Changing the Landscape of Environmental and Energy Research Through Novel Nanoscale Materials

By: Santanu Chaudhuri, Material Matters, 2012 v7, n4

ISP/Applied Sciences Laboratory, Washington State University, WA 99210-1495

Santanu Chaudhuri

Introduction

Energy and environmental catalysts (EECs) form a major group of materials that will continue to drive the growth of industry and research investments. The EEC market is estimated to be between $10−16 billion and continues to favor new systems with low operational and recycling costs. The stability of catalytic systems is highly dependent on maintaining controlled operating conditions and the ability to regenerate the catalyst bed to retain the catalytic efficiency. Many of the noble and precious metals are in low supply, and sustainable practices in catalysis are becoming more important along with the search for alternative technologies. Key regulatory developments and upcoming changes in automobile emissions will also heighten the demands on current commercial catalytic systems and push the traditional catalysts to their performance limit. Especially for the control of NOx, hydrocarbon and CO/CO2 emissions under very lean burning conditions, new systems will be required for efficient vehicle technologies. For energy storage and conversion, the catalysis community is seeking game changers with novel means of turning abundant materials into active catalysts with potential regeneration pathways to extend their lifetime. From conversion of solar radiation, hydrogen storage, and fuel cell applications to improving the conversion of biofuels from different feedstocks, nanoscale catalysts with the ability to be incorporated in reactors and energy conversion/storage systems are becoming key drivers for catalysis research. It seems paramount that nanoscale materials with exceptional activity, selectivity, and stability are needed to meet these demanding performance objectives.

This article provides a perspective on important advances in nanoscale catalysis and identifies key areas needing further development. In particular, the combination of traditional synthesis and testing protocols for catalyst design will need to be augmented by precise surface science experiments and theoretical modeling of possible catalytic reaction pathways. Results of in situ spectroscopy on single crystal surfaces and supported catalysts are often difficult to compare with polycrystalline or supported nanoscale catalysts. A sustained effort is needed to combine promising experimental results on precise mechanisms of catalytic reactions with first-principles modeling/simulations and high-throughput screening of catalyst/support combinations. Validated experimental/modeling frameworks for material design can significantly shorten the development times for major classes of catalysts.

Automotive and Environmental Applications

Modern vehicular and industrial operations are critically dependent on the performance of catalytic systems. Important classes of catalytic systems have made progress using nanoscale technologies in laboratory-scale research efforts. There are great opportunities for scale-up, stabilization, and reduced-cost preparation methods in such catalytic systems for technology maturation and commercial use. The current technology in catalytic conversion depends on combinations of precious metals such as Pt (Aldrich Product No. 520780), Pd (Aldrich Product No. 267082), and Rh (Aldrich Product No. 204218) for the reduction of CO, NOx, and hydrocarbons (HC). As an alternative to such traditional methods with platinum group metals (PGM), promising zeolite-based catalysts and transition metal-based catalysts are also explored. The promise of nanoscale catalysts is reducing the net cost of PGM in a catalyst bed due to their higher surface area and kinetics while managing challenges regarding their chemical/thermal stability. The use of ultra-small metal clusters of Pt, Pd, Au (Aldrich Product No. 265772), Ag (Aldrich Product No. 265500), and Cu has been pursued in selective catalytic reduction (SCR) research. There is also a strong correlation of reactivity with support; such as zeolites, Al2O3 (Aldrich Product No. 319767), ZrO2 (Aldrich Product No. 230693), MgO (Aldrich Product No. 203718), SiO2 (Aldrich Product No. 342831), or TiO2 (Aldrich Product No. 204757). For example, catalysts on aluminosilicates, such as Pt/MCM-41 (Aldrich Product No. 205915 or Product No. 643653), have activity superior to a Pt/SiO2 or Pt/Al2O3 catalyst due to an ability to improve surface area and pore size ratios to obtain optimal performance. Pt/BaO/Al2O3-based catalysts also show high performance and high selectivity in NOx/CH4 streams. Stability of small metal clusters on different supports and in the presence of other reducing agents such as CO, H2, NH3, and hydrocarbons is a challenging area for catalyst stabilization. Currently, reasonably stable Pd-Rh/BaO/CeO2−ZrO2 catalysts are somewhat more economical than Pt-based catalysts. However, the search for a Pt replacement is difficult with the increasing need to reduce CO and hydrocarbons in exhaust due to tougher environmental regulations in the US and Europe. Strontium-doped perovskites (La0.9Sr0.1MnO3 and La0.9Sr0.1CoO3) have shown promising performance in the SCR environment.1 The partial substitution of La3+ by Sr2+ creates a charge imbalance that can be balanced by creation of oxygen vacancy. Although in these systems, the addition of precious metals such as Pd−Rh will still be required for hydrocarbon and CO oxidation.

