Inhibition Effect of Surface Oxygenated Species on Ammonia

ARTICLE pubs.acs.org/JPCC

Inhibition Effect of Surface Oxygenated Species on Ammonia Oxidation Reaction Wei Peng, Li Xiao,* Bing Huang, Lin Zhuang,* and Juntao Lu College of Chemistry and Molecular Sciences, Hubei Key Lab of Electrochemical Power Sources, Wuhan University, Wuhan 430072, People's Republic of China ABSTRACT: The ammonia oxidation reaction (AOR) cannot proceed on Pt surface at potentials corresponding to the formation of surface oxygenated species (Oads and OHads), it is thus suspicious that, in addition to the Nads, the surface oxygenated species may also play an inhibitive role. Yet this is difficult to be proven in aqueous media where the AOR is always accompanied with the water oxidation reaction. In the present work, we carry out differential electrochemical mass spectroscopy (DEMS) studies of the AOR in nonaqueous media where no surface oxygenated species is involved, and the results turn out be remarkably different from those obtained in KOH solution. It is evident that, without the blocking of oxygenated species, the Pt surface can remain continuously active for AOR, and N2 is the dominant product. More strikingly, Pd becomes highly active for the AOR in nonaqueous media, a result enormously different from that in KOH solution where Pd exhibits a very low catalytic activity because of severer surface passivation by oxygenated species. This work provides compelling evidence for the inhibition effect of surface oxygenated species on the AOR, and illuminates our understanding of relevant reaction mechanisms.

’ INTRODUCTION The ammonia oxidation reaction (AOR, Reaction 1) is an anodic process related to energy conversion1
7 and environmental protection.8
12 2NH3 f N2 þ 6Hþ þ 6e


ð1Þ

As a hydrogen carrier, NH3 possesses attractive features such as its high hydrogen content (17.6 wt %) and ease of liquification. However, the kinetics of AOR is disappointedly slow, even catalyzed by Pt, the most effective metal thus found.13
21 Efforts have been devoted to further improving the catalytic activity of Pt toward the AOR,22
35 mostly by making Pt alloys, Pt
M (M = Ir, Ru, Ni, Cu, Pd, Rh, etc.),22
29 among which Pt
Ru30 and Pt
Rh30 have exhibited interesting enhancements. In-depth studies also showed that the AOR is a structure-sensitive reaction and takes place favorably on the Pt(100) facet,23 thus better catalytic activity can be expected through making Pt nanoparticles with rich (100) surfaces.26 Albeit encouraging, the progress is still not groundbreaking. Challenges seem to stem from the difficult control of the catalytic selectivity. On the one hand, the dehydrogenation processes of NH3 require an active surface with high hydrogen affinity; on the other hand, the combination between surface nitrogenated species (e.g., Nads and NHads) for N2 production favors a surface with relatively low reactivity. A basic experimental observation is that the steady anodic current for N2 production on Pt surface can only be seen with large potential polarization (e.g., above 0.3 V). This is believed to be ascribed to the slow dehydrogenation of NH3 on Pt; in other words, NH3 has only been r 2011 American Chemical Society

partially dehydrogenated in the low potential region.36 One may thus expect to use a metal with higher hydrogen affinity, such as Pd; but, confusedly enough, Pd turned out to show a very low catalytic activity toward the AOR in KOH solution.37 In addition to the dehydrogenation of NH3, the following process of N2 production is also puzzling: the N2 production cannot proceed on Pt with increasing the potential. It is thought that NH3 is fully dehydrogenated in this potential region,38 and the resulting Nads binds strongly to the Pt surface, thus hindering the N2 production.39 Yet things could be more complicated: de Vooys et al.39 suggested the influence of the coadsorbed OH
should be taken into account, and Vidal-Iglesias et al.29 proposed that the surface oxide could be another inhibitor to the AOR since the water oxidation reaction is involved in this potential region. It is imaginable that the surface oxygenated species (OHads and Oads) would play a bilateral role in the AOR. On the one side, the surface oxygenated species can play a reactive role, it can react with the surface nitrogenated species36 to produce nitrogen oxides (N2O and NO); on the other side, the surface oxygenated species is a competitive adsorbate and could hinder the adsorption of NH3 and the combination between surface nitrogenated intermediates. There has hitherto been no experimental proof about the real role of surface oxygenated species in AOR. In the present work, we attempt to unravel the impact of surface oxygenated species on the AOR. Our idea is to study the AOR in nonaqueous media where surface oxygenated species is absent, and Received: August 23, 2011 Revised: October 14, 2011 Published: October 18, 2011 23050

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Figure 1. Schematic illustrations of the DEMS setup for measurements in aqueous (A) and nonaqueous (B) media.

then to compare with results obtained in aqueous media so as to find out the effects of surface oxygen species. The experimental method we employed is the differential electrochemical mass spectroscopy (DEMS), which is usually applied in aqueous systems, but is modified in the present work for measurements in nonaqueous systems.

