Letter pubs.acs.org/acscatalysis
Highly Efficient Catalysis of Preferential Oxidation of CO in H2‑Rich Stream by Gold Single-Atom Catalysts Botao Qiao,†,§ Jiaxin Liu,† Yang-Gang Wang,‡ Qingquan Lin,§,∥ Xiaoyan Liu,§ Aiqin Wang,§ Jun Li,*,‡ Tao Zhang,*,§ and Jingyue (Jimmy) Liu*,† †
Department of Physics, Arizona State University, Tempe, Arizona 85287, United States State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy Sciences, Dalian 116023, China ‡ Department of Chemistry and Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Tsinghua University, Beijing 100084, China §
S Supporting Information *
ABSTRACT: Preferential oxidation of CO (PROX) in H2-rich stream is critical to the production of clean H2 for the H2-based fuel cells, which provide clean and efficient energy conversion. Development of highly active and selective PROX catalysts is highly desirable but proved to be extremely challenging. Here we report that CeO2-supported Au single atoms (Au1/CeO2) are highly active, selective, and extremely stable for PROX at the PEMFC working temperature (∼80 °C) with >99.5% CO conversion over a wide temperature window, 70−120 °C (or 50−100 °C, depending on the Au loading). The high CO conversion realized at high temperatures is attributed to the unique property of single-atom catalysts that is unable to dissociatively adsorb H2 and thus has a low reactivity toward H2 oxidation. This strategy is proven in general and can be extended to other oxide-supported Au atoms (e.g., Au1/FeOx), which may open a new window for the efficient catalysis of the PROX reaction. KEYWORDS: single-atom catalysis, gold, preferential oxidation, carbon monoxide, electron microscopy
H
activation,15 only occurs on sites that contain more than two metal atoms/aggregates.16,17 In our recent work, we demonstrated that oxide-supported Au single-atom catalysts (SACs) are highly active for CO oxidation even at ambient temperatures.18,19 Therefore, if the activation barrier of H2 is much higher on supported Au single atoms than that of CO, then oxide-supported Au SACs should provide an effective approach to developing highly active and selective Au catalysts for the PROX reaction.20 In this Letter, we, for the first time, report that CeO2 supported Au single atoms (Au1/CeO2) are highly active, selective, and stable for the PROX reaction. We further demonstrate that the strategy of suppressing H2 oxidation by modifying the active sites for CO oxidation is general and can be extended to other oxide supported metal atoms (e.g., FeOxsupported Au1 SACs). The CeO2 support was synthesized by coprecipitation of Ce(NO3)3·6H2O and Na2CO3 solution and calcination of the precipitate solids at 400 °C for 5 h (see sample preparation details in the Support Information (SI)). CeO2-supported Au1 SACs with 0.05 and 0.3 wt % Au were prepared by a facile adsorption method (denoted as 0.05Au1/CeO2 and 0.3Au1/
2-based
fuel cells (i.e., proton exchange membrane fuel cells, (PEMFCs)) are crucial for efficient energy conversion and play a central role in the H2 economy.1−3 Production of H2 is generally accomplished by a multistep procedure including catalytic reforming of hydrocarbons followed by water gas-shift (WGS) processes.4,5 Although these processes can reduce the CO content to about 1 vol %, such residual CO can severely poison the Pt anode and thus decrease the performance of fuel cells. Preferential oxidation of CO (PROX) in H2-rich stream has been recognized as the most straightforward and cost-effective solution to remove the residual CO to an acceptable level (e.g., below 50 ppm).6,7 Among the various catalysts developed for the PROX reaction,6−9 oxide-supported Au catalysts seem to be the most suitable and promising10 because they are highly active for CO but less active for H2 oxidation at low temperatures.11,12 Very few supported Au catalysts, however, can reduce CO to the target level at the PEMFC working temperature window (e.g., 50−100 °C). The critical challenge is the significant drop of CO conversion with reaction temperature,10,13 which is due to the competitive oxidation of H2 at elevated temperatures. Therefore, the selectivity toward CO oxidation can be manipulated by lowering the reactivity toward H2 oxidation.14 It has been proposed and subsequently proved that the dissociative adsorption of H2 on a metal, a key step for H2 © XXXX American Chemical Society
Received: June 2, 2015 Revised: September 17, 2015
6249
DOI: 10.1021/acscatal.5b01114 ACS Catal. 2015, 5, 6249−6254
Letter
ACS Catalysis
signal is too weak to give a robust fitting it can be deduced that the majority of the Au probably existed as Au+. PROX reaction measurements were performed with 40 vol % H2 + 1 vol % CO + 1 vol % O2 and helium balance. Figure 2
CeO2, respectively). The 0.05Au1/CeO2 with further calcination at 200 °C was denoted as 0.05Au1/CeO2−C200. For comparison, a 2 wt % Au/CeO2, which primarily consisted of Au NPs, was prepared by a deposition−precipitation (DP) method (denoted as 2Au/CeO2-DP). In addition, a 1 wt % Au/ CeO2 (2RRCe, denoted as Au/CeO2-2RRCe) reference catalyst (provided by Haruta Gold Inc.) was also tested for comparison. The X-ray diffraction pattern of the synthesized CeO2 powders is presented in Figure S1. It exhibits typical diffraction peaks of CeO2. The BET surface area of the synthesized CeO2 was measured to be 38.1 m2 g−1. Representative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the 0.05Au1/CeO2 are shown in Figure 1 and S2. Isolated single Au atoms (highlighted by the
Figure 2. CO conversion as a function of reaction temperature for PROX on Au/CeO2 catalysts. Reaction condition: 1 vol % CO + 1 vol % O2 + 40 vol % H2 balanced with He. Weight hourly space velocity (WHSP) = 25 000 mL gcat−1 h−1.
shows CO conversion with reaction temperature. On the reference sample of Au/CeO2 -2RRCe, CO conversion increased gradually with reaction temperature and reached the maximum of 92% at 80 °C and then decreased slightly. Comparatively, the 2Au/CeO2-DP was highly active and yielded 100% CO conversion at 40 °C. However, the CO conversion decreased with increasing reaction temperature. Such a temperature-dependent behavior is caused by the competitive oxidation of H2 with increasing temperature and has been observed on supported Au catalysts.23−26 The 0.05Au1/CeO2 catalyst reached 100% CO conversion at 70 °C, maintained such a performance at temperatures up to 100 °C, and achieved a stellar conversion of >99.5%, even at 120 °C. The 0.3Au1/CeO2 performed as good as the 2Au/CeO2-DP and realized 100% CO conversion at a temperature as low as 50 °C, clearly demonstrating its high atom efficiency. Moreover, the 0.3Au1/CeO2 achieved total CO conversion in the temperature range of 50−100 °C, the typical temperature window for practical PEMFC operations. As shown in Figure S6, both 0.05 and 0.3 wt % Au1/CeO2 exhibited 50% CO2 selectivity after the CO conversion achieving 100%. The selectivity of the 2% Au/CeO2-DP catalyst decreased gradually with reaction temperature after CO conversion achieving 100%, suggesting that the active centers in the Au particle catalyst behave differently from those of the isolated Au atom catalysts. The CeO2 support, as a control, was almost inactive with CO conversation
