Palladium Catalysts for Methane Oxidation: Old Materials, New Challenges.

ConspectusMethane complete oxidation is an important reaction that is part of the general scheme used for removing pollutants contained in emissions from internal combustion engines and, more generally, combustion processes. It has also recently attracted interest as an option for the removal of atmospheric methane in the context of negative emission technologies. Methane, a powerful greenhouse gas, can be converted to carbon dioxide and water via its complete oxidation. Despite burning methane being facile because the combustion sustains its complete oxidation after ignition, methane strong C-H bonds require a catalyst to perform the oxidation at low temperatures and in the absence of a flame so as to avoid the formation of nitrogen oxides, such as those produced in flares. This process allows methane removal to be obtained under conditions that usually lead to higher emissions, such as under cold start conditions in the case of internal combustion engines. Among several options that include homo- and heterogeneous catalysts, supported palladium-based catalysts are the most active heterogeneous systems for this reaction. Finely divided palladium can activate C-H bonds at temperatures as low as 150 °C, although complete conversion is usually not reached until 400-500 °C in practical applications. Major goals are to achieve catalytic methane oxidation at as low as possible temperature and to utilize this expensive metal more efficiently.Compared to any other transition metal, palladium and its oxides are orders of magnitude more reactive for methane oxidation in the absence of water. During the last few decades, much research has been devoted to unveiling the origin of the high activity of supported palladium catalysts, their active phase, the effect of support, promoters, and defects, and the effect of reaction conditions with the goal of further improving their reactivity. There is an overall agreement in trends, yet there are noticeable differences in some details of the catalytic performance of palladium, including the active phase under reaction conditions and the reasons for catalyst deactivation and poisoning. In this Account we summarize our work in this space using well-defined catalysts, especially model palladium surfaces and those prepared using colloidal nanocrystals as precursors, and spectroscopic tools to unveil important details about the chemistry of supported palladium catalysts. We describe advanced techniques aimed at elucidating the role of several parameters in the performance of palladium catalysts for methane oxidation as well as in engineering catalysts through advancing fundamental understanding and synthesis methods. We report the state of research on active phases and sites, then move to the role of supports and promoters, and finally discuss stability in catalytic performance and the role of water in the palladium active phase. Overall, we want to emphasize the importance of a fundamental understanding in designing and realizing active and stable palladium-based catalysts for methane oxidation as an example for a variety of energy and environmental applications of nanomaterials in catalysis.

Sulfur-treated TiO2 shows improved alcohol dehydration activity and selectivity.

The dehydration of alcohols is an important class of reactions for the development of fossil-free fuel and chemical industries. Acid catalysts are well known to enhance the reactivity of alcohols following two main pathways of either dehydration to olefins or dehydrogenation to ketones/aldehydes. TiO2 surfaces have been well documented for primary and secondary alcohol dehydration with selectivity ranging from 1-100% towards dehydration products based on process conditions and catalyst structure. In this work we document the effects of various sulfur treatments of TiO2 surfaces which induce higher activity and, more importantly, higher selectivity for alcohol dehydration than untreated surfaces. The increase in activity and >99% dehydration selectivity is coupled with demonstrated stability for several hours on stream at high conversion. Using temperature programmed reaction studies, XPS and FT-IR spectroscopy, we identify Lewis acidic sites correlated with sulfate species on TiO2 surfaces as active sites for the reaction.

Steam-created grain boundaries for methane C-H activation in palladium catalysts.

Defects may display high reactivity because the specific arrangement of atoms differs from crystalline surfaces. We demonstrate that high-temperature steam pretreatment of palladium catalysts provides a 12-fold increase in the mass-specific reaction rate for carbon-hydrogen (C­H) activation in methane oxidation compared with conventional pretreatments. Through a combination of experimental and theoretical methods, we demonstrate that an increase in the grain boundary density through crystal twinning is achieved during the steam pretreatment and oxidation and is responsible for the increased reactivity. The grain boundaries are highly stable during reaction and show specific rates at least two orders of magnitude higher than other sites on the palladium on alumina (Pd/Al2O3) catalysts. Theoretical calculations show that strain introduced by the defective structure can enhance C­H bond activation. Introduction of grain boundaries through laser ablation led to further rate increases.

Transparent Pressure Sensor with High Linearity over a Wide Pressure Range for 3D Touch Screen Applications.

