Unraveling the H2 Promotional Effect on Palladium-Catalyzed CO Oxidation Using a Combination of Temporally and Spatially Resolved Investigations

The promotional effect of H2 on the oxidation of CO is of topical interest, and there is debate over whether this promotion is due to either thermal or chemical effects. As yet there is no definitive consensus in the literature. Combining spatially resolved mass spectrometry and X-ray absorption spectroscopy (XAS), we observe a specific environment of the active catalyst during CO oxidation, having the same specific local coordination of the Pd in both the absence and presence of H2. In combination with Temporal Analysis of Products (TAP), performed under isothermal conditions, a mechanistic insight into the promotional effect of H2 was found, providing clear evidence of nonthermal effects in the hydrogen-promoted oxidation of carbon monoxide. We have identified that H2 promotes the Langmuir–Hinshelwood mechanism, and we propose this is linked to the increased interaction of O with the Pd surface in the presence of H2. This combination of spatially resolved MS and XAS and TAP studies has provided previously unobserved insights into the nature of this promotional effect.


XRD
The powder X-Ray diffraction (PXRD) measurement was carried out using a PANalytical X'Pert Pro X-ray diffractometer. The X-ray source used was copper Kα with a wavelength of 1.5405 Å. Diffractograms were collected from 15° to 90° with a step size of 0.02°. In situ diffractograms were obtained using the same experimental conditions used for the SPACI-FB-XAFS experiments.
There were no observable changes in the diffractograms over the course of the in situ experiments and a representative X-ray diffraction pattern is shown below in Figure S1, which has reflections associated with γ-Al2O3 (37.7, 39.9, 45.9, 61.2 and 66.9°). 1,2 No reflections characteristic of PdO (33.5, 33.8 or 54.7°) 2 or metallic Pd (40.2, 46.8 or 68.3) 2 are observed.
Assuming a detection limit of 3 nm, we assume our Pd catalyst has particles smaller than this. The intensity of the STEM HAADF image is proportional to the local thickness and atomic number of specimen. Pd nanoparticles are therefore observable as small regions of higher intensity on the lower intensity Al2O3 support. The blue circles in Figure S2 highlight some small Pd particles with diameters of between 1.0 and 1.5 nm ( Figure S3).

XAFS XAFS experimental
XAFS measurements were performed at the Pd K-edge on the B18 beamline at the Diamond Light Source, Didcot, UK. Measurements were performed in transmission mode using a QEXAFS setup with fast scanning Si (311) crystal monochromator. The time resolution of the spectra was ~3.5 min/spectrum (kmax= 16 Å -1 ). All XAFS spectra were acquired concurrently with the appropriate foil placed between It and Iref. Spectra were collected over 15 min every 1 mm of the reactor bed to obtain sufficient signal to noise and spatial resolution.

S11
Tables S4 and S5 collate the linear combination data as shown in Figure 3, using the spectra of a PdO reference and the Pd foil reference.

TAP Knudsen Diffusion:
In a TAP experiment the use of a small number of molecules combined with the constant evacuation of the micro-reactor leads to the elimination of any gas-gas interactions. Therefore convectional transport is eliminated and the transport of gaseous molecules in the micro-reactor occurs solely by Knudsen diffusion, the diffusivity of which is defined in Equation S1 .

Equation S1
The diffusional flow rate through the reactor cross section is proportional to the cross sectional area of the reactor, the effective diffusivity and the gas concentration gradient, as shown in

Equation S2
The rates of chemical transformation can be measured in the diffusion + reaction experiment based on information about diffusion transport. In a diffusion-reaction case: this entails the inclusion of an inert gas with the reaction mix. The use of the inert molecules allows characterization of the Knudsen diffusion transport for the specific micro-reactor packing, including the catalyst. As shown in Equation S2, the Knudsen diffusivity for the same packing at the same temperature is proportional to the inversed square root of molecular weight. Using this relationship, the diffusivity (or residence time) obtained for the inert molecules can be renormalized to characterize other reactant/product molecules.
In a pulse-response TAP experiment, the rates of chemical transformation or gas concentrations are not measured directly. In the observed responses (i.e. exit flow time dependencies) reaction rates are coupled with Knudsen transport. Thus, in order to obtain the rate constants of chemical S14 transformation, a theoretical model for Knudsen diffusion combined with reaction rate needs to be applied for the analysis of the data gathered in a TAP experiment.

Moment Based Analysis:
In TAP data analysis, the intensity (area) and the shape of the experimental response curves are compared with the theoretical model in order to extract kinetic information. A TAP pulse response defines the exit flows of specific molecules from the micro-reactor as a function of time. A TAP response starts from zero (the reactor is initially evacuated) passes through a maximum and reaches zero again (all molecules are evacuated in the end). Both intensity and shape of such curve can be analysed using moments, which are integrals of the observed TAP pulse responses weighted with a different power of time as demonstrated in Equation S3.

Equation S3
The 0 th moment (M0) is the integral of observed exit flow and has the units of moles. It is used to determine the total number of molecules leaving the reactor after a single pulse (used to calculate conversion).

Equation S4
According to Equation S4, the diffusional response is determined by only one parameter, which can be used for fitting experimental curves. This parameter can be determined by calculating the first moment of the response, M1, which gives the residence time in the reactor, as defined in Equation S5:

Equation S5
The purely diffusional residence time (first moment) depends on temperature and molecular weight of the probe molecule according to the Knudsen diffusion theory as is defined by The thin zone TAP reactor (TZTR) configuration, where the catalyst zone is made very narrow with respect to the entire length of the micro-reactor thereby allowing a near uniform catalyst composition 6 , was adopted for the experiments. The specifics of the model are reported elsewhere 7 , as is the methodology for the derivation of the basic kinetic coefficients, and how these are used to determine the adsorption rate constant, ka . 8