’ EXPERIMENTAL SECTION Preparation of Working Electrodes. Pt and Pd working electrodes were prepared by electrochemical deposition on a Au rotating disk electrode (5 mm in diameter, Pine Research Instruments) at a rotation rate of 1600 rpm in a deaerated KCl solution (0.1 M) containing 5  10
5 M H2PtCl6 and 10
5 M PdCl2, respectively. A sheet of carbon paper was used as the counter electrode and a Ag/AgCl (0.1 M KCl) served as the reference electrode. The deposition potential was fixed at
0.7 V (vs Ag/AgCl) for Pt electrode and
0.3 V (vs Ag/AgCl) for Pd electrode, where the reduction processes were both under diffusion control. In order to prepare Pt and Pd electrodes with similar electrochemical surface area, 10 000s and 7000s were chosen as the electrochemical deposition time for Pt and Pd, respectively. All experiments were performed in a conventional three-electrode cell at room temperature. The potentiostat was a CHI-660 potentiostat. All reagents were of GR grade and solutions were prepared using ultrapure water (18 MΩ 3 m). CO Stripping. CO stripping experiments were performed to determine the electrochemical surface area of the working electrodes. After electrochemical deposition, the working electrode was rinsed with ultrapure water repeatedly prior to the following test. CO stripping was carried out in a 0.5 M H2SO4 solution deaerated by

Figure 2. The CO stripping on Pt (A) and Pd (B) in a deaerated 0.5 M H2SO4 solution at a potential scan rate of 20 mV/s.

purging Ar. For each working electrode, a new sheet of carbon paper was used as the counter electrode in order to exclude any possible metal contamination introduced from the previous electrochemical operations. The reference electrode was a reversible hydrogen electrode (RHE) in the same solution. Each working electrode was preadsorbed with CO. For Pt electrode, the potential was held at 0.1 V (vs RHE) while the solution was bubbled with CO for 10 min and subsequently with Ar for another 15 min. Then the potential was swept between 0.05 and 1.3 V (vs RHE) for two cycles. As for Pd electrode, the whole procedure was almost the same as that for Pt electrode except that the CO adsorption potential was set at 0.4 V (vs RHE) and the potential sweeping range was between 0.1 and 1.2 V (vs RHE). Cyclic Voltammetry. After CO stripping, the working electrode was rinsed with ultrapure water repeatedly prior to the following test. Cyclic voltammograms (CVs) were recorded in a deaerated 1 M KOH solution. For electrochemical measurements in KOH solution, a Hg/HgO electrode was used as the reference electrode, while the recorded electrode potential was converted afterward to be versus the reversible hydrogen electrode (RHE) in the same solution. The cyclic voltammetry for AOR was carried out in a deaerated 1 M KOH solution containing 0.1 M NH3. For electrochemical measurements in nonaqueous media, anhydrous acetonitrile was used in most cases as the solvent (otherwise will be specified in the context), and recrystallized tetrabutyl ammonium tetrafluoroborate (TBATF) was used as the supporting electrolyte (0.06 M). A Ag/Ag+ electrode was employed as the reference electrode, filled with an acetonitrile solution containing 23051

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Figure 3. DEMS measurements in aqueous medium for the AOR on Pt at a potential scan rate of 5 mV/s. (A) CV in 1 M KOH, (B) CV in 1 M KOH with 0.1 M NH3, (C) MSCV of N2 (m/z = 28), (D) MSCV of N2O (m/z = 44), and (E) MSCV of NO (m/z = 30).

5 mM AgNO3 and 0.06 M TBATF. The cyclic voltammetry for AOR was performed in an ammonia-saturated solution obtained by blowing dried ammonia gas (99.9%) into the nonaqueous solution in the cell. Differential Electrochemical Mass Spectroscopy (DEMS) Measurements. In this work, a special DEMS setup was used, equipped with an AMETEK Dycor mass spectrometer (MS). As shown in Figure 1, the main feature is that a face-up disk electrode was used as the working electrode, immediately above which a MS sampling probe was located. The electrolyte layer above the disk electrode was controlled to be as thin as possible so as to increase the local concentration and to facilitate the evaporation of volatile species. A capillary (2 μm in diameter and 1 cm in length) was sealed at the end of a stainless steel pipe to serve as the MS sampling probe. For studies in aqueous media, the capillary sampling probe was wrapped with a sheet of porous PTFE membrane and then submerged in the electrolyte layer to get as close as possible to the surface of the working electrode (Figure 1A). For studies in nonaqueous media, the capillary probe was placed at the center of a polyethylene (PE) mini-hood capping over the working electrode (Figure 1B), such that a notable part of the volatile products can be collected before diffusing to the bulk solution.