The demand for display technology is expected to increase with the continuous spread of portable electronics and with the expected emergence of flexible, wearable, and transparent display devices. A touch screen is a critical component in display technology that enables user interface operations, and the future generation of touch screens, the so-called 3D touch screens, is expected to be able to detect multiple levels of pressure. To enable 3D touch screens, transparent pressure sensors with high linearity over a working range that encompasses the pressure range of human touch (10-100 kPa) are required. In this work, a transparent and linear capacitive pressure sensor is reported with a transmittance over 85% and high linearity (R2 = 0.995) over 5-100 kPa of pressure. To render the sensor transparent, a microstructured "hard" elastomer layer was filled in with a refractive index matching a "soft" elastomer layer, through which light scattering was minimized. High linearity was attained from the sensor's unique architecture that increases the effective area of the capacitor with applied pressure. These attributes render our sensor highly suitable for future 3D touch screen applications.

Highly Uniform and Low Hysteresis Piezoresistive Pressure Sensors Based on Chemical Grafting of Polypyrrole on Elastomer Template with Uniform Pore Size.

Sensor-to-sensor variability and high hysteresis of composite-based piezoresistive pressure sensors are two critical issues that need to be solved to enable their practical applicability. In this work, a piezoresistive pressure sensor composed of an elastomer template with uniformly sized and arranged pores, and a chemically grafted conductive polymer film on the surface of the pores is presented. Compared to sensors composed of randomly sized pores, which had a coefficient of variation (CV) in relative resistance change of 69.65%, our sensors exhibit much higher uniformity with a CV of 2.43%. This result is corroborated with finite element simulation, which confirms that with increasing pore size variability, the variability in sensor characteristics also increases. Furthermore, our devices exhibit negligible hysteresis (degree of hysteresis: 2%), owing to the strong chemical bonding between the conductive polymer and the elastomer template, which prevents their relative sliding and displacement, and the porosity of the elastomer that enhances elastic behavior. Such features of the sensor render it highly feasible for various practical applications in the near future.

Microstructured Porous Pyramid-Based Ultrahigh Sensitive Pressure Sensor Insensitive to Strain and Temperature.

An ultrahigh sensitive capacitive pressure sensor based on a porous pyramid dielectric layer (PPDL) is reported. Compared to that of the conventional pyramid dielectric layer, the sensitivity was drastically increased to 44.5 kPa-1 in the pressure range <100 Pa, an unprecedented sensitivity for capacitive pressure sensors. The enhanced sensitivity is attributed to a lower compressive modulus and larger change in an effective dielectric constant under pressure. By placing the pressure sensors on islands of hard elastomer embedded in a soft elastomer substrate, the sensors exhibited insensitivity to strain. The pressure sensors were also nonresponsive to temperature. Finally, a contact resistance-based pressure sensor is also demonstrated by chemically grafting PPDL with a conductive polymer, which also showed drastically enhanced sensitivity.

Highly Ordered 3D Microstructure-Based Electronic Skin Capable of Differentiating Pressure, Temperature, and Proximity.

Electronic skin are devices that mimic the functionalities of human skin, which require high sensitivity, large dynamic range, high spatial uniformity, low-cost and large-area processability, and the capacity to differentiate various external inputs. We herein introduce a versatile droplet-based microfluidic-assisted emulsion self-assembly process to generate three-dimensional microstructure-based high-performance capacitive and piezoresistive pressure sensors for electronic skin applications. Our technique can generate uniformly sized micropores that are self-assembled in an orderly close-packed manner over a large area, which results in high spatial uniformity. The size of the micropores can easily be tuned from 100 to 500 µm, through which sensitivity and dynamic range were controlled as high as 0.86 kPa-1 and up to 100 kPa. Our device can be printed on curvilinear surfaces and be molded into various shapes. We furthermore demonstrate that by simultaneously utilizing capacitive and piezoresistive pressure sensors, we can distinguish between pressure and temperature, or between pressure and proximity. These demonstrations make our process and sensors highly useful for a wide variety of electronic skin applications.

Pressure Insensitive Strain Sensor with Facile Solution-Based Process for Tactile Sensing Applications.

Tactile sensors that can mechanically decouple, and therefore differentiate, various tactile inputs are highly important to properly mimic the sensing capabilities of human skin. Herein, we present an all-solution processable pressure insensitive strain sensor that utilizes the difference in structural change upon the application of pressure and tensile strain. Under the application of strain, microcracks occur within the multiwalled carbon nanotube (MWCNT) network, inducing a large change in resistance with gauge factor of ∼56 at 70% strain. On the other hand, under the application of pressure to as high as 140 kPa, negligible change in resistance is observed, which can be attributed to the pressure working primarily to close the pores, and hence minimally changing the MWCNT network conformation. Our sensor can easily be coated onto irregularly shaped three-dimensional objects (e.g., robotic hand) via spray coating, or be attached to human joints, to detect bending motion. Furthermore, our sensor can differentiate between shear stress and normal pressure, and the local strain can be spatially mapped without the use of patterned electrode array using electrical impedance tomography. These demonstrations make our sensor highly useful and important for the future development of high performance tactile sensors.