Role of Nanoarchitecture

One of the important aspects of support/nanoparticle interactions is the exposed catalyst facets and their activity on a particular support as the reactants diffuse between different exposed facets and the support. In addition, thermal stability of the nanoparticles inside of a support depends on reducing mobility of the nanoparticles using encapsulation and other immobilization methods. Examples of the use of a nanoporous support and matching architecture of support/nanoparticle components are abundant. However, many of the systems have faced challenges keeping the catalytic activities high through the thermal, adsorption/desorption cycles of impure feeds and oxidative sintering tests. As a result, use of PGM still remains high with some positive changes in the practice of reuse and a growing culture favoring sustainable use of precious metal catalysts.

Role of Facets in Nanoparticle

For most nanoscale catalysts on oxide supports, understanding the morphology of the nanoparticles and their changes is important. Atomistic imaging techniques under realistic (non-UHV) reactive environments start to show the dynamic nature of nanoparticle surfaces due to interactions with support and reactive gas phase. Hansen et al. clearly demonstrated that the dynamic nature of faceting, shape, and size of nanoscale catalysts can be followed using atomistic imaging under realistic conditions.2 Moreover, the close proximity of facets in a nanoparticle is an ideal testing ground for understanding the differences and the cooperative nature of chemistry in Pt particles, with the possibility of translating concepts from single crystal surface science studies to real catalytic materials on oxide supports.3-5 For example, on platinum single crystal surfaces, the (100) surface has a higher affinity for oxygen than the (111) surface. As NOx reduction is dependent on O coverage, understanding the ratio between different facets in a nanoscale catalyst is important for understanding the balance of reactive interfaces. In fact, Komanicky et al. found that there is a “division of labor” between (111) and (100) facets in Pt nanoparticles.4 The (100) nanofacet primarily supplies oxygen to the (111) for efficient reduction.

Core/Shell Nanocatalysts

In recent developments in reducing Pt content, core/shell particles of Cu−Pt (Cu@Pt, Cu shell, Pt core and Pt@Cu Pt shell, Cu core) supported on γ-Al2O3 have shown better NO selectivity (>90%) in the presence of H2 and show promise to lower Pt content. In fact, between 250−400 °C, the Pt@Cu nanoparticles performed better than monometallic Pt.6 The fundamental aspects of the near-surface alloys and the shifts in the d-band center have been described using Density Functional Theory (DFT) calculations. There are some conflicting reports in the literature about the true stability of core/shell nanostructures and all combinations of core/shell ordering are not stable under thermal cycling and repetitive reaction cycles. In reality, many core/shell structures face challenges at higher temperatures. The inter-diffusion of core/shell atoms, and the segregation of monometallic and/or stoichiometric regions are hard to control in a catalytic system.

