NOVELTY - System, comprises: a catalyst vessel holding a catalyst system comprising catalytically active gold deposited onto a support (26), where the support comprising many nanoparticles, which are present as clusters of aggregated nanoparticles onto which the catalytically active gold is deposited, where the nanoparticles have been thermally treated before gold deposition; a supply of gas feed fluidly coupled to an inlet of the catalyst vessel, the gas feed comprising carbon monoxide and hydrogen; and an electrochemical cell downstream from and fluidly coupled to an outlet of the catalyst vessel.    USE - The system is useful for: generating electricity; useful for the selective oxidation of carbon monoxide relative to hydrogen (all claimed); and useful in selective oxidation reactions for removing carbon monoxide from reformed hydrogen to be used in the operation of a fuel cell or other carbon monoxide sensitive device e.g. fuel cell power systems, to purify carbon monoxide-contaminated hydrogen feedstock via catalytic oxidation of carbon monoxide to carbon dioxide.    ADVANTAGE - The system: provides excellent catalytic performance with respect to carbon monoxide oxidation; has high carbon monoxide oxidation activity relative to hydrogen; is very stable, has long shelf life, and provides high levels of catalytic activity for extended time periods; and are effective in humid environments and work over a wide temperature range including room temperature (22-27 degrees C) and much cooler temperature (less than 5 degrees C).    DETAILED DESCRIPTION - System comprises: a catalyst vessel holding a catalyst system comprising catalytically active gold deposited onto a support, where the support comprising many nanoparticles, which are present as clusters of aggregated nanoparticles onto which the catalytically active gold is deposited, where the nanoparticles have been thermally treated before gold deposition in a manner such that the catalyst system selectively oxidizes carbon monoxide relative to hydrogen to a greater degree than an identical catalyst system without the thermal treatment; a supply of a gas feed fluidly coupled to an inlet of the catalyst vessel, the gas feed comprising carbon monoxide and hydrogen; and an electrochemical cell downstream from and fluidly coupled to an outlet of the catalyst vessel. INDEPENDENT CLAIMS are also included for:    (1) selectively oxidizing carbon monoxide relative to hydrogen comprising: providing a catalyst vessel; and causing a supply of a gas feed to contact the catalyst system under conditions such that the carbon monoxide is selectively oxidized with respect to the hydrogen;    (2) a catalyst system for selectively oxidizing carbon monoxide relative to hydrogen, comprising catalytically active gold deposited onto the support;    (3) making a catalyst system comprising either: providing many nanoparticles, incorporating the nanoparticles into the support, and using physical vapor deposition techniques to deposit catalytically active gold onto the support, where the nanoparticles have been subjected to a thermal treatment; or providing information indicative of how a thermal treatment of a support comprising many nanoparticles that are present as clusters of aggregated nanoparticles impacts a characteristic of a catalyst system for oxidizing carbon monoxide relative to hydrogen, and using the information to make a catalyst system that selectively oxidizes carbon monoxide relative to hydrogen;    (4) generating electricity comprising: contacting a fluid mixture comprising carbon monoxide and hydrogen gases with a catalyst system; and using the gas to create electricity; and    (5) preparing a catalyst comprising either: providing metal oxide nanoparticles, thermally treating the nanoparticles, incorporating the nanoparticles onto a host to provide a catalyst support in a manner such that the nanoparticles are present on a host surface as aggregates of the nanoparticles, and depositing catalytically active gold onto the catalyst support that incorporates the thermally treated nanoparticles; or providing a support comprising a surface having crystalline and amorphous surface regions, thermally treating the support, and depositing physical vapor catalytically active gold onto the thermally treated support.    DESCRIPTION OF DRAWING(S) - The figure shows schematic side view of an apparatus for carrying out a physical vapor deposition process for depositing catalytically active gold onto a support.    Apparatus (10)    Housing (12)    Vacuum chamber (14)    Vacuum gate valve (22)    Support (26)