Support Layer Influencing Sticking Probability: Enhancement of

Article pubs.acs.org/JPCC

Support Layer Influencing Sticking Probability: Enhancement of Mercury Sorption Capacity of Gold Ylias M. Sabri, Samuel J. Ippolito,* and Suresh K. Bhargava* Centre for Advanced Materials & Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, Melbourne, VIC 3001 Australia S Supporting Information *

ABSTRACT: Elemental mercury (Hg0) sorption on Au thin films deposited over SiO2 (SiO2:Au) and Ti (Ti:Au) support layer is investigated by quartz crystal microbalance (QCM) technique. The Hg0 sorption capacity of Au thin films is shown to be greatly influenced by the underlying support layer, where an enhancement of 267% was observed for SiO2:Au over Ti:Au films for the same Au film thickness. Furthermore, by increasing the Au film thickness, the total Hg0 captured was observed to increase even though the mercury sorption efficiency (or mass ratio of Hg0 to Au) was observed to decrease for a 8 h Hg0 exposure period. Sticking probability calculations showed that the SiO2:Au substrate maintains a sticking probability of ∼10−6, which is at least 2 orders of magnitude greater than that of the Ti:Au surface, thus indicating that Hg0 sorption capacity of a material can be greatly influenced by the choice of underlying support layer. to be efficient Hg0 scavengers;15−17 however, the uptake of such materials has been limited. In this work we describe the dramatic influence the underlying support material has on the Hg−Au sticking probability and hence Hg0 sorption capacity of Au ultrathin film coatings on both SiO2 and Ti support materials. By understanding the effect the underlying support material, it is proposed that better Hg0 scavenging efficiencies can be obtained from materials used for removing Hg0 from gas phase industry stack effluents or within laboratory based preconcentration equipment. The use of Au based sorption materials would also have the added advantage of potentially being significantly more economical than conventional sorption based processes as it can be regenerated to a certain extent following mere dry nitrogen exposure18 or fully regenerated using heat treatment processes,19−21 thus reducing landfill requirements for conventional spent sorbent based materials which either have limited, or no regenerable capabilities. Additionally the effect a support material is also studied in respect to the Hg0 sorption capacity as a function of Au film thickness in real-time. Monitoring of the Hg−Au interaction process was performed by fabricating quartz crystal microbalance (QCM) with Ti and SiO2 supported Au thin films of different thickness. The QCM measurements made it possible to measure the Hg0 capacity of each film during the Hg0 exposure process in real-time by utilizing the linear relationship (eq 1) between the frequency shift (Δf) and the mass change (Δm) observed on the electrode surface given by

1. INTRODUCTION Among various pollutants, mercury (Hg) has a significant impact on many biological processes and the environment in general. Elemental mercury (Hg0) represents 90−99% of atmospheric mercury emissions, which is primarily emitted from burning coal and other industrial process. Due to its high vapor pressure and low solubility in water, gas phase Hg0 is difficult to remove using conventional pollution control scrubbing systems.1−4 Consequently, the lack of an environmentally friendly and economically viable mercury control process has led to the closure of several cement plants within the USA and Europe.5−7 A current challenge faced by industry is the availability of suitable sorbent materials which meet rigorous requirements (being highly recyclable and costeffective) to capture Hg0 from industry effluent streams.8−10 Currently, the most widely used process for removing Hg0 from industry stack gases is by activated carbon (AC) adsorption, metal oxides loaded AC, and metal/metal oxides supported on ceramic and silicate particles.11−13 It has been estimated that the cost involved to treat a Hg0 contaminated stream with carbon based sorbents can exceed US$50,000 per kg of Hg removed.14 The main downside to using such processes is the requirement to minimize moisture build-up from the gas stream on the sorbent material and the fact that the bed can pose a serious fire hazard due to the exothermic adsorption of organic compounds overtime. Furthermore, once the sorbent material has expired it must be relegated as toxic waste, further burdening landfill sites. In order to overcome these issues, metal and metal oxides on different supports developed by ADA Technologies for example have been used in gas, petroleum, and alumina industries as they have been shown © 2013 American Chemical Society

