Precious Group Metals Research Articles

Non-Noble Metal Catalysts As Oxygen Depolarized Cathodes in Chlor Alkali Electrolyzers

Chlorine is an essential element of chemical industries, and especially has paramount importance in polymer industry. A typical route for chlorine generation is electrolysis of brine solution where chlorine is generated at the anode; producing caustic soda and hydrogen at cathode. However, the relevant state of the art membrane reactors is one of the most energy intensive processes in industry. Moreover, the membrane technology is believed to have reached the theoretical limit, and energy consumption cannot be reduced further. In the present chlor-alkali technology, sodium chloride solution is electrolyzed at a constant current density, and the decomposition voltage ΔE of the overall reaction is 2.186 V as per thermodynamic calculation. If the hydrogen evolution reaction at the cathode side is replaced with oxygen reduction reaction (ORR), the decomposition voltage will be significantly reduced, and, as a consequence, the energy consumption will be reduced simultaneously. Introduction of such oxygen depolarized cathode results in the following chemical reactions: Anode: 2Cl- —— Cl2+2e- (E=1.358 V vs. NHE) Cathode: 4Na+ + 2H2O + O2 +4e- —— 4NaOH (E=0.401 V vs. NHE) Overall Reaction: 4NaCl + 2H2O + O2 —— 4NaOH + 2Cl2 (ΔE=-0.957 V) Even though the voltage requirement can be minimized by oxygen depolarized cathode, there is still significant overpotential loss from the cathodic reaction. This is due to the fact that ORR is kinetically a very sluggish reaction. Platinum is the best catalyst known for ORR in acid media. However, Pt is poisoned in concentrated NaOH solution resulting in significant shifts in onset potential and limiting current values. Early in 1970s, heating of non-noble metal along with carbon and a source of nitrogen atoms in presence of ammonia resulted in formation of electrocatalysts with ability to reduce oxygen. Since then, there has been a gradual development of such “non-PGM” catalysts, where PGM stands for Precious Group Metals. The introduction of metal organic frameworks (MOFs) as sacrificial templates has brought a revolution in electrocatalysis research. Owing to the large surface area, preferable porosity and high nitrogen content, the non-PGM catalysts obtained from MOF precursors have shown excellent ORR activity. However, most of the MOF synthesis methods are based on the traditional solution reactions, which need either excessive amount of ligand or a huge amount of solvent, or both. Thus, the low yielding synthesis route and comprehensive separation methods of MOF seriously constrain the application of the MOF based catalysts for ORR. In the presentation, we will be discussing about the new in-house synthesized MOF based catalyst using solid-state reaction. In addition to advantages like simple preparatory route and high reproducibility, this catalyst has high yield and scalability which are uncommon traits for the more conventional MOF based catalysts. This catalyst has high ORR onset potential even in extreme corrosive conditions such as concentrated alkali and acid. Herein, we propose a facile, simplified approach to obtain the MOF based catalysts for ORR with low cost and high reproducibility. The as-prepared catalysts have been tested as oxygen depolarized cathodes in chlor-alkali electrolysis cells and have been operated at significantly lower voltages compared to commercial catalysts like Pt/C and Ag/C.

LNT–SCR dual-layer catalysts optimized for lean NOx reduction by H2 and CO

Monolithic catalysts consisting of a layer of selective catalytic reduction (SCR) catalyst deposited on top of a lean NOx trap (LNT) catalyst were optimized to provide high NOx conversion at both low and high temperatures with minimal precious group metal (PGM) loading using H2/CO reductant mixtures. The optimized dual-layer catalyst circumvents the need for urea feed and has the potential to reduce the expensive PGM loading by up to 38% from that of LNT only catalyst under laboratory test. We investigated the impact of catalyst design variables, such as SCR and LNT zoning, the ceria level in LNT, as well as SCR zeolite type (ZSM-5, SSZ-13) layer thickness. Zoning of either or both the SCR and LNT in the dual-layer catalysts enables an increase of the low-temperature NOx conversion, and minimizes the high temperature (300–400°C) conversion loss caused by the SCR diffusion resistance and undesired NH3 oxidation by the LNT. High ceria loading of the LNT enhanced NH3 generation, NOx adsorption and mitigated CO poisoning at low temperatures (150–250°C). Commercial Cu-SSZ-13 exhibited a higher NH3 storage capacity and better low-temperature SCR activity than the in-house synthesized Cu-ZSM-5, and improved the low-temperature NOx conversion of the dual-layer catalysts. The diffusion resistance in the top active Cu-zeolite layer inhibited the overall NOx reduction as shown by replacing it with an inert Na-ZSM-5 layer with a high Si/Al ratio. Washcoat diffusion limitations adversely affect the high temperature performance more than the NH3 oxidation to NOx. The experiments revealed that diffusion limitations in the top SCR layer loading of 1.0g/in.3 started at 150°C using a pure H2 feed and at 250°C using a CO/H2 feed.

