Patent Application: US-3977101-A

Abstract:
catalyst compositions are provided that are useful in selectively removing carbon monoxide from a hydrogen - containing gas . these catalyst compositions preferably have the formula : nn / ce 1 − a x a ′ y a ″ z o 2 − δ , where a , a ′, a ″ are independently selected from the group consisting of : zr , gd , la , sc , sr , co , cr , fe , mn , v , ti , cu and ni ; n is one or more members of the group consisting of : pt , pd , and au ; n is a weight percent between 0 and 25 ; x , y and z are independently 0 to 0 . 9 ; x + y + z is 0 . 1 to 0 . 9 ; and δ is a number which renders the composition charge neutral ; or nn / y 1 − y , where m is one or more members of the group selected from : zr , co , cr , fe , mn , v , ti , ni and cu ; n is one or more members of the group selected from : pt , pd , and au ; n is a weight percent between 0 and 25 ; y is 0 . 1 to 0 . 9 ; and x and δ make the compositions charge neutral .

Description:
one application for the materials and methods described herein is selective removal of carbon monoxide from reformate fuel for hydrogen powered polymer electrolyte membrane fuel cells ( pemfc ). since even low levels of carbon monoxide can degrade hydrogen pemfc performance , use of these catalysts will improve fuel cell operation when using fuel sources other than pure hydrogen . the specific components of these catalysts listed above can be separated into four categories . the first category includes ce , either as ceo 2 or doped ceo 2 . this component acts both as a support in synergy with noble metals and an active metal oxide oxidation catalyst . an equilibrium between ce 3 + and ce 4 + results in an exceptionally high oxygen storage and release capacity that facilitates catalytic combustion by providing oxygen directly to catalytically active sites . furthermore , ceo 2 - containing catalysts are less susceptible to deactivation from water vapor and more resistant to sintering than catalysts employing inactive metal oxide supports such as al 2 o 3 . the second category includes zr , gd , la , sc and sr . doping ceo 2 with zr 4 + is believed to create defects in the fluorite oxygen sublattice and results in an increase in oxygen mobility and diffusivity . furthermore , small quantities of gd , la , sc and sr in the composition are believed to produce mobile oxygen vacancies that take part in the catalytic combustion mechanism . the third category of additives imparts a more substantial effect on the catalyst qualities and includes the transition metals co , cr , fe , mn , v , ti , cu and ni . in addition to lowering the temperature of the ce 4 + - ce 3 + redox couple , most of these metals have multiple oxidation states and metal oxygen bond strengths conducive to oxidation reactions . accordingly , when combined with ceo 2 , they also act in synergy to provide additional catalytic active sites and increase oxygen mobility as well as reactive surface oxygen species . the fourth category includes the noble metals pt , pd and au . both pt and au are present in the metallic state whereas pd can be present as a metal or an oxide . although inclusion of this fourth category in catalyst compositions improves catalyst oxidation activity , it is not necessary and is deemphasized due to the high cost of these metals . high oxidation activity of these materials results from judicious selection of specific compositions . the compositions are selected based on the qualities described above , as well as results of catalyst testing , such as that described herein , or known in the art . however , catalyst activity must be balanced with selectivity for preferential interaction of co with catalyst active sites . one class of catalysts consist of finely dispersed au and / or cu on nanocrystalline multi - component metal oxides . for cu - containing analogs , the cu is believed to be present as groups of atoms forming clusters , or stabilized surface cu ions . the composite catalyst can be denoted by the general formula m x ( m ′ o y ) z ( ceo 2 − δ ) 1 −( x + z ) , where m represents particles or clusters of cu or au , m ′ is a transition metal oxide or mixtures of transition metal oxides selected from zr , co , cr , fe , mn , v , ti , ni , and cu , and 0 . 5 ≦( x + z )≦ 1 . proposed mechanism . although applicants do not wish to be bound by theory , and the mechanism for selective oxidation of carbon monoxide on these materials is not known , it is believed that the first step involves adsorption of oxygen from the feedstream onto a surface metal . cation to form a superoxide ( o 2 − ). the superoxide likely converts to a peroxide species ( o 2 2 − ), which has been suggested to be the active form of surface oxygen for similar materials . the peroxide ion may associate with two reduced - state metal cations or an oxygen vacancy . simultaneously , carbon monoxide can selectively adsorb from the feedstream to a metal particle , cluster , or ion , and migrate to the three - phase boundary between the specific active site , metal oxide surface , and gas phase . at this three - phase boundary , reaction with o 2 2 − can occur to produce co 2 . this process will remove deleterious carbon monoxide from the reformate fuel without combusting hydrogen . catalyst preparation . the coprecipitation method may be used to prepare catalysts . the coprecipitation method involves dissolution of metal salt precursors ( typically nitrates ) in the appropriate molar ratios in water and / or alcohol . the precursor solution is slowly mixed with a precipitating reagent ( e . g ., nh 4 oh , naoh , koh , na 2 co 3 ) with constant stirring . the resulting