Rh/AlPO4 catalyst has a much higher resistance to SO2 poisoning than the Rh/Al2O3 catalyst. When the sulfur poisoning was performed at 500 °C, the extent of poisoning was very similar to the case of Figure 5. 3.3. Effect of Phosphorus Poisoning. XRD patterns of Rh/AlPO4 and Rh/Al2O3 after exposure to a gaseous mixture of DMP and 10 vol % O2/N2 at 800 °C showed no detectable changes (Supporting Information). Nevertheless, the deposition of phosphorus species onto both catalysts was obvious from the X-ray fluorescence analysis results; although the extent of deposition was strongly dependent on the type of catalyst support, as shown in Table 3. The amount of phosphorus Table 3. Deposition of Phosphorus Oxide onto the Catalysts at 800 °C SBETa/ m2 g−1

exposure time/h

P uptakeb/ mmol g−1

P uptakec/ mmol m−2

Rh/ AlPO4

60

0.33

0.07

0.001

Rh/ Al2O3

71

1.0 0.33

0.74 1.1

0.012 0.015

1.0

2.3

0.032

sample

Figure 6. FT-IR spectra of Rh/AlPO4 and Rh/Al2O3 (a) before and (b) after phosphorus poisoning at 800 °C for 1 h. Asterisks indicate the nitrate contamination of KBr.

BET surface area after thermal aging at 900 °C for 25 h in a stream of 10 vol % H2O/air. b0.9 vol % (CH3O)2POH, 10 vol % O2, N2 balance, 800 °C. cPhosphorus uptake divided by SBET.

a

phosphorus poisoning test, suggesting that DMP supplied to the catalyst was oxidized to form phosphate overlayers. Summarizing these results, it is considered that the DMP vapor was decomposed in the presence of O2, leading to the deposition of phosphorus oxide species on the catalysts according to the following reaction:

species deposited on Rh/Al2O3 increased to 2.3 mmol-P g−1 after 1 h of exposure. Assuming homogeneous deposition over the entire surface of the Rh/Al2O3 catalyst (71 m2 g−1), this quantity corresponds to full coverage by 4.5 monolayers of phosphorus oxide (P2O5). These are higher than the values calculated for Rh/AlPO4 (60 m2 g−1), which were 0.74 mmol-P g−1 and 1.7 monolayers. Another point to be noted is that phosphorus deposition did not increase linearly with time-onstream. For instance, a 10-fold increase of the uptake was observed for Rh/AlPO4 when the exposure time was changed from 0.33 to 1 h. This behavior is probably due to different deposition rates of phosphorus oxide over the fresh and poisoned surface of catalysts, although a detailed mechanism is not clear at this stage. The structure of the phosphorus overlayers was studied using FT-IR in the region from 1500 to 400 cm−1, as shown in Figure 6. The exposure of Rh/Al2O3 to the mixture of DMP/O2/N2 caused the appearance in its FT-IR spectrum of a broad band centered at approximately 1150 cm−1, which was assigned to P−O stretching vibrations (νP−O).40 This band may also include contributions from characteristic bands due to P3O9 (1250 cm−1), P2O7 (1000−900 cm−1), PO4 (1100 cm−1), and PO3 (1300 cm−1), although precise assignment is difficult because of the overlap with alumina absorption bands below 1000 cm−1. No obvious change in the FT-IR spectrum of Rh/ AlPO4 was observed, because the bands for the deposits overlapped with those of the intrinsic phosphate species. Two peaks centered at approximately 1150 and 500 cm−1 were, respectively, assignable to P−O stretching (νP−O) and O−P−O bending (δO−P−O) vibration modes of tetragonal orthophosphate (PO4) species,30,41 while the weak band centered at 730 cm−1 was assigned to the asymmetric and symmetric stretching frequencies of Al−O−P bonds in nonstoichiometric aluminum phosphates.42 These peaks were slightly intensified after the

2(CH3O)2 POH + 7O2 → 4CO2 + P2O5 + 7H 2O

(1)

Because as-produced P2O5 is a gas at 800 °C, it can pass through the heated catalyst bed. However, larger quantities of phosphorus were deposited on Rh/Al2O3 compared with Rh/ AlPO4 for each exposure time (Table 3). Such different deposition behaviors can be rationalized by assuming the following gas−solid reaction: P2O5(g) + Al 2O3 → 2AlPO4

ΔG° = − 423kJ mol−1 (800°C) (2)

Reaction 2 is thermodynamically favorable, produces an AlPO4 overlayer that covers the Al2O3 surface, and may explain the greater extent of phosphorus poisoning on Rh/Al2O3 compared with Rh/AlPO4. A similar reaction sequence is known to be involved in the phosphorus poisoning of automotive catalysts; the engine oil additive ZDDP is oxidized in a combustion process and deposited on the catalyst as glassy overlayers of zinc phosphates.2−5 The zinc phosphate overlayers are then converted to AlPO4 via interactions with the Al2O3 support. Therefore, the reaction with Al2O3 plays a key role in facilitating the deposition of phosphate overlayers. This mechanism is not available for the Rh/AlPO4 catalyst, and thus, the phosphate support does not promote phosphate deposition. Figure 7 compares the catalytic light-off curves for NO, CO, and C3H6 over the Rh/AlPO4 and Rh/Al2O3 catalyst before and after phosphorus poisoning for 1 h. The Rh/Al2O3 catalyst suffered a significant loss in activity after exposure to the DMP 7237