Modeling Studies on Lean NOx Reduction by a Sequence of LNT–SCR Bricks

Several experimental studies have been conducted to determine the NOx reduction by a series of LNT (lean NOx trap) and SCR (selective catalytic reduction) catalytic bricks. An important goal is to minimize the required precious metal loading in the LNT while keeping the NOx emission below a specified level. We present a mathematical model of this system using hydrogen as the reductant. Simulations are used to determine the influence of the architecture of the LNT–SCR bricks, nonuniform precious metal loading in the LNT bricks, and the cycle time at temperatures in the range of 200–350 °C. The simulations lead to the following observations: (a) Low temperature reduction is the limiting step in the optimization of precious group metal (PGM) loading in LNT. (b) The NOx conversion increases as the number of the sequential bricks (with total length fixed) increase and reaches an asymptotic limit. From a practical point of view, there is little incentive in using more than two sequential pairs. (c) Nonuniform p...

Thermodynamic estimation of minor element distribution between immiscible liquids in Fe–Cu-based metal phase generated in melting treatment of municipal solid wastes

Waste electrical and electronic equipment (WEEE) has become an important target in managing material cycles from the viewpoint of not only waste management and control of environmental pollution but also resource conservation. This study investigated the distribution tendency of trace elements in municipal solid waste (MSW) or incinerator ash, including valuable non-ferrous metals (Ni, Co, Cr, Mn, Mo, Ti, V, W, Zr), precious group metals (PGMs) originated from WEEE (Ag, Au, Pd, Pt), and others (Al, B, Pb, Si), between Fe-rich and Cu-rich metal phases by means of simple thermodynamic calculations. Most of the typical alloying elements for steel (Co, Cr, Mo, Nb, Ni, Si, Ti, V, and W) and Rh were preferentially distributed into the Fe-rich phase. PGMs, such as Au, Ag, and Pd, were enriched in the Cu-rich phase, whereas Pt was almost equally distributed into both phases. Since the primary metallurgical processing of Cu is followed by an electrolysis for refining, and since PGMs in crude copper have been industrially recovered from the resulting anode slime, our results indicated that Ag, Au, and Pd could be effectively recovered from MSW if the Cu-rich phase could be selectively collected.

Investigation Using Simulations for the Development of Low Cost Catalytic Converter from Non-Precious Metals

This paper discusses the uses of non-precious metals in catalytic converters to bring down the cost without sacrificing the performance capability. Copper powder and nickel catalyst were chosen as the alternative catalysts to reduce the use of precious group metals (PGMs) platinum, palladium, and rhodium. Simulation by COMSOL has shown that Nickel and copper were very effective in reducing NOx during rich condition of air-fuel mixture while oxidizing CO and HC during lean condition. Simulations using FLUENT and COMSOL have shown the actual characteristics of the catalytic converter performance. The flow throughout catalytic converter and the backpressure have successfully determined. Furthermore, catalyst conversion efficiency also has been shown clearly. On the other hands, the experimental results have excellently validated the simulation results in terms of the nature and trends of the catalytic converter performance as well as its efficiency. Catalyst distribution and application of the non-zoning monolith substrates have further contributes to cut down the production cost. It was found that the low cost catalytic converter was able to meet the EURO 2 emission regulation control and has optimum backpressure at full throttle.

Investigation Using Simulations for the Development of Low Cost Catalytic Converter from Non-Precious Metals

This paper discusses the uses of non-precious metals in catalytic converters to bring down the cost without sacrificing the performance capability. Copper powder and nickel catalyst were chosen as the alternative catalysts to reduce the use of precious group metals (PGMs) platinum, palladium, and rhodium. Simulation by COMSOL has shown that Nickel and copper were very effective in reducing NOx during rich condition of air-fuel mixture while oxidizing CO and HC during lean condition. Simulations using FLUENT and COMSOL have shown the actual characteristics of the catalytic converter performance. The flow throughout catalytic converter and the backpressure have successfully determined. Furthermore, catalyst conversion efficiency also has been shown clearly. On the other hands, the experimental results have excellently validated the simulation results in terms of the nature and trends of the catalytic converter performance as well as its efficiency. Catalyst distribution and application of the non-zoning monolith substrates have further contributes to cut down the production cost. It was found that the low cost catalytic converter was able to meet the EURO 2 emission regulation control and has optimum backpressure at full throttle.