precipitates are filtered and washed several times in distilled water . the washed precipitates are dried at ˜ 100 ° c ., followed by calcining in air between 300 and 850 ° c . for several hours . the resulting dried powder is ground and passed through a 150 - μm sieve . to decrease particle size , the powder is thoroughly milled to nanometer dimensions using a spex certiprep 8000 mixer / mill . other preparation methods may be used , also . hydrothermal synthesis also involves precipitation of an oxide from precursor salts with a base ; however , the reaction is carried out at temperatures between 120 ° c . and 180 ° c . in a sealed vessel . the hydrothermal process tends to produce ultra fine powders with a very narrow distribution of particle sizes and shapes . ceramic processing is a solid state synthesis that requires physically mixing the constituent metal oxides followed by high - temperature calcination . this process eliminates problems associated with selective precipitation that might be encountered with coprecipitation ; however , the high processing temperatures result in catalysts with much lower surface areas . impregnation involves mixing a catalyst precursor solution with a powder prepared by one of the methods above . after removal of the solvent , the mixture is oxidized in an oven as appropriate . catalyst calcination and pretreatment . catalysts are calcined in air prior to any additional treatment , and the calcination temperature and duration is used to adjust the crystallographic properties of the material as described above . in particular , the calcination temperature affects the ratio of surface metal to the metals in the bulk oxide , as well as the surface cluster size . however , for cu - containing analogs reductive pretreatment is used to affect the nature of the cu active sites . for example , tschöpe et al . showed that after reduction of cu 0 . 15 ce 0 . 85 o 2 − δ at 200 ° c ., the cu content was 100 % cu + 52 . increasing the reduction temperature to 300 ° c . divided the cu content to 70 % cu + and 30 % cu 0 , and reduction at 400 ° c . produced 100 % cu 0 . since cu + has been suggested to be the active site for co oxidation , low - temperature reduction is likely favored for similar systems . however , the above discussion only relates to activity for co oxidation and not selectivity over h 2 . since high activity already is easy to achieve , improvements in selective co oxidation will be gained through alternative treatments that enhance selectivity . accordingly , promising catalyst compositions are tested after exposure to both oxidative and reductive conditions over the temperature range of 100 ° to 850 ° c . catalyst characterization . the catalysts are characterized by methods known in the art , including i ) scanning electron microscopy ( sem ), ii ) energy dispersive x - ray spectroscopy ( edx ), iii ) x - ray diffraction ( xrd ), iv ) surface area and pore size analysis , v ) particle size measurement , vi ) metal dispersion , vii ) fourier - transform infrared spectroscopy ( ftir ), and viii ) atomic absorbance spectroscopy ( aas ). catalyst screening apparatus . a schematic diagram of a single reactor to be used for catalyst screening is shown in fig1 . one actual catalyst screening apparatus in use incorporates six reactors , which enables rapid catalyst screening . important components of the reactor include an electric furnace surrounding a 4 - mm i . d . quartz tube containing the catalyst sample . the catalyst is held in place by a plug of quartz wool and a narrower inner - tube , and the temperature is monitored and adjusted using a controller paired with a thermocouple touching the top the catalyst bed . the quartz reactor tube is coupled to ⅛ ″ stainless steel inlet and outlet tubes using swagelok fittings with teflon ferrules . simulant reformate fuel is prepared by mixing h 2 , co , and o 2 from cylinders with a balance of n 2 or ar . flow rates are controlled by needle valves and monitored using both ball - type and bubble flow meters . sampling of the inlet and outlet gas stream is achieved through ports located on opposite sides of the reactor . catalyst screening conditions and procedure . approximately 0 . 2 g of catalyst is used for each evaluation , which corresponds to a cylindrical catalyst volume of ˜ 0 . 13 cm 3 . catalysts are screened at temperatures between 50 - 200 ° c . and space velocities between 10 , 000 - 20 , 000 hr − 1 ( contact times between 0 . 4 - 0 . 2 seconds ), which requires flow rates between 20 - 40 ml / min . the co concentration in the feedstream is maintained at ˜ 2000 ppm . to simulate h 2 - rich reformate , the h 2 concentration is between 10 , 000 - 50 , 000 ppm . the o 2 concentration is a variable in catalyst performance evaluation , and ranges between 0 - 6 , 000 ppm . the concentrations of species in the inlet and outlet streams ( i . e ., co , h 2 , and co 2 ) are determined by chromatographic analysis of 1 - ml samples using a hewlett - packard 5890 gc with thermal - conductivity detection . chromatographic separation is performed with an alltech ctr - 1 column , and peak areas are determined using a hewlett - packard 3396a integrator . calibration is performed by measuring chromatographic peak area as a function of concentration over the relevant concentration ranges for each gas . for each catalyst , activity and selectivity are determined over a range of temperatures for a fixed o 2 concentration approximately equal to the co concentration . the temperature required for 50 % and 100 % co oxidation is determined from the data , and the catalysts are screened at these temperatures for a