Received: January 29, 2013 Revised: March 18, 2013 Published: March 19, 2013 8269

dx.doi.org/10.1021/jp4009724 | J. Phys. Chem. C 2013, 117, 8269−8275

The Journal of Physical Chemistry C

Article

2.2. Hg0 Exposure and Monitoring. The same experimental setup described in our prior work was used, details of which can be found elsewhere.26,27 Briefly, mercury vapor concentrations were achieved by using mercury permeation tubes which were purchased from VICI, TX, USA. Different Hg0 concentrations were generated using a PID temperature controller to heat the permeation tubes at different set points. The Hg0 concentration generated at each set point was calibrated using an acidic KMnO4 wet trapping method, which was adapted from the Ontario Hydro method.26,28 The Hg0 trapped in the KMnO4/H2SO4 solution was quantified using Agilent Technologies HP7700 series inductively coupled plasma mass spectroscopy. The change in resonant frequency (Δf) of the QCMs was measured using a Maxtek RQCM (10 MHz phase locked oscillator, USA) with resolution of ±0.01 Hz. The QCMs were exposed to dry N2 for a 100 min period while monitoring its response in order to ensure the devices were stable prior to any Hg0 exposure experiments. The QCMs were then tested toward three Hg0 concentrations of 3.65, 5.70, and 10.55 ± 0.05 mg/ m3 (balance N2) using 8 h exposure times for each Hg0 concentration followed by a 5 h recovery under dry N2 atmosphere. The purpose of the 5 h recovery period was to remove any loosely attached Hg0 atoms from the surface of the Au thin film based QCM prior to removing the device from the gas chamber. A freshly made, unexposed QCM was used for each Hg0 exposure experiment. The total gas flow rate into the chamber was kept constant at 200 sccm using a custom built four channel mass flow controller system. A constant pressure of ∼1200 Torr was maintained throughout the entire experiment. The approximate volume of the chamber housing the QCM sensors was ∼0.1 L. 2.3. Surface Characterization. Scanning electron microscope (SEM) characterization was performed using a FEI Nova NanoSEM instrument operating at an accelerating voltage of 10 kV, spot size of 3 and working distance of 4.5 mm. Atomic force microscopy (AFM) tapping mode measurements were performed on the relevant films using a NanoScope IIIa Multimode AFM (Vecco, USA) under air-ambient conditions (20 °C and 40% RH). Rotated Monolithic silicon probes (TAP300-G) with a symmetric tip shape, a resonant frequency of 300 kHz, a spring constant of 40 N m−1, and a radius of curvature of tip 0.995 as shown in Figure 5. This type of model was the best fit for all tested operating temperatures and Hg0 concentrations. Here, (nHg(t)) represents the amount of Hg0 undergoing sorption on the Au film during Hg0 exposure and the constants a1, a2, b, c, and d are dependent on temperature and Hg0 concentration. The constants a1 and a2 indicate the magnitude of the role each exponential component has on the overall model, whereas the constants b and c determine how quickly that effect deteriorates with time. The constant d determines the maximum amount of mercury that can undergo sorption on the surface. The rate k(t) at which Hg0 undergoes sorption on the Au surface is evaluated as in eq 3 and derived from eq 2 a a k(t ) = 1 et / b + 2 et / c (3) b c

Figure 6. Sticking probability of Au films having a thickness of 40 nm deposited on either a 20 nm SiO2 or Ti support layer exposed to Hg0 concentration of 10.55 mg/m3 for 8 h at an operating temperature of 28 °C.

observed that the sticking probability of Ti:Au reduces significantly reaching from 1.0 to a value of ∼10−8 which is 2 orders of magnitude lower than that of the SiO2:Au surface. The vast reduction in S for the Ti:Au may be due to a lack of diffusion of Hg into the surface. Therefore, it is highly likely that the Hg diffuses right through the Au thin-film deposited on SiO2, thus allowing Hg0 sorption sites to become available for more Hg0 atoms to undergo sorption and diffusion processes at the surface. That is, Hg diffusion processes is expected to be better favored on SiO2:Au over the Ti:Au surface. This is

