Al2O3 Sorbents for Elemental Mercury Capture at High

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Pd/Al2O3 Sorbents for Elemental Mercury Capture at High Temperatures in Syngas Wenhui Hou, Jinsong Zhou, Chunjiang Yu,* Shulin You, Xiang Gao, and Zhongyang Luo State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: To capture elemental mercury (Hg0) at high temperatures in simulated syngas, Pd/Al2O3 sorbents were developed using an impregnation method. The Pd/Al2O3 sorbents with a Pd loading value of 8% exhibited high Hg0 capture activity from 200 to 270 °C. Over 90% of the mercury capture by 8Pd/Al2O3 was achieved for operating temperatures up to 270 °C. The superior performance of Pd/Al2O3 sorbents could be primarily ascribed to the formation of a Pd−Hg amalgam. H2 and CO were found to be promotional on Hg0 capture because of the reduction of PdO to Pd metal. H2S significantly inhibited Hg0 capture due to the consumption of the active palladium. HCl promoted Hg0 capture, which could be ascribed to the catalytic oxidation of Hg0 to HgCl2. H2O showed a prohibitive effect on Hg0 removal. The regeneration experiments indicated that the spent Pd/Al2O3 sorbents could be facilely revivified and reused.

1. INTRODUCTION Mercury, which is a highly toxic heavy metal, is considered a global threat to human and environmental health.1,2 In 2010, the worldwide anthropogenic mercury emission was estimated to be 1960 tons, and coal burning (all uses) contributed approximately 24% of this total.3,4 The world’s first legally binding treaty on limiting the emission and use of hazardous mercury (Minamata Convention on Mercury) was adopted by delegates from over 140 countries and regions in January 2013, after four years of negotiation.5 Coal gasification has gained considerable attention as a promising use of coal due to its mitigated effects on the environment. However, mercury can also be released during the gasification process. Hg0 has been reported to be the predominant form of mercury in the syngas generated from coal gasification, accounting for approximately 50−97% of the total mercury. Additionally, Hg0 is more difficult to capture as compared to oxidized (Hg2+) or particulate-bound mercury (Hgp) because of its low solubility.6−8 In recent years, many researchers have studied the Hg0 capture in flue gas. By contrast, the removal of Hg0 from syngas is rarely investigated. As compared to the capture observed in combustion flue gas, mercury capture is preferred at higher temperatures in gasification systems for power generation because increased temperatures can improve the overall thermal efficiency.9 Thus, the removal of elemental mercury at higher temperatures from syngas is essential for the efficient gas-cleaning of a coal gasification system that is used for power generation. Various sorbents, particularly activated carbons, have been observed to exhibit high mercury removal efficiencies. However, these sorbents are extremely restricted in coal gasification applications because of their decreased efficiency at elevated temperatures (>200 °C).10 Many supported noble metals, including Pd, Rh, Ir, Pt, and Au, have been used to test their mercury adsorption abilities, due to their regeneration potential and good stability at high temperatures. In particular, Pd has been considered the most attractive option for use in hightemperature removal of Hg0.11,12 Density functional theory (DFT) calculations also predicted that the amalgamation © 2014 American Chemical Society

enthalpy of Pd was significantly higher than that of the other pure metals that are potential candidates for mercury capture at high temperatures.13 In addition, the lower metal requirement (in volume) and longer usage period could offset the higher cost of this precious metal.6 Al2O3 is usually used as a support for palladium, due to its optimal mechanical properties and thermal stability, which can enhance the gas−solid contact. Baltrus et al.14,15 investigated the relative affinities between palladium and Hg0 and showed evidence of Hg amalgamation on the Pd/Al2O3 sorbent. Poulston et al. used X-ray diffraction (XRD) to demonstrate the formation of a Pd−Hg solid solution.16 Unfortunately, with respect to either the role of individual syngas components in Hg0 capture by Pd/Al2O3 or the regenerability of the sorbent, no investigation has been reported to date. In the research reported herein, Hg0 capture over Pd/Al2O3 sorbents that were synthesized using an impregnation method was evaluated at high temperatures. The optimal Pd loading of the prepared sorbents was explored. The effects of individual syngas components on Hg0 capture were also examined. Moreover, the regenerability of the sorbent was investigated. The ultimate goal of this study was to develop a powerful, regenerable material that was favorable for removing Hg0 from syngas at high temperatures.

