Atomically dispersed Pd on nanostructured TiO₂ for NO removal by solar light

Abstract

Reducing the particle size of noble metals on ceramic supports can maximize noble metal performance and minimize its use. Here Pd clusters onto nanostructured TiO2 particles are prepared in one step by scalable flame aerosol technology while controlling the Pd cluster size from a few nanometers to that of single atoms. Annealing such materials at appropriate temperatures leads to solar photocatalytic NOx removal in a standard ISO reactor up to 10 times faster than that of commercial TiO2 (P25, Evonik). Such superior performance can be attained by only 0.1 wt.% Pd loading on TiO2. Annealing these flame‐made powders in air up to 600 °C decreases the amorphous TiO2 fraction and increases its crystal and particle sizes as observed by x‐ray diffraction (XRD) and N2 adsorption. The growth of single Pd atoms to Pd clusters on TiO2 prepared at different Pd loading and annealing conditions was investigated by scanning transmission electron microscopy and XRD. The single Pd atoms and clusters on TiO2 are stable up to, at least, 600 °C for 2 h in air but at 800 °C they grow into PdO nanoparticles whose fraction is comparable with the nominal Pd loading. Hence, most of Pd atoms are on the TiO2 surface where at 800 °C they diffuse and coalesce. Diffuse reflectance infrared Fourier transform spectroscopy reveals NO adsorption on single, double, three and fourfold coordinated Pd atoms depending on their synthesis and annealing conditions. The peak intensity of NO adsorption sites involving multiple Pd atoms is substantially lower in TiO2 containing 0.1 wt.% than 1 wt.% Pd but that intensity from single Pd atoms is comparable. This indicates the dominance of isolated Pd atoms compared to clusters in Pd/TiO2 containing 0.1 wt.% Pd that match or exceed the photocatalytic NOx removal of Pd/TiO2 of higher Pd contents. © 2016 American Institute of Chemical Engineers AIChE J, 63: 139–146, 2017

Influences of particle size and crystallinity of highly loaded CuO/ZrO2 on CO2 hydrogenation to methanol

“…Decrease in the precursor feed rate reduces the flame temperature and the particle residence time in the flame, which hinders particle growth, as shown in Figure . The precursor feed rate also affects the crystallization of metal oxide particles; specifically, the higher flame temperature and longer residence time at the higher feed rate promote the conversion of amorphous phase into crystalline phases …”

“…In this study, 65.2 wt% CuO on ZrO 2 support, which corresponds to 60 wt% in the metallic copper state, was prepared by FSP. In FSP, the size of CuO and ZrO 2 as well as the crystallinity of ZrO 2 was controlled by changing the precursor feed rate that alters the concentration and residence time of Cu and Zr species in the high‐temperature region in the flame. We studied the influences of particle size and crystallinity of the CuO/ZrO 2 catalysts on CO 2 hydrogenation to methanol.…”

A Review of Photocatalytic Materials for Urban NOx Remediation

“…Doping the TiO 2 surface, or forming a nano-composite with other species, such as noble metals, metal-oxides and carbon, is generally aimed at increasing the activity of the material, chiefly through shifting the band gap towards the visible region and preventing electron/hole recombination. There has been extensive research into this, with studies investigating doping with Fe [96,98,99], Fe 2 O 3 [100], Cu [98] , Sn [101], Zn [102], Zn-S [103], Au-N [104], Al 2 O 3 [105], Pd [88,106], Pt [107], Pt/Au [108], Ag [109,110], g-C 3 N 4 [111,112], graphene [113], Sn and Ce/Mn-graphene [114], other forms of carbon [53,115] and minerals from clay [116]. Many of these have previously been summarised in tables in Martinez et al [98,101].…”

Which is Better for Nanomedicines: Nanocatalysts or Single‐Atom Catalysts?

“…• High temperature and vacuum conditions • High requirements for equipment [92] Liquid phase synthesis…”