Mechanistic Insights on NOx Reduction

The accurate mechanism of NOx reduction is still a challenge for experimental and theoretical studies.7-9 A commonly encountered 4-step Langmuir-Hinshelwood type mechanism describes the steps in the following way:

1. Adsoprtion: NO + Pt → NO−Pt
2. Oxygen chemisorption: O2 + 2Pt → 2O−Pt
3. NO oxidation: NO−Pt + O-Pt → NO2−PT + PT
4. Desorption: NO2−Pt → NO2 + Pt

NO Reduction on Pure Metal

Optimizing the NO adsorption on a catalyst surface requires further understanding of the NO molecule and target catalyst surfaces. The first step is NO adsorbing onto Pt(111) and other low-energy terraces, steps, defects, or similar microfacets in nanoparticles. NO has three fully occupied (in mixture of 5σ and 1π orbitals) and a singly occupied electron in the antibonding 2π* orbital. First-principles calculated bonding geometry and adsorption energy show a complex mixing and participation of σ and π* orbitals with respect to Pt-d orbitals.10 At high NO coverage, fcc, hcp, and atop binding sites for NO are populated in Pt-, Pd-, and Rh-based catalysts, and the ground state DFT calculations are helpful in understanding the fundamental surface chemistry and binding geometry. However, further development of kinetic theories with pressure/temperature-dependent rates, the role of spin multiplicity, and the transport of reactive species on metal surfaces for the SCR are needed to identify the full complexity and the dynamic nature of catalytic processes on PGMs at high temperatures. For example, NO and NO2 are in a thermodynamic equilibrium and their sticking coefficients are surface coverage dependent. The oxygen coverage of metals and diffusion of oxygen also play an important role in determining the rate of desorption of products.

Mechanism of NO Reduction on Supported Catalyst

The model 4-step mechanism shown (Equation 1) is not fully relevant for modern supported platinum systems wtih alkaline earth oxide promoters. For catalysts such as Pt/BaO/Al2O3, the BaO + NO + O2 leads to nitrite species, Ba(NO2)2, which is oxidized by the Pt/O2 to Ba(NO3)2, and the nitrates subsequently decompose, releasing NO2. Such processes of NOx trapping in multiphase systems is critically dependent on catalyst morphology, dispersion, and diffusion and needs careful calibration of synthesis methods to optimize performance. Beyond PGM, many base oxides/metals (e.g., Al2O3, TiO2, ZrO2, MgO promoted by Co, Ni, Cu, Fe, Sn, Ga, In, and Ag compounds) are active catalysts for the selective reduction of NOx (NO and NO2) with hydrocarbons (HC-SCR).11 For zeolite- and perovskite-based catalysis, the mechanism of NO adsorption followed by desorption of N2 and O2 is more complex. In Cu-ZSM-5, first-principles calculations revealed the Cu-site can bind N2, O2, and NO (both mono- and di-nitrosyl) with stronger binding of NO and less strongly bound N2 and O2.

Modifying Electronic Structure of Support

Although there have been tremendous developments in synthetic techniques and a greater control of nanoparticle dispersion, size, and morphology, understanding the effect of the local electronic structure of the support remains under-utilized beyond the well-known oxide supports. It is evident that the electronic structure of the supporting oxide, its surface acidity/basicity, and the nature of its defects control the performance of a catalytic system. Once the exact mechanism and role of support is understood using first-principles modeling and experiments, many critically important aspects of the electronic structure of oxides such as MgO, Al2O3, and ZrO2 can be altered using dilute transition metal and rare-earth dopants. We recently discussed the role of vacancies and oxygen chemisorption kinetics in core/shell (partially oxidized) Zr/ZrO2 particles for 15 transition metal elements (Figure 1).12 The role of oxide ion diffusion in the oxide matrix and trapping by impurity element doping can be utilized to improve catalyst performance. Similar efforts to change the role of the support using cationic doping of MgO by rare-earth elements showed a significant change in the defect density and the interaction with water using first-principles calculations.13

Figure 1. Altering the energetics and diffusion barrier in supports and high-temperature catalysts such as ZrO2 can provide options of co-optimizing reactivity, defect density, and surface/bulk transport: A) the impurity induced changes in the defect states calculated with respect to native band gap, and B) volcano curve showing optimum of defect formation, oxygen chemisorption, and transport kinetic for different transition metal elements.12