For the purpose of monitoring the formation of N2, N2O, and NO, we chose three m/z values, 28 (N2ø+)/44 (N2Oø+)/30 (NOø+), for recording. The electrochemical conditions of the DEMS measurements were identical to those of ordinary cyclic voltammetry described above.

’ RESULTS AND DISCUSSION CO Stripping Measurements. Prior to the AOR experiments, the electrochemical surface area (ESA) of the working electrode was determined by CO stripping experiments. Figure 2A,B shows results for Pt and Pd electrodes, respectively. The ESA was determined by calculating the charge corresponding to the CO stripping peak, using 420 and 326.7 μC/cm2 as the unit capacity for the Pt and Pd surface, respectively.40
43 The ESA of the electrodeposited electrode is about 2.8 cm2. In the following electrochemical measurements, all recorded currents will be converted to current density using the ESA of the corresponding electrode. Also indicated in Figure 2 is that the Pt and Pd surfaces thus obtained are normally polycrystalline and dominated by (100) and (111) facets,40
43 thus no special morphology effects are involved in the present work.44 23052

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Figure 4. DEMS measurements in aqueous medium for the AOR on Pd at a potential scan rate of 5 mV/s. (A) CV in 1 M KOH, (B) CV in 1 M KOH with 0.1 M NH3, (C) MSCV of N2 (m/z = 28), (D) MSCV of N2O (m/z = 44), and (E) MSCV of NO (m/z = 30).

DEMS Studies of the AOR in KOH Solution. As aforementioned, it was thought that, in addition to the Nads, the surface oxides could also be an inhibitor to the AOR, based on the DEMS observations on Pt(100) surface.29 We try to verify this concern by performing DEMS measurements on polycrystalline Pt and Pd in KOH solution. The cyclic voltammograms (CVs) and the corresponding MS voltammograms (MSCVs) were recorded simultaneously and are displayed in Figures 3 and 4. Figures 3B and 4B are the CVs of the AOR catalyzed by Pt and Pd, respectively, while Figures 3A and 4A are the corresponding CVs without ammonia. As for the MSCVs, each MS ion current has been subtracted by its own background signal so as to underline the signal change. The correlation between the CVs and the MSCVs in Figures 3 and 4 are apparent, despite a small delay in the MS response. It can be seen from Figure 3B that the anodic peak potential in the CV of Pt-catalyzed AOR is almost identical to the onset potential of the formation of surface oxide in KOH solution without ammonia (Figure 3A). Moreover, as revealed in Figure 3C
E, the MS ion current of N2ø+ decreases with the anodic current of AOR, accompanied by the increase in the MS signals of nitrogen oxides (N2Oø+ and NOø+). Such

observations seem to suggest that the formation of surface oxides on Pt is correlated to the declining process of N2 production and to the formation of nitrogen oxides.39 In comparison to Pt, Pd exhibits a much lower catalytic activity toward the AOR (Figure 4B), but, somewhat unexpectedly, the MS signal of N2, albeit very weak, can also be observed in this case at positive potentials (Figure 4C), which, to our knowledge, has not yet been reported in the literature. It seems that the AOR has been inhibited to a greater extend on Pd surface, which may be due to its higher surface reactivity.45 DEMS Studies of The AOR in Nonaqueous Media. It seems both Nads and oxygenated species could play an inhibitive role in the AOR, but it is difficult to distinguish them in aqueous solutions where the water oxidation reaction is inevitable. In the present work, we carry out DEMS studies of the AOR in nonaqueous media which, in principle, does not contain any oxygen sources, such that we may be able to ascertain the impact of oxygenated species on the AOR in aqueous media. The DEMS studies of the AOR on Pt and Pd were performed in an acetonitrile solution saturated with NH3, and the results turned out to be very different from those in aqueous solutions. As illustrated in Figures 5A and 6A for the CVs of AOR on Pt and 23053

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Figure 5. DEMS measurements in an ammonia-saturated acetonitrile solution for the AOR on Pt at a potential scan rate of 10 mV/s. (A) CVs with (red) and without NH3 (black), (B) MSCV of N2 (m/z = 28), and (C) MSCV of N2O (m/z = 44).