Nanostructured Thin Film Electrocatalysts - Current Status and Future Potential

3M's nanostructured thin film (NSTF) catalyst technology platform is the only practical example of an extended surface area catalyst which significantly reduces many performance, cost and durability barriers facing electrodes for H2/air PEM fuel cells for vehicles. In this paper we first discuss the recent gains in NSTF alloy ORR activities from both materials and process improvements, followed by accelerated durability tests of MEA's with these catalyst improvements. Next we discuss recent progress in tailoring NSTF anode catalysts with ultra-low precious group metal oxygen evolution reaction (OER) catalysts for high tolerance to automotive start up/shut down and cell reversal events. We follow with our progress in developing strategies and anode GDL's for optimized fuel cell water management for automotive low temperature operation. Finally we discuss a path forward to achieve the entitlement NSTF activity and comment on critical durability issues intrinsic to ultra-low loading catalysts.

APPLICATION OF RESET CONTROL TO A BATCH REACTOR IN THE PRECIOUS GROUP METALS INDUSTRY

Application of reset control to the temperature control loop of an exothermic batch reactor in the Precious Group Metals industry is investigated in this paper. Through modelling and simulation of the process temperature response to cooling, a comparison is drawn between the existing PI controller, an optimized linear PI controller, an optimized linear PI controller with lead, and a reset controller. The simulations confirm the ability of reset control to significantly reduce temperature overshoot as well as decrease the settling time in this process control loop. In addition, the simulations uncover the necessity of applying a non-zero initial condition to the reset controller in this specific control application. The main purpose of the paper is to demonstrate the potential for novel application of reset control with non-zero initial conditions to an existing industrial process in the hydrometallurgical industry. In the literature actual industrial applications of reset control appear to be scarce, and the application of a non-zero initial condition to the reset controller may well be unique. The paper also highlights some typical practical issues, including effects of dead time and noise, which are involved with the application of theoretical control solutions in an industrial environment.

Ag doped WO 3-based powder sensor for the detection of NO gas in air

WO 3-based materials as sensors for the monitor of environmental gases such as NO x (NO+NO 2) have been rapidly developed for various potential applications (stationary and mobile uses). It has been reported that these materials are highly sensitive to NO x with the sensitivity further enhanced by adding precious group metals (PGM such as Pt, Pd, Au, etc.). However, there has been limited work in revealing the sensing mechanism for these gases over the WO 3-based sensors. In particular, the role of promoter is not yet clear though speculations on their catalytic, electronic and structural effects have been made in the past. In parallel to these PGM promoters here we report, for the first time, that Ag promotion can also enhance WO 3 sensitivity significantly. In addition, this promotion decreases the optimum sensor temperature of 300 °C for most WO 3-based sensors, to below 200 °C. Characterizations (XRD, TEM, and impedance measurement) reveal that there is no significant bulk structure change nor particle size alteration in the WO 3 phases during the NO exposure. However, it is found that the Ag doping creates a high concentration of oxygen vacancies in form of coordinated crystallographic shear (CS) planes onto the underneath WO 3. It is thus proposed that the Ag particle facilitates the oxidative conversion of NO to NO 2 followed by a subsequent NO 2 adsorption on the defective WO x sites created at the Ag–WO 3 interface; hence, accounting for the high molecular sensitivity.

The Nature of the Active Metal Surface of Catalysts for the Clean Combustion of Biogas Containing Ammonia

The nature of the active surface of supported metal catalysts for the clean combustion of ammonia-containing simulated biogas has been investigated to understand the very high N2 selectivity of some catalysts. A comparison of the activities of Al2O3-supported precious group metals has shown that under conventional operating conditions the fuel is fully combusted but NH3 is converted predominantly into NOx. In contrast, it is shown that it is possible by operating under O2-deficient conditions to attain essentially zero emissions of NOx with Rh- or Ir-based catalysts, whereas Pt or Pd gave markedly lower selectivities to N2. These variations in selectivity are attributed to the differences in the state of the metal surface during reaction. For Rh- and Ir-based catalysts it has been shown that under fuel-rich conditions, surface carbon, derived from the dissociative adsorption of CO, results in self-poisoning toward CO and H2 oxidation at low temperatures. However, at higher temperatures a preference toward methanation of the Cads is observed. It is thought that this surface reaction between the Cads and Hads scavenges both reductants so that Oads can react with NH3 in a highly specific manner, to give NOx. The NOx thus formed is subsequently reduced by reaction with excess CO to give N2. A model is presented that shows the way in which the chemistry of the surface of the active metal changes as the reaction parameters are varied.