The fact that the summation of two exponential terms best describes the Hg0 sorption rate suggests that there are at least 8273

dx.doi.org/10.1021/jp4009724 | J. Phys. Chem. C 2013, 117, 8269−8275

The Journal of Physical Chemistry C

Article

for allowing the use of their comprehensive facilities and services. S.J.I. acknowledges ARC for APDI fellowship.

because the Au on the SiO2 support is expected to be more mobile than on a Ti support due to the better adhesion of Au on Ti over the SiO2 support layer.34,35 Therefore, the better Hg diffusion in the SiO2:Au surface is expected to be due to the better mobility of the Au on the SiO2 as there was no evidence of any porosity on the surface from our AFM and SEM characterization studies.

■

(1) Qui, J. Tough talk over mercury treaty. Nature 2013, 493, 144− 145. (2) Presto, A. A.; Granite, E. J. Survey of catalysts for oxidation of mercury in flue gas. Environ. Sci. Technol. 2006, 40 (18), 5601−5609. (3) James, J. Z.; Lucas, D.; Koshland, C. P. Gold Nanoparticle Films As Sensitive and Reusable Elemental Mercury Sensors. Environ. Sci. Technol. 2012, 46 (17), 9557−9562. (4) Ramesh, G. V.; Radhakrishnan, T. P. A Universal Sensor for Mercury (Hg, HgI, HgII) Based on Silver Nanoparticle-Embedded Polymer Thin Film. ACS Appl. Mater. Interfaces 2011, 3 (4), 988−994. (5) Paone, P. In Mercury controls for the cement industry; Cement Industry Technical Conference, March 28, 2010; pp 1-12. (6) Impact of EPA’s Regulatory Assault on Power Plants: New Regulations to Take 34 GW of Electricity Generation Offline and the Plant Closing Announcements Keep Coming; Institute for Energy Research: 2012; pp 1-40. (7) Sullivan, E. Overview Impact of Esisting and Proposed Regulatory Standards on Domestic Cement Capacity. http://www.cement.org/ Econ_Anal_%202-1-11.pdf (accessed 2012). (8) Reddy, B. M.; Durgasri, N.; Kumar, T. V.; Bhargava, S. K. Abatement of Gas-Phase MercuryRecent Developments. Catal. Rev., Sci. Eng. 2012, 54 (3), 344−398. (9) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Novel Sorbents for Mercury Removal from Flue Gas. Ind. Eng. Chem. Res. 2000, 39 (4), 1020−1029. (10) Granite, E. J.; Myers, C. R.; King, W. P.; Stanko, D. C.; Pennline, H. W. Sorbents for Mercury Capture from Fuel Gas with Application to Gasification Systems. Ind. Eng. Chem. Res. 2006, 45 (13), 4844−4848. (11) Mei, Z. J.; Shen, Z. M.; Zhao, Q. J.; Wang, W. H.; Zhang, Y. J. Removal and recovery of gas-phase element mercury by metal oxideloaded activated carbon. J. Hazard. Mater. 2008, 152 (2), 721−729. (12) Fan, X.; Li, C.; Zeng, G.; Gao, Z.; Chen, L.; Zhang, W.; Gao, H. Removal of Gas-Phase Element Mercury by Activated Carbon Fiber Impregnated with CeO2. Energy Fuels 2010, 24 (8), 4250−4254. (13) Yuan, C. S.; Lin, H. Y.; Wu, C. H.; Liu, M. H. Partition and size distribution of heavy metals in the flue gas from municipal solid waste incinerators in Taiwan. Chemosphere 2005, 59 (1), 135−145. (14) Turchi, C. S. Novel Process for Removal and Recovery of Vapor Phase Mercury; ADA Technologies, Inc.: USA, 2000; pp 1−57. (15) Yan, T. Y. Removal of mercury from natural gas and liquid hydrocarbons utilizing silver on alumina adsorbent. Patent number: 5,053,209, 1989. (16) Granite, E. J.; Hargis, R. A.; Pennline, H. W. Sorbents for mercury removal from flue gas; Report Number: DOE/FETC/TR--98−01; Federal Energy Technology Center: Pittsburgh, 1998; p 47. (17) Roberts, D. L.; Albiston, J.; Broderick, T.; Greenwell, C.; Stewart, R. Novel Process for Removal and Recovery of Vapor Phase Mercury; ADA Technologies, Inc.: USA, 1998; p 92. (18) Sabri, Y. M.; Ippolito, S. J.; Atanacio, A. J.; Bansal, V.; Bhargava, S. K. Mercury vapor sensor enhancement by nanostructured gold deposited on nickel surfaces using galvanic replacement reactions. J. Mater. Chem. 2012, 22 (40), 21395−21404. (19) Morris, T.; Sun, J.; Szulczewski, G. Measurement of the chemical and morphological changes that occur on gold surfaces following thermal desorption and acid dissolution of adsorbed mercury. Anal. Chim. Acta 2003, 496 (1−2), 279−287. (20) Fialkowski, M.; Grzeszczak, P.; Nowakowski, R.; Holyst, R. Absorption of Mercury in Gold Films and Its Further Desorption: Quantitative Morphological Study of the Surface Patterns. J. Phys. Chem. B 2004, 108 (16), 5026−5030. (21) Nowakowski, R.; Kobiela, T.; Wolfram, Z.; Dus, R. Atomic force microscopy of Au/Hg alloy formation on thin Au films. Appl. Surf. Sci. 1997, 115 (3), 217−231.