Pd, respectively, in both the forward (positively going) and backward (negatively going) potential scans, anodic currents are observed and continuously increase with the potential, and no current peak appears in the high potential region. Meanwhile, N2ø+ is the dominant signal detected by DEMS (Figures 5B and 6B), which shows a clear correlation with the anodic current of AOR (Figures 5A and 6A). The faint MS signals of N2O (Figures 5C and 6C) should be caused by the trace amount of water in the system. These CV and MSCV features are remarkably different from those in aqueous media, and strongly indicate that the AOR, once taking place, has not been inhibited in nonaqueous media. The distinct behaviors of AOR in aqueous and nonaqueous media can most reasonably be ascribed to the difference in the surface state of the catalysts. When anhydrous acetonitrile is used as the electrolyte, no oxygenated species will be created on the catalyst surface, thus the adsorption of NH3 and the subsequent dehydrogenation steps can proceed without interference from the competing adsorption of oxygenated species. In this case, the only possible inhibitor for the AOR would be the strongly

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Figure 6. DEMS measurements in an ammonia-saturated acetonitrile solution for the AOR on Pd at a potential scan rate of 10 mV/s. (A) CVs with (red) and without NH3 (black), (B) MSCV of N2 (m/z = 28), and (C) MSCV of N2O (m/z = 44).

adsorbed intermediate, NHads and/or Nads.28 It seems, however, that in nonaqueous media, there is no poisoning intermediate that can effectively block the metal surface, given the fact that the anodic current of AOR increases continuously with the electrode potential. Further increasing the electrode potential (i.e., exceeding the positive extreme of potential in Figures 5 and 6) will leave the electrode surface covered with N2 bubbles. To exclude uncertain effect caused by particular organic solvent adopted, we also conducted the DEMS experiments in anhydrous N,N-dimethylformamide (DMF), and the result turned out to be essentially the same. Inhibition Effect of Surface Oxygenated Species. On the basis of the above comparative DEMS studies of the AOR in aqueous and nonaqueous media, it is evident that the surface oxygenated species does cause an inhibition effect on the AOR in KOH solution. In addition to the competitive adsorption, the possible inhibition mechanism could also be a spatial effect: the coadsorbed oxygenated species isolates the surface nitrogenated species and thus reduces their combination for N2 production, 23054

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The Journal of Physical Chemistry C leading to the observed decrease in the MS signal of N2 (Figure 3C). Meanwhile, the reaction between surface-nitrogenated species and surface-oxygenated species to produce NxO is not fast enough, resulting in the decline in the anodic current at high potentials (Figure 3B). Whereas in nonaqueous media the above inhibition effect disappears, therefore no current peak is observed in either the CVs or the MSCVs upon increasing the electrode potential (Figure 5). One cannot conclude, however, that the Nads is not an inhibitor for AOR in KOH solution, because it has not been proven in the present work whether the Nads has been involved in the AOR in nonaqueous media (although it has been believed so in KOH solution). It is also difficult to compare the reaction rate of AOR in nonaqueous media with that in aqueous media, because the AOR is a pH-dependent reaction and the potential reference “RHE” is ill defined in nonaqueous media, such that we cannot compare the anodic currents of AOR on the same footing. Another striking change upon switching the AOR from aqueous to nonaqueous media is the activation of Pd. Distinctly different from that in KOH solution, the AOR on Pd in nonaqueous media is vigorous; the anodic current density on Pd (Figure 6A) is even greater than that on Pt (Figure 5A) at the same potential. Such a peculiar change in the catalytic activity of Pd toward the AOR is now understandable by taking into account the inhibitive role played by the oxygenated species. Whereas in KOH solution the Pd surface is of higher surface reactivity, in comparison to the Pt surface, and is passivated by the oxygenated species to a greater extent, such an inhibition effect does not exist in nonaqueous media, thus the Pd surface becomes highly efficient for the dehydrogenation of NH3, giving rise to the observed high catalyticactivity toward the AOR.

’ CONCLUSIONS To reveal the role of surface oxygenated species in the AOR, we have conducted comparative DEMS studies in aqueous and nonaqueous media. The special DEMS setup in this work is designed to enable measurements in nonaqueous media. It is found that, in nonaqueous media where surface oxygenated species is absent, the AOR on Pt can proceed continuously without any inhibition, and N2 is the dominant product detected. More strikingly, without surface blocking of oxygenated species, Pd turns out to be highly active for the AOR, probably because of its higher activity for the dehydrogenation of NH3. It is thus evident that the surface oxygenated species does cause an inhibition effect on the AOR in aqueous media, most probably through the competing adsorption with NH3 and the blocking to the combination between surface nitrogenated species. It also implies that the reaction between the surface oxygenated species and the surface nitrogenated species is not fast in kinetics, leading to the passivation of the metal surface. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the National Science Foundation of China (20933004, 20773096), the National Basic

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Research Program of China (2012CB932800, 2012CB215503), the National Hi-Tech R&D Program (2011AA050705), and the Fundamental Research Funds for the Central Universities (203275662, 203275672).

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