variable o 2 concentration . conversion values , % c , for co are determined based on co removal and co 2 production according to , %   c = [ co 2 ] outlet [ co ] inlet  ( 100 )   and ( 2 ) %   c = [ co ] inlet - [ co ] outlet [ co ] inlet  ( 100 ) ( 1 ) where [ co ] inlet , [ co ] outlet , and [ co 2 ] outlet are the inlet and outlet co concentration , and the outlet co 2 concentration , respectively . consumption of h 2 is determined using an equation analogous to equation 1 . for each set of data , selectivityis highlighted by plotting % c versus h 2 consumption . examples of selective removal of carbon monoxide from a hydrogen - containing gas are shown in fig2 - 5 . catalyst optimization . catalyst optimization is performed by careful variation of catalyst constituents and concentrations in the material , as well as preparation and pretreatment conditions . optimization is also performed using statistical methods known in the art . catalyst compositions are also tested on traditional cordierite monolith and alumina pellet supports . the ceramic monoliths are obtained from commercial sources ( corning ) and have a cell density of 400 / in 2 . alumina pellets also are obtained from commercial sources ( aldrich ) and have a particle diameter of ˜ 6 mm . since the method of incorporating the catalysts onto the support structures is anticipated to have a dramatic effect on the resulting activity , several methods are investigated : i ) slurry coating , ii ) decomposition of precursors , and iii ) deposition - precipitation . each of these methods , as well as the procedures for evaluating structurally - supported catalysts , is described below . slurry coating . catalyst powder slurries are prepared by mixing approximately 2 g of 0 . 2 to 2 - μm catalyst powder in 1 , 4 - butanediol . the 1 , 4 - butanediol has sufficient viscosity to maintain suspension of the catalyst particles , while allowing thorough coating of the support surface . the uncoated support is immersed in the slurry , then removed and dried at ˜ 100 ° c . for 12 hours , followed by calcining in air for 2 hours at temperatures between 200 and 900 ° c . as with the powder catalysts , exact calcination temperatures and pretreatment conditions are determined experimentally , and depend on the specific catalyst being used . to obtain the desired loading , this process may be repeated several times , and variation of the slurry thickness is another way to control the catalyst loading and microstructure . furthermore , the surface area of the monolith may be increased before application of the catalyst . this task is achieved by wash - coating with al 2 o 3 from a bohemite solution , followed by drying and heating at 400 ° c . decomposition of precursors . for this method , an aqueous solution of metal precursors in the appropriate stoichiometric ratios is prepared at a concentration of at least 1m . depending on the solubility of the precursors , it is desirable to have the concentration as high as possible . as with the slurry - coating technique , the support is submerged into the solution for a defined period of time , then removed , dried , and weighed . this process is repeated until there is no weight gain ( usually 3 or 4 applications ), then calcined in air to produce the catalyst coating . alternatively , the sample can be calcined between catalyst precursor applications . deposition - precipitation . this method combines the decomposition technique above with the coprecipitation technique . specifically , the support is submerged in a solution of the appropriate precursors in the necessary stoichiometric ratio . next , nh 4 oh ( or other precipitating reagent ) is added slowly with mixing to produce the corresponding metal hydroxides , which coats all surfaces of the support materials . since this step normally results in production of a thick gel , the concentration of the precursor solution and nh 4 oh need to be low to avoid pore clogging with monoliths . the metal hydroxide layer is then decomposed to the oxide by calcining in air . for noble metal - containing catalysts , the metal oxide can be deposited first , then the noble metal can be deposited using the procedure . evaluation . for monolith supports , small sections of monolith are cut to fit inside a 13 . 8 - mm i . d . quartz tube , and the tube is incorporated into the reactor assembly shown in fig1 . a photograph of a section of bare monolith is shown next to a catalyst - coated monolith within the reactor tube in fig6 . the length of the monolith ( catalyst bed depth ) is ˜ 25 mm . approximately 0 . 2 - 0 . 4 g of catalyst can be deposited onto a monolith of these dimensions using the techniques above . for pellet supports , the catalyst - coated pellets simply replace the monolith in the assembly . evaluation of the supported catalysts is performed according to the conditions described above . 1 . gardner , s . d . ; hoflund , g . b . ; davidson , m . r . ; laitinen , h . a . ; schryer , d . r . ; upchurch , b . t . langmuir 1991 , 7 , 2140 - 2145 . 2 . gardner , s . d . ; hoflund , g . b . ; schryer , d . r . ; schryer , j . ; upchurch , b . t . ; kielin , e . j . langmuir 1991 , 7 , 2135 - 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preferred embodiments of this invention . those of ordinary skill in the art will appreciate that methods , techniques , compositions and components other than those specifically described herein can be employed in the practice of the invention without departing from its spirit scope . all references included herein are hereby incorporated by reference to the extent not inconsistent with the disclosure herein .