4. CONCLUSIONS Using the QCM technique we have shown that the Hg0 sorption capacity of an Au thin film can be significantly enhanced by choosing the correct support material, even though AFM measurements might show similar roughness and surface area properties between the Au films regardless of the underlying layer. The Hg0 sorption capacity of the Au films were shown to have increased up to 4-fold by using SiO2 instead of Ti as the support material. The increase is attributed to the SiO2:Au surface maintaining more than 2 orders of magnitude higher Hg−Au sticking probability than the Ti:Au surface during the Hg0 exposure period. The effect of Au film thickness was also assessed to show that the Hg0 scavenging efficiency of the Au film increases with increasing Au film thickness; however, the overall sorption capacity does not necessarily increase by simply incorporating more sorption material (Au) on either the SiO2 or Ti support. Of the tested Au thin film combinations, the 10 nm Au film deposited over a SiO2 support layer was observed to be the most efficient Hg0 scavenger based on Hg0 absorbed per mass of Au used. It is envisaged that, by understanding the influence of the support layer using this approach, the Hg0 sorption capacity and efficiency of materials such as Au and possibly other noble metals can be enhanced with the aim of reducing the required quantity of the sorbent layer whiles still maintaining a high sorption capacity properties.

■

ASSOCIATED CONTENT

S Supporting Information *

Figure S1 - SEM images showing 40 nm thick Au film deposited on 20 nm SiO2 support layer (a) and 20 nm Ti support layer (b). Scale bars represent 500 nm. The image in (a) shows much smaller grains and “cracks” on the surface which could potentially have high number density of surface defects and therefore Hg0 sorption sites resulting in its enhanced mercury sorption capacity. Figure S2 - QCM response showing the elemental mercury sorption capacity of Au films having a thickness of 10, 20, 30, and 40 nm with SiO2 as the support material, exposed to Hg0 concentrations of (a) 3.65 mg/m3 and (b) 5.70 mg/m3 at and operating temperature of 90 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; suresh.bhargava@rmit. edu.au. Phone: +61 3 9925 2330. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the Australian Research Council (ARC) for supporting this project (Grant No. LP100200859) and the RMIT microscopy and microanalysis facility (RMMF) 8274

