1. Atomic Tailoring of Catalyst Surfaces for High Selectivity Partial Oxidation of Propane J. Gleaves, R. Fushimi, G. Yablonaky, M. Rude, D. French, P. Buzzeta, S. Mueller, J. Swisher, J. Searcy Washington University A. Gaffney The Rohm and Haas Company Funded by the NSF’s GOALI (Grant Opportunities for Academic Liaison with Industry) Initiative Heterogeneous Kinetics and Particle Chemistry Laboratory Changing the Surface Transition Metal Composition of Bulk Catalysts Creating Nanoscale Concentration Gradients of Transition Metal Species on Bulk Metal Oxide Catalysts Transition metal source Catalyst particles Atomic beam Laser beam Sample holder in transfer arm (Vacuum - 10 -8 torr) Vibrate bed Nanoscale concentration gradients of transition metal species on the surface of metal oxide catalyst particles can be created by immersing the particles in a dilute beam of transition metal atoms. The atoms are produced by focusing the light from a pulsed excimer laser onto the surface of a metal target which is contained in a vacuum chamber. The laser pulse ablates the metal surface producing a pulsed beam of metal atoms. The particles are contained in a shallow tubular reactor that is continuously agitated so that the surface of the particles are randomly exposed to the atomic beam. After deposition, the catalyst particles are transferred, under vacuum, to a microreactor where they can be tested. The selectivity of a metal oxide catalyst is a function of its bulk structure and surface composition. A variety of metal oxide crystal structures exhibit catalytic activity, but their selectivity depends strongly on the preparation procedure, which in turn influences the surface composition and structure. At present, there is no way to predict how a change in the preparation procedure will affect the surface composition, and no way to predict which surface composition will give the highest selectivity. To develop a more detailed understanding of how the surface composition of a catalyst influences its activity-selectivity we will focus on changing the surface and near surface composition of mixed metal oxide catalysts by changing either the oxygen concentration or the concentration of one or more metal constituents. In effect, the bulk crystal structure and its attendant electronic properties will remain intact while the surface composition is altered. Scientific Motivation Conclusions This project is concerned with the development of highly selective catalysts for the selective conversion of short chain hydrocarbons. To date the focus has been on the development of a novel new synthesis technique that uses atomic beam deposition to precisely alter the surface composition of bulk industrial catalysts. It is important to note that the new synthesis technique is very general in nature, and that any metal, including refractory metals (e.g., tungsten) can be deposited on practically any substrate particle. Substrate materials include, but are not limited to metal oxide, and supported metal catalysts, polymeric particles, ceramic particles, and particles with semiconducting, superconducting, or photocatalytic properties, and particles that are biologically active. Conclusions Atom Deposition Chamber Cu pulses.1s Transition metal source Laser beam path Gatevalve (separates deposition chamber and reactor chamber) Sample vibrator assembly Research Objective The focus of our current research is on the development of highly selective catalysts and the optimum process conditions for the selective conversion of propane to acrylic acid. The project uses a novel new synthesis approach to create nanoscale gradients in the surface composition of bulk industrial catalysts. The new approach will be generally applicable to catalytic processes involving mixed metal oxide and supported metal catalysts. Reaction of Butene over Surface Modified VPO Butene Furan + O 2 P Ox, T Rx, t Rx (VO) 2 P 2 O 7 Oxygen-enriched nanolayer + M (VO) 2 P 2 O 7 Metal-enriched nanolayer (VO) 2 P 2 O 7 + O 2 Oxygen, metal-enriched nanolayer (VO) 2 P 2 O 7 Butene Furan Atomic tailoring of technical catalysts particles Submonolayer Change in Surface Composition Physical characterization Kinetic characterization RHROH Metal Oxide Particle RHROH Key Challenges 1. Uniform, precise coverage change 2. Kinetic analysis changes composition Pulsed Oxidation of (VO) 2 P 2 O 7 under Vacuum Conditions 0.0 0 4.0 0 Time (s) Pulse Number 500 Oxygen pulse response curves (≈1 x 10 15 O atoms/pulse) T = 450 °C Oxygen Conversion Reactor equilibrated VPO After several oxidation-reduction cycles Butene Reaction over VPO based Catalysts 0.0 Time (s) 1.0 Pulse Number 10 0 0.0 Time (s) 1.0 Pulse Number 100 VPO Butene conversion Furan production Time (s) 0.0 1.0 100 Pulse Number 0.0 Time (s) 1.0 100 Pulse Number VPO - Cu deposition (Total coverage <.005 monolayers of Cu atoms) Butene conversion Furan production Furan Yield versus Pulse Number Normalized yield Pulse Number Two copper samples Normalized yield Pulse number Catalytic Selective Oxidation-Reduction Cycle C 3 H 8 + 2 O 2 C 3 H 4 O 2 + 2H 2 O Selective oxidation of propane to acrylic acid Phase B Surface phase MaMa MbMb O2-O2- n2n2 n4n4 O2O2 n e - Propane Acrylic acid H+H+ a+ b+ Propane activation site Oxygen activation site R. K. Grasselli, Surface properties and catalysis by nonmetals, 1983, 273 -288 Preliminary Experiments: Combining surface synthesis and kinetic characterization Preliminary experiments combining atomic beam deposition and TAP pulse response experiments were performed using (VO) 2 P 2 O 7 as a substrate. The base catalyst consisted of 200  m VPO particles that had been equilibrated for several hundred hours in a butane-oxygen feed at reaction conditions. Surface modified catalysts were prepared by depositing copper and tellurium atoms on a portion of the base catalyst. During the deposition process the catalyst was maintained at room temperature. Coverages ranged from ~10 15 to 10 17 atoms per sample, but the maximum coverage was always less than 1/100 of a monolayer. Kinetic tests of unmodified and modified catalysts were performed by pulsing oxygen-argon and butene-argon mixtures over 140 mg samples hled at 430  C. Catalyst samples were first exposed to between 200 and 1000 oxygen pulses (pulse size 10 15 molecules per pulse), and then to 50 to 500 butene pulses. Each sample was exposed to a series of oxidation-reduction cycles. After oxidizing a catalyst sample it was then exposed to a series of butene-argon pulses and the transient response of either butene or one of several reaction products was monitored. Comparison of the normalized furan yield as a function of pulse number for reactor equilibrated VPO, VPO modified with copper, and VPO modified with tellurium. Furan production was determined by calculating the moments of individual pulse response curves. The furan yield changes with each butene pulse as the VPO surface oxygen is depleted. The rate of change is clearly altered by the addition of metal atoms, and the maximum in the furan yield occurs earlier in the reduction cycle on the metal modified samples. Comparison of two different VPO- Cu samples prepared from the same base sample and approximately the same number of Cu atoms. Transition Metal Concentration Changing the surface concentration of a transition metal species in a bulk catalyst in a precise controllable manner is a much more difficult problem. The addition of a metal by standard methods (e.g., incipient wetness, CVD) generally involves a number of reaction steps that are not well defined. With standard methods the change in the catalytic properties of a catalyst cannot be directly related to the change in the transition metal surface concentration. The key experimental problem is to develop a method to add different transition metal atoms to the surface of a catalyst in precisely known amounts so that the change in the concentration of the transition metal can be directly related to changes in catalyst performance.

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