CO Oxidation on Au Catalyst

For Au nanoparticles used for CO oxidation, TiO2 is the most common support. It is suggested that CO adsorbs at the edges of the Au nanoparticles and O2 is actually activated around defect sites in the support. Improving defect density and accessible surface area is part of any rational approach toward improving catalytic performance. Spectroscopic studies of Au on nanocrystalline CeO2 (Aldrich Product No. 700290) showed that CO was bonded to Au3+, Au+, and neutral Au species, whereas the active O2 bonded to CeO2 as superoxide η1-O2.14 From a mechanistic standpoint, DFT calculations on supported Au clusters are still a daunting task. Au is a late transition metal with 6s14f145d10 in the valence shell and calculations need to account for relativistic effects efficiently.15

Energy Applications

Catalysts for Fuel Cell Research

For the fuel cell industry, the most important reaction is oxygen reduction reaction (ORR). ORR is a multielectron half-cell reaction (2O2 + 2H+ + 2e- = H2O) that may include a number of elementary steps involving different reaction intermediates, including inner sphere and outer sphere electron transfer mechanisms. The rate of electrocatalytic reactions such as ORR can be simplified (Equation 1) as related to the O2 concentration in the solution (CO2), the surface coverage of the catalyst (Pt-based catalysts) by different species (θad) and their change of standard free energy of adsorption (ΔGad).

The current challenge is finding nanoscale materials that out-perform commercial catalysts at a lower Pt-loading level. The recent trend is to use carbon-supported binary alloys of Pt with Cr, Mn, Co, and Ni. In such bimetallic alloys, the surface structure of the stoichiometric Pt3M phase (M=Ni, Co, Fe, V, Ti) is assumed. In particular, if the d-band center of the surface electronic state of Pt3M catalyst is known from calculations or experiments, the activity of the material can be well understood.16 The (1−θad) term in the rate expression is a rough measure of the number of O2 adsorption sites available on a Pt3M catalyst surface with already-adsorbed OH from water dissociation (2H2O = OHad + H3O+ + 2e-). The preferential growth directions and shape of Pt nanoparticles also change the trends of electrocatalytic reactions. A better control of nanocatalysts dispersed on C-based support is possible if their strengths and vulnerabilities are known under controlled conditions.5

On the anode side, the fuel cell industry is plagued by the need of high-purity hydrogen with low CO content. The Pt-loading levels can be reduced using nanoscale engineering techniques and stable bimetallic catalyst surfaces. New catalysts created using novel synthetic and immobilization techniques result in increased performance and higher CO tolerance. Thus, the current trend is to identify catalysts that are capable of CO tolerances of 200 ppm and higher. Such operating conditions can be achieved in high-temperature fuel cells (HT-PEMFC) which need different nanoscale catalysts that can operate well above 100 °C and different construction of the fuel cell stacks/membrane for better water management since the Pt/C and Pt3M systems can aggregate, migrate, and reduce activity at high temperatures. As discussed in the context of NOx SCR processes, finding smarter materials for supports is critically important for low Pt-loading catalytic systems such as core/shell or bimetallic nanoscale catalysts. Novel immobilization and catalyst stabilization pathways are currently being developed to address the catalyst stability issues for HT-PEMFC.17

The energy applications for nanoscale catalysts are abundant. From photovoltaic cells to fuel cell/battery applications, the role of nanoscale catalysis in alternative energy applications is increasing. Here, we will limit our discussion to the nanoscale metal cluster catalysts in PEM fuel cells and hydrogen storage applications.

Smaller than Nanoscale

As particle size becomes smaller, the materials research community looks at small metal clusters and identifying single-site catalysts on a support for energy and environment-related applications. The search for green fuels such as hydrogen connects environmental and energy-related catalysts in the subnanoscale to interesting domains of chemistry while blurring the lines between homogeneous and heterogeneous catalysis. The ability to isolate and manipulate the electronic structures and active molecular/atomic orbitals of individual catalytic sites is one of the promising approaches to avoid some of the stability issues of multi-phase nanoparticle-based catalysts.