2. EXPERIMENTAL SECTION 2.1. Preparation of Sorbents. Sorbents were developed using an incipient wetness impregnation method. First, palladium nitrate was dissolved in deionized water, and γAl2O3 was then impregnated in the palladium nitrate solution with continuous stirring for 30 min. This mixture was dried at 100 °C for 12 h, followed by calcination at 500 °C in air for 2 h. Finally, the samples were pressed and then sieved to 80−100 Received: Revised: Accepted: Published: 9909

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mesh size. The samples are denoted as xPd/Al2O3, where x is in the range of 1−20 and denotes the loading values (wt %) in the sorbent. 2.2. Characterization of Sorbents. X-ray diffraction (XRD) tests were obtained on an X’Pert PRO system employing Cu Kα radiation (λ = 1.541 Å) to examine the crystal structure. The Brunauer−Emmett−Teller (BET) surface area was measured by a nitrogen adsorption system (Micromeritics 2020 mintrument). X-ray photoelectron spectroscopy (XPS) analysis was made on a thermo ESCALAB MARK II instrument with Al Kα radiation to detect the binding energies of Pd 3d and S 2p. The C 1s line at 284.6 eV was used as a reference to calibrate the binding energies. Temperatureprogrammed reductions by H2 (H2-TPR) were carried out using a Micromeritics Autochem 2920 instrument, and 0.1 g of sample was positioned in the sample holder. Before the reduction, the samples were heated in Ar flow at 200 °C for 2 h to remove water from the surface. The flow then was switched to 10% H2 in He (30 cm3/ min), and the temperature of the sample was increased from 50 to 900 °C at a rate of 10 °C/min. 2.3. Mercury Removal Measurement. The bench-scale experimental system used for Hg0 capture was described previously.17 All individual syngas components, including H2, CO, H2S, HCl, and dry N2 as the balance, were provided from gas cylinders. A heated water bubbler was used to generate the water vapor. A stable feed of Hg0 concentration (∼70 μg/m3) was obtained by passing 200 mL of N2 through the permeation tube. In each test, 0.15 g of sample was loaded into a quartz reactor that was maintained at the desired temperature by a tubular furnace. The Hg0 concentrations were continuously recorded by A DM-6A/MS-1A mercury analyzer (Nippon Inc., Japan). Three sets of experiments were performed in the present study. The operating temperature and simulated syngas compositions are listed in Table 1. The experiments in set I

continuously using the same mercury analyzer. In set III, the regenerability of the 8Pd/Al2O3 sorbent was evaluated under simulated syngas, as described in section 2.4. In the present experiments, only negligible amounts of Hg2+ can be observed in the gas stream. Thus, the loss of Hg0 was ascribed to adsorption onto the Pd/Al2O3 sorbent, and the average Hg0 capture efficiency (Ecap) over CeTi sorbents was calculated as follows: ⎛ [Hg T]out ⎞ ⎟ × 100% Ecap (%) = ⎜1 − [Hg T]in ⎠ ⎝ T

(1)

T

where [Hg ]in and [Hg ]out represent the total amount of input and output Hg0, which can be calculated by integrating the realtime Hg0 concentration at the inlet and outlet of the reactor over 2 h, respectively. 2.4. Sorbent Regeneration. After the mercury removal test under simulated syngas, sorbents were regenerated by heating to 400 °C in pure N2 carrier gas for 1 h. At this temperature, the release of the captured Hg0 was considered to be nearly complete by the decomposition of the Pd−Hg amalgam. Although the mercury release could be accelerated at higher temperatures, the morphology of the Pd supported on the Al2O3 might also be negatively impacted. Several capture− regeneration cycles were conducted under the same simulated syngas to evaluate the regeneration performance of Pd/Al2O3 sorbents along cycles.

3. RESULTS AND DISCUSSION 3.1. Sample Characterization. XRD patterns of different Pd/Al2O3 sorbents are shown in Figure 1. For pure Al2O3,

Table 1. Experimental Conditions sample set I set II set III

Al2O3, 1Pd/Al2O3, 5Pd/ Al2O3, 8Pd/Al2O3, 10Pd/ Al2O3, 20Pd/Al2O3 8Pd/Al2O3 8Pd/Al2O3

gas components (1.2L min−1) 30% H2, 20% CO, 100 ppm of H2S, 10 ppm of HCl N2 + H2, CO, H2S, HCl; SSG(dry); SSG(8% H2O) 30% H2, 20% CO, 100 ppm of H2S, 10 ppm of HCl

temp (°C) 200−350 270 270

Figure 1. XRD profiles of (a) pure Al2O3, (b) 1Pd/Al2O3, (c) 5Pd/ Al2O3, (d) 8Pd/Al2O3, (e) 20Pd/Al2O3, and (f) PdO.