dx.doi.org/10.1021/jp4009724 | J. Phys. Chem. C 2013, 117, 8269−8275

The Journal of Physical Chemistry C

Article

(22) Janshoff, A.; Galla, H.-J.; Steinem, C. Piezoelectric Mass-Sensing Devices as Biosensors - An Alternative to Optical Biosensors? Angew. Chem. 2000, 39 (22), 4004−4032. (23) Janshoff, A.; Steinem, C. Quartz Crystal Microbalance for Bioanalytical Applications. Sensors Update 2001, 9 (1), 313−354. (24) Haskell, R. L. B.; Caron, J. J.; Duptisea, M. A.; Ouellette, J. J.; Vetelino, J. F. In Effects of film thickness on sensitivity of SAW mercury sensors; Ultrasonics Symposium, 1999. Proceedings, Orono, ME, IEEE: Orono, ME, 1999; pp 429−434, Vol. 1. (25) Sabri, Y. M.; Ippolito, S. J.; Al Kobaisi, M.; Griffin, M. J.; Nelson, D.; Bhargava, S. K. Investigation of Hg Sorption and Diffusion Behavior on Ultra-thin Films of Gold Using QCM Response Analysis and SIMS Depth Profiling. J. Mater. Chem. 2012, 22 (39), 20929− 20935. (26) Sabri, Y. M.; Ippolito, S. J.; O’Mullane, A. P.; Tardio, J.; Bansal, V.; Bhargava, S. Creating gold nanoprisms directly on quartz crystal microbalance electrodes for mercury vapor sensing. Nanotechnology 2011, 22 (30), 305501−09. (27) Sabri, Y. M.; Kojima, R.; Ippolito, S. J.; Wlodarski, W.; Kalantarzadeh, K.; Kaner, R. B.; Bhargava, S. K. QCM Based Mercury Vapor Sensor modified with polypyrrole supported Palladium. Sens. Actuators, B 2011, 160 (1), 616−622. (28) Laudal, D.; Nott, B.; Brown, T.; Roberson, R. Mercury speciation methods for utility flue gas. Fresenius J. Anal. Chem. 1997, 358 (3), 397−400. (29) Sabri, Y. M.; Ippolito, S. J.; Tardio, J.; Bhargava, S. K. Study of Surface Morphology Effects on Hg Sorption-Desorption Kinetics on Gold Thin-Films. J. Phys. Chem. C 2012, 116 (3), 2483−2492. (30) Scheide, E. P.; Taylor, J. K. Piezoelectric sensor for mercury in air. Environ. Sci. Technol. 1974, 8 (13), 1097−1099. (31) Scheide, E. P.; Taylor, J. K. A piezoelectric crystal dosimeter for monitoring mercury vapor in industrial atmospheres. Am. Ind. Hyg. Assoc. J. 1975, 36 (12), 897−901. (32) Levlin, M.; Niemi, H. E. M.; Hautojärvi, P.; Ikävalko, E.; Laitinen, T. Mercury adsorption on gold surfaces employed in the sampling and determination of vaporous mercury: a scanning tunneling microscopy study. Fresenius J. Anal. Chem. 1996, 355 (1), 2−9. (33) Sabri, Y. M.; Ippolito, S. J.; Tardio, J.; Atanacio, A. J.; Sood, D. K.; Bhargava, S. K. Mercury diffusion in gold and silver thin film electrodes on quartz crystal microbalance sensors. Sens. Actuators, B 2009, 137 (1), 246−252. (34) Sexton, B. A.; Feltis, B. N.; Davis, T. J. Characterisation of gold surface plasmon resonance sensor substrates. Sens. Actuators, A 2008, 141 (2), 471−475. (35) Homola, J.; Yee, S. S.; Gauglitz, G. Surface plasmon resonance sensors: review. Sens. Actuators, B 1999, 54 (1−2), 3−15.

8275

dx.doi.org/10.1021/jp4009724 | J. Phys. Chem. C 2013, 117, 8269−8275