Isolated Atom-center Systems

The ability to probe chemical activity of isolated atom centers using theory and experiments is strengthening rapidly, with new applications in energy storage and conversion in mind. For example, in hydrogen storage, the search for catalysts that can activate hydrogen in ambient conditions is critically important for automobile applications. At the same time, most such systems depend on complex electrocatalytic activation and precious metals rendering them ineffective for cost-conscious automotive applications. Isolated atom centers and single-site catalysts are, therefore, sought to find catalytic centers that can act as a pump to activate hydrogen that can then subsequently be stored in a liquid or a solid for reversible use in hydrogen storage tanks. For example, AlH3 is a solid with a great potential for hydrogen storage only if catalytic centers can be found that activate hydrogen on Al allowing its use as a solid-state storage system. In our previous work, we showed using a combined theoretical and experimental approach where surfaces of abundant metals such as Al can be activated using transition-metal catalysts with metal atoms in isolation or combination, and provide a unique environment for reactions such as hydrogen activation which is otherwise impossible on Al surfaces.18 This method is the first report of hydrogen activation on an Al(111) surface doped with Ti atoms at low temperatures. The unique reactive nature of surface or subsurface Ti atoms in Al is mainly due to the ability to bind and donate electrons to atomic hydrogen, leading to hydrogen chemisorption. Although theoretically predicted as a possibility, isolating Ti centers on Al supports experimentally is difficult at higher temperatures as Ti prefers subsurface and step edges in Al(111) and Al(100) surfaces. Eventually it was identified, using a combination of in situ surface IR spectroscopy on single crystal Al(111) doped with Ti atoms and CO as a molecular probe, that it is possible to activate hydrogen on Ti-doped Al at temperatures as low as 90 K (Figure 2).19 Hydrogen storage is critically dependent on finding such cheaper alternatives to precious metals that are known to activate hydrogen at room temperature. Isolated atom-center based catalysts and single-site catalysts can also be stabilized and immobilized in a host−guest framework such as MOF or applied as a coating on electrode surfaces with greater control over nanoparticles. As such, the highest limit for dispersion of catalysts is a single catalyst on an accessible guest framework. The trajectory of catalyst development is, therefore, to design low-cost, highly dispersed catalysts with an easy regeneration route to reduce our use of precious metal catalysts in the next decade. Moreover, new catalysts are being developed to combine hydrogen storage and CO2 sequestration in a new twist to environmental catalyst and energy storage applications. For example, in a promising example from homogeneous catalysis, Ir-containing organometallic clusters have been shown to reversibly store and release H2/CO2 at specific pH via formation of formic acid (H2 (aq.) + CO2(aq.) ↔ OHCOOH (aq.)) from an aqueous solution.20 New methods to separate the two gases in a one-step process can revolutionize hydrogen storage possibilities in a liquid.

Figure 2. CO as a probe for identifying an isolated catalytic center: image shows an Al(111) surface with one of the Ti containing active sites. This surface can activate H2 at the surprisingly low temperature of 90 K. CO can be used to identify active sites and hydrogen spillover using the blue-shift of surface IR spectrum.19

Conclusion

With reduction in the size of gas conversion systems for automotive and environmental applications, the trends that started with nanoscale catalysts are now being tested in single-site and isolated atom-center based catalytic systems with increasing importance. There is a growing recognition of the need to combine organic/inorganic supports with stability, selectivity, and tunable reactivity in catalytic centers. Many new methods for manipulating activity and controlling catalytic lifetime have grown in complexity, along with our growing understanding of the fundamental role of electronic structure of active sites in realistic environments using combinations of theory and experiments. This trend will continue to deliver some of the great discoveries of our time in energy and environmental applications using clever utilization of nanoscale understanding of catalytic processes.

Materials

     

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