were intended to determine the optimal operating temperature and loading value of Pd on Al2O3. These removal experiments using Pd/Al2O3 sorbents with different loading values of Pd were conducted under simulated syngas (SSG: 30% H2, 20% CO, 100 ppm of H2S, 10 ppm of HCl, and 70 μg/m3 Hg0) at reaction temperatures ranging from 200 to 350 °C. The set II experiments performed in this study were intended to investigate the roles of individual syngas components in Hg0 removal over the optimal 8Pd/Al2O3 sorbents at 270 °C. Following the Hg0 capture experiments using pure N2 and 1 ppm of HCl plus N2, temperature-programmed desorption (TPD) experiments were performed under N2 flow at 1 L/min to identify the captured mercury. The mercury-adsorbed sample was heated to 650 °C at a rate of 10 °C/min. The mercury species desorbed from the sample were also measured

diffraction peaks at 37.6°, 45.9°, and 67.0°, which are indicative of γ-Al2O3, were detected. For 1Pd/Al2O3, no diffraction peaks corresponding to the PdO phase were observed, indicating that Pd existed as amorphous or extremely dispersed species.18 As the loading value of Pd increased from 5% to 20%, the diffraction line of PdO (2θ = 33.8, 42.0, 54.7, 60.2) became apparent and intense, which could be ascribed to the change in particle size and the formation of a larger amount of PdO crystallites.19 Furthermore, the diffraction lines indicative of the Pd metal phase were not observed in all Pd/Al2O3 samples, although palladium has been considered to exist in at least two different states.20 Consequently, the Pd metals were also in a highly dispersed form. 9910

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3.2. Activity of Pd/Al2O3 Sorbents. The Ecap values of Pd/Al2O3 sorbents with different loading values over a range of temperatures from 200 to 350 °C are presented in Figure 2. For

Figure 2. Mercury capture performance over Pd/Al2O3 sorbents under simulated syngas at different temperatures.

Figure 3. Effect of individual syngas components on Hg0 removal at 270 °C.

pure Al2O3, less than 10% Ecap in the whole temperature range was obtained, and it was considered to be completely ineffective in this study. With the addition of Pd onto Al2O3, the Hg0 removal ability was significantly enhanced; for example, 1Pd/ Al2O3 resulted in approximately 45.7% Ecap at 230 °C. Pd/ Al2O3 sorbents with higher Pd loading showed higher activities until the loading value of Pd reached 8%, resulting in an Ecap greater than 90% at temperatures ranging from 200 to 270 °C. This capability at elevated temperatures is also superior to that of commercial activated carbon (AC) under similar conditions.21 With the further addition of Pd, Ecap had a slight decrease. 8Pd/Al2O3 showed the best activity, which could be attributed to better dispersion of Pd on the surface of Al2O3. Table 2

even higher Ecap of 98.5% was obtained by an additional increase in the H2 concentration to 30%. It is possible that PdO was reduced to elemental Pd in the H2 atmosphere and that the amount of the active component available to capture Hg0 increased, which promoted the removal of Hg0. In this work, TPR-H2 was performed to demonstrate the reduction of PdO by H2. As shown in Figure 4, only one main

Table 2. Physical Properties of Various Sorbents sample

surface area (m2/g)

pore volume (m3/g)

average pore size (nm)

A12O3 1Pd/Al2O3 5Pd/Al2O3 8Pd/Al2O3 20Pd/Al2O3

225.83 214.02 216.61 199.36 172.10

0.426 0.420 0.424 0.391 0.322

7.544 7.849 7.822 7.836 7.489

Figure 4. H2-TPR profiles of 8Pd/Al2O3.

reduction peak was obtained at 66 °C, which could be due to the reduction of PdO to Pd. Similar reduction peaks were also obtained in a previous investigation on the Pd/Al2O3 sample.22 The results demonstrated that the reduction of PdO on the Al2O3 support occurred at low temperatures under the experimental conditions. The possible reduction reactions can be written as follows:

displays that high loadings of Pd (>8%) caused a significant reduction in both surface area and pore volume of the Pd/ Al2O3 sorbent, thus leading to the slight decrease of Ecap. This finding was similar to those obtained in previous investigations,15 in which a change in Pd dispersion occurred when the Pd loading exceeded 8.5%. For all Pd/Al2O3 sorbents, the Hg0 removal ability was relatively constant at temperatures ranging from 200 to 270 °C, and it then dropped significantly as the temperature further increased from 270 to 350 °C. 3.3. Effect of H2. As shown in Figure 3, when the pure N2 gas stream passed through the fresh 8Pd/Al2O3 sorbent, the Ecap at 270 °C was observed to be 73.5%. This large loss of Hg0 could be ascribed to the Hg amalgamation on the 8Pd/Al2O3.9 It is well-known that H2 is one of the staple components in syngas; therefore, it is necessary to evaluate its effect on Hg0 removal over Pd/Al2O3 sorbents. In this study, H2 was found to be promotional on Hg0 capture. The average Ecap increased to approximately 95.2% when 10% H2 was introduced, and an

2PdO + H 2 → Pd 2O + H 2O

(2)

Pd 2O + H 2 → 2Pd + H 2O

(3) 0

3.4. Effect of CO. CO was found to enhance Hg removal over Pd/Al2O3 sorbents. As illustrated in Figure 3, 5% CO resulted in an Ecap of 90.5%, which is higher than 73.5%, that occurred under pure N2 conditions. Increasing the CO concentration to 20% increased the Ecap to 98.1%. As demonstrated with H2, the promotional effect of CO on mercury capture is correlated with the reduction of PdO to its metallic state, which can also be demonstrated by the CO-TPR 9911

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Figure 5. XPS spectra of (A) Pd 3d for fresh Pd/Al2O3 sorbent and (B) S 2p for Al2O3 sorbent after the test.

experiments. Similar results have been reported in the previous literature.23 The reaction can be summarized as follows: PdO + CO → Pd + CO2

sorbents. The addition of 1 ppm of HCl into the pure N2 gas flow increased the Ecap from 73.5% to 84.6%. When the HCl concentration further increased to 10 ppm, Ecap remained at a similar level, indicating that the promotional effect of HCl was limited and that only a fraction of HCl had been consumed during the experimental period. In this work, TPD experiments were performed following the Hg removal experiments using pure N2 and 1 ppm of HCl plus N2, respectively, to identify the captured mercury on the Pd/ Al2O3 sorbents. As shown in Figure 6, two shoulder peaks are

(4)

3.5. Effect of H2S. Although H2S is present in syngas, the role of H2S on the Hg0 capture over Pd/Al2O3 is unclear. In this study, H2S alone was found to significantly inhibit Hg adsorption. The Ecap decreased from 73.5% under a pure N2 atmosphere to 15.6% and 10.3% when 40 and 100 ppm of H2S were added into the gas stream, respectively. It is likely that H2S consumed the active component palladium, which is responsible for Hg0 removal.24 In addition, it has been reported that the interaction between Pd and H2S is strong.25 The consumption of the active component can also be verified by the XPS pattern of Pd 3d and S 2p shown in Figure 5. Two intense peaks ascribed to the 3d5/2 and 3d3/2 states of Pd 3d were observed.26 The doublets with larger binding energy attributed to plasmon losses are not discussed in this study. The Pd 3d peaks at approximately 335.4 and 337.0 were assigned to Pd metal (reduced state) and PdO (oxidized state), respectively.20 After the test in the presence of H2S, the Pd/ Al2O3 sorbent was analyzed by XPS. It can be observed from Figure 5 that the S peaks are mainly centered at approximately 161.9 and 163.9 eV, which may be assigned to S2− and elemental S, respectively. The S2− was in accordance with the values for PdS, implying the existence of PdS. The possible reactions are proposed to be as follows:27 Pd + H 2S → PdS + H 2

(5)

PdO + H 2S → PdS + H 2O

(6)

H 2S + O* → S + H 2O

(7)

S + Pd → PdS

(8)

Figure 6. Hg-TPD profiles after adsorption in pure N2 and 1 ppm of HCl plus N2 atmosphere, respectively.