DOI: 10.1021/acs.iecr.5b01720 Ind. Eng. Chem. Res. 2015, 54, 7233−7240

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Industrial & Engineering Chemistry Research

Figure 7. Catalytic light-off curves for NO, CO, and C3H6 before (solid lines) and after (dotted/dashed lines) phosphorus poisoning. The catalyst was aged in a stream of 10 vol % H2O/air at 900 °C for 25 h and subsequently in a stream of 0.9 vol % (CH3O)2POH, 10 vol % O2, and He as balanced by N2 at 800 °C for 1 h prior to the catalytic test. The catalytic test conditions are described in Figure 5.

4. CONCLUSION The Rh/AlPO4 catalyst was shown to exhibit higher resistance to sulfur and phosphorus poisoning compared with the Rh/ Al2O3 catalyst. The acidity of the phosphate surface is effective for suppressing the catalyst deactivation caused by adsorption of SO2 at 200 and 500 °C. Conversely, the Al2O3 surface has Lewis acid/base sites that are reactive to SO2 and thus the SO2 dominant Rh surface. Rh/AlPO4 also demonstrated a higher resistance to phosphorus poisoning at 800 °C using DMP, (CH3O)2POH, as a model phosphorus source. This test led to significant deactivation of the Rh/Al2O3 catalyst, because the oxidation of DMP produced phosphorus oxides which reacted with the Al2O3 surface to form amorphous AlPO4 overlayers that occluded the active Rh sites. In the Rh/AlPO4 catalyst, the AlPO4 surface was less reactive to the phosphorus oxides, and therefore, their deposition was suppressed. The combination of high catalytic activity, high thermal stability, and poisoning resistance render phosphate-supported Rh catalysts interesting for potential practical applications in automobiles.

vapor. In contrast, the light-off characteristic was preserved for Rh/AlPO4, although its temperature was shifted approximately 80 °C higher. Notably, the deactivation observed for Rh/Al2O3 was greater than expected on the basis of the quantity of phosphorus deposits, thus suggesting a deleterious effect on the Rh species. To elucidate this point, XPS spectra of the P 2s, Al 2s, O 1s, and Rh 3d levels were obtained to determine the surface Rh concentration before and after phosphorus poisoning (Supporting Information). The calculated surface compositions are shown in Table 4. Clearly, the surface concentration Table 4. Surface Composition of the Catalysts after Phosphorus Poisoning at 800 °C surface concentrationa/atom % sample

exposure time/h

Rh

Al

P

Rh/AlPO4

0 0.33 1 0 0.33 1

0.20 0.24 0.28 0.38 0.22 0.15

48.57 47.36 46.36 99.62 97.43 91.71

51.23 52.40 53.35 null 2.35 8.14

Rh/Al2O3

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ASSOCIATED CONTENT

S Supporting Information *

a

Determined by XPS analysis. Normalized by the sum of Rh, Al, and P.

NH3-TPD, CO2-TPD, FT-IR of adsorbed pyridine, SO2 adsorption, catalytic activity, XRD, and XPS. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01720.

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of Rh on Al2O3 decreased to a greater extent relative to AlPO4. This result suggests that physical blocking of the active sites by the phosphate overlayers served as the prevailing mechanism for Rh/Al2O3 catalyst deactivation. The conversion of Al2O3 to AlPO4 is considered to lead to significant surface reconstruction and thus occlusion of Rh. The poisoning is irreversible and negatively impacts catalyst performance. Such a deactivation mechanism due to phosphorus poisoning was suppressed to a significant extent on Rh/AlPO4 because its phosphate surface is less reactive to the phosphate deposits. Although Rh/AlPO4 after poisoning showed slightly increasing values of the Rh concentration, they should be nearly constant considering an experimental error tolerance of ±0.05 atom %.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Elements Science and Technology Project from the Ministry of Education, Culture, Sports, Science and Technology (MEXT). A part of this work was performed under management of the Elements Strategy Initiative for Catalysts & Batteries (ESICB) supported by MEXT. 7238

DOI: 10.1021/acs.iecr.5b01720 Ind. Eng. Chem. Res. 2015, 54, 7233−7240

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DOI: 10.1021/acs.iecr.5b01720 Ind. Eng. Chem. Res. 2015, 54, 7233−7240

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Industrial & Engineering Chemistry Research (en)3Al3P4O16·3H2O and [NH4]3[Co(NH3)6]3[Al2(PO4)4]2·2H2O. Spectrochim. Acta, Part A 2000, 56, 2715−2723.

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DOI: 10.1021/acs.iecr.5b01720 Ind. Eng. Chem. Res. 2015, 54, 7233−7240