observed at approximately 280 and 390 °C for the Hg-TPD profiles after adsorption under the HCl atmosphere, as compared to only one single peak at approximately 390 °C under the N2 atmosphere. These results suggest that the temperature slightly higher than 390 °C is appropriate for sorbent regeneration. The low mercury desorption peak at approximately 280 °C appeared to be consistent with that of HgCl2,29,30 indicating that the Pd−Hg amalgam is not the unique mechanism for Hg0 removal and that another mechanism, in which a small amount of HgCl2 forms on the surface of the sorbents, is involved. The high mercury desorption peak for both cases was ascribed to the result of the decomposition of the Pd−Hg amalgam.31 However, the Pd/Al2O3 sorbent began to significantly desorb Hg0 at approximately 280 °C, which was much closer to the optimal reaction temperature of 270 °C in this study, indicating that further increases in the reaction temperature would not result

where O* denotes the surface oxygen on the Pd/Al2O3 sorbent. It was worth mentioning that this test merely intended to evaluate the effect of H2S on Hg0 removal; therefore, only H2S was used. The inhibitory influence of H2S can be alleviated when 30% H2 is introduced into the 100 ppm of H2S gas flow. The presence of gas-phase H2 resulted in the reduction of PdS and the regeneration of the active palladium component, leading to an increase in Ecap from 10.3% to 90.6%.28 3.6. Effect of HCl. The study also investigated the single factor of HCl on Hg0 removal over Pd/Al2O3. As illustrated in Figure 3, HCl also promoted the Hg0 removal over Pd/Al2O3 9912

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in a high Hg0 removal efficiency. It was also concluded that the formation of Pd−Hg was primarily responsible for the removal of Hg0 because the area under the first peak was much smaller than that of the second peak. The overall oxidation reaction of Hg to HgCl2 over Pd/Al2O3 is proposed to be as follows:6,32 Hg + 2HCl → HgCl 2

second cycle, the Ecap increased to 70.3% and remained constant thereafter over several regeneration cycles. Similar tendencies can also be observed for the mercury capacity. For the experiments in this study, mercury capacities represent the amount of adsorbed mercury on the sorbent per mass unit when mercury breakthrough exceeded the value of 20%. Nevertheless, the mercury capture capacity can remain constant over several cycles, which makes Pd/Al2O3 a prospective candidate for Hg0 control at elevated temperatures.

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3.7. Effect of H2O. H2O is unavoidably present in syngas and is found to inhibit Hg0 adsorption/oxidation over various sorbents/catalysts, which could be ascribed to competitive adsorption on the active sites.33 In this work, a prohibitive effect of H2O on Hg0 capture was also observed over the Pd/Al2O3 sorbent. The use of 8% H2O resulted in a slight decline in Ecap, as shown in Figure 3. Similar findings could be obtained with other precious metals, such as Au- or Ag-based sorbents.34 H2O can be weakly adsorbed onto the surfaces of precious metals.34 Therefore, it is likely that the inhibitive effect of H2O was also ascribed the competitive adsorption. It should be noted that the reduction of Ecap over the Pd/Al2O3 sorbent was smaller than that over carbon or metal-based materials,35 indicating that the Pd/Al2O3 sorbent has good H2O-resistance properties. 3.8. Performance of the Sorbents after Regeneration. Figure 7 shows the mercury removal performance of the Pd/

4. CONCLUSIONS A series of Pd/Al2O3 sorbents with different Pd loading values were prepared to evaluate their mercury removal activities at high temperatures in simulated syngas. The Pd/Al2O3 sorbent was extremely active for Hg0 capture at high temperatures. 8Pd/Al2O3 outperformed the tested sorbents, with Ecap values exceeding 90% at temperatures from 200 to 270 °C in simulated syngas. H2 and CO were found to be promotional on Hg0 capture because of the reduction of PdO to Pd metal. H2S alone was observed to significantly inhibit Hg adsorption due to its reaction with active palladium. HCl was also found to be promotional, which can be attributed to the catalytic oxidation of Hg0 to HgCl2. H2O showed a prohibitive effect on Hg0 removal. In addition, the mercury capture capacity remained constant over several regeneration cycles. Although palladium is more expensive than activated carbon, the ease with which regeneration can occur using the Pd/Al2O3 sorbent makes it economically competitive. The Pd/Al2O3 sorbent prepared in this study is quite favorable in capturing mercury from syngas at high temperatures.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-571-87952041. Fax: +86-571-87951616. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Fund of China (no. 51176171) and the National Science Foundation for Distinguished Young Scholars of China (no. 51125025).

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REFERENCES

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dx.doi.org/10.1021/ie501292a | Ind. Eng. Chem. Res. 2